A molecular-statistical theory of the temperature-dependent pitch in cholesteric liquid crystals van der Meer, B. W.; Vertogen, G.; Dekker, A. J.; Ypma, J. G. J.An interaction between chiral molecules is derived in terms of the orientations of their long molecular principal axes. In analogy with Maier-Saupe and Goossens the derivation is based on the electric multi pole expansion. The molecules are assumed to behave as if they were cylindrically symmetric, i.e., the system is locally nematic. As an example the chiral molecules are represented by Kuhn models, and the relevant coupling constants are calculated explicitely. An expression for the free energy is obtained in the molecular field approximation. It is shown that, if only the induced dipole-dipole and the dipole-quadrupole dispersion energies are taken into account, a temperature-independent cholesteric pitch is obtained. This result is traced back to the symmetric character of the interaction. In order to explain the experimental situation one has to introduce, in analogy with Keating, an asymmetry producing interaction. The proposed model is discussed and its thermodynamic properties are calculated in the molecular field approximation. It is found that the magnitude of the reciprocal pitch varies nearly linearly with temperature in agreement with experiment. Interpretational difficulties related to the use of the. multi pole expansion are discussed.

Un modèle de la phase smectique C est proposé dans lequel l'inclinaison est supposée avoir son origine dans les forces entre dipôles et dipôles induits. On a supposé que la rotation autour du grand axe moléculaire est libre. Trois phases alignées ont été trouvées correspondant à la phase smectique C , la phase smectique A et la phase nématique. L'entropie d'entassement joue un rôle important dans le mécanisme de la transition smectique A-smectique C. Cette transition n'est pas du type connu ordre-désordre. Abstract.-A model of the smectic C phase is proposed in which the tilt is thought to originate from the forces between dipoles and induced dipoles. The rotation about the long axis is assumed to be free. Three ordered phases are found corresponding with the smectic C phase, the smectic A phase and the nematic phase. The entropy of packing plays an important role in the mechanism of the smectic C-smectic A phase transition. This transition is not of the usual order-disorder type.

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