Ion–Polymer Interactions Control Energetic Elasticity in TetraPEG Ion Gels

Energetic elasticity arising from polymer–solvent interactions governs the mechanical response of polymer networks; however, its molecular origin in ion gels, i.e., polymer networks swollen with ionic liquids (ILs), remains poorly understood. Here, we investigated the role of ion–polymer interactions in tetra-arm poly(ethylene glycol) (TetraPEG) ion gels prepared in ILs. By separating the elastic modulus into energetic ( G E ) and entropic ( G S ) contributions, we found that the ion gels exhibit negative energetic elasticity, similar to TetraPEG hydrogels. Comparative experiments using imidazolium- and pyrrolidinium-based ILs revealed that hydrogen bonding between PEG ether oxygen atoms and the imidazolium cations enhances the energetic contribution, whereas non-hydrogen-bonding pyrrolidinium cations provide only a minor contribution. Addition of Li salt decreased the shear modulus and changed both G E and G S through preferential Li + –PEG complexation, replacing hydrogen-bonding solvation and yielding more compact polymer conformations. At high Li salt concentrations, coordination of most ether oxygens to Li + suppresses conformational changes of the polymer chains upon deformation, leading to nearly constant G E and G S . These findings strongly support the view that the elasticity of ion gels is governed by molecular-level ion–polymer interactions and provide guidelines for tuning the mechanical properties of polymer gel electrolytes through ion coordination.