Relaxation processes in poled electro-optic polymer films

Combining several experimental techniques for monitoring chromophore orientational relaxation in dye doped nonlinear optical polymer films within the theoretical framework of the Dissado-Hill and Adam-Gibbs relaxation models, we have developed a method for characterizing the useful lifetime of devices based on nonlinear optical polymers. In order to monitor the chromophore relaxation process over 13 orders of magnitude in time, it was necessary to perform both time and frequency domain relaxation experiments. In the time domain experiments both the linear and second order nonlinear optical susceptibilities of nonlinear optical polymer films were monitored. The linear susceptibility was probed by monitoring the depolarization current created from the reorientation of the dipolar chromophore molecules, while the second order susceptibility was probed by monitoring the decay of the second harmonic light generated from the relaxing chromophores. In the frequency domain, the linear and second order nonlinear optical susceptibilities of nonlinear optical polymer films were monitored, using dielectric relaxation and dielectric spectroscopy respectively. Dielectric spectroscopy, which is frequency domain electric field induced second harmonic generation, was developed to study the nonlinear optical response near the glass transition temperature at short time scales. These measurements, along with polymer structural relaxation measurements, which were performed using differential scanning calorimetry, were used to characterize the decay of poling-induced electro-optic properties of guest-host and side-chain methacrylate polymers having glass transition temperatures in the range of 90 < [T_g] < 125^o C. The temperature dependence of the relaxation time constants, τ, from the polymeric systems were then compared and a scaling model for predicting useful lifetimes of poled electro-optic media is discussed within the framework of the Adam-Gibbs entropic model. We have also verified that chromophore relaxation is coupled primarily to the /alpha, or main chain relaxation of the host polymer. In addition, after scaling the temperature to the glass transition temperature, T_g, and the relaxation time constant, /tau, to the glass transition temperature time constant, τ_g, the calculated values for τ for all three types of polymeric systems in this study, seem to have the same reduced temperature dependence which indicates chromophore orientational relaxation behavior may be universal.