Designing ultratough single-network hydrogels with centimeter-scale fractocohesive lengths via inelastic crack blunting

Jie Ma, Xizhe Zhang, Daochen Yin, Yijie Cai, Zihang Shen, Zhi Sheng, Jiabao Bai, Shaoxing Qu, Shuze Zhu*, Zheng Jia*, Designing ultratough single-network hydrogels with centimeter-scale fractocohesive lengths via inelastic crack blunting, Advanced Materials, 2311795, 2024

Fractocohesive length, defined as the ratio of fracture toughness to work of fracture, measures the sensitivity of materials to fracture in the presence of flaws. The larger the fractocohesive length, the more flaw-tolerant and crack-resistant the hydrogel. For synthetic soft materials, the fractocohesive length is short, often on the scale of 1 mm. Here, we design highly flaw-insensitive (HFI) single-network hydrogels containing an entangled inhomogeneous polymer network of widely distributed chain lengths. The HFI hydrogels demonstrate a centimeter-scale fractocohesive length of 2.21 cm, which is the highest ever recorded for synthetic hydrogels, and an unprecedented fracture toughness of ~13300 J/m2. The uncommon flaw insensitivity results from the inelastic crack blunting inherent to the highly inhomogeneous network. When the HFI hydrogel is stretched, a large number of short chains break while coiled long chains can disentangle, unwind and straighten, producing large inelastic deformation that substantially blunts the crack tip in a plastic manner, thereby deconcentrating crack-tip stresses and blocking crack extension. The flaw-insensitive design strategy is applicable to various hydrogels such as PAAm and PDMA hydrogels and enables the development of HFI soft composites.