Polymer Composites Obtained by Polymerization of Microemulsions

Department of Chemical Engineering, The University of Akron, Akron, Ohio ... An analagous surfactant-free system was also studied and compared with th...
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Langmuir 1997, 13, 617-622

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Polymer Composites Obtained by Polymerization of Microemulsions Formed with Inorganic and Organic Monomers Ramachandra Mukkamala and H. Michael Cheung* Department of Chemical Engineering, The University of Akron, Akron, Ohio 44325-3906 Received August 23, 1995. In Final Form: November 27, 1996X Phase behavior studies of microemulsions containing the monomers methyl methacrylate (MMA), acrylic acid (AA), and tetraethoxysilane (TEOS), the cross-linking agent ethylene glycol dimethacrylate (EGDMA), and water (W), stabilized by the surfactant cetyltrimethylammonium bromide (CTAB), were carried out. An analagous surfactant-free system was also studied and compared with the surfactant-based system. Macroscopically single-phase, transparent microemulsions were formed over a considerable portion of the composition domain for the surfactant-based system. The effects of hydrochloric acid (HCl) and base (NH4OH) on the gel times were examined. Selected compositions in the surfactant-based system were chosen to examine the effect of the water to TEOS ratio (R) on the thermal stability and pore morphology of the polymer composite formed from the hydrolysis and condensation reactions of TEOS and the polymerization of organic-monomer-containing microemulsions. The polymer composites thus obtained exhibit high thermal stability and uniform pore morphology. Thermal stability and pore morphology studies were carried out using differential scanning calorimetry and scanning electron microscopy, respectively. The effect of nitrogen purging on gel time and the effect of EGDMA (a cross-linking agent) on pore morphology are also discussed.

Introduction The sol-gel process is a well-known route for the formation of prepolymers with viscosities that are suitable for various coating techniques, for example dipping and spraying. A variety of coatings and films have also been developed using sol-gel methods.1-3 This process is a relatively new method for preparing ceramic composites.4 Although this process is advantageous in obtaining potentially higher purity and homogeneity products at lower processing temperatures compared with traditional glass-melting or ceramic powder methods, the shrinkage and densification of the gel layer during drying or sintering leads to cracks with layer thicknesses more than a few micrometers.1,4 Schmidt et al.1 and Chujo et al.5 found that the introduction of organics causes a decrease of the network connectivity and increases relaxation ability which can help in overcoming these problems. They also mentioned that the hardness or abrasion resistance is not significantly affected by organics. Yamauchi et al.,6 Arriagada et al.,7 and Espiard et al.8 found that it is possible to obtain nanometer-sized, monodisperse, spherical particles of hybrid silica solids by the hydrolysis of TEOS (tetraethoxysilane) in oil-in-water (o/w) or waterin-oil (w/o) microemulsions stabilized by a suitable surfactant. In a related development, recent investigations * Author to whom correspondence should be addressed. E-mail: [email protected]. X Abstract published in Advance ACS Abstracts, February 1, 1997. (1) Schmidt, H.; Rinn, G.; Nab, R.; Sporn, D. In Better Ceramics Through Chemistry III; Brinker, C. J., Clark, D. E., Ulrich, D. R., Eds.; Materials Research Society Proceedings 121; Materials Research Society: Pittsburgh, PA, 1988; pp 743-754. (2) Chatelon, J. P.; Terrier, C.; Bernstein, E.; Berjoan, R.; Roger, J. A. Thin Solid Films 1994, 247, 162-168. (3) Bertoluzza, A.; Fagnano, C.; Morelli, M. A.; Gottardi, V.; Guglielmi, M. J. Non-Cryst. Solids 1982, 48, 117-128. (4) Ulrich, D. R. Chemtech 1988, April, 242-249. (5) Chujo, Y.; Saegusa, T. Adv. Polym. Sci. 1991, 100, 11-29. (6) Yamauchi, H.; Ishikawa, T.; Kondo, S. Colloids Surf. 1989, 37, 71-80. (7) Arriagada, F. J.; Osseo-Asare, K. J. Dispersion Sci. Technol. 1994, 15 (1), 59-71. (8) Espiard, P.; Mark, J. E.; Guyot, A. Polym. Bull. 1990, 24, 173179.

