DIELECTRIC-RELAXATION AT THE GLASS-TRANSITION OF CONFINED N-METHYL-EPSILON-CAPROLACTAM

The effects of confinement on the glass transition of the nonassociati ng glass-forming liquid N-methyl-epsilon-caprolactam were studied in d etail by means of broadband dielectric relaxation spectroscopy, 5 Hz-2 GHz, and thermally stimulated depolarization current measurements, 77 -300 K. The liquid was two dimensionally confined in the pores of cont rolled porous glasses with mean pore diameter d=2.5, 5.0, 7.5, and 20. 0 nm (Gelsil glasses) and d=4.0 nm (Vycor glass) and three dimensional ly confined in butyl rubber with mean droplet diameter d=7.6 nm. The c onfined liquid is classified into two fractions: a relatively immobile interfacial layer close to the wall and the inner layer (volume liqui d). For the volume liquid the ct relaxation associated with the glass transition becomes faster and the glass transition temperature decreas es compared to the bulk liquid. These effects increase with decreasing d and are stronger for three- than for two-dimensional confinement. T hey can be understood on the basis of the cooperativity concept and th e configurational entropy model of Adam and Gibbs. The systematic vari ation of d allows the determination of the cooperativity length xi at T-g to xi less than or equal to 10-12 nm.