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Preparation of Ultrafiltration Membranes by Direct Microemulsion Polymerization Using Polymerizable Surfactants T. D. Li, L. M. Gan,* C. H. Chew, W. K. Teo,† and L. H. Gan‡ Department of Chemistry, National University of Singapore, Singapore 119260, and Division of Chemistry, NIE, Nanyang Technological University, Singapore Received March 25, 1996. In Final Form: August 21, 1996X A novel method has been successfully used for the preparation of ultrafiltration (UF) membranes by direct polymerization of bicontinuous microemulsions. One of these microemulsions consisted of a polymerizable cationic surfactant of ((acryloyloxy)undecyl)trimethylammonium bromide (AUTMAB), methyl methacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA), and water. The other was a zwitterionic microemulsion composed of a polymerizable surfactant of ((acryloyloxy)undecyl)dimethylammonio acetate (AUDMAA), MMA, HEMA, and water. In addition, a small amount of the cross-linker ethylene glycol dimethacrylate (EGDMA) was also included in each microemulsion polymerization for improving its UF membrane strength. The uniqueness of the microemulsion system is that all organic components can readily be copolymerized at room temperature using a redox initiator. The transparent membranes thus formed showed excellent strength and performance on the separation of different molecular weights of poly(ethylene glycol) (PEG). The pore sizes of the membranes were calculated to be within the range 1-5 nm in diameter, and they could be regulated by varying microemulsion compositions.
Introduction Ultrafiltration (UF) membrane processes are increasingly applied in many separation processes in food, beverage, and dairy industries. They are also widely used for effluent treatment and in biotechnology and medical applications.1 The most common UF membranes are based on polysulfone, cellulose acetate, polyamide, and various fluoropolymers.2a They are typically made by the phase inversion method in which a homogeneous polymer solution is converted to a porous polymer framework through the exchange of a solvent with a coagulant which is a strong nonsolvent, resulting in the formation of a thin skin and a porous asymmetric substructure.3 New approaches for the preparation of hydrophobic-hydrophilic composite membranes via concentrated emulsion polymerization have been developed by Ruckenstein et al.4-10 These membranes can be made by sandwiching concentrated emulsions between two glass plates for polymerization at 50-60 °C, or after polymerization of concentrated emulsions, the composite polymers will be ground, washed, dried, and then transferred into membranes by hot pressing at 150 °C. The advantage of using a concentrated emulsion is that high dispersed phase volume fractions as large as 0.99 can be obtained for making efficient membranes for pervaporation use. The preparation of membranes without the need of organic solvent and nonsolvent can be achieved by the * To whom correspondence should be addressed. † Department of Chemical Engineering. ‡ Nanyang Technological University. X Abstract published in Advance ACS Abstracts, November 1, 1996. (1) Kulkarni, S. S.; Funk, E. W.; Li, N. N. In Membrane Handbook: Introduction and Definitions; Ho, W. S. W., Sirkar, K. K., EDs.; Van Nostrand Reinhold: New York, 1992; p 394. (2) Sourirajan, S.; Matsuura, T. In Reverse Osmosis/Ultrafiltration Process Principle; National Research Council of Canada: Ottawa, 1985; (a) p 680; (b) p 79; (c) p 688. (3) Wijmans, J. G.; Kant, J.; Mulder, H. V.; Smolders, C. A. Polymer 1985, 26, 1539. (4) Ruckenstein, E.; Sun, F. J. Appl. Polym. Sci. 1992, 46, 1271. (5) Ruckenstein, E.; Li, H. Polymer 1994, 35, 4343. (6) Ruckenstein, E. Colloid Polym. Sci. 1989, 267, 792. (7) Ruckenstein, E.; Chen, H. H. J. Membr. Sci. 1992, 66, 205. (8) Ruckenstein, E.; Sun, F. J. Membr. Sci. 1993, 81, 191. (9) Sun, F.; Ruckenstein, E. J. Membr. Sci. 1995, 99, 273. (10) Ruckenstein, E.; Sun, F. J. Membr. Sci. 1995, 103, 271.
