Methyl group dynamics in a confined glass

TL;DR

This study investigates how confinement in mesoporous silicates affects methyl group dynamics in glassy toluene, revealing a significant reduction in rotational barriers and identifying surface and bulk-like contributions to the dynamics.

Contribution

It introduces a barrier distribution model to analyze confined methyl group dynamics, highlighting the impact of structural disorder and confinement on rotational barriers.

Findings

Confinement decreases average rotational barriers.

Distinction between surface-like and bulk-like methyl groups.

Estimated interaction range of 7 Å, larger than nearest-neighbor distance.

Abstract

We present a neutron scattering investigation on methyl group dynamics in glassy toluene confined in mesoporous silicates of different pore sizes. The experimental results have been analysed in terms of a barrier distribution model, such a distribution following from the structural disorder in the glassy state. Confinement results in a strong decreasing of the average rotational barrier in comparison to the bulk state. We have roughly separated the distribution for the confined state in a bulk-like and a surface-like contribution, corresponding to rotors at a distance from the pore wall respectively larger and smaller than the spatial range of the interactions which contribute to the rotational potential for the methyl groups. We have estimated a distance of 7 Amstrong as a lower limit of the interaction range, beyond the typical nearest-neighbour distance between centers-of-mass (4.7 Amstrong).