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This is to certify that the

dissertation entitled

THERMOSET POLYMER-LAYERED

SILICIC ACID NANOCOMPOSITES
presented by

Zhen Wang

has been accepted towards fulfillment
of the requirements for

 

 

Ph.D. degree in Chemistry

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ma alumnus-p.14

THERMOSET POLYMER-LAYERED SILICIC
ACID NANOCOMPOSITES

By

Zhen Wang

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry

1 997

ABSTRACT
THERMOSET POLYMER-LAYERED SILICIC ACID NANOCOMPOSITES
By

Zhen Wang

Nanocomposites are formed when phase mixing occurs on a nanometer length
scale. Due to the improved phase morphology and interfacial properties, nanocomposites
exhibit mechanical properties superior to conventional composites. Toyota researchers
first demonstrated that organoclay could be exfoliated in a nylon-6 matrix to greatly
improve the thermal and mechanical properties of the polymer, which has resulted in a
practical application in the automobile industry. A great deal of research has been
conducted on organic-inorganic hybrid composites in which smectite clays are used as
reinforcement agents. However, little work has been devoted to derivatives of other
layered inorganic solids. In the present work, the first examples of organic polymer-
layered silicic acid nanocomposites have been prepared by formation of a cured epoxy
polymer network in the presence of organo cation exchange forms of magadiite.

The exfoliation of silicate nanolayers in the epoxy matrix was achieved by in-situ
intragallery polymerization during the thermosetting process. In general, the tensile
properties, solvent resistance, barrier properties and chemical stability of the polymer
matrix are greatly improved by the embedded silicate nanolayers when the matrix is
flexible (sub-ambient Tg). The improvement of properties are dependent on the silicate
loading, the degree of nanolayer separation and interfacial properties. Interestingly, the
exfoliation also affects the polymer elasticity in a favorable way. The mechanism leading
to nanocomposite formation is proposed. One exfoliated epoxy-magadiite

nanocomposite/composition possessed unique transparent optical properties

The exfoliation chemistry was successfully extended to the other members of the
layered silicic acid family. A new approach also was developed to prepare thermoset
epoxy polymer-layered silicate nanocomposites in which curing agents can be directly
intercalated into the intragallery without the need for alkylammonium ions on the
exchange sites. This new development has resulted in a greater improvement in the
overall properties of thermoset polymer—clay nanocomposites.

The exfoliation chemistry was extended further to other thermoset silicone
polymer systems. The new polysiloxane-layered silicic acid nanocomposites were
prepared with promising mechanical properties.

Some fundamental chemistry and physics issues regarding nanocomposite
formation were elucidated by this research work, particularly with regard to the
relationship of microstructure and interfacial factors to the mechanical properties of the

nanocomposites.

To my parents, my sister and my wife

for all their love and support.

iv

ACKNOWLEDGMENTS

First and foremost, I would like to thank my advisor, Prof. Thomas J. Pinnavaia
for his support and guidance over the years. I am grateful for his patience and inspiration
during the experimental work and the shaping of this thesis. I would also like to thank
Prof. Smith, Prof. Baker, Prof. Cukier and Prof. Giacin for serving on my committee.

I would like to express my true appreciation to all the Pinnavaia group members,
past and present. I would especially like to thank Dr. Tie Lan for his friendship and
helpful discussions, Prof. J. Wang for his assistance with TGA measurements, W. Zhang

and Dr. Kim for their assistance with N2 adsorption-desorption measurements, Dr.

Massam for revising part of this thesis, Dr. Severin for her assistance with formatting my
thesis, and Beth for her help with improving my English skills.

I am grateful to the staff of the MSU Composite Materials & Structures Center for
their assistance in obtaining the mechanical testing data. The technical assistance of Dr.
Huang of the x-ray powder diffraction facility is also greatly appreciated.

Financial support given by the Department of Chemistry Michigan State
University, the Center for Fundamental Materials Research. National Science Foundation
and N anocor is gratefully acknowledged and appreciated.

My deepest gratitude goes to my parents and my sister. From a distance, I have
always felt and received their support and encouragement over the last five years.

Finally, I am grateful to my wife Tao for all the love and encouragement she has
given me over the years. Her patience and understanding during my over-night lab work

were essential for the completion of this work.

TABLE OF CONTENTS
LIST OF TABLES ............................................................................................................ xi
LIST OF FIGURES .......................................................................................................... xiii
CHAPTER 1 INTRODUCTION .................................................................................... l
1.1 Nanocomposite Materials ......................................................................... 1
1.1.1 Nanocomposite Concept ............................................................... 1
1.1.2 Performance Properties of Nanocomposites ................................. 1
1.1.3 Formation of Nanocomposite Materials Through Sol-Gel
Approach ....................................................................................... 2
1.2 Polymer-Clay Nanocomposites ................................................................. 2
1.2.1 Layered Structures Suited for Nanocomposite Formation ............ 2
1.2.2 Introduction to Smectite Clay Structure ........................................ 3
1.2.3 Organo Clay Structures and Properties ......................................... 5
1.2.4 Types of Polymer-Clay Composites ............... 7
1.2.5 Intercalated Polymer-Clay Nanocomposites ................................. l 1
1.2.6 Exfoliated Nylon-6-Clay Nanocomposites ................................... 13
1.2.7 Recent Advances on Polymer-Exfoliated Clay
Nanocomposite Formation ............................................................ 16
1.3 Research Objectives .................................................................................. 19
1.3.1 Current Issues and New Directions ............................................... 19
1.3.2 Layered Silicic Acid Structures .................................................... 22
1.3.3 Research Goals .............................................................................. 26
1 .4 References ..................................................................................... 28
CHAPTER 2 HYBRID ORGANIC-INORGANIC NANOCOMPOSITES
FORMED FROM AN EPOXY POLYMER AND A LAYERED
SILICIC ACID (MAGADIITE) ................................................................ 3]
2.1 Introduction ............................................................................................... 3 1

vi

 

2.2
2.3

2.4
2.5

CHAPTER 3

3.1
3.2
3.3

3.4
3.5

Experimental ............................................................................................. 32

Results and Discussion .............................................................................. 35
2.3.1 Synthesis of Organo Magadiites ................................................... 35
2.3.2 Exfoliation of Magadiite Nanolayers in an Epoxy Polymer

Matrix ............................................................................................ 40

2.3.3 Performance Properties of Exfoliated Epoxy-Magadiite
Nanocomposites ............................................................................ 43

Conclusions ............................................................................................... 52

References ................................................................................................. 54

MECHANISM OF LAYERED SILICIC ACID NAN OLAYER

EXFOLIATION IN EPOXY-MAGADIITE NANOCOMPOSITES ....... 56
Introduction ............................................................................................... 56
Experimental ............................................................................................. 57
Results and Discussion .............................................................................. 60
3.3.1 Synthesis of Organo Magadiites With Paraffin Structures ........... 60
3.3.2 Swelling Properties of Organo Magadiites ................................... 65
3.3.3 Exfoliation of C18A1M-Magadiite in an Epoxy Matrix .............. 69
3.3.4 Mechanism of Silicate Nanolayer Exfoliation For Organo
Magadiites ............... 71
3.3.5 Effect of Gallery Acidity of Organo Magadiites on
Formation of Nanostructures ........................................................ 73
3.3.6 Relationship Between Nanostructures and Performance
Properties ...................................................................................... 74
3.3.7 Effect of Secondary Alkylamine and Alkylammonium ions
on Mechanical Properties .............................................................. 81
3.3.8 Effect of Alkylammonium ions on Thermal Stability of
Nanocomposites ............................................................................ 85
3.3.9 More Evidences for Morphological Differences Between
Intercalated and Exfoliated Nanocomposites ................................ 87
Conclusions ............................................................................................... 90
References ................................................................................................. 91

vii

 

CHAPTER 4 HYBRID ORGAN IC-INORGANIC NAN OCOMPOSIT ES
FORMED FROM AN EPOXY POLYMER AND LAYERED

4.1
4.2

4.3

4.4
4.5

SILICIC ACIDS (KENYAITE AND ILERITE) ...................................... 92
Introduction ............................................................................................... 92
Experimental ............................................................................................. 93
4.2.1 Materials ........................................................................................ 93
4.2.2 Synthesis of Layered Silicic Acids and Their Derivatives
4.2.3 Preparation of Epoxy-Layered Silicic Acid (Kenyaite and
Ilerite) Composites ........................................................................ 95
4.2.4 Characterization Methods ............................................................. 96
Results and Discussion .............................................................................. 96
4.3.1 Exfoliation of Kenyaite Nanolayers in an Epoxy Polymer
Matrix ............................................................................................ 96
4.3.1.1 Synthesis of Organo Kenyaites ......................................... 96
4.3.1.2 Exfoliation of Cl8-Kenyaite-PF in an Epoxy
Polymer Matrix ................................................................. 103
4.3.1.3 Exfoliation of CnH2n+1NH3+ICnH2n+1NHz-kenyaites
in an Epoxy Polymer Matrix ............................................. 106
4.3.1.4 Effect of Gallery Acidity of Organo Kenyaites on
Exfoliatlon ............... 109
V 4.3.2 Exfoliation of Ilerite Nanolayers in an Epoxy Polymer
Matrix ............................................................................................ I 12
4.3.2.1 Synthesis of Organo Ilerites .............................................. l 12
4.3.2.2 Exfoliation of C18-Ilerite in an Epoxy Polymer
Matrix ..... . .......................................................................... 1 14
4.3.3 Performance Properties of Exfoliated Epoxy-Kenyaite and
Ilerite Nanocomposites ................................................................. 1 17
4.3.3.] Effect of Chain Length on Tensile Properties ................... 1 17
4.3.3.2 Effect of Extent of Layer Separation on
Performance Properties ..................................................... 1 18
4.3.3.3 Effect of Nanolayer Thickness on Performance
Properties .......................................................................... 12]
Conclusions ............................................................................................... l 25
References ................................................................................................. l 26

viii

In.

 

CHAPTER 5

5.1
5.2

5.3

5.4
5.5

CHAPTER 6

6.1
6.2

6.3

HYBRID ORGANIC-IN ORGANIC NANOCOMPOSITES
FORMED FROM AN EPOXY POLYMER AND AN AMINE
CURING AGENT (JEFFAMINE) INTERCALATED IN

PROTON FORMS OF LAYERED SILICIC ACIDS .............................. 127
Introduction ................................ 127
Experimental ............................................................................................. 130
5.2.1 Materials ........................................................................................ 130
5.2.2 Synthesis of Layered Silicic Acids and Their Derivatives ........... 130
5.2.3 Preparation of Epoxy-Layered Silicic Acid Composites

Using Jeffamine-H-Layered Silicic Acid Intercalates .................. 136
5.2.4 Characterization Methods ............................................................. 136
Results and Discussion .............................................................................. 137
5.3.1 Synthesis of Jeffamine-H-Layered Silicic Acid Intercalates ........ 137

5.3.2 Exfoliation of Layered Silicic Acid N anolayers in an Epoxy
Polymer Matrix Using Jeffamine—H-Layered Silicic Acid
Intercalates .................................................................................... 1 49

5.3.3 Performance Properties of Exfoliated Epoxy-Layered
Silicic Acid Nanocomposites Prepared by the Proton

Exchanged Pathway ...................................................................... 161
Conclusions ............................................................................................... I 70
References ................................................................................................. l7 1

HYBRID NANOCOMPOSITES FORMED FROM AN
ORGAN OFUNCT IONAL POLY(DIMETHYLSILOXANE)
POLYMER AND PROTON FORMS OF LAYERED SILICIC
ACIDS; AND THEIR USE FOR THE FORMATION OF HIGH

SURFACE AREA SILICAS ..................................................................... 173
Introduction ........ . ...................................................................... . ............... 173
Experimental ............................................................................................. 175
6.2.1 Materials ....................................................................................... 175
6.2.2 Synthesis of Layered Silicic Acids ............................................... 176
6.2.3 Preparation of PDMS-Layered Silicic Acid Composites .............. 177
6.2.4 Characterization Methods ............................................................. 179
Results and Discussion ............... . .............................................................. 179

ix

 

6.4
6.5

6.3.1 Synthesis of PDMS-H-Layered Silicic Acid Intercalates ............. 179
6.3.2 Exfoliation of Layered Silicic Acid Nanolayers in a Cured

PDMS Matrix ................................................................................ 182
6.3.3 Performance Properties of Exfoliated PDMS-Layered

Silicic Acid Nanocomposites ........................................................ 191
Conclusrons ................................ 194
References ................................................................................................. 195

--_m

'J'J'.4

 

LIST OF TABLES

Table 1.1 Polymers Reported to Form Intercalated Polymer-Clay
Nanocomposites ........................................................................................ 12

Table 1.2 Mechanical and Thermal Properties of Nylon 6-Clay Composites .......... 14

Table 2.1 Intercalates Formed by Reaction of Na+-Magadiite with
C18H37NH3+CI' and C13H37NH2 ....................... 36

 

Table 2.2 Chemical and Solvent Resistance of Epoxy-Exfoliated Magadiite
Nanocomposites Prepared From C18-magadiite-PF (9.1 wt %).
Values are the Immersion Weight Gain (wt %) after a Certain
Period. ....................................................................................................... 50

Table 3.1 Intercalates Formed by Reaction of Na+-magadiite with
CH3(CH2)17NH3-n(CH3)n+Cl' andCH3(CH2)17NH2-n(CH3)n (n =
0, 1, 2) or CH3(CH2)17N(CH3)3+Br (n = 3) ............................................ 63

Table 3.2 Basal spacings (d001, A) for CH3(CH2)17NH3-n(CH3)n+-magadiite
, Paraffin Structures Under Air-dried Conditions and Solvated by
Epoxy Resin (EPON 828), Poly(oxypropy1eneamine) (D-2000)
Curing Agent, and a Stoichiometric Mixture of These Polymer
Precursors .................................................................................................. 66

Table4.1 Intercalates Formed by Reaction of K+-Kenyaite with
CnH2n+1NH3+C1' and CnHzmlNHz ................................... 100

Table 4.2 Effect of Chain Length of CH3(CH2)n-1NH3+/CH3(CH2)n-lNHz
Gallery Surfactant on Tensile Properties of Epoxy-Kenyaite
Nanocomposites. The organo kenyaite loading is 15 wt % for each
composite. ................................................................................................. 118

Table 4.3 Chemical and Solvent Resistance of Epoxy-Layered Silicic Acid
Nanocomposites Prepared From C18-magadiite-PF (9.1 wt %),
Cl8-kenyaite-PF (10.4 wt %) and C18-ilerite (10.0 wt %). Values
are the Immersion Weight Gains (wt %) after a Certain Period. .............. 124

Table 5.1 Synthesis of Alkali-Metal Forms of Layered Silicic Acids ...................... 131

xi

 

I, 1;. .

Table 5.2 Synthesis of JEFFAMINE—H-Magadiite Intercalates Useful for
Preparation of Epoxy-Exfoliated Layered Silicic Acid
N anocomposites ........................................................................................ l 33

Table 5.3 Synthesis of JEFFAMINE-H-Kenyaite Intercalates Useful for
Preparation of Epoxy-Exfoliated Layered Silicic Acid
Nanocomposites ........................................................................................ 1 34

Table 5.4 Synthesis of JEFFAMINE-H-Ilerite Intercalates Useful for
Preparation of Epoxy-Exfoliated Layered Silicic Acid
Nanocomposites ........................................................................................ 1 35

Table 5.5 Basal Spacings (A) of As-Synthesized Intercalates Formed From

Layered Silicic Acids and JEFFAMINE D-Series and T-Series .............. 146
Table 5.6 Synthesis Conditions for Formation of Powdered Forms of As-
Synthesized JEFFAMINE-H-Layered Silicic Acid Intercalates ............... 148

Table 5.7 Average Silicate Layer Separation (A) for Regularly Exfoliated
Epoxy-Magadiite Nanocomposites Prepared form D2000-H-
magadiite Intercalates ............................................................................... 156

Table 5.8 Chemical and Solvent Resistance of Epoxy-Exfoliated Layered
Silicic acid Nanocomposites Prepared From D2000-H-silicic acid
intercalates. The silicate (SiOz) loadings for each composite are
4.4 wt %, 6.4 wt % and 6.6 wt % for ilerite, magadiite and kenyaite
respectively. Values are the Immersion Weight Gain (wt %) after
Certain Uptake Periods. ............................................................................ 169

Table 6.1 Synthesis of DMSAlZ-H-layered Silicic Acid Intercalates ...................... 178

xii

 

13"? J“!

LIST OF FIGURES

Figure 1.1 The layered framework of smectite clays. Each layer consists of
two tetrahedral sheets cross-linking a central octahedral sheet.

M“+-xH20 represents the interlayer exchangeable cation with its

coordination water molecules. Each silicate nanolayer (sheet) has
a 200 nm ~ 2 pm of length in a x b dimensions, and about 1 nm of
layer thickness ........................................................................................... 4

Figure 1.2 Orientations of alkylammonium ions in the galleries of layered
silicates with different layer charge densities. .......................................... 6

Figure 1.3 Schematic illustrations of (A) a conventional; (B) an intercalated;
and (C) an exfoliated polymer-clay nanocomposites. The clay
layers adopt an aggregated, intercalated, and exfoliated
morphology, respectively, in each type of composite. The clay
interlayer spacing is fixed in an intercalated nanocomposite, on the
other hand, the average gallery height is determined by clay
silicate loading in an exfoliated nanocomposite. ...................................... 8

Figure 1.4 Schematic illustration of (A) a disordered exfoliated and (B) an
ordered exfoliated polymer—clay nanocomposites. The onium ion
is represented by 6 and the polymer chain by
(Polym.). he is the average gallery height for the exfoliated
polymer-clay nanocomposites. ho is the gallery height expected
for a lipid-like bilayer of onium ions. ....................................................... 10

 

Figure 1.5 Injection-molded NCH (nylon 6-clay hybrid) timing belt cover. ............. 15

Figure 1.6 Proposed model for the torturous zigzag diffusion path in an
exfoliated polymer-clay nanocomposite when used as a gas barrier. ....... 18

Figure 1.7 Proposed model for the fracture of (A) a glassy and (B) a rubbery
polymer-clay exfoliated nanocomposite with increasing strain ................ 18

Figure 1.8 Interfacial interactions in a polymer-layered silicate
nanocomposite: (A) Electrostatic binding of onium ion to the clay
surfaces; (B) "Dissolution" of the alkyl chains into the polymer
matrix by van der Waals forces; (C) Physical sorption of polymer
to the siloxane oxygen atoms of the basal surfaces; (D) Polymer
sorption to hydroxyl-terminated edge sites. .............................................. 21

xiii

 

fi—m-‘f- ' 3

higure 1.9 Magadiite structure postulated by Garces et al. The presence of a

bilayer of interlayer water molecules is based on the basal spacing
of 15.6 A for Na+-magadiite. .................................................................... 23

Figure 1.10 Schematic representation (edge-view) of the layered silicic acids.
The silicate layers are formed by sharing of apical oxygen atoms
between tetrahedral SiO4 sheets for ilerite, magadiite and kenyaite.
The thickness of the layers differs depending on the degree of
cross-linking between stacked sheets ........................................................ 25

Figure 2.1 X-ray powder diffraction patterns of (A) air-dried Na+-magadiite;

(B) CH3(CH2)17NH3+-magadiite with a lateral monolayer (LM)
structure. Patterns (C) and (D) are for mixed
CH3(CH2)17NH3+/CH3(CH2)17NH2-magadiite with a paraffin
(PF) and a lipid bilayer (BL) gallery structure, respectively. The
LM structure contains no free amine (0.45 onium ions per Si14029
formula unit), whereas PF and BL structures contain neutral
primary amine molecules, in addition to primary onium ions. ................. 37

Figure 2.2 Therrnogravimetric analysis curves of (A) air-dried N a+-magadiite;
(B) CH3(CH2)17NH3+-magadiite with a lateral monolayer (LM)

structure. Patterns (C) and (D) are for mixed
CH3(CH2)17NH3+/CH3(CH2)17NH2-magadiite with a paraffin
(PF) and a lipid bilayer (BL) gallery structure, respectively. ................... 38

Figure 2.3 X-ray diffraction pattern of the initial mixture product in an effort
to obtain C18-magadiite-PF. The mixture was formed when Na+-
magadiite was ion exchanged by
CH3(CH2)17NH3+ICH3(CH2)17NH2 followed by ethanol washing.
The tabulated values in the insert are (1001 values for the lipid
bilayer and paraffin phases. ...................................................................... 439

Figure 2.4 X-ray diffraction patterns of Cl8-magadiite-PF in different
physical states: (A) pristine organo magadiite; (B) 10 wt % organo
magadiite solvated by epoxide resin (EPON 828) at 75 0C for 90
min. ........................................................................................................... 41

Figure 2.5 X-ray diffraction patterns of products prepared by reaction of (A)
C18-magadiite-PF (15 wt % loading) with epoxide resin and
poly(oxypropyleneamine) curing agent at an epoxide group to
amine group molar ratio of 2:1. Reaction conditions were as
follows: (B) 25 0C, 60 min; (C) 75 0C, 5 min; (D) 75 0C, 15 min;

(B) 75 oC, 3 h; (F) 75 0C, 3 h and 125 0C, 5 min; (G) 75 0C, 3 h
and 125 0C, 10 min. The 001 lines marked with an asterisk are
from the initial paraffin structure of the organo magadiite. ...................... 42

F'lgure 2.6 X-ray diffraction pattern of a cured epoxy polymer-magadiite

nanocomposite prepared from C l8-magadiite-PF. The organo
srlrcate content was 15 wt %. Polymer curing was carried out at 75

xiv

 

r‘_ d ‘-

0C for 3 h, and followed by 3 h at 125 0C. ............................................... 44

Figure 2.7 Comparison of the tensile strengths of an epoxy-exfoliated
magadiite nanocomposite prepared from C18-magadiite-PF and

conventional magadiite composites prepared from Na+-magadiite
and Cl8—magadiite-LM. The inorganic silicate loading was

determined by calcining the composites in air at 650 0C for 4 hours

using a heating rate of 2 0C/rnin. The onium ion and amine
content of the organo magadiite was counted as contributing to the
stoichiometry of epoxide cross-linking ..................................................... 45

Figure 2.8 X-ray diffraction patterns of products prepared by reaction of (A)
C18-magadiite-BL (15 wt % loading) with epoxide resin and
poly(oxypropyleneamine) curing agent at an epoxide group to
amine group molar ratio of 2:1. Reaction conditions were as

follows: (B) 75 0C, 60 min; (C) 75 0C, 120 min. The inset is a
pattern for the cured epoxy polymer-magadiite nanocomposite
prepared from C18-magadiite-BL. The marked sharp lines in
pattern (B) are derived from the octadecylarnine ...................................... 47

Figure 2.9 Effect of octadecylammonium and octadecylarnine on the tensile
modulus of epoxy-exfoliated magadiite nanocomposites prepared
from C18-magadiite-PF. The solid line is for composites formed
by including the onium ion and neutral amine content of the
organo magadiite as a curing agent for epoxide crosslinking and
reducing the amount of poly(oxypropyleneamine) accordingly; the
dash line is for composites formed by disregarding the cross-
linking reactivity of the gallery onium ion and amine in the initial
intercalate and curing the resin with 1 equiv of
poly(oxypropyleneamine). The values given in parenthesis for
curve B are the molar fractions of excess alkylamine functional
groups contributed by the onium ions and free amine relative to
poly(oxypropyleneamine). ........................................................................ 48

Figure 2.10 Effect of octadecylammonium and octadecylarnine on the strain-at-
break values of epoxy-exfoliated magadiite nanocomposite
prepared from C18-magadiite-PF. The dashed line is for
composites formed by not counting onium ion and amine content
of the organo magadiite as contributing to the stoichiometry for
epoxide crosslinking, the solid line is reversed. The values in
parenthesis give the amount of excess amine curing agent due to
the organo magadiite component. ............................................................. 49

Figure 2.11 Chemical reagent and solvent uptake of (A) 30% sulfuric acid; (B)
5% acetic acid; (C) methanol; and (D) toluene by epoxy-exfoliated
magadiite nanocomposites prepared from C18-magadiite-PF (9.1
wt %). ........................................................................................................ 51

1“gm 2.12 Comparison of the optical properties among (A) a pristine epoxy
polymer; (B) an epoxy-exfoliated magadiite nanocomposite
prepared form C18-magadiite-PF; (C) an epoxy-exfoliated smectite

clay nanocomposite prepared from CH3(CH2)17NH3+ ion-

XV

.U “up.“ A"!

 

Figure 3.1

Figure 3.2

Figure 3.3

Figure 3.4

Figure 3.5

Figlu‘e 3.6

exchange montmorillonite (from Wyoming, cf. reference 21). The
thickness of each sample is ~ 1 mm. Polymer curing was carried

out at 75 0C for 3 h, and followed by 3 h at 125 0C. .............................

X-ray powder diffraction patterns of air-dried Na+-magadiite and
CH3(CH2)17NH3-n(CH3)n+~intercalated magadiites (n = 0, 1, 2, 3).
The organo magadiites have paraffin-like gallery structures with

onium ion and free amine compositions given in Table 1. ....................

Effect of washing on the formation of C18AlM-magadiite. The
product in Pattern (A) was obtained by centrifuging the secondary
alkylammonium exchanged reaction mixture without further
treatment. The product in pattern (B) was formed when the onium
ion exchanged reaction mixture was washed with an equal volume
of ethanol. The paraffin phase in pattern (C) was obtained by

washing the product of pattern (B) with 50% EtOH ..............................

X-ray diffraction patterns of products prepared by reaction of
C18A2M-magadiite (15 wt % loading) with a stoichiometric
mixture of epoxide resin and poly(oxypropyleneamine) curing
agent under the reaction conditions as follows: (A) 75 0C, 10 min;

(B) 75 0C, 90 min; (C) 75 0C, 120 min; (D) 75 0C, 3 h and 125 0C,

20 rmn. ...................................................................................................

X-ray diffraction patterns of products prepared by reaction of
Cl8A3M-magadiite (20 wt % loading) with a stoichiometric
mixture of epoxide resin and poly(oxypropyleneamine) curing

agent under the reaction conditions as follows: (A) 75 0C, 10 min;

(B) 75 0C, 30 min; (C) 75 0C, 45 min ....................................... ‘ .............

X-ray diffraction patterns of products prepared by reaction of
C18A1M-magadiite (15 wt % loading) with a stoichiometric
mixture of epoxide resin and poly(oxypropyleneamine) curing
agent (epoxide group to amine group molar ratio of 2:1). Reaction
conditions were as follows: (A) 25 0C, 30 min; (B) 75 0C, 10 min;
(C) 75 0C, 15 min; (D) 75 0C, 30 min; (E) 75 0C. 60 min; (F) 75

0C, 70 min; (G) 75 0C, 90 min ...............................................................

Proposed pathway for epoxy-magadiite exfoliated nanocomposite
formation: (A) An organo magadiite with paraffin-like gallery
structure of onium ions and neutral amine (see Table 3.1 for
compositions). (B) An intercalate formed when long chain
alkylammoniums reorienting from a paraffin to a lipid bilayer
orientation to accommodate the co—intercalation of epoxide and
curing agent. (C) The gallery height exceeds the value
characteristic of a lipid bilayer due to the rapid intragallery
polymerization rate. The silicate platelets start to tilt. The chain
formation (gelation) is the main reaction for the epoxide and amino
groups at this stage of the process. (D) Silicate nanolayers are
completely exfoliated in a fully cross-linked epoxy polymer

...53

...62

...64

...67

68

...70

network ...................................................................................................... 72

xvi

 

Egure 3.7 X-ray diffraction patterns of cured epoxy polymer-magadiite

nanocomposites prepared from CH3(CH2)17NH3-n(CH3)n+-
magadiites with a paraffin gallery structure (cf. Table 3.1). The
organo magadiite loading is 15 wt % (n = 0) or 20 wt % (n = 1, 2,
3). Polymer curing was carried out at 75 0C for 3 h, followed by 3
h at 125 OC. The primary and secondary onium ions (11 = 0, 1)
give highly exfoliated (disordered) nanocomposites (d > 90 A).
The tertiary onium ion derivative (11 = 2) also gives an exfoliated
nanocomposite, but the layer separation is highly ordered ((1 = 78

). The quaternary onium ion exchange form (n = 3) forms an
intercalated nanocomposite (d = 41 A). .................................................... 76

Figure 3.8 A comparison of the tensile strengths of epoxy-magadiite
nanocomposites prepared from CH3(CH2)17NH3_n(CH3)n+-
magadiites. The magadiite silicate loading (expressed on wt %
SiOz) was determined by calcining the composites in air at 650 0C
for 4 hours using a heating rate of 2 oC/min. The secondary
alkylammonium (n = 2) and the free amine content of

CH3(CH2)17NH2CH3+-magadiite was counted as contributing to

the stoichiometry for epoxide cross—linking at magadiite loadings >
10 wt %. .................................................................................................... 77

Figure 3.9 X-ray diffraction patterns of CH3(CH2)n-1N(CH3)3+-magadiites in
different physical states: (A) organo magadiite; (B) intercalated
magadiite (10 wt %) in an cured epoxy nanocomposite. .......................... 78

Figure 3.10 A comparison of the tensile strengths of epoxy-magadiite
intercalated nanocomposites prepared from CH3(CH2)n_

1N(CH3)3+-magadiites. The magadiite silicate loading was
determined by calcining the composites in air at 650 0C for 4 hours
using a heating rate of 2 oC/rnin. .............................................................. 79

I:‘igure3.ll Toluene uptake by nanocomposites prepared from

CH3(CH2)17NH3-n(CH3)n+-magadiites (r1 = 1, 2, 3). The tabulated
values in the insert are equilibrium data determined by the
immersion weight gain after 24 hr. The loading of organo-
magadiite is 10 wt % for each epoxy—magadiite composite ...................... 80

1:lgure 3.12 A comparison of (A) the tensile strengths and (B) the tensile
moduli between epoxy-magadiite exfoliated nanocomposites
prepared from C18AlM-magadiite and Cl8-magadiite-PF, and
epoxy—smectite clay exfoliated nanocomposite prepared from
CH3(CH2)17NH3+ ion-exchange montmorillonite (from
Wyoming). The silicate loading was determined by calcining the

composites in air at 650 0C for 4 hours using a heating rate of 2

xvii

 

klgure 3.13 Effect of methyloctadecylarnmonium and methyloctadecylamine
on (A) the tensile strengths; (B) the tensile moduli; (C) the strain-
at-break values of epoxy-exfoliated magadiite nanocomposites
prepared from C18A1M-magadiite. The solid line is for
composites formed by including the secondary onium ion and
neutral amine content of the organo magadiite as curing agents for
epoxide cross-linking and reducing the amount of
poly(oxypropyleneamine) (D-2000) accordingly; the dash line is
for composites formed at an epoxy-D-2000 stoichiometry that
disregards the reactivity of the gallery secondary onium ion and
amine in the initial intercalate. The values in parenthesis for the
dash curves are the molar fraction of excess secondary amine
functional groups relative to D-2000. ....................................................... 83

Figure 3.14 Comparison of the stain-at-break values among the exfoliated
epoxy-magadiite nanocomposites prepared from C18A1M-
magadiite, the intercalated nanocomposites prepared from
C18A3M-magadiite, and the conventional composites prepared
from C18-magadiite-LM with a lateral monolayer structure .................... 84

Figure 3.15 Thermogravimetric analysis curves for (A) epoxy-exfoliated
magadiite nanocomposite prepared from C18A1M-magadiite and
(B) intercalated nanocompsoite formed from C18A3M-magadiite.
The loading of organo magadiite is 20 wt % for each epoxy-
magadiite nanocomposite. Curves (C) and (D) are for the pristine
organo magadiites of C18A1M-magadiite and C18A3M—magadiite,
respectively. The analysis was carried out in N 2 atmosphere with a

heating rate of 5 oC/min. ........................................................................... 86

Figure 3.16 X-ray powder diffractions for the silica formed fromvupon
calcination of (A) the exfoliated nanocomposite prepared form
C18A-magadiite-PF; and (B) the intercalated nanocomposite
prepared from C18A3M-magadiite. The expanded inset for the
d001 peaks was obtained using a step scan mode. The calcination

condition is at 650 0C for 4 h in air using a heating rate of 2
oC/rnin ....................................................................................................... 88

Figme 3.17 N2 adsorption-desorption isotherms for the silicas obtained by

calcination of an epoxy-exfoliated magadiite nanocomposite
prepared from C18-magadiite-PF (top curve) and an intercalated
nanocomposite prepared from C18A3M—magadiite (bottom curve).
The organo magadiite loading for each composite is 10 wt %. The

calcination was carried out at 650 0C for 4 hr in air using a heating
. rate of 2 0C/rnin ......................................................................................... 89
Flgure 4.1 X-ray powder diffraction patterns of air-dried K+-kenyaite and

CH3(CH2)n-1NH3+ICH3(CH2)n- 1N Hz-kenyaites. The gallery
structures and compositions are given in Table 4.1. ................................. 99

Figure 4.2 Thermogravimetric analysis curves for air-dried K+-kenyaite and
CH3(CH2)n-1NH3+/CH3(CH2),,-1N Hz-kenyaites. The gallery

xviii

 

structures and compositions derived from these curves are give in
Table 4.1 .................................................................................................... 101

Figure 4.3 X-ray powder diffraction patterns of (A) C8—kenyaite-GB (gauche
block structure) obtained by applying a film of the wet product on
a glass slide right after centrifuging the reaction mixture; (B) air-
dried C8-kenyaite-GB with a less ordered gauche block gallery
structure; (C) CH3(CH2)7NH3+-kenyaite with a lateral bilayer
gallery structure obtained by thorough washing of the C8-kenyaite-
GB to remove excess amine ...................................................................... 102

Figure 4.4 X-ray diffraction patterns of (A) pristine C18-kenyaite-PF and (B-
E) the partially cured composites prepared by reaction of C18-
kenyaite-PF (14.6 wt % loading) with a stoichiometric mixture of
epoxide resin (EPON 828) and poly(oxypropyleneamine)
(Jeffamine D-2000) curing agent. The primary alkylammonium
and the free amine content of C18-kenyaite-PF was counted as
contributing to the stoichiometry for epoxide cross-linking.
Reaction conditions were as follows: (B) 25 0C, 60 min; (C) 75 0C,

30 min; (D) 75 0C, 40 min; (B) 75 0C, 120 min. ...................................... 104

Figure 4.5 X-ray diffraction pattern of a cured epoxy polymer-kenyaite
nanocomposite prepared from C18-kenyaite—PF. The organo
kenyaite loading was 14.6 wt %. Polymer curing was carried out

at 75 0C for 3 h, and followed by 3 h at 125 0C. The sharp lines
are the in-plane reflections of the exfoliated kenyaite layers .................... 105

Figure 4.6 XRD patterns of partially cured composites prepared by reaction of
CH3(CH2)n_1NH3+ICH3(CH2)n-1NH2-kenyaites (n = 6, 8, 10, 12
and 18) with a stoichiometric mixture of epoxide resin and

poly(oxypropyleneamine) curing agent. The organo kenyaite
loading is 15 wt % for n = 6, 10, 12, 18, and 30 wt % for n = 8.

