MICHIGAN STATEU msven

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This is to certify that the
thesis entitled
Development and Characterization of Porous,

Optically Clear, Metalorganic
Thin-film Coatings

presented by

Charles D. Gagliardi

has been accepted towards fulfillment
of the requirements for

 

M. S . degree in Chem. Eng .

 

\Z/ [7. Vex/11‘1“?

Major professor/

 

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MSU Is An Affirmative Actlon/Equal Opportunity Institution
chG-nt

 

 

DEVELOPMENT AND CHARACTERIZATION OF
POROUS, OPTICALLY CLEAR, METALORGANIC

THIN-FILM COATINGS

BY

Charles D. Gagliardi

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

MASTER OF SCIENCE

Department of Chemical Engineering

1992

ABSTRACT
DEVELOPMENT AND CHARACTERIZATION OF

POROUS, OPTICALLY CLEAR, HETALORGANIC,
THIN-FILM COATINGS

BY

Charles D. Gagliardi

The controlled hydrolysis of group IV 'd' block metal alkoxide
carboxylates (metal alkoxoacylates) at room temperature can provide
excellent coating materials for applications requiring porous, optically
clear, thin-film coatings. This work demonstrates a method for making
water soluble films from hafnium isopropoxide acetate complexes, and
compares the films and processing behavior associated with titanium,
zirconium, and hafnium.alkoxide acetates. Raman and IR spectroscopy are
used to characterize the materials. A series of titanium isopropoxide
carboxylates are also examined to determine the effect of carboxylic
acid chain length on film quality. This study shows that the titanium
isopropoxide valerate complexes produced stable, organic solvent and
water insoluble filmu. The Raman spectra for the various coating
solutions demonstrate their strong chemical similarity and dramatically
show the chemical effect of adding water. Slight differences among the
spectra of the hydrolyzed solutions may be correlated with differences

in film quality.

I dedicate this work to Charles and Leola Gagliardi,

two of the most patient, loving, and generous people I have ever known.

11

I am most grateful to my advisor, Dr. Kris A. Berglund, for his
invaluable guidance. The generous funding for my work provided by the
Center for Fundamental Materials Research (CPMR) and the Composite
Materials and Structures Center (CMSC) at Michigan State University, and
by the United States Department of Agriculture (USDA), is acknowledged
with much gratitude. The Michigan State University LASER Lab facility
provided me with the use of excellent LASER and spectroscopy equipment
which I depended upon for many of my experiments.

The work of Dilum.Dunuwila as an undergraduate research assistant
was a great and much appreciated contribution to this project. I would
also like to thank Dr. Tony Oertling and Dr. Mike Cerreta for their help
in learning Raman spectroscopy and proper laser operation and
maintenance, and Dr. Ron Lessard, Dr. Joel Dulebon, and Dr. Beatrice Van

Vlierberge-Torgerson for their consultations and advice.

iii

TABLE OF CONTENTS

Page
INTRODUCTIONOOOOOO0.0.000...OOOOOOOOOOOOOOOOOOO00000000000...0.00.000001

CHAPTER 1: MATERIALS AND PROCESSING METHODS..........................3
1.1 TITANIUM, ZIRCONIUM, AND HAFNIUM................................3
1.2 COLLOIDAL SUSPENSIONS AND GELS..................................5
1.3 SOL—GEL PROCESSING..............................................8

GROUP IV TRANSITION METAL ALKOXIDE SYSTEMS...................8
1.4 METAL ALKOXIDES.................................................9
1.5 METAL CARBOXYLATES.............................................13
1.6 ALCOHOLS, CARBOXYLIC ACIDS, AND ESTERS.........................14
1.7 METAL ALKOXIDE CARBOXYLATE DERIVED COATINGS....................16
1.8 COATING METHOD AND PRECAUTIONS.................................19

CHAPTER 2: SPECTROSCOPIC ANALYTICAL TECHNIQUES......................20
2.1mm AND RESONANCE RAMAN SCATTERING...........................20
2.2 INFRARED ABSORPTION............................................22

ATTENUATED TOTAL REPLECTANCE (ATR)22

CHAPTER 3: SOLUBLE COATINGS.........................................24
3.1 FILMS WITH ACETIC ACID.........................................24
3.2 COMPARISON OF GROUP IV METAL ALKOXIDE ACETATES.................24

”HAFNIUM METALLO-ORGANIC FILMS AND GELS
PRODUCED BY SOL-GEL PROCESSING”.............................25

CHAPTER 4: INSOLUBLE COATINGS.......................................35
4.1 THE ADVANTAGE OF TITANIUM FOR INSOLUBLE COATINGS...............35
4.2 PRE-RESONANCE RAMAN CHARACTERIZATION AND THE EFFECT OF

VARYING THE CARHOXYLIC ACID CHAIN-LENGTH ......................35
”CHARACTERIZATION OF METAL-ORGANIC FILMS FROM
TITANIUM ALKOXIDE CARBOXYLATE COMPLEXES”....................36

CHAPTER SWY ANDRBCOWNDATIONS.OOOOOOOOOOOOOOOOOO00......0.00.042

LIST orMERENGSOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO0.0.0.00000000045

iv

Table

Table

Table

Table

LIST OF TABLES

Page

Some properties of group IV transition metals.................4
Oxidation state, coordination, and stereochemistry............6
Boiling and points and degree of molecular association.......12

Structures and normal boiling points for selected alcohols,
carboxylic acids, and esters.................................15

LIST OF FIGURES

Page

Figure 1. SCI-gel reaCtionmchanismBOOOOOOOOO0.0.0.0000...00.0.0.0...10

vi

IITRODUCTIOI

This work investigates the controlled hydrolysis of titanium,
zirconium, and hafnium.metal alkoxide carboxylates at room temperature
to provide coating materials suitable for applications requiring highly
porous, small pore size, optically clear, thin-film coatings. The
significance of this research rests on the usefulness and variety of the
applications. The films could be used as separation process membranes
or as supports for catalysts or photocatalysts. Optically clear
catalyst support films could be used to allow vibrational spectroscopy
studies of the catalyzed reaction. Due to the ambient processing
temperature of these materials, many delicate organic macromolecules
could be incorporated into these films which would be destroyed by
higher processing temperatures. Thus, these materials could be used to
create a variety of optically probed chemical sensors.

The hydrolysis of metal alkoxide carboxylates may be viewed as the
sol-gel processing of a metal alkoxide using a carboxylic acid as a
chemical modifying agent. However, since the presence of the carboxylic
acid creates fundamental changes in the processing characteristics of
the precursor and the properties and chemical composition of the
product, I have chosen to treat metal alkoxide carboxylates in a
category of their own. Nevertheless, since the hydrolysis of metal
alkoxide carboxylates is clearly included within the very broad realm of

sol-gel chemistry, a brief review of this topic will be given. Metal

alkoxide carboxylates, otherwise known as metal alkoxoacylates, share in
the chemistry of both metal alkoxides and metal carboxylates; therefore,
relevant information concerning both these types of compounds will be
reviewed. Similarly, information concerning alcohols, carboxylic
acids,and esters will also be presented. The processing of the metal
alkoxide carboxylates can lead to materials which are highly soluble in
water (produced with acetic acid) or materials which demonstrate
insolubility in water and increased resistance to many organic solvents
(produced with valeric acid). Because of fundamental differences in the
processing methods for soluble and insoluble films, the two topics will
be treated separately.

On the topic of water soluble materials, a.mothod for making
films from hafnium isopropoxide acetate complexes will be demonstrated,
and the films and processing behavior associated with titanium,
zirconium, and hafnium alkoxide acetates will be compared.
Characterization of the materials will be accomplished with Raman and IR
spectroscopy.

The investigation of water insoluble films will involve an
examination of a series of titanium isopropoxide carboxylates to
determine the effect of carboxylic acid chain length on film quality.
This will show that the titanium isopropoxide valerate complexes
produced stable, organic solvent resistant and water insoluble films.
The Raman spectra for the various coating solutions will demonstrate
their strong similarity and will dramatically show the chemical effect
of adding water. Slight differences among the spectra of the hydrolyzed

solutions will then be correlated with differences in film quality.

CHAPTERI.

MATERIALS AND PROCESSING METHODS

Since this investigation is designed to characterize the soluble
and insoluble films which can result from the controlled partial
hydrolysis of metal alkoxide carboxylates, it is reasonable to begin by
assessing the basic materials and methods which control the properties
of the films. I will begin with the transition metals themselves, and
then proceed to briefly review the nature of colloids and gels, sol-gel
processing, alkoxides, carboxylates, and the alcohols, carboxylic acids,
and esters which are always involved in the production of our coating
solutions. Finally, I will discuss the coating solutions and methods

themselves.

W

Each of the coating materials generated and studied for this work
involves titanium, zirconium, or hafnium. These elements comprise the
group IV transition or 'd' block metals in the periodic table. To
distinguish the transition metal groups from the main element groups,
the CAS nomenclature uses an 'A' to designate the main groups and a 'B'
to designate the transition groups; the older IUPAC nomenclature is in

conflict with this convention. Throughout the remainder of this

document I will refer to the transition group IV elements simply as
'group IV' without further distinction.

Titanium is the most abundant element of the group, making up 63%
of the earth's crust. Among all the transition metals, titanium is the
second most abundant. Zirconium.represents roughly 1.6% of the crust,
while hafnium.comprises less than 0.03%. However, when compared to
other elements, hafnium is no more rare than cesium or bromine
(Greenwood and Earnshaw, 1984).

Some of the physical properties of Ti, Zr, and 8f are presented in
Table 1. Due to the ”lanthanide contraction”, the ionic radii of Zr and
Hf are virtually identical; the only significant difference between
these elements are their densities, transition temperatures, and neutron

absorbing abilities (Greenwood and Barnshaw, 1984).

Table 1. Some properties of group IV transition metals.

 

Property Ti 2r 8f
Atomic number 22 40 72
Number of natural isotopes 5 5 6
Atomic weight 47.88 91.22 178.49
Electronic Configuration [Ar] [Kr] [xgl4f14
3:124:32 «125:2 3424.2
Electronegativity 1.5 1.4 1.3
Metal radius/pm 147 160 159
Ionic radius (6 coord.)/pm M(IV) 60.5 72 71
M(III) 67.0
run) 86
up/Oc 1667 1857 2222
ap/Oc 3285 4200 4450
Bnthalpy of fusion/kJ/mole 18.8 19.2 (25)
Enthalpy of vaporization/kJ/mole 425 567 571
Density (25°C) g/cm3 4.5 6.5 13.28
Electrical resistivity (25°C) pohm cm. 42-0 4°00 35-1

 

(Greenwood and Barnshaw, 1984)

The metals are highly corrosion resistant. Except for hydrofluoric
acid, mineral acids have little effect on the dense oxide layer which
forms over the metal's surface. The group IV transition metals also
exhibit a large range of oxidation states and coordination numbers which
are shown in Table 2 (Greenwood and Barnshaw, 1984).

