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Poly(vinylidene fluoride) with Grafted Zwitterionic Polymer Side Chains for Electrolyte-Responsive Microfiltration Membranes Guangqun Zhai, S. C. Toh, W. L. Tan, E. T. Kang,* and K. G. Neoh Department of Chemical and Environmental Engineering, National University of Singapore, Kent Ridge, Singapore 119260
C. C. Huang and D. J. Liaw Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan 106 Received March 14, 2003. In Final Form: May 23, 2003 Thermally induced molecular graft copolymerization of the zwitterionic monomer, N,N′-dimethyl(methylmethacryloyl ethyl)ammonium propanesulfonate (DMAPS), with the ozone-preactivated poly(vinylene fluoride) (PVDF) was carried out in a mixed solvent of N-methyl-2-pyrrolidone (NMP) and dimethyl sulfoxide (DMSO). The chemical composition of the resulting PVDF with grafted DMAPS polymer (PDMAPS) side chains, or the PDMAPS-g-PVDF copolymers, was analyzed by elemental analysis. An increase in the [DMAPS]/[-CH2CF2-] molar feed ratio used for graft polymerization gave rise to an increase in the graft concentration of the DMAPS polymer in the copolymer. Microfiltration (MF) membranes were prepared from the DMSO solutions of the copolymers by phase inversion in aqueous media of different ionic strength and temperature. The surface composition and morphology of the MF membranes were investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), respectively. The mean pore size of the membrane decreased with the increase in graft concentration of the PDMAPS-g-PVDF copolymer. However, it increased with an increase in the ionic strength of the casting bath. Due to the anti-polyelectrolyte effect, the permeability of aqueous solutions through the PDMAPS-g-PVDF MF membranes exhibited a dependence on electrolyte concentration. The flow rate was observed to decrease as the electrolyte concentration of the permeate was increased.
1. Introduction Although poly(vinylidene fluoride) (PVDF) has been widely studied as a membrane material,1 its potential has yet to be fully realized in the lack of its stimuliresponsiveness to the changing environment. To overcome this drawback, a number of studies have been dedicated to the surface modification of existing PVDF membranes,2-4 as well as other polymeric membranes prepared from poly(ethylene-co-vinyl alcohol),5 poly(tetrafluoroethylene),6 and polycarbonate (PC),7 to improve their responsiveness to stimuli, such as changes in pH and temperature of the environment. An alternative approach involves the molecular graft copolymerization of PVDF with a functional monomer, such as methoxypoly(ethylene glycol) monomethacrylate (MPEGMA),8 acrylic acid (AAc),9 4-vinylpyridine(4VP),10 and N-isopropylacrylamide * To whom all correspondence should be addressed. Fax (65)6779-1936. E-mail:
[email protected]. (1) Li, K. Chem. Eng. Technol. 2002, 25, 2. (2) Tarvainen, T.; Nevalainen, T.; Sundell, A.; Svarfvar, B.; Hyrsyla, J.; Paronen, P.; Jarvinen, K. J. Control. Release 2000, 66, 19. (3) Akerman, S.; Viinikka, P.; Svarfvar, B.; Putkonen, K.; Jarvinen, K.; Kontturi, K.; Nasman, J.; Urtti, A.; Paronen, P. Int. J. Pharm. 1998, 164, 29. (4) Svarfvar, B. L.; Ekman, K. B.; Sundell, M. J.; Nasman, J. H. Polym. Adv. Technol. 1996, 7, 839. (5) Shieh, M. J.; Lai, P. S.; Young, T. H. J. Membr. Sci. 2002, 204, 237. (6) Ito, Y.; Ochiai, Y.; Park, Y. S.; Imanishi, Y. J. Am. Chem. Soc. 1997, 119, 1619. (7) Ito, Y.; Park, Y. S.; Imanishi, Y. J. Am. Chem. Soc. 1997, 119, 2739. (8) Wang, P.; Tan, K. L.; Kang, E. T.; Neoh, K. G. J. Mater. Chem. 2001, 11, 783.
(NIPAm),11 to produce PVDF with grafted side chains prior to the membrane fabrication by phase inversion. This latter approach provides an effective means for controlling the physiochemical characteristics, such as surface composition, morphology, pore-size distribution, and surface functionalities of the resulting membranes. In comparison with the pH-sensitive and thermoresponsive membranes, there have been relatively few reports on the electrolyte-sensitive polymeric membranes. Previous studies on water-soluble polymers and copolymers have suggested that a simple linear polyelectrolyte, such as poly(sodium acrylate) or poly (4-vinylpyridinium perchloric salt), ionizes in aqueous solution to result in repulsive interactions. The interactions, in turn, give rise to an extended conformation of the macromolecular backbones. Upon addition of a small amount of low molecular weight electrolyte, such as sodium chloride, to the solution, the polyelectrolyte chains will undergo contraction and aggregation (the “polyelectrolyte effect”).12-14 On the other hand, for polybetaines, i.e., polymers prepared from zwitterionic monomers, the electrostatic attraction among opposite charges will give rise to a (9) Ying, L.; Wang, P.; Kang, E. T.; Neoh, K. G. Macromolecules 2002, 35, 673. (10) Zhai, G. Q.; Ying, L.; Kang, E. T.; Neoh, K. G. J. Mater. Chem. 2002, 12, 3508. (11) Ying, L.; Kang, E. T.; Neoh, K. G. Langmuir 2002, 18, 6416. (12) Lowe, A. B.; McCormick, C. L. In Stimuli-Responsive Water Soluble and Amphiphilic Polymers; 2nd ed.; McCormick, C. L., Ed.; American Chemical Society: Washington, D.C., 2000; Vol. 780, pp 1-13. (13) Kee, R. A.; Gauthier, M. Macromolecules 2002, 35, 6526. (14) McCormick, C. L.; Kathmann, E. E. In The Polymeric Materials Encyclopedia; Salamone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996; Vol. 7, pp 7189-7201.
