The sample preparation pathway of solid polymer electrolytes (SPEs) influences their thermal properties, which in turn governs the ionic conductivity of the materials especially for systems consisting of a crystallizable constituent. Majority of poly(ethylene oxide) (PEO)‐based SPEs with molar masses of PEO well above 104 g mol−1 (where PEO is crystallizable and should reach an asymptote in thermal behaviour) display molar mass dependence of the thermal properties and ionic conductivities in non‐equilibrium conditions, as reported in the literature. In this study, PEO of different viscosity‐molar masses (Mη = 3 × 105, 6 × 105, 1 × 106, 4 × 106 g mol−1) and LiClO4 salt (0 to 16.7 wt%) were used. The SPEs were thermally treated under inert atmosphere above the melting temperature of PEO and then cooled down for subsequent isothermal crystallization for sufficient experimental time to develop morphology close to equilibrium conditions. The thermal properties (e.g. glass transition temperature, melting temperature, crystallinity) according to differential scanning calorimetry and the ionic conductivity obtained from impedance spectroscopy at room temperature (σDC ∼ 10−6 S cm−1) demonstrate insignificant variation with respect to the molar mass of PEO at constant salt concentration. These findings are in agreement with the PEO crystalline structures using X‐ray diffraction and ion − dipole interaction by Fourier transform infrared results. © 2017 Society of Chemical Industry

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Lack of precise thermal control in poly(ethylene oxide) (PEO)‐based solid polymer electrolytes (SPEs) with molar masses well above M > 104 g mol−1 during preparatory stage often demonstrates molar mass dependence on thermal properties and ionic conductivities. In the earlier study, PEO‐based SPEs were heated under inert atmosphere above the melting temperature of PEO and then cooled down for subsequent isothermal crystallization. The system demonstrates insignificant variation with respect to molar mass of PEO at constant salt concentration for thermal properties, ionic conductivity and intermolecular interaction. Here, we report the subsequent results of rheology and microscopic heterogeneity as a function of PEO molar mass (Mɳ = 3 × 105 and 4 × 106 g mol−1) and lithium salt concentration (0–13 wt.%). Above the solubility limit of PEO, the glass transition studied using differential scanning calorimetry shows the presence of microscopic heterogeneity of the systems as indicated by the change in enthalpy of endothermic overshoot near the endset of glass transition. Rheological parameters were measured over a wide range of frequency (0.01 to 100 Hz) at different temperatures (30, 60 and 80 °C) using parallel plate geometry. Melt rheology system reveals that neat PEO at 80 °C exhibits restriction in the long‐range motion of the chains. When incorporated with salt, it exhibits further deviation from the terminal viscoelastic relaxation behaviour. This study agrees with our previous findings that influence of molar mass of PEO in this range is insignificant on the melt rheology and microscopic heterogeneity at constant salt concentration.

Thermal properties and ionic conductivity of epoxidized natural rubber (ENR)‐based solid polymer electrolytes (SPEs) are influenced by their sample preparatory pathways. In literature, there are inconsistencies reported on conductivity values in the range of 10−8–10−9 S cm−1 due to different sample preparatory pathways for same systems at constant salt concentration. In this study, simple thermal treatment was adopted on SPE films for regulating the thermal properties of the materials which in turn to govern the conductivity of the materials. ENR (with 25 and 50 mol% epoxidation) and lithium perchlorate salt (0–20 wt.%) were used. The ENR‐based SPE films were thermally treated with three different annealing times (i.e. t = 0, ½, 24 h) at 80 °C under nitrogen atmosphere. Differential scanning calorimetry and impedance spectroscopy were utilized to analyze the thermal properties and ionic conductivity at room temperature, respectively. The glass transition parameters such as glass transition temperature, change in heat capacity, width of glass transition and change in enthalpy of endothermic overshoot were elucidated. Higher glass transition temperature values are noted for ENR‐based SPEs films with longer annealing time, which may imply higher extent of intermolecular interaction between ENR and salt, thus leading to an increase in ionic conductivity of ∼10−10–10−8 S cm−1 for t = 0 and 24 h, respectively at mass fraction of salt, WS = 0.107. Consequently, it is deemed important to have precise control in the thermal treatment of ENR‐based SPEs during their preparatory stage.

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