Hydration and swelling of dry polymers for wet adhesion

Whereas two dry surfaces can adhere to each other upon contact, it is challenging to form adhesion between two wet surfaces separated by interfacial water. A commonly used strategy for adhesion of wet surfaces such as wet tissues is to let adhesives such as monomers, macromers, polymers, and particles diffuse across the interfacial water and subsequently form physical and/or chemical crosslinking with the surfaces. In contrast, a recently developed strategy for wet adhesion is to make adhesives such as dry polymer networks quickly absorb the interfacial water and then crosslink with the surfaces. Since the absorption of interfacial water requires hydration and swelling of the dry polymer networks, understanding the physics and mechanics of this concurrent hydration and swelling process will potentially facilitate the design of future adhesives for wet adhesion. In this paper, we present a combined theoretical and experimental study on dry polymer networks that absorb interfacial water for wet adhesion. We observe that the absorption of interfacial water creates a sharp hydration front in the initially dry polymer network, where the hydrated part swells by further absorption of water and the un-hydrated part maintains in the dry state. We develop a quantitative model for the coupled hydration and swelling of polymer networks governed by the Case-I diffusion and validate the model with experimental data. We finally present a guideline for the design of dry polymer networks to achieve fast, stable, and tough adhesion with wet surfaces.