The reactions were carried out at 25 0C for 5 min to 60 min for n S

12, and at 75 0C, 30 min for n = 18. The sharp lines in the patterns
for n = 10, 12 are attributed to phase-segregated amine. .......................... 107

Figure 4.7 X-ray diffraction patterns of cured epoxy-kenyaite nanocomposites
prepared from CH3(CH2)n-1NH3+/CH3(CH2)n-1NH2-kenyaites (n
= 6, 8, 10, 12 and 18). The organo kenyaite loading for each
composite was 15 wt %. Polymer curing was carried out at 75 0C
for 3 h, and followed by 3 h at 125 0C ...................................................... 108

Figure 4.8 X-ray powder diffraction patterns of CH3(CH2)17NH3-n(CH3)n+-

kenyaites (n = 1, 2, 3). All of the organo kenyaites have paraffin-
like gallery structures. ............................................................................... 1 10

F'lgure 4.9 X-ray diffraction patterns of cured epoxy polymer-kenyaite

nanocomposites prepared from CH3(CH2)17NH3-n(CH3)n+—
kenyaites with a paraffin gallery structure. The organo kenyaite
loading was 20 wt % (n = 1, 3) and 10 wt % (n = 2). Polymer

xix

 

curing was carried out at 75 0C for 3 h, followed by 3 h at 125 0C.
The secondary onium ions (11 = 1 give highly exfoliated
(disordered) nanocomposites (d > 90 ). The tertiary onium ion
derivative (11 = 2) also gives an exfoliated nanocomposite, but the
layer separation is highly ordered ((1 = 72 A). The quaternary
onium ion exchange form (n = 3) forms an intercalated
nanocomposite (d = 44 A) ................................. 111

Figure 4.10 X-ray powder diffraction patterns of air-dried (A) Na+-ilerite; (B)
H+-ilerite; and (C) CH3(CH2)17NH3+-ilerite (Cl8-ilerite). ..................... 1 13

Figure 4.11 X-ray diffraction patterns of the partially cured composites
prepared by reaction of C18-ilerite (15 wt % loading) with a
stoichiometric mixture of epoxide resin and
poly(oxypropyleneamine) curing agent. The primary
alkylammonium and the free amine content of C18-i1erite was
counted as contributing to the stoichiometry for epoxide cross—
linking. Reaction conditions were as follows: (A) 25 0C, 20 min;

(B) 75 0C, 40 min; (C) 75 0C, 100 min; (D) 75 0C, 150 min. .................. l 15

Figure 4.12 X-ray diffraction pattern of a cured epoxy polymer-ilerite
nanocomposite prepared from C18-ilerite. The organo ilerite
loading was 15 wt %. Polymer curing was carried out at 75 0C for
3 h, and followed by 3 h at 125 0C. The sharp peaks at 3.69 A and
1.87 A are the in-plane reflections of ilerite. The broad peaks at

26 = 20°, 42° are due to the polymer matrix ............................................ l 16

Figure 4.13 A comparison of the tensile strengths of epoxy-kenyaite

nanocomposites prepared from CH3( CH2)17NH3-n(CH3)n+-
kenyaites. The kenyaite silicate loading (expressed on wt % SiOZ)

was determined by calcining the composites in air at 650 0C for 4

h using a heating rate of 2 oC/min. The secondary
alkylammonium and the free amine content of

CH3(CH2)17NH2CH3+-kenyaite (n = 2) was counted as
contributing to the stoichiometry for epoxide cross-linking. .................... 1 19

Figure 4.14 Toluene uptake curves for nanocomposites prepared from

CH3(CH2)17NH3-n(CH3)n+-kenyaites (n = 1, 2, 3). The tabulated
values in the insert are equilibrium data obtained from the
immersion weight gain after 24 hr. The loading of organo-
kenyaite is 10 wt % for each epoxy-kenyaite composite. ......................... 120

Figure 4.15 Comparison of (A) tensile strengths and (B) tensile moduli for
epoxy-exfoliated ilerite nanocomposites prepared from C18-ilerite
and epoxy-intercalated ilerite composites prepared from C18A3M-
ilerite. The primary onium ion and amine content of the organo
ilerite was counted as contributing to the stoichiometry of epoxide
cross-linking .............................................................................................. 122

XX

 

lr—v-Afid' -

Figure 4.16 A comparison of the tensile strengths between epoxy-magadiite
and epoxy-kenyaite nanocomposites prepared from (A) primary;
(B) secondary; (C) tertiary; (D) quaternary alkylammonium
exchanged CH3(CH2)17NH3-n(CH3)n+-layered silicic acids. .................. 123

Figure 5.1 XRD patterns of air-dried proton exchanged forms of layered
silicic acids: (A) H+-i1erite; (B) H+-magadiite; (C)-H+-kenyaite. ............ 138

Figure 5.2 XRD patterns for the as-synthesized (unwashed) intercalates

formed from H+-magadiite and poly(oxypropyleneamines): (A)
Jeffamine D-series; and (B) Jeffamine T-series. ........................................ 140

Figure 5.3 XRD patterns for the as-synthesized intercalates (unwashed)

formed from H+-kenyaite and poly(oxypropyleneamines): (A)
Jeffamine D-series; and (B) Jeffamine T-series ........................................ 141

Figure 5.4 XRD patterns for the as-synthesized (unwashed) intercalates

formed from H+-ilerite and poly(oxypropyleneamines) Jeffamine
T-series ...................................................................................................... 142

Figure 5.5 XRD patterns of the intercalates formed from H+-kenyaite and the
poly(oxypropyleneamine) Jeffamine D4000: (A) as—synthesized
(unwashed); (B) washed one time with ethanol; and (C) washed
multiple times with ethanol ....................................................................... 144

Figure 5.6 TGA curves for H-kenyaite and D4000-H-kenyaite intercalates
with different basal spacings (cf. Figure 5.5) ............................................ 145

Figure 5.7 XRD patterns of (A) the initial D2000-H-magadiite intercalate and
the nanocomposites formed at 20 wt % D2000-H-magadiite
intercalate loading by reaction of stoichiometric mixtures of
epoxide and the poly(oxypropyleneamine) under the following
reaction conditions: (B) 25 0C, 60 min; (C) 75 0C, 60 min; (D) 75
0C, 120 min; (B) 75 0C, 150 min. ............................................................. 150

Figure 5.8 XRD patterns of (A) the initial D2000-H-kenyaite intercalate and
the nanocomposites formed at 20 wt % D2000-H-kenyaite
intercalate loading by reaction of stoichiometric mixtures of
epoxide and the poly(oxypropyleneamine) under the following
reaction conditions: (B) 25 0C, 2 min; (C) 25 0C, 10 min; (D) 75
0C, 30 min. ................................................................................................ 151

F1gure 5.9 XRD patterns of (A) the initial D2000-H-ilerite intercalate and the
nanocomposites formed at 20 wt % D2000-H-i1erite intercalate
loading by reaction of stoichiometric mixtures of epoxide and the
poly(oxypropyleneamine) under the following reaction conditions:

(B) 25 0C, 5 min; (C) 75 oC, 5 min; (D) 75 0C, 135 min. ........................ 152
F'lgure 5.10 XRD patterns for the epoxy-layered silicic acid nanocomposites

formed from the following intercalates: (A) D2000-H-ilerite; (B)
D2000-H-magadiite; (C) D2000-H-kenyaite. The D2000-layered

xxi

_ ' _Ir!"l'

 

silicic acid intercalate loadings are 20 wt %, 10 wt % and 15 wt %
for ilerite, magadiite and kenyaite, respectively. The intercalated
curing agent was counted as contributing to the stoichiometry for
epoxide cross-linking. ............................................................................... 154

Figure 5.11 XRD patterns of cured epoxy-magadiite nanocomposites (curing

conditions: at 75 0C for 3 h, and followed by 3 h at 125 0C)
prepared from D2000-H-magadiite intercalates. The D2000-H-
magadiite intercalate loadings were as follows: (A) 5 wt %; (B) 10
wt %; (C) 20 wt %; (D) 30 wt %; (E) 40 wt %; and (F) 50 wt %. ............ 155

Figure 5.12 XRD patterns for the silica formed from upon calcination of (A)
D2000-H-magadiite intercalate with a 54.6 A basal spacing and
containing 50 wt % H-magadiite; and (B) an exfoliated epoxy-
magadiite nanocomposite prepared form the D2000-H-magadiite
intercalate with a 44.0 A basal spacing and containing 5 wt % H-
magadiite. The expanded insert for the d001 peaks was obtained

using a step scan mode. The calcinations were carried out at 650
0C for 4 h in air using a heating rate of 2 °C/min. ................................... 157

Figure 5.13 N2 adsorption-desorption isotherms for the silicas obtained by

calcination of an epoxy-exfoliated magadiite nanocomposite
prepared from D2000—H-magadiite intercalate with a 44.0 A basal
spacing and containing 5 wt % H-magadiite (to curve); and the
D2000-H-magadiite intercalate with a 54.6 A basal spacing
containing 50 wt % H-magadiite (bottom curve). The calcinations

were carried out at 650 0C for 4 h in air using a heating rate of 2
°C/min. ...................................................................................................... 159

Figure 5.14 Possible types of morphologies for silicate nanolayer aggregates
formed upon calcination. The card house structure is predominant
for the magadiite recovered by calcination of an exfoliated epoxy-
magadiite nanocomposite. Restacked layers are more predominant
in the magadiite obtained by calcination of a well ordered D2000-
H-magadiite intercalate. ............................................................................ 160

Figure 5.15 Comparison of tensile properties for exfoliated nanocomposites
prepared from D2000-H-magadiite intercalates and organo
magadiite intercalates with a paraffin structure (C18-magadiite-PF
and C18AlM-magadiite). ......................................................................... 162

Flgure 5.16 Comparison of methanol uptake curves for epoxy-exfoliated
magadiite nanocomposites prepared from C18-magadiite-PF
intercalates and D2000-H-magadiite intercalates. The tabulated
values are the loadings (wt %) of C18-magadiite-PF intercalate and
D2000-H-magadiite intercalate for each composite. ................................ 164

F1gure 5.17 Comparison of toluene uptake curves for epoxy-exfoliated
magadiite nanocomposites prepared from C18-magadiite-PF
intercalates and D2000-H-magadiite intercalates. The tabulated
values are the loadings (wt %) of C18-magadiite-PF intercalate and
D2000-H-magadiite intercalate for each composite. ................................ 165

xxii

_____—

m."g‘"" ..I-‘ 7

 

1 ‘—
E
'9 I; '

Figure 5.18 Comparison of methanol and toluene uptake at equilibrium vs.
magadiite silicate (SiOZ) loading for epoxy-exfoliated magadiite
nanocomposites prepared form C18-magadiite—PF and D2000-H-
magadiite intercalates ................................................................................ 166

Figure 5.19 Tensile properties for the epoxy-kenyaite nanocomposites prepared
from D2000-H-kenyaite intercalates before and after having been
swollen in toluene. In the solvent swelling experiment the
nanocomposites were soaked in toluene for 12 h, and subsequently
dried in air for 24 h before the tensile properties were measured ............. 167

Figure 5.20 A comparison of the tensile strength vs. silicate loading for epoxy-
layered silicic acid nanocomposites prepared from DZOOO-H-
layered silicic acid intercalates. ................................................................ 168

Figure 6.1 XRD patterns of intercalates formed by reaction of aminopropyl
terminated polydimethylsiloxane (DMSA12) and (A) H-ilerite; (B)
H-magadiite; and (C) H-kenyaite in ethanol/H20 (cf. Table 6.1)
and air-dried without washing ................................................................... 181

Figure 6.2 XRD patterns of (A) the initial DMSA12-H-kenyaite intercalate
and the partially cured composites formed at 15 wt % H-kenyaite
loading by reaction of a stoichiometric mixture of DMSA12 and
DMSE12 under the following reaction conditions: (B) 50 0C, 30
min; (C) 50 0C, 22 h; (D) 50 0C, 24 h and 125 0C, 10 min. The
pattern (E) is for the fully cured PDMS-kenyaite composite that
was cured first at 50 0C for 24 h and then at 125 0C for 6 h ..................... 183

Figure 6.3 XRD patterns of the cured PDMS-exfoliated layered silicic acid
nanocomposites prepared from the intercalates formed between
DMSA12 and (A) H-magadiite; and (B) H-kenyaite. The H-silicic
acid loading for each composite is 10 wt %. Polymer curing was
carried out at 50 0C for 24 h, and followed by 6 h at 125 OC. .................. 184

Figure 6.4 XRD patterns for the silica-intercalated kenyaites obtained by
calcination of the PDMS-exfoliated kenyaite nanocomposites at

540 0C for 10 h with a heating rate of 1 0C/min. The H-kenyaite
loading was (A) 8.2 wt %; and (B) 15 wt %. Pattern (C) is for the
DMSA12-H-kenyaite intercalate after the same calcination
conditions. ................................................................................................. 1 86

Figure 6.5 N2 adsorption-desorption isotherm for the silica residue obtained

by calcining the PDMS-exfoliated kenyaite nanocomposite
prepared from a DMSA12-H-kenyaite intercalate. The H—kenyaite

loading was 8.2 wt %. The calcination was carried out at 540 0C
for 10 hr in air using a heating rate of 1 °C/min. ...................................... 187

Figure 6.6 XRD atterns of kenyaite intercalates: (A) DMSA12-H-kenyaite

(36.5 ); (B) a PDMS-intercalated kenyaite (10 wt % loading of
H-kenyarte) nanocomposite (72.8 A) ........................................................ 189

xxiii

Figure 6.7 Comparison of BET surface area for the silica residues obtained by
calcining exfoliated PDMS-kenyaite nanocomposites with
different loadings (wt %) of H-kenyaite. The open circle is the
surface area for the silica residue obtained by calcining an
intercalated PDMS-kenyaite nanocomposite with a 10 wt %

loading of H-kenyaite. The calcination was carried out at 540 0C
for 10 hr in air using a heating rate of 1 OC/rnin. ...................................... 190

Figure 6.8 Toluene uptake by the exfoliated PDMS-kenyaite nanocomposites
prepared from DMSA12-H-kenyaite intercalates. The tabulated
values in the insert are equilibrium data determined by the
immersion weight gain after 24 hr. The H-kenyaite loadings for
the composites are 5 wt % and 10 wt %. .................................................. 192

Figure 6.9 Thermogravimetric analysis curves for (A) a pristine PDMS
polymer; and (B) an exfoliated PDMS-kenyaite nanocomposite.
The loading of H-kenyaite for the composite is 10 wt %. The
analysis was carried out in N2 atmosphere with a heating rate of 5

°C/min. ...................................................................................................... 193

xxiv

Chapter 1
INTRODUCTION

1.1 Nanocomposite Materials
1.1.1 Nanocomposite Concept

Nanostructured hybrid organic-inorganic composites have attracted great attention

from both a fundamental research and applications point of view."4 In general,
composite materials are formed when at least two distinctly dissimilar materials are
mixed to form a monolith. The overall properties of a composite material are determined
not only by the parent components, but also by the composite phase morphology and
interfacial properties.2 The fiber-reinforced polymer composites are very typical and
successful examples of this composite concept. A nanocomposite is formed when phase

mixing occurs on a nanometer length scale. For conventional composites, phase mixing

occurs on a macroscopic (um) length scale. Nanocomposites are usually superior to

conventional composites owing to their unique phase morphology and improved
interfacial properties.
1.1.2 Performance Properties of N anocomposites

Improvement of strength, stiffness and toughness are the primary characteristic
Structural properties of organic-inorganic nanocomposites. Desirable properties such as
barrier properties, thermal stability, moisture stability, solvent resistance and fire
rCtardancy generally accompany the reinforcement benefit. When special inorganic

bUilding blocks and/or polymer materials are applied, the secondary characteristics of

"uUc.

2

improvement such as optical transparency, dielectric strength, nonlinear optical
properties, quantum confinement effects and electrical conductivity can also be
observed.2
1.1.3 Formation of Nanocomposite Materials Through Sol-Gel Approach
One approach to preparing organic-inorganic nanocomposites is by the sol-gel
processing method.5'8 In this method the inorganic phase is formed by the hydrolysis and
condensation of metal oxide precursors (Scheme 1) in the presence of preformed polymer
or polymer precursors which can be polymerized simultaneously. The size of inorganic
building blocks is controllable and the interfacial properties can be improved by
introducing covalent bonding between organic and inorganic phases. However, the sol-
gel approach is limited by the evolution of volatile by-product and shrinkage when the
hybrid is molded at elevated temperature. The controlling of microstructure formation
also needs great effort to be achieved and phase segregation is another issue that has to be
solved especially when a high loading of the inorganic phase is involved.
Scheme 1

Hydrolysis:

M(OR)4 + H20 —> (RO)3M-OH + ROH

Condensation:

-:M-OH + HO-M:- —> -:M-O-M:- + 1130

and/or

\ I- —-> —\M O M’— O
— _ _ - - +
’M OH + R0 M\ , \ R H

M = Si, Al, Ti, Zr

1.2 Polymer-Clay N anocomposites
1.2.1 Layered Structures Suited for Nanocomposite Formation
Inorganic materials with structures that can be broken down into nanoscale

building blocks are attractive alternatives to the sol-gel approach in the preparation of

3

hybrid organic-inorganic nanocomposites. Lamellar structures become good candidates
because of their diverse intercalation chemistry and platy morphology. Layered
structures such as M082, V205, FeOCl, layered phosphates and layered silicates are all
plausible inorganic materials which have potential applications. in the field of organic-
inorganic nanocomposites.3~9'“

Clay silicate nanolayers have a chemically stable siloxane surface with a high
surface area. They also possess high aspect ratios and high strength which are very
important indexes for use as reinforcing agents when compared with conventional
fibers.12 More importantly, their interlayer surface is easily modified by ion-exchange
reaction and the gallery can be intercalated by organic cations or polymer precursors. All
of these technical factors along with the economic considerations suggest that layered
silicate clays are particularly well suited to the design of platelet—reinforced organic-
inorganic nanocomposites.

1.2.2 Introduction to Smectite Clay Structure

Clays are 2-dimensional layered silicates of particle size < 2 pm. The 2:1 mica-

type silicates consist of 10 A-thick layers stacked face-face to form turbostratic tactoids.
Each silicate nanolayer (~ 2000 A diameter) is made up of a central sheet of edge-shared

octahedra sandwiched by two sheets of comer-shared SiO4 (Si can be replaced by Al)
tetrahedral layers. The substitution of metal ions in the octahedral or tetrahedral layer by
low valence metal ions ( e.g., 814+ by Al3+; Al3+ by Mg2+ or Fe2+; Mg2+ by Lit) results in
negative charges to the clay layers which are neutralized by gallery cations, such as N at

K+ or Ca2+. A typical smectite clay structure is represented in Figure 1.1.'2 The
hydrophilic inorganic cations in the galleries can be replaced by ion exchange reaction

with both inorganic and organic cations. Many polymers, both hydrophilic and

hydrophobic, have been intercalated into the clay gallery. 13,14

 
    

 
 

.‘ .—
'—-II- V‘
--r 72......" ._._.

M‘UA
\YA

  

 

Figure 1.1 The layered framework of smectite clays. Each layer consists of two
tetrahedral sheets cross-linking a central octahedral sheet. M“+-xH20 represents the

interlayer exchangeable cation with its coordination water molecules. Each silicate
nanolayer (sheet) has a 200 nm ~ 2 pm of length in a x b dimensions, and about 1 nm of
layer thickness.

5

1.2.3 Organo Clay Structures and Properties

The intercalation chemistry of layered silicate clays plays an important role in the
formation of polymer-clay nanocomposites. Because many thermoplastic and thermoset
polymers or their precursors are hydrophobic, the hydrophilic inorganic clay surface
usually has to be modified to accommodate the incoming organic species. Generally, an
organic cation exchange reaction is used to form a hydrophobic organo clay. Depending
in part on the layer charge density of silicate clays and the dynamic size of organic
cations, the structures adopted by organo clay are quite different. A schematic
illustration of intragallery structures for organo clays is represented in Figure 1.2,
wherein primary alkylammonium ions are used as intragallery cations. When long chain
alkylammonium ions are the exchange cations, the alkyl chain axes will lie parallel to the

siloxane surface for low charge density clays and a paraffin structure is preferred for high

charge density clays.” When the onium ion chain length becomes shorter, the chain
orientation changes from a paraffin to a lateral monolayer structure for the same charge
density clay. Interestingly, when additional neutral organic molecules such as
alkylamines co-occupy the gallery along with the cationic surfactant, which are needed to
balance the layer charge, a lipid-like structure (Figure 1.2 F-H) is formed for some
layered silicate clays.l6

For most short chain organo clays, the intragallery region is not swellable by a
polymer or polymer precursor due to strong electrostatic forces between the layers and
the exchange cations. The driving force for swelling may be balanced by the intragallery
electrostatic forces. Depending on the nature of the organic cations, mixed
organic/inorganic exchanged clays can be formed. Normally, multiple exchanged
reactions can overcome this competitive binding of inorganic ions to the exchange sites
to give a completely organic cation exchanged product. The presence of metal ions on

even a small fraction of the exchange sites can significantly affect the swelling properties

of organo clays in some cases, especially when a very hydrophobic system is involved. ‘7

W

A: Lateral Monolayer

 

 

 

 

 

 

 

F: Inclined Bilayer

 

 

 

 

 

vvv‘

 

 

 

‘AAMI

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

fir/
I
JJ '4‘ 1‘ (
1 1 1 1 1‘ 1
1‘ ‘>
— - - _.l r -- —- —. —l l
G: Gauche Block H: Lipid Bilayer

Figure 1.2 Orientations of alkylammonium ions in the galleries of layered silicates

with different layer charge densities. ”'16

7

For long chain organo clays that are swellable, the alkyl chains can rearrange their
orientation to accommodate the incoming intercalated species. Depending on the
mobility of intragallery organic cations, the increased gallery space can be varied. For
instance, quaternary alkylammonium exchanged clay can only provide a limited gallery
space for the incoming organo species, because a lipid-like monolayer (Figure 1.2E) is
the optimal structure that can be formed.

1.2.4 Types of Polymer-Clay Composites

From a structural point of view, polymer-clay composites can be generally

classified into “conventional composites” and “nanocomposites”.3 In a conventional .
composite, as illustrated in Figure 1.3A, the registry of clay tactoids are retained when
mixed with the polymer, and there is no intercalation of the polymer into the clay
structure. In these conventional composites the clay plays no major functional role and
acts mainly as a filler for volumetric and economic considerations. An improvement in
modulus can be achieved in some cases, but this reinforcement benefit is usually

accompanied by a sacrifice in other properties such as strength or elasticity.

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9

Two types of polymer-clay nanocomposites are possible. Intercalated
nanocomposites (Figure 1.3B) are formed when one or a few molecular layers of polymer
are inserted into the clay galleries with fixed interlayer spacings. The polymer occupies
the clay gallery in a crystallographically regular fashion, regardless of the overall ratio of
clay to polymer. When the extragallery polymer phase is not considered, the clay
component can be described as an intercalation compound with definite composition and
structure.

In contrast, an exfoliated polymer-clay nanocomposite is a new phase formed
between polymer and layered silicate nanolayers (Figure 1.3C). The individual 10 A-
thick clay layers are separated in a continuous polymer matrix by average distances that
depend on loading. From a microstructural point of View, the domain size of the polymer
phase has been dramatically reduced, and the registry of silicate layers is no longer
maintained in most cases. Due to these structural factors, the interactions between
silicate nanolayers are weak in relation to the interactions between polymer chains (or
network) and layered silicate. So, a nonadditive behavior for the exfoliated polymer-clay
nanocomposites is observed.

Interestingly, the interlayer spacing for an exfoliated nanocomposite may be

uniform (ordered) or variable (disordered).18 Structural differences between these two
phases are schematically illustrated in Figure 1.4. It is noteworthy to point out that an
ordered exfoliated polymer-clay nanocomposite is formed when a synthetic layered
silicate clay is involved. The mechanistic reason behind this particular structural feature

will be discussed in more detail in chapter 3.

10

 

 

P 1 he > hO
(Polym) ( O ym)

 

 

 

A: Disordered Exfoliated B: Ordered Exfoliated

Figure 1.4 Schematic illustration of (A) a disordered exfoliated and (B) an ordered
exfoliated polymer-clay nanocomposites.l 8 The onium ion is represented
by 6 and the polymer chain by (Polym.). he is the average gallery height

for the exfoliated polymer-clay nanocomposites. h0 is the gallery height expected for a
lipid-like bilayer of onium ions.

 

11

1.2.5 Intercalated Polymer-Clay Nanocomposites

Polymer-intercalated clay phases have been known for decades”.‘4 But their

potential as reinforcing and electrically conducting phases in nanocomposite structures

has only recently been recognized.”20 So, the field of synthesis of intercalated polymer-
clay nanocomposites is still very active. Table 1.1 lists the typical polymers that have
been reported to be intercalated into clay galleries. The polymer can be either
hydrophilic or hydrophobic. When oligomers are used, the polymer can be formed by
radical, ionic and redox polymerization. The improvement of dielectric strength.
nonlinear optical properties, quantum confinement effect and electrical conductivity are
of major interest.

Intercalated polymer-clay nanocomposites can be readily synthesized by direct

polymer intercalationl4 or by in situ intercalative polymerization of monomers. ”-21 For
these preformed hydrophilic polymers such as poly(vinyl alcohol), poly(vinyl acetate)
and poly(ethylene oxide), the intercalation can be performed in aqueous solution, wherein
clay layers exist in a highly swollen or exfoliated state. An organic solvent can be used

when a hydrophobic polymer is involved. The removal of the solvent, especially if it is

toxic, is a major limitation of this approach. Molten polymers, such as nylon-6, poly(e-

caprolactone), and polystyrene have been reported recently that can be intercalated into

organo clay galleries without the facilitation of solvent,27 however this method can not
be applied to a very hydrophobic thermoplastic polymer such as polyethylene or

polypropylene, and thermoset polymers.

12

Table 1.1 Polymers Reported to Form Intercalated
Polymer-Clay Nanocomposites

 

 

oligomers/polymers formula ref.
1?
Poly(e-caprolactam) '(‘CH2(CH2)4— C- 111-); 13
C") H
Poly(e-caprolactone) {CH2(CH2)4" C—O)—n 21
Polystyrene +012- (EH-)3 25
Ph
CH3
P l (meth lmethac late) (CH — (II—)— 24
0 Y Y TY 2 I n
cozcn3
Poly(vinyl alcohol) {- CH2- $H '1'] 23
OH .
Poly(ethylene oxide) -(- CH2- CH2-0—)n— 20
Polyaniline (‘Q'Hi 19
H
N
Polypyrrole fr I 26

Polythiophene ‘ m 26

 

13

In situ intercalative polymerization method is a very successful and more general
approach to prepare an intercalative polymer-clay nanocomposite. First at all, the
polymer precursors (monomers or oligomers) are adsorbed or intercalated into clay
galleries. Then, the intercalated polymer precursors are polymerized inside clay gallery.
Solvent can also be involved when necessary in the entire process such as the formation
of PMMA-intercalated clay nanocomposites.24 In most cases, organo clay is intercalated
by the polymer precursors at the liquid state (at the molten state for crystalline
monomers), and the initial viscosity of the system is optimized by the nonviscous organic
species to form a homogenous intercalated phase. Another advantage of this approach is
that the intragallery cations can perform as a catalyst to initiate the polymerization, so
that a driving force for the monomer diffusion exists which can play a very important role
to achieve a high degree of homogeneity. This method works very well for both
thermoplastic and thermoset polymer systems. The limitation of this approach is that a
suitable polymer precursor for this specific intercalation reaction is not always available.
1.2.6 Exfoliated Nylon-6-Clay Nanocomposites

Owing to the platy morphology of the silicate layers and the improved
microscopic homogeneity, the exfoliated clay nanocomposites can exhibit dramatically
improved properties that are not available for the conventional and the intercalated
composite materials. Toyota researchers first demonstrated that organo clays exfoliated
in a thermoplastic nylon-6 polymer matrix greatly improved the thermal, mechanical,

barrier and even the flame retardant properties of the polymer.”31
By replacing the hydrophilic exchange cations of Na+-montmorillonite with a

hydrophobic ammonium cations of (o-amino acid they were able to conduct the ring

opening polymerization of e-caprolactam in the interlayer gallery region. The

intercalative polymerization process resulted in almost complete delamination

(exfoliation) of the stacked clay layers into the nylon-6 polymer matrix. The overall

14

performance properties of the nylon-6-clay nanocomposites substantially exceeded the

composites prepared by the conventional method.32 For instance, the exfoliation of only
4.2 wt % montmorillonite clay in nylon-6 increased the tensile strength and modulus,
respectively, from 69 MPa and 1.1 GPa for the pristine polymer, to 107 MPa and 2.1 GPa
for the composite. More importantly, the thermal and rheological properties of the nylon-

6 were improved dramatically by exfoliation. An increase in heat distortion temperature

from 65 0C for nylon-6 to 145 °C for the nylon-6-clay hybrid was achieved with as low as
4.2 wt % clay loading. The performance properties of nylon-6-clay nanocomposites are
summarized in Table 1.2. This spectacular improvement in heat distortion temperature
has made it possible to extend the commercial use of this relatively cheap polymer to
high temperature environments, such as specialized under-the-hood applications in the
automobile industry. A picture for the timing belt cover made of nylon-6-clay hybrid is

shown in Figure 1.5.

