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EFFECTS OF COUPLING AGENTS ON DISPERSION AND
STRUCTURE IN POLYPROPYLENE / CLAY
NANOCOMPOSITES

presented by
Sharad Kumar

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EFFECTS OF COUPLING AGENTS ON DISPERSION AND STRUCTURE 1N
POLYPROPYLENE / CLAY NANOCOMPOSITES
By

Sharad Kumar

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
DOCTOR OF PHILOSPHY

Department of Chemical Engineering and Materials Science

2005

ABSTRACT
EFFECTS OF COUPLING AGENTS ON DISPERSION AND STRUCTURE IN
POLYPROPYLENE / CLAY NANOCOMPOSITES
By

Sharad Kumar

This work focuses on improving the interactions of layered silicates with bulk
polypropylene to optimize the nanolayer exfoliation. Silane coupling agents were
selected based on their affinity with the bulk polypropylene: this was obtained by
solubility parameter calculations. Maleated polypropylenes with different extents of
bound maleic anhydride were also evaluated for their effectiveness in nanolayer
exfoliation.

To study the interactions with Silane coupling agents, the clay pretreated with a
C18-Surfactant was treated additionally with phenyl-trimethoxy Silane. 5 wt% of this clay
was melt blended with 10 wt% maleated polypropylene PP-g-MA and with 85 wt% of
two different polypropylene grades having widely different molecular weights. The
resulting nanocomposites were analyzed for microstructure and mechanical performance
using X-ray diffraction (XRD), transmission electron microscopy (TEM), melt rheology
and tensile modulus measurements. Nanocomposites with the most exfoliated structure
were Obtained when the chosen Silane was located at the edges of the organoclay. This
was confirmed with higher (almost twice) relative viscosity of the composites with Silane
modification at the organoclay edges as compared to composites with organoclay. The

analysis of several TEM images also supports the rheology data. The improvements in

tensile modulus were greatest for the composites with this clay, indicative of improved
coupling. The effects of shear and extensional flows on nanolayer morphology were
investigated by extrusion through uniform bore and converging dies. Nanolayer
restacking was observed; the levels for which were reduced for the nanocomposites
reinforced with edge silated clay particularly at higher residence times in shear flows, as
quantified by image analysis of the nanocomposite extrudates. Our results also show that
compatibilizers having greater amounts of covalently bound maleic anhydride yield the

most exfoliated structure.

ACKNOWLEDGMENTS

I would like to sincerely thank my advisor Dr K. J ayararnan for his help through
the different stages of the Ph.D. work. He was always available to answer my queries and
helped me understand the fundamental concepts behind the research work. His mentoring
has given me the ability to handle challenging assignments with ease. I would also like to
thank my committee members Dr Baker, Dr Lee and Dr Ofoli for serving on my
committee and providing valuable guidance.

I would also take this opportunity to thank my parents, my sister Deepshikha and
her family and Pooja for the immense love and support they have extended to me always.
Without their support, this task would not have been possible. Their faith in me provided
the required courage to cope with the stresses involved during the difficult stages of the
research work and I am highly thankful to them.

I would also like to express my gratitude to my fellow colleagues, who provided
me with the technical skills during the early stages of my research work. I would
especially like to thank Giri, Darrell and Bhaskar, who helped me a lot in shaping my
experiments.

Finally yet significantly, I would also like to thank my friends Hernant, Madhu,
Anish, Srivatsan, Krishnan, Shireesh, Patil, Tanrnay and John. It was an enjoyable and
rewarding experience, which I had during my five years at Michigan State in your
company. Thanks are also due to my other friends: Kartik, Nehru and Ritesh. You all

have provided me with your support, whenever I was in need and I thank you.

iv

TABLE OF CONTENTS

LIST OF TABLES ..................................................................... vii

LIST OF FIGURES .................................................................... viii

CHAPTER 1
INTRODUCTION
1.1 BACKGROUND ..........................................................
1.2 RESEARCH OBJECTIVES .............................................. 13

CHAPTER 2
POLYPROPYLENE NANOCOMPOSITE PREPARATION AND
CHARACTERIZATION

2.1 NANOCOMPOSITE PREPARATION ............................... 15
2.2 SILANE TREATMENT FOR CLAYS ................................ 18
2.3 X-RAY DIFFRACTION ................................................ 18
2.4 TRANSMISSION ELECTRON MICROSCOPY ................... 20
2.5 RHEOLOGY ............................................................. 21
2.5.1 Oscillatory Shear Measurements ................................. 21
2.5.2 Creep Compliance Measurements ................................ 24
2.6 NANOLAYER MORPHOLOGICAL CHANGES IN
EXTRUSION ............................................................ 25
2.7 MECHANICAL PERFORMANCE .................................. 27
CHAPTER 3

THE USE OF SILAN E COUPLING IN MELT MIXED POLYPROPYLENE
CLAY NAN OCOMPOSITES

3. 1 INTRODUCTION ...................................................... 3 1
3.1.1 Objective ............................................................. 31
3.1.2 Background .......................................................... 33

3.2 EXPERIMENTAL DETAILS ........................................ 36
3.2.1 Materials ............................................................ 36
3.2.2 Procedures .......................................................... 37

3.2.2.1 SILA TION ................................................... 37
3.2.2.2 MELT PROCESSING ..................................... 38
3.2.2.3 EXTRUSION ................................................ 39
3.2.3 Structural Characterization ....................................... 40
3.2.4 Rheological and Mechanical Characterizations. . .. . . . . . . . 41

3.3 RESULTS AND DISCUSSION ..................................... 41
3.3.1 Silation and Clay Structure... 41
3.3.2 Morphology of Nanocomposites ................................. 42

3.3.2.1 COMPRESSION MOLDED SPECIMENS ............ 42
3.3.2.2 EXTRUDED SPECIMENS 44

3.3.3 Oscillatory Shear Rheolog/ ........................................
3.3.4 Mechanical Performance ..........................................
3.4 CONCLUSIONS ........................................................

CHAPTER 4
CHARACTERIZATION OF COMPATIBILIZER EFFECTIVENESS
ON RHEOLOGY AND STRUCTURE
4. 1 INTRODUCTION ......................................................
4.1.1 Objective .............................................................
4.1.2 Background ...........................................................
4.2 EXPERIMENTAL ......................................................
4.2.] Materials .............................................................
4.2.2 Melt Mixing ..........................................................
4.2.3 Structural Characterization .......................................
4.2.4 Rheological Characterization .....................................
4.3 RESULTS AND DISCUSSION .......................................

4.4 CONCLUSION ...........................................................

CHAPTER 5

CONCLUSIONS AND RECOMMENDATIONS
5.1 CONCLUSIONS ........................................................
5.2 RECOMMENDATIONS ...............................................

BIBLIOGRAPHY ....................................................................

vi

47
49
50

69
69
7O
73
73
74
74
75
76
82

101
104

108

Table 3.1:

Table 3.2:

Table 3.3:

Table 4.1:

Table 4.2:

Table 4.3:

LIST OF TABLES

Composition of various composite samples by weight %..... . ..
Uniform bore and profile dies used for extrusion studies ..........
Distribution of particle structures for composites before

and after extrusion through uniform bore die (Die-1) at
different rates from TEM Image analysis............

Molecular Information on Compatibilizers ..........................

Composite identification and compatibilizer grade in
each composite ..........................................................

Nature and concentration of maleic anhydride in maleated

polypropylenes obtained by titration and average length
of clay platelet ...........................................................

vii

52

53

54

84

85

86

Figure 2.1.

Figure 2.2.

Figure 3.1.

Figure 3.2.

Figure 3.3.

Figure 3.4.

Figure 3.5.

Figure 3.6.

Figure 3.7.

Figure 3.8.

LIST OF FIGURES

Representative DSC scans for melting transitions for
polypropylene and corresponding silicate free melts:

(——) Homopolymer PP, (—) Silicate free melt ................

Capillary Rheometer setup for Extrusion Studies with

Die Profiles ..........................................................................

Coupling between the matrix and the nanolayer faces
is achieved with PP-g—MA chains and coupling
between the nanolayer edges and the matrix can be

achieved with selected silanes ............................................. .

XRD patterns of organoclays with different Silane
treatment Showing no change in d-spacing with
phenyl trimethoxysilane (SiPh) but a noticeable
increase in d-spacing with a different Silane

used in 1.3 1 PS. The organoclay I.30P was heated for

12 hours at 100°C to match the drying step in Silation ........

FTIR Spectra OfI.30P and I.30PSiPh.............................

XRD patterns for polypropylene/clay nanocomposites
with 5 wt% of different clays and 10 wt% of the same

PP-g-MA (AC 950)

Transmission electron micrographs of the composites
(a) PPl-I.30P, (b) PPl-I.30PSiPh, (c) PP2-I.30P,
(d) PP2-I.30PSiPh (The scale bar in each

picture is 200 nm)

Different particle structures identified in

transmission electron micrographs ...................................... .

Length distributions of "single particles" in
nanocomposites (—) PPl-I.30P, (— —)PP1-I.30PSiPh,

(----) PP2—I.30P, (— - -) PP2-I.30PSiPh ......................... .

Transmission Electron Micrographs for the composite
extrudates through Die-1 (a) PP2-I.30P-195 sec“ ,

(b) PP2-I.3OPSiPh—195 sec", (c) PP2-I.3OP-3650 sec",
((1) PP2-I.30PSiPh-3650 sec". The scale bar in each

picture is 500 nm ................................................................ .

viii

29

30

55

56

57

58

59

60

61

62

Figure 3.9.

Figure 3.10.

Figure 3.11.

Figure 3.12.

Figure 3.13.

Figure 3.14.

Figure 4.1:

Figure 4.2:

Figure 4.3:

Transmission Electron Micrographs for the composite

extrudates through Die-2 (a) PP2-1.30P-1 sec

(b) PP2-1.30PSiPh-1 sec .The scale bar In

each picture is 500 nm . . . .. 63

Dynamic viscosity curves for three composites

and the corresponding silicate free melt of PP6323

and PP-g-MA: (A) PPl-I.30P, (O) PPl-I.3OPSiPh,

(I) PPl and PP-g-MA 64

Dynamic viscosity curves for two composites
and the corresponding silicate free melt of PP6523
and PP-g-MA at 180°C: (V) PP2-1.3OP,

(a) PP2-1.3OPSiPh, (o) PP2 and PP-g-MA ........................ . 65

Storage Modulus (6') curves versus frequency (w)
for the composites with different clays at 180°C:
(A) PPl-I.3OP, (O)PP1-I.30PSiPh ..................................... . 66

5 versus Complex Shear Modulus (G*) for the
composites with different clays at 180°C:
(A) PPl-I.30P, (O) PPl-I.30PSiPh ..................................... . 67

Tensile Moduli of PP6323 based nanocomposites with
5 wt% of different clays and 10 wt% of the same
PP-g-MA (AC 950) .......................................................... . 68

XRD patterns for polypropylene/clay nanocomposites:

Curve A: 1.3OP organoclay, curve B: NC] 1, curve C:

NC12, curve D: NC13, curve B: NC 14. All composites

have a fix loading of 85wt% PP6323, 10 wt% PP-g—MA

and 5wt% I.30P Clay. The composite NCll is based on

PP-g—MAI (Acid Number =7), the composite NC12 is based

on PP-g-MA2 (Acid Number=10), the composite NC13 is

based on PP-g-MA3 (Acid Number=9) and the composite

NC14 is based on PP-g—MA4 (Acid Number =23) . . . .. 87

Transmission Electron Micrograph for the composite

NC12. The composite NC12 consists of 85wt% PP6323,

10wt% PP- -g-MA2 (Acid Number= 10) and 5wt% I. 30P clay.

The scale bar In the picture is 200 nm.. 88

Transmission Electron Micrograph for the composite

NC13. The composite NC13 consists of 85wt% PP6323,

10wt% PP-g—MA3 (Acid Number =9) and 5wt% I.3OP

clay. The scale bar in the picture is 200 nm ........................ 89

ix

Figure 4.4:

Figure 4.5:

Figure 4.6:

Figure 4.7:

Figure 4.8:

Figure 4.9:

Figure 4.10:

Transmission Electron Micrograph for the composite

NC14. The composite NC14 consists of 85wt% PP6323,
10wt% PP-g-MA4 (Acid Number =23) and 5wt% I.30P clay
The scale bar in the picture is 200 nm..............

Length distributions of single particles for composites:

(— —) NC12, (----) NC13 and (—) NC14. All composites
have a fix loading of 85wt% PP6323, 10 wt% PP-g—MA
and 5wt% I.30P Clay. The composite NC12 is based on
PP-g—MA2 (Acid Number=10), the composite NC13 is
based on PP-g-MA3 (Acid Number=9) and the composite
NC14 is based on PP-g-MA4 (Acid Number =23)

Dynamic viscosity curves for composite NCll (V) and

the Silicate free melt of PP6323 and PP-g—MAl (V) at
180°C. The composite NCll consists of 85wt% PP6323,
10wt% PP-g-MAl (Acid Number =7) and 5wt% I.3OP
clay ...................................................................................

Dynamic viscosity curves for composite NC12 (A) and

the silicate free melt of PP6323 and PP-g—MA2 (A) at
180°C. The composite NC12 consists of 85wt% PP6323,
10wt% PP-g-MA2 (Acid Number =10) and 5wt% I.30P
clay ................................................................................. .

Dynamic viscosity curves for composite NC13 (D) and

the silicate free melt of PP6323 and PP-g-MA3 (I) at
180°C. The composite NC13 consists of 85wt% PP6323,
10wt% PP-g-MA3 (Acid Number =9) and 5wt% I.30P
clay ................................................................................. .

Dynamic viscosity curves for composite NC14 (O) and

the silicate free melt of PP6323 and PP-g-MA4 (O) at
180°C. The composite NC14 consists of 85wt% PP6323,
10wt% PP-g-MA4 (Acid Number =23) and 5wt% I.30P
clay ................................................................................. .

Relative viscosity curves obtained for the composites
with different compatibilizers at 180°C: (V) NC] 1,

(A) NC12, (El) NC13, (O) NC14. All composites

have a fix loading of 85wt% PP6323, 10 wt% PP-g-MA
and 5wt% 1.30P Clay. The composite NC11 is based on
PP-g-MAI (Acid Number =7), the composite NC12 is
based on PP-g-MA2 (Acid Number=10), the composite
NC13 is based on PP-g-MA3 (Acid Number=9) and

the composite NC14 is based on PP-g-MA4

9O

91

92

93

94

95

(Acid Number =23) 96

Figure 4.11: Storage Modulus (6') curves versus frequency ((1)) for
the composites with different compatibilizers at 180°C:
(V) NC11, (A) NC12, (El) NC13, (O) NC14. All
composites have a fix loading of 85wt% PP6323,
10 wt% PP-g-MA and 5wt% I.3OP Clay. The composite
NC11 is based on PP-g—MAI (Acid Number =7), the
composite NC12 is based on PP-g—MA2 (Acid
Number=10), the composite NC13 is based on PP-g—MA3
(Acid Number=9) and the composite NC14 is based on
PP-g-MA4 (Acid Number =23) . . . . .. 97

Figure 4.12: Phase Angle (6) versus Complex Shear Modulus (6*) for
the composites with different compatibilizers at 180°C:
(V) NC11, (A) NC12, (El) NC13, (O) NC14. All composites
have a fix loading of 85wt% PP6323, 10 wt% PP-g-MA
and 5wt% I.3OP Clay. The composite NC11 is based on
PP-g—MAI (Acid Number =7), the composite NC12 is based
on PP-g-MA2 (Acid Number=10), the composite NC13
is based on PP-g—MA3 (Acid Number=9) and the
composite NC14 is based on PP-g-MA4
(Acid Number =23) 98

Figure 4.13: Phase Angle (8) versus Complex Shear Modulus (G*) for
the silicate free composite matrices at 180°C: (V) NC11,

(A) NC12, (I) NC13, (O) NC14. All composites have a

fix loading of 85wt% PP6323, 10 wt% PP-g-MA and

5wt% I.3OP Clay. The composite NC11 is based on

PP-g-MAI (Acid Number =7), the composite NC12 is

based on PP-g-MA2 (Acid Number=10), the composite

NC13 is based on PP-g-MA3 (Acid Number=9) and

the composite NC14 18 based on PP- -g-MA4

(Acid Number =23)... 99

Figure 4.14: Creep Compliance J (t) as a function of time at a stress of
10 Pa at 180°C: (V) NC] 1, (A) NC12, (El) NC13, (O) NC14
All composites have a fix loading of 85wt% PP6323, 10 wt%
PP-g-MA and 5wt% I.3OP Clay. The composite NC11 is
based on PP-g-MA] (Acid Number =7), the composite
NC12 is based on PP-g—MA2 (Acid Number=10), the
composite NC13 is based on PP-g—MA3 (Acid Number=9)
and the composite NC14 is based on PP-g-MA4
(Acid Number =23) . . . .. 100

xi

Figure 5.1:

Proposed mechanism for the enhancement in the stability

of nanolayers by careful control of the interactions between

the different system components. This is achieved

by favorable interactions of the phenyl end-capped Silane

with the bulk polypropylene and also increased

hydrogen bonding interactions between the PP-g-MA

and the hydroxyl groups. 107

xii

INTRODUCTION

 

CHAPTER-1
1 .1 BACKGROUND

Polymer-layered silicate nanocomposites have been a focus of great interest over
the last decade after the pioneering work done by researchers at Toyota for nylon-6 based
materials[1-5]. The high aspect ratio of delaminated or exfoliated platelets has the
potential for producing striking improvements in a variety of polymer properties with the
incorporation of small amounts of nanoclays[6-14]. These nanoclays have aspect ratios of
the order of 200 and provide considerable enhancement in stiffness at a much lower
loading as compared to conventional fillers (mica and talc), which need to be used at a
rather high loading of 20 to 40 wt% in order to provide similar improvements in
mechanical performance. The final part weight with nano-clays also has a lower density
as compared to traditional talc filled composites owing to low filler loadings. Another
main advantage of nano-clays is that they are impervious to gases and when they are well
dispersed and oriented, can yield greatly improved barrier properties[15] as well as flame
retardancy [16-20] in the composite. The earliest success in nanocomposite production
was restricted to polar polymers, i.e. nylon and epoxy[21]. Limited success has been
achieved with the most common commodity polymer polypropylene, which is non-polar
in nature. Polypropylene (PP) is used in a wide range of products ranging from
automotive bumpers to packaging applications such as pouches for meals-ready-to-eat

(MRE’S) and other food containers and therefore the prospects of improved nano-clay

dispersion in PP in order to perk up the properties needs careful and detailed
investigations.

The silicate clay widely used for reinforcing polymers is Montrnorillonite
(MMT). The silicate clays exists naturally in a tactoid structure comprised of several tens
of stacked layers (nanolayers) with a typical lateral dimension of 100-200 nm, a layer
thickness of 1 nm, and an interlayer spacing of about 1 nm and have surface areas of
greater than 700 mZ/g [22, 23]. Each nanolayer is comprised of an aluminum-oxygen-
hydroxyl octahedral sheet sandwiched between two silicon-oxygen tetrahedral sheets for
the 2:1 smectite clays used for this work. The nanolayers can be separated by the
introduction of water or other swelling agents into the Van-der-Waals gap between the
clay layers. The gap contains exchangeable cations (usually sodium) to compensate the
unbalanced charge in the interior of the clay layers due to isomorphous substitutions. This
exchange capacity of the clays (cation exchange capacity) is an important variable, which
dictates the effectiveness of the clay to be used as reinforcements for different polymer
matrices [24]. Another important property of these clays is that they have hydroxyl
groups at the Slightly positive edges of the platelets (as opposed to fibrous clays like
sepiolite).

The clay nano-layers need to be separated and dispersed in the polymer matrix;
the procedure termed as clay exfoliation or clay delamination. In some cases, the polymer
chains swell (intercalate) the clay galleries, but don’t lead to complete separation of the
layers. The resulting composite contains ordered clay tactoids with increased layer

spacing and are termed as intercalated composites. Exfoliated clay composites yield

enhanced mechanical and barrier performance as compared to intercalated composites
and therefore constitutes the desired structure.

