Highly Mechanical and Fatigue‐Resistant Double Network Hydrogels by Dual Physically Hydrophobic Association and Ionic Crosslinking

Abstract

Double network (DN) hydrogels with high strength and toughness are considered as promising soft materials. Herein, a dual physically cross‐linked hydrophobic association polyacrylamide (HPAAm)/alginate‐Ca2+ DN hydrogel is reported, consisting of a HPAAm network and a Ca2+ cross‐linked alginate network. The HPAAm/alginate‐Ca2+ DN hydrogel exhibits excellent mechanical properties with the fracture stress of 1.16 MPa (3.0 and 1.7 times higher than that of HPAAm hydrogel and HPAAm/alginate hydrogel, respectively), fracture strain of 2604%, elastic modulus of 71.79 kPa, and toughness of 14.20 MJ m−3. HPAAm/alginate‐Ca2+ DN hydrogels also demonstrate self‐recovery, notch‐insensitivity, and fatigue resistance properties without any external stimuli at room temperature through reversible physical bonds consisting of hydrophobic association and ionic crosslinking. As a result, the dual physical crosslinking would offer an avenue to design DN hydrogels with desirable properties for broadening current applications of soft materials.

Nanoclay Reinforced Self‐Cross‐Linking Poly(N‐Hydroxyethyl Acrylamide) Hydrogels with Integrated High Performances

“…Polymer hydrogels, which are 3D cross-linked polymer networks with abundant water, have received great attention in various fields, including tissue engineering, [1][2][3] sensors and actuators, [4][5][6] water treatment, [7] drug delivery, [8] and so on. However, conventional hydrogels, cross-linked by chemical cross-linkers, usually suffer from weak mechanical properties (i.e., poor mechanical strength, low stretchability, bad toughness, and/or low recoverability) owing to their heterogeneous network structures and lack of effective energy dissipation mechanisms, [9] which largely limits their applications in the load-bearing fields.…”

Self‐recovery and fatigue of double‐network gels with permanent and reversible bonds

“…Observations in a multi‐cycle test with a fixed (5 min) interval of recovery between cycles are reported in Ref. . The above studies dealt with the response of DN gels in cyclic tests with a strain‐controlled (a fixed maximum elongation ratio per cycle k max ) programs and relatively small (below 10) numbers of cycles.…”

“…The following conclusions are drawn from the results of numerical analysis reported in Figures : In agreement with available experimental data, introduction of short intervals of self‐recovery between subsequent cycles of deformation reduces damage accumulated in DN gels and leads to an increase in their lifetime. This result is obtained for cyclic deformations with strain‐controlled (Fig.…”

“…). An advantage of our constitutive model is that it allows the lifetime under fatigue conditions to be predicted based on observations in tests with small number of cycles. The difference between one‐stage and two‐stage kinetics of self‐recovery is explained in Section Fitting of Observations by slowing down of the recovery process driven by evolution of micro‐structure of a DN gel, eq . Simulation reveals another mechanism for transition from the exponential to non‐exponential decay in residual strain when recovery is performed after multi‐cycle deformation (Figs.…”

“…DN gels able to recover within a few minutes demonstrate an exponential decay in residual strain with recovery time t rec . When the recovery process requires more time, it involves two stages: elongation ratio k decreases rapidly with t rec at the first stage, and reduces slowly at the final stage. Comparison of observations on as‐prepared and fully swollen gel specimens shows that the duration of total recovery is weakly affected by degree of swelling The growth of maximum elongation ratio under stretching k max induces an appropriate increase in recovery time .…”