Table 1.2 Mechanical and Thermal Properties of Nylon 6-Clay companies32

 

Composite wt % Tensile Strength Tensile Modulus Impact HDT (0C)

 

Type Clay (MPa) (GPa) (Id/m?) at 18.5 kg/cm2
"NanoscopiC" 4.2 107 2.1 2 8 14
(Exfoliated) ' 5
"Micro" 50 61 1.0
(Tactoids) 2'2 89
Pristine o 69 1.1 2.3 65

Polymer

 

 

 

 

Figure 1.5 Injection-molded NCH (nylon 6-clay hybrid) timing belt cover.32

16

1.2.7 Recent Advances on Polymer-Exfoliated Clay Nanocomposite Formation

Over the pass few years, this revolutionary nanocomposite chemistry has been
successfully extended to other polymer systems such as polyimide,33 acrylonitrile

rubber,34 polyether,35 epoxy 36,37 and polysiloxane.38 Among these, the epoxy-clay
nanocomposites with a sub-ambient Tg exhibited exceptionally strong reinforcing effects.
For instance, 7.5 vol. % of the exfoliated 10 A-thick silicate layers improve the strength
of the polymer matrix by more than 10-fold. Mass transport studies of polyimide-clay

nanocomposites revealed a great reduction in the permeability of small gases, e. g., C02,
02, H20, and the organic vapor ethyl acetate.” More recently, silicate nanolayers have

been dispersed in a very hydrophobic polymer, such as polypropylene.40~41 Although, the
barrier properties of this polypropylene-clay nanocomposite have not yet been reported,
the application of this nanocomposite in the field of packaging is very promising.
Approaches to the exfoliation of clay nanolayers have been investigated by
several different research groups using both thermoplastic and thermoset polymers.
However, the desired exfoliated polymer-clay nanocomposites are much more difficult to
prepare. Nevertheless, in situ polymerization is still a very powerful approach to
preparing exfoliated polymer-Clay nanocomposites.42 In this approach, a strong driving
force, which can be a chemical potential and/or a mechanical force such as the shear
blending, has to be applied in order to achieve the exfoliated phase of the silicate
nanolayers. The driving force of polymerization to overcome the intragallery
electrostatic forces can be especially effective. In utilizing this driving force, controlling
the relative intra- and extragallery polymerization rates is perhaps the key factor. For
instance, the acidic intragallery cations can function as catalyst centers and cause the
intragallery polymerization rate to be competitive with the extragallery polymerization
rate. This competitive polymerization results in the exfoliation of the silicate nanolayers

in a monolithic polymer matrix. However, the rapidly increasing viscosity of the

17

polymerizing media can prevent the silicate nanolayers from exfoliating to a high extent.
If the migration of the polymer precursors into clay galleries is slow early, then an
intercalated clay nanocomposite will be formed, which is always not as a good performer
as an exfoliated polymer-clay nanocomposite.

The mechanisms and factors governing the reinforcement of barrier and
mechanical properties of polymer-clay nanocomposites were illustrated by some
researchers. For instance, the larger aspect ratios and the turbostratic stacking fashion of

the exfoliated clay platelets significantly increase the transportation path (Figure 1.6) for

the gas molecules, and simultaneously reduce the permeability.43 Some results also
revealed that the improvement of mechanical properties is quite dependent on the glass
transition temperature of the pristine polymer used. For the polymer matrix with a sub-
ambient Tg, the reinforcement by the exfoliated clay is more significant than the high Tg

system. A theory of platelet alignment under strain was used to explain this spectacular

improvement which is illustrated in Figure 1.7.36

l8

torturous path

platelet

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1.6 Proposed model for the torturous zigzag diffusion path in an exfoliated

polymer-clay nanocomposite when used as a gas barrier.43

A: Glassy Matrix B: Rubbery Matrix

 

 

 

 

     
   

 

 

 

 

“111.1 ‘3 ”'1' “lull Ill I'll

“H /| $7 ‘1 /| ||\|

// / x / uIll

// \\ ”/ \\\ /// \\ 11H 1

\\\\\ /\ \ \ \\\\\¢ \\ l I I H
\\ ///

 

 

 

 

 

 

 

 

Increasing Strain

>

 

Figure 1.7 Proposed model for the fracture of (A) a glassy and (B) a rubbery

polymer-clay exfoliated nanocomposite with increasing strain.36

19

1.3 Research Objectives
1.3.1 Current Issues and New Directions

The breakthrough represented by nylon-6-clay nanocomposite introduced a new
concept in the field of organic-inorganic nanocomposites. However, some of the
fundamental chemistry and physics issues regarding nanocomposite formation are not
well elucidated. For instance, what are the most effective aspect ratios of silicate
nanolayers? What extent of exfoliation is needed for optimized mechanical properties?
Is the extent of layer exfoliation totally controllable? The mechanisms responsible for
the unprecedented reinforcement properties of this exciting new class of nanocomposite
materials are not well understood, because the models developed for the conventional
fiber-reinforced composites can not be simply transferred into this new field. New
models for nanostructural composites, which can be used to explain the issues, such as
the effect of nanolayer exfoliation on polymer elasticity, are needed.

The importance of interfacial properties of organic-inorganic composites, which
can govern composite performance properties, has been realized. The suitable interface
is almost a prerequisite to achieve a nanostructural phase, otherwise a segregated
conventional phase will be formed. However, several factors can play important roles in

determining the performance properties, especially the mechanical behavior for the

composites. A general model '17 is proposed by Pinnavaia et al. for the possible
interactions occurring at polymer-clay interfaces which is schematically illustrated in
Figure 1.8. Earlier work has revealed that the physical adsorption of the polymer to the
siloxane-like oxygen atoms of the clay (ie. Type C interactions) are more important in
governing the mechanical behavior of the composite than the interactions between the
alkyl chains and polymer network (ie. Type B interactions). This result suggests that
exfoliated forms of layered silicate clays would be good reinforcement agents for epoxy
matrices even in the absence of onium-exchange cations on the gallery surfaces.

However, it is not easy to investigate the effect of only one interfacial factor while the

20

other factors are maintained unchanged. So, expanding the exfoliated chemistry
developed for semctite clays to other layered inorganic materials is essential in order to
help address these issues.

The majority of engineering thermoplastic and thermoset polymers have not yet
been fully explored for nanocomposite formation. Benefits similar to those obtained for
nylon-6 should be possible for other polymer systems through nanolayer reinforcement.
A deeper understanding of the fundamental chemistry and physics regarding
nanocomposite formation, the relation between structure and properties, and the
interfacial factors governing the mechanical behavior will help to advance nanocomposite

science and technology.

21

 

Polymer

 

 

 

 

Figure 1.8 Interfacial interactions in a polymer-layered silicate nanocomposite: (A)
Electrostatic binding of onium ion to the clay surfaces; (B) "Dissolution" of the alkyl
chains into the polymer matrix by van der Waals forces; (C) Physical sorption of polymer
to the siloxane oxygen atoms of the basal surfaces; (D) Polymer sorption to hydroxyl-
terminated edge sites.l7

22 '

1.3.2 Layered Silicic Acid Structures

Although, a great deal of research has been conducted on organic-inorganic
hybrid composites in which smectite clays are used as reinforcement agents, layered
silicic acids have drawn little attention in this field. Layered silicic acids are potentially
good candidates not only because they have platy phase morphology and intercalation
chemisz similar to smectite clays, but also because they possess high purity and
structural properties which are not available to smectite clays. Also, they can be
synthesized in high purity.

The alkali-metal-ion forms of this layered silicate family include as members
kanemite (NaHSi205-nH20), makatite (Na28i409-nH20), ilerite (also called octosilicate,
Na28i3017-nH2 O), magadiite (Na28i14Ozg-nHzO) and kenyaite

(Na28i20041-nH20).16’44'45 The alkali metal silicate layers contain Q3 (5810') silicon

sites in which the terminal oxygen is neutralized by alkali ions and protons. Layered
silicic acid derivatives are formed when the alkali metal ions are replaced completely by

protons. Except for makatite, the detailed crystal structures of these layered silicates are
still unknown. In the makatite structure, continuous sheets of edged-shared Q3 tetrahedra

are condensed to form six-memberred rings. A proposed structure for magadiite,47 which
possesses a layer thickness (11.2 A) similar to smectite clay (9.6 A), is shown in Figure
1.9. In general, the structure of these layered silicates is built up of one or more sheets of
SiO4 tetrahedra.

Most of the alkali-metal forms of layered silicic acids can be easily synthesized
by hydrothermal methods. By changing the ratio of starting sodium hydroxide,
amorphous silica and water, and by controlling the reaction conditions, such as reaction

temperature and time, these alkali-metal layered silicates can be prepared with different

layer thickness.

23

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24

The intercalation chemistry of layered silicic acid is very similar to smectite clays.

The alkali ions are exchangeable, and the intragalleries can be intercalated by various

organic molecules 1943-49 and even pillared by inorganic pillaring agents.50a51 However,
the physical states of silicate layers in aqueous solution are quite different. The layers of
Na+-smectite clay are exfoliated in water, whereas the layers of layered silicic acids are
stacked in an aqueous suspension. The special properties of layered silicates, which are
not available to smectite clays include the presence of basal plane hydroxyl groups 53-53
and the variable thickness of the silicate layers 46 from 5.0 A for makatite to 17.7 A for
kenyaite, as shown in Figure 1.10. '

Much research has been conducted on transforming layered silicic acids into

catalysts and pillared microporous derivatives or molecular sieves.50.51 A few examples
have been reported using layered silicic acids for nanocomposite formation. This limited
knowledge is mainly due to a lack of structural information and a complete knowledge of
the intercalation chemistry for layered silicic acids. For instance, the conditions needed
for the synthesis of alkylammonium exchanged layered silicic acids are much more

restrictive than those for the synthesis of smectite clay analogs.

 

 

 

 

Kenyaite: N328I20041(0H)2'XH20

Figure 1.10 Schematic representation (edge-view) of the layered silicic acids.46 The
silicate layers are formed by sharing of apical oxygen atoms between tetrahedral SiO4

sheets for ilerite, magadiite and kenyaite. The thickness of the layers differs depending
on the degree of cross-linking between stacked sheets.

26

1.3.3 Research Goals

The active hydroxyl sites of layered silicic acids may provide enhanced bonding
to gallery intercalants. Also, the variable layer thickness may provide insights on the
relationship between nanocomposite properties and aspect ratios. Although a large
number of organic compounds have been intercalated into layered silicic acid derivatives,
the exfoliation of this family of materials in a polymer matrix has not yet been achieved.
In the present work we will first extend the exfoliation chemistry developed for smectite
clays in an epoxy matrix to include the layered silicate magadiite with a layer thickness
of 11.2 A. The chemistry elucidated for magadiite is likely to be applicable to other
members of this mineral family for the formation of polymer-inorganic nanolayer
composites.

Some fundamental chemistry and physics issues regarding nanocomposite
formation will be elucidated to some degree by this research work. The main focus will
be on the relationship of microstructures and properties, and the interfacial factors which
govern the mechanical properties of nanocomposites. The interactions between the
silicate phase and the polymer phase will be emphasized over the other interactions. New
polymer systems, especially other thermoset polymers, are also of interest in this work.

Epoxy-clay nanocomposites prepared by in situ intercalative polymerization
starting from an organo clay compromise the mechanical performance properties of the
resulting composites. New approaches to polymer-layered silicate nanocomposites will
be also explored in the present work so that greatly improved mechanical properties can
be achieved using proton exchanged forms of layered silicic acids.

Once the exfoliation chemistry of layered silicic acids has been established, then
long term future work can be proposed. Since the layered silicic acids possess active
functional hydroxyl groups for potential covalent linking to the polymer phase, a great

opportunity is provided to investigate the potential benefit of covalent interfacial forces

27

between organic and inorganic phases in improving the mechanical properties of
polymer-inorganic nanolayer composites.

The polymer-layered silicic acid nanocomposites described in the present work
are characterized by X-ray powder diffraction, which is the most powerful technique to
determine the orientation and stacking of layers and the average basal spacing between
silicate layers. Tensile tests will be used to obtain the mechanical properties of the
composite materials. Sample coupons with a dog-bone shape will be prepared by the
molding methods. The performance properties will be further characterized in terms of
chemical stability, solvent resistance and thermal stability. The technique of surface area

measurement by N2 adsorption-desorption will also be used to deepen our understanding

of the phase morphology of silicate nanolayers.

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28

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Sugahara, Y.; Sugimoto, K.; Yanagisawa, T.; Nomizu, Y.; Kuroda, K.; Kato, C.
Yogyo Kyokai Shi 1987, 95, 117.

Yanagisawa, T.; Yokoyama, C.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn.
1990, 63, 47.

Dailey, J. S.; Pinnavaia, T. J. Chem. Mater. 1992, 4, 855.
Wong, S. T.; Cheng, S. Chem. Mater. 1993, 5, 770.
Rojo, J. M.; Ruiz-Hitzky, E.; Sanz, J. Inorg. Chem. 1988, 27, 2785.

Pinnavaia, T. J.; Johnson, I. D.; Lipsicas, M. J. Solid State Chem. 1986, 63,
l 18.

Chapter 2

HYBRID ORGANIC-INORGANIC NANOCOMPOSIT ES FORMED FROM AN
EPOXY POLYNIER AND A LAYERED SILICIC ACID (MAGADIITE)

2.1 Introduction

Toyota researchers first demonstrated that organo clays exfoliated in a
thermoplastic nylon-6 polymer matrix greatly improved the thermal, mechanical, barrier
and even the flame retardant properties of the polymer. These composites are now used
in under-the-hood applications in the automobile industry.“4 Although considerable
research has been conducted on organic-inorganic hybrid composites in which smectite
clays are used as reinforcement agents,"8 relatively little work has been devoted to
derivatives of layered silicic acids that possess layered morphology and ion-exchange
properties similar to smectite clays. The alkali-metal-ion forms of this layered silicate
family include as members kanemite (NaHSizos-nHZO), makatite (Na281409-nH20),
ilerite (Na2S13017-nH2 O), magadiite (Na2Si14029-nH2 O) and kenyaite
(Na2SizoO41-nH20).9’” The detailed crystal structures of these layered silicates are still
unknown, except for makatite.l2 In general, the structure of these layered silicates is built
up of one or more sheets of SiO4 tetrahedra with an abundant hydroxyl siloxane
surface.”'”

The alkali-metal silicate layers contain Q3 (5810“) silicon sites in which the
terminal oxygen is neutralized by alkali ions and protons. The alkali ions are

exchangeable, and the intragalleries can be intercalated by various organic molecules and

even pillared by inorganic pillaring agents.‘5"° Layered silicic acid derivatives are

31

32

formed when the alkali-metal ions are replaced completely by protons. The special
properties of layered silicic acids include their basal plane hydroxyl groups and the
variable thickness of the silicate layers (from 5.0 A for makatite to 17.7 A for kenyaite).l2
The active hydroxyl sites may provide enhanced bonding to the gallery intercalates.
Also, the variable layer thickness may provide insights into the relationship between
nanocomposite properties and aspect ratios. Although a large number of organic
compounds have been intercalated into layered silicic acid derivatives,9"7'18 the
exfoliation of this family of materials in a polymer matrix has not yet been achieved.

In the present work we will extend the exfoliation chemistry of smectite clay in an
epoxy matrix to include the layered silicate magadiite with a layer thickness of 11.2 A.
The chemistry elucidated for magadiite is likely to be applicable to other members of this

mineral family for the formation of polymer-inorganic nanolayer composites.

2.2 Experimental

Materials. The epoxide resin used to for epoxy-magadiite hybrid composite
formation was poly(bisphenol A-co-epichlorohydrin) (Shell, EPON 828), with MW
~377:

CH OH CH
A I 3 l I 3 A
n

CH3 CH3
n = O (88%); n = l (10%); n = 2 (2%).
The curing agent was poly(propylene glycol) bis(2-aminopropyl ether) (Huntsman
Chemical, JEFFAMINE D-2000), with MW ~2000:
HZNCHCHZ -{ OCHZCIIH —} N H2
(IIH3 CH3 X
x=33.1

The above monomers afford a rubbery-like epoxy polymer matrix with a sub-ambient Tg
of -40 0C. All the other chemicals used in this work were purchased from Aldrich

Chemical Co. and used without further purification.

33

Synthesis of Na*-magadiite. Nat-magadiite was synthesized according to the
published methods.19 A suspension of 60.0 g of amorphous silica gel (1.0 mol) in 300
mL of 1.11 M NaOH solution (0.33 mol) was heated at 150 0C for 42 h with stirring in a
Teflon-lined stainless steel 1.0 L Parr reactor. The suspension containing Nat-magadiite
was centrifuged, and the solid product was washed twice with 600 mL of deionized water
to remove excess N aOH, and air-dried at room temperature.

Synthesis of Organo Magadiites. Primary long chain alkylammonium
exchanged C18-magadiite-PF was prepared by the following approach. An aqueous
suspension containing 15.0 g of Na’t-magadiite was combined with ethanol/water
solution containing 0.141 mol of alkylammonium chloride and alkylamine in a 6:1 molar
ratio to form a suspension in 1.0 L of 1:1 (vzv) ethanol/water total solution. The
suspension was stirred at 65 0C for 48 hours. The pH of the reaction mixture was in the
range of 8 - 9. The product mixture was added to an equal volume of ethanol and
centrifuged. The wet solid product was washed consecutively with one 7 SO-mL portion
of 50 % EtOH, two 750-mL portions of 25 % EtOH and then with water until free of Cl',
and air-dried.

Octadecylamine-solvated octadecylammonium exchanged magadiite (C l 8-
magadiite-BL) was prepared using Lagaly‘s method.9 An aqueous suspension containing
4.0 g of Na+-magadiite was combined with ethanol/water solution containing 2.63 x 10'2
mol of alkylammonium chloride and alkylamine in a 3:2 molar ratio to form a suspension
in 400 mL of 1:9 (vzv) ethanol/water total solution. The suspension was stirred at 65 0C
for 48 hours. The pH of the reaction mixture was in the range > 9. The product mixture
was added to 200 mL of ethanol and centrifuged. The wet solid product was washed with
one 250-mL portion of 10 % EtOH and then with water until free of Cl', and air-dried.

Octadecylammonium exchanged magadiite (Cl8-magadiite-LM) with a lateral
monolayer structure could be prepared by two methods. (1) C18-magadiite-PF or C18-

magadiite-BL was washed with ethanol extensively until a lateral monolayer phase

34

appeared (characterized by XRD) and air-dried. (2) An aqueous suspension containing
4.0 g of Na+-magadiite was combined with ethanol/water solution containing 2.25 x 10-2
mol of alkylammonium chloride to form a suspension in 400 mL of 3:2 (vzv)
ethanol/water total solution. The suspension was stirred at 65 °C for 48 hours. The pH
of the reaction mixture was maintained in the range of 6 - 7 by adding 0.1 M HCl
solution. The product mixture was added to an equal volume of ethanol and centrifuged.
The wet solid product was washed with ethanol extensively until a lateral monolayer
phase appeared and free of Cl'.

All of the air-dried organo magadiite was ground to a powder with a particle size
smaller than 270 mesh (53 um) and stored for further use.

Preparation of Epoxy-Magadiite Composites. Equivalent amounts of epoxide
resin and poly(oxypropyleneamine) curing agent were mixed at room temperature for 30
min. The desired amount of organo magadiite was added to the epoxide-
poly(oxypropyleneamine) mixture and stirred for another 60 min. This mixture was
outgassed in a vacuum oven and poured into a stainless steel mold for curing at 75 0C
for 3 h and, subsequently, at 125 °C for an additional 3 h.

X-ray Powder Diffraction (XRD). XRD patterns were recorded on a Rigaku
rotaflex 200B diffractometer equipped with a rotating anode, Cu Kc: x-ray radiation (1» =
1.541838 A) and a curved crystal graphite monochromator. The x-ray was operated at 45
KV and 100 mA. Diffraction patterns were collected with 0.010 29 interval between 1
and 10° 20 using a scanning rate of 2° 29 per minute, and DS and SS slit widths of 1/6.
Samples of epoxy-solvated magadiite or uncured epoxy-magadiite composites were
prepared by applying thin films on glass slides. Cured composite specimens were
prepared by mounting a flat rectangular sample into an aluminum holder.

Thermal Analysis. Thermogravimetric analyses (TGA) were performed using a

Cahn TG System 121 thermogravimetric analyzer. Samples were heated to 750 °C at a
heating rate of 5 °C/min under N2 atmosphere.

35

Mechanical Measurement. Tensile testing was performed at ambient
temperature according to ASTM procedure D3039 using a SFM-20 United Testing
System.

Chemical and Solvent Resistance. The resistance of the composite materials to
solvent swelling was obtained according to ASTM procedure D543. The specimens were
immersed in the desired reagent and removed periodically to measured the weight gain

until equilibrium was reached.

2.3 Results and Discussion
2.3.1 Synthesis of Organo Magadiites
Intercalation chemistry has played a very important role in the formation of

polymer-clay nanocomposites."20

Most polymer precursors require a hydrophobic
environment to be intercalated into the clay galleries. Our first goal was to synthesize
suitable organo magadiites which can be used to prepare epoxy-magadiite
nanocomposites. Na+-magadiite has been reported to undergo ion-exchange reaction
with primary alkylammonium ions. However, a lipid-like bilayer is the most favored
intragallery structure.9 The lipid-like bilayer is formed from neutral amine and the
necessary amount of alkylammonium needed to balance the negative charge of the
silicate layer (cf. Figure 1.2H). The presence of a large amount of neutral amine with
mono-functional end groups is not suitable in the chemistry of epoxy polymer curing. In
this work, we have developed a new approach to synthesize a paraffin-like intercalated
phase of organo magadiite by using long chain primary alkylammonium surfactants and a
neutral amine as gallery guests.

The Na+ form of the magadiite was converted to a previously unknown mixed
CH3(CH2)17NH3+-CH3(CH2)17NH2 intercalate by ion exchange reaction in the presence
of neutral amine. A basal spacing of 38.2 A was consistent with the paraffin structure of

onium ions and neutral amine with an inclined angle about 65°.9 This intercalated

paraffin-like phase was designated C l8-magadiite-PF. Elemental analysis and TGA

36

results indicated that the sodium ions were completely replaced by onium ions. This
product also contained a small fraction of neutral primary amine which was essential for
forming the paraffin structure, in addition to the primary onium ions.

Two previously known organo magadiite intercalates were also synthesized for
comparison purposes. One was an organo magadiite with a 63.1 A basal spacing.
prepared using Lagaly's method.9 The gallery of this organo magadiite contains a lipid-
like bilayer structure of onium ions and neutral amine molecules. The sample was
designated ClS-magadiite-BL. This product should be regarded as octadecylamine-
solvated octadecylammonium magadiite which contains near 100% neutral amine, except
the alkylammonium fraction needed to balance the negative charge of the silicate layer.
The other known organo magadiite phase was a 14.0 A phase intercalated by a lateral
monolayer of CH3(CH2)17NH3+ ions. This monolayer phase was denoted C18-
magadiite-LM. In this lateral monolayer structure the onium ion chains lie parallel to the
interlayer siloxane surface.

The unit cell compositions, basal spacings and gallery structures for the three
organo magadiite derivatives are summarized in Table 2.1. The powder XRD patterns
and TGA curves for each phase, along with that for Na+-magadiite are given in Figure

2.1 and Figure 2.2 respectively.

Table 2.1 Intercalates Formed by Reaction of Na+-Magadiite with C18H37NH3+C1'

 

 

and C13H37NH2
material intragallery compositiona gallery door
designation per 81140292‘ unit cell structure (A)
ClS-magadiite-LM (C13H37NH3+)0,45H+1,55 lateral monolayer 14.0
C18-magadiite-PF (C18H37NH3+)2(C18H37NH2)0.48 paraffin 38.2
C18magadiite-BL (C18H37NH3+)2(C18H37NH2)1.83 lipid bilayer 63.1

 

aThe compositions were determined by CHN and Si analyses.

 

 

 

 

 

 

 

 

 

 

 

 

63.1A
a?
(D
C
G)
4—0
.E
G)
.2
4—1
5..“
G)
I
@Wfiafwfifl" .
«m-Qmfismgvi’r‘hém»
A
lllllllllllllllllllllllllllllllllllllll
O ‘10 20 30 40

28 (Degrees)

Figure 2.1 X—ray powder diffraction patterns of (A) air-dried Na+-magadiite; (B)
CH3(CH2)17NH3+-magadiite with a lateral monolayer (LM) structure. Patterns (C) and
(D) are for mixed CH3(CH2)17NH3+/CH3(CH2)17NH2-magadiite with a paraffin (PF)
and a lipid bilayer (BL) gallery structure, respectively. The LM structure contains no free
amine (0.45 onium ions per Si14029 formula unit), whereas PF and BL structures contain

neutral primary amine molecules, in addition to primary onium ions.

38

 

   
   
    
   

C18-magadiite-LM

 

NaI-magadiite

 

 

80

$3

:70

.C

.9

0’ C18-magadiite-PF

3 60
50 C18-magadiite-BL
4G llLllllllIllllllllIIllllIllllllllllll

 

0 100 200 300 400 500 600 700

Temperature (°C)

Figure 2.2 Thermogravimetric analysis curves of (A) air-dried Na+-magadiite; (B)
CH3(CH2)17NH3+—magadiite with a lateral monolayer (LM) structure. Patterns (C) and

(D) are for mixed CH3(CH2)17NH3+ICH3(CH2)17NH2-magadiite with a paraffin (PF)
and a lipid bilayer (BL) gallery structure, respectively.

39

It is noteworthy that the initial product obtained by the ion exchange reaction for
C18-magadiite-PF is a mixture of the lipid bilayer and the paraffin magadiite phases with
its XRD pattern shown in Figure 2.3. To obtain this paraffin phase it is essential to
execute washing according to the conditions specified in the experimental section.
Otherwise, most of the organo species will be removed from the gallery by the ethanol

washing process, resulting in the formation of C18-magadiite-LM with a lateral

monolayer structure.

 

(001)

      
    

Ii id bilayer araftin
80161.8A 01'38.7A
002 31.3A 002 '19.3A
003 20.3 A

Intensity

(002)"
(002) (003)

 

 

 

llllllLUllllllllllllllllllllLlllll

1 2 3 4 5 6 7 8 9 10
29 (Degrees)

 

Figure 2.3 X-ray diffraction pattern of the initial mixture product in an effort to
obtain ClB-magadiite-PF. The mixture was formed when Na+-magadiite was ion

exchanged by CH3(CH2)17NH3+/CH3(CH2)17NH2 followed by ethanol washing. The
tabulated values in the insert are (1001 values for the lipid bilayer and paraffin phases.

40

2.3.2 Exfoliation of Magadiite N anolayers in an Epoxy Polymer Matrix
Owing to the strong intragallery electrostatic interactions in Cl8-magadiite-LM,

this structure can not be further intercalated by epoxide resin or even by small polymer

precursor molecules such as e-caprolactam. However, both C18-magadiite-PF and C18-

magadiite-BL with 38.2 A and 63.1 A basal spacing are readily swelled by polymer
precursors and can be used to form hybrid composites. C18—magadiite-PF is much
preferred over the BL analog due to a much smaller amount of neutral amine (cf. Table
2.1), which can participate and interfere with the desired thermoset curing process.
Unless otherwise indicated, all of the following experiments were carried out by using
C18-magadiite-PF as a reinforcement agent. All effort to form a paraffin-like intercalate
with onium ions and amines with chain length shorter than C16 resulted in intercalates

with a lipid bilayer or lateral monolayer structure.

C18-magadiite-PF when mixed with epoxide resin at 75 0C undergoes a 4.3 A
increase in basal spacing from 38.2 A to 42.5 A shown in Figure 2.4. This indicates that
the gallery onium ions change their orientation from an inclined to a vertical orientation
to accommodate epoxide penetration into the gallery. Furthermore, the X-ray diffraction
patterns in Figure 2.5 show the gallery can also be expanded by co-intercalation of the
epoxide and poly(oxypropyleneamine) mixtures. At room temperature, the gallery is
only partially solvated due to the slow gallery diffusion of those macromolecules. This

results in the presence of a 63 A intercalate and the unintercalated initial phase. At

elevated temperature (75 °C), the gallery is totally solvated within 15 minutes to form an
intercalate with a very sharp dom peak near 63 A. This latter value is consistent with the
reorientation of the onium ions into a bilayer structure when solvated by the resin and the
curing agent (Compare Figure 2.5A and 2.5D). The broadening of the first order peak

upon further heating at 75 °C (Figure 2.5E) suggests that the gallery is further expanded

by cross-linking polymerization. Upon further heating at 125 °C, the gallery continues to

41

undergo expansion and the solvated phase basal spacing of 63 A is replaced by the nearly

amorphous phase characteristic of the exfoliated composite (Figure 2.5F and 2.5G).

 

 

 

 

 

 

42.5A
.é‘ 38.2A
U)
C
53
5
Q,
.g
g
g .. at ”MA B
LA A
11_LLIJIJ_IIIIIIILIIllllllIlJllILlll

 

1 2 3 4 5 6 7 8
2(-) (Degrees)

Figure 2.4 X-ray diffraction patterns of ClB-magadiite-PF in different physical states:
(A) pristine organo magadiite; (B) 10 wt % organo magadiite solvated by epoxide resin
(EPON 828) at 75 °C for 90 min.

42

 

 

 

 

 

 

 

 

 

 

(OO1)62.7A (001)*38.7A
(00‘) (002)31.3A (002)*19.3A
1 (003)21.0A
. F
1%; E
c (002) ~ A
o
E
.2 D
E
in
(I:
C
B-
002*
( ) A
lllJLllllIlLLlllllllllllllllllllLl

 

 

 

1 2 3 4 5 6 7 8
28 (Degrees)

Figure 2.5 X-ray diffraction patterns of products prepared by reaction of (A) C18-
magadiite-PF (15 wt % loading) with epoxide resin and poly(oxypropyleneamine) curing
agent at an epoxide group to amine group molar ratio of 2:1. Reaction conditions were as
follows: (B) 25 0C, 60 min; (C) 75 °C, 5 min; (D) 75 °C, 15 min; (B) 75 °C, 3 h; (F) 75

°C, 3 h and 125 °C, 5 min; (G) 75 °C, 3 h and 125 0C, 10 min. The 001 lines marked
with an asterisk are from the initial paraffin structure of the organo magadiite.

43

The X-ray diffraction pattern of epoxy-exfoliated magadiite composite completely

cured at 125 °C for 3 h is shown in Figure 2.6. The absence of the 001 diffraction peaks
provides strong evidence that the silicate layers of magadiite have been exfoliated in the
thermoset curing process. However, the magadiite 2-dimensional structure is still
retained in the exfoliated state, as indicated by a d014 reflection at 3.4 A. As indicated by
the XRD data in Figure 2.5, the key to achieving an exfoliated composite structure is to
first load the magadiite gallery with mobile onium ions and amine guest species and then
expand the gallery region to a "critical gallery height" by replacing the amine with
epoxide and poly(oxypropyleneamine) precursors. This expansion of the gallery by the
precursors allows the intragallery and extragallery curing of the polymer to occur at
comparable cross-linking rates.
2.3.3 Performance Properties of Exfoliated Epoxy-Magadiite Nanocomposites

The benefit of magadiite exfoliation in polymer reinforcement is illustrated by the
tensile strength vs loading curves in Figure 2.7. The tensile strengths of the exfoliated

magadiite nanocomposites are superior to the conventional composites prepared from

both Na+-magadiite and ClS-magadiite-LM. In obtaining these curves we adjusted the
stoichiometry of epoxide and poly(oxypropyleneamine) by including the intragallery
onium ion and amine as curing agents. The results are very predictable. Since the single
silicate nanolayers have been dispersed into the polymer matrix, the exfoliated
nanocomposite is more or less microscopically homogeneous relative to the macroscopic
homogeneity of the conventional composites. Thus each nanolayer of the exfoliated

composite contributes to the reinforcement effect.

 

 

Intensity

 

 

 

nlrlrlrlrlrlrlrlrli

20 22 24 26 28 30
29 (Degrees)

 

Intensity

 

 

itllllllilililltillllilllirlllill1

1 2 3 4 5 6 7 8
26) (Degrees)

Figure 2.6 X-ray diffraction pattern of a cured epoxy polymer-magadiite
nanocomposite prepared from C18-magadiite-PF. The organo silicate content was 15 wt

%. Polymer curing was carried out at 75 °C for 3 h, and followed by 3 h at 125 °C.

45

 

   

 

 

 

4
: A: C18-magadiite-PF, exfoliated
' B: Nai-magdiite, aggregated
E 3 _ C: C18-magadiite-LM, aggregated
E, I
g - A
E 2-
35 .
a) .
E - __----<> B
C _ "’7 ’_ .— -A C
m ’-
1— 1-
L
a; __ __ "A’
0llllllllllllJJllllLIlllll‘llll
0 5 ' 10 15

Magadiite Silicate Loading (wt °/o)

Figure 2.7 Comparison of the tensile strengths of an epoxy-exfoliated magadiite
nanocomposite prepared from ClS-magadiite-PF and conventional magadiite composites

prepared from Nit-magadiite and Cl8-magadiite-LM. The inorganic silicate loading
was determined by calcining the composites in air at 650 0C for 4 hours using a heating

rate of 2 °C/min. The onium ion and amine content of the organo magadiite was counted
as contributing to the stoichiometry of epoxide cross-linking.