10.1021/la034440q CCC: $25.00 © 2003 American Chemical Society Published on Web 07/08/2003
PVDF with Zwitterionic Polymer Side Chains
collapsed, globule-like conformation in an aqueous solution. The presence of sodium chloride will shield the intrachain and interchain attraction, allowing the polymer chains to adopt a more extended conformation (the “antipolyelectrolyte effect”).12,14-17 In addition to the electrolyte-dependent behavior, another interesting characteristic of the polybetaines is their temperature-dependent phase transition. Polybetaines aggregate in aqueous solutions at room temperature and dissolve abruptly when heated to over their respective upper critical solution temperature (UCST).18-22 N,N′-dimethyl(methylmethacryloyl ethyl) ammonium propane sulfonate (DMAPS) is a sulfobetaine monomer. Surface graft copolymerization of DMAPS with polymer substrates, such as polyethylene (PE),23 polyaniline (PANi),24 and PC,25 has been carried out to enhance their surface functionalities. The DMAPS polymer was found to provide excellent hemocompability.26 The absorbency of poly(sodium acrylate) superabsorbent in saline solution can be improved further by the introduction of zwitterionic DMAPS units.27,28 This phenomenon can be attributed to the expansion of the DMAPS polymer chains (PDMAPS) in the electrolyte solution, i.e., the anti-polyelectrolyte effect, compared to the contraction of the sodium acrylate polymer chains, i.e., the polyelectrolyte effect. Attempts have also been made to study the solution properties of the DMAPS polymers.29,30 It was found that the UCST of the aqueous solution of PDMAPS is dependent on the polymer concentration, and the added low molecular weight salt (sodium chloride) has a complicated effect on the UCST. The consistency among these results is yet to be studied in more detail.31 In the present work, we report on the synthesis and characterization of PVDF with grafted DMAPS polymer side chains (PDMAPS-g-PVDF copolymers) from molecular graft copolymerization of DMAPS with the ozonepreactivated PVDF in solutions. The microfiltration (MF) membranes were cast from the dimethyl sulfoxide (DMSO) solutions of the copolymers in aqueous media by phase inversion. The effects of electrolyte concentration and temperature of the coagulation bath on the physicochemical characteristics of the resulting copolymer membranes are also investigated. The PDMAPS-g-PVDF copolymers (15) Kubota, T.; Tochiyana, O.; Tanaka, K.; Niibori, Y. Radiochim. Acta 2000, 88, 579. (16) Kwon, O. P.; Im, J. H.; Kim, J. H.; Lee, S. H. Macromolecules 2000, 33, 9310. (17) Kudaibergenov, S. E.; Sigitov, V. B. Langmuir 1999, 15, 4230. (18) Heinz, B. S.; Laschewsky, A.; Rekai, E. D.; Wischerhoff, E.; Zacher, T. In Stimuli-Responsive Water Soluble And Amphiphilic Polymers; 2nd ed.; McCormick, C. L., Ed.; American Chemical Society: Washington, D.C., 2000; Vol. 780, pp 162-181. (19) Arotcarena, M.; Heise, B.; Ishaya, S.; Laschewsky, A. J. Am. Chem. Soc. 2002, 124, 3787. (20) Georgiev, G. S.; Mincheva, Z. P.; Georgieva, V. T. Macromol. Symp. 2001, 164, 301. (21) Virtanen, J. Ph.D. Dissertation; University of Helsinki, Finland, 2002. (22) Virtanen, J.; Arotcarena, M.; Heise, B.; Ishaya, S.; Laschewsky, A.; Tenhu, H. Langmuir 2002, 18, 5360. (23) Kang, E. T.; Neoh, K. G.; Li, Z. F.; Tan, K. L.; Liaw, D. J. Polymer 1998, 39, 2429. (24) Li, Z. F.; Kang, E. T.; Neoh, K. G.; Tan, K. L.; Huang, C. C.; Liaw, D. J. Macromolecules 1997, 30, 3354. (25) Chen, W.; Neoh, K. G.; Kang, E. T.; Tan, K. L.; Liaw, D. J.; Huang, C. C. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 357. (26) Yuan, Y. L.; Al, F.; Zhang, J.; Zang, X. B.; Shen, J.; Lin, S. C. J. Biomater. Sci.,Polym. Ed. 2002, 13, 1081. (27) Lee, W. F.; Tu, Y. M. J. Appl. Polym. Sci. 1999, 72, 1221. (28) Lee, W. F.; Wu, R. J. J. Appl. Polym. Sci. 1997, 64, 1701. (29) Liaw, D. J.; Huang, C. C. Macromol. Symp. 2002, 179, 209. (30) Chen, L.; Honma, Y.; Mizutani, T.; Liaw, D. J.; Gong, J. P.; Osada, Y. Polymer 2000, 41, 141. (31) Shulz, D. N.; Peiffer, D. G.; Agarwal, P. K.; Laladas, J.; Soni, L.; Handwerker, B.; Garner, R. T. Polymer 1986, 102, 5246.
