Nonlinear optical (NLO) polymers. IV. Second‐order optical nonlinearity of NLO polyurea and copolyurea with NLO dipole moments aligned transverse to the main backbone

Abstract

This article presents the molecular orientation and second-order optical nonlinearity of newly prepared polyureas and copolyurea with dipole moments aligned transverse to the main backbone. Nonlinear optical (NLO) polyureas, TPU and TPU2, were prepared from 2,4-diamino-4′-nitroazobenzene (2R-DIAMINE) with 4,4′-diphenylmethane diisocyanate (DMDI) and tolylene 2,4-diisocyanate, respectively. NLO copolyurea was prepared from DMDI with 2R-DIAMINE and m-phenylene diamine. TPU and TPU2 gave d33 values of 12.5 and 9.8 pm/V, respectively, under optimum poling conditions. A time-dependent decay curve of second-order nonlinear susceptibility was fitted well with a Kohlrausch–Williams–Watts stretched exponential function. The relaxation time of TPU2 was 4.2 × 108 s at 100 °C. Copolyurea was uniaxially drawn in ratios of 1.5 and 2.0. The average molecular angles ΦX, ΦY, and ΦZ in three laboratory frames were evaluated from the refractive indices. ΦY decreased and ΦX and ΦZ increased with an increasing draw ratio. The dependence of the second-order harmonic intensity on the incidence angle, that is, the Maker fringe pattern, was fitted with two independent tensor components, d33 and d31, for undrawn film and five independent tensor components, d33, d32, d31, d15, and d24, for drawn films. For drawn films, Kleinman symmetry was not satisfied: d31 ≠ d15 and d32 ≠ d24. An increase in the draw ratio gave rise to a large increase in the tensor component d33. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 247–255, 2001