Dynamics of twist point defects with stretching in a polymer crystal

A molecular-dynamics simulation of the behavior of a twist point defect wit h stretching in a chain of an equilibrium polymer crystal ("united" atoms a pproximation for polyethylene) is performed for immobile and mobile neighbo ring chains. It is shown that such a defect in a cold polymer crystal posse sses soliton-type mobility. The upper limit of the spectrum of soliton velo cities is found, and it is the same for both cases. The maximum possible ve locity of defects is three times lower than the theoretical limit of the sp ectrum (which is equal to the velocity of "torsional" sound in an isolated chain). An explanation of the reason for this discrepancy is proposed: beca use of the interaction of two "degrees of freedom" of the defect (twisting and stretching) the energy of a nonlinear wave is dissipated in the linear modes of the system, which results in effective friction whose magnitude de pends strongly on the velocity of the defect. The "boundary of the spectrum of soliton velocities" determines the transition between regimes of strong and weak braking of defects. (C) 2000 MAIK "Nauka/ Interperiodica".