| Abstract: | Nonsegmented and segmented polyurethanes (PURs) containing 4,4′-di(ω-hydroxyalkoxy)biphenyls were synthesized and the effect of the spacer length of the mesogenic diol on thermal and mechanical properties was investigated. DSC measurements showed similar thermal transitions for the segmented and the nonsegmented PURs. However, for the segmented PURs a shift to lower temperatures was observed. Polarization microscopy indicated the existence of LC phases in the nonsegmented PURs. The moduli of films of segmented PURs increased with growing length of the spacer, particularly at temperatures ≤40°C, due to decoupling of motions of the mesogenic and the urethane part. Decoupling also promotes formation of larger domains of the mesogenic groups. Flow was observed above a temperature dependent critical stress value. Chemical cross-linking below the thermal transitions improved the mechanical film properties as the physical network, due to domains of the mesogenic groups, superposes the chemical network. This was demonstrated with a PUR containing COOH groups crosslinked with a melamine resin at temperatures below or above the thermal transitions. |
