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.

Polyimide (PI) derived from 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA) with trans-1,4-cyclohexanediamine (CHDA), i.e., s-BPDA/CHDA was investigated from the viewpoint of ordered structure and intermolecular interaction. Thermodynamic parameters of the model compounds for melting behavior suggested significantly restricted conformational changes in the trans-1,4-cyclohexylene unit and the presence of strong BPDI—BPDI interaction in s-BPDA/CHDA. The effect of diamine structure on the fluorescence yield also supported the presence of the BPDI—BPDI interaction or the BPDI dimer in s-BPDA-based semi-cycloaliphatic PIs. The results of the fluorescence depolarization measurements can be rationalized by a proposed mechanism assuming the presence of the BPDI dimer sites, where the fluorescence of s-BPDA/CHDA occurs by excitation of the lower energy trap sites consisting of the BPDI dimer via excitation energy migration or direct excitation of the dimer. The structure-sensitive infrared band around 550 cm-1 gradually shifted toward higher frequency with simultaneous narrowing with increasing cure temperature, suggesting gradual ordered structure formation in s-BPDA/CHDA. Thermal imidization at 400 °C caused splitting of the C—H stretching band around 2940 cm-1, corresponding to the disappearance of distinct glass transition for the s-BPDA/CHDA system.

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 degrees of molecular orientation, film densities, and dynamic mechanical properties of these PIs were compared with those of familiar PIs based on symmetric 3,4,3',4'biphenyltetracazboxylic dianhydride (s-BPDA). PI(a-BPDA/PDA) cured at 350°C showed a Tg close to that of PI(s-BPDA/PDA) prepared under the same condition in spite of the bent chain structure of the former. Comparison of the PIs cured at 400°C manifested that PI(a-BPDA/PDA) has a Tg at 410°C (abrupt decrease in E') whereas PI(s-BPDA/PDA) show no distinct Tg. Similar result was also observed for the ODA systems. The unexpectedly high Tgs of a-BPDA-based PIs were explained in terms of the restricted conformational change around the phenyl-phenyl bond in the a-BPDA unit. The difference of the E' decrement at the Tg for both type of PIs are probably attributed to the difference of the intensity of intermolecular interactions. In PI(a-BPDA/ PDA) thin film cured on a substrate, the segments unexpectedly align slightly to the film plane.

ABSTRACT:Asymmetric biphenyl type polyimides (PI) derived from 2,3,3Ј,4Ј-biphenyltetracarboxylic dianhydride (a-BPDA) and p-phenylenediamine (PDA) or 4,4Ј-oxydianiline (ODA) show higher T g s, and much better thermoplasticity than the corresponding isomeric PIs from symmetric 3,3Ј,4,4Ј-biphenyltetracarboxylic dianhydride (s-BPDA). In addition, a-BPDA-derived PIs are completely amorphous owing to their bent chain structures and highly distorted conformations, whereas the PIs from s-BPDA are semicrystalline. a-BPDA-derived PIs possessing these properties or the a-BPDA monomer were used as a flexible blend component or a comonomer to improve the insufficient thermoplasticity of semirigid s-BPDA/PDA homo polymer. The blends composed of s-BPDA/PDA (80%) with a-BPDA-derived PIs (20%), as well as the s-BPDA/PDA-based copolymer containing 20% a-BPDA, showed a certain extent of thermoplasticity above the T g s without causing a decrease in T g . In addition, these blends and copolymer provided comparatively low thermal expansion coefficient (ca. 18 ppm). The improved film properties for the blends are related to good blend miscibility. On the other hand, when s-BPDA/ODA was used as a flexible matrix polymer instead of a-BPDAderived PIs, the 80/20 blend film annealed at 400°C exhibited no prominent softening at the T g . This result arises from annealing-induced crystallization of the flexible s-BPDA/ODA component. Thus, these results revealed that a-BPDA-derived PIs are promising candidates as matrix polymers for semirigid s-BPDA/PDA for the present purpose.