Motion of twist defects in crystalline polyethylene: A molecular-dynamics study

We study the behavior of three types of twist defects in polyethylene crystals, namely, the π twiston with chain elongation, the π twiston with chain compression, and the 2π twiston with no net chain translation. Using molecular-dynamics techniques and a realistic polyethylene model, we show that at low temperatures all these twistons can propagate smoothly like solitary waves. When they arrive at a free chain end, they cannot re-emerge but will be annihilated. At high temperatures, π twistons with chain compression and 2π twistons are unstable and they can be easily broken into sharp twists that contain one or several gauche torsion angles. π twistons with chain elongation are relatively stable against thermal fluctuation. In short polyethylene chains may move to a chain end in a few picoseconds, but in long chains they diffuse slowly like Brownian particles.