Effect of Relative Strength of Two Networks on the Internal Fracture Process of Double Network Hydrogels As Revealed by in Situ Small-Angle X-ray Scattering

Abstract

Double network hydrogels (DN gels) exhibit extraordinarily high strength and toughness by interplay of the two contrasting networks: the rigid, brittle network serves as a sacrificial bond that fractures at a relatively low strain, while the soft, stretchable network serves as hidden length that sustains stress by large extension afterwards. The internal fracture process of the brittle network strongly depends on the relative strength of the two networks. In this study, we study the internal fracturing process of typical DN gels that show yielding or necking under uniaxial stretching, using in situ small-angle X-ray scattering.Two samples consisting of the same brittle first network from poly(2-acrylamido-2methylpropanesulfonic acid) but stretchable second network from poly(N,N-dimethylacrylamide) of different concentrations were adopted. We found that (1) the brittle network shows non-affine deformation even far below the yield strain by local fracture; (2) for the sample of low second network concentration, significant strain amplification occurs around the submicron-scale voids (defects) preexisting in the brittle network, which induces the fracture percolation of brittle network from voids to show the necking phenomenon; (3) the strain amplification at voids is suppressed in the sample of high second network concentration, and fracture of brittle network occurs dispersedly, showing yielding without necking.

Mechanochemistry unveils stress transfer during sacrificial bond fracture of tough multiple network elastomers

“…1. 33,35,57,58 Such hypothesis, also based on the very heterogeneous structure of the gel ller network of poly(2acrylamido-2-methyl-1-propanesulfonic acid) as detected by light scattering, 59 implies that most of the ller network chains in the island should be unloaded aer necking. Our own group discovered that some multiple network elastomers also undergo elastic necking in uniaxial tension 19 and addressed the question of bond scission, and indirectly load transfer.…”

Recent Strategies for Strengthening and Stiffening Tough Hydrogels

“…In the dense region, polymer chains with specific structures or functional groups often form crystalline physically crosslinked by the highly increased number of intra/interchain bonds, which leads to further increase in the mechanical properties compared with those of general hydrogels without crystalline. Despite the multiple phenomenological studies on the phase separation and spatial inhomogeneities in polymer gels at nanoscales, [ 146–153 ] the effect of the solvent exchange‐induced phase separation on the enhancement of hydrogel mechanical properties was very recently started to be investigated.…”

Microscopic insights into the failure of elastic double networks

“…7) [12,13]. In addition, scattering experiments show evidence of the delocalization of stress by probing the length scale of the nonaffine response [41] and the expansion of microscopic defects [42]. In particular, Ref.…”

“…In particular, Ref. [42] suggests that a higher fraction of matrix monomer leads to the creation of new microcracks instead of the expansion of existing defects (Fig. 8).…”