Dielectric studies of glass transition in confined propylene glycol

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Abstract
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 decreases compared to the bulk liquid. The shifts increase with decreasing d, are larger in butyl rubber than in controlled porous glasses (three-dimensional versus two-dimensional confinement) and vanish for 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 at is determined to be 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.

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