Understanding the mechanisms of lithium-ion transport in polymers is crucial for the design of polymer electrolytes. We combine modular synthesis, electrochemical characterization, and molecular simulation to investigate lithium-ion transport in a new family of polyester-based polymers and in poly(ethylene oxide) (PEO). Theoretical predictions of glass-transition temperatures and ionic conductivities in the polymers agree well with experimental measurements. Interestingly, both the experiments and simulations indicate that the ionic conductivity of PEO, relative to the polyesters, is far higher than would be expected from its relative glass-transition temperature. The simulations reveal that diffusion of the lithium cations in the polyesters proceeds via a different mechanism than in PEO, and analysis of the distribution of available cation solvation sites in the various polymers provides a novel and intuitive way to explain the experimentally observed ionic conductivities. This work provides a platform for the evaluation and prediction of ionic conductivities in polymer electrolyte materials.

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We perform a joint experimental and computational study of ion transport properties in a systematic set of linear polyethers synthesized via acyclic diene metathesis (ADMET) polymerization. We measure ionic conductivity, σ, and glass transition temperature, T g, in mixtures of polymer and lithium bis­(trifluoro­methane­sulfonyl)­imide (LiTFSI) salt. While T g is known to be an important factor in the ionic conductivity of polymer electrolytes, recent work indicates that the number and proximity of lithium ion solvation sites in the polymer also play an important role, but this effect has yet to be systematically investigated. Here, adding aliphatic linkers to a poly­(ethylene oxide) (PEO) backbone lowers T g and dilutes the polar groups; both factors influence ionic conductivity. To isolate these effects, we introduce a two-step normalization scheme. In the first step, Vogel–Tammann–Fulcher (VTF) fits are used to calculate a temperature-dependent reduced conductivity, σr(T), which is defined as the conductivity of the electrolyte at a fixed value of T – T g. In the second step, we compute a nondimensional parameter f exp, defined as the ratio of the reduced molar conductivity of the electrolyte of interest to that of a reference polymer (PEO) at a fixed salt concentration. We find that f exp depends only on oxygen mole fraction, x 0, and is to a good approximation independent of temperature and salt concentration. Molecular dynamics simulations are performed on neat polymers to quantify the occurrences of motifs that are similar to those obtained in the vicinity of isolated lithium ions. We show that f exp is a linear function of the simulation-derived metric of connectivity between solvation sites. From the relationship between σr and f exp we derive a universal equation that can be used to predict the conductivity of ether-based polymer electrolytes at any salt concentration and temperature.

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Polymer multifunctionality can be designed through the incorporation of chemical groups termed "mechanophores" that have a specific chemical transformation in response to applied force. The behavior of mechanophorelinked polymers depends on synthetic factors such as the choice of the mechanophore, the polymer chemistry, and the mechanophore linking architecture and on externally imposed factors such as temperature, loading mode, and loading rate. While many papers have explored changing polymer architecture, relatively few have systematically looked at these external factors, particularly temperature and loading mode. These external factors are critical for practical application of mechanophore-linked polymers, particularly for damage detection in engineering materials. Here, we use a single synthetic system to quantify the influence of these externally imposed factors. In particular, the mechanophore spiropyran (SP) is covalently bonded into lightly cross-linked poly(methyl methacrylate) (PMMA). SP is a mechanophore that has a distinct color and fluorescence change when activated through force to the merocyanine state, making it ideal for in situ studies. We monitor and analyze the full field fluorescence of SP-PMMA samples during mechanical loading under tension and compression, over 3 decades of strain rate, and over a 60°C range in temperature. Typical SP mechanoactivation response exhibits three distinct regimes: minimal change through yield, followed by rapid intensity increase, and approach to a steady state. Stress has a strong influence on the rate of increase in SP activation, where stress increase by temperature decrease or strain rate increase substantially raises the SP activation rate. Uniaxial compression displays a qualitatively similar response to that of uniaxial tension. However, a longer flat region is observed in the case of compression as compared to tension corresponding with the larger yield strain.

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