Tough Hydrogels with Dynamic H-Bonds: Structural Heterogeneities and Mechanical Performances

The recently developed tough and self-healing hydrogels, containing a large number of physical bonds, have found widespread applications in bioengineering and soft electronics. Owing to the sophisticated physical interactions and organization, these hydrogels often demonstrate structural heterogeneities, which strongly influence their mechanical performances. Using poly(N,N-dimethyl acrylamide-co-methacrylic acid) hydrogels (P(DMAA-co-MAAc)) with dynamic hydrogen bonds (H-bonds) as a model system, the structural characterization has been carried out in this study using small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) analyses, along with exploring the influence of the structure on the mechanical properties. By systematically tuning the polymer composition, including the monomer fraction and chemical cross-linking density, two different hydrogel regions with distinct mechanical properties were observed: the swollen regions and shrunk regions. The former was mechanically weak, whereas the latter exhibited a tough behavior. Both types of hydrogels were noted to be highly heterogeneous, which results from the nanoscale spatial polymer concentration fluctuations at ∼100 nm scale. Combined with the contrast variation utilizing SANS, the structure parameters, including the polymer volume fraction of the dense and sparse regions, volume fraction occupied in the space, and average correlation length of the long-range and short-range heterogeneous structures, were explored using a scaling model based on a two-phase system composed of the densely and sparsely cross-linked regions. Using in situ SAXS, the microscopic deformation of the tough and shrunk hydrogels was noted to follow the affine deformation, while a significantly non-affine deformation was observed in the swollen hydrogels, which might have led to the different mechanical performances of these materials.