Dielectric relaxation at the glass transition of confined N-methyl-ε-caprolactam

The effects of confinement on the glass transition of the nonassociating glass-forming liquid N-methyl-epsilon-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=4.0 nm (Vycor glass) and three dimensionally confined in butyl rubber with mean droplet diameter d=7.6 nm. 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 ct 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 xi at T-g to xi less than or equal to 10-12 nm.