DOI: 10.1021/la901839j A Langmuir XXXX, XXX(XX), XXX–XXX pubs.acs.org/Langmuir © XXXX American Chemical Society Morphologies of Planar Polyelectrolyte Brushes in a Poor Solvent: Molecular Dynamics Simulations and Scaling Analysis Jan-Michael Y. Carrillo and Andrey V. Dobrynin* Polymer Program, Institute of Materials Science and Department of Physics, University of Connecticut, 2152 Hillside Road, U-3046, Storrs, Connecticut 06269 Received May 22, 2009. Revised Manuscript Received July 14, 2009 Using molecular dynamics simulations and scaling analysis, we study the effect of the solvent quality for the polymer backbone, the strength of the electrostatic interactions, the chain degree of polymerization, and the brush grafting density on conformations of the planar polyelectrolyte brushes in salt-free solutions. Polyelectrolyte brush forms: (1) vertically oriented cylindrical aggregates (bundles of chains), (2) maze-like aggregate structures, or (3) thin polymeric layer covering a substrate. These different brush morphologies appear as a result of the fine interplay between electrostatic and short-range monomer-monomer interactions. The brush thickness shows nonmonotonic dependence on the value of the Bjerrum length. It first increases with the increasing value of the Bjerrum length, and then it begins to decrease. This behavior is a result of counterion condensation within a brush volume. 1. Introduction Polyelectrolyte brushes consist of charged polymers end grafted to substrates of different geometries. 1-6 In polar solvents, the ionizable groups on the polymer backbone dissociate by releasing the counterions into solution and leaving uncompensated charges on the polymer chains. The morphology of the grafted polyelectro- lyte layers depends on the solvent quality for the polymer back- bone, the fraction of the charged groups, the chain’s degree of polymerization, the polymer grafting density, and the salt concen- tration. By varying these parameters one can control both brush thickness and structure (see for review refs 1-6). The tremendous interest in these polymeric systems was dictated by their applications for colloidal stabilization, drug delivery, biocom- patible coatings, pH-controlled gate devices (filters), “smart surfaces”, and biosensor technology. 1,2,4,5,7-12 While the properties of the polyelectrolyte brushes in good and θ-solvent conditions for the polymer backbone were extensively studied over the years, the analysis of the phase diagram of the poly- electrolyte brushes in poor solvents is still incomplete. 1-5 There were only a few attempts to analyze this type of polymeric system. 6,13-17 Recently, we used molecular dynamics simulations in combi- nation with scaling analysis to study the effects of the solvent quality for the polymer backbone and the strength of the electro- static interactions on the morphology of the spherical polyelectro- lyte brushes in salt-free solutions. 14 We have shown that the morphology of the spherical polyelectrolyte brush is controlled by a fine interplay between the long-range electrostatic interactions between charged groups and the short-range monomer-mono- mer interactions. It was demonstrated that the spherical polyelec- trolyte brush could be in a star-like spherical conformation, a “star of bundles” conformation in which polyelectrolyte chains self-assemble into clusters of pinned cylindrical micelles, a micelle- like conformation with a dense core and charged corona, or could form a thin polymeric layer uniformly covering the particle surface. Counterions play an important role in controlling brush properties. Counterion condensation inside the brush results in nonmonotonic dependence of the layer thickness on the strength of the electrostatic interactions which is controlled by the value of the Bjerrum length. We have found that the thickness of the brush layer first increases with the increasing value of the Bjerrum length then it begins to decrease. The decrease of the brush thickness was explained by a combination of two effects associated with the counterion condensation. 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