4850
Langmuir 1996, 12, 4850-4856
Molecular Imprint Membranes Prepared by the Phase Inversion Precipitation Technique Hong Ying Wang, Takaomi Kobayashi,* and Nobuyuki Fujii Department of Chemistry, Nagaoka University of Technology, Kamitomioka, Nagaoka, Niigata 940-21, Japan Received March 14, 1996X Molecular imprint membranes of theophylline (THO) were prepared by the phase inversion technique with acrylonitrile-acrylic acid copolymer. The copolymer cast solution of dimethyl sulfoxide (DMSO) with the THO template was used and coagulated in poor solvent, water. After removal of the template from the solidified copolymer membrane, THO in aqueous solution was permeated as a solute through the resultant membrane by means of pressure driven filtration. It was shown that the uptake of the solute molecule strongly depends on the template concentration in the DMSO cast solution. The increase of the THO concentration in the cast solution caused an increase in the solute taken into the copolymer membrane. Further, the selectivity of the THO-imprinted copolymer membrane was examined by the uptake experiment of caffeine (CAF) solute and was compared with that of THO solute. The total amounts of CAF taken into the membrane are much lower than that of THO substrate. This result supported the conclusion that the membrane records the shape of THO molecule during the phase inversion process. Evidence of the interaction between THO and copolymer was also shown by characterization of the THO-imprinted membrane.
Introduction Molecular imprinting has received much attention in recent years, because it has practical application in bioselective chromatograph materials.1-3 The imprinting technique was found to be an effective means of recording molecular information in polymeric materials like crosslinked gels. In this way, progress has been made in building the specific binding sites into the cross-linked polymer matrix obtained by copolymerization of methacrylic acid (MA) monomer with the template molecule. After removal of the template from the gel matrix, these polymer matrices were applied to chromatographic uses such as amino acid derivative separation4,5 and chiral recognition.6 In these uses, the copolymerization of the functional MA monomer with the cross-linked monomer ethylene glycol dimethacrylate (EDMA) gave a rigid network polymer matrix. For example, in recent works reported by Mosbach and co-workers,1,7 the molecular imprinted polymer of both MA and EDMA monomers can be prepared by convenient radical polymerization in the presence of the template molecule theophylline (THO). It was shown that the rigid insoluble polymer records the molecule shape and its chemical functionality in the matrix network after the removal of the template from the matrix. But these studies of the imprint material are mainly concerned with chromatographic uses, because the imprinting techniques have been suitable only for crosslinked polymer matrices. On the other hand, to date, little is known concerning a class of the membrane with imprinting functionality. This is due to the problem of handling the rigid gel type matrices prepared by the copolymerization. More recently, we reported the preliminary result of our investigations on the molecular X Abstract published in Advance ACS Abstracts, September 1, 1996.
(1) Mosbach, K. Trends Biochem. Sci. 1994, 19 (1), 9. (2) Wulff, G. In Molecular Interaction in Bioseparations; Ngo, T., Ed.; Plenum Press: New York, 1993; p 363. (3) Norrlon, O.; Glad, M.; Mosbach, K. J. Chromatogr. 1984, 299, 29. (4) Sellergren, B.; Ekberg, B.; Mosbach, K. J. Chromatogr. 1985, 347, 1. (5) Andersson, L.; Sellergren, B.; Mosbach, K. Tetrahedron Lett. 1984, 25, 5211. (6) Sellergren, B.; Shea, K. J. J. Chromatogr., A 1993, 654, 17. (7) Vlatakis, G.; Andersson, L. I.; Muller, R.; Mosbach, K. Nature 1993, 361, 645.
