Dielectric relaxation at the glass transition of confinedN-methyl-ɛ-caprolactam

The effects of confinement on the glass transition of the nonassociating glass-forming liquid N-methyl-{ε}-caprolactam were studied in detail 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 controlled porous glasses with mean pore diameter $d=2.5,$ 5.0, 7.5, and 20.0 nm (Gelsil glasses) and d=4.0nm (Vycor glass) and three dimensionally confined in butyl rubber with mean droplet diameter d=7.6nm. The confined liquid is classified into two fractions: a relatively immobile interfacial layer close to the wall and the inner layer (volume liquid). For the volume liquid the {α} relaxation associated with the glass transition becomes faster and the glass transition temperature decreases compared to the bulk liquid. These effects increase with decreasing d and are stronger for three- than for two-dimensional confinement. They can be understood on the basis of the cooperativity concept and the configurational entropy model of Adam and Gibbs. The systematic variation of d allows the determination of the cooperativity length {ξ} at Tg to ξ<~10--12nm.