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

Hybrid organic-inorganic materials based on polyimide-silica systems h ave been produced by the sol-gel route from solution mixtures of hydro lysed tetraethoxysilane (TEOS) and an aromatic polyamic acid. Compatib ilization of the two components was achieved with the addition of smal l amounts of gamma-glycidyloxypropyltrimethoxysilane, and the evolutio n of the morphology was controlled by the partial substitution of TEOS with dimethylethoxysilane. In all cases imidization and network forma tion for the two respective components were carried out simultaneously on cast films after evaporating the solvent through successive temper ature rises in steps up to a maximum of 300 degrees C. The results ill ustrate the role of the interconnected silica-rich particles within th e polyimide-rich matrix in depressing the alpha-relaxations and reduci ng accordingly the thermal expansion coefficient of the film or coatin g by an extent substantially greater than can be expected from the usu al additivity rules. Forcing the reoccurrence of a particulate morphol ogy through desolubilization of the siloxane component and simultaneou sly preserving the adhesion between the two phases gives rise to subst antial improvements in both tensile strength and elongation at break. For systems exhibiting a co-continuous two-phase morphology the observ ed changes in properties can be partially attributed to residual orien tation within the polyimide phase resulting from the internal constrai nts imposed on its shrinkage during removal of the solvent and the imi dization reactions.