Voltage Creep in Holographic PDLC Gratings

In holographic Bragg gratings formed by anisotropic photopolymerization of free-radical monomers, an electric field is required to switch the grating between a diffracting and a transmitting state. The voltage necessary to turn the grating completely off is defined as the switching voltage. In this work, we report on the gradual increase of this switching voltage with time after initial fabrication. The switching field increases approximately 60% over a seven-day period for a pentaacrylate/E7 material system. Using dielectric measurements, it is observed that the resistance of the cell does not change over time while the capacitance decreases. FT-IR spectroscopy was also used to explore the continued post-polymerization after fabrication of the gratings. Increases in switching field were observed for samples fully illuminated with light as well as those kept in the dark. We speculate that the rise in the switching voltage is caused by post polymerization of residual reactive moieties located near the polymer/nematic droplet interface. An increase in the amount of interfacial area generated by post-polymerization yields more surface area for the LC molecules to bind; thus a larger field is needed to switch.