Effect of ethylene carbonate on proton conducting polymer electrolyte based on poly(ε-caprolactone) (PCL)

• Addition of EC reduces the DSC melting enthalpy and glass transition temperature.
• High dielectric constant value of EC causes the high dissociation rate of ions.
• Close coupling between the conductivity and the polymer segment mobility
• XRD shows that plasticizing effect reduces the degree of crystallinity.

Polymer electrolytes (PE) comprised of poly(ε-caprolactone) (PCL), ethylene carbonate (EC) and ammonium thiocyanate (NH4SCN) were prepared by solution casting. Thermal properties, conductivity and structural studies were characterized by differential scanning calorimetry (DSC), impedance spectroscopy and X-ray diffraction (XRD), respectively. EC is intended to be used as plasticizing solvent to enhance the conductivity. It is found later that EC can also promote ion dissociation in polymer electrolyte system. From DSC studies, the introduction of EC as additive into PCL-NH4SCN complexes reduces the Tm from 56.4 °C to 44.1 °C, drops Tg from − 59.7 °C to − 76.7 °C and suppresses the crystallinity. The conductivity of PE improves from 4.6 × 10− 8 S cm− 1 to 3.8 × 10− 5 S cm− 1 at 50 wt.% EC. Relative permittivity studies show that EC contributes to higher dissociation rate of charge ions. In the temperature dependence conductivity analysis, VTF equation fits very well with regression value close to unity for all the samples demonstrating a close coupling between the conductivity and the polymer segment mobility. The pre exponential factor A increases as more EC is added, reflecting the growth in the number of charge carriers. Structural analysis using XRD displays an apparent reorganization of polymer matrix due to the EC effect. This is in agreement with DSC and EIS observations. Degree of crystallinity calculated using XRD two phase model shows a decrease from 32.7% of zero EC to 16.0% of 50 wt.% EC. These results are comparable to the DSC analysis.