The clay in its pristine form is hydrophilic and so has to be modified before it can
be used as reinforcement for different polymer systems. For a nanocomposite formation,
the polymer chain has to first penetrate between the clay galleries and then by employing
certain processing strategies, the clay layers are separated far apart resulting in
exfoliation of the nanolayers. Due to the 1 nm spacing between the closely packed sheets,
there is a large entropic barrier, which the polymer has to overcome in penetrating this
gap and hence, becoming intermixed with the clay. In order to enhance the interaction
between the mineral and the organic polymer, the hydrophilic face of the clay platelets is
modified by pre-intercalating long chain alkyl ammonium ions as surfactants. The onium
ion is tethered to the surface of the clay and the hydrocarbon tails swell the clay and
improve its dispersibility in organic materials. Theoretical studies by Vaia and
coworkers[25, 26] and by Balazs and coworkers[27—42] suggest that the hydrocarbon tail
makes interactions between the gallery faces and the polymer more enthalpically
favorable and thereby compensates for the entropic reduction owing to polymer chain
confinement in the clay galleries. Here the length of the surfactant chain is an important
variable that influences the level of exfoliation. Reichert and coworkers[43] investigated
the effectiveness of the chain length by exchanging synthetic sodium fluoromica with
various protonated alkyl amines such as butyl (C4), hexyl (C6), octyl (C8), dodecyl
(C12), hexadecyl (C16), and octadecyl (C18) amine. They concluded that the degree of
dispersion went up with the alkyl chain length up to C18. Manias et al.[18] have reported

that clay treated with a surfactant having two C18 tails mixes readily with a variety of

fimctionalized polypropylenes. Ton-that et al.[44] have compared the performance of
polypropylene nanocomposites prepared with 2 wt % of two different surfactant treated
clays - Cloisite 308 with one tallow tail and Cloisite 15A with two hydrogenated tallow
tails (tallow composition is typically ~65% C18; ~30% C16; ~5% C14). They found that
the nanocomposite with Cloisite 15A had a noticeably higher tensile modulus and tensile
strength. Recent studies done by Hotta and Paul[45] indicate that the surfactant treatment
with two alkyl tails yields better exfoliation in a polyethylene system as compared to a
surfactant with just one long alkyl tail. The results from these studies with polyolefins are
opposite to the results obtained for a Nylon-6 clay nanocomposite, where a surfactant
treatment with 1 tallow tail was found to yield better dispersion levels as compared to a
treatment with 2 tallow tails[46]. Surfactant with one tallow tail allowed efficient contact
with the clay faces and the polyamide chains, while maintaining enough platelet spacing.
The surfactant treatment with 2 tallow tails although instrumental in providing enough
platelet spacing, weakened the platelet and polyamide chain interactions and led to poor
clay delarnination in the final composite. Manias et al.[18] have also indicated that the
polymer-clay interactions should be more favorable than the surfactant-clay interactions
in order to get improved dispersion levels.

Thermal characteristics of the organoclay play an important role in the final
microstructure of the composite, since the melt intercalation is carried out usually at
higher temperature. The loss of the interlayer surfactant present in the clay galleries
during high temperature has been studied by Lee et al.[47] for two different surfactant
treated clays- Cloisite 308 with one tallow tail and Cloisite 20A with two hydrogenated

tallow tails and found that the amount of surfactant degradation was greater for Cloisite

3GB and subsequently the reduction in the basal spacing was greater for the Cloisite 308.
The final dispersion levels of this clay were also found to be lower as compared to the
Cloisite 20A for Similar loadings in the final polypropylene nanocomposite. A recent
numerical simulation study[48] on thermodynamic miscibility of a nanocomposite system
also suggests that the surfactant loss can lead to the presence of a bare clay surface,
which is highly detrimental for the exfoliation process.

In-situ polymerization techniques were initially used to fabricate clay/polymer
hybrids. In the first step, the monomer was intercalated in the clay galleries followed by
the polymerization in the second step, which led to an expansion of the clay galleries and
resulted in an exfoliated morphology. The first polymer clay composite was synthesized
in Nylon-6 using a similar monomer intercalation method by the Toyota group. The
organophilic montrnorillonite was ion exchanged with arninododecanoic acid, which was
also the catalyst for the ring opening polymerization of the monomer e-caprolactam. The
polymerization of Nylon-6 occurs in the presence of MMT after e-caprolactam is
intercalated into the galleries. The resulting material provided significant improvements
in mechanical, thermal and gas barrier properties at loadings of only 2-5wt% MMT. The
in-situ polymerization technique for fabricating exfoliated nanocomposites was extended
to epoxy systems as well. Pinnavaia et al.[49, 50] accomplished this by controlling the
catalyst concentration in the galleries to obtain higher polymerization rates in the clay
galleries. Recently an in-situ polymerization technique has also been reported[51] for
making high performance polyolefin nanocomposites.

Melt intercalation in mixing instruments like twin-screw extruder still remains the

convenient and the commercial inexpensive way for nanocomposite production. The

technique involves intercalating the polymer directly in the clay interlayer galleries
directly through heating the mixture and then applying Shear to separate the clay platelets.
This method can be applied to a range of commodity polymers from essentially non-polar
polypropylene to weakly polar polyethylene terephthalate, to strongly polar nylon. The
melt mixing requires effective stress transmission between the matrix and the layered
silicates particularly while the layers are still stacked to facilitate layer sliding and
thereby have high exfoliation levels. Fornes et al.[52] studied melt processed
nylon/montrnorillonite nanocomposites and Showed that if the molecular weight is lower
than a critical value, the nanocomposite had a significant fraction of intercalated
structures. This is because, only with a polymer of molecular weight higher than the
critical molecular weight for entanglement can a sufficient level of shear stresses be
transmitted from the matrix to a stack of nanolayers for sliding them apart.

In case of non-polar polypropylene, the dispersion of polar silicates is facilitated
by the addition of functionalized polyolefin oligomers (compatibilizers), where the polar
functionality (usually maleic anhydride) wets the clay surface and the oligomers are
soluble in the bulk matrix. Using this approach, Okada and co-workers[53] synthesized
the first PP-clay hybrids using maleic anhydride functionalized PP oligomers. The
procedure involved the dissolution of functionalized polyolefin oligomer in toluene
followed by intercalation of this mix in the clay galleries. The toluene was then
evaporated and this mixture of functionalized polyolefin and clay was then melt blended
with bulk PP to prepare the polymer clay hybrid. A solvent free procedure was also
demonstrated to work for preparing PP-clay intercalation compounds using two different

functionalized PP oligomers: maleic anhydride (MA) functionalized and a polypropylene

oligomer with telechelic OH groups (PP-OH)[9, 10, 54]. The need to have a significant
level of polar functionality to interact with the nanolayer faces has since been established
by Hasegawa et al.[54-57], Gopakumar et al.[58] for polyethylene and by Ishida et al.[59]
for a wide range of polymers. Theoretical studies done by Balazs et al.[38, 40] have also
established that adding functionalized polymers improves the clay exfoliation levels.
Strong interactions between the maleic anhydride moieties and the organoclay results in
an increase in system enthalpy, which compensates for the reduction in entropy due to the
polymer chain intercalating in the 1 nm gap between the clay platelets and leads to
greater separation of the clay nanolayers. A recent molecular dynamics study by Minisini
and Tsobnang[60] calculates the interaction energy between functionalized
polypropylenes and the organoclay surface. The interaction energy was highly favorable
with the presence of maleic anhydride moieties and therefore should lead to improved
organoclay dispersion.

The loading of the maleic anhydride functionalized polyolefins (PP-g-MA), the
molecular weight of the PP-g-MA and the molar ratio of the maleic anhydride
functionality[6l] dictate the choice of compatibilizer for polyolefin nanocomposites. It is
well established that functionalization with maleic anhydride leads to chain scission of
the polypropylene[62]. Therefore large amounts of low or modest molecular weight PP-
g-MA will lead to unacceptable mechanical properties. Wang et al[63, 64] have earlier
investigated the effects of different molecular weight compatibilizers, maleic anhydride
contents and the compatibilizer loadings on the clay exfoliation in polypropylene
nanocomposites. Their results showed that the compatibilizer with lower molecular

weight and higher MA content could lead to good clay dispersion in PP/clay composites,

yet there was some deterioration in both mechanical and thermal properties. A high
maleic anhydride content in the maleated polypropylene while favorable for improved
interactions with the clay surface results in a lower molecular weight wax, which will not
entangle adequately with the bulk polypropylene during compounding[6l]. Okada and
coworkers[9, 10] used a 3:1 ratio and found that with the higher MA content, this ratio
led to phase separation of the polar and non-polar polypropylenes.

The nature of the maleic anhydride functionality itself influences the exfoliation
levels of the nanoclay. The maleic anhydride moieties in these materials may exist in
three distinct forms. First as an un-reacted small molecule, second as a maleic anhydride
oligomer and third as maleic anhydride bound to a polymer chain [65, 66]. The stronger
bond between the clay and the compatibilizer will allow more effective stress transfer
from the polymer to the clay in shear and ease the separation of the clay nanolayers.
Hence a greater fraction of bound maleic anhydride should yield a greater degree of
exfoliation. In one part of this study, four nanocomposite formulations with a 10 wt%
loading of compatibilizers and 5 wt% organoclay were investigated for rheological and
mechanical improvements, wherein the chosen compatibilizers had different bound
maleic anhydride contents from 0.37-2.6 wt%. The detailed rheological analysis and
detailed image analysis of the transmission electron micrographs revealed that the
distribution of maleic anhydride in maleated polypropylene plays an important role in the
process of clay delarnination and the clay exfoliation levels were higher for
compatibilizers with more bound maleic anhydride.

A few research groups have also focused on alternate methods to exfoliate the

clay in PP. Intercalation of olefinic monomers followed by in-situ polymerization[67] and

in-situ melt phase ultrasonication[68] have been used to delaminate the clay in
polypropylene. Manias and coworkers[18, 69] have used functionalized polypropylenes
to improve the polymer/clay interactions; they also investigated a semi-fluorinated
surfactant having a poor affinity for the clay surface in order to decrease the
surfactant/clay interactions. In both cases, they obtained a mix of intercalated/exfoliated
structures with improved mechanical and flame retardant properties. Liu and Wu[70]
used a co-intercalation clay modification, which involved treating the clays with alkyl-
ammoniurn surfactants followed by treatment with epoxy-propyl methacrylate, the latter
to tether to the polypropylene. They obtained an intercalated morphology and improved
mechanical properties with increased silicate loading. In a recent work, Kato and
coworkers[71] use a water based slurry treatment of the clay, which doesn’t involve alkyl
ammonium surfactant treatment; this clay was compounded with PP and PP-g-MA in a
twin-screw extruder. They obtained nanocomposites with high levels of clay exfoliation,
which had comparable properties to a conventionally prepared nanocomposite.

Another way to increase the interactions between polymer and the filler is to have
favorable interactions of the bulk polymer with the hydroxylated edges of the silicate
layer[72]. The OH groups on the clay edges can be modified with appropriate Silane
coupling agents, which Should provide the necessary interfacial coupling during the
process of melt compounding and facilitate the nano-layer sliding, when the layers are
still stacked. Previous studies[73] have used Silane-modified clays, which have resulted
in nanocomposites with improved mechanical properties. However with the layered
morphology of clays, some silanes can actually move into the interlayer galleries. Fluoro-

alkyl silanes were used by Manias and coworkers, which led to Silane intercalation in the

clay galleries and increased the basal spacing form 1.98 to 2.4 nm. Other silanes have
been shown to react with the edges mainly[74-77], which is evident by similar interlayer
spacing before and after Silane treatment. One of the objectives of this work was to
investigate the effectiveness of the latter class of silanes in enhancing exfoliation levels
of clay in polypropylene nanocomposites.

The silanes used in this study were selected based on thermodynamic
compatibility between the filler and the polymer matrix. In previous studies, the filler-
matrix compatibility and its resulting effects on interfacial adhesion have been
investigated. Miller and coworkers [78-80] have worked with different organo-functional
silanes and indicate that there is an enhanced interfacial strength in systems with more
favorable thermodynamic mixing. Their study includes both crystalline and amorphous
polymer composites and suggests that thermodynamic compatibility as quantified by the
free energy of mixing (—AGmix) provides a convenient means for predicting the interfacial
strength between Silane treated surfaces and polymers. A solubility parameter match
between the polymer and the Silane organo-fimctional group has been suggested as a
criterion for compatibility.

Investigating the nanolayer morphological stability to firrther melt processing was
the other objective of this work. Once the nanoclay is exfoliated in the polymer matrix, it
is used for applications ranging from automotive[8l, 82] to food packaging to flame
retardant materials[l6-20, 83]. This production step is accomplished by additional melt
processing steps like injection molding and extrusion: both involving different extents of
straight and converging-diverging flows. Additional melt blending can affect the

nanolayer morphology as illustrated by the work of Reichert et al.[84] and Manias et

10

al.[18]. Shear induced flocculation of nanolayers has also been observed by Okamoto et
al.[85]. Reichert et al.[84] conducted annealing studies of injection molded
polypropylene nanocomposites and observed a coarsening of the nanolayers after
annealing. The ability of the nanocomposites to withstand additional processing effects
indicates improved coupling between the clay and the bulk polymer. Similar studies were
conducted by Manias et al.[18] for two polypropylene nanocomposite systems with the
same organoclay for each case but with two different matrices: neat PP alone for one case
and PP-g-MA alone for the second. The structure before and after annealing was studied
by X-ray diffraction and it was observed that the composite with the neat PP matrix
turned fi'om an exfoliated structure to strongly intercalated after annealing cycle of 15
min. The other structure with PP-g-MA as matrix was stable over a period of 30 min
indicating better coupling for this case.

The clay platelets being anisotropic orient differently based on the type of flow,
and therefore processing induced orientation assumes a lot of significance[86]. Recent
studies indicate that along with exfoliation, the orientation of the nanolayers is also an
important variable, which dictates the levels of improvements in material properties of
the polymer[87]. This study assumes vital importance when these materials have to be
fabricated for use as barrier materials or for flammability reductions[l6, 83], where
greater alignments of Silicate nanolayers yield better properties. Qian et a1. did
permeability measurement on cast films as well as blown films of a 7-wt% clay
reinforced polypropylene nanocomposite[15]. The reduction in permeability for the
blown film was about 60% for the blown film and 30% for the cast film as compared to

the silicate free polypropylene film. These nanocomposites had a large fraction of

11

intercalated structures. Greater orientation of the nanolayers in the plane of the film was
obtained in the film blowing process, which led to lower relative permeability. The
degree of crystallinity in the matrix polymer also goes up in the presence of the
nanolayers particularly as they are oriented. Increased crystallinity leads to further
reduction in permeability to gases[88]. Greater reductions in permeability can be obtained
with full exfoliation of the nanolayers in polypropylene, followed by even greater
orientation of the high aspect ratio particles. Different studies have been conducted
previously to investigate the nanolayer orientation in. both shear flows[3, 89-92] as well
as elongational flows[93]. Kojima et al.[3, 90] in their work with nylon-clay
nanocomposite films observed that the clay platelets align parallel to the surface of the
film and along the flow axis. Bafna et al.[89] observed a similar kind of behavior with
PP-clay nanocomposite films. Varlot et al.[92] have observed that the clay platelets are
aligned with normals both parallel and perpendicular to the thickness direction of the
sample for injection molded nylon—clay nanocomposites. Schmidt et al.[91] have
investigated the influence of shear on viscoelastic polymer-clay solutions by means of
small-angle neutron scattering (SANS) and found that at higher shear rates, the scattering
anisotropy increased due to the enhanced orientation of the clay platelets in the shear
field. Okamoto et al.[93] conducted uniaxial elongational tests on their elongational flow
OptO-rheometer (Similar to Meissner’s elongational rheometer) in the melt state at
constant Hencky strain rate on samples of PP-clay nanocomposites and observed a

perpendicular alignment of the clay platelets to the stretch direction.

12

1.2 RESEARCH OBJECTIVES

The goal here is to develop a novel and more controllable approach to prepare
polypropylene nanocomposites with mostly exfoliated clay structures and investigate the
resulting nanocomposite for rheological and mechanical improvements. This approach is
motivated by the need for better coupling between the matrix and the nanolayers. The
task of coupling the matrix and the clay can be accomplished by providing improved
interactions at the clay faces and at the hydroxylated clay edges. The incorporation of
maleated polypropylenes serves to provide favorable interactions at the clay faces. The
edge sites can also be functionalized with carefully chosen Silane coupling agents in order
to enhance the bulk polypropylene/clay interactions. The morphological stability of the
anisotropic silicate layers to various flows was also investigated. The information hence
obtained will allow us to design materials with high levels of clay exfoliation and also
lead to evaluation of the nanolayer morphological stability during post processing
compounding.

The various research tasks, which this work attempts to answer, are as follows:

0 To investigate the effects of compatibilizers with varying maleic anhydride
distribution on the rheological and mechanical performance of melt mixed
polypropylene nanocomposites.

0 Additional coupling provided on the hydroxylated clay edges with carefully chosen
Silane coupling agents in order to improve the interactions between the silicate layers
and the bulk polypropylene. To investigate the nanolayer exfoliation levels and the

concomitant improvements in flow and mechanical properties.

13

To investigate the effects of extrusion through uniform bore and profile dies on the
nanolayer morphology. The composites extruded for this study had fixed loadings of
clay and compatibilizer and were verified to have a high degree of clay exfoliation
initially by XRD, TEM and oscillatory shear rheology. The additional task was to
investigate if the edge functionalization with Silane coupling agents provides more
stability to the silicate structure? A high morphological stability to additional melt
processing will further provide evidence for improved interactions between the clay

and the bulk polypropylene.

l4

POLYPROPYLENE NANOCOMPOSITE PREPARATION AND

CHARACTERIZATION

 

CHAPTER-2
2.1 NANOCOMPOSITE PREPARATION

All nanocomposites in this study were melt mixed using a batch mixing
technique. The residence time in the mixing technique should be selected to provide
enough shear while avoiding the degradation of surfactant; this can be manipulated by
adjusting the throughput rate and the screw rpm[94]. For small batches, batch mixers like
a DACA mixer[95], Gelimat[96] or a Banbury[6l] can be used., where the shear rates in
the batch mixers ranged from 50 to 200 S'1 and the residence time was between 2 to 10
minutes.

The melt mixing technique for this research involved initial homogenization of
the materials in the solid form by shaking them in a sample bag vigorously for 2 minutes.
The homogenous powder-pellet blend is fed to a preheated mixer bowl in a batch Size of
47 grams for 1 run. The bowl containing 2 Banbury impellers has a volume of 75 cm3 and
was manufactured by CW Brabender Instruments Inc. A nitrogen purge port was also
incorporated along with the mixing bowl to ensure minimum polymer degradation.
Solomon et al.[97] and Lee et al.[47] have previously used similar mixing instrument for
preparation of polypropylene nanocomposites.

The homogenous powder melt was melt blended in the pre-heated bowl at a

temperature of 180°C at 150 rpm for 10 minutes. The rationale behind choosing the

15

mixing protocol was to maximize shearing time, minimize polymer degradation and
limited availability of materials. The chosen temperature of 180°C was above the melting
temperature of the polypropylene as well as the corresponding nanocomposites. The
melting temperature of the polypropylene chosen for this study was greater than 160°C
and had a specific gravity between 0.88-0.92, as indicated by the company literature.
Representative DSC scans on the polypropylene and PP-maleated PP blends shown in
Figure 2.1 further confirm that the melting point range for all the materials is between
160°C and 170°C. The temperature of 180°C was chosen to maximize polymer melt
viscosity, so that enough shear stresses are generated during the process of melt mixing.
Fomes et al.[52] have worked with a range of nylon-6 molecular weights and have
concluded that a higher melt viscosity is much effective for sliding the clay platelets apart
and therefore maximize clay exfoliation.

The time period of mixing should be sufficient enough to completely melt and
mix the polymer. Higher melt mixing times have been established to be better in
promoting clay exfoliation[94], however excessive mixing time can lead to possible
material degradation as well. The mixing time for the instrument has been optimized by a
previous work in our lab, in which constant torque readout from the instrument was
observed in a time span of 5 minutes thereby indicating the material has been melted and
mixed uniformly. The mixing speed was determined by the maximum permissible speed
on the instrument and thus to impart the maximum possible shear into the material using
the instrument.

The “clump” of the materials afier the mixing was scrapped from the mixer bowl

using a brass scraper and the clump was fed to a mini granulator to make small pellets,

16

the melt was chopped in a mini-granulator. The resulting granules were then compression
molded to make 50 mm disks for rheological analysis and XRD measurements.

The compression molding procedure involved a Wabash Compression Molder,
which was preheated to 200°C. The temperature was chosen to ensure homogenous
melting of the polymer. The samples are molded using 2 thick steel plates, a multi cavity
mold and a polyimide release film. The choice of the release film was based on its
inertness to the material and its high temperature stability.

