Some novel polysiloxane elastomers and inorganic-organic composites

Abstract

This review describes the use of polysiloxanes in developing two novel types of materials. In the first approach, polysiloxane elastomers were prepared so as to have unusual network chain length distributions, thereby improving their ultimate properties. The technique involved end linking mixtures of very short and relatively long functionally terminated chains of poly(dimethylsiloxane) to give bimodal networks. Such (unfilled) elastomers show very large increases in reduced stress or modulus at high elongations because of the very limited extensibility of the short chains present in the networks. This non-Gaussian behavior also appears in compression or biaxial extension, as obtained by inflation of sheets of the material. Non-Gaussian theories taking into account this limited chain extensibility were found to be in good agreement with experiment. The composites were prepared using techniques very similar to those employed in the sol-gel pproach to ceramics. Alkoxysilanes or related metaloorganic materials were hydrolyzed in the presence of polymer chains, for example, polysiloxanes and polyoxides, that have reactive end groups such as hydroxyls. The end groups bond the polymer chains into the silica or related ceramic material formed in the hydrolysis, thus forming inorganic-organic composites. When the polymer chains are in excess, they constitute the continuous phase, with ceramic-type material appearing as reinforcing domains. When present in smaller amounts, the polymer is dispersed in the continuous ceramic phase, to give a polymer-modified ceramic. Under some conditions, bicontinuous systems are obtained. The composites thus prepared were characterized by stress-strain measurements, density determinations, differential scanning calorimetry, electron microscopy, X-ray and neutron scattering, and NMR spectroscopy.