- Email: [email protected]
Effect of side chain length on the separation performance of poly(alkyl methacrylate) ionomer membrane
Journal of Membrane Science 167 (2000) 67–77
Effect of side chain length on the separation performance of poly(alkyl methacrylate) ionomer membrane Jong-Woo Lee a , Hee-Tak Kim a , Jung-Ki Park a,∗ , Kew-Ho Lee b a
Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusung-dong, Yusung-gu, Daejon 305-701, South Korea b Korea Research Institute of Chemical Technology, 100 Jang-dong, Daejon, 305-343, South Korea Received 16 February 1999; received in revised form 5 August 1999; accepted 5 August 1999
Abstract Poly(alkyl methacrylate-co-sodium methacrylate) ionomer membranes with different alkyl side chain length were prepared and performances were characterized by the permeability method with poly(ethylene glycol) (PEG) 200 and 1000 aqueous solution to investigate the structural effects of polymer materials on the separation performances of membrane. The flux and rejection of PEG 200, PEG 1000 aqueous solution for the ionomer membrane showed maximum values at a certain side chain length of the ionomer (maximum flux and rejection of PEG 1000 is 15.5 kg/m2 h, 84.5%, respectively). The number of water molecules participating in the hydration shell surrounding the ionic aggregates was decreased with increasing the side chain length of the ionomer. The volume of the hydration shell decreased and the flexibility of the matrix polymer chain increased with increasing the side chain length of the ionomer membrane. The separation performance of the ionomer membrane shows optimum value due to the enhanced flexibility of matrix polymer and the reduced volume of hydration shell with the side chain length of the ionomer membrane because the flux of PEG solution increases while the rejection decreases with increasing the volume of hydration shell and the flexibility of matrix polymer chain. Both the flux and the rejection were increased with increasing the molecular weight of the solute. The surface interaction between the solute and the membrane was postulated to mainly affect the separation characteristics. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Ionomer membrane; Alkyl methacrylate; Nanofiltration; Hydration
1. Introduction Nanofiltration is a pressure driven membrane process for retaining substances with molecular weight higher than about 180 g/mol and multivalent ions [1]. Many industrial effluents contain macromolecules as well as small organic compounds that should be removed when reuse of the water is required, but in ∗ Corresponding author. Tel.: +82-42-869-3925; fax: +82-42-869-3910. E-mail address: [email protected] (J.-K. Park).
many cases monovalent ions contained in the effluents do not have to be removed. Ultrafiltration and reverse osmosis are not appropriate membrane processes for this purpose because ultrafiltration cannot retain smaller size molecules (<1000 g/mol) and reverse osmosis consumes higher energy [2]. Nanofiltration is a good alternative to ultrafiltration and reverse osmosis for such diverse applications as the textile industries, the pulp and paper industries as well as food industries [3–6]. The membranes based on many different polymers such as aromatic polyamides, polysulfones,
0376-7388/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 9 9 ) 0 0 2 8 2 - 3
68
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
poly(phenylene oxide), TFC have been investigated as a nanofiltration membrane [7–8]. Ionomer membrane is also one of the potential candidates for nanofiltration because of its hydrophilic character [9]. The ionomer membranes with optimized ionic content do not require covalent cross-linking to retain water insolubility. These films are generally flexible and tough, which can enhance the puncture and tear resistance. The characteristics of ionomer membranes can be easily altered by loading the same membrane with different ion content [10]. Due to these excellent hydrolytic and physical properties, ionomer membranes have been widely studied as membranes for pervaporation, reverse osmosis and electrolytic applications such as brine electrolysis [10–14]. However, studies on the ionomer membrane for nanofiltration are relatively rare [9,15]. It has been reported that the wide spectrum of flux can be obtained by varying the ionic concentration of the ionomer membrane [9]. For the comprehensive understanding of separation mechanism in ionomer membranes, more investigations with various ionomer membranes must be carried out. In this work we prepared a series of poly(alkyl methacrylate-co-sodium methacrylate) ionomer membranes and investigated the relation between the structural membrane properties such as flexibility and ion aggregates, and the separation performance of the ionomer membrane systematically. The effect of side chain length on the separation performance of the ionomer membrane was thoroughly studied.
dried under vaccum at 70◦ C. The functionality of the precipitated polymer was determined by the titration with 0.1 N NaOH/methanol solution using phenolphthalein as an indicator. Solutions of copolymer in a mixture of tetrahydrofuran and methanol were neutralized by NaOH solution in methanol. The neutralized polymer was dried in a vaccum oven at 100◦ C. The reaction scheme for the synthesis of poly(alkyl methacrylate-co-sodium methacrylate) ionomer was shown in Fig. 1. 2.2. Membrane preparation The porous polysulfone membrane was used as a support. The solution of polysulfone/N-methyl-2pyrrolidone (15/85 g/g) was cast onto non-woven
2. Experimental 2.1. Synthesis of polymer and ionomer The alkyl methacrylate–methacrylic acid copolymer was prepared by solution polymerization of alkyl methacrylate and methacrylic acid using AIBN as an initiator. To the reactor fitted with a mechanical stirrer, condenser, nitrogen inlet and outlet, the mixed monomer and solvent (tetrahydrofuran) were added, and then the initiator was injected. The mixtrure was heated to 70◦ C with stirring for 24 h under nitrogen atmosphere. After polymerization was completed, the product solution was precipitated in water and filtered. The product was washed with methanol and
Fig. 1. Synthetic scheme of poly(alkyl methacrylate-co-sodium methacrylate).
