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Preparation of ion-exchange fiber fabrics by electrospray deposition
Journal of Colloid and Interface Science 293 (2006) 143–150 www.elsevier.com/locate/jcis
Preparation of ion-exchange fiber fabrics by electrospray deposition Hidetoshi Matsumoto, Yuji Wakamatsu, Mie Minagawa, Akihiko Tanioka ∗ Department of Organic and Polymeric Materials, and International Research Center of Macromolecular Science, Tokyo Institute of Technology, Mail Box S8-27, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan Received 11 May 2005; accepted 6 June 2005 Available online 12 July 2005
Abstract Ion-exchange fiber (IEF) fabrics were prepared by electrospray deposition (ESD) and post-deposition chemical modification of their surfaces. Nonwoven fibrous fabrics were obtained from the solutions of synthetic polymers—polystyrene (PS) and poly(4-vinylpyridine) (P4VP)—of various concentrations. The diameter of the fiber in the fabrics ranged from 600 nm to 1.70 µm. Cation- and anion-exchange fiber (CEF and AEF) fabrics were obtained from the sulfonation of PS fabrics and the quaternization of P4VP fabrics, respectively. These fabrics were thoroughly characterized by a series of techniques, such as scanning electron microscopy (SEM), permporometry, nitrogen adsorption measurements, and potentiometric titrations. The SEM images showed that the fabrics had a porous structure after their chemical modification. The mean pore size, porosity, and specific surface area of the flow-through pores were 1.67–3.53 µm, about 80%, and 13 m2 /g, respectively. The ion-exchange capacity was in the range from 0.78 to 1.34 mmol/g. The AEF fabric, on the other hand, showed a high specific surface area, i.e., the Brunauer–Emmett–Teller (BET) surface area of 600 m2 /g, due to the formation of much smaller pores on the surface of the fiber structure in the fabric. The secondary chemical modification of the nano-microfiber fabrics by ESD provides novel functional materials with a large adsorption capacity and a high catalytic activity. 2005 Elsevier Inc. All rights reserved. Keywords: Electrospray deposition; Ion exchange; Nanofiber fabric; Polystyrene; Poly(4-vinylpyridine)
1. Introduction Electrospray deposition (ESD) is a straightforward and versatile method for forming thin films. This method has the following advantages: (i) applicability to solute molecules which have a wide range of molecular weights (e.g., inorganic molecules, synthetic polymers, proteins, and DNA); and (ii) ability to deposit polymer thin films with nanomicroscaled structures, which range from spheres to fibers [1–5]. The ESD methods consist of the following steps: (i) a strong electric field is applied between a nozzle containing polymer solution and a conductive substrate; (ii) when the voltage reaches a critical value, the electrostatic forces overcome the surface tension of the solution; (iii) charged droplets (or jets) are sprayed from the tip of the nozzle in a dry atmosphere; and (iv) the dried droplets (or jets) are fi* Corresponding author. Fax: +81 3 5734 2876.
E-mail address: [email protected] (A. Tanioka). 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.06.022
nally collected on the substrate to form a thin film. There have been many studies on electrostatic fiber formation, called “electrospinning” [6–8]. Electrostatic fiber formation has drawn significant attention in many fields including biomaterial scaffolds for tissue engineering [9], drug delivery systems [10], and high-performance filter media [11]. The scope of ion-exchange technology has widened into the areas of biotechnology, pharmaceutical processing, production of ultrapure water for the semiconductor industry and has extended to surface modified materials for catalytic conversion processes [12]. Particularly, various ionexchange fibers (IEFs) have been successfully used due to their large specific surface, good mechanical properties, good handling characteristics, and flexibility to be processed in diverse forms [13–17]. The introduction of ionic functional groups to nano-microscaled fibrous fabrics obtained by ESD is a promising option to provide novel ion exchangers with a high adsorption capacity and a high catalytic activity. Our previous papers reported the
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preparation of ion-exchange nanofiber fabrics by ESD from biological polymers—chitosan (basic mucopolysaccharide) and chondroitin sulfate (acidic mucopolysaccharide) with poly(ethylene oxide) [5]. Polyelectrolyte solutions, however, generally show a low ESD processability, because the charging of a polyelectrolyte solution with high conductivity at the tip of the nozzle under an applied voltage tends not to occur [3]. In the present work, we attempt to prepare novel ion exchangers by the secondary chemical modification of the nano-microfiber assemblies by ESD from nonionic synthetic polymers. The aims of this study are: (i) to prepare cation- and anion-exchange fiber fabrics by the sulfonation and quaternization of polystyrene and poly(4-vinylpyridine) fiber fabrics, respectively; and (ii) to characterize their pore structure and ion-exchange capacity.
was 15 kV, and the flow rate was 0.02 ml/min. Deposition was carried out at 25 ◦ C and at less than a 30% relative humidity.