For molding the specimens, the assembly was as follows. One sheet of the
polyimide release film was placed on the top of one of the thick steel plates. This was
followed by the multi cavity mold on the top of the release film. The 9 cavities were
filled with the required amount of pellets, which were calculated based on the thickness
of the mold (about 1 mm), the mold diameter (close to 50 mm) and the density of the
material (value chosen was 1 gm/cm3). The samples were then covered with another
release film followed by the second thick steel plate. The complete mold assembly was
transferred to the preheated compression-molding machine. The platens of the machine
were closed and the material was lightly compression molded for 5 minutes at a low
pressure of 2 tons. This was followed by another cycle of compression at 10 tons for an
additional 5 minutes at the same temperature. Following this, the cooling water was
introduced into the platens and the set point temperature was also reduced to 30°C to
quench the assembly as quickly as possible, while maintaining the pressure at 10 tons.
The assembly was taken out at a temperature around 65°C and the samples were removed
from the molds carefully. These disks were used for further rheological analysis as well

as X-ray diffraction measurements.

17

2.2 SILANE TREATMENT FOR CLAYS

For doing the Silane treatment on the organoclay I.3OP, 20 grams of the
organoclay was stirred in a 2wt% Silane solution in methanol for 2 minutes. The solution
was allowed to stand for 10 minutes and then decanted. The particles were kept under
ambient conditions for 24 hrs to cure the Silane layer. A series of three wash cycles with
methanol were done to remove any un-reacted Silane. The resulting clay cake was dried
in vacuum for a period of twelve hours at 100°C. The dried clay clumps were lightly
ground and the resulting clay (referred to as I.30PSiPh in the subsequent discussion) was
used for melt blending with the polypropylene and the compatibilizer. All clays were
characterized by XRD for basal spacing. A different commercially available silated clay
I.31PS was received from Nanocor and compared with the I.3OP clay as well as with the

I.30PSiPh.

2.3 X-RAY DIFFRACTION

One of the structural characterization techniques used for this study is X-ray
diffraction. X-ray diffraction is based on the fact that ordered structures act as scattering
sites for x-ray radiation. Also since the wavelength of the X-ray is of the same order as
the inter-atomic distance in a crystal, the diffraction from these sites can be analyzed
using a detector and can provide insight into the microstructure. The mathematical
expression for this diffraction phenomenon was established by Bragg and is given by

equation 2.1.

ml 2 2d Sin 6 (2.1)

18

Here “n” stands for the order of diffraction, “d” is the spacing between atoms or
planes, and “0” is the angle of diffraction. For this study, the first order peak was
considered for analysis[98].

The natural montmorillonite clay has a basal spacing of about 0.95 nm. Afier the
clay modification with octadecyl ammonium surfactants, the basal spacing for the
organoclay I.3OP increases to 2.3 nm. For polypropylene, the X-ray peak is obtained at a
20 value of about 14 degrees. The powder x-ray diffraction of the clays is carried out by
initially compacting the clay in a brass fixture with a l"-1" cavity. For all the
nanocomposite specimens, the compression-molded disks were used. These were first
pasted onto glass slides to provide enough rigidity to the samples, so as to get enough
diffraction intensities with the X-ray beam. The X-ray diffractometer used for the study
was a Rigaku Rotaflex Ru-ZOOBH X-ray diffractometer, which is equipped with a Ni-
filtered Cu Ka radiation source and operated at 45 kV and 100 mA. The sample is
scanned over a 20 range of 05° to 15° at a rate of 0.5°/min and measurements are
recorded at equal increments of 001°.

The chosen voltage and current were the recommended parameters for the
instrument. The scan range was chosen to encompass the various diffraction peaks (clay,
polypropylene) required for the research. The lower scan angle was chosen as 0.5° and
the higher scan angle (15°) was chosen, since the polypropylene peak occurs around 14°.
The scan speed was found to be optimum for the analysis, since a higher scan speed
might not Show the diffraction intensities accurately at low 20 values. The sampling
interval of 001° was chosen to provide efficient resolution in the quality of diffracted

data over the entire investigated range. The other instrument setting was the choice of

19

diffraction slits. There are 4 slits overall, one between the x-ray source (diffraction slit)
and the sample; two afier the sample on the goniometer and the last slit is on the detector.
The first two slits are of the same width by convention and l/6° slits were used for this
work. The 1/60 slits were the smallest available and were used to obtain diffraction
intensities at low angles with accuracy. The other two slits were both 0.3° and were

chosen based on the outcomes of a few trial runs.

2.4 TRANSMISSION ELECTRON MICROSCOPY

Transmission electron microscopy (TEM) is a valuable analytical tool, which is
based on scattering of electrons. Since the electron wavelength is much smaller than
light, the TEM resolution is far better than that from a light microscope. Therefore TEM
micrographs can provide information about the finest details of internal structure. The
microscope involves an electron beam (produced by an electron gun), which passes
through a series of lenses (condenser lens, objective lens) and then through the sample.
The sample image is passed through a set of intermediate and projector lenses, being
enlarged all the way. Finally the image is formed on a phosphorescent screen, which is
then digitized using a digital camera. The dark areas (clay in our work) represent those
areas of the sample through which fewer electrons were transmitted, whereas the lighter
areas correspond to the bulk polymer phase.

The technique however probes very small areas of the samples and is a big
limitation. Also to get a uniform passage of electrons through the sample, it has to be thin
(about 90 nm) and of uniform thickness, which is another big limitation of the technique.

For polypropylene samples, which have glass transition temperatures below room

20

temperature, cryosectioning is required to ensure enough polymer rigidity. The sectioning
was done using the following procedure. Initially a small trapezoid was carved on a small
compression molded section of the specimen at room temperature using a razor blade.
This was followed by placing the sample in a cryo-assembly maintained at —120°C using
a liquid nitrogen reservoir. Sections of thickness close to 90 nm were chipped from the
trapezoidal structure using a diamond knife and transferred to a copper grid of 300-mesh
size. For some specimens, a copper grid of 450-mesh size was also used. The choice of
the grid was governed by the fact, that they should provide enough rigidity to the thin
sections and should not be damaged due to the electron beam. The cryomicrotomed
sections for extruded strands were prepared carefully from the strand core to get a much
more representative estimate of the composite morphology. The thin sections were then
viewed under a microscope J EOL 100 CX equipped with a 120 KeV electron accelerator.
The digital images obtained using a digital camera were analyzed using commercial
image analysis software (Sigma Scan Pro 5.0, SPSS Inc) to calculate the percentage of
single platelet structures, stacked structures as well as agglomerates in the sample. The

procedure is laid out in detail in Chapters 3-4.

2.5 RHEOLOGY

2.5.1 Oscillatory Shear Measurements

The rheological properties of a polymer yield useful information about the
material microstructure, which dictates its end product performance and processing ease.

The viscoelastic (a combination of viscous and elastic) response of polymers as a

21

function of frequency (time) and temperature can be obtained by using rotational
rheometers. The structure of a material can typically by accessed by its response to small

strain amplitude oscillatory strains.

y = 70 sin wt (2.2)

r = r sin(a)t + 6) (2.3)

0

The stress can be decomposed into two components — an elastic component in
phase with the strain and a viscous component 90° out of phase with strain. This yields
two dynamic moduli — G' (in-phase elastic or storage modulus) and G" (out-of-phase
viscous or loss modulus). The phase tangent provides information regarding the melt

elastic nature (equation 2.4)

tan6=

 

(2.4)

I

A complex viscosity may be defined, with its magnitude is given by,

 

, . 2 , 2
'77 |=(n'2+n"2)1/2 {[G—j +(fi) = — (2.5)

22

The rheometer used for this work was an RMS 800 instrument fi'om Rheometrics
Inc. The strain limit to be used for oscillatory testing has to be in the linear viscoelastic
limit. This limit is obtained by running a strain sweep test. The strain sweep test involves
varying the strains for the material at a fixed frequency and monitoring the viscoelastic
properties (G', n') as a function of strain. The strain at which the chosen viscoelastic
property shows a decrease indicates the onset of non-linearity. The strains utilized in this
research were confirmed to be in the linear viscoelastic regime. Typically a 2 % strain for
the composites was in the linear viscoelastic regime and was used for the oscillatory
shear measurements. The chosen strain percentage also resulted in torque values greater
than 2 g-cm, which is the lowest limit for acceptable torque values for the instrument.
The lower strain values also preclude any nano-layer alignment, which has been earlier
observed to occur with large amplitude oscillatory strains[99-101]. The chosen frequency
range was 0.01-100 rad/s for all the composite specimens, whereas a fiequency range of
05-100 rad/s was used for the silicate free matrices. The lower frequencies were used for
the composites to investigate any possible pseudo-yield behavior, typically observed with
nanocomposites[97, 100, 102]. A lower frequency of 0.05 rad/S was sufficient for the
silicate free matrices, Since all the materials reached a steady plateau in the viscosity
curve. All the data reported in this work were only considered if the corresponding torque

values were above the acceptable lower torque-sensing limit of the instrument.

23

2.5.2 Creep Compliance Measurements

The creep compliance measurements are an efficient tool to monitor the low
frequency micro-structural changes and yield phenomenon associated typically with
filled materials[103]. Creep is a slow, progressive deformation of a material under
constant stress. A creep test involves application of a constant shear stress and monitoring
the resulting shear strain as a function of time. The creep compliance is the ratio of the
time dependent strain to the constant stress. For a viscoelastic liquid, the creep

compliance J (t) can be defined by the equation 2.6.

, —t/Z

J(t)=—+ZJ.(l-e 1) (2.6)

’70 i l

A higher value for the creep compliance is typically a characteristic of a liquid
like response and a lower creep compliance value is typically observed for materials
having a solid like response. For sufficiently higher creep times, the zero shear viscosity
can be obtained by making a plot of t/J(t) versus time and reading the steady plateau
value of the curve slope[104].

The creep compliance measurements for this research are obtained with an AR
2000 rheometer at 180°C from TA Instruments. 40mm steel plates were utilized for this
research. The 50 mm compression molded disks were reduced in diameter to 40mm to be
used in the instrument by cutting the disk carefully with a sharp razor blade. The samples
were initially homogenized for 10 minutes before starting the run. The creep tests were
performed at a stress of 10 Pa. Similar values of stress have been previously used by

other researchers for creep compliance measurements[103, 104] with polypropylene and

24

polypropylene clay hybrids as well. The chosen stress value of 10 Pa was confirmed to be
in the linear viscoelastic regime, since similar compliance values were obtained at a
higher stress of 15 Pa. The tests were performed at a temperature of 180°C and the
samples were melted for 10 minutes before starting the test. The gap spacing used for the
runs ranged from 0.8 mm-0.9 mm. The spacing was chosen so as to get enough normal

force and for making sure that both plates are in sufficient contact with the sample.
2.6 NANOLAYER MORPHOLOGICAL CHANGES IN EXTRUSION

To investigate the morphology of the silicate nanolayers in different flows, the
granulated pellets were passed through a uniform bore die and a converging die providing
logarithmic tensile (Hencky) strain of 5. The specifications of the two dies and the
extrusion parameters have been shown in Figure 2.2.

The dies were mounted in a Dynisco LCR6000 Capillary rheometer (Figure 2.2)
and extrusion was carried out at 200°C. The procedure for extruding with the straight and
profile dies involves the following sequence of steps: (1) the capillary barrel of the
rheometer was heated to the test temperature (200°C) and the die was attached to it; (2) a
total of 6 g of nanocomposite pellets was loaded into the capillary barrel in 2+2+2 gms
increments, afier adding each 2 gm batch, the sample was allowed to melt for 2 minutes
to ensure homogenous melting, (6) the test was programmed in and run from the
instrument sofiware window. Two different rates of extrusion were used with Die 1- 195
sec'1 (piston rate of 16 mm/min) and 3650 sec'l (piston rate of 300 min/min) in order to
contrast the effects of low and high strain rates. These rates were obtained based on the

following equations for straight shear flows.

25

r'= 49 (2.7)

 

7 = Apparent Shear Rate, Q= Volumetric Flow Rate, Re= Die Exit Radius

V anz

_ P
Q — 4 (2-8)

Rb = Inlet Die Radius, VI) = Piston Speed

 

E: Q (2.9)

E = Average velocity

=I|t~

(2.10)

t = Residence Time, L= Die Length

The converging die used in this study has a semi-hyperboloidal profile, which

provides a fixed extensional (Hencky) strain, a” on the melt.

, = Ln(—B (2.11)

26

In the above equation, R3 is the inlet radius of the semi-hyperboloidal die (same
as barrel diameter) and RE is the exit radius of the die. The capital letters “B” and “E”
used as subscripts are used to distinguish the semi-hyperboloidal die (Die 2) with the
straight die (Die 1) as used in this work. The semi-hyperboloidal die provides an

extensional strain rate (5" ), over its entire length (L) for a given piston velocity (Vp).
V
é=TPexp(sH) (2.12)

Both the dies used in these studies have a length of 20 mm. In order to draw the
distinctions between the morphology in straight and converging flows, a similar piston
ram rate was used for the converging die (Die 2), after adjusting it for the difference in
the exit diameters. The equivalent ram speed turns out to be about 9.2 mm/min, which

translates to an extensional strain rate of 1 sec".
2.7 MECHANICAL PERFORMANCE

Exfoliated clay platelets result in large improvements in the mechanical
performance of a composite. The composites in this study were evaluated for
improvements in tensile modulus. The specimens for this evaluation were prepared by
using a mini DSM injection molder. The choice of the instrument is based on the small
nanocomposite batch size. Typically 1 kg of material is required with a conventional
injection molder. Such amount of material wasn’t possible to obtain with the Banbury
Mixer and therefore the mini-injection molder was used for this research. The procedure

involves initially feeding 12 gms of pellets into a DSM 15 cm3 Micro-Extruder. The

27

extruder was operated at a uniform temperature of 180°C for all the zones and a run cycle
of 5 rpm for 2 minutes was used to homogenize and melt the specimens. The 5 rpm screw
speed was chosen to impart minimum shear to the instrument, so that the microstructure
of the composite remains more or less intact. XRD scans run on specimens before and
after DSM micro-extruder compounding are almost similar, which corroborate the idea
that the clay microstructure remains almost the same. The melt from the DSM after the
two minutes cycle is taken out by opening a side gate and collected in a melt holder,
which is maintained at 180°C. The melt holder is loaded in the DSM Injection Molding
machine and injection molded at 100 psi to make tensile coupons. The coupons obtained
were devoid of any scratches and were mostly smooth.

The 1% tangent tensile moduli were obtained at a testing speed of 0.7 in/min
using a United Universal Testing Machine. The tensile testing was carried out following
the procedure ASTM D3039 with a 20 K load cell. The tensile specimens prepared by the
DSM compounding didn’t comply with ASTM standards, but were useful to provide a
relative estimate of the improvements in tensile modulus as compared to the base
polypropylene. At least 3 samples were tested for each measurement, and approximately

5% error was associated with the obtained values.

28

 

 

 

 

 

0
-2 .
E -4 -
LE,
2» -6 -
IL
‘5
:2 -8 -
— PP
-10 - —Silicate Free Melt
_12 l I l l
100 120 140 160 180 200

temperature (°C)

Figure 2.1. Representative DSC scans for melting transitions for polypropylene and
corresponding silicate free melts: (—) Homopolymer PP, (—) Silicate free
melt

29

   
 
 
  
 

piston

heated
rheometer
barrel

polymer
melt

uniform
bore die

converging die

 

 

Die- 1 Die-2

 

 

p.20mm—bl
[a— 20mm»

 

 

 

1.0mm 0.831 mm

 

 

 

 

Figure 2.2. Capillary Rheometer Setup for Extrusion Studies with Die Profiles

30

THE USE OF SILANE COUPLING IN MELT MIXED POLYPROPYLENE

CLAY NANOCOMPOSITES

 

CHAPTER 3

3. 1 INTRODUCTION

3.1.] Objective

In this chapter, we have investigated the effects of additional coupling at the clay
edges on the clay exfoliation levels for melt mixed polypropylene nanocomposites and
the associated improvements in flow and mechanical properties. Melt processing of
nanocomposites requires effective stress transmission between the matrix and the layered
silicates particularly while the layers are still stacked to facilitate layer sliding and result
in high exfoliation levels. Montrnorillonite pretreated with a C18-surfactant was treated
additionally with a carefully chosen silane coupling agent and X-ray diffraction was used
to confirm if the silane was located predominantly at the clay edges. A different
commercially available silated clay I.31PS was also used for nanocomposite preparation.
5 wt% of each clay was compounded with 10 wt% maleated polypropylene PP-g-MA
(acid number =23 and Mw=22000) and 85 wt% of 2 grades of polypropylene (MFR= 4
and 11). The resulting nanocomposites were analyzed with X-ray diffraction,
transmission electron microscopy (TEM), and melt rheology. The nanolayer stability to
additional processing was also studied by monitoring the morphology of nanocomposite
extrudates through different flow geometries and at different strain rates. The questions

addressed in the present work are as follows.

31

Does the incorporation of carefully chosen silane coupling agent enhance the
interactions between the bulk polypropylene and the clay and thereby improve the
delamination of the clay?

How are the rheological response and the mechanical performance of the
polymer/clay hybrids affected by different silane treatments on the clay?

Afier effective dispersion of clay has been obtained initially, what is the
morphological change when using clays with and without silane treatment on

extrusion through straight and converging dies?

32

3 . I . 2 Background

The potential for significant improvements in mechanical[7, 12, 53, 73, 105-107],
flame retardancy[16, 17, 83] and gas barrier properties[15] with small loadings (as low as
3 wt%) of layered silicates in various polymer matrices has stimulated the growth of
polymer nanocomposites[6, 13, 81, 82] and has attracted both industrial and academic
interest. However attempts to exfoliate the clay in non-polar systems like polypropylene
have met with limited success. Absence of enough interfacial contacts between the
nanoclay and non—polar polymers like polypropylene impairs the efficiency of stress
transfer from the continuous polymer matrix and results in incomplete clay exfoliation.
This efficiency is critical for the exfoliation process, since the clay platelets separate only
when the attractive Van-der—Waals force between them is exceeded by the transmitted
stress during melt blending[108]. More favorable interactions of the polymer/clay as
compared to the surfactant/clay result in a favorable enthalpy of mixing[25, 26], and
leads to better clay exfoliation.

Frequently with polypropylene nanocomposites, fimctionalized polyolefin
oligomers (compatibilizers) are used[9, 10, 54-57, 63, 64], where the polar functionality
(usually maleic anhydride) wets the clay surface and the oligomers are soluble in the bulk
matrix. A high level of grafting (more than 5 wt% MA), which although desirable for
favorable interaction with the clay surface causes PP chain scission[62] and is a primary
limitation of using maleated PP as a polymeric compatibilizer.

Another way to increase the interactions between polymer and the filler is to have

favorable interactions of the bulk polymer with the hydroxylated edges of the silicate

33

layer[72] by using silane coupling agents. The efficiency of the multifunctional silane-
coupling agents is based on their bridging ability: the hydrolyzable moiety of the silane
reacts with the hydroxyls present on the clay edges and the organo-functional group of
the silane interacts with the bulk polymer. The latter interaction can be in the form of
covalent bonding or the formation of interpenetrating networks (IPN’s)[109] for non-
reactive systems like polypropylene. Figure 3.1 shows a schematic of the various
interaction sites with clay nanolayers. The maleic anhydride moiety in the PP-g—MA has
favorable interactions with the clay faces. Even though the edge area is much smaller as
compared to the nanolayer face area for a single clay platelet, the un-separated clay
stacks should provide enough functionalization sites, which can be coupled to the bulk
polypropylene. As is shown in the present work, use of carefully chosen silane coupling
agents to functionalize the hydroxylated clay edges improve its interactions with the bulk
polypropylene, and translates into enhanced clay delamination and improved
nanocomposite mechanical performance.

Previous studies[73] have used silane-modified clays, which have resulted in
nanocomposites with improved mechanical properties. However with the layered
morphology of clays, some silanes can actually move into the interlayer galleries. Fluoro-
alkyl silanes were used by Manias and coworkers[l 8], which led to silane intercalation in
the clay galleries and increased the basal spacing form 1.98 to 2.4 nm. Other silanes have
been shown to react with the edges mainly[74-77], which is evident by similar interlayer
spacing before and afier silane treatment. Song and Sandi[74] have investigated the
surface properties of smectite clays modified with two different long chain organo-silanes

and found a reduction in the surface area for the modified clays owing to the formation of

34

a hydrophobic coating as a result of the condensation reaction between the silane and the
hydroxylated clay edges. Morris and coworkers[110] indicate that for layered clays,
interlayer hydroxyls are normally inaccessible to silane coupling agents and found
reactions between the clay edge hydroxyls and trimethylsilyl chloride for their system
using Al-NMR. One of the objectives of this work was to investigate the effectiveness of
silanes, which functionalize the clay edge hydroxyls in enhancing exfoliation levels of
clay in polypropylene nanocomposites.

