Estimation of Free Volumes of Ionomers by Positron Annihilation Technique

Abstract

Positron annihilation lifetime measurement was performed from room temperature to meltimg temperature for ethylene-methacrylic acid (EMAA) and the ionomers, whose 60% of acid groups were neutralized with metallic cations such as Zn²⁺, Na⁺, and Mg²⁺, using melt reaction. The lifetime spectra were analyzed by the methods of non-linear least squares and inverse Laplace transformation. The free volume radii of EMAA and the ionomers expanded with temperature from about 0.34nm (24°C) to 0.37nm (110°C). The expansion of EMAA and the ionomers was close to that of polyethylene, but smaller than that of polypropylene, whose free volume radius expanded from about 0.34nm (24°C) to 0.38nm (60°C) and 0.40nm (100°C). Bridging structure of the ionomers prevented thermal hysterisis recognized in the relative intensity of free volume. Differences in the relative intensity of free volume were the smallest for ionomers neutralized with Zn²⁺. This proves that bridging structures of EMAA-0.6Na and EMAA-0.6Mg are more stiff than that of EMAA-0.6Zn and that the variation of free volume concentration become greater upon the elevation of temperature.