Rotational Reorientation Dynamics of Nonlinear Optical Chromophores in Rubbery and Glassy Polymers: α-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(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 s 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, χ⁽²⁾, is shown to be sensitive to 〈cos θ〉, where θ 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 χ⁽²⁾ can be represented by a Kohlrausch-Williams-Watts equation, from which average rotational reorientation time constants, 〈τ〉, may be determined. By comparison of SHG and dielectric relaxation measurements in PIBMA, which exhibits only an α-relaxation, and PEMA, which exhibits distinct α- and β-relaxations at temperatures T ≤ 1.15–1.17T_g, the rotational, reorientation dynamics of DR1 are shown to be coupled with the α-relaxation dynamics in these polymers. 〈τ〉 values are obtained over a seven decade range and fit well to WLF equations above T_g but deviate below T_g; scaling the 〈 τ〉 data using the reduced variable Tg/T reveals good overlap for the two polymers, indicating similarity in the cooperativity of the α-relaxations in PIBMA and PEMA. The implications of the coupling of nonlinear optical dopant reorientation to the polymer α-relaxation for the design of temporally stable SHG polymeric materials are discussed.