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Abstract
Linking molecular interactions to the macroscopic mechanical properties of soft materials has remained a major challenge in soft-matter science. In this work, we developed metal–polymer soft framework (MPF) ion gels, in which metal–ligand coordination complexes act as dynamic cross-linking points within a structurally homogeneous polymer network. Terpyridine-terminated TetraPEG prepolymers were cross-linked with first-row transition-metal ions in an ionic liquid medium to form uniform coordination-bonded gel networks. The experimentally observed elastic modulus (G′) increased in the order Zn2+ < Co2+ < Ni2+, which was quantitatively reproduced by modeling the equilibrium distribution of bis-terpyridine complexes, M(tpy)22+, acting as cross-linking points. By integrating complexation thermodynamics with the classical rubber elasticity relation, we established a quantitative model that not only determines the stepwise equilibrium constants (K1 and K2) governing the formation of mono- and bis-terpyridine complexes but also accurately predicts the non-monotonic dependence of G′ on metal-ion concentration. Furthermore, the stress relaxation time (τ) exhibited an exponential dependence on the coordination bond strength. An analysis based on an Arrhenius-type relation, in which the activation energy (Ea) scales with the bond strength, indicates that macroscopic stress relaxation in the MPF ion gels is governed by a bond-activation process, leading to M–tpy bond dissociation. These findings establish a quantitative molecular framework connecting coordination thermodynamics and kinetics with macroscopic gel elasticity, offering molecular design principles for coordination-bonded soft materials with programmable elasticity.
Supplementary materials
Title
Supporting Information
Description
This Supporting Information provides detailed experimental procedures, analytical methods, and supplementary data for the Metal–Polymer Soft Framework (MPF) ion gels. Included materials are: 1H NMR spectrum of TetraPEG-tpy (Figure S1); time-dependent evolution of G′ and G″ during gelation (Figure S2); SAXS profiles (Figure S3); frequency-dependent viscoelastic moduli (Figure S4); photograph of TetraPEG-tpy gelation with Li ions (Figure S5); equilibrium species distribution curves (Figure S6); calculated concentration profiles of bis-terpyridine complexes (Figure S7); storage modulus of the chemically cross-linked ion gel (Figure S8); time-dependent UV–vis spectra and kinetic analysis (Figure S9); strain-dependent oscillatory shear measurements (Figure S10); and optimized geometries of M–tpy complexes (Figure S11). All data and methods required to reproduce the results in the main manuscript are provided.
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