P-79 / P. A. Kossyrev P-79: Model of Freedericksz Transition and Hysteresis Effect in Polymer Stabilized Nematic Liquid Crystal Configurations for Display Applications Pavel A. Kossyrev, Jun Qi, Nikolai V. Priezjev, Robert A. Pelcovits and Gregory P. Crawford Department of Physics, Brown University, Providence, Rhode Island, USA Abstract The critical field of the Freedericksz transition and optical hysteresis is investigated for Polymer Stabilized Nematic Liquid Crystals as a function of polymer concentration. The results are described by a simple phenomenological model, which accounts for the change in polymer network morphology as the concentration of polymer is increased. 1. Introduction Polymer Stabilized Liquid Crystals (PSLC) are composed of a polymer network dispersed in a nematic liquid crystal. They are promising materials for many display configurations. Small amounts of reactive monomer polymerized in a nematic liquid crystal create an aligned polymer network that under some circumstances can provide a memory effect [1]. The polymer network structure captures the details of the nematic director prior to photo-polymerization. In this paper we present our results on the influence of the polymer network on the switching behavior of a homogeneously aligned nematic liquid crystal. We show that the critical field of the Freedericksz transition, as well as the width of optical hysteresis, increases with the polymer network concentration, and that this dependence on polymer concentration can be described by a phenomenological model. Previous models described the critical field of the Freedericksz transition in PSLC systems, which to some extent took into account the polymer morphology as collection of planes [2] or bundles [3] parallel to the substrates, or collectively as an effective field [4]. These models nicely describe the behavior of the critical field in the low concentration limit. The model of the polymer matrix as a series of planes perpendicular to the x-, y-, and z-axes of the Cartesian coordinate system was introduced in [5]. Current models do not explain the dependence of the critical field on the polymer concentration especially in the high concentration limit. We explain the behavior of the critical field of the Freedericksz transition in liquid crystal-polymer systems by introducing a phenomenological model, which takes into account the morphology of the polymer network. Also we explain the optical hysteresis effect and its dependence on concentration in such systems. 2. Result Consider a planar cell of PSLC sandwiched between two transparent indium tin oxide (ITO) coated glass plates with a 5µm cell gap thickness. The glass substrates are coated with rubbed polyimide such that the nematic directors on both surfaces are anchored parallel to the plates. Two types of PSLC were prepared from low molecular weight nematic liquid crystal BL038 (EM Industries), diacrylate monomers LC242 (BASF) for the first type and RM257 (EM Industries) for the second type, and a low percentage of photo-initiator Darocur1173 (Ciba) was used so that polymerization can be carried out in the UV. When a diacrylate monomer is polymerized in a liquid crystal solvent, the orientation and order of the resultant network depends on the orientation and order of the liquid crystal. So we assume that the polymer network has planar alignment, which is identical to the alignment of liquid crystal. In our studies we investigated the concentration range where the monomer is totally dissolved in liquid crystal at room temperatures (<6% of monomer by weight). We also make sure that the polymerization is complete by UV curing the samples for sufficiently long time. Figure 1 The experimental set up for measuring the transmission properties of a PSLC cell. The cell is situated between the cross polarizers oriented at 45 0 with respect to the nematic director inside the cell. A schematic illustration of the experimental set up for measuring the transmission properties of a PSLC cell is presented in Figure 1. We consider the arrangement of a cell between a pair of parallel polarizers. The nematic director in the cell is oriented at 45 o relative to the transmission axes of the polarizers. ITO glass plates serve as the electrodes to apply 1kHz square ac-voltage V. In the field-off state, the cell exhibits a phase retardation Γ(=2π/λ(n e -n o )d=4π), where n e (=1.799), n o (=1.527) are the extra-ordinary and ordinary refractive indices of the BL038 respectively, d(=5µm) is the cell gap thickness, and λ(=633nm) is the wavelength of the incident laser light. Due to the uniform alignment of the liquid crystal and the polymer network, the transmission light intensity of the cell I(=1/2cos 2 (Γ/2)) is maximum in the field-off state with such arrangement of polarizers. ISSN/0002-0966X/02/3301-0506-$1.00+.00 © 2002 SID 506 • SID 02 DIGEST