Dielectric properties and conductivity of styrene-methacrylic acid copolymers and partially neutralized salts

Dielectric and conductivity measurements have been carried out on styrene-methacrylic acid copolymers and their salts in the glass transition region, stressing the comparison of polymers neutralized to different extents. A simple monomer-dimer equilibrium between carboxylic acid groups is applicable to the pure acid polymers, which explains the increase of both the glass transition temperature and of the dielectric strength with concentration of methacrylic acid. The dielectric data indicate that very few sodium carboxylate groups can dissolve in the matrix of the pure salt, whereas the presence of carboxylic acid groups in partly neutralized polymers enhances the dissolution of the salt groups in the matrix. Considerable differences are observed between the partly neutralized polymer and the blend consisting of pure acid and pure salt copolymers, which suggests that exchange of cations between acid and salt groups is quite slow even at high temperatures. The conductivity of the ionomers appears to be related to the diffusion of carriers through the matrix.