Pervaporation Characteristics of Organic−Inorganic Hybrid

Swelling of poly(vinyl alcohol-co-acrylic acid) (P(VA-co-AA)) membranes in aqueous alcohol solutions operated under pervaporation conditions leads to ...
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Macromolecules 2005, 38, 8440-8446

Pervaporation Characteristics of Organic-Inorganic Hybrid Membranes Composed of Poly(vinyl alcohol-co-acrylic acid) and Tetraethoxysilane for Water/Ethanol Separation Tadashi Uragami,* Hiroshi Matsugi, and Takashi Miyata Unit of Chemistry, Faculty of Engineering and High Technology Research Center, Kansai University, Suita, Osaka 564-8680, Japan Received July 5, 2005; Revised Manuscript Received August 8, 2005

ABSTRACT: Swelling of poly(vinyl alcohol-co-acrylic acid) (P(VA-co-AA)) membranes in aqueous alcohol solutions operated under pervaporation conditions leads to low water/ethanol selectivity. To reduce swelling, organic-inorganic hybrid membranes composed of P(VA-co-AA) and tetraethoxysilane (TEOS) were prepared. However, when an aqueous ethanol solution was permeated through the P(VA-co-AA)/ TEOS hybrid membranes by pervaporation, the permeation rate increased and the water/ethanol selectivity decreased with increasing TEOS content. The increase in the permeation rate and the decrease in the water/ethanol selectivity were caused by an increase in the degree of swelling of the membrane and a decrease in the membrane density with increasing TEOS content. These effects resulted from insufficient formation of hydrogen bonds between the silanol groups by hydrolysis of TEOS and the hydroxyl and carboxyl groups of P(VA-co-AA). When the P(VA-co-AA)/TEOS hybrid membranes were annealed, the water/ethanol separation factor increased with increasing annealing time and TEOS content. Longer annealing time promoted the dehydration-condensation reaction between P(VA-co-AA) and TEOS in P(VA-co-AA)/TEOS hybrid membranes, leading to enhanced water/ethanol selectivity of the hybrid membranes.

Introduction Alcohol is a clean energy source that can be made by fermentation of biomass. However, it is produced at low concentration and must be concentrated before industrial use. In general, an aqueous solution of dilute ethanol can be concentrated by distillation; however, ethanol and water form an azeotrope (96.5 wt % ethanol) that cannot be separated by distillation. On the other hand, membrane technology can be applied to separate an azeotropic water/ethanol mixture. Membrane separation techniques have attracted considerable attention in a wide variety of medical, food, industrial, energy, and environmental applications. Pervaporation is very useful for the separation of azeotropic mixtues1,2 and close-boiling-point chemicals3,4 and, in addition, can also offer advantages in energy savings. The chemical and physical properties of polymer membranes for pervaporation can be tailored by several methods to improve their performance. Organic-inorganic hybrid materials are expected to be the next generation, high-performance membranes in a variety of fields because they have both the functionality of organic compounds and the stability of inorganic compounds.5 Recently, it has been demonstrated that the size of the dispersed phase can be controlled on a nanometer- or molecular-size scale in multicomponent materials.6-9 These organic-inorganic hybrid materials have the merits of both the lightness and pliability of organic materials and, in addition, the heat resistance and stability of inorganic compounds. The organic-inorganic hybrid membranes are made by a low-temperature, solution-based sol-gel method.10-12 Using this sol-gel method, it is possible to homogeneously hybridize the organic and inorganic compo* To whom correspondence should be addressed.