The study of nanolayer morphological changes to additional melt processing was
the other objective of this research. Additional melt processing steps affect the nanolayer
morphology as illustrated by the work of Reichert et al.[84] and Manias et al.[18].
Reichert et al.[84] conducted annealing studies of injection molded polypropylene
nanocomposites and observed a coarsening of the nanolayers after annealing. The
capacity of the nanocomposites to withstand additional processing effects is a measure of
the improved coupling between the clay and the bulk polymer. Similar studies were
conducted by Manias et al.[18] for two polypropylene nanocomposite systems with the
same organoclay for each case but with two different matrices: neat PP alone for one case
and PP-g—MA alone for the second. The structure before and after annealing was studied
by X-ray diffraction and it was observed that the composite with the neat PP matrix
turned from an exfoliated structure to strongly intercalated after annealing cycle of 15
min. The other structure with PP-g-MA as matrix was stable over a period of 30 min
indicating better coupling for this case. Wang and co-workers[111] investigated the role

of shear stress on dispersion of PP/clay nanocomposites by using dynamic packing

35

injection molding (DPIM) and concluded that the clay assumes an intercalated and
oriented morphology in the skin region, whereas it is largely exfoliated in the core region.

In this work, we have investigated the effects of additional coupling using silane
coupling agents on clay exfoliation levels for melt mixed polypropylene nanocomposites
and the associated improvements in flow and mechanical properties. The nanolayer
stability to additional processing was also studied by monitoring the morphology of

nanocomposite extrudates through different flow geometries and at different strain rates.

3.2. EXPERIMENTAL DETAILS

3.2.1 Materials

The polypropylenes utilized in this research were Basel] Profax 6323 with a
molecular weight (Mw) of 248,000 and melt flow index (MFI) of my 10min and Profax
6523 with a molecular weight (MW) of 390,000 and MFI of 4g/ 10min (molecular weight
information provided by Basell). The maleated polypropylene AC950 (acid number =23
and Mw=22000) from Honeywell was used as a compatibilizer. The chosen compatiblizer
with a total maleic anhydride content of 4wt% has 2.6 wt% bound maleic anhydride and
has been established as an effective candidate for exfoliation of clay in
polypropylene[l 12].

The basic organoclay used here was Nanocor I.3OP, modified by the exchange of
95-98% of the sodium cations in the clay gallery with octadecyl-ammonium ions. The
specific gravity of the clay was 1.9 and a gallery spacing of 2.3 nm. The silane employed

for modifying I.3OP was phenyltrimethoxy silane (trade name Z-6124) from Dow

36

Coming. The choice of the silane-coupling agent was based on a solubility parameter
match between the polymer and the organo-functional silane group (typical silane
structure is (RO),,-Si-Y, where RO is a hydrolyzable group, such as methoxy or ethoxy
and Y is an organo-firnctional group, such as amino, phenyl or isobutyl). This should
result in the lowest system free energy, which directly correlates with the experimentally
observed interfacial adhesion as observed by Miller and coworkers[78-80]. The solubility
parameter (8) was calculated using equation (3.1) by using the group molar attraction

constants (G) for each group as discussed by Sperling[1 l3].

:-——pZG
M

6 (3-1)

‘6 ‘1’

Here p is the density and “M” is the molecular weight (for the polymer, M is
the monomer molecular weight). The 8 values calculated for the polypropylene was 8.0
(cal/cm3)“2 and 8.3 (cal/cmz')1/2 for the phenyl group in the chosen silane for this work,
which indicates that clay functionalized with the phenyltrimethoxy silane should have
favorable interactions with the bulk polypropylene. A different commercially available
silated clay I.31PS was received from Nanocor and compared with the I.3OP clay as well

as with the I.3OP silated in this work.

3. 2. 2 Procedures
3.2.2.1 SILATION

For doing the silane treatment on the organoclay I.3OP, 20 grams of the
organoclay was stirred in a 2wt% silane solution in methanol for 2 minutes. The solution

was allowed to stand for 10 minutes and then decanted. The particles were kept under

37

ambient conditions for 24 hrs to cure the silane layer. A series of three wash cycles with
methanol were done to remove any un-reacted silane. The resulting clay cake was dried
in vacuum for a period of twelve hours at 100°C. The dried clay clumps were lightly
ground and the resulting clay (referred to as I.30PSiPh in the subsequent discussion) was

used for melt blending with the polypropylene and the compatibilizer.
3.2.2.2 MELT PROCESSING

The bulk PP, PP-g—MA and clays were initially bag mixed for two minutes to
insure a homogeneous mixture and the mix was then fed to a lab scale Banbury Mixer,
where it was compounded at 180°C at 150 rpm for 10 min. The melt mixing was carried
out under a nitrogen blanket to ensure minimum polymer degradation. The “clump” of
material was removed from the mixer and pelletized in a mini-granulator. The composite
formulations are laid out in Table 3.1. A fixed loading of 5wt% clay and 10wt%
compatibilizer was used for this study. The pellets fiom the granulator were melted in a
DSM 15 cm3 Micro-Extruder at 5 rpm with a minimum input of additional shear and then
injection molded at 100 psi in a DSM Injection Molding Machine to make tensile
coupons for mechanical testing.

The pellets were compression-molded at 200°C in a Wabash Compression
molder. The materials were loaded on the hot plates and kept at 5 minutes for
homogenous melting. This was followed by a 2 tons pressure cycle for 5 minutes and an
additional 10 tons cycle for 5 min. Circular disks of 50 mm diameter and 1.0-1 .2 mm in
thickness were made and were used for X-ray diffraction, transmission electron

microscopy and rheological characterization.

38

3.2.2.3 EXTRUSION

The stability of the nanocomposite morphology to further melt processing was
determined by extruding the granulated pellets at two different strain rates through a
uniform bore die and then through a converging die which provided a logarithmic tensile
strain or Hencky strain of 4.9 at the center line. The specifications of the two dies and the
extrusion parameters have been shown in Table 3.2. The converging die offers a fixed
Hencky strain (81,) of 4.9 and the die profile is designed to provide a uniform extensional

strain rate (4%) for a given piston velocity (V p).

- P
£=—ex a 3.2
L p ( )

Here L is the die length. The two composites PP2-I.3OP and PP2-I.30PSiPh are
used for the extrusion studies to contrast the effects of silane functionalization of the clay
on the morphological stability.

The dies were mounted in a Dynisco LCR6000 Capillary rheometer and extrusion
was carried out at 200°C. The extrusion procedure involves the following sequence of
steps: (1) the capillary barrel of the rheometer was heated to the test temperature (200°C)
and then the die was attached to it; (2) 6 g of nanocomposite pellets were loaded into the
capillary barrel in 2+2+2 gm increments, afier adding each 2 gm batch, the sample was
allowed to melt for 2 minutes to ensure homogenous melting, (6) the test is programmed
in and run from the instrument software window. Two different rates of extrusion were
used with Die 1- 195 sec'1 (piston rate of 16 mm/min) and 3650 sec”l (piston rate of 300

mm/min) in order to contrast the effects of low and high strain rates. In order to draw the

39

distinctions between the morphology in straight and converging flows, a similar piston
ram rate was used for the converging die (Die 2), after adjusting it for the difference in
the exit diameters of the two dies. The equivalent ram speed turns out to be about 9.2
mm/min, which translates to an extensional rate of l sec'l. The detailed experimental
setup and the extrusion layout for the various experiments have been discussed in

Chapter 2 (section 2.5).

3.2. 3 Structural Characterization

All nanocomposites were analyzed for morphology using TEM and XRD. The
characterization of the structure by XRD is performed on the compression-molded disks
with a Rigaku Rotaflex Ru-ZOOBH X-ray diffractometer, which is equipped with a Ni-
filtered Cu Kor radiation source and operated at 45 kV and 100 mA. The sample is
scanned over a 29 range of 05° to 10° at a rate of 0.5°/min and measurements are
recorded at equal increments of 001°.

The TEM used in this study is a JEOL 100 CX with a 120 KeV electron
accelerator. Ninety nanometer thin sections are prepared from the compression molded
disk. The sections are cryomicrotomed at a temperature of —125 °C in order to create the
required rigidity to produce the uniform thin sections required to obtain clear
reproducible images. Several of these micrographs were analyzed using image analysis
software (Sigma Scan Pro 5.0, SPSS Inc.) to get a length distribution for the clay
platelets. Particles were catalogued in several images for each composite so that a total of
close to 100 single particles were analyzed for each composite. For imaging the

nanocomposite extrudates, thin sections were prepared carefully from the core of the

40

extruded strand in order to get a better idea of a representative nanolayer microstructure.

The FT IR spectra for the clay samples were obtained using compressed KBr pellets.

3.2.4 Rheological and Mechanical Characterizations

The oscillatory shear measurements were performed in a parallel plate fixture
(50mm diameter plates) with a Rheometrics RMS 800 at 180°C and subjected to
oscillatory shear over a frequency range of 0.05-100 rad/s and a strain amplitude of 2%,
which was confirmed to be within the linear viscoelastic regime by performing a strain
sweep. The data are recorded only when the torque amplitude is above the recommended
threshold of 2 g-cm.

The tensile modulus was obtained at testing speeds of 0.7 in/min using a United
Universal Testing Machine. At least 3 samples were tested for each measurement, and a
5% error was associated with the obtained values. For improvements in mechanical
performance, the composites PPl-I.30P, PPl-I.30PSiPh and PPl-I.3lPS were used,

whereas the composites PP2-I.3OP and PP2-I.3OPSiPh were used for the extrusion study.

3.3. RESULTS AND DISCUSSION

3.3.1 Silation and Clay structure

Figure 3.2 shows the XRD patterns for the organoclay I.3OP, I.31PS and the
I.3OPSiPh. The I.3OPSiPh clay treated with the phenyl trimethoxy silane has similar basal
spacing of 2.2 nm as compared to the unsilated organoclay (1301’), indicating that the
silane has functionalized the edge hydroxyls and doesn’t intercalate the clay galleries.

Song and Sandi[74] also obtained unchanged basal spacing after the treatment of the clay

41

with a different long alkyl trimethoxy silane. Additionally, they used the Near Edge X-
Ray Absorption Fine Structure Spectroscopy to confirm that the silane has reacted with
the clay edge hydroxyls.

For comparing the two clays in this study for basal spacing, the organoclay I.3OP
was heated under vacuum for 12 hrs at 100°C. The basal spacing was found to reduce
from 2.3 nm to 2.2 nm for the I.3OP after the heating cycle for 12 hours. The loss of
interlayer water could have resulted in this decrease in basal spacing. A similar loss of
0.1 nm in the basal spacing was observed, when the I.3OP was dried for 12 hrs under
vacuum at 130°C. The other clay used in this work: I.31PS clay has a basal spacing of
2.7mm, thereby indicating that the silane treatment for this clay intercalates the clay
galleries owing to an increase in the spacing. A comparison of the IR spectra shown in
Figure 3.3 between the I.3OP and I.3OPSiPh shows the characteristic phenyl ring
deformation modes in the range 560-420 cm'1 for the I.3OPSiPh, indicating that the
phenyl grafting has been successfully performed on the hydroxylated edges of the

organoclay I.3OP.

3. 3.2 Morphology of Nanocomposites
3.3.2.1 COMPRESSION MOLDED SPECIMENS

Figure 3.4 shows the XRD patterns for the five composites. Lack of distinct
features in the XRD curve implies the existence of random-well dispersed clay platelets.
The presence of distinct peaks indicates the presence of ordered clay stacks and the
corresponding 20 values allows us to calculate the spacing of the stacked platelets. The

X-ray diffraction pattern for the composite PPl-I.31PS shows an intercalation peak at

42

29=2.3°, which translates to an intercalated composite with 3.8 nm as the basal spacing,
an increase of 1.1 nm over the base clay I.31PS (XRD spacing calculated from Figure
3.2) used for this composite. An intercalated morphology with the I.31PS have been also
observed by Ellis and D'Angelo[105], who melt blended the polypropylene
nanocomposites with the help of a masterbatch concentrate termed C.31PS that contained
50-60 wt% of the I.31PS along with the polymer and compatibilizer. The XRD pattern
for the composite PPl-I.30P lacks distinct intercalation peaks and displays a “shoulder”,
whose length indicates the population of intercalated as opposed to exfoliated species.
The composites PPl-I.30PSiPh, PP2-I.3OP and PP2-I.3OPSiPh are the most featureless
curves implying that the nanolayers have undergone the largest extent of separation in
these composites.

Careful distinctions in morphology of the polypropylene clay hybrids can be
made by analysis of the transmission electron micrographs (TEM’s). The micrographs
were obtained for the composites, which appear from the XRD results to be significantly
exfoliated: PPl-I.30P, PPl-I.30PSiPh, PP2-I.3OP and PP2-I.3OPSiPh (Figure 3.5). The
micrograph for the composite PPl-I.3OPSiPh in Figure 3.5(B) has the most distinct and
random particle structures as compared to all the other composites. A good amount of
single platelet structures can be clearly seen in all the micrographs, implying a high level
of clay exfoliation. Clay platelets sliding out of a stack when subjected to shear are seen
in all micrographs analogous to the formation termed “skewed stack” by Fomes et al.[52]
in their work with nylon-6-1ayered silicate nanocomposites.

Three different types of particle structures on several such images of these four

composites were identified and are shown in Figure 3.6. A structure where no gaps can

43

be detected and that is at least half a particle length away from neighboring particles is
termed a single particle. A stack refers to a structure containing particles arranged with
significant face-to-face overlap and several discernible gaps ranging from 3 to 20 nm.
The remaining structures can be categorized as agglomerates. A quantitative comparison
of the degree of exfoliation in these composites can be obtained from an analysis of the
length distribution of such particles. The results of this analysis are shown in Figure 3.7.
The composite PPl-I.3OP has a bi-modal distribution of single clay platelets with almost
equal contributions from two most probable lengths of 125 nm and 210 nm. Although the
composite PPl-I.30PSiPh has a most probable length around 180 nm, it also has a
significant proportion of clay platelets with lengths >200 nm. Similar results were
obtained for the composites PP2-I.3OP and PP2-I.3OPSiPh, with the latter having slightly
greater platelet lengths. This clearly identifies that the composites prepared with the
phenyltrimethoxy silane treated clay had improved levels of exfoliation as compared to
the composites with the organoclay. Also the exfoliation levels of the clay with the lower
molecular weight polypropylene PP6323 are slightly better than the composites with the

higher molecular weight PP6523 as the matrix.

3. 3 .2.2 EXT R UDED SPECIMENS
Shear Flow

Representative micrographs for the nanocomposite extrudates through shear flow
in a uniform bore die (Die 1) are shown in Figure 3.8 (A, B, C, D). The micrographs

3.8(A) and 3.8(C) correspond to the composite PP2-I.3OP extruded at rates of 195 sec'1

44

and 3650 sec‘l respectively, whereas 3.8(B) and 3.8(D) correspond to the composite PP2-
I.30PSiPh extrudates at analogous rates.

A comparison of the as-mixed organoclay composite PP2-I.3OP (earlier shown in
Figure 3.5(C)) with 3.8(A,C) indicates that the additional extrusion step results in
occurrence of stacking at both extrusion rates. The lower extrusion rate of 195 sec"l leads
to a large concentration of clay stacks [Figure 3.8(A)] as compared to the higher
extrusion rate of 3650 sec’l (Figure 3.8(C)). This could be attributed to the greater
residence time (~ 0.82 sec) for the lower extrusion rate as compared to 0.04 sec for the
higher rate. The extrudates for the composite PP2-I.3OPSiPh with the edge silated clay
also indicate restacking after extrusion, especially at the lower extrusion rate of 195 sec",
as compared to the as-mixed composite (Figure 3.5(D)). Visual inspection of the
micrographs indicate lower restacking at similar extrusion rates for the composite PP2-
I.30PSiPh as compared to the extrudates corresponding to the composite PP2-I.3OP. The
composite extrudate for PP2-I.3OPSiPh at 3650 sec'1 (Figure 3.8(D)) also indicates a
large number of single platelet structures. Wang and co-workers[l l l] have indicated the
positive influence of shear on the levels of clay exfoliation in the core region of injection
molded nanocomposite specimens.

An analysis of the area fractions for the three structures: single platelets, stacks
and aggregates shows that the aggregate structures present in the as-mixed composites
from the Banbury mixer are broken into ordered stacks and single platelets after extrusion
(Table 3.3). This is indicated by a decrease in the area fraction for the aggregate
structures for both composites after extrusion. The tendency to re-stack upon extrusion is

observed for both composites at both rates as evident by an increase in the area fraction,

45

however the area fractions of the stacks are much higher for the composite PP2-I.3OP as
compared to PP2-I.3OPSiPh for both extrusion rates. This could be attributed to better
interfacial coupling for the latter case with the edge silated clay, which results in

additional stability to the clay morphology and limits the restacking levels.

Extensional Flow

In addition to flow through straight channels, extensional flows are also
encountered in various processing techniques like injection molding and blown film
extrusion. To examine the effects of extensional flows on clay nano-structure, the
composites PP2-I.3OP and PP2-I.3OPSiPh were extruded through a semi-hyperboloidal
profile die: Die 2 (specifications laid down in Table 3.2). The representative micrographs
from the core region are shown in Figures 3.9(A) and 3.9(B). Figure 3.9(A) for the
composite PP2-I.3OP shows a high percentage of clay stacks in addition to the single
platelets; on the contrary the morphology is largely preserved for the extrudate PP2-
I.30PSiPh, which shows a high concentration of mostly single clay platelets in figure

3.9(B).

3.3.3 Oscillatory Shear Rheology

The dynamic viscosities for the composites based on the PP6323 as well as the
corresponding polypropylene-compatibilizer matrix are shown in Figure 3.10. The
composite PPl-I.30PSiPh has higher dynamic viscosity over the entire 6* range, with all
the composites showing an abrupt increase in viscosity with decreasing stresses, thereby
indicating the presence of a yield stress for these systems. Xu and coworkers[l 14] have

analyzed the melt state viscoelastic properties of an exfoliated in-situ polymerized and

46

intercalated solution blended PS/organoclay composite. For intercalated composites, they
obtained a limiting value for the complex viscosity up to 3wt% clay loading; a divergence
in the complex viscosity was observed for higher loadings of 6.7 and 9wt%, consistent
with the presence of a yield stress. For exfoliated composites, the yield stress was
observed for loadings of 3.5 wt% and 3.9 wt%.

The dynamic viscosity curves are more conveniently related to the mean aspect
ratio and therefore to the degree of exfoliation[6l] because the intrinsic viscosity
increases with the average aspect ratio of the particles as has been established for dilute
suspensions[115] as well as for filled polymer melts[116]. The dynamic viscosity curves
for the composites PP2-I.3OP, PP2-I.3OPSiPh and the corresponding silicate free melt are
shown in Figure 3.11. The viscosity values are slightly higher for the composite PP2-
I.30PSiPh as compared to the composite PP2-I.3OP over the entire G* range, indicative of
better exfoliation.

The dynamic storage modulus (G') plotted in Figure 3.12 shows a plateau at low
frequencies, which is consistent with the presence of a yield stress. Matsumoto and
coworkers[l 17] observed a second plateau at low frequencies for high loadings of 1030
wt% of spherical particles (styrene-divinylbenzene copolymers in polystyrene) in
diethylphthalate (DEP), which indicated a yield stress. Aubry and coworkers[102] have
worked with 0.5-10 wt% organoclay loadings with polyamide-12 as the bulk matrix. A
second plateau was observed at lower frequencies for clay volume fractions higher than
1.5%. A percolated network structure for the anisotropic clay platelets at low frequencies
results in this solid like behavior at larger time scales. A modified Carreau-Yasuda model

has been fitted to the complex viscosity curves of PP nanocomposites in a recent study by

47

Lertwilmolnun and Vergnes[l l8] and it was found that the estimated yield stress value
was around 800 Pa for a nanocomposite system prepared with 5-wt % of organoclay and
a PP-g-MA containing 1 wt % maleic anhydride.

The plots of phase angle (8) versus complex modulus (G*) are shown in Figure
3.13 for the nanocomposite formulations, which indicate that the lowest phase angle is
obtained for the composite PPl-I.30PSiPh. This plot was first used by Van Gurp and
Palmen[1 19] to provide additional verification of the time temperature superposition.
Trinkle and coworkers[120] have used it to elucidate the effects of molecular weight and
polydispersity for polystyrene, poly (methylmethacrylate) and metallocene polyolefins
and indicate that the minima in the plot can be used to identify the rubbery plateau
modulus. The same authors[121] also extended the plot utility to study the viscoelastic
response of long chain branched polymers.

