Glassy dynamics of simulated polymer melts: Coherent scattering and van Hove correlation functions: Part II: Dynamics in the α-relaxation regime

Whereas the first part of this paper dealt with the relaxation in the β-regime, this part investigates the final relaxation (α-relaxation) of a simulated polymer melt consisting of short non-entangled chains in the supercooled state above the critical temperature Tc of ideal mode-coupling theory (MCT). The temperature range covers the onset of a two-step relaxation behaviour down to a temperature merely 2% above Tc. We monitor the incoherent intermediate scattering function as well as the coherent intermediate scattering function of both a single chain and the melt over a wide range of wave number q. Upon approaching Tc the coherent α-relaxation time of the melt increases strongly close to the maximum qmax of the collective static structure factor Sq and roughly follows the shape of Sq for q ≳ qmax. For smaller q-values corresponding to the radius of gyration the 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. Deviations are found very close to and far above Tc, the onset of which depends on q. The time-temperature superposition principle of MCT is clearly borne out in the whole range of reciprocal vectors. An analysis of the α-decay by the Kohlrausch-Williams-Watts (KWW) function reveals that the collective KWW stretching exponent and KWW relaxation time show a modulation with Sq. Furthermore, both incoherent and coherent KWW times approach the large-q prediction of MCT already for q > qmax. At small q, a q-3 power law is found for the coherent chain KWW times similar to that of recent experiments.