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
polyester (the backing material) by using a casting knife, and the cast membrane was immediately immersed into a water bath to produce the porous support. The prepared support was dipped into the hexane/ethanol solution of poly(alkyl methacrylate-cosodium methacrylate). The dip coated membrane was then dried for 5 h at room temperature. 2.3. Determination of type and amount of water absorbed in the ionomer The equilibrium water content (degree of swelling) of the ionomer was determined by the weight gain after immersion in water at room temperature for 1 day. The water molecules in the polymer membrane can be classified into freezing, freezing bound and non-freezing ones [9]. The relative amount of three types of water was estimated by DSC. [Du Pont 9900] DSC curves were obtained in the temperature range from −100◦ C to 100◦ C with a heating rate of 10◦ C/min under nitrogen atmosphere. The enthalpy of melting of water in the swollen ionomer was calibrated using pure water as a standard to obtain the water content in the swollen ionomer. The water content is given by the mass of water in the ionomer divided by the dry mass of the sample, expressed in units of g/g.
69
2.5. Nanofiltration experiments The nanofiltration experiments were carried out in a flat sheet laboratory module in which the four membranes were installed in series. The area of each membrane was 19.63 cm2 . The membranes were first stabilized with pure water for about 2 h with a pressure of 20 bar at 25◦ C, and the pure water flux of the membranes was measured. The water flux and the flux for the aqueous solution of poly(ethylene glycol) (PEG) whose molecular weight was 200 and 1000 were then tested at different pressures (p) between 5 and 25 bar at room temperature. The solute concentration of the feed of the PEG aqueous solution was 2000 ppm, and the feed was circulated through the feed chamber of the permeation cell. The solute rejection R is defined as R=
Cf − Cp Cf
(1)
where Cf and Cp are the solute concentration of the feed and the permeate, respectively. The concentration of the solute was determined by the peak area observed from liquid chromatography.
3. Results and discussion
2.4. NMR measurements
3.1. Characterization of polymer and membrane
Cross-polarization solid-state 13 C NMR spectra of the hydrated ionomer were obtained at 47 kG on a Varian XL-200NMR. The spectrometer was equipped with an auxiliary high power amplifier and a solid state probe for magic-angle spinning. The spin–spin relaxation time, T2 , was calculated from the NMR line widths.
The abbreviated names of the copolymer together with their molecular weight and composition studied in this work are summarized in Table 1. For brevity, the copolymer poly(alkyl methacrylate-co-sodium methacrylate) with different alkyl side chain length will be designated as PASn, where n indicates the number of carbon atoms in the pendant alkyl
Table 1 The compositions, molecular weights of PAS series and the casting solvent for top layer coating
PAS4 PAS6 PAS8 PAS10 PAS13
Number of carbon in side chain
MAA content (mol%)
Molecular weight (Mw )
Poly dispersity
Casting solvent (hexane/ethanol)
4 6 8 10 13
15.9 15.7 15.9 15.0 15.5
89 000 83 000 82 000 89 000 75 000
1.91 1.94 1.67 1.78 1.69
0/100 10/90 20/80 80/20 90/10
70
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
group (PAS6, for example, means poly(hexyl methacrylate-co-sodium methacrylate) with the ion content 16 mol%). For all the PAS series with different alkyl side chain length, the molecular weight and ion content showed almost the same values. The weight average molecular weight measured by gel permeation chromatography was around 80,000 and the acid content obtained by titration was around 16 mol% for each PAS. The neutralization of poly(alkyl methacrylate-comethacrylic acid) was evidenced by FT-IR spectra, which was shown in Fig. 2. The neutralization of carboxylic acid is indicated by the reduction in the intensities of the characteristic bands at 1700 cm−1 (asymmetric COO− stretching peak of the acid polymer) and the appearance of the characteristic bands at 1560 cm−1 (asymmetric COO− stretching peak of the ionomer). The composite membrane was prepared by dipping polysulfone support into the hexane/ethanol solution where the PAS series was dissolved. The composition of these cosolvents are also summarized in Table 1. As the number of carbon atoms in the pendant alkyl group increases, the copolymer dissolved in less polar solvent due to the decrease of ion content based on the weight percent. The thickness of the polysulfone support was about 100 m and the thin top layer was
Fig. 2. FT-IR spectra of the sodium ionomer and the unneutralized copolymer (a) ionomer (b) copolymer.
Fig. 3. The swelling ratio of the PAS series as a function of the number of carbon atoms in the side chain.
1 m, which was determined by the scanning electron microscopy image of the cross-section. 3.2. Type and amount of water absorbed in ionomers In Fig. 3 the swelling ratios of the PAS series with different side chain length were shown. As the number of carbon atoms in the pendant alkyl group increased, swelling ratio decreased due to the decrease of ion content based on the weight percent. To investigate the type and amount of water absorbed in the top dense layer, the DSC thermograms of the water swollen PAS with various side chain lengths were obtained and analyzed. The water molecules in the polymer membrane can be classified into three types: freezing water, freezing bound water and non-freezing water [9,16]. Water whose melting temperature and enthalpy of melting are not significantly different from those of normal water is called freezing water. Water species that are very closely associated with the ions in the polymer are referred to as non-freezing water. The less closely associated water species that exhibit large differences in transition temperatures are called freezing bound water. The amount of freezing and non-freezing water was shown in Fig. 4 as a function of side chain length of the PAS series. The amount of non-freezing and
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
71
Table 2 The glass transition temperatures of the PAS series Tg (◦ C) PAS4 PAS6 PAS8 PAS10 PAS13
Fig. 4. Freezing and non-freezing water content of the PAS series as a function of the number of carbon atoms in the side chain.
