1986
Chem. Mater. 1999, 11, 1986-1989
Characteristics of Porous Anion-Exchange Membranes Prepared by Cografting of Glycidyl Methacrylate with Divinylbenzene Keisuke Sunaga, Min Kim, Kyoichi Saito,* and Kazuyuki Sugita Department of Materials Technology, Chiba University, Inage, Chiba 263-8522, Japan
Takanobu Sugo Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute, Takasaki, Gunma 370-1292, Japan Received July 13, 1998. Revised Manuscript Received May 26, 1999
Porous anion-exchange membranes of a flat-sheet form were prepared by cografting of an epoxy group containing monomer, glycidyl methacrylate (GMA) and a cross-linker, divinylbenzene (DVB), onto a porous polyethylene membrane, followed by the introduction of a diethylamino (DEA) group. A monomer mixture of GMA and DVB containing 1-10 mol % of DVB provided a DVB-cross-linked graft chain ranging from 15 to 45 mol % of DVB. After the introduction of the DEA group, the swelling of the membrane in water was reduced. The results of determinations of protein binding capacity and the elution percentage of bovine serum albumin (BSA) demonstrated that the GMA/DVB graft chains hinder protein binding in the multilayer and capture the protein via a hydrophobic interaction. The stability of the size of the DVB-cross-linked anion-exchange membrane was proven.
Introduction A functionalized porous membrane is a promising material for the removal of toxic metal ions and the recovery of valuable proteins in various fields, such as semiconductor and pharmaceutical industries. Convection-aided separation using functionalized porous membranes has advantages over diffusion-controlled separation using functionalized beads, in that the convective flow of target ions and molecules to the neighborhood of the functional groups of the polymer chains grafted onto the pore minimizes the diffusional masstransfer resistance.1,2 We have so far collected metal ions and proteins using porous chelate-forming 3 and ion-exchange4-6 membranes of a hollow-fiber form. Yamagishi et al.3 developed a porous iminodiacetatetype chelating membrane for removing trace amounts of heavy metal ions from ultrapure water. Kubota et al.7 experimentally demonstrated that porous ionexchange membranes had a higher dynamic, i.e., prac* Corresponding autho: Kyoichi Saito, Department of Materials Technology, Faculty of Engineering, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan. Telephone/fax: +81-43-290-3439. Email:
[email protected]. (1) Brandt, S.; Goffe, R. A.; Kessler, S. B.; O’Connor, J. L.; Zale, S. E. Bio/Technology 1988, 6, 779. (2) Shinano, H.; Tsuneda, S.; Saito, K.; Furusaki, S.; Sugo, T. Biotechnol. Prog. 1993, 9, 193. (3) Yamagishi, H.; Saito, K.; Furusaki, S.; Sugo, T.; Ishigaki, I. Ind. Eng. Chem. Res. 1991, 30, 2234. (4) Tsuneda, S.; Shinano, H.; Saito, K.; Furusaki, S.; Sugo, T. Biotechnol. Prog. 1994, 10, 76. (5) Tsuneda, S.; Saito, K.; Furusaki, S.; Sugo, T. J. Chromatogr. A 1995, 689, 211. (6) Matoba, S.; Tsuneda, S.; Saito, K.; Sugo, T. Bio/Technology 1995, 13, 795. (7) Kubota, N.; Miura, K.; Saito, K.; Sugita, K.; Watanabe, K.; Sugo, T. J. Membr. Sci. 1996, 117, 135.
