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.

arXiv comment62 pages, 7 main figures; Supporting Information included. Revised version with additional simulations and analyses; title and interpretation updated