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are shown to be promising materials for fabricating MF membranes with electrolyte-sensitive flux behavior. 2. Experimental Section 2.1. Materials and Reagents. Poly(vinylidene fluoride) (Kynar K-761) powders (MW ) 441 000) were obtained from Elf Atochem of North America Inc. The monomer, N,N′-dimethyl(methylmethacryloyl ethyl) ammonium propane sulfonate was prepared according to the methods reported earlier.29,32 The chemical structure of DMAPS is shown as follows:
The solvent, N-methyl-2-pyrrolidone (NMP, reagent grade) was obtained from Merck Chemical Co. The solvent for the DMAPS powders, dimethyl sulfoxide, was obtained from LabScan Asia Ltd., Bangkok, Thailand. They were used as received. 2.2. Thermally Induced Graft Copolymerization of DMAPS with the Ozone Preactivated PVDF (the PDMAPSg-PVDF Copolymer). PVDF powders were dissolved in NMP to form a 7 wt % solution. A continuous O3/O2 mixture stream was bubbled through 30 mL of the NMP solution of PVDF at room temperature (∼25 °C) for about 15 min. The O3/O2 stream was generated from an Azcozon RMU16-40EM ozone generator. The flow rate was adjusted to 300 L/h to result in an ozone concentration of about 0.027 g/L in the gaseous mixture. Previous studies13 had reported a peroxide concentration of about 10-4 mol/g of PVDF under these conditions. After the ozone preactivation, the polymer solution was cooled in an ice bath. An argon stream was introduced for about 30 min to degas the ozone and oxygen dissolved in the solution. A predetermined quantity of the DMAPS monomer was dissolved in 20 mL of DMSO. The monomer solution was added to the degassed solution of ozone-preactivated PVDF to achieve the desirable [DMAPS]/[-CH2CF2-] molar ratio. After an additional 15 min of argon purging, the temperature of the water bath was raised to 60 °C to induce the decomposition of peroxide groups on the PVDF chains and to initiate the graft copolymerization of DMAPS. A constant flow of argon was maintained during the 3 h of thermal graft polymerization. After the reaction, the reaction mixture was cooled in an ice bath and the resultant PDMAPS-g-PVDF copolymer was precipitated in an excess amount of doubly distilled water. After recovery by filtration, the copolymer was washed repeatedly in ethanol and water for 48 h. The solvent was changed every 8 h. The copolymer sample was recovered and dried by pump under reduced pressure for subsequent characterization. 2.3. Free Radical Homopolymerization of DMAPS (PDMAPS). A 50 mL aliquot of doubly distilled water, 10 g of DMAPS, and 0.05 g of ammonium peroxydisulfate ((NH4)2S2O8) were introduced into a flask. The temperature of the reaction mixture was raised to 60 °C to induce the polymerization reaction. After 24 h of reaction, the flask was cooled in an ice bath and the reaction mixture was precipitated in excess NMP. The DMAPS homopolymer (PDMAPS) sample was recovered by filtration and dried in a vacuum oven. 2.4. UV-Visible Absorption Spectroscopy of the PDMAPS Solution. Known amounts of PDMAPS were added into aqueous media of different ionic strength. After the homopolymer had been dissolved completely at 70 °C, the absorbance of the solution at 500 nm was measured on a UVvis-near-IR spectrophotometer (Shimadzu UV-3101PC scanning spectrophotometer, Kyoto, Japan) under the time course mode. 2.5. Fabrication of the Microfiltration Membranes. The phase inversion technique was used to prepare the MF membranes. The PDMAPS-g-PVDF copolymer powders were dissolved in DMSO to a concentration of 12 wt % at room temperature. The copolymer solution was cast on a glass plate, which was subsequently immersed in an aqueous coagulation bath of a predetermined temperature and ionic strength. After the mem(32) Liaw, D. J.; Lu, W. F.; Whung, Y. C.; Lin, M. C. J. Appl. Polym. Sci. 1987, 34, 999.
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brane had detached from the glass plate, it was transferred into a second bath of doubly distilled water at 70 °C for 10 min. This procedure was to stabilize the pore structure and to refine the pore size distribution. The purified MF membrane was dried by pump under reduced pressure for the subsequent characterization. 2.6. Thermogravimetric Analysis. The thermal stability of the PDMAPS-g-PVDF copolymers was studied by thermogravimetric analysis (TGA). The samples were heated from room temperature to about 700 °C at a rate of 10 °C/min under a dry nitrogen atmosphere in a Du Pont Thermal Analyst 2100 system, fitted with a TGA 2050 thermogravimetric thermal analyzer. 2.7. X-ray Photoelectron Spectroscopy. The surface composition of the MF membranes was measured by X-ray photoelectron spectroscopy (XPS). XPS measurements were carried out on a Kratos AXIS HSi spectrometer with a monochromatized Al KR X-ray source (1486.71 eV photons) at a constant dwell time of 100 ms, a pass energy of 40 eV, and an anode current of 15 mA. The conditions for the sample measurement and data analysis were similar to those described earlier.10 2.8. Composition Analysis. The carbon, nitrogen, and hydrogen elemental contents were determined using a PerkinElmer 2400 element analyzer. The bulk graft concentration is defined as the number of DMAPS repeat units per repeat unit of PVDF or the ([-DMAPS-]/ [-CH2CF2])bulk ratio. Taking into account the elemental stoichiometries of both the graft and the fluoropolymer chains, the ([-DMAPS-]/[-C2CF2-])bulk ratio can be calculated from eq 1 below:
([-DMAPS-]/[-CH2CF2-])bulk ) 2[N]/([C] - 11[N])
(1)
where the factors 2 and 11 are introduced to account for the fact that there are 2 and 11 carbon atoms per repeat unit of PVDF and DMAPS polymer, respectively. 2.9. Scanning Electron Microscopy. The surface morphology of the resulting copolymer MF membranes was studied on a JEOL 6320 scanning electron microscope. The membranes were mounted on the sample studs by means of double-sided adhesive tapes. A thin layer of platinum was sputtered onto the membrane surface prior to the scanning electron microscopy (SEM) measurement. The measurements were performed under an accelerating voltage of 15 kV. 2.10. Pore Size Measurement. The pore size and the pore size distribution of the PDMAPS-g-PVDF membranes were measured on a Coulter Porometer II apparatus, manufactured by Coulter Electronics Ltd., Buckinghamshire, U.K. The commercial product, Porofil, for the Coulter Porometer instrument was used as the wetting agent. 2.11. Electrolyte-Sensitive Flux through the MF Membranes. The PDMAPS-g-PVDF MF membrane was immersed in the doubly distilled water for several minutes. It was then mounted on the microfiltration cell (Toyo Roshi UHP-25, Tokyo, Japan). An aqueous NaCl solution of a specific concentration was added to the cell. The flux was calculated from the volume of solution permeated per unit time under a specific fixed pressure.
