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

Abstract

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 (N_w) and dispersity (Đ) alter the pH-responsive conformation of poly(acrylic acid) (PAA) brushes grafted to silica nanoparticles. We quantified the average brush length (l_b) at various pH using dynamic light scattering. The l_b of low-N_w PAA brushes (N_w = 45) increased as Đ was increased from 1.09 to 1.69, but l_b for the high-N_w PAA brushes (N_w ≈ 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 l_b when N_w is low. Additionally, the extent of pH-response in l_b/l_b,max increased with N_w or Đ (when N_w is low), where the maximum brush length (l_b,max) was l_b measured at pH 10. The scaling of l_b/l_b,max with degree of dissociation (α), however, was influenced by N_w but not Đ, indicating the low- and high-N_w 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 l_b at low N_w arose from subtle differences in dissociation behaviors among brushes of varying Đ. At low Đ, the low-N_w brush adopted a pH-independent extended conformation due to strong excluded volume interactions, whereas the conformation of the high-N_w 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 N_w.