INFLUENCE OF SILOXANE COMPOSITION AND MORPHOLOGY ON PROPERTIES OF POLYIMIDE-SILICA HYBRIDS

Hybrid organic-inorganic materials based on polyimide-silica systems have been produced by the sol-gel route from solution mixtures of hydrolysed tetraethoxysilane (TEOS) and an aromatic polyamic acid. Compatibilization of the two components was achieved with the addition of small amounts of gamma-glycidyloxypropyltrimethoxysilane, and the evolution of the morphology was controlled by the partial substitution of TEOS with dimethylethoxysilane. In all cases imidization and network formation for the two respective components were carried out simultaneously on cast films after evaporating the solvent through successive temperature rises in steps up to a maximum of 300 degrees C. The results illustrate the role of the interconnected silica-rich particles within the polyimide-rich matrix in depressing the alpha-relaxations and reducing accordingly the thermal expansion coefficient of the film or coating by an extent substantially greater than can be expected from the usual additivity rules. Forcing the reoccurrence of a particulate morphology through desolubilization of the siloxane component and simultaneously preserving the adhesion between the two phases gives rise to substantial improvements in both tensile strength and elongation at break. For systems exhibiting a co-continuous two-phase morphology the observed changes in properties can be partially attributed to residual orientation within the polyimide phase resulting from the internal constraints imposed on its shrinkage during removal of the solvent and the imidization reactions.