EFF ECT OF COMPA ATIBILIT ENHA TY ANCEME ENT ON STRUCTU S URE, MO ORPHOL LOGY AN THE RMO-M ECHANI ND ICAL PROPER P RTIES OF POLYI F IMIDE/S SILICA N NANOCO OMPOSI TES

Abstract

Polyimide/silica (PI/silica) and poly(hydroxyl imide)/silica (PHI/silica) nanocomposites containing pure as well as organically modified silica network structures have been prepared through the sol-gel process from solution mixtures of poly(amic acid) (PAA) and alkoxysilanes. Two types of PAAs were employed. The first was obtained from the reaction of pyromellitic dianhydride (PMDA) with oxydianiline (ODA) in dimethylacetamide as a solvent. The second was prepared from the reaction of PMDA with a mixture of 3,3′- diamino-4,4′-dihydroxybiphenyl and ODA. The hybrid films obtained by solvent elution technique were thermally cured to carry out the imidization process and the silica network formation. The effect of compatibility enhancement achieved through the modification of polymer backbone as well as through the use of non-silane and silane-terminated imide coupling oligomers on structure and properties of hybrids was studied and compared with the systems where reinforcement of the matrix was achieved using pure silica. The presence of pendant hydroxyl groups on polymer backbone played an important role in dictating the morphology of the PHI/silica hybrids. A model namely “Retain and React” based on two-step process has been proposed to explain the formation of polymer- modified silica clusters composed of co-continuous nano-domains with diffused boundaries as the previously proposed models failed to explain the observed morphology. The introduction of coupling oligomers in hybrid systems impeded the agglomeration of nano- sized silica domains and promoted the formation of novel fibril structures connecting the organic and inorganic phases together. Significant reduction in the roughness of the surfaces in the hybrid materials having modified matrix and silica networks has also been attributed to effect of enhanced coupling. A large shift in the glass transition temperature (Tg) of the hybrid material to higher temperatures confirmed the restricted segmental motion of polymer backbone due to increased interfacial interaction. The high surface-area of the interconnected silica domains stabilized the materials and shifted the decomposition profiles to higher temperatures by providing barriers to heat and mass transport in the hybrid material and thus slowing down the diffusion of oxygen in the matrix. The enhanced polarity coupled with organically modified silica offer an opportunity for improving Tg, reducing the coefficient of thermal expansion and increasing thermal-oxidative stability of polyimide/silica hybrids. The co-continuous and interpenetrating network structure thus produced also improves mechanical properties. Key words: Organic/inorganic hybrids; nanocomposites; polyimide; silica; sol-gel process; compatibilization; morphology; visco-elastic and thermal properties