Structure and properties of novel asymmetric biphenyl type polyimides. Homo- and copolymers and blends

Asymmetric biphenyl type polyimides (PI) were prepared by thermal imidization of polyamic acids (PAA) derived from 2,3,3',4'-biphenyltetracarboxylic dianhydride (a-BPDA) and p-phenylenediamine (PDA) or 4,4'-oxydianiline (ODA). The dynamic mechanical properties of these PIs were compared with those of the isomeric PIs derived from symmetric 3,4,3',4'-biphenyltetracarboxylic dianhydride (s-BPDA). a-BPDA/PDA polyimide has a considerably bent chain structure compared to semirigid s-BPDA/PDA. Nevertheless, the a-BPDA/PDA film annealed at 350 degrees C showed a higher Tg than the s-BPDA/PDA film prepared under the same conditions. When these PIs were annealed at 400 degrees C, a-BPDA/PDA exhibited an abrupt E' decrease at the T-g (=410 degrees C) as well as the counterpart annealed at 350 degrees C, whereas s-BPDA/PDA showed no distinct T-g in the E' curve. Similar annealing effects were also observed for the ODA systems. The unexpectedly higher T-g's of a-BPDA-based PIs could be explained in terms of the more restricted conformational change through the crank shaft-like motion. The difference between the extents of the E' decrease at the T-g for a- and s-BPDA-based PIs is attributed to the difference of the intensity of intermolecular interactions. The blends of s-BPDA/PDA with a-BPDA-based PI (80/20) and the corresponding copolyimide improved the insufficient thermal processability of homo s-BPDA/PDA without causing a decrease in T-g . The results revealed that, for semirigid s-BPDA/PDA, a-BPDA-based PIs are better matrix polymers than s-BPDA/PDA.