S0743-7463(95)00712-8 CCC: $14.00

have shown that uniform nanometer-sized porous polymeric solids can be obtained by polymerizing suitable monomer-containing microemulsions.9,10 Microemulsions are isotropic, thermodynamically stable mixtures of oil, water, and surfactant.11,12 The characterization studies done by Raj et al.10 on the microemulsion system of methyl methacrylate (MMA), acrylic acid (AA), and water showed that uniform porous solids can be obtained by polymerizing the Winsor-IV regions (macroscopically single-phase bicontinuous w/o or o/w microemulsions) with sodium dodecyl sulfate (SDS) as surfactant. We investigated the possibility of preparing precursors for obtaining composites formed by the hydrolysis reaction of the sol-gel process coupled with polymerization of an organic-monomer-containing microemulsion system and obtained products with controlled microstructure, high thermal stability, and homogeneity. To achieve our goal, the phase behavior of a microemulsion system containing two organic monomers (MMA and AA), an inorganic monomer (TEOS), and water, using cetyltrimethylammonium bromide (CTAB) as the surfactant, was studied. The phase behavior of a similar system without using surfactant was also studied, and the natures of the phase behaviors were compared. The reason for selecting the cationic surfactant CTAB for this system is 2-fold. It increases the flocculating action in a silica sol and also stabilizes the microemulsion system.13 Macroscopically single-phase, transparent regions in a large area of the ternary phase diagram were observed when the surfactant CTAB was used, unlike the surfactant-free microemulsions which showed a large area of two-phase, opaque regions in the phase diagram. Though the exact chemistry and mechanism of the solgel process is not known, the general reaction of the sol(9) Stoffer, J. O.; Bone, T. J. Dispersion Sci. Technol. 1980, 1 (4), 393-412. (10) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. Langmuir 1991, 7, 2586-2591. (11) Borkovec, M.; Eicke, H.-F.; Hammerich, H.; Gupta, B. D. J. Phys. Chem. 1988, 92, 206-211. (12) Friberg, S. E.; Rong, G.; Yang, C. C.; Yang, Y. In Polymer Assoc. Struct. Microemulsions and Liquid Crystals; El-Nokaly, M., Ed.; ACS Symposium Series 384; 1989; pp 34-46. (13) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979.

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gel process to form highly porous silica oxide (SiO2) involves at least hydrolysis, condensation, and densification steps.3,14-18,29 cat.

nSi(OC2H5)4 + 4nH2 98 nSi(OH)4 + 4nC2H5OH (hydrolysis) cat.

nSi(OH)4 98 nSiO2(gel) + 2nH2O (polycondensation) ∆T

nSiO2(gel) 98 nSiO2(glass)

(densification)

