| The dynamics of glass-forming materials were explored near their glass transition in various geometric confinements: soft confinement, hard confinement, and intermediate situations in terms of polymer gel and polymer solution. The length scales of these geometrical restrictions were of the order of several nanometers. Solvation dynamics of a triplet state probe was used as the technique and provided very consistent results regarding the response of these viscous solvents. The dynamics was accelerated in soft spatial confinement in the case of glass-forming 'water' in 'oil' microemulsions, where the extramicellar phase was more fluid than the intramicellar sample. On the other hand, hard confinement by porous solids resulted in slower dynamics and an increased glass transition temperature for all samples of different sizes with pore diameters between 4 nm and 7.5 nm. Via the attachment of the probe molecules to the inner surface of porous glasses filled with 3-methylpentane, the interfacial layer was measured selectively and showed an increase of the relaxation time by more than three orders of magnitude over that of the bulk liquid. This frustration was most pronounced at the pore boundary and decreased gradually across the first few nanometers distance away from the interface. Confinement by polymers was analogous to the hard confinement situation, which resulted in slower dynamics and more disperse relaxation times while increasing the polymer concentration. These findings were rationalized on the basis of the predominant interfacial dynamics which differ from bulk behavior, while other finite size effect could remain disregarded. The penetration depth of this surface effect into the liquid was governed by the length scale of cooperativity of the viscous liquid, typically 1--3 nm. It was also observed that the extent of the surface effects depended on how much of the cooperative volume was involved. |
