A Structural Classification of Smectic Liquid Crystals

Abstract

A systematic classification of thermotropic smectic phases has been developed, with fourteen different phase types. First, the assumption is made that inall smectic phases the molecular packing within the layers is based on a herringbone arrangement of the molecular planes. The specific packing models for the various phases are derived from this herringbone arrangement by the introduction of certain kinds of orientational disorder of the molecular planes. It appears that four different packing models are possible. Second, a distinction is made between phases in which the director is normal to the smectic plane and those in which the director is tilted. Combining this distinction with the four packing models referred to above, one arrives at eight possible layer structures. Third, a distinction is made based on the presence or absence of certain X-ray reflections related to the existence of a well-developed three-dimensional lattice. Incorporation of this distinction leads to a system of fourteen possible smectic phase types. All known layered smectic phases can be classified within this system.