Rheological criteria for distinguishing self-healing and non-self-healing hydrogels

Owing to the structural resemblance of synthetic hydrogels to biological tissues, numerous self-healing wet materials have recently emerged, based on either chemical or physical motifs. With a plethora of novel synthetic hydrogels, a reliable characterisation strategy to assess self-healing is necessary. Herein, we therefore propose rheological criteria to distinguish self-healing hydrogels by comparing two types of poly (vinyl alcohol) hydrogels prepared via freezing/thawing (FX) or borax addition (BX). Non-self-healing FX and self-healing BX hydrogels exhibit incompatible features for three rheological characteristics: 1) Bingham behaviour without a lower Newtonian flow region (LNFR) (FX) versus pseudoplastic behaviour with a LNFR (BX), 2) infinite chain flow relaxation time (τf) versus finite τf on a reasonable time scale, and 3) drastic gel-sol transition with heating versus negligible transition. The plotting of modulus master curves confirms the terminal flow behaviour of the self-healing BX hydrogels, in contrast to the rubbery nature of the FX gels. The flowability of the self-healing hydrogels is ascribed to the pseudo-state of the gel network, due to the dynamic nature of the borate-diol bonds. Thus, the rheological criteria provide a sound theoretical background for self-healing material characterisations.