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 (Tgα≥250°C). By dielectric loss measurements at different frequencies we put into evidence a subglass transition temperature Tgβ around 120°C. 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°C. All the decay curves of SHG could be fitted by a KWW-stretched exponential I2ϖ(t)=I2ϖ(t=0) exp(−t/τ)β. An Arrhenius plot of τ clearly shows that there is a transition temperature nearby Tgβ with two different activation energies, i.e. 140 kJ mol−1 above 120°C and 28 kJ mol−1 below 120°C for PI-DR1. An extrapolation of the low-temperature regime to room temperature allows to predict a relaxation time τ≈1.25 years. Tgβ 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 Tgβ leads to almost the same stability as thermally assisted poling with lower degradation.