Molecular model for cholesteric liquid crystals

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Abstract
As a first step toward constructing a general molecular model for cholesteric liquid crystals, we propose here a planar model in which the intermolecular potential is required to contain chiral contributions as a result of symmetry considerations. By means of a mean-field analysis, we identify the microscopic basis for the existence of the cholesteric phase. The latter is shown to be a consequence of chirality. The temperature dependence of the cholesteric pitch is discussed, and several numerical examples are given. Our theoretical deductions are consistent with experimental observations. We indicate, however, that under exceptional circumstances two unobserved phenomena may be found to exist. First, particular systems made up of chiral molecules may exist in the nematic phase. Next, as temperature is varied, a particular pure cholesteric substance may undergo untwisting and reverse its helicity after passing through a divergence in pitch.

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