Rotational reorientation dynamics of disperse red 1 in polystyrene: α-relaxation dynamics probed by second harmonic generation and dielectric relaxation

Using a novel experimental protocol involving second harmonic generation (SHG), the rotational, reorientation dynamics of disperse red 1 (DR1) chromophores doped at 2 wt. % in polystyrene (PS) have been monitored over 11 decades in time, from 1O^-6 s onwards. This nonlinear optical (NLO) technique allows quantification of the average rotational, reorientation time constant 〈τ〉 over a broad range of temperatures, 28 to 138 °C, above and below the glass transition temperature T_g. Good agreement was obtained between 〈τ〉's determined from SHG and dielectric relaxation measurements. This is expected since both SHG and dielectric relaxation measurements of PS+2 wt. % DR1 monitor the rotational reorientation of DR1 chromophores. Values of 〈τ〉 fit well to a Williams-Landel-Ferry equation above T_g, indicating that the rotational reorientation of DR1 is coupled to the α relaxation of PS; below T_g the values of 〈τ〉 show an apparent Arrhenius temperature dependence with an activation energy of 45-50 kcal/mol. Above T _g, the SHG data are in agreement with the literature values of the temperature dependence of the α-relaxation dynamics for homopolymer PS obtained using NMR, viscosity, compliance, and photon correlation spectroscopy measurements; at T_g the absolute values of 〈τ〉 obtained by SHG are also in good agreement with the literature values obtained for relaxation times in PS using NMR, photon correlation spectroscopy, and enthalpy relaxation. Upon scaling the data using a reduced parameter, T_r/T, where T_r is close to the calorimetrically determined T_g, an excellent overlap of SHG results for the rotational reorientation dynamics of DR1 and thereby the α transition in PS, poly(ethyl methacrylate), and poly(isobutyl methacrylate) is observed below T_g; however, above T_g, the PS data show a much stronger temperature dependence, suggesting that PS may be a more "fragile" glass-former than the methacrylate-based polymers. Physical aging greatly shifts the relaxation to longer times, resulting in a significant increase in the temporal stability of SHG properties in these NLO polymers.