Eur. Phys. J. E (2006) 20: 327-333
https://doi.org/10.1140/epje/i2006-10022-4

Regular Article

Distortion and unwinding of the helical structure in polymer-stabilized short-pitch ferroelectric liquid crystal

¹ Laboratoire de Thermophysique de la Matière Condensée, Equipe de l'UMR-CNRS 8024, Université du Littoral Côte d'Opale, 145, Avenue Maurice Schumann, 59140, Dunkerque, France
² Laboratoire de Dynamique et Structure des Matériaux Moléculaires, UMR-CNRS 8024, Université des Sciences et Technologies de Lille, Bâtiment P5, 59655, Villeneuve d'Ascq Cedex, France

^* e-mail: daoudi@univ-littoral.fr

Received: 13 February 2006
Accepted: 7 July 2006
Published online: 25 July 2006

Abstract

We report the effect of an anisotropic polymer network formed from an achiral photoreactive monomer in a short-pitch chiral SmC* phase on the distortion and the unwinding of the helical structure of the ferroelectric phase. The electro-optical behaviour and ferroelectric properties were experimentally determined for films containing various polymer concentrations. The critical field, E_u, for the transition from the distorted structure to the homogeneous state was measured as a function of polymer concentration. A linear increase of E_u versus polymer concentration was observed, showing that the helical structure of the short-pitch SmC* phase was stabilized by the polymer network. This behaviour was expected to be a consequence of the increase of the apparent elastic constants of the ferroelectric liquid crystal stabilized by the anisotropic polymer network films. The polymer network morphology was investigated using atomic-force microscopy, revealing a twisted structure of the polymer fibers. This twisted structure was transferred onto a polymer network during the polymerization process within a short-pitch SmC* phase. The increase of the apparent elasticity can then be interpreted by a strong interaction between polymer network and the liquid-crystal molecules. From our experimental data, the coupling coefficient, W_p, characterizing this interaction was evaluated for all studied polymer concentrations.