Polyacrylamide hydrogels. II. elastic dissipater

A rubber band is an elastic dissipater. It has narrow hysteresis on load and unload, but dissipates all its elastic energy on snap. We hypothesize that the dissipation of elastic energy can be a potent toughening mechanism. A polyacrylamide hydrogel has narrow hysteresis, but appreciable toughness, providing an ideal candidate to test the hypothesis of elastic dissipater. We peel the polyacrylamide hydrogel of various thicknesses, and record the steady peel forces. A transition thickness exists, below which the steady peel force increases linearly with thickness, and above which the steady peel force is independent of thickness. The transition thickness is comparable to the fractocohesive length, defined by the ratio of toughness over the work of fracture, and is ∼1 mm for the polyacrylamide hydrogel. The linear slope is comparable to the work of fracture. The linear extrapolation of the steady peel force to vanishing thickness defines the peel threshold, and the thickness-independent steady peel force defines the toughness. The former is much below the latter. We interpret these experimental findings in terms of elastic dissipater. A crack not only cuts a layer of polymer chains, but also breaks some polymer chains in a zone of thickness comparable to the fractocohesive length. Even though only a small fraction of the chains in the zone rupture, all the elastic energy stored in this zone dissipates and contributes to toughness. The entire zone acts as an elastic dissipater, not just a layer of individual polymer chains. We discuss the importance of the elastic dissipater in creating materials of high toughness, high threshold, and low hysteresis.