Orientational Phase Transition in the System of Flexible Molecules

By assuming an attractive potential between molecules which depends on their relative orientations and respective molecular configurations, the phase transitions in the system of flexible molecules are studied in a mean field approximation. For simplicity, a two state model is adopted, where all configurations of a molecule are simply classified into "long" and "short" cases. It is found that the orientational transition is accompanied with a certain change in the molecular configuration for the case of long and flexible molecules (type I), whereas with a much smaller change in the case of short and rather rigid molecules (type II). In the intermediate case, transitions of the type I and II occur successively. Transitions observed in lipid membranes and in nematogenic homologous series can be understood on the basis of these analyses.

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