2. Experimental
2.4. Introduction of ion-exchange groups to as-deposited fabrics
2.1. Materials Atactic polystyrene (PS, Mw = 320,000) and poly(4vinylpyridine) (P4VP, Mw = 150,000–200,000) were from Idemitsu Kosan, Japan, and Polysciences, USA, respectively. Tetrahydrofuran (THF), dimethylformamide (DMF), ethanol (EtOH), sulfuric acid, dibromopropane (DBP), and n-hexane were from Wako, Japan. These reagents were of extrapure or pure grade and were used without further purification. Deionized water was prepared using a water purifier (Pure-line WL100, Yamato, Japan). 2.2. Preparation of polymer solutions PS and P4VP were dissolved in the THF/DMF mixture (50/50 v/v) and the EtOH/water mixture (92/8 v/v), respectively, at concentrations between 10 and 20 wt%. The solution viscosities were measured with an oscillating-type viscosity meter (VM-100A, CBC Materials, Japan). Surface tensions of the solution were determined by the Whilhelmy plate method using a fully automatic surface tensiometer (CBVP-Z, Kyowa Interface Science, Japan). All measurements were carried out at 25 ◦ C. 2.3. Electrospray deposition A scheme of the ESD device is shown in Fig. 1. Polymer solutions were contained in a syringe with a stainless steel nozzle (1.0 mm internal diameter). The nozzle was connected to a high-voltage regulated DC power supply (HDV-20K 7.5STD, Pulse Electronic Engineering, Japan). A constant volume flow rate was maintained using a syringetype infusion pump (MCIP-III, Minato Concept, Japan). The grounded target used for the counter electrode was an aluminum plate (10 × 10 cm2 area). The distance between the nozzle tip and the substrate surface was 15 cm for the PS solutions and 10 cm for the P4VP solutions, the applied voltage
Fig. 1. Schematic diagram of the instrumentation for ESD.
Cation-exchange fiber (CEF) fabrics and anion-exchange fiber (AEF) fabrics were prepared by the sulfonation of the PS fabrics and the quaternization of the P4VP fabrics, respectively. Prior to the introduction of the ion-exchange groups, the fabrics were placed between two poly(tetrafluoroethylene) (PTFE) plates and heated at 110 ◦ C for the PS fabrics and at 150 ◦ C for the P4VP fabrics, respectively (these temperatures correspond to Tg for PS and Tm for P4VP, respectively, obtained from the DSC analysis), for 2 h to improve the dimensional stability. The PS fabrics were sulfonated with 80% H2 SO4 at 60 ◦ C for 30–120 min and then sufficiently washed with deionized water. The P4VP fabrics were quaternized with a 40 wt% DBP/n-hexane solution shielded from light at 25 ◦ C for 30–120 s and then thoroughly washed with n-hexane and deionized water. 2.5. Characterization of fabrics The surface morphologies of the electrospray-deposited fabrics before and after the introduction of the ion-exchange groups were observed using a scanning electron microscope (SM-200, Topcon, Japan) operated at 10 or 15 kV. All samples were sputter-coated with Au. The thermal analysis of the as-deposited fabrics was carried out using a differential scanning calorimeter (DSC 6100, Seiko Instruments, Japan). The samples were heated from 0 to 200 ◦ C at the rate of 10 ◦ C/min. The pore size distribution, the mean pore size, porosity, and specific surface area of the flow-through pores in the IEF fabrics were characterized by the bubble-point method (ASTM F316-86, JIS K3832) using a measuring instrument (Perm-Porometer, PMI, USA). The Brunauer– Emmett–Teller (BET) specific surface areas of the IEF fabrics were determined from the N2 adsorption isotherms measured at 77 K using an adsorption apparatus (BELSORP 28SA, BEL Japan, Japan).
H. Matsumoto et al. / Journal of Colloid and Interface Science 293 (2006) 143–150
The ion-exchange capacity of the IEF fabrics was estimated by the potentiometric titrations. The potentiometric titration was performed with an automatic titrator (DMS Titrino 716, Metrohm, Switzerland) [18]. The combined pH glass electrode (No. 6.0218.010, Metrohm, Switzerland) and the combined Ag electrode (No. 6.0450.100, Metrohm, Switzerland) were used for the titration of the CEF and AEF fabrics, respectively. Two pieces of CEF and AEF fabrics (each fabric area was 4 × 4 cm2 ) were immersed in 2 mol/l HCl and 2 mol/l KCl for 12 h in order to convert the counterion to the H+ and Cl− forms, respectively. They were then sufficiently washed with deionized water. Thereafter, the CEF fabrics were immersed in 50 ml of 2 mol/l KCl with stirring under N2 gas flow for 1 h, and the AEF fabrics were immersed in 50 ml of 2 mol/l NaNO3 with stirring for 1 h. These treatments were repeated four times in order to elute H+ or Cl− from the membranes thoroughly, and all eluents were collected. The collected solutions were titrated with 0.1 mol/l KOH and 0.1 mol/l AgNO3 , respectively. The titers of the KOH and AgNO3 correspond to the amount of the ion-exchange groups in the CEF and AEF fabrics, respectively, NX (mol). The ion-exchange capacity, IEC (mol/g-dry fabric), was determined by IEC = NX /wdry ,
(1)
where the weight of dried fabric, wdry (g), was measured after vacuum-drying at 100 ◦ C for 6 h.