From Table 2, it is apparent that the group IV metals are capable
of showing a wide range of complex chemical behavior. It is clearly the
richness of this chemistry that will enable us to adjust the properties

of our coating materials to suit the needs of specific applications.

W

A colloid has been defined as a suspension of a finely dispersed
phase (~1 - 1,000 nm) not significantly effected by gravitational
settling, with interaction dominated by short-range forces such as von
der Waals attraction and surface charges. The dispersed phase also
characteristically exhibits Brownian motion, due to random collisions
with molecules of the supporting medium (Brinker and Scherer, 1990).

Colloidal suspensions in a gaseous medium are termed aerosols;
colloidal liquid droplets in a liquid medium are emulsions. A sol,
distinct from.both aerosols and emulsions, is defined as a colloidal
suspension of a dispersed solid phase in a liquid medium, where the
"solid“ can be either particulate (non—polymeric) or polymeric (Brinker
and Scherer, 1990).

Many polymeric systems could also be legitimately considered true

solutions rather than sols, and so the terminology chosen often depends

Table 2. Oxidation state, coordination, and stereochemistry

 

Oxida-
tion Coordi- Stereochemistry Ti Zr/Bf
state nation
-1 (d5) 6 Octahedral [Ti(bipy)3]‘ --
0 (d4) 5 Octahedral lTi(bipy)3l [Zr(bipy)3l
2 (d2) 6 Octahedral TiC12 Layer structures
and clusters
12 " [Tilfls-C535I2
(C°)2l
3 (d1) 3 Planer [T1{N(31M03)2}2
(COIZI
5 Trigonal bipyr. [TiBr3(NMe3)2]
6 Octahedral [T1(urea)6]3+ zrx3 (Cl: Br,
I),BfI3
4 (do) 4 Tetrahedral TiCl4 ZrCl4(g) (solid
is octahedral)
5 Trigonal bipyr. [T10C12(N“93I21 --
Square pyramidal [TiOC1412‘ --
5 Octahedral [TiPSIZ- [er612-
,ZrC14(s)
7 Pentagonal [T1C1ISZCNH92I31 --
bipyramidal
Capped trigonal [TiP5(02)]3' [2:231315-
prismatic
8 Dodecahedral [Ti(fl2-NO3)4 [2r(c204)4]4-
Square antipris- -- Zr(acac)4
matic
11 -- [Ti(fl5-C535)(Sz
CNM92)3]
12 -- --

[M(n3-BH4)4

 

(Greenwood and Earnshaw, 1984)

upon the context. The kinetics must also be considered; a solution of
monomers and oligomers may gradually evolve into a polymeric sol, and
eventually become a particulate sol or gel. There is no exact time when
the polymer becomes large enough for the solution to be considered a
sol, and the choice of terminology in describing the initial stage may
well depend on the kinetics of the reaction.

A gel is a substance characterized by the presence of a continuous
solid skeleton wet by a continuous liquid phase. Gels can result from
the growth of branched or cross-linked polymers, the entanglement of
polymeric molecules, or the formation of particulate networks held
together by von der Waals forces. Gels can be clear, cloudy, or opaque,
and the liquid phase of the gel can exist as a solution, polymeric sol,
or particulate sol. The process of gelation generally begins with the
formation of fractal aggregates that grow into clusters that link
together, as sometimes described by the theory of percolation, until a
spanning cluster extends throughout the sol (Brinker and Scherer, 1990).
In polymeric systems, the theory of bond percolation seems most
applicable to simple cases where a monomer forms bonds at random between
neighboring molecules. Complex chemical systems may not be so easily
adaptable to this method of modelling.

When gels are allowed to dry by the evaporation of the liquid phase
into a surrounding gas phase, the process is accompanied by significant
shrinkage and xerogels (xero 8 dry) result. These xerogels are often
highly porous and have been used as catalyst supports and filtering
media. Gels can also be dried under supercritical conditions, producing
aerogels which are significantly less dense than the corresponding

xerogels.

8
W

A variety of systems which exhibit sol formation and gelation can
be very useful in the manufacturing of homogeneous powders, radioactive
oxide particles, ceramics (including fibers and multicomponent glasses),
films, coatings, and monoliths. The recent popularity of the sol-gel
approach is attributed to the demonstration by Yoldas (Yoldas, 1975 and
1977) and Yamane and coworkers (Yamane, et. al., 1978) that monoliths
can be made by carefully drying gels. The promise of a structurally
solid and useful glass produced at room temperature proved to be more
dream than reality, and monoliths remain the least important sol-gel
products (Brinker and Scherer, 1990).

Sol-gel systems have been made by a diverse array of inorganic and
metalorganic precursors including transition metal esters and salts,
aluminates, borates, and silicates. Silicates are the single most
common precursors, and alkoxides, in general, are used more than any
other starting material for the sol-gel process. The general usefulness
of alkoxides results from their reactivity with water; the partially
hydrolyzed alkoxides can then react together to form dimers, chains, and
rings through condensation reactions which free additional water and

allow further hydrolysis (Brinker and Scherer, 1990)

GROUP IV TRANSITION METAL ALKOXIDE SYSTEMS

The sol-gel process proceeds through the effect of hydrolysis,
alcoxolation, olation, oxolation, alkoxy bridging, and other
nucleophilic association mechanisms. Due to the lower electronegativity
of these transition metals, the group IV transition metal alkoxides are

more susceptible to hydrolysis and condensation than silicon alkoxides.

The suggested mechanisms for these reactions have been summarized in
Figure 1 (Brinker and Scherer, 1990). At this point, it should also be
noted that olation, as shown in Figure 1, only occurs when the
coordination number of the metal has been expanded beyond its oxidation

number.

WIPES

It has already been noted that industrial applications of metal
alkoxides (also named as metal esters) include sol-gel processing of
ceramic and metalorganic powders, films, and fibers. However, these
important metalorganic compounds are also used as cross-linking agents
and catalysts for a variety of organic reactions. Of the group IV
transition metal alkoxides, the titanates have undoubtedly been used
most frequently for catalysis. Esters and polyesters (weber, 1962;
Jaruzelski and Sheppard, 1963; Stevens and Gardner, 1965; Case,1968;
Marzocchi,1972; Haslam, 1958; and Temin and Baum, 1967), polysiloxanes
(Gaillissen and Gancberg, 1956; Madaras, 1958; Kohn and Guez, 1961;
Hartlein and Olson, 1972; Rauner and Tyler, 1962), polyolefins (Anderson
et. al., 1958; Arnold and Foster, 1960; Coover, 1960; Dawes and
Winkler,1964; Sianesi and Caporiccio, 1966; Orzechlowski and MacKenzie,
1967; Hogan and Witt, 1971; Hiroyuki, 1972), polyformamide (Beek and
Pijpers, 1972), polycarbonate (Reynolds and Dunham, 1972), and
polyurethanes (Lawham, 1965; Smelts, 1969) have all been made via
titanate catalyzed reactions (Du Pont, TYZOR Bulletin).

The catalysis of ester formation from alcohols and carboxylic acids

plays an important role in the processing of coating materials derived

10

H R:
3—0 + M-O-R —. l3:0: _. M—OR _. HO-M- ._ 0<3
I .

1-Q»IF43H + RCDH

mm
M\ R-
M—o+N-o-R—s 3:—oM—OR_.M—O—M.—o<a
a
_p M—O—M +ROH
meme
fix a-
M—o+M-o-a—s gz—oM—OHflM-o—MO—Ka
a

‘M—O—M'I'HO

 

H

B |
N—oa+M.—o<: qM-O-M+ R—OH

H

I

M—OH+Mo—O<:-IO“-°-“+ 3°

 

 

 

Figure 1. Sol-gel reaction mechanisms.

11

from metal alkoxide carboxylates. It is especially important to note
that along with the ester, water is generated which can further
hydrolyze the metalorganic titanium.

Many group IV 'd' block transition metal alkoxides can be produced
by reacting the metal chloride with an alcohol in the presence of

ammonia:

ac14 + 430R + 4M3 —> “-03” + 4NH4C1
One or more of the alkoxy groups can be changed on an alkoxide by an
alcoholysis reaction which is often employed to synthesize alkoxides
with long or bulky alkoxy groups; alkoxy groups can also be switched by
transesterification, in which the new alkoxy group is supplied by an

organic ester rather than a alcohol. The overall effect of alcoholysis,

N(-0R)4 + HOR' _, (R'O-)M(-OR)3+ HOR or

M(-OR)4+4BOR' _, M(-OR')4 + 480R

can be equivalent to the effect of transesterification:

- + . _ - +
M( 0R)4 4C83COOR _, in OR )4 4CH3COOR
When used in alkoxide synthesis, alcoholysis or transesterification is
generally driven to completion by removing the product alcohol or ester
by azeotropic distillation in an organic solvent. However, it is
important to realize that these reactions take place to some degree

whenever alcohols or esters are mixed with an alkoxide.

12

In comparing the isopropoxides of titanium, zirconium, and hafnium,
it should be noted that the titanate is a liquid at room temperature,
whereas the zirconium and hafnium isopropoxides are solids. It should
also be mentioned that, among tertiary alkoxides, volatilities increase
in the order Ti < Zr < Hf; due to the bulkiness of their alkoxy groups,
these compounds exist as monomeric species -- showing a drastic
reduction in the tendency toward coordination expansion and bridging
associations (Bradley et. al., 1952 (b)). Normal alkoxides (Bradley et.
al., 1953) and shorter chain secondary alkoxides (Bradley et. al., 1952
(c)) show higher values of molecular complexity (degree of oligomer—
ization). The boiling points and degree of molecular association are

shown in Table 3 for several alkoxides:

Table 3. Boilingfiand points and degree of molecular association.

 

 

Alkoxide Boiling Molecular
point Association
°C/5 mm
Ti(OEt) 138.3 2.4
Ti(opri) 91.3 1.4
Ti(OBut) 93.8 —-
Ti(OAmt) 142.7 1.0
2r(OEt) 234.8 3.6
2r(opri) 203.8 3.0
Zr(OBut) 89.1 1.0
Zr(OAmF) 138.4 1.0

 

(Mehrotra, 1967)

When comparing alkoxides to determine the effect of the central metal on
molecular complexity, zirconium and hafnium were found to produce a
greater tendency toward oligomer formation than titanium, This was
attributed to the larger size and maximum covalency of the heavier atoms

since the oxygen-metal intermolecular binding is thought to be quite

13

similar for these metals (Bradley et. al., 1952). The characteristics
of the titanium, zirconium, and hafnium alkoxides have been studied and
reviewed by Bradley and coworkers in several early sources (Bradley and
Wardlaw, 1951; Bradley et. al., 1952 (a, b, c), 1953, and 1978).
Excellent reviews by Mehrotra (1967) and Livage and coworkers (1988)

also contribute to this area.

