Abstract
ABSTRACTA series of copolyimides were prepared by incorporating pyromellitic dianhydride (PMDA) into the homopolyimide derived from 5,4′‐diamino‐2‐phenyl benzimidazole (DAPBI) and 3,3′,4,4′‐biphenyl tetracarboxylic dianhydride (s‐BPDA), with the purpose of enhancing backbone rigidity of the homopolyimide. A systematic change in thermal, mechanical and optical properties was observed by changing the ratios of PMDA and s‐BPDA components. Glass transition temperatures were in the range of 404–425°C, depending on the pyromellitimide content. Mechanical properties were significantly enhanced: Tensile modulus and strength ranged from 5.8 to 8.5 GPa and 249 to 263 MPa, respectively, and much higher than that of the homopolyimide (s‐BPDA/DAPBI). Wide‐angle X‐ray diffractions showed that the copolymer films were amorphous, although locally ordered regions were observed with long periodicity larger than that of the homopolyimide. The properties enhancement was attributed to the increased overall rigidity of the polymer backbones and enhanced molecular in‐plane orientation. Possible structure–property relationships were also discussed. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1653–1658, 2013
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