Designing extremely resilient and tough hydrogels via delayed dissipation

While high resilience of a material requires low mechanical dissipation of the material under deformation, high toughness requires significant mechanical dissipation during crack propagation. Here we reconcile this pair of seemingly contradictory properties to design extremely tough and resilient hydrogels. We propose a resilient domain for hydrogels' deformation, below which hydrogels are deformed with low mechanical dissipation, but above which the deformation is highly dissipative. Therefore, hydrogels will appear resilient under moderate deformation within the resilient domain, but materials around crack tips will be deformed beyond the resilient domain and thus dissipate significantly to toughen the hydrogels. We implement the resilient domain by pre-stretching an interpenetrating-network hydrogel to damage the short-chain network to a controlled degree. The resultant hydrogel is highly resilient if deformed within the pre-stretched range (i.e., resilient domain), but highly dissipative if deformed beyond the resilient domain because of further damage of the short-chain network-achieving both high resilience of 95% and high toughness of 1900 J/m(2). To quantitatively explain the experimental results, we further adopt an interpenetrating-network model with network alteration, which can guide the design of future resilient and tough hydrogels. (C) 2014 Elsevier Ltd. All rights reserved.