Supersonic motion of vacancies in a polyethylene crystal

The possibility of supersonic motion of vacancies in a polyethylene crystal is revealed by means of analytical investigation and numerical simulation. It is demonstrated that in the crystalline field of immovable neighbors, a vacancy with a core size of about 70 CH2 groups and a velocity in the range of 1.02-1.05 sound velocity preserves itself for the time scale of about 1 ns. It is demonstrated that this type of structural defect is similar to coupled supersonic solitons described earlier in the one-dimensional chains with combined gradient and nongradient nonlinearity. An analytic approach is proposed for prediction of their shape and velocity. The simulation of the crystal with all degrees of freedom released demonstrates that the supersonic vacancy is still distinguishable. Its lifetime is less than 5 ps but still may be significant for physical applications.