There are many parameters that affect the microstructure of the precursors formed by the sol-gel process and polymerization of microemulsions. Among them, pH, the R ratio (water to TEOS molar ratio), gelation time, temperature, nature of electrolyte, electrolyte concentration, nature of solvent, and type of alkoxide are the variables of major importance.2,13-16,19-22 The present study investigates the effects of acid and base on gelation time and examines whether the introduction of organic monomers (MMA, AA) has any adverse effect on the chemistry of the sol-gel process. The study also analyzes the effect of the R ratio on thermal stability, microstructure, and the homogeneity of the inorganic- and organicbased polymer composite formed by polymerizing these microemulsions stabilized by CTAB. Experimental Section TEOS, AA, and MMA were obtained from Aldrich and were of purity greater than 98%. The water was triple-distilled. CTAB, 95% purity, was obtained from Aldrich. Hydrochloric acid (HCl) with normality 12.1 and ammonium hydroxide (NH4OH) with normality 14.8 were obtained from Fisher Scientific. Samples for phase behavior and characterization studies were prepared by weighing or pipetting the required amounts of various components into clean glass tubes which were then sealed. The amount of CTAB used for the stabilization of surfactantcontaining microemulsion was always 15 wt % of the total sample prepared. Acrylic acid/methyl methacrylate solution was prepared in the ratio of 3:1 by weight, respectively. The phase behavior studies were done at 25 + 0.1 °C and 1 atm. The viscosity measurements were done using a Brookfield LVT digital viscometer having a small sample adapter with provision to control (14) Kaiser, A.; Schmidt, H. J. Non-Cryst. Solids 1984, 63, 261-271. (15) Bechtold, M. F.; Vest, R. D.; Plambeck, L., Jr. J. Am. Chem. Soc. 1968, 90, 4590-4598. (16) Hench, L. L.; West, J. K. Chem. Rev. 1990, 90, 33. (17) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press, Inc.: New York, 1990. (18) Dislich, H.; Hinz, P. J. Non-Cryst. Solids 1982, 48, 11-16. (19) Brinker, C. J.; Keefer, K. D.; Schaefer, D. W.; Ashley, C. S. J. Non-Cryst. Solids 1982, 48, 47-64. (20) Mukherjee, S. P. J. Non-Cryst. Solids 1980, 42, 477-488. (21) Chang, N. J.; Kaler, E. W. Langmuir 1986, 2, 184-190. (22) Aelion, R.; Loebel, A.; Eirich, F. J. Am. Chem. Soc. 1950, 72, 5705-5712. (23) Wunderlich, B. Thermal Analysis; Academic Press, Inc.: New York ,1990. (24) Sakka, S.; Kamiya, K.; Yoko, Y. ACS Symp. Ser. 1988, 360, 345-353. (25) Adams, J.; Baird, T.; Braterman, P. S.; Cairns, A. J.; Segal, D. L. In Better Ceramics Through Chemistry III; Brinker, C. J., Clark, D. E., Ulrich, D. R., Eds.; Materials Research Society Proceedings 121; Materials Research Society: Pittsburgh, PA, 1988; pp 361-366. (26) Debsikdar, J. C. Adv. Ceram. Mater. 1986, 1 (1), 93-98. (27) Mizuno, T.; Phalippou, J.; Zarzycki, J. Glass Technol. 1985, 26 (1), 39-45. (28) Nogami, M.; Moriya, Y. J. Non-Crystalline Solids 1980, 37, 191201. (29) Scherer, G. W. In Better Ceramics Through Chemistry III; Brinker, C. J., Clark, D. E., Ulrich, D. R., Eds. Materials Research Society Proceedings 121; Materials Research Society: Pittsburgh, PA, 1988; pp 179-197.

