The polyimide hybrids that have been most widely studied by the authors contain a silica phase produced from tetraethoxysilane by the sol-gel method. Despite the expected strong interactions, through H-bonds, between the polyamic acid precursor for the polyimide and the large number of hydroxyl groups from the silica phase, it is rather difficult to prevent phase separation and rapid coarsening of the morphology upon removal of the solvent from the original solution mixture. These systems can be compatibilized, however, with the selective use of functionalized trialkoxysilane coupling agents, such as glycidyl and isocyanate types.
In the present work we have established that compatibilty is achieved only upon exceeding a critical concentration of coupling agent, which increases linearly with the total silica content up to a certain level. This was found to be connected with the ability of the coupling agent to displace the strongly associated solvent molecules with the amic acid units of the polymer chains in order to develop the required level of interactions across the interface between the organic and inorganic components.