Theory of a helicoidal cholesteric phase induced by an external field

• Published in: Liquid crystals Vol. 43; no. 6; pp. 783 - 795
• Main Author: Matsuyama, Akihiko
• Format: Journal Article
• Language: English
• Published: Abingdon Taylor & Francis 02.05.2016; Taylor & Francis Ltd
• Subjects: cholesteric phase; Crystals; Free energy; Helicoid; Liquid crystals; Nematic; Nematic crystals; Order parameters; Phase diagrams; phase separation; Phase transformations; Phase transitions; Strength; twist-bend nematic
• ISSN: 0267-8292; 1366-5855
• DOI: 10.1080/02678292.2016.1144810

We present a mean field theory to describe a helicoidal cholesteric phase for mixtures of a chiral nematic liquid crystal (LC) and a polymer chain as well as for pure chiral nematic LC molecules in the presence of a longitudinal external field parallel to the pitch axis of a cholesteric (Ch) phase. The free energy of the helicoidal Ch phase (Ch H ) is derived as a function of a usual orientational order parameter and an order parameter of the Ch H phase. On increasing the strength of the external field, we find that the Ch phase changes to the nematic (N) phase through the Ch H phase. Depending on the temperature and the strength of the external field, we find the second-order N-Ch H and Ch H -Ch phase transitions and the first-order paranematic (pN)-N, pN-Ch H and pN-Ch phase transitions. We also predict phase diagrams in mixtures of a flexible polymer and a Ch LC molecule under the external field.