46

Examples of epoxy hybrid composites based on smectite clay intercalates have
been reported previously?"22 In comparison to the results obtained for epoxy-smectite
clay nanocomposites, the reinforcement provided by magadiite is not as significant at
higher loading. The difference in the charge density between magadiite and smectite
results in a different loading of alkylammonium ions. It is quite likely that the onium
exchange ions function as curing agents in the process of polymer cross-linking. This
hypothesis was supported by an experiment wherein C18-magadiite-BL was swelled by
epoxide and poly(oxypropyleneamine). The series of sharp XRD reflections present at
the swelling stage disappear upon complete curing (Figure 2.8). This result signifies that
C18-magadiite-BL can also be exfoliated in an epoxy polymer matrix. Therefore, it was
necessary to investigate the effect of alkylammonium ion chain length on the mechanical
behavior of our nanocomposites.

The dashed line (Curve B) in Figure 2.9 shows the results obtained if we do not
compensate for the alkylammonium and alkylamine content of the organo magadiite
curing agent and use the stoichiometry amounts of epoxide resin and
poly(oxypropyleneamine). The tensile moduli of the resulting nanocomposites exhibit a
maximum. At low organo magadiite loading, the effect was minimal due to
reinforcement contribution of the layers, but at high loading, the effect of the gallery
onium ion and amine is significant. Although the polymer network maintains its
continuity, the excess amine weakens the composite and reduces the benefit derived from
exfoliation of the silicate layers. However, we still obtain some benefit from the effect of
excess alkylamine in nanocomposite formation. The strain-at-break is significantly
improved by the excess alkylamine provided by the organo magadiite (Figure 2.10). A
very elastic nanocomposite with higher tensile strength and modulus is obtained.
Significantly, this plasticizing effect can not be achieved by adding an excess of

poly(oxypropyleneamine) in the absence magadiite reinforcement.

47

 

 

   

 

 

 

 

 

 

3‘
"-1
‘8 ,
ill11111111111L11_Lllllllllu
% 3 4 5 6 7 8
H 26 (Degrees) C
a) u -
6
C3
0"
Q)
°‘ B
63.1 A ‘
'A

.‘

 

 

 

lLlllillilllilllllllllilll11111111

1 2 3 4 5 6 7 8
29 (Degrees)

 

Figure 2.8 X-ray diffraction patterns of products prepared by reaction of (A) C18-
magadiite-BL (15 wt % loading) with epoxide resin and poly(oxypropyleneamine) curing
agent at an epoxide group to amine group molar ratio of 2: 1. Reaction conditions were as
follows: (B) 75 °C, 60 min; (C) 75 0C, 120 min. The inset is a pattern for the cured

epoxy polymer-magadiite nanocomposite prepared from ClS-magadiite-BL. The marked
sharp lines in pattern (B) are derived from the octadecylarnine.

48

 

 
   

    
 

 

 

 

12
7310'
a_ .-
g _
(D .-
A
e 8-
'0 l-
O .—
2 " - ‘\
%j 6 - (22.4%» \
5 :_ (10.6%)
I— _ \A B
4- (35.6%)
2iiiiiiiiiliiiiiiililLiiiLiiii
0 5 10 15

Magadiite Silicate Loading (wt %)

Figure 2.9 Effect of octadecylammonium and octadecylarnine on the tensile modulus
of epoxy-exfoliated magadiite nanocomposites prepared from C18-magadiite-PF. The
solid line is for composites formed by including the onium ion and neutral amine content
of the organo magadiite as a curing agent for epoxide crosslinking and reducing the
amount of poly(oxypropyleneamine) accordingly; the dash line is for composites formed
by disregarding the cross-linking reactivity of the gallery onium ion and amine in the
initial intercalate and curing the resin with 1 equiv of poly(oxypropyleneamine). The
values given in parenthesis for curve B are the molar fractions of excess alkylamine
functional groups contributed by the onium ions and free amine relative to
poly(oxypropyleneamine).

49

 

       

 

 

 

 

90 _
3 (35.6 %)V B
80 :- A: C13 onium ion ,
A : and amine counted ,’
o\° 7o 1 ,
g E B: C18 onium ion ”
<3 60 '_ and amine not counted I
m h I
{a = ,
.- l
g 505' (22.4%) ,’
:3 - v A
40 : , ’
E (10.6%) , , ’
30 r , V
2011L114LllllllllllLlJLllllllll
O 5 10 _ 15

Magadiite Silicate Loading (wt °/o)

Figure 2.10 Effect of octadecylammonium and octadecylamine on the strain-at-break
values of epoxy-exfoliated magadiite nanocomposite prepared from C18-magadiite-PF.
The dashed line is for composites formed by not counting onium ion and amine content
of the organo magadiite as contributing to the stoichiometry for epoxide crosslinking, the
solid line is reversed. The values in parenthesis give the amount of excess amine curing
agent due to the organo magadiite component.

 

50

The exfoliated nanocomposites are superior not only in their tensile properties.
We also measured the chemical stability and solvent resistance for the exfoliated

nanocomposites prepared from C18-magadiite-PF. The results are listed in Table 2.2.

Table 2.2 Chemical and Solvent Resistance of Epoxy-Exfoliated Magadiite
Nanocomposites Prepared From C18-magadiite-PF (9.1 wt %).
Values are the Immersion Weight Gain (wt %) after a Certain

 

 

Period.
materials 10% distilled 30% 5% methanolb toluenec
NaOHa Hzoa H2804a acetic acida
pristine polymer 1.6 2.5 16.7 13.4 76.5 189
C18-magadiite-PF 1.5 1.6 7.2 9.6 58.5 136

 

aweight gain after 15 days. bweight gain after 48 hr. cweight gain after 24 hr.

Since the pristine epoxy polymer has already offered good inertness to basic and aqueous
solution uptake, we do not expect an impressive improvement for nanocomposites in
these two cases. For inorganic acid and organic acid solutions, the exfoliated
nanocomposite containing 9.1 wt % C18-magadiite-PF did show a significant reduction
in uptake. The uptake of methanol and toluene was reduced substantially for the
exfoliated nanocomposites. On the other hand, we observed that the barrier to solvent
uptake by conventional and intercalated composites was not as significant as for the
exfoliated nanocomposite. These barrier properties parallel tensile properties. Figure
2.11 shows the kinetic solvent uptake data for the nanocomposites prepared from C18-

magadiite-PF relative to the pristine epoxy polymer.

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52

The most significant result we derive from the epoxy-magadiite system relative to
epoxy-smectite nanocomposite is the unique transparent optical properties. As shown in
Figure 2.12, the magadiite composites are much more transparent than the corresponding
smectite composites at the same loading. This result suggests that the refraction index of
magadiite nearer matches that of the organic matrix or that the magadiite is fully
exfoliated more than the smectite clay. Future studies will address the transparent
properties, which represent a significant milestone in the development of polymer-clay

nanocomposites.

2.4 Conclusions

In conclusion, three organo magadiite intercalates have been synthesized. The
lateral monolayer intercalate, which contains only intercalated onium ions and no free
amine, is not organophilic enough to be intercalated by epoxy monomer and curing agent.
The lipid bilayer intercalate can be exfoliated into the epoxy matrix, but the high gallery
concentrations of onium ion and free amine compromise the properties of the matrix by
participating in the curing process. Thus, the reinforcing benefit of the nanolayers is
diminished. The paraffin intercalate is best suited for exfoliation into an epoxy matrix,
because the gallery concentration of onium ion and free amine is sufficient to impart a
hydrophobic environment and allow monomer intercalation without greatly disrupting the
integrity of the polymer network. The transparent properties of the composites, together
with the barrier film properties of these materials should make them especially attractive

for packaging materials and protective films.

S3

‘ l
Pristine I
" Polymer

 

 

vi.— --,_.__ 2 _.._A

A B C

Figure 2.12 Comparison of the optical properties among (A) a pristine epoxy polymer;
(B) an epoxy-exfoliated magadiite nanocomposite prepared form C18-magadiite-PF; (C)
an epoxy-exfoliated smectite clay nanocomposite prepared from CH3(CH2)17NH3+ ion-
exchange montmorillonite (from Wyoming, cf. reference 21). The thickness of each
sample is ~ 1 mm. Polymer curing was carried out at 75 0C for 3 h, and followed by 3 h
at 125 0C.

2.5

P?°>‘?‘P‘:“

ll.
12.

13.
14.
15.
l6.
17.
18.

19.
20.

21.

54

References
Usuki, A.; Kawasumi, M.; Kojima, Y.; Okada, A.; Kurauchi, T.; Kamingaito, O.
J. Mater. Res. 1993, 8, 1174.

Usuki, A.; Kojima, Y.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T.;
Kamigaito, O. J. Mater. Res. 1993, 8, 1179.

Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T.;
Kamigaito, O. J. Mater. Res. 1993, 8, 1185.

Okada, A.; Usuki, A. Mater. Sci. Eng. 1995, C3, 109.

Messersmith, P. B.;Gianne1is, E. P. Chem. Mater. 1993, 5, 1064.

Wang, M. S.; Pinnavaia, T. J. Chem. Mater. 1994, 6, 468.

Lan, T.; Kaviratna, P. D.; Pinnavaia, T. J. Chem. Mater., 1995, 7, 2144.
Giannelis, E. P. Adv. Mater. 1996, 8, 29.

Lagaly, G.; Beneke, K.; Weiss, A. Am. Mineral. 1975, 60, 642.

Beneke, K.; Lagaly, G. Am. Mineral. 1977, 62, 763.

Beneke, K.; Lagaly, G. Am. Mineral. 1983, 68, 818.

Schwieger, W.; Heidemann, D.; Bergk, K. H. Revue de Chimie Mine’rale 1985,

f 12,639.

Rojo, J. M.; Ruiz-Hitzky, E.; Sanz, J. Inorg. Chem. 1988, 27, 2785.
Pinnavaia, T. J.; Johnson, I. D.; Lipsicas, M. J. Solid State Chem. 1986, 63,
Dailey, J. S.; Pinnavaia, T. J. Chem. Mater. 1992, 4, 855.

Wong, S. T.; Cheng, S. Chem. Mater. 1993, 5, 770.

Ruiz-Hitzky, E.; Rojo, J. M. Nature 1980, 287, 28.

Sugahara, Y.; Sugimoto, K.; Yanagisawa, T.; Nomizu, Y.; Kuroda, K.; Kato, C.
Yogyo Kyokai Shi 1987, 95, 117.

Fletcher, R. A.; Bibby, D. M. Clays Clay Miner. 1987, 35, 318.

Fukushima, Y.; Okada, A.; Kawasumi, M.; Kurauchi, T.; Kamigaito, 0. Clay
Miner. 1988, 23, 27.

Lan, T.; Kaviratna, P. D.; Pinnavaia, T. J. Chem. Mater., 1994, 6, 573.

55

22. Messersmith, P. B.; Giannelis, E. P. Chem. Mater. 1994, 6, 1719.

Chapter 3

MECHANISM OF LAYERED SILICIC ACID NANOLAYER EXFOLIATION IN
EPOXY-MAGADIITE NAN OCOMPOSITES

3.1 Introduction
The breakthrough represented by nylon-6-clay nanocomposites presages the

development of a new area of composite science and technology based on organic

polymer-inorganic nanolayer interactions. 1'3 In the passed few years, this revolutionary
nanocomposite chemistry has been successfully extended to the other polymer systems
such as polyimide,4v5 epoxy 6’7 and polysiloxane.8 Among these, the epoxy-clay
nanocomposites with a sub-ambient Tg exhibited exceptionally strong reinforcing effects.

For instance, 7.5 vol. % of the exfoliated 10 A-thick silicate layers improve the strength

of polymer matrix stronger by more than 10—fold.9

Approaches to the exfoliation of clay nanolayers have been investigated by

several different research groups using both thermoplastic and thermoset polymers.3’10'l2
The hydrophilic inorganic clay surface usually has to be modified to accommodate the
incoming organic precursors. Generally, an organic cation exchanged reaction is used to
form a hydrophobic organo clay. Then, the organo clay is intercalated by polymer
precursors which are either organic monomers or prepolymers. In situ intercalative

polymerization is a very successful approach for the preparation of exfoliated polymer-

clay nanocomposites.13'15 In this approach, the acidic intragallery cations function as
catalyst centers and cause the intragallery polymerization rate to be competitive with the

extragallery polymerization rate. This competitive polymerization results in the

56

57

exfoliation of the silicate nanolayers in a monolithic polymer matrix. However, this
exfoliation chemistry has only been developed for smectite clay, few other layered

inorganic materials have demonstrated this similar chemistry. It is recognized that clay

nanolayer exfoliation generally improves mechanical properties,7’9vl6 quantitative studies
of the relationship between performance properties and the degree of clay layer
exfoliation are unavailable. In order to facilitate the development and practical utilization
' of this new family of nanocomposite materials, several fundamental chemistry and
physics issues regarding nanocomposite formation need to be elucidated.

In last chapter, we have successfully extended the exfoliation chemistry
developed for smectite clays to magadiite, a typical member of layered silicic acid
family. Layered silicic acids possess some special properties, which are not available to
smectite clays, so another modeling system has been provided to address the issues
relative to the exfoliation. Optimistically, the exfoliation of layered silicic acids in
polymer matrices may contribute further understanding of the nanolayer exfoliation
processes, the relationships between the structure, composition, and performance

properties, and the basic physics behind the reinforcement.

3.2 Experimental
Materials. The epoxide resin used to for epoxy-magadiite hybrid composite

formation was poly(bisphenol A-co-epichlorohydrin) (Shell, EPON 828), with MW
~377:

A (CH3 9” 9&3 )0\
CH2- CHCH20+© lc—Q- OCHZCHCHZOLO— p—Q OCHzCH-Cllz
CH3 CH3

n = O (88%); n = 1 (10%); n = 2 (2%).
The curing agent was poly(propylene glycol) bis(2-aminopropyl ether) (Huntsman
Chemical, JEFFAMINE D-2000), with MW ~2000:

58

HZNCHCHz + OCH2|CH -} NH2
(IIH3 CH3 x
x=33.l
The above monomers afford a rubbery-like epoxy polymer matrix with a sub-ambient Tg
of -40 0C. All the other chemicals used in this work were purchased from Aldrich
Chemical Co. and used without further purification.

Synthesis of Nat-magadiite. Nat-magadiite was synthesized according to the
published methods.17 A suspension of 60.0 g of amorphous silica gel (1.0 mol) in 300
mL of 1.11 M NaOH solution (0.33 mol) was heated at 150 0C for 42 h with stirring in a
Teflon-lined stainless steel 1.0 L Parr reactor. The suspension containing Nat-magadiite
was centrifuged, and the solid product was washed twice with 600 mL of deionized water
to remove excess N aOH, and air-dried at room temperature.

Synthesis of Organo Magadiite. Quaternary alkylammonium exchanged
magadiites were prepared in general by the reaction of 10.0 g of Na+-magadiite with 800
mL of 0.117 M CH3(CH2)n-1N(CH3)3+Br' (n = 12, 16, 18) aqueous solution at 65 0C for
48 hr. The products were centrifuged and washed by deionized water until free of Br'.
For reaction with n = 12, 16, the reaction was stopped after 24 hr, and resumed for an
additional 24 hr after the mother liquor had been replaced with a fresh solution of onium
ions. This treatment could be repeated until free of the Na+-magadiite phase, which
could be checked by the x-ray diffraction technique.

Primary, secondary and tertiary long chain alkylammonium exchanged
CH3(CH2)17NH3-n(CH3)n+-magadiites (n = O, l, 2) were prepared by the following
approach. An aqueous suspension containing 15.0 g of Na+-magadiite was combined
with ethanol/water solution containing 0.141 mol of alkylammonium chloride and
alkylamine in a 6:1 molar ratio to form a suspension in 1.0 L of 1:1 (vzv) ethanol/water
total solution. This suspension was stirred at 65 0C for 48 hours. The pH of the reaction

mixture was in the range of 8 - 9. The product mixture was added to an equal volume

59

ethanol and centrifuged. The wet solid product was washed consecutively with one 750-
mL portion of 50 % EtOH, two 750-mL portions of 25 % EtOH and then with water until
free of Cl', and air-dried. The air-dried organo magadiite was ground to a powder with a
particle size smaller than 270 mesh (53 um) and stored forfurther use.

Preparation of Epoxy-Magadiite Composites. Equivalent amounts of epoxide
resin and poly(oxypropyleneamine) curing agent were mixed at room temperature for 30
min. The desired amount of organo magadiite was added to the epoxide-
poly(oxypropyleneamine) mixture and stirred for another 60 min. This mixture was
outgassed in a vacuum oven and poured into a stainless steel mold for curing at 75 0C
for 3 h and, subsequently, at 125 0C for an additional 3 h.

X-ray Powder Diffraction (XRD). XRD patterns were recorded on a Rigaku
rotaflex 200B diffractometer. Samples of wet solid organo magadiite products, epoxy-
solvated magadiites, and uncured epoxy-magadiite composites were prepared by applying
thin films on glass slides. Cured composite specimens were prepared by mounting a flat
rectangular sample into an aluminum holder.

Chemical Analysis. CHN analysis is determined by combustion of samples in an
oxygen atmosphere at 980 oC. Na and Si are determined by ICP analysis.

Thermal Analysis. Thermogravimetric analyses (TGA) were performed using a

Cahn TG System 121 thermogravimetric analyzer. Samples were heated to 750 0C at a

heating rate of 5 °C/min under N2 atmosphere.

Surface Area Measurement. N2 adsorption-desorption isotherms were
determined on a ASAP 2010 Sorptometer at liquid N2 temperature using a static sorption
mode. Samples were outgassed at 150 0C and 10'5 Torr for 12 h. Surface areas were
determined using BET plots.

Mechanical Measurement. Tensile testing was performed at ambient

temperature according to ASTM procedure D3039 using a SFM-ZO United Testing

System.

60

Chemical and Solvent Resistance. The resistance of the composite materials to
solvent swelling was obtained according to ASTM procedure D543. The specimens were
immersed in the desired reagent and removed periodically to measured the weight gain

until equilibrium was reached.

3.3 Results and Discussion
3.3.1 Synthesis of Organo Magadiites With Paraffin Structures

In the approach of in situ intercalative polymerization for the synthesis of
polymer-clay nanocomposites, the intragallery acidic catalytic centers play a very
important role to afford a competitive intragallery polymerization rate.6 By using onium
ions of different Bronsted acidity, it is possible to achieve a series of polymer-layered
silicate nanocomposites with different degree of layer separation.

Recently, we developed a new approach to paraffin-like intercalated phases of
organo magadiite using long chain primary alkylammonium surfactants and a neutral
amine as gallery guests. This significantly reduced the amount of neutral amine needed
to form the intercalate. In the present work, we have extended this new synthesis
approach to prepare the other organo magadiites with paraffin-like gallery structures
using long chain secondary and tertiary alkylammonium ions in place of primary onium
ions. A quaternary alkylammonium ion exchanged magadiite was also prepared using
the previous known method. 13 Figure 3.1 shows the powder x-ray diffraction patterns for
CH3(CH2)17NH3-n(CH3)n+-magadiite (n = O, 1, 2, 3). Table 3.1 lists their intragallery
compositions, dom basal spacings and gallery structures. The primary and secondary
alkylammonium exchanged magadiites with n = 0 and 1, require a small amount of
neutral amine to achieve this particular paraffin-like structure. We also find that when
amine head groups are bigger, less neutral amine is needed for achieving this paraffin-
like structure. Consequently, the tertiary alkylammonium exchanged magadiite is more
truly an ion exchanged product without additional neutral amine molecules. Due to the

hydrophobic nature of CH3(CH2)17NH3-n(CH3)n+ (n = 0, l, 2) salts and their amine

61

analogue, the sodium ions are rarely retained in these paraffin structures as judged by
both elemental analysis and TGA results. The residue of sodium ions in the gallery for
the quaternary alkylammonium exchanged magadiite can be overcome by applying
multiple ion-exchange reactions. The small fraction of. protons existed in C18A3M-
magadiite is due to multiple washing by water in an effort to remove the extra salts. It is
noteworthy that the initial products obtained by ion-exchange reaction with
CH3(CH2)17NH3-n(CH3)n+-magadiite (n = 0, 1) are a mixture of lipid bilayer and
paraffin phases as shown in Figure 3.2 (also see Figure 2.3). The washing process can
unify the phases existing in reaction media, however, this process has to be adjusted so
that a lateral monolayer structure can be avoided except for n = 3. It is has been proved
that organo magadiites with the lateral monolayer structure are not expandable by
polymer precursors due to the high electrostatic interactions between their silicate

nanolayers and the exchange cations.

62

 

 

 

 

Q

 

 

Relative Intensity

 

 

37.4 A
4 n=1
36.2 A
n=O
\ “15.6 A Nat-magadiite

I'llIFIIU'IrTTTIII'jII'jIUllj

O 5 1O 1 5
29 (Degrees)

 

 

 

Figure 3.1 X-ray powder diffraction patterns of air-dried Na+-magadiite and
CH3(CH2)17NH3_n(CH3)n+-intercalated magadiites (n = 0, 1, 2, 3). The organo

magadiites have paraffin-like gallery structures with onium ion and free amine
compositions given in Table 1.

63

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37.4 A

Relative Intensity

A A
IJJJJIIIIlllllllllllJlUJlllllllll
1 2 3 4 5 6 7 8
26) (Degrees)

 

 

 

Figure 3.2 Effect of washing on the formation of C18A1M-magadiite. The product
in Pattern (A) was obtained by centrifuging the secondary alkylammonium exchanged
reaction mixture without further treatment. The product in pattern (B) was formed when
the onium ion exchanged reaction mixture was washed with an equal volume of ethanol.
The paraffin phase in pattern (C) was obtained by washing the product of pattern (B) with
50% EtOH.

65

3.3.2 Swelling Properties of Organo Magadiites

All of the paraffin structures described in Table 3.1 can be intercalated by epoxide
resin at 75 0C to give intercalates with about 42 A basal spacings (Table 3.2). In this
epoxide intercalation reaction, the onium ions reorient from an inclined angle to a
perpendicular orientation so that extra intragallery space is formed to accommodate the
intercalated epoxide resin. The solvation of these paraffin structures by the
poly(oxypropyleneamine) (D-2000) curing agent gives products with different gallery
structures. For instance, C18-magadiite-PF and C18AlM-magadiite can be intercalated
by the D-2000 curing agent. For C18-magadiite-PF, the basal spacing increases from
38.2 A to 63.3 A; whereas for C18A1M-magadiite, the basal spacing increases from 37.4
A to 52.4 A initially. This intercalation process is facilitated by the proton transfer
reaction between the curing agent and gallery onium ion. However, the solvation of
C18A2M-magadiite and C18A3M-magadiite by D-2000 is not thermodynamically
favored due to the lower acidity of the onium ions and the presence of metal ions for
C18A3M-magadiite. The co-intercalation of the mixture of epoxide resin and D-2000
curing agent was quite successful for all of the organo magadiites. The different
increased basal spacings are derived from the solvation abilities of D-2000 curing agent
and epoxide resin. All these solvation results are summarized in Table 3.2 and verified
by our XRD studies. The XRD results for the co-intercalation of Cl8A2M-magadiite and
C18A3M-magadiite by the mixture of epoxide resin and D-2000 curing agent at an

elevated temperature are shown in Figure 3.3 and Figure 3.4, respectively.

66

Table 3.2 Basal Spacings (d001, A) for CH3(CH2)17NH3-n(CH3)n+-magadiite
Paraffin Structures Under Air-dried Conditions and Solvated by Epoxy Resin
(EPON 828), Poly(oxypropy1eneamine) (D-2000) Curing Agent, and a
Stoichiometric Mixture of These Polymer Precursors

 

n air-dried epoxy D-2000 epoxy & D-2000

 

solvated solvated solvated
0 38.2 42.5 63.3 62.7
1 37.4 41.9 52.4 51.4
2 32.0 42.5 32.4 43.1
3 34.1 41.7 34.2 39.1

 

67

 

 

 

 

 

 

 

 

 

 

 

 

a

C

G)

.0—0

s

Q)

.2 C

4—0

a

Q)

‘I B
IIITrlTTlllIIIIIIIIIIIITIIIIIIIlll

1 2 3 4 5 6 7 8
26) (Degrees)

Figure 3.3 X-ray diffraction patterns of products prepared by reaction of C18A2M-
magadiite (15 wt % loading) with a stoichiometric mixture of epoxide resin and
poly(oxypropyleneamine) curing agent under the reaction conditions as follows: (A) 75

°C, 10 min; (B) 75 °C, 90 min; (C) 75 °C, 120 min; (D) 75 °C, 3 h and 125 0C, 20 min.

68

 

 

 

 

 

 

 

 

39.1A
34.0A
.3
2
93 \ C
E W
(D
.2
‘66
a:
A
....-...,....,.........,........-

1 2 3 4 5 6 7 8
26) (Degrees)

Figure 3.4 X-ray diffraction patterns of products prepared by reaction of C18A3M—
magadiite (20 wt % loading) with a stoichiometric mixture of epoxide resin and
poly(oxypropyleneamine) curing agent under the reaction conditions as follows: (A) 75

°C, 10 min; (B) 75 °C, 30 min; (C) 75 °C, 45 min.

69

3.3.3 Exfoliation of CHAIM-Magadiite in an Epoxy Matrix

The co-intercalation of C18A1M-magadiite by a 2:1 molar mixture of epoxy resin
and curing agent as a function of time is investigated by XRD (Figure 3.5). The solvation
of C18AlM-magadiite by resin and curing agent is initially very slow at room
temperature (Figure 3.5A), since the viscosity of the system is high and diffusion process
is very low. Upon heating at 75 °C, the C18A1M-magadiite is rapidly solvated as
evidenced by a broad x-ray peak at low angle (Figure 3.5B). This two-phase mixture is
quickly replaced by a single (1001 peak at 51.4 A (Figure 3.5C) corresponding to the
onium ions totally reoriented as an inclined bilayer conformation to accommodate the
intercalated epoxy resin and curing agent. The 51.4 A peak is eventually replaced by
another peak at 57.0 A (Figure 3.5D) to provide more intragallery space, presumably
through more extension of alkyl chains. Finally, a phase with 62.7 A basal spacing
appears, corresponding to the onium ions with a lipid bilayer orientation (Figure 3.5B).
This phase is crucial to achieving an exfoliated state instead of an intercalated state of the
layer silicates. In this phase, the organo onium ions adopt an optimized orientation to
afford maximum intragallery space for the intercalated species. The interactions between
silicate nanolayers and polymer precursors are balanced by the electrostatic forces
between silicate layers. Beyond this stage, the interactions between intercalates and
silicate layers are more favored than electrostatic interactions between silicate layers.
This hypothesis is supported by the following XRD results. Figure 3.5F indicates that the
platelets do not have to keep their original stacking registry when intragallery region has
been expanded beyond that critical region. The final x-ray amorphous phase (Figure
3.5G) suggests that an exfoliated state had been achieved in which silicate layer
exfoliated dispersed in organic matrix. Our previous results have showed that the
primary alkylammonium exchanged C18-magadiite-PF also goes through this

intercalation-exfoliation process with slight difference.

 

 

 

62.7 A

f

57.0 A

f

 

51.4A

Relative Intensity

 

38.2 A
B .
A

IllrlfinllltIFIIIIIIIIIIIIIIIUIII

2 3 4 5 6 7 8
29 (Degrees)

 

 

 

 

_-L

Figure 3.5 X-ray diffraction patterns of products prepared by reaction of C18A1M-
magadiite (15 wt % loading) with a stoichiometric mixture of epoxide resin and
poly(oxypropyleneamine) curing agent (epoxide group to amine group molar ratio of
2:1). Reaction conditions were as follows: (A) 25 °C, 30 min; (B) 75 °C, 10 min; (C) 75
0C, 15 min; (D) 75 0C, 30 min; (E) 75 °C, 60 min; (F) 75 °C, 70 min; (G) 75 °C, 90 min.

71

3.3.4 Mechanism of Silicate Nanolayer Exfoliation For Organo Magadiites

Figure 3.6 illustrates a pathway for the formation of an epoxy-exfoliated
magadiite nanocomposite formed from an acidic CH3(CH2)17NH3-n(CH3)n+-magadiite
with n = 0, l. The organophilic magadiite with a paraffin-like structure (Figure 3.6A) is
the starting point which can be easily co-intercalated by stoichiometric ratio of the
epoxide resin and D-2000 curing agent at an elevated temperature. An intercalate is
formed with the gallery height expected from a lipid-like bilayer formation of onium lions
(Figure 3.6B). The intragallery acidic onium ions catalyze the epoxide polymerization to
afford a competitive intragallery polymerization rate relative to extragallery rate, so the
gallery will expand beyond a bilayer region (Figure 3.6C). The relative rates of reagent
intercalation, chain formation and network cross-linking must be controlled to form the
final epoxy-magadiite exfoliated nanocomposite (Figure 3.6D). It is seems that the
intercalation is thermodynamically favored especially at elevated temperature, because
the overall exfoliation process requires an entropy increase. The onium ion protons play
an essential role in this in-situ polymerization process to satisfy all these requirements.
The D-2000 curing agent has a mild reactivity to give chain formation privilege over
cross-linking formation, in another word, the gel time is also an important factor to

decide the morphology of the final product.

72

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73

3.3.5 Effect of Gallery Acidity of Organo Magadiites on Formation of

Nanostructures

The XRD patterns (Figure 3.7) of the cured composites prepared from
CH3(CH2)17NH3-n(CH3)n+-magadiite (n = O, l, 2, 3) show that three kinds of
nanocomposite structures have been prepared. Two exfoliated epoxy-magadiite
nanocomposites have been obtained from C18-magadiite-PF and C18A1M-magadiite for
which the galleries are occupied by acidic primary or secondary
alkylammonium/alkylamine with a paraffin structure. These exfoliated nanocomposites
are characterized by the absence of XRD peaks at low angle. An intercalated
nanocomposite is obtained from quaternary alkylammonium exchanged C18A3M-
magadiite, wherein the galleries are non-acidic. The formation of an intercalated
nanocomposite for this exchange derivative is quite expected because of the absence of
protons to catalyze the intragallery polymerization reaction and the strong interaction of
the quaternary alkylammonium ions with the silicate surface. Interestingly, an ordered
exfoliated nanocomposite is achieved by using C18A2M-magadiite with mild Brdnsted
acidity. In this ordered exfoliated nanocomposite, the registry of the silicate nanolayer
stacking is sufficient to give x-ray scattering along 001, but the gallery height is far larger
than the value expected for an intercalated nanocomposite with a bilayer of long chain
onium ions (cf., Figure 1.4). This result further supports our previous hypothesis
concerning the exfoliation of silicate nanolayers. Because tertiary alkylammonium ions
provide weak acidity to catalyze the intragallery epoxy polymerization, the gallery
expansion will be mainly determined by the initial monomer loading in the gallery.
Further expansion to the exfoliated state is precluded by the high viscosity and slow
diffusion of additional monomers into the gallery. Our x-ray results show that the 2-
dimensional structure of magadiite is retained for all nanocomposite structures, as
indicated by the several strong in-plane peaks. It is noteworthy that the ordered

exfoliated nanocomposite is generally limited to synthetic layered silicates. The

74

homogenous distribution of layer charges in a synthetic clay results in a homogeneous
intragallery polymerization rate. Most nature clays with a heterogeneous charge
distribution will give broad x-ray peaks at low angle, because intragallery polymerization
rate is not uniform from gallery to gallery.
3.3.6 Relationship Between Nanostructures and Performance Properties

It is interesting to note the relationship between mechanical performance and the
extent of layer separation for epoxy-magadiite nanocomposites. The mechanical
properties for the epoxy nanocomposites prepared from ClS-magadiite-PF have
investigated in chapter 2. A comparison of tensile properties for the nanocomposites
prepared form CH3(CH2)17NH3-n(CH3)n+-magadiites (n = 1, 2, 3) is shown in Figure
3.8. The tensile strengths for all three types of nanocomposites show a dependence on
magadiite silicate loading. However, the silicate loading is substantially more effective
for the exfoliated nanocomposites than the intercalated nanocomposites. The intercalated
nanocomposites give a trend similar to the conventional composites even they have
multiple molecular layers of epoxy polymer inserted into magadiite galleries. It is also
known that the loading percentage of reinforcing agents will not be quite effective for
phase segregated composites. This suggests that the tensile reinforcement properties for
nanocomposites will not become additive until the layer separation has reached a certain
threshold or at least part of silicate nanolayers reached that stage. The ordered exfoliated
nanocomposites prepared from C18A2M-magadiite behave more or less like the
disordered exfoliated nanocomposites prepared from ClSAlM-magadiite. However, the
performance of ordered exfoliated nanocomposites is not as quite good as the disordered
exfoliated nanocomposites. The average layer separation for disordered exfoliated
nanocomposites is more than 100 A and substantially larger than 78 A for the ordered
exfoliated nanocomposites. We can conclude that the tensile properties are dependent on
the extent of layer separation. There is a structural reason for this effect. For an ideal

exfoliated polymer-layered silicate nanocomposite, the average distance between silicate

75

nanolayers will be dependent on the silicate loading. Therefore, alternation of the
organic and inorganic phases in an exfoliated nanocomposite is more uniform than in an
intercalated nanocomposite. From a microscopic point of view, the homogeneity of an
exfoliated nanocomposite is higher than an intercalated nanocomposite. This improved
homogeneity for exfoliated nanocomposites optimizes a strong molecular interactions
between the silicate layers and the polymer network. So, the improved interfacial
properties of exfoliated nanocomposites is an important feature of this superior tensile
performance. To further illustrate the importance of layer separation on properties, we
prepared a series of intercalated nanocomposites with different layer separation (Figure
3.9). By simple variation the chain lengths of quaternary alkylammoniums, the basal
spacing of intercalated nanocomposites can differ from 35.5 A to 41.3 A. Figure 3.10
showed that all of these intercalated nanocomposites exhibit a similar dependence on
silicate loading. This consequence suggests that a dramatic improvement in tensile
properties will not happen until the layer separation greatly exceeds the value for a onium
ion bilayer in the galleries.

