Rapid Recovery Double Cross-Linking Hydrogel with Stable Mechanical Properties and High Resilience Triggered by Visible Light

Abstract

The designed tough hydrogels, depending on energy dissipation mechanism, possess excellent biocompatibility, stimuli-responsiveness, and outstanding mechanical properties. However, the application of hydrogels is greatly limited in actuators and sensors for the lack of instantaneous recovery and resilience. In this work, we synthesized a double cross-linking poly(acrylic acid) hydrogel via a simple, one-pot, visible-light-trigger polymerization, with carboxymethyl cellulose as initiator and the first cross-linker, N,N'-methylene bis(acrylamide) (MBA) as the second cross-linker. The tensile strength and elastic modulus are in the range of 724-352 kPa and 115-307 kPa, respectively, depending on the MBA content. The swelling ratio of hydrogels dramatically decreased with increasing the MBA content. DMA results indicate that the internal friction between molecules within the hydrogel decreases with the increase of MBA content. Cyclic tensile tests show that after the structure stabilizes, the resilience, maximum stress, and residual strain of Gel-2 maintains over 93% (95% for successive cyclic tensile test), 115 kPa and less than 3%, respectively, at a strain of 125%. The values of resilience and residual strain are almost constant in both successive and intermittent cyclic tensile tests. Moreover, the swollen hydrogel has higher resilience and lower residual strain than the same hydrogel in the as-prepared state.

Mechanical Strengths of Hydrogels of Poly(N,N‐Dimethylacrylamide)/Alginate with IPN and of Poly(N,N‐Dimethylacrylamide)/Chitosan with Semi‐IPN Microstructures

“…Including the recovery curve, remarkable hysteresis loop could be found in each cycle. The hysteresis loop means energy dissipation, and could be resulted from breaking and re‐arrangement of the multi‐interactions in the composite hydrogels during stressing . The dissipation energies for the IPN hydrogel were, respectively, evaluated to be 3.05, 6.05, and 12.4 kJ m −3 for the first, the second, and the third cycle, which are much greater than those of the semi‐IPN hydrogel (0.23, 0.48, and 0.95 kJ m −3 ).…”

“…It has been reported that the storage modulus at relatively low frequency is related to effective chain density ( N ), that is where λ is a characteristic constant for hydrogel, and R and T are the gas constant and absolute temperature, respectively. For the present hydrogels, the E′ data at the frequency of 11.3 rad s −1 were extracted to reveal their chain densities.…”

“…It has been suggested that there are some microstructure changes including disentanglements and sliding of polymer chains during the energy dissipation process, which results in decrease in strength of hydrogel . Therefore, the hydrogels were, respectively, compressed to 50% strain and to higher stain of 75%, and then recovered naturally to zero stress.…”

“…The higher strengthening and toughening effect brought by the IPN structure has been carefully analyzed. For practical application, hydrogel is often subjected to cyclic stress, so that the stability of hydrogel is very important . Therefore, cyclic loading and unloading of the hydrogels were carried out.…”

Possible Application of Tough Hydrogel in Machinery

“…We have successfully prepared a modified CMC/PAA hydrogel by adding molecule crosslinker (N, N'-methylene bis(acrylamide), MBA) [6]. The hydrogel exhibited remarkable resilience (over 93% and 95% for successive and intermittent cyclic tests) and anti-fatigue properties (residual strain less than 3%).…”