Anomalous optical properties of photoactive cholesteric liquid crystal doped with single-walled carbon nanotubes

Abstract

<p>Optical transmission and selective reflection data are reported for suspensions of single-walled carbon nanotubes (CNTs) in photoactive nematic material ZhK-440 with a mesogenic chiral dopant M5. At small concentrations of CNTs (<i>C</i> ≈ 0.01–0.05%), the preferential localisation of CNTs at oily sticks (cholesteric topological defects) and suppression of the network of oily streaks by CNTs were observed. At the same time, the optical density <i>D</i> was shown to be essentially non-linear and a minimum at certain concentration of CNTs, <i>C</i> ≈ 0.05–0.08%, was observed. This anomalous behaviour was explained by the presence of the structural transition from the loose (ramified) aggregates with highly anisotropic shape (oriented along the anchoring direction on rubbed polyvinyl alcohol) to the compact aggregates with denser packing. The location of this minimum, as well as the selective reflection maximum (helical pitch), was sensitive to partially reversible UV-induced <i>trans–cis–trans</i> isomerisation effects. The UV-controlled helical pitch variation was shown to be only slightly affected by introduction of CNTs.</p