Molecular packing and properties of poly(benzoxazole-benzimidazole-imide) copolymers

Novel random poly(benzoxazole-benzimidazole-imide) copolymers were synthesized via the poly(amic acid)s from the reaction of 5-amino-2-(4-aminobenzene)benzoxazole (BOA) and 5-amino-2-(4-aminobenzene)benzimidazole (BIA) with 3,3′,4,4′-biphenyl tetracarboxylic dianhydride (BPDA). The corresponding homopolyimides were also prepared for comparison. The resulting polyimides were characterized by different techniques to obtain their molecular packing and properties. The copolyimide films not only showed ordered arrangement of molecular chains, high packing coefficients and low water absorption, they also exhibited extremely high tensile strength (233.64–325.79 MPa) and modulus (5.18–5.75 GPa) without any stretching. The glass transition temperatures (Tgs) and 5% weight-loss temperatures in nitrogen of the copolyimides were in the range of 339–387 °C and 573–577 °C, respectively. The excellent mechanical and thermal properties of these copolyimides have been verified to be mainly attributed to the improving regularity of the interchain packing by the introduction of benzoxazole moieties as well as the enhancement of the hydrogen bonding by the incorporation of benzimidazole moieties. In addition, the copolyimide films also showed excellent adhesion to copper (637.0–960.4 N m−1) without any adhesion promoters. Meanwhile, the deviation of the experimental values of the molecular packing and properties from the expected values indicated the cooperative effects of BOA and BIA moieties on the copolyimide backbone at the molecular level.