3. Results and Discussion 3.1. Ozone Preactivation of PVDF and Graft Copolymerization of DMAPS with the PVDF (the PDMAPS-g-PVDF Copolymers). The direct oxidation of polymer chains by ozone is a well-known method for introducing peroxides and hydroperoxides for the subsequent radical-initiated graft polymerization.8,33,34 The peroxides can be used to initiate the free radical polymerization of DMAPS to produce the graft copolymers. The activation energy and Arrhenius coefficient (ln A) for the decomposition reaction of the initiators on the ozone preactivated PVDF chains are reported to be about 39 kJ/mol and 5.8, respectively.34 On the basis of these data, (33) Boutevin, B.; Robin, J. J.; Torres, N.; Casteil, J. Polym. Eng. Sci. 2002, 42, 78. (34) Fargere, T.; Abdennadher, M.; Delmas, M.; Boutevin, B. J. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 1337.
Figure 1. Effect of the [DMAPS]/[-CH2CF2-] molar feed ratio on the ([N]/[C])bulk ratio and the bulk graft concentration (([-DMAPS-]/[-CH2CF2-])bulk ratio) of the PDMAPS-g-PVDF MF membrane.
the half-life of the peroxide groups at 60 °C is calculated to be about 45 min. Thus, it is probably sufficient to fix the reaction time at 3 h for the thermally induced initiator decomposition and graft copolymerization in this work. 3.1.1. Composition Analysis of the PDMAPS-g-PVDF Copolymers. Elemental analysis was used to determine the carbon and nitrogen contents in the copolymer powders. The bulk graft concentration, or the ([-DMAPS-]/ [-CH2CF2-])bulk ratio, is calculated from the ([N]/[C])bulk ratio and eq 1 (see Experimental Section). The ([N]/[C])bulk ratio and the corresponding bulk graft concentration, or the ([-DMAPS-]/[-CH2CF2-])bulk ratio, as a function of the monomer to PVDF feed ratio ([DMAPS]/[-CH2CF2-] ratio) is shown in Figure 1. The results indicate that the graft concentration increases with the increase in monomer to polymer feed ratio, or the DMAPS monomer concentration used for graft copolymerization. 3.1.2. Thermogravimetric Analysis (TGA) of the PDMAPS-g-PVDF Copolymers. One of the unique properties of PVDF is its excellent thermal stability. The thermal stability of the PDMAPS-g-PVDF copolymer is studied by TGA. Compared to the PVDF and DMAPS homopolymer, a distinct two-step degradation process was observed for the graft copolymers. The onset of the first major weight loss at about 330 °C corresponds to the decomposition of the DMAPS side chains of the copolymers. The second major weight loss commences at about 400 °C, corresponding to the decomposition of the PVDF main chain. TGA results also indicate that the extent of the first major weight loss of each graft copolymer coincides approximately with the DMAPS polymer content in the copolymer, as determined from elemental analysis. 3.2. Thermoresponsive Behavior of the DMAPS Homopolymer. After synthesis and purification, the DMPAS homopolymer (PDMAPS) was dissolved in an aqueous medium at 70 °C to achieve a specific polymer concentration. UV-visible absorption spectroscopy was used to measure the absorbance of the PDMAPS aqueous solution at the wavelength of 500 nm under different temperatures.30,35-38 3.2.1. Effect of Polymer Concentration on the Upper Critical Solution Temperature (UCST) of PDMAPS. Figure 2a shows the absorbance of the aqueous solutions of PDMAPS of different concentrations as a function of (35) Zareir, H. M.; Bulmus, E. V.; Gunning, A. P.; Hoffman, A. S.; Piskin, E.; Morrise, V. J. Polymer 2000, 41, 6723. (36) Zareir, H. M.; Dincer, S.; Piskin, E. J. Colloid Interface Sci. 2002, 251, 424. (37) Dincer, S.; Tuncel, A.; Piskin, E. Macromol. Chem. Phys. 2002, 203, 1460. (38) Kim, E. J.; Cho, S. H.; Yuk, S. H. Biomaterials 2001, 22, 2495.
