(758c) Nanostructure Formation on Collapse of Polyelectrolyte Brushes

Polyelectrolyte brushes have been used to modify surfaces for applications in lubrication, colloidal stabilization and tailoring interactions with biological substances. A majority of these applications rely on the extended state of the polyelectrolyte brush, where the polymer chains extend from the surface due to strong osmotic pressure from counterions trapped within the brush volume. However, polyelectrolytes are sensitive to many stimuli in their local environment including pH, ionic strength, solvent quality and counterion valency, which can disrupt the extended morphology of the polyelectrolyte brush. Using a combination of theory, simulations and experiments, we demonstrate that the polyelectrolyte brush collapses into nanoscale “pinned micelles” in the presence of multivalent ions. Previous studies of polyelectrolyte brush interactions with multivalent ions demonstrated a sharp collapse in the brush height and attractions between opposing brushes when multivalent ions are present, which is hypothesized to be due to the bridging of polyelectrolyte chains by the multiple charges of multivalent ions. However, due to high levels of counterion condensation for multivalent ions, which can effectively neutralize the polyelectrolyte, solvophobic effects from the polyelectrolyte backbone must also be considered. The interplay between the multivalent ion bridging and solvophobic interactions leads to formation of nanoscale structures on brush-coated surfaces in the presence of multivalent ions. This fundamental understanding leads to capabilities for rational design of new, stimuli-responsive materials that take advantage of this alternate polyelectrolyte brush morphology, nanoscale pinned micelles.