Time-dependent mechanical properties of tough ionic-covalent hybrid hydrogels

Hybrid gels featuring interpenetrating covalent and ionic crosslinked networks have recently been shown to exhibit both high toughness and recoverability of strain-induced network damage. The high toughness results from the energy dissipated as entropically strained network strands are released by the dissociation of ionic crosslinks. As in the so-called double network hydrogels, the toughening process is inherently linked to network damage. This damage, however, can be recovered to a large degree in hybrid gels due to the reformation of ionic associations when the gel is unloaded. The stability of the ionic network under load is here investigated and it is shown that these networks show large stress relaxation at constant strain, time dependent stress-strain behaviour and rate-dependent toughness. A double exponential model is invoked to mathematically describe the stress relaxation of the hybrid gels indicating at least two relaxation mechanisms. The rate-dependent toughness and the relaxation behaviour of the hybrid gels are attributed to the labile unzipping of the ionic crosslinks which is assumed to be load and time dependent.