Understanding the Dissociation of Hydrogen Bond Based Cross-Links In Hydrogels Due to Hydration and Mechanical Forces

The submersion of a polymer network with a high density of hydrogen bond based cross-links in an aqueous bath results in the formation of a rubberlike hydrogel. The cross-links, which connect chains and maintain the structure of the network, can dissociate as a result of two main factors: (1) the interactions between the hydrogen bonds and the surrounding water molecules and (2) the forces that are exerted on the cross-link from the interconnected chains. We recall that these forces depend on the ratio between the end-to-end distance and the overall contour length of the chains. The application of an external loading translates into the stretching of chains in the network and, as a consequence, an increase in the forces that are exerted. Sufficiently large external loadings can break cross-links and alter the microstructure of the network. From a macroscopic viewpoint, the breaking of cross-links leads to significant microstructural changes such as a reduction in the chain density and an entropic gain. In this work, we derive a microscopically motivated and energy-based model that captures the effects of hydration and external loadings on the overall changes in properties and mechanical response of hydrogels. We provide a systematic method to estimate the percentage of cross-links that dissociate due to hydration and application of a force and provide representative examples to highlight the influence of the orientations of the interconnected chains with respect to a cross-link. To validate the model, we carry out uniaxial tension experiments on poly(methacrylic acid) hydrogels with five different water contents and fit the model parameters. We show that the model is capable of qualitatively and quantitatively capturing the experimental results. The model is also used to approximate the number of cross-links that dissociate due to the water and the externally applied uniaxial force. This work paves the way to the efficient design of multifunctional synthetic hydrogels with programmable properties and behaviors.