PVDF with Zwitterionic Polymer Side Chains
Figure 2. (a) UV-visible absorbance of aqueous solutions of PDMAPS of different concentrations as a function of temperature. (b) UV-visible absorbance of aqueous solutions of PDMAPS of different electrolyte concentration as a function of temperature.
temperature. All PDMAPS aqueous solutions exhibit an abrupt increase in absorbance with the decrease in solution temperature from 70 °C. This phenomenon corresponds to the transparent-opaque transition30 of the solution during the temperature declines. Such a transition is attributed to the phase separation of PDMAPS and water. At a high temperature, DMAPS polymer chains adopt an extended conformation and dissolve completely in the aqueous medium. With the temperature decreasing to its UCST, the intra- and interchain interactions of the DMAPS polymer lead to the formation of a well-defined micelle (dissolution-micellization transition). The polymer chain itself undergoes a hydrophilic-hydrophobic transformation. Figure 2a also indicates that the UCST of the PDMAPS solution exhibits a distinct dependence on the polymer solution concentration. When the concentration is very low (1-2 wt %, curves 1 and 2), the UCST decreases very gradually to about 60 °C. It decreases rapidly to about 50 °C when the polymer concentration increases to 5 wt % (curve 3). On the other hand, when the polymer concentration increases to over 10 wt % (curves 4 and 5), the UCST remains almost constant at about 45 °C. Thus, a high concentration of the polymer chains retards the phase separation of the PDMAPS solution. The stabilization of the PDMAPS solution at a high polymer concentration probably has resulted from the increased spatial and intermolecular interactions among the chains in solution. 3.2.2. Effect of Electrolyte Concentration on the UCST of PDMAPS. The dependence of the UCST of the PDMAPS aqueous solution on the electrolyte concentration (ionic strength) is also analyzed, as shown in Figure 2b. The absorbance data in Figure 2b suggest that the addition of a low molecular weight salt (sodium chloride in this study) can greatly decrease the UCST of the PDMAPS aqueous solution. Thus, the phase separation is retarded,
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especially when the electrolyte concentration of the aqueous medium is over 0.01 M. The UCST of the PDMAPS solution decreases from over 65 °C to less than 40 °C, when the electrolyte concentration of the aqueous medium increases from about 10-7 to 10-2 mol/L. For the PDMAPS solutions with electrolyte concentration above 10-1 mol/L, no phase separation was observed even when the solution was cooled to the ice bath temperature. The data in Figure 2b suggest that addition of electrolytes helps to stabilize the zwitterionic polymer solution and to prevent the phase separation. The stabilization of the PDMAPS aqueous solution in the presence of an electrolyte can be attributed to the anti-polyelectrolyte effect. For PDAMPS solution of low ionic strength, the electrostatic attraction of the oppositely charged ions on DMAPS polymer chains is predominant, and the polymer chains tend to adopt a coiled conformation, leading to micellization in the solution when the temperature is decreased. In the solution of high electrolyte concentration, the electrostatic attraction in the polymer chains is shielded. As a result, the polymer chains adopt a more extended conformation via the Debye-Huckel shielding effect.12 It should be noted that the data in Figure 2 are influenced by both thermodynamic and kinetic effects and should only be considered as rough estimates of the UCST’s. They only provide semiquantitative information with respect to the impact of solution conditions, such as graft and electrolyte concentration and temperature, on the membrane structure and on the membrane permeability. 3.3. Fabrication of MF Membranes from the PDMAPS-g-PVDF Copolymers. After the PDMAPSg-PVDF MF membranes were cast by phase inversion in aqueous media of different ionic strength and temperature from the 12 wt % DMSO solution of the respective copolymers, the surface composition and morphology of the membrane were analyzed by XPS and SEM, respectively. 3.3.1. XPS Analysis of the MF Membranes. The surface composition of the PDMAPS-g-PVDF MF membranes was studied by XPS. Taking into consideration the thermoresponsive nature of the DMAPS polymer side chain, XPS analysis of the surface composition of the copolymer membranes cast from copolymer solutions at different temperatures was first carried out. However, the XPS C 1s core-level line shape of the copolymer membrane was found to undergo obvious changes only when the temperature of the casting bath was increased to about 100 °C. The changes in the C 1s core-level spectra of the pristine PVDF membrane and three PDMAPS-g-PVDF MF membranes of different graft concentrations cast at room temperature and at about 100 °C are compared in Figure 3. For the pristine PVDF membrane cast at room temperature, the C 1s core-level spectrum can be curve-fitted with two peak components, having binding energies (BE’s) at about 285.8 eV for the CH2 species and 290.5 eV for the CF2 species, respectively.9 The ratio for the two species, determined from the spectral peak component area, is about 1.04, which is in good agreement with the structure of PVDF. On the other hand, the C 1s core-level spectrum of the PVDF membrane cast at 100 °C can be curve-fitted with three peak components. In addition to the two main components mentioned above, the minor peak component at the BE of about 284.6 eV is attributed the neutral hydrocarbon, arising from the branching sites and end groups of the PVDF chains.39 The C 1s core-level spectra of the PDMAPS-g-PVDF MF membranes are curve-fitted
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Figure 4. Effect of [DMAPS]/[-CH2CF2-] molar feed ratio on the ([N]/[C])surface ratio and the surface graft concentration ([-DMAPS-]/[-CH2CF2-])surface ratio) of the PDMAPS-g-PVDF MF membrane cast at room temperature and at 100 °C, respectively.
