Ensemble and single-molecular studies of polymer dynamics near the glass transition

Rotational dynamics of polymers are studied by both ensemble and single-molecule spectroscopy. Polarized fluorescence recovery after photobleaching (FRAP) is used to measure bulk rotational diffusion of probe molecules in poly(cyclohexyl acrylate) matrix at various temperatures above the glass transition temperature of the polymer. The anisotropy decay is fit by a stretched exponential function: f (t) = exp[- (t /τ ) β ] , and is a nonexponential decay with small β values ~ 0.6. The dependence of rotational time on temperature follows the well-established WilliamsLandel-Ferry equation, which describes the primary relaxation of polymers, and therefore demonstrates that the rotational times of probes are indeed reflective of their host material. The same polymer is also investigated by single-molecule fluorescence spectroscopy. The rotational motion of the probe molecule can be elucidated by the auto-correlation function of the reduced linear dichroism signals. Each auto-correlation function is fit to the stretched exponential function, and the results from all single molecules show wide distributions of correlation times and β. The average rotational time from the single-molecule experiment agrees with that measured by the ensemble technique, and the sum of all correlation functions forms a nonexponential decay that is almost identical to the bulk anisotropy decay. Both results suggest that the polymer system has inhomogeneous dynamics that are not pure diffusion.