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 ∝ 1/d in perfect agreement with recent DSC measurements. Thermal expansion measurements of empty and salol filled mesoporous samples revealed that the contribution of negative pressure to the downshift of Tg is small (< 30%) and the main effect is due to the suppression of dynamically correlated regions of size ξ when the pore size d approaches ξ.

We present novel low-frequency (0.1 Hz-50 Hz) measurements of the complex elastic susceptibility of the glass-forming liquid salol confined to nanoporous Vycor glass. Our data can be perfectly interpreted with the assumption of a radial distribution of Vogel-Fulcher temperatures T0(r) inside the pores, resulting from an increase of the molecular relaxation time with decreasing distance from the rough pore surface as recently found by computer simulations (Scheidler et al., Europhys. Lett. 59, 701 (2002)). The results show for the first time, that the dynamic elastic response is extremely sensitive for separating confinement-induced acceleration effects of the molecular dynamics and surface-induced slowing-down due to rough pore interfaces.

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