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

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