1i§i___lIIAL_§ASIQXXLATEfi

Metal carboxylates are also known as soaps. Titanium, zirconium,
and hafnium carboxylates can be produced by reacting the carboxylic acid
anhydrides with an appropriate metal alkoxide (Mehrotra, 1967).
Generally, tetrasoaps cannot be obtained from the reaction of a
carboxylic acid with these alkoxides because of the tendency to form
oligomers. As alcohol is liberated, an ester and water are formed which
initiates the oligomer formation. In the case of titanium, oxide-
carboxylates tend to ultimately result from this reaction (Mehrotra,
1967).

zirconium tetrasoaps can also be derived from the reaction of the
metal chloride with a carboxylic acid. Since the carboxylates are
bidentate ligands, the zirconium is expected to be eight coordinate.
When allowed to react with zirconium isopropoxide, carboxylic acids

produce a dimer by the following reaction (Mehrotra, 1967):

2214029), 4» 7RCOOH —- (Rcoohzsrozz-(oocn)3 + 79908 + Rcoom:1

14

Some interesting zirconium IV carboxylate complexes include Zr(OZCR)4,
ZrO(02CR)2(H20)x, an~ ZrO(OH)(02CR)(HZO)x. Not surprisingly, the
solubility decreases in water and increases in hydrocarbons as the alkyl
chain-length increases (Mehrotra, 1983).

Evidently, Ti(IV) tetracarboxylates do not form easily since a
large number of attempts to synthesize them had failed before success

was eventually attained. Several mixed carboxylates have also been

isolated: (o-OC5H4C02)Ti(OPri)2, (o-HOC5H4C02)2Ti(OPri)(OZCR), and (o-
HOCGH4C02)Ti(OPri)(OZCR)2, where R-C11823, C15831, and C21843.
Heterocyclic Ti(IV) carboxylates have also been reported having a
structure described by: (PriO)4_nTiLn, where n=1-3 (Mehrotra, 1983).
Both Ti(II) and Ti(III) carboxylates also exist, and many of these

compounds are darkly colored (Mehrotra, 1983).

W

The reactions used to generate the alkoxide carboxylate coating
solutions involve, as reactants or products, several organic compounds.
These compounds are alcohols, carboxylic acids, and esters, and some of
their important properties should be discussed.

It is commonly known that in aqueous solution the acidity of an
alcohol decreases as the alkyl chain length increases. Although this
phenomenon has been incorrectly attributed to an inductive effect in
some older organic chemistry textbooks, it can more properly be
explained by differences in the solvation energy of the anion. In
dimethyl sulfoxide, a poor solvating agent for anions, the acidities are

equivalent (Allinger et. al., 1976). It is also interesting to note the

15

intrinsic acidity of alcohols in the gas phase shows that the acidity

increases slightly with the length of the alkyl chain (Brauman and

Blair, 1968).

A similar trend can be observed among carboxylic acids.

The intrinsic acidity in the gas phase increases from acetic to

propionic to butyric acid.

While, in aqueous solution, acetic acid is

strongest and propionic acid is weakest among the three acids (Jen and

Thomas, 1975; Yamdagni and Kebarle, 1973).

Furthermore, it is important

to note that the insoluble metalorganic coating solutions are

predominantly solvated by excess carboxylic acid and are not expected to

act like aqueous solutions.

The structures, names, and boiling points of some common alcohols,

carboxylic acids, and esters have been given in Table 4.

The boiling

Table 4. Structures and normal boiling points for selected alcohols,
carboxylic acids, and esters.

 

 

Name Structure Normal Boil Point
Methanol CH3OH 64.7
Ethanol CB3C32OH 78.3
n—propanol CH3CH2CHZOH 97.2
iso-propanol C33C3(03)C33 82.3
n-butanol CH3(CH2)3OH 117.7
iso-butanol ((333)2030320a 107.9
sec-butanol C33C32CHIOBIC33 99.5
t-butanol (CH3)3COH 82.5
acetic acid CH3COOH 118.
propionic acid (3330323308 141.
butyric acid C53(C32)2C°°a 163.
valeric acid CH3(CH2)3COOH 187.
hexanoic acid C33IC32I4C°°3 205.
octanoic acid CH3(CH2)5COOH 237.
n-propyl acetate CH3COOC82CH2CH3 101.6
iso-propyl acetate C33C°°C3(C33)2 88.4
n-propyl n- C33IC32)3C°°C32C32C33 167.5
valerate

iso-propyl n- CH3(C52)3C00C3(C33)2 --------
valerate

 

Data on alcohols and carboxylic acids taken from Allinger (1976); data
on esters taken from Lange's Handbook of Chemistry, 11th ed. (1974).

16

points of the alcohols, carboxylic acids, and esters are of general
importance to the synthesis of metal alkoxide carboxylates and
polyalkoxoacylates because the unwanted reaction products and excess
reactants are often removed by distillation. In our work, however, the
metalorganic coatings were produced at ambient temperatures and no side

products were removed.

 

Among group IV transition metal alkoxide carboxylates, Ti complexes
have been studied the most. Two Du Pont patents describe the chemistry
involved when the carboxylic acids are mixed with the titanium alkoxides
and allowed to react extensively, with or without the addition of water
(Feld and Cowe, 1965). Without the addition of water, sufficient
reaction leads to metalorganic polymers with 1, 3/2, or 2 carboxylate

groups per titanium atom (Langkammerer, 1952):

 

 

on
. . I .
n(RO)4'I| + 2:112 coon—u —o- “n- + nR coon + 2nRon
(Ion
— .I n
0R ocon'

l I
2 n(R0)4Ti + 5 nR'COOH—‘ - o—ri - o-ITi- 0- + 2 nR'COOR + 5 nROH
000w 000w

 

 

ocon'
I
n(RO)4Ti +3nR'coon—- —Ti-O- + nR'COOR + 3nROH
I
ocox'
n

17

The ratio of the combined reactants determines the number of isopropyl
groups to be replaced with carboxyl groups. With the addition of a
sufficient quantity of water, only one carboxyl side group is retained
per titanium atom and the isopropyl groups are replaced by hydroxyl
groups (Haslam, 1952):

OH

-'OF'TV_

OCOR'

 

 

In both patents, higher chain-length carboxylic acids are preferred and
the waxy products are soluble in organic solvents. When films are cast
from these materials, the coating is transparent but soft. The films
scratch easily when wiped with a Kimwipe tissue.

The interaction of titanium alkoxides with acetic acid has received
special attention due to the interest in using the products as a Ti02
coating precursor. These films have limited utility in direct
applications requiring the unfired film to contact water or organic
solvents; however, the films can be used with dry gases. Films made
from alkoxide acetates are ideal for applications requiring easily
removable or soluble membranes. The formation of these materials are
often treated in the context of sol-gel chemistry in which the acetic
acid is regarded as a modifier of the metal alkoxide precursor (Livage,
1986; Doeuff, et. al.,1987; Livage, et. al., 1990; Sanchez, et. al.,
1988 (a, b) and 1990, Sanchez and Livage, 1990). The acetic acid
expands the coordination number of the Ti to 5 and 6, where the acetate
groups act as bridging and chelating bidentate ligands (Sanchez, C., et.

al., 1988 (a)). Sanchez and coworkers postulate the formation of the

18

Ti(OR)4(AcOH) transition state from the nucleophilic addition of the

acid followed by the subsequent removal of an alcohol molecule, leaving
Ti(OR)3(OAc) (Sanchez et. al., 1988 (a)). The presence of acetic acid
favors polycondensation rather than particulate sol formation. Since
the alcohol group is more easily removed by hydrolysis than the acetate
group, a greater number of acetates leads to a slower rate of
hydrolysis. The first stage of hydrolysis may remove —OR groups while
the second stage breaks bridging acetate bonds to form chelating
bidentate and monodentate ligands. The final stage of hydrolysis would
leave only -OH groups and monodentate acetate groups on the polymer
backbone (Doeuff, et. al.,1987).

The formation of complex oxo—alkoxides also proceeds from the
reactions of alkoxides and acetic acid. These oxo-alkoxides are thought
to develop as side-reaction byproducts which do not participate in the
polymerization process (Sanchez et. al., 1990). One such structure

which has been isolated and crystallized from the reaction of Ti

isopropoxide and acetic acid is Ti5(u-O)2(u3-O)2(uz-OAc)4(u2-
OPri)4(OPri)5. This compound is believed to originate from the
Ti3O(OPri)9(OAc)3 trimer which is then believed to release a CH3COOPri
molecule leaving Ti3(OPri)9(OAc)2. The further removal of a -OPri by
hydrolysis would then be followed by oxolation in which two
Ti3(OPri)7(OAc)ZOH species combine to release two isopropanol molecules
and the final oxo-alkoxide product (Sanchez, et. al., 1990). These
studies demonstrate the type of chemistry which can be involved in

alkoxide and carboxylic acid reactions.

19
W

The coating solutions are applied to dry, chemically cleaned glass
or quartz (Micro cleaning solution) and spun dry at approximately 1100
rpm. This spin-coating technique has been used to produce excellent
films from a variety of coating formulations. The coating material
probably bonds to the silicon in the surface of the glass via Ti-O-Si
linkages; if the coating solution is allowed to age too long before use
or if it is prepared at significantly elevated temperatures, the
resulting film is softer and shows poor adhesion. Thus, kinetics and
processing temperatures are very important. Some properties of the
films may also be affected by exposure to UV radiation. Therefore care
should be taken to protect the films from long term exposure to light

for some applications.

CHAPTER2.

SPECTROSCOPIC ANALYTICAL TECHNIQUES

Raman and IR spectroscopies have been very useful in characterizing
the coating solution. In this section a brief introduction will be
provided for these powerful and complimentary vibrational spectroscopy

techniques.

W

Raman spectroscopy is still less ccnmon than IR absorption, so it
will be discussed in somewhat greater detail. Raman scattering provides
information which is qualitatively similar to IR absorption: the bands
correspond to rotational and vibrational transitions. However, some of
the transitions which are quantum mechanically allowed for IR absorption
are not allowed for Raman scattering, and vice versa. Thus, the two
techniques can be regarded as complimentary rather than redundant. This
is especially true for simple, centrosymmetric molecules because their
selection rules are mutually exclusive; no transition which is active
for one of these two spectroscopy techniques is also active for the
other.

