The effect of water content on the elastic modulus and fracture energy of hydrogel

Both elastic and fracture behavior of hydrogel are affected by its water content. As shown by extensive experimental data, currently prevalent models, which are primarily based on the Flory–Rehner theory (F–R theory), are unable to correctly capture the effect of water content (or conversely polymer fraction) on the elastic modulus of hydrogels. Lake–Thomas theory cannot provide correct predictions on fracture toughness with different water content conditions as well. In this work, we carry out experiments on polyacrylamide (PAAm) gel and discover scaling-laws that differ significantly in the swollen and dehydrated state in addition to contradicting F–R model. We also derive scaling laws that are consistent with our experiments. Intriguingly, we find that the application of the scaling theory to fracture problems of the hydrogel can also provide a better theoretical prediction. An intriguing implication of this result is that the study of the fracture threshold of soft matter may be replaced to some extent by merely the studying of their elastic modulus.