Interfacial fracture of hydrogen bonded double network and ionically bonded polyampholyte hydrogels

Understanding hydrogel-hydrogel adhesion is critical for designing materials used in biomedical, electronic, and energy applications, where precise interfacial bonding is essential. Traditional adhesion tests (peel, pull, lap shear) often fail to accurately measure adhesion strength and toughness due to significant energy dissipation within the soft hydrogel substrates, underscoring the need for more reliable evaluation methods. Unlike these conventional methods, in this study fracture toughness experiments determine the true energy required to initiate and propagate a crack at the interface, offering a more fundamental and quantitative measure of interfacial bonding. Hence, the proposed study conducts a comprehensive fracture investigation of hydrogel-hydrogel adhesion due to hydrogen bonding and ionic bonding under both mode-I and mixed-mode conditions. A novel fracture specimen and J-integral approach are employed under both loading conditions. Hydrogels are commonly described as nonlinear elastic materials. J-integral fracture mechanics accommodates nonlinear materials and provides a more precise evaluation of the fracture energy required to initiate and propagate a crack under both loading conditions. Amylopectin/polyacrylamide (Amy/PAAm) and Amy/P(N-hydroxyethyl acrylamide) (Amy/PHEAAm) serve as model DN gels. This study examines the effect of monomer concentration (20, 25, 30 wt.% for Amy/PAAm and 20, 22.5, 25 wt.% for Amy/PHEAAm) on fracture initiation toughness. Results show that decreasing monomer concentration enhances fracture toughness. For Amy/PAAm, 20 wt.% achieves the highest toughness of 4.73 J/m², a 78% increase over 30 wt.%. A similar trend is observed in Amy/PHEAAm, where 20 wt.% yields a 2.15-fold increase over 25 wt.%. Both hydrogels exhibit higher fracture toughness under mixed-mode loading than under mode-I loading. This study also extends the analysis to polyampholyte (PA) hydrogel fabricated with anionic sodium p-styrenesulfonate (NaSS) and cationic 2-acryloyloxyethyltrimethylammonium chloride (DAC), P(NaSS-co-DAC). The interfacial fracture toughness of ionically bonded P(DAC-co-NaSS) hydrogels made of varying NaSS:DAC molar ratios (0.9:1.1, 1:1, 1.1:0.9, and 1.5:0.5) are determined and found that increasing anionic monomer contents led to significantly higher fracture toughness, with the 1.5:0.5 ratio exhibiting the highest 12 J/m², a 1.7-fold increase over the lowest, 4.5 J/m², of 0.9:1.1. Additionally, the influence of sodium chloride (NaCl) on the interfacial ionic bonding is explored using P(NaSS-co-DAC) of 1:1 NaSS:DAC molar ratio. NaCl treated interface exhibited significantly enhanced interfacial strength, resulting in a maximum 100% increase in fracture toughness compared to that without the NaCl treatment. Lastly, the fracture toughness of all P(NaSS-co-DAC) PA and NaCl treated hydrogels are enhanced under mixed-mode loading, compared to mode-I conditions. Overall, ionically bonded hydrogels outperform hydrogen-bonded gels in fracture toughness, especially under mixed-mode loading, highlighting the stronger role of ionic interactions over hydrogen bonding. Digital Image Correlation (DIC) is employed to map the strain distribution at the crack tip, providing insight into the opening and shear strain fields within the fracture process zone and to correlate with fracture toughness values.