Raman experiments require a very bright monochromatic light source
which is ideally provided by a laser. Fluid samples are usually

illuminated with a light beam oriented 90° with respect to the path

20

21

toward the collection optics; solid samples are often studied in a
180°, backscattering configuration. Typically, the scattered radiation
is directed through a scanning monochrometer to a sensitive detector,
such as a photo multiplier tube (PMT), connected to a recording device.
Alternatively, the scattered light is taken through a narrow band pass
filter (such as a double monochrometer with gratings in subtractive
dispersion), then dispersed to illuminate an optical multichannel array
(OMA) detector which can analyze the entire spectral region illuminating
its surface.

The wavenumbers of the incident radiation ( q; ), a Raman band
V”), and the associated molecular vibrational transition ( V’u ) are
related such that V":- V’, t \T .3. When V" is greater than V", , V" is
an anti—Stokes band; otherwiseV" is a Stokes band. Stokes Raman
scattering is generally much more intense than anti-Stokes because the
Stokes transitions originate from the well-populated ground state;
whereas anti-Stokes scattering originates from vibrational excited
states which are poorly populated at room temperature in the absence of
electromagnetic stimulation. Raman scattering occurs due to an
oscillating molecular dipole which is induced by the incident light.
Raman active vibrational transitions occur when the vibration involves a
change in the polarizability tensor a. The induced dipole moment, P,
depends upon the electric field of the incident radiation, E, and a

such that P = a ' E. If Q): represents the kth normal coordinate, then

the th normal mode will be Raman active only if

6a..
In - I] i 0'
M)" (60 I

k

 

22

for some element (1 U of a. The subscript '0' denotes evaluation at
the equilibrium position, and the term ((1 (3),‘ represents an element in

the derived polarizability tensor for the kth normal coordinate, a," .

Resonance Raman scattering occurs when the incident radiation is
coincident with an electronic absorption band of the sample. The
resulting enhanced signal is 103-10‘ times more intense than normal
Raman scattering. An explanation of this phenomena using Kramers-

Heisenberg-Dirac dispersion theory has been reviewed by Dophin (1978).

W

When light is absorbed in the infrared (IR) region, the energy of
the absorbed radiation coincides with the energy required for rotational
and vibrational transitions. When observing vibrational transitions in
fluid samples, the vibrational bands appear broadened by the closely
spaced rotational transitions which occur at each vibrational state.

IR spectroscopy is widely used in the identification of organic
compounds. The various functional groups have characteristic absorption
bands which aid chemists in determining the structure of an unknown

molecular species.

ATTENUATED TOTAL REFLECTANCE (ATR)

Attenuated total reflectance (ATR) spectroscopy allows an IR
absorption spectrum to be taken at the surface of an ATR crystal or
internal reflection element (IRE). The IR light passes through the
crystal, being ”internally reflected" back and forth between opposing

faces. However, with each ”internal reflection”, the light protrudes

23

slightly from the surface allowing it to sample the external
environment. The depth of the light's penetration depends upon the
entry angle and the relative indices of refraction for the ATR crystal
and the surrounding medium. This is a very useful technique for
examining many samples which would otherwise be difficult to analyze.
The ”circle cell“ configuration is especially useful for testing aqueous
solutions with minimal IR absorbance from the water. The theory of ATR
spectroscopy and practical information about various ATR configurations

can be found in a text by Harrick (1967).

CEAPTER3.

SOLUBLE COATINGS

3a1a___IILN§_IIEI_AGITISLAGID

Soluble coatings have been made from mixtures of acetic acid with
titanium.isopropoxide, zirconium n-propoxide, and hafnium isopropoxide.
The coating solutions were formed by first reacting the alkoxide with
acetic acid, and then dissolving the product in water. The aqueous
mixtures were then used to coat glass substrates. In general, it was
possible to make fairly stable sols from the zirconium and hafnium
alkoxides which produced excellent films. The titanate based coating
solutions showed a greater tendency to polymerize, and only produced

good films when not excessively aged.

 

Although hafnium and zirconium alkoxides with identical alkoxide
groups should behave almost identically, it is not surprising that the
zirconium.n-propoxide showed significant differences from the hafnium
isopropoxide isopropanol solvate or isopropanolate which was used to
prepare the hafnium-based films. Alcohols are not inert solvents, and
they significantly change the reactivity of the alkoxide. Furthermore,

the less bulky n-propoxide groups offer less protection to the metal

24

25

atom allowing a greater tendency toward intermolecular association and
coordination expansion and a lower tendency toward linear
polymerization. Both the zirconium and hafnium preparations produced

films which were superior to titanate derived films.

SHAZNIHM METALLQ:QBGANIQ.EILH§.AND.§EL§.EBQDHQED.HX fiflLzfiflh EBQQBSEINQL
A comparison of these soluble, sol-gel processed coating solutions

and their Raman and IR characterization is contained in a previously

published paper (Gagliardi and Berglund, 1989) and is reprinted here in

its entirety with the permission of the original publisher:

HAFNIUM METALLO-ORGANIC FILMS AND GELS
PRODUCED BY SOLoGEL PROCESSING

CHARLES D. GAGIJARDI AND KRIS A. BERGLUND _
Michi State University, Department of Classical Engineering and
Carta'for mrdamsntalMatcrialsRcscarch,Eastlansing.W 48824

ABSTRACT

Mcnflougardcmtuiflsamoficnuscdupmnmfmhiglflynfiwmryoxidcsandhigh-
temperature composites. The feasibility of producing hafnium uremic-organic films and gels by
sol-gel techniques is demonstrated in the present study. Since hafnium alkoxides are not
commercially available. their preparation is an obstacle in the development of their sol-gel
processing applications. A common synthesis technique was used to produce hafnium
isopropoxide. which was subsequently hydrolyzed undcr acid catalyzed conditions to produce
films and gels. The films were prepared by spin casting and remained optically transparent
Initially, the gels were also optically transparent. but became translucent upon drying. Raman,
IR. and light scattering spectroscopies were used to characterize this sol-gel process. and it is
comparedwithdrcanalogouspmccssingofdtaniumanda’rconiumalkoxidea

INTRODUCTION

Alkoxide: of titanium and zirconium have received increasing interest as precursors for
powdered ceramics. sols. gels, and film coatings. Substantially fewer studies have been
undertaken with hafnium alkoxides, partly due to the lack of any commercial som'cc. Hafnium
oxide films have been made from a hafnium alkoxide using low-pressure chemical va
deposition (CVD) [l]. and ion-sputtering [2]. Our crurcnt work is aimed at producing are
organic films which can be prepared by sol-gel processing at room pressures and temperatures.
and characterizing the process.

Livagc has shown that optically clear titanium monoliths can be made from TKO-Bu“), when
hydrolyzed with butanol and acetic acid [3]. Clear solutions have been made from the hydrolysis
ofazirconiumalkoxidcwithaccticacid[4],andtransparcnt.colorlcssgclshavcbeatmadsfrom
mixtures of titanium and zirconium alkoxides with acetic acid. water, and metal acctam [5]. In
addidon,acedcacidhasalsobocnuscdformakingfilmswithmixnncsof1cad, zirconium.and
titanium alkoxides [6]. Since acetic acid has been used successfully in these similar alkoxide
systems, we investigated the effect of acetic acid on the hydrolysis of hafnium isopropoxide.
The isopropoxide was chosen for direct comparison to titanium isopropoxide [3] and for its
established synthesis [7-10].

The hafnium isopropoxide synthesis is presented. along with a description of the sol-gel
processing for the formation of the sols, gel monoliths, and films. The acetic acid and metal
alkoxide systems were compared for hafnium. zirconium. and titanium alkoxides. Light
scattering was used to characterize sol stability and gelation rates. Infrared and Raman
specn'oscopicswcrcusedtoprobcthcstrucnncofsolsandgcls.

MATERIALS AND METHODS

Hafnium isopropoxide was prepared b the direct action of ammonia on hafnium
mnachloridcindtcprsscnccofisopmpanolastzsaibodbyMcIuonlll]:

MCI. + nROI-l + nNI-l, --> M(OR), + MC!

26

27

The reaction was carried out in excess benzene to keep the product in solution. as in previous
syntheses [8.9]. Twenty-five grams of HfCl‘ were dissolved in 350 ml benzene and 35 ml
isOpropanol. The benzene and the isopropanol were dried as described below. and the benzene
was added to the fifth prior to the addition of isopropanol. An excess of NH3 was bubbled
through a gas dispersion tube while rapidly stirring the solution. The white NH4Cl was removed
by filtration through a fine glass frit. rinsing the solid with two ~30 ml portions of benzene. The
excess NH; and solvents were driven off under vacuum with the gradual application of heat.
The product was refluxed at ~170°C/0.35mm Hg. but was not purified by distillation. The
mixture was then cooled and returned to atmospheric presure. About 60 ml of dry isopropanol
was then added and the flask was slowly reheated until the solid melted. As the product was
slowly cooled. it formed a wet slurry which could be easily removed from the flask. Although
this slurry/paste was used directly in some initial experiments. for subsequent preparations it was
transferred to other containers and dried under vacuum to give a white solid. The entire
synthesis was performed under vacuum or argon atmosphere to avoid moisture. and the product
was stored under argon. The hafnium isopropoxide produced in the manner described still
contains isopropanol. Complete removal of the isopropanol requires drying at 120-130 °C under
vacuum of 0.5mm Hg [8]. Since the material used in the current study was not dried under these
conditions. it should more correctly be referred to as hafnium isopropoxide isopropanolate to
refiecr the residual isopropanol content.

Titanium isopropoxide. NH3. and HfCl‘ were used as purchased from Aldrich Chemical
Company with no further purification. The NH, was anhydrous. 99.99% pure. and packaged
in a lecture bottle. Zirconium n-propoxide was purchased from Alpha Products of Morton
Thiokol. with no further purification. However. the zirconium compound had an amber color
which was possibly due to an impurity. as further evidenced" by the observation of a fluorescent
baseline in its Raman spectrum. lsopropanol was refluxed over fieshly prepared CaO. distilled.
refluxed with magnesium filings. and redistilled just prior to its use. Benzene was refluxed over
Na metal until the benzoquinone indicator appeared grayish blue. then distilled as needed.
Glacial acetic acid was purchased from Fisher Scientific and used without further purification.
Kimax brand microscope slides and commercial grade fused silica windows (ll8”T x 3/4"D)
were used as substrates for films and were pretreated by rinsing with acetone. De-ionized water
witharesistanceof18 Mohmwasusedthroughout.

