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# Simulated glass-forming polymer melts : Dynamic scattering functions, chain length effects, and mode-coupling theory analysis

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Type of Publication: | Journal article |

Author: | Frey, Stephan; Weysser, Fabian; Meyer, Hendrik; Farago, Jean; Fuchs, Matthias; Baschnagel, Jakob |

Year of publication: | 2015 |

Published in: | The European Physical Journal E ; 38 (2015). - 11. - ISSN 1292-8941. - eISSN 1292-895X |

DOI (citable link): | https://dx.doi.org/10.1140/epje/i2015-15011-x |

Summary: |
We present molecular-dynamics simulations for a fully flexible model of polymer melts with different chain length N ranging from short oligomers (N = 4) to values near the entanglement length (N = 64). For these systems we explore the structural relaxation of the supercooled melt near the critical temperature T
_{c} of mode-coupling theory (MCT). Coherent and incoherent scattering functions are analyzed in terms of the idealized MCT. For temperatures T > T _{c} we provide evidence for the space-time factorization property of the β relaxation and for the time-temperature superposition principle (TTSP) of the α relaxation, and we also discuss deviations from these predictions for T ≈ T _{c}. For T larger than the smallest temperature where the TTSP holds we perform a quantitative analysis of the dynamics with the asymptotic MCT predictions for the late β regime. Within MCT a key quantity, in addition to T _{c}, is the exponent parameter λ. For the fully flexible polymer models studied we find that λ is independent of N and has a value (λ = 0.735 ) typical of simple glass-forming liquids. On the other hand, the critical temperature increases with chain length toward an asymptotic value T _{c} ^{∞} . This increase can be described by T _{c} ^{∞} − T _{c}(N) ∼ 1/N and may be interpreted in terms of the N dependence of the monomer density ρ, if we assume that the MCT glass transition is ruled by a soft-sphere-like constant coupling parameter Γ _{c} = ρ _{c} T _{c} ^{−1/4}, where ρ _{c} is the monomer density at T _{c}. In addition, we also estimate T _{c} from a Hansen-Verlet-like criterion and MCT calculations based on structural input from the simulation. For our polymer model both the Hansen-Verlet criterion and the MCT calculations suggest T _{c} to decrease with increasing chain length, in contrast to the direct analysis of the simulation data. |

Subject (DDC): | 530 Physics |

Keywords: | Soft Matter, Polymers, Polyelectrolytes |

Bibliography of Konstanz: | Yes |

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FREY, Stephan, Fabian WEYSSER, Hendrik MEYER, Jean FARAGO, Matthias FUCHS, Jakob BASCHNAGEL, 2015. Simulated glass-forming polymer melts : Dynamic scattering functions, chain length effects, and mode-coupling theory analysis. In: The European Physical Journal E. 38, 11. ISSN 1292-8941. eISSN 1292-895X. Available under: doi: 10.1140/epje/i2015-15011-x

@article{Frey2015Simul-31009, title={Simulated glass-forming polymer melts : Dynamic scattering functions, chain length effects, and mode-coupling theory analysis}, year={2015}, doi={10.1140/epje/i2015-15011-x}, volume={38}, issn={1292-8941}, journal={The European Physical Journal E}, author={Frey, Stephan and Weysser, Fabian and Meyer, Hendrik and Farago, Jean and Fuchs, Matthias and Baschnagel, Jakob}, note={Article Number: 11} }