A secondary minima was also observed at lower values of G* for some of the
nanocomposite specimens. From the secondary minima in the van-Gurp-Palmen (vGP)
plots, we can calculate the yield stress values for the nanocomposite formulations. This
allows reliable estimation of the yield stress than is possible by reading the plateau value
of the storage modulus curve in the low frequency regime. The composite PPl-I.30PSiPh
is around 2500 Pa, which is more than double as compared to the composites PPl-l.30P
(~950 Pa). This may be attributed to better coupling between the phenyltrimethoxy silane
modified clay and the polymer matrix as well as to a high concentration of single

platelets in the composite PPl-I.30PSiPh.

48

3 .3. 4 Mechanical Performance

The mechanical performance for the three composites has been discussed in
Figure 3.14. The more exfoliated composite PPl-I.30PSiPh shows the maximum increase
in tensile modulus (~ 37%), whereas the nanocomposite PPl-I.31PS with I.31PS having
significant intercalated structures shows the least improvement in tensile modulus
(~18%). Typically composites with largely exfoliated clay platelets have an improved
mechanical performance as compared to an intercalated composite. Ton—that et al.[44]
worked with a fixed loading (2 wt%) of two different clays: Cloisite 15A and Cloisite
30B and found a better mechanical performance for the predominantly exfoliated
nanocomposites obtained with the Cloisite 15A. For composites with largely exfoliated
clay platelets, the increases in tensile modulus of up to 40 % have been obtained with
clay loadings of 5wt%[43, 73], whereas about 25% increase in tensile modulus has been
observed for similar clay loadings with intercalated composites[105, 107]. Ellis and
D’Angelo obtained intercalated structures with 131 PS, an organoclay with additional
silane treatment. They also prepared composites with the help of a masterbatch
concentrate termed C.3l PS that contained 50 to 60wt % of the 131 PS with the other two
components. By both compounding methods, a 5wt% intercalated nanocomposite
exhibited a 25% improvement in tensile modulus.

Enhanced mechanical performance further illustrates the effect of increased
coupling obtained at the clay edges with the phenyltrimethoxy silane treatment. Although
di-alkoxy and tri-alkoxy silanes have been established to be superior to mono-alkoxy
silanes[122] in terms of adhesion improvements owing to extensive interphases, care

should be taken that the chosen silane for clay modification shouldn’t participate in any

49

reactions with the maleic anhydride moieties of the PP-g-MA, since this will rule out
favorable interactions between PP-g-MA/clay as compared to the surfactant/clay:
essential for clay exfoliation. In previous studies[73], the silane coupling agent amino-
propyl—trimethoxy silane has been used for clay treatment. The amine group can
chemically bond with the PP-g-MA as shown by Nygard et al.[123] and Bikiaris et
al.[124]. The silane treatment with the phenyltrimethoxy silane in our case serves two
important functions. It is mostly located at the clay edges and provides additional
coupling between the clay and the bulk polypropylene during melt blending, which
facilitates nanolayer sliding and results in high exfoliation levels. Additional proof of
improved coupling is obtained with an improved mechanical and rheological
performance for the composites reinforced with this clay, with the yield stress values
almost double as compared to the organoclay composites. Secondly it also leads to
morphological stabilization to additional processing and limits the clay re-stacking. This
provides further evidence of better interfacial contacts between the clay and PP and can
serve as a useful tool to render morphological stability to the clay nanolayers in

additional melt processing steps.
3.4. CONCLUSIONS

This study reveals that carefully chosen silane-coupling agents can improve the
interfacial interactions between the edges of the nanolayer stack and the bulk polymer.
This facilitates sliding of individual nanolayers from the clay stack during melt blending.
The organoclay with additional edge functionalization using phenyltrimethoxy silane was
exfoliated to a greater extent than the base organoclay in a melt processed polypropylene

nanocomposite system, as quantified by rheological findings and image analysis of

50

various transmission electron micrographs. The silane-coupling agent for this study was
chosen based on a solubility parameter match between the bulk polymer and the non-
hydrolyzable group of the silane. The edge silated clay led to better improvements in
mechanical performance, which was attributed to better coupling between the silicate
layers and the bulk PP. The van-Gurp-Palmen plots were used to identify the yield stress
for the nanocomposites and the yield stress values were higher (almost double), when the
edge silated clay was used for compounding. A high level of nanolayer re-stacking was
observed in extrusion studies particularly at higher residence times in shear flows, and
these levels were found to be less for the nanocomposites reinforced with the edge silated

organoclay.

51

 

 

 

 

 

 

Table 3.1: Composition of various composite samples by weight %
Material Id PP6323 PP6523 PP-g-MA I.3OP I.3OP-SiPh 1.3 1PS
PPl-I.30P 85 — 10 5 — —
PPl-I.30PSiPh 85 — 10 — 5 —
PPl-I.31PS 85 — 10 —— — 5
PP2-I.3OP —— 85 10 5 — —
PP2-I.3OPSiPh — 85 10 — 5 —

 

 

 

 

 

 

 

 

52

 

Table 3.2: Uniform bore dies and profile dies used in extrusion studies

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Die-l Die-2
9.55 m
f t 1
a E
i __':h_ i .4...
1.0 mm 0.831 mm
Inlet radius 0.5 4,775
(mm)
Outlet radius 0. 5 0,4155
(mm)
Length 20 20
(mm)
Extrusronéfgnperature 200 200

 

53

 

Table 3.3: Distribution of particle structures for composites before and after
extrusion through uniform bore die (Die-l) at different rates from TEM
image analysis

 

 

 

 

 

 

 

 

Area Fraction

Sample Single Stacks Aggregates
PP2-I.3OP before extrusion 0.30 0.19 0.51
PP2-I.3OP-195 sec" 0.20 0.62 0.18
PP2-I.30P-3650 sec" 0.45 0.47 0.08
PP2-I.3OPSiPh before extrusion 0.37 0.20 0.43
PP2-I.3OPSiPh-195 sec" 0.44 0.49 0.07
PP2-I.3OPSiPh-3650 sec" 0.77 0.23 0.00

 

 

 

 

 

 

54

--
r‘ ‘\

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

*4
33"]

 

\/\/\ PP Chain Silane Coupling

--"—e PP-g-MA

Figure 3.1. Coupling between the matrix and the nanolayer faces is achieved with PP-
g-MA chains and coupling between the nanolayer edges and the matrix
can be achieved with selected silanes

55

 

 

‘22 nm

 

 

 

 

 

.E‘
(I)
C
(D
q—a
E
2.2 nm
I.3OPSiPh
Va . m
# I.3OP

 

12 3 4 5 6 7 8 910

Figure 3.2. XRD patterns of organoclays with different silane treatment showing no
change in d-spacing with phenyl trimethoxysilane (SiPh) but a noticeable
increase in d-spacing with a different silane used in I.31PS. The
organoclay I.3OP was heated for 12 hours at 100°C to match the drying
step in silation

56

Absorbance

Figure 3.3.

    
 

I.30PSiPh

 

1000 900 800 700 600
Wavenumbers (cm’l)

FTIR Spectra of I.3OP and I.3OPSiPh

57

 

PP2-I.3OPSiPh

 

 

 

 

 

 

 

 

3‘
17)
C PP2-I.3OP
3
E
- PP1-I.30PSiPh
PP1-l.3OP
* PP1-l.31PS
I.3OP
1 2 3 4 5 6 7 8 9 10

Figure 3.4. XRD patterns for polypropylene/clay nanocomposites with 5 wt% of
different clays and 10 wt% of the same PP-g-MA (AC 950)

58

 

Figure 3.5. Transmission electron micrographs of the composites (a) PPl-I.30P, (b)
PPl-I.30PSiPh, (c) PP2-I.3OP, (d) PP2-I.3OPSiPh (The scale bar in each
picture is 200 nm)

59

\
/

Single Particles

 

.4/
= |\/

Stacks Agglomerates

 

 

Figure 3.6. Different particle structures identified in transmission electron
micrographs

60

0.3

.0
N

.0
A

Relative Frequency

.0
0

Figure 3.7.

 

~—PP1-l.30P , .x.

— - -PP1-l.3OPSiPh ; /' ;\,, ~
-- - - PP2-I.3OP .- /' ‘(‘. ‘\
—---PP2-l.30PSiPh- ’ - \

 

 

 

 

 

- \ d
I Q
0 50 1 00 1 50 200 250
Length (nm)
Length distributions of "single particles" in nanocomposites (—) PP] -

I.3OP, (— —) PPl-I.3OPSiPh, ("I") PP2-I.3OP, (— - —) PP2-I.3OPSiPh

61

 

Figure 3.8. Transmission Electron Micrographs for the composite extrudates through
Die-1 (a) PP2-I.30P-l95 sec", (b) PP2-I.3OPSiPh-l95 sec'l, (c) PP2-I.3OP-
3650 see“, (d) PP2-I.3OPSiPh-3650 sec". The scale bar in each picture is
500 nm

62

 

Figure 3.9. Transmission Electron Micrographs for the composite extrudates through
Die-2 (a) PP2-I.30P-1 see", (b) PP2-I.3OPSiPh-l sec“. The scale bar in
each picture is 500 nm

63

 

 

 

 

 

E . .4...... o. . ....n, . .4.....3
I o 1
I AA 0° I
Xmas,
A 104 I:- AA 00 E
‘1’ AAAAOO :
Dc? _ AA QC>0 '
v e I I I I I I I I I AA 00 ‘
-C- ' I I I AA 00 d
3 I- I.- AA 00 q
10 : I A O :
i I AAOd
: A PP1-I.3OP II. A :
' PP1-I.3OPSiPh I. i
I PP1 and PP-g-MA '
2 . .4.....r . . ......r e . ......
1O 2 3 4
10 1O 10
6* (Pa)

Figure 3.10. Dynamic viscosity curves for three composites and the corresponding
silicate free melt of PP6323 and PP-g-MA: (A) PP1-I.3OP, (O) PP1-

I.3OPSiPh, (I) PP1 and PP-g-MA

64

 

 

 

 

105 5 ~ - ' ' ' '
E annfi'ee
_ a
$365
4. 665 -
,0? 1O 5 565 E
I ' . . O . . 95
Dc-U . ’ o 666‘
v . . fie J
-5:- O O ‘96.]
103 - 0, 9i
V PP2-I.3OP O '
e PP2-1.30PSiPh ’
e PP2 and PP-g-MA
102 . . ......r . . ......r . .41...“
102 103 104 105

G* (Pa)

Figure 3.11. Dynamic viscosity curves for two composites and the corresponding
silicate free melt of PP6523 and PP-g-MA at 180°C: (V) PP2-I.3OP, (D)
PP2-I.3OPSiPh, (O) PP2 and PP-g-MA

65

 

105 'l . n w - ..e ‘
328§ E
22 I '
00222 I.-
104 ooOXXAA ..I g
000 AA '
A 0000000 AA .-
(U o 0 AAA I-
93103 AAAAAA '- ‘l
I. l
2
1O .' A PP1-I.3OP “,4
_-' o PP1-I.3OPSiPh j
I PP1 and PP-g-MA '

 

101 ...unn ...“nfl ...nnn ...HH“ 5
10" 10'1 10° 101 102
00 (rad/s)

 

Figure 3.12. Storage Modulus (G') curves versus frequency (00) for the composites with
different clays at 180°C: (A) PP1-I.3OP, (O) PP1-I.3OPSiPh

66

 

"U" U T I 'UVIU' U I ' 'U'UU'

01
O
I
D
D
D
D
D
D
I

 

 

 

A OOOQAAC9
A O O
A A o0 M0
g; 40 ' A o a
e A 00
a . o
E 30 . AA 0 .
to AAA 0
O
O O
0630 A PP1-I.3OP
20 ' o PP1-I.30PSiPh '
...;l3 1 . Allllll . n ...-1.15
10 104 10
(3* (Pa)

Figure 3.13. 8 versus Complex Shear Modulus (0*) for the composites with different
clays at 180°C: (A) PP1-I.3OP, (O) PP1-I.3OPSiPh

67

2.5 '

PP1-1.30PSiPh

  
 
 
   

PP1-I.3OP
PP1-1.31 PS

PP6323
1.5

Tensile Modulus (GPa

 

0.5

Figure 3.14. Tensile Moduli of PP6323 based nanocomposites with 5 wt% of different
clays and 10 wt% of the same PP-g-MA (AC 950)

68

CHARACTERIZATION OF COMPATIBILIZER EFFECTIVENESS ON

RHEOLOGY AND STRUCTURE

 

CHAPTER 4

4. 1 INTRODUCTION

4.1.1 Objective

The object of this chapter is to investigate the importance of maleic anhydride
distribution in maleated polypropylene (PP) compatibilizers for dispersion of nanolayers
in PP melts. Selected grades of compatibilizers with a total maleic anhydride content of
more than 1 wt% and different bound maleic anhydride contents were used for this study.
10 wt% of each of these compatibilizers were melt blended with 5 wt% nanoclay with the
rest being bulk polypropylene. The structure of the resulting nanocomposites was
investigated with XRD, TEM, controlled strain and controlled stress rheometry.

The main questions addressed in this chapter are as follows:

0 For a fixed compatibilizer loading of 10wt% and 5wt% organo-clay and the rest being
bulk polypropylene, how is the clay exfoliation levels affected by the maleic
anhydride distribution: free, oligomeric and bound in four different PP-g-MA
compatibilizers having total maleic anhydride contents varying fi'om 1.2-4 wt%?

0 To investigate the rheological response of composites with different levels of clay
exfoliation using dynamic oscillatory shear as well as creep compliance

measurements?

69

4.1.2 Background

The dispersion of layered silicates in polymeric matrices is of great interest in the
polymer industry after the pioneering work done by researchers at Toyota for nylon-6
based materials [1-5]. Once the layered clay has been delaminated by different processing
techniques, the polymer-clay hybrids exhibit unparalleled mechanical, thermal and gas
barrier properties as compared to the base polymer matrices [7, 9-11, 13]. The nanoscopic
fillers score over the conventional reinforcements in their ability to provide greater
improvements in properties with a much reduced filler loading. These attributes of
nanocomposites have made them suitable for applications ranging from automotive to
food packaging to flame retardant materials.

Functionalized polyolefin oligomers (compatibilizers) are used in aiding clay
dispersion for non-polar polymers like polypropylene and polyethylene[54-57], where the
polar functionality (usually maleic anhydride) wets the clay surface and the oligomers are
soluble in the bulk matrix. Using this approach, Okada and co-workers [53] synthesized
the first PP-clay hybrids using maleic anhydride functionalized PP oligomers. The need
to have a significant level of polar functionality has since been established by Hasegawa
et al.[54-56], Gopakumar et al. [58] for polyethylene and by Ishida et al.[59] for a wide
range of polymers. Theoretical studies done by Balazs et al. [27, 38-40] and molecular
simulation studies by Minisini and Tsobnang [60] have also established that adding
functionalized polymers leads to improved clay exfoliation levels. Strong interactions
between the maleic anhydride moieties and the organoclay increase the system enthalpy,

which compensates for the reduction in entropy due to the polymer chain intercalating in

70

the 1 nm gap between the clay platelets and leads to greater separation of the clay
nanolayers.

The loading of the maleic anhydride functionalized polyolefins (PP-g-MA), the
molecular weight of the PP-g-MA[64, 107] and the molar ratio of the maleic anhydride
functionality [61] are the important variables, which dictate the choice of a
compatibilizer. It is well established that functionalization with maleic anhydride leads to
chain scission of the polypropylene [62] and therefore large amounts of low or modest
molecular weight PP-g-MA will lead to unacceptable low mechanical properties.
Marchant and Jayaraman[61] have investigated the polypropylene nanocomposite
structure with varying amounts of different maleated polypropylenes and concluded that
an optimum concentration of a compatibilizer with higher molar ratios of maleic
anhydride moieties led to the most exfoliated structure. They also established a low shear
relative viscosity calculated from a ratio of the plateau values of the composites and the
silicate free melts as an index for clay exfoliation. Wang et al. [63, 64] investigated the
effects of different molecular weight compatibilizers, maleic anhydride contents and the
compatibilizer loadings on the clay exfoliation in polypropylene nanocomposites. Their
results showed that the compatibilizer with lower molecular weight and higher MA
content could lead to efficient clay dispersion in PP/clay composites.

The nature of the maleic anhydride functionality itself and its influence on the
compatibilizer effectiveness to exfoliate the clay in polyolefin nanocomposites is one of
the main objectives of this work. The maleic anhydride moieties in these materials may
exist in three distinct forms. First as an unreacted small molecule, second as a maleic

anhydride oligomer and third as maleic anhydride bound to a polymer chain [65, 66]. The

71

“bound” maleic anhydride moieties are grafted at the end of the polymer chains using a
free radical reaction between polypropylene, maleic anhydride and an organic peroxide
initiator[125]. The most probable structure from this reaction is an end group consisting
of either a single maleic anhydride (succinic graft) or a multifunctional grafi (poly maleic
anhydride graft) as a result of the macro radical attacking another maleic anhydride
molecule. De Roover and coworkers [125] have used titration and FTIR to identify these
structures in 4 different modified polypropylenes. The percentage of bound maleic
anhydride in the PP-g—MA turns out to be a critical variable and as is shown in the
present work that a compatibilizer will a higher bound MA content results in higher
levels of exfoliation in melt mixed polypropylene nanocomposites.

A detailed study of the low frequency viscoelastic response of nanocomposites
with varying levels of clay exfoliation is the other important objective of this paper.
Solomon and co-workers[97] have evaluated the melt phase properties of PP/clay hybrids
and found that above a loading of 2wt% silicate, the nanocomposites exhibited a low
frequency plateau in the storage modulus curve, which is indicative of a yield stress. The
presence of an apparent yield stress for a polypropylene clay hybrid has also been
observed by Galgali et al.[103], who attributed it to frictional interactions between the
anisotropic clay platelets. In the present paper, when maleated polypropylenes with
higher bound maleic anhydride are used, the nanocomposites showed a similar plateau
for the storage modulus in the low frequency regime. The plot of the phase angle (8)
versus the complex shear modulus (G*) is used to detect the subtle micro-structural
differences in polypropylene nanocomposites with different levels of clay exfoliation.

This plot ofien called the van-Gurp-Palmen (vGP) plot has been previously used for

72

verification of the time temperature superposition principle[l l9] and to elucidate the
effects of polydispersity and long chain branching[120, 121]. The creep compliance
measurements were also obtained in the linear viscoelastic range at a stress of lOPa in

order to get detailed information about the solid like response of these materials.

4.2 EXPERIMENTAL

4.2.1 Materials

This study uses organically modified montrnorillonite clay, Nanocor I.3OP,
polypropylene homopolymer-Basell 6323 (MFR=12) and various grades of maleated
polypropylenes. The original montrnorillonite clay is sodium montmorillonite with a
cation exchange capacity of over 120 meq/ 100 grams. The modified clay has 95-98% of
the sodium ions exchanged with octadecyl ammonium ions and a specific gravity of 1.9.
The modified clay has a basal spacing .of 2.3 run before compounding. This value was
confirmed with X-ray diffraction patterns. The selected maleic anhydride modified
polypropylenes had over lwt% total maleic anhydride as confirmed by the suppliers.
Two grades of maleic anhydride modified polypropylene (PP-g-MA) compatibilizers
with acid numbers of 10 and 7 have been chosen from a set of Eastman Epolenes, as well
as two grades from Honeywell A-C waxes with acid numbers of 9 and 23. The molecular
weights, polydispersity and the acid number for all maleated polypropylenes are
presented in Table 4.1. The chosen compatibilizers provide us a wide range of molecular
weights and varying levels of maleic anhydride grafting. The results for the maleic

anhydride distribution for the four PP-g-MA compatibilizers as obtained by using

73

titration were obtained from the previous work of Marchant[126] and have been included

in Table 4.3.

4.2.2 Melt Mixing

The bulk PP, PP-g-MA’s and clay were mixed in a lab scale Banbury mixer and
compression molded following the mixing protocol as discussed in Chapter 2 (section
2.1). The compatibilizer and the organoclay loadings were kept fixed at 10wt% and 5wt%
respectively. The compatibilizer grade and sample identification for each composite is

given in Table 4.2.

4.2.3 Structural Characterization:

All nanocomposites were analyzed using TEM and XRD. The characterization of
the structure by XRD was performed on the compression-molded disks with a Rigaku
Rotaflex Ru-200BH X-ray diffractometer, which was equipped with a Ni-filtered Cu Kor
radiation source and operated at 45 kV and 100 mA. The sample was scanned over a 20
range of 05° to 10° at a rate of 0.5°/min and measurements were recorded at equal
increments of 001°.

