Molecular weight distribution of network strands in double network hydrogels estimated by mechanical testing

The remarkably high toughness of double network (DN) hydrogels is related to the energy dissipation that occurs due to the scission of network strands in the most tightly crosslinked of its two interpenetrating networks. The loading curve during tensile testing of DN hydrogels usually takes a complex shape and sometimes shows yielding. Wang and Hong recently proposed a model [1] based on the successive scission of the shortest remaining elastically active strands during tensile loading that predicts the shape of the loading curve of one DN hydrogel reasonably well. In the present study, the model was modified slightly and applied to five different DN gels prepared with different crosslink densities in the first (tight) network. The model fit parameters provide a description of the network strand length distribution in the tight network and show that the network strand concentration increases and the network strand length decreases when a higher crosslinker concentration was used to prepare the tight network. Such network strand distributions estimated from analysis of the tensile loading and unloading curves may be one of the first successful attempts to estimate the molecular weight distribution of a crosslinked polymer.