Cation-Exchange Porous Hollow-Fiber Membranes Prepared by

Strongly acidic cation-exchange porous hollow-fiber membranes with a high water permeability and a high affinity for sodium ion were prepared. Cross-l...
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Ind. Eng. Chem. Res. 2002, 41, 5686-5691

Cation-Exchange Porous Hollow-Fiber Membranes Prepared by Radiation-Induced Cografting of GMA and EDMA Which Improved Pure Water Permeability and Sodium Ion Adsorptivity Kaori Saito, Kyoichi Saito,* and Kazuyuki Sugita Department of Materials Technology, Chiba University, Inage, Chiba 263-8522, Japan

Masao Tamada and Takanobu Sugo Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute, Takasaki, Gunma 370-1292, Japan

Strongly acidic cation-exchange porous hollow-fiber membranes with a high water permeability and a high affinity for sodium ion were prepared. Cross-linked polymer chains, consisting of ethylene dimethacrylate (EDMA) as a cross-linker and glycidyl methacrylate (GMA) as a precursor monomer for sulfonation, were appended onto a porous hollow-fiber membrane by radiation-induced graft polymerization. The degree of cross-linking, which is defined by the molar content of EDMA in a monomer solution of EDMA and GMA, ranged from 3 to 12%. For example, the cation-exchange membrane immobilizing un-cross-linked graft chains exhibited a negligible pure water flux, whereas that immobilizing cross-linked graft chains with a degree of crosslinking of 6% had a 1.2-fold higher pure water flux than the original hollow-fiber membrane. Sodium ions were captured during the permeation of the sodium chloride solution through the cation-exchange membranes. The capturing efficiency for sodium ion, which is defined as the ratio of breakthrough capacity to equilibrium capacity, increased with an increase in the degree of cross-linking. Introduction The quality of ultrapure water used in IC manufacturing affects the yield and reliability of the products obtained. With the advancement in the degree of packaging density, a higher quality of ultrapure water, the alkali ion content of which is lower, is desired for fewer defects and less contamination; therefore, the development of functional membranes with a better performance is necessary. Hori et al.1 suggested the application of microfilters with functional groups to the removal of trace amounts of ions dissolved in ultrapure water through chelate formation and ion-exchange reactions. Imino diacetate, sulfonic acid, and quaternary ammonium groups as the functional group were introduced to polymer chains grafted onto the pore surface of a porous polyethylene hollow-fiber membrane, which is commercially available for the microfiltration of colloidal substances and microorganisms. The introduction of sulfonic acid groups as the functional group to the graft chains causes a drastic decrease in the permeability of pure water because of the expansion of the charged graft chains induced by electrostatic repulsion. Cross-linking of the charged graft chains is a possible countermeasure to this drawback. The methods for cross-linking grafted chains include ionic crosslinking (Figure 1a), radiation cross-linking (Figure 1b), and cografting of cross-linkers with precursor monomers for functionalization (Figure 1c). Ionic cross-linking is achieved by reacting a solution containing di- and trivalent metal ions with the negatively charged sulfonic acid groups of the graft chain; * Corresponding author. Tel/fax: +81-43-290-3439. Email: [email protected].

for example, Sasagawa et al.2 cross-linked graft polymer chains with magnesium ions. Osada et al.3 demonstrated that the poly(methacrylic acid) branches grafted onto a porous flat-sheet membrane were cross-linked by the addition of di- and trivalent metal ions such as Cu2+ and Cr3+. However, ionic cross-linking is not favorable for ultrapure water production for the following two reasons: (1) sodium ions cannot be removed from pure water by the graft chains because of its lower selectivity than di- and trivalent ions bound to the SO3H groups of the graft chains, and (2) heavy metal ions repel the di- and trivalent ions from the graft chains to dissolve in pure water. On the other hand, radiation cross-linking is not practical in that irradiation for cross-linking the graft chains, in addition to that for initiating the graft polymerization, lowers the mechanical strength of the polymer membranes even at a low cross-linking density.4 When the charged graft chains are prepared by the graft polymerization of reactive monomers followed by a polymer reaction to introduce ion-exchange groups such as phosphoric acid5 and diethylamino,6 cografting of cross-linkers representative of divinylbenzene (DVB) improves liquid permeability and reduces swelling. Sodium ion adsorptivity of the cation-exchange porous membrane is required to produce a higher quality of ultrapure water for IC manufacturing. As for the cationexchange beads based on styrene-DVB copolymers, a polymer network structure with a higher density of DVB cross-linking achieved a higher selectivity of sodium ions over hydrogen ions. Cross-linked structures provide more accessible space for hydrated cations.7 Therefore, the cografting of a cross-linker and a precursor monomer

