Nonexponential Relaxation of Poly(cyclohexyl acrylate): Comparison of Single-Molecule and Ensemble Fluorescence Studies

• Published in: The journal of physical chemistry. B 2009-02, Vol.113 (8), p.2253-2261
• Main Authors: Wei, Chia-Yin Joyce, Vanden Bout, David A
• Format: Article
• Language: English
• Subjects: B: Macromolecules, Soft Matter
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp806293x

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 (T g) of the polymer host. FRAP measurements of the ensemble anisotropy decay show a nonexponential decay with β 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 β shows no temperature dependence over this range. The same system was also studied by single-molecule spectroscopy at 2 °C above the T g 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 (τ) and β 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 β values are smaller than the average β 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.