the sample temperature. The sample temperature was maintained at 25 + 0.1 °C, and the measurements were done at a shear rate of 79.2/s. The same instrument was used for examining the effect of acid and base on the gelation rate. However, due to limitations in the range of values the instrument can display, a procedure which involved lowering the shear rate in steps as the viscosity increased was used. As the display reached its maximum the next lower shear rate was selected. This procedure was used for all of the gel time samples. Ten gram samples containing 20 wt % AA:MMA mixture (3:1), 20 wt % water, and 60 wt % of TEOS were prepared, a fixed number of moles of acid or base were added, and the sample was thoroughly stirred prior to use in the gelation time study. Several HCl to TEOS molar ratios (r), 0.01, 0.10, 0.15, 0.20, and 0.25, were studied, and the viscosity was recorded as a function of time with the shear rate shifting as described above. The time where the viscosity of the sample reached 100 poise was considered as the gelation time. The same procedure, with the same molar ratios, was used for examining the effect of base, NH4OH, on gelation time. The pH of the samples was measured using an Orion 420-A pH meter. Samples for polymerization were prepared by purging the microemulsion samples with dry nitrogen gas at a rate of 0.56 L/h at 1 atm for 15 min. The cross-linking agent, ethylene glycol dimethacrylate (EGDMA), and initiator, 2,2-dimethoxy-2-phenylacetophenone (DMPA), of 99% purity or above were obtained from Aldrich. The amount of DMPA used to initiate the free radical polymerization reaction was 0.02 g for a 10 g microemulsion sample. The amount of EGDMA used was always 8 wt % of the total MMA and AA present. The samples were thoroughly mixed using a Scientific Industries model G-560 Vortex Genie-2 and equilibrated in a constant-temperature bath at 25 + 0.1 °C, for a period of about 24 h until the hydrolysis reaction between the inorganic monomer, TEOS, and water had taken place and gel was formed. These samples were prepared in 30 mL glass tubes. Following the gelation of the inorganic phase, the samples were photopolymerized in a custom visible light photoreactor. The photoreactor was maintained at a controlled temperature by circulating water from a refrigerated circulator through it and uses a 500 W quartz-halogen lamp. The polymerization was carried out for a duration of 1 h at a temperature of 25 + 0.1 °C. At this point the samples were organic/inorganic polymer composites. The thermal stability of the polymer composite at different R ratios were analyzed using a Dupont Instruments-910 DSC system along with a Dupont thermal analyst 2100 system. The polymer composite was powdered and dried at 35 °C for 2 days. Then 10-20 mg of the sample was hermetically sealed in a highpressure sample pan. The sample was placed in the DSC cell and was heated to 500 °C at a ramp rate of 25 °C/min. The degradation temperature of the polymer composite was observed from the sharp endothermic peak in the plot of heat flow versus temperature.23 The polymer surface morphology for the polymer composite with different R ratios was examined under a ISI SX-40 scanning electron microscope (SEM). The samples were dried at room temperature for 7 days before being analyzed under SEM.

Results The pseudoternary phase diagrams of the surfactantfree microemulsion system of TEOS, AA, MMA, and water equilibrated at 25 ( 0.1 °C observed 24 and 120 h after sample preparation are shown in Figures 1 and 2, respectively. The phase behavior of the surfactant-free system after 24 h shows three distinct regions labeled A, B, and C. Compositions from region A are two-phase samples with the bottom phase being opaque precipitate and the top phase being a translucent sol. Compositions from region B are macroscopically single-phase transparent sols, and those in region C are two-phase transparent sols. The phase behavior of the same system 120 h after preparation showed four distinct regions labeled D, E, F, and G. Region D consists of two-phase samples with the bottom phase being opaque precipitate and the top phase being a translucent sol but with larger phase separation than was observed for region A. The very small region E was a transparent gel. The region F was a two-phase

Polymerization of Microemulsions

Figure 1. Pseudoternary phase diagram for the surfactantfree system TEOS, water, AA/MMA (3:1) 24 h after preparation. The samples were equilibrated at 25 ( 0.1 °C. Compositions are on a weight basis. Domains: A, two macroscopic phases with an opaque lower phase and a translucent upper phase; B, single macroscopic, transparent phase; C, two macroscopic, transparent phases.

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Figure 4. Pseudoternary phase diagram for the surfactantcontaining system TEOS, water, AA/MMA (3:1) 120 h after preparation. CTAB content was 15 wt% based on the total sample mass. The samples were equilibrated at 25 ( 0.1 °C. Compositions are on a weight basis. Domains: A′, two macroscopic phases with a gelled lower phase and a transparent upper phase; B′, translucent gel; C′, transparent liquid phase and a small amount of precipitate; D′, transparent gel; E′, transparent liquid phase; F′, opaque liquid phase. Table 1. Sample Data for Gellation of the Acid-Catalyzed Systema S no. 1 2 3 4 5

Figure 2. Pseudoternary phase diagram for the surfactantfree system TEOS, water, AA/MMA (3:1) 120 h after preparation. The samples were equilibrated at 25 ( 0.1 °C. Compositions are on a weight basis. Domains: D, two macroscopic phases with an opaque lower phase and a translucent upper phase; E, transparent gel; F, two macroscopic, transparent phases; G, two macroscopic phases with an opaque lower phase and a transparent gel as the upper phase.

