Electro-optic characterization of photopolymerization induced polymer dispersed cholesteric liquid crystals

Abstract

Polymer dispersed liquid crystals (PDLC) are composites of liquid crystal droplets dispersed in a polymer matrix. PDLCs have been widely used for large area optically variable windows, window shutters, high contrast colored reflective displays, light valves, and optical power limiters, among others. The size, concentration, and birefringence of the LC droplets would determine the intensity of the off-state scattering. Several factors influence the size and shape of the droplets: solvent, temperature, and polymer.
PDLCs usually make use of nematic liquid crystals whose molecules are aligned in a general direction, that is, having their director, n, pointed in the same direction. Polymer dispersed cholesteric liquid crystals (PDCLC) are PDLCs which make use of cholesteric or chiral nematic LCs. Cholesteric liquid crystals have the same properties as nematic LCs however, its molecules align in a helical pattern. The chirality of the LC on PDLC systems will result to a different property.
In the Polymerization Induced Phase Separation (PIPS) technique, a homogeneous mixture of liquid crystal, prepolymer and photoinitiator is irradiated with ultraviolet light to initiate curing. During the curing process, the molecular weight of the polymer increases, leading to a decrease in the solubility of the liquid crystal and polymer. As the solubility becomes low enough, phase separation is induced when the liquid crystals come out of solution and form droplets. These droplets grow until the solidifying matrix entraps them. Droplet morphology is affected by the intensity of UV light and LC:polymer ratios. PDLC films cured with high intensity lead to faster polymerization and thus, produce smaller droplets.
Polymer Dispersed Liquid Crystal films scatter light when there is no applied electric field. The directors of the droplets are oriented randomly so minimum transmittance is observed. When an electric field is applied, the directors align themselves in the direction of the field. As light passes through the film, optimum transmittance is noticed. Contrast ratio is the ratio between the intensity of the on state and off state ofthe PDLC. The objectives of this study are to fabricate PDCLC cells via the PIPS method using the thermochromic liquid crystal mixture E48:TM74A and polymer polymethylmethacrylate, to characterize the PDCLC systems and investigate the dependence of the electro-optic properties with formulation and sample preparation conditions.