Barrier Properties of Ethylene Ionomers after Sodium and Barium Ion Exchange in Aqueous Solution

Abstract This study investigates gas transport through ethylene ionomers when a high degree of neutralization characterizes the pristine material. Poly(ethylene-co-methacrylic acid) (or PEMAA) is chosen as the precursor and is converted into an ionomer with sodium or barium counterions in aqueous solutions of the corresponding metal hydroxides. The ion exchange protocol follows a diffusion-reaction mechanism. FTIR spectroscopy and calorimetry are used to characterize the polymer’s degree of neutralization and crystallinity. It is shown that the preparation method does not deplete the degree of crystallinity as it would occur during cooling from the melt or during solvent evaporation after dissolution. In the case of barium, the degree of crystallinity with respect to the pristine copolymer is only reduced by 13% at the most. The thermal analysis confirms that the divalent metal ion stabilizes the primary crystalline structure after annealing at 65 °C. Mass transport is investigated at 1 atm and 25, 30, and 35 °C following the classic solution-diffusion model. Metal ions reduce the permeability of pure gases such as carbon dioxide, oxygen, and nitrogen. The diffusivity and solubility contributions to the permeability are evaluated to describe the permeability reduction. Carbon dioxide interacts with the metal ions and is partially immobilized during permeation. The diffusivity reduction overcomes the solubility increase, causing a decrease in permeability. For homonuclear species such as oxygen and nitrogen, the interaction is absent and the decrease in solubility reduces the permeability. The proposed preparation method can be applied to ethylene ionomer coatings in the packaging industry to enhance the barrier properties while also improving mechanical properties.