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Biomicrofluidics / Volume 5 / Issue 4 / REGULAR ARTICLES
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Biomicrofluidics 5, 044119 (2011); http://dx.doi.org/10.1063/1.3672190 (12 pages)

Effects of chain stiffness and salt concentration on responses of polyelectrolyte brushes under external electric field

Qianqian Cao, Chuncheng Zuo, Lujuan Li, and Guang Yan

College of Mechanical Science and Engineering, Jilin University, Changchun 130022, China

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(Received 25 October 2011; accepted 5 December 2011; published online 21 December 2011)

We report a molecular dynamics study on non-equilibrium dynamics of polyelectrolyte brushes under external electric fields. In this work, the effects of chain stiffness and salt concentration on static and dynamic responses of the brushes are addressed in detail. Our simulations indicate that varying these parameters induce rich electro-responsive behavior of the brushes. The increase of salt concentration results in the enhancement of an opposite electric field formed by non-equilibrium distribution of cations and anions, which resists stretching or shrinkage of grafted chains. At strong positive electric fields, the flexible brushes are more sensitive to the change of salt concentration. When reversing the electric field, the stiff brushes undergo a conformational transition from collapse to complete stretching. At high salt concentrations, dynamic responsive magnitude of the brush thickness to added electric field is strongly reduced. It was found that the fall time for the stiff brush becomes much shorter than that for the flexible brush. Additionally, increasing ion concentration leads to an excess extension or shrinkage of flexible brushes. For strongly stiff brushes, such phenomenon occurs in the presence or absence of salt.

© 2011 American Institute of Physics

Article Outline

  1. INTRODUCTION
  2. MODEL AND SIMULATION METHOD
  3. RESULTS AND DISCUSSION
  4. CONCLUSIONS

RELATED DATABASES

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KEYWORDS and PACS

Keywords

biochemistry, bioelectric phenomena, biological effects of fields, molecular dynamics method, polymer electrolytes, shrinkage

PACS

  • 87.15.rp

    Polymerization

  • 87.15.ap

    Molecular dynamics simulation

  • 02.60.-x

    Numerical approximation and analysis

  • 87.50.C-

    Static and low-frequency electric and magnetic fields effects

  • 82.35.Rs

    Polyelectrolytes

ARTICLE DATA

Digital Object Identifier
http://dx.doi.org/10.1063/1.3672190

PUBLICATION DATA

ISSN

1932-1058 (online)

Publisher

$abstract.society.value is a Member of CrossRef American Institute of Physics

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Figures (click on thumbnails to view enlargements)

FIG.1
Snapshot of the initial model system. Grafted chains are in an extended state. An electric field E is applied normal to the wall to control the stretching/shrinkage of the polyelectrolyte brush. Red and blue beads represent charged and neutral monomers, respectively. Cations and anions dissociated from salt are shown in orange and cyan, respectively. Counterions from polyelectrolytes are represented as green beads. Grafted monomers are shown in grey.

FIG.1 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.2
Density profiles ρm(z) of monomers from: (a) flexible polyelectrolyte brushes and (b) stiff polyelectrolyte ones with chain stiffness kθ/kθ* = 450 at different salt concentrations. The insets show monomer density in the absence of salt. Simulation data are obtained at E/E* = 2.0.

FIG.2 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.3
Density profiles ρm(z) of monomers from: (a) flexible polyelectrolyte brushes and (b) stiff polyelectrolyte ones with chain stiffness kθ/kθ* = 450 at different salt concentrations. Simulation data are obtained at E/E* = -2.0.

FIG.3 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.4
Typical snapshots of the brush system at different salt concentrations and electric fields. (a)-(d) correspond to flexible brushes: (a) E/E* = 0 and cσ3 = 0; (b) E/E* = 2.0 and cσ3 = 0.03; (c) E/E* = -2.0 and cσ3 = 0.03; (d) E/E* = -2 and cσ3 = 0.08. (e)-(h) correspond to stiff brushes with chain stiffness kθ/kθ* = 450; (e) E/E* = 0 and cσ3 = 0; (f) E/E* = -2.0 and cσ3 = 0.01; (g) E/E* = -2.0 and cσ3 = 0.03; (h) E/E* = -2.0 and cσ3 = 0.08.

FIG.4 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.5
Density profiles of cations ρc(z) and anions ρa(z) for: (a) the flexible brush at cσ3 = 0.03 and (b) the flexible brush and the stiff one with chain stiffness kθ/kθ* = 450 at cσ3 = 0.08. Simulation data are obtained at E/E* = -2.0.

FIG.5 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.6
Average thickness h of: (a) flexible brushes and (b) stiff ones with chain stiffness kθ/kθ* = 450 as a function of salt concentration at different electric fields.

FIG.6 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.7
Average thickness h of the brush as a function of electric field at different chain stiffness.

FIG.7 Download High Resolution Image (.zip file) | Export Figure to PowerPoint

FIG.8
Dynamic response of brush thickness h under an alternate square-wave electric field for: (a) flexible brushes and (b) stiff ones with chain stiffness kθ/kθ* = 450, in the absence of salt and at cσ3 = 0.08. (c) zooms on the dashed region in (b).

FIG.8 Download High Resolution Image (.zip file) | Export Figure to PowerPoint



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