B. Chen, I. Saito, R.J. Sheridan, K.J. Kwon, L.C. Brinson, M.D. Dickey
Duke University,
United States
Keywords: glass, gel, viscoelasticity, AFM, mechanical mapping
Summary:
Glassy gels—organic polymer networks swollen by ionic liquids—have recently attracted attention due to their unique combination of glass-like rigidity and network-like elasticity.[1] They exhibit high strength, a modulus on the order of gigapascals, and unexpectedly strong adhesion. Atomic force microscopy (AFM) provides a powerful platform for probing the mechanical behavior of complex soft materials such as nanocomposites, biological tissues, and hydrogels. To characterize the distinctive surface properties of glassy gels, we conducted systematic AFM-based mechanical measurements across a range of temperatures, indentation speeds, and force setpoints. Force–volume mapping revealed that the surface modulus of the glassy gel exceeds 1 GPa—an order of magnitude higher than that of thiol-based photopolymers. Moreover, the glassy gel exhibited a unique force–distance response characterized by pronounced time-dependent behavior and strong adhesion forces, features absent in conventional glassy polymers such as polystyrene or PMMA and in thiol-crosslinked photopolymers. Additionally, we found that the surface of the glassy gel becomes soft and viscoelastic upon contact with various substrates, regardless of surface chemistry. We propose that this dynamic interfacial softening—arising from unprecedented interactions between the polymer and ionic liquid—balances strong adhesion with high bulk stiffness. Our findings highlight the distinctive mechanical adaptability of glassy gels and open new questions about the complex interplay between polar polymers and ionic liquids. [1] Wang, M., Xiao, X., Siddika, S., Shamsi, M., Frey, E., Qian, W., Bai, W., O’Connor, B. T., & Dickey, M. D. (2024). Glassy gels toughened by solvent. Nature, 631(8020), 313–318. https://doi.org/10.1038/s41586-024-07564-0