Self-motion in glass-forming polymers: A molecular dynamics study

Abstract

We present results of molecular dynamics simulations of an undercooled polymer melt, performed to study the validity of mode-coupling theory (MCT) for realistic polymer melts in general. The mean square displacements of the chain segments are computed to study the diffusion constant of the Rouse-like motion. It is shown that this diffusion constant follows a power law behavior as a function of the temperature, as predicted by the MCT. In addition, we studied the incoherent part of the intermediate scattering function and show that these functions obey the second scaling law of the MCT. We also calculated the relaxation times of the alpha-relaxation and found that they follow the same power law (gamma=2.9) as the diffusion constant. Using gamma, and the relationships given by MCT, we obtain values for a (0.27) and b (0.46) and use these exponents to describe the beta-relaxation regime. We find that the long time part of the beta-relaxation can be described accurately by the Von Schweidler relaxation over a wide range of wave numbers. In the short time regime of the beta-relaxation, no critical decay is observed.