Super-tough hydrogels from shape-memory polyurethane with wide-adjustable mechanical properties

Recoverable hydrogels with high strength and toughness have been fabricated from hydrophilic and thermoplastic polyurethane (HTPU), which chains consist of hydrophilic polyethylene glycol (PEG) segment of high crystallinity and hydrophobic segment with strong hydrogen-bonding groups. This HTPU absorbed high amount of water, during which the PEG crystals swollen and dissolved, while the hydrophobic segments still held the adjacent chains together, forming a stable hydrogel. Even at equilibrium swelling state (89 wt% water), the HTPU hydrogel exhibited high modulus (0.4 MPa), high strength (2.6 MPa), and large strain at break (~500%). The effect of water content on the tensile properties of HTPU hydrogels was carefully studied at different levels of swelling. Interestingly, the hydrogels demonstrated a transition from a typical tough plastic to a tough elastomer when the water content reached 35 wt% of the hydrogel, with breaking strength of 10.0 MPa and fracture energy of 59.7 MJ/m³ at maximum strain over 1600%. The results from differential scanning calorimetry, Fourier transform infrared spectroscopy, and microscope measurements showed that the wide adjustability of this HTPU mechanical property was a result of the changes in its crystallinity, hydrogen-bonding, and hydrophobic association. Furthermore, the shape-memory performance of the HTPU was studied with heat and water stimuli and found faster at heating to 70 °C than that by immersion in water: 10 s versus 10 min. This study may widen the application of HTPU biodegradable polymers and provide new frontiers for the design of tough hydrogels by network structure control.