Figure 3. Pseudoternary phase diagram for the surfactantcontaining system TEOS, water, AA/MMA (3:1) 24 h after preparation. CTAB content was 15 wt% based on the total sample mass. The samples were equilibrated at 25 ( 0.1 °C. Compositions are on a weight basis. Domains: A′, two macroscopic phases with a gelled lower phase and a transparent upper phase; B′, translucent gel; C′, transparent liquid phase and a small amount of precipitate; D′, transparent gel; E′, transparent liquid phase; F′, opaque liquid phase.

transparent solution similar to region C, and the region G was a two-phase system with the bottom phase being opaque precipitate and the top phase being a translucent gel. The change in the phase behavior for the two different times is due to the hydrolysis and condensation reactions of TEOS. The corresponding phase diagrams for the microemulsion system containing CTAB (15 wt % of the total sample) as the surfactant equilibrated at 25 + 0.1 °C and observed 24 and 120 h after sample preparation are shown in Figures 3 and 4, respectively. The phase diagram after 24 h showed six distinct regions labeled A′, B′, C′, D′, E′, F′, and G′. Apart from the regions A′ and F′, which were two-phase samples with the bottom phase gelled and the

b

AA:MMA (3:1) TEOS water HCl:TEOS pH of the (% by wt) (% by wt) (% by wt) (molar ratio) r solution 20 20 20 20 20

60 60 60 60 60

20 20 20 20 20

0.01 0.10 0.15 0.20 0.25

0.29 -b -

a CTAB content was 15 wt % based on the total sample mass. “-” indicates the data beyond the pH range 0-14 scale.

top phase as a clear transparent solution and an opaque sol, respectively, all the other regions were found to be macroscopically single-phase transparent gels or sols. The details of the regions are shown in Figure 3. The phase diagram after 120 h shows the same regions with the exception that the gelled regions after 24 h were extended into the sol regions and the sol regions were decreased. This is due to the hydrolysis and condensation reactions of TEOS. On the whole, from the surfactant-free and surfactant-based phase behavior studies it was observed that a large area of macroscopically single-phase microemulsion regions was obtained when 15 wt % of cationic surfactant CTAB was used, unlike the surfactant-free system which resulted in a large area of two-phase, opaque regions. Even though ordinary care was used in our sample preparations, we observed that the gelation times of the microemulsions varied (in the range of 10-24 h) when different batches of AA, MMA, and TEOS were used, possibly because of small variations in the overall pH of the solution. To avoid these variations in the gelation times, the same batch of AA, MMA, and TEOS, EGDMA, DMPA, and HCl, NH4OH, and triple-distilled water with pH ) 6.97 was used throughout the phase behavior and characterization studies. For studying the effects of acid and base on this microemulsion system, a sample composition with the stoichiometric ratio of water to TEOS (1:3 by wt, 4:1 by mol) for the hydrolysis reaction was chosen. All studies were conducted at 25 ( 0.1 °C. The composition of the sample selected was 20% monomer by wt (AA and MMA) with an AA to MMA ratio of 3:1, 20 wt % water and 60 wt % TEOS. The CTAB added was 15 wt % of the total sample prepared. The acid and base used were 12.1 N HCl and 14.8 N NH4OH, respectively. The molar ratios r [HCl to TEOS or NH4OH to TEOS] 0.01, 0.10, 0.15, 0.20, and 0.25 were chosen, and a gelation time study for both acid and base were performed. The data are shown in Table 1 and Table 2, and the results are shown in Figures 5 and 6, respectively. For acid, the gel time decreases from 728 to 130 min as the r ratio increases and the

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Table 2. Sample Data for Gellation of the Base-Catalyzed Systema S no.

