Langmuir 1999, 15, 1829-1832
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Fourier Transform Infrared Spectroscopic Study of Flat Surfaces of Amphoteric-Charged Poly(acrylonitrile) Membranes: Attenuated Total Reflection Mode Toshihiko Jimbo,† Akihiko Tanioka,*,† and Norihiko Minoura‡ Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan, and National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba-shi, Ibaraki 305-8565, Japan Received September 22, 1998. In Final Form: December 1, 1998
Amphoteric-charged poly(acrylonitrile) (PAN) membranes possessing both carboxyl and tertiary amine groups were prepared by radical graft polymerization. By varying the molar ratio of acidic to basic monomers, we could obtain four kinds of amphoteric-charged membranes. These membranes were systematically analyzed by Fourier transform infrared spectroscopy/attenuated total reflection (FT-IR/ATR) regarding the effects of feed monomer composition and pH change. Also, the spectra acquired were qualitatively compared with our previous electrokinetic data. From these results, it was found that the spectral difference among the amphoteric-charged PAN membranes can be characterized by salt formation between the carboxyl group and the adjacent tertiary amine group on the graft chains and/or sodium ions; the larger the tertiary amine group fraction, the more carboxyl groups take the ionized form.
1. Introduction Synthetic polymer membranes used in dialysis and ultrafiltration have been modified with various kinds of hydrophilic agents to minimize membrane-fouling phenomena. Staude et al.1,2 and Ulbricht et al.3-5 extensively studied the modified ultrafiltration membranes and proved the effectiveness of the pore-surface modification in terms of low-protein-adsorption phenomena. Recently, according to a series of studies by Saito et al.,6-9 weak amphoteric polymer membranes based on poly(vinyl alcohol) and succinyl chitosan showed interesting phenomena concerning ionic behavior in pH change. Such an amphoteric material can be expected to be useful not only as antifouling material but also in biomedical devices such as artificial organs and drug delivery systems because the material shows a pH response.10 So far, we prepared poly(acrylonitrile) (PAN) based amphoteric-charged membranes via graft polymerization from a mixed solution of acidic and basic monomers.11 Their characterization was accomplished by an electrokinetic method based on streaming potential, and the results were theoretically examined by site dissociation modeling to determine the ratio of acidic to basic groups at the pore surface. However, * To whom correspondence should be addressed. Telephone: +81-3-5734-2426, Fax: +81-3-5734-2876, E-mail: atanioka@ o.cc.titech.ac.jp. † Tokyo Institute of Technology. ‡ National Institute of Materials and Chemical Research. (1) Du¨putell. D.; Staude, E.; Wyszynski, D. Desalination 1994, 95, 75. (2) Hosch J.; Staude, E. J. Membr. Sci. 1996, 121, 71. (3) Ulbricht, M.; Oechel, A.; Lehmann, C.; Tomaschewski, G.; Hicke, H.-G. J. Appl. Polym. Sci. 1995, 55, 1707. (4) Ulbricht, M.; Matuschewski, H.; Oechel, A.; Hicke, H.-G. J. Membr. Sci. 1996, 115, 31. (5) Ulbricht, M.; Richau, K.; Kamusewitz, H. Colloids Surf. A 1998, 138, 353. (6) Saito, K.; Tanioka, A. Polymer 1996, 37, 5117. (7) Saito, K.; Tanioka, A. Polymer 1996, 37, 2299. (8) Saito, K.; Ishizuka, S.; Higa, M.; Tanioka, A. Polymer 1996, 37, 2493. (9) Ramı´rez, P.; Mafe´, S.; Tanioka, A.; Saito, K. Polymer 1997, 38, 4931. (10) Chen, J.; Minoura, N.; Tanioka, A. Polymer 1994, 35, 2853. (11) Jimbo, T.; Tanioka, A.; Minoura, N. Langmuir 1998, 14, 7112.
