Single molecules probe polymer dynamics in the supercooled regime
Dr. Renaud Vallee (Laboratory of molecular dynamics and spectroscopy, Katholieke Universiteit Leuven)

1) Using single molecule spectroscopy, we show that the fluorescence lifetime trajectories of single probe molecules embedded in a glass-forming polymer melt exhibit strong fluctuations of a hopping character. Using molecular dynamics simulations targeted to explain these experimental observations, we show that the lifetime fluctuations correlate strongly with the average square displacement function of the matrix particles. The latter observable is a direct probe of the meta-basin transitions in the potential energy landscape of glass-forming liquids. We thus show here that single molecule experiments can provide detailed microscopic information on system properties that hitherto have been accessible via computer simulations only.
2) We show that recent developments of Single Molecule Spectroscopy techniques promote their uses to the in-depth study of the complex behavior of glass forming liquids. Specifically, by following the time Molecular Dynamics evolution of a single dumbbell representing the probe molecule in a coarse-grained model of a glass forming polymer melt, we provide a comprehensive analysis of various dynamic correlation functions of this single molecule. The orientational time correlation functions of different rank l exhibit a stretching behavior depending both on l and on temperature. Furthermore, although the time-temperature superposition principle is violated in case l = 4, the correlation function relaxes on the same time scale as that of the incoherent intermediate scattering function, both types of relaxation times following nevertheless the same mode-coupling power law as do the the bulk melt. These results shed light on a recent debate concerning the origin of non-exponentially decaying orientational correlation functions of single molecules. They also suggest strongly that single molecule measurement can provide very detailed informations on the local motion mechanisms leading to the glass transition, allowing one to probe very specific features of the mode coupling theory.