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direct use of certain microemulsion polymerization systems.11-18 These microemulsion systems are usually composed of methyl methacrylate (MMA) or styrene and are stabilized by nonpolymerizable surfactants, such as anionic sodium dodecyl sulfates (SDSs) or cationic nalkyltrimethylammonium halides. For the use as separation membranes, the nonpolymerizable surfactant embedded in these microporous polymer composites should first be removed by extraction. Currently, we have investigated the preparation of transparent and microporous polymer composites by microemulsion polymerization using some readily polymerizable surfactants.19,20 In this paper, we report two polymerizable microemulsion systems (except water, of course) that have been successfully used for the preparation of UF membranes. One of these systems is a cationic microemulsion consisting of a polymerizable ((acryolyloxy)undecyl)trimethylammonium bromide (AUTMAB), MMA, 2-hydroxylethyl methacrylate (HEMA), and water, and the other is a zwitterionic microemulsion consisting of a polymerizable ((acryloyloxy)undecyl)dimethylammonio acetate (AUDMAA), MMA, HEMA, and water. Experimental Section Materials. Methyl methacrylate (MMA), 2-hydroxylethyl methacrylate (HEMA), and ethylene glycol dimethacrylate (EGDMA) were obtained from Aldrich and purified under reduced pressure. Ammonium persulfate (APS) from Fluka and N,N,N′,N′-tertramethylethylenediamine (TMEDA) from Aldrich (11) Menger, F. M.; Tsuno, T.; Hammond, G. S. J. Am. Chem. Soc. 1990, 112, 1263. (12) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. Langmuir 1991, 7, 2586. (13) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. Polymer 1993, 3305. (14) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. J. Appl. Polym. Sci. 1993, 47, 499. (15) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. Polymer 1995, 36, 2637. (16) Qutubuddin, S.; Lin, C. S.; Tajuddin, Y. Polymer 1994, 35, 4606. (17) Chieng, T. H.; Gan, L. M.; Chew, C. H.; Ng, S. C. Polymer 1995, 36, 1941. (18) Chieng, T. H.; Gan, L. M.; Chew, C. H.; Lee, L.; Ng, S. C.; Pey, K. L.; Grant, D. Langmuir 1995, 11, 3321. (19) Gan, L. M.; Chieng, T. H.; Chew, C. H.; Ng, S. C. Langmuir 1994, 10, 4022. (20) Gan, L. M.; Li, T. D.; Chew, C. H.; Teo, W. K.; Gan, L. H. Langmuir 1995, 11, 3316.
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Table 1. Microemulsion Compositions for Making Membranes sample series and no.
water (wt %)
MMA/HEMA (7:3) (wt %)
EGDMA (wt %)
AUDMAA (wt %)
A-1 A-2 A-3 A-4 B-1 B-2 B-3 B-4
20 30 40 50 20 30 40 50
48 42 36 30 48 42 36 30
4 3.5 3 2.5 4 3.5 3 2.5
28 24.5 21 17.5
a
AUTMAB (wt %)
approximate gel time at 30 °Ca (min)
28 24.5 21 17.5
10 7 6 5 15 10 8 7
Redox initiator (equimolar APS/TMEDA) used was 20 mM based on the weight of water.