extensive studies to examine the potentially excellent functionality of amphoteric-charged membranes have not yet been sufficiently carried out. In this respect, it is considered of great importance to investigate well the overall properties of amphoteric-charged membranes from a fundamental standpoint from which a new high performance membrane can be designed. The flat membrane surfaces were well characterized by the Fourier transform infrared spectroscopy/attenuated total reflection (FT-IR/ ATR) method to examine fouling phenomena12,13 and surface modification.3-5 To obtain much more information with respect to the amphoteric-charged layer grafted to the membrane surfaces, here we report on FT-IR/ATR characterization of four kinds of amphoteric-charged PAN membranes which have different acid-to-base ratios. Also, the results are compared with those electrokinetically characterized in our previous research. 2. Experimental Section 2.1. Materials. PAN powder (Mw ) 130,000) was provided by Mitsubishi Rayon Co., Ltd., Japan. Poly(vinylpyrrolidone) (PVP), N,N-dimethylformamide (DMF), acrylic acid (AAc), sodium bisulfite, ammonium persulfate (APS), and N,N,N′,N′-tetramethylethylenediamine (TMEDA) were purchased from Wako Pure Chemical Co., Ltd., Japan. N,N-(dimethylamino)propyl acrylamide (DMAPAA) was provided by Kohjin Co., Ltd., Japan. These reagents were used without further purification. 2.2. Preparation of PAN Porous Membrane. Porous membranes were prepared from a mixed solution consisting of 12 wt % PAN, 3 wt % PVP (pore-forming agent), and DMF as a solvent. These polymers were dissolved by stirring for 5 h at 70 °C. The polymer solution was cooled to 25 °C and thereafter cast onto a glass plate guided by a 150-µm thick spacer. The evaporation period was 30 s at 25 ( 1 °C, and the humidity was 60 ( 2%. The glass plate was then immersed in a coagulation medium consisting of a 40 vol % glycerol and water mixture for 20 min at 10 °C. The membrane was washed sufficiently with abundant deionized water for 3 days. The PAN membrane obtained had a thickness of 110 µm and a hydraulic permeability (12) Zhu, H.; Nystro¨m, M. Colloids Surf. A 1998, 138, 309. (13) Pihlajama¨ki, A.; Va¨isa¨nen, P.; Nystro¨m, M. Colloids Surf. A 1998, 138, 323.
10.1021/la9813073 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/29/1999
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Table 1. Reaction Conditions and Hydraulic Permeability sample membrane
total monomer conc. (wt %)
TMEDA conc. (wt %)
hydraulic permeability (mL‚m-2‚h-1‚cm-1 H2O)b
amphoteric-1 (80:20)a amphoteric-2 (70:30)a amphoteric-3 (60:40)a amphoteric-4 (55:45)a
1.5 1.5 2.5 2.5
0.35 0.35 0.60 0.60
652 582 557 570
a Parentheses show feed molar ratios of monomers, AAc/DMAPAA. b Hydraulic permeability at a pressure of 75.0 cm H O. Data are 2 averaged over 5 points.
Table 2. Characteristic Parameters of the Pore Surface of Amphoteric-Charged PAN Membranes Determined from Zeta Potential Measurement sample membrane (80:20)a
amphoteric-1 amphoteric-2 (70:30)a amphoteric-3 (60:40)a amphoteric-4 (55:45)a
iepb
pKac
pKbc
molar ratiod
3.9 4.5 5.4 6.3
4.2 4.2 4.2 4.2
6.3 6.3 6.3 6.3
74.0:26.0 59.5:40.5 49.0:51.0 35.0:65.9
a Parentheses show feed molar ratios of monomers (before grafting), AAc/DMAPAA. b Isoelectric point. c Dissociation constants at the pore surface. Subscripts a and b denote acid and base, respectively. d Molar ratios determined from zeta potential measurements (after grafting), which correspond to [(acidic group)/ (basic group)]. All data shown are cited from ref 11.