freezing water was observed to be decreased with the increase of side chain length. A hydration shell is known to be composed of water molecules surrounding the ionic groups. Near the ionic group, water molecules are strongly bound to the ionic group producing a primary hydration shell. Water molecules outside the primary hydration shell, denoted as the secondary hydration shell, are relatively free compared to the water molecules in the primary hydration shell. The amount of non-freezing water can be an indicator of the size of the hydration shell, because the non-freezing water molecules would be a major element in the primary hydration shell. The decrease in the amount of non-freezing water molecules with increasing alkyl side chain length is indicative of the smaller hydration shell. 3.3. Flexibility of hydrated ionomer The glass transition temperatures of the PAS series measured by DSC were shown in Table 2. In dry state, Tg of PAS was moderately decreased with the alkyl side chain length when the number of carbon atoms in the alkyl side chain was changed from 4 to 10. However, Tg decreased dramatically as the number of carbon atoms in the alkyl side chain was above 10. The lower Tg of the ionomer with longer alkyl side
63 54 48 41 −70
chain indicates higher flexibility of the main chain of the ionomer. As already shown in Fig. 4, non-freezing water content of the ionomer with shorter side chain was higher than that of the ionomer with longer side chain. Because the plasticizing effect of non-freezable bound water induces a strong decrease of the glass transition temperature, the flexibility of the hydrated ionomer could not be simply predicted by the Tg of the dried ionomer or the swelling ratio of the hydrated ionomer. In order to have a good understanding of the flexibility of the hydrated ionomer, relaxation time for each functional group of the hydrated ionomer was quantified by the half line width of 13 C solid NMR. It is generally accepted that 13 C nuclei relax mainly through the dipolar interactions between the carbon and the neighboring protons, especially intramolecular interactions between the nuclei separated by a few bonds. The 13 C NMR spectra of PAS4 and PAS8 are shown in Fig. 5. The NMR line widths is affected by relaxation processes and the spin–spin relaxation time T2 is defined as 1 = 1v π T2
(2)
where 1v is the line width associated with relaxation processes. The longer spin–spin relaxation time of the ionomer means greater mobility of the local segment. The spin–spin relaxation time values for the carbonyl (178 ppm) and ␣-methyl carbon (15 ppm) groups of the hydrated PAS4 and PAS8 are listed in Table 3. The ionomer was hydrated using the calculated equilibrium amount of deuterated water (D2 O). It is of interest in the data that the value of T2 for the carbonyl carbon of PAS4 is greater than that of PAS8 while the value of T2 for the ␣-methyl carbon of PAS4 is smaller than that of PAS8.
72
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
Fig. 5. Typical
13 C
solid NMR spectra of PAS4 and PAS8.
The carbonyl group in the hydration shell has enhanced mobility due to the pasticizing effect of deuterated water. Even though the hydration shell size of PAS4 is larger than that of PAS8, the values of T2 for the ␣-methyl carbon of PAS8 is greater than that of PAS4. It is because the flexibility of hydrophobic alkyl side chain increases with side chain length irrespective of the amount of hydration shell. From the comparison of Tg of the matrix in the dry state and the 13 C solid NMR data of the hydrated sample, it is concluded that the flexible region composed of the hydration shell decreases with the increasing alkyl side chain length, but the flexibility of hydrophobic alkyl side chains increases with the side chain length. 3.4. Effect of pressure on performance To investigate the characteristics of solute transport in these ionomer membranes, separation performances Table 3 Spin–spin relaxation times of PAS4 and PAS8 T2 (m s)
–COO– –CH3
PAS4
PAS8
0.7 2.7
0.4 6.4
were characterized by the permeability method with the PEG 200 and 1000 aqueous solutions. The operating pressure was varied from 5 to 30 bar. The pressure dependency of flux and rejection through the PAS membranes with different alkyl side chain length can reveal the mechanism governing the transport characteristics. Fig. 6 shows the dependency of flux on pressure for the PAS6 membrane. The flux of water and PEG 1000 solution is nearly proportional to pressure, while that of PEG 200 solution deviates from the pure water flux. The linear increase of the flux with pressure for pure water is a general tendency for pressure driven transport. When the solute is contained in the feed, the concentration polarization phenomena are frequently observed. The more deviation of the flux of PEG solution from the pure water flux with increasing pressure indicates the presence of the layer of high solute concentration, frequently referred to as the gel layer. As shown in Fig. 6, the fouling of PEG 200 on the PAS6 membrane is greater than the concentration polarization of PEG 1000. The higher flux for the solution containing higher molecular-weight solute represents the lower concentration polarization and the consequent lower additional resistance to water transport by the gel layer. This phenomenon was already explained by Kim et al. in terms of surface interaction between the solute and the membrane [9]. The
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
73
Fig. 6. The flux of PEG solution for PAS6 as a function of pressure.