AA:MMA (3:1) TEOS water NH4OH:TEOS pH of the (% by wt) (% by wt (% by wt) (molar ratio) r solution

1 2 3 4 5 aCTAB

20 20 20 20 20

60 60 60 60 60

20 20 20 20 20

0.01 0.10 0.15 0.20 0.25

5.83 7.72 8.10 8.33 9.41

content was 15 wt % based on the total sample mass.

Figure 7. Viscosity versus time as a function of acid content as measured by the HCl to TEOS mole ratio (r ratio) for the TEOS, AA/MMA (3:1), water, CTAB (15 wt % of total sample) system with nitrogen purging.

Figure 5. Viscosity versus time as a function of acid content as measured by the HCl to TEOS mole ratio (r ratio) for the TEOS, AA/MMA (3:1), water, CTAB (15 wt % of total sample) system without nitrogen purging.

Figure 8. Viscosity versus time as a function of base content as measured by the NH4OH to TEOS mole ratio (r ratio) for the TEOS, AA/MMA (3:1), water, CTAB (15 wt % of total sample) system with nitrogen purging.

Figure 6. Viscosity versus time as a function of base content as measured by the NH4OH to TEOS mole ratio (r ratio) for the TEOS, AA/MMA (3:1), water, CTAB (15 wt % of total sample) system without nitrogen purging.

reaction is exothermic. For base, the gel time decreases from 218 to 53 min. It was found that the gel time decreases for both acid and base as the r ratio increases. Both acid and base catalyze the hydrolysis and condensation reaction in proportion to the r ratio. It was also observed that the gel time using base as catalyst is shorter than that observed for acid at the same r ratio. A similar experiment was conducted to study the effect of purging on the gel time using the same r ratios for the same sample composition. Our motivation for this part of the work was mainly due to our experience that purging was often necessary prior to polymerizing organicmonomer-containing microemulsions. Dry nitrogen gas

was purged through the sample for 15 min, and gel times were determined using a viscometer. The gel time plots with nitrogen purging are shown in Figures 7 and 8, respectively. It was found that nitrogen purging has an insignificant effect on the gelling of the sample except at the highest r ratios studied, where it increased the gel times. Nitrogen purging did not have any effect on the pH of the samples. For studying the effect of water (W) to TEOS molar ratio R on thermal stability and pore morphology, the following wt % compositions of the system with molar R ratios 1.65, 3.81, 5.77, and 19.18 were chosen: 20% AA/ MMA (3:1), 70% TEOS, and 10% W (system I); 20% AA/ MMA (3:1), 60% TEOS, and 20% W (system II); 20% AA/ MMA (3:1), 40% TEOS, and 40% W (system III); and 20% AA/MMA (3:1), 30% TEOS, and 50% W (system IV). The compositions were on weight basis. The samples were polymerized, and the polymer composites were obtained as per the procedure discussed in the Experimental Section. The reason for selecting these R ratios was to analyze the samples above and below the stoichiometric composition of water and TEOS, i.e., R ) 4 (from hydrolysis reaction). The cross-linking agent EGDMA was 8 wt % of the total AA and MMA monomer used in the system,

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Table 3. Degradation Temperatures as Measured by Differential Scanning cClorimetry system

