X-ray analysis and molecular modelling of the structure of aromatic copolyimides

Abstract

X-ray diffraction and molecular mechanics modelling have been used to investigate the structures of two families of aromatic copolyimides. The first of these is synthesized by the reaction of 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) with a mixture of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA); the second is synthesized from BPDA and a mixture of o-tolidine (OTOL) and p-phenylene diamine (PPD). The X-ray fibre diagram of 70/30 copoly(BPDA-PFMB/PMDA-PFMB) is very similar to that of homopoly(BPDA-PFMB), except that it is more diffuse. It appears that the structure is highly blocky, and the main effect of introduction of comonomer is to reduce the crystallinity of the poly(BPDA-PFMB) blocks. In contrast, the X-ray data for the copoly(BPDA-OTOL/BPDA-PPD) are non-periodic along the fibre direction, and the layer-line positions are reproduced by a model consisting of parallel arrays of extended chains of completely random sequence. Interestingly, the correlation length for the extended chain is larger in the copolymer than in homopolymer BPDA-OTOL, in contrast to those seen for the copoly(BPDA-PFMB/PMDA-PFMB) series. It is suggested that this is due to the fact that ‘kink’ distortions are less disruptive in copoly(BPDA-OTOL/BPDA-PPD).