The TEM used in this study was a JEOL 100 CX with a 120 KeV electron
accelerator. 90 nm thin sections were prepared from a disk formed for the rheological
characterization. The sections were cryomicrotomed at a temperature of —120°C in order
to create the required rigidity to produce the uniform thin sections required to obtain clear
reproducible images. Several of these micrographs were analyzed using commercial

image analysis software (Sigma Scan Pro 5.0, SPSS Inc.) to get a length distribution for

74

the clay platelets. The boundary of the single clay platelets in the micrograph was
manually outlined by using the software, which then returned its major length. Particles
were catalogued in several images for each composite so that a total of close to 100 single
particles were analyzed for each composite. About four images were required for the

composite NC 14 and three images for the rest.

4.2.4 Rheological Characterization

Oscillatory shear measurements were performed with a Rheometrics RMS 800;
the compression-molded disk was held between two parallel plates (50 mm) at 180°C and
subjected to oscillatory shear over a frequency range of 0.05-100 rad/s at strain amplitude
of 2%. The data were recorded only when the torque amplitude is above the
recommended threshold of 2 g-cm. The chosen strain amplitude was confirmed to be
within the linear viscoelastic regime by performing a strain sweep at a fixed frequency of
1 rad/sec at 180°C over a range of 0.5-30%. The samples were melted for 10 minutes
before starting the test for homogenization.

The creep measurements were obtained at a stress of 10 Pa with an AR 2000
rheometer from TA Instruments. The chosen stress value of 10 Pa was confirmed to be in
the linear viscoelastic regime, since similar compliance values were obtained at a higher
stress of 15 Pa. The tests were performed at a temperature of 180°C and the samples were
melted for 10 minutes before starting the test. A time sweep was performed at the test
temperature to verify the absence of any micro-structural changes occurring in the
experimental time frame during the annealing process as has been observed by Reichert

et a1. [84] and Solomon et al.[97].

75

4.3 RESULTS AND DISCUSSION

A primary focus for this study is the fraction of bound maleic anhydride and how
it dictates the compatibilizer effectiveness in exfoliating the organoclay. The titration
results summarized in Table 4.3 (modified from Reference [126]) indicate that different
compatibilizers have different amounts of all three chemical forms of maleic anhydride.
The compatibilizer PP-g-MA2 has a greater amount of total maleic anhydride than PP-g—
MA3, but PP-g-MA3 has a larger amount of bound maleic anhydride. This turns out to be
important for exfoliation. The compatibilizers PP-g-MA3 and PP-g-MA4 contain higher
percentages of bound maleic anhydride with the latter containing the highest percentage
of bound (2.6%) as well as total amount of maleic anhydride (4%)

The XRD patterns for the four composites along with the organoclay have been
shown in Figure 4.1. The organoclay I.3OP (curve A) shows a peak at 20 values of 2.8
corresponding to a basal spacing of 2.3 nm. Based on the degree of dispersion, this peak
can either shift to lower 20 values for an intercalated composite or might yield a
featureless curve indicative of the absence of any ordered platelet structures in an
exfoliated nanocomposite. There is a small intercalation peak for the nanocomposite
(NC11) with the lowest bound MA content as compared to the other three
nanocomposites. This would indicate that the degree of exfoliation is the least for NC11.
Further evaluation of the composite structure is possible by considering “shoulders” in
the diffraction pattern. The length of this shoulder will indicate the population of
intercalated as opposed to exfoliated species. The NC11 composite (Curve B) exhibits the
longest shoulder of all the composites. The curves C and D for NC12 and NC13

respectively are very similar and no differences between the two can be readily discerned

76

from their XRD patterns. The composite NC14 has the most featureless XRD pattern
implying the absence of ordered platelet structures.

Subtle differences in the structure of the four nanocomposites can be obtained
from analysis of the transmission electron micrographs. The micrographs were obtained
for composites NC12, NC13 and NC14, which appear from the XRD results to be
significantly exfoliated. The representative micrographs are shown in Figures 4.2-4.4.
The micrograph for the composite NC12 in figure 4.2 shows the presence of some
intercalated clay tactoids in the composite. The composite micrographs for NC13 and
NC14 have much fewer intercalated clay structures and have a large concentration of
single clay platelets, implying that the nanolayers have been significantly separated in
these two composites. Here a single platelet here is one, where no visible gaps can be
located and it is at least half a particle length away from a neighboring particle.

The clay exfoliation levels can be compared quantitatively from an analysis of the
fraction of single particles and the corresponding length distribution for such particles.
Several transmission electron micrographs were used for this purpose. The results of this
analysis are presented in Figure 4.5. The composites NC12 and NC13 have a uni-modal
distribution with most probable length of 100 nm and 150 nm respectively. The
composite NC14 has a bi-modal distribution with almost equal contributions from two
most probable lengths of 125 nm and 210 nm, which identifies NC14 prepared with the
PP-g-MA having the greatest bound MA fraction clearly as the most exfoliated sample
with the highest average platelet length.

Small amplitude oscillatory shear rheology can yield useful information about the

nanocomposite microstructure, since both storage modulus and the loss modulus of the

77

nanocomposite melts increase with the clay loadings [100]. For fixed silicate loadings
and for nanocomposites with an intercalated morphology, the increase in dynamic storage
modulus is highly sensitive to the intercalation levels and the flow induced alignment of
the clay nanolayers[100, 127]. However for exfoliated nanocomposites for a fixed silicate
loading, the relative viscosity of the composite provides a good index for the extent of
clay delamination [61], since the relative viscosity is highly sensitive to the filler aspect
ratio as has been well established for dilute suspensions [115] as well as for filled
polymer melts [116].The relative viscosity can be calculated by the ratio of the dynamic
viscosity of the composite to the corresponding silicate free mixture at a fixed stress.

The dynamic viscosity for the composites and the corresponding silicate free
melts containing both PP and PP-g-MA in the same proportion as the composites are
plotted in figures 4.6-4.9. An analysis reveals two distinct types of viscosity curves.
Figures 4.6 and 4.7 for the composites NC11 and NC12 fall into the first category, where
the composite viscosity reaches a plateau value at low stresses. The other type of
viscosity curve is observed for the composites NC13 and NC14, as shown in Figures 4.8
and 4.9. At low frequencies (or equivalently low stresses), the viscosity shows an abrupt
increase with decreasing stresses, thereby indicating the presence of a yield stress for
these systems.

The relative viscosity is plotted in Figure 4.10 over a wide G* range. The
composites NC14 and NC13 with higher contents of bound MA have higher relative
viscosities as compared to the composites NC12 and NC11, thereby indicating that the
levels of clay delamination are more pronounced in the former two cases. The composite

NC11 with the compatibilizer PP-g-MAI shows the least relative viscosity amongst the

78

four composite formulations. The rheology results are in agreement with the earlier
reported XRD and TEM observations, with the exception of NC13. The relative viscosity
difference between NC 13 and NC14 is hardly discernible and doesn’t clearly illustrate
the different levels of clay exfoliation.

The dynamic storage modulus (6') curves have been plotted in Figure 4.11. The
composites NC13 and NC14 have higher storage modulus values as compared to the
composites NC12 and NC11 over most of the frequency range. A low frequency plateau
in the G' plot is observed for the composites NC13 and NC14, which provides further
evidence for the presence of a yield stress. Such viscoelastic behavior (liquid at shorter
time scales and solid like at larger time scales) has been previously observed for other
polymer-clay nanocomposite systems and has been attributed to the formation of a
percolated network structure of the anisotropic clay platelets [102]. In a recent study with
melt mixed polypropylene nanocomposites involving Cloisite-20A clay, PP-g-MA with 1
wt% maleic anhydride and MFI=6 PP, Lertwimolnum and Vergnes[118] have fitted a
Carreau-Yasuda model to the complex viscosity curves to estimate the yield stress for
these systems. They obtained excellent fits between the predicted and the experimental
data, and found that the yield stress values reached a maximum of 800 Pa at a
compatibilizer loading of 25wt% beyond which the value was almost constant.

A better understanding of the yield stress and its connection with the
microstructure can be obtained by plotting the phase angle (5) versus the complex shear
modulus (G*). The plots for the four nanocomposites are shown in Figure 4.12. Lowering
in phase angle for most of the 6* range was the lowest for the highly exfoliated

composites NC13 and NC 14, which use the compatibilizer with the greatest amount of

79

bound maleic anhydride, thereby indicating the highly elastic nature for these composites.
The lowering in the phase angle was the least for the intercalated composite NC11. There
is a hint of the presence of a secondary minimum for the phase angle for the composites
NC13 and NC14, however due to limited data in the lower G* it cannot be conclusively
established. Matsumoto et al.[117] have analyzed the long time viscoelastic response of
polystyrene suspensions with spherical particles. The secondary minima in the low
frequency regime was closely related to the yield stress, since both having similar
relaxation mechanisms. The 6* value at the secondary minima can be interpreted as the
yield stress for the system. The values are of the order of 960 Pa for the composites NC13
and NC14, with the former having slightly greater values. The van-Gurp-Palmen plots for
all composites merge at large 6* values (of the order of 105 Pa), which is indicative of
the plateau modulus for the polymer (Gno). Since all composites converge at the same
point, it appears that the plateau modulus is unaffected by clay incorporation. However, it
cannot be conclusively established in absence of data at higher 6* values. The
corresponding plots for the silicate free matrices are shown in Figure 4.13. All the plots
are essentially similar in nature and illustrate the effects of incorporation of low
molecular weight maleated polypropylenes in the composite matrices. Trinkle and
Friedrich[120] have also observed similar plots for three different molecular weights
polystyrenes. It should be pointed here that the dynamic rheology studies with the
nanocomposite formulations at 180°C yield an incomplete picture of the yield stress for
the nanocomposite formulations. A better representative idea of the composite yield
behavior can be obtained by conducting the tests at higher temperatures, which will yield

the viscoelastic response of the material in the lower G* regime.

80

Controlled stress rheometry is ideally suited to monitor the low frequency
response of viscoelastic materials. Galgali et al.[103] used creep compliance
measurements to monitor the hybrid formation kinetics for PP/clay hybrids upon addition
of compatibilizer and also to elucidate the effects of annealing on the microstructure.
They observed that the creep compliance for a compatibilized hybrid was progressively
higher than the un-compatibilized hybrid for a 6wt% silicate loading. The creep
compliance for the four composite formulations is plotted in Figure 4.14. The composite
NC13 and NC14 have low compliance values as compared to the composites NC11 and
NC12, which is a typical characteristic of a solid like response. The creep response at
higher times also demonstrates the subtle structural differences between the composites
NC13 and NC14, where in the creep compliance is progressively lower for the composite
NC14 as compared to the composite NC13.

An average single platelet length was calculated fiom the earlier reported TEM
analysis and is reported in Table 4.3. The greatest length (142 nm) is obtained for the
composite NC14, which utilizes the compatibilizer with the highest amounts of both total
as well as bound maleic anhydride. A greater average length is obtained for the
composite NC13 (129), as compared to the composite NC12 (92nm). The nature of
maleic anhydride grafting plays a crucial role here, since the total amount of maleic
anhydride graft is higher for the compatibilizer PP-g-MA2 (1.8% versus 1.59%), but the
bound maleic anhydride content is smaller than the compatibilizer PP-g-MA3
compatibilizer (0.94% versus 1.3%).

Overall the clay exfoliation levels scale as NC14>NC13>NC12>NC11 and are

progressively higher, when compatibilizers containing higher grafiing percentage of

81

bound maleic anhydride are used for nanocomposite preparation. This can be explained
by considering the strength of the bond between the clay and the compatibilizer chains.
When more than one maleic anhydride group is attached to a chain end, the potential for
multiple interactions with the clay surface will enhance the ability of the compatibilizer
to enter the clay gallery and also create a stronger bond between the clay and the
compatibilizer. This will allow effective stress transfer from the polymer to the clay in
the high shear melt mixing process and result in enhanced clay platelet separation. This is
analogous to the polyamide work [52], where higher nanolayer exfoliation levels were

achieved with higher molecular weight matrices.

4.4 CONCLUSION

This study reveals that the distribution of maleic anhydride in maleated PP-
compatibilizers plays an important role in the process of clay delamination in a melt
processed polypropylene nanocomposite system. The clay exfoliation levels were
quantified with rheological analysis coupled with detailed image analysis of the
composite micrographs. The three forms of maleic anhydride: bound, oligomeric and free
were calculated by titration for four different compatibilizers and a higher fraction of
bound maleic anhydride in the compatibilizer results in higher clay exfoliation for a fixed
10 wt% loading of the compatibilizer. This was explained by the formation of a stronger
bond between the clay and the compatibilizer, which should facilitate transfer of stresses
from the PP matrix chains to slide the clay nanolayers during melt blending.

The dynamic storage modulus curves approached a steady plateau at low

frequencies indicating the presence of a yield stress. The relative viscosity for the

82

composites was calculated from the dynamic viscosities for the composites and the
corresponding silicate free melts in order to quantify clay delamination levels, and it
indicated a progressive increase with incorporation of compatibilizers with higher bound
maleic anhydride grafting. The average length for the single clay platelet was obtained by

analysis of several TEM images for each specimen and it also followed the same trend.

83

Table 4.1: Molecular information on compatibilizers

 

 

 

 

 

 

 

 

 

 

Mw Mn Polydispersity Acid Number
PP-g-MAla 52,000 27,400 1.9 7
PP-g-MA2° 47,000 24,800 1.9 10
PP-g-MA3b 28,250 7,700 3.7 9
PP-g-MA4b 22,000 3,700 5.9 23

 

a) Molecular weight values obtained from Eastman Chemicals
b) Molecular weight and MAH functionality obtained from Honeywell International, Inc.

84

 

Table 4.2: Composite identification and compatibilizer grade in each composite

 

 

 

 

 

Composite ID PP-g-MA grade
NC] 1 PP-g-MAI
NC12 PP-g-MA2
NC13 PP-g-MA3
NC14 PP-g-MA4

 

 

 

 

85

Table 4.3: Nature and concentration of maleic anhydride in maleated polypropylenes
obtained by titration and average length of clay platelet

 

 

 

 

 

 

 

 

 

 

Wt % maleic anhydride in PP-g-MA (a) 161113362311)
Small Molecule Oligomeric Bound Total
PP-g-MAI 0.69 0.14 0.37 1.20 —
PP-g-MA2 0.65 0.21 0.94 1.80 92
PP-g-MA3 0.00 0.29 1.30 1.59 129
PP-g-MA4 0.83 0.56 2.61 4.00 142

 

 

 

 

a) Titration results obtained from Marchant [126]

86

 

Intensity

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4.1:

2 3 4 5 0 7 8 910

XRD patterns for polypropylene/clay nanocomposites: Curve A: I.3OP
organoclay, curve B: NC11, curve C: NC12, curve D: NC13, curve E:
NC14. All composites have a fix loading of 85wt% PP6323, 10 wt% PP-g-
MA and 5wt% I.3OP Clay. The composite NC11 is based on PP-g-MAl
(Acid Number =7), the composite NC12 is based on PP-g-MA2 (Acid
Number=10), the composite NC13 is based on PP-g-MA3 (Acid
Number=9) and the composite NC14 is based on PP-g-MA4 (Acid
Number =23)

87

 

Figure 4.2: Transmission Electron Micrograph for the composite NC12. The
composite NC12 consists of 85wt% PP6323, 10wt% PP-g-MA2 (Acid
Number =10) and 5wt% I.3OP clay. The scale bar in the picture is 200 nm

88

 

Figure 4.3: Transmission Electron Micrograph for the composite NC13. The
composite NC13 consists of 85wt% PP6323, 10wt% PP-g-MA3 (Acid
Number =9) and 5wt% I.3OP clay. The scale bar in the picture is 200 nm

89

 

Figure 4.4: Transmission Electron Micrograph for the composite NC14. The
composite NC14 consists of 85wt% PP6323, 10wt% PP-g-MA4 (Acid
Number =23) and 5wt% I.3OP clay. The scale bar in the picture is 200 nm

90

 

.0
or
A

 

 

 

 

>5
8
0) 0.4 '-
3
C.
9 0.3 -
LL
g 0.2 -
E
(D 0.1 -
m
0.0 ,’ 1 l 1 \ I __
0 50 100 150 200 250

Length (nm)

Figure 4.5: Length distributions of single particles for composites: (— —) NC12, (....)
NC13 and (—) NC14. All composites have a fix loading of 85wt%
PP6323, 10 wt% PP-g-MA and 5wt% I.3OP Clay. The composite NC12 is
based on PP-g-MA2 (Acid Number=10), the composite NC13 is based on
PP-g-MA3 (Acid Number=9) and the composite NC14 is based on PP-g-
MA4 (Acid Number =23)

91

 

 

 

 

5
10: . ....... .4....... e.......:
E NC11 ,
. v Silicate Free Melt;
' v Vvv
4 V
| : VVV :
(U v vvvv VVV '
D. ' VVVVV VV
V VVV
— 3 7" VV
:10 E- V'V VVV -.-:,
5 vs
. v ,
. (a) .
2 n n 1 nun-ll n n n Ill-ll - n - LII-L
10

10 G* (PalO

Figure 4.6: Dynamic viscosity curves for composite NC11 (V) and the silicate free
melt of PP6323 and PP-g-MAl (V) at 180°C. The composite NC11
consists of 85wt% PP6323, 10wt% PP-g-MAl (Acid Number =7) and
5wt% I.3OP clay

92

 

I t ...-'1

 

 

 

ul
AAAA Silicate Free Melt-
AA '
A
4 AAA
:- A 1
A 10 : AAA :
(1) AAA :
| :- AA .
(U - A A A A A A ‘ AA AA J
a II A AA AA q
AA A
y 3 _ AAA AA 4
f: 10 ' AA AA:
A
A .1
. ‘A
2 ”4-. . . ...... ....r
10 2 l 3 ' 4 5
10 10 * 10 10
G (Pa)
Figure 4.7: Dynamic viscosity curves for composite NC12 (A) and the silicate free

melt of PP6323 and PP-g-MA2 (A) at 180°C. The composite NC12
consists of 85wt% PP6323, 10wt% PP-g-MAZ (Acid Number =10) and
5wt% I.3OP clay

93

 

 

 

 

“EDD Silicate Free Melt-

A 104- a '.-.

(9 UDDDDD .
(U I DU

9; 3bll III-.....Ill Dunno .

— I E] .-

C 10 5 ”Dan:

".I :

102 . .eLhur h l . .....I . . ......

102 105

10G* (Pa)10

Figure 4.8: Dynamic viscosity curves for composite NC13 (Cl) and the silicate free

melt of PP6323 and PP-g-MA3 (II) at 180°C. The composite NC13
consists of 85wt% PP6323, 10wt% PP-g-MA3 (Acid Number =9) and
5wt% I.3OP clay

94

 

 

 

 

o NC14 I
- o . . -
: 0000,; Silicate Free Melt:
104 -” 0°00 '-
71? 5 0°C 5
| . 00 u
(U : e e e 0 e e . 0000 1
D- . . I . . . . 000 .
:I/ 3 - ... 00 .1
C 10 5 .0. c>0 3
: e 0 :
e. 0

b I. i
102 . . ......l . . h..hul . . Lh....

102 4 105

103 10
G* (Pa)

Figure 4.9: Dynamic viscosity curves for composite NC14 (O) and the silicate free
melt of PP6323 and PP-g-MA4 (O) at 180°C. The composite NC14

consists of 85wt% PP6323, 10wt% PP-g-MA4 (Acid Number =23) and
5wt% I.3OP clay

95

N
O

 

 

 

V NC11
> o A NC12
1'5 15. D D NC13 .
O 8 o NC14
0
.‘L’ (E)
> cu
(D 10- AA m -
.2 A CD
C.“ AA
(D A .
‘1 5" ”We. ““Afffirrhaeremrfit

VVVVVVVVVVVVVVVVV
O . . . I...” . . . .. I I . I

 

103 104
G* (Pa)

Figure 4.10: Relative viscosity curves obtained at similar G* for the composites with
different compatibilizers at 180°C: (V) NC11, (A) NC12, (El) NC13, (O)
NC14. All composites have a fix loading of 85wt% PP6323, 10 wt% PP-g-
MA and 5wt% I.3OP Clay. The composite NC11 is based on PP-g-MAl
(Acid Number =7), the composite NC12 is based on PP-g-MA2 (Acid
Number=10), the composite NC13 is based on PP-g-MA3 (Acid
Number=9) and the composite NC14 is based on PP-g-MA4 (Acid
Number =23)

96

Figure 4.11:

 

10'" 10'2 10‘1 10° 101 102

0) (rad/s)

Storage Modulus (G') curves versus frequency (00) for the composites with
different compatibilizers at 180°C: (V) NC11, (A) NC12, (D) NC13, (O)
NC14. All composites have a fix loading of 85wt% PP6323, 10 wt% PP-g-
MA and 5wt% I.3OP Clay. The composite NC11 is based on PP-g-MAl
(Acid Number =7), the composite NC12 is based on PP-g-MAZ (Acid
Number=10), the composite NC13 is based on PP-g-MA3 (Acid
Number=9) and the composite NC14 is based on PP-g—MA4 (Acid
Number =23)

97

(D
O
i

 

 

 

v NC11
80 - A NC12 -
1:1 NC13
TI? 70 - o NC14 -
(D
m 60 _ VvVVVVvV _
a v VVVVVV AAAAAAZXXREN
a) 50 - AAA 919999990qu474. -
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(,0 40 - (£6 -
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20 . . ......I . . ...J..I . . I...“

 

103 10
G* (Pa)

Figure 4.12: Phase Angle (5) versus Complex Shear Modulus (G*) for the composites
with different compatibilizers at 180°C: (V) NC11, (A) NC12, (El) NC13,
(O) NC14. All composites have a fix loading of 85wt% PP6323, 10 wt%
PP-g-MA and 5wt% I.3OP Clay. The composite NC11 is based on PP-g-
MAl (Acid Number =7), the composite NC12 is based on PP-g-MA2
(Acid Number=10), the composite NC13 is based on PP-g-MA3 (Acid
Number=9) and the composite NC14 is based on PP-g-MA4 (Acid
Number =23)

98

 

(D
O

 

 

 

I. ~~
80 ' e : : {ff-I, '
O . r”
1,? 70 - If? '
'0,
CD I".
(D 60 - ,~ .
- -:~
8) 50 . .z..
'0 I 1,. 1
Z 40 . v NC11 Matrix ..
A NC12 Matrix
30 . I NC13 Matrix 1
I NC14 Matrix
20 I . ......I . . ...luI . . ......

