Revealing the pure confinement effect in glass-forming liquids by dynamic mechanical analysis

TL;DR

This study uses dynamic mechanical analysis to distinguish between two effects causing the lowering of glass transition temperature in confined glass-forming liquids, confirming the core relaxation reduction as the main factor.

Contribution

It provides direct experimental evidence that the pure confinement effect, specifically the reduction of core relaxation time, is responsible for Tg shifts in confined glass-forming liquids.

Findings

Core relaxation time decreases with pore size.

Tg shift aligns with core relaxation reduction.

Surface interactions cause a secondary, slower relaxation.

Abstract

Many molecular glass forming liquids show a shift of the glass transition Tg to lower temperatures when the liquid is confined into mesoporous host matrices. Two contrary explanations for this effect are given in literature: First, confinement induced acceleration of the dynamics of the molecules leads to an effective downshift of Tg increasing with decreasing pore size. Secondly, due to thermal mismatch between the liquid and the surrounding host matrix, negative pressure develops inside the pores with decreasing temperature, which also shifts Tg to lower temperatures. Here we present novel dynamic mechanical analysis measurements of the glass forming liquid salol in Vycor and Gelsil with pore sizes of d = 2.6, 5.0 and 7.5 nm. The dynamic complex elastic susceptibility data can be consistently described with the assumption of two relaxation processes inside the pores: A surface induced slowed down relaxation due to interaction with rough pore interfaces and a second relaxation within the core of the pores. This core relaxation time is reduced with decreasing pore size d, leading to a downshift of Tg in perfect agreement with recent DSC measurements.