Revealing spatially heterogeneous relaxation in a model nanocomposite

TL;DR

This study combines experimental, simulation, and theoretical methods to reveal how nanoparticle interfaces influence the spatially heterogeneous relaxation dynamics in glycerol-silica nanocomposites, showing increased relaxation times and interfacial densification effects.

Contribution

It provides a microscopic understanding of the interfacial slowing down in nanocomposites by integrating atomistic simulations with statistical mechanical theory.

Findings

No 'glassy' layer observed near nanoparticles.

Alpha relaxation time near nanoparticles increases significantly upon cooling.

Interfacial layer thickness expands from 1.8 nm to 3.5 nm near Tg.

Abstract

The detailed nature of spatially heterogeneous dynamics of glycerol-silica nanocomposites is unraveled by combining dielectric spectroscopy with atomistic simulation and statistical mechanical theory. Analysis of the spatial mobility gradient shows no 'glassy' layer, but the alpha relaxation time near the nanoparticle grows with cooling faster than the alpha relaxation time in the bulk, and is ~ 20 times longer at low temperatures. The interfacial layer thickness increases from ~ 1.8 nm at higher temperatures to ~ 3.5 nm upon cooling to near Tg. A real space microscopic description of the mobility gradient is constructed by synergistically combining high temperature atomistic simulation with theory. Our analysis suggests that the interfacial slowing down arises mainly due to an increase of the local cage scale barrier for activated hopping induced by enhanced packing and densification near the nanoparticle surface. The theory is employed to predict how local surface densification can be manipulated to control layer dynamics and shear rigidity over a wide temperature range.