Ionic Heterogeneity Regulates Mechanical Deformation of Lithium Salt-Containing Polyacrylates at Comparable Glass Transition Temperatures

Polymer networks near the glass transition temperature (Tg) exhibit mechanical behavior governed by intrinsic chain dynamics and local structural heterogeneity. In polymers derived from multiple monomer systems, Tg spans a broad temperature range due to differences in molecular structure and intermolecular interactions, making it difficult to compare deformation behavior under identical thermal proximity to Tg. Since Tg reflects the characteristic timescale of segmental relaxation, controlling Tg provides a direct way to position materials within a comparable dynamical regime relative to experimental timescales. Here, we show that incorporation of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) enables chemically distinct polyacrylate networks to converge to a narrow near-ambient Tg window, despite an initial Tg variation of approximately 20 °C. This Tg convergence establishes a unified segmental-dynamics regime across different polymer compositions. Within this framework, copolymerization between ethyl acrylate and 2-[(butylcarbamoyl)oxy]ethyl acrylate is used to systematically tune ion coordination and local structural heterogeneity without significantly affecting Tg. Although Tg is comparable, the materials exhibit pronounced differences in tensile deformation, including multi-stage yielding and wide variations in yield and fracture stresses and ductility. In situ SAXS reveals void formation at early deformation stages followed by structural reorganization associated with secondary yielding and stress whitening. These results demonstrate that local structural heterogeneity governs deformation behavior in near-glassy polymer networks when Tg is constrained, providing a basis for structure–property correlation under a fixed dynamical window.