103 10“ 105
6* (Pa)

Figure 4.13: Phase Angle (6) versus Complex Shear Modulus (G*) for the silicate free

composite matrices at 180°C: (V) NC11, (A) NC12, (I) NC13, (O)
NC14. All composites have a fix loading of 85wt% PP6323, 10 wt% PP-g-
MA and 5wt% I.3OP Clay. The composite NC11 is based on PP-g-MAl
(Acid Number =7), the composite NC12 is based on PP-g-MAZ (Acid
Number=10), the composite NC13 is based on PP-g-MA3 (Acid
Number=9) and the composite NC14 is based on PP-g-MA4 (Acid
Number =23)

99

.0
o
01

 

0.04” ‘0‘“. 1
. v NC11
A NC12
0.03- n NC13 ,
o NC14

.0

o

N
I

   

A A A A A A
/IA A A A A A A A
////////////////

/////////////

.»’/////////

_O
O
.3

 

compliance (Pa'l)

llIIIllllIllllllllllllllllllllllllllIllllglvlyvlvlvl'lvl!lll
((((((((((((((((((((((((((((((((((((((((‘.CC...........

500 1000 1500 2000
time (sec)

 

 

.0

o

o
c

Figure 4.14: Creep Compliance J(t) as a function of time at a stress of 10 Pa at 180°C:
(V) NC] 1, (A) NC 12, (Cl) NC13, (O) NC14. All composites have a frx
loading of 85wt% PP6323, 10 wt% PP-g-MA and 5wt% I.3OP Clay. The
composite NC11 is based on PP-g-MAl (Acid Number =7), the composite
NC12 is based on PP-g—MA2 (Acid Number=10), the composite NC13 is
based on PP-g-MA3 (Acid Number=9) and the composite NC14 is based
on PP-g-MA4 (Acid Number =23)

100

CONCLUSIONS AND RECOMMENDATIONS

 

CHAPTER-5

5.1 CONCLUSIONS

This study was focused towards improving the interactions between the different
components of a polypropylene nanocomposite system — bulk polypropylene, maleated
polypropylene and the nanoclay intercalated with long chain alkyl ammonium
surfactants, in order to boost the clay exfoliation levels. There were two primary issues

addressed in this work.

0 How can the interactions between the organoclay and the bulk
polypropylene be improved to facilitate effective clay dispersion?

- How is the nanolayer morphology affected by additional processing and
does a careful control of the interactions between the different system

components provide additional stability to the nanocomposite structure?

The silicate fillers offer two interaction sites: the negatively charged clay faces,
which have long chain alkyl ammonium surfactants tethered to them and the
hydroxylated clay edges. The early research attempts were targeted towards improving
the extent of coupling by adding maleated polypropylenes (compatibilizer), where the
maleic anhydride moiety has favorable interactions with the clay and facilitates the

polymer intercalation in the clay galleries. Previous authors[6] have attributed it to the

101

strong hydrogen bonding interactions of the maleic anhydride with the polar clay
surfaces, which provides the required coupling between the clay and the bulk polymer.
The strength of the hydrogen bond is an order of magnitude higher than the attractive
Van-der-Waals forces and therefore dominates over the attractive forces between the clay
platelets. This also explains why the distribution of maleic anhydride in maleated
polypropylene plays an important role in the process of clay delamination (as discussed
in Chapter 4) and the clay exfoliation levels were higher when compatibilizers with more
bound maleic anhydride were used. Recent studies by Eastwood et al.[128] indicate that
the clay exfoliation levels are higher in the presence of optimum amounts of
intermolecular hydrogen bonding and these interactions provide a driving force for the
polymer to exfoliate the layered silicate. The clay edges also have a few hydroxyl groups
on the edges, which can serve as anchoring points for the maleic anhydride moiety.
Another way to increase the interactions between polymer and the filler is to use
silane coupling agents to provide the necessary interfacial coupling during the process of
melt compounding. The choice of the silane coupling agent is important and a solubility
parameter match between the bulk polymer and the non-hydrolyzable group of the silane
can be used for silane selection. Our results show that nanocomposites with the most
exfoliated structure are obtained, when the chosen silane is located at the edges of the
organoclay and doesn’t intercalate the clay galleries. A schematic of the interactions
between the different components after the silane treatment has been discussed in Figure
5.1. It lays down the interactions, when organoclay is modified with phenyltrimethoxy
silane and subsequently the silane treated organoclay is melt blended with PP-g-MA and

bulk polypropylene. We have previously established that the phenyltrimethoxy silane

102

functionalizes the edge hydroxyls and doesn’t intercalate the clay galleries (Chapter 3).
Treatment of the organoclay with phenyltrimethoxy silane results in the formation of a
strong covalent bond between the clay and the silicon atom of the silane coupling agent
and the phenyl functionality improves the interactions with the bulk polypropylene.
Previous studies with silane coupling agents have attributed this improved interfacial
coupling to the formation of an interpenetrating network (IPN). The phenyltrimethoxy
silane treatment also results in the creation of two additional hydroxyl functionalities per
coupling edge site, which offer additional anchoring points for hydrogen bonding with
the maleic anhydride. This should further facilitate the sliding of the clay nanolayers
during melt blending.

The other object of this study was to investigate the clay nanolayer morphology in
shear and extensional flows. The idea was to investigate if improved interfacial coupling
limits the extent of restacking/re-aggregation. Strong intermolecular interactions will
limit the polymer chain mobility and provide stable structures. A high level of nanolayer
re-stacking was observed in extrusion studies (Chapter 3) particularly at higher residence
times in shear flows. However the levels of re-aggregation and restacking were reduced
for the nanocomposites reinforced with phenyltrimethoxy silane functionalized clay
particularly at higher residence times in shear flows. This could be attributed to two
factors: the first owing to better interfacial interactions between the phenyl group and the
bulk polypropylene. The second factor might be improved hydrogen bonding interactions
between the PP-g-MA and the clay edge hydroxyls when the phenyltrimethoxy silane
treated organoclay is used for melt blending, which provide additional morphological

stability to the clay structure. On the other hand, the presence of a large amount of

103

hydrogen bonding interactions between the PP-g-MA and the clay faces seems to result
in more clay restacking and aggregation. The other factor, which needs to be carefully
investigated in this regard is the effect of temperature on hydrogen bonding interactions
between the filler and the compatibilizer. The reduced restacking levels obtained in this
study with the appropriate silane treatment provides evidence of better interfacial
contacts between the clay and PP and can serve as a useful tool to render morphological

stability to the clay nanolayers.

5.2 RECOMMENDATIONS

Based on the results of this investigation, the following recommendations can be
made for future research efforts:

1. The overall goal of this study was to systematically evaluate the interactions
between the multi-component polypropylene/PP-g-MA/organoclay systems
and optimize it to boost the clay exfoliation levels. Two additional
recommendations can be made in this field. The use of longer alkyl chain
silane coupling agents needs to be evaluated in order to study the effects of
chain length on the coupling between the clay and the bulk polypropylene.
The second recommendation can be made with regard to the surfactant
treatment of the clay. Recent studies evaluate the positive effects of using
polyrnerically modified surfactants to facilitate clay exfoliation in melt
mixed systems. Wilkie and coworkers[129, 130] have prepared two new
organically modified clays that contain an oligomeric styrene or
methacrylate, which were melt-blended with different nanocomposite

systems. In the case of polypropylene[130], they obtained good levels of

104

clay exfoliation without using any maleated PP. Similar studies were carried
out successfully by Kurian et al.[l3l], where polystyrene (PS)/clay
composites by using a PS surfactant modified clay.

In this study, the surfactant treatment for the organoclay involved a single
long alkyl tail. A recent study with polyethylene/clay hybrids suggests that
the presence of two alkyl tails gives better exfoliation, owing to the
preferential affinity of the non-polar polyolefin with the alkyl tails and not
with the clay faces. This needs to be evaluated for the polypropylene system
and should lead to the most optimum structure.

This study was based on a batch mixing technique, where all ingredients
were mixed together and melt blended. Owing to the positive effects of
masterbatching, a comparative study needs to be undertaken to contrast the
effects of masterbatching to batch compounding.

The sequence of feeding the system components (clay, polypropylene and
PP-g-MA) also needs careful consideration. Effects of adding clay in a
secondary step after the PP and PP-g-MA have been mixed in the first step
needs to be evaluated, because recent studies point out towards this protocol
as being beneficial for effective clay dispersion.

In the nanoclay morphological stability studies, the core of the extruded
strand was analyzed for effects of extensional flow on nanostructure. To
complete eliminate wall shear contamination; a core-skin flow has to be

used. Here the skin can layer can be comprised of a linear low-density

105

polyethylene (LLDPE), which will offer the desired lubrication to the
composite during extrusion through the semi-hyperboloidal dies.

The van-Gurp-Palmen plot as used in this research work can be used to
characterize the yield behavior of the nanocomposite materials. In order to
further establish it, the data should be supplemented with yield stress
measurements obtained from alternate techniques. The yield stress
measurements can be obtained by monitoring the creep response[103] to
progressive increase in stress. The stress at which the zero shear viscosity
drops by a magnitude can be identified as the yield stress of the material.
For some of the nanocomposite materials, the rheology data in the low 6*
regime could not be obtained and therefore the yield stress information
obtained by the van-Gurp-Palmen plots was insufficient. For those cases,
the rheology experiments should be performed at slightly higher

temperatures (200°C is recommended).

106

OH
M l -H,o
/ + OH — Sli ——>
OH OH

O/ /\
\
Sr

Phenyl group having favorable aflinity with
' the bulk polypropylene chains. It also
renders additional stability to the clay
since it is tightly bound to the bulk polymer

 

Two additional hydroxyl groups per PTMS
molecule created by the silylation process,
which offer increased hydrogen bonding
interaction sites with PP-g-MA

Figure 5.1: Proposed mechanism for the enhancement in the stability of nanolayers by
careful control of the interactions between the different system
components. This is achieved by favorable interactions of the phenyl end-
capped silane with the bulk polypropylene and also increased hydrogen
bonding interactions between the PP-g-MA and the hydroxyl groups.

107

10.

ll.

BIBLIOGRAPHY

Usuki, A., Kawasumi, M., Kojima, Y., Okada, A., Kurauchi, T., and Kamigaito,
O., “Swelling Behavior of Montmorillonite Cation Exchanged for Omega-Arnino
Acids by Epsilon-Caprolactam.” J. Mater. Res., 8, (5), 1174-1178 (1993)

Usuki, A., Kojima, Y., Kawasumi, M., Okada, A., Fukushima, Y., Kurauchi, T.,
and Kamigaito, 0., “Synthesis of Nylon 6-Clay Hybrid.” J. Mater. Res., 8, (5),
1179-1184 (1993)

Kojima, Y., Usuki, A., Kawasumi, M., Okada, A., Fukushima, Y., Kurauchi, T.,
and Kamigaito, O., “Mechanical-Properties of Nylon 6-Clay Hybrid.” J. Mater.
Res., 8, (5), 1185-1189 (1993)

Kojima, Y., Usuki, A., Kawasumi, M., Okada, A., Kurauchi, T., and Kamigaito,
0., “Synthesis of Nylon-6-Clay Hybrid by Montmorillonite intercalated with
epsilon-caprolactam.” J. Polym. Sci. Pol. Chem, 31, (4), 983-986 (1993)

Kojima, Y., Usuki, A., Kawasumi, M., Okada, A., Kurauchi, T., Kamigaito, O.,
and Kaji, K., “Fine-Structure of Nylon-6-Clay Hybrid.” J. Polym. Sci. Pol. Phys.,
32, (4), 625-630 (1994)

Giannelis, E., Krishnamoorti, R., and Manias, E., “Polymer-silicate
nanocomposites: Model systems for confined polymers and polymer brushes.”
Adv. Polym. Sci, 138, 107-147 (1999)

Giannelis, E.P., “Polymer layered silicate nanocomposites.” Adv. Mater., 8, (1),
29-35 (1996)

Giannelis, E.P., “Polymer-layered silicate nanocomposites: Synthesis, properties
and applications.” Appl. Organomet. Chem, 12, (10-11), 675-680 (1998)

Kato, M., Usuki, A., and Okada, A., “Synthesis of polypropylene oligomer-clay
intercalation compounds.” J. Appl. Polym. Sci, 66, (9), 1781-1785 (1997)

Kawasumi, M., Hasegawa, N., Kato, M., Usuki, A., and Okada, A., “Preparation

and mechanical properties of polypropylene-clay hybrids.” Macromolecules, 30,
(20), 6333-6338 (1997)

Yasue, K., Kathira, S., Yoshikawa, M., and Fujimoto, K., “In Situ Polymerization

Route to Nylon 6-Clay Nanocomposites.” Polymer Clay Nanocomposites, T.J.
Pinnavaia and G.W. Beall, Editors. (2000)

108

12.

13.

14.

15.

l6.

17.

18.

19.

20.

21.

22.

23.

Alexandre, M. and Dubois, P., “Polymer-layered silicate nanocomposites:
preparation, properties and uses of a new class of materials.” Mat. Sci. Eng, 28,
(1-2), 1-63 (2000)

Ray, S. and Okamoto, M., “Polymer/layered silicate nanocomposites: a review
from preparation to processing.” Prog. Polym. Sci., 28, (11), 1539-1641 (2003)

Messennith, PB. and Giannelis, E.P., “ Synthesis and Barrier Properties of
Poly(Epsilon-Caprolactone)-Layered Silicate Nanocomposites.” J. Polym. Sci.
Pol. Chem, 33, (7), 1047-1057 (1995)

Qian, (3., Cho, J.W. and Lan, T., “Preparation and Properties of Polyolefin
Nanocomposites.” Polyolefin_s 2001, Houston, TX (2001)

Bartholmai, M. and Schartel, B., “Layered silicate polymer nanocomposites: new
approach or illusion for fire retardancy? Investigations of the potentials and the
tasks using a model system.” Polym. Advan T echnol., 15, (7), 355-364 (2004)

Gilman, J., Jackson, C., Morgan, A., Harris, R., Manias, E., Giannelis, E.,
Wuthenow, M., Hilton, D., and Phillips, 8., “Flammability properties of polymer -
Layered-silicate nanocomposites. Polypropylene and polystyrene
nanocomposites.” Chem. Mater., 12, (7), 1866-1873 (2000)

Manias, E., Touny, A., Wu, L., Strawhecker, K., Lu, B., and Chung, T.,
"Polypropylene/Montmorillonite nanocomposites. Review of the synthetic routes
and materials properties.” Chem. Mater., 13, (10), 3516-3523 (2001)

Tang, Y., Hu, Y., Li, B., Liu, L., Wang, Z., Chen, Z., and Fan, W.,
"Polypropylene/montmorillonite nanocomposites and intumescent, flame-

retardant montrnorillonite synergism in polypropylene nanocomposites.” J. Poly.
Sci. Poly. Chem, 42 (23), 6163-6173 (2004)

Tang, Y., Hu, Y., Wang, S., Gui, Z., Chen, Z., and Fan, W., “Intumescent flame
retardant-montmorillonite synergism in polypropylene-layered silicate
nanocomposites.” Poly. Int, 52 (8), 1396-1400 (2003)

Pinnavaia, T.J., Lan, T., Kaviratna, P.D., Wang, Z. and Shi, H.H., “Clay-
Reinforced Epoxy Nanocomposites- Synthesis, Properties and Mechanism of
Formation.”Abstr. Pap. Am. Chem. S., 210, 61 (1995)

Van Olphen, H., An Introduction to Clay Colloid Chemistry; Interscience
Publishers: New York, 1962

 

Theng, B.K.G., Soils with Variable Charge, New Zealand: Offset Publications
(1980)

109

24.

25.

26.

27.

28.

29.

30.

31.

32.

33.

34.

35.

Delozier, D.M., Orwoll, R.A., Cahoon, J.F., Ladislaw, J.S., Smith, 1.6., and
Connell, J.W., “Polyimide nanocomposites prepared with a novel aromatic
surfactant.” High. Perform. Polym., 15, (3), 329-346 (2003)

Vaia, R. and Giannelis, E., “Lattice model of polymer melt intercalation in
organically-modified layered silicates.” Macromolecules, 30, (25), 7990-7999
(1997)

Vaia, R. and Giannelis, E., “Polymer melt intercalation in organically-modified
layered silicates: Model predictions and experiment.” Macromolecules, 30, (25),
8000-8009 (1997)

Balazs, A., Singh, G, and Zhulina, E., “Modeling the interactions between
polymers and clay surfaces through self-consistent field theory.” Macromolecules,
31, (23), 8370-8381 (1998)

Kuznetsov, D. and Balazs, A., “Phase behavior of end-functionalized polymers
confined between two surfaces.” J. Chem. Phys., 113, (6), 2479-2483 (2000)

Huh, J., Ginzburg, V., and Balazs, A., “Thermodynamic behavior of
particle/diblock copolymer mixtures: Simulation and theory.” Macromolecules,
33, (21), 8085-8096 (2000)

Gee, R., Maxwell, R., and Balazs, B., “Molecular dynamics studies on the effects
of water speciation on interfacial structure and dynamics in silica-filled PDMS
composites.” Polymer, 45, (1 1), 3885-3891 (2004)

Lee, J., Shou, Z., and Balazs, A., “Predicting the morphologies of confined
copolymer/nanoparticle mixtures.” Macromolecules, 36, (20), 7730-7739 (2003)

Buxton, G., Lee, J., and Balazs, A., “Computer simulation of morphologies and
optical properties of filled diblock copolymers.” Macromolecules, 36, (25), 9631-
9637(2003)

Lee, J., Thompson, R., Jasnow, D., and Balazs, A., “Entropically driven formation
of hierarchically ordered nanocomposites.” Phys. Rev. Lett., 89, (15), (2002)

Buxton, G. and Balazs, A., “Lattice spring model of filled polymers and
nanocomposites.” J. Chem. Phys., 117, (16), 7649-7658 (2002)

Thompson, R., Ginzburg, V., Matsen, M., and Balazs, A., “Predicting the

mesophases of copolyrner—nanoparticle composites.” Science, 292, (5526), 2469—
2472 (2001)

110

36.

37.

38.

39.

40.

41.

42.

43.

45.

46.

