Bipolar Ion-Exchange Membranes Prepared from Blends of Graft Copolymers

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Abstract
Bipolar ion-exchange membranes were prepared from graft copolymers of polyethylene (PE). Powders of styrene-grafted PE (I) and 4-vinylpyridine-grafted PE (II), or powders of ethyl acrylate-grafted PE (III) and II were blended in various proportions and melt-pressed to films. Ion-exchange groups were then introduced by the sulfonation (100%), alkaline hydrolysis (100%), or N-methylation (80–90%) of the blend polymer films. Although, in the titration curves of bipolar membranes, pH regions which apparently resemble the isoelectric point of polyampholytes appeared, no behavior based on the interaction between acidic and basic groups was indicated in the survey of their physico-chemical properties. The dependency of the concentration potential of the membranes (Em), containing one kind of ionized group, on the mole fraction of the group (Fi) is not remarkable. On the other hand, the Em of the membranes containing both positively- and negatively-charged groups is strongly dependent on Fi, because of the additive contribution of the oppositely-charged groups to the potential. The bi-ionic potential of the membrane, EK–Li, was found to be a function of Em⁄|E0|(|E0| is the absolute value of the concentration potential for ideal permselective membranes) in the cases of both I-II and II-III.

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