Molecular mechanism of abnormally large nonsoftening deformation in a tough hydrogel

Abstract

Tough soft materials usually show strain softening and inelastic deformation. Here, we study the molecular mechanism of abnormally large nonsoftening, quasi-linear but inelastic deformation in tough hydrogels made of hyperconnective physical network and linear polymers as molecular glues to the network. The interplay of hyperconnectivity of network and effective load transfer by molecular glues prevents stress concentration, which is revealed by an affine deformation of the network to the bulk deformation up to sample failure. The suppression of local stress concentration and strain amplification plays a key role in avoiding necking or strain softening and endows the gels with a unique large nonsoftening, quasi-linear but inelastic deformation.

Keywords: hyperconnective network; large quasi-linear deformation; molecular glue; nonsoftening; tough hydrogel.

Fig. 1.

Molecular structure and mechanical performance of B and glue-B gels. (A) Schematic diagram depicting the molecular structures of B and glue-B gels. B gel is formed from PBMA-b-PMAA-b-PBMA triblock copolymers. The PBMA end-blocks form micelles in water and serve as physical cross-linkers. The d₀ represents the average neighboring intermicelle distance. The glue-B gel is formed by polymerizing PAAm inside the B gel, and the linear PAAm forms hydrogen bonding with the PMAA midblock of the B gel. (B) Nominal stress-stretch curves for the B and glue-B gels in uniaxial tension. Samples have a rectangular shape with a gauge length of 20 mm and a width of 7.5 mm, and the initial strain rate was 0.01 s⁻¹. The inset in B shows the enlarged view of small stretch ratio.

Fig. 2.

Structure evolution of B and glue-B gels subjected to uniaxial tension as observed by in situ SAXS measurement. The microscopic deformation ratio (d/d₀) in the parallel (//) and perpendicular (⊥) directions of stretching versus λ for (A) B gel and (B) glue-B gel. The insets are the 2D SAXS patterns at representative λ for B and glue-B gels. Initial strain rate: 0.01 s⁻¹. Stretching direction is horizontal.

Fig. 3.

Experiments depicting the irreversible internal breakage that occurred by stretching in glue-B gel. The irreversible internal breakage means there is damaged structure that cannot recover after a waiting time of 10⁵ s. (A) Cyclic tensile tests at different waiting times between the first and the subsequent cycles at a stretching ratio $\lambda =1.7$. The initial strain rate was 0.01 s⁻¹. (B) Waiting time dependencies of residual strain and recovery ratio defined as the area ratio of the second hysteresis loop to the first one. (C) λ-dependence of the irreversible energy determined from the difference between the first hysteresis loop and the one that was obtained after a waiting time of 10⁵ s. (D) The fraction of the irreversible energy relative to the total work of stretch determined from the stress–strain curve.

Fig. 4.

Schematic to show the molecular mechanism of deformation and fracture of B and glue-B gels. (A) For B gel, once the shortest strand is pulled out from micelle, the neighboring strands will be overloaded. The stress concentration amplifies the local strain and induces the catastrophic failure of the sample. Thus, the gel shows nonaffine deformation in its network and failures at small stretching. The bolded chains highlight the pullout chain and the overstressed neighboring chains. (B) For the glue-B gel, when the shortest strand of network is pulled out, the load will be transferred to the glue polymers, which effectively suppress local stress concentration. With increasing deformation, further rupture does not occur at the neighboring strands but occurs randomly in the skeleton network. At the same time, the hyperconnectivity retains the integrity of the skeleton network. Consequently, the gel always shows affine deformation in its network and quasi-linear stress–strain response. The bolded chains highlight the pullout chain and the load transfer from the pullout chain to glue polymer.