surface concentration of the solute would be lower for the higher molecular weight solute whose dimension is larger than that of the hydration shell, because the repulsive interaction between hydrophilic solute and hydrophobic chain of the membrane occurs. The rejection of PEG solution with flux for the PAS6 membrane is shown in Fig. 7. The rejection of PEG 200 solution through the PAS6 membrane increased with flux, while that of PEG 1000 solution slightly decreased. The enhancement in rejection of PEG 200 solution with increasing flux is a general tendency from the viewpoint of the relationship between rejection and flux. However, the rejection of PEG 1000 is slightly reduced with flux. This phenomenon may be closely related to the chain flexibility and morphology of the hydrated ionomer membrane. While the ionomer membranes do not have real visible pores (which is shown in Fig. 8), water molecules highly interacting with ion clusters in the ionomer membrane generate free space around the ion aggregate sites which depend on their hydration degree. The
solute retention characteristics depend much on how much free space for solute transport is there in the membranes, which can for some membranes be related to the flux. Hence, the permselectivity of the ionomer membrane depends on the formation of a percolation path through the membrane as water swells the ionic aggregates. The polar solute like PEG is transported through the percolated hydration shell whose polarity is higher than that of surrounding hydrophobic chains. However, this percolation path is interrupted with hydrophobic alkyl side chains, and small channels connecting the water pools, which makes the movement of solute molecules very difficult, can be formed. The polar solutes greater than the hydration shell in size would be repelled at the entrance of the hydration shell. The solutes whose size is comparable to that of hydration shell would be entrapped and sorbed in the hydration shell of the ionomer membrane. When the matrix polymer surrounding the hydration shell is rigid at room temperature, the hydration shell irrespective of pressure hinders the transport of solute. However, for the ionomer membrane whose matrix is quite
74
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
Fig. 7. The rejection of PEG solution for PAS6 as a function of pressure.
flexible, the solute repelled at the entrance of percolation path or entrapped in the isolated small channels could pass through the size changeable hydration shell with pressure. Therefore, the rejection of larger molecules compared with the size of hydration shell could be decreased with pressure when it passes through the polymer matrix which is quite flexible. Contrarily, the smaller solute compared with the size of hydration shell passes through the hydration shell freely, and the rejection will thus be increased with pressure irrespective of polymer matrix flexibility around the hydration shell.
flux and rejection showed maximum values at a certain side chain length of the ionomer. As shown in Fig. 9(a) the flux of water and PEG 200, PEG 1000 would be decreased for the ionomer membrane whose carbon atom number in the side chain is over 8.
3.5. Effect of side chain length on separation performance The separation performances of the PAS ionomer membranes with different alkyl side chain length were characterized by the permeability method with PEG 200 and 1000 aqueous solutions at 20 bar. The flux and rejection of PEG 200, PEG 1000 aqueous solutions for the ionomer membrane are shown in Fig. 9. The
Fig. 8. SEM image of PAS6 composite membrane (surface of top layer, ×10 000).
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
Fig. 9. (a) The flux and (b) rejection for the PAS series as a function of the number of carbon atoms in the side chain.
75
76
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
These changes in flux may be caused by the solubility and tortuousity changes for the PAS ionomer membrane with different alkyl side chain. The transport of liquid through a dense, non-porous membrane is greatly influenced by the solubility and diffusivity of the organic solute. For the PAS membranes, as already shown in Fig. 4, the amount of water molecules participating in the hydration shell surrounding the ionic aggregates is decreased with increasing the side chain length of the ionomer, which results in the reduced flux for the ionomer membrane with longer side chain of the matrix polymer. The lower flux for the ionomer membrane made of the matrix polymer with short side chain may result from the morphology and the tortuousity of the hydration shell. More studies on the morphology and tortuousity of the ionomer membrane with flexible alkyl side chain are needed for better understanding of the flux behavior for the ionomer membrane with short side chain of the matrix polymer. It was observed in Fig. 9(b) that the rejection of PEG 200 and PEG 1000 showed the maximum values when the number of carbon atoms in the alkyl side chain of the ionomer was around 8. The changes in rejection of PEG 200 and PEG 1000 results from the amount of hydration shell and the flexibility of the matrix polymer surrounding the hydration shell. The lower rejection for the membrane made of the matrix polymer with shorter alkyl side chain is mainly attributed to the large amount of the hydration shell. As the side chain length increases, the amount of hydration shell decreases, which is already indicated by the swelling ratio and non-freezing water content. Even though small amount of hydration shell for the ionomer membrane made of the matrix polymer with longer side chain is expected to cause higher rejection, the rejection for the ionomer membrane whose matrix polymer having the number of carbon atoms in the alkyl side chain greater than 8 is found experimentally to decrease with increase of the side chain length. This decrease of rejection for the ionomer membrane made of the matrix polymer with longer side chain resulted from the flexibility of matrix polymer surrounding the hydration shell. As the side chain length of the ionomer increases, the flexibility of the matrix polymer increases. When the larger solute passes through the hydration shell surrounded by the flexible polymer chain, with increase of the
flexibility of the ionomer membrane, the solute repelled at the entrance of percolation path or entrapped in the isolated small channels could come to pass through the size changeable hydration shell. Thus, the flexibility of the ionomer membrane made of the matrix polymer with longer side chain reduces the rejection of PEG solute. This consideration is depicted in Fig. 10. The polar solutes whose size is smaller than that of the hydration shell, like PEG 200, can pass through the path in the ionomer membrane irrespective of pressure. However, the polar solutes greater than the hydration shell in size, like PEG 1000, would be repelled at the entrance or entrapped in the hydration shell of the ionomer membrane. When the matrix polymer surrounding the hydration shell is rigid at room temperature, the transport of solute is hindered by hydration shell irrespective of pressure. For the ionomer membrane whose matrix is quite flexible, the solute repelled at the entrance of percolation path or entrapped in the isolated small channels at low pressure could pass through the size changeable hydration shell at high pressure.
Fig. 10. Schematic illustration of separation mechanism of PEG solution for the PAS ionomer membrane.
