Molecular Motions in Polymer Films near the Glass Transition: a Single Molecule Study of Rotational Dynamics

Single molecule spectroscopy was used to measure the rotations of fluorescent probe molecules in thin films of poly(methyl acrylate) and poly(n-butyl methacrylate) just above their glass-transition temperatures. By collecting the polarized fluorescence from isolated probe molecules, the rotational diffusion of single molecules was followed in real time. The autocorrelation of these transients yields a nonexponential decay from which the rotational correlation time can be calculated. Molecules reveal a broad distribution of correlation times, which showed a clear dependence on the length of observation. At short times, the spatially heterogeneous nature of these films was reflected in their wide range of correlation times. At longer times, environmental exchanges caused the correlation times to converge on a limiting bulk value. The dynamics were characterized by three time scales:  a rotational correlation time (τc), an environmental exchange time (τex), and the time scale upon which the distribution of time-averaged single-molecule correlation times converged to the ensemble-averaged limit (τbulk). Both τex and τbulk were much longer than τc, with the ensemble average τex being approximately 20 times longer than the average τc and τbulk roughly 125 times τc. All three time scales were found to have the same relatively weak temperature dependence when measured at temperatures 5, 10, and 15 K above the glass transition.