The relationship between properties and extent of layer separation is further
demonstrated in Figure 3.11. The results show that both kinetic and equilibrium solvent
uptake data for the nanocomposites prepared from CH3(CH2)17NH3-n(CH3)n+-magadiite
(n = 1, 2, 3) relied on their nanophase morphology. These results further prove that the

properties of polymer-layered silicate nanocomposites are closely linked to the extent of

layer separation.

76

 

 

Relative Intensity

 

 

 

 

 

IlllllllllllllllllllfilllIII

1 2 3 4 5 6 7 8
28 (Degrees)

Figure3.7 X-ray diffraction patterns of cured epoxy polymer-magadiite

nanocomposites prepared from CH3(CH2)17NH3-n(CH3)n+-magadiites with a paraffin
gallery structure (cf. Table 3.1). The organo magadiite loading is 15 wt % (n = 0) or 20
wt % (n = 1, 2, 3). Polymer curing was carried out at 75 0C for 3 h, followed by 3 h at
125 °C. The primary and secondary onium ions (n = 0, 1) give highly exfoliated
(disordered) nanocomposites (d > 90 A). The tertiary onium ion derivative (n = 2) also
gives an exfoliated nanocomposite, but the layer separation is highly ordered ((1 = 78 A).

The quaternary onium ion exchange form (n = 3) forms an intercalated nanocomposite (d

=41 A).

77

 

 

 

 

 

4
- n=1
5‘33-
g u-
5 I _/v “=2
a h
C
22-
(‘5 .—
2 : --~'fln=3
‘é’ .
1)—
" L
olllllllllllllllIJJALLIIILILLII
0 5 10 15

Magadiite Loading (wt % SiOz)

Figure 3.8 A comparison of the tensile strengths of epoxy-magadiite nanocomposites
prepared from CH3(CH2)17NH3-n(CH3)n+-magadiites. The magadiite silicate loading
(expressed on wt % SiOz) was determined by calcining the composites in air at 650 0C
for 4 hours using a heating rate of 2 oC/min. The secondary alkylammonium (n = 2) and

the free amine content of CH 3(CH2)17NH2CH3+-magadiite was counted as contributing
to the stoichiometry for epoxide cross-linking at magadiite loadings > 10 wt %.

 

A 34.1 A

 

 

 

.5. / \ n=18
8
‘_E 32.0 A
g
g
g n=16
28.3 A
n=12

 

 

 

 

 

B 41.3A

 

Relative Intensity
E
m

 

 

 

llIllllllllllllllllllllllljlllllllllllllell

12345678910
29(DegreeS)

 

 

Figure 3.9 X-ray diffraction patterns of CH3(CH2)n-1N(CH3)3+-magadiites in

different physical states: (A) organo magadiite; (B) intercalated magadiite (10 wt %) in
an cured epoxy nanocomposite.

79

 

 
  
 
 

 

 

 

2.0
r
1; -
m h
.5,
51.5—
6 1-
C _
9 h
(7)
2
210 ——O—n=12
11’ —<>---n=16
----A—-~n=18
0.5llllllllllllllllllllllelLlll
° 5 10 ' 15

Magadiite Silicate Loading (wt %)

Figure 3.10 A comparison of the tensile strengths of epoxy-magadiite intercalated
nanocomposites prepared from CH3(CH2)n-1N(CH3)3+-magadiites. The magadiite

silicate loading was determined by calcining the composites in air at 650 0C for 4 hours

using a heating rate of 2 °C/min.

8O

 

 

 

 

200
d o
- o
. o o
1 50" o O A A
O A
. A A
. o A o o
g ' O A o o
E - O A 0 v V v V
8 100: o A o 3 V
.. A
.c . 0 V
.g’ _ Q A O V
a . o v
50- A V o pristine 189%
4 8 A n=3 159%
. 0 ”=2 1400/0
4 V n=1 118%
O-‘TL I I r I I T—I I I l I I I I I I I I I I I I I I
0 100 200 300 400 500

Time (Minutes)

Figure 3.11 Toluene uptake by nanocomposites prepared from CH3(CH2)17NH3-

“(CH3)n+-magadiites (n = 1, 2, 3). The tabulated values in the insert are equilibrium data
determined by the immersion weight gain after 24 hr. The loading of organo-magadiite is
10 wt % for each epoxy-magadiite composite.

81

3.3.7 Effect of Secondary Alkylamine and Alkylammonium ions on Mechanical

Properties

The comparison of the tensile properties for epoxy-magadiite and epoxy-
exfoliated smectite clay exfoliated nanocomposites is. shown in Figure 3.12. At low
silicate loading, they show similar reinforcing performance. At high silicate loading, the
epoxy-smectite clay nanocomposites are better than magadiite in terms of tensile strength
and modulus. The difference in the charge density between magadiite and
montmorillonite will result in different loading of onium ions. Our previous studies have
shown that the gallery alkylammonium exchange cations and alkylamine are incorporated
into epoxy network to form dangling polymer chains. The formation of these dangling
chains compromises the advantages of silicate layer exfoliation, particularly, at high
magadiite loading. Both the primary and secondary ammonium and become involved in
the cross-linking reaction.

The effect of methyloctadecylamine on tensile properties is shown in Figure 3.13.
The dashed line shows that both the tensile strengths and the moduli will exhibit a
maximum if we do not compensate for the curing by the gallery species by reducing the
amount of D-ZOOO curing agent. Interestingly, the polymer elasticity is affected in a
favor way (Figure 3.14A) for the epoxy-magadiite exfoliated nanocomposites. The effect
of intercalation on the elasticity for the epoxy-magadiite intercalated nanocomposites is
not as significant as exfoliation (Figure 3.143). This unique behavior is totally opposite
from conventional composites in which a reinforcement in modulus is accompanied by a

sacrifice in the polymer elasticity (Figure 3.14C).

82

 

>

IIII'I’III'IIIj

      

Tensile Strength (MPa)

- - -o- - - C18-montmorillonite
___v__ C18A1M-magadiite
—-A— C18-magadiite-PF

 

 

lLLllllllllllllllJlllll

O 2 4 6 8 10 12
Silicate Loading (wt %)

 

   

 

 

 

16 P
' B
'- O
14 _-
r 2 "
a ‘2 :'
(I) .-
2 10 -
a I
8 I
2 8 ':
g ..
6 .-
" I ---0 .-- CtB-montmorillonite
4 ——v—— C18A1M-magadiite
—A— C18—magadiite-PF
2 4 J 4 l l l 1 l L l l L J l 1 l L l I l l l l

 

O 2 4 6 * 8 10 12
Silicate Loading (wt %)

Figure 3.12 A comparison of (A) the tensile strengths and (B) the tensile moduli
between epoxy-magadiite exfoliated nanocomposites prepared from C18A1M-magadiite

and C18-magadiite-PF, and epoxy-smectite clay exfoliated nanocomposite prepared from
CH3(CH2)17NH3+ ion-exchange montmorillonite (from Wyoming). The silicate loading

was determined by calcining the composites in air at 650 0C for 4 hours using a heating

rate of 2 OC/rnin.

83

 

    
    
 
 
    
       
  

   

 

 

 

 

 

 

711‘
n.
3 (11.6%)
g A.
5 2 “.‘(1a.4%)
5 3‘ _
g _ “~.(26.1%)
'as ‘n
8 1
1..
o , nnnnnnnnnnnnnnnnnn I nnnnnnnnn
O 5 1O 15
Magadiite Silicate Loading (wt %)
12
’_ B
A r
d? 10 -
g -
- (11.6%)
g 6_ ‘. (13.4%)
2 ‘A\
O \
I'- 4 \‘
‘x (26.1%)
‘A
2 111111111111111111 l 11111111

 

 

 

0 5 1O 15
Magadiite Silicate Loading (wt %)
80
C

,.. 70 (26.1%)
a ,A
35 6° .
(9 (18.439' ’
:50 (11.6%)_,./A
.- A.
5'7; 40

30

20

 

Magadiite Silicate Loading (wt %)

Figure 3.13 Effect of methyloctadecylammonium and methyloctadecylamine on (A)
the tensile strengths; (B) the tensile moduli; (C) the strain-at-break values of epoxy-
exfoliated magadiite nanocomposites prepared from C18AlM-magadiite. The solid line
is for composites formed by including the secondary onium ion and neutral amine content
of the organo magadiite as curing agents for epoxide cross-linking and reducing the
amount of poly(oxypropyleneamine) (D-ZOOO) accordingly; the dash line is for
composites formed at an epoxy-D-2000 stoichiometry that disregards the reactivity of the
gallery secondary onium ion and amine in the initial intercalate. The values in
parenthesis for the dash curves are the molar fraction of excess secondary amine
functional groups relative to D-2000.

84

 

60

    

 

 

 

 

L.
I A: exfoliated
so_L
e: 40 :-
gg - B: intercalated
2 : ,0----—---------o
e 30: '
m )-
.é
m
g; 20 ’ x .
a) : x‘ C: Conventional
- “~-__v____-_-—-""V'""'------_- -—V
10 :
llllllliLllllllllllllllllilll
0
0 5 10 15

Magadiite Silicate Loading (wt %)

Figure 3.14 Comparison of the stain-at-break values among the exfoliated epoxy-
magadiite nanocomposites prepared from C18AlM-magadiite, the intercalated
nanocomposites prepared from C18A3M-magadiite, and the conventional composites
prepared from C18-magadiite-LM with a lateral monolayer structure.

85

3.3.8 Effect of Alkylammonium Ions on Thermal Stability of N anocomposites

We also have investigated by TGA methods the effect 'of long chain
alkylammonium on the thermal stability of the nanocomposites prepared from
CH3(CH2)17NH3-n(CH3)n+-magadiites (n = O, 1, 2,.3). Figure 3.15 compares the
behavior for the exfoliated nanocomposite prepared from C18A1M-magadiite and the
intercalated nanocomposite prepared from C18A3M-magadiite. The exfoliated
nanocomposite prepared from C18-magadiite-PF is similar to ClSAlM-magadiite, and
the exfoliated nanocomposite prepared from C18A2M-magadiite is similar to C18A3M-
magadiite. The thermal stability for the exfoliated nanocomposites prepared from C18-
magadiite-PF and C18A1M-magadiite is as good as pristine epoxy polymer. This has
also been observed in the case of nylon 6-c1ay exfoliated nanocomposites. The lower
temperature weight lose for the intercalated nanocomposite is attributed to the
decomposition of isolated quaternary alkylammonium cations from the cross-linked
polymer network. An analogous weight lose is observed for C18A3M-magadiite (Figure
3.15D). The TGA results also verify that the gallery onium ions are incorporated into the
polymer network; otherwise the exfoliated ClSAlM-magadiite nanocomposite should

show a similar low temperature decomposition of onium ions as Figure 3.15C.

86

 

 

100

 

80

Weight Loss (%)

 

 

 

lllLllllllLllllll_LlllJlllllllllLllll

100 200 300 400 500 600 700
Temperature (°C)

 

o IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII

Figure 3.15 Thermogravimetric analysis curves for (A) epoxy-exfoliated magadiite
nanocomposite prepared from C18AlM-magadiite and (B) intercalated nanocompsoite
formed from Cl8A3M-magadiite. The loading of organo magadiite is 20 wt % for each
epoxy-magadiite nanocomposite. Curves (C) and (D) are for the pristine organo
magadiites of Cl8A1M-magadiite and C18A3M-magadiite, respectively. The analysis

was carried out in N 2 atmosphere with a heating rate of 5 0C/rnin.

87

3.3.9 More Evidences for Morphological Differences Between Intercalated and

Exfoliated Nanocomposites

It is interesting to note the differences between intercalated and exfoliated
nanocomposites using the other techniques such as surface area measurement by N2
adsorption-desorption. The intercalated nanocomposite prepared from C18A3M-
magadiite and the exfoliated nanocomposite prepared from C18-magadiite-PF were
calcined at 650 0C for 4 hours. The X-ray results (Figure 3.16) showed strong in-plane
magadiite peaks in both cases. However, the layer restacking upon calcination was quite

different in these two cases. The silica from the intercalated nanocomposite showed a

narrow (1001 peak coheres the (1001 peak for the exfoliated nanocomposite was a very

broad shoulder, which indicated less restacking order. The N2 adsorption-desorption

measurements for calcined residues of the exfoliated and the intercalated
nanocomposites, shown in Figure 3.17, verify the above XRD results. A much larger
texture porosity and surface area were observed for the magadiite recovered form the
exfoliated nanocomposite than from the intercalated nanocomposite. Most of the
exfoliated silicate nanolayers will retain their exfoliated morphology to form the familiar
card-house structure upon calcination. The card-house structure will result in a
significant increase in texture porosity and surface area, in addition to a very broad 001
XRD reflection. These latter experiments allow us to explore the overall phase
morphology for intercalated and exfoliated nanocomposites. The results help us to

deepen understanding the relationship between nanophase morphology and properties.

88

 

 

”T 11.911
5001'

 

    
 
   

Exfoliated

 

Intercalated

 

AJAAAAAAA‘LAAAAAAALIAA—LAAJ AAAAAAAAAAAAA

 

 

 

 

E
(D
C
Q)
E 5 e 7 a 9
3 I 29(Dogroos)
z I 1 A
% I Exfoliated
I
I l
l
| l B
1 ~ Intercalated
I
llllllLllllllllllllllllllllLJ
0 10 20 30 40 50 60

26) (Degrees)

Figure 3.16 X-ray powder diffractions for the silica formed from upon calcination of
(A) the exfoliated nanocomposite prepared form ClSA-magadiite-PF; and (B) the
intercalated nanocomposite prepared from C18A3M-magadiite. The expanded inset for
the d001 peaks was obtained using a step scan mode. The calcination condition is at 650

0C for 4 h in air using a heating rate of 2 °C/min.

89

 

250

200‘

.5
0|
ciD

Volume Adsorbed (cm3/g. STP)
ES
0
I l

 

 

 

 

 

0 0.2 0.4 0.6 0.8 1
Relative Pressure (P/Po)

Figure 3.17 N2 adsorption-desorption isotherms for the silicas obtained by calcination

of an epoxy-exfoliated magadiite nanocomposite prepared from C18-magadiite-PF (top
curve) and an intercalated nanocomposite prepared from C18A3M-magadiite (bottom
curve). The organo magadiite loading for each composite is 10 wt %. The calcination

was carried out at 650 0C for 4 hr in air using a heating rate of 2 OC/min.

90

3.4 Conclusions

Organo magadiites intercalated by RNH3-n(CH3)n+ forms of onium ions with
paraffin-like structures allow us to prepare a series of epoxy-magadiite nanocomposites
with different extent of layer separation. The mechanism leading to exfoliation of
magadiite nanolayers is understood by investigating the swelling properties of the each
intercalate. The dependence of the performance properties of epoxy-magadiite
nanocomposites on magadiite loading and the extent of layer separation are the two major
factors governing the mechanical properties of the composites. The exfoliated
nanocomposites are superior to intercalated and conventional composites for their overall
performance due to their specific phase morphology and improved interfacial properties.
However, the exfoliated nanocomposites are more difficult to form than intercalated
nanocomposites. By optimizing reaction condition, the exfoliation chemistry will be
likely to be extended to the other members of layered silicic acid family, and more

sophisticated polymer systems.

3.5

>°9°>'.O‘SJ'

11.

12.

l3.
14.
15.
16.

17.
18.

91

References
Fukushima, Y.; Okada, A.; Kawasumi, M.; Kurauchi, T.; Kamigaito, 0. Clay
Miner. 1988, 23, 27.

Usuki, A.; Kojima, Y.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T.;
Kamigaito, O. J. Mater. Res. 1993, 8, 1179.

Okada, A.; Usuki, A. Mater. Sci. Eng. 1995, C3, 109.

Yano, K.; Usuki, A.; Okada, A.; Kurauchi, T.; Kamigaito, O. J. Polym. Sci., Part
A: Polym. Chem. 1993, 31, 2493.

Lan, T.; Kaviratna, P. D.; Pinnavaia, T. J. Chem. Mater. 1994, 6, 573.

Lan, T.; Kaviratna, P. D.; Pinnavaia, T. J. Chem. Mater., 1995, 7, 2144.
Messersmith, P. B.; Giannelis, E. P. Chem. Mater. 1994, 6, 1719.

Burnside, S. D.; Giannelis, E. P. Chem. Mater. 1995, 7, 1597.

Lan, T.; Pinnavaia, T. J. Chem. Mater. 1994, 6, 2216.

Messersmith, P. B.; Giannelis, E. P. J. Polym. Sci., Part A: Palym. Chem. 1995,
33, 1047.

Pinnavaia, T. J .; Lan, T.; Wang, Z.; Shi, H.; Kaviratna, P. D. ACS Symp. Ser.
1996, 622, 250.

Vaia, R. A.; Jandt, K. D.; Kramer, E. J.; Giannelis, E. P. Chem. Mater., 1996,
8, 2628.

Kato, C.; Kuroda, K.; Misawa, M. Clays Clay Miner. 1979, 27, 129.
Fukushima, Y.; Inagaki, S. J. Inclusion Phenom. 1987, 5, 473.
Messersmith, P. B.; Giannelis, E. P. Chem. Mater. 1993, 5, 1064.

Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T.;
Kamigaito, O. J. Mater. Res. 1993, 8, 1185.

Fletcher, R. A.; Bibby, D. M. Clays Clay Miner. 1987, 35, 318.

Sugahara, Y.; Sugimoto, K.; Yanagisawa, T.; Nomizu, Y.; Kuroda, K.; Kato, C.
Yogyo Kyokai Shi 1987, 95, 117.

c.

US

Chapter 4

HYBRID ORGANIC-INORGANIC NANOCOMPOSITES
FORMED FROM AN EPOXY POLYMER AND LAYERED SILICIC ACIDS
(KENYAITE AND ILERITE)

4.1 Introduction

Layered silicic acids have drawn little attention in the field of organic-inorganic
composites. Even less work has been done for members other than magadiite for this
layered inorganic family of silicates in efforts to use them as reinforcements. In the last
two chapters, we have fully investigated the exfoliation chemistry of magadiite. It should
in principle be possible to realize similar benefits for other layered silicic acids based on

nanolayer reinforcement.
The family of layered silicic acids includes five members in as-synthesized N a+-
exchanged form: kanemite (NaHSi205-nH20), makatite (Na28i409-nH20), ilerite

(Na28i3017-nH20), magadiite (Na28i14029-nH20) and kenyaite (Na28120041-nH20). 1'3
They all can be easily synthesized by hydrothermal methods. By changing the ratio of
starting materials such as sodium hydroxide, amorphous silica and water, and by
controlling the reaction conditions such as reaction temperature and time, one can

produce these alkali-metal-ion layered silicates with similar layer structure, but with
different layer thickness.4'6 The variable aspect ratios of the silicate nanolayers provide
us extra opportunities to elucidate some issues governing the mechanical properties of the
polymer-layered silicate nanocomposites. In this chapter, we extend the exfoliation

chemistry developed for magadiite to the other members of this layered inorganic family.

92

93

At the same time, we address the fundamental issues regarding nanocomposite formation,
such as the nanolayer exfoliation processes, the relationships between structure,

composition, and performance properties, and the basic physics behind the reinforcement.

4.2 Experimental
4.2.1 Materials

The epoxide resin used to for epoxy-magadiite hybrid composite formation was
poly(bisphenol A-co-epichlorohydrin) (Shell, EPON 828), with MW ~377:

CH3 on CH3
Ci/io\ {@h—Q OCH CiiCH o-l-O 'c—Q— OCH c/gfm
2- CHCHzO ' 2 2 n I 2 - 2

CH3 CH3
n = O (88%); n = 1 (10%); n = 2 (2%).
The curing agent was poly(propylene glycol) bis(2-aminopropyl ether) (Huntsman
Chemical, JEFFAMINE D-ZOOO), with MW ~2000:
HZNCHCHZ -[ OCHZC|IH -} NHZ
CH3 CH3 X
=33.1

4.2.2 Synthesis of Layered Silicic Acids and Their Derivatives

Synthesis of Kt-kenyaite. K*-kenyaite was synthesized according to published
methods.5 A suspension of 75.0 g of amorphous silica gel (1.25 mol) in 300 mL of 1.39
M KOH solution (0.42 mol) was heated at 140 0C for 72 h with stirring in a Teflon-lined
stainless steel 1.0 L Parr reactor. The suspension containing K*-kenyaite was
centrifuged, and the solid product was washed twice with 600 mL of deionized water to
remove excess KOH, and air-dried at room temperature.

Synthesis of Nat-ilerite. Na*-ilerite was synthesized according to published
methods.6 A suspension of 142.9 g of amorphous silica gel (2.38 mol) in 300 mL of 3.97
M NaOH solution (1.19 mol) was heated at 105 0C for 9 days with stirring in a Teflon-

lined stainless steel 1.0 L Parr reactor. The suspension containing Nat-ilerite was

94

centrifuged, and the solid product was washed three times with 700 mL of deionized
water to remove excess N aOH, and air-dried at room temperature.

Synthesis of Ht-ilerite. Ht-ilerite was obtained by titration of synthetic Nat
ilerite with dilute hydrochloric acid. A 500 mL of aqueous suspension containing 15.0 g
of Na*-ilerite was titrated by 0.1 M HCl at a rate of 3 mUmin to lower the pH to 1.9. The
suspension with pH = 1.9 was stirred for 24 hours. H*-ilerite was separated by
centrifugation and washed with deionized water until free of Cl', and then air-dried at
room temperature.

Synthesis of Organo Kenyaites. CnH2n+1NH3+ICnH2n+1NHz-kenyaite (n = 6, 8)
was prepared by the following approach. An aqueous suspension containing 15.0 g of
K+-kenyaite was combined with an aqueous solution containing 0.102 mol of
CnH2n+1NH3+Cl' to form a suspension in 1.0 L of total solution. The suspension was
stirred at 50 0C for 24 hours. The pH of the reaction mixture was in the range of 7 - 8.

The product mixture was centrifuged and the wet solid product was washed once with

500 mL of H20 and then air-dried.

CnH2n+1NH3+ICnH2n+1NHg-kenyaite (n = 10, 12) was prepared by the following
approach. An aqueous suspension containing 15.0 g of K+-kenyaite was combined with
ethanol/water solution containing 0.102 mol of CnH2n+1NH3+Cl' to form a suspension in
1.0 L of 7:1 (11 = 10) or 3:1 (n = 12) (v/v) ethanol/water total solution. The suspension
was stirred at 50 0C for 24 hours. The pH of the reaction mixture was in the range of 7 -
8. The product mixture was centrifuged. The wet solid product was washed once with
500 mL of H20 and then air-dried.

Primary, secondary and tertiary long chain alkylammonium exchanged
CH3,(CH2)17NH3.n(CH3)n+/CH3(CH2)17NH2.n(CH3)n -kenyaites (n = O, 1, 2) were
prepared by the following approach. An aqueous suspension containing 15.0 g of K+-
kenyaite was combined with ethanol/water solution containing 0.102 mol of

alkylammonium chloride and alkylamine in a 4:1 molar ratio to form a suspension in 1.0

95

L of 1:1 (v:v) ethanol/water total solution. The suspension was stirred at 65 0C for 24
hours. The pH of the reaction mixture was in the range of 8 - 9. The product mixture
was added to an equal volume ethanol and centrifuged. The wet solid product was
washed consecutively with two 750-mL portions of 50 % EtOH, one 750-mL portion of
30 % EtOH and then with water until free of Cl', and air-dried.

Quaternary alkylammonium exchanged CH3(CH2)17N(CH3)3+-kenyaite was
prepared by the reaction of 10.0 g of K+-kenyaite with 800 mL of 5.08 x 10'2 M
CH3(CH2)17N(CH3)3+Br' aqueous solution at 65 0C for 48 hr. The product was
centrifuged and washed with deionized water until free of Br‘, and then air-dried.

All of the air-dried organo kenyaites were ground to a powder with a particle size
smaller than 270 mesh (53 um) and stored for further use.

Synthesis of Organo Ilerites (Octosilicates). Quaternary alkylammonium
exchanged CH3(CH2)17N(CH3)3+-ilerite was prepared by the reaction of 10.0-g quantity
of air-dried Na+-ilerite with 31.2 g (7.95 x 10"2 mol) of CH3(CH2)17N(CH3)3+Br’ in 800
mL of 3:5 (v:v) ethanol/water solution at 65 0C for 48 hr. The reaction mixture was
centrifuged and the wet solid product was washed with three 750-mL portions of 50 %
EtOH and then with water until free of Br', and air-dried.

CH3(CH2)17NH3+-ilerite, abbreviated C18-ilerite, was obtained by conversion of
the H+ form of ilerite to CH3(CH2)17NH2-ilerite intercalate in the presence of neutral
amine. A 2.0—g quantity of air-dried H+-ilerite was added to 100 mL of 1:4 (v:v)
ethanol/water solution containing 1.8 g (6.68 x 10'3 mol) of CH3(CH2))7NH2 and stirred
at 50 0C for 4.5 hr. The final product was obtained by evaporation of the ethanol/water
solvent and air—dried.

4.2.3 Preparation of Epoxy-Layered Silicic Acid (Kenyaite and Ilerite)
Composites
Equivalent amounts of epoxide resin and poly(oxypropyleneamine) curing agent

were mixed at room temperature for 30 min. The desired amount of organo layered

96

silicate was added to the epoxide—poly(oxypropyleneamine) mixture and stirred for
another 60 min. This mixture was outgassed in a vacuum oven and poured into a
stainless steel mold for curing at 75 0C for 3 h and, subsequently, at 125 0C for an
additional 3 h.

4.2.4 Characterization Methods

X-ray Powder Diffraction (XRD). XRD patterns were recorded on a Rigaku
rotaflex 200B diffractometer. Diffraction patterns were collected with 0.010 29 interval
between 1 and 10° 20 using a scanning rate of 2° 20 per minute, and DS and SS slit
widths of 1/6. Samples of epoxy-solvated layered silicic acids or uncured epoxy-layered
silicic acid composites were prepared by applying thin films on glass slides. Cured
composite specimens were prepared by mounting a flat rectangular sample into an
aluminum holder.

Thermal Analysis. Thermogravimetric analyses (TGA) were performed using a
Cahn TG System 121 thermogravimetric analyzer. Samples were heated to 750 0C at a
heating rate of 5 °C/rnin under N2 atmosphere.

Mechanical Measurement. Tensile testing was performed at ambient
temperature according to ASTM procedure D3039 using a SFM-20 United Testing
System.

Chemical and Solvent Resistance. The resistance of the composite materials to
solvent swelling was obtained according to ASTM procedure D543. The specimens were
immersed in the desired reagent and removed periodically to measured the weight gain

until equilibrium was reached.

4.3 Results and Discussion
4.3.1 Exfoliation of Kenyaite Nanolayers in an Epoxy Polymer Matrix
4.3.1.1 Synthesis of Organo Kenyaites

The synthesis of swellable organo kenyaites was our first goal in our effort to

prepare epoxy-kenyaite nanocomposites. Our starting layered inorganic solid was the K+

97

form of kenyaite which has a better crystallinity and contains less impurities in term of
amorphous silica or quartz than the Na+ form of kenyaite},7 Kenyaite has been reported
to undergo ion-exchange reaction with primary CnH2n+1NH3+Cl' (n = 10, 12) and
quaternary CH3(CH2)H-1N(CH3)3+X' (n = 10, 12, X : Cl, Br) ammonium ions.3 In the
present work we studied the intercalation of primary alkylammonium/alkylamine
mixtures of the type CnH2n+1NH3+Cl‘/CDH2M1NH2 (n = 6, 8, 10, 12, 18). The solvent
used for the intercalation reaction was varied from an aqueous solution for n = 6, 8 to an
ethanol/water solution for n = 10, 12, 18, the concentration of ethanol was varied from
12.5 % to 50% (volume), depending on the chain length of the alkylammonium involved.
For the long chain alkylammonium, the intercalation method used for kenyaite
intercalation is the same as that previously described in Chapter 2 & 3 for the preparation
of organo magadiite.

Figure 4.1 shows the powder x-ray diffraction patterns for kenyaites intercalated
by mixtures of protonated and neutral primary amines with chain length n = 6, 8, 10, 12,
18. Table 4.1 lists the intragallery compositions, dog) basal spacings and gallery
structures. For chain length with n = 6, the intercalated alkylammonium forms only a
lateral monolayer structure. This result also agrees with the well-known intercalation
chemistry of 2:1 smectite clays.899 For n = 8, 10, 12, the basal spacings increase from
19.6 A for K-kenyaite to ~ 40 A for the organo kenyaites. On the basis of the observed
spacings, we propose that the intragallery alkylammonium/alkylamine chains have a
gauche-block structure, wherein the alkyl chains adopt a bilayer arrangement with a
mixed trans and cis conformation of the chains (cf. Figure 1.2G). This conformational
arrangement of the chains is consistent with Lagaly’s model with only a slight
difference.10 Our long chain alkylammonium (n = 18) exchanged kenyaite showed a
different gallery structure from those with n = 6, 8, 10, 12. The 43.1 A basal spacing for
air-dried CH3(CH2)17NH3+ICH3(CH2)17NH2-kenyaite was consistent with the paraffin

structure of the intercalated alkyl chains, and is designated to Cl8-kenyaite-PF. The

98

TGA results given in Figure 4.2 and Table 4.1 show that all these primary
alkylammonium exchanged kenyaites, except for n = 6 must contain a mixture of

protonated and neutral amine in order to explain the observed gallery structures.

It is noteworthy that the broad do 0 1 peak at 35.9 A for
CH3(CH2)7NH3+/CH3(CH2)7NHz-kenyaite (Figure 4.3B) most likely arises due to the
evaporation of the neutral amine molecules from the gallery. This short chain amine
possesses a relatively high vapor pressure. However, the wet and as-synthesized
CH3(CH2)7NH3+/CH3(CH2)7NHz-kenyaite gave a high intensity and narrow peak at
35.9 A as shown in Figure 4.3A. Excessive washing resulted in the formation of another
kind of gallery structure with a 23.1 A basal spacing (Figure 4.3C). The gallery height of
5.4 A (layer thickness of kenyaite is 17.7 A) is consistent with a lateral bilayer structure
of alkyl chains (see Figure 1.2A). As we have already demonstrated for the intercalation
chemistry of organo magadiites with a lateral monolayer structure, the organo kenyaites
with lateral monolayer and lateral bilayer structures are not expandable by epoxy resin
and other polymer precursors due to the high electrostatic interactions between the onium

ions and the silicate nanolayers.

99

 

43.1 A

n_18
42.3 A
#_7 n=12
40.0 A

37.6 A
n=8

 

 

Relative Intensity

 

 

 

 

 

 

 

20.7A
n=6
A 19-5 A K+-kenyaite
1 1 1 1 l 1 1 1 LJ 1 1 1 1 I 14 1 1
0 5 10 15 20

29 (Degrees)
V
Figure 4.1 X-ray powder diffraction patterns of air-dried K+-kenyaite and
CH3(CH2)n-1NH3+ICH3(CH2)n-1NHz-kenyaites. The gallery structures and

compositions are given in Table 4.1.

100

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100:
95_b
: K*-kenyaite
90:-
: __n=6
A 85:
o\° -
E 80:
.51.)» : J:
3 : '1 7
i— “
75_ , \\__-
: '._\ \'--\_-‘n=1OJ
.. \-.. H”
70:- \ ........ 11:12
.- \ -----
65:—
: ~-~_s_n=18
60111111111111L11111il1111111111114111—

 

0 100 200 300 400 500 600 700
Temperature (°C)

Figure 4.2 Thermogravimetric analysis curves for air-dried K+-kenyaite and

CH3(CH2)n-1NH3+ICH3(CH2)n-(NHz-kenyaites. The gallery structures and compositions

derived from these curves are give in Table 4.1.