3. Results and discussion 3.1. Effect of polymer concentration on morphology of As-deposited fabrics To clarify the optimal conditions for the fiber formation, we grew PS and P4VP fabrics by ESD from the respective polymer solutions at various concentrations. The properties of the polymer solutions are summarized in Table 1. The viscosity of the polymer solution increased with an increase of the polymer concentration. Figs. 2 and 3 show the Table 1 Properties of polymer solution for ESD Polymer
Solvent
Concentration (wt%)
Viscosity (mPa s)
Surface tension (mN/m)
PS PS PS PS PS
THF/DMF (50/50 v/v) THF/DMF (50/50 v/v) THF/DMF (50/50 v/v) THF/DMF (50/50 v/v) THF/DMF (50/50 v/v)
10 12.5 15 17.5 20
61.6 95.5 150.3 261.7 411.7
38.2 37.0 37.4 36.5 37.2
P4VP P4VP P4VP P4VP P4VP
EtOH/water (92/8 v/v) EtOH/water (92/8 v/v) EtOH/water (92/8 v/v) EtOH/water (92/8 v/v) EtOH/water (92/8 v/v)
10 12.5 15 17.5 20
67.0 105.3 198.3 307.0 450.3
25.1 24.6 26.3 25.5 23.1
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surface SEM images and the fiber diameter distribution of the as-deposited fabrics, respectively. All polymer solutions showed a good ESD processability. A smooth fibrous structure without beads was formed above 12.5 wt% for PS and above 15 wt% for P4VP. The higher solution viscosity tends to form a thicker fibrous structure: the mean fiber diameter was 900 nm–1.32 µm for the PS fabrics and 620 nm–1.70 µm for the P4VP fabrics. These results correspond to the effect of the solution viscosity described in our previous paper [3]. Here, we used the nano-microfiber fabrics with the comparatively narrow fiber diameter distributions, which were from 15 wt% PS and 15 wt% P4VP solutions, for the next surface chemical modification. 3.2. Physicochemical properties of ion-exchange fiber fabrics Since the as-deposited fabrics were easy to fuzz out, heat treatments were carried out before the chemical modification in order to improve the dimensional stability. Figs. 4a and 4c show the surface SEM images of the PS and P4VP fabrics after the heat treatments, respectively. The fuzz-free fabrics after the heat treatments maintained their fibrous structure. In order to introduce the ion-exchange groups onto the surface of the fibrous fabrics, we sulfonated the surface of the PS fabric and quaternized that of the P4VP fabric. We then used the sulfuric acid aqueous solution and DBP/n-hexane solution for the introduction of the cationand anion-exchange groups, respectively. The former can introduce the sulfonic acid groups into the vicinity of the PS fabric surface, because sulfuric acid is difficult to permeate into the hydrophobic PS [19]. The latter, on the other hand, can simultaneously cross-link and introduce quaternary pyridinium groups (without performing the cross-linking modification, the quaternized P4VP fabric is soluble in water) [20]. The ion-exchange capacity (IEC) of the CEF and AEF fabrics as a function of the chemical reaction time is shown in Figs. 5a and 5b, respectively. For the CEF fabric, it tooks about 90 min to reach a constant capacity. For the AEF fabric, on the other hand, the IEC decreased with an increase in the reaction time. This tendency is dominated by the competition of the reaction rates between the crosslinking by DBP and the dissolution of the fabric in DBP. However, even a 30-s reaction produced some capacity content. The physicochemical properties of the IEF fabrics are listed in Table 2. The IEC of the CEF and AEF fabrics was 1.34 (sulfonated PS fabric for 120 min) and 0.78 mmol/g (quaternized P4VP fabric for 30 s), respectively, which compares with the reported IEC of 1–4 mmol/g for the conventional IEFs produced by coating or grafting fibrous substrates with functionalized resins or monomers [21]. Surface and cross-sectional SEM images showed that the CEF and AEF fabrics have microscaled pore structures (Figs. 4b and 4d–4f). The size distribution of the flow-through pores also demonstrated that these fabrics have micropores with a nar-
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Fig. 2. Surface SEM images of the as-deposited PS and P4VP fabrics from the solutions with various concentrations: (a) 10 wt%, (b) 12.5 wt%, (c) 15 wt%, (d) 17.5 wt%, and (e) 20 wt% PS in the mixture of THF and DMF; (f) 10 wt%, (g) 12.5 wt%, (h) 15 wt%, (i) 17.5 wt%, and (j) 20 wt% P4VP in the mixture of EtOH and water.
row distribution (Fig. 6a): the average pore diameter of the CEF and AEF fabrics was 1.87 and 3.53 µm, respectively
(Table 2). In addition, the porosity and the specific surface area of the flow-through pore were approximately 80% and
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Fig. 3. Fiber diameter distribution of as-deposited PS and P4VP fabrics from the solutions of various concentrations: (a) 12.5 wt%, (b) 15 wt%, (c) 17.5, and (d) 20 wt% PS in the mixture of THF and DMF (50/50 v/v); (e) 15 wt%, (f) 17.5 wt%, and (g) 20 wt% P4VP in the mixture of EtOH and water (98/2 v/v).
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Fig. 4. Morphologies of IEF fabrics: surface SEM images of PS fabrics (a) before (but after heat treatment) and (b) after the sulfonation, and P4VP fabrics (c) before (but after heat treatment) and (d) after being quaternized. Cross-sectional SEM images of (e) sulfonated PS (CEF) fabrics and (f) quaternized P4VP (AEF) fabrics.
