Interaction-Dependent Sampling of Dynamic Heterogeneity by Molecular Probes in Polymer Melts

Single-molecule probes are widely used to characterize dynamic heterogeneity in glass-forming materials. Here, we use molecular dynamics simulations of dilute probe dimers embedded in a supercooled polymer melt to examine how probe–host interaction strength affects the relationship between probe rotational relaxation and host dynamics. We find that changing the probe–host interaction does not substantially perturb the overall polymer matrix dynamics, but alters the local environments sampled by the probes. Weakly interacting probes preferentially sample relatively mobile environments and are partially decoupled from local cage constraints, whereas strongly interacting probes are more closely associated with slower, cage-like regions of the matrix. To relate these observations to the intrinsic dynamics of the host, we compare probe rotational relaxation with wavevector-dependent structural relaxation and dynamic susceptibility of the polymer melt. This comparison suggests a scale-dependent correspondence between probe dynamics and host dynamic heterogeneity, in which the apparent probe response depends on both the interaction strength and the spatial scale of matrix relaxation being sampled.