Orientational relaxation of transversely aligned nonlinear optical dipole moments to the main backbone in the linear polyurethane

This paper presents the orientational relaxation of nonlinear optical (NLO) chromophore in a linear polyurethane of T-polymer based on 4-[(2-hydroxyethyl)amino]-2-(hydroxymethyl)-4'-nitro-azobenzene (T-AZODIOL) and aromatic diisocyanate of tolylene 2,4-diisocyanate (TDI). T-polymer has the feature that NLO dipole moment is aligned transverse to the main chain. A remarkable difference of the orientational relaxation of NLO dipole moments was observed between the samples corona-poled at 80 and 94 degrees C. The time-dependent decay curve of second-order nonlinear susceptibility is fitted well by a Kohlrausch-Williams-Watts stretched exponential function. The relaxation time of T-polymer poled at 94 degrees C is 3 orders of magnitude larger than that poled at 80 degrees C, and the relaxation time for the sample poled at 94 degrees C is over 50 years at room temperature. The second-order nonlinear susceptibility of the sample poled at 94 degrees C is twice larger than that poled at 80 degrees C. Thermally stimulated discharge current measurement for the sample poled at 94 degrees C clearly indicates the broad current flow due to the segmental molecular motion and the sharp current flow due to the reorientation of NLO dipole moment at higher temperature which is significantly related to the effective transition temperature for SHG activity. The remarkable stability of second harmonic generation activity for the sample poled at 94 degrees C is ascribed to the smaller free volume for the sample poled at 94 degrees C and/or the orientational retaining of the aligned NLO chromophores by space charges formed in terms of the charges injected at 94 degrees C.