RELAXATION PROCESSES IN NONLINEAR-OPTICAL POLYMERS - A COMPARATIVE-STUDY

Relaxation processes in nonlinear optical (NLO) polymers with glass tr ansition temperatures in the range of 125 degrees C < T-g < 176 degree s C have been studied. The relaxational mechanisms of these side-and m ain-chain polymers have been investigated above and below the glass tr ansition by second-harmonic decay, dielectric relaxation, and differen tial scanning calorimetry measurements, and the results obtained have been compared with a variety of nonlinear optical polymer systems cite d in the literature. The nonexponential relaxation in both the time an d frequency domain was modeled by the Kohlrausch-Williams-Watts functi on whereas the nonlinear relaxational behavior of these polymers was m odeled in terms of the Tool-Narayanaswamy description of the glassy st ate using the Adam-Gibbs expression for the relaxation time. This proc edure allows for the nonlinear extension of the liquid equilibrium sta te behavior into and below the glass transition region with an accurat e prediction of the relaxation times over more than 15 orders of magni tude in time. Time-temperature scaling of the relaxation times with re spect to T-g/T as the relevant scaling parameter is observed below the glass transition. The effect of annealing was investigated using diff erential scanning calorimetry with the result that a single set of par ameters is sufficient to describe a wide range of thermal histories wi th as well, as without annealing. Optimum annealing temperatures/annea ling times for best orientational stability in NLO polymers were calcu lated according to the same relaxation scheme.