MOLECULAR-DYNAMICS SIMULATION OF AN AMORPHOUS POLYMER UNDER TENSION .1. PHENOMENOLOGY

Molecular dynamics computer simulation has been used to study the properties of a linear polymer model resembling polyethylene over a wide range of temperatures. A loose coupling constant pressure method was used to facilitate examination of mechanical properties between 10 and 500 K. By applying uniaxial tension to the samples, we have characterized elasticity, yield, and plastic flow at low temperatures and viscoelasticity at high temperatures. Qualitative comparisons show that there are strong similarities between these short time (approximately 1 ns) simulations and laboratory measurements obtained on time scales orders of magnitude longer. The extensional (Young's) modulus has values typical of a glassy solid for T less-than-or-equal-to 100 K and shows a significant decrease as the temperature is raised. The high temperature of the glass transformation, as characterized by the disappearance of a yield stress, and the breadth of the transformation region, as characterized by density and modulus, are both explained in terms of the ultrashort time scale of the computer experiments.