Dynamics of supercooled liquids confined to the pores of sol-gel glass: a dynamic light scattering study.

Abstract

Dynamics of low molecular weight and polymeric glass forming liquids in confined geometries has been studied by means of depolarized dynamic light scattering: photon correlation spectroscopy and Fabry-Perot interferometry. The pore size of the glassy matrix amounted to 2.5, 5.0, and 7.5 nm. The glass transition temperature T(g) of these liquids in confined geometries has been measured using differential scanning calorimetry. A systematic decrease of T(g) (up to 25 K) with decreasing pore size has been observed. The relaxation times of the alpha process at constant temperature were decreasing with decreasing pore size (up to 6 orders of magnitude at T(g)), while the width of their distribution was increasing. The change of the relaxation times can be assigned to the change of T(g) in confined geometries. After correcting the activation plots for the shift of T(g) a master curve was obtained for all pore sizes and the bulk material. The effect of chemical modification of the surface of the porous matrix on the dynamics of ortho-terphenyl has also been studied. These dramatic changes of the T(g) and the relaxation time of confined liquids can be explained by simple thermodynamic arguments. There is no indication that they are related to the change of the correlation length of cooperative dynamics.