S0743-7463(96)00243-0 CCC: $12.00
Chart 1
imprinting of THO in a microporous membrane of poly(acrylonitrile-co-acrylic acid) (P(AN-co-AA)) (Chart 1).8 The copolymer contains both acrylic acid (AA) residues as the functional sites and acrylonitrile (AN) residues as membrane formation sites. Because the AN segments in the copolymer strongly coagulate in a poor solvent such as water, the membrane type of the imprint matrix was developed by the phase inversion precipitation method.9,10 The phase inversion involves the polymer transforming from the liquid phase of the cast solution to the solid state, and the process of polymer solidification is very often initiated by polymer coagulation in a poor solvent. It has been well-known that the technique is applied to produce typical separation membranes11-13 for ultrafiltration, microfiltration, and reverse osmosis filtration. In order to control the permeability of the resulting membrane, especially an ultrafiltration membrane, some water soluble additives such as LiNO3,14 LiCl,15 poly(vinyl alcohol),16,17 and poly(ethylene glycol)18 have been added to the polymer cast solution. Then, the solidification has taken place in the presence of the additive. Hence, the resulting polymeric membrane contains their volumetric (8) Kobayashi, T.; Wang, H. Y.; Fujii, N. Chem. Lett. 1995, 927. (9) Mulder, M. Basic Principles of Membrane Technology; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991; p 58. (10) Cabasso, I. In Encyclopedia of Polymer Science and Engineering; Mark, H. F., Bikales, N. M., Overberger, C. G., Menges, G., Kroschwitz, J. I., Eds.; John Wiley & Sons: Toronto, 1987; Vol. 9, p 509 and references therein. (11) Loeb, S.; Sourirajan, S. Adv. Chem. Ser. 1962, 38, 117. (12) Munari, S.; Bottino, A.; Capanneli, G. J. Membr. Sci. 1983, 16, 181. (13) Koenhen, D. M.; Mulder, M. H. V.; Smolder, C. A. J. Appl. Polym. Sci. 1977, 21, 199. (14) Nakao, S.; Osada, H.; Kurata, H.; Tsuru, T.; Kimura, S. Desalination 1988, 70, 191. (15) Hinke, E.; Staude, E. J. Appl. Polym. Sci. 1991, 42, 2951. (16) Miyama, H.; Tanaka, K.; Nosaka, Y.; Fujii, N. J. Appl. Polym. Sci. 1988, 36, 925. (17) Kobayashi, T.; Nagai, T.; Suzuki, T.; Nosaka, Y.; Fujii, N. J. Membr. Sci. 1994, 86, 47. (18) Kobayashi, T.; Miyamoto, T.; Nagai, T.; Fujii, N. J. Appl. Polym. Sci. 1994, 52, 1519.
© 1996 American Chemical Society
Preparation of Molecular Imprint Membranes Chart 2
size after washing the additive away with water. These approaches give us the possibility of preparation on the imprinted membrane by the phase inversion technique, if the polymer material has a functional group which interacts with the template. We report the imprinting investigation of THO in P(ANco-AA) porous membranes by the phase inversion method. Included are membrane characteristics and substrate selectivity results in comparison with the performance of the uptake of THO and caffeine (CAF) (Chart 2) by the copolymer membranes. The emphasis in this work is mainly placed on the effect of the THO template in the cast solution on the imprint properties of the phase inversion membrane of P(AN-co-AA). Experimental Methods Materials. All reagents used in the experiment were of reagent grade unless otherwise described. Acrylonitrile, AA, and dimethyl sulfoxide (DMSO) were distilled before use.8,19 THO and CAF were products of Tokyo Kasei and used without further purification. Various molecular weight (MW) dextrans in the range 1 × 104 to 2 × 106 (Pharmacia) were employed without further purification as probes20 for estimating the pore size of the copolymer membranes.21 Water was distilled and then purified by passing it through ion exchange resin. Copolymerization. Copolymerization of AN and AA was carried out in DMSO solution as previously reported.8,19 Identification of the resulting copolymer, P(AN-co-AA), was carried out using an FT-IR spectrometer (FT-IR 8200, Shimadzu) and an 1H-NMR spectrometer (JNM-GX 270 FT-NMR; DMSO-d6). The IR peak intensity ratio of the CN stretching of PAN to the CdO stretching of polyacrylic acid was used for the calibration to estimate the AA segments in the copolymer. The content of the AA segment in the P(AN-co-AA) copolymer was also determined from the 1H-NMR spectrum of the copolymer in DMSO-d6. The values of the mole fraction of the copolymer estimated from the FT-IR and 1H-NMR data were 0.16 and 0.14, respectively. The viscosity of the copolymer in dimethylformamide solution was measured at 30 °C using an Ubbelohde viscometer. The value of the intrinsic viscosity, [η], estimated by extrapolating the linear ηsp/C vs concentration plot, was 2.25 cm3/g. The molecular weight of the copolymer was estimated as 7 × 104 according to the literature.22 Membrane Preparation by Phase Inversion. Preparation of the P(AN-co-AA) membranes by the phase inversion method was carried out according to Scheme 1, as well as our previous procedure.7,16-18,20 Here, DMSO and water were selected as the cast solvent of the copolymer and the coagulation medium, respectively. The THO template was dissolved into the DMSO cast solution in the range 0-4.7 wt %. At concentrations higher than 4.7 wt %, the solubility of the template molecule in th DMSO cast solution is not good, because of its saturation. DMSO cast solutions containing 10 wt % P(AN-co-AN) copolymer and various weight percents of the template were well mixed at 50 °C for 20 h. The solutions were then cast on a glass plate warmed at 50 °C and coagulated in water at 23 °C. It was reported by Mosbach that the removal of the THO template from the gel matrix consisting of MA and EDMA was carried out by washing it with 0.1 wt % aqueous acetic acid (AcOH). (19) Kobayashi, T.; Miyamoto, T.; Nagai, T.; Fujii, N. Chem. Lett. 1993, 663. (20) (a) Granath, K. A.; Kvist, B. E. J. Chromatogr. 1967, 28, 69. (b) Walther, D. H.; Sin, G. M.; Blanch, H. W.; Pransnitz, J. M. J. Macromol. Sci. Phys. 1994, B33, 267. (21) Kobayashi, T.; Kumagai, K.; Nosaka, Y.; Miyama, H.; Fujii, N. J. Appl. Polym. Sci. 1991, 43, 1037. (22) Onyon, P. F. J. Phys. Soc. 1959, 37, 315.
