Polymer Dispersity Affects Conformation of Brushes Grafted on Nanoparticles

Polyelectrolyte-grafted nanoparticles have been applied in controlled drug delivery systems, sensors, and water purification membranes. Their efficacy depends, in part, on the conformational response of polyelectrolyte brushes to pH changes. Although some brush properties controlling conformational pH-response, such as brush length l_b, are well-studied, the impact of brush dispersity remains underexplored. Here, we show that the effect of dispersity on the conformational pH-response of polyelectrolyte brushes depends on the molecular weight. We synthesized poly(tert-butyl acrylate)-grafted silica nanoparticles with varying dispersities and subsequently hydrolyzed them to poly(acrylic acid) (PAA) brushes. At low weight-average degree of polymerization N_w, l_b of low-dispersity PAA brushes was weakly dependent on pH, whereas l_b of high-dispersity PAA brushes was greater and drastically increased with pH. At a higher N_w, however, l_b of the PAA brushes showed similar pH-response regardless of dispersity. We hypothesize that the differences in l_b are related to differences in conformation among low and high dispersity brushes. Variation in response of l_b to pH, originating in conformational changes, via control over dispersity can be leveraged to improve dispersion in complex media.