Influence of the Degree of Swelling on the Stiffness and Toughness of Microgel‐Reinforced Hydrogels

• Published in: Macromolecular rapid communications. 2023-08, Vol.44 (16), p.e2200864-n/a
• Main Authors: Kessler, Michael, Yuan, Tianyu, Kolinski, John M., Amstad, Esther
• Format: Article
• Language: English
• Subjects: Fabrication; Fracture toughness; granular materials; Heat treating; Hydrogels; mechanical properties; Microgels; microgel‐reinforced hydrogels; Microparticles; soft matters; Stiffness; Swelling
• ISSN: 1022-1336; 1521-3927
• DOI: 10.1002/marc.202200864

The stiffness and toughness of conventional hydrogels decrease with increasing degree of swelling. This behavior makes the stiffness‐toughness compromise inherent to hydrogels even more limiting for fully swollen ones, especially for load‐bearing applications. The stiffness‐toughness compromise of hydrogels can be addressed by reinforcing them with hydrogel microparticles, microgels, which introduce the double network (DN) toughening effect into hydrogels. However, to what extent this toughening effect is maintained in fully swollen microgel‐reinforced hydrogels (MRHs) is unknown. Herein, it is demonstrated that the initial volume fraction of microgels contained in MRHs determines their connectivity, which is closely yet nonlinearly related to the stiffness of fully swollen MRHs. Remarkably, if MRHs are reinforced with a high volume fraction of microgels, they stiffen upon swelling. By contrast, the fracture toughness linearly increases with the effective volume fraction of microgels present in the MRHs regardless of their degree of swelling. These findings provide a universal design rule for the fabrication of tough granular hydrogels that stiffen upon swelling and hence, open up new fields of use of these hydrogels. The stiffness and toughness of conventional hydrogels decrease with increasing degree of swelling, limiting their use for load‐bearing applications. This shortcoming can be addressed by reinforcing them with microgels that introduce the double network toughening effect into hydrogels. In this work, the microstructure of microgel‐reinforced hydrogels and in particular their degree of swelling is correlated to their mechanical properties.