Solubility of Methacrylic Acid Alkali Metal Salts in Polyethylene Oxide and Cation Conductivity after Polymerization

Methacylic acid alkali metal salts (MAAM; M = Li, Nu, K, Rb, or Cs) were dissolved in polyethylene oxide, and in situ polymerization was carried out. The solubility of these salt monomers in PEO oligomers increased with the ionic radius of the cation. Their solubility decreased with an increase in the average molecular weight of PEO oligomers, suggesting the considerable contribution of terminal hydroxyl groups of PEO to the solubilization of these salt monomers. The polymerization of MAAM in PEO induced a slight increase of turbidity. The PEO (molecular weight = 200) system containing P(MAAM) shows an ionic conductivity of 1-5 x 10(-4) S/cm depending on the cation species. The P(MAAM)/PEO systems having larger cations such as Rb or Cs as the mobile ion show higher ionic conductivity, and a clear relationship was obtained between conductivity and cation radius. This tendency cannot be explained by the ordinary migration model of ions as being either naked or solvated. The fact that the systems having larger cations show higher conductivity can be explained by the following two reasons: the interaction force with the polymer matrix is weaker, and the dissociation energy is lower. The former reason should be dominant, because considerable differences in the ionic conductivity cannot be explained fully by differences in the lattice energy only. Similar experiments were carried out in polyl(oligo-oxyethylene) methacrylatel (P(MEO)). However, the ionic conductivity of the product remained at around S/cm, the observed tendency between the ionic conductivity and the cation radius was quite similar, suggesting the same mechanism as in PEO.