Polymer electrolytes through functionalization of poly (poly (ethylene glycol) methacrylate) with zwitterionic pendant groups: The role of ion clusters upon conductivity

Abstract

The present study proposes a complete (pMPS8) and partial (pMPS8-co-PEGDMA) functionalization of poly(poly(ethyleneglycol) methyl ether methacrylate (pPEGMA) with zwitterionic pendant groups to improve the mobility of polymer chains and ion-ion associations during Li+ conduction (weak electrolyte). Structural, thermal and electrochemical characterizations of the polymer electrolytes are conducted to analyze the dependence of conductivity on LiPF6 concentration. The ionic conductivity of pMPS8 (3.22 × 10−5 S cm−1 at 25 °C) in the presence of LiPF6 is due to the incorporation of the zwitterionic pendant groups into the pPEGMA. In pMPS8, a more significant interaction of Li+ with the sulfobetaine group at low LiPF6 concentrations, decreases its ion movement. While at higher salt concentrations, the saturation of sulfonate groups with Li+ makes possible its interaction with ethoxy sites, thus, increasing ionic conductivity. pMPS8 is less sensitive (i.e. ionic conductivities remain constant) to the increases of salt concentration compared to the pMPS8-co-PEGDMA (larger number of ethoxy sites per pendant groups) due to its complete functionalization (short range mobility). The ionic conductivity for pMPS8-co-PEGDMA polymer starts to be significant at molar ratios higher than 1.0: 1.0, suggesting that above this stoichiometry the interactions between zwitterion pendant groups and lithium salt become suppressed, enabling its use at higher LiPF6 concentrations. The analysis of TGA, DSC, XRD and EIS exhibit that ionic conductivity is coupled with the mobility of chains, hence the transport mechanism occurs through dynamic percolation theory where the translational dynamics of the ions depends on segmental dynamics and by renewal of hopping probabilities.