A series of polyimide/silica hybrid composites have been synthesized by simultaneous polycondensation of the organic polyimide (PI) phase and the sol-gel reactions (hydrolysis and condensation) of the inorganic silica phase. Sol-gel parameters such as pH value, solvent, water/Si ratio, were systematically varied so that their effect on the microstructure of silica could be explored. Emphasis has been placed on the interactions between the organic and the inorganic phases by FTIR with the introduction of two coupling agents, i.e., g-glycidyloxypropyl- trimethoxysilane (GOPTMS) and 3-amino-propyltriethoxysilane (ATPES). Thermal and electrical properties of the hybrid composites were examined by TGA, DSC, and AC impedance and the morphology by SEM; these properties were correlated to their synthesis chemistry.
FTIR results indicate complete imidization of PI, not affected by the presence of the catalyst, solvent ethanol, and silica particles of the inorganic phase. The hybrid films with lower silica content give higher transparency. TGA results indicate ~ 25 ℃increase of decomposition temperature for the hybrid composites compared to the neat PI; it shows a minimum when pH = 4. The thermal stability of the APTES system is generally better than the GOPTMS system. SEM results indicate that maximum particle size is obtained for the system with pH = 4. AC impedance results indicate that dielectric constant increases with the silica particle size and the silica content. The coupling agents help to reduce the silica particle size (~ 100 nm for GOPTMS, ~ 200 nm for ATPES, and > 400 nm with no coupling agent) and to distribute evenly the silica particles in the PI matrix.