Crosslinker-Free Polyacrylamide Hydrogels Exhibit High Strength, Fracture Resistance, and Reprocessability

Abstract

Polyacrylamide (PAAm) hydrogels are commonly prepared by incorporating covalent crosslinkers to establish a stable network structure. Here, we report crosslinker-free, high-water-content (>93%) PAAm single-network hydrogels that exhibit unexpectedly high mechanical performances. These hydrogels are prepared from highly concentrated acrylamide monomer solution through free-radical polymerization initiated by ammonium persulfate. Despite the absence of intentionally introduced covalent crosslinkers, these hydrogels remain as self-supporting gels during swelling in water for several weeks and exhibit high mechanical strength and crack resistance, comparable to or even exceeding those of conventional chemically crosslinked PAAm hydrogels. This behavior is attributed to a physically constrained network arising from extensive chain entanglements and possible branching generated during radical polymerization. Interestingly, these crosslinker-free hydrogels gradually weaken during prolonged swelling, with some eventually losing their self-supporting shapes and transforming into polymer solutions that can be reprocessed. This work expands the processing window for preparing tough and reprocessable hydrogels and provides a simple strategy for designing robust soft materials without adding chemical crosslinkers.