13 m2 /g for both fabrics (Table 2). The BET specific surface area, however, showed a remarkably different value between the AEF and CEF fabrics: the BET surface area was 1.7 for the CEF fabric and 599.7 for the AEF fabric (Table 2). The BET surface area of the AEF fabric is much higher than the values that have been reported for electrospun mats (i.e., 2.5–51 m2 /g) [22–24] and equal to the values of activated carbon fibers [25]. This can be explained by the assumption that there are very small pores with diameters of several nanometers on the surface of the AEF fabrics. The pore size distribution for IEF fabrics (Fig. 6b) from N2 adsorption–desorption isotherm [26] suggested the assumption. We found that the chemical modification of the P4VP fabrics using DBP was highly effective for forming intra-fiber pores with a diameter of several nanometers. The control of the conditions for the ESD and secondary processing enables us to improve the ion-exchange capacity and
the specific surface area of the IEF fabrics. (For example, Rabolt and co-workers reported that the humidity and molecular weight strongly affected the formation of the pores on the surface of the electrospun PS fibers [27].)
4. Conclusion In the present work, we succeeded in preparing IEF fabrics by ESD from synthetic polymer solutions and secondary chemical modification of the fabric surface, and characterized their ion-exchange capacity and pore characteristics. Nonwoven nano-microfiber fabrics were prepared by ESD from PS/(THF–DMF) and P4VP/(EtOH–water) solutions. The CEF and AEF fabrics with microscaled interconnected flow-through pores were obtained by the sulfonation of the PS fabric in a sulfuric acid solution and the simultaneous
H. Matsumoto et al. / Journal of Colloid and Interface Science 293 (2006) 143–150
(a)
149
(a)
(b) Fig. 5. Time dependences of chemical modification on ion-exchange capacity (IEC) for (a) CEF fabrics and (b) AEF fabrics.
Table 2 Physicochemical properties of ion-exchange fiber fabrics IECc (mmol/g-dry fabric) Average pore diameterd (µm) Porosityd (%) Through-pore specific surface aread (m2 /g) BET specific surface areae (m2 /g) Thickness (µm)
CEF fabrica
AEF fabricb
1.34 1.87 75.1 13.43 1.7 52
0.78 3.53 80.4 13.76 599.77 40
a Prepared by ESD from 15 wt% PS/(THF–DMF) solution and postdeposition sulfonation for 120 min. b Prepared by ESD from 15 wt% P4VP/(EtOH–water) solution and postdeposition quaternization for 30 s. c Ion-exchange capacity from potentiometric titration. d Estimated by permporometry measurements. e Estimated by N adsorption experiments. 2
quaternization and cross-linking of the P4VP fabric in the DBP/n-hexane solution, respectively. In addition, the chemical modification of P4VP using DBP could form intra-fiber pores with a diameter of several nanometers. These results indicate that the secondary chemical modification of the nano-microfiber fabrics by ESD provides novel functional materials with large adsorption capacities and high catalytic activities. Further studies on the catalytic effect of IEF fabrics by ESD and secondary processing are now in progress and the results are going to be reported in a later paper.
(b) Fig. 6. The size distribution of pores in IEF fabrics from (a) permporometry measurements and (b) N2 adsorption–desorption experiments. r is the pore radius, and V is the volume of condensed N2 .
Acknowledgments The authors thank Mr. Takehiko Yaza, Seika Corporation, and Mr. Toshiya Tanioka, Gun-ei Chemical Industries Co., Ltd., for the permporometry measurements and the nitrogen adsorption experiments, respectively. Y.W. expresses special appreciation to Ms. Mie Kikuchi for her assistance and encouragement.
References [1] O.V. Salata, Curr. Nanosci. 1 (2005) 25. [2] I. Uematsu, H. Matsumoto, K. Morota, M. Minagawa, A. Tanioka, Y. Yamagata, K. Inoue, J. Colloid Interface Sci. 269 (2004) 336. [3] K. Morota, H. Matsumoto, T. Mizukoshi, Y. Konosu, M. Minagawa, A. Tanioka, Y. Yamagata, K. Inoue, J. Colloid Interface Sci. 279 (2004) 484. [4] H. Matsumoto, T. Mizukoshi, K. Nitta, M. Minagawa, A. Tanioka, Y. Yamagata, J. Colloid Interface Sci. 286 (2005) 414. [5] H. Seo, H. Matsumoto, S. Hara, M. Minagawa, A. Tanioka, H. Yako, Y. Yamagata, K. Inoue, Polym. J. 37 (2005) 391. [6] D.H. Reneker, I. Chun, Nanotechnology 7 (1996) 216. [7] Y. Dzenis, Science 304 (2004) 1917. [8] D. Li, Y. Xia, Adv. Mater. 16 (2004) 1151.