10.1021/ie010438n CCC: $22.00 © 2002 American Chemical Society Published on Web 10/15/2002

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Figure 1. Cross-linking of a polymer chain grafted onto the pore surface: (a) ionic cross-linking; (b) radiation cross-linking; (c) cografting of cross-linkers.

will increase both pure water permeability and sodium ion adsorptivity also in the case of cation-exchange porous membranes. Two advantages of the porous hollow-fiber membranes over the conventional polymeric beads made of styrene-DVB copolymer are as follows: (1) ions are transportable to the neighborhood of the functional group via convective flow or permeation flow driven by a transmembrane pressure, and (2) the length required for the ions to diffuse into the cross-linked graft chains of about 0.1 µm is much shorter than the radius of the polymeric beads. The objective of this study was threefold: (1) to prepare sulfonic-acid-group-containing graft chains crosslinked with a cross-linker onto a porous hollow-fiber membrane by radiation-induced cografting, (2) to determine pure water permeability as a function of sulfonic acid group density, and (3) to verify the increase in sodium ion selectivity of the resultant cationexchange porous hollow-fiber membrane in the permeation mode. In this study, ethylene dimethacrylate and glycidyl methacrylate were used as the hydrophilic cross-linker and the precursor monomer for sulfonation, respectively. Experimental Section Reagents. A porous hollow-fiber membrane (Asahi Kasei Corp.) was used as the trunk polymer for grafting. This membrane made of polyethylene had inner and outer diameters of 2.0 and 3.0 mm, respectively, with a pore diameter of 0.36 µm and a porosity of 71%. Technical-grade glycidyl methacrylate (GMA, CH2d CCH3COOCH2CHOCH2) and ethylene dimethacrylate (EDMA, CH2dCCH3COOCH2CH2OOCCCH3dCH2) were purchased from Tokyo Kasei Co. and Shin-Nakamura Chemical Industrial Co. and used without further purification. Sodium sulfite was purchased from Nacalai Tesque. Other reagents were of analytical grade or higher. Preparation of SO3H-Group-Containing CrossLinked Graft Chains. The preparation scheme of the cross-linked polymer chains containing a sulfonic acid group (-SO3H) grafted onto a porous hollow-fiber membrane is shown in Figure 2. This scheme consists of the following four steps. (1) Irradiation of an electron

beam: the original hollow-fiber membrane was irradiated with an electron beam of 200 kGy dose in a nitrogen atmosphere at ambient temperature. (2) Cografting of EDMA with GMA: the irradiated hollowfiber membrane was immersed in a solution of EDMA and GMA at 313 K for 5-17 min. Methanol was used as the solvent. The composition of EDMA/GMA/methanol was set at a volume ratio of VEDMA/10/90, where VEDMA ranged from 0 to 2.0. The volume ratio of the monomer liquid to the original hollow-fiber membrane was in excess. The degree of cross-linking (x) and the degree of cografting (dg) are defined as

x ) 100(WEDMA/198)/(WGMA/142 + WEDMA/198) dg ) 100(W1 - W0)/W0

(1) (2)

where WEDMA and WGMA are the weights of EDMA and GMA, respectively, in a mixture of EDMA/GMA/ methanol. The numbers 198 and 142 are the molecular masses of EDMA and GMA, respectively. W0 and W1 are the weights of the original and EDMA/GMAcografted hollow-fiber membranes, respectively. The resultant porous hollow-fiber membrane is referred to as the GMA(x/dg) membrane. (3) Introduction of the sulfonic acid group:8 the GMA(x/dg) membrane was immersed in a solution of Na2SO3/2-propanol/water (in a weight ratio of 10/15/75). The SO3H group density (ds) was calculated from the weight gain as follows:

ds ) [(W2 - W1)/82]/W2

(3)

where W2 is the weight of the hollow fiber after sulfonation and the number 82 is the molecular mass of H2SO3. (4) Hydrophilization of the remaining epoxy group: the hollow-fiber membrane was immersed in 0.5 M H2SO4 at 353 K for 2 h. The resultant porous hollowfiber membrane is referred to as the SS(x/dg/ds) membrane. After the GMA(x/dg) membrane was dried under reduced pressure, the IR spectra were measured with a micro FT-IR spectrophotometer (Perkin-Elmer Co., Spectrum One) to examine the distribution of poly-GMA and -EDMA chains across the membrane thickness. The aperture size was 100 × 100 µm2, and the measurement region was scanned in 100 µm steps.

