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

Whereas the first part of this paper dealt with the relaxation in the beta -regime, this part investigates the final relaxation (alpha -relaxation) of a simulated polymer melt consisting of short non-entangled chains in the s upercooled state above the critical temperature T-c of ideal mode-coupling theory (MCT). The temperature range covers the onset of a two-step relaxati on behaviour down to a temperature merely 2% above T-c. We monitor the inco herent intermediate scattering function as well as the coherent intermediat e scattering function of both a single chain and the melt over a wide range of wave numbers q. Upon approaching T-c the coherent alpha -relaxation tim e of the melt increases strongly close to the maximum q(max) of the collect ive static structure factor S-q and roughly follows the shape of S-q for q greater than or similar to q(max). For smaller q-values corresponding to th e radius of gyration the relaxation time exhibits another maximum. The temp erature dependence of the relaxation times is well described by a power law with a q-dependent exponent in an intermediate temperature range. Deviatio ns are found very close to and far above T-c, the onset of which depends on q. The time-temperature superposition principle of MCT is clearly borne ou t in the whole range of reciprocal vectors. An analysis of the alpha -decay by the Kohlrausch-Williams-Watts (KWW) function reveals that the collectiv e KWW stretching exponent and KWW relaxation time show a modulation with S- q. Furthermore, both incoherent and coherent KWW times approach the large-g prediction of MCT already for q > q(max). At small q, a q(-3) power law is found for the coherent chain KWW times similar to that of recent experimen ts.