Rotational reorientation dynamics of disperse red 1 in polystyrene: a-relaxation dynamics probed by second harmonic generation and dielectric relaxation. Dhinojwala, Ali; Wong, George K.; Torkelson, John M. Northwestern Univ., Evanston, IL, USA. J. Chem. Phys. (1994), 100(8), 6046-54. CODEN: JCPSA6 ISSN: 0021-9606. Journal written in English. CAN 120:299786 AN 1994:299786 CAPLUS

Above Tg, the SHG data are in agreement with the literature values of the temp. dependence of the a-relaxation dynamics for homopolymer I obtained using NMR, viscosity, compliance, and photon correlation spectroscopy measurements; at Tg the abs. values of átñ obtained by SHG are also in good agreement with the literature values obtained for relaxation times in I using NMR, photon correlation spectroscopy, and enthalpy relaxation. Upon scaling the data using a reduced parameter, Tr/T, where Tr is close to the calorimetrically detd. Tg, an excellent overlap of SHG results for the rotational reorientation dynamics of DR1 and thereby the a transition in I, poly(Et methacrylate), and poly(iso-Bu methacrylate) is obsd. below Tg; however, above Tg, the I data show a much stronger temp. dependence, suggesting that I may be a more "fragile" glass-former than the methacrylate-based polymers. Phys. aging greatly shifts the relaxation to longer times, resulting in a significant increase in the temporal stability of SHG properties in these NLO polymers. Using a novel exptl. protocol involving second harmonic generation (SHG), the rotational, reorientation dynamics of disperse red 1 (DR1) chromophores doped at 2-wt.% in polystyrene (I) have been monitored over 11 decades in time, from 10-6 s onwards. This nonlinear optical (NLO) technique allows quantification of the av. rotational, reorientation time const. átñ over a broad range of temp., 28-138°, above and below the glass-transition temp. Tg. Good agreement was obtained between átñ's detd. from SHG and dielec. relaxation measurements. This is expected since both SHG and dielec. relaxation measurements of I + 2 wt.% DR1 monitor the rotational reorientation of DR1 chromophores. Values of átñ fit well to a Williams-Landel-Ferry equation above Tg, indicating that the rotational reorientation of DR1 is coupled to the a relaxation of I; below Tg the value of átñ show an apparent Arrhenius temp. dependence with an activation energy of 45-50 kcal/mol.