nents. Previously, we prepared organic-inorganic hybrid membranes by the sol-gel reaction of (i) poly(vinyl alcohol) (PVA) and tetraethoxysilane (TEOS),13 (ii) quaternized chitosan and TEOS,14,15 and (iii) copolymers of butyl methacrylate and vinyltriethoxysilane (P(BMAco-VTES) and TEOS16,17 to improve the selectivity of polymer membranes. In this study, to improve the water/ethanol selectivity of PVA/TEOS hybrid membranes reported in a previous report,13 copolymers of PVA and poly(acrylic acid), P(VAco-AA), were prepared. In addition, P(VA-co-AA)/TEOS hybrid membranes were made, and their pervaporation characteristics for the separation of water and ethanol were determined. Experimental Section Materials. Poly(vinyl alcohol) (PVA), supplied by Nippon Synthetic Chemical Industry Co., Ltd., with an average degree of polymerization of 1650 and a degree of saponification of 99.7 mol %, was employed as the organic membrane component. Poly(vinyl alcohol-co-acrylic acid) (P(VA-co-AA)) membranes were synthesized by the following method: first, copolymers of vinyl acetate (VAc) with acrylic acid (AA) were polymerized using benzoyl peroxide as an initiator at 60 °C for 6 h; second, P(VA-co-AA) copolymers were prepared by saponification of poly(vinyl acetate-co-acrylic acid), P(VAc-co-AA), dissolved in 1,4-dioxane with an aqueous sodium hydroxide solution. Tetraethoxysilane (TEOS), purchased from Shin-Etsu Chemical Co., Ltd., was used as the inorganic component. All other solvents and reagents were purchased from Wako Pure Chemical Industries, Ltd., were of analytical grade and were used without further purification. Preparation of PVA Membranes. PVA powder was dissolved in DMSO at 80 °C to prepare a 5 wt % casting solution. After removal of the insoluble impurities using a glass filter, the solution was stirred for 1 h at 25 °C. The PVA membranes were made by pouring the casting solution onto stainless steel plates and then allowing the solvent to evaporate completely at 80 °C for 14 h.

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Preparation of P(VA-co-AA)/TEOS Hybrid Membranes. After a prescribed amount of TEOS was mixed with P(VA-coAA) to form a 5 wt % solution in DMSO at 25 °C, 1 M HCl was added to the P(VA-co-AA)/TEOS mixture as an acid catalyst for the sol-gel reaction. The P(VA-co-AA)/TEOS hybrid membranes were prepared by pouring the casting solutions onto stainless steel plates and then allowing the solvent to evaporate completely at 80 °C for 14 h. The membrane thickness used in this study was about 40-55 µm. Permeation Measurements. The pervaporation experiments were carried out using the apparatus described in previous studies18-27 under the following conditions: permeation temperature, 40 °C; permeate side pressure, 1 × 10-2 Torr (1.33 Pa). The effective membrane area was 13.8 cm2. An aqueous solution of 85 wt % ethanol was used as the feed solution. The permeate was collected in a U-tube at liquid nitrogen temperature. The permeation rates of aqueous alcohol solutions during pervaporation were determined from the weight of the permeate collected in the U-tube, the permeation time, and the effective membrane area. To facilitate a comparison of the permeation rates of membranes with different thickness, the normalized permeation rate (mkg/(m2 h)), which is the product of the permeation rate and the membrane thickness. The compositions of the feed solution and permeate were determined using a gas chromatograph (Shimadzu GC9A) equipped with a flame ionization detector (FID) and a capillary column (Shimadzu Co. Ltd.; Shimalite F) heated to 200 °C. The permeation results were reproducible, and the errors inherent in the permeation measurements were on the order of a few percent. The pervaporation separation factor, Rsepa.H2O/EtOH, was calculated from eq 1:

Rsepa.H2O/EtOH ) (PH2O/PEtOH)/(FH2O/FEtOH)

(1)

where FH2O and FEtOH, PH2O and PEtOH are the weight fractions of water and ethanol in the feed solution and the permeate, respectively. It is assumed that the separation factor for each membrane is not dependent on the membrane thickness. Membrane Density. The density of the membranes was measured by the flotation method28,29 using a mixed solution of benzene and carbon tetrachloride at 40 °C. Degree of Swelling of Membranes. The P(VA-co-AA) and P(VA-co-AA)/TEOS hybrid membranes were dried completely under reduced pressure at 40 °C and weighed. Thereafter, the membranes were immersed into an aqueous solution containing 85 wt % ethanol in a sealed vessel at 40 °C. After the weight of the membranes became constant, they were taken out of the vessel, wiped quickly with filter paper, and weighed. The degree of swelling (DS) of the membrane was determined by eq 2:

DS ) Ws/Wd

Figure 1. Effects of the AA content on the normalized permeation rate (O) and the water/ethanol separation factor (b) of an aqueous solution of 85 wt % ethanol through P(VAco-AA) membranes during pervaporation.

