Solvent Exchange-Induced Microphase Separation and Structural Arrest to Form Glassy Hydrogels

Abstract

Glassy hydrogels with high stiffness and toughness have been developed in recent years by forming dense associative interactions in a gel matrix. There are several reports on forming robust hydrophobic associations with microphase separation during the solvent exchange process to convert elastic organogels to glassy hydrogels. However, the microstructure formation during the solvent exchange process and the mechanism accounting for rubbery-to-glassy transition of the gels remain unclear. In this study, we copolymerize hydrophobic ethylene glycol phenyl ether acrylate and hydrophilic methacrylic acid in dimethyl sulfoxide, followed by solvent exchange with water to form glassy hydrogels with microphase-separated structures. Ultrasmall- and small-angle X-ray scattering measurements are performed on the gel during the solvent exchange process at various temperatures, and structural parameters are ascertained to trace the structural evolution of the gel. A two-stage structural formation mechanism is proposed for the varying microstructure and properties of the gel during the solvent exchange process. At the initial stage, segregation of hydrophobic segments leads to microphase separation that creates a bicontinuous structure with a high-viscosity polymer-rich phase. At the late stage, the polymer-rich phase becomes vitrified, which arrests the microphase separation and produces a glassy hydrogel far from the thermodynamic equilibrium state. The metastability nature of glassy gel can be harnessed to mediate the microstructure and properties by hydrothermal treatment to reactivate the phase separation. This study provides insights into the interaction between microphase separation and vitrification that determines the structure and properties of glassy gels, which will merit the design of high-performance soft materials with phase-separated structures.