Modeling of Thermo-Responsive Stiffening of Poly(oligo(ethylene glycol)methacrylate)–Cellulose Nanocrystal Hydrogels

Understanding the phase transition-induced stiffening of thermo-responsive hydrogels is crucial for their practical applications. Here, we used theoretical models to describe the stiffening during the isochoric thermal transitions of a series of model thermo-responsive hydrogels consisting of hydrazide-functionalized poly(oligo(ethylene glycol)methyl ether methacrylate) (POEGMA-H) and oxidized cellulose nanocrystals. Beyond the transition temperature, theoretical models showed that the magnitude of the force required to extend a collapsed chain in a poor solvent was determined by the exposed surface area of the chain and interfacial tension between the polymer and solvent. Storage modulus measurements of the hydrogels with different molar percentage of OGEMA300 monomers confirmed the role of the exposed surface area of the chain in the solvent on the thermal stiffening of thermo-responsive hydrogels. The reduction in the magnitude of thermal stiffening of hydrogels in the presence of isopropyl alcohol confirmed the effect of interfacial tension on the thermal stiffening of thermo-responsive hydrogels. The ideas presented in this study will facilitate the development of stimuli-stiffening hydrogels for biomedical applications.