Polymer dispersed liquid crystal film devices, and method of forming the same

#1. Field of the Invention

#This invention relates to polymer dispersed liquid crystal films and devices, and to methods of forming the same by controlling the conditions of photopolymerization to control the properties of the film.

#2. Description of the Related Art

#FIG. 1 is a simplified diagram of a system used for controlled photopolymerization of a PDLC in accordance with the invention;

#FIG. 2 is a plan view of a mask used to introduce periodic variations in liquid crystal bubble size into the PDLC;

#FIG. 3 is a combined diagram and chart relating a periodic variation in liquid crystal bubble size with a corresponding periodic variation in the exposure intensity, and the applied voltage necessary for field alignment of the resulting liquid crystal bubbles;

#It has been discovered that numerous characteristics of a PDLC film can be altered by varying the size of the LC bubbles within the film, and that in turn LC bubble size can be controlled by carefully selecting the environment in which film polymerization takes place. In particular, unique types of PDLC films with properties not previously available can be achieved by varying the polymerization conditions over the film area, and thereby achieving a non-uniform LC bubble size. The properties of the film will then depend upon the manner in which the non-uniformity is implemented.

#The amount of light which PDLC films will transmit varies in accordance with a voltage signal applied to the film. In general, the films exhibit high scattering and low transmissivity at low voltage levels, a high transmissivity at high voltage levels, with a transition region in-between. Characteristics which have been determined to be at least partially dependent upon the LC bubble size within the film are the threshold voltage for optical transmission (defined as the voltage at which the film's transmission is 10% of full transmission); the contrast ratio (the ratio between maximum and minimum transmission levels); the absolute transmission levels; the wavelengths at which significant off-state transmission takes place; and the response times for the film to gain or lose transmissivity when an applied voltage signal is turned on or off.

#For a PDLC film polymerized by exposure to ultraviolet light, the LC bubble size has been found to increase with lower exposure intensities, and to decrease with greater exposure intensities. In turn, the film's threshold and operating voltages for optical transmission can be increased by reducing the bubble size (increasing the exposure intensity) or decreased by increasing the bubble size (reducing the exposure intensity); its contrast ratio and absolute transmissivity can be increased by decreasing the bubble size (increasing the exposure intensity) or decreased by increasing the bubble size (decreasing the exposure intensity); its optical response times can be increased by increasing the bubble size (reducing the exposure intensity) or decreased by decreasing the bubble size (increasing the exposure intensity). The temperature at which polymerization takes place and the LC concentration can also be selected to determine the film's electro-optic properties, as described in some of the examples below. In general, smaller LC bubbles are formed at higher temperatures and with lower LC molecule concentrations.