Free volume and internal structural evolution during creep in model amorphous polyethylene by Molecular Dynamics simulations

All-atom Molecular Dynamics (MD) simulations were employed to investigate the structural and free volume evolution (correlated with damage) during creep of model amorphous polyethylene (PE) at various applied stress states (tension, shear, compression), stress levels (10-200 MPa), and temperatures (175-325 K). The Modified Embedded-Atom Method for saturated hydrocarbons is applied to show that the phenomenological macroscale creep response of PE can be captured through MD simulations. The model adequately predicts the three classical stages of creep (primary, secondary, and tertiary) and provides detailed insight into the underlying molecular mechanisms. The calculated glass transition temperature (T-g) was found to be very close to the experimental T-g. Simulations were performed at temperatures below T-g (175 K) to above T-g (325 K) and demonstrate that the transition from glassy to rubbery state is reflected in the chain dynamics and damage evolution. Under all the stress states and temperatures simulated, the evolution of void volume, nucleation, growth, and coalescence are shown to directly correlate with specific stages of the creep response and the underlying chain dynamics within each stage. A correlation between the steady-state creep rate and steady-state void nucleation rate is found, suggesting that secondary creep is heavily driven by void nucleation, while tertiary creep is driven by void growth and coalescence.