Designing Ultrahigh-Water-Content, Tough, and Crack-Resistant Hydrogels by Balancing Chemical Cross-Linking and Physical Entanglement

Hydrogels are cross-linked hydrophilic polymer structures containing large amounts of water. However, high water content or a high swelling of the gels can degrade the mechanical properties of most existing hydrogels. In this work, a series of polyacrylamide (PAAm) hydrogels with diverse cross-linker contents are synthesized by incorporating an acrylate-functionalized cross-linker, N, N′-methylenebis (acrylamide), into polymer networks through free radical polymerization. These PAAm gels with different cross-linker contents are also embedded as soft layers in a layered hydrogel containing alternating poly(dodecyl glyceryl itaconate, PDGI) as hard layers. The effects of the cross-linking content on the mechanical properties and adhesion performance of PAAm and PDGI/PAAm gels are investigated. A strategy of balancing chemical cross-linking and physical entanglement is proposed to develop ultrahigh-water -content, tough, and crack-resistant hydrogels. The balance is achieved by introducing a very small amount of chemical cross-linker, which significantly improves the water content (>98 wt %), mechanical, and adhesion properties of PAAm gels. Additionally, a secondary balance between the soft and hard interfaces coupled with the first balance in the soft layers allows the design of high-water-content (>95 wt %) and tough PDGI/PAAm layered gels. The chemical cross-linking here tailors not only the mechanical properties of the soft layers but also the mobility of the polymer chains with functional groups at the interfaces. This study provides some insights into the effects of chemical cross-linking on the mechanical properties and adhesion performance of soft materials.