A Constitutive Model for Binary-Solvent Gels

Abstract

A hydrogel is a network of polymeric chains hosting a large amount of the single solvent, namely, water. The high degree of hydration not only endows hydrogels with desired attributes such as superb biocompatibility but it also yields disadvantages, including high volatility and inability to host hydrophobic drugs. The need for enhancing the versatility of hydrogels to meet requirements of diverse applications has led to the fabrication of binary-solvent gels (e.g., gels in aqueous ethanol) with the hope to capitalize on both the merits of water and other organic solvents. In this paper, to understand the fundamental mechanics of binary-solvent gels, we develop a constitutive model by formulating the free energy function based on the extended Flory–Huggins lattice theory and deriving the equilibrium equations. We then apply the model to examine the mechanical behaviors of binary-solvent gels under mechanical forces, or subject to geometric constraints. The model can consistently capture some experimental findings on binary-solvent gels such as the cononsolvency effect. In particular, we employ the model to analyze a bilayer soft actuator consisting of a binary-solvent gel film attaching to a passive polymer substrate. The proposed model may provide insights into the design of novel soft machines based on binary-solvent gels.