Effect of Ionization on Aqueous Phase Radical Copolymerization of Acrylic Acid and Cationic Monomers

Positive Charge Influences on the Surface Interactions and Cohesive Bonding of a Catechol-Containing Polymer

Achieving robust underwater adhesion remains challenging. Through generations of evolution, marine mussels have developed an adhesive system that allows them to anchor onto wet surfaces. Scientists have taken varied approaches to developing mussel-inspired adhesives. Mussel foot proteins are rich in lysine residues, which may play a role in the removal of salts from surfaces. Displacement of water and ions on substrates could then enable molecular contact with surfaces. The necessity of cations for underwater adhesion is still in debate. Here, we examined the performance of a methacrylate polymer containing quaternary ammonium and catechol groups. Varying amounts of charge in the polymers were studied. As opposed to protonated amines such as lysine, quaternary ammonium groups offer a nonreactive cation for isolating effects from only charge. Results shown for dry bonding demonstrated that cations tended to decrease bulk cohesion while increasing surface interactions. Stronger interactions at surfaces, along with weaker bulk bonding, indicate that cations decreased the cohesive forces. When under salt water, overall bulk adhesion also dropped with higher cation loadings. Surface attachment under salt water also dropped, indicating that the polymer cations could not displace surface waters or sodium ions. Salt did, however, appear to shield bulk cation-cation repulsions. These studies help to distinguish influences upon bulk cohesion from attachment at surfaces. The roles of cations in adhesion are complex, with both cohesive and surface bonding being relevant in different ways, sometimes even working in opposite directions.

Superlubricity of pH-responsive hydrogels in extreme environments

Poly(acrylamide-co-acrylic acid) (P(AAm-co-AA)) hydrogels are highly tunable and pH-responsive materials frequently used in biomedical applications. The swelling behavior and mechanical properties of these gels have been extensively characterized and are thought to be controlled by the protonation state of the acrylic acid (AA) through the regulation of solution pH. However, their tribological properties have been underexplored. Here, we hypothesized that electrostatics and the protonation state of AA would drive the tribological properties of these polyelectrolyte gels. P(AAm-co-AA) hydrogels were prepared with constant acrylamide (AAm) concentration (33 wt%) and varying AA concentration to control the amount of ionizable groups in the gel. The monomer:crosslinker molar ratio (200:1) was kept constant. Hydrogel swelling, stiffness, and friction behavior were studied by systematically varying the acrylic acid (AA) concentration from 0-12 wt% and controlling solution pH (0.35, 7, 13.8) and ionic strength (I = 0 or 0.25 M). The stiffness and friction coefficient of bulk hydrogels were evaluated using a microtribometer and borosilicate glass probes as countersurfaces. The swelling behavior and elastic modulus of these polyelectrolyte hydrogels were highly sensitive to solution pH and poorly predicted the friction coefficient (µ), which decreased with increasing AA concentration. P(AAm-co-AA) hydrogels with the greatest AA concentrations (12 wt%) exhibited superlubricity (µ = 0.005 ± 0.001) when swollen in unbuffered, deionized water (pH = 7, I = 0 M) and 0.5 M NaOH (pH = 13.8, I = 0.25 M) (µ = 0.005 ± 0.002). Friction coefficients generally decreased with increasing AA and increasing solution pH. We postulate that tunable lubricity in P(AAm-co-AA) gels arises from changes in the protonation state of acrylic acid and electrostatic interactions between the probe and hydrogel surface.