Use of polymeric catalyst in synthesis of sol-gel derived ceramic materials

Polymeric catalysts (e.g., polymeric acid catalysts such as poly(styrene)sulfonic acid) can be used in the synthesis of sol-gel derived ceramic materials from metal alkoxides, for example, organic/inorganic hybrid materials formed by reaction of a metal alkoxide and a reactive endcapped polymeric modifier. The use of a polymeric catalyst, rather than a conventional monomeric (inorganic or organic) acid catalyst, has a non-deleterious effect on the aging behavior and offers potential for control of rheological properties (spinnability, coatability, etc.) of the hybrid materials.

BACKGROUND OF THE INVENTION 
Several previous studies have demonstrated the successful incorporation of 
various functionalized oligomers into a sol-gel network to produce novel 
organic/hybrid materials referred to as "ceramers". References which 
describe such previous work include: Huang, H. et al., Macromolecules 
1987, 20(6), 1322; Huang, H. et al., Polym. Bull., 14(6), 557 (1985). 
These initial studies involved a sol-gel reaction using 
tetramethylorthosilicate (TMOS) or tetraethylorthosilicate (TEOS) and 
silanol-terminated poly(dimethylsiloxane), for instance, under acidic 
conditions. The next systems investigated were hybrids based upon TEOS and 
TMOS, or related metal alkoxides, reacted with an oligomer of 
poly(tetramethylene oxide) endcapped with isocyanatopropyltriethoxysilane. 
The molecular weight of the oligomers was varied from 650 grams/mole to 
2900 grams/mole. The silane-endcapped PTMO oligomers were also reacted 
with the silane under acidic conditions. The novel hybrid materials 
produced were monoliths with good mechanical properties and were optically 
clear. 
The acids or bases typically employed in these sol-gel reactions for 
preparation of the ceramers, as well as conventional sol-gel ceramic 
materials, include hydrochloric acid (HC1) and glacial acetic acid. The 
presence of such species in the final material may lead to an accelerated 
aging effect. The ability of the chloride species to migrate in the 
material would also be expected to influence the ionic conductivity. in 
addition, there is a potential interest in fiber drawing or spinning 
applications for sol-gel derived materials. 
A literature reference by K. Nakanishi et al. appears in J. Non-Crystalline 
Solids 108 (1989) 157-162, which speaks of the crystallization of silica 
gels containing sodium poly-4-styrene sulfonate which is not deemed to 
have functioned as a polymeric catalyst in such a system since the purpose 
of the sodium poly-4-styrene sulfonate was to initiate crystallization. In 
addition, a mineral acid was added to catalyze the sol-gel reactions 
instead of utilizing a polymeric acid catalyst.

