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 ethyl )amino]-2-(hydroxymethyl)-4'-nitro-azobenzene (T-AZODIOL) and aromatic diisocyanate of tolylene 2,4-diisocyanate (TDI). T-polymer has the fe ature 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 deg rees C. The time-dependent decay curve of second-order nonlinear susce ptibility is fitted well by a Kohlrausch-Williams-Watts stretched expo nential 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, an d the relaxation time for the sample poled at 94 degrees C is over 50 years at room temperature. The second-order nonlinear susceptibility o f the sample poled at 94 degrees C is twice larger than that poled at 80 degrees C. Thermally stimulated discharge current measurement for t he sample poled at 94 degrees C clearly indicates the broad current fl ow due to the segmental molecular motion and the sharp current flow du e to the reorientation of NLO dipole moment at higher temperature whic h 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 small er free volume for the sample poled at 94 degrees C and/or the orienta tional retaining of the aligned NLO chromophores by space charges form ed in terms of the charges injected at 94 degrees C.