Tirrell Group: Surface Forces and Electrochemical Measurements of Polyelectrolyte Brushes

Understanding polyelectrolyte brush behavior has a great deal of practical interest in the realm of novel materials and interactions with physiological systems. In part, this interest is due to growing desire for polymeric materials, which can respond to changes in their environment. The polyelectrolyte brushes studied in this project are composed of the strong polyanion poly(sodium styrene sulfonate) and are highly responsive to changes in the ionic environment of their surrounding solution.

The surface forces apparatus (SFA) measures the forces of interactions between two surfaces coated with polyelectrolyte brushes via compressions and separations at speeds as slow as 1 nm/s. These compression/separation cycles provide both structural (i.e. polyelectrolyte brush height) and behavioral information (i.e. whether the two brushes are adhesive or purely repulsive with each other). In particular, these polyelectrolyte brushes demonstrate very interesting behavior when in the presence of multi-valent ions. When multi-valent ions are present in the surrounding solution, brushes readily collapse and adhere strongly to one another. This collapse/adhesion is a direct result of multi-valent ions being up taken by a brush (as measured via electrochemistry). When the multi-valent ions are released by the brush, the chains once again extend themselves and show only repulsive forces when brought into contact. This reversibility depends highly on the amounts of both mono-valent and multi-valent ions present in a given system.

Schematic representations of polyelectrolyte brushes are shown. a) When only mono-valent counterions are present, the brushes interact with one another in a reversible manner upon approach and separation (no hysteresis is seen during SFA measurements). b) As multi-valent ions are added to the system, their high affinity drives them to replace mono-valent counter ions and the brushes eventually become electrostatically saturated with multi-valent counterions. Schematic of diblock copolymer chemical structure is also included. c) Polyelectrolyte brushes containing multi-valent counterions exhibit hysteresis and adhesion when brought into contact. Multi-valent bridging between the brush chains is the mechanism behind adhesion.

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