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
4. Conclusions The flux and rejection of PEG aqueous solution for poly(alkyl methacrylate-co-sodium methacrylate) ionomer membrane showed maximum values at a certain side chain length of the ionomer. As the side chain length of the ionomer increases, the amount of hydration shell decreases while the flexibility of the matrix polymer increases. The amount of hydration shell and the flexibility of the matrix polymer chain may affect the performance of the ionomer membrane.
Acknowledgements This work was supported by the Korean Ministry of Science and Technology in 1997. References [1] M. Nystrom, L. Kaipia, S. Luque, Fouling and retention of nanofiltration membranes, J. Membr. Sci. 98 (1995) 249–262. [2] K. Scott, Handbook of Industrial Membranes, Elsevier, Oxford, 1995. [3] R. Levenstein, D. Hasson, R. Semiat, Utilization of the Donnan effect for improving electrolyte separation with nanofiltration membranes, J. Membr. Sci. 116 (1996) 77–92. [4] S. Wadley, C.J. Brouckaert, L.A.D. Baddock, C.A. Buckley, Modelling of nanofiltration applied to the recovery of salt from waste brine at a sugar decolourisation plant, J. Membr. Sci. 102 (1995) 163–175.
77
[5] J.M.K. Timmer, H.C. van der Horst, T. Robbertsen, Transport of lactic acid through reverse osmosis and nanofiltration membranes, J. Membr. Sci. 85 (1993) 205–216. [6] A.E. Childress, M. Elimelech, Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes, J. Membr. Sci. 119 (1996) 253– 268. [7] R.J. Petersen, Composite reverse osmosis and nanofiltration membranes, J. Membr. Sci. 83 (1993) 81–150. [8] P. Eriksson, Water and salt transport through two types of polyamide compositr membranes, J. Membr. Sci. 36 (1988) 297–313. [9] H.T. Kim, J.K. Park, K.H. Lee, Preparation, characterization, and performance of poly(acrylamidocaproic acid) partially neutralized with calcium for use in nanofiltration, J. Appl. Polym. Sci. 60 (1996) 1811–1819. [10] M.R. Tant, K.A. Mauritz, G.L. Wilkes, Ionomer, Blackie, New York, 1997. [11] A.A. Gronowski, M. Jiang, H.L. Yeager, G. Wu, A. Eisenberg, Structure and properties of hydrocarbon ionomer membranes 1. Poly(methyl methacrylate-co-methacrylic acid), J. Membr. Sci. 82 (1993) 83–97. [12] C.D. Ihm, S.K. Ihm, Pervaporation of water-ethanol mixtures through sulfonated polystyrene membranes prepared by plasma graft-polymerization, J. Membr. Sci. 98 (1995) 89–96. [13] H. Matsuyama, M. Teramoto, M. Tsuchiya, Percolation permeability in styrene-methacrylic acid ionomers, J. Membr. Sci. 118 (1996) 177–184. [14] A. Eisenberg, H.L. Yeager (Eds.), Perfluorinated Ionomer Membranes, ACS Symp. Ser. No. 180, American Chemical Society, Washington, DC, 1982. [15] J. Ramkumar, B. Maiti, T.S. Krishnamoorthy, Transport of some nitrogen heterocyclic and aromatic compounds through metal ion containing Nafion ionomer membrane, J. Membr. Sci. 125 (1997) 269–279. [16] T. Hatakeyama, Thermal Analysis: Fundamentals and Applications to Polymer Science, Wiley, West Sussex, 1994.
Effect of side chain length on the separation performance of poly(alkyl methacrylate) ionomer membrane Jong-Woo Lee a , Hee-Tak Kim a , Jung-Ki Park a,∗ , Kew-Ho Lee b a
Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusung-dong, Yusung-gu, Daejon 305-701, South Korea b Korea Research Institute of Chemical Technology, 100 Jang-dong, Daejon, 305-343, South Korea Received 16 February 1999; received in revised form 5 August 1999; accepted 5 August 1999
Abstract Poly(alkyl methacrylate-co-sodium methacrylate) ionomer membranes with different alkyl side chain length were prepared and performances were characterized by the permeability method with poly(ethylene glycol) (PEG) 200 and 1000 aqueous solution to investigate the structural effects of polymer materials on the separation performances of membrane. The flux and rejection of PEG 200, PEG 1000 aqueous solution for the ionomer membrane showed maximum values at a certain side chain length of the ionomer (maximum flux and rejection of PEG 1000 is 15.5 kg/m2 h, 84.5%, respectively). The number of water molecules participating in the hydration shell surrounding the ionic aggregates was decreased with increasing the side chain length of the ionomer. The volume of the hydration shell decreased and the flexibility of the matrix polymer chain increased with increasing the side chain length of the ionomer membrane. The separation performance of the ionomer membrane shows optimum value due to the enhanced flexibility of matrix polymer and the reduced volume of hydration shell with the side chain length of the ionomer membrane because the flux of PEG solution increases while the rejection decreases with increasing the volume of hydration shell and the flexibility of matrix polymer chain. Both the flux and the rejection were increased with increasing the molecular weight of the solute. The surface interaction between the solute and the membrane was postulated to mainly affect the separation characteristics. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Ionomer membrane; Alkyl methacrylate; Nanofiltration; Hydration
1. Introduction Nanofiltration is a pressure driven membrane process for retaining substances with molecular weight higher than about 180 g/mol and multivalent ions [1]. Many industrial effluents contain macromolecules as well as small organic compounds that should be removed when reuse of the water is required, but in ∗ Corresponding author. Tel.: +82-42-869-3925; fax: +82-42-869-3910. E-mail address: [email protected] (J.-K. Park).
