Synthesis and mesomorphism of thermotropic liquid crystalline polyurethanes based on meta-diisocyanates with 4,4 '-bis(omega-hydroxyalkoxy) biphenyls

There are two series of novel thermotropic liquid crystalline polyurethanes (LCPUs), obtained from the polyaddition reaction of 4,4'-bis(omega-hydroxy alkoxy) biphenyls (HBn, n: the number of methylene units in hydroxyalkoxy g roup) with 1,3-bis(isocyanatomethyl) benzene (XDI) and 1,3-bis(isocyanatome thyl) cyclohexane (H6XDI), respectively. The Fourier transform infrared spe ctroscopy, differential scanning calorimetry, wide angle Xray diffraction, polarizing optical microscope and thermogravimetric analysis were used for characterizing these polyurethanes. All of the liquid crystalline polyureth anes exhibited enantiotropism. Except that XDI/HB11 showed trimorphism, all the other polyurethanes presented monomorphism in this study. The H-bonded urethane domain is stronger when the polyurethanes are composed of the ben zene ring urethane moiety and longer alkoxy chain, and they show a higher t ransition temperature associated with the better stability in mesophase. Al l the thermal degradation of LCPUs are higher than 330 degrees C. The polyu rethanes of H6XDI/HB2 and XDI/HB2 show one-step degradation, while the othe rs show two-step degradation. (C) 2000 Elsevier Science Ltd. All rights res erved.