Are domains in liquid crystalline polymers arrays of disclinations?

Abstract

Interaction of mobile disclinations in the liquid crystalline state results in various disclination arrays. The mesophase state of liquid crystalline polymer (LCPs) samples has been assumed to consist of a polydomain texture, each domain having an internal structure of uniform orientation bounded by regions where the molecular director field undergoes reorientation from one domain to its neighbour. However, the detailed nature of the boundary region (in particular whether the molecular director reorients in a continuous or discontinuous fashion) is largely unknown. We have observed groupings of disclinations of alternating sign which resemble Bouligand's serrated wall structure¹ and ‘foursomes’ of two positive and two negative disclinations which bear a likeness to the ‘effective domain’ structure recently hypothesized by Marrucci² to account for certain rheological aspects of LCPs. The arrangements of disclinations in our thin-film samples give rise to an apparent polydomain texture consisting of regions of uniform molecular orientation bounded by disclinations where the molecular director undergoes continuous rotation about an axis parallel to the disclination lines (Neel walls). The specific disclination arrays observed will be influenced by the molecular weight and stiffness constants of the particular LCP and by the flow history, thickness and boundary conditions of the film³; nevertheless, except in the vicinity of the disclination cores, the field of molecular director trajectories throughout the polydomain sample texture is both continuous and translationally parallel.