Smectic A–cholesteric transition in a side chain cooligomer–CE1 blend. A particular confined geometry for the TGB phase?

Abstract

The blend of an industrial cooligomer side chain cholesteric material with a polar small molecule liquid crystal of reversed chirality induces a low temperature smectic A phase [1]. We present here the change in the textural characteristics at the S_A-Ch transition observed by optical microscopy between untreated glass slides: particular fingers, different from those recently described [2], are obtained for a slow increase of temperature, starting from an unperturbed spontaneous homeotropic S_A domain. These fingers, coexisting with large S_A domains, are understandable as 180° Bloch walls introducing progressively the helicity in the medium. Over a large temperature domain, these chiral fingers coexist in an apparent thermodynamic equilibrium with the normal S_A phase, suggesting the occurrence of an analogue [3,4] of the Shubnikov superconducting phase: probably due to the presence of the glass slides, the geometry of the screw dislocations, comparable to the vortex of type 2 superconductors, is different from the recently described case [5] of the twisted grain boundary phases of the A type.