Multiscale Energy Dissipation Mechanism in Tough and Self-Healing Hydrogels

Understanding the energy dissipation mechanism during deformation is essential for the design and application of tough soft materials. We show that, in a class of tough and self-healing polyampholyte hydrogels, a bicontinuous network structure, consisting of a hard network and a soft network, is formed, independently of the chemical details of the hydrogels. Multiscale internal rupture processes, in which the double-network effect plays an important role, are found to be responsible for the large energy dissipation of these hydrogels.

Figure 1

Multiscale structure of polyampholytes (PA) hydrogels. (a) SAXS spectrum and Teubner-Strey (T-S) model fitting [23, 24] for a PA hydrogel, $\mathrm{P}(\mathrm{NaSS}\text{−}co\text{−}\mathrm{MPTC})\text{−}2.1\text{−}0$, (b) 2D Young’s modulus distribution obtained by nanomechanical mapping, and (c) a sectional profile. Inset in (a) shows an optical image of the gel. (d)–(f) Illustrations of the multiscale structure of PA gels: bicontinuous network structure consists of hard, dense regions (green) and soft, sparse regions (red) with a $d_0$ in hundred nanometers (e), aggregated polymer chains of dense (d) and sparse (f) regions at nanoscale, and ionic bonds at monomer scale [insets in (d) and (f)].

Figure 2

Combined stress and structural measurements in a tensile test. (a) Nominal stress versus stretch ratio $\lambda$ and the corresponding time-resolved 2D SAXS patterns at several representative $\lambda$. (b) Microscopic deformation ratio ($d/d_0$) in the parallel ($⫽$) and perpendicular ($\perp$) directions of stretching versus $\lambda$. The dashed vertical lines mark the boundaries between three deformation regimes and the arrows indicate the two $\lambda$ used in the recovery test of Fig. 3. Sample: $\mathrm{P}(\mathrm{NaSS}\text{−}co\text{−}\mathrm{DMAEA}\text{−}\mathrm{Q})\text{−}2.4\text{−}0.1$ ($d_0=102\mathrm{nm}$). True strain rate: $0.05{\mathrm{s}}^{-1}$. Stretching direction: vertical.

Figure 3

Self-recovery behavior of gels under different deformations. (a),(b) Loading-unloading curves of the cyclic tensile test at different waiting times between the first and second cycles, for $\lambda$ of 3.3 (a) and 7.7 (b). (c),(d) Waiting-time dependences of recovery ratio defined as the ratio of the second loading hysteresis loop to the first (c) and residual strain of sample after the first loading (d). Sample: $\mathrm{P}(\mathrm{NaSS}\text{−}co\text{−}\mathrm{DMAEA}\text{−}\mathrm{Q})\text{−}2.4\text{−}0.1$ ($d_0=102\mathrm{nm}$).

Figure 4

Illustration of the multiscale fracture process of PA gels for high toughness. (a) The bicontinuous hard (green) and soft (red) networks with a length scale $d_0$ in the range of hundred nanometers were isotropic, and the polymer chains are in globule conformation due to ionic associations. (b) Under stretching, the ionic bonds break and the aggregated polymer chains unfold, permitting the bicontinuous network a very large reversible affine deformation. (c) With increasing deformation, some hard strands start to rupture, and the load is transferred to neighboring hard strands via soft strands. As the soft strands remain intact, the gel softens but still reverts to the original shape at unloading. (d) After rupture of most of the hard strands, the soft strands start to rupture, leading to global failure of the gel. Through such a multiscale rupture process, the gel dissipates a significant amount of energy, to show a high toughness.