tical, binding capacity for proteins and exhibited a higher throughput for protein recovery compared to a conventional column charged with ion-exchange beads. In addition, a linear scale-up of protein recovery was readily achieved by bundling the hollow fibers to form a hollow-fiber membrane module.8 Functional porous membranes of a flat-sheet form instead of a hollow-fiber form are promising in terms of ease of module fabrication and economic feasibility. Introduction of a high density of functional groups into the flat-sheet membranes is required, while maintaining the physical and chemical stability of the material. The radiation-induced graft polymerization of precursor monomers and the subsequent functionalization step are suitable preparation schemes to meet this requirement.9 Graft chains containing ion-exchange groups expand or shrink depending on the ionic strength, pH, and kind of solvent used.10-13 However, size changes due to swelling are unsuitable for flat-sheet ion-exchange membranes; the stability of the membrane size to various environmental changes contributes to the ease of module fabrication. An appropriate degree of crosslinking of the graft chains will be effective in reducing the swelling.14 (8) Kubota, N.; Konno, Y.; Saito, K.; Sugita, K.; Watanabe, K.; Sugo, T. J. Chromatogr. A 1997, 782, 159. (9) Kim, M.; Kiyohara, S.; Konishi, S.; Tsuneda, S.; Saito, K.; Sugo, T. J. Membr. Sci. 1996, 117, 33. (10) Osada, Y.; Honda, K.; Ohta, M. J. Membr. Sci. 1986, 27, 327. (11) Ito, Y.; Kotera, S.; Inaba, M.; Kono, K.; Imanishi, Y. Polymer 1990, 31, 2157. (12) Mika, A. M.; Childs, R. F.; Dickson, J. M.; McCarry, B. E.; Gagnon, D. R. J. Membr. Sci. 1995, 108, 37. (13) Mika, A. M.; Childs, R. F.; Dickson, J. M.; McCarry, B. E.; Gagnon, D. R. J. Membr. Sci. 1997, 135, 81.
10.1021/cm980490i CCC: $18.00 © 1999 American Chemical Society Published on Web 07/24/1999
Porous Anion-Exchange Membranes
Chem. Mater., Vol. 11, No. 8, 1999 1987
Figure 1. Preparation scheme for a porous anion-exchange membrane by radiation-induced cografting of GMA with DVB, and subsequent chemical modifications. Table 1. Reaction Conditions of Cograft Polymerization composition of monomer solution molar ratio of DVB to GMA in monomer mixture volume ratio of monomer mixture to methanol reaction temperature reaction time
1/99-10/90 40/60 313 K 1-24 h
The objective of this study was 2-fold: (1) to prepare novel porous anion-exchange membranes by the cografting of an epoxy group containing monomer and a crosslinker onto a porous flat-sheet membrane followed by the introduction of an anion-exchange group and (2) to investigate the swelling and protein binding characteristics of the resultant membranes. Experimental Section Materials. A porous flat-sheet membrane (Asahi Chemical Industry Co., Tokyo, Japan) was used as a trunk polymer for grafting. This membrane was made of high-density polyethylene (PE), 100 µm thick. The membrane had a threedimensional pore network with a porosity of 70% and a nominal pore diameter of 0.4 µm. Glycidyl methacrylate (GMA, CH2dCCH3COOCH2CHOCH2) and divinyl benzene (DVB, CH2dCHC6H4CHdCH2) were purchased from Tokyo Chemical Industry Co. (Tokyo, Japan) and used without further purification. Bovine serum albumin (BSA) was obtained from Sigma (St. Louis, MO), and dissolved in 0.02 M Tris-HCl buffer (pH 8). The other chemicals used were of analytical grade. Cografting of GMA with DVB. The preparation scheme for a porous anion-exchange membrane is illustrated in Figure 1. The reaction conditions for cograft polymerization are summarized in Table 1. A 10 cm × 10 cm PE porous membrane was irradiated with an electron beam from a cascade-type accelerator (Dynamitron, IEA 3000-25-2, Radiation Dynamics Co.) in a nitrogen atmosphere at ambient temperature. The total irradiation dose was 50 kGy. The irradiated membrane was immersed in a monomer solution which had previously been deaerated with nitrogen. The monomer solution contained GMA or a mixture of GMA and DVB in methanol. After cograft polymerization, the DVB mole % in the monomer mixture remained almost constant because the amount consumed by the cograft polymerizaton was negligible compared to the amount of the feed monomer. (14) Egawa, H.; Nonaka T. J. Appl. Polym. Sci. 1985, 30, 3239.
