Evidence of surface softening in polymers and their nanocomposites as determined by spontaneous particle embedment

In the present work, we report results from particle embedment experiments which give the surface modulus of neat epoxy, epoxy/POSS composites and polystyrene films. The embedment of the particles was determined from atomic force microscope measurements and the modulus estimated from the Johnson, Kendall and Roberts (JKR) model. The work of adhesion between submicron particle and the polymer surface was used as the driving force for particle embedment. For neat epoxy, epoxy/POSS composite systems and PS films, the surface modulus value was found softer than the macroscopic glassy modulus. The maximum embedment depth obtained for all surfaces was low enough so that it did not cause plastic deformation on the surface. The maximum stress values on all surfaces induced by the particle embedment were estimated to verify the expected response to be close to the linear regime.