Confinement effects on glass forming liquids probed by dynamic mechanical analysis

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 ${T}_{g}\ensuremath{\propto}1/d$ 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 $\ensuremath{\xi}$ when the pore size $d$ approaches $\ensuremath{\xi}$.