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Figure 2. Preparation scheme of cation-exchange porous hollow-fiber membranes by radiation-induced cograft polymerization of EDMA with GMA and subsequent modifications.

Measurement of Pure Water Flux (PWF). The permeability of pure water through the porous hollowfiber membrane was determined using the experimental apparatus illustrated in Figure 3. The hollow-fiber membrane was positioned in a U-shaped configuration. Pure water prepared through the ultrafiltration membrane was permeated radially outward through the pores of the membrane at a constant transmembrane pressure of 0.1 MPa at ambient temperature. The effluent penetrating the hollow-fiber membrane was continuously collected. In the permeation experiment, the PWF based on the inside surface area of the hollowfiber membrane was calculated as follows:

PWF ) (permeation rate)/ (inside surface area of the hollow fiber) (4) where the inner diameter of the membrane was measured with a microscope. The PWF ratio was defined as the ratio of PWF of the SS(x/dg/ds) membrane to that of the original porous hollow-fiber membrane. Determination of Breakthrough Curves. Breakthrough curves, i.e., changes of the sodium ion concentration as a function of the accumulative effluent volume, were determined using an apparatus similar to that shown in Figure 3. A 15 mg of Na/L of sodium

Figure 3. Experimental apparatus for the determination of PWF and the breakthrough curve of sodium ion.

chloride solution was permeated from the inside surface of the SS(x/dg/ds) membrane of about 3 cm length outward through the pores of the membrane at a constant permeation pressure of 0.1 MPa. The effluent penetrating the membrane was continuously collected using fraction vials. The sodium ion concentration in each fraction was determined using an atomic absorption spectrophotometer (Hitachi A-2000). From the curves, the breakthrough ion-exchange capacity (qB), equilibrium ion-exchange capacity (qE), and capturing efficiency (η) of the sodium ion were

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Figure 4. Degree of cografting of EDMA and GMA vs reaction time. Figure 6. Distribution of polymer chains grafted onto the porous hollow-fiber membrane across the membrane thickness.

Figure 5. Sulfonation of EDMA/GMA-cografted hollow-fiber membranes. Figure 7. PWF ratio as a function of the SO3H group density.

calculated as

qB )

∫0V (C0 - C) dV/W2

(5)

qE )

∫0V (C0 - C) dV/W2

(6)

η ) 100qB/qE

(7)

B

E

where C0 and C are the sodium ion concentrations of the feed and the effluent, respectively. V is the effluent volume, and VB and VE are the effluent volumes when C reaches 10% of C0 and C0, respectively. Results and Discussion Properties of SO3H-Group-Containing Porous Hollow-Fiber Membranes. The degree of cografting is shown in Figure 4 as a function of the reaction time in a monomer mixture of EDMA and GMA. Swelling of the membrane with increasing reaction time enhanced the diffusion of the monomers into the matrix, resulting in a convex curve of the degree of cografting vs reaction time. The cografting rate decreased with an increase in the amount of EDMA in the monomer mixture. Similarly, in a cografting system of GMA and DVB, the crosslinker DVB retarded the cografting rate.6 The epoxy group of the graft chain is converted into the SO3H group by reacting with sodium sulfite. Time courses of the SO3H group density, defined by eq 3, for various degrees of cross-linking are shown in Figure 5. Sulfonation was impeded by the increase in the degree of cross-linking. This can be explained by the facts that

