Fracture of dual crosslink gels with permanent and transient crosslinks: Effect of the relaxation time of the transient crosslinks

Abstract

We investigate the fracture properties of poly(acrylamide- co-1-vinylimidazole) dual crosslink hydrogels [P(AAm- co-VIm)-M2+ gels] containing a small fraction of covalent bonds and a majority of dynamic bonds based on metal coordination bonds (Ni2+ or Zn2+). Unlike a previous study on a different dual crosslink hydrogel system having slower dynamic bonds based on poly(vinylalcohol) and borate ions (PVA-Borax gels), the presence of these faster dynamic coordination bonds has two main effects: They significantly toughen the P(AAm- co-VIm)-M2+ gels even at high stretch rates, where the dynamic bonds should in principle behave as covalent bonds at the crack tip, and they toughen the gels at very low stretch rates, where the dynamic bonds are invisible during the loading stage. We propose two additional molecular mechanisms to rationalize this behavior of P(AAm- co-VIm)-M2+ gels: we hypothesize that fast exchanging dynamic bonds remain slow compared to the characteristic time of bond scission and are, therefore, able to share the load upon covalent bond scission even at low loading rates. We also argue of the existence of longer-lived clusters of dynamic bonds that introduce a stretch rate-dependent strain hardening in uniaxial tension and stabilize and increase the size of the dissipative zone at the crack tip, thereby introducing a strain-dependent dissipative mechanism.

Self-healable fiber-reinforced vitrimer composites: overview and future prospects

“…49,51 Similar to known dynamic networks, there is a strong inuence of the networks density, the nature of the crosslinking points on the association/ dissociation of the individual bonds. 52 This in turn is directly related to the overall stress recovery and the relation between bond exchange rates and stress-strain relationships. 53 Ran Ni and co-workers have analysed the role of entropy on the phase behaviour of the linker-mediated vitrimers, 54 where the effect of linker concentration makes a huge impact reentering their gel-sol transition.…”

“…49,51 Similar to known dynamic networks, there is a strong influence of the networks density, the nature of the crosslinking points on the association/dissociation of the individual bonds. 52 This in turn is directly related to the overall stress recovery and the relation between bond exchange rates and stress–strain relationships. 53…”

A highly tough, fatigue-resistant, low hysteresis hybrid hydrogel with a hierarchical cross-linked structure for wearable strain sensors

“…26 Zn 2+ ion is able to form tetra-ligand structures while possessing a low dissociation energy, contributing to a much faster association/dissociation dynamics and low hysteresis compared to other metal ions. 27,28 Thus, Zn 2+ is the ideal metal ion to construct a hierarchical cross-linked hydrogel possessing outstanding mechanical properties.…”