Structural Dependence of Salt-Responsive Polyzwitterionic Brushes with an Anti-Polyelectrolyte Effect

Some polyzwitterionic brushes exhibit a strong "anti-polyelectrolyte effect" and ionic specificity that make them versatile platforms to build smart surfaces for many applications. However, the structure-property relationship of zwitterionic polymer brushes still remains to be elucidated. Herein, we aim to study the structure-dependent relationship between different zwitterionic polymers and the anti-polyelectrolyte effect. To this end, a series of polyzwitterionic brushes with different cationic moieties (e.g., imidazolium, ammonium, and pyridinium) in their monomeric units and with different carbon spacer lengths (e.g., CSL = 1, 3, and 4) between the cation and anion were designed and synthesized to form polymer brushes via the surface-initiated atom transfer radical polymerization. All zwitterionic brushes were carefully characterized for their surface morphologies, compositions, wettability, and film thicknesses by atomic force microscopy, contact angle measurement, and ellipsometry, respectively. The salt-responsiveness of all zwitterionic brushes to surface hydration and friction was further examined and compared both in water and in salt solutions with different salt concentrations and counterion types. The collective data showed that zwitterionic brushes with different cationic moieties and shorter CSLs in salt solution induced higher surface friction and lower surface hydration than those in water, exhibiting strong anti-polyelectrolyte effect salt-responsive behaviors. By tuning the CSLs, cationic moieties, and salt concentrations and types, the surface wettability can be changed from a highly hydrophobic surface (∼60°) to a highly hydrophilic surface (∼9°), while interfacial friction can be changed from ultrahigh friction (μ ≈ 4.5) to superior lubrication (μ ≈ 10^-3). This work provides important structural insights into how subtle structural changes in zwitterionic polymers can yield great changes in the salt-responsive properties at the interface, which could be used for the development of smart surfaces for different applications.