Subglass transition and relaxation of oriented chromophores in polyimides for second order nonlinear optics

We synthesized and characterized side-chain polyimides bearing DR1 or NPP groups (PI-DR1 and PI-NPP). These copolymers are soluble in organic solvents and they have a very high glass transition temperature (T-g(alpha) greater than or equal to 250 degreesC). By dielectric loss measurements at different frequencies we put into evidence a subglass transition temperature T-g(beta) around 120 degreesC. We also measured relaxation kinetics of polar order of nonlinear optical (NLO) chromophores by second harmonic generation (SHG) at different temperatures over a wide range extending from 80 to 180 degreesC. All the decay curves of SHG could be fitted by a KWW-stretched exponential rootI(2 infinity)(t) = rootI(2 infinity)(t = 0) exp(-t/tau)(beta). An Arrhenius plot of tau clearly shows that there is a transition temperature nearby T-g(beta) with two different activation energies, i.e, 140 kJ mol(-1) above 120 degreesC and 28 kJ mol(-1) below 120 degreesC for PI-DRL. An extrapolation of the low-temperature regime to room temperature allows to predict a relaxation time tau approximate to 1.25 years. T-g(beta) value also coincides with the temperature at which an oriented PI-DR1 film starts to disorient. when heated gradually from room temperature. A physical aging procedure is described which allows to increase the thermal stability. Finally, it is shown that photoassisted poling made at temperatures higher than T-g(beta) leads to almost the same stability as thermally assisted poling with lower degradation. (C) 2000 Elsevier Science S.A. All rights reserved.