Organo-soluble segmented rigid-rod polyimide films. Part 4.—Anisotropic structure and properties

A series of fully aromatic copolyimides have been synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), pyromellitic dianhydride (PMDA), and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) in m-cresol, (BPDA–PFMB)x-(PMDA–PFMB)y. The molar composition ratios are 100 : 0, 85 : 15, 70 : 30 and 50 : 50, respectively. The unoriented polyimide films with a thickness ranging between 10 and 20 µm have been characterized by an in-plane orientation of the c axis in the crystals as well as the molecular chain axis relative to the film surface. This in-plane orientation has been studied by transmission and reflection geometrical modes of wide-angle X-ray diffraction (WARD) and polarized Fourier-transform infrared spectroscopy (FTIR), and can be attributed to the rigidity and linearity of the chain molecules. The structural anisotropy leads to anisotropic behaviour in the glass-transition temperatures (Tg), the linear coefficients of thermal expansion (α), the dielectric behaviour (ε′ and ε″) as well as the refractive indices (n‖ and n⊥) parallel and perpendicular to the copolyimide film surface. Possible structure–property relationships are also discussed.