Structure and mass transport in constrained polymer crystals via molecular dynamics simulations

To study the plastic deformation of polymer crystals, the structure and molecular motion in surface-constrained polymethylene crystals containing 192 C₅₀H₁₀₀ chains were followed using molecular dynamics simulations. Total energy, temperature and density were evaluated as a function of time up to 100 ps. Structure changes, longitudinal and transverse mass transport, changes in setting angle of the zigzag chains and the concentration and distribution of gauche conformations were noted. Selected chains were followed during the plastic deformation by tracing their centre-of-mass coordinates and end-to-end distances. The plastic deformation involves affine expansion and contraction, development of crystallographic fault lines (slip planes), edge dislocations, curved lattice planes and longitudinal diffusion of chains out of the crystal. The time-scale for all these motions is in the picosecond range. The actual chain motion reaches about 10% of the speed of sound.