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Graft copolymers of rigid-rod polymers as single-component molecular composites☆

Abstract End-capped poly(p-phenylenebenzobisthiazole) structures containing randomly distributed 2,6-dimethylphenoxy pendants along the rigid-rod backbone were synthesized through the copolycondensation of 2,5-diamino-1,4-benzenedithiol dihydrochloride with terephthaloyl chloride and 2-(2,6-dimethylphenoxy)-terephthaloyl chloride in polyphosphoric acid. End-capping was accomplished through the addition of 2,4,6-trimethylbenzoic acid to the reaction mixture. Through control of reaction stoichiometry, copolymer intrinsic viscosities were limited to 8.6 dl g−1 (methanesulphonic acid, 30°C) or less. The 2,6-dimethylphenoxy moiety served as a graft site for grafting reactions with 3-phenoxybenzoic acid in a methanesulphonic acid/phosphorus pentoxide medium. Incorporation of the resultant thermoplastic poly(ether ketone) side-chains into the rigid-rod poly(p-phenylenebenzobisthiazole) structures was substantiated through infra-red spectroscopy and analysis of model reactions.

Influence of molecular structure on processing conditions and mechanical properties of graft rigid-rod copolymers

Abstract Dynamic and tensile mechanical properties of a series of graft rigid-rod copolymers were studied. The copolymers were single-component rigid-rod molecular composites consisting of a rigid-rod poly( p -phenylenebenzobisthiazole) backbone grafted with flexible-coil poly(oxy-1,3-phenylenecarbonyl-1,4-phenylene) side-chains. Dynamic storage moduli of the graft copolymers showed three characteristic regions: an initial plateau region, a transition zone and a second plateau region. The peak transition temperatures of the dynamic loss modulus curves were independent of the rigid-rod backbone length but decreased with increasing side-chain lengths, suggesting that the transition was a secondary transition associated with localized motions of the flexible side-chains. The graft rigid-rod copolymers were compression moulded into tensile test specimens at temperatures above their peak transition temperatures. Although the specimens appeared well consolidated as evidenced by the scanning electron micrographs of tensile-fractured surfaces, their tensile properties were relatively poor compared with those reported for rigid-rod molecular composite blends. The significance of poor tensile properties is discussed based on the structural characteristics of the graft copolymers, namely the length of rigid-rod backbone, the frequency of graft sites, and the average contour length of flexible-coil side-chains.

Multidimensional benzobisoxazole rigid-rod polymers. II. Processing, characterization, and morphology

A high-torque rheometer was used to facilitate the polycondensation of 4-[5-amino-6-hydroxybenzoxazol-2-yl]benzoic acid (ABA) with trimesic acid and 1,3,5,7-tetrakis(4-carboxylatophenyl)adamantane to yield two- and three-dimensional benzobisoxazole polymers, respectively. Although the resultant polymer dopes exhibited improved homogeneity compared to polymer dopes previously prepared in glassware, improved polymer solution viscosities were not achieved. Fibers spun from the two- and three-dimensional polymers did not show a significant increase in compressive strength compared to fibers of the linear or one-dimensional benzobisoxazole polymer derived from the homopolymerization of ABA. Morphological studies of the polymer fibers and films by wide-angle X-ray scattering and scanning electron microscopy strongly indicated more lateral disorder and a more isotropic character for the three-dimensional structures compared to the one-dimensional structures.

Rod aggregation in graft rigid-rod copolymers for single-component molecular composites

Graft copolymers of rigid-rod polymers, specially synthesized for single-component molecular composites, were investigated by wide-angle X-ray scattering. The copolymers consist of a flexible poly(ether ketone) (PEK) side chain attached to a rigid-rod poly(p-phenylene benzobisthiazole) (PBZT). Emphasis was especially placed on the extent of rod aggregation in the copolymers before and after heat treatments. Results were compared among the graft copolymers of various structural characteristics and the blend of PBZT and PEK homopolymers. The results confirmed that the flexible side chains effectively suppress the thermally induced rod aggregation. The frequency of the graft sites along the rigid-rod backbone was found to be the key structural parameter limiting the extent of rod aggregation. A certain minimum frequency of the graft sites appeared to be required for the copolymers to be stable against the thermal aggregation. The side-chain length showed only a minimal effect, while the rigid-rod backbone length exhibited no effect on the stability of the copolymer morphology in association with the thermal aggregation.