ROTATIONAL REORIENTATION DYNAMICS OF NONLINEAR-OPTICAL CHROMOPHORES IN RUBBERY AND GLASSY-POLYMERS - ALPHA-RELAXATION DYNAMICS PROBED BY 2ND-HARMONIC GENERATION AND DIELECTRIC-RELAXATION

Second harmonic generation (SHG) is used to monitor rotational, reorientation dynamics of disperse red 1 (DRI) doped at 2 wt % in poly(isobutyl methacrylate) (PIBMA) and poly(ethyl methacrylate) (PEMA). A delay-trigger approach is employed in conjunction with conventional measurements to monitor dynamics from 10(-4) a to as long as necessary, permitting characterization of rotational reorientation above and below T(g). The dynamics of the orientation component of the second-order macroscopic susceptibility, chi(2), is shown to be sensitive to [cos theta], where theta is the angle between the direction vector of the applied dc field and that of the chromophore dipole moment. Both in poling-onset-mode and temporal decay experiments, the time dependence of the orientational component of chi(2) can be represented by a Kohlrausch-Williams-Watts equation, from which average rotational reorientation time constants, [tau], may be determined. By comparison of SHG and dielectric relaxation measurements in PIBMA, which exhibits only an alpha-relaxation, and PEMA, which exhibits distinct alpha- and beta-relaxations at temperatures T less-than-or-equal-to 1.15-1.17T(g), the rotational, reorientation dynamics of DR1 are shown to be coupled with the a-relaxation dynamics in these polymers. [tau] values are obtained over a seven decade range and fit well to WLF equations above T(g) but deviate below T(g) scaling the [tau] data using the reduced variable TIT reveals good overlap for the two polymers, indicating similarity in the cooperativity of the alpha-relaxations in PIBMA and PEMA. The implications of the coupling of nonlinear optical dopant reorientation to the polymer alpha-relaxation for the design of temporally stable SHG polymeric materials are discussed.