the cografting of EDMA reduces the epoxy group density of the graft chain and that the resultant cross-linked polymer network grafted onto the porous hollow-fiber membrane obstructs the access of the reagents. The IR profile across the membrane thickness is shown in Figure 6. The peak at 2918 cm-1 is assigned to the C-H bonds of polyethylene, GMA, and EDMA. The peak at 1148 cm-1 corresponds to the ester bonds of GMA and EDMA. A uniform profile of the ester bond was observed for the degrees of cross-linking of 3 and 6%, whereas a higher degree of cografting was observed only on both sides of the GMA(12/100) membrane. The retardation of the grafting rate in the presence of EDMA can induce a nonuniform distribution of the graft chain across the membrane thickness. Permeability of Cation-Exchange Porous Hollow-Fiber Membranes. The high permeability of pure water is one of the critical requirements for effective metal removal during liquid permeation across the cation-exchange porous membrane. The grafting of polymer chains in the absence of cross-linkers onto a microfiltration membrane and the subsequent introduction of ion-exchange groups induced a drastic decrease in liquid permeability.3 The PWF ratios of the SS(x/dg/ ds) membrane to the original porous hollow-fiber membrane are shown in Figure 7 as a function of the SO3H group density. The SS(0/130/ds) membrane, i.e., uncross-linked cation-exchange membrane, exhibited a negligible PWF ratio at SO3H group densities of 0.4 mol/ kg and higher. In contrast, the PWF ratios of the crosslinked cation-exchange membranes decreased as the

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Table 1. Previous Studies on Cross-Linked Ion Exchangers cross-linker

degree of cross-linkinga (mol %)

matrix

functional group

analyte

researcher

DVBb TAICc-DVB DVB glutaraldehyde EDMAf

3-13 15-20 4-16 0.5-4.2 13-12

polystyrene poly-GMAd polystyrene chitosan PEg-GMA

sulfonic acid diethylamino phosphoric acid amine sulfonic acid

cation BSAe Eu3+, Fe3+ Pd2+ Na+

Bonner and Smith7 Yu and Sun10 Alexandratos and Natesan11 Ruiz et al.12 this study

a Degree of cross-linking is equivalent to the molar content of the cross-linker in the monomer solution. b Divinylbenzene. c Triallyl isocyanurate. d Glycidyl methacrylate. e Bovine serum albumin. f Ethylene dimethacrylate. g Polyethylene.

Figure 8. Breakthrough curves of sodium ion for the SS(x/dg/ds) membranes.

SO3H group density increased in the range of 0-0.7 mol/ kg to reach a minimum and increased again thereafter; for example, the SS(6/100/1.5) membrane had a PWF ratio of 1.2. The liquid permeability is governed by the balance between a decrease in the effective pore size caused by the extension of the graft chain from the pore surface and an increase in the pore size caused by the swelling of the membrane matrix. We reported the same tendency in the flux vs the degree of GMA grafting without cross-linking for the chelating porous hollow-fiber membrane.9 This result can also be explained by the balance between the pore volume reduction and the matrix volume swelling accompanied by grafting. Sodium Ion Uptake during Liquid Permeation. Breakthrough curves for the sodium ion were determined in the permeation mode. Dimensionless breakthrough curves are shown in Figure 8 to compare the breakthrough performance among the SS(x/dg/ds) membranes with various qE values. The dimensionless time, τ, is the ratio of the amount of sodium ion fed from the inside surface of the membrane to that of sodium ion bound in equilibrium with the feed concentration; the ordinate is the relative sodium ion concentration of the effluent to the feed, C/C0.

τ ) (C0Qt)/(W2qE)

(8)

where Q is the flow rate of the solution. The higher degree of cross-linking resulted in a more favorable breakthrough curve for practical application of the cation-exchange porous hollow-fiber membrane to the removal of metal ions from ultrapure water. Breakthrough ion-exchange capacity qB, equilibrium ion-exchange capacity qE, and capturing efficiency η, defined by eqs 5-7, respectively, are shown in Figure 9. The value of qE was constant up to the degree of crosslinking of 6% and then decreased, whereas qB increased

Figure 9. Breakthrough and equilibrium ion-exchange capacities and capture efficiency of sodium ion vs degree of cross-linking.