the desired concentration of PEG was then passed through the test cell for permeation rate and solute rejection measurements. The performance of the membranes was characterized by their PWP (pure water permeation rate in g/(cm2 h)), PR (membrane permeated production rate in g/(cm2 h)), and Ra (the percentage of solute rejection (%) of PEG solutes at the average operating pressure of 28 kg/cm2 at a flow velocity of 100 L/h). The PEG concentration in the feed solution was maintained at 200 ppm in all testing experiments. Solution concentrations in the feed and in the product streams were determined using a Simadzu total carbon analyzer Model500. The percentage of solute separation or apparent rejection, Ra, was calculated from the following relation:23
Ra ) [(Cb - Cp)/Cb] × 100%
Figure 1. Microemulsion regions (shaded areas) for water/ MMA/HEMA/EGDMA with either AUDMAA or AUTMAB at 30 °C. were used as received. Millipore water of electrical conductivity ca. 1.0 µS cm-1 was used. ((Acryloyloxy)undecyl)trimethylammonium bromide (AUTMAB) and ((acryloyloxy)undecyl)dimethylammonio acetate (AUDMAA) were synthesized as reported earlier.21,22 Poly(ethylene glycol)s (PEGs) from Aldrich were used as received in UF evaluations. Polymerization and Sample Preparation. Some compositions of transparent microemulsions, along the P-water line of Figure 1, were systematically chosen for the polymerization to obtain membranes. The concentration of the redox initiator (APS/ TMEDA in an equimolar ratio) was 20 mmol/L on the basis of the water used. The microemulsion compositions containing APS in test tubes were purged with dry nitrogen gas at a flow rate of 0.50 L/h for about 15 min in a glove chamber under nitrogen atmosphere, and then TMEDA was injected into the microemulsion system with good mixing. The polymerization proceeded readily at room temperature. When the viscosity of the sample increased, it was introduced immediately and spread evenly onto one of the glass plates and then gently covered with the other one. The glass plate assembly was left standing overnight. The assembly was then immersed in water, and the membrane formed was removed from the glass plates. The cast membrane was resuspended in a water bath for about 1 week before testing. Characterization of Membranes. Ultrafiltration experiments were performed in test cells made of 316 stainless steel and have the same configuration as those used by Sourirajan2b except the effective membrane used in this study was 14.85 cm2. The membranes removed from the water bath were mounted on stainless steel porous plates housed in the test cells. A porous filter paper was placed between the membrane and the porous plate to protect the membrane. The upper part of a test cell was a high-pressure chamber provided with inlet and outlet openings for the flow of the feed solution. The lower part of the test cell had an outlet opening for the collection of the permeation product. Prior to UF experiments, Millipore water was circulated in the test loop for washing purposes. The feed solution containing (21) Chew, C. H.; Li, T. D.; Gan, L. M.; Teo, W. K. J. Macromol. Sci., Pure Appl. Chem. 1995, A32 (2), 211. (22) Li, T. D.; Chew, C. H.; Ng, S. C.; Gan, L. M.; Teo, W. K.; Gu, J. Y.; Zhang, G. Y. J. Macromol. Sci., Pure Appl. Chem. 1995, A32 (5), 969.
where Cp is the permeate product concentration, while Cb is the bulk concentration in the feed.
Results Microemulsion Systems and Membrane Formation. Besides water and a polymerizable surfactant (either cationic AUTMAB or zwitterionic AUDMAA), the microemulsion systems also contained a mixture of monomers of MMA, HEMA, and EGDMA as a cross-linker. The weight ratio of MMA to HEMA was fixed at 7 to 3, and the composition of the mixed monomers was 95 wt % of MMA and HEMA with the remaining 5 wt % of the cross-linker EGDMA. The very large microemulsion regions formed by these components are shown in Figure 1. The transparent microemulsion region containing AUTMAB is denoted by the shaded area enclosed by the solid line, while that of AUDMAA is represented by the dotted line. In this study, the composition ratio of mixed monomers to either AUTMAB or AUDMAA was fixed at point P (65:35). The various microemulsion compositions were then prepared by adding different amounts of water to the systems along the P-water line. Table 1 lists the actual compositions of these microemulsions which had been polymerized to form transparent membranes. Except water, all other components in each microemulsion sample are polymerizable. The polymerization was readily initiated by a very active redox initiator composed of an equimolar mixture of APS and TMEDA at about 30 °C or room temperature. The fast gelation time for series A samples containing AUDMAA decreased from about 10 (A-1) to 5 min (A-4) as the composition of the total polymerizable components decreased from 80 to 50 wt %, respectively. Similarly, the gelation time for series B containing AUTMAB also decreased from about 15 to 7 min for samples B-1 to B-4. It seems that the zwitterionic (AUDMAA) microemulsions polymerized faster than cationic microemulsions. All the membranes obtained from both series, were transparent and hard. Once the water content exceeded (23) Kim, K. J.; Fane, A. G.; Aim, R. B.; Liu, M. G.; Jonsson, G.; Tessaro, I. C.; Broek, A. P.; Bargeman, D. J. Membr. Sci. 1994, 87, 35.
