We analyze dynamical heterogeneities in a simulated ''bead-spring'' model of a nonentangled, supercooled polymer melt. We explore the importance of chain connectivity on the spatially heterogeneous motion of the monomers. We find that when monomers move, they tend to follow each other in one-dimensional paths, forming strings as previously reported in atomic liquids and colloidal suspensions. The mean string length is largest at a time close to the peak time of the mean cluster size of mobile monomers. This maximum string length increases, roughly in an exponential fashion, on cooling toward the critical temperature T MCT of the mode-coupling theory, but generally remains small, although large strings involving ten or more monomers are observed. An important contribution to this replacement comes from directly bonded neighbors in the chain. However, mobility is not concentrated along the backbone of the chains. Thus, a relaxation mechanism in which neighboring mobile monomers along the chain move predominantly along the backbone of the chains, seems unlikely for the system studied.

FIG. 8. Temperature dependence of the ratio of ͗s seg (t str max)͘ and ͗s(t str max)͘. t str max is the peak time of ͗s seg ͘ and ͗s͘ at different temperatures. T MCT ϭ0.45.

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 T c, 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 S q . 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. PACS. 64.70.Pf Glass transitions -61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling -61.20.Ja Computer simulation of liquid structure 246 The European Physical Journal E