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 numbers 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. Futhermore, both incoherent and coherent KWW-times approach the large-q prediction of MCT already for q > qmax. At small q, a q -power law is found for the coherent chain KWW-times similar to that of recent experiments. PACS. 64.70.Pf Glass transitions – 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling – 61.20.Ja Computer simulation of liquid structure