Investigation of bifurcated hydrogen bonds within the thermotropic liquid crystalline polyurethanes

Abstract The polyurethanes are synthesized from the biphenyl-4,4′-diol (mesogenic biphenol) and 1,3-Bis(isocyanatomethyl) cyclohexane, using (CH 2 ) 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.