R ratio

degradation temperature

I II III IV

1.65 3.81 5.77 19.18

373 °C 364 °C 333 °C 346 °C

the surfactant CTAB was 15 wt % of the total sample, and the initiator DMPA was 0.2 wt % of the total sample. The organic monomer composition is fixed, and only the effect of R ratios was studied. It was observed that the polymerized samples for systems III and IV (higher R ratios) were opaque and those of systems I and II (lower R ratios) were transparent. Effect of R Ratio on Thermal Stability. The DSC endotherm peaks observed for all four systems indicated that the degradation of the organic phase in the polymer composite occurs between 330 and 370 °C. The degradation temperatures observed from the DSC plots for systems I-IV are shown in Table 3. The degradation temperatures were higher at low R ratios than at high R ratios. These degradation temperatures are considerably higher than those obtained for AA/MMA copolymers, which were in the range of 275 °C. Effect of R Ratio on Pore Morphology. SEM examination of the polymer composites showed a substantial influence of R ratio on the morphology. Systems I and II (R ratio range 1-4) exhibited finer cell structure (in the micron size range) than systems III and IV (R ratio range 5.5-20) which exhibited cell structure in the 10 µm size range. The SEM micrographs for systems I-IV are shown in Figure 9a-9d, respectively. Discussion As has been previously observed, the organic monomer AA exhibits limited surfactant-like characteristics10 in the MMA/AA/water system. The pseudophase behavior for the surfactant-free and surfactant-based study was done to ensure the effect of the amphiphilic nature of AA on the system. It was found that the solubilizing effect of AA on the system TEOS/MMA/water is not significant as the system resulted in a very small region of macroscopically single-phase transparent systems, E, at very low TEOS and high water and AA/MMA compositions. The study showed that transparent, macroscopically single-phase microemulsions can be formed over a considerable portion of the composition domain with a surfactant-based microemulsion system. We noted that even though ordinary care was taken in their preparation, different batches of AA, MMA, TEOS, and CTAB exhibited varying gelation times, possibly due to small variations in the sample pH. The polymerization studies for making composites were done only after the gelation of the sample took place. The DSC thermal degradation results and SEM results obtained for representative samples prepared using different batches were reproducible when polymerization was done after the sample gelled, although the gel times varied in the range 10-24 h. The acid and base catalytic effects on gel time have been extensively studied.14,16,24-28 It is known that acid promotes the rate of hydrolysis reaction more than the polycondensation reaction and base promotes the condensation reaction more than the hydrolysis reaction. Polycondensation creates additional bridging bonds and increases the viscosity. Hence the gelation time is shorter for a base-catalyzed system compared to that for an acidcatalyzed system. To examine whether the incorporation of an organic phase into the reactive TEOS and water system would

affect the chemistry of the gelation process, a qualitative gel time study on the present system was performed using HCl and NH4OH. Parallel samples were studied: one prepared with purging of the sample with dry nitrogen and the other without nitrogen purging (Figures 5-8). The results indicated that the chemistry was not affected by the incorporation of organics, and the nitrogen-purging studies indicated that the effect of nitrogen purging is significant only at high concentrations of acid or base. At high acid or base concentrations the gel time was increased by nitrogen purging, indicating that both the hydrolysis and condensation reaction were delayed. Photopolymerization of the precursor microemulsions to obtain polymer composites requires nitrogen purging as oxygen is an inhibitor for the free radical reaction30 and will lead to low molecular weight polymers. In this perspective, nitrogen purging at low r ratios will be effective for the characterization studies, and this leads to a conclusion that nitrogen purging has no significant effect as long as the r ratios are small, i.e., 0.01-0.15. From the gel time studies for the present system, high gelation rates were observed for pH above 8.0; this agrees with the observations of Mukherjee.20 A high AA:MMA ratio (3:1 by wt) was selected to form a copolymer which will have high glass transition temperature, flexibility, and mechanical properties better than those of the individual homopolymers.10,31 The reason for the opacity of the polymerized samples for systems III and IV at high R ratios could be because of the effect of cross-linking agent EGDMA which would be in excess as the monomer concentrations were less and affects the transparency of the composite. This is in agreement with the observation reported by Raj et al.10 that the amount of EGDMA used also plays an important role in increasing the transparency of the polymer. Hiemann et al.32 reported that agitation of the sample will improve the homogeneity of the ceramic composites. To study this phenomena, some preliminary experiments were done on representative samples, and the results indicated that the transparency of the sample is improved when the sample is agitated until the viscosity of the sample reaches about 10 poise. This gives an indication that the effect of EGDMA on polymer transparency can be controlled by agitating the sample. The endothermic peaks around 350 °C for the systems I-IV were indicative of the thermal degradation of the copolymer AA and MMA (organic phase). The thermal degradation temperatures of the copolymer AA and MMA obtained without the presence of inorganic phase are around 275 °C.33 The high degradation temperatures of the composites compared to the degradation temperature of the copolymer were due to the very strong network composite material formed by the gelation of TEOS and polymerization of the organic phase. This thermally stable composite material can now be fused into porous ceramic solids by removing surfactant from the gel by extraction with excess water and by burning off the organic phase and evaporating the residual alcohol, water, and other inorganic solvents which find a very important role in making porous films and also in fiber-coating technology.34 (30) Odian, G. Principles of Polymerization; Wiley: New York, 1981. (31) Billmeyer, F. W. Text Book of Polymer Science; Wiley: New York, 1981. (32) Hiemann, P. J.; Hurwitz, F. I.; Rivera, A. L. Adv. Compos. Mater. 1992, 19, 27-33. (33) Palani Raj, W. R. Development of a Microemulsion Route to Microporous Polymeric Materials. M.S. Thesis, University of Akron, Akron, OH, 1992. (34) Friberg, S. E.; Yang, C. C. In Innovations in Materials Processing Using Aqueous, Colloid and Surface Chemistry, Doyle, F. M., Raghavan, S., Somasundaram, P., Warren, G. W., Eds. The Minerals, Metals & Materials Society: Warrendale, PA, 1988; pp 181-191.