Langmuir, Vol. 12, No. 20, 1996 4851 Scheme 1
According to the method, we tried to remove the THO template by washing with a large excess of AcOH solution in the present work. Then, the membrane was rinsed with a large excess of water to remove the AcOH. For a reference experiment, the cast solution without the THO template was used in 10 wt % P(AN-co-AA). In addition, a PAN homopolymer membrane was also used for the unimprinted membrane obtained by a similar procedure to that for the THO imprinted membrane. The viscosity of the DMSO cast solution containing various concentrations of the THO template was measured with a B-type viscometer (Model BL, Tokimec Inc., Roter No 3 at 6 and 12 rpm) at 50 °C. Membrane Characterization. For the FT-IR sample, the copolymer membrane was obtained by the cast on the glass plate with about 1 µm thickness and then the thin cast copolymer solution was immersed in water to coagulate in the presence of THO. After the membrane was washed well with water, it was freeze-dried to remove the water in the membrane. The FT-IR spectra of the copolymer membranes were measured with the transmittance setup with 40 accumulations for the thin film obtained. The morphology of the cross section of the copolymer membranes was observed by a scanning electron micrograph (SEM), JXA-733 (Jeol). The wet sample of the membrane was lyophilized. The cross section was obtained by fracturing the membrane at liquid nitrogen temperature. Gold sputter on the membrane was carried out by use of an SPM-112 (Anelva) sputter gun. In order to estimate the pore size of the copolymer membranes, the molecular weight cutoff (MWCO) of the membranes was measured by permeation of dextran solution through the membranes. The apparatus for the dextran permeation is similar to that previously used.8,20 Rejection, R, of the dextran was calculated as
R ) (Cf - Cp)/Cf
(1)
where Cf and Cp are the concentration of feed and permeated solutes, respectively. Dextran in the feed and the permeate solutions was analyzed with a GPC apparatus (type CCPE-II RI 8000 of TOYO SODA with 30 cm columns of TSKgel G5000 PWXL). The pore radius (rp) of the copolymer membranes was estimated by using the dextran permeation data with the sieve slit model (eq 2).23,24
rs ) rp/R
(2)
Here, the molecular weight (MW) of dextran, which is the cutoff value at R ) 0.9, was converted to molecular radius, rs, according to the Stokes-Einstein equation25
rs ) kT/6πηD
(3)
where k, T, η, and D are the Boltzmann constant, the absolute temperature, the viscosity, and the diffusivity (cm3/s). (23) Sarboloauki, M. N. Sep. Sci. Technol. 1982, 17, 381. (24) Sarboloauki, M. N. J. Appl. Polym. Sci. 1984, 29, 743. (25) Deen, W. M.; Bohrer, M. P.; Brenner, B. M. Kidney Int. 1979, 16, 353.
