Preparation of anion-exchange membrane based on block copolymers: Part 1. Amination of the chloromethylated copolymers

doi:10.1016/S0376-7388(97)00283-4

Journal of Membrane Science

Volume 140, Issue 2, 18 March 1998, Pages 195-203

https://doi.org/10.1016/S0376-7388(97)00283-4 Get rights and content

Abstract

The anion-exchange membrane was prepared by a two-step synthesis: chloromethylation of the homemade block copolymers of polysulfone (PSf) and polyphenylenesulfidesulfone (PPSS) in the first step with subsequent amination by means of trimethylamine in the second step. The lowest measured area resistivities, equilibrated in 2 mol/dm³ KCl aqueous solution, were 4.5–3.30 Ω cm². The anion-exchange capacity reached up to 0.92 meq/(g-dry-resin), and the transport number of chlorine anion was in the 0.6–0.7 range.

Introduction

The ion-exchange membrane is now widely used in the electrodialysis for the desalination of brackish water, the production of table salt, recovery of valuable metals from the effluents of metal-plating industry, and for many other purposes.

For the purpose of applying electrodialysis process under severe conditions such as high temperatures and strongly oxidizing conditions, a more stable ion-exchange membrane should be developed. Perfluorocarbon ion-exchange membrane has been developed and successfully applied under those conditions. However, it has found only a few major industrial applications, other than in the chloralkali industry, primarily because of its high cost.

It was, therefore, the objective of this research to develop new types of ion-exchange membrane which would be cheap but also have good electrochemical properties and excellent resistance to degradation by heat and chemical attack.

Recently, engineering plastics such as polysulfone and polyethersulfone have been widely used as a base polymer for ultrafiltration, reverse osmosis and gas separation because of their excellent workability and mechanical strength 1, 2, 3, 4, 5, 6, 7, 8.

Several workers 9, 10 had earlier prepared ion-exchange membranes using polysulfone as a base polymer of the membrane. These ion-exchange membranes are likely to undergo a dimensional change during flocculation and tend to have defects, and as the ion-exchange capacity increases, the affinity to water increases, whereby it tends to hardly flocculate. Hence, it appears difficult to obtain a membrane having sufficient mechanical strength, and the dimensional stability tends to be poor [11]. The high degree of swelling to water leads to low mechanical strength because of increase in the affinity toward water. Water tends to concentrate in molecular clusters around the ionic group and, therefore, the affinity of the membrane to water increases. The high affinity to water leads to low cohesive forces, and the membrane swells quite strongly in water 11, 12, 13. The ion-exchange membrane should prevent extensive swelling in water to avoid decreasing mechanical strength.

Terada et al. 12, 13 have reported a new type of ion-exchange membrane. Their study demonstrated that an ion-exchange membrane formed by using a block copolymer having segments, to which ion-exchange groups can be introduced readily, had a higher mechanical strength as compared with the type of ion-exchange membrane formed by segments, to which ion-exchange groups can hardly be introduced. The block structure in the membrane has the following advantages: the cohesive force of the part of polymer without ionic groups controls the swelling of the membrane.

The aims of this paper are to prepare an anion-exchange membrane using block copolymer of the homemade polysulfone (PSf) and polyphenylenesulfidesulfone (PPSS), and to study its properties.

Section snippets

Copolymerization apparatus

As shown in Fig. 1, the copolymerization apparatus was composed of the following: a four-necked round-bottomed flask; an impeller of 6 blades which was used as a stirrer; a thermometer; an oil bath which was used as heat source; a condenser which was to maintain the temperature of a round-bottomed flask.

Polysulfone (PSf)

The procedure for synthesis was the same as in J.P. Pat. No. 61-168629 [14]: 0.06 mol of bisphenol A [2,2-bis(4-hydroxyphenyl)propane] was dissolved in 40 ml of N-methylpyrolidone (NMP), and then

Block copolymer preparation

Fig. 4 shows the IR spectrums of the homemade polysulfone and block copolymers. Polysulfone generally shows that the particular transmittance by the antisymmetric vibration of C–O of

is 1275–1200 cm⁻¹, the particular transmittance by skeletal vibration of –SO₂– is 1242, 1165 and 1100 cm⁻¹, and the particular transmittance by skeletal vibration of C(CH₃)₂ is 1175–1165 cm⁻¹. The particular transmittance by skeletal vibration of

of PPSS is 1502 cm⁻¹ [18].

From the IR spectrums of Fig. 4, the homemade

Conclusions

1. The anion-exchange membrane was prepared by a two-step synthesis, consisting of chloromethylation of the homemade block copolymers of polysulfone (PSf) and polyphenylenesulfidesulfone (PPSS) in the first step with subsequent amination by means of trimethylamine in the second step.

2. The lowest measured area resistivities equilibrated in 2 mol/dm³ KCl aqueous solution were in the 4.5–3.30 Ω cm² range.

3. The anion-exchange capacity reached up to 0.92 meq/g-dry-resin, and the transport number of

List of symbols

r	area resistivity in 2 M KCl aqueous solution, Ω cm²
r₁, r₂	electric resistance in 2 M KCl aqueous solution of the cell including membrane and excluding membrane, respectively, Ω
S	effective membrane area, cm²
M_O,HCl	moles of HCl in the flask at the start of titration
M_E,HCl	moles of HCl after equilibration
W	weight of dry resin, g
W_W	weight of wet resin, g
IEC	Ion-exchange capacity, meq/g-dry-resin
A_f	fixed-ion concentration, meq/g-H₂O
W_C	water content, g-H₂O/g-dry-resin
t₋	static transport number

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Preparation of anion-exchange membrane based on block copolymers: Part 1. Amination of the chloromethylated copolymers

Abstract

Introduction

Section snippets

Copolymerization apparatus

Polysulfone (PSf)

Block copolymer preparation

Conclusions

List of symbols

J. Membr. Sci.

J. Membr. Sci.

J. Memb. Sci.

J. Membr. Sci.

Polysulfone membrane. IV. Performance evaluation of radel A/PVP membranes

J.M.S.

Asymmetric membrane preparation from nonsolvent casting systems

Desalination

Microporous polyethersulphone membrane

Revue Roumaine de Chemie

Ion transport across charged ultrafiltration membrane

Memoirs of the Faculty of Science, Kyushu University Ser. C