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 d 33 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 × 10 8 s at 100 °C. Copolyurea was uniaxially drawn in ratios of 1.5 and 2.0. The average molecular angles Φ X , Φ Y , and Φ Z in three laboratory frames were evaluated from the refractive indices. Φ Y decreased and Φ X and Φ Z 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, d 33 and d 31 , for undrawn film and five independent tensor components, d 33 , d 32 , d 31 , d 15 , and d 24 , for drawn films. For drawn films, Kleinman symmetry was not satisfied: d 31 ¬= d 15 and d 32 ¬= d 24 . An increase in the draw ratio gave rise to a large increase in the tensor component d 33 .