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Many molecular glass forming liquids show a shift of the glass transition Tgto lower temperatures when the liquid is confined into mesoporous hostmatrices. Two contrary explanations for this effect are given in literature:First, confinement induced acceleration of the dynamics of the molecules leadsto 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, whichalso shifts Tg to lower temperatures. Here we present novel dynamic mechanicalanalysis measurements of the glass forming liquid salol in Vycor and Gelsilwith pore sizes of d = 2.6, 5.0 and 7.5 nm. The dynamic complex elasticsusceptibility data can be consistently described with the assumption of tworelaxation processes inside the pores: A surface induced slowed down relaxationdue to interaction with rough pore interfaces and a second relaxation withinthe core of the pores. This core relaxation time is reduced with decreasingpore size d, leading to a downshift of Tg in perfect agreement with recent DSCmeasurements.

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