Relaxation processes in nonlinear optical polyimide side-chain polymers

Relaxation processes in nonlinear optical modified polyimide polymers with side-chain azo chromophores having glass transition temperatures in the range of 140 < T-g < 170 degrees C have been studied. The relaxational mechanisms of the side-chain chromophores in these polymers have been investigated above and below the glass transition by second-harmonic decay, dielectric relaxation, and differential scanning calorimetry measurements. The nonexponential relaxation in both the time and frequency domains was modeled by the Kohlrausch-Williams-Watts (KWW) function. The nonlinear relaxational behavior of these polymers can be modeled in terms of the Tool-Narayanaswamy description of glassy state behavior. It 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 (T-g - T)/T as the relevant scaling parameter is observed below the glass transition.