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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
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 | ||||||||
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| Subjects: | Q Science > QC Physics Q Science > QD Chemistry |
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| Divisions: | Faculty of Science > Physics | ||||||||
| Journal or Publication Title: | Macromolecules | ||||||||
| Publisher: | American Chemical Society | ||||||||
| ISSN: | 0024-9297 | ||||||||
| Official Date: | 20 February 2015 | ||||||||
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| 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: |
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