ORGANO-SOLUBLE SEGMENTED RIGID-ROD POLYIMIDE FILMS .3. EFFECTS OF COPOLYMER COMPOSITION ON THERMAL EXPANSIVITY AND ON RELAXATION PROCESSES

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(g)s have been found to follow the molar-additivity rules. Dynamic mechanical experiments manifest two relaxation processes above room temperature. A subglass beta relaxation, which is independent of the copolymer composition relative to the varying dianhydride composition, is found in the copolyimides studied. This beta transition exhibits the same temperature and frequency dependence in all the copolyimides. An alpha 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 beta relaxation is also discussed.