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[9] W.-J. Li, C.T. Laurencin, E.J. Caterson, R.S. Tuan, F.K. Ko, J. Biomed. Mater. Res. 60 (2002) 613. [10] K. Kim, Y.K. Luu, C. Chang, D. Fang, B.S. Hsiao, B. Chu, J. Control. Release 98 (2004) 47. [11] P. Gibson, H. Schreuder-Gibson, D. Rivin, Colloids Surf. A 187–188 (2001) 469. [12] M. Streat, Chem. Ind. Issue 13 (2004) 20. [13] S. Buckley, Spec. Chem. Mag. 22 (2002) 12. [14] T. Jaskari, M. Vuorio, K. Kontturi, A. Urtti, J.A. Manzanares, J. Hirvonen, J. Control. Release 67 (2000) 179. [15] T. Jaskari, M. Vuorio, K. Kontturi, J.A. Manzanares, J. Hirvonen, J. Control. Release 70 (2001) 219. [16] J. Lilja, J. Aumo, T. Salmi, D.Yu. Murzin, P. Mäki-Arvela, M. Sundell, K. Ekman, P. Peltonen, H. Vanio, Appl. Catal. A 228 (2002) 253. [17] P.K. Pääkkönen, A.O.I. Krause, React. Funct. Polym. 55 (2003) 139. [18] H. Matsumoto, Y.-C. Chen, R. Yamamoto, Y. Konosu, M. Minagawa, A. Tanioka, J. Mol. Struct. 739 (2005) 99.
[19] R. Yamamoto, H. Matsumoto, A. Tanioka, J. Phys. Chem. B 107 (2003) 10506. [20] T. Aritomi, T. Isomura, K. Fukuta, K. Sakata, J. Ion Exchange 14 (2003) 193. [21] L. Dominguez, K.R. Benak, J. Economy, Polym. Adv. Technol. 12 (2001) 197. [22] Y.J. Ryu, H.Y. Kim, K.H. Lee, H.C. Park, D.R. Lee, Eur. Polym. J. 39 (2003) 1883. [23] B. Ding, J. Kim, E. Kimura, S. Shiratori, Nanotechnology 15 (2004) 913. [24] L. Zhang, Y.-L. Hsieh, Polym. Prepr. 46 (2005) 59. [25] A. Oya, S. Yoshida, J. Alcaniz-Monge, A. Linares-Solano, Carbon 33 (1995) 1085. [26] M. Mulder, Basic Principles of Membrane Technology, second ed., Kluwer Academic, Dordrecht, 1998. [27] C.L. Casper, J.S. Stephens, N.G. Tassi, D.B. Chase, J.F. Rabolt, Macromolecules 37 (2004) 573.
Preparation of ion-exchange fiber fabrics by electrospray deposition Hidetoshi Matsumoto, Yuji Wakamatsu, Mie Minagawa, Akihiko Tanioka ∗ Department of Organic and Polymeric Materials, and International Research Center of Macromolecular Science, Tokyo Institute of Technology, Mail Box S8-27, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan Received 11 May 2005; accepted 6 June 2005 Available online 12 July 2005
Abstract Ion-exchange fiber (IEF) fabrics were prepared by electrospray deposition (ESD) and post-deposition chemical modification of their surfaces. Nonwoven fibrous fabrics were obtained from the solutions of synthetic polymers—polystyrene (PS) and poly(4-vinylpyridine) (P4VP)—of various concentrations. The diameter of the fiber in the fabrics ranged from 600 nm to 1.70 µm. Cation- and anion-exchange fiber (CEF and AEF) fabrics were obtained from the sulfonation of PS fabrics and the quaternization of P4VP fabrics, respectively. These fabrics were thoroughly characterized by a series of techniques, such as scanning electron microscopy (SEM), permporometry, nitrogen adsorption measurements, and potentiometric titrations. The SEM images showed that the fabrics had a porous structure after their chemical modification. The mean pore size, porosity, and specific surface area of the flow-through pores were 1.67–3.53 µm, about 80%, and 13 m2 /g, respectively. The ion-exchange capacity was in the range from 0.78 to 1.34 mmol/g. The AEF fabric, on the other hand, showed a high specific surface area, i.e., the Brunauer–Emmett–Teller (BET) surface area of 600 m2 /g, due to the formation of much smaller pores on the surface of the fiber structure in the fabric. The secondary chemical modification of the nano-microfiber fabrics by ESD provides novel functional materials with a large adsorption capacity and a high catalytic activity. 2005 Elsevier Inc. All rights reserved. Keywords: Electrospray deposition; Ion exchange; Nanofiber fabric; Polystyrene; Poly(4-vinylpyridine)
1. Introduction Electrospray deposition (ESD) is a straightforward and versatile method for forming thin films. This method has the following advantages: (i) applicability to solute molecules which have a wide range of molecular weights (e.g., inorganic molecules, synthetic polymers, proteins, and DNA); and (ii) ability to deposit polymer thin films with nanomicroscaled structures, which range from spheres to fibers [1–5]. The ESD methods consist of the following steps: (i) a strong electric field is applied between a nozzle containing polymer solution and a conductive substrate; (ii) when the voltage reaches a critical value, the electrostatic forces overcome the surface tension of the solution; (iii) charged droplets (or jets) are sprayed from the tip of the nozzle in a dry atmosphere; and (iv) the dried droplets (or jets) are fi* Corresponding author. Fax: +81 3 5734 2876.
