Strengths have been measured for silica gels prepared with poly(ethylene oxide) (PEO) and poly(vinyl acetate) (PVAc). The modulus of rupture was determined using standard three-point bending in a mechanical tester. The samples were prepared by mixing tetraethylorthosilicate with the polymers dissolved in water (PEO) or water/ethanol(PVAc). Molecular weights between 200 and 3400 were used for PEO. Substitutions of the polymer for silica represented between 5 and 25% on a weight basis. In these systems, the interactions between the polymer and the silica network are largely through hydrogen bonding, between ether oxygens in PEO and silanols and between hydroxyls in PVAc and silanols. The extent of the bonding increases with increasing aging time, as indicated by an almost linear increase in the modulus of rupture. The increase for PEO was greater than for PVAc. Among PEO samples, the strengths were higher for higher molecular weights at the same weight fraction.

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Thermogravimetric analyses of sol-gel derived silica and silica-poly(vinyl acetate) (PVAc) materials show that the loss in weight between 35 and 900~ can be attributed to three distinct reactions. Samples were prepared by dissolving the reactants tetraethyl orthosilieate (TEOS), poly(vinyl acetate ) (PVAc), and water in mixtures of ethanol and formamide. The lowest temperature weight loss is due to the decomposition/removal of the solvents, while the intermediate weight loss corresponds to decomposition of the PVAc. The highest temperature weight loss is related to the dehydroxylation of the silica surface. The relative amounts of ethanol and formamide have a considerable effect on processing time, drying behavior, and the resulting thermal behavior of the gels.

No abstract

Sol-gel processing of organic/inorganic composite materials has attracted much study in recent years. The low temperature sol-gel process and its inherent versatility allow microstructural control on the nanoscale, leading to many interesting materials. In this preliminary study, we have synthesized silica/poly(vinyl acetate) (SiO 2 /PVAc) composite materials and investigated the effect of formamide substitutions for ethanol as the solvent. The interaction between SiO 2 and PVAc is primarily hydrogen bonding between silanols on the silica surface and the carbonyl groups of the PVAc. The time to gel, skeletal density, and the linear shrinkage were significantly dependent on the presence and quantity of formamide. Nitrogen sorption was used to evaluate the xerogels structures. The thermal behavior was studied with thermogravimetry/Fourier transform infrared spectroscopy (TG/FTIR).Sol-gel processing has been used to prepare inorganic oxides by hydrolyzing alkoxides. In addition, sol-gel processing lends itself quite well to the synthesis of inorganic-organic hybrid materials. First, the low temperature aspect of sol-gel processing can be exploited to create many inorganic-organic composite materials unattainable with typical high temperature ceramic processing (7). Second, sol-gel processing can provide intimate mixing of chemically dissimilar materials at room temperature.There are several ways in which organic materials may be incorporated into inorganic matrices. The strength of the interactions between the phases can divide these types of hybrid materials into two classes (7). Class I hybrid materials correspond to weak phase interactions such as van der Waals interactions, hydrogen bonding (2), or simple mechanical blending of the inorganic and organic phases (3). Class Π hybrid materials possess strong covalent or iono-covalent bonds, usually via -Si-C-linkages (1,4), between the inorganic and organic phases. Class II hybrids are discussed first because they represent the larger group. Both of these groups contain some subdivisions, usually differing by way of preparation or specific function (5).