The influence of lateral substituents on the phase behaviour of nematogenic liquid crystals

Abstract

The influence of lateral substituents or side chains attached to thermotropic nematogens is considered on the basis of the Flory lattice model. The rigid core of the nematogen is characterized by its axial ratio x and the characteristic temperature T∗ which measures the strength of the anisotropic dispersion forces operative between the cores. For the side chain an additional parameter zm which is proportional to its hard core volume enters the partition function. Given these quantities and the free volume as obtained from the thermal expansion coefficient, the nematic-isotropic transition temperature T_NI and the temperature dependence of the order parameter may be calculated. The results show that the lowering of T _NI by lateral substituents is caused by (1) their interference with molecules packing and (2) the dilution of the anisotropic dispersion forces between the central nematogenic cores. As an additional consequence of this the order parameter is lowered considerably. A survey of recent experimental data shows that the decrease of T _NI is correlated solely with the hard core volume of the substituent regardless of its chemical structure or polarity, thus corroborating the theoretical predictions.