Mechanical Properties of Soluble Polymer/Silica Gel Hybrids
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337 Mat. Res. Soc. Symp. Proc. Vol. 576 © 1999 Materials Research Society
and the resulting microstructures [22-25]. When it comes to transparency in the physical hybrids, important factors, in addition to the volume fraction of polymer, are the presence or absence of phase separation, and the presence or absence of porosity [26]. The physical hybrids we have prepared contain PVAc or PEO. Polymers containing carbonyl groups, such as PVAc can be incorporated into sol-gel processes because they are soluble in alcohol-water mixtures due to partial esterification of the side chains. Polymers containing ether oxygens, such as PEO, are soluble in water because the ether oxygens readily form hydrogen bonds. The presence of linear polymers in the gels makes it easier to form crack-free samples. Poly(vinyl acetate) PVAc -[-CH2-CHOCOCH3-]n is a thermoplastic commonly used in coatings, paints and adhesives. It has a density of about 1.2 g/cm 3 . It has no tendency to form covalent bonding with polymerizing TEOS. PVAc has a refractive index well matched to silica (1.45), where most polymers have a refractive index considerably greater than 1.45. PVAc by itself has excellent optical transparency. PVAc can be added directly to typical sol-gel formulations. Its solubility in ethanol-water mixtures makes it easy to produce organic-inorganic hybrids over a wide range of compositions [15]. That is, hydrolysis and condensation reactions of TEOS occur in the presence of dissolved PVAc, which suggests that the organic polymer chains are mixed on the molecular scale with TEOS. The polymerization of the inorganic network proceeds around the polymer domains, meaning that the system forms sequential interpenetrating networks. Depending on the volume fractions of the organic and inorganic polymers, phase separation can give rise to translucent or opaque hybrids. PVAc may interfere with the development of the silica network, acting to isolate or separate the silica. Alternatively, it is possible that some PVAc chains may be entrapped within the silica network. In slowly dried bulk gels with a range of volume fractions, there is no indication that phase separation occurs [14,18]. There is, however, a characteristic morphology in gel coatings that indicates phase separation occurs, most likely immediately after dipping. Analogous to polyacrylic acid (PAA)-silica gels [24, 25], there are several possible explanations for the differences between bulk gels and coatings in PVAc-silica gels. When gelation occurs before phase separation, homogeneous solutions are preserved at the sol-gel transition. On the other hand, in thin film formation, the relation between gelation time and phase sepation time is reversed. The rapid change of solvent composition due to the evaporation of alcohol promotes phase separation before gelation. Depending on the speed of drying, nanometer to submicron features appear in this system. Poly(ethylene oxide) PEO [-(O-CH2-CH2-)-]n is a polymer that is commonly used in polymer electrolytes. It is a semicrystalline polymer,
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