of 1200 ( 50 (mL‚m-2‚h-1‚cm-1 H2O) under a static pressure of 75.0 cm H2O (see also ref 14). 2.3. Preparation of Amphoteric-Charged Membranes. Graft polymerization onto a PAN membrane surface was performed by a radical reaction in aqueous solution containing AAc and DMAPAA monomers. TMEDA was used as an additive.15 The solution pH was precisely adjusted to 5.6. Some of the dissolved oxygen was removed by bubbling nitrogen gas for 30 min. Ten pieces of the PAN membrane, each 5 × 5 cm and 110 µm thick, were immersed in 200 mL of monomer solution for 1 h. Graft polymerization was then initiated by the addition of APS (0.4 g per 200 mL monomer solution).11 All reactions were carried out in a reciprocating shaker for 2 h at 25 °C. The reaction conditions which are summarized in Table 1 were determined to have the same hydraulic permeability for all grafted membranes.11,14 After the graft polymerization, the membranes were washed with abundant deionized water for 24 h at 25 °C and then sequentially with 1 mN HCl and 0.1 mN NaOH for 24 h at 25 °C to remove the homopolymer produced simultaneously. The characteristic values of the grafted membranes are listed in Table 2. 2.4. FT-IR/ATR Measurements. FT-IR/ATR spectra were acquired with a model FT/IR-410 (JASCO, Japan) equipped with an ATR unit ATR-500/M (KRS-5 crystal, 45°, JASCO). FT-IR measurements of powder type and thin film type samples were carried out using the same apparatus as described above by a conventional procedure. The membranes were freeze-dried for FT-IR/ATR measurements and thereafter pressed carefully on one side of the crystal (9 × 30 mm) to ensure the sensitivity of the spectra obtained. All spectra were taken by 300 scans at a nominal resolution of 4 cm-1. To obtain pH-dependent IR spectra, the grafted membranes were immersed for 3 h in 0.01 M phosphate buffer solution precisely adjusted to the desired pH by the addition of either 1 N HCl or NaOH. The membranes were subsequently immersed in abundant pure ethanol for 3 h, so that the counterions bound to the fixed charge groups are not significantly exchanged during freeze-drying. This process allows the counterions to bind tightly with the fixed charge groups because of an ion-pair formation between them in the low dielectric constant region.16,17 The membranes were then freezedried by a conventional procedure. We confirmed that the freezedrying of the membranes from an aqueous (pH-buffered) solution (14) Jimbo, T.; Higa, M.; Minoura, N.; Tanioka, A. Macromolecules 1998, 31, 1277. (15) Burow, M.; Minoura, N. Biochem. Biophys. Res. Commun. 1996, 227, 419. (16) Fuoss, R. M. J. Am. Chem. Soc. 1958, 80, 5059. (17) Mafe´, S.; Ramı´rez, P.; Tanioka, A.; Pellicer, J. J. Phys. Chem. B 1997, 101, 1851.
Figure 1. FT-IR spectra of membrane constituents and unmodified PAN membrane. The unmodified PAN membrane was measured by the ATR method. results in IR spectra that show a rather poor reproducibility of data, in particular, for the absorption at 1560 cm-1, the intensity of which is quite sensitive for pH change. Instead, for the freezedrying from ethanol, the results offered good reproducibility, proving that this procedure is reasonable for the preparation of dried membranes for the study of the spectral variations regarding the effects of pH change. The pH values indicated in Figures 2, 3, and 4 correspond to those of the buffer solution in which the membranes are immersed before being freeze-dried with ethanol.
3. Results and Discussion 3.1. FT-IR/ATR Spectra of Amphoteric-Charged PAN Membranes and the Effects of Varying Feed Monomer Ratio. Figure 1 shows FT-IR spectra of membrane constituents and unmodified (as-cast) PAN membrane. It is obvious that the unmodified PAN membrane displays a complex spectrum attributed to both pure PAN and PVP materials. This allows us to confirm that PVP as a pore-forming agent could not be sufficiently removed even after many washings (the remaining PVP was within 1 wt % of the elemental analysis result). Characteristic peaks of the unmodified PAN membrane can be observed at 2243 cm-1 (CtN stretching vibration), 1674 cm-1 (this peak consists of the CdN valency bond vibration of the oxime group from PAN and the CdO stretching vibration from PVP), and 1454 cm-1 (CH2 scissoring vibration).14 On the other hand, FT-IR/ATR spectra for surfaces of amphoteric-charged PAN membranes originate from both the graft chains at the surface and the membrane substrate. Figure 2 shows the FTIR/ATR spectra of unmodified and amphoteric-charged PAN membranes, and the frequencies and assignments of noticeable peaks are listed in Table 3. These assignments are based on refs 14 and 18-20. These spectra can (18) Bellamy, L. J. In The Infra-red Spectra of Complex Molecules; Wiley: New York, 1975. (19) White, R. G. In Handbook of Industrial Infrared Analysis; Plenum Press: New York, 1964. (20) Barbucci, R.; Casolaro M.; Magnani, A. Makromol. Chem. 1989, 190, 2627.
