Random zwitterionic amphiphilic copolymers (r-ZACs) are random/statistical copolymers that combine hydrophobic and zwitterionic units. r-ZACs with glassy hydrophobic repeat units have been studied as thin film composite membrane materials with extremely high resistance to organic fouling. This makes them promising for use as anti-fouling coatings in other demanding applications such as ship hulls, building materials, and biomaterials. These applications, however, require different mechanical properties to be resilient, both in dry state and when exposed to aqueous media these systems would be exposed to during operation (e.g. seawater, saline). r-ZACs have been documented to self-assemble to create disordered interconnected nanodomains of hydrophobic and zwitterionic phases, which can influence both mechanical properties and swelling in such situations. In this study, we aimed to characterize the mechanical properties of r-ZAC materials under conditions relevant to these applications, and link our findings with material chemistry and water-zwitterion interactions. For this purpose, we have synthesized a cohort of r-ZACs at varying zwitterionic monomer chemistries as well as monomer ratios. These r-ZACs combined one of two commercially available zwitterionic monomers, 2-methacryloyloxyethyl phosphorylcholine (MPC) or sulfobetaine methacrylate (SBMA), with the hydrophobic monomer methyl methacrylate (MMA), chosen due to its strong mechanical properties. We then analyzed their water uptake behavior and mechanical properties and examined how salts can change the properties of these materials. The elastic moduli of all r-ZACs were higher than the PMMA homopolymer, likely due to strong interactions between zwitterionic groups creating rigid domains that also act as physical cross-links. Upon exposure to water, the elastic moduli of r-ZACs decreased whereas elongation at break and toughness increased as the zwitterionic domains took up water and were plasticized. The presence of salts led to changes in water uptake, and in mechanical properties such as elongation at break and toughness. We examined the binding of water through Fourier transform infrared (FTIR) spectroscopy, which showed that interactions between r-ZACs and water additionally change based on ionic species and strength, as well as the chemical structure of the zwitterionic unit. Through our increased understanding of the bulk mechanical properties of r-ZACs, we speculate that these materials could be utilized as mechanical water sensors, cartilage replacements, and ion sensors.