4852 Langmuir, Vol. 12, No. 20, 1996
Wang et al. Scheme 2
Figure 1. FT-IR spectra of P(AN-co-AA) membranes prepared from the cast solution (a) with 4.7 wt % THO template, (b) after washing with 0.1 wt % AcOH aqueous solution, and (c) with THO. Substrate Permeation Experiments. Permeation experiments on an aqueous solution containing 3.6 µM THO or CAF solute were carried out with the pressure driven setup at 2.5 kPa using the Amicon type 8050 cell with a 50 mL volume of the solution. The substrate concentrations in feed and permeate solutions were analyzed by HPLC (CCPD, Toyo Soda Co.) with a TSKgel-ODS80 column (1.5 × 20 cm2). The eluent solution was MeOH:H2O ) 7:3 (v/v), and the eluent rate was 0.5 mL/min. The absorbance of the permeate solution was monitored at 270 nm with a UV-vis detector (UV8000) connected to the HPLC apparatus.
Results and Discussion Characteristics of P(AN-co-AA) Membranes. Before the THO substrate permeation across the copolymer membrane obtained by the extraction of the template, FT-IR spectra of the membrane were measured in order to examine the interaction between the THO template and the resultant copolymeric membrane. Comparison of FT-IR spectra was carried out between the copolymer membranes before and after washing with AcOH aqueous solution. Figure 1 shows representative FT-IR spectra of the copolymer membranes cast using a 4.7 wt % THO template solution and THO in KBr. We can find the characteristic peaks of CN stretching at 2242 cm-1 for PAN segments26 and those of CdO stretching near the 1720-1730 cm-1 region for the AA segments of the copolymer.27 Also, the spectra have IR peaks for CH stretching near 1449 and 2937 cm-1. The removal of the template molecule from the P(AN-co-AA) membrane was checked by comparison of spectra a and b of Figure 1 before and after washing. We noted that the peak intensity ratio of the CdO stretching to the CN stretching shows slightly different values in both spectra. The values obtained for spectra a and b are 1.36 and 1.41, respectively. This means that, because the IR peak for CdO stretching of the THO molecule for the template overlaps with that for the (26) Yamadera, R. Koubunshi Kagaku 1964, 21, 362. (27) Lee, Y. M.; Oh, B. J. Membr. Sci. 1995, 98, 183. (28) Reference 9, pp 58-68. (29) Friendrich, C.; Driancourt, A.; Noel, C.; Monnerie, L. Desalination 1981, 36, 39. (30) Guillotin, M.; Lemoyne, C.; Noel, C.; Monnerie, L. Desalination 1977, 21, 165. (31) Bottino, A.; Capannelli, G.; Munari, S. J. Appl. Polym. Sci. 1985, 30, 3009. (32) Kobayashi, T.; Nagai, T.; Ono, M.; Wang, H. Y.; Fujii, N. J. Appl. Polym. Sci., in press.
copolymer as shown in Figure 1c, the washing can remove the template from the membrane. It is noted that the CdO peak appears near 1724 and 1730 cm-1 for the copolymer membranes with and without the washing treatment, respectively. The CdO peak wavenumber for the washed membrane is identical with that for the membrane prepared from the cast solution without the THO template. Accordingly, a slight shift toward the lowwavenumber side implies the interaction between the THO molecule and COOH segments in the membrane. Note that the OH stretching vibration in the 3200-3500 cm-1 region has two peaks in spectra a and b. In general, carboxylic acids such as R-COOH exist as dimers due to strong hydrogen bonding, as shown in Scheme 2a. The carboxylic acid dimers display very broad OH stretching in the region 2500-3300 cm-1. Hence, the broad peaks in the region for spectra a and b imply the presence of hydrogen bonding between the COOH groups in the copolymer membrane. In addition, a free OH stretching vibration is observed near 3500 cm-1.33,34 As the THO template is present in the membrane, the appearance of the broad IR peak near 3522 cm-1 in spectrum a, therefore, results from the free OH group of AA segments. That is, the added THO template disrupts the inter- or intramolecular hydrogen bonding between AA segments in the membrane. Then, as shown in Scheme 2b, the THO template hydrogen bonds with the AA segments. Figure 2 shows SEM photographs of the cross section of the P(AN-co-AA) membranes prepared from various cast solutions containing the THO template at concentrations of (a) 0.5, (b) 2.5, and (c) 4.7 wt %. It is obvious from the SEM photographs that the copolymer membranes have an asymmetric structure, which consists of a dense top layer supported by a large porous sublayer with a fingerlike structure in the total thickness (about 70 µm). We saw that the SEM photograph for the copolymer membrane prepared from the cast solution without the template has a similar morphology of the cross section structure to that of photograph a. This comparison suggests that the THO addition into the cast solution is regardless of the morphology of the resultant membrane at lower than 0.5 wt % THO. These membranes have a very thin dense top layer (
