Relaxation Processes in Nonlinear Optical Polymers: A Comparative Study

Abstract

Relaxation processes in nonlinear optical (NLO) polymers with glass transition temperatures in the range of 125 °C < Tg < 176 °C have been studied. The relaxational mechanisms of these side- and main-chain polymers have been investigated above and below the glass transition by second-harmonic decay, dielectric relaxation, and differential scanning calorimetry measurements, and the results obtained have been compared with a variety of nonlinear optical polymer systems cited in the literature. The nonexponential relaxation in both the time and frequency domain was modeled by the Kohlrausch−Williams−Watts function whereas the nonlinear relaxational behavior of these polymers was modeled in terms of the Tool−Narayanaswamy description of the glassy state using the Adam−Gibbs expression for the relaxation time. This procedure allows for the nonlinear extension of the liquid equilibrium state behavior into and below the glass transition region with an accurate prediction of the relaxation times over more than 15 orders of magnitude in time. Time−temperature scaling of the relaxation times with respect to Tg/T as the relevant scaling parameter is observed below the glass transition. The effect of annealing was investigated using differential scanning calorimetry with the result that a single set of parameters is sufficient to describe a wide range of thermal histories with as well as without annealing. Optimum annealing temperatures/annealing times for best orientational stability in NLO polymers were calculated according to the same relaxation scheme.