Single molecule probing of the glass transition phenomenon: simulations of several types of probes

Molecular dynamics simulations of a system of short bead-spring chains containing an additional dumbbell are presented and analyzed. This system represents a coarse-grained model for a melt of short, flexible polymers containing fluorescent probe molecules at very dilute concentration. It is shown that such a system is very well suited to study aspects of the glass transition of the undercooled polymer melt via single molecule spectroscopy, which are not easily accessed by other methods. Such aspects include data which can be extracted from a study of fluctuations along a trajectory of the single molecule, probing the rugged energy landscape of the glass-forming liquid and transitions from one metabasin of this energy landscape to the next one. Such an information can be inferred from "distance maps" constructed from trajectories characterizing the translational and orientational motion of the probe. At the same time, determining autocorrelation functions along such trajectories, it is shown for several types of probes (differing in their size and/or mass within reasonable limits) that this time-averaged information of the probe is fully compatible with ensemble averaged information on the relaxation of the glass-forming matrix, accessible from bulk measurements. The analyzed quantities include the fluorescence lifetime, linear dichroism, and also various orientational correlation functions of the probe, in order to provide guidance to experimental work. Similar to earlier findings from simulations of bulk molecular fluids, deviations from the Stokes-Einstein and Stokes-Einstein-Debye relations are observed.