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Table 2. Some Characteristics of Membranes Made by Direct Polymerization of Precursor Microemulsions precursor Microemulsions 20% water characteristics membrane performance PWP solute separationa (%) PEG-200 PEG-600 PEG-1000 PEG-2000 PEG-4000 PEG-6000 PEG-10 000 MWCO (g/mol) a
(g/(cm2
h))
30% water
40% water
50% water
A-1
B-1
A-2
B-2
A-3
B-3
A-4
B-4
0.100
0.160
0.195
0.362
0.343
0.486
0.502
0.686
82.0 90.1 95.2 97.1 98.2 98.3 98.4 600
73.1 82.1 90.1 95.2 97.2 98.6 98.8 1000
72.2 84.0 90.0 94.0 96.5 98.2 98.6 1000
63.1 73.0 82.1 91.1 95.5 96.7 98.7 2000
61.9 72.0 80.0 90.2 93.2 96.3 97.3 2000
53.3 63.2 73.0 82.4 90.6 92.5 96.5 4000
48.2 60.0 71.0 85.2 90.2 95.7 96.2 4000
44.9 54.2 62.0 73.7 85.5 90.8 95.4 6000
Operating pressure at 28 kg/cm2.
30%, the transparent membranes became slightly softer, but they were still strong enough for use as membranes. Membrane Characterization. The results of the membrane evaluation in terms of pure water permeation rate (PWP) and the separation of PEG solutes of different molecular weights for both microemulsion series A and B are summarized in Table 2. The compositions for both series are identical, except the zwitterionic surfactant CH2dCHCOO(CH2)11N+(CH3)2CH2COO- was used in series A, but the cationic surfactant CH2dCHCOO(CH2)11N+(CH3)3Br- was used in series B. In general, PWP was higher for series B than series A. In addition, PWP increased with the increase of the water content in each precursor microemulsion within each series. The PEG separations as a function of PEG molecular weight for both types of membranes are presented in Figure 2. Within series A, over 96% of the PEGs of MW higher than 6000 could be separated by the membranes A-1 to A-4. However, the efficiency of these membranes to separate PEGs of MW lower than 2000 was strikingly different. A similar trend for the PEG separation was also observed for membranes of series B. But the percentages of different PEG separations were generally lower than those of the corresponding membranes of series A. This can be better illustrated by the separation of two PEG samples (PEG-1K and PEG-2K) as shown in Figure 3. The percentage of PEG separation by each membrane decreased linearly with the increase of the water content in each precursor microemulsion, irrespective of whether zwitterionic surfactant (series A) or cationic surfactant (series B) was used. But the molecular weight cutoffs (MWCOs) were higher for membranes of series B than those of series A at the same water contents in the corresponding precursor microemulsions. The influence of PEG molecular weight on the permeation rates for membranes of series A and series B is shown in Figure 4. It is clearly indicated that membranes made from precursor microemulsions with higher water contents (50% for A-4 and B-4) exhibited much higher permeation rates than those with lower water contents (20%) as for membranes A-1 and B-1. On the other hand, the permeation rates through membranes B was generally faster than those through membranes A for corresponding compositions of precursor microemulsions. A significant decrease in the permeation rate with an increase of the molecular weight of PEG can also be seen for the membranes A-4 and B-4 but not for membranes A-1 and B-1. The effect of operating pressure on the membrane performance for separating PEG-2K by membrane series A and B is shown in Figure 5. The percentage of separation for the membranes made from precursor microemulsions with water contents less than 30 wt % was hardly affected by the operating pressure up to 42 kg/cm2. Only the
Figure 2. Separation of different MW PEGs from a 200 ppm PEG aqueous solution at 28 kg/cm2: (A) membranes of series A containing AUDMAA; (b) membranes of series B containing AUTMAB. Their precursor microemulsion compositions are listed in Table 1.
