Molecular structure and reentrant phases in polar liquid crystals

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Abstract
The spin-gas model of liquid crystals is employed to describe and explain single, double and quadruple reentrances of nematic and smectic phases, as well as reentrances below the monolayer smectic A1 phase, all of which are observed experimentally. New sextuple (and octuple) reentrances are predicted in the sequence nematic-smectic Ad-nematic-smectic Ad-nematic-smectic A1-nematic-smectic Ad(-nematic-smectic A1), as temperature is lowered. The multiple reentrances are very sensitive to molecular structure, in particular to the molecular tail length. The microscopic information available from the statistics of microscopic configurations is scrutinized. Statistical weights of classes of positional configurations are monitored. Two distinct mechanisms of nematic reentrance feature : microscopic frustration of dipoles and competing local antiferroelectric and ferroelectric orders. Frustration is relieved by atomic and librational permeations, causing distinct smectic reentrances. The continuum limit of librational permeation is taken. The subtle role played by molecular dimers is outlined and the concept of a dipolar-pair length is critically examined