In this study, diffusion of water into ethylene ionomers neutralized with sodium or zinc cations (Na‐ionomer and Zn‐ionomer, respectively) was studied by in situ Fourier transform near infrared (NIR) spectroscopy. Water ingress was measured by NIR from the edge of the laminated glass toward the center and the measured water concentration profiles were analyzed by Fick's second law or the modified Jacobs–Jones model, which describes diffusion into a two‐phase system of different density and polarity. It was found that the modified Jacobs–Jones model agrees well with the measured water concentration profiles of ionomers, while the Fickian model fits well to the poly(vinyl‐butyral) (PVB) results. The modified Jacobs–Jones model was also confirmed to apply to water vapor transmission rate (WVTR) data of another sodium ionomer (Na‐ionomer‐HTM) with a higher melting temperature than Na‐ionomer. The water diffusion coefficient for Na‐ionomer, Zn‐ionomer, and PVB are compared, and the diffusion parameters are discussed considering the higher‐ordered structures of ionomers studied by small‐angle X‐ray scattering (SAXS). Simulating conditions that PV modules encounter in field applications, model projections show that Zn‐ionomer and Na‐ionomer‐HTM give significantly lower water permeation than PVB, and due to its low diffusion coefficient, Na‐ionomer will have significantly less water ingress than PVB. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48929.

This study investigates the properties and structure of poly(ethylene‐co‐methacrylic acid) (EMAA) ionomers co‐neutralized by zinc and sodium, and compares them with ionomers neutralized by a single metal: zinc (EMAA‐Zn) and sodium (EMAA‐Na). The bending stiffness and the degree of crystallization for the co‐neutralized ionomer (Na:Zn = ~1:1 on a molar basis) gives 16.5% and 9.8% increase compared to the simple average of their single cation counterparts. Such unusual properties are seen in mechanical, thermal, and rheological properties and water uptake rate in this study. Infrared spectra of the co‐neutralized ionomer shows changes at the carboxylate antisymmetric stretching band, which correspond to the formation of carboxylates bridging zinc and sodium ions. X‐ray scattering identified that the number density of ionic multiplets for the co‐neutralized ionomers are equivalent to EMAA‐Na and ~ 1.7 times higher than EMAA‐Zn. The fraction of side groups forming ionic multiplets, β, in co‐neutralized ionomers were 0.7–0.8, close to EMAA‐Na (1.0) and higher than EMAA‐Zn (0.3), indicating that sodium promote zinc to form ionic multiplets. The synergistic properties induced by co‐neutralization are ascribed not only to a formation of new ionic multiplets where both cations are included but also to the change in the number density of ionic multiplets.

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