Abstract: As filtration and separation technologies continue to improve, an emerging bottleneck arises in the differentiation between similarly sized nanometer-scale particles. Polymer brushes are densely grafted polymer chains and are viable as a surface coating to achieve improved nanometer-scale separations. Due to their stimuli-responsive nature to voltage, pH, salt concentration, etc., weak polyelectrolyte brushes are especially interesting for controlling adsorption/desorption or separation phenomena. Understanding in situ adsorption necessitates understanding the fundamental physics governing the interplay among the brush, particle, and solution environments.

This dissertation aims to elucidate the dynamics and kinetics of nanoparticle adsorption to weak polybasic brushes. The following parameters were investigated: solution pH, brush molecular weight, brush grafting density, and nanoparticle geometry. Weakly polybasic poly(2-vinyl pyridine) (P2VP) brushes were utilized due to their protonated structure at low pHs and neutrality at high pHs. The P2VP brushes were prepared from two different end-functionalization chemistries, carboxyl (-COOH) and amine (-NH2). These brush surfaces were challenged with nanometer-sized particles- either isotropic citrate-coated gold nanospheres (AuNPs) or anisotropic sulfonated polystyrene (PSS) coated gold nanorods (AuNRs). These nanoparticles were prepared in 1 mM citrate buffer at pH = 4.0 or 6.2.

Here, quartz crystal microbalance with dissipation (QCM-D) and ex situ microscopy techniques measure adsorption behaviors in the bulk and on the surface, respectively. The pH was the most impactful on overall particle adsorption. At pH 4.0, particle uptake was promoted by electrostatic interactions, and adsorption maxima were determined to be independent of brush molecular weight. At pH 6.2, smaller diameter NPs experienced increased selectivity relative to larger diameter NPs, indicating the prominent role of steric interactions. When differentiating between mixed-diameter NPs, increasing brush grafting density selectively adsorbed large-diameter particles at pH 4.0 and small-diameter particles at pH 6.2. Generally, nanoparticle geometry determines penetration depth into the brushes, and with increasing grafting density at high pH, P2VP brushes increased alignment out-of-plane of AuNRs. Overall, these studies strive to elucidate the impact of brush structure in relation to solution environment and nanoparticle geometry to expand the applications of polyelectrolyte brushes in separation technologies of the future.