Highly Tough, Stretchable, Self-Healing, and Recyclable Hydrogels Reinforced by in Situ-Formed Polyelectrolyte Complex Nanoparticles

It remains a challenge to fabricate healable and recyclable polymeric materials with simultaneously enhanced tensile strength, stretchability, and toughness. Herein, we report a simple approach to fabricate high-performance polymer hydrogels that not only integrate high tensile strength, stretchability, and toughness but also possess self-healing and recycling capabilities. The polymer hydrogels are fabricated by mixing a positively charged polyelectrolyte mixture of poly(diallyldimethylammonium chloride) (PDDA)/branched poly(ethylenimine) (PEI) with a negatively charged polyelectrolyte mixture of poly(sodium 4-styrenesulfonate) (PSS)/poly(acrylic acid) (PAA) in an aqueous solution followed by molding, drying, and rehydration. The (PDDA/PEI)–(PSS/PAA) hydrogels with in situ-formed PDDA–PSS nanoparticles have a tensile strength, strain at break, and toughness of 1.26 ± 0.06 MPa, 2434.2 ± 150.3%, and 19.53 ± 0.48 MJ/m3, respectively. The toughness of the (PDDA/PEI)–(PSS/PAA) hydrogels is ∼5.2 and ∼108 times higher than that of the PEI–PAA and PDDA–PSS hydrogels, respectively. Benefiting from the high reversibility of the hydrogen-bonding and electrostatic interactions, the (PDDA/PEI)–(PSS/PAA) hydrogels can efficiently heal from physical damage to restore their original mechanical properties at room temperature in water. Moreover, the (PDDA/PEI)–(PSS/PAA) hydrogels after being dried and ground can be recycled under a pressure of ∼3 kPa at room temperature in the presence of water to reuse the damaged hydrogels.