Thermal properties of liquid crystalline polyurethanes obtained from an asymmetric mesogenic diol

Novel main-chain liquid crystal polyurethanes were synthesized by the polyaddition of an asymmetric mesogenic diol and diisocyanates. The asymmetric mesogenic diol (MD62), 1-hydroxy-6-(4-(4-(2-(hydroxy)ethyl)phenylazo)phenoxy)hexane, was synthesized by a combination of an azo coupling method and the Williamson ether synthesis. Tolylene-2,4-diisocyanate (TDI), 1,3-bis(isocyanatomethyl)cyclohexane (13CH), 1,3-bis(isocyanatomethyl)benzene (13X) and 1,6-diisocyanatohexane (16H) were used as diisocyanates. The liquid crystalline polyurethane (PU62-TDI) obtained from MD62 and TDI formed a nematic phase on heating and cooling processes. PU62-TDI exhibited glass-nematic and nematic-isotropic phase transition. The liquid crystalline polyurethane (PU62-13CH) synthesized from 13CH instead of TDI also formed the nematic phase and revealed the same phase behavior. By a variable-temperature IR spectroscopy, hydrogen bonds between the urethane units are linked to the phase transitions of the liquid crystalline polyurethane.