Electro-Optic Effects in Nanophase Polymer Dispersed Liquid-Crystal Systems
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ABSTRACT Research into electro-optic effects in nanophase polymer dispersed liquid crystal (PDLC) materials has highlighted their potential as materials for a new class of tuneable filters. The structures, based on UV cured phase separated composites, contain liquid crystal both as discrete nano-scale droplets, and as material dissolved in the polymeric host. The essential difference between these materials and more conventional PDLC's is the scale of the refractive index inhomogeneity which is considerably smaller than the wavelength of visible light. Based upon effective medium approximations, the composite thus acts as a single isotropic medium, whose average refractive index is dependant on the level of applied electric field. Tuneable filters have been fabricated using the composite material for use in the visible spectral band.
INTRODUCTION Electrooptical systems based on composites of liquid crystals (LCs) and polymeric hosts have been widely investigated by many workers [1-4]. Such systems are commonly known as polymer dispersed liquid crystals (PDLCs). The driving force for much of this work is the possibility of cheap and effective large area visible light shutters [5]. Today such systems are readily available, and, indeed, large electrooptic PDLC privacy windows are available that can be switched from opacity to a transparent state by applying a small electric field across the PDLC. PDLCs are formed when an intimate mixture or solution of liquid crystal and [suitable] polymeric host are made incompatible [6]. This occurs as the interaction energy between the polymer (monomer or oligomer) and the LC is increased, or as the temperature or pressure
applied to the system is reduced. This effect is described by the Flory-Huggins Model which derives the unitless interaction parameter X. As the interaction parameter is increased the solubility of the LC in the polymer phase is reduced thereby driving the LC out of solution and allowing it to form the discrete areas of LC that give the PDLC its unique dispersed phase properties. It is thus apparent that the final properties of a given PDLC system will depend strongly on the initial solubility of the LC in the polymer precursor, the degree of change of solubility and the rate at which the change occurs (i.e. the rate of polymerisation or freezing). Current systems are generally based on phase separation processes which result in LC droplet sizes of the order of a few microns in dimension. This paper is concerned with material in which the droplet size is reduced to the order of a few tens of nanometers, by control of the kinetics of the phase separation process.
MATERIALS AND CELL MANUFACTURE The composite system investigated is based on the photocurable monomer di-pentaerythritol hexa acrylate (DPEHA) and up to 40% of fluorinated liquid crystal (Merck BL036). The system undergoes the photocuring reaction illustrated in figure 1.
441 Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
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