Dynamic mechanical analysis of confined glass-forming liquids

Acoustic spectroscopy is a valuable tool, when the degrees of freedom of a material couple to strain. Since this is the case for molecular glass-forming liquids the dynamic mechanical response is a quantity of primary interest here. Dynamic mechanical analysis measurements (0.1-100 Hz) have been performed for salol, toluene and ortho-terphenyl confined in mesoporous (2.5-10 nm) silica matrices. Important parameters (relaxation times of the slow -relaxation, change of Tg with pore size, number and size of dynamically correlated molecules, etc.) characterizing the glass freezing in the three glass-forming liquids have been extracted from the measurements for various confining pore sizes. A comparison with literature data shows excellent agreement and demonstrates that mechanical measurements provide a very sensitive tool to probe the dynamics of molecular glass forming liquids in confined systems.