Equilibrium and Out-of-Equilibrium Adherence of Hydrogels against Polymer Brushes

At low pH, underwater adherence between poly(N,N-dimethylacrylamide) hydrogels and poly(acrylic acid) brushes is due to the formation of multiple hydrogen bonds. The effect of the key parameters controlling the formation of these interactions (contact time and composition of the hydrogel) was investigated with a contact mechanics test using a flat probe. We specifically quantified the difference in adherence observed between the gel and the brush as gels were in their preparation state or in their equilibrated state. The progressive swelling to equilibrium of the gels results (for a fixed contact time) in a significant decrease in adherence even in the case of relatively low dilution factors. This adherence loss was attributed to the slowdown of the kinetics of formation of multiple H-bond interactions as the gel approached its equilibrated state. In both equilibrated and nonequilibrated conditions the energy of adherence scaled with the polymer concentration, independent of the cross-linking density of the hydrogel, suggesting that the Lake and Thomas amplification factor is not relevant for these weak bonds.