Figure 3. XPS C 1s core-level spectra of the membranes cast by phase inversion at 25 °C and at about 100 °C from 12 wt % DMSO solutions of (a) the pristine PVDF homopolymer and the PDMAPS-g-PVDF copolymers prepared from the [DMAPS]/ [-CH2CF2-] molar feed ratios of (b) 0.05, (c) 0.11, and (d) 0.23.
with five peak components using the following approach. The two peak components of about equal intensities at the BE’s of about 285.8 and 290.5 eV are assigned to the CH2 and CF2 species of PVDF, respectively. The component at the BE of about 288.5 eV is assigned to the -O-CdO species of the grafted DMAPS polymer chains.9,40,41 The component with the BE at 284.6 eV, on the other hand, is attributed to the hydrocarbon backbone of the grafted DMAPS polymer chain, or the CH species. The peak component at the BE of about 287 eV arises from the combined contribution of the C-O species,9 C-N+ species,25 and C-SO3- species42 of the DMAPS polymer side chains. The C 1s core-level spectra in Figure 3 show that, after ozone preactivation, graft copolymerization and phase inversion in water at room temperature, the spectral area ratio of the CH component at 284.6 eV to that of the CF2 component at 290.5 eV increases from 0 (pristine PVDF membrane) to over 65% when the feed ratio used for graft copolymerization increases from 0 to 0.23. The XPS spectra in Figure 3 also show that both the pristine PVDF membrane and the PDMAPS-g-PVDF MF membranes cast at 100 °C always have a higher [CH]/[CF2] ratio than that of the corresponding MF membrane cast at room tem(39) Russo, S.; Pianca, M.; Giovanni, M. In The Polymeric Materials Encyclopedia; Salamone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996; Vol. 7, pp 7123-7138. (40) The Handbook of X-ray Photoelectron Spectroscopy, 2nd ed.; Moulder, J. F., Stickle, W. F., Sobol, P. E., Bomben, K., Eds.; PerkinElmer Corp. (Physical Electronics): Wellesley, MA, 1992; pp 227-228. (41) Fraser, S.; Short, R. D.; Barton, D.; Bradley, J. W. J. Phys. Chem. B 2002, 106, 5596. (42) Ruangchuay, L.; Schwank, J.; Sirivat, A. Appl. Surf. Sci. 2002, 199, 128.
perature. The enhancement in surface graft concentration of the PDMAPS-g-PVDF MF membranes is attributable to the thermoresponsive nature of the DMAPS polymer side chains. At a high temperature, the DMAPS polymer becomes more hydrophilic than at a low temperature. When the polymer solution undergoes phase inversion in an aqueous bath, the migration of the DMAPS polymer side chains from the bulk matrix to the surface occurs. For the PDMAPS-g-PVDF MF membranes cast at the two temperature extremes, their surface graft concentration, or the ([-DMAPS-]/[-CH2CF2-])surface ratio, can be determined from the ([N]/[C])surface ratio and eq 1, in a manner similar to that used to determine the bulk graft concentration. Figure 4 shows that the surface graft concentration of the MF membranes cast at different temperatures increases gradually with the increase in ([DMAPS]/[-CH2CF2-]) molar feed ratio used for graft copolymerization. The surface graft concentration of the MF membrane determined by XPS and the bulk graft concentration determined by elemental analysis are compared (compared Figure 1 and Figure 4). It is unambiguous that the bulk graft concentration of the copolymer is always higher than the surface graft concentration of the corresponding MF membrane, despite the fact that a higher temperature of the casting bath can increase the surface graft concentration of the MF membrane. For other MF membranes involving PVDF with grafted hydrophilic side chains and cast in an aqueous medium, the surface graft concentration is always much higher than the corresponding bulk graft concentration, as a result of surface enrichment of the side chains.8-10 Thus, the surface migration or rearrangement of the zwitterionic graft chains in the PDMAPS-gPVDF MF membranes is entirely different from that of the MF membranes prepared from PVDF with grafted acrylic acid (AAc), 4-vinylpyridine (4VP), ethylene glycol, and N-isopropylacrylamide (NIPAm) side chains. Arising from reduced hydrodynamic interactions, especially for membranes cast below the UCST of DMAPS polymer in aqueous (nonelectrolyte) media (see Figure 2), the micellization of the zwitterionic polymer in the surface and near-surface regions, as a result of the phase inversion process, probably has reduced substantially the effective surface concentration of the DMAPS polymer probed by XPS. The low degree of surface aggregation of the DMAPS side chains can also be attributed to their relatively short chain length. The average degree of polymerization of DMAPS, defined as the average number of repeat units of DMAPS per initiation site, is estimated to be in the
PVDF with Zwitterionic Polymer Side Chains
Figure 5. XPS C 1s core-level spectra of PDMAPS-g-PVDF MF membrane (([-DMAPS-]/[-CH2CF2-])bulk ) 0.20) cast from 12 wt % DMSO solution at room temperature by phase inversion in aqueous media of different electrolyte strength: (a) doubly distilled water and (b) 10-4, (c) 10-3, and (d) 10-1 mol/L electrolyte.
range of 4-16 when the monomer to polymer feed ratio used for graft copolymerization is increased from 0.05 to 0.23. The XPS C 1s core-level spectra of the MF membranes cast from a 12 wt % DMSO solution of the PDMAPS-gPVDF copolymer (([-DMAPS-]/[-CH2CF2-])bulk ) 0.20) in aqueous medium of different electrolyte concentrations (I’s) at room temperature are shown in Figure 5. The spectral area ratio of the (CH)DMAPS component to the (CF2)PVDF component increases initially with the electrolyte concentration of the casting bath up to about 10-4 mol/L. The ratio, however, undergoes a gradual decrease when the electrolyte concentration of the casting bath is increased further. Low molecular weight electrolyte has a complicated effect on the behavior of DMAPS polymer chains in an aqueous medium.30 When the ionic strength of the aqueous solution becomes large, the electrostatic attraction of the polymer chains is gradually shielded by the ions in the aqueous medium, and the distribution of DMAPS polymer side chains on the membrane surface, partially driven by the electrostatic attraction, is reduced. As a result, the spectral area ratio of the (CH)DMAPS species to the (CF2)PVDF species decreases at a high electrolyte concentration of the casting bath. 3.3.2. Surface Morphology of the MF Membranes. The surface morphologies of the PDMAPS-g-PVDF MF membranes and pristine PVDF membrane were revealed by SEM. Figure 6 shows the SEM images obtained at a magnification of ·5000 (reproduced at 85% of original size) for the MF membranes cast from 12 wt % DMSO solution by phase inversion in doubly distilled water at room temperature. SEM images reveal that the incorporation of the DMAPS side chains increases the porosity of the PDMAPS-g-PVDF MF membranes, while the mean pore size decreases with the increase in bulk graft concentra-
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Figure 6. SEM micrographs of the MF membranes cast by phase inversion from the 12 wt % DMSO solutions of (a) the pristine PVDF and the PDMAPS-g-PVDF copolymers of different bulk graft concentrations of (b) 0.10, (c) 0.12, and (d) 0.20.
