(651b) The Effect of Trivalent Counterions to the Structure of Highly Dense Polystyrene Sulfonate Brushes

Surface tethered polyelectrolyte brushes are scientifically interesting and technologically relevant to many applications, ranging from colloidal stabilization to responsive and tunable materials to lubrication. Many applications operate in environments containing multi-valent ions, media in which our scientific understanding is not yet well-developed. We synthesized high-density polystyrene sulfonate (PSS) brushes via surface initiated atom-transfer radical polymerization, and performed neutron reflectivity (NR) and surface forces apparatus (SFA) measurements to investigate and compare the effects of mono-valent Na⁺, and tri-valent Y³⁺ counterions to the structure of the densely tethered PSS brushes. NR and SFA results demonstrate that in monovalent salt solution, the behavior PSS brush agrees with scaling theory well, exhibiting two distinct regimes, the osmotic and salted brush regimes. Introducing trivalent Y³⁺ cations causes an abrupt shrinkage of the PSS brush due to the uptake of Y³⁺ counterions. The uptake of Y³⁺ counterions and shrinkage of the brush are reversal up on increasing the concentration of monovalent salt. Our findings demonstrate that the presence of multi-valent counterions can significantly alter the structure of polyelectrolyte brushes, in a manner different from mono-valent ions, which has implications for the functionality of the brushes.