Rejuvenation, aging, and confinement effects in atactic-polystyrene films subjected to oscillatory shear

Molecular-dynamics simulations of 5 nm-thick atactic-polystyrene films have been used to study the influence of cyclic-shear deformation on the stress–strain behavior and local segmental mobility. Upon cyclic yield the stress–strain behavior of the films slowly evolves towards a steady state which is characterized by a decrease of the maximum stress and by an enhanced dissipative process. Immediately after plastic deformation the storage modulus is decreased and the loss modulus is increased as compared with their initial values. Such changes in the viscoelastic moduli reflect the mechanical rejuvenation of a polymer glass. This mechanical rejuvenation of polymers is connected to the increase in the simulated segmental mobility, which is calculated for the entire film as well as in different layers.