DIELECTRIC-RELAXATION STUDIES ON WATER-ABSORPTION OF ETHYLENE IONOMERS

Sodium and zinc salts of ethylene-methacrylic acid (5.4 mol %) copolymer were studied by dielectric spectroscopy to clarify how water molecules are absorbed in these ionomers. The samples are denoted EMAA-xNa(or Zn)-yH2O, where x and y are degree of neutralization and molar ratio of H2O to COONa, respectively. The dielectric relaxations of EMAA and EMAA-0.6Zn, whose water absorption is usually small, were scarcely affected by water uptake. On the other hand, the dielectric relaxations of EMAA-xNa, particularly highly neutralized sodium salts, sensitively responded to water uptake. In EMAA-0.90Na-yH2O, for example, a water peak appeared near 270 K at 1 kHz in addition to the alpha and beta relaxations and gradually increased with increasing water content (y). Interestingly, the dielectric increment (DELTAepsilon) of this water peak increased more than 100-fold when y was increased from 2 to 3. At y = 3, a new peak appeared near 320 K and was independent of measuring frequency. The temperature exhibiting this peak corresponded to the order-disorder transition of ionic clusters. These results indicate that three water molecules per sodium cation form a primary hydration shell and that the excess water molecules are absorbed around that shell. Consequently, we can conclude that the ionic clusters are probably transformed into a structure reminiscent of an inverse micelle upon water absorption.