(2)

where Ws is the weight of the membrane swollen in an aqueous solution of 85 wt % ethanol and Wd is the weight of the dried membrane. Annealing of Membranes. The P(VA-co-AA) and P(VAco-AA)/TEOS hybrid membranes were placed between filter papers and annealed under a nitrogen atmosphere at 130 °C for 6, 10, and 15 h. Transmission Electron Micrographs (TEM). The P(VAco-AA) and P(VA-co-AA)/TEOS hybrid membranes were embedded in epoxy resin and sliced into thin films (thickness ≈50 nm) with a microtome (Leica; Reichert Ultracut E). The structure of the membranes was observed by a transmission electron microscope (TEM) (JEOL JEM-1210) at an accelerating voltage of 80 kV.

Results and Discussion Effect of AA Content. Figure 1 shows the effects of the AA content in the P(VA-co-AA) copolymer membranes on the water/ethanol separation factor and the

Figure 2. Effects of the TEOS content in the P(VA-co-AA)/ TEOS hybrid membranes on the normalized permeation rate (O) and the water/ethanol separation factor (b) of an aqueous solution of 85 wt % ethanol through the P(VA-co-AA)/TEOS hybrid membranes during pervaporation.

normalized permeation rate for an aqueous ethanol feed solution during pervaporation. The normalized permeation rate is the product of the permeation rate and the membrane thickness. As can be seen in Figure 1, both the normalized permeation rate and the separation factor showed a maximum at an AA content of 10 mol %. The normalized permeation rate and the separation factor of P(VA-co-AA) copolymer membranes were greater than those of a pure PVA membrane. This result suggests that PAA enhanced the performance a PVA membrane. On the other hand, permeation and separation characteristics of P(VA-co-AA) copolymer membranes depend significantly on the AA content. With increasing AA content in the P(VA-co-AA) copolymer, the increase in both the normalized permeation rate and the separation factor for membranes with less than 10 mol % content of AA can be attributed to an increase in the affinity of water for P(VA-co-AA). The decrease in both the normalized permeation rate and the separation factor for membranes containing more than 10 mol % AA can be attributed to a significant increase in the degree of swelling of the P(VA-co-AA) membrane. Because the P(VA-co-AA) copolymer membrane with an AA content of 10 mol % had the highest water/ ethanol separation factor, we used this polymer to prepare hybrid membranes with TEOS and investigated their permeation and separation properties for an aqueous ethanol solution during pervaporation. Effect of the TEOS Content on Permeation Characteristics. Figure 2 shows the effect of the TEOS content in P(VA-co-AA)/TEOS hybrid membranes on

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Figure 3. Effects of the TEOS content in the P(VA-co-AA)/ TEOS hybrid on the degree of swelling (O) of the P(VA-coAA)/TEOS hybrid membranes in an aqueous solution of 85 wt % ethanol and their density (b).

Figure 4. Effects of the TEOS content in the P(VA-co-AA)/ TEOS hybrid on the sorption selectivity (O) and the diffusion selectivity (b) for an aqueous solution of 85 wt % ethanol through P(VA-co-AA)/TEOS hybrid membranes.

