Compatibilizing Polyimide/Silica Hybrids by Alkoxysilane Terminated Oligoimides: Morphology-Properties Relationships

A fluorinated polyimide (PI), synthesized from 4,4’-(hexafluoroisopropylidene) diphthalic anhydride and 4,4’-diaminodiphenyl ether, was used as matrix for the preparation of PI/silica hybrids. The inorganic phase was obtained in situ by a sol-gel route with tetraethoxysilane as precursor. Both micron- and nano-scale hybrids were obtained depending on the interfacial interaction between the organic and inorganic phases. To promote such interaction a compatibilizing agent was synthesized, in the form of an alkoxisilane-terminated oligoimide. Both the PI and the coupling agent were characterized by FTIR and Raman spectroscopies and by GPC measurements. The effect of the coupling agent on the morphology of the hybrids and on the size of the silica particles was investigated by scanning electron microscopy. The viscoelastic, mechanical, and thermal properties of hybrid composites were studied. It was shown that by adding appropriate amounts of the compatibilizer it was possible to control the morphology and to obtain homogeneous nanostructured systems. A general improvement of the mechanical performances and of the thermal stability was demostrated, together with an increase of Tg, which was found more pronounced for the nanocomposites than for the microcomposites.