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