many cases monovalent ions contained in the effluents do not have to be removed. Ultrafiltration and reverse osmosis are not appropriate membrane processes for this purpose because ultrafiltration cannot retain smaller size molecules (<1000 g/mol) and reverse osmosis consumes higher energy [2]. Nanofiltration is a good alternative to ultrafiltration and reverse osmosis for such diverse applications as the textile industries, the pulp and paper industries as well as food industries [3–6]. The membranes based on many different polymers such as aromatic polyamides, polysulfones,
0376-7388/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 9 9 ) 0 0 2 8 2 - 3
68
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
poly(phenylene oxide), TFC have been investigated as a nanofiltration membrane [7–8]. Ionomer membrane is also one of the potential candidates for nanofiltration because of its hydrophilic character [9]. The ionomer membranes with optimized ionic content do not require covalent cross-linking to retain water insolubility. These films are generally flexible and tough, which can enhance the puncture and tear resistance. The characteristics of ionomer membranes can be easily altered by loading the same membrane with different ion content [10]. Due to these excellent hydrolytic and physical properties, ionomer membranes have been widely studied as membranes for pervaporation, reverse osmosis and electrolytic applications such as brine electrolysis [10–14]. However, studies on the ionomer membrane for nanofiltration are relatively rare [9,15]. It has been reported that the wide spectrum of flux can be obtained by varying the ionic concentration of the ionomer membrane [9]. For the comprehensive understanding of separation mechanism in ionomer membranes, more investigations with various ionomer membranes must be carried out. In this work we prepared a series of poly(alkyl methacrylate-co-sodium methacrylate) ionomer membranes and investigated the relation between the structural membrane properties such as flexibility and ion aggregates, and the separation performance of the ionomer membrane systematically. The effect of side chain length on the separation performance of the ionomer membrane was thoroughly studied.
dried under vaccum at 70◦ C. The functionality of the precipitated polymer was determined by the titration with 0.1 N NaOH/methanol solution using phenolphthalein as an indicator. Solutions of copolymer in a mixture of tetrahydrofuran and methanol were neutralized by NaOH solution in methanol. The neutralized polymer was dried in a vaccum oven at 100◦ C. The reaction scheme for the synthesis of poly(alkyl methacrylate-co-sodium methacrylate) ionomer was shown in Fig. 1. 2.2. Membrane preparation The porous polysulfone membrane was used as a support. The solution of polysulfone/N-methyl-2pyrrolidone (15/85 g/g) was cast onto non-woven
2. Experimental 2.1. Synthesis of polymer and ionomer The alkyl methacrylate–methacrylic acid copolymer was prepared by solution polymerization of alkyl methacrylate and methacrylic acid using AIBN as an initiator. To the reactor fitted with a mechanical stirrer, condenser, nitrogen inlet and outlet, the mixed monomer and solvent (tetrahydrofuran) were added, and then the initiator was injected. The mixtrure was heated to 70◦ C with stirring for 24 h under nitrogen atmosphere. After polymerization was completed, the product solution was precipitated in water and filtered. The product was washed with methanol and
Fig. 1. Synthetic scheme of poly(alkyl methacrylate-co-sodium methacrylate).
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
polyester (the backing material) by using a casting knife, and the cast membrane was immediately immersed into a water bath to produce the porous support. The prepared support was dipped into the hexane/ethanol solution of poly(alkyl methacrylate-cosodium methacrylate). The dip coated membrane was then dried for 5 h at room temperature. 2.3. Determination of type and amount of water absorbed in the ionomer The equilibrium water content (degree of swelling) of the ionomer was determined by the weight gain after immersion in water at room temperature for 1 day. The water molecules in the polymer membrane can be classified into freezing, freezing bound and non-freezing ones [9]. The relative amount of three types of water was estimated by DSC. [Du Pont 9900] DSC curves were obtained in the temperature range from −100◦ C to 100◦ C with a heating rate of 10◦ C/min under nitrogen atmosphere. The enthalpy of melting of water in the swollen ionomer was calibrated using pure water as a standard to obtain the water content in the swollen ionomer. The water content is given by the mass of water in the ionomer divided by the dry mass of the sample, expressed in units of g/g.
69
2.5. Nanofiltration experiments The nanofiltration experiments were carried out in a flat sheet laboratory module in which the four membranes were installed in series. The area of each membrane was 19.63 cm2 . The membranes were first stabilized with pure water for about 2 h with a pressure of 20 bar at 25◦ C, and the pure water flux of the membranes was measured. The water flux and the flux for the aqueous solution of poly(ethylene glycol) (PEG) whose molecular weight was 200 and 1000 were then tested at different pressures (p) between 5 and 25 bar at room temperature. The solute concentration of the feed of the PEG aqueous solution was 2000 ppm, and the feed was circulated through the feed chamber of the permeation cell. The solute rejection R is defined as R=
Cf − Cp Cf
(1)
where Cf and Cp are the solute concentration of the feed and the permeate, respectively. The concentration of the solute was determined by the peak area observed from liquid chromatography.
3. Results and discussion
2.4. NMR measurements
3.1. Characterization of polymer and membrane
Cross-polarization solid-state 13 C NMR spectra of the hydrated ionomer were obtained at 47 kG on a Varian XL-200NMR. The spectrometer was equipped with an auxiliary high power amplifier and a solid state probe for magic-angle spinning. The spin–spin relaxation time, T2 , was calculated from the NMR line widths.
