Dielectric Response of Polymer Dispersed Liquid Crystal Films.

The dielectric response of three types of PDLC films has been studied as a function of frequency of the applied field and temperature. Each type of film contains nematic liquid crystal E7 in a different polymer binder:epoxy (Epon 828), thermoplastic (PMMA) and UV curable adhesive (Norland 65). A model is developed using an effective medium technique in a self consistent field approximation that reasonably describes the dielectric response of PDLC films as a function of frequency. The electrical properties of the constituent phases are estimated. In the E7/PMMA and E7/Norland 65 films the conductivity of E7 is found to be much greater than PMMA or Norland 65 resulting in charge buildup close of 100 Hz at the droplet/polymer interface. For the E7/Epon film the conductivity of Epon is found to be close to E7 so that there is no charge buildup at the droplet/polymer interface. The effect of charge buildup is more clearly seen in the shielding of the applied field with frequency in the optical transmission measurements. For the PMMA and Norland 65 films the turn on voltage increases as the frequency is decreased below 100 Hz. The optical measurements correlate reasonably well with the dielectric response. The shielding of the droplet field in the PMMA film saturates below a certain frequency. By measuring the shielding as a function of droplet size it is shown that this is due to charge depletion in the droplet. In the Norland film, unlike the PMMA film, the shielding does not completely saturate, tending to increase at very low frequency. These results are compared to the model prediction of the field in the droplet. Using Debye-Huckel theory, the charge carrier concentration and electric field are shown to vary significantly over the volume of the droplet for different values of the applied field. From the temperature dependence of the dielectric response the activation energy associated with side group motion (beta transition) in pure PMMA is calculated. Also the activation energy associated with conduction mechanisms of E7 is calculated for the E7/Norland and E7/PMMA films. (Abstract shortened with permission of author.).