Synthesis and characterization of chemically cross-linked polyimide–siloxane hybrid films

Chemically cross-linked polyimide and silica hybrid films were prepared through the sol–gel processing. PI matrix was prepared by the reaction of pyromellitic dianhydride with a mixture of diamines e.g., oxydianiline and 2,5-diaminohydroquinon (2,5-DAHQ) to include pendant hydroxyl groups on the chain. These groups were reacted further with isocyanatopropyltriethoxysilane. An appropriate amount of tetraethoxysilane was then added and the sol–gel process was carried out to condense ethoxy groups from both types of silanes thus producing chemically bonded composite films. The films with different silica contents were evaluated by a variety of techniques including FTIR, 29Si NMR, SEM, tensile, thermal, mechanical and thermogravimetric analyses. The chemical interaction between the phases brought about an intimate dispersion of the two phases, which resulted in the formation of nano-sized co-continuous domains. The tensile modulus of such films was higher and thermal expansion coefficient was much lower than those with similar silica contents without inter-phase bonding.