Synthesis, characterization, fiber spinning, and mechanical properties of poly(benzobisthiazole)s with substituted biphenyl moieties in the main chain

The rigid-rod polymers, such as poly(benzobisoxazole) (PBO) and poly(benzobisthiazole) (PBZT), exhibit excellent physicochemical and thermal properties, but have relatively low compressive strength. The low compressive properties of the rigid-rod polymers has greatly restrained their potential commercial applications as reinforcement materials. The objective of this research is to enhance the compressive properties of the rigid-rod polymers by chemical modification of the polymer chain structure. The concept of "multidimensional crosslinking" between polymer chains is proposed to be the possible solution for the problem, and therefore the multidimensionally crosslinkable units have been designed and incorporated in the PBZT backbone by polycondensation.; 2,2{dollar}spprime{dollar}-Dimethyl-6,6{dollar}spprime{dollar}-dinitrobiphenyl-4,4{dollar}sp prime{dollar}-dicarboxylic acid and 2,2{dollar}spprime{dollar},6,6{dollar}sp prime{dollar}-tetramethylbiphenyl-4,4{dollar}spprime{dollar}-dicarboxylic acid or its acid chloride, have been synthesized as PBZT diacid monomers, and designed to be multidimensional crosslinkable units in the polymer main chain. Two phenyl rings in the two fully ortho-substituted biphenyl molecules are nearly perpendicular to each other, and due to their prominent non-coplanar configurations the methyl groups point at the different directions in a three-dimensional view. Induced by the thermal degradation of the methyl group at elevated temperature, crosslinking will occur multidimensionally in the polymer chains with a fully ortho-substituted biphenyl moiety in the backbone. The polymerization technique of the fully ortho-substituted biphenyl monomers with 2,5-diamino-1,4-benzenedithiol dihydrochloride, another PBZT monomer, has been explored, and a series of homopolymers and copolymer were synthesized.; It has been shown that the polycondensation of 2,2{dollar}spprime{dollar}-dimethyl-6,6{dollar}spprime{dollar}-dinitrobiphenyl-4,4{dollar}sp prime{dollar}-dicarboxylic acid or its acid chloride with 2,5-diamino-1,4-benzenedithiol dihydrochloride results in only low to moderate molecular weight polymers in solutions of poly(phosphoric acid) (PPA) and tetramethylenesulfone (sulfolane). A high molecular weight polymer can be obtained by the reaction of 2,2{dollar}spprime{dollar},6,6{dollar}spprime{dollar}-tetramethylbiphenyl-4,4{dollar}sp prime{dollar}-dicarbonyl chloride with 2,5-diamino-1,4-benzenedithiol dihydrochloride. The tetramethylbiphenyl PBZT/PBZT (25/75) copolymer in PPA has been spun into fibers by solution spinning. The data of solid state NMR, TGA and TG-MS indicate that crosslinking occurs when the fiber is free annealed. Although the tensile strength decreases, the fiber compressive strength increases in free annealing both at 500 and at 520{dollar}spcirc{dollar}C for ten minutes. The tension annealing of the fiber increases the fiber tensile strength, but results in a decrease in the fiber compressive strength.