Amphoteric-Charged Poly(acrylonitrile) Membranes
Figure 2. FT-IR/ATR spectra of unmodified and amphotericcharged PAN membranes relative to the effects of varying the feed molar ratio of monomers. The vertical lines represent the positions of characteristic peaks which, in turn, are located at 1660, 1560, and 1400 cm-1 from the left side. Feed molar ratios of monomers, AAc/DMAPAA, are indicated in parentheses. Table 3. Characteristic Wave Numbers and Assignments for Amphoteric-Charged PAN Membranes wave number (cm-1) 1726 1650-1670 1560 1454 1390-1400
assignments
{ {
CdO stretching of COOH group CdO stretching of secondary amide group CdO stretching from PVPa CdN stretching from PAN membrane substrate NsH deformation of secondary amide group COO- (ionized) asymmetric stretching CH2 scissoring COO- (ionized) symmetric stretching
a PVP remaining in the PAN membrane substrate, which could not be completely extracted during washing procedure.
be characterized by the bands of the asymmetric and symmetric CdO stretching of the COO- group at 1560 cm-1 and 1390-1400 cm-1, respectively, and the N-H deformational vibration of the secondary amide group at 1560 cm-1 which overlaps the asymmetric CdO stretching of the COO- group (refer to the vertical lines in the figure).18,19 The C-N stretching vibration of the tertiary amine group was difficult to identify from these spectra as well as in the case of the pure DMAPAA monomer spectrum (see p 288 in ref 18). It is reported that the CdO stretching vibration of COOH type can be seen at 17001720 cm-1.19 However, it cannot be observed distinctly in these spectra. The carboxyl group in majority takes the ionized form, i.e., the COO- type, because of salt formation with the adjacent tertiary amine group.21 Note that for the amphoteric-1 membrane, a slight shoulder near 1725 cm-1 can be observed, implying that the COOH type still remains because the number of COOH groups is expected to be in excess compared with that of the tertiary amine groups (see column 5 in Table 2). This fact can also be confirmed by the pH-dependent IR spectra of the AAcgrafted membrane as shown in Figure 3 (see refs 14 and 22 for details concerning the AAc-grafted PAN membrane). This reveals that the COOH type spectrum at 1726 cm-1 shifts to the bands at 1560 cm-1 and slightly to 13901400 cm-1 to form COO-Na+ as the pH elevates.19,20 In Figure 2, the strong peak in the 1650-1670 cm-1 region includes the CdO stretching vibration of the secondary amide group, CdO from PVP, and the oxime group (CdN) (21) Burns, N. L.; Holmberg, K.; Brink, C. J. Colloid Interface Sci. 1996, 178, 116. (22) Jimbo, T.; Tanioka, A.; Minoura, N. J. Colloid Interface Sci. 1998, 204, 336.
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Figure 3. FT-IR/ATR spectra of AAc-grafted PAN membrane at different pH values.
from PAN. As for the latter two bands that arise from the PAN membrane substrate, we already confirmed these by the IR spectra of the respective pure PVP and pure PAN materials (see Figure 1). The band of the N-H deformational vibration of the secondary amide group at 1560 cm-1 overlaps the asymmetric stretching vibration of the COO- group, and on the other hand, the corresponding symmetrical vibration can be observed at 13901400 cm-1. If one focuses on these characteristic bands, in particular, a considerable enhancement of the band at 1560 cm-1 can be observed as the feed molar fraction of DMAPAA monomer for amphoteric-charged PAN membranes increases. As mentioned above, two contributions, i.e., the N-H and COO- groups, cause the band at 1560 cm-1. Here, we have to examine which groups dominate to enhance this band, by determining the pH dependence of the IR spectra of the membranes because the absorption bands of the carboxyl group vary depending on pH, as shown in Figure 3. 3.2. FT-IR/ATR Spectra of Amphoteric-Charged PAN Membranes on Change in pH. Figures 4a and b show the FT-IR/ATR spectra for two kinds of amphotericcharged membranes at different pH values. In both cases, the bands at 1560 cm-1 (asymmetric COO- stretching vibration) and at 1390-1400 cm-1 (symmetric COOstretching vibration) are simultaneously enhanced as the pH increases (refer to the vertical lines in each figure). This may be responsible for salt formation of ionized COOgroups with adjacent tertiary amine groups and/or sodium ions. To support this explanation, the IR spectrum of a mixture of AAc and DMAPAA monomers, whose molar ratio is 50:50, should be examined and is shown in Figure 5 (note that the mixture was completely dried before measurement). The strong absorption of the protonated COOH band at 1701 cm-1 which is (characteristic of the AAc monomer) cannot be seen, and instead, the bands at around 1550 cm-1 and at 1390 cm-1 can be identified. The spectral analysis reveals that AAc and DMAPAA monomers on mixing are expected to form salts in which the COOH group of AAc is changed to the ionized form, i.e., COO-. Thus, the enhancement of the band at 1560 cm-1 for the amphoteric-charged PAN membranes (see Figures 2 and 4) can be considered to arise from the salt formation of the COO- group rather than the contribution of the N-H group. This result explains well the spectral variation of the amphoteric-charged PAN membranes on change in pH. 3.3. Comparison between FT-IR/ATR and Zeta Potential Studies for Amphoteric-Charged PAN Membranes. Table 2 lists the parameters of amphotericcharged PAN membranes determined from the pH-
