SIDE-CHAIN LIQUID-CRYSTAL POLYURETHANES WITH AZOBENZENE MESOGENIC MOIETIES - INFLUENCE OF SPACER LENGTH ON HYDROGEN-BONDING AT DIFFERENT TEMPERATURES

A series of side chain liquid crystal polyurethanes (CnCNPs), in which the spacer length was varied from 2 to 12 methylene units, were synth esized by the addition polymerization of yl)amino]-omega-(4-cyanoazobe nzene-4'-oxy)-alkanes (CnCN-diols) with hexamethylene diisocyanate. Th e liquid crystalline properties of CnCNPs were characterized by means of differential scanning calorimetry, polarizing optical microscopy, a nd X-ray diffraction. Polyurethanes with spacer length 4 or higher exh ibited mesomophic properties. C4CNP and C5CNP exhibited an enantiotrop ic nematic mesophase, while C6-C12CNPs exhibited enantiotropic bilayer smectic mesophases. CnCNPs have a high tendency to crystallize; cryst allization is kinetically controlled. Polyurethane's backbone crystall ization is closely related to hydrogen bonding. To establish the role of hydrogen bonding in mesophase formation as well as crystallization, Fourier transform infrared spectroscopy studies of CnCNPs were carrie d out at different temperatures focusing on H-bonds between the N-H an d C=O groups of the urethane backbone. With increasing temperature, C= O and N-H stretching bands were evenly shifted to higher wavenumbers, with two exceptions (C4CNP and C5CNP) discussed in detail in the text. (C) 1998 John Wiley & Sons, Inc.