Kuznetsov, D. and Balazs, A., “Scaling theory for end-functionalized polymers
confined between two surfaces: Predictions for fabricating polymer/clay
nanocomposites.” J. Chem. Phys., 112, (9), 4365-4375 (2000)

Ginzburg, V., Singh, G, and Balazs, A., “Theoretical phase diagrams of
polymer/clay composites: The role of grafted organic modifiers.”
Macromolecules, 33, (3), 1089-1099 (2000)

Ginzburg, V. and Balazs, A., “Calculating phase diagrams for nanocomposites:
The effect of adding end-functionalized chains to polymer/clay mixtures.” Adv.
Mater., 12, (23), 1805-1809 (2000)

Zhulina, E., Singh, C., and Balazs, A., “Attraction between surfaces in a polymer
melt containing telechelic chains: Guidelines for controlling the surface
separation in intercalated polymer-clay composites.” Langmuir, 15, (11), 3935-
3943 (1999)

Ginzburg, V. and Balazs, A., “Calculating phase diagrams of polymer-platelet
mixtures using density functional theory: Implications for polymer/clay
composites.” Macromolecules, 32, (17), 5681-5688 (1999)

Lyatskaya, Y. and Balazs, A., “Modeling the phase behavior of polymer-clay
composites.” Macromolecules, 31, (19), 6676-6680 (1998)

Singh, C., Pickett, G., Zhulina, E., and Balazs, A., “Modeling the interactions
between polymer-coated surfaces.” J. Phys. Chem. B, 101, (50), 10614-10624
(1997)

Reichert, P., Nitz, H., Klinke, S., Brandsch, R., Thomann, R., and Mulhaupt, R.,
“Poly(propylene)/organoclay nanocomposite formation: Influence of

compatibilizer functionality and organoclay modification.” Macromol. Mater.
Eng, 275, (2), 8-17 (2000)

Ton-That, M., Perrin-Sarazin, F., Cole, K., Bureau, M., and Denault, J.,
“Polyolefin nanocomposites: Formulation and development.” Polym. Eng. Sci.,
44, (7), 1212-1219 (2004)

Hotta, S. and Paul, D., “Nanocomposites formed from linear low density
polyethylene and organoclays.” Polymer, 45, (22), 7639-7654 (2004)

Fomes, T., Hunter, D., and Paul, D., “Nylon-6 nanocomposites from
alkylammonium-modified clay: The role of alkyl tails on exfoliation.”
Macromolecules, 37, (5), 1793-1798 (2004)

111

47.

48.

49.

50.

51.

52.

53.

54.

55.

56.

57.

58.

Lee, J ., Lim, Y., and Park, 0., “Thermal characteristics of organoclay and their
effects upon the formation of polypropylene/organoclay nanocomposites.” Polym.
Bull, 45, (2), 191-198 (2000)

Kim, K., Utracki, L.A., and Kamal, M.R., “Numerical simulation of polymer
nanocomposites using self-consistent mean-field model.” J. Chem. Phy., 121,
(21), 10766-10777 (2004)

Lan, T., Kaviratna, RD, and Pinnavaia, T.J., “Mechanism of Clay Tactoid
Exfoliation in Epoxy-Clay Nanocomposites.” Chem. Mater., 7, (11), 2144-2150
(1995)

Lan, T. and Pinnavaia, T., “Clay-Reinforced Epoxy Nanocomposites.” Chem.
Mater., 6, (12), 2216-2219 (1994)

Sun, T. and Garces, J., “High-perforrnance polypropylene-clay nanocomposites
by in-situ polymerization with metallocene/clay catalysts.” Adv. Mater., 14, (2),
128-130 (2002)

Fomes, T.D., Yoon, P.J., Kesklcula, H., and Paul, D.R., “Nylon 6 nanocomposites:
the effect of matrix molecular weight.” Polymer, 42, (25), 9929-9940 (2001)

Usuki, A., Kato, M., Okada, A., and Kurauchi, T., “Synthesis of polypropylene-
clay hybrid.” J. Appl. Polym. Sci., 63, (1), 137-139 (1997)

Hasegawa, N., Kawasumi, M., Kato, M., Usuki, A., and Okada, A., “Preparation
and mechanical properties of polypropylene-clay hybrids using a maleic
anhydride-modified polypropylene oligomer.” J. Appl. Polym. Sci., 67, (1), 87-92
(1998)

Hasegawa, N., Okamoto, H., Kato, M., and Usuki, A., “Preparation and
mechanical properties of polypropylene-clay hybrids based on modified
polypropylene and organophilic clay.” J. Appl. Polym. Sci., 78, (l 1), 1918-1922
(2000)

Hasegawa, N., Okamoto, H., Kawasumi, M., Kato, M., Tsukigase, A., and Usuki,
A., “Polyolefin-clay hybrids based on modified polyolefins and organophilic
clay.” Macromol. Mater. Eng., 280, (7-8), 76-79 (2000)

Hasegawa, N. and Usuki, A., “Silicate layer exfoliation in polvolefin/clay
nanocomposites based on maleic anhydride modified polyolefins and organophilic
clay.” J. Appl. Polym. Sci., 93, (1), 464-470 (2004)

Gopakumar, T., Lee, J., Kontopoulou, M., and Parent, J., “Influence of clay
exfoliation on the physical properties of montrnorillonite/polyethylene
composites.” Polymer, 43, (20), 5483-5491 (2002)

112

59.

60.

61.

62.

63.

64.

65.

66.

67.

68.

69.

70.

Ishida, H., Campbell, 8., and Blackwell, J., “General approach to nanocomposite
preparation.” Chem. Mater., 12, (5), 1260-1267 (2000)

Minisini, B. and Tsobnang, F., “Molecular mechanics studies of specific
interactions in organomodified clay nanocomposite.” Campos. Part A-Appl. S.,
36, (4), 531-537 (2005)

Marchant, D. and Jayaraman, K., “Strategies for optimizing polypropylene-clay
nanocomposite structure.” Ind. Eng. Chem. Res., 41, (25), 6402-6408 (2002)

Minoura, Y., Ueda, M., Mizunmna, S., and Oba, M., “The Reaction of
Polypropylene with Maleic Anhydride.” J. Appl. Polym. Sci., 13, 1625 (1969)

Wang, Y., Chen, F ., Li, Y., and Wu, K., “Melt processing of polypropylene/clay
nanocomposites modified with maleated polypropylene compatibilizers.”
Compos. Part B-Eng., 35, (2), 111-124 (2004)

Wang, Y., Chen, F., and Wu, K., “Twin-screw extrusion compounding of
polypropylene/organoclay nanocomposites modified by maleated
polypropylenes.” J. Appl. Polym. Sci., 93, (1), 100-112 (2004)

Sclavons, M., Carlier, V., DeRoover, B., Franquinet, P., Devaux, J., and Legras,
R., “The anhydride content of some commercial PP-g-MA: FTIR and titration.” J.
Appl. Polym. Sci., 62, (8), 1205-1210 (1996)

Sclavons, M., Franquinet, P., Carlier, V., Verfaillie, G., Fallais, I., Legras, R.,
Laurent, M., and Thyrion, F., “Quantification of the maleic anhydride grafted
onto polypropylene by chemical and viscosimetric titrations, and FTIR
spectroscopy.” Polymer, 41, (6), 1989-1999 (2000)

Sun, T. and Garces, J ., “High-performance polypropylene-clay nanocomposites
by in-situ polymerization with metallocene/clay catalysts.” Adv. Mater., 14, (2),
128-130 (2002)

Lee, E., Mielewski, D., and Baird, R., “Exfoliation and dispersion enhancement in
polypropylene nanocomposites by in-situ melt phase ultrasonication.” Polym.
Eng. Sci., 44, (9), 1773-1782 (2004)

Wang, Z.M., Nakajima, H., Manias, E., and Chung, T.C., “Exfoliated PP/clay
nanocomposites using ammonium-terminated PP as the organic modification for

montrnorillonite.” Macromolecules, 36, (24), 8919-8922 (2003)

Liu, X. and Wu, Q., “PP/clay nanocomposites prepared by grafting-melt
intercalation.” Polymer, 42, (25), 10013-10019 (2001)

113

71.

72.

73.

74.

75.

76.

77.

78.

79.

80.

81.

82.

Kato, M., Matsushita, M., and Fukumori, K., “Development of a new production

method for a polypropylene-clay nanocomposite.” Polym. Eng. Sci., 44, (7),
1205-1211 (2004)

Shi, H.Z., Lan, T., and Pinnavaia, T.J., “Interfacial effects on the reinforcement

properties of polymer-organoclay nanocomposites.” Chem. Mater., 8, (8), 1584-
1587 (1996)

Lan, T. and Qian, 6., “Preparation of high performance polypropylene
nanocomposites.” Additives 2000. Clearwater Beach, FL (2000)

Song, K. and Sandi, G., “Characterization of montrnorillonite surfaces after
modification by Organosilane.” Clay Clay Miner., 49, (2), 119-125 (2001)

Le Pluart, L., Duchet, J ., Sautereau, H., and Gerard, J .F., “Surface modifications
of montrnorillonite for tailored interfaces in nanocomposites.” J. Adhesion, 78,
(7), 645-662 (2002)

Le Pluart, L., Duchet, J ., Sautereau, H., and Gerard, J.F., “Tailored interfaces in
nanocomposites.” Macromol. Symp., 194, 155-160 (2003)

Zhao, C., Feng, M., Gong, F., Qin, H., and Yang, M., “Preparation and
characterization of polyethylene-clay nanocomposites by using chlorosilane-
modified clay.” J. Appl. Polym. Sci., 93, (2), 676-680 (2004)

Miller, A. and Berg, J ., “The prediction of adhesion between polymer matrices
and silane-treated glass surfaces in filled composites.” J. Adhes. Sci. T echnol., l6,
(5), 495-507 (2002)

Miller, A. and Berg, J ., “Predicting adhesion between a crystalline polymer and
silane-treated glass surfaces in filled composites.” J. Adhes. Sci. T echnol., 16,
(14), 1949-1956 (2002)

Miller, A., Knowlton, M., and Berg, J., “The use of UNIFAC for the estimation of
adhesion enhancement between polymers and mineral surfaces treated with silane
coupling agents.” J. Adhes. Sci. T echnol., 14, (12), 1471-1484 (2000)

Garces, J.M., Moll, D.J., Bicerano, J., Fibiger, R., and McLeod, D.G., “Polymeric
nanocomposites for automotive applications.” Adv. Mater., 12, (23), 1835-1839
(2000)

Hong, C., Lee, Y., Bae, J., Jho, J., Nam, B., and Hwang, T., “Performance of

polypropylene/clay nanocomposites for automotive parts applications.” J. Ind.
Eng. Chem, 11, (1), 76-82 (2005)

114

83.

84.

85.

86.

87.

88.

89.

90.

91.

92.

93.

94.

Gilman, J ., “Flammability and thermal stability studies of polymer layered-silicate
(clay) nanocomposites.” Appl. Clay Sci., 15, (1-2), 31-49 (1999)

Reichert, P., Hoffinann, B., Bock, T., Thomann, R., Mulhaupt, R., and Friedrich,
C., “Morphological stability of poly(propylene) nanocomposites.” Macromol.
Rapid Comm, 22, (7), 519-523 (2001)

Okamoto, M., Sato, H., Taguchi, H., and Kotaka, T., “Shear-induced aggregation
behavior in lipophilized smectite clay/styrene suspension.” Nihon Reoroji. Gakk.,
28, (4), 199-200 (2000)

Kumar, S. and Jayaraman, K., “Process Induced Orientation of nanolayers in
Polyolefin Nanocomposites.” Society of Plastics Engrs. Annual Tech. Conf., 62,
1498-1502. Chicago, IL (2004)

Zhang, Q., Wang, Y., and Fu, Q., “Shear-induced change of exfoliation and
orientation in polypropylene/montmorillonite nanocomposites.” J. Polym. Sci.
Pol. Phys., 41, (1), 1-10 (2003)

Sekelik, D., Stepanov, E., Nazarenko, S., Schiraldi, D., Hiltrrer, A., and Baer, E.,
“Oxygen barrier properties of crystallized and talc-filled poly(ethylene
terephthalate).” J. Polym. Sci. Pol. Phys., 37, (8), 847-857 (1999)

Bafna, A., Beaucage, G., Mirabella, F., and Mehta, S., “3D Hierarchical
orientation in polymer-clay nanocomposite films.” Polymer, 44, (4), 1103-1115
(2003)

Kojima, Y., Usuki, A., Kawasumi, M., Okada, A., Kurauchi, T., Kamigaito, O.,
and Kaji, K., “Novel Preferred Orientation in Injection-Molded Nylon 6-Clay
Hybrid.” J. Polym. Sci. Pol. Phys., 33, (7), 1039-1045 (1995)

Schmidt, G., Nakatani, A.I., Butler, P.D., Karim, A., and Han, C.C., “Shear
orientation of viscoelastic polymer-clay solutions probed by flow birefringence
and SANS.” Macromolecules, 33, (20), 7219-7222 (2000)

Varlot, K., Reynaud, E., Kloppfer, M.H., Vigier, G., and Varlet, J., “Clay-
reinforced polyamide: Preferential orientation of the montrnorillonite sheets and
the polyamide crystalline lamellae.” J. Polym. Sci. Pol. Phys., 39, (12), 1360-1370
(2001)

Okamoto, M., Nam, P.H., Maiti, P., Kotaka, T., Hasegawa, N., and Usuki, A., “A
house of cards structure in polypropylene/clay nanocomposites under elongational

flow.” Nano Lett., 1, (6), 295-298 (2001)

Fasulo, P.D., Rodgers, W.R., and Ottaviani, R.A., “Extrusion Processing of TPO
Nanocomposites.” Polym. Eng. Sci., 44, (6), 1036-1045 (2004)

115

95.

96.

97.

98.

99.

100.

101.

102.

103.

104.

105.

106.

Dolgovskij, M.K., Fasulo, P.D., Lortie, F ., Macosko, C.W., Ottaviani, R.A., and
Rodgers, W.R., “Effect of Mixer type on exfoliation of Polypropylene
Nanocomposites.” SocietyI of Plastics Engrs. AnnuAaI Tech. Conf., 2255-2259.
2003. Nashville, TN

Gopakumar, T. and Page, D., “Compounding of nanocomposites by thennokinetic
mixing.” J. Appl. Polym. Sci., 96, (5), 1557-1563 (2005)

Solomon, M., Almusallam, A., Seefeldt, K., Somwangthanaroj, A., and Varadan,
R, “Rheology of polypropylene/clay hybrid materials.” Macromolecules, 34, (6),
1864-1872 (2001)

Cullity, B.D., “Elements of X-rav Diffraction. 2nd ed., Massachusetts: Addison-
Wesley (1978)

Krishnamoorti, R., “Rheology and structure of polymer layered-silicate
nanocomposites.” Abstr. Pap. Am. Chem. S., 218, U558-U558 (1999)

Krishnamoorti, R. and Giannelis, E., “Rheology of end-tethered polymer layered
silicate nanocomposites.” Macromolecules, 30, (14), 4097-4102 (1997)

Krishnamoorti, R. and Giannelis, E., “Strain hardening in model polymer brushes
under shear.” Langmuir, 17, (5), 1448-1452 (2001)

Aubry, T., Razafinimaro, T., and Mederic, P., “Rheological investigation of the
melt state elastic and yield properties of a polyamide-12 layered silicate
nanocomposite.” J. Rheol., 49, (2), 425-440 (2005)

Galgali, G., Ramesh, C., and Lele, A., “A rheological study on the kinetics of
hybrid formation in polypropylene nanocomposites.” Macromolecules, 34, (4),
852-858 (2001)

Auhl, D., Stange, J., Munstedt, H., Krause, B., Voigt, D., Lederer, A., Lappan, U.,
and Lunkwitz, K., “Long-chain branched polypropylenes by electron beam

irradiation and their rheological properties.” Macromolecules, 37, (25), 9465-
9472 (2004)

Ellis, TS. and D'Angelo, J.S., “Thermal and mechanical properties of a
polypropylene nanocomposite.” J. Appl. Polym. Sci., 90, (6), 1639-1647 (2003)

Oya, T., Polypropylene-Clay Nanocomposites, in Polymer Clay Nanocomposites,
T.J. Pinnavaia and G.W. Beall, Editors. (2000)

116

107.

108.

109.

110.

111.

112.

113.

114.

115.

116.

117.

118.

119.

Svoboda, P., Zeng, C., Wang, H., Lee, L., and Tomasko, D., “Morphology and
mechanical properties of polypropylene/organoclay nanocomposites.” J. Appl.
Polym. Sci., 85, (7), 1562-1570 (2002)

Cho, Y.G. and Kamal, M.R., “Estimation of stress for separation of two platelets.”
Polym. Eng. Sci., 44, (6), 1187-1195 (2004)

Milczewska, K., Voelkel, A. and Jeczalik, J. “The use of Flory-Huggins
parameters to characterization of polymer/filler interactions.” Macromol. Symp.,
194, 305-311 (2003)

Voelkel, A., Milczewska, K. and J eczalik, J. “ Characterization of the interactions

in polymer/silica systems by inverse gas chromatography.” Macromol. Symp.,
169, 45-55 (2001)

Wang, K., Liang, S., Du, R.N., Zhang, Q. and Fu, Q. “The interplay of
thermodynamics and shear on the dispersion of polymer nanocomposite.”
Polymer, 45, (23), 7953-7960 (2004)

Marchant, D. and Jayaraman, K., “Effectiveness Of PPMA Compatibilizers For
Nanolayer Dispersion In PP: Bound Vs Free Anhydride.” Society of Plastics
Engrs. Annual Tech. Conf., 61, 2200-2204. Nashville, TN (2003)

Voeks, J .F ., “Cohesive Energy Density and Internal Pressure of High Polymers.”
J. Polym. Sci. PartA, 2, 5319-5325 (1964)

Xu, L., Reeder, S., Thopasridharan, M., Ren, J., Shipp, DA. and Krishnamoorti,
R. “Structure and melt rheology of polystyrene-based layered silicate
nanocomposites.” Nanotechnology, 16, $514-$521 (2005)

Luciani, A., Leterrier, Y., and Manson, J., “Rheological behaviour of dilute
suspensions of platelet particles.” Rheol. Acta, 38, (5), 437-442 (1999)

Utracki, L., Favis, B., and Fisa, B., “Dynamic and Steady-state flow of
Polypropylene Systems.” Polym. Composite, 5, (4), 277-288 (1984)

Matsumoto, T., Hitomi, C., and Onogi, S., “Rheological Properties of Disperse
Systems of Spherical-Particles in Polystyrene Solution at Long Time-Scales.” T.
Soc. Rheol., 19, (4), 541-555 (1975)

Lertwilmolnun, W. and Vergnes, B., “Influence of compatibilizer and processing
conditions on the dispersion of nanoclay in a polypropylene matrix.” Polymer, 46,
(10), 3462-3471 (2005)

Van Gurp, M. and Palmen, J., “Time temperature superposition for polymeric
blends.” Rheology Bulletin, 67, 5-8 (1998)

117

120.

121.

122.

123.

124.

125.

126.

127.

128.

129.

130.

131.

Trinkle, S. and Friedrich, C., “Van Gurp-Palmen-plot: a way to characterize
polydispersity of linear polymers.” Rheol. Acta, 40, (4), 322-328 (2001)

Trinkle, 8., Walter, P., and Friedrich, C., “Van Gurp-Palmen Plot II -
Classification of long chain branched polymers by their topology.” Rheol. Acta,
41, (1-2), 103-113 (2002)

Galgali, G., Agarwal, S., and Lele, A., “Effect of clay orientation on the tensile
modulus of polypropylene-nanoclay composites.” Polymer, 45, (17), 6059-6069
(2004)

Miller, A. and Berg, J ., “Effect of silane coupling agent adsorbate structure on
adhesion performance with a polymeric matrix.” Compos. Part A-Appl. S., 34, (4),
327-332 (2003)

Nygard, P., Grundke, K., Mader, E., and Bellmann, C., “Wetting kinetics and
adhesion strength between polypropylene melt and glass fibre: influence of
chemical reactivity and fibre roughness.” J. Adhes. Sci. T echnol., 16, (13), 1781-
1808 (2002)

Deroover, B., Sclavons, M., Carlier, V., Devaux, J., Legras, R., and Momtaz, A.,
“Molecular Characterization of Maleic Anhydride-Functionalized
Polypropylene.” J. Polym. Sci. Pol. Chem, 33, (5), 829-842 (1995)

Marchant, D. “PhD Dissertgtion” Michigan State University, (2003)

Krishnamoorti, R., Ren, J., and Silva, A., “Shear response of layered silicate
nanocomposites.” J. Chem. Phys., 114, (11), 4968-4973 (2001)

Eastwood, E., Viswanathan, S., O’Brien, C.P., Kumar, D., and Dadmun, M.D.,
“Methods to improve the properties of polymer mixtures: optimizing

intermolecular interactions and compatibilization.” Polymer, 46, 3957-3970
(2005)

Su, 8., Jiang, D., and Wilkie, C., “Novel polymerically-modified clays permit the
preparation of intercalated and exfoliated nanocomposites of styrene and its
copolymers by melt blending.” Polym. Degrad. Stabil, 83, (2), 333-346 (2004)

Su, S., Jiang, D., and Wilkie, C., “Poly(methyl methacrylate), polypropylene and
polyethylene nanocomposite formation by melt blending using novel
polymerically-modified clays.” Polym. Degrad. Stabil, 83, (2), 321-331 (2004)

Kurian, M., Dasgupta, A., Beyer, FL, and Galvin, M.E., “Investigation of the

effects of silicate modification on polymer-layered silicate nanocomposite
morphology.” J. Polym. Sci. Pol. Phys., 42, (22), 4075-4083 (2004)

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