Figure 2. Determination of epoxy group density in the graft chain. After a predetermined length of time, the membrane was taken out and washed repeatedly with methanol. The degree of grafting is defined as follows:
degree of grafting (%) ) (weight of GMA/DVB-cografted polymer chain) 100 ) (weight of trunk polymer) (W1 - W0) (1) 100 W0 where W0 and W1 are the weights of the trunk and GMA/DVBcografted membranes, respectively. The epoxy group was determined by the HCl-dioxane method. For the GMA-grafted membranes with different degrees of grafting, the epoxy group density determined by the HCl-dioxane method is compared with that calculated by the weight gain in Figure 2. An excellent agreement between the two values validated the quantitative determination of the epoxy group density of the graft chain. The density of DVB in the graft chain was calculated by subtracting the epoxy group density from the total amount. The resultant GMA/DVBcografted membrane is referred to as a G/D(dg, x) membrane,
1988 Chem. Mater., Vol. 11, No. 8, 1999
Figure 3. Degree of grafting as a function of reaction time for various DVB mole % in the monomer mixture.
Sunaga et al.
Figure 4. Molar densities of GMA and DVB in the graft chain vs degree of grafting.
where dg and x in parentheses represent the degree of grafting and the DVB mole % in the graft chain, respectively. Introduction of Anion-Exchange Group and Determination of Swelling Ratio. The G/D(160, 45) membrane was immersed in a 50 v/v % diethylamine aqueous solution at 303 K up to 24 h. The number of DEA groups introduced was determined by titration. The resultant anion-exchange membrane is referred to as the G/D-AE(160, 45, y) membrane, where y is the conversion of the epoxy group to a diethylamino (DEA) group. For comparison, the GMA-grafted membrane, the G(170) membrane, was reacted with diethylamine. The resultant membrane is referred to as the G-AE(170, y) membrane. The conversion of the epoxy group to the DEA group is defined as
conversion (%) ) (moles of the DEA group after reaction) 100 (2) (moles of the epoxy group before reaction) The remaining epoxy groups of the G/D-AE and G-AE membranes were reacted with ethanolamine at 303 K for 1 h. The swelling ratio is defined as the surface area ratio of the wet membrane size to the dry membrane size, where the wet membrane size was measured with a scale after 1 h immersion of the dry membrane in water at ambient temperature. BSA Adsorption and Elution. BSA binding capacity was determined in the batch mode: the G/D-AE or G-AE membrane was immersed in a 3 g/L BSA buffer solution (pH 8.0) at 303 K. After equilibration, the BSA concentration in the solution was determined by measuring UV absorbance at 280 nm. The amount of that BSA adsorbed onto the membrane was evaluated from the decrease in BSA concentration in the solution. After repeated washing of the membrane with water, the membrane was soaked in 0.5 M NaCl to elute the protein. The elution percentage is defined as
elution percentage (%) ) (amt of BSA eluted) 100 (3) [(amt of BSA adsorbed) - (amt of BSA washed)]
Results and Discussion DVB Mole % in the Graft Chain. The degree of grafting (dg) defined by eq 1 ranged from 76 to 160% for grafting times ranging from 4 to 24 h for the 5 mol % DVB monomer mixture. An increase in divinylbenzene (DVB) mole % in the monomer solution decreased the cografting rate, as shown in Figure 3. An example of the molar densities of glycidyl methacrylate (GMA)
Figure 5. The DVB content in the graft chain vs that in the monomer mixture.
