Near-surface mechanical properties of amorphous polymers

Polymeric material near a free surface can have properties which deviate considerably from the bulk properties. Many researchers have reported a reduced glass transition temperature in thin polymeric films and attributed this effect to an enhanced segmental mobility near a free surface. It was also reported that sufficiently thin polymeric structures show a higher ductility than the bulk material. In this paper, we therefore investigate the hypothesis that the near-surface mechanical properties of amorphous polymers differ from the bulk properties owing to the presence of an absolute length scale. Microindentations and nanoindentations are performed on polystyrene, using a range of indenter sizes and indentation loads. In addition, numerical simulations are carried out with an advanced material model for polystyrene. A comparison between the experimental and numerical results indeed indicates that a length-scale effect is present near the surface. Simulations performed at an elevated temperature indicate that our results are consistent with the observations of a reduced T g .