2012 Impact factor 1.824
EPJ E - Soft Matter and Biological Physics
Soft Matter and Biological Physics


Eur. Phys. J. E 5, 245-256

Glassy dynamics of simulated polymer melts: Coherent scattering and van Hove correlation functions

Part II: Dynamics in the ${\alpha}$-relaxation regime
M. Aichele1 and J. Baschnagel2

1  Institut für Physik Johannes Gutenberg-Universität Mainz, Staudinger Weg 7, 55099 Mainz, Germany
2  Institut Charles Sadron, 6 rue Boussingault, 67083 Strasbourg, France

(Received 16 January 2001)

Abstract
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 supercooled state above the critical temperature $\ensuremath{T_{\mathrm{c}}} $ 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 $\ensuremath{T_{\mathrm{c}}} $. 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 $\ensuremath{T_{\mathrm{c}}} $ the coherent ${\alpha}$-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 $q
\gtrsim q_{max}$. 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 $\ensuremath{T_{\mathrm{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 ${\alpha}$-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.

PACS
64.70.Pf - Glass transitions.
61.25.Hq - Macromolecular and polymer solutions; polymer melts; swelling.
61.20.Ja - Computer simulation of liquid structure.


© EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2001