Generalized Van der Waals Analysis of Thermodynamic Properties of Side Chain Liquid Crystalline Polymers

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Abstract
Dilatometric studies were performed on the nematic and isotropic phases of a low molar mass and two polymeric liquid crystals, containing identical mesogenic groups. The polymers were found to display higher transition temperatures, increased densities and lower expansion coefficients relative to the corresponding values of the low molar mass model system. The thermodynamic properties of the low molar mass and polymeric liquid crystals were analyzed on the basis of a generalized Van der Waals approach, incorporating geometric parameters characteristic of the mesogenic groups as well as anisotropic and isotropic interaction parameters. The results indicate that the increase of the density characteristic of the transition from a low molar mass to a polymer system is an important factor in controlling the shift of the transition temperature as well as of the thermal expansion coefficient. In addition, however, we found that, within the frame-work of the theory, the effective molecular volume of the mesogenic units within the polymers are increased and the effective axial ratios of the mesogenic units are decreased relative to the corresponding values of the low molar mass system. These variations are obviously caused by the coupling of the mesogenic units to the non-mesogenic chain backbone.