E-mail address: [email protected] (A. Tanioka). 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.06.022
nally collected on the substrate to form a thin film. There have been many studies on electrostatic fiber formation, called “electrospinning” [6–8]. Electrostatic fiber formation has drawn significant attention in many fields including biomaterial scaffolds for tissue engineering [9], drug delivery systems [10], and high-performance filter media [11]. The scope of ion-exchange technology has widened into the areas of biotechnology, pharmaceutical processing, production of ultrapure water for the semiconductor industry and has extended to surface modified materials for catalytic conversion processes [12]. Particularly, various ionexchange fibers (IEFs) have been successfully used due to their large specific surface, good mechanical properties, good handling characteristics, and flexibility to be processed in diverse forms [13–17]. The introduction of ionic functional groups to nano-microscaled fibrous fabrics obtained by ESD is a promising option to provide novel ion exchangers with a high adsorption capacity and a high catalytic activity. Our previous papers reported the
144
H. Matsumoto et al. / Journal of Colloid and Interface Science 293 (2006) 143–150
preparation of ion-exchange nanofiber fabrics by ESD from biological polymers—chitosan (basic mucopolysaccharide) and chondroitin sulfate (acidic mucopolysaccharide) with poly(ethylene oxide) [5]. Polyelectrolyte solutions, however, generally show a low ESD processability, because the charging of a polyelectrolyte solution with high conductivity at the tip of the nozzle under an applied voltage tends not to occur [3]. In the present work, we attempt to prepare novel ion exchangers by the secondary chemical modification of the nano-microfiber assemblies by ESD from nonionic synthetic polymers. The aims of this study are: (i) to prepare cation- and anion-exchange fiber fabrics by the sulfonation and quaternization of polystyrene and poly(4-vinylpyridine) fiber fabrics, respectively; and (ii) to characterize their pore structure and ion-exchange capacity.
was 15 kV, and the flow rate was 0.02 ml/min. Deposition was carried out at 25 ◦ C and at less than a 30% relative humidity.
2. Experimental
2.4. Introduction of ion-exchange groups to as-deposited fabrics
2.1. Materials Atactic polystyrene (PS, Mw = 320,000) and poly(4vinylpyridine) (P4VP, Mw = 150,000–200,000) were from Idemitsu Kosan, Japan, and Polysciences, USA, respectively. Tetrahydrofuran (THF), dimethylformamide (DMF), ethanol (EtOH), sulfuric acid, dibromopropane (DBP), and n-hexane were from Wako, Japan. These reagents were of extrapure or pure grade and were used without further purification. Deionized water was prepared using a water purifier (Pure-line WL100, Yamato, Japan). 2.2. Preparation of polymer solutions PS and P4VP were dissolved in the THF/DMF mixture (50/50 v/v) and the EtOH/water mixture (92/8 v/v), respectively, at concentrations between 10 and 20 wt%. The solution viscosities were measured with an oscillating-type viscosity meter (VM-100A, CBC Materials, Japan). Surface tensions of the solution were determined by the Whilhelmy plate method using a fully automatic surface tensiometer (CBVP-Z, Kyowa Interface Science, Japan). All measurements were carried out at 25 ◦ C. 2.3. Electrospray deposition A scheme of the ESD device is shown in Fig. 1. Polymer solutions were contained in a syringe with a stainless steel nozzle (1.0 mm internal diameter). The nozzle was connected to a high-voltage regulated DC power supply (HDV-20K 7.5STD, Pulse Electronic Engineering, Japan). A constant volume flow rate was maintained using a syringetype infusion pump (MCIP-III, Minato Concept, Japan). The grounded target used for the counter electrode was an aluminum plate (10 × 10 cm2 area). The distance between the nozzle tip and the substrate surface was 15 cm for the PS solutions and 10 cm for the P4VP solutions, the applied voltage
Fig. 1. Schematic diagram of the instrumentation for ESD.
Cation-exchange fiber (CEF) fabrics and anion-exchange fiber (AEF) fabrics were prepared by the sulfonation of the PS fabrics and the quaternization of the P4VP fabrics, respectively. Prior to the introduction of the ion-exchange groups, the fabrics were placed between two poly(tetrafluoroethylene) (PTFE) plates and heated at 110 ◦ C for the PS fabrics and at 150 ◦ C for the P4VP fabrics, respectively (these temperatures correspond to Tg for PS and Tm for P4VP, respectively, obtained from the DSC analysis), for 2 h to improve the dimensional stability. The PS fabrics were sulfonated with 80% H2 SO4 at 60 ◦ C for 30–120 min and then sufficiently washed with deionized water. The P4VP fabrics were quaternized with a 40 wt% DBP/n-hexane solution shielded from light at 25 ◦ C for 30–120 s and then thoroughly washed with n-hexane and deionized water. 2.5. Characterization of fabrics The surface morphologies of the electrospray-deposited fabrics before and after the introduction of the ion-exchange groups were observed using a scanning electron microscope (SM-200, Topcon, Japan) operated at 10 or 15 kV. All samples were sputter-coated with Au. The thermal analysis of the as-deposited fabrics was carried out using a differential scanning calorimeter (DSC 6100, Seiko Instruments, Japan). The samples were heated from 0 to 200 ◦ C at the rate of 10 ◦ C/min. The pore size distribution, the mean pore size, porosity, and specific surface area of the flow-through pores in the IEF fabrics were characterized by the bubble-point method (ASTM F316-86, JIS K3832) using a measuring instrument (Perm-Porometer, PMI, USA). The Brunauer– Emmett–Teller (BET) specific surface areas of the IEF fabrics were determined from the N2 adsorption isotherms measured at 77 K using an adsorption apparatus (BELSORP 28SA, BEL Japan, Japan).
