Nonionic Double and Triple Network Hydrogels of High Mechanical Strength

Among the hydrogels prepared in recent years, double network (DN) hydrogels exhibit the highest compression strength, toughness, and fracture energies. However, synthesis of DN hydrogels with extraordinary mechanical properties is limited to polyelectrolyte networks, which hinders their widespread applications. Herein, we prepared nonionic DN and triple network (TN) hydrogels based on polyacrylamide (PAAm) and poly(N,N-dimethylacrylamide) (PDMA) with a high mechanical strength by sequential polymerization reactions. The TN approach is based on the decrease of the translational entropy of the second monomer upon its polymerization in the first network, so that additional solvent (third monomer) can enter into DN hydrogel to assume its new thermodynamic equilibrium. The first network of TN hydrogels comprises chemically cross-linked PAAm or PDMA while the second and third networks are linear polymers. To increase the degree of inhomogeneity of the first network hydrogel, an oligomeric ethylene glycol dimethacrylate was used as a cross-linker in the gel preparation. Depending on the concentration of the first network cross-linker and on the molar ratio of the second and third to the first network units, TN hydrogels contain 89–92% water and exhibit high compressive fracture stresses (up to 19 MPa) and compressive moduli (up to 1.9 MPa).