Organo-soluble segmented rigid-rod polyimide films. Part 3.—Effects of copolymer composition on thermal expansivity and on relaxation processes

Abstract

A series of copolyimides based on 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), pyromellitic dianhydride (PMDA)(1,2,4,5,-benzenetetracarboxylic dianhydride) and 2,2′-bis(trifluoromethyl) benzidene (PFMB) have been synthesized with varying BPDA–PFMB and PMDA–PFMB compositions. A systematic change in the linear coefficient of thermal expansion (CTE) and the glass-transition temperature (T_g) is observed by changing the ratios of the BPDA–PFMB and PMDA–PFMB compositions. Both the CTEs and T_gs have been found to follow the molar-additivity rules. Dynamic mechanical experiments manifest two relaxation processes above room temperature. A subglass β relaxation, which is independent of the copolymer composition relative to the varying dianhydride composition, is found in the copolyimides studied. This β transition exhibits the same temperature and frequency dependence in all the copolyimides. An α transition (corresponding to the glass-transition temperature) is observed and is dependent on the PMDA–PFMB composition within the system. The possible molecular origin of the β relaxation is also discussed.