Reorientation Dynamics of Polymer Dispersed Nematic Liquid Crystal Films

Abstract

The electro-optical dynamics, hysteresis effects, and microscopic structure of polymer-dispersed films of nematic liquid crystal are probed in order to gain insight into the operation of this new class of liquid crystal light valves. In tracking the rise and decay response times of these devices, it appears that there are both ‘fast’ (0·1·l.0ms) and ‘slow’ (10-1000ms) processes that occur in the film. A model is proposed which explains these results, in which the reorientation of the nematic droplets takes place in two stages: a fast reorientation by the nematic within the bulk of the droplet, followed by a slower rotation of the nematic nearer the droplet surface (including the point disclinations). This model agrees with a similar proposal made by Doane et al in a previous study of related films. This model is also used to explain both the behaviour of the films in response to short voltage pulses and hysteresis effects present in the film. The response time of these films can be tailored by adjusting the droplet size within the film, as well as the choice of the drive waveform and voltage. The non-spherical shape of the nematic droplets in the film is proposed to be the most important factor controlling the electro-optic properties of these devices. Data is presented which shows that the more distorted the nematic cavity, the more quickly the film decays, and the higher the field required for reorientation. It is proposed that the minimization of deformation energy of the nematic in a non-sperical cavity is the primary driving force for relaxation in these films, rather than previously postulated ‘surface interactions’