Nonexponential relaxation of poly(cyclohexyl acrylate): comparison of single-molecule and ensemble fluorescence studies.

Rotational motions of probe molecules in poly(cyclohexyl acrylate) (PCA) were investigated by both ensemble fluorescence recovery after photobleaching (FRAP) and single-molecule spectroscopy at temperatures near the glass transition temperature (Tg) of the polymer host. FRAP measurements of the ensemble anisotropy decay show a nonexponential decay with beta values of 0.5-0.6 when fit by a stretched exponential function. The relationship between the average relaxation time and temperature follows the Williams-Landel-Ferry equation, whereas beta shows no temperature dependence over this range. The same system was also studied by single-molecule spectroscopy at 2 degrees C above the Tg of PCA. The rotational dynamics of the probe molecule can be measured by the autocorrelation function of the linear dichroism signals. Each single-molecule correlation function was fit to the stretched exponential function. The results from all single-molecule data yield broad distributions of both the correlation times (tau) and beta values. The average of the single-molecule correlation times agrees with the ensemble relaxation time, and the sum of all single correlation functions has a nonexponential decay that is almost identical to the ensemble anisotropy decay. The ensemble beta values are smaller than the average beta values in the single-molecule experiments, demonstrating that the system exhibits heterogeneous dynamics. However, the dynamics are not described by an ensemble of molecules that all have single-exponential correlation functions with different time constants.