tion. The formation of the microporous membrane structure probably can be attributed to the hydrodynamic interactions of the grafted DMAPS chains with the aqueous medium during the phase inversion process. The hydrodynamic interactions arise from the dissolutionmicellization effect of the DMAPS graft chains, as well as the increase in hydrophilicity of PVDF after graft copolymerization with DMAPS. The water contact angle of the DMAPS-g-PVDF copolymers decreases from about 110 to about 90° when the graft concentration (([-DMAPS-]/ [-CH2CF2-])bulk ratio) increases from 0.05 to 0.2. The morphology of the PDMAPS-g-PVDF MF membrane (([-DMAPS-]/[-CH2CF2-])bulk ) 0.20) cast from 12 wt % DMSO solution in aqueous media of different I’s at room temperature. The SEM results reveal that the pore dimension of the PDMAPS-g-PVDF MF membrane increases gradually with the ionic strength of the casting bath. When cast in the bath of low electrolyte concentration, electrostatic attraction will result in a reduction in pore dimension. On the other hand, when cast in the aqueous bath of high electrolyte concentration, the interaction of the zwitterionic graft chains with the aqueous electrolyte during the phase inversion process shields the electrostatic attraction among the graft chains (the anti-polyelectrolyte effect). The anti-polyelectrolyte effect forces the DMAPS chains in the surface and nearsurface regions to assume the fully extended conformation. The subsequent dissolution-micellization transition during the phase inversion process gives rise to the increased pore dimension in the “dried” membrane. 3.4. Pore Size Measurements of the MF Membranes. The pore sizes and pore-size distribution of the PDMAPS-g-PVDF membranes cast from copolymer solutions of different graft concentrations in aqueous media of different electrolyte concentrations are measured on the Coulter Porometer II, according to the procedures reported earlier.9-11
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Zhai et al.
Table 1. Pore Size Distribution of the PDMAPS-g-PVDF MF Membranesa [DMAPS]/[-CH2CF2-] molar feed ratio
0.06 0.11 0.17 0.23
0.23 0.23 0.23 0.23
bulk graft concn
electrolyte temp of strength pore size (µm) casting of casting soln (°C) bath (mol/L) min max mean
(a) Effect of Bulk Graft Concentration on the Pore-Size Distributionb 0.05 25 0 0.29 0.10 25 0 0.35 0.12 25 0 0.33 0.20 25 0 0.23
1.30 1.12 0.89 0.82
0.64 0.52 0.48 0.31
(b) Effect of Ionic Strength of the Casting Bath on the Pore-Size Distributionc 0.20 25 1.0 × 10-7 0.23 0.82 0.20 25 1.0 × 10-4 0.22 0.88 0.20 25 1.0 × 10-2 0.24 0.96 0.20 25 1 0.23 1.14
0.31 0.32 0.35 0.44
a The commercial PVDF MF membrane (obtained from Millipore Corp., Bedford, MA) has a standard pore diameter designation of d ) 0.22 µm. The actual pore characteristics are as follows: maximum pore size ) 0.72 µm; minimum pore size ) 0.52 µm; mean pore size ) 0.57 µm. b Cast in doubly distilled water. c Cast in aqueous NaCl solution of different concentrations.
