Water-Induced Brittle-Ductile Transition of Double Network Hydrogels

In the previous studies, we have developed double network hydrogels (DN gels) with extra-ordinarily high mechanical strength and toughness although the gels contain 90 wt % of water. In this study, we investigated the effect of water content on the mechanical behavior of the DN gel consists of poly(2-acrylamido-2-methylpropanesulfonic acid) gels (PAMPS gels) as the first network and polyacrylamide gels (PAAm gels) as the second network. When the DN gel was dried gradually from its equilibrium swelling state (90 wt % water content), it exhibited a ductile behavior with a very high fracture stress and strain at an intermediate water content c (65 wt % < c < 75%), and then became brittle upon further decreasing in water content (c < 60 wt %). The behavior of the DN gel was compared with the corresponding single network gels, PAMPS and PAAm. It is found that at the intermediate water content, the fracture stress and strain of PAAm gel reached maximum, and the brittle−ductile change of DN gel upon with the change in water content was the results of the change in the mechanical properties of PAAm network.