102

 

 

 

23.1 A
35.9 A
.é‘
2 H c
9, 1- We»... . .
5
<1)
.2
E
a)
a:
B
m
A

1.“: __ ‘A—nAA‘

“AA A

 

 

11111111111111111L111111111l11Ll11

1 2 3 4 5 6 7 8 9 10
26) (Degrees)

 

Figure 4.3 X-ray powder diffraction patterns of (A) C8-kenyaite-GB (gauche block
structure) obtained by applying a film of the wet product on a glass slide right after
centrifuging the reaction mixture; (B) air-dried C8-kenyaite-GB with a less ordered

gauche block gallery structure; (C) CH3(CH2)7NH3+-kenyaite with a lateral bilayer

gallery structure obtained by thorough washing of the C8-kenyaite-GB to remove excess
amine.

103

4.3.1.2 Exfoliation of C18-Kenyaite-PF in an Epoxy Polymer Matrix

The intercalation of C18-kenyaite-PF by a mixture of epoxy resin and curing
agent as a function of time was investigated by XRD (Figure 4.4). The solvation of C 18-
kenyaite-PF by a stoichiometric mixture of resin and curing agent was initially very slow
at room temperature, because the viscosity of the system is high and the diffusion process
is very slow. The 46.7 A spacing for the solvated intercalate (Figure 4.4B) is consistent
with the perpendicular orientation of onium ions and amine molecules with a paraffin
monolayer structure (cf. Table 4.1). The intercalation may be limited primarily to the
epoxide resin only at this stage, because the change in intragallery spacing is very small
and the spacing agrees with that for epoxide solvation without the presence of the curing
agent. Upon heating at 75 °C, the C18-kenyaite-PF is rapidly solvated, as evidenced by a
sharp increase in spacing to a value of 62.8 A (Figure 4.4C). The 62.8 A peak is replaced
by another peak at 69.0 A (Figure 4.4D) corresponding to the onium ions with a lipid
bilayer orientation (cf. Table 4.1). As we have discussed in Chapter 2 & 3, the formation
of this intermediate phase is crucial for achieving an exfoliated state instead of an
intercalated state for the silicate nanolayers. In this phase, the organic onium ions adopt
an orientation that allows optimal intragallery space for the intercalated species. Curing
beyond this stage allows the exfoliated state of silicate nanolayers to be achieved, as
shown in the XRD Figure 4.4E. The final x-ray amorphous phase (Figure 4.5) suggests
that an epoxy-exfoliated kenyaite nanocomposite has been achieved. The formation of an
epoxy-exfoliated kenyaite nanocomposite supports the pathway that we have previously
proposed for the exfoliation of organo magadiites in an epoxy polymer matrix, even

though the layer thickness increases from 11.2 A for magadiite to 17.7 A for kenyaite.

 

 

 

 

 

69.0
E" 62.8
8
.9 E
E _ M mg i - -
a) D
.5, 46.7A 1
2
(D
a:
-11 -__ _ C
B
43.1A W" “W" ‘
A

 

1111L1111|111111111111111111111111

1 2 3 4 5 6 7 8
29 (Degrees)

 

 

 

Figure 4.4 X-ray diffraction patterns of (A) pristine C18-kenyaite-PF and (B-E) the
partially cured composites prepared by reaction of C18-kenyaite-PF (14.6 wt % loading)
with a stoichiometric mixture of epoxide resin (EPON 828) and
poly(oxypropyleneamine) (Jeffamine D-2000) curing agent. The primary
alkylammonium and the free amine content of C18-kenyaite-PF was counted as
contributing to the stoichiometry for epoxide cross-linking. Reaction conditions were as

follows: (B) 25 °C, 60 min; (C) 75 °C, 30 min; (D) 75 °C, 40 min; (B) 75 0C, 120 min.

105

 

 

Intensity

 

Intensity

 

 

 

1o 20 so 40 so so
26 (DegreGS)

 

 

llLllllllllllllJlllllllJllllllJlllJllLlIllll

1 2 3 4 5 6 7 8 9 1 O
29 (Degrees)

 

Figure 4.5 X-ray diffraction pattern of a cured epoxy polymer-kenyaite
nanocomposite prepared from C18-kenyaite-PF. The organo kenyaite loading was 14.6

wt %. Polymer curing was carried out at 75 0C for 3 h, and followed by 3 h at 125 0C.
The sharp lines are the in-plane reflections of the exfoliated kenyaite layers.

106

4.3.1.3 Exfoliation of CnH2n+1NH3+ICnH2n+1NH2-kenyaites in an Epoxy Polymer
Matrix

The intercalation of CnH2n+1NH3+ICnH2n+1NH2—kenyaites (n = 6, 8, 10, 12) by a
stoichiometric mixture of epoxide resin and curing agent was also investigated by XRD
(Figure 4.6). The intercalation of the resin/curing agent mixture was very successful for
chain length n 2 8. The basal spacings for n = 8, 10 and 12 all exceeded the values that
the lipid-like bilayer structures can afford (cf. Table 4.1). The sharp x-ray peaks for n =
10, 12 arise from the phase segregated extragallery amine molecules. This result
suggests that the pristine intragallery alkylammonium/alkylamine molecules have lost
their original registry and preferred orientations after the co-intercalation of epoxide resin
and poly(oxypropyleneamine) curing agent. The alkylammonium ions can undergo
proton transfer reaction and, consequently, the small mobile amine molecules can be
excluded from intragallery region to provide extra intragallery space for the intercalation
and intragallery polymerization.

The XRD patterns of fully cured epoxy-kenyaite nanocomposites prepared from
CnH2n+1NH3+ICnH2n+1NH2-kenyaites (n = 6, 8, 10, 12, 18) are shown in Figure 4.7. The
absence of the 001 diffraction peaks for n > 6 provides strong evidence that the silicate
nanolayers of kenyaite have been exfoliated during the thermosetting process. A small
contraction in d-spacing is observed for the resulting aggregated epoxy-kenyaite
composite prepared from C6-kenyaite-LM. This is probably due to the loss of interlayer
water molecules during the thermosetting process. The low angle background scattering
for the epoxy-kenyaite nanocomposites prepared from Cann+1NH3+/CnH2n+1NH2-
kenyaites (n = 8 and 10) is more intense than for C18-kenyaite-PF. This suggests that the
extent of nanolayer exfoliation for the nanocomposites prepared from different

intercalates may vary due to the nature of alkylammonium ions. This issue will be

discussed in detail latter.

107

 

62.8 A

74

 

 

 

 

Relative lntensuty
f3
O
T
:3
1L
N

 

 

 

 

 

12 3 4 5 67 8 910
28 (Degrees)

Figure 4.6 XRD patterns of partially cured composites prepared by reaction of
CH3(CH2)n-1NH3+ICH3(CH2)n-1N Hz-kenyaites (n = 6, 8, 10, 12 and 18) with a
stoichiometric mixture of epoxide resin and poly(oxypropyleneamine) curing agent. The
organo kenyaite loading is 15 wt % for n = 6, 10, 12, 18, and 30 wt % for n = 8. The
reactions were carried out at 25 0C for 5 min to 60 min for n S 12, and at 75 0C, 30 min

for n = 18. The sharp lines in the patterns for n = 10, 12 are attributed to phase-segregated
amine.

108

 

 

 

 

 

 

n=18
k n=12
.é‘
U)
8
E n=10
Q) Mr- Mg #2.. - -._
.2
E
Q)
n: 19.5A
n:
n:
lililllllllllllllllllllllllllllllLJ

 

 

1 2 3 4 5 6 7 8 910
28 (Degrees)

Figure 4.7 X-ray diffraction patterns of cured epoxy-kenyaite nanocomposites
prepared from CH3(CH2)n-1NH3+ICH3(CH2)n-lNHz-kenyaites (n = 6, 8, 10, 12 and 18).
The organo kenyaite loading for each composite was 15 wt %. Polymer curing was

carried out at 75 0C for 3 h, and followed by 3 h at 125 °C.

109

4.3.1.4 Effect of Gallery Acidity of Organo Kenyaites on Exfoliation

Organo kenyaites with paraffin-like gallery structures were prepared by
intercalation of long chain secondary and tertiary alkylammonium ions in place of
primary onium ions. The methodology used to achieve. intercalation was the same as
described above for C18-kenyaite-PF. A quaternary alkylammonium ion exchanged
kenyaite was also prepared using the previous reported method of Lagaly.lo Figure 4.8
shows the XRD patterns of CH3(CH2)17NH3-n(CH3)n+-kenyaite (n = l, 2, 3). The XRD
patterns (Figure 4.9) of the cured composites prepared from these intercalates show that
three kinds of nanocomposite structures have been achieved. A disordered exfoliated, an
ordered exfoliated and an intercalated epoxy—kenyaite nanocomposites were obtained for
n = l, 2, 3, respectively. The different gallery acidities provided by onium ions resulted
in the different intragallery polymerization rates, which is the decisive factor governing
the final morphology of silicate nanolayers in the polymer matrix. This dependence of
gallery exfoliation on onium ion acidity was also shown in the system of
CH3(CH2)17NH3-n(CH3)n+-magadiite (n = l, 2, 3), which was fully investigated in
Chapter 3. So, we have developed a general approach, wherein by varying the Brbnsted
acidity of the intragallery catalytic centers, it is possible to control the extent of
intragallery separation. Interestingly, the extent of layer separation for the intercalated
and ordered exfoliated composites is slightly less for epoxy-kenyaite nanocomposites
than for magadiite. This difference in the extent of exfoliation may reflect differences in

layer charge density.

110

 

 

 

 

 

 

 

 

 

 

 

39.8A
E
U)
C
93
5 n=1
9
g 38.2A
OJ
0:
n:
39.3A
n=3
lirlLrihrrlriJliriiT—r—T-li1111111111

 

1 2 3 4 5 6 7 8 910
26) (Degrees)

Figure 4.8 X-ray powder diffraction patterns of CH3(CH2)17NH3-n(CH3)n+-kenyaites
(n = 1, 2, 3). All of the organo kenyaites have paraffin-like gallery structures.

lll

 

 

 

.é‘
(D
C
E 71.8 A
a) =
.2 1
q—o
59
Q)
a:
n:

 

-

_
A-‘A_-. 4-.. A A.... A A
wv—v wv-v-wWV—v v—rrv v" ""

 

 

 

llJlllllllllllllLllllllllllllllll’l

1 2 3 4 5 6 7 8
26) (Degrees)

Figure4.9 X-ray diffraction patterns of cured epoxy polymer-kenyaite

nanocomposites prepared from CH3(CH2)17NH3-n(CH3)n+-kenyaites with a paraffin
gallery structure. The organo kenyaite loading was 20 wt % (n = 1, 3) and 10 wt % (n =

2). Polymer curing was carried out at 75 0C for 3 h, followed by 3 h at 125 OC. The
secondary onium ions (n = 1) give highly exfoliated (disordered) nanocomposites (d > 90
A). The tertiary onium ion derivative (n = 2) also gives an exfoliated nanocomposite, but
the layer separation is highly ordered ((1 = 72 A). The quaternary onium ion exchange
form (n = 3) forms an intercalated nanocomposite (d = 44 A).

112

4.3.2 Exfoliation of Ilerite Nanolayers in an Epoxy Polymer Matrix
4.3.2.1 Synthesis of Organo Ilerites

The Na+ form of ilerite has been reported only in a few cases to undergo cation-
exchange reaction by quaternary or short chain primary alkylammonium ions.6 Our
effort to convert Na+-ilerite to long chain alkylammonium exchanged ilerite with a
paraffin structure was not successful using our previously developed method for
converting Na+-magadiite and K+-kenyaite to their organic analogs. However, it was
possible to convert Na+-ilerite into organo intercalates with very high basal spacings
from 61 A - 75 A. The variable interlayer spacings and the orientations of intragallery
onium ions are not understood. Our results showed that washing processes removed
some of the intragallery organic species and lowered the amine content. However, a
single phase with a paraffin gallery structure was very difficult to achieve.

A new approach was developed to achieve organo ilerite intercalate with a
reasonable content of intragallery long chain onium ions. N a+-ilerite was fist converted
into the proton exchanged form, then the H—ilerite was reacted with small amount of
neutral amine. The experimental section provides specific details for the synthesis of this
CH3(CH2)17NH3+-ilerite which is designated as C18-ilerite. The XRD patterns for both
the inorganic forms and the organic form of ilerite are shown in Figure 4.10. The basal
spacing of 36.4 A is consistent with a perpendicular monolayer of alkyl chains. The
XRD pattern show no peaks for the neutral amine phase. Our TGA results shows that the
molar ratio of CH3(CH2)17NH2 to Si30172' unit is about 1.6. It is very reasonable to
assume the amine molecules have been transferred into gallery region by the acid-base

reaction.

113

 

 

 

 

 

 

 

 

 

 

 

 

 

"2A 75A
3e4A
.~§‘
(I)
I:
.03
5
CD
.2
5
,IA
B
A
L i '
lllllllllllllllllllllJiJlllllllllllllll
0 10 20 30 40

28 (Degrees)

Figure 4.10 X-ray powder diffraction patterns of air-dried (A) Na+-ilerite; (B) W‘-
ilerite; and (C) CH3(CH2)17NH3+-ilerite (C18-ilerite).

114

4.3.2.2 Exfoliation of C18-Ilerite in an Epoxy Polymer Matrix

The co-intercalation of C18-ilerite by a stoichiometric mixture of epoxide resin
and curing agent as a function of time was investigated by XRD (Figure 4.11). At 25 0C,
the C18-ilerite is partially solvated, as evidenced by a broad x-ray peak at low angle
(Figure 4.11A). This two-phase mixture was quickly replaced by a single door peak at
60.1 A (Figure 4.11B) corresponding to an orientation of the onium ions with a lipid-like
bilayer structure to afford an optimal intragallery space for the intercalated epoxide resin
and curing agent. Figure 4.11 C and D indicate that the intragallery region has been
expanded beyond that the critical height which the lipid-bilayer structure can afford. The
final x-ray amorphous phase (Figure 4.12) suggests that an exfoliated epoxy-ilerite
nanocomposite has been obtained. The ilerite 2-dimensional structure is still retained in
the exfoliated state, as indicated by strong high angle in-plane peaks. The formation
pathway for the epoxy-exfoliated ilerite nanocomposites shows a similarity to magadiite

and kenyaite.

115

 

 

 

{

 

Relative Intensity

 

 

 

 

 

lllljllllllllllllJlJlllllllllllllll

 

1 2 3 4 5 6 7 8 9 1O
29(Degrees)

Figure 4.11 X-ray diffraction patterns of the partially cured composites prepared by
reaction of C18-ilerite (15 wt % loading) with a stoichiometric mixture of epoxide resin
and poly(oxypropyleneamine) curing agent. The primary alkylammonium and the free
amine content of C18-ilerite was counted as contributing to the stoichiometry for epoxide
cross-linking. Reaction conditions were as follows: (A) 25 °C, 20 min; (B) 75 0C, 40

min; (C) 75 °C, 100 min; (D) 75 °C, 150 min.

116

 

 

 

 

 

 

3.69A
F
.E‘
2
2* 2
.2 E
a) 1.8711
.0-9
5
10 20 30 40 50 60
29 (Degrees)
lllllllJllJlLJlJilllllllllIll]llllllLlllllll

 

 

1 2 3 4 5 6 7 8 . 9 10
29 (Degrees)

Figure 4.12 X-ray diffraction pattern of a cured epoxy polymer-ilerite nanocomposite
prepared from C18-ilerite. The organo ilerite loading was 15 wt %. Polymer curing was

carried out at 75 0C for 3 h, and followed by 3 h at 125 °C. The sharp peaks at 3.69 A

and 1.87 A are the in-plane reflections of ilerite. The broad peaks at 29 = 20°, 420 are
due to the polymer matrix.

117

4.3.3 Performance Properties of Exfoliated Epoxy-Kenyaite and Ilerite

Nanocomposites
4.3.3.1 Effect of Chain Length on Tensile Properties

It was interesting to know the benefits of kenyaite and ilerite nanolayer
exfoliation in the epoxy polymer matrices. Table 4.2 illustrates the tensile properties of
the epoxy-exfoliated kenyaite nanocomposites prepared from
CnH2n+1NH3+ICnH2n+1NH2-kenyaites (n = 8, 10, 12, 18). All of the exfoliated
nanocomposites have shown a tremendous increase in tensile strength. Even the tensile
elongation at break is improved when the matrix has a sub-ambient Tg. However, the
composites prepared from the intercalates with a chain length n S 12 behave not as good
as C18-kenyaite-PF (n = 18) at the same organo kenyaite loading. As we have
demonstrated before, CnH2n+1NH2 amines with mono-functional end groups can be
incorporated into a cross-linked epoxy network to form the dangling chains. This is the
major factor in deciding the improvement properties for each composite. From the
thermogravimetric analysis we recognized that for the C10H21NH3+IC10H21NH2-kenyaite
intercalate, has been loaded with 20.7% more amino groups than ClS-kenyaite-PF.
Therefore, there are more dangling chains in the composite prepared from
C10H21NH3+ICIOHZINHg-kenyaite than ClS-kenyaite-PF. Consequently, the benefit of
nanolayer exfoliation was compromised more when the short chain onium exchanged
kenyaites (n = 8, 10, 12) are used to form the epoxy-kenyaite nanocomposites. Also,
there are other factors that govern the performance properties of the final composites. All
of the nanocomposites prepared from CnH2n+1NH3+/CnH2n+1NHz-kenyaites show
amorphous XRD patterns, but the patterns for the composites prepared from short chain
onium exchange kenyaits show greater low angle background scattering than C18-
kenyaite-PF. This suggests that the degree of exfoliation in latter case is higher. The

extent of layer separation always governs the overall performance properties of the

118

polymer-layered silicate nanocomposites, and in some cases is more decisive than the

number of dangling network chains caused by the amine intercalated.

Table 4.2 Effect of Chain Length of CH3(CH2)n-1NH3+ICH3(CH2)n-1NH2
Gallery Surfactant on Tensile Properties of Epoxy-Kenyaite
Nanocomposites. The organo kenyaite loading is 15 wt % for
each composite.

 

 

chain length n = pristine polymer 8 10 12 18
tensile strength (MPa) 0.63 1.63 1.94 1.74 2.63
elongation at break (%) 23.4 28.2 38.8 39.8 41.6

 

4.3.3.2 Effect of Extent of Layer Separation on Performance Properties

A comparison of tensile properties for the nanocomposites prepared form
CH3(CH2)17NH3-n(CH3)n+-kenyaites (n = l, 2, 3) is shown in Figure 4.13. Although,
the tensile strengths for all three types of nanocomposites (cf. Figure 4.9) show a
dependence on kenyaite silicate loading, the silicate loading is substantially more
effective for the exfoliated nanocomposites (n = 1) than the ordered exfoliated ( n = 2)
and the intercalated nanocomposites (n = 3). This result further supports the conclusion
that the tensile properties are closely linked to the extent of layer separation and the
effective behavior for the exfoliated silicate nanolayers. The structural reason behind this
effect has been discussed in detail in Chapter 3. Figure 4.14 shows the dependence of
solvent uptake for the nanocomposites prepared from CH3(CH2)17NH3_n(CH3)n+-

kenyaites (n = l, 2, 3) on their nanophase morphology.

119

 

 

 

 

 

5’
1342'
o. .
g -
c I.
53.
C
a) l-
: .—
U) .
22"
.5 :
C
a) .-
l— -
1—
lllllllllllllllllllllllllllll
0,
0 5 10 15

Silicate Loading (wt % SiOZ)

Figure 4.13 A comparison of the tensile strengths of epoxy-kenyaite nanocomposites
prepared from CH3(CH2)17NH3-n(CH3)n+-kenyaites. The kenyaite silicate loading
(expressed on wt % SiOz) was determined by calcining the composites in air at 650 0C
for 4 h using a heating rate of 2 0Chain. The secondary alkylammonium and the free

amine content of CH3(CH2)17NH2CH3+-kenyaite (n = 2) was counted as contributing to
the stoichiometry for epoxide cross-linking.

120

 

 

 

 

200
n O O
.1 O
.. o o
O

’0‘ 150: o o e e e

t . o 6 6 6

'E . O 6 6 v V

(U - 0 V

22 100- 3 6 v V V v

..C - 0 V

9 . 8 V

o . o 8 v V

3 .

50- o 6 v o pristine 189%
- V A n=3 152%
.1 e 0 =2 1480/0
- v n=1 114%
O_l I I I I T I I I I I I I I I I I I I I I r I I I I I I I I

0 1 00 200 300 400 500 600

Time (Minutes)

Figure4.l4 Toluene uptake curves for nanocomposites prepared from

CH3(CH2)17NH3-n(CH3)n+-kenyaites (n = 1, 2, 3). The tabulated values in the insert are
equilibrium data obtained from the immersion weight gain after 24 hr. The loading of
organo-kenyaite is 10 wt % for each epoxy-kenyaite composite.

121

The mechanical performance properties of epoxy-ilerite nanocomposites was also
investigated. An intercalated epoxy-ilerite nanocomposite(d-spacing = 39.0 A) was
prepared from C18A3M-ilerite, which is a quaternary CH3,(CH2)17N(CH3)3’r ion
exchanged ilerite with a 29.5 A basal spacing. A comparison of tensile properties
between the exfoliated and intercalated epoxy-ilerite nanocomposites is shown in Figure
4.15. The exfoliated nanocomposites performed better in the terms of tensile strength
and tensile modulus. All of kenyaite and ilerite results show the exceptional
performance, owing primarily to the presence of exfoliated silicate nanolayers.
4.3.3.3 Effect of N anolayer Thickness on Performance Properties

After we have extended the intercalation and exfoliation chemistry to the other
members of the layered silicic acid family, there remains an important question: Which
member in term of nanolayer thickness is more suitable for nanocomposite formation and
optimal performance properties?

A comparison of the dependence of tensile properties on layered silicate loading
for all three types nanocomposites (exfoliated, ordered exfoliated and intercalated) is
shown in Figure 4.16 for magadiite and kenyaite. Interestingly, the curves for magadiite
and kenyaite for each type of composite are almost identical in terms of slope and value
at the same loading. We also measured the chemical stability and resistance to solvent
swelling for the exfoliated nanocomposites prepared from C18-magadiite-PF, Cl8—
kenyaite-PF and C18-ilerite. The results are given in Table 4.3. Since the silicate (SiOz)
loading determined by calcination is difficult to control in equal for each case, each
composite possess only a closed silicate loading. Nevertheless, all of these composites
showed a similar performance. The effect of different layer thickness is hardly to be

distinguished based on solvent uptake experiment.

122

 

 

   
      

 

 

4 A
. —O—C18-ilerite
A _ ""A'---C18A3M-ilerite
Q
Q. 31— ‘
g +
'c I
'5:
c I-
9 2—
(7) ..
2 " A
‘8 - ”A .......... A .......
i— 11.-
oLII.llllllillJLlllLlllLJllel
O 5 1° 15

Ilerite Silicate Loading ( wt °/o)

 

Tensile Modulus (MPa)
0’

—o— ClB-ilerite
- - -A— - - C18A3M-ilerite

 

I III III III III III
,I I I I I
\
i
\
I
I

 

 

llllllllllllll4llllllll

0 2 4 6 8 10 12
Ilerite Silicate Loading (wt °/o)

Figure 4.15 Comparison of (A) tensile strengths and (B) tensile moduli for epoxy-
exfoliated ilerite nanocomposites prepared from C18-ilerite and epoxy-intercalated ilerite
composites prepared from Cl8A3M-ilerite. The primary onium ion and amine content of

the organo ilerite was counted as contributing to the stoichiometry of epoxide cross-
linking.

123

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Table 4.3 Chemical and Solvent Resistance of Epoxy—Layered Silicic Acid
Nanocomposites Prepared From C18-magadiite-PF (9.1 wt %), C18-
kenyaite-PF (10.4 wt %) and Cl8-ilerite (10.0 wt %). Values are the
Immersion Weight Gains (wt %) after a Certain Period.

124

 

materials 10% distilled 30%

NaOHa 1‘120a H2804al acetic acida

methanolb tolueneC

 

pristine polymer 1.6 2.5
C18-magadiite-PF 1.5 1.6
C18-kenyaite-PF 1.7 2.1
C18-ilen'te 2.4 2.6

5%
16.7 13.4
7.2 9.6
8.1 9.1
8.0 10.7

76.5
58.5
57.1
56.6

189
136
132
133

 

a Weight gain after 15 days. ‘3 Weight gain after 48 hr. 0 Weight gain after 24 hr.

Additional factors could govern the overall performance properties of the
composites prepared from different layered silicic acids, such as the aspect ratios, the
amine loading (which can form dangling chains), the amount of amorphous silica
presented (which probably always accompanies the crystalline products), and the degree
of nanolayer exfoliation. At this stage, we can conclude only that all of the organo

layered silicic acid nanolayers exhibit similar composite performance properties when

their silicate nanolayers are exfoliated in epoxy matrix with a sub-ambient Tg.

125

4.4 Conclusions

The exfoliation chemistry of magadiite was successfully extended to the other
members of the layered silicic acid family. The mechanism leading to the exfoliation of
silicate nanolayers for magadiite is shared by kenyaite and ilerite. The approach
developed for organo layered silicic acids, which allows control of the extent of
nanolayer separation, make it possible to investigate the relationship between the
nanolayer properties and material performance properties. Factors such as silicate
loading, extent of nanolayer separation and the degree of cross-linking also govern the
performance of composites prepared from organo kenyaites and ilerites. A large amount
of alkylamine should be avoided in the synthesis of polymer-layered silicic acid
nanocomposites to minimize the fraction of dangling chain in the polymer network. The
overall performance properties of the nanocomposites prepared from different organo

layered silicic acids show no significant dependence on the nanolayer thickness.

P
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126

References

Lagaly, G.; Beneke, K.; Weiss, A. Am. Mineral. 1975, 60, 650.
Beneke, K.; Lagaly, G. Am. Mineral. 1977, 62, 763.

Beneke, K.; Lagaly, G. Am. Mineral. 1983, 68, 818.

Fletcher, R. A.; Bibby, D. M. Clays Clay Miner. 1987, 35, 318.
Beneke, K.; Kruse, H. H.; Lagaly, G. Z. anorg. allg. Chem. 1984, 518, 65.
Kosuge, K.; Tsunashima, A. J. Chem. Soc., Chem. Commun. 1995, 2427.
Beneke, K.; Lagaly, G. Am. Mineral. 1989, 74, 224.

Lan, T.; Kaviratna, P. D.; Pinnavaia, T. J. Chem. Mater. 1995, 7, 2144.
Shi, H.; Lan, T.; Pinnavaia, T. J. Chem. Mater. 1996, 8, 1584.

Lagaly, G.; Beneke, K.; Weiss, A. Am. Mineral. 1975, 60, 642.

Chapter 5

HYBRID ORGANIC-INORGANIC NANOCOMPOSITES FORMED FROM AN
EPOXY POLYMER AND AN AMINE CURING AGENT (J EF FAMINE)
INTERCALATED IN PROTON FORMS OF LAYERED SILICIC ACIDS

5.1 Introduction

A great deal of research has been conducted on organic-inorganic hybrid
composites in which layered silicate clays are used as reinforcement agents."” Layered
silicate clays become good candidates not only because their silicate nanolayers have a
chemically stable siloxane surface, high aspect ratios and high strength, but more
importantly,” because the rich intercalation chemistry of clay silicates can be used to
facilitate exfoliation of silicate nanolayers into the polymer network.”15 The exfoliation
can optimize the interfacial contact between the organic and inorganic phases for
improved nanocomposite properties. '6

Approaches to the exfoliation of clay nanolayers have been investigated by
several different research groups using both thermoplastic and thermoset polymers.7"7'3°
Most polymer precursors require a hydrophobic environment for intercalation into clay
galleries. The hydrophilic inorganic clay surface usually has to be modified first to
accommodate the incoming organic precursors. Generally, an organic cation exchanged
reaction is used to form a hydrophobic organo clay. Then, the organo clay is intercalated
by polymer precursors which are either organic monomers or prepolymers. Direct
intercalation of a preformed polymer into the clay galleries by melt processing is also

possible.2|

127

128

The most commonly used organic species for clay cation exchange reaction are

ammonium ions of the type R1R2R3R4N+, wherein the R groups contain 1-18 carbon

atoms and up to three R groups are replaced by protons. An intercalated nanocomposite
is likely to be formed when only non-acidic quaternary alkylammonium ions are used.
The overall performance properties of the intercalated polymer-clay nanocomposites are
inferior to their exfoliated analogs. In most cases, the presence of long chain (11 2 8)
alkyl groups and strong intragallery acidity is required to form an exfoliated polymer-clay
nanocomposite; otherwise, the resulted organo cation exchange clay products are not
sufficiently hydrophobic to facilitate the intercalation of the incoming polymer
precursors.8 However, the introduction of larger alkyl groups will simultaneously cause a
steric or structural problem, because most of the organic cations can not be incorporated
into the polymer network and will independently occupy space in the polymer matrix
without contributing positive structural properties. When onium ions dilute the polymer
network structure, especially for high charge density layered silicate clays with high
surface concentrations of organo species loaded, the advantages of silicate nanolayer
exfoliation will be compromised. Although, some organic cations can have functional
groups for cross-linking into the polymer chains with minimal loss of performance
properties, unteathered onium ions act as a plasticizer and performance is not as good as a
homopolymer matrix.

As discussed in Chapters 2, 3 and 4, primary alkylammonium exchanged layered
silicic acids have been successfully used as the starting layered hosts to form epoxy-
exfoliated layered silicic acid nanocomposites. The mono-functional amino groups cause
the formation of a significant number of dangling chains and lower the extent of network

formation as illustrated schematically bellow:

129

Although, onium ions derived from aliphatic diamines would minimize dangling chain
formation, clays interlayered by such onium ions form lateral monolayers, which is the
most favorable structure, and the strong intragallery bonding prevents swelling by the
polymer precursors.

The proton exchanged forms of layered silicic acids can be obtained easily by the

treatment of alkali-metal forms of layered silicic acids using a dilute acid.”27 The H+
forms of layered silicic acids have a layered structure similar to their alkali-metal
exchange forms, and can be used as acid catalysts and as absorbents for alkylamine.28'29
The present work demonstrates that poly(oxypropyleneamine) curing agents acting as
strong bases can form onium ions by a simple acid-base reactioniin the galleries of a
silicate clay. Thus, these curing agents can be intercalated into silicate galleries to form

an intercalate that functions as an alkylammonium exchange layered silicic acid.

 

 

 

(5.1)

 

 

 

NH;

 

A new approach to the preparation of thermoset polymer-layered silicic acid
nanocomposites is discussed based on this chemistry, thus eliminating the need to pre-
intercalated the layered silicate galleries with undesired onium ions. This new processing
development has resulted in a greater improvement of the overall performance properties

of polymer-layered silicic acid nanocomposites.

130

5.2 Experimental
5.2.1 Materials
The epoxide resin used to for epoxy-layered silicic acid hybrid composite

formation was poly(bisphenol A-co-epichlorohydrin) (Shell, EPON 828), with MW

~377:
/0\ CH3 CH3
6H,- CHCH,61@6_@ ocnzchcnzo+©-c—©- 66m CH,
CH3 CH3

DGEBA type, n = O (88%); n = l (10%); n = 2 (2%).

The curing agent was poly(oxypropyleneamine) (Huntsman Chemical, JEFFAMINE D-

series and T-series):

JEFFAMINE D Series JEFFAMINE T-Series

HzNCHCH {OCHZCH l—NH2 $H210CH2CH<CH3HPH2
[m3 1H3 CH3CH2CCH2‘E' OCH2CH(CH3)-}yNH2
CHZ-f OCHZCH(CH3)-}ZNH2
MW x -
13-2000 33.1 T403 5-3
D-4000 67.7 3% 2552-1;

The above monomers can afford epoxy polymer matrices with a broad Tg range from a

sub-ambient Tg of -40 oC (D-2000) to a Tg above room temperature of 95 0C (T-403).
All the other chemicals used in this work were purchased from Aldrich Chemical Co. and
used without further purification.
5.2.2 Synthesis of Layered Silicic Acids and Their Derivatives

Synthesis of Alkali-Metal Forms of Layered Silicic Acids. Alkali-metal forms
of layered silicic acids were synthesized by hydrothermal methods.”32 In general, a

suspension of amorphous silica gel was added to a MOH solution (M = Na or K) and

heated at a temperature above 100 0C for a few days with stirring in a Teflon-lined Parr

131

reactor. The suspension containing the alkali-metal form of layered silicic acid was
centrifuged, and the solid product was washed with deionized water to remove excess
MOH, and air-dried at room temperature. The synthesis conditions used for each layered

silicic acid are specified in Table 5.1.