INSTRUMENTION

The light scattering experiments were performed on a Coulter sub-micron particle size
analyzer. model N4MD. Polystyrene and acrylic cuvetts were used to hold the samples. Raman
spectra were collected on a Spex 1877 triple spectrometer using an OMA detector. The Raman
spectrawererecordedusingthe514.5 nmlinefromanargon laser. TheIRtransmission spectra
were taken on a Perkin-Elmer model 1750 F'l'lR. The samples were held in a short-path liquid
cell holder with NaCl windows.

PROCESSING

(a) SolsandGels

Gels made from titanium isopropoxide. acetic acid. and water were compared with the
zirconium n-propoxide and hafnium isopropoxide analogues. In each case. the alkoxide was
first mixed with the acetic acid. producing a strongly exothermic reaction which resulted in a
colorless. transparent liquid. These mixtures were usually used immediately in the next step of
the sol preparation; however. if allowed to age. they all solidified. The hafnium mixture
solidified most quickly (< 20 minutes). the zirconium complexes solidified next (~l hour). and
the titanium complexes were slowest to solidify (a few days). The solids formed emit a
characteristic esrer odor; the generation of an ester during the sol-gel processing of Ti(O-Bu'm
with acetic acid was also noted by Livage [3].

28

The next step of the process involves
combining these complexes with water. If the
complex is still in a liquid phase at the time of
this addition. it my rapidly solidify. become a
white paste. form translucent clumps. or
remainaclearliquidadependingonthetype
of complex and the molar ratios. As
additional wata is added. the solidcan usually
be redissolved. The liquid was passed
througha0.44 micron filterandstoredinvials
orincappedcuvetts. ‘lhebasic stepsinthe
sol-gel process are summarized infig. 1.

(Malta:

The films were made by spin-casting the
solutions described in the previous section.
The coating liquid was applied to the surface
of the substrate. which was then spun to
remove the excess. resulting in a relatively
even film. Films were also made by
drOpwise addition of the liquid onto a
spinning substrate. or by angling the surface
of the substrate and allowing the excess-
coating material to drip off.

   
   
 

 

  

Alkoxide
Carboxylat
Complexes

 
 

 

 

 

 

 

quuld

 

 

 

Figure 1. Schematic of Sol-Gel Process

RESULTS AND DISCUSSION

When sufficientacetic acidwasmixedwith titanitnnorhafniumisopropoxide.orzireonium
n—propoxide. a water soluble product resulted. For titanium alkoxides. Livage has desuibed this
general complex as ‘l’i(OR),(Ac), [3]. In the case of zirconium. several carboxylate complexes
have been made by reacting alkoxides with carboxylic acids [ll]: ((RC02)3Zr)20.
21(02CR)(OPr‘)3. and Zr(02CR )2(OPr')2. These compounds are metal carboxylates. and the
last two compounds listed are also metal alkoxide carboxylate complexes [1 1]. For simplicity,
the complexes which result from mixing acetic acid with an alkoxide will be regarded as metal
alkoxide acetates. TtPA. ZnPA. and HiPA will be used as acronyms for titanium isopropoxide
acetates. zirconium n-propoxide acetates. and hafnium isopropoxide acetates. respectively.

The titanium-containing sols. produced from TiPA and water. gelled in times ranging from
twodaystomorethanaweek. 11tesolsderivedfromhafniumandzirooniumneverformedsolid
monoliths if a solid phase was created and then redissolved. providing the sols were made from
only alkoxides. acetic acid. and water. Sols from these latter two alkoxides have remained stable
for months showing little or no change in particle size as detected by light-scattering. Examples
of sols of this type are given in Tables I-Ill. The acid/alkoxide and the water/alkoxide ratios will
be designated r. and r... respectively. throughout the discussion.

Titanium-containing sols with the r. values of 5.20 and 10.4 evolved into transparent gels
for r. values of 46.3 and 66.1. The compositions of these four sols are sumrnarind in Table l.
The solutions with the smaller r. values gelled several days sooner. and for a given r. . the
solution with the smaller r. gelled more quickly. The particle growth was apparent from light
scattering experiments.
Table II shows the samples studied for the zirconium-type sols. For the sample with an
r. of 1.00. the complex gelled immediately when exposed to a little water and never fully
redissolved forar. of51.9. The gel apparently softened andresetatthe bottomofthevial. For
sols with r. values of 1.50 and 1.75. the solid complex eventually redissolved. except for some
small translucent particles which were only apparent when the vials were gently shaken. The
remaining samples. with r. values ranging from 1.79 to 5.20. all formed enorely clear sols with
novisibleparticles;except forthe sol with an r. of 1.79. none of these samples formed a

29

TABLE I. Dependance of Gelation Rates
on Composition for Sols from TiPA‘

R.b

 

R": Gelation Average

 

Time Particle Size
5.2 46.3 ~2 Days ~50 nm after
the first hour.
5.2 66.1 ~5 Days ~22 nmafta'
the first hour.
10.4 46.3 me than ~12 nm after
1 week first hour
~38 nm after
68 hours.
10.4 66.1 longest ~12 nm after
68 hound.

 

aFormed solid phase with initial addition of
water.bAcetic acid/alkoxide molar ratio.

cWater/alkoxide molar ratio. dCounts/sec
were initially too low to permit the particle
size measurement.

TABLE n. Dependance of 801 Stability on

 

 

Composition for ZnPA Derived Sols‘
R.b ch Comments

1.00 51.9 No maid-e
1.50 51.9 No measurementd-e
1.75 51.9 No measurementd-e
1.79 20.8 (:01me 21.304
1.82 13.9 Counts/sec :2.8e4
1.88 13.9 Counts/sec :2.2e4
2.“) 17.3 COMB/£31.75“
3.00 13.9 Counu/sec.:1.IXIe4
5.20 16.5 COMB/80045563
5.20 46.3 No measurementf

 

aWe have never observed gelation in Zr
alkoxide/acetic acid sols of this type. except
when it occurs immediately. l’Acetic acid]
alkoxide molar ratio. Water/alkoxide molar
ratio. dFormed solid phase upon exposure to
water. cNever entirely redissolved. fCounts!
see. were too low to allow measurement.

TABLE III. Dependanoe of Sol Stability on

 

 

Composition for l-liPA Derived Sols‘l

11.9 ch Comments

11.8 106 particle size: ~60 nm. stable
with aging.

5.30 46.5 no size measurement:
counts/sec were too low

3.76 108 particle size: ~3IX) nm.

showed slight decrease with
aging (~20nm Imonth)

 

‘Formed solid phase with initial addition of
water. I’Acetic acid/alkoxide molar ratio.
Water/alkoxide molar ratio.

significant solid phase which then entirely
dissolved.

The hafnium sol-gel solutions behaved
similarly to those from the zirconium system
with the exception that a solid phase always
formed during the initial moment of hydration.
Significantly higher values of r. and/or r. were
used for hafnium sols except for the sol with r.
and r. values of 5.30 and 46.5. respectively.
The behavior of both the hafnium and
zirconium based sols contrasted sharply with
that of the titanium-type sols. which gel over a
wide range of compositions.

I-lafnium-containing gel monoliths were
prepared from hafnium isopmpoxide. acetic
acid. hydrochloric acid. and isopropanol.
Initially. a solid phase was formed which
subsequently redissolved. The resulting sols
became clear gels within one to two weeks.
Thesegelstookonabluishtintastheyaged.
presumably due to the greater scattering
efi'tciencyof blue light offthe small particles.

The titanium-derived gels also took on a
bluish tint as they aged and became cloudier
than the hafnium gels. Aging of these gels at
70°Cinsea1edvia1s.overtheperiodofseveral
days. caused a further whitening of the gels
and the formation of a colorless liquid phase
surrounding the gels. When shaken. the solid
structure was easily destroyed. forming a white
emulsion. This emulsion thickened over time.
but never set so firmly that vigorous shaking
could not produce dramatic shear-thinning
behavior.

The sols created were also used in making
filmsonglassandfusedsilica substrates. The
two hafnium-containing solutions with the
largest r. values (11.8 and 5.30. shown in
TablelIl)workedwellasacoatingmateria1

30

characteristics of these solutions did not change appreciably over time. Man ofthe zirconium-
containing mixtures were too viscous to produce an even coating. but the ms showed good
adhesion. The zirconium sol with r. and r. values of 5.20 and 46.3. respectively. provided
good films when diluted to slightly lower the viscosity. The titanium-type sols produced very
thin films when freshly prepared. However. these sols were in the initial stages of gelation. and
the quality of the film was dependent on the age of the sarrrple. The best titanium films from this
system were aged for several hours and then diluted slightly with water before being applied.
Samples aged for significantly longer periods uced flaky films with poor adhesion. These
systems have not been carefully optimized for coating; however. the hafnium and zirconium
films appear to be easier to make. just as the titanium system appears to be better suited for the
formation of gel rrtonoliths over a wide range of compositions.

The ester formation which was observed may play an important role in the sol-gel process.
lfthesolidificationofthecomplexesforrnedfromrhealkorddeandtheaoeticacidrequireswater.
it should be noted that water is generated at the same rate as the ester. Alcohols and carboxylic
acidsareknowntoreact inthepresenceofmineralacidswhenheatedlhereactivityofprimary

alcoholsis thanthatofsecondaryalcohols.andthe1essbulkycarboxylicacidsaremme
reactive [12 :
H+
RCOOl-l + R'OH = RCOOR' + 1120 (2)

A plausible mechanism for this acid catalyzed reaction has been proposed[12]; however. in the
presence of the alkoxide. and in the absence of a mineral acid. this reaction does nor necessarily
have the same mechanism. If the water is consumed by reaction with the metal. it would drive
the reacrion forward by removing one of the products and generating one of the two reactants
through displacement of either an acetate ion or an ester group from the metal. Water may be a
necessary reactant in this solidification since a small amount of watercauses immediate furnation
ofasolidphaseinmanyrmxunesofumrumnorhafiuumisopropoxidemdweucacid Although
water may be necessag for solidification. the presence of too much water may actually prevent
the formation ofa sol' phase. This can be seen with initially liquid ZnPT samples that solidify
in less than an hour. but do not solidify. or even cloud. when water is added.