membranes made from precursor microemulsions with 50% water content were slightly affected by operating pressures above 28 kg/cm2. Hence, the results of the separation listed in Table 2 were all obtained at 28 kg/ cm2. Discussion Though MMA, water, and AUDMAA or AUTMAB can form ternary microemulsions,20,22 the addition of the
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Figure 3. Separation of PEG (1K & 2K) by membranes made from two series of precursor microemulsions with different water contents: line 1, series B for PEG-1K; line 2, series A for PEG1K; line 3, series B for PEG-2K; line 4, series A for PEG-2K.
cosurfactant HEMA enhances the solubilization of the system markedly, resulting in enlarging the microemulsion region. This enables us to choose microemulsion compositions with a wider range of water content for polymerization. The sharp increase of electrical conductivity as shown in Figure 6 is due to the change of the microemulsion structures from water-in-oil droplets at low water content (20%) to bicontinuous conducting structures (ca. 20-60%), followed by the formation of oilin-water droplets at high water contents (70%). This type of microstructural transformation in microemulsions is well-known.24-26 It is thus believed that all the compositions listed in Table 1 are bicontinuous microemulsions. The incorporation of a few percentages of the cross-linker EGDMA into these bicontinuous microemulsions is to improve the mechanical strength of the formed membranes after polymerization. The uniqueness of these bicontinuous microemulsions is that all organic components (monomers) are readily polymerizable. These monomers possess the common reactive group (CH2dC(R)COO-), which most likely carries out cross-polymerizations in bicontinuous microemulsions. With the use of the reactive redox initiator APS/TMEDA, the fast gelation of these bicontinuous microemulsions was thus achieved within 5-15 min. Since the aqueous phase contained 20 mM of the initiator, the rate of gelation thus increased with the aqueous content in each microemulsion due to the increasing amount of initiator used. The fast gelation of the microemulsions containing polymerizable surfactants is important for producing transparent polymer composites without phase separation. This is because the possible rearrangements of the microstructures of bicontinuous microemulsions are thus minimized by the fast gelation during the early stage of polymerization. (24) Clausse, M.; Zradba, A.; Nicholas-Morgantini, L. In Microemulsion Systems; Rossano, H. L., Clausse, M., Eds.; Marcel Dekker, Inc.: New York, 1987; p 387. (25) Chen, S. J.; Evans, D. F.; Ninham, B. W. J. Phys. Chem. 1984, 88, 1631. (26) Geoges, J.; Chen, J. W. Colloid Polym. Sci. 1986, 264, 896.
Figure 4. Influence of different MW PEGs on the permeatino rates of membranes: (A) membranes of series A; (B) membranes of series B. Their precursor microemulsion compositions are shown in Table 1.
