Suppression of the reentrant nematic and stabilization of the smectic phases by carbon nanotubes

We have carried out the first optical, dielectric and Xray investigations on composites of a liquid crystal (LC) exhibiting the nematic-smectic-nematic reentrant (N-SmA-RN) sequence, with small concentrations of carbon nanotubes (CNT). Despite the fact that both components are rod-like anisotropic structures, the presence of CNT, even though in the dilute concentration limit, significantly influences the thermal behaviour of the host LC system. The large aspect ratio (similar to 250) CNT favour the smectic phase over the reentrant nematic: for the 0.5% CNT composite the SmA-RN transition temperature gets lowered by 11 K, whereas the high temperature N-SmA boundary is hardly affected. The carbon nanotubes encourage a stronger antiparallel coupling between the neighbouring dipoles, a feature that gets reflected in the dielectric data in the mesophase as well as in the isotropic phase, and corroborated by Xray studies. Magnetic field driven Freedericksz transformation measurements bring out the influence of CNT on the splay and bend Frank elastic constants, with the former exhibiting a large enhancement. The magnetic field-changeable electrical conductivity of the medium is proposed as a possible conductivity switch. Two different scenarios are offered to explain the preference for the smectic phase. A Landau-de Gennes model which brings out this prevalence of the layered phase is also presented. (C) 2018 Elsevier B.V. All rights reserved.