A Novel Concept for Modeling the Time‐Temperature Dependence of Polar Order Relaxation in Nonlinear Optical Active Polymers

Abstract

AbstractThe widely accepted concept of introducing a fictive temperature (Tf) to describe the structural relaxation below the glass transition temperature is transposed to explain the orientational dynamics of nonlinear optical (NLO) active chromophores in poled polymer systems. The influence of different thermal histories on the results of differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC) gives evidence for a direct connection between the structural relaxation as measured by DSC and the relaxation of polar order as measured by TSDC. With the fictive temperature being determined from the DSC experiments it is possible to relate the equilibrium dynamics of the NLO chromophores above the glass transition temperature as determined by dielectric spectroscopy to the nonequilibrium relaxation times of polar order relaxation below Tg over 12 decades in time. The obtained time‐temperature dependence for the relaxation times, based on Scherer's extension of the Adam‐Gibbs theory (AGV), shows a Vogel‐Fulcher behavior above Tg and an Arrhenius‐like behavior far below Tg. Below Tg the temperature dependence of the relaxation times of polar order is in excellent agreement with an empirically found modified Vogel‐Fulcher equation, which modeled the relaxation of polar order of different poled polymer systems in an universal manner. The AGV theory further offers a direct approach to introduce the influence of the thermal history on the time‐temperature characteristics of polar order relaxation.