Molecular Dynamics in Confining Geometries

摘要

Broadband dielectric studies (10~(―3)Hz - 10~9 Hz) on the molecular dynamics of low molecular weight glass forming liquids in confining geometries are reviewed. In these systems an interplay between surface- and confinement effects is observed: Due to the interaction of the guest molecules with the host system their dynamics is slowed down and hence the glass transition temperature is increased. This effect can be counterbalanced by lubricating the inner surfaces of the pores causing the dynamics of the molecules inside the pores to decouple from the solid walls. ―Confinement effects cause the molecular dynamics of an embedded liquid to be orders of magnitude faster than the bulk liquid. The glass transition temperature is also observed to dramatically decrease with the strength of confinement. It reflects the inherent length scale of the dynamic glass transition of the embedded liquid which increases with decreasing temperature. If it becomes comparable with the size of the confining space the molecular dynamics changes from a Vogel-Fulcher-Tammann- to an Arrhenius-type temperature dependence. This, in turn, proves the cooperative nature of the dynamic glass transition.