Hydrogels are cross-linked polymers containing water and are used in medical fields, robotics, and energy storage. Hydrogels tend to be soft materials due to their high-water content which has motivated research to find ways to tune their mechanical properties. Poly-vinyl alcohol (PVA) is one common hydrogel where its mechanical properties can be changed by using different salt solutions. This is accomplished by a freeze-thaw method where the PVA is initially frozen and then thawed in differing salt solutions which influences the overall structure.<br/><br/>In this study, we used the ReaxFF reactive forcefield to investigate PVA soaked in lithium chloride and potassium chloride. ReaxFF is based on a bond-order formalism and trained against DFT data allowing the forcefields to accurately describe chemical reactions while performing molecular dynamics simulations. ReaxFF has successfully been used to describe polymers and salt-water systems making it appropriate to use in this study. Based on the Hoffmeister effect and experimental results, we know that the PVA soaked in potassium chloride will result in a tougher hydrogel. It was hypothesized that lithium might promote a proton transfer from the PVA hydroxyl group, therefore not allowing the PVA to hydrogen bond with itself forcing the PVA to dissolve back into solution. Potassium, on the other hand, would not promote a proton transfer, allowing a stronger network of hydrogen bonds to form therefore creating a tougher PVA hydrogel. By using ReaxFF based molecular dynamics simulations, we were able to show a proton transfer was favorable in the lithium case and unfavorable in the potassium case. We then performed a compression and expansion test to determine the mechanical properties of the salt soaked PVA structures. The ReaxFF results agree with experimental results and have been able to provide atomistic detail to the tunable mechanical properties in PVA hydrogels based on salt solutions.

Differing mechanical properties in PVA hydrogels due to lithium and potassium ions described by reactive forcefield based molecular dynamics simulations

ACS Fall 2021

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