High-strength semi-crystalline hydrogels with self-healing and shape memory functions

We present a non-covalent approach and bulk photopolymerization method of hydrophilic and hydrophobic monomers to generate high-strength self-healing hydrogels with shape-memory effect. The hydrogels consist of linear poly(N,N-dimethylacrylamide) (PDMA) or polyacrylic acid (PAAc) chains containing (meth)acrylate units with long alkyl side chains. The existence of crystalline domains and hydrophobic associations formed by side alkyl chains produces hybrid-crosslinked PDMA and PAAc hydrogels with particularly high fracture energy of 20 ± 1 kJ m^-2 and Young's modulus up to 308 ± 16 MPa. The mechanical properties of the hydrogels could be tailored by varying the degree of crystallinity. By choosing suitable comonomer pairs and compositions, one may vary the degree of crystallinity between 3 and 33%, which leads to two orders of magnitude change in the modulus, elongation at break, and toughness. The hydrogels undergo up to 1000 fold change in their elastic moduli by changing the temperature between below and above the melting temperature of the crystalline regions. They also exhibit self-healing and shape memory functions triggered by heating above the melting temperature. Healed hydrogels sustain up to 138 ± 10 MPa compressive stresses, which are around 87% of the compressive stress of the virgin gel samples.