Interaction Controlled Molecular Probing of Length Scale Dependent Glassy Dynamics in Polymer Melts

TL;DR

This study uses molecular dynamics simulations to show how probe--host interaction strength influences the ability of molecular probes to reflect the dynamic heterogeneity and length scale-dependent glassy dynamics in polymer melts.

Contribution

It reveals the microscopic basis of how probe--host interactions affect the interpretation of probe dynamics in glassy polymers, highlighting scale-dependent heterogeneity.

Findings

Weakly interacting probes access more dynamic environments.

Strongly interacting probes are confined to less mobile cages.

Probe dynamics encode spatial scale and heterogeneity information.

Abstract

Single molecule probes are widely used to characterize dynamic heterogeneity in glass forming liquids, but interpreting probe dynamics remains challenging because the measured response depends on how the probe couples to its host environment. Using molecular dynamics simulations of dilute probe dimers embedded in a supercooled polymer melt, we show that the probe--host interaction strength determines which heterogeneous environment of the host matrix is reflected in the probe dynamics. Weakly interacting probes partially decouple from their local cages and remain able to access dynamically active environments, whereas strongly interacting probes are more constrained within less mobile, cage-like environments. This interaction-dependent response provides a microscopic basis for the variation in fragility inferred from the probe dynamics, even though the intrinsic host dynamics remains essentially unperturbed. By comparing probe rotational relaxation with the wavevector-dependent structural relaxation and dynamic susceptibility of the host, we establish a scale-dependent correspondence between probe dynamics and host dynamic heterogeneity. Our results show that molecular probes do not simply report the bulk host relaxation, but instead encode the spatial scale and heterogeneous environment associated with the probe--host interaction.