Plastic-like Hydrogels with Reversible Conversion of Elasticity and Plasticity and Tunable Mechanical Properties

The development of hydrogels with excellent mechanical properties is highly desirable in both fundamental studies and practical applications. But it is difficult to construct hydrogels that are both tough and stiff at the same time as these properties often contradict each other. Here, we report a facile and efficient method for producing ultrastiff and tough poly(N-isopropylacrylamide) (PNIPAM)/clay plastic-like hydrogels (PHs) by immersing PNIPAM/clay hydrogel into NaCl aqueous solution. The optimized PH-2-6 presented superior strength, modulus, and toughness (4.1 ± 0.2 MPa, 41.6 ± 8 MPa, and 15.85 ± 0.8 MJ m^-3, respectively). The unique mechanical properties are attributed to the synergistic effect of the osmotic pressure and the strong affinity between Na⁺ ion and the PNIPAM chain, which lead to a high degree of PNIPAM chain entanglement and fixing. Note that the PHs were molded into any required shape under an applied force, and retained permanently their shapes even if the load was removed, thus displaying typical plasticity. However, the deformed PHs could return to their original size and softness of hydrogel when immersed in pure water, which is a kind of shape-memory effect. The reversible conversion of elasticity and plasticity and shape memory arise from a kind of dynamic physical across-linking of Na⁺ and PNIPAM molecular chains, which could exist in the salt aqueous and disintegrate in water reversibly. Moreover, the mechanical properties of hydrogel can be tuned by adjusting the salt concentration and immersion time. The facile strategy may provide further avenue in developing hydrogels with such versatile dynamic behaviors to expand their applications.