Complex Hydrogels Based on Multiply Interpenetrated Polymer Networks: Enhancement of Mechanical Properties via Network Multiplicity and Monomer Concentration

We present the preparation and study of the compressive mechanical and swelling properties of multiply interpenetrated, from double to quintuple (5-fold), polymeric hydrogels based on N,N-dimethylacrylamide (DMAAm) cross-linked with N,N′-methylenebis­(acrylamide) (MBAAm), using five different monomer concentrations, from 1 to 5 M. Our results showed enhancement of the fracture stress, fracture energy density, and Young’s modulus with network multiplicity and DMAAm monomer concentration. Fracture strain was found to substantially improve only when increasing the monomer concentration from 1 to 3 M and network multiplicity from 1 to 2. The best-performing multiple polymer hydrogel was the quintuple network prepared at 5 M DMAAm concentrations, which exhibited a record fracture stress of 51 MPa, a fracture strain of 88%, a fracture energy density of 5.5 MJ m–3, and a Young’s modulus of 2.1 MPa. Normalized values of the fracture stress and fracture energy density calculated by multiplication of the corresponding original values by the aqueous swelling degrees also increased with network multiplicity and monomer concentration, suggesting a net gain in mechanical property enhancement, primarily originating from network multiplicity. Combined analysis of the swelling and Young’s modulus measurements on the multiply interpenetrated polymeric hydrogels allowed the calculation of the concentration of the elastically effective polymer chains in the bulk, which was independent of monomer concentration but increased linearly with network multiplicity, suggesting a corresponding increase in the concentration of trapped entanglements and/or strain hardening