Glassy Dynamics of Simulated Polymer Melts: Coherent Scattering and Van Hove Functions Part II: Dynamics in the alpha-Relaxation Regime

Abstract

Whereas the first part of this paper dealt with the relaxation in the beta-regime, this part investigates the final (alpha) relaxation of a simulated polymer melt consisting of short non-entangled chains above the critical temperature Tc of mode-coupling theory (MCT). We monitor the intermediate incoherent as well as the coherent chain and coherent melt scattering functions over a wide range of wave numbers q. Upon approaching Tc the coherent alpha-relaxation time of the melt increases strongly close to the maximum of the static structure factor of the melt. At q corresponding to the radius of gyration of the chain the melt relaxation time exhibits another maximum. The temperature dependence of the relaxation times is well described by a power-law with a q-dependent exponent in an intermediate temperature range. The time-temperature superposition principle of MCT is clearly bourne out in the whole range of wave numbers. An analysis of the alpha-decay using Kohlrausch-Williams-Watts (KWW) functions reveals that the collective melt KWW-stretching exponent and KWW-relaxation times are modulated with the structure factor. Furthermore, both incoherent and coherent KWW-times approach the large-q prediction of MCT at q comparable to the maximum of the structure factor. At small q a power law with exponent -3 is found for the coherent chain KWW-times similar to that of recent experiments.