The effects of hydrogen bonding on the liquid crystalline behavior of semiflexible poly(urethane-ester)s

The peculiar liquid crystalline behavior of two poly(urethane-ester)s TDI-C6C4 and TDI-C6C8 is reported. Evidence is provided of the formation of two nematic mesophases, a cybotactic nematic and a conventional nematic mesophase, in TDI-C6C8 and of one cybotactic nematic mesophase in TDI-C6C4. IR spectroscopy indicates that different hydrogen bonded and non-bonded structures occur with varying temperature. The transition from the nematic cybotactic mesophase to the nematic mesophase, or to the isotropic phase, is accompanied by a substantial decrease of the strength of the hydrogen bonds. This is also reflected in the dynamic-mechanical behavior of these poly(urethane-ester)s which is similar to the one of slightly crosslinked thermoreversible networks.