ORGANO-SOLUBLE SEGMENTED RIGID-ROD POLYIMIDE FILMS .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 mum 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 (WAXD) 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 (T(g)), the linear coefficients of thermal expansion (alpha), the dielectric behaviour (epsilon' and epsilon'') as well as the refractive indices (n(parallel-to), and n(perpendicular-to)) parallel and perpendicular to the copolyimide film surface. Possible structure-property relationships are also discussed.