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 °C showed a higher T g than the s-BPDA/PDA film prepared under the same conditions. When these PIs were annealed at 400 °C, a-BPDA/PDA exhibited an abrupt E‘ decrease at the T g (=410 °C) as well as the counterpart annealed at 350 °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/ODA.

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A completely amorphous polyimide (PI) derived from 2,3,3′,4 ′-biphenyltetracarboxylic dianhydride (a-BPDA) with 4,4′ -oxydianiline (4,4′-ODA) (i.e. a-BPDA/ODA) was used as a matrix polymer for a rod-like polyimide structure derived from pyromellitic dianhydride (PMDA) with p-phenylenediamine (PDA) (i.e. PMDA/PDA) to improve the toughness without sacrificing its ultra-low coefficient of thermal expansion (CTE) characteristics. A matrix effect of a-BPDA/ODA was investigated by comparing with an isomer PI system, s-BPDA/ODA (s-BPDA: 3,3′,4,4′-biphenyltetracarboxylic dianhydride). Neither of the PMDA/PDA-based blend systems with a minor fraction of these flexible PIs showed any distinct glass transitions during dynamic mechanical thermal analysis. The unique fluorescence behavior of perylenetetracarboxydiimide (PEDI), which became almost non-fluorescent by intimate intermolecular contact with the PMDA/PDA chains, was applied to study the miscibility of the PMDA/PDA-based blend systems. For this purpose, a- and s-BPDA/ODA was labeled by copolymerization using a trace amount of difunctional PEDI. The results revealed that the a-BPDA/ODA-containing blend system was miscible over the entire blend composition whereas the s-BPDA/ODA-containing counterpart was essentially immiscible. The a-BPDA/ODA was much more effective as a flexible component than s-BPDA/ODA for reducing the crystallinity of PMDA/PDA and, as a result, significantly improved the film toughness. Blending of only small amounts of a-BPDA/ODA (5—10 wt.%) into PMDA/PDA caused an unexpected further decrease in the ultra-low CTE (2.8 ppm/K) of homo PMDA/PDA film. A mechanism is proposed to reasonably explain the results obtained in the present study. The blend system composed of PMDA/PDA (90 wt/%) and a-BPDA/ODA (10 wt.%) achieved an ultra-low CTE of 0.9 ppm K—1 in addition to sufficient film flexibility.