Fabrication of Tough Hydrogels from Chemically Cross-Linked Multiple Neutral Networks

A simple method was developed to fabricate tough hydrogels from a chemically cross-linked neutral hydrogel. Loosely cross-linked pseudo- and true-interpenetrating polymer network (IPN) hydrogels with double-, triple-, and quadruple-network structures were synthesized from acrylamide (AAm), and their mechanical properties were studied. Increasing the number of polymeric networks significantly changed the mechanical properties of pseudo-IPN hydrogels even though the chemical composition and polymerization procedure of each individual network was the same. The SN and DN hydrogels showed behavior similar to extensible soft tissue, but the TN and QN hydrogels exhibited strain localization during tensile deformation. Loading–unloading–reloading tensile experiments indicated that tensile loading causes no damage to the SN and DN hydrogels. For pseudo-IPN hydrogels where strain localization occurred, unloading before strain localization resulted in no damage, but unloading after strain localization showed a large...