Dipolar Correlations in 1,4-Polybutadiene Across the Timescales : a Numerical Molecular Dynamics Simulation Investigation

Research Program SPP 1369 (German Science Foundation)International audienceBroadband dielectric spectroscopy is able to follow the time correlation of its observable, the polarization of a sample, over an unprecedented range of frequency or timescales, respectively. Features in the dielectric susceptibility as a function of frequency are assigned to different molecular motions, which, for polymers in the melt, range from high-frequency vibrations over a possible Johari-Goldstein β-relaxation, the segmental or α-relaxation, a possible normal mode for chains with a net dipole moment along the end-to-end vector of the chain to conductivity contributions on the low-frequency side. This assignment is a statement about correlations between the local dipole moments making up the sample polarization and their behavior across length and timescales. Such correlations can depend on temperature and on density as well as on confinement effects. A molecular dynamics simulation of a chemically realistic model of 1,4-polybutadiene confined between graphite walls allows for a study of such correlations over a broad temperature range in the bulk-like center of the film, as well as in the wall regions where confinement -induced correlations are present. We have already shown that confinement induces a new type of normal mode into this polymer which does not possess such a mode in the bulk. Here, we discuss the scaling of the dielectric relaxation process in this system and additionally analyze the temperature and confinement dependence of high-frequency vibrations