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Title: Salt- and pH-induced swelling of a poly(acrylic acid) brush via quartz crystal microbalance w/dissipation (QCM-D)
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 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 p K 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 p K 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 +1/3 C s −1/3 of brush height H , while in the low-salt “osmotic brush” regime ( C s < 10 mM salt), we find H ∼ Nσ +1/3 C 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.  more » « less
Award ID(s):
1707640
PAR ID:
10175374
Author(s) / Creator(s):
; ;
Date Published:
Journal Name:
Soft Matter
Volume:
15
Issue:
39
ISSN:
1744-683X
Page Range / eLocation ID:
7838 to 7851
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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