Interaction-Dependent Molecular Probing of Heterogeneous Glassy Dynamics in Polymer Melts

Abstract Single-molecule probes are widely used to characterize dynamic heterogeneity in glass-forming liquids, but their measured dynamics can depend on how the probe couples to its host environment. Using molecular dynamics simulations of dilute dimer probes embedded in a supercooled polymer melt, we examine how probe–host affinity influences the correspondence between probe rotational relaxation and host dynamics. Increasing the probe–host interaction strength systematically modifies the relative temperature dependence of probe and host relaxation while leaving the intrinsic host dynamics essentially unperturbed. Lower-affinity probes exhibit stronger non-Gaussian translational motion and larger displacement fluctuations, whereas higher-affinity probes remain more localized and are statistically less exposed to the most dynamically active host environments. Comparison with wavevector-dependent host structural relaxation further identifies an effective matching wavevector at which the host structural relaxation exhibits the temperature dependence most similar to that of the probe, providing a phenomenological measure of the probe–host dynamical correspondence. Together with real-space analyses of the local environments sampled by the probe, these results show that probe–host affinity systematically influences the quantitative relationship between molecular-probe relaxation and heterogeneous host dynamics.