EXAMPLES 
Triethoxysilane endcapped poly(tetramethylene oxide) PTMO was synthesized 
by the following procedure. A 1:1.1 equivalent mole ratio of PTMO to 
3-isocyanatopropyltriethoxysilane was mixed in a three neck round bottle 
flask under nitrogen. This system was under constant stirring and was 
maintained at a temperature of 60.degree. C. The reaction was monitored by 
infrared spectroscopy using the absorptions for the functional groups of 
--OH and --N.dbd.C'O which were reduced in intensity and which eventually 
dispersed. The formation of the urethane linkage was monitored as well. 
The reaction was allowed to run 100 hours until substantially complete. 
The poly(styrene sulfonic acid) PSS (70,000 gram/mole) was supplied as a 
30 wt % solution (Polysciences, Inc., Cat. No. 8770). This was used as is 
without further purification. There were 0.0026 equivalents/gram of the 
PSS solution by titration with standardized NAOH. The density of the PSS 
solution was determined to be 1.2 gram/ml. The TEOS was obtained from the 
Fluka Company and was used as received. 
PROCEDURES 
Six milliliters of isopropanol (IPA) and four milliliters of 
tetrahydrofuran (THF) were mixed in a 50 ml flask at room temperature. A 5 
gram portion of TEOS and 5 grams of the silane endcapped oligomer were 
weighed into the IPA/THF solvent and were stirred for approximately ten to 
fifteen minutes. The distilled-deionized water was then added 
volumetrically with stirring. One milliliter of the PSS solution (about 
0.9 gtam of PSS) was then added dropwise to the solution with vigorous 
stirring. The mixture (a sol) was stirred for one to two minutes after 
addition of the acid, was then cast into a petri dish coated with 
fluoropolymer (TEFLON brand), and was then covered to reduce evaporation. 
The sol was then allowed to gel undisturbed at ambient conditions for five 
days and was then opened to the air to remove excess solvent. The age of a 
gel was designated from the date of preparation of the sol. 
CHARACTERIZATION METHODS 
The dynamic mechanical data were obtained using an automated Toyo 
Rheovibron Dynamic Viscoelastometer Model DDV-IIC. Most samples were 
analyzed between -150.degree. C. and 100.degree. C. at a rate of 1.degree. 
to 3.degree. C. per minute. All spectra represent data that was obtained 
at 11 Hz. 
The mechanical properties were measured with an Instron Model 1122 using an 
initial strain rate of 2 mm/min at 25.degree. C. The dogbone specimens 
ranged in thickness from 15 to 35 mils with a 10 mm initial gage length. 
The reported values represent an average of at least four and typically 
five specimens. 
A Siemens Kratky camera system was utilized for small angle x-ray 
scattering (SAXS) measurements in conjunction with a M. Braun 
position-sensitive detector from Innovative Technology Inc. 
RESULTS AND DISCUSSION 
The storage moduli and tan delta behavior are represented in FIG. 2 for two 
TEOS(50)-PTMO(2K)-100-0.014 ceramers aged for ten days. One sample was 
catalyzed with HCl and the other with PSS. The storage modulus is similar 
for the two systems throughout the temperature range scanned. The broad 
tan delta peak indicates by its maximum that the T.sub.g (-15.degree. C.) 
of the PSS ceramer was slightly lower than the T.sub.g (-10.degree. C.) of 
the HC1-catalyzed ceramer. This behavior is believed to be caused by a 
higher degree of reaction. This is contrary to the expected tan delta 
behavior for a network forming system. A model, proposed for a related 
system describes two types of motional restrictions on the PTMO oligomers 
that give rise to the tan delta peak (see H. Huang et al., (Polym. 
Preprints, 28(2), 1987). A TYPE 1 restriction would occur as the PTMO 
segments are reacted at both ends into the silicate network. This 
restriction would increase the amount of thermal energy necessary for 
inducement of notion in the oligomer and thus raise its T.sub.g slightly. 
The second restriction, TYPE 2, would be the result of encapsulation or 
mixing of the PTMO oligomer within the condensed TEOS network. The TYPE 1 
restriction would be expected to predominate in a system with poor phase 
mixing. The TYPE 2 restriction would be prevalent as both the level of 
phase mixing and the extent of reaction increase, respectively. The PSS 
tan delta peak's magnitude is decreased and the modulus increases compared 
to the HC1. These observations suggest a slightly higher level of 
incorporation of the PTMO segments using PSS as a catalyst. The peak tan 
delta temperature indicates an increased condensation of the silicate 
structure (see S. Sakka et al., J. of Non-Crystall. Solids 24 (1986). 
To further investigate the type of structure developing in these systems, 
the effect of higher levels of TEOS on the modulus and tan delta behavior 
were studied. The dynamic mechanical behavior, as a function of TEOS 
concentration, for a PSS-type ceramer based on the 2000 gram/mole 
endcapped PTMO oligomer is presented in FIG. 3. The storage moduli in the 
rubbery region increases from about 10.sup.7 to 10.sup.8 Pa with an 
increasing level of TEOS from 40 to 70 wt %. Note also the shift in the 
maximum amount of TEOS along with a decrease in its magnitude. All the tan 
delta peaks appear to start around -80.degree. C. then become quite broad 
as the TEOS is increased. This agrees with previous literature studies on 
similar formulations with HCl as the catalyst. Similar trends are observed 
for the lower molecular weight oligomers of silane endcapped PTMO. 
The corresponding stress-strain behavior of this series of PSS catalyzed 
ceramers (FIG. 4) clearly demonstrates an increase in the modulus at 
25.degree. C. corresponding to an increase in the level of TEOS. The 
modulus for the 40 wt % TEOS ceramer is 25 MPa compared to 250 MPa for the 
modulus of the 70 wt* TEOS sample. These values, measured ten days after 
casting, compare to equivalent compositions of the HCl ceramers which were 
tested thirty days after casting. This further supports the observation 
the PSS catalyzed systems proceed to a somewhat higher extent of reaction 
than the HC1 catalyzed ceramer of comparable compositions for the same 
time period. 
The SAXS behavior of a PSS ceramer and a HCl ceramer is presented in FIG. 5 
as a plot of the smeared intensity versus s, the scattering vector. The 
scattering vector is defined as 2 sin theta lambda, where theta is 
one-half the radial scattering angle and lambda is the wavelength. The 
existence of a maximum in the scattering profiles indicates there is a 
characteristic distance in the scattering source. This suggests a 
correlation distance may exist as a result some degree of microphase 
separation. 
The reciprocal of s which can be used as an estimate of the correlation 
length, and is similar for the two samples, implies their morphologies are 
not significantly different. The somewhat higher intensity for the PSS 
ceramer compared to the HCl catalyzed material is however further support 
of a higher level of reaction in the PSS catalyzed sample. Poly(styrene 
sulfonic acid), PSS, is an effective catalyst for the preparation of novel 
hybrid materials prepared by the incorporation of oligomers into a sol gel 
derived network. The elastic modulus at 25.degree. C. increased from 25 to 
250 MPa as a function of an increasing level of TEOS from 40 to 70 wt %. 
The stress at break is higher for the PSS catalyzed ceramer as compared to 
the HCl catalyzed material of comparable degree of reaction. The PSS 
ceramer had a maximum tan delta peak 15.degree. C. lower than the HCl 
catalyzed ceramer. The SAXS behavior was similar for both systems. 
Overall, we have noted no significant changes in the ultimate mechanical 
properties or structure (morphology) for a comparable degree of reaction. 
In summary, the catalytic nature of the polymeric acid catalyst in those 
novel hybrid materials indicates they may apply equally well as a catalyst 
for classical sol-gel reactions and possibly offer control of rheological 
behavior.