LATTICE MODEL STUDIES OF EFFECT OF CHAIN FLEXIBILITY ON NEMATIC-ISOTROPIC TRANSITION .2. RIGID CORES HAVING SEMI-FLEXIBLE TAILS

A steric, mean field, three-dimensional, simple cubic lattice model (using DiMarzio lattice statistics), with and without attractive forces, for molecules composed of rigid cores and semiflexible pendant tails is presented. The calculated trends and absolute values of the following properties at the nematic-isotropic transition for cores of length-to-breadth ratios of 4 and 5 with tails of varying length are compared with relevant experimental data: temperature, core order parameter, intramolecular and intermolecular tail order parameters, densities, relative density change, and entropy change. With results intermediate between those for the completely flexible tail model and those for the rigid rod models, this semiflexible tail model reasonably describes the general features and trends observed experimentally in homologous series of liquid crystals at the transition. Tail chain flexibility (important in the transition) at a given temperature is rather adequately treated as an entirely internal property in this model. Steric repulsions are sufficient to generate the transition; attractive interactions, while not essential, do allow some adjustments in trends and some improvements in energetics. Preliminary calculations indicate that a softened repulsive potential leads to somewhat improved values for the core order parameter and the relative density change.