A Hypothesis for the Molecular Organisation in the SDand S4Phases

Abstract

The structures of the S_D phase and of the birefringent phase formed at the S_AS_D or I S_D transition (provisionally called the S₄ phase) are discussed. It is suggested that the S₄ phase is discotic and that its basic structural unit is a more or less flat ring of dimeric molecules (this assembly being stabilised by interactions between the strong lateral dipoles of the molecules). It is suggested that the S₄S_D transition entails the aggregation of these rings into cylindrical stacks with pointed ends. These units then form a complex jointed network with overall cubic symmetry. Such a structure is similar to a previously proposed model for the S_D phase but has lattice dimensions of twice the size.