and DVB units in the graft chain is shown in Figure 4 as a function of dg. A linear increase in both units was observed for the 5 mol % DVB monomer mixture: the DVB content in the graft chain was constant irrespective of dg. The DVB content in the graft chain is shown in Figure 5 as a function of the DVB content in the monomer mixture. A larger amount of DVB is incorporated into the polymer chain grafted onto a polyethylene (PE) matrix than that contained in the monomer mixture; for example, 5 mol % of DVB in the monomer mixture provided 45 mol % of DVB in the graft chain. Conversion of Epoxy Group to a Diethylamino Group. The time course of conversion of the epoxy group to the diethylamino (DEA) group for the GMA/ DVB-cografted, G/D(160, 45) membrane is shown in Figure 6 along with the results for the GMA-grafted, G(170) membrane. The presence of DVB as a crosslinker among the graft chains retarded the introduction of DEA groups because of steric hindrance. The G/DAE(165, 45, 50) membrane had a DEA group density of 1.7 mol per kg, which was comparable to that of a conventional nonporous ion-exchange membrane. The resultant membranes can be useful as size-stable porous anion-exchange membranes. Swelling Ratio. In Figure 7 the swelling ratio is compared between G/D-AE(160, 45, y) and G-AE(120, y)
Porous Anion-Exchange Membranes
Figure 6. Comparison of introduction of diethylamino (DEA) group between cross-linked and non-cross-linked GMA-grafted membranes.
Figure 7. Swelling ratio of membrane as a function of conversion.
membranes as a function of conversion, y. Apparently, the size change of the G/D-AE membrane in the wet state was reduced: the maximum swelling of the G/DAE membrane was 3%, whereas the G-AE membrane exhibited a swelling ratio of 10%. Cografting of a precursor monomer, namely GMA, for the introduction of the functional group, and a cross-linker, i.e., DVB, was effective in reducing the swelling. The cross-linking of the graft chains by vapor-phase cografting of GMA and DVB was reported to be capable of retaining the pure-water permeability of the porous hollow-fiber membrane after the conversion of some epoxy groups into phosphoric acid groups.15 Crosslinking contributes to the suppression of expansion of the polymer chains emerging from the pore surface toward the pore interior as well as a reduction of swelling of the polymer chains invading the matrix. BSA Adsorption Characteristics. To examine the expansion and shrinkage behavior of the graft chains, the BSA binding capacity was determined, because the expansion of the graft chains induces multilayer binding of the protein. Moreover, the elution percentage of the adsorbed protein with 0.5 M NaCl is a measure of the hydrophilicity/hydrophobicity of the graft chain-protein interface. (15) Saito, K.; Kaga T.; Yamagishi, H.; Furusaki, S.; Sugo, T.; Okamoto, J. J. Membr. Sci. 1989, 43, 131.
Chem. Mater., Vol. 11, No. 8, 1999 1989
Figure 8. Comparison of BSA binding capacity between crosslinked and non-cross-linked anion-exchange membranes.
Figure 9. Comparison of elution percentage of BSA between cross-linked and non-cross-linked anion-exchange membranes.
First, the binding capacity of the G-AE(140, y) membrane for BSA increased with increasing conversion, whereas that of the G/D-AE(160, 45, y) membrane remained constant irrespective of the conversion (Figure 8). This result demonstrates that the graft chains of the G-AE membrane expand due to mutual electrostatic repulsion and hold more protein molecules than those which could be adsorbed as a monolayer on the interface5,6 and that the graft chains of the G/D-AE membrane do not permit entry of the protein. Second, the elution percentages of G-AE(140, y) and G/D-AE(160, 45, y) membranes show a marked difference (Figure 9): the BSA adsorbed onto the G-AE membrane was quantitatively eluted with 0.5 M NaCl, whereas almost no BSA adsorbed onto the G/D-AE membrane was eluted. This is ascribed to the difference in the interactions between the graft chains and BSA. BSA was adsorbed based on electrostatic and hydrophobic interactions onto the G-AE and G/D-AE membranes, respectively. Acknowledgment. We thank Dr. Noboru Kubota of the Industrial Membranes Development Department of Asahi Chemical Industry Co., Ltd., Japan, for providing the starting original porous polyethylene membrane. CM980490I
