Modification of transport properties of ion exchange membranes: XIV. Effect of molecular weight of polyethyleneimine bonded to the surface of cation exchange membranes by acid–amide bonding on electrochemical properties of the membranes

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Abstract

In order to prepare a cation exchange membrane having excellent monovalent cation permselectivity, a thin polyethyleneimine (PEI) layer was formed on one surface of the membrane by sulfonyl–amide bonding. The electrochemical properties of the cation exchange membranes, which had been prepared by the reaction of an aqueous PEI solution with chlorosulfonylated membranes, were examined in accordance with the number of the successive PEI reaction batches. The monovalent cation permselectivity decreased with the increase in the number of reaction times of the reused PEI solution. The smaller molecular weight of PEI was found to be consumed during the reaction, which means that PEI with higher molecular weight has difficulty reacting with the chlorosulfonylated membrane.

Introduction

Ion exchange membranes have been widely used in various industrial fields, electrodialytic concentration of seawater to produce edible salt, desalination of brackish water, diffusion dialysis to recover acids and alkalis from waste acids and alkalis, separators for electrolysis such as chlor-alkali production, etc.

In the electrodialytic concentration of seawater, ion exchange membranes having a low electrical resistance and a monovalent ion permselectivity are required to save energy and to avoid scaling of alkaline earth metal compounds, e.g. CaSO4. Especially, the most effective and simplest method to prepare a monovalent cation permselective membrane is to adsorb or ion-exchange a cationic polyelectrolyte such as PEI on the surface of the cation exchange membranes [1], [2], [3], [4], [5], [6], [7], [8], [9]. However, the monovalent cation permselectivity of the membranes gradually deteriorates during electrodialysis because the PEI was desorbed from the membrane surface. Thus, fixation of the cationic polyelectrolyte on the membrane surface by covalent bonding has been actively studied. One of the effective methods is to fix PEI on the membrane surface by acid–amide bonding [10], [11], [12], [13], [14]. Namely, after introducing sulfonyl chloride groups into the membranous polymer composed of styrene–divinylbenzene by reaction with chlorosulfuric acid, thin PEI layers were formed on the membrane surfaces by sulfonyl–amide bonding. A copolymer membrane composed of styrenesulfonyl chloride and divinylbenzene was also used as a precursor for this reaction [15]. Though these membranes showed excellent monovalent cation permselectivity, the current efficiency in electrodialytic seawater condensation was low, i.e. about 5–8% compared with that of the membranes without PEI layers and the electrical resistance was also high [16]. These properties were thought to be due to the existence of the PEI layers on both surfaces of the membrane. Also a broad molecular weight distribution of PEI might also affect the electrochemical properties of the membrane. In general, the reactivity between the cross-linked chlorosulfonylated membranes and an aqueous PEI solution is thought to be restricted by the properties of the membrane (the density of reactive groups on surfaces, the pore size of the membrane, the swelling degree, etc.) and the PEI (molecular weight distribution, branching). These factors determine the degree of the formation of acid–amide bonding on the membrane surface and consequently determine the monovalent cation permselectivity, the durability and the electrical resistance of the membrane. A portion of a polymer having a higher molecular weight is reported to adsorb selectively on the solid surface from the polymer solution when the polymer has a wide molecular weight distribution. On the other hand, others have reported that a polymer having a low molecular weight initially adsorbs on the solid surface. PEI is highly branched and has a wide molecular weight distribution. When PEI is reacted with the cross-linked polymeric membrane containing sulfonyl chloride groups, the effect of molecular weight on the reaction rate is interesting to investigate. In this study, to clarify the effective molecular weight of PEI for providing monovalent cation permselectivity to the cation exchange membrane, the same PEI solution was used several times to modify the chlorosulfonylated membrane surface. The change in the molecular weight distribution of PEI in the used solution and the electrochemical properties of the obtained cation exchange membranes were then evaluated.

Section snippets

Membrane

The precursor membrane for the cation exchange membrane was prepared by the ‘paste method’ [17], [18]. That is, a mixture of styrene (90 parts) and commercial divinylbenzene (10 parts) was copolymerized in the presence of dioctylphthalate (30 parts), benzoylperoxide (1 part), polyvinyl chloride (paste resin, 50 parts) and polyvinylchloride fabric as the support material. The membrane thickness was about 0.11–0.16 mm.

Reagents

Chlorosulfuric acid, sulfuric acid, sodium hydroxide, sodium chloride, calcium

Relationship between the electrochemical properties of the cation exchange membranes and the number of reactions using the same PEI reaction solution

Fig. 3 shows the relationship between the electrochemical properties of the resultant membranes and the number of the reaction batches. The electrical resistance of the cation exchange membrane was 3–4 Ω cm2 and decreased with the number of the reactions. The reaction of sulfonyl chloride groups with PEI was thought to be difficult with the increase in the number of reactions. The electrical resistance of the unmodified cation exchange membrane was about 2.3 Ω cm2. The increase in the electrical

Conclusions

Chlorosulfonylated membranes were selectively reacted with a smaller molecular weight of PEI and the larger molecular weight portion remained in the reaction solution. Therefore, the smaller molecular weight PEI determined the electrochemical properties of the membrane. The molecular weight of 18,000–20,000 of PEI (the converted molecular weight of PEG) was found to be essential for modifying the chlorosulfonylated membrane for excellent monovalent permselectivity.

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