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The ion-exchange capacity of the IEF fabrics was estimated by the potentiometric titrations. The potentiometric titration was performed with an automatic titrator (DMS Titrino 716, Metrohm, Switzerland) [18]. The combined pH glass electrode (No. 6.0218.010, Metrohm, Switzerland) and the combined Ag electrode (No. 6.0450.100, Metrohm, Switzerland) were used for the titration of the CEF and AEF fabrics, respectively. Two pieces of CEF and AEF fabrics (each fabric area was 4 × 4 cm2 ) were immersed in 2 mol/l HCl and 2 mol/l KCl for 12 h in order to convert the counterion to the H+ and Cl− forms, respectively. They were then sufficiently washed with deionized water. Thereafter, the CEF fabrics were immersed in 50 ml of 2 mol/l KCl with stirring under N2 gas flow for 1 h, and the AEF fabrics were immersed in 50 ml of 2 mol/l NaNO3 with stirring for 1 h. These treatments were repeated four times in order to elute H+ or Cl− from the membranes thoroughly, and all eluents were collected. The collected solutions were titrated with 0.1 mol/l KOH and 0.1 mol/l AgNO3 , respectively. The titers of the KOH and AgNO3 correspond to the amount of the ion-exchange groups in the CEF and AEF fabrics, respectively, NX (mol). The ion-exchange capacity, IEC (mol/g-dry fabric), was determined by IEC = NX /wdry ,
(1)
where the weight of dried fabric, wdry (g), was measured after vacuum-drying at 100 ◦ C for 6 h.
3. Results and discussion 3.1. Effect of polymer concentration on morphology of As-deposited fabrics To clarify the optimal conditions for the fiber formation, we grew PS and P4VP fabrics by ESD from the respective polymer solutions at various concentrations. The properties of the polymer solutions are summarized in Table 1. The viscosity of the polymer solution increased with an increase of the polymer concentration. Figs. 2 and 3 show the Table 1 Properties of polymer solution for ESD Polymer
Solvent
Concentration (wt%)
Viscosity (mPa s)
Surface tension (mN/m)
PS PS PS PS PS
THF/DMF (50/50 v/v) THF/DMF (50/50 v/v) THF/DMF (50/50 v/v) THF/DMF (50/50 v/v) THF/DMF (50/50 v/v)
10 12.5 15 17.5 20
61.6 95.5 150.3 261.7 411.7
38.2 37.0 37.4 36.5 37.2
P4VP P4VP P4VP P4VP P4VP
EtOH/water (92/8 v/v) EtOH/water (92/8 v/v) EtOH/water (92/8 v/v) EtOH/water (92/8 v/v) EtOH/water (92/8 v/v)
10 12.5 15 17.5 20
67.0 105.3 198.3 307.0 450.3
25.1 24.6 26.3 25.5 23.1
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surface SEM images and the fiber diameter distribution of the as-deposited fabrics, respectively. All polymer solutions showed a good ESD processability. A smooth fibrous structure without beads was formed above 12.5 wt% for PS and above 15 wt% for P4VP. The higher solution viscosity tends to form a thicker fibrous structure: the mean fiber diameter was 900 nm–1.32 µm for the PS fabrics and 620 nm–1.70 µm for the P4VP fabrics. These results correspond to the effect of the solution viscosity described in our previous paper [3]. Here, we used the nano-microfiber fabrics with the comparatively narrow fiber diameter distributions, which were from 15 wt% PS and 15 wt% P4VP solutions, for the next surface chemical modification. 3.2. Physicochemical properties of ion-exchange fiber fabrics Since the as-deposited fabrics were easy to fuzz out, heat treatments were carried out before the chemical modification in order to improve the dimensional stability. Figs. 4a and 4c show the surface SEM images of the PS and P4VP fabrics after the heat treatments, respectively. The fuzz-free fabrics after the heat treatments maintained their fibrous structure. In order to introduce the ion-exchange groups onto the surface of the fibrous fabrics, we sulfonated the surface of the PS fabric and quaternized that of the P4VP fabric. We then used the sulfuric acid aqueous solution and DBP/n-hexane solution for the introduction of the cationand anion-exchange groups, respectively. The former can introduce the sulfonic acid groups into the vicinity of the PS fabric surface, because sulfuric acid is difficult to permeate into the hydrophobic PS [19]. The latter, on the other hand, can simultaneously cross-link and introduce quaternary pyridinium groups (without performing the cross-linking modification, the quaternized P4VP fabric is soluble in water) [20]. The ion-exchange capacity (IEC) of the CEF and AEF fabrics as a function of the chemical reaction time is shown in Figs. 5a and 5b, respectively. For the CEF fabric, it tooks about 90 min to reach a constant capacity. For the AEF fabric, on the other hand, the IEC decreased with an increase in the reaction time. This tendency is dominated by the competition of the reaction rates between the crosslinking by DBP and the dissolution of the fabric in DBP. However, even a 30-s reaction produced some capacity content. The physicochemical properties of the IEF fabrics are listed in Table 2. The IEC of the CEF and AEF fabrics was 1.34 (sulfonated PS fabric for 120 min) and 0.78 mmol/g (quaternized P4VP fabric for 30 s), respectively, which compares with the reported IEC of 1–4 mmol/g for the conventional IEFs produced by coating or grafting fibrous substrates with functionalized resins or monomers [21]. Surface and cross-sectional SEM images showed that the CEF and AEF fabrics have microscaled pore structures (Figs. 4b and 4d–4f). The size distribution of the flow-through pores also demonstrated that these fabrics have micropores with a nar-
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Fig. 2. Surface SEM images of the as-deposited PS and P4VP fabrics from the solutions with various concentrations: (a) 10 wt%, (b) 12.5 wt%, (c) 15 wt%, (d) 17.5 wt%, and (e) 20 wt% PS in the mixture of THF and DMF; (f) 10 wt%, (g) 12.5 wt%, (h) 15 wt%, (i) 17.5 wt%, and (j) 20 wt% P4VP in the mixture of EtOH and water.
