X-Ray Diffraction Study of the Mesophase of Octaphenylcyclotetrasiloxane

Abstract

Octaphenylcyclotetrasiloxane (OPCTS) was investigated by Keyes and Daniels [1], who concluded that this compound presents a plastic phase in the range between 188°C and 205°C during the heating process and before the transition to the isotropic liquid phases. They used differential thermal analysis to observe the appearance of this mesophase and of a solid-solid phase transition at %76°C. During the reverse process a supercooling effect was observed: the mesophase exists from 203°C to 155°C. The arguments in favour of the plastic crystal nature of the mesophase were the assumed globular shape of the molecule and the observed optical isotropy, which seemed to exclude a liquid crystal nature of the mesophase. In fact liquid crystals are in-general characterized by optical birefringence and by either rod-like or disc-like molecular shapes.