Thermotropic liquid-crystalline systems containing methylated rings

Abstract

The thermal properties of low molecular mass and polymeric liquid crystals of similar chemical structure have been investigated in order to see if a parallel structure-properties relationship exists between the two groups of compounds. Two series of models and of the corresponding polymers were prepared, bearing the same mesogenic moiety obtained by reacting 4-hydroxybenzoic acid with different methylated hydroquinones. Ether or ester linkages have been used to bridge the rigid core to the flexible polymethylene spacer. The thermal stability and the nature of the mesophases were examined by different techniques. An interpretation of the results on the basis of the axial ratio and the strength of the orientation-dependent attractions, is attempted for the model compounds. As far as polymers are concerned thermodynamic parameters follow the expected trend if compared with those of low molecular mass analogues. The dependence of the liquid crystal to isotropic transition temperature on the degree of substitution shows the same trend as for the model compounds. Geometric and electronic effects must be taken into account to understand the experimental results.