‘Ihe acetic acid and alkoxide derived complexes for titanium. zirconium. and hafnium have
been analyzed with infrared spectroscopy before and after the addition of water (Fig. 2-4). The
acetate anion may form ionic associations with a metal. or it may bind as a monodentate.
chelating or bridging ligand [11.13]. The difference between symmetric and asymmetric 002'
stretching vibrations. [v.(coz'w,(coz')l - A, occurs within a characteristic range for each type of
acetate ligand [13]. his characteristic has been used by Livage to conclude that the complexes
formed by rrtixing acetic acid and Ti(O-Bu‘m are probably characterized by chelating and
bridging acetate ligands [3]. After hydrolysis. the samples in Livage's study showed a decrease
in A. indicating a decrease in the number of bridging acetate ligands [3]. In our study, the
titanium complexes. prior to hydration. showed a strong asymmetric stretch at 1599 cm'l. with
weak shoulders at 1566 cm: 1 and 1552 cm: 1 as shown in Fig. 2a. The symmetric stretch gave a
broad band with three peaks at 1447. 1413. and 1379 cm: 1. The 21': thus occurred at 220. 153.
and 105cm:1.hrdicafingmuandrmetypesofacemtefigarrdbmdingwaepmbablypresenautd
that the number of monodentate ligands was probably greater than for the conditions studied by
Livage [3]. After hydration. strong bands in Fig. 2b appeared at 1646 and 1388 cm:I indicating
thatA hasincreasedto258cm°1. 'l'hisshowedthattheadditionofwamrfurtherincreasedthe
relative number of monodentate ligands. probably by breaking some bridging acetate bonds.

The non-hydrated complexes. of zirconium and hafnium, both demonstrated
predominantly bidentate character in their IR spectra shown in the lower sections of Figures 3a
and 4a. The peak assignments are given in Table IV. For the hafnium complex. as were found
at 100. 150. and 207 cm'l. and for the zirconium. A's were found at 96 and 178 cm". In the
case of zirconium. two different hydrated samples have been examined which yield different
results. A sample with an r.of4 was mixed with waterin a 1:1 volumetricratio. yi ' g A'sof
109and227cm".wherethefirsta hasbeencalculatedfromthemoredominantpeaksfig. 3b).
‘lltis indicates that both monodentate and bidentate ligands are probably present. with the
bidentate ligands predominating. The IR spectrum of the second sample (r. a 2.00 and r. -
17.3)showsthemonodentateligandpredominatingwithA . 225cm4uheweaka'peaksgave a

31

 

 

DTIANSMISSION
QTR ANSMISSION

 

 

 

 

 

 

 

‘TRANSHISSION

 

 

 

 

 

I“ 17]. I“ I!“ 3“

rumour-ea. cat-l

 

"I. “a II‘ I“ "I. “I ll“

WAVINUNIER. CM-l WAVENUMIER. CM"

Figures 24. IR spectra of TiP'l‘. ZnPA. and l-liPA. before and after the addition of water.
TtPA: 2(a) r. a 5.2. r. a 0; 2(b) r. a 5.2. r. = 46.3. ZnPA: 3(a) r. s 5.2. r" a 0; 3(b) r. =- 5.2.
r. a 46.3: 3(c) r. =- 1.75. r. a 51.94. I-IiPA: 4(a) r. a 5.3. r. a 0; 4(b) r. a 5.3, r. a 46.5.
‘1'he"""indicatesbandsthatareg_iveninTableIVanduscdin A calculations.

A value calculated at 110 cm1 (Fig.
3c). The symmetric stretch is very
weak in this spectrum. The water-
treated hafnium sample ( r. = 5.30
and a r. = 46.5) showed a dominant
A . 233 curl. with weaker bands
giving A . 129cm°1(Fig. 4b). In
comparing these systems we see that
acetate ligands of all possible types
are probably present. in various
relative amounts. in most of these
samples. One should bear in mind
that if this analysis is being applied to
sols under highly acidic conditions.
or with a high ion concentration. the
non-bonded acetate ion would
probably have a A similar to that of
the bridging acetate [13]. Even in a
system as complex as this. with three
ligand types and free acetic acid
present in a single sample. it is
relatively easy to study the effect of
the composition on the relative
predominance of a given acetate
ligand. From the zirconium srudy. it
appears that increasing the amount of
water. or decreasing the amount of
acid. lowers the concentration of
bidentate ligands. Neither r, nor r.
are the same between the two samples
which correspond to the spectra in
Figures 3b and 3c. However. the
relative difference between the r.
values is much greater than that for
the r.; therefore. one could argue that
the observed differences betwoen the

TABLE IV. Absorbance Peaks for Calculation of
W Frequency Separation for the Acetate

v, vs Aava(COZ-)-vs(m2-)
asymmetric symmetric characteristic
stretcha stretcha frequency separationb

 

 

 

 

(fiPA)

2(a) 1599 (s) 1379 (s) 220 (1)
1566 (sh) 1413 (s) 153 (2)
1552 (sh) 1447 (s) 105 (3)

2(b) 1646 (s) ' 13mg 258 (1)

(RM)

3(a) 1594 (sh) 1358 (sh) 17s (2)
1548 (m) 1452 (m) 96 (1)

 

3(b) 1643 (w) 1416 (sh) 22f (2)
1561 (m) 1452(m) 109 (1)

3(c) 1644 (s) 1419 (w) 225 (1)
1562 (sh) 1452 (w) 110 (2)
(HiPA)
4(a) 1587 (sh) 1380 (sh) 207 (3)
1569 (sh) 1419 (sh) 150 (2)
1556 (s) 1456 (s) 100 (1)

4(1)) 1645 (s) 1412 (w) 233 (l)
1584 ($11) 1455 (w) 129 (2)

aTheintensityofthepealtisindicatedbythesyrnbolin
parentheses: s=su'ong. msmedium. w-weak.
sh=shoulder. bThe relative dominance of the 113m-
typecorrespondingtorheA isshownbythenumberin
parentheses: lsmost dominant. 3=least dominant.

 

 

 

 

spectra shown in Figures 3b and 3c
are predominantly due to the
difference 1n r. . while the differences
between 3a and 3b are predominantly
due to the difference 1n r... Increasing
r had the same effect on the hafnium
and zirconium systems.

The Raman s ectra for the
titanium complex. ore and after the
addition of water. are compared with
the various solvent bands in Fig. 5.
The reaction with acetic acid liberates
isopropanol and generates isopropyl
acetate. Bothoftheseorganicshavea
strong band near 820 cm 1 and this
band appears in the spectra of the
non-hydrated mixture and the sols.
The broad peak in the spectrum near
630 cm- 1 . in Fig. 5c for the non-
hydrated sample. probably receives
contributions from the acetate 5(0-
CaO) and the v(M-0) of the alkoxide.
When water is added. the intensity of
this peak is severely reduced as is
clearly shown by Fig. 5d.
presumably because additional ester
groups have been displaced from the
titanium by water.

A comparison of the Raman
spectra of sols from hafnium
and titanium (be ore it has gelled).
neat isopropanol. and acetic acid are
presented in Fig. 6. The similarity of
thespectrafortheuvosolsissuiking
(Fig. 6a and 6b). and emphasizes the
chemical similarity between the two
systems. The slight differences in
relative band height for the three
peaks between 800-10mcm1maybe
due to the additional isopropanol 1n
the hafnium system.

The Raman spectrum of the
zirconium complex. before and after
the addition of water. was compared
with the spectrum of neat acetic acid
(Fig. 7); the presence of water did
nOt naticeably change the appearance
of the spectrum for the zirconium
complex. which is especially
interesting because this sample did
not form a solid when water was
added. The generation of water which
accompanies the ester generation my
offer some explanation for this
strange result: the generated water
may have already reacted with the
system to the extent that additional
water caused no further observable
changes. It would be interesting to
examine the hafnium and acetic, acid

32

A A

A

IAMAN IN I ENSI‘I'Y

SIM

see we seesee reeeeeeeereee
meanness-l

 

Figure 5. Raman spectra of TiPA. before and after
the addition of water. are compared with spectra of
some processing reagents and side products: (a) acetic
acid: (b) isopropyl acetate; (c)T'1PA. r. a 5.2. r. =- 0;
(d) TiPA. r. a 5.2. r, a 46.3; (e) isopropanol.

“OI

 

 

“DI

IAIIAN INTENSITY

“II

 

 

see aee see one wee see eee reee
rattan strtrr.cat-|

Figure 6. Raman spectra of TiPA and IIiPA are
comparedwithoneanotherand with isopropyl alcohol
and acetic acid: (a) acetic acid: (b) HiPA + water. (r,
I 5.3. r... =- 46.5); (c) TiPA + water, (r. c 5.2, r, a

46.3); (d) isopropanol.

complex. but the solidification
occurred too rapidly. In the case of
zirconium. the complex had begun to
whiten and solidify soon after the
spectrum was finished.

The Raman spectra of the gel
monoliths of hafnium and titanium are
shown in Fig. 8. These spectra are
quite similar. just as the Raman

of the sols were similar. The

s near 820 cm'1 indicate that
1sopropanol and probably isopro l
acetate are trapped in these gels.
metal alkoxide v(M(O-C)) band near
1029 cm1 is weak in the titanium gel
and net apparent in the hafnium gel.
indicating that few metal ester groups
remain.

Figure 8. Comparison of Raman
spectra for the titanium and hafnium
gels derived from TiPA and I-liPA .
respectively: (a) titanium gel; (b)
hafnium gel.

33

j)

RAMAN INTENSITY

 

 

 

see aee see sis 76o see eee reee
name strainers-l

Figure 7. RamanspectraonnPA.beforeandafter
theadditionofwater.arecomparedwiththe spectrum
of acetic acid: (a) acetic acid: (b) ZnPA. r. a 5.2. r. s
0; (c) ZnPA. r. s 5.2. r, - 46.3.

IAMAN INTENSITY

 

 

 

1"LLJ111

res m use use

RAMAN srtrrr. cut-l

34
CONCLUSIONS

The sol-gel processing of hafnium isopropoxide can lead to translucent. monolithic gels
(when dried) and transparent films. However. this hafnium alkoxide appears to be better stuted

for the making of films. which are quite sensitive to the exact composition of the sol. Sols made
from ZnPA and liiPA by the resolvation of an initially formed solid are quite stable ova time.
showing no tendency for significant particle grovnh.

Increasing r. orreducingr.appears toreduce .herelative nurnbaofbidentate ligands.
Increasing r. decreases the gelation time for the titanium system which 1s contrary to the behavior
observed for other similar system [3].

The spectroscopic evidence for all three systems indicates that acetic acid 1s active in the
coordinationchemistryofthemetal. Furthermore. thedifl'erencesinthegel-formin versusfilm—
forming characteristics are probably directly related to the role played by acetic acid. The
extension of the current work to organic acids of longer chain and different ligand behavior
shouldallowengineeringofsystemswith widelyvarying attributes.

