4,4′-diisocyanatoazobenzene a mesogen for liquid crystalline polyurethanes?

Abstract

The mesomorphic properties of 4,4′-diisocyanatoazobenzene were investigated by polarizing microscopy and DSC. It shows a nematic phase when thoroughly purified by distillation. The nematic range, however, is smaller than that of other liquid crystalline diisocyanates like 4,4′diisocyanatophenyl benzoate. A number of binary hard segment type polyurethanes was made with several diols as chain extenders and in addition ternary copolyruethanes containing two different mesogenic diisocyanates. They are crystalline materials with poor solubility. The melt temperatures of these polyurethanes are higher than those of the corresponding polymers with 4,4′-diisocyanatophenyl benzoate. Liquid crystalline behaviour could not be observed in any of the polymers probably because of the high melting transitions and relatively lower izotropization temperatures which result from the higher rigidity and symmetry of the azobenzene structure.