their permeation and separation characteristics. As can be seen in Figure 2, with increasing TEOS content, the normalized permeation rate increased remarkably; however, the water/ethanol separation factor decreased. In general, cross-linking of a polymer membrane leads to an increase in separation factor and a decrease in permeation rate. However, in P(VA-co-AA)/TEOS hybrid membranes, the separation factor decreased and the normalized permeation rate increased with increasing TEOS content. This result indicates that TEOS does not act as a simple cross-linker. Thus, we determined the degree of swelling and the density of the P(VA-co-AA)/ TEOS hybrid membranes to clarify the effects of TEOS on the permeation and separation characteristics of P(VA-co-AA)/TEOS hybrid membranes. Figure 3 shows the effects of the TEOS content on the degree of swelling and the density of P(VA-co-AA)/ TEOS hybrid membranes. As can be seen in this figure, the degree of swelling of the P(VA-co-AA)/TEOS hybrid membrane increased with increasing TEOS content. Because swelling of the P(VA-co-AA)/TEOS hybrid membrane increased with higher TEOS content, this result suggests that the condensation reactions between the hydroxyl groups of the P(VA-co-AA) and the ethoxy groups of the TEOS and the formation of hydrogen bonds between the hydroxyl groups in the P(VA-co-AA) and the silanol groups in the hydroxyl silane molecules, which are produced by hydrolysis of TEOS molecules, did not proceed satisfactorily. The density of P(VA-co-AA)/TEOS hybrid membranes decreased with increasing TEOS content. This result is a further indication that the condensation reactions between P(VA-co-AA) and TEOS did not proceed to a significant extent, and consequently a cohesive structure between the silanol groups in the hydroxylsilane, which has a rough structure with a low density, was formed in the membrane. Analysis of Water/Ethanol Selectivity. On the basis of the solution-diffusion model, differences in the solubility of the permeants and differences in the diffusivity of the permeants in a polymer membrane determine its permselectivity30,31 Therefore, it is very important to evaluate the sorption selectivity and the diffusion selectivity to elucidate the separation mechanism of an aqueous ethanol solution through the P(VAco-AA)/TEOS hybrid membranes. Thus, the water/ ethanol selectivity of an aqueous ethanol solution through the P(VA-co-AA)/TEOS hybrid membranes was investigated from the viewpoint of the solution-diffu-

Scheme 1. Hydrolysis and Condensation Scheme for TEOS

sion mechanism. Both the sorption selectivity and the diffusion selectivity were determined as follows: the sorption selectivity, Rsorp.H2O/EtOH, was determined from the composition sorbed into the membrane using eq 3:

Rsorp.H2O/EtOH ) (MH2O/MEtOH)/(FH2O and FEtOH)

(3)

where MH2O and MEtOH, FH2O and FEtOH are weight fractions of water and ethanol in the membrane and the feed solution, respectively. The diffusion selectivity was calculated from the separation factor determined from the results in Figure 2 using eq 1 and the sorption selectivity using eq 3, as expressed by

Rdiff.H2O/EtOH ) Rsorp.H2O/EtOH/Rsepa.H2O/EtOH

(4)

Figure 4 shows the sorption selectivity and the diffusion selectivity of the P(VA-co-AA)/TEOS hybrid membranes as a function of the TEOS content. The diffusion selectivities were higher than the sorption selectivities. This result supports that permeation and separation of an aqueous ethanol solution through the P(VA-co-AA)/TEOS hybrid membranes depend significantly on the diffusion process of the permeants. The sorption selectivity decreased with an increase of the TEOS content, and the diffusion selectivity had a maximum at a TEOS content of 10 wt %. The decrease in the water/ethanol separation factor with increasing TEOS content is due to the fact that the selectivity decreased for both the sorption process and the diffusion process. Structure of P(VA-co-AA)/TEOS Hybrid Membranes. On the basis of the above results, the structure of P(VA-co-AA)/TEOS hybrid membranes is illustrated in Scheme 1, showing the process of the polycondensation reaction of TEOS.32 In the first step of the process of preparing P(VA-co-AA)/TEOS hybrid membranes,

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Scheme 2. Tentative Illustration for the Interaction between P(VA-co-AA) and TEOS