ACKNOWLEDGMENTS

Thesu forthisworkbytheCompositeMaterialsandStrucutresCenta (CMSC)and
the Centeror Fundamental Materials Research (CFMR) at Michigan State University is
gratefully acknowledged. All Raman spectra were taken at the Michigan State University

LASERLaboratory. 'I'hanksislikewiseextendedtoDr. R.B. LessardandDr. W. A. Oertling
fortheirhelpfuldiscussions.

REFERENCES

l. M.Yoshue. Jpn Patent 63236335 (30ctoba 1988).

2- AF Stew-ml) R TallinLKI-o Hissinsin mlasealnducednamninmtical.
W. ediwdbyILE. Bennet.A. l-I..Guentha D. Milam.B.E.Newnam(NBS
U..S Spec. Publ. 746. .1.985)pp 362-365

3- I EvasemmmmmmmnnediwdbyCJ Drinker D EClu-k.

.R. Ulrich (Mater. Res. Soc. Proc. 13,. Pittsburgh. PA l986)pp. 717-724

.EYoldas. J. MatSci.. 211080 (1986).

.Tomandl. 11 Rosch. A. sucgelschtnin In W edited

...1 Brinka. D. E.Clark.D. R. Ulrich(Mater. Res. Soc. Proc. 1Z1. Pittsburgh.PA

)pp. 665- 670

Lipcl . D- 1- Coleman. M- 8- Leone in Wall.

by C. J. Brinker. D. E. Clark. D. R. Ulrich (Mater. Res. Soc. Proc. 13, Pittsburgh.

6)pp. 665-670

.Brad19e 2.OR. C. Mehrotra. D. P. Gaur. WM (Acad. Press. New York.

1

radle y.R.C. Mehrotra.W. Wardlaw, I. (Item. Soc.. 1634 (1953)

1.1. Singh.R.I(.Multani.andB. D.Jain,lsrael.l. C11em.1_17l (1969).

ehrotra..lnorg.Chim.ActaRev. 199(1967).

..241\(1)ehrooa. and R. Bohra. Wm (Acad. Press. New York. 1983) pp.

12. R. ‘1‘. Morrison. and R. N. Boyd. W319 ed.(A11yn and Bacon. Boston.
1981). pp. 602-603. pp. 680-682.

l3.l(.Nakarr10to. llll' 1' 411 1-1‘1'!‘ I'll" .

(John Wiley and Sons. New York. 1986) pp. 231-23 .

01
5.305139

d
"U11 a:
snai>ao
oo

“’18

E

10.
ll.

PPVp
ppflp
z

83
of)

  
 
 

m4.

IISOLUBLE COATINGS

 

Although titanium produced the poorest water soluble films, it
clearly makes superior water insoluble and organic solvent resistant
coatings. The same characteristics which made it poorly suited for
producing stable aqueous eole, allow it to form harder films when
processed by other methods. These coating mixtures are distinguished
from the water soluble coatings by the use of greater quantities of
carboxylic acids and much emaller quantities of water. In these coating

mixtures, the carboxylic acid acts as the solvent as well as a reactant.

 

In studying the effect of the acid chain length, it became readily

apparent that valeric acid produced superior films. However, propionic
and butyric acids can also be used, although they are not quite as

strong.

35

36

ZEBE:EE§QNAN§B.BAHAH CHABBQIEBIZAIIQH Qt HlmbhngGBNIQ IILHS £393
IIIAHIHH ALKQXIDB QBBEQXXLAIE QQHELEXESL

Our study of the effect of the different carboxylic acids on the
quality of the films and on the processing of the materials is also
contained in a previously published paper (Gagliardi and Berglund, 1990)

and is reprinted here with the permission of the publisher:

PRE-RESONANCE RAMAN CHARACTERIZATION OF METAL-ORGANIC
FILMS FROM TITANIUM ALKOXIDE CARBOXYLATE COMPLEXES

CHARLES D. GAGIJARDI, DILUM DUNUWILA, AND KRIS A. BERGLUND
Michigan State University. Department of Orenrical Engineering and Center for Fundamental
Materials Research. East Lansing. MI 48824.

ABSTRACT

Metal-organic films were produced at ambient temperatures and pressures by the controlled
hydrolysis of carboxylic acid and titanium isopropoxide mixtures. Pre-resonance Raman
spectroscopy was used to study the effect of the organic acid chain length upon the nature of the
resulting film. Propionic, butyric, valeric, hexanoic, and octanoic acids were studied. resulting in
a proposed correlation between film quality and certain spectroscopic features of the alkoxide
carboxylates. 'I‘hechoiceofthecarboxylic acidand thefpreseneeofanappmpriateamountofwater
were shown tobectiticalinthedevelopmentofagood tlm.

INTRODUCTION

Coating materials produced from the controlled hydrolysis of titanium alkoxide carboxylates
offer additional versatility and performance over standard sol-gel films. Two patents exist for the
manufacturing of acyl-gmup-containing polymeric titanium compounds from titanium alkoxide and
carboxylic acid precursors [1]. In the first of these patents [2], the alkoxide and carboxylic acid
mixture is heated under reflux, followed by the removal of alcohol and alkyl ester by distillation.
Under these conditions the only water present is that produced by the generation of the alkyl ester,
and the resulting polymer is dependent upon the molar ratio (Ra) of alkoxide to acid [1.2]. Thus,
[-O-Ti(OR)(OCOR')-]n, [-O-Ti(OR)(OCOR')-O—Ti(OCOR')2-]n, and [-O-Ti(OCOR')2-]n represent the
polymeric structtn'es resulting from R, values of 2.0. 2.5, and 3.0, respectively. In the second
patent [3]. the alkoxide is first reamed with the carboxylic acid. and is then hydrolyzed with
additional water. The byproducts of the reaction are removed by distillation as in the first patent.
and the polymeric product is reported to have the structure [-O-Tr(OH) (OOOR'Hn. The mterials
produced by these methods have found wide application as highly refractive coatings for glass and
other substrates. and water-resistant. protective films.

The materials currently under investigation do not require refluxing at elevated temperatures or
the removal of reaction byproducts by distillation; the procedure is thus more direct and cost
effective. The resulting films are likewise optically transparent and water resistant. In the two
patents previously referenced. longer chain length carboxylic acids are preferred. In the current
worhmladvelyshatchainlengthacidsaretheobjectsofsmdy.

MATERIALS AND INSTRUMENTATION

The titanium isopropoxide and the carboxylic acids (propionic, butyric, valeric, hexanoic.
and octanoic) were obtained from the Aldrich Chemical Company and used without further
purificatimt. The microscope slides, which were used as substrates for the films. were standard,
pee-cleaned slides manufactured by VWR Scientific Inc.. All water was de-ionized, with a
resistance of 18 Mohm. '

Raman spectra were collected with a Spex 1877 triple spectrometer equipped with a diode
array detector using the 363.8 nm line from an argon laser. Samples were held in quartz cuvettes
fitted with Teflon caps.

37

3 8
EXPERIMENTAL PROCEDURE

The liquid carboxylic acid (propionic. butyric, valeric, hexanoic, or octanoic) and the
titanium" werefirstcombinedinaglassscindllationvialanddtomughlymixedwidra
va'texmixing ' . ThesampleaweremadewithdtemolarratiosdescribedinTablelsuchthat
eachsamplehadatotalvolumebetween3and5ml. Forthosesamplesrequiringwater.thewater
wasaddednextandthesamplewasagainthoroughlymixed. Filmswerethenspin-castonglass
substratesbywettingthetopsm'faceoftheglassandthenspinningitaboutanaxisperpendicular
to the wetted surface. The glass slides were always centered about the axis of rotation. and they
were allowed to spin for at least 5 minutes for sufficient drying.

Laurie acid and titanium isopropoxide mixtures were also studied todetermine the effect of
adding small amounts of lauric acid to the coating solutions previously described In this study.
vuyingammmofhmicwidwaeaddedmmedbmmmnspecmwuenkenofme
resultingaolution.

TABLE 1. Composition of coating solutions and description of resulting films'.

Acid 3.11 awe Film Description
Propionic 2.5 0.0 extensive cracking. flaking

 

 

 

 

 

Butyric 13.3 8.8 extensive cracking. flaking
Valerie 2:5 0:0 extensive cracking. flaking

 

Hexanoic 2.5 0.0 extensive cracking. flaking

Octanoic 2.5 0.0 slight cracking, moderate shrinkage at edges, soft
anoii: 10.0 0.0 Wm:

Propionic 2.5 1.4 extensive cracking. flaking .

 

 

          

H.L'r’i'ii ”In, .‘

 

 

 

 

Bch 2.5 1.4 extensive cracking. flaking

m 10.0 1 ' ' °

Valerie 2.5 1.4 extensive cracking

Xalerii: 10 0 1.4 W
Hexanoic 2.5 1.4 no cracking, extensive shrinkage at edges, soft

Octanoic 2.5 1.4 slight cracking, moderate shrinkage at edges, soft

W 10.0 4.4
'fledesaibedchmtahdcsappeumuchmaequicflyfamewlummmoutwatufik.
isthemolarratioofearboxylicacidtoalkoxide. chisthemolarratioofwater to alkoxide.

 

RESULTS AND DISCUSSION

'l'hepre-resonanceRamanspectrapresentedinFig. 1 showtheresultsofaddingsmall
quantities of lauric acid to TM. The vibrational band assignments for TiPT have been discussed
in several previous papers [4.5.6]. However, due to coupling between 00 and CC vibrational
modes. the band assignments have been difficult to make [6]. The strongest TiPT peaks occur at
1026 cm‘ and 1182 cm°1. The peak at 1026 cm°l has been associated with the (C-O)Ti stretching
vibration [4]. The infrared assignment for this stretch was given as 1005 cm'1 [6]. The band at
1182cm'l hubeennkenaspredominandyaskeleulsnetchsimilutothelkpeakrepmedu

39

1170 cm1 [5]. It is likely that both the 1026 cm" and 1182 cm" bands represent coupled
vibrational modes; therefore. they should not be expected to agree exactly with literature values for
the pure vibrational modes. Discrepancies between the IR and Rarnan peaks can also be expected
due to the different selection rules. The peaks of intermediate intensity at 565 cm1 and 612 car-1

maybetakenasthesymmetricand
antisymmetric stretch of Ti-O.
respectively [4.6]. The weak
peaks at 1129 cm'1 and 852 cm"
agree closely with the 1R reported
values of 1131 cm'1 and 851 cm"
for a coupled 00 stretch and
skeletal vibration. and a pure
skeletal stretch. respectively. The

band at 1182 cm" diminishes with
increasing amounts of acid. and the
isopropanol peak at 819 cm'l
begins to ap as the iPr-O-
groups on the titanium are displaced
by carboxylate ligands. The
symmetric Ti-O stretch at 565 cm"

so decreases. leaving the other

Ti-O stretch at 612 cm"
undiminished. The relative peak
intensities of the 565 cm’l and 612
cm" vibrations appear to change
morewiththeadditionoflamicacrd
than with shorter chain-length
acids. This could account for the
stabilizing effect of adding small
amounts of lauric acid to the other

oating solutions and
descriptions of the resulting films
are presented in Table 1. The best
film without water resulted from
hexanoic acid. R. a 10.0. With

water. the best film resulted from
valeric acid. also with R. I 10.0.
The film solutions containing no
added water produced less stable
films. in general. than those from
solutions with added water.
However. a particular acid at a
given R. may produce a boner film
without water than with water as
exemplified by hexanoic acid at R.
a 10.0. In general. it might also be
said that higher R. values produce
better films than lower R. values.

yet octanoic acid produces better
films at the lower R. . with or

without water.

