Macromolecules, Vol.52, No.10, 3852-3862, 2019
From Molecular Electrostatic Interactions and Hydrogel Architecture to Macroscopic Underwater Adherence
We investigate the macroscopic adhesion energy (W-a) in pure water between a positively charged hydrogel of varying cross-link densities made from (methacryloyloxyethyl)trimethylammonium chloride and acrylamide [poly(MAETAC-co-AAm)] and a negatively charged and cross-linked poly(acrylic acid) (PAA) thin film gel grafted on a silicon wafer. Adhesion tests were carried out on a custom-built probe-tack setup fully immersed in pure water. The interfacial charge density on the PAA hydrogel thin film was estimated from streaming potential measurements and the molecular architecture of the thick hydrogel was obtained from mechanical testing. For a fixed interfacial charge density, W-a increased weakly with contact time (in stark contrast with the case where adhesion is due to H-bonds) but strongly with debonding rate. For a given gel, the work of adhesion increased linearly with the interfacial charge density of the thin PAA film, whereas at constant interfacial charge density, W-a was found to decrease with the modulus of the gel. The results were analyzed with a simple kinetic bond model proposed by Chaudhury for weak adhesion of elastomers. Using realistic values of the spring constant of the polymer chain and of the characteristic time of bond dissociation, we demonstrate that the work of adhesion can be understood by a combination of a strain rate-dependent bond breaking kinetics and a pH-dependent areal density of electrostatic interactions.