FORMATION OF INTERCONNECTED GLOBULAR STRUCTURE OF SILICA PHASE IN POLYIMIDE SILICA HYBRID FILMS PREPARED BY THE SOL-GEL PROCESS

Poly(amic acid) (PAA), a precursor of polyimide, derived from pyromellitic dianhydride and bis(4-aminophenyl) ether, was converted to a poly(amic acid) triethylamine salt (PAA-NEt3) that was soluble in methanol, which is a common solvent used in the sol-gel processing of silica. Tetramethoxysilane was hydrolyzed and condensed in a methanol solution of PAA-NEt3. Films were cast from the resulting mixtures, and the solvent was evaporated at 20 or 60-degrees-C. Finally, the polyimide-silica hybrid films were obtained by heating the precursor films to 300-degrees-C. The hybrid films containing up to 50 wt % silica were self-standing and transparent. The formation of an interconnected globular structure of silica with uniform diameter was confirmed by scanning electron microscopy (SEM). This is in contrast to hybrid films previously prepared by the sol-gel reaction of tetraethoxysilane in a N,N-dimethylacetamide (DMAc) solution of PAA that had dispersed silica particles with average diameters of approximately 5 mum. The diameters of the silica spheres were 0.2 and 0.08 mum when hybrid films prepared using methanol as the solvent were dried at 20 and 60-degrees-C, respectively. According to the results of dynamic mechanical analysis (DMA), the storage modulus became larger, and the decrease of the modulus at the glass transition temperature (T(g)) was reduced by increasing the silica content in the hybrid films. A single peak corresponding to the glass transition temperature of the polyimide was observed in the tan delta spectra. The intensity of the maximum peak was lower for the films prepared by the low-temperature drying method than the peaks exhibited by the films prepared with high-temperature drying. The hybrid films in the present study showed higher storage modulus values, a reduction in the decrease of the glass transition temperature, and lower tan delta maximum intensities as compared to the films previously prepared with DMAc that contained the same amount of silica.