Salt- and pH-induced swelling of a poly(acrylic acid) brush via quartz crystal microbalance w/dissipation (QCM-D)

Abstract

We infer the swelling/de-swelling behavior of weakly ionizable poly(acrylic acid) (PAA) brushes of 2–39 kDa molar mass in the presence of KCl concentrations from 0.1–1000 mM, pH = 3, 7, and 9, and grafting densities σ = 0.12–2.15 chains per nm² using a Quartz Crystal Microbalance with Dissipation (QCM-D), confirming and extending the work of Wu et al. to multiple chain lengths. At pH 7 and 9 (above the pK_a ∼ 5), the brush initially swells at low KCl ionic strength (<10 mM) in the “osmotic brush” regime, and de-swells at higher salt concentrations, in the “salted brush” regime, and is relatively unaffected at pH 3, below the pK_a, as expected. At pH 7, at low and moderate grafting densities, our results in the high-salt “salted brush” regime (C_s > 10 mM salt) agree with the predicted scaling H ∼ Nσ^+1/3C_s^−1/3 of brush height H, while in the low-salt “osmotic brush” regime (C_s < 10 mM salt), we find H ∼ Nσ^+1/3C_s^+0.28–0.38, whose dependence on C_s agrees with scaling theory for this regime, but the dependence on σ strongly disagrees with it. The predicted linearity in the degree of polymerization N is confirmed. The new results partially confirm scaling theory and clarify where improved theories and additional data are needed.