Synthesis and characterization of poly(ethylene glycol)-based single-ion conductors

A series of ionomers was synthesized by melt polycondensation of poly(ethylene glycol) (PEG) oligomers and dimethyl 5-sulfoisophthalate sodium salt. The molar mass of the PEG spacer was either 400, 600, or 900, and the cation was exchanged from sodium to lithium or cesium by dialysis. Since the anions are covalently attached to the ionomer chains and are essentially immobile relative to the cations, these ionomers are excellent models for polymeric single-ion electrolytes. The experimental evidence to date suggests that the cations do not aggregate to form the usual ion clusters seen in other ionomers. No relaxation time associated with ion clusters was observed in rheological measurements, nor was an "ionomer peak" observed in small-angle X-ray scattering measurements. The ionic conductivity increases significantly with increasing PEG spacer molecular weight, although the total cation content decreases at the same time. At room temperature, the highest conductivity (10 ^-6 S/cm) was achieved for the sodium ionomer with PEG spacer M _n = 900, similar to other single-ion polymer electrolytes. The various ionomers, although with significantly different cation type and concentration, have almost the same ionic conductivity at the same temperature interval from the corresponding glass transition temperature. This strongly supports that ion migration in these ionomers is closely correlated with the segmental mobility of the polymer matrix.