Kinetic insights into glassy hydrogels with hydrogen bond complexes as the cross-links

Revealing the structure formation kinetics is a long-term challenging issue in the development of tough hydrogels, although they are significant for understanding structure-property relationship and toughening mechanism. Here, we report a series of tough and stiff poly(methacrylamide-co-methacrylic acid) hydrogels, with a focus on the structure-property relationship and structure formation kinetics. These hydrogels in a glassy state possess moderate water content and excellent mechanical properties with Young's modulus up to 200 MPa. The microstructure of the gels changes from uniform to bicontinuous and then to colloidal network as the fraction of methacrylamide, fam, increases, accounting for the ductile-brittle transition of the mechanical performances. Sequential gelation and vitrification take place in the systems with relatively low fam to form transparent gels with a homogeneous matrix, whereas colloidal jamming and coarsening occur in the systems with high fam to form opaque gels with a colloidal network structure. The structure and properties of the glassy gels are determined by the hydrogen bond complexation and the microphase separation that are strengthened by the increase in fam. Based on these findings, the mechanical properties of hydrogels with high fam can be improved by suppressing the microphase separation during the gel synthesis. Understanding the structure-property relationship and regulation strategy of both microstructure and macroscopic performance of these glassy hydrogels should be inspirative for designing other tough materials with diverse applications as structural elements in biomedical and engineering fields.