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Figure 9. (a, top left) SEM micrograph for system I, R ) 1.65. (b, top right) SEM micrograph for system II, R ) 3.81. (c, bottom left) SEM micrograph for system III, R ) 5.77. (d, bottom right) SEM micrograph for system IV, R ) 19.18.

The surface morphology studies indicate that the pore sizes for systems III and IV were considerably larger than the dimensions expected for microemulsion. This may be attributed to the phase separation during gelation and polymerization or may be an artifact from the SEM sample preparation procedures. We have observed considerable discrepancy between the pore sizes observed using SEM and those inferred from freezing point depression measurements in polymerized microemulsions. The transition from closed cell structure to open cell structure as the R ratio is increasing indicates that the microemulsions formed at low R values (systems I and II) were w/o type microemulsions and those formed at high R ratios were bicontinuous microemulsions.35 Conclusions A large domain of transparent macroscopically singlephase microemulsions were obtained using 15 wt % CTAB for the system containing both inorganic and organic monomer species, TEOS and AA/MMA with water. Both hydrochloric acid and ammonium hydroxide catalyze the hydrolysis and polycondensation reactions. Higher acid to TEOS and base to TEOS molar ratios resulted in an (35) Sasthav, M.; Palani Raj, W. R.; Cheung, H. M. J. Colloid Interface Sci. 1992, 152 (2), 376-385.

increase in the rate of gelation. A gel time study indicated that there is no effect of the incorporation of organic monomers (AA and MMA) on the chemistry of the hydrolysis and condensation reaction of TEOS. The thermal stability of the polymer composite obtained in these studies was higher than that of the copolymer obtained without the inorganic monomer TEOS. The study also reveals that the homogeneity and pore morphology of the polymer composite are comparable to those observed in organic-monomer-polymerized microemulsion systems. The nitrogen purging was found to increase gel time for high r ratios. Surface morphology studies indicated that it is possible to obtain polymer composites of uniform pore size and homogeneity by controlling R ratio, r ratio, EGDMA composition, and agitation. Work is currently in progress in our laboratory to characterize and investigate the effects of cross-linking agent, agitation, and acid and base concentrations on the properties of the polymer composites formed. Acknowledgment. Many thanks to Dr. W. R. Palani Raj, Kimberly Clark Corp., Neenah, WI, and Dr. Mohan Sasthav, Westvaco Corp., Columbia, MD, for their helpful discussions. LA950712G