Comparative study of the dielectric and optical response of PDLC films

The dielelectric response of three types of PDLC films were measured as a function of frequency (1 mHz to 10 kHz) at room temperature. PDLC films containing nematic liquid crystal droplets in different polymer binders -- epoxy (Epon 828), thermoplastic (PMMA), and a UV curable adhesive (Norland 65) -- were studied for a range of droplet sizes. Transmission at normal incidence as a function of the frequency (0.1 Hz to 200 Hz) of the applied field (0 to 120 Vrms) was also measured. The dielectric data show that charge is built up at both the electrode (epoxy films) and droplet/polymer interface (UV, PMMA films). This charge buildup causes a shielding of the field in the droplets, resulting in a much higher switching voltage as shown by the optical transmission. In the PMMA-based film the extent of the shielding is strongly dependent on droplet size.