(225b) Polyelectrolyte Brush Behavior in the Presence of Ruthenium Hexamine

This presentation will describe the behavioral and structural properties of end-tethered polyelectrolyte brushes in the presence of multi-valent and mono-valent counterions. Combining electrochemical experiments using cyclic voltammetry (CV) and surface force experiments using the surface forces apparatus (SFA), a detailed study of the amphiphilic diblock copolymer poly(t-butyl styrene)20 ? poly(styrene sodium-sulfonate)420 (PtBS-PSSNa) was performed. Starting with a hydrophobic modified surface, the hydrophobic block of this copolymer is then anchored to the surface allowing this investigation to be based purely on the thermodynamic interactions of the polyelectrolyte block. Past studies have detailed polyelectrolyte brush behavior using solely mono-valent counterions providing the ground work for this current analysis using the multi-valent counterion of ruthenium hexamine and the mono-valent sodium. Through combining CV results and SFA results, all at corresponding identical ionic environments, a detailed analysis of confined ruthenium hexamine counterion charges (Q) and equilibrium brush height (L0) was made possible. Brush structure and behavior is highly dependant upon the amount of ruthenium hexamine inside the brush due to its strong affinity to replace the sodium ions as the primary counterions inside the brush. These studies were performed varying both the ionic strength and the ionic ratio (ruthenium hexamine to sodium) of the surrounding environment. As the ionic ratio increases, the brush becomes more saturated with ruthenium hexamine. Simultaneously the brush also moves towards its more entropically favored collapsed state while also developing adhesive properties, not present without multi-valent counterions, when brushes are brought together using the SFA. The attached graph details brush behavior using two different ionic strengths. The y-axis shows the brush height (L0) while the x-axis shows the normalized amount of ruthenium hexamine confined within the brush (Q/Qmax). The amount of ruthenium hexamine inside the brush increases as the amount of ruthenium hexamine in solution is increased. This increase is done by fixing the ionic strength but varying the ratio of mono-valent sodium to multi-valent ruthenium hexamine. This graph shows one very distinct brush collapse of the 0.003 M ionic strength experiment which occurs at a concentration of ruthenium hexamine of approximately 9E-7 M. The experiment at 0.3 M ionic strength has a less distinct collapse at a concentration of 2E-4 M ruthenium hexamine (~0.8 Q/Qmax).