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 fluoresce nt 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 d iffusion of single molecules was followed in real time. The autocorrelation of these transients yields a nonexponential decay from which the rotationa l correlation time can be calculated. Molecules reveal a broad distribution of correlation times, which showed a clear dependence on the length of obs ervation. At short times, the spatially heterogeneous nature of these films was reflected in their wide range of correlation times. At longer times, e nvironmental exchanges caused the correlation times to converge on a limiti ng bulk value. The dynamics were characterized by three time scales: a rota tional correlation time (tau (e)), an environmental exchange time (tau (ex) ), and the time scale upon which the distribution of time-averaged single-m olecule correlation times converged to the ensemble-averaged limit (tau (bu lk)). Both tau (ex) and tau (bulk) were much longer than tau (c). with the ensemble average tau (ex) being approximately 20 times longer than the aver age tau (c) and tau (bulk) roughly 125 times tau (c). All three time scales were found to have the same relatively weak temperature dependence when me asured at temperatures 5, 10, and 15 K above the glass transition.