Investigation of bifurcated hydrogen bonds within the thermotropic liquid crystalline polyurethanes

The polyurethanes are synthesized from the biphenyl-4,4'-diol (mesogenic biphenol) and 1,3-Bis(isocyanatomethyl) cyclohexane, using (CH2) of 2, 6 and 11 units as flexible alkylene spacer, respectively. FTIR detects the hydrogen bond in the thermotropic liquid crystalline polyurethane. FTIR spectra show a new C=O absorption with lower wavenumber at around 1658 cm(-1) is assigned to "bifurcated" hydrogen bonded C=O group a C=O with higher strength hydrogen bonds. The distributions of "bifurcated" hydrogen bonded C=O are increased substantially along with increasing the flexible spacer length in polymer backbone. The "bifurcated" hydrogen bond existed not only at the temperature below T-g, but also existed at the temperature far higher than T-m and T-i. It almost is independent of temperature and exhibits a stable interaction (or strucuture) throughout a wide temperature range, differences from the normal liquid crystalline polyurethanes. It is worthy of predicting the thermotropic liquid crystalline polyurethane with "bifurcated" hydrogen bond would enhance its performances. (C) 2011 Elsevier Ltd. All rights reserved.