The characteristics of solute rejection of different MW PEGs by the membranes prepared are attributed to their different pore sizes which were estimated from the nominal MWCO value of a membrane using the Stokes-Einstein equation:23
ra ) (kT)/(6πηDAB) where ra is the radius of different solutes of PEG, k is Boltzmann’s constant, T is absolute temperature (K), η is the viscosity (Pa‚s) of water, and DAB is the diffusivity of the solute in water (cm2/s), which can be found in the reference.2c The average pore size diameters for membranes of series A and B were calculated from the results of MWCO. Figure 7 shows that the pore sizes of both types of membranes increased with the increase of water content in precursor microemulsions, but membranes of series B had larger pore sizes than the corresponding membranes of series A. These results account for the PEG solute rejections as shown in Figure 2. These membrane pores were extremely small, as they only increased from about 1 to 5 nm in diameter when the water contents in precursor microemulsions were raised from 20 to 50 wt %. However, these pore sizes may only relate to the widths of bicontinuous structures which are originally occupied by water. But the lengths of the bicontinuous structures may be long and randomly
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Figure 7. Effect of the water content in precursor microemulsions on the formation of different pore sizes of membranes.
Figure 5. Effect of operating pressure on the membrane performance for separating PEG-2K: (A) membranes of series A; (B) membranes of series B. Their precursor microemulsion compositions are shown in Table 1.
Figure 6. Change of electrical conductivity of precursor microemulsions as a function of water content along the P-water line of Figure 1.
distributed, as has been observed previously20 by scanning electron microscopy. In this study, we are unable to use
the similar SEM technique to examine the pore sizes of these membranes incorporated with HEMA. It seems that the pore sizes of these membranes had been considerably reduced in the presence of HEMA. This effect is currently under study. HEMA may not only orient together with AUDMAA or AUTMAB at the interfaces of MMA and water but also presents significantly in the water channels of bicontinuous microemulsions due to its high water solubility. Once the precursor microemulsions were polymerized, the deposition of water-swollen poly(HEMA) or its copolymers in the water channels might narrow the pore size of the membranes. This may account for the extremely small pore sizes of membranes observed in this study. It is noted from Figure 7 that the pore sizes of membranes of series B are significantly larger than those of membranes of series A at the corresponding water contents in their precursor microemulsions. This may arise from some different orientations of monomers at the interfaces of bicontinuous structures due to the different charges of cationic AUTMAB and neutral zwitterionic AUDMAA used. Thus, it is not surprising to observe larger pore sizes for membranes consisting of cationic AUTMAB due to its charge repulsion. In order to produce a given average pore size for both membranes A and B, about 10% more water content is required for series A, as indicated by the dotted lines in Figure 7. The large different permeation rates among the various membranes may be due to different pore sizes and porosities of the membranes. The effect of the pore size of the membranes on the permeation rate for PEG-6K is shown in Figure 8. It is expected that the permeation rate increases with the pore size of the membranes within either series A or B. With the comparison between membranes of series A and series B at any given average pore size, the permeation rate for the former (series A) is significantly higher than that for the latter. This may be attributed to the presence of different porosities of both types of membranes at any given average pore size. As pointed out earlier for Figure 7, membrane A contained a higher water content in each precursor microemulsion than the corresponding membrane B of the similar pore size. The different population of pores may be derived from the different water contents in both types of precursor microemulsions. The effect of operating pressure on the solute rejection is marginal, especially for the membranes prepared from
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pressure, particularly for membranes of series B prepared from higher water contents, is most probably due to compactness and deformation of pore structures at elevated pressures. Conclusions
Figure 8. Effect of the pore sizes of membranes on the permeation rate for PEG-6K.
Two series of membranes have been prepared from direct polymerization of bicontinuous microemulsions containing either polymerizable zwitterionic AUDMAA or cationic AUTMAB using a redox initiator at room temperature. These UF membranes thus prepared retained the transparent feature as in precursor microemulsions. They exhibited good water permeation flux and solute PEG rejection. Their pore size were calculated to be within the range 1-5 nm on the basis of the results of their MWCO values. The pore size increased as a function of increasing water content in each precursor microemulsion. The UF membranes derived from AUDMAA systems (series A) produced smaller pore sizes and narrower pore size distributions than those from the cationic AUTMAB systems (series B).
lower water contents of precursor microemulsions. The decline in the solute rejection with increasing operating
LA960284U
