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

摘要

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#)>= 250deg C). By dielectric loss measurements at different frequencies we put into evidence a subglass transition temperature T_g~(#beta#)around 120deg 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 180deg C. All the decay curves of SHG could be fitted by a KWW-stretched exponential (I_(2#omega#-bar))~(1/2)=(I_(2#omega#-bar)(t=0))~(1/2) 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 120deg C and 28 kJ mol~ (-1) below 120deg C for PI-DRI. An extrapolation of the low-temperature regime to room temperature allows to predict a relaxation time #tau#≈1.25 years. T_g~#beta# value also coincides with the temperature at which an oriented PI-DRI 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.