Salt- and pH-induced swelling of a poly(acrylic acid) brush via quartz crystal microbalance w/dissipation (QCM-D)
Year: 2019
Journal: Soft Matter, Volume 15, OCT 21, page 7838–7851
Authors: Hollingsworth, Nisha R.; Wilkanowicz, Sabina, I; Larson, Ronald G.
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 sigma = 0.12-2.15 chains per nm(2) 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) similar to 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 similar to N sigma C-+1/3(s)-1/3 of brush height H, while in the low-salt osmotic brush regime (C-s < 10 mM salt), we find H similar to N sigma C-+1/3(s)+0.28-0.38, whose dependence on C-s agrees with scaling theory for this regime, but the dependence on sigma 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.