row distribution (Fig. 6a): the average pore diameter of the CEF and AEF fabrics was 1.87 and 3.53 µm, respectively
(Table 2). In addition, the porosity and the specific surface area of the flow-through pore were approximately 80% and
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Fig. 3. Fiber diameter distribution of as-deposited PS and P4VP fabrics from the solutions of various concentrations: (a) 12.5 wt%, (b) 15 wt%, (c) 17.5, and (d) 20 wt% PS in the mixture of THF and DMF (50/50 v/v); (e) 15 wt%, (f) 17.5 wt%, and (g) 20 wt% P4VP in the mixture of EtOH and water (98/2 v/v).
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Fig. 4. Morphologies of IEF fabrics: surface SEM images of PS fabrics (a) before (but after heat treatment) and (b) after the sulfonation, and P4VP fabrics (c) before (but after heat treatment) and (d) after being quaternized. Cross-sectional SEM images of (e) sulfonated PS (CEF) fabrics and (f) quaternized P4VP (AEF) fabrics.
13 m2 /g for both fabrics (Table 2). The BET specific surface area, however, showed a remarkably different value between the AEF and CEF fabrics: the BET surface area was 1.7 for the CEF fabric and 599.7 for the AEF fabric (Table 2). The BET surface area of the AEF fabric is much higher than the values that have been reported for electrospun mats (i.e., 2.5–51 m2 /g) [22–24] and equal to the values of activated carbon fibers [25]. This can be explained by the assumption that there are very small pores with diameters of several nanometers on the surface of the AEF fabrics. The pore size distribution for IEF fabrics (Fig. 6b) from N2 adsorption–desorption isotherm [26] suggested the assumption. We found that the chemical modification of the P4VP fabrics using DBP was highly effective for forming intra-fiber pores with a diameter of several nanometers. The control of the conditions for the ESD and secondary processing enables us to improve the ion-exchange capacity and
the specific surface area of the IEF fabrics. (For example, Rabolt and co-workers reported that the humidity and molecular weight strongly affected the formation of the pores on the surface of the electrospun PS fibers [27].)
4. Conclusion In the present work, we succeeded in preparing IEF fabrics by ESD from synthetic polymer solutions and secondary chemical modification of the fabric surface, and characterized their ion-exchange capacity and pore characteristics. Nonwoven nano-microfiber fabrics were prepared by ESD from PS/(THF–DMF) and P4VP/(EtOH–water) solutions. The CEF and AEF fabrics with microscaled interconnected flow-through pores were obtained by the sulfonation of the PS fabric in a sulfuric acid solution and the simultaneous
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(a)
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(a)
(b) Fig. 5. Time dependences of chemical modification on ion-exchange capacity (IEC) for (a) CEF fabrics and (b) AEF fabrics.
Table 2 Physicochemical properties of ion-exchange fiber fabrics IECc (mmol/g-dry fabric) Average pore diameterd (µm) Porosityd (%) Through-pore specific surface aread (m2 /g) BET specific surface areae (m2 /g) Thickness (µm)
CEF fabrica
AEF fabricb
1.34 1.87 75.1 13.43 1.7 52
0.78 3.53 80.4 13.76 599.77 40
a Prepared by ESD from 15 wt% PS/(THF–DMF) solution and postdeposition sulfonation for 120 min. b Prepared by ESD from 15 wt% P4VP/(EtOH–water) solution and postdeposition quaternization for 30 s. c Ion-exchange capacity from potentiometric titration. d Estimated by permporometry measurements. e Estimated by N adsorption experiments. 2
quaternization and cross-linking of the P4VP fabric in the DBP/n-hexane solution, respectively. In addition, the chemical modification of P4VP using DBP could form intra-fiber pores with a diameter of several nanometers. These results indicate that the secondary chemical modification of the nano-microfiber fabrics by ESD provides novel functional materials with large adsorption capacities and high catalytic activities. Further studies on the catalytic effect of IEF fabrics by ESD and secondary processing are now in progress and the results are going to be reported in a later paper.
(b) Fig. 6. The size distribution of pores in IEF fabrics from (a) permporometry measurements and (b) N2 adsorption–desorption experiments. r is the pore radius, and V is the volume of condensed N2 .
Acknowledgments The authors thank Mr. Takehiko Yaza, Seika Corporation, and Mr. Toshiya Tanioka, Gun-ei Chemical Industries Co., Ltd., for the permporometry measurements and the nitrogen adsorption experiments, respectively. Y.W. expresses special appreciation to Ms. Mie Kikuchi for her assistance and encouragement.
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