Molecular weight and dispersity affect chain conformation and pH-response in weak polyelectrolyte brushes

The impact of brush molecular weight distribution on the conformation and response of weak polyacid brushes was investigated. We show that weight-average degree of polymerization (Nw) and dispersity (Đ) alter the pH-responsive conformation of poly(acrylic acid) (PAA) brushes grafted to silica nanoparticles. We quantified the average brush length (lb) at various pH using dynamic light scattering. The lb of low-Nw PAA brushes (Nw = 45) increased as Đ was increased from 1.09 to 1.69, but lb for the high-Nw PAA brushes (Nw ≈ 813) did not vary substantially when Đ was increased from 1.23 to 1.76. This result indicates that the presence of a small fraction of long chains in a broad dispersity brush has a greater impact on lb when Nw is low. Additionally, the extent of pH-response in lb/lb,max increased with Nw or Đ (when Nw is low), where the maximum brush length (lb,max) was lb measured at pH 10. The scaling of lb/lb,max with degree of dissociation (α), however, was influenced by Nw but not Đ, indicating the low- and high-Nw PAA brushes were in the quasi-neutral brush (q-NB) and salted brush (SB) regimes, respectively. Differing behaviors in the pH-response and α-response of lb at low Nw arose from subtle differences in dissociation behaviors among brushes of varying Đ. At low Đ, the low-Nw brush adopted a pH-independent extended conformation due to strong excluded volume interactions, whereas the conformation of the high-Nw brush varied from collapsed to stretched with increasing pH arising from electrostatic interactions. At high-Đ, we suggest the brush conformation also varied from collapsed to stretched with increasing pH regardless of Nw.