Effects of confinement on relaxation in glass-formers : Magnitude, trends and dependence on the Kohlrausch exponent

We show that the Adam-Gibbs theory cannot by itself explain the reduction of τα and Tg when a supercooled liquid is confined in nano-meter pores. However, when combined with the coupling model, the modified theory can explain the finite-size effects of small molecule glass-formers in pores and ultra thin polymer films. The modified theory also predicts two additional behaviors. First, magnitude of the finite-size effect is larger in liquids that have smaller exponent, β, in the Kohlrausch expression for the structural relaxation of the unconfined liquid at Tg. Second, the dependence of the structural relaxation time of the liquid on Tg/T becomes increasingly weaker with decreasing pore size. These predicted behaviors are found in the experimental data. A previously proposed mechanism for the glass transition reduction in thin freely-standing polymer films [K.L. Ngai et al., J.Non-Cryst.Solids 235-237, 435 (1998)] is revisited. Recent Monte Carlo simulation results support the mechanis