up to the degree of cross-linking of 6% and then decreased. With an increasing degree of cross-linking, the value of η increased and leveled off at the degree of cross-linking of 6%. The quantity of η reflects the selectivity of sodium ions over hydrogen ions at an almost identical SO3H group density. The cross-linked polymer network prepared by cografting EDMA with GMA and subsequent sulfonation produced micropores that are more selective for sodium ion than hydrogen ion, compared to the un-cross-linked polymer structure. This phenomenon has been reported in conventional ionexchange beads based on cross-linked polymer networks such as the styrene-DVB copolymer, as summarized in Table 1. For example, Bonner and Smith7 reported that the selectivity for various cations was improved by an increase in the degree of cross-linking. Here, we succeeded in preparing a highly selective polymer network grafted onto a porous hollow-fiber membrane while maintaining the permeability of the resultant membrane such that it is comparable to that of an original membrane for microfiltration. Conclusions To remove trace amounts of sodium ions from ultrapure water, cation-exchange porous hollow-fiber membranes containing sulfonic acid groups were prepared by cografting a cross-linker with a precursor monomer for sulfonation. Sulfonation of poly(glycidyl methacrylate) chains grafted onto a porous hollow-fiber membrane allowed the expansion of the polymer chains, which, in turn, decreased the permeability drastically. In contrast, cografting of ethylene dimethacrylate as a cross-linker with glycidyl methacrylate restricted the expansion of the graft chains even after sulfonation. Cross-linked graft chains containing sulfonic acid groups with a higher degree of cross-linking exhibited higher selectivity for sodium ions than hydrogen ions while maintaining the high permeability of pure water.

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Acknowledgment The authors thank Noboru Kubota and Kohei Watanabe of the Industrial Membranes Divison of Asahi Kasei Corp. for providing the original porous hollowfiber membrane. Helpful discussion with Takahiro Hori of Asahi Kasei Corp. is gratefully acknowledged. Literature Cited (1) Hori, T.; Hashino, M.; Omori, A.; Matsuda, T.; Takasa, K.; Watanabe, K. Synthesis of novel microfilters with ion-exchange capacity and its application to ultrapure water production systems. J. Membr. Sci. 1997, 132, 203. (2) Sasagawa, N.; Saito, K.; Sugita, K.; Sugo, T. Ionic crosslinking of SO3H-group-containing graft chains helps to capture lysozyme in a permeation mode. J. Chromatogr. A. 1999, 848, 161. (3) Osada, Y.; Honda, K.; Ohta, M. Control of water permeability by mechanochemical contraction of poly(methacrylic acid)grafted membranes. J. Membr. Sci. 1986, 27, 327. (4) Saito, K.; Kaga, T.; Yamagishi, H.; Furusaki, S.; Sugo, T.; Okamoto, J. Phosphorylated hollow fibers synthesized by radiation grafting and cross-linking. J. Membr. Sci. 1989, 43, 131. (5) Saito, K.; Ito, M.; Yamagishi, H.; Furusaki, S.; Sugo, T.; Okamoto, J. Novel hollow-fiber membrane for the removal of metal ion during permeation: preparation by radiation-induced cografting of a cross-linking agent with reactive monomer. Ind. Eng. Chem. Res. 1989, 28, 1808.

(6) Sunaga, K.; Kim, M.; Saito, K.; Sugita, K.; Sugo, T. Characteristics of porous anion-exchange membranes prepared by cografting of glycidyl methacrylate with divinylbenzene. Chem. Mater. 1999, 11, 1986. (7) Bonner, O. D.; Smith, L. Selectivity scale for divalent cations on Dowex 50. J. Phys. Chem. 1957, 61, 326. (8) Kim, M.; Saito, K. Radiation-induced grafted polymerization and sulfonation of glycidyl methacrylate on to porous hollow-fiber membranes with different pore sizes. Radiat. Phys. Chem. 2000, 57, 167. (9) Yamagishi, H.; Saito, K.; Furusaki, S.; Sugo, T.; Ishigaki, I. Introduction of a high-density chelating group into a porous membrane without lowering the flux. Ind. Eng. Chem. Res. 1991, 30, 2234. (10) Yu, Y.; Sun, Y. Macroporous poly(glycidyl methacrylatetriallyl isocyanurate-divinylbenzene) matrix as an anion-exchange resin for protein adsorption. J. Chromatogr. A 1999, 855, 129. (11) Alexandratos, S. D.; Natesan, S. Ion-selective polymersupported reagents: the principle of bifunctionality. Eur. Polym. J. 1999, 35, 431. (12) Ruiz, M.; Sastre, A. M.; Guibal, E. Palladium sorption on glutaraldehyde-cross-linked chitosan. React. Funct. Polym. 2000, 45, 155.

Received for review May 14, 2001 Revised manuscript received December 19, 2001 Accepted August 13, 2002 IE010438N