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Complex interactions with the surroundings dictate a tagged chain’s dynamics in unentangled polymer melts

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Panja, Debabrata, Barkema, Gerard T. and Ball, Robin (2015) Complex interactions with the surroundings dictate a tagged chain’s dynamics in unentangled polymer melts. Macromolecules, 48 (5). pp. 1442-1453. doi:10.1021/ma502523p

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Official URL: http://dx.doi.org/10.1021/ma502523p

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Abstract

For more than half a century the theoretical landscape for single chain dynamics for dense polymeric solutions and melts below the entanglement threshold has been dominated by the Rouse model for independent phantom chains, supported by ideas of hydrodynamic screening. There exists, however, a large body of literature from experiments, Monte Carlo, and molecular dynamics simulations on the deviations from the Rouse behavior for unentangled homopolymer melts, showcased mostly in the subdiffusive behavior of center-of-mass of tagged chains at intermediate times, with the subdiffusion exponent reported in the range 0.75–0.85. The influence of the surrounding chains of length Ns on the motion of a single tagged chain of length N is a key test, by which, through high-precision numerical simulation of unentangled melts, we show that the Rouse model fails. Our central results are that at intermediate times the tagged chain’s center-of-mass moves subdiffusively, ⟨Δr2cm⟩ ∝ tα with subdiffusion exponent α = 0.87 ± 0.03 as opposed to αRouse = 1, and that its crossover time to Fickian behavior is directly controlled by the relaxation time of the surrounding chains when the latter are shorter. The terminal relaxation time for the tagged chain and the long time diffusion coefficient are then sensitive to Ns. Both measured exponent flow, that is plots of dα(t)/d ln (t) vs α(t) where α(t) is the effective exponent between ⟨Δr2cm⟩ and t, and successful blob scaling arguments support the anomalous value of α as a true exponent. We find the same exponent in the scaling of Rouse mode amplitude correlation functions and directly related exponent for the monomeric diffusion. We show that the consequences of these results on the dynamics of a tagged monomer and the chain’s segmental orientation autocorrelation function agree very well with rheological measurements and NMR relaxometry experiments. We reflect back on a history of related experimental anomalies and discuss how a new theory might be developed.

Item Type: Journal Article
Subjects: Q Science > QC Physics
Q Science > QD Chemistry
Divisions: Faculty of Science > Physics
Journal or Publication Title: Macromolecules
Publisher: American Chemical Society
ISSN: 0024-9297
Official Date: 20 February 2015
Dates:
DateEvent
20 February 2015Published
9 February 2015Accepted
15 December 2014Submitted
Volume: 48
Number: 5
Page Range: pp. 1442-1453
DOI: 10.1021/ma502523p
Status: Peer Reviewed
Publication Status: Published
Access rights to Published version: Restricted or Subscription Access
RIOXX Funder/Project Grant:
Project/Grant IDRIOXX Funder NameFunder ID
EP/I01358X/1[EPSRC] Engineering and Physical Sciences Research Councilhttp://dx.doi.org/10.13039/501100000266

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