Collapse of polyelectrolyte brushes: Scaling theory and simulations

Abstract

We investigate polyelectrolyte brushes using both scaling arguments and molecular dynamics simulations. As a main result, we find a novel collapsed brush phase. In this phase, the height of the brush results from a competition between steric repulsion between ions and monomers and an attractive force due to electrostatic correlations. As a result, the monomer density inside the brush is independent of the grafting density and the polymerization index. For small ionic and monomer radii (or for large Bjerrum length) the brush undergoes a first-order phase transition from the osmotic into the collapsed state.PACS. 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling -68.45.Ws Other nonelectronic properties -61.20.Qg Structure of associated liquids: electrolytes, molten salts, etc.

Effects of counterion fluctuations in a polyelectrolyte brush

“…Though we find a first order transition similar to that of Csajka et al [26], we also find a critical point in the phase diagram and find that, generically, the decrease in brush height is continuous.…”

“…We find that for strongly charged brushes, there is a collapse transition in which the brush height decreases with increasing charge on the polyelectrolyte chains. Unlike the scaling theory [26], we find that the transition is similar to the liquid-gas transition with a line of first-order phase transition terminating at a critical point. Surprisingly, the valence of the counterions plays an important role: the transition to the collapse regime is discontinuous for monovalent counterions, while it is continuous for multivalent counterions.…”

Neutral and charged polymers at interfaces

“…This has been recently rationalized in terms of another scaling regime, the collapsed regime. In this regime one finds that correlation and fluctuation effects, which are neglected in the discussion in this section, lead to a net attraction between charged monomers and counterions [332]. Similarly, two charged surfaces, one decorated with a charged brush, the other one neutralized by counter ions, attract each other at large enough grafting densities [333].…”

Theoretical Approaches to Neutral and Charged Polymer Brushes

“…This has been recently rationalized in terms of another scaling regime, the collapsed regime. In this regime one finds that correlation and fluctuation effects, which are neglected in the discussion in this section, lead to a net attraction between charged monomers and counter-ions [88,89].…”