Enhanced Free Surface Mobility Facilitates Release of Free-volume Holes in Thin-film Polymer Glasses

The existence of holes in glass-former liquid and the concept of free volumes have been recognized as the underpinning of molecular dynamics of polymer glasses. Annihilation of the holes, through various mechanisms, is relevant to the volume contraction and vitrification of the supercooled liquid. In the nanoconfined geometries, the process of free volume loss through diffusion to the surface is very much appealing, as the finite size scale eases hole removal by reducing the diffusional distance. Herein, we explored the free volume diffusion dynamics by investigating the glass transition temperature (T_g) changes in thin poly (ethylene terephthalate) (PET) films due to variations in the fraction of free area at the film surfaces — associated with enhanced molecular mobility, and by modeling using the surface-mediated diffusion model. Our experimental and modeling results showed that the enhanced mobility in the free area on the surface promotes the hole diffusion in thin films, resulting in a decrease of T_g of PET film with an increase of the fractions of these areas. Also, the activation energy for free volume diffusion decreased with the reduction of film thickness, demonstrating that free volume diffusion in thin films can be modulated by free surface mobility.