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

This article presents the molecular orientation and second-order optical no nlinearity of newly prepared polyureas and copolyurea with dipole moments a ligned transverse to the main backbone. Nonlinear optical (NLO) polyureas, TPU and TPU2, were prepared from 2,4-diamino-4'-nitroazobenzene (BR-DIAMINE ) with 4,4'-diphenylmethane diisocyanate (DMDI) and tolylene 2,4-diisocyana te, 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 x 10(8) s at 100 degreesC, Copolyurea was uniaxially drawn in rati os of 1.5 and 2.0. The average molecular angles Phi (x), Phi (Y), and Phi ( Z) in three laboratory frames were evaluated from the refractive indices. P hi (Y), decreased and Phi (X) and Phi (Z) increased with an increasing draw ratio. The dependence of the second-order harmonic intensity on the incide nce angle, that is, the Maker fringe pattern, was fitted with two independe nt tensor components, d(33), and d(31), for undrawn film and five independe nt tensor components, d(33), d(32), d(31), d(15), and d(24), for drawn film s. For drawn films, Kleinman symmetry was not satisfied: d(31) not equal d( 15) and d(32) not equal d(24). An increase in the draw ratio gave rise to a large increase in the tensor component d(33). (C) 2000 John Wiley & Sons, Inc.