New polyimide-silica nano-composites from the sol-gel process using organically-modified silica network structure

The polyimide (PI) matrix prepared by the reaction of pyromellitic anhydride (PMDA) and oxydianiline (ODA) has been reinforced with organically-modified silica network structure using the sol-gel process. Aminophenyltrimethoxysilane (APTMOS) and PMDA were reacted in 2:1 molar ratio in dimethylacetamide (DMAc) as solvent to generate amic acid oligomeric species with alkoxy groups at the chain ends. Specific amounts of these along with tetraethoxysilane (TEOS) were mixed in the high molecular weight polyamic acid solution in DMAC to carry out the sol-gel process. Imidization was carried out by successively heating the hybid films up to 300 degrees C. Morphology, thermal and mechanical properties of these hybrids using imide-modified silica network were studied as a function of silica content and compared with the one in which reinforcement of the PI matrix was achieved using pure silica network generated from TEOS only. SEM studies show a drastic decrease in the silica particle size when imide oliogmers were introduced in the silica network as spacer group. The diameter of the silica particles was in the range of 40 nm in the compatibilized system with no significant change observed on increasing silica content in the matrix. With the un-compatibilized system where silica network was generated directly from TEOS the particle diameter varied from 0.2 to 3 mu m for 5 to 40% silica content respectively. Higher thermal stability and mechanical strength and improved transparency were observed in case of compatibilized hybrid system. The imide active silica therefore was found to be a better reinforcement for PIs than the pure silica obtained from TEOS. (c) 2006 Springer Science + Business Media, Inc.