(577a) Responsiveness of Polyelectrolyte Brushes to Multi-Valent Counterions

While polyelectrolyte brushes are more classically known for their colloidal stabilization or high lubrication properties, their uses have grown as a result of their unique ability to respond to changes in their environment.  Potential stimuli, which range from electrical and mechanical to environmental changes in pH and salt concentrations, have propelled applications of polyelectrolyte brushes into fields of multilayer formation and behavior, chemical gating, and drug delivery among others.  The responsiveness of a polyelectrolyte brush to a given environmental change is dependent upon its structure.  This work will discuss polyelectrolyte brushes comprised of the strong polyelectrolyte, poly(sodium styrenesulfonate) which has shown a strong dependence upon the quantity and valency of salt in a surrounding solution.

When these charged polymer chains are attached to a surface at one end and have a high enough tethering density, the chains of the polymers extend into solution creating a “brush-shaped” structure.  The chain elasticity of the polyelectrolytes is resisted by both electrostatic interactions and the osmotic pressure of counterions associated with the charged monomer segments.  When these counterions inside the brush, which begin as mono-valent sodium ions, are replaced by multi-valent counterions, polyelectrolyte brush properties are dramatically changed.  Brushes which were originally extended, exhibiting purely repulsive forces when forced into contact, become collapsed and adhesive when multi-valent counterions replace their mono-valent counterparts.  These results combine intermolecular force data measured when two brushes were brought together using the Surface Forces Apparatus (SFA), and the quantity of multi-valent ions inside a polylelectrolyte brush at a given time and salt concentration determined via cyclic voltametry.