Influence of confinement on the fragility of antiplasticized and pure polymer films.

We investigate by molecular dynamics simulation how thin film confinement modifies the fragility of a model glass-forming liquid characterized previously in the bulk. Film confinement is found to reduce the relative fragility of the polymer fluid, leading to effects similar to simulations of the addition of an antiplasticizer additive. A reduction of fragility is not observed for the antiplasticized polymer film. These effects are interpreted in terms of variation in the string (cooperatively moving segments) concentration with film confinement and the addition of antiplasticizing additives.