silanol groups were formed by hydrolyzing TEOS in the presence of an acid catalyst (A). The resulting silanol groups yielded siloxane bonds due to dehydration or dealcoholysis reactions with other silanol groups or ethoxy groups during the membrane drying process (B). These reactions led to cohesive bodies between siloxane groups in the membrane. Because the siloxane groups were dispersed in the membrane, the silanol groups in the siloxane and the hydroxyl groups in the P(VA-coAA) formed hydrogen and covalent bonds, which are the cross-linking points, as illustrated in Scheme 2. However, dehydration and dealcoholysis reactions occurred predominantly rather than the formation of hydrogen bonds and covalent bonds. Thus, a rough cohesive structure of TEOS was formed, and hence, a membrane with a more open structure was produced, leading to a decrease in the water/ethanol selectivity. The decrease in the membrane density and the increase in the degree of swelling of the membrane with increasing TEOS content, shown in Figure 3, can be attributed to the formation of hydrogen and covalent bonds. Effect of Annealing of P(VA-co-AA)/TEOS Hybrid Membranes. PVA is a crystalline polymer, and its crystallinity results from strong hydrogen bonds between the hydroxyl groups and increases with increasing annealing temperature. We have previously reported that the molecular structure of PVA membranes can be controlled by annealing under various conditions, and that their water/ethanol selectivity for an aqueous ethanol solution by pervaporation can be controlled by the annealing conditions.13 On the other hand, it has been reported that the polycondensation reaction of TEOS strongly depends on the annealing temperature and time,33 and the structure of the resulting membranes was significantly influenced by these factors. Therefore, annealing of the P(VA-co-AA)/TEOS hybrid membranes resulted in an improvement of the permeation and separation characteristics. Thus, the effects of the annealing temperature and time on the permeation and separation characteristics were carefully examined by annealing the P(VA-co-AA)/TEOS hybrid membranes. Figure 5 shows the normalized permeation rate and the water/ethanol separation factor of an aqueous ethanol solution of 85 wt % ethanol through P(VA-coAA)/TEOS hybrid membranes annealed at 130 °C for 6, 10, and 15 h as a function of the TEOS content. For all membranes, the separation factors of the annealed P(VA-co-AA)/TEOS hybrid membranes were greater than those of untreated P(VA-co-AA)/TEOS hybrid membranes. The normalized permeation rates in the former membranes were smaller than those in the latter ones. As can be seen from Figure 5, the separation factors of annealed P(VA-co-AA)/TEOS hybrid mem-

Figure 5. Effects of the TEOS content on the normalized permeation rate and the water/ethanol separation factor of an aqueous solution of 85 wt % ethanol through the annealed P(VA-co-AA)/TEOS hybrid membranes as a function of the annealing time. Annealing time: (O) 0, (b) 6, (0) 10, and (9) 15 h. The annealing temperature was at 130 °C.

branes were grated than those of annealed P(VA-co-AA) membrane. These results suggest that the annealing treatment of the P(VA-co-AA)/TEOS hybrid membranes leads to denser membranes due to an increase in hydrogen bonding. Figure 6 shows the sorption selectivity and the diffusion selectivity of the membranes. As can be seen, the sorption selectivity and the diffusion selectivity increased with increasing annealing time. The increase in the water/ethanol separation factor due to the annealing of the P(VA-co-AA)/TEOS hybrid membranes can be attributed to an increase in both the sorption and diffusion selectivity. Figure 7 shows the effect of the annealing time on the degree of swelling of the P(VA-co-AA)/TEOS hybrid membranes containing various amounts of TEOS. As can be seen from Figure 7, annealing of the P(VA-coAA)/TEOS hybrid membranes decreased the degree of swelling of the membranes with increasing TEOS content. In addition, with increasing annealing time, the degree of swelling of all P(VA-co-AA)/TEOS hybrid membranes decreased remarkably. Figure 8 shows the density of the P(VA-co-AA)/TEOS hybrid membranes containing various TEOS contents as a function of the annealing time. The membrane density increased with increasing TEOS content and annealing time. From these results, it is presumed that with increasing annealing time the cross-linking reaction between the P(VA-co-AA) copolymer and TEOS proceeded more complete. Hence, the membranes become denser, and consequently, the degree of swelling of the annealed P(VA-co-AA)/TEOS hybrid membranes is depressed. Both the depression of swelling of the membrane and the increase in the membrane density enhance the

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Figure 8. Effect of the annealing time on the density of the annealed P(VA-co-AA)/TEOS hybrid membranes as a function of the TEOS content. TEOS content: (O) 0, (b) 10, (0) 20, and (9) 30 wt %. The annealing temperature was at 130 °C.

Figure 6. Effects of the annealing time on the normalized permeation rate and the water/ethanol separation factor of an aqueous solution of 85 wt % ethanol through the annealed P(VA-co-AA)/TEOS hybrid membranes as a function of the TEOS content. TEOS content: (O) 0, (b) 10, (0) 20, and (9) 30 wt %. The annealing temperature was at 130 °C.

Figure 9. Transmission electron micrographs of cross sections of the untreated P(VA-co-AA)/TEOS hybrid and annealed P(VA-co-AA)/TEOS hybrid membranes. The dark regions represent the TEOS component.

