Rotational Reorientation Dynamics of Nonlinear Optical Chromophores in Rubbery and Glassy Polymers: a-Relaxation Dynamics Probed by Second Harmonic Generation and Dielectric Relaxation

Second harmonic generation (SHG) is used to monitor rotational, reorientation dynamics of disperse red 1 (DR1) doped at 2 wt % in poly(isobuty1 methacrylate) (PIBMA) and poly(ethy1 methacrylate) (PEMA). A delay-trigger approach is employed in conjunction with conventional measurements to monitor dynamics from lo-‘ s to as long as necessary, permitting characterization of rotational reorientation above and below To’ The dynamics of the orientation component of the second-order macroscopic susceptibility, x ( ~ ) , is shown to be sensitive to (cos e), where 0 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 ~ ( 2 ) can be represented by a Kohlrausch-WilliamsWatts equation, from which average rotational reorientation time constants, ( T ) , may be determined. By comparison of SHG and dielectric relaxation measurements in PIBMA, which exhibits only an a-relaxation, and PEMA, which exhibite distinct aand 8-relaxations at temperatures T 5 1.15-1.17TE, the rotational, reorientation dynamics of DR1 are shown to be coupled with the a-relaxation dynamics in these polymers. (I) values are obtained over a seven decade range and fit well to WLF equations above Tg but deviate below T,; scaling the (I) data using the reduced variable T,/T reveals good overlap for the two polymers, indicating similarity in the cooperativity of the a-relaxations in PIBMA and P E W . The implications of the coupling of nonlinear optical dopant reorientation to the polymer a-relaxation for the design of temporally stable SHG polymeric materials are discussed.