Table 5.1 Synthesis of Alkali-Metal Forms of Layered Silicic Acids

 

d001(A) M0H333i02=H20 temp time H20 washings

 

materials molar ratio (°C) (h) vol (mL) x times
Na+-magadiite 15.6 1:3:50 150 42 600 x 2
K+-kenyaite 19.6 1:3:40 140 72 600 x 2
Na+-ilerite 11.2 1:2:14 105 216 700 x 3

 

3M = Na (magadiite and ilerite), K (kenyaite)

Synthesis of Proton Forms of Layered Silicic Acids. Ht forms of layered silicic
acids were obtained by titration of synthetic alkali-metal forms of layered silicic acids
with dilute a hydrochloric acid.23 A 500-mL of aqueous suspension. containing 15.0 g of
layered silicic acid (magadiite, kenyaite and ilerite) was titrated with 0.1 M HCl at a rate
of 3 mljmin to lower the pH to 1.9. For kenyaite the final pH was 1.8. The suspensions
at pH = 1.9 (1.8 for kenyaite) were stirred for 24 hours (48 h for kenyaite). The proton

exchange forms of layered silicic acids were separated by centrifugation and washed with
deionized water until free of Cl', and then air-dried at room temperature.

Synthesis of Jeffamine-H-Layered Silicic Acid Intercalates. An ethanol
solution containing the Jeffamine (D2000, D4000, T3000 and T5000) was added to an

aqueous suspension of the H+ form of the layered silicic acid and stirred at 60 0C for a
certain period. The products were separated by centrifugation and air-dried. The ratios

of Jeffamine to H-layered silicic acid, the solvent composition, reaction time, and post

132

reaction washing process used for each synthesis, along with the basal spacings for air-
dried products for each intercalate, are specified in Table 5.2-5.4. The powdered products
for composite preparation were ground, and the fine particles were collected and stored
for further use.

The intercalates formed between Jeffamine D400 or T403 and the H-layered
silicic acids were synthesized by the following method. An aqueous solution containing
Jeffamine was combined with an aqueous suspension of a layered silicic acid to form a

mixture containing 3 g of Jeffamine in 80 mL of water per gram of layered silicic acid.

The reaction is carried at 50 0C for 24 h. The reaction mixture was centrifuged, and the

product was washed twice with an equal volume of water and air-dried.

133

 

 

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5.2.3 Preparation of Epoxy-Layered Silicic Acid Composites Using Jeffamine-H-
Layered Silicic Acid Intercalates

Stoichiometric amounts of epoxide resin and Jeffamine curing agent in the form

of the free amine and the Jeffamine-H-layered silicate intercalate were mixed at 25 0C.

This mixture was prepared by adding the desired amount of Jeffamine-H-layered silicic

acid intercalates to an epoxide-Jeffamine curing agent mixture at 25 0C and stirring for
30 min. The Jeffamine in intercalated form was counted as contributing to the
stoichiometry for epoxide cross-linking. This mixture was outgassed in a vacuum oven

and poured into a stainless steel mold for curing at 75 0C for 3 h and, subsequently, at

125 0C for an additional 3 h.
5.2.4 Characterization Methods
X-ray Powder Diffraction (XRD). XRD patterns were recorded on a Rigaku

rotaflex ZOOB diffractometer equipped with a rotating anode, Cu Ka x-ray radiation (7t =

1.541838 A) and a curved crystal graphite monochromator. The x-ray was operated at 45

KV and 100 mA. Diffraction patterns were collected with 0.010 '20 interval between 1
and 100 20 using a scanning rate of 2° 20 per minute, and DS and SS slit widths of 1/6.

For patterns obtained at starting angle of 0.50 20, a SS width of 0.05 was used instead of

1/6. XRD patterns at high angles were collected using DS and SS slit widths of l. The
samples of gel-like Jeffamine intercalated H-layered silicic acids or uncured epoxy-
layered silicic acid composites were prepared by applying thin films on glass slides.

Cured composite specimens were prepared by mounting a flat rectangular sample into an

aluminum holder.

Thermal Analysis. Thermogravimetric analyses (TGA) were performed using a
Cahn TG System 121 thermogravimetric analyzer. Samples were heated to 750 0C at a

heating rate of 5 °C/min under N2 atmosphere.

137

Surface Area Measurement. N2 adsorption-desorption isotherms were

determined on a ASAP 2010 Sorptometer at liquid N2 temperature using a static sorption

mode. Samples were outgassed at 150 0C and 10'5 Torr for 12 h. Surface areas were
determined using BET plots.

Mechanical Measurement. Tensile testing was performed at ambient
temperature according to ASTM procedure D3039 using a SFM-ZO United Testing
System.

Chemical and Solvent Resistance. Resistance to solvent swelling was obtained
according to ASTM procedure D543. The specimens were immersed in the desired
reagent and removed periodically to measured the weight gain until equilibrium was

reached.

5.3 Results and Discussion
5.3.1 Synthesis of Jeffamine-H-Layered Silicic Acid Intercalates

Alkali-metal forms of layered silicic acids can be easily converted to their proton
analogs using the titration technique. Figure 5.1 shows the x-ray diffraction patterns for
each H+ form of layered silicic acid. The proton forms normally contain less structural
water molecules than their alkali-metal forms. The presence of the interlayer water
molecules is very important to prevent the silicate layers from cross-linking. The proton
forms can undergo very similar intercalation chemistry as their alkali forms as long as

they are not converted into anhydrous forms by high temperature treatment.23

138

 

 

 

 

 

 

 

 

18.8A
7.5A
.é‘
U)
C
2
E
,3 . 12.0A
E
m .
r: ‘ . H-kenyalte
| H-magadiite
H-ilerite A
illllljllllllLllllllllllllLLlellllllll

 

0 5 10 15 20 25 30 35 40
29 (Degrees)

Figure 5.1 XRD patterns of air-dried proton exchanged forms of layered silicic acids:

(A) H+-ilerite; (B) H+-magadiite; (C) H+-kenyaite.

139

Our efforts to synthesize Jeffamine intercalated layered silicic acids were very
successful. Because the higher molecular weight Jeffamines are immiscible with aqueous
solution, a co-solvent of water and ethanol was generally necessary to obtain these
particular types of intercalates. The amount of ethanol used was dependent on the
solubility of Jeffamine curing agent; however, a high volume percentage of ethanol was
avoided. The synthesis conditions given in Table 5.2-5.4 provide only typical reaction
conditions for the synthesis of each intercalate. However, the range for ethanol can be
varied form 0% for D400 and T403 to 50% (vol.) for a higher molecular weight
Jeffamine. Also, the reaction can be completed in 4 h to 24 h at an elevated temperature
over a broad range form 35 0C to 75 0C.

Interestingly, two kinds of intercalates can be formed between high molecular
weight Jeffamines and layered silicic acids. As-synthesized intercalates normally
exhibited high basal spacings and a gel-like physical state. The post synthesis washing of
the products with a mixture of ethanol and water removes part of the intercalated
Jeffamine from gallery and results in a powered form with decreased basal spacings.

Figure 5.2-5.4 show XRD patterns for some of the intercalates formed between
layered silicic acids and Jeffamines. Basically, the basal spacings are dependent on the
molecular weight and functionality (number of amino end groups) of the Jeffamine, or,
more precisely, on the equivalent weight. For instance, Jeffamine D2000 and T3000 give
very similar gallery height when they are intercalated into the galleries of layered silicic
acids. If we subtract the layer thickness for the different member of the layered silicic
acids from the observed d-spacings (especially in the cases of magadiite and kenyaite),
the obtained intercalates normally have a very similar gallery height. Apparently, each

negative charge in the layer occupies an equal surface area, independent of the difference

in layer thickness.

140

 

 

 

 

 

54.6 A A
.3;
g 16.1 A
Q
E
0
E.
g
Q
C
02000
D400
11111111111liirliiiLililililiiilinr

 

1 2 3 4 5 6 7 8 9 1O
29(Degrees)

 

.7A B

 

Relative Intensity

 

 

 

L141ll1111llldllJLLJLLJJJLLLlllL‘

1 2 3 4 5 6 7 8
29 (Degrees)

 

Figure 5.2 XRD patterns for the as-synthesized (unwashed) intercalates formed from

H+-magadiite and poly(oxypropyleneamines): (A) Jeffamine D-series; and (B) Jeffamine
T-series.

141

97.2A
65.4A
Dmxm
02000
20.9A

lllllllllllllllllllllllllllllllllLJ

 

 

Relative Intensity

 

 

 

 

e
329 (Degr698)6

85.0A
66.1A
Taxm
Tmmo

 

 

Relative Intensity

 

 

 

 

21.5A
T-403
111111llzllllllLLumllllllllllll[1111
8

329 4(0995799~‘3)6

Figure 5.3 XRD patterns for the as-synthesized intercalates (unwashed) formed from

H+-kenyaite and poly(oxypropyleneanfines): (A) Jeffamine D-series; and (B) Jeffamine
T-series.

142

 

 

 

 

71.1A
'5‘ T5000
C
.93
g 65.4A
0)
.2
<3
0)
I
T3000
11.5A
T403
lllllllllllllllllulllllllllLlllJU

 

1 2 3 4 5 6 7 8 9 10
28(Degrees)

Figure 5.4 XRD patterns for the as-synthesized (unwashed) intercalates formed from

H+-ilerite and poly(oxypropyleneamines) Jeffamine T-series.

143

The formation of intercalates using water soluble Jeffamine D230, D400 and
T403 exhibited only slightly expanded (~ 4 A increase) gallery heights. This suggests
that only a monolayer of Jeffamine has been intercalated with the amine in a strong
electrostatic interaction with the layers.

We have mentioned in the beginning that washing the as-synthesized intercalates
can result in a decrease in basal spacing and a powdered physical state. However, the
washing process has to be controlled, otherwise, a collapsed gallery can be formed as
shown by the XRD results in Figure 5.5. The as-synthesized D4000-H-kenyaite
intercalate has a basal spacings of 97.2 A, whereas an intercalate with a 69.0 A basal
spacing is formed when it is washed by the mixture of ethanol and water (cf. Table 5.3).
An intercalate with a 20.9 A basal spacing is likely to be formed when further washing is
applied. This latter intercalate has a lateral monolayer gallery structure which can not be
intercalated by any other polymer precursors.

TGA results for a series of intercalate of D4000-I-I-kenyaite intercalates with
different basal spacings are shown in Figure 5.6. The weight loss showed even for the
intercalate with the highest basal spacing corresponds to a molar ratio of amino groups to
intragallery protons of only about 0.45. It is likely that all of the amino groups have been
protonated and that further intercalation by neutral Jeffamine might be possible at higher
concentrations, except for the inaccessible intercalate with a 20.9 A basal spacing.

The basal spacings for each intercalate formed between the layered silicic acids
and the Jeffamines are summarized in Table 5.5. An ordered peak was not observed for
the intercalate formed between H-magadiite and Jeffamine D4000, suggesting that this

product may contain exfoliated silicic acid nanolayers.

 

144

 

 

 

97.2A
.4?
2
2 69.0
E
(D
.2
E A
(D
m
- - B
20.9A
C

 

 

illiiiiliiiiliiiililiiliiiiliiiiLiiir

1 2 3 4 5 6 7 8
26 (Degrees)

 

Figure 5.5 XRD patterns of the intercalates formed from H+-kenyaite and the
poly(oxypropyleneamine) Jeffamine D4000: (A) as-synthesized (unwashed); (B) washed
one time with ethanol; and (C) washed multiple times with ethanol.

145

 

 

 

 

 

 

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g 60: \
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: ——H-kenyaite
- —----20.9A \\ ‘
4o:— -------- -69.0A ‘--_‘___
: —--97.2A
30_lllllllll_LlllllllllllllllllllLLlllll

 

0 100 200 300 400 500 600 700
Temperature (°C)

Figure 5.6 TGA curves for H-kenyaite and D4000-H-kenyaite intercalates with
different basal spacings (cf. Figure 5.5).

146

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147

Although, the approach developed for the synthesis of Jeffamine-H-layered silicic
acid intercalates was successful, most of products required a washing process in order to
obtain intercalates in a powdered physical state. The post washing process can cause two
problems. First of all, the washing process has to be controlled to avoid the formation of
a collapsed lateral monolayer gallery structure or the formation of mixed phases.
Secondly, Jeffamine is deintercalated and a significant amount of ethanol have to be
used, although both chemicals can be recycled. However, we developed an alternative
method that can avoid both problems. In this approach, the precise amount of Jeffamine
and the minimum amount of ethanol need to form a powdered intercalate are used. The
powdered intercalates are formed directly after the solvent is removed after air drying.
Typical experimental conditions are summarized in Table 5.6 for the synthesis of
different intercalates wherein Jeffamine D2000 is the intercalate. The intercalation
reaction were completed in less than 5 hours. Some of the intercalates may contain less
than 3 wt % of water, which can be removed by drying in an oven at 100 0C for
overnight. This approach is very general for Jeffamine and other organic species that can
react with the proton forms of layered silicic acids. For instances, the preparation of

C18-magadiite-PF and C18-kenyaite-PF is easily obtained by this universal approach.

148

 

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149

5.3.2 Exfoliation of Layered Silicic Acid Nanolayers in an Epoxy Polymer Matrix

Using Jeffamine-H-Layered Silicic Acid Intercalates

Except for those intercalates with a lateral monolayer or lateral bilayer structures,
the Jeffamine-layered silicic acid intercalates are reactive toward intercalation by the
mixture of Jeffamine and epoxide resin. The co-intercalation of D2000-H-magadiite
intercalate with a 44.0 A basal spacing, D2000-H-kenyaite intercalate with a 47.8 A basal
spacing, and D2000-H-ilerite intercalate with a 53.0 A basal spacing by a stoichiometric
mixture of epoxide and D2000 curing agent were investigated by XRD as a function of
time (see Figure 5.7, Figure 5.8 and Figure 5.9, respectively). The advantages using
powdered forms of these intercalates are that the diffusion process is much quicker and
more silicate can be used to form a composite with a high silicate loading. As shown in
these figures, the co-interrelation of epoxide and D2000 is very readily achieved. An
intercalate with a higher basal spacing is quickly formed even at room temperature with
about a 20 A increase of gallery height for magadiite and kenyaite, and a 12 A increase
for ilerite. We also observed that the diffusion process is much faster for D2000-H—
layered silicic acid intercalates than the alkylammonium exchange analogs, suggesting a
more open gallery structure. The initial intercalates were replaced quickly by expanded
phases that exhibit only second and third order peaks. In all cases, a rapid intragallery
polymerization was observed with the formation of exfoliation (amorphous) phases. The
strong intragallery acidity affords a very favorable intragallery environment for

delamination of silicate nanolayers.

150

 

 

 

 

 

 

 

38.7A
E
34.2A
2‘ \‘N
.2 D
a: 61A
4—0
5
0
.g 30.5A
.59
§
C
30.5A
44.0A B
22.0A
a- A
lJJllJLlllllLLllllllllllILLLLILLLL

 

 

 

1 2 3 4 5 6 7 8
29 (Degrees)

Figure 5.7 XRD patterns of (A) the initial D2000-H-magadiite intercalate and the
nanocomposites formed at 20 wt % D2000-H-magadiite intercalate loading by reaction of
stoichiometric mixtures of epoxide and the poly(oxypropyleneamine) under the following
reaction conditions: (B) 25 °C, 60 min; (C) 75 0C, 60 min; (D) 75 °C, 120 min; (B) 75
°C, 150 min.

151

 

 

67.4 A

 

47.8 A

Relative Intensity
K
0 U

A

lllllllllllllllllllllllllLllLllllJ

1 2 3 4 5 6 7 8
29 (Degrees)

 

 

 

 

Figure 5.8 XRD patterns of (A) the initial D2000-H-kenyaite intercalate and the
nanocomposites formed at 20 wt % D2000-H-kenyaite intercalate loading by reaction of

stoichiometric mixtures of epoxide and the poly(oxypropyleneamine) under the following

reaction conditions: (B) 25 °C, 2 min; (C) 25 °C, 10 min; (D) 75 °C, 30 min.

152

 

 

 

 

 

 

 

 

a?
(D
C
.2
E
Q)
.2
E
a)
I C
65.3A
B
53.0A
__ -__ A
iiLiLunmiilliiiilliiiiiiilhililliilliii;
l 2 3 4 5 6 7 8 9 10

29 (Degrees)

Figure 5.9 XRD patterns of (A) the initial D2000-H-ilerite intercalate and the
nanocomposites formed at 20 wt % D2000-H-ilerite intercalate loading by reaction of

stoichiometric mixtures of epoxide and the poly(oxypropyleneamine) under the following

reaction conditions: (B) 25 °C, 5 min; (C) 75 °C, 5 min; (D) 75 °C, 135 min.

153

The XRD patterns for the fully cured composites prepared from D2000-H-layered
silicic acid intercalates are shown in Figure 5.10. All these patterns are characterized by
the absence of XRD peaks at low angle which suggests that the exfoliated epoxy-layered
silicic acid nanocomposites have been obtained. The x-ray results also show that the 2-
dimensional structures of layered silicic acids are retained for all of these exfoliated
nanocomposites, as indicated by the strong in-plane peaks. We point out that the other
Jeffamine-H-layered silicic acid intercalates can also undergo very similar intercalation
and exfoliation chemistry.

A series of epoxy nanocomposites with different D2000-H-magadiite intercalate
loadings were also prepared. The XRD patterns for the nanocomposites are shown in
Figure 5.11. Interestingly, as the magadiite loading is increased, broad x-ray reflections
appear, which signifies that the average layer separation decreases with increasing
loading. The high basal spacing peaks at low 26 angles are observed when the loading of
the intercalate reaches 40 wt %. These results show that the average layer separation is
related to the loading of silicate nanolayers. Table 5.7 lists the results of average
nanolayer separation expected for the theoretical calculation and the values observed by
XRD. The calculated and observed values agree relatively well in the range of high
loading for D2000-H-magadiite intercalates. It is noteworthy that peaks at 121 A and 95
A are of relatively low intensity. So, the magadiite layers may be dispersed in the
polymer a mixed disordered and ordered exfoliated fashion. It is not surprising that an
ordered exfoliated phase is present, because the synthetic layered silicate normally has a
very homogenous distribution of charges. The uniform distribution in layer charge favors

uniform polymerization rates and the formation of regularly ordered gallery heights

between nanolayers.

 

154

 

 
  
    

kenyaite _ _C

Relative Intensity

magadiite

 

 

 

 

 

29 (Degrees)

 

Relative Intensity

 

 

 

1o 20 30 40 so 60
26 (Dogma)

Figure 5.10 XRD patterns for the epoxy-layered silicic acid nanocomposites formed
from the following intercalates: (A) D2000-H-ilerite; (B) D2000-H-magadiite; (C)
D2000-H-kenyaite. The D2000-layered silicic acid intercalate loadings are 20 wt %, 10
wt % and 15 wt % for ilerite, magadiite and kenyaite, respectively. The intercalated
curing agent was counted as contributing to the stoichiometry for epoxide cross-linking.

155

 

 

 

 

 

 

 

E‘
(D
C
23

E 10% B
(D
.5

2.3 20% C
c:

121A 30% D

95 A ~ -- {107; E

50°/o#¢ F

2 4 e 8 1o

26) (Degrees)

Figure 5.11 XRD patterns of cured epoxy-magadiite nanocomposites (curing
conditions: at 75 0C for 3 h, and followed by 3 h at 125 0C) prepared from D2000-H-

magadiite intercalates. The D2000-H-magadiite intercalate loadings were as follows: (A)
5 wt %; (B) 10 wt %; (C) 20 wt %; (D) 30 wt %; (E) 40 wt %; and (F) 50 wt %.

 

156

Table 5.7 Average Silicate Layer Separation (A) for Regularly
Exfoliated Epoxy-Magadiite Nanocomposites Prepared form
D2000-H-magadiite Intercalates

 

 

Wt 7?. 92°00'11“ 5 10 20 3o 40 50
magadiite intercalate

calculated 1360 670 3 10 205 149 l 13

observed — — — — 121 95

 

It was of interest to investigate the nature of the silica formed upon calcination of
the nanocomposites. The morphology of the nanolayers in the nanocomposite should be
reflected in the surface area and XRD patterns of the silica recovered upon calcination
accordingly, the exfoliated nanocomposite prepared from D2000-H-magadiite intercalate
with a 44.0 A basal spacing was calcined at 650 0C for 4 h to form silica. The D2000-H-
magadiite intercalate with a 54.6 A basal spacing was also calcined at the same
conditions. The XRD results are shown in Figure 5.12. Strong in-plane magadiite peaks
are observed in both cases. However, the extent of layer restacking upon calcination was

quite different in these two cases. The silica formed from the D2000-H-magadiite

intercalate shows an intense and narrow dOOI peak indicative of extensive layer

restacking, whereas the C1001 peak for the silica from the exfoliated nanocomposite is very

weak and broad, indicating much less restacking order.

 

IFIquOJF' - l'. . _
L

 

157

 

 

 

 

 

 

 

7
29 (Degrees)

 

Relative Intensity

A

 

 

llllllllllllllllllllllllllLL

0 1 0 20 30 40 50 60
29 (Degrees)

Figure 5.12 XRD patterns for the silica formed from upon calcination of (A) D2000-
H-magadiite intercalate with a 54.6 A basal spacing and containing 50 wt % H-magadiite;
and (B) an exfoliated epoxy-magadiite nanocomposite prepared form the D2000-H-
magadiite intercalate with a 44.0 A basal spacing and containing 5 wt % H-magadiite.
The expanded insert for the d001 peaks was obtained using a step scan mode. The

calcinations were carried out at 650 0C for 4 h in air using a heating rate of 2 OC/min.

158

The N2 adsorption-desorption measurements for the silica obtained from the

exfoliated nanocomposite and D200-H-magadiite intercalate, are shown in Figure 5.13.
The N2 adsorption results verify the above XRD results. A much larger texture porosity
and surface area are observed for the silica recovered form the exfoliated nanocomposite
than from D2000-H-magadiite intercalate. Most of the exfoliated silicate nanolayers in
the nanocomposite retain their exfoliated morphology to form the familiar card-house
structure upon calcination. The card house structure is schematically illustrated in Figure
5.14 along with the restacked layer aggregation possibilities for the silica obtained by
calcination of the D2000-H-magadiite intercalate. The card-house structure will result in
a significant increase in texture porosity and surface area. In addition a weak and very
broad 001 XRD reflection is expected, as observed. Interestingly, the surface area for the
exfoliated nanocomposite prepared from D2000-H-magadiite intercalate is about 50%
larger than the value obtained by calcination of the exfoliated nanocomposite formed
from C18-magadiite-PF, although they have a similar magadiite silicate (SiOz) loading
(cf. Figure 3.17). This result suggests that the extent of magadiite exfoliation is higher
for the nanocomposite formed from D2000-H-magadiite than from C18-magadiite-PF.
So, it would not be surprising to expect a superior performance properties for the

nanocomposite prepared from D2000-H-magadiite intercalate than Cl8-magadiite-PF.

___m:,

 

159

 

 

 

 

 

 

ff 400—
.—
m d
9.3
E 300-
E — sw=192m2g'
8 200‘
'C
<
m .—
g
E 100‘
_ SBET=61m’g“
0 1 1 1 l 1 1 1 l 1 1 1 l 1 1 1 l 1 1 1
0 0.2 0.4 0.6 0.8 1

Relative Pressure (P/Po)

Figure 5.13 N 2 adsorption-desorption isotherms for the silicas obtained by calcination

of an epoxy-exfoliated magadiite nanocomposite prepared from D2000-H-magadiite
intercalate with a 44.0 A basal spacing and containing 5 wt % H-magadiite (top curve);
and the D2000-H-magadiite intercalate with a 54.6 A basal spacing containing 50 wt %

H-magadiite (bottom curve). The calcinations were carried out at 650 0C for 4 h in air

using a heating rate of 2 oC/min.

 

 

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161

5.3.3 Performance Properties of Exfoliated Epoxy-Layered Silicic Acid

Nanocomposites Prepared by the Proton Exchanged Pathway

A comparison of the tensile properties for epoxy—exfoliated magadiite
nanocomposites prepared from D2000-H-magadiiteintercalates and alkylammonium
exchanged magadiite intercalates is shown in Figure 5.15. For both the tensile strength
and the tensile modulus, the nanocomposite prepared from D2000-H-magadiite
intercalates out-performs the nanocomposites prepared from C18A1M-magadiite and
C18-magadiite-PF (cf. Chapter 2 & 3). The increase with loading is nearly linear for the
nanocomposites prepared by the new proton-exchange layered silicic acid pathway. On
the other hand, the nanocomposites prepared by alkylammonium exchange pathway show
a tendency to yield, especially for the tensile modulus.

At least two factors play important roles in the improvement of the mechanical
performance of the nanocomposites obtained by proton exchange pathway. The first
factor is that the formation of dangling chains has been eliminated by the pathway using
Jeffamine intercalated H-layered silicic acids as the starting materials. As was shown in
Chapter 2 & 3, the formation of the dangling chains in the polymer matrix compromises
the reinforcement advantages of the exfoliated silicate nanolayers, particularly, at high
magadiite loading when alkylammonium exchanged magadiite intercalates are used to
achieve exfoliation. Secondly, as we have demonstrated in section 5.3.2, the extent of
silicate nanolayer exfoliation is superior for the proton-exchange pathway. The degree of
exfoliation always play a dominate role in deciding the performance properties of

polymer-layered silicate clay nanocomposites.

 

 

162

 

DZOOO—H-magadiite

   
  
   

,0

t

18A1M-magadiite

C18-magadiite-PF

Tensile Strength (MPa)

I I I I I I I I I I I I l I T I

 

 

 

lllllllllllulllllllllllllIll

0 5 10 15
Magadiite Silicate Loading (wt % SiOz)

 

 

DZOOO—H-magadiite

C18A1 M-magadiite
- - O

01 8-magadiite-PF

Tensile Modulus (MPa)
8

I—IIITIIITIUITIIIIII[IIIIIII

 

 

1111111111111111_L14l111111111

0 5 10 1 5
Magadiite Silicate Loading (wt % SiOZ)

 

Figure 5.15 Comparison of tensile properties for exfoliated nanocomposites prepared
from D2000-H-magadiite intercalates and organo magadiite intercalates with a paraffin
structure (C18-magadiite-PF and ClSAlM-magadiite).

 

163

The exfoliated nanocomposites prepared by this new pathway are superior not
only in their tensile properties. We also compared the solvent resistance for the
exfoliated nanocomposites prepared from different pathways. The solvent uptake curves
are plotted in Figure 5.16 and Figure 5 . 17 for the nanocomposites prepared from D2000-
H-magadiite intercalate and C18-magadiite-PF. The plots of equilibrium uptake value vs.
magadiite silicate loading are shown in Figure 5.18. The uptake of methanol and toluene
was reduced substantially by both pathways. However, the nanocomposites prepared
from this new H-magadiite pathway show a much greater reduction of solvent uptake
than those prepared from alkylammonium exchanged magadiites. The formation of
dangling chains in the polymer matrix is still a dominant factor in compromising the
solvent swelling properties of nanocomposites formed from organo magadiites.

More significantly, we have found that the nanocomposites prepared by this H-
magadiite pathway can be almost completely restored to their original strength after
solvent swelling. Show in Figure 5.19 are the tensile properties of epoxy-kenyaite
composites in the original state and after swelling in toluene and drying. In contrast, the
pristine epoxy polymer, as well as most of the nanocomposites prepared by the
alkylammonium exchange pathway, are completely disintegrated upon drying, especially
at low magadiite silicate loading.

We already discussed in Chapter 4 the effect of nanolayer thickness on the
performance properties of nanocomposites prepared by the alkylammonium exchange
pathway. However, the effect of alkylammonium and alkylamine in introducing dangling
chains in the polymer matrix should not be ignored. This factor has been largely
eliminated using the Jeffamine-H-layered silicic acid intercalates as starting materials. A
comparison of tensile strength for nanocomposites prepared from different proton-
exchange members of the layered silicic acid family is given in Figure 5.20. We only
observed marginal differences between each member. This applies to the results of

chemical stability and solvent resistance, which are summarized in Table 5.8.

 

80
C18-magadiite-PF o
70 o
O
60 o o
A o
°\° o
‘c’ 50 0 o
. O o
g o .
.1:
.Q’ o o A A
0 30 o A
g 0 0° A A A
20 ° ° AA 0 pristine
o A
X A o 4.8%
10 A 14.6%)
0 fI T l I rI I l I I I I I I I I I I I I I I1 I I I I
0 100 200 300 400 500 600
Time (Minutes)
80
: DZOOO-H-magadiite o
702‘ o
i o
60‘: o o
A 3 °
25 so: o °
.C 1 o o 0
a 2 o 0
S2 40: o o o
5 ' o
.9 3 ° 0 A
a: 30: 0 ° A
3 i 0 0° A A A
w€o ° 1AA“ ..
: AA 0 pristine
10; ° A A o 5.1%
3 A A 16.2%
.1
O j—I I I I I I I I I T I I I I I I I I I I I I [fl I I
0 100 200 300 400 500 600

Figure 5.16 Comparison of methanol uptake curves for epoxy-exfoliated magadiite
nanocomposites prepared from C18-magadiite-PF intercalates and D2000-H-magadiite
intercalates. The tabulated values are the loadings (wt %) of C18-magadiite-PF

164

 

 

 

 

 

 

 

Time (Minutes)

intercalate and D2000-H-magadiite intercalate for each composite.

—-.r.. A“

 

 

\I'I

165

 

 

 

 

 

 

 

 

 

200
‘ CtB-magadiite—PF
-I O O
. O
. o O
A 150': o 0
9\: " o o 0 2
C .I r, 0 0 A A
'36 - o J A A
(5 ° -
100- o A
E .4 ’J o A .
.9 A o A
2 J . z A hl
" Z 0 pristine
50: G 3 o 4.8%
- X A 14.6%
1 _
c I I I I I I I I I l I I I I I I 1' I I I W I I I 1 I I I I 2 f '
0 100 200 300 400 500 600 .
Time (Minutes) "
200
D2000-H-magadiite o
i o o
q o 0 0
150: o o
"o‘ .. o
2\" ..J O o O
c: o
.66 -i C 0 o
(5 1004 0 °
E ‘ ° °
C» .4 ° A
'5 q 0 ° A A
3 -i 0 A A A A
o
501 ° 9 A A A o pristine
‘ A A 16.2%
o¢jI1I1IfifIIIIIIIIrleIII'IIII
O 100 200 300 400 500 600

Time (Minutes)

Figure 5.17 Comparison of toluene uptake curves for epoxy—exfoliated magadiite
nanocomposites prepared from C18-magadiite-PF intercalates and D2000-H-magadiite
intercalates. The tabulated values are the loadings (wt %) of C lS—magadiite-PF
intercalate and D2000-H--magadiite intercalate for each composite.

 

166

 

 

 

 

 

 

 

65
:Methanol
60"
35 .
g ' C18—magadiite-PF
g 55—
% - \ DZOOO-H-magadiite
3 : \~
50:- .\‘
’ ‘o
45.111111111llllLllllllLllllllllllllllllllllllllll
0 2 4 6 8 10 12

Magadiite Silicate Loading (wt °/o SiOZ)

 

150
Toluene

‘\Q\. CiB-magadiite-PF
\ .

\.~ DZOOO—H-magadiite
.\ ~

140

130

120

110

Weight Gain (%)

100
‘9

80 iiiiiiiLLiiiiiilininiiillinijiilniijiiijiiiiimi

0 2 4 6 8 1O 12
Magadiite Silicate Loading (wt % SD)

90

IIIIIIIII'IIII'IIII'IIIIIIIIIIIIII

 

 

 

Figure 5.18 Comparison of methanol and toluene uptake at equilibrium vs. magadiite

silicate (SiOz) loading for epoxy-exfoliated magadiite nanocomposites prepared form
C18-magadiite-PF and D2000-H-magadiite intercalates.

167

 

3.5

3.0

 

2.5

2.0

1.5

 

Tensile Strength (MPa)

1.0 —0— original composites jg.

. . . - - - preswollen in toluene

[#lllllllllllllllllll 1'
0.50 2 4 6 8 1O 12 E3

Kenyaite Silicate Loading (wt °/o SiOz)

 

IIIIIIIII'IIIIIIIII'IIIIIIIII

 

 

 

 

...A
O

—0— original composites

 

(D

- - 0 - - - preswollen in toluene

Tensile Modulus (MPa)
01 m \l on

.h

oWII'III'III'IIITIII'III'III

 

 

llllllllllllllllllllllj

2 4 6 8 10 12
Kenyaite Silicate Loading (wt °/o SiOZ)

 

03

Figure 5.19 Tensile properties for the epoxy-kenyaite nanocomposites prepared from
D2000—H-kenyaite intercalates before and after having been swollen in toluene. In the
solvent swelling experiment the nanocomposites were soaked in toluene for 12 h, and
subsequently dried in air for 24 h before the tensile properties were measured.

