Synthesis and characterization of fully rodlike poly(4,4 '-biphenylene pyromellitimide)s with various short side groups

Abstract

Fully rodlike poly(4,4-biphenylene pyromellitimide) (PMDA-BZ) is so brittle in spite of its extremely high modulus. In this study, the brittleness was attempted to be improved without a significant sacrifice of the high modulus by incorporating short side groups. For this, benzidine monomers, which contain methyl, methoxy, fluoro, and trifluoromethyl at the 2,2'-positions, were synthesized and then used for polycondensation reactions with pyromellitic dianhydride in N-methyl-2-pyrrolidone, producing soluble poly(amic acids)s. The synthesized poly(amic acid)s were converted to the fully rodlike polyimides in films by a conventional spin-coating on substrates, soft bake, and thermal imidization. The brittleness of PMDA-BZ was successfully healed with a small portion of sacrifice in the modulus by incorporating methyl, methoxy, and trifluoromethyl groups but could not be healed by the fluoro side group. The improvement in the brittleness might be contributed from the chain mobility and lateral chain packing order enhanced by the incorporation of the side groups, which are evident on the measured structures and properties. The structure and other properties were detected to be influenced by the incorporated side groups. The detailed structures and properties were interpreted by considering roles of side groups and the correlation between structure and properties, respectively.