(651e) Polyelectrolyte Brush Conformations in Multivalent Ion-Driven Brush Collapse

Charged polymers grafted at one end to a surface and packed densely will extend away from the surface, forming a brush-like structure. These polyelectrolyte brushes have found significant applications in stabilizing dispersed particles and modifying surface properties. Of great interest has been the response of the polyelectrolyte brush structure to changes in the environment, namely the solvent quality and salt concentration. The distance the polymer chains extend from the surface, the brush height, will depend on the environmental conditions and is instrumental to the performance of the surfaces modified with these brushes. In the presence of salt ions, the brush has been shown to contract due to a decrease in the brush osmotic pressure, and in the presence of a poor solvent, the brush undergoes a rapid and strong shrinkage to a collapsed brush state. Less well-understood is the response of the brush conformation to the presence and concentration of multivalent ions. Previous studies have shown that the brush height undergoes a rapid and strong shrinkage at a critical concentration of multivalent ions, behavior that is suggestive of brush collapse in a poor solvent, but expected to be caused by attractions between polyelectrolyte chains due to bridging by the multivalent ions. We have used an energy balance approach to provide a theoretical understanding of bridging-driven brush collapse, considering the sum of the electrostatic, polymeric and entropic mean field terms and including additional parameterized phenomenological terms for counterion condensation and bridging. The theory examines the dependence of the brush collapse on the multivalent ion concentration as well as the potential structures of the brush in the collapsed state, considering homogeneous, pinned surface micelles and bundled brush conformations. Understanding the role multivalent ions play in inducing brush collapse and the structure of the collapsed state is essential to application of these materials in the complex environments commonly found in industrial formulations and biological systems and may lead to the use of multivalent ions as a tuning parameter for new stimuli-responsive materials.