The Raman s us of 'water
free' coating solutions. shown in
Figures 2 and 3. look very similar;
itappearsthatallofthesecarboxylic
acids interact with the TtPT in a
similar way. The lower R.‘

RAMAN I NTENS I TY
I

 

 

260 ' 600 ' 1000 '14bo V1800
WAVENUMBER, CM"

Figure 1. Raman spectra showing the effects of
increasing concentrations of lauric acid on titanium
isopropoxide (TiPT). (A) Pure TiPT, R. a 0, (B)
'I'iI’I‘+lsuricacid.R.-0.12.(C)'I‘il’l‘+lauricacid.
R. m 0.62. (R. a the molar ratio of acid to alkoxide.)

RAMAN INTENS ITY
r U

 

 

200 600 1000 '14b0 81360
WAVENUMBER, CM“

Figure 2. Raman spectra of coating solutions having
a low acid molar ratio (R. s 2.6) and no water. (Ru 8

0.0.). Acids used: (A) propionic. (B) butyric. (C)
valeric. (D) hexanoic. and (E) octanoic. (R. a the
molar ratio of acid to alkoxide and R. a the molar
ratio of water to alkoxide.)

solutions are distinguished from the
higher R. solutions by different
relative peak intensifies for the band
at 1182 cm'1 . This band is
generally less dominant at the
higher R. values. However. there

are no features which strongly
distinguish between the different
carboxylic acids. which shows the
great similarity of their reactions
with the TM .

In Fig. 4. the higher Rw

(molar ratio of water to alkoxide)
solutions are clearly distinguished
from those of the other solutions.
In the 'water-free' mixtures there is
a fairly symmetrical. broad band
near 600 cm'1 which contains both

the 565 cm" and 612 cm" Ti-O
Stretching vibrations. After the
addition of water. this broad band
becomes highly unsymmetrical as

the 612 cm'1 vibration dominates

the 565 cm'1 vibration. This result
shows the dramatic impact that
water has on the alkoxide
carboxylates and demonstrates part
of the role of water in creatin a
stable film. The band near

cm'l appears increasingly unsym-
metrical as the chain length
decreases, except for the mixture
containing propionic acid. which
had the least asymmetry. The band
appearing near 430 cm'1 is also
interesting. It appears that the
integrated intensity of this band is
greatest for butyric and valeric acid
mixtures. Thus. the bands near 430
cm'1 and 600 cm“ may both be
correlated with film quality. The
development of these bands may
also indicate the degree of
polymerintion. Another band of
interest is the isopropanol band at

819 cm”. This band appears
slightly larger in the butyric and
valeric acid coating solutions.
indicating that the alkoxide
carboxylate complexes formed from
these acids may have a greater
tendency to release isopropanol.
perhaps through greater polymer-
ization. It is also possible that the
iPr-O- groups on these complexes

 

RAMAN_INTENSITY
E?

 

200 600 1000 01400 '1000
WAVENUMBER, CM"

Figure 8. Raman spectra of coating solutions having
a high acid molar ratio (R. s 10.0) and no water. (R.
a 0.0.). Acids used: (A) propionic. (B) butyric, (C)
valeric. (D) hexanoic, and (E) octanoic. (R. s the
molar ratio of acid to alkoxide and R, sthe molar
ratio of water to alkoxide.)

)4 A

9'

H B

U)

z

CIJ

5" C

2

H .
g E

 

 

200 ' 600 '1000 '1400 "1800
WAVENUMBER, CM"

Figure 4. Roman spectra of coating solutions having
a high acid molar ratio (R. s 10.0) and added water

(R. s 1.4). Adds used: (A) propionic. (B) butyric. (C)
valeric. (D) hexanoic. and (E) octanoic. (R. a: the
molar ratio of acid to alkoxide and R. a the molar
ratio of water to alkoxide.)

are more easily removed by hydrolysis. Thus. it appears that the complexes formed from different
«moxyficmidsdifiamcemmupecuofmcumacdmwimwataJnddwscspxnoscoptcauy
observabledifi'erencesnnybycorrelated with filmquality.

41

CONCLUSIONS

Pre-rcsonancecnhancedRamanspectroscopyisvery usefulinstudyingthedifferencesinthe
alkoxidecarbox ylatessinceitcmphasizesthechemistryofthccomplex overthedifferencesinthe
vibrational bands of the individual acids. Although the reactions between the acids and the TiPT
are very similar. greater differences among the complexes appear after the addition of water. The
film-making quality of the solution may be correlated with the asymmetry of the broad band near
600crrt'1 and with the integrated intensity ofthe band near 430 cm”. The results demonstrate the
critical choice of the carboxylic acid and the role of water in achieving good film quality.

ACKNOWLEDGMENTS

'Ihesup‘pu'tforthisworkby ytheComposithatcrialsandSmtcnrresCenter (CMSC)anddre
Center for undamental Materials Research (CFMR) at Michigan State University rs gratefully
acknowledged. All Raman spectra were taken at the Michigan State University LASER

laboratory.

REFERENCES

p—a
I

R Feld. P L Cowe. WWW (Butterwonh and Co..
. Washington. 1965) pp. 84-85.

C. M. langkammerer. U. 8. Patent No. 2 621 193 (p December 1952).

1.11. Haslam. U. S. Patent No. 2 621 195 (9 December 1952).

K. A. Berglund, D. R. Tallant. R. G. Dosch. in '

edited by 1.. L. Hench. D. R. Ulrich (John Wiley and Sons, New York. 1986) pp. 94-99.

V. A. Zeitler,C. A. Brown. J. Phys. Chem.§_1_. 1174 (1957).

D. C. Bradley, R. C. Mehrotra. D. P. Gaur. W (Acad. Press. New York.

1978)pp. 117.119-121.

12?..40C. Mehrotra. and R. Bohra. Mm] magma (Acad. Press. New York. 1983) pp. 233-

J thseinncncLchamichhmuahflhsmimn editedby C J Brinker.D. EClark.

D. R. Ulrich (Mater. Res. Soc. Proc. 13. Pittsburgh. PA l986)pp. 717-724.

CD 038M K-A- Bflslund mWflMflmwfibfl

A. AksayisG. l... McVay. D. R. Ulrich (Mater. Res. Soc. Proc. 115, Pittsburgh. PA. 1986)

pp. 127-1

‘0 oo NI OHJI hUN

CHAPTERS.

m1 AID murals

Coating solutions derived from the partial hydrolysis of group IV
transition metal alkoxide carboxylates were compared and characterized
by spectroscopic analysis. The general sol-gel procedure involved
reacting the alkoxide and carboxylic acid, and then forming a sol
through the partial hydrolysis of the alkoxy acylate complexes.

Acetic acid was used to produce water soluble coatings. The
processing of these materials used larger amounts of water and
carboxylic acid than typically incorporated in the production of similar
sols, and often involved the rapid formation of an opaque gel which was
then resolvated by the further addition of water. This method resulted
in stable zirconium.and hafnium alkoxoacylate sols which produced
excellent films. The titanate produced inferior films and less stable
sols which tended to gel. Spectroscopic IR analysis showed that the
carboxyl groups were present as monodentate and bidentate bridging and
chelating ligands. Larger quantities of water seemed to favor the
population of monodentate ligands as was observed by Livage (1986).
Light scattering experiments with a particle size analyzer indicated
that higher concentrations of water reduced the rate of gelation and the
particle size over the concentration range studied for the titaniump
based system. This result is contrary to the trends observed by Livage

for titanium n-butoxide and acetic acid (Livage, 1986), and offers

42

43

further evidence that the processing behavior of each system may be
highly dependent upon the concentration range of the reactants and the
choice of alkoxy ligands. The Raman data from our samples further
emphasized the great chemical similarity among coating solutions made
from titanium and hafnium isopropoxides. The spectra of the zirconium-
based solutions indicate that the zirconium n-propoxide probably
contained a highly fluorescing impurity which effectively masked the
weaker Raman signal.

Using substantially smaller amounts of water and slightly longer-
chain carboxylic acids, water insoluble coatings were produced with
titanium isopropoxide. Zirconium n-propoxide produced similar, but less
solvent resistant coatings. On this basis, titanium.isopropoxide was
chosen for a further study on the effect of the carboxylic acid chain
length. A survey of carboxylic acids with chain lengths from 3 to 8
carbons indicated that valeric acid produced the best films, although
propionic and butyric acids also produced water insoluble coatings of
fair quality. Pro-resonance Raman spectroscopic analysis demonstrated
the chemical similarity of the materials and showed dramatic changes
upon the addition of water. The band associated with the (C-O)Ti
vibration (1029 cm'l) is sharply reduced upon the addition of water
relative to the largely increased band at 612 cm.‘1 which is associated
with the asymmetric Ti-O stretch. This drastic change occurs fairly
rapidly upon the addition of water. The degree of asymmetry in the Ti-O
stretch and the integrated intensity of a band near 430 cm'1 may
indicate structural differences which affect the quality of the film.
However, it must also be emphasized that the spectral differences are

q

relatively slight, indicating that the major effect of increasing the

44

acid chain length may be an increase in mechanical stability due to the
alkyl chain itself. The larger chain may offer support to the matrix
like reinforcing fibers in a composite. In this sense, our film
materials may be excellent examples of a successful molecular composite.
The next step in this research should be an investigation of the
kinetics for the hydrolysis reaction and the associated condensation
reactions which follow. An improved understanding of the reactions
would also result if the reaction products could be isolated and
analyzed at various times during the reaction. Such separation may be

achievable through a properly selected column chromatography technique.

10.

11.

12.

13.

14.

15.

16.

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