Characterization of polymer dispersed liquid-crystal shutters by ultraviolet/visible and infrared absorption spectroscopy

Abstract

Incorporation of dichroic dyes substantially improves the contrast of polymer dispersed liquid-crystal (PDLC) shutters. The efficiency of dichroic dyes in a PDLC shutter depends on the scattering efficiency of the film, the dichroic ratio of the dye, the order parameter of the dye in the liquid crystal, and the fraction of dye dissolved in the droplets. Ultraviolet/visible spectroscopy can be effectively used to determine the scattering efficiency of PDLC films, and the distribution of dichroic dye dissolved in the liquid-crystal droplets. To achieve high contrast the dichroic dye must be segregated in the droplet. We show that the segregation of dye in the PDLC droplets is dependent on the type of dye used and ranges from 19% to 100%. The scattering efficiency of the PDLC shutter provides insight into the effect of the dichroic dye on the contrast of the film. Production of a black PDLC shutter requires a mixture of several dichroic dyes. By careful consideration of the relative amounts of the components of the dye mixture, a black PDLC shutter can be constructed with a uniform contrast over the entire visible region. The dichroic absorption in the infrared of the liquid crystal used to make a PDLC can be analyzed in a manner analagous to the dichroic dye to determine the fraction of liquid-crystal phase separated in droplets and dissolved in the polymer binder.