Determination of fracture energy of high strength double network hydrogels

The fracture energy G of double network (DN) gels, consisting of poly(2-acrylamido-2-tnethylpropanesulfonic acid) (PAMPS) as the first network and poly(acrylamide) (PAAm) as the second network, was measured by the tearing test as a function of the crack velocity V. The following results were obtained: (i) The fracture energy G ranges from 10(2) to similar to 10(3) J/m(2), which is 100-1000 times larger than that of normal PAAm gels (10 J/m(2)) or PAMPS gels (10(-1) J/m(2)) with similar polymer concentrations to the DN gels. (ii) G shows weak dependence on the crack velocity V. (iii) G at a given value of V increases with decreasing of cross-linking density of the 2nd network. The measured values of G were compared with three theories that describe different mechanisms enhancing the fracture energy of soft polymeric systems. A mechanism relating to a heterogeneous structure of the DN gel is convincing for the remarkable large values of G.