Effect of physical ageing in thin glassy polymer films

We have studied the effect of physical ageing in thin supported glassy polystyrene films by using ellipsometry to detect overshooting in the expansivity-temperature curve upon heating of aged samples. Films with thickness 10–200 nm have been aged at $70\un{^\circ C}$ and $80\un{^\circ C}$ (below the bulk glass transition temperature). We observe clear relaxation peaks in the expansivity-temperature curve for films thicker than 18 nm but not for the 10 nm film. The intensity of the relaxation peak is inversely proportional to the film thickness, while the temperatures characteristic to the relaxation peak are almost independent of the film thickness. These observations are successfully interpreted by the idea that the surface layer of the order of 10 nm has liquid-like thermal properties