Mechanical and thermal properties of Polyimide/Silica hybrids with imide-modified silica network structures

A series of hybrid materials incorporating imide-modified silica (IM-silica) network structures into a polyimide (PI) matrix were produced with a sol-gel technique from solution mixtures of poly(amic acid) and tetraethoxysilane (TEOS) containing alkoxysilane-terminated amic acids with various degrees of polymerization. The hybrid films, obtained by solvent evaporation, were heated successively to a maximum temperature of 300 degrees C to carry out the imidization process and silica network formation in the PI matrix. The morphology and mechanical properties of these hybrids with IM-silica networks were studied and compared with the properties of one in which reinforcement of the matrix was achieved with a pure silica network generated from TEOS. The introduction of longer imide spacer groups into the silica network led to a drastic decrease in the silica particle size. Improved tensile modulus was observed in such compatibilized hybrid systems. Comparative thermogravimetric measurements of these hybrids showed improved thermooxidative resistance. A PI hybrid with 30% IM-silica had a thermal decomposition temperature nearly 260 degrees C higher than that of the pure PI matrix. The high surface area of the interconnected silica domains and increased interfacial interaction were believed to restrict the segmental motion of the polymer and thus slow the diffusion of oxygen in the matrix, thereby slowing the oxidative decomposition of the polymer. The reinforcement of existing and new PIs by this method offers an opportunity for improving their thermooxidative stability without degrading their mechanical strength. (C) 2007 Wiley Periodicals, Inc.