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to a qualitative correspondence in tendency; the molar fraction of the basic group at the surface increases with increasing feed DMAPAA monomer, and thereby the absorption bands at 1560 cm-1 and 1390-1400 cm-1 are simultaneously enhanced because of the salt formation between the carboxyl and tertiary amine groups (see Figure 2). Note, however, that the zeta potential results as shown in Table 2 provide information on pore-surface charge, namely, the interior portions of the grafted membranes. Conversely, information obtained from FTIR/ATR spectroscopy is limited only to portions of the surface and near the surfaces. Despite different situations for both measurements, a comparison between their results will be possible under the condition that a difference in charge state between the pore surface and the flat surface can be negligible. Here, some degree of overlap among the characteristic bands from the graft chains and the PAN membrane substrate considerably complicates spectral analysis.14 If we can separate these spectra precisely, a much more quantitative discussion will be achieved in regard to the functional groups attached to the graft chain, so that the acid-to-base ratio can be determined from a spectroscopic aspect.
Figure 4. FT-IR/ATR spectra of amphoteric-charged PAN membranes at different pH values: (a) amphoteric-1; (b) amphoteric-3. The vertical lines represent the positions of characteristic peaks which, in turn, are located at 1660, 1560, and 1400 cm-1 from the left side. Feed molar ratios of monomers, AAc/DMAPAA, and pH values are indicated in parentheses.
Figure 5. FT-IR spectrum of a mixture of AAc and DMAPAA monomers. The molar ratio is 50:50. The vertical lines represent the positions of characteristic peaks which, in turn, are located at 1660, 1560, and 1390 cm-1 from the left side.
dependent zeta potential and its theoretical modeling.11 The molar ratio of acid to base is varied depending on the feed monomer composition. Parentheses in column 1 show the feed molar ratio of AAc and DMAPAA monomers, the values of which however, do not coincide with the molar ratio derived from the zeta potential study (see column 5). Both ratios show only a similar tendency in which the molar fraction of the basic group (determined from the zeta potential) increases with increasing feed DMAPAA monomer. This fact can be explained by the different reactivity of the AAc and DMAPAA monomers. Comparison between zeta potential and FT-IR/ATR results leads
4. Conclusions Amphoteric-charged PAN membranes were prepared by surface graft polymerization and characterized systematically with FT-IR/ATR spectroscopy regarding the effects of acid-to-base ratio and change in pH. It was successfully shown that the spectroscopic analysis of top surfaces of amphoteric-charged PAN membranes provides much information concerning the functional groups bound to the graft chain. FT-IR/ATR spectra of four kinds of amphoteric-charged PAN membranes are largely affected by the different composition of acidic and basic groups on the graft chains. In pH-dependent IR spectra, we can determine qualitatively the state of charge groups; for instance, the extent of salt formation between the carboxyl group and the adjacent tertiary amine group and/or sodium ions. Moreover, the variation in IR spectra among the amphoteric-charged PAN membranes prepared from different monomer compositions is correspondent in tendency to the data, i.e., the acid-to-base ratio, derived from a zeta potential study. From these aspects, the FTIR/ATR study of amphoteric-charged groups on the membrane surface can be promising for characterization of the charge states as well as the zeta potential measurement. It is already known that the zeta potential study provides macroscopic information such as isoelectric point, dissociation constant, surface charge density, and, in this case, acid-to-base ratio. These parameters which reflect membrane properties are of importance for clarifying and describing phenomenological performances of the charged membranes; for instance, the transport properties of electrolytes and protein adsorption in membrane fouling. In contrast, the advantage of the FT-IR/ATR measurement is attributable to an approach from a microscopic view point for characterizing the charged surface groups. From both macroscopic and microscopic standpoints, we consider it essential and meaningful to determine the detailed properties of amphoteric-charged membranes targeted for practical use. Acknowledgment. We express our great appreciation to Dr. Seiji Hayashi and Mr. Jun Okumura (Mitsubishi Rayon Co., Ltd., Japan) for kindly providing poly(acrylonitrile) powder. We would also like to thank Dr. Hideaki Oike for the FT-IR/ATR apparatus. LA9813073
