Thermal and rheological properties of a liquid-crystalline polyurethane

Abstract

A novel liquid-crystalline polyurethane (LCPU) was synthesized by the condensation of2,4-toluenediisocyanate and 4,4'-bis(6-hydroxyhexoxy)biphenyl. Solution polymerization in dimethylformamideproduced intrinsic viscosities in the range 0.30-0.57 dL/g. Differential scanning calorimetry shows a crystallinemelting transition that is highly dependent on molecular weight and thermal history and a clearing transitionthat is virtually independent of both. Parallel-plate rheometry results are consistent with the presence ofa liquid-crystalline phase, coexisting with a small amount of three-dimensional crystallinity, between 152 and166 °C. Dynamic mechanical measurements reveal an unusually high elastic modulus of 180 MPa in the plateauregion above Tg, compared with 6 MPa for an amorphous polyurethane with a chemical structure based on1,4-butanediol.