Dielectric studies of glass transition in confined propylene glycol

The dynamical behaviour of the glass transition of propylene glycol confined in droplets in butyl rubber (three-dimensional confinement, mean droplet diameter d=8-11 nm) and in pores in controlled porous glasses (two dimensional confinement, mean pore diameter d=2.5-7.5 nm) has been studied in detail by means of broadband dielectric spectroscopy (5 Hz-2 GHz) and of thermally stimulated depolarization current measurements. Effective medium theory corrections of the data are discussed. The results indicate the existence of a relatively immobile interfacial layer close to the wall. For the volume liquid the dynamics of the glass transition becomes faster and the glass transition temperature T sub g decreases compared to the bulk liquid. The shifts Delta T sub g increase with decreasing d, are larger in butyl rubber than in controlled porous glasses (three-dimensional versus two-dimensional confinement) and vanish for dapprox = 10-12 nm. These results are discussed in relation to those obtained with polymers confined in thin polymeric films (one-dimensional confinement) and in semicrystalline polymeric samples and are explained on the basis of the cooperativity concept and the model of Adam and Gibbs. The cooperativity length xi < = 5-6 nm in both butyl rubber and controlled porous glasses. Interesting effects of confinement are observed on the shape of the dielectric response of the process associated with the glass transition.