(363d) History-Dependent Swelling of Poly(acrylic acid) Brushes and Layer-By-Layer Films Via Quartz Crystal Microbalance w/ Dissipation (QCM-D)

Polyelectrolytes are charged (ion-containing) polymers that can exist in many different morphologies. Polyelectrolyte brushes (PEBs) have a wide variety of uses in areas such as waterborne lubrication and oil separation. Polyelectrolyte multilayers (PEMs), also known as Layer-by-Layer (LbL) films, have gained tremendous attention, specifically in the fields of electronics and drug delivery. While the uses of PEBs and PEMs are many, their fundamental dynamics still remains unclear, which makes it difficult to use them in applications where specific swelling profiles are required, especially in environments of varied pH. For this reason, we investigated the swelling/de-swelling behavior of PEBs of the weakly ionizable polyanion poly(acrylic acid) (PAA) using Quartz Crystal Microbalance with Dissipation (QCM-D). The PEBs were fabricated as previously described¹ using gold-thiol linkages to deposit polymer chains directly on the gold QCM-D crystal in situ as a self-assembled monolayer (SAM), and diluted to achieve various grafting densities. Next, these PEBs were complexed with the polycation poly(ethylene imine) (PEI) to form PEMs with a brush underlayer, and these films were investigated by QCM-D under a stepwise pH cycle to observe their swelling dynamics. Previously, researchers have demonstrated hysteretic swelling with respect to changes in pH in experimental studies of both PEMs and PEBs. However, the extent of this hysteretic response with respect to various film characteristics has not been thoroughly characterized. We find that both PEB and PEM structures may or may not exhibit history-dependent swelling/de-swelling with respect to pH depending on their chain length and grafting density. At low grafting densities in longer, more elastic chains, we see hysteretic swelling behavior at longer timescales. By contrast, at higher grafting densities and in shorter, more rigid chains, we see a rapid swelling/deswelling profile with little to no hysteresis. We also see an intermediate regime of intermediate chain length and grafting density where probable hysteresis might be present at short timescales but depreciates at longer timescales.

Finally, we revisit previous QCM-D studies of a standalone PEB of PAA and comparisons to scaling theory. PEBs swell and de-swell with respect to salt concentration corresponding to the well-known characteristic osmotic and salted brush swelling regimes predicted by mean-field theory.^2,3 Previously, we found only partial agreement with theory for weakly ionizable brushes for our brushes with respect to grafting density,¹ but noted that the pH values investigated would make the brushes strongly charged. We also noted that the brushes investigated were rather short, and as such, inelastic, when theory assumes the chains are Gaussian. From our hysteresis studies at varied pH for an individual brush layer, we find a pH value where the brushes are partially collapsed. We take this pH value for a long chain, dilute brush to then investigate the swelling behavior with varied salt and grafting density once again, assuming that now the chains can be considered to be both elastic and weakly ionized.

References

1. N. R. Hollingsworth, S. I. Wilkanowicz and R. G. Larson,Soft Matter, 2019,15, 7838–7851.

2. E. B. Zhulina, T. M. Birshtein and O. V. Borisov,Macromolecules, 1995,28, 1491–1499.

3. R. Israels, J. F. A. M. Leermakers, J. Fleer and E. B. Zhulina,Macromolecules, 1994,27, 3249–3261.