The pore size and pore-size distribution of the MF membranes cast from the 12 wt % DMSO solution of the respective copolymers in doubly distilled water are summarized in Table 1. The data suggest that the mean pore size decreases gradually with the increase in bulk graft concentration of the PDMAPS-g-PVDF copolymer. The dependence of the pore size and pore-size distribution of the PDMAPS-g-PVDF MF membrane on the electrolyte strength of the casting bath is also shown in Table 1. Thus, the mean pore size of the PDMAPS-g-PVDF MF membrane increases with the electrolyte concentration of the casting bath. This phenomenon is attributed to the anti-polyelectrolyte effect as mentioned above. The pore-size measurement also indicates that the PDMAPS-g-PVDF MF membranes have an effective mean pore size comparable to that of the commercial hydrophilic PVDF MF membrane with a standard pore diameter of 0.22 µm. Thus, the latter was used for the comparative study of the electrolyte-dependent flux behavior with the present PDMAPS-g-PVDF MF membranes. 3.5. Flux Behavior Through the PDMAPS-g-PVDF MF Membranes. The electrolyte-dependent permeability of aqueous solutions through the PDMAPS-g-PVDF MF membranes, defined as the ratio of flux to pressure drop across the membrane, is shown in Figure 7. Sodium chloride was added to achieve the specific ionic strength of the solution. The flux of the aqueous solution through the pristine PVDF membrane cast from the 12 wt % DMSO solution by phase inversion exhibits an electrolyteindependent behavior (curve 4). However, the flux of the aqueous media through the commercial hydrophilic PVDF MF membrane of comparable effective pore size (obtained from Millipore Corp., Bedford, MA) shows a weak dependence on the electrolyte concentration of the medium, especially in the high electrolyte concentration range (curve 5). On the other hand, the flux of the aqueous solution through the PDMAPS-g-PVDF MF membranes decreases with the increase in electrolyte concentration from 10-7 to 10-1 mol/L, with the most drastic change in flux being observed at electrolyte concentrations between 10-7 and 10-3 mol/L (curves 1-3). The dependence of permeation rate through the PDMAPS-g-PVDF MF membrane on the electrolyte concentration is attributable to the conformational change of the DMAPS polymer side chains on the membrane
Figure 7. Electrolyte concentration-dependent permeability of aqueous solution through the PDMAPS-g-PVDF MF membranes. Curves 1 and 2 are the permeability through the MF membranes cast from PDMAPS-g-PVDF copolymer (([-DMAPS-]/[-CH2CF2-])bulk ) 0.10) in the coagulation bath with an electrolyte strength of 10-7 and 10-4 mol/L, respectively, at room temperature. Curve 3 is the permeability through the MF membranes cast from the PDMAPS-g-PVDF copolymer (([-DMAPS-]/[-CH2CF2-])bulk ) 0.20) in doubly distilled water at room temperature. Curve 4 is the permeability through the membrane cast from the PVDF homopolymer. Curve 5 is the permeability through the commercial hydrophilic PVDF MF membrane with a standard pore diameter of d ) 0.22 µm. The permeability is defined as the ratio of flux (in ml/min) to pressure drop across the membrane (bar/cm).
surface and subsurface, especially on the pore surface and subsurface regions. When the electrolyte concentration is very low, the electrostatic attraction among the ammonium cations and sulfate anions forces the DMAPS side chains to adopt a coiled conformation. On the other hand, due to the electrostatic screening effect, a high electrolyte concentration will shield the electrostatic attraction among the ammonium cations and sulfate anions. The DMAPS graft chains in the surface and subsurface regions adopt a more expanded conformation as a result of the anti-polyelectrolyte effect. The effective pore dimension is reduced, resulting in a reduced flux through the MF membrane. The conformational change of the DMAPS chains in the subsurface regions will result in a marked contraction or expansion of the micrometersize pores. This mechanism is particularly important to the “valve effect” of the present membrane having relatively short graft chains with no surface aggregation and trans-membrane pore sizes in the micrometer region. Although the swelling of the graft chains on the surface as a function of electrolyte concentration has a maximum at an electrolyte concentration around 10-4 mol/L, the permeability of the electrolyte continues to decrease at higher electrolyte concentrations. The phenomenon can probably be attributed to the fact that the anti-polyelectrolyte effect of the DMAPS chains in the subsurface regions of the pores continues to increase at electrolyte concentrations well above 10-4 mol/L. As a result, the effective pore size, and thus the permeability, continues to decrease gradually at electrolyte concentrations approaching 1 mol/L. Since there are no ionic functional groups on the surface of the pristine PVDF membrane, the conformation of the PVDF chains remains unchanged when exposed to aqueous media of different electrolyte concentrations. The weakly electrolyte-dependent flux through the commercial hydrophilic PVDF MF membrane may have resulted from the functional groups tethered on the membrane surface after surface modification. Although the surface graft concentration of the DMAPSg-PVDF MF membrane cast in an aqueous medium at 100 °C has been enhanced to some extent (Figure 3), the
PVDF with Zwitterionic Polymer Side Chains
membrane exhibits an electrolyte-independent and much larger flow rate in comparison to other MF membranes prepared in this study. This phenomenon can be explained from the morphological point of view. For the membrane cast in boiling water, the SEM image reveals the presence of both the microporous and macroporous structures. The conformational change of the DMAPS side chains exerts only a minimal effect on the effective pore dimension. As a result, the membrane exhibits an electrolyte-independent and a much larger flow rate. Finally, it is conceivable that the present microporous membrane is not the best membrane morphology to be used for studying the stimulisensitive behavior. Enhanced stimuli-responsiveness can be expected from membranes having ultrafiltration (UF) characteristics. Attempts are currently being made to prepare UF membranes from the graft copolymers. 4. Conclusions A new graft copolymer, PDMAPS-g-PVDF, was successfully synthesized through the molecular graft copolymerization of the zwitterionic DMAPS with the ozonepreactivated PVDF backbone. Elemental and thermogravimetric analyses reveal that the bulk graft concentration of the copolymer increases with the monomer to
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polymer feed ratio used for graft copolymerization. The MF membranes were cast by phase inversion in aqueous media of different electrolyte concentration and temperature. Arising from the interaction of the zwitterionic DMAPS graft chains with the aqueous electrolyte during phase inversion, the mean pore size of the PDMAPS-gPVDF MF membranes decreased with the increase in the graft concentration and increased with the increase in electrolyte strength of the casting bath. The flux of the aqueous solution through the PDMAPS-PVDF MF membranes exhibited a strong dependence on the electrolyte concentration of the solution, as a result of the interaction of the DMAPS polymer chains on the pore surface and subsurface region with the electrolyte solution (the antipolyelectrolyte effect). However, the temperature-dependent perameability of the aqueous solution through the PDMAPS-g-PVDF MF membranes was not observed. The present study has shown that molecular functionalization by graft copolymerization prior to membrane fabrication could be a relatively versatile approach to the preparation of membranes with controllable pore size, uniform surface composition (including the composition of pore surfaces), and electrolyte-sensitive permeability. LA034440Q