Figure 7. Effect of the annealing time on the degree of swelling of the annealed P(VA-co-AA)/TEOS hybrid membranes as a function of the TEOS content. TEOS content: (O) 0, (b) 10, (0) 20, and (9) 30 wt %. The annealing temperature was at 130 °C.

water/ethanol selectivity for an aqueous ethanol solution in both the sorption and diffusion process. Physical Structure of P(VA-co-AA)/TEOS Hybrid Membranes. Figure 9 shows transmission electron micrographs of P(VA-co-AA) membranes, untreated P(VA-co-AA)/TEOS hybrid membranes, and annealed (P(VA-co-AA)/TEOS hybrid membranes. In these micrographs, the black background shows the TEOS component, whereas the white part represents the P(VA-co-AA). As can be seen from these micrographs, the P(VA-co-AA)/TEOS hybrid membranes containing a low TEOS content have a structure in which TEOS was dispersed homogeneously within the P(VA-co-AA) membrane because the TEM image shows exclusively a white image. On the other hand, in the hybrid membranes with a TEOS content of 20 wt %, cohesive

bodies of TEOS with diameters in the rage of 400-600 nm can clearly be observed. The TEM results support that the organic component and the inorganic component are homogeneously mixed in the hybrid membrane for low TEOS content, whereas cohesive bodies of TEOS are formed and a microphaseseparated structure is obtained for membranes having high TEOS content. From these results, the structure of P(VA-co-AA)/ TEOS hybrid membranes can be illustrated as shown in Scheme 3. In untreated P(VA-co-AA)/TEOS hybrid membranes, the hydrogen bonds between the hydroxyl groups in the P(VA-co-AA) and the silanol groups in the hydroxyl silane and the covalent bonds between the hydroxyl groups in the P(VA-co-AA) and the ethoxy groups in the TEOS molecule did not form satisfactorily. Hence, a cohesive structure of TEOS was formed, and consequently, the membrane density did not increase, resulting in no improvement in selectivity. However, in the annealed P(VA-co-AA)/TEOS hybrid membranes, the cross-linking reaction between the P(VA-co-AA) copolymer and TEOS was accelerated effectively, and a denser membrane structure was formed, which signifi-

Macromolecules, Vol. 38, No. 20, 2005 Scheme 3. Illustration of the Structural Changes of the Untreated P(VA-co-AA)/TEOS Hybrid Membranes to the Annealed P(VA-co-AA)/TEOS Hybrid Membranes by Annealing

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AA) membrane increased the swelling of the membrane and decreased the density of the membrane. Therefore, the water/ethanol selectivity of the P(VA-co-AA)/TEOS hybrid membrane did not improve. These results could be due to the fact that condensation reactions between P(VA-co-AA) and the TEOS did not occur significantly, and membranes with a more open structure were formed. On the other hand, when the P(VA-co-AA)/ TEOS hybrid membranes were annealed under various conditions, the water/ethanol selectivity in the P(VAco-AA)/TEOS hybrid membranes increased significantly with increasing annealing time and TEOS content. These improvements can be attributed to the fact that the acceleration of the cross-linking reaction between the P(VA-co-AA) copolymer and the TEOS by thermal annealing resulted in the formation of denser membranes and reduced swelling of the membranes. In this study, we demonstrated that membranes with high water/ethanol selectivity can be designed by hybridization of P(VA-co-AA) copolymer as an organic component and TEOS as an inorganic component using the sol-gel reaction. Annealing of P(VA-co-AA)/TEOS hybrid membranes offers potential for improved selective separation of water/ethanol. Acknowledgment. This work was supported by a Grant-in Aid for Scientific Research in Priority Areas (B) ‘‘Novel Smart Membranes Containing Controlled Molecular Cavities” from the Ministry of Education, Culture Science, Sports and Technology, Japan (MEXT), and ‘‘High-Tech Research Center” Project for Private Universities: matching fund subsidy from MEXT, 20002004. References and Notes

cantly reduced swelling of the membrane. As a result, membranes with enhanced selectivity were successfully prepared. Conclusions Organic-inorganic hybrid pervaporation membranes were prepared from P(VA-co-AA) and TEOS using the sol-gel reaction. The addition of TEOS in the P(VA-co-

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