The abbreviated names of the copolymer together with their molecular weight and composition studied in this work are summarized in Table 1. For brevity, the copolymer poly(alkyl methacrylate-co-sodium methacrylate) with different alkyl side chain length will be designated as PASn, where n indicates the number of carbon atoms in the pendant alkyl
Table 1 The compositions, molecular weights of PAS series and the casting solvent for top layer coating
PAS4 PAS6 PAS8 PAS10 PAS13
Number of carbon in side chain
MAA content (mol%)
Molecular weight (Mw )
Poly dispersity
Casting solvent (hexane/ethanol)
4 6 8 10 13
15.9 15.7 15.9 15.0 15.5
89 000 83 000 82 000 89 000 75 000
1.91 1.94 1.67 1.78 1.69
0/100 10/90 20/80 80/20 90/10
70
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
group (PAS6, for example, means poly(hexyl methacrylate-co-sodium methacrylate) with the ion content 16 mol%). For all the PAS series with different alkyl side chain length, the molecular weight and ion content showed almost the same values. The weight average molecular weight measured by gel permeation chromatography was around 80,000 and the acid content obtained by titration was around 16 mol% for each PAS. The neutralization of poly(alkyl methacrylate-comethacrylic acid) was evidenced by FT-IR spectra, which was shown in Fig. 2. The neutralization of carboxylic acid is indicated by the reduction in the intensities of the characteristic bands at 1700 cm−1 (asymmetric COO− stretching peak of the acid polymer) and the appearance of the characteristic bands at 1560 cm−1 (asymmetric COO− stretching peak of the ionomer). The composite membrane was prepared by dipping polysulfone support into the hexane/ethanol solution where the PAS series was dissolved. The composition of these cosolvents are also summarized in Table 1. As the number of carbon atoms in the pendant alkyl group increases, the copolymer dissolved in less polar solvent due to the decrease of ion content based on the weight percent. The thickness of the polysulfone support was about 100 m and the thin top layer was
Fig. 2. FT-IR spectra of the sodium ionomer and the unneutralized copolymer (a) ionomer (b) copolymer.
Fig. 3. The swelling ratio of the PAS series as a function of the number of carbon atoms in the side chain.
1 m, which was determined by the scanning electron microscopy image of the cross-section. 3.2. Type and amount of water absorbed in ionomers In Fig. 3 the swelling ratios of the PAS series with different side chain length were shown. As the number of carbon atoms in the pendant alkyl group increased, swelling ratio decreased due to the decrease of ion content based on the weight percent. To investigate the type and amount of water absorbed in the top dense layer, the DSC thermograms of the water swollen PAS with various side chain lengths were obtained and analyzed. The water molecules in the polymer membrane can be classified into three types: freezing water, freezing bound water and non-freezing water [9,16]. Water whose melting temperature and enthalpy of melting are not significantly different from those of normal water is called freezing water. Water species that are very closely associated with the ions in the polymer are referred to as non-freezing water. The less closely associated water species that exhibit large differences in transition temperatures are called freezing bound water. The amount of freezing and non-freezing water was shown in Fig. 4 as a function of side chain length of the PAS series. The amount of non-freezing and
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
71
Table 2 The glass transition temperatures of the PAS series Tg (◦ C) PAS4 PAS6 PAS8 PAS10 PAS13
Fig. 4. Freezing and non-freezing water content of the PAS series as a function of the number of carbon atoms in the side chain.
freezing water was observed to be decreased with the increase of side chain length. A hydration shell is known to be composed of water molecules surrounding the ionic groups. Near the ionic group, water molecules are strongly bound to the ionic group producing a primary hydration shell. Water molecules outside the primary hydration shell, denoted as the secondary hydration shell, are relatively free compared to the water molecules in the primary hydration shell. The amount of non-freezing water can be an indicator of the size of the hydration shell, because the non-freezing water molecules would be a major element in the primary hydration shell. The decrease in the amount of non-freezing water molecules with increasing alkyl side chain length is indicative of the smaller hydration shell. 3.3. Flexibility of hydrated ionomer The glass transition temperatures of the PAS series measured by DSC were shown in Table 2. In dry state, Tg of PAS was moderately decreased with the alkyl side chain length when the number of carbon atoms in the alkyl side chain was changed from 4 to 10. However, Tg decreased dramatically as the number of carbon atoms in the alkyl side chain was above 10. The lower Tg of the ionomer with longer alkyl side
63 54 48 41 −70
chain indicates higher flexibility of the main chain of the ionomer. As already shown in Fig. 4, non-freezing water content of the ionomer with shorter side chain was higher than that of the ionomer with longer side chain. Because the plasticizing effect of non-freezable bound water induces a strong decrease of the glass transition temperature, the flexibility of the hydrated ionomer could not be simply predicted by the Tg of the dried ionomer or the swelling ratio of the hydrated ionomer. In order to have a good understanding of the flexibility of the hydrated ionomer, relaxation time for each functional group of the hydrated ionomer was quantified by the half line width of 13 C solid NMR. It is generally accepted that 13 C nuclei relax mainly through the dipolar interactions between the carbon and the neighboring protons, especially intramolecular interactions between the nuclei separated by a few bonds. The 13 C NMR spectra of PAS4 and PAS8 are shown in Fig. 5. The NMR line widths is affected by relaxation processes and the spin–spin relaxation time T2 is defined as 1 = 1v π T2
(2)
where 1v is the line width associated with relaxation processes. The longer spin–spin relaxation time of the ionomer means greater mobility of the local segment. The spin–spin relaxation time values for the carbonyl (178 ppm) and ␣-methyl carbon (15 ppm) groups of the hydrated PAS4 and PAS8 are listed in Table 3. The ionomer was hydrated using the calculated equilibrium amount of deuterated water (D2 O). It is of interest in the data that the value of T2 for the carbonyl carbon of PAS4 is greater than that of PAS8 while the value of T2 for the ␣-methyl carbon of PAS4 is smaller than that of PAS8.
