Effect of pendent groups on the structure of polyimides

X-ray diffraction and molecular mechanics modeling have been used to investigate the structures of aromatic polyimides. The polyimides are synthesized by the reaction of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB), 3,3′-dimethyl-4,4′-diaminobiphenyl or o-tolidine (OTOL) and 2,2′-dimethyl-4,4′-diaminobiphenyl (DMB). PFMB, OTOL and DMB contain the same biphenyl structure, but the pendent groups are different or are at different positions. In the present paper, we describe the effect of pendent groups on the crystal structure and the drawability for these polymers. The crystal structures are triclinic unit cell with a=17.49 Å, b=19.94 Å, c=20.43 Å, α=40.7°, β=74.5° and γ=79.4° for homopoly(BPDA-OTOL) and triclinic unit cell with a=16.46 Å, b=9.60 Å, c=40.6 Å, α=90°, β=46°, and γ=79.4° for homopoly(BPDA-DMB). The homopoly(BPDA-OTOL) fibers can be drawn into ×2. Higher draw ratio of homopoly(BPDA-OTOL) fibers were not obtained because of thermal degradation. The other two homopolymers can be drawn from ×2 upto ×10. The increasing draw ratios are accompanied with a decrease of paracrystalline distortion and an increase of correlation length as well as degree of orientation. These are probably due to the presence of more flexible polymer chains of homopoly(BPDA-PFMB) and homopoly(BPDA-DMB), which increase the perfection of polymer chain arrangements during drawing.