Viscoelastic recovery behavior following Atomic Force Microscope nanoindentation of semi-crystalline poly(ethylene)

The residual imprint left behind by the AFM nanoindentation of polymers has been seldom studied in the past. In this work, the evolution of indentations at room temperature performed on a semicrystalline poly- (ethylene) in a broad range of experimental conditions is presented. The study shows that the recovery after 24 h is substantial, although not complete. Moreover, the dynamics of the recovery process is not seen to depend on the magnitude of the applied load for the nanoindentation, but instead on the rate of the indentation used. This points out that viscoelastic processes are minimized when performing fast nanoindentations, while at low loading rates there seems to be a residual viscoelastic energy stored in the system. Although, the recovery process involves complex displacements and bending of crystalline lamellae, which is difficult to model theoretically, the present work attempts to quantify the magnitudes as well as the directions of such displacements for indentations involving from two to seven lamellae.