Complicated phase behavior and ionic conductivities of PVP- co-PMMA-based polymer electrolytes

Abstract

We have used DSC, FTIR spectroscopy, and ac impedance techniques to investigate the interactions that occur within complexes of poly(vinylpyrrolidone- co-methyl methacrylate) (PVP- co-PMMA) and lithium perchlorate (LiClO 4) as well as these systems' phase behavior and ionic conductivities. The presence of MMA moieties in the PVP- co-PMMA random copolymer has an inert diluent effect that reduces the degree of self-association of the PVP molecules and causes a negative deviation in the glass transition temperature ( T g). In the binary LiClO 4/PVP blends, the presence of a small amount of LiClO 4 reduces the strong dipole–dipole interactions within PVP and leads to a lower T g. Further addition of LiClO 4 increases T g as a result of ion–dipole interactions between LiClO 4 and PVP. In LiClO 4/PVP- co-PMMA blend systems, for which the three individual systems—the PVP- co-PMMA copolymer and the LiClO 4/PVP and LiClO 4/PMMA blends—are miscible at all compositional ratios, a phase-separated loop exists at certain compositions due to a complicated series of interactions among the LiClO 4, PVP and PMMA units. The PMMA-rich component in the PVP- co-PMMA copolymer tends to be excluded, and this phenomenon results in phase separation. At a LiClO 4 content of 20 wt% salt, the maximum ionic conductivity occurred for a LiClO 4/VP57 blend (i.e., 57 mol% VP units in the PVP- co-PMMA copolymer).