Spectral Properties of Tunable Privacy Window Films Made of Polymer-Dispersed Liquid Crystals

Modern applications of polymer-dispersed liquid crystals (PDLC) made of liquid crystal droplets dispersed in polymer matrices continue to grow. They include privacy and smart windows, flexible diffusers, advanced displays, energy storage and energy harvesting devices, to name a few. An electrically controlled switching between opaque and transparent states of PDLC films enables their numerous applications. An applied electric field reorients an average director of a liquid crystal droplet resulting in a gradual transition from an opaque to a transparent state. A fully transparent state is observed if the refractive index matching is achieved. As a rule, electro-optical characterization of polymer dispersed liquid crystals is performed using a laser. As a result, basic physical parameters such as switching times, the steepness of the switching curve, contrast ratio, and transmittance are obtained for a single wavelength of light. Even though the spectral dependence of electro-optical properties of PDLC films was mentioned in several papers, systematic studies reporting the dependence of contrast ratio, transmittance, and steepness of the switching curve of PDLC films on the wavelength of light are still missing. In this paper, electrically controlled spectral properties of PDLC films are reported. The obtained dependence of the contrast ratio and transmittance on the wavelength of light can be used for the optimization of electro-optical performance of polymer-dispersed liquid crystals.