Temperature-domain analysis of primary and secondary dielectric relaxation phenomena in a nonlinear optical side-chain polymer

The investigation of dipole relaxation processes in nonlinear optical (NLO) polymers containing chromophore dipoles with large hyperpolarizabilities is important for optimizing the poling process and for predicting the long-term stability of the poling-induced order. The primary or alpha relaxation is difficult to assess by dielectric spectroscopy in polymers with high glass transition temperature due to thermally induced chromophore degradation. A fast experimental procedure is developed for the investigation of dielectric relaxation processes in NLO polymers, without severely inducing chromophore degradation. The procedure is based on the measurement of the dielectric function epsilon (T)= epsilon '(T)-i epsilon "(T) at a few frequencies from 30 Hz to 30 kHz, while heating the polymer at a constant rate. The complex plane representation of the temperature-dependent dielectric function is used to determine the distribution of relaxation times, while the temperature-dependent mean relaxation time tau alpha (t) is numerically determined from the dielectric loss epsilon "(T). With only three decades in frequency, information on five decades in time or 90 K in temperature is gained above the glass transition temperature. The strong alpha and the weak beta relaxation below the glass transition are separately investigated by thermally stimulated depolarization after a suitable two-step poling procedure. The method has been applied to a typical polyimidelike side-chain nonlinear optical polymer with modified Disperse Red 1 chromophores. Even below room temperature, a gamma -relaxation process is observed, demonstrating significant mobility of the chromophores much below the glass transition. From the results of thermally stimulated depolarization it is concluded that the initial fast decay of the electro-optical response to a temporally stable value is related to the partial depolarization caused by the beta relaxation.