Atomistic molecular dynamics simulations of a chemically realistic model of atactic short-chain polystyrene between gold surfaces (111) and positron annihilation lifetime spectroscopy experiments on similar polystyrene thin films on gold were performed. Results from both approaches show that the free volume voids in the film have a slightly smaller average size than in bulk polystyrene. In agreement to that the existence of an interphase of higher density at the polymersolid substrate interface is shown both by the simulation as well as in the experiment. The average shape of the voids is similar in the bulk and the film.