168

 

    

 

  

 

 

5-
A 4+—
m .-
m .-
E 3L
.C -
5 I
C .-
2 3:
C0 .
2 i-
C ..
'03 Z —O—magadute
I —<>---ilerite
1 _ ----A---- kenyaite
OILJALllllJllllllllllill
0 2 4 6 8 10 12

Silicate Loading (wt % SiOz)

Figure 5.20 A comparison of the tensile strength vs. silicate loading for epoxy-layered
silicic acid nanocomposites prepared from D2000-H-layered silicic acid intercalates.

169

Table 5.8 Chemical and Solvent Resistance of Epoxy-Exfoliated Layered
Silicic acid Nanocomposites Prepared From D2000-H-silicic
acid intercalates. The silicate (SiOz) loadings for each composite
are 4.4 wt %, 6.4 wt % and 6.6 wt % for ilerite, magadiite and
kenyaite respectively. Values are the Immersion Weight Gain
(wt %) after Certain Uptake Periods.

 

 

materials 10% distilled 30% 5% methanolb tolueneC
NaOHa H203 H2804a acetic acida
pristine polymer 1.6 2.5 16.7 13.4 76.5 189
ilerite 1.6 2.7 12.4 10.4 58.7 1 18
magadiite 1.7 2.4 12.5 8.8 53.2 102
kenyaite 1.8 2.4 13.3 9.0 53.7 104

 

a Weight gain after 15 days. b Weight gain after 48 hr. C Weight gain after 24 hr.

Theoretically, the aspect ratios of silicate nanolayers should affect the mechanical
properties of each nanocomposite. But, the number of silicate nanolayer presented is
varied when the silicate loading is a constant. We already know that the silicate loading
has a significant effect on the performance properties. Also, the interfacial properties can
also influence the overall performance. The nanocomposites we prepared so far have a
relatively weak interface, wherein ionic bonding and van der Waals forces are the main
interactions between the polymer matrix and the layered silicate phases. If interfacial
interactions are more important than aspect ratios, then we will not see the effect of
aspect ratios. Covalent bonding between the polymer and the layered silicate phases
could address this issue. However, forming covalent linkages will not be easy. Most
silylating agents which can be potentially grafted on interlayer surface of layered silicic
acids are not large enough to force formation of the paraffin or lipid bilayer structures
that are needed to achieve exfoliation in the final nanocomposite. The more commonly
encountered lateral monolayer structure is not swellable by our polymer precursors.

Also, the conditions needed for grafting reactions are not suited to our organo layered

 

 

170

silicic acids with a paraffin structure, because most intragallery organo species are
desorbed under organic solvent refluxing conditions. Nevertheless, grafting is still an
attractive option, provided that the proper reaction conditions and silylating agents, can

be formed.

5.4 Conclusions

A new approach has been developed for the preparation of thermoset polymer-
layered silicate clay nanocomposites in which curing agents can be directly intercalated
into the intragallery without the need for alkylammonium ions on the exchange sites of
the clay. This new development has resulted in a greater improvement of the overall
properties of the polymer-layered silicic acid nanocomposites. This approach is general
and could be applied to a lot of polymer systems when the polymer precursors can react
with the intragallery protons of layered silicate clays. Nanocomposites based on both
thermoplastic and thermoset polymers should be possible by this method. The
interactions between the polymer precursors and protons can lead either to ionic bonding
or the hydrogen bonding. The polymer precursors with amino, hydroxyl, amide, urea and
urethane end or side groups are all good candidates for intercalation into proton

exchanged clays for nanocomposite formation.

 

5.5

swung.»

ll.

12.
13.
14.
15.

16.
17.

18.

19.

20.

21.

171

References
Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T.;
Kamigaito, O. J. Mater. Res. 1993, 8, 1185.

Kojima, Y.; Fukumori, K.; Usuki, A.; Okada, A.; Kurauchi, T. J. Mater. Sci.
Lett. 1993, 12, 889.

Yano, K.; Usuki, A.; Okada, A.; Kurauchi, T.; Kamigaito, O. J. Polym. Sci., Part
A: Polym. Chem. 1993, 31, 2493.

Messersmith, P. B.; Giannelis, E. P. Chem. Mater. 1993, 5, 1064.

 

Wang, M. 8.; Pinnavaia, T. J. Chem. Mater. 1994, 6, 468.
Messersmith, P. B.; Giannelis, E. P. Chem. Mater. 1994, 6, 1719.
Okada, A.; Usuki, A. Mater. Sci. Eng. 1995, C3, 109.

 

Lan, T.; Kaviratna, P. D.; Pinnavaia, T. J. Chem. Mater. 1995, 7, 2144.
Giannelis, E. P. Adv. Mater. 1996, 8, 29.
Kurokawa, Y.; Yasuda, H.; Oya, A. J. Mater. Sci. lett. 1996, 15, 1481.

Usuki, A.; Kato, M.; Okada, A.; Kurauchi, T. J. Appl. Polym. Sci. 1997, 63,
137.

Pinnavaia, T. J. Science 1983, 220, 365.
Kato, C.; Kuroda, K.; Misawa, M. Clays Clay Miner. 1979, 27, 129.
Fukushima, Y.; Inagaki, S. J. Inclusion Phenom. 1987, 5, 473.

Fukushima, Y.; Okada, A.; Kawasumi, M.; Kurauchi, T.; Kamigaito, 0. Clay
Miner. 1988, 23, 27.

Shi, H.; Lan, T.; Pinnavaia, T. J. Chem. Mater. 1996, 8, 1584.

Usuki, A.; Kojima, Y.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T.;
Kamigaito, O. J. Mater. Res. 1993, 8, 1179.

Messersmith, P. B.; Giannelis, E. P. J. Polym. Sci., Part A: Polym. Chem. 1995,
33, 1047.

Pinnavaia, T. J.; Lan, T.; Wang, Z.; Shi, H.; Kaviratna, P. D. ACS Symp. Ser.
1996, 622, 250.

81212821; A.; Jandt, K. D.; Kramer, E. J .; Giannelis, E. P. Chem. Mater. 1996,

Vaia, R. A.; Ishii, H.; Giannelis, E. P. Chem. Mater. 1993, 5, 1694.

 

22.
23.
24.

25.
26.
27.
28.
29.
30.
31.
32.

172

Lagaly, G.; Beneke, K.; Weiss, A. Am. Mineral. 1975, 60, 642.
Lagaly, G.; Beneke, K.; Weiss, A. Am. Mineral. 1975, 60, 650.

Pinnavaia, T. J .; Johnson, I. D.; Lipsicas, M. J. Solid State Chem. 1986, 63,
118.

Rojo, J. M.; Ruiz-Hitzky, 13.; Sanz, J. Inorg. Chem. 1988, 27, 2785.
Beneke, K.; Lagaly, G. Am. Mineral. 1983, 68, 818.

Beneke, K.; Lagaly, G. Am. Mineral. 1989, 74, 224.

Kruse, H. H.; Beneke, K.; Lagaly, G. Colloid Polym. Sci. 1989, 267, 844.
Doring, J .; Lagaly, G. Clay Mineral. 1993, 28, 39.

Fletcher, R. A.; Bibby, D. M. Clays Clay Miner. 1987, 35, 318.

Beneke, K.; Kruse, H. H.; Lagaly, G. Z anorg. allg. Chem. 1984, 518, 65.
Kosuge, K.; Tsunashima, A. J. Chem. Soc., Chem. Commun. 1995, 2427.

 

Chapter 6

HYBRID NANOCOMPOSITES FORMED FROM AN ORGANOFUNCTIONAL
POLY(DIMETHYLSILOXANE) POLYMER AND PROTON FORMS OF
LAYERED SILICIC ACIDS; AND THEIR USE FOR THE FORMATION OF
HIGH SURFACE AREA SILICAS

6.1 Introduction

Polymer-clay nanocomposites can exhibit performance properties superior to their
parent materials},2 Two types of nanostructured polymer-clay nanocomposites have
been synthesized},4 Intercalated nanocomposites are formed when one or a few
molecular layers of polymer are inserted into the clay galleries with fixed interlayer
spacings. Exfoliated nanocomposites are formed only when the silicate nanolayers are
exfoliated in the polymer matrix with their average layer separation dependent on the
clay silicate loading. The interlayer spacing for the exfoliated nanocomposite may be
uniform (regular) or variable (disordered).5 Although the intercalated phase of layered
silicates can improve some structural properties,6 the exfoliated phase of silicate
nanolayers is more effective in improving the overall performance properties of the
resulting composite materials?9

However, organic polymers more readily form intercalated polymer-clay
nanocomposites than exfoliated polymer-clay nanocomposites upon intercalation in
organo clays. Ever since the nylon 6-clay hybrid material, which is truly an exfoliated
polymer-clay nanocomposite, was synthesized by Toyota researchers,‘0"3 only a few
other polymer systems such as polyimides, 14 acrylonitrile rubber,15 polyether,‘6 epoxy 1‘9

and polysiloxanel7 have been successfully used to extend this revolutionary chemistry

173

 

174

over the pass ten years. In most systems, the factors required to facilitate the exfoliation
of clay nanolayers were not well understood. The process of exfoliation is most difficult

when an extremely hydrophobic polymer is involved, such as polyethylene and

polypropylene”,19

Polysiloxane polymers are unique polymeric materials because of their special

properties in a variety of physical forms (fluids, resins, and elastomers). They exhibit the
lowest Tg of (-125 0C) and Tm of (-40 oC) among useful polymers. Siloxane polymers

have very low surface tension and good dielectric strength. They possess good thermal
stability, oxidative resistance and chemical stability. All of these physical and chemical

properties decide that they can be widely used as lubricants, greases, releasing agents,

surfactants, water-repellents, adhesives, heat-transfer fluids, sealants and so on.20~21

Polysiloxane elastomers are commonly reinforced by fillers such as silica to

improve abrasion resistance, tear strength and tensile properties.22 The technique of in

situ formation of precipitated silica particles (sol-gel technique) has been recently used in

order to achieve the formation of highly dispersed silica fillers};23 In this method, the
inorganic phase is formed by the hydrolysis and condensation of metal oxide precursors

such as tetraethylorthosilicates:

Si(OEt)4 + 2H20 —) Si02 + 4EtOH

The metal oxide precursors are first absorbed into the polymer network, then the swollen
polymer is placed into water containing the catalyst ( an acid or base) which can facilitate
the above hydrolysis and condensation reaction. The average diameter of the resulted
silica particles is about 20 ~ 30 nm. The resulted composites showed improved tensile
properties.

Layered silicate nanolayers can be used as alternative inorganic components in

the synthesis of polysiloxane-silicate nanocomposites. The clay silicate nanolayers

possess high particle aspect ratios comparable to the in situ formed silica particles.24 It

175

has been reported that the dielectric strength of siloxane polymer was improved by

simply imbedding the surfactant treated silicate clays such as polygorskite and

montmorillonite into the siloxane polymer matrix.25 The intercalation chemistry of
layered silicate clays by silicone polymers can play an important role in further
improving the performance properties of resulted composite materials, especially when
10 A-thick nanolayers of clay are exfoliated in the polymer matrix.

The synthesis of a polysiloxane polymer-exfoliated clay nanocomposite was

reported recently.17 In that synthesis, a silanol terminated poly(dimethylsiloxane)
(PDMS) was cross-linked by tetraethylorthosilicate (TEOS) in the presence of an organo
clay. The resulting nanocomposite exhibited an improved solvent resistance and thermal
stability. However, the organo clay used to achieve silicone intercalation contained long
chain quaternary alkylammonium ions on the gallery exchange sites. The introduction of
nonfunctional onium ion surfactants is not preferred in this chemistry, because they
compromise the performance properties of the polymer.

In the present work, the new approach has been developed for preparing
thermoset silicone polymer-layered silicate clay nanocomposites in which curing agents
can be directly intercalated into clay galleries and cross-linked without the need of
organic onium ions on the clay exchanged sites. We also show that the calcination of the
PDMS-layered silicic acid nanocomposites leads to porous, high surface area silicas in

which the silicate layers are partially re-stacked with silica separating the layers.

6.2 Experimental
6.2.1 Materials

The organofunctional siloxane precursors (Gelest) used for the cross-linked

polymer matrix formation were as follow:

 

 

176

Epoxypropoxypropyl Terminated PDMS (DMSE12)
8‘3 8‘3 H3 \
n
CH3 CH3 .CH3
MW = 900-1100 n = 7.3-10.0

Aminopropyl Terminated PDMS (DMSA12)

FH3 9‘3 in
H

CH3 CH3 CH3
MW = 900-1000 11 = 8.8-10.2
The above siloxane precursors were cross-linked by the reaction of epoxy groups with
primary amine groups to afford a polysiloxane elastomer matrix.
6.2.2 Synthesis of Layered Silicic Acids
Synthesis of Alkali-Metal Forms of Layered Silicic Acids. Alkali-metal forms

of layered silicic acids were synthesized by hydrothermal methods.”28 In general, a

suspension of amorphous silica gel was added to a MOH solution (M = Na or K) and

heated at a temperature above 100 0C for a few days with stirring in a Teflon-lined Parr
reactor. The suspension containing the alkali-metal form of layered silicic acid was
centrifuged, and the solid product was washed with deionized water to remove excess
MOH, and air-dried at room temperature. The exact synthesis conditions used for each
layered silicic acid were specified previously in Chapter 5 (of, Table 5.1).

Synthesis of Proton Forms of Layered Silicic Acids. H+ forms of layered silicic
acids were obtained by titration of synthetic alkali-metal forms of layered silicic acids
with dilute hydrochloric acid.29 A SOO-mL of aqueous suspension containing 15.0 g of
layered silicic acid (magadiite, kenyaite and ilerite) was titrated with 0.1 M HC] at a rate

of 3 mUmin to lower the pH to 1.9. For kenyaite the final pH was 1.8. The suspensions

 

 

 

177

at pH = 1.9 (1.8 for kenyaite) were stirred for 24 hours (48 h for kenyaite). The proton

exchange forms of layered silicic acids were separated by centrifugation and washed with
deionized water until free of Cl', and then air-dried at room temperature.

Synthesis of DMSA12-H-Layered Silicic . Acid Intercalates. An ethanol
solution containing the aminopropyl terminated PDMS (DMSA12) was added to an

aqueous suspension of the H+ form of layered silicic acid and stirred at 60 0C for a
certain period. The ratios of DMSA12 to H-layered silicic acid, the solvent composition,
reaction time, and post reaction washing process used for each synthesis, along with basal
spacing for each intercalate, are specified in Table 6.1. The solid products were obtained
by either centrifugation when a washing process was applied or by evaporation of the

EtOH/HZO solvent in hood. The DMSA12-H-layered silicic acid intercalates were dried

in an oven at 100 0C for 24 h before they were used for composite preparation.
6.2.3 Preparation of PDMS-Layered Silicic Acid Composites

Stoichiometric amounts of DMSE12 and DMSA12 in the form of the free amine
and the DMSA12-H-layered silicate intercalate were mixed at room temperature for 10

min. The desired amount of DMSA12-H-layered silicic acid intercalates was added to

the DMSE12-DMSA12 mixture and stirred at room temperature or at 50 0C for an
additional 24 h. The DMSA12 in intercalated form was counted as contributing to the

stoichiometry for DMSE12-DMSA12 cross-linking. This mixture was outgassed in a

vacuum oven and poured into a stainless steel mold for curing at 125 0C for 6 h.

 

 

178

 

 

 

 

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8E 8 «a omen 8a 23 3m
28: Co 3. anm 32 Esq fine ofixeox-m-§<mga
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ONEEOAm 93v Gov A5 Cum—EOE Boa 06:6 68992-: we 88m 2V Sou 03380::
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moan—3.8:.— Eu< 395m gaoma_.=.§<m29 we flmofiim u.» wink.

179

6.2.4 Characterization Methods
X-ray Powder Diffraction (XRD). XRD patterns were recorded on a Rigaku

rotaflex 200B diffractometer equipped with a rotating anode, Cu Ka x-ray radiation (A =

1.541838 A) and a curved crystal graphite monochromator. The x-ray was operated at 45

KV and 100 mA. Diffraction patterns were collected with 0.010 26 interval between 1
and 10° 29 using a scanning rate of 2° 20 per minute, and DS and SS slit widths of 1/6.

For patterns obtained at starting angle of 0.50 29, a SS width of 0.05 was used instead of

1/6. XRD patterns at high angles were collected using DS and SS slit widths of l. The
gel-like samples were prepared by applying thin films on glass slides. Cured composite
specimens were prepared by mounting a flat rectangular sample into an aluminum holder.

Surface Area Measurement. N2 adsorption-desorption isotherms were

determined on a ASAP 2010 Sorptometer at liquid N2 temperature using a static sorption

mode. Samples were outgassed at 150 0C and 10'5 Torr for 12 h. Surface areas were
determined using BET plots.

Thermal Analysis. Thermogravimetric analyses (TGA) were performed using a
Cahn TG System 121 thermogravimetric analyzer. Samples were heated to 750 0C at a
heating rate of 5 °C/min under N2 atmosphere.

Chemical and Solvent Resistance. Resistance to solvent swelling was obtained
according to ASTM procedure D543. The specimens were immersed in the desired
reagent and removed periodically to measured the weight gain until equilibrium was

reached.

6.3 Results and Discussion
6.3.1 Synthesis of PDMS-H-Layered Silicic Acid Intercalates
We have demonstrated previously that the proton forms of layered silicic acids

can react with the polymer precursors with amino end groups. The present work shows

 

 

180

that this approach can be extended to the aminofunctional PDMS siloxane precursors

wherein a very hydrophobic system is involved:

 

 

H+ + HzN-R-(Si(Me2)-O)n-R-NH2 ——> +H;.N-R-(Si(M<>2)-O)..-R-1‘IH3+ (6.1)

 

The quantitative nature of this reaction is shown by the XRD patterns in Figure 6.1. The
intercalates are formed between DMSA12 and H-layered silicic acids. A co-solvent of
water and ethanol, however, is still necessary for this intercalation reaction. Generally, a
high volume percentage of ethanol has to be applied due to the poor solubility of
DMSA12 in water. The typical synthesis conditions along with basal spacings for each
intercalate are specified in Table 6.1. Basically, a post reaction washing is not necessary
in order to obtain a powdered form of the product, and synthesis conditions can be varied
in a certain range without changing the basal spacings of the resulted intercalates.

The washing is applied only when an intercalate with a decreased basal spacing
(less PDMS) is desired. As listed in Table 6.1, the basal spacing of the DMSA12-H-
kenyaite intercalate decreased from 63.1 A to 36.5 A when the as-synthesized product
was washed by a 1:1 (v/v) mixture of water and ethanol. , Interestingly, both the
intercalates of DMSA12-H—kenyaite with different d-spacings can be further swollen by
the mixture of DMSA12 and DMSE12, but the structures for the final composites formed

by these two intercalates are quite different, as discussed in next section.

 

 

 

181

 

 

 

 

  
  

 

63.1 A
kenyaite C
56.3 A
.é‘
U)
C
93
s
a:
'2 diite
"‘ ma a
% 42.6 A g B
a:

ilerite

   

A

 

 

lllllllllllllll'lllllllllllllllllll

1 2 3 4 5 6 7 8 910
2®(Degrees)

 

Figure 6.1 XRD patterns of intercalates formed by reaction of aminopropyl
terminated polydimethylsiloxane (DMSA12) and (A) H-ilerite; (B) H-magadiite; and (C)
H-kenyaite in ethanol/P120 (cf. Table 6.1) and air-dried without washing.

182

6.3.2 Exfoliation of Layered Silicic Acid Nanolayers in a Cured PDMS Matrix
The reaction of DMSA12-H-kenyaite intercalate with a 63.1 A basal spacing by a
stoichiometric mixture of DMSE12 and DMSA12 as a function of time was investigated

by XRD (Figure 6.2). At a loading of 15 wt % H-kenyaite, an intercalate with a 75 A d-

spacing was quickly formed at 50 °C, corresponding to about a 12 A increase of gallery
height. Unlike the co-intercalation chemistry observed previously for organo layered
silicic acids and mixtures of Jeffamine and epoxide, an even higher d-spacing peak

observed with about an additional 9 A gallery height increase can be achieved by

increasing the curing time at 50 0C to 22 h. This is probably due to the slow diffusion
process and the low reactivity for these polymer precursors with high molecular weight,
consequently, more intermediate intercalates are observed. The absence of a first order
peak is observed when the reaction mixture is heated to a higher temperature of 125 °C.

Finally, an amorphous XRD pattern is obtained (Figure 6.2E) after a sequential cure at 50

0C and 125 °C, which suggests an exfoliated structure has been achieved in the synthesis
of PDMS-kenyaite nanocomposite.

The DMSA12-H-magadiite intercalate undergoes very Vsimilar intercalation and
exfoliation chemistry under the same reaction condition. The XRD patterns for the cured
PDMS-layered silicic acid nanocomposites are shown in Figure 6.3. The retention of the
2-D layered silicate structures upon exfoliation is confirmed by the presence of the in-
plane peaks. It is noteworthy that the peak intensity for these patterns is lower than the
system of epoxy-layered silicic acid nanocomposites. This is probably due to the

interference scattering (contrast matching) by the siloxane polymer backbone.

 

 

 

183

 

 

 

Relative Intensity

 

 

 

 

 

rililijilllniiliillllilllirlilinll

 

1 2 3 4 5 6 7 8
29 (Degrees)

Figure 6.2 XRD patterns of (A) the initial DMSA12-H-kenyaite intercalate and the
partially cured composites formed at 15 wt % H-kenyaite loading by reaction of a
stoichiometric mixture of DMSA12 and DMSE12 under the following reaction
conditions: (B) 50 °C, 30 min; (C) 50 °C, 22 h; (D) 50 °C, 24 h and 125 0C, 10 min. The
pattern (E) is for the fully cured PDMS-kenyaite composite that was cured first at 50 0C
for 24 h and then at 125 0C for 6 h.

 

 

184

 

  
      

 

'—

gu‘.‘|n. AMA~A ~~*I|
_ v v I 1

Relative Intensity

A

 

   
 

magadiite

 

LllllllllllllllljlllJJlllllll4llll4

1 2 3 4 5 6 7 8 9 1 0
20 (Degrees)

 

 
 
   
    

kenyane

 

Relative Intensity

 

magadiite

 

llllllllllLJl—Lll

10 20 30 4O 50 60
29 (Degrees)

Figure 6.3 XRD patterns of the cured PDMS-exfoliated layered silicic acid
nanocomposites prepared from the intercalates formed between DMSA12 and (A) H-
magadiite; and (B) H-kenyaite. The H-silicic acid loading for each composite is 10 wt %.

Polymer curing was carried out at 50 0C for 24 h, and followed by 6 h at 125 °C.

 

 

185

The exfoliation of silicate nanolayers in the polysiloxane matrix is further proved

by the following experiments. The PDMS-kenyaite nanocomposites were calcined at 540

0C for 10 h in order to remove the cured polymer and obtain the calcined silica residue,
the DMSA12-H-kenyaite intercalate with a 63.1 A d-spacing was also calcined as a
reference sample. The XRD patterns for these calcined samples are shown in Figure 6.4.
Interestingly, the DMSA12-H-kenyaite intercalate shows a strong asymmetric peak with
about a 32 A d-spacing, whereas, the PDMS nanocomposites prepared from this
DMSA12-H-kenyaite intercalate show broad peaks at higher spacings with the peak
position dependent on the loading of H-kenyaite. Since the composite samples are
calcined in air, the polymer matrix is converted into amorphous silica. The kenyaite
layers become partially restacked with amorphous silica trapped between the layers. The
is supported the weight loss after calcination and by XRD patterns for the calcined
samples. The kenyaite galleries are loaded with the different amounts of polysiloxane
polymer when the ratio of polymer to H-kenyaite is varied. Therefore, once the
intragallery polysiloxane polymer is converted into the amorphous silica phase, the
gallery height becomes proportional to the loading of intragallery polymer. The XRD
results for these calcined samples ((1001 = ~ 100 A at 8.2 wt %, ~ 76 A at 15 wt % loading
of H-kenyaite) are consisted with this concept, and with the presence of exfoliated silicate
nanolayers in the initial composite.

The technique of surface area measurement by N2 adsorption-desorption was also

used to investigate the partial restacking of silicate nanolayers for the silica-intercalated
kenyaites formed by calcination. which is shown in Figure 6.5. A large texture porosity
and surface area were observed for the silica obtained from the calcined nanocomposite
with a 8.2 wt % loading of H-kenyaite (Figure 6.5). This result agrees with the formation
of a silica in which some of the kenyaite is re-stacked with amorphous silica between the
layers and remainder is exfoliated. This provides a significant increase in texture

porosity and surface area, especially when the amorphous silica is presented.

 

186

 

Relative Intensity

IIIWIITIIIIIIIIIIIIIII'IIIITIUIIIIIIII

 

 

 

 

 

Figure 6.4

lllrlllllllllllllllljjllllLlLll11111111

 

0 2 4 6 8

28 (Degrees)

10

XRD patterns for the silica-intercalated kenyaites obtained by calcination

of the PDMS-exfoliated kenyaite nanocomposites at 540 0C for 10 h with a heating rate

of l °C/min. The H-kenyaite loading was (A) 8.2 wt %; and (B) 15 wt %. Pattern (C) is
for the DMSA12-H-kenyaite intercalate after the same calcination conditions.

 

187

 

350

300 -

250 -

200 -

150-

100-

 

Volume Adsorbed (cat/g. STP)

3355339 m2‘g-1

5Od

 

 

 

O 0.2 0.4 0.6 ‘ 0.8 1
Relative Pressure (P/Po)

Figure 6.5 N2 adsorption-desorption isotherm for the silica residue obtained by

calcining the PDMS-exfoliated kenyaite nanocomposite prepared from a DMSA12-H-
kenyaite intercalate. The H-kenyaite loading was 8.2 wt %. The calcination was carried

out at 540 0C for 10 hr in air using a heating rate of l °C/min.

188

As we mentioned in last section, the washed DMSA12-H-kenyaite intercalate
with a 36.5 A d-spacing showed a behavior different from the unwashed 63.1 A
intercalate (see Table 6.1) when swollen by a mixture of DMSE12 and DMSA12. An
intercalated nanocomposite with a 72.8 A d-spacing is formed instead of an exfoliated
nanocomposite. Figure 6.6 compares the XRD patterns for the 72.8 A intercalated
nanocomposite and the DMSA12-H-kenyaite intercalate from which the intercalated
nanocomposite was prepared. The lower gallery height causes the intragallery diffusion
process to be slow and the extragallery cross-linking rate is faster than the intragallery
polymerization rate.

The intercalated PDMS-kenyaite nanocomposite with a 72.8 A d-spacing was
used to compare the structural difference between the silica obtained from intercalated
and exfoliated nanocomposite phases. A plot of surface area vs. H-kenyaite loading for
the silicas obtained from PDMS-exfoliated kenyaite nanocomposites is given in Figure
6.7. Included in the figure is the surface area of the silica obtained by calcination of a
PDMS-intercalated kenyaite nanocomposite at 10 wt % loading. At the same 10 wt % H-
kenyaite loading, the surface area for the intercalated nanocomposite is substantially
lower than the silica obtained from exfoliated analog. The registry of restacked silicate
nanolayers is greater in the case of the intercalated nanocomposite, so the fraction of
exfoliated structure is limited. On the other hand, the surface area increases with the
kenyaite nanolayer loading for the silica obtained from exfoliated nanocomposites. This

effect is explained by the formation of a greater fraction of exfoliated kenyaite silica with

increased loading.

_ r- ' «Hm. :_W.1

 

189

 

 

 

 

 

 

72.8A
36.5A
E
(D
C
.9
E
Q)
.2
E
Q)
E
B
A
llllllIJlllllLlLlllllllllJllllllll
1 2 3 4 5 6 7 8

26 (Degrees)

Figure 6.6 XRD patterns of kenyaite intercalates: (A) DMSA12-H-kenyaite (36.5 A);

(B) a PDMS-intercalated kenyaite (10 wt % loading of H—kenyaite) nanocomposite (72.8
A).

190

 

  
  

 

 

 

400
: Exfoliated
350 -:
«E 300 .5
cu : O
a) ..
E 250 1 Intercalated
a, : (d = 72.8 A)
o _ 001
g 200 -_
:3
(D
|— 150 -
Lu -
CD
1 00
50 l I I I I I T I I I I I l I I I I I. I I I I I I
0 2 4 6 8 1 O 12

H-kenyaite Loading (wt %)

Figure 6.7 Comparison of BET surface area for the silica residues obtained by
calcining exfoliated PDMS-kenyaite nanocomposites with different loadings (wt %) of
H-kenyaite. The open circle is the surface area for the silica residue obtained by
calcining an intercalated PDMS-kenyaite nanocomposite with a 10 wt % loading of H-

kenyaite. The calcination was carried out at 540 0C for 10 hr in air using a heating rate of

1 °C/min.

.PMI“*‘A.II._.I< .t' ‘V “v‘ .' I '
i1. . .
“‘H3~

191

6.3.3 Performance Properties of Exfoliated PDMS-Layered Silicic Acid

Nanocomposites

The exfoliated PDMS-kenyaite nanocomposites show superior solvent resistance
properties relative to pristine PDMS (see Figure 6.8). With a 10 wt % of H-kenyaite
loading, the toluene uptake is substantially reduced by a factor of greater than 2. The
effect of kenyaite loading on reduction is very significant. This highly desired
improvement in swelling resistance is attributed to the interfacial forces that are higher
than the intramolecular forces in the pristine polymer. The polysiloxane polymer has a
very low glass transition temperature, so that a large free volume exists in the pristine
polymer. The enhanced polymer-kenyaite interactions strengthen the polymer and make
it more resistant to solvent swelling. One also would predict an increase in Tg. Future
studies will investigate this point.

The thermal stability of the exfoliated nanocomposite is compared with the
pristine polysiloxane polymer shown in Figure 6.9. Unlike the result that Burnside and
Giannelis has claimed previously for PDMS-smectite clay nanocomposites,l7 substantial
improvement in thermal stability is not observed in this work. ‘The difference behavior is

probably due to the introduction of the organic linkages in this system.

 

192

 

 

 

 

 

 

300
250: o 0 O O O . . .1
2 o o pristine
? : O A A A
2‘, 200- 0 A A A
c 1 o A A 5 wt %
g - A
E 150: 0 A ‘ if
2’ - J
o .1 A .. -—-1
g j v v v v v v v v
‘00 : 883 V V 10 wt %
3 ° v pristine 260%
5°“. °va 5% 212%
10% 111%
01-1!IIIIIIIIIIfiIIlfit1til|llf1lll|lll

0 50 100 150 200 250 300 350 400 450
Time (Minutes)

Figure 6.8 Toluene uptake by the exfoliated PDMS-kenyaite nanocomposites
prepared from DMSA12-H-kenyaite intercalates. The tabulated values in the insert are
equilibrium data determined by the immersion weight gain after 24 hr. The H-kenyaite
loadings for the composites are 5 wt % and 10 wt %.

193

 

100

 

 

 

O)
O

1,

Weight (0/0)

.b
O

I I I l I I I I I I I l I I I I I I I I

N
O

 

 

lllllllllllllllllllIlllllLlllIllll 11‘

O 100 200 300 400 500 600 700
Temperature (°C)

 

 

Figure 6.9 Thermogravimetric analysis curves for (A) a pristine PDMS polymer; and
(B) an exfoliated PDMS-kenyaite nanocomposite. The loading of H-kenyaite for the
composite is 10 wt %. The analysis was carried out in N2 atmosphere with a heating rate

of 5 °C/min.

194

6.4 Conclusions

The approach developed for the formation of thermoset polymer-layered silicate
clay nanocomposites wherein curing agents can be directly intercalated into the clay
galleries without the need for chemically inert quaternary alkylammonium ions on the
exchange sites is successfully extended to the polysiloxane system. The technique
provides nanocomposites with greatly improved solvent resistance with an expected
improvement of the mechanical properties. This approach may also be expected to apply
to the preparation of nanocomposites form a polysiloxane precursor with hydroxyl end
groups. The PDMS-layered silicic acid nanocomposites also are chemically interesting
precursors for the preparation of high surface area silica powders for potential adsorption

and catalytic applications.

6.5

PP!"

S"

10.
11.

12.

13.

14.

15.

16.
17.
18.
19.

20.

21.

195

References

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Okada, A.; Usuki, A. Mater. Sci. Eng. 1995, C3, 109.
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