Confinement effects on glass forming liquids probed by DMA

TL;DR

This study investigates how confinement in mesoporous materials affects the glass transition of salol, revealing that reduced pore size primarily causes a downshift in T-g through core relaxation dynamics, with minor influence from negative pressure.

Contribution

The paper provides a comprehensive analysis combining DMA and thermal expansion measurements to distinguish between confinement effects and negative pressure on T-g shifts in glass forming liquids.

Findings

Core relaxation time decreases with pore size, aligning with T-g downshift.

Negative pressure contributes less than 30% to T-g shift.

Suppression of dynamic correlations is a key factor in confinement effects.

Abstract

Many molecular glass forming liquids show a shift of the glass transition T-g 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 T-g increasing with decreasing pore size. Second, due to thermal mismatch between the liquid and the surrounding host matrix, negative pressure develops inside the pores with decreasing temperature, which also shifts T-g to lower temperatures. Here we present 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 T-g proportional to 1/d in perfect agreement with recent differential scanning calorimetry (DSC) measurements. Thermal expansion measurements of empty and salol filled mesoporous samples revealed that the contribution of negative pressure to the downshift of T-g is small (<30%) and the main effect is due to the suppression of dynamically correlated regions of size xi when the pore size xi approaches.