Swelling dynamics of zwitterionic copolymers: The effects of concentration and type of anion and cation

• Swelling dynamics of polyzwitterionic copolymers are measured with ellipsometry.
• A clear anti-polyelectrolyte effect is observed.
• Swelling dynamics are strongly dependent on the ionic strength.
• The maximum swelling degree is dependent on both the cation and the anion.
• At high concentrations of LiCl an anomalous swelling behavior is found.

The effect of different salts and their concentration on the swelling of zwitterionic copolymers has been investigated for bulk polymer samples as well as for thin films. Relatively low ratios of the zwitterionic monomer already radically change the swelling properties of the copolymer. Increasing the NaCl concentration results in a significant increase of the swelling, which is indicative of the so-called anti-polyelectrolyte effect. The dynamic swelling experiments on the thin film show that only at high NaCl concentrations, the swelling is limited by solute diffusion. These swelling characteristics are distinctly affected by the type of ions present in the aqueous solution. The changes in maximum swelling degree are most sensitive to the type of anion, where the order of the extent of the effect of the anions follows the Hofmeister lyotropic series. For different cations, smaller changes in maximum swelling degree are found that only partially comply with the Hofmeister series. In most cases similar semi-Fickian swelling behavior is found, except in the case of LiCl concentrations above 0.8 M, where anomalous sigmoidal swelling curves are obtained. This behavior in high LiCl concentrations can be explained by the loss of interchain molecular interactions during swelling. This work provides meaningful insights on the behavior of zwitterionic copolymer films for applications such as membrane filtration that utilize different ions and ionic strengths.

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