A Thiophene-Based Liquid Crystalline Aromatic Polyamide

Abstract

We have synthesized a liquid crystalline, aromatic polyamide from p-phenylene diamine (PPD) and 2, 5-thiophene dicarboxylic acid (Th). The new polymer, poly(p-phenylene thiophenylamide), abbreviated PPD-Th, like its terephthalic acid precursor, poly(p-phenylene terepthalamide) (PPD-T), exhibits a lyotropic mesophase in concentrated sulphuric acid solutions. Polarizing optical microscopy and deuterium NMR show that concentrated PPD-Th solutions exhibit nematic-to-isotropic phase transitions at a much lower temperature than comparable PPD-T solutions. More importantly, unlike PPD-T, lyotropic PPD-Th solutions do not form a crystallo-solvate complex gel phase at room temperature, i.e. the fluid PPD-Th mesophase persists below 300 K enabling the spinning of PPD-Th fibres from a lyotropic dope at ambient temperatures. Thermogravimetric analysis shows that the 2, 5-thiophene heterocycle in the PPDTh backbone does not significantly diminish its thermal stability relative to that of PPD-T. Preliminary x-ray diffraction results show that PPD-Th crystallizes in a tentative two-chain, orthorhombic unit cell with dimensions a = 3.73 Å, b = 5.04 Å and c = 25.20 Å (fibre axis). The fibre repeat corresponds to two chemical repeats related by a twofold screw axis. Mechanical properties (tensile strength, modulus and elongation to break) are presented for as-spun and heat-treated PPD-Th fibres.

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