72
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
Fig. 5. Typical
13 C
solid NMR spectra of PAS4 and PAS8.
The carbonyl group in the hydration shell has enhanced mobility due to the pasticizing effect of deuterated water. Even though the hydration shell size of PAS4 is larger than that of PAS8, the values of T2 for the ␣-methyl carbon of PAS8 is greater than that of PAS4. It is because the flexibility of hydrophobic alkyl side chain increases with side chain length irrespective of the amount of hydration shell. From the comparison of Tg of the matrix in the dry state and the 13 C solid NMR data of the hydrated sample, it is concluded that the flexible region composed of the hydration shell decreases with the increasing alkyl side chain length, but the flexibility of hydrophobic alkyl side chains increases with the side chain length. 3.4. Effect of pressure on performance To investigate the characteristics of solute transport in these ionomer membranes, separation performances Table 3 Spin–spin relaxation times of PAS4 and PAS8 T2 (m s)
–COO– –CH3
PAS4
PAS8
0.7 2.7
0.4 6.4
were characterized by the permeability method with the PEG 200 and 1000 aqueous solutions. The operating pressure was varied from 5 to 30 bar. The pressure dependency of flux and rejection through the PAS membranes with different alkyl side chain length can reveal the mechanism governing the transport characteristics. Fig. 6 shows the dependency of flux on pressure for the PAS6 membrane. The flux of water and PEG 1000 solution is nearly proportional to pressure, while that of PEG 200 solution deviates from the pure water flux. The linear increase of the flux with pressure for pure water is a general tendency for pressure driven transport. When the solute is contained in the feed, the concentration polarization phenomena are frequently observed. The more deviation of the flux of PEG solution from the pure water flux with increasing pressure indicates the presence of the layer of high solute concentration, frequently referred to as the gel layer. As shown in Fig. 6, the fouling of PEG 200 on the PAS6 membrane is greater than the concentration polarization of PEG 1000. The higher flux for the solution containing higher molecular-weight solute represents the lower concentration polarization and the consequent lower additional resistance to water transport by the gel layer. This phenomenon was already explained by Kim et al. in terms of surface interaction between the solute and the membrane [9]. The
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
73
Fig. 6. The flux of PEG solution for PAS6 as a function of pressure.
surface concentration of the solute would be lower for the higher molecular weight solute whose dimension is larger than that of the hydration shell, because the repulsive interaction between hydrophilic solute and hydrophobic chain of the membrane occurs. The rejection of PEG solution with flux for the PAS6 membrane is shown in Fig. 7. The rejection of PEG 200 solution through the PAS6 membrane increased with flux, while that of PEG 1000 solution slightly decreased. The enhancement in rejection of PEG 200 solution with increasing flux is a general tendency from the viewpoint of the relationship between rejection and flux. However, the rejection of PEG 1000 is slightly reduced with flux. This phenomenon may be closely related to the chain flexibility and morphology of the hydrated ionomer membrane. While the ionomer membranes do not have real visible pores (which is shown in Fig. 8), water molecules highly interacting with ion clusters in the ionomer membrane generate free space around the ion aggregate sites which depend on their hydration degree. The
solute retention characteristics depend much on how much free space for solute transport is there in the membranes, which can for some membranes be related to the flux. Hence, the permselectivity of the ionomer membrane depends on the formation of a percolation path through the membrane as water swells the ionic aggregates. The polar solute like PEG is transported through the percolated hydration shell whose polarity is higher than that of surrounding hydrophobic chains. However, this percolation path is interrupted with hydrophobic alkyl side chains, and small channels connecting the water pools, which makes the movement of solute molecules very difficult, can be formed. The polar solutes greater than the hydration shell in size would be repelled at the entrance of the hydration shell. The solutes whose size is comparable to that of hydration shell would be entrapped and sorbed in the hydration shell of the ionomer membrane. When the matrix polymer surrounding the hydration shell is rigid at room temperature, the hydration shell irrespective of pressure hinders the transport of solute. However, for the ionomer membrane whose matrix is quite
74
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
Fig. 7. The rejection of PEG solution for PAS6 as a function of pressure.
flexible, the solute repelled at the entrance of percolation path or entrapped in the isolated small channels could pass through the size changeable hydration shell with pressure. Therefore, the rejection of larger molecules compared with the size of hydration shell could be decreased with pressure when it passes through the polymer matrix which is quite flexible. Contrarily, the smaller solute compared with the size of hydration shell passes through the hydration shell freely, and the rejection will thus be increased with pressure irrespective of polymer matrix flexibility around the hydration shell.
flux and rejection showed maximum values at a certain side chain length of the ionomer. As shown in Fig. 9(a) the flux of water and PEG 200, PEG 1000 would be decreased for the ionomer membrane whose carbon atom number in the side chain is over 8.
3.5. Effect of side chain length on separation performance The separation performances of the PAS ionomer membranes with different alkyl side chain length were characterized by the permeability method with PEG 200 and 1000 aqueous solutions at 20 bar. The flux and rejection of PEG 200, PEG 1000 aqueous solutions for the ionomer membrane are shown in Fig. 9. The
Fig. 8. SEM image of PAS6 composite membrane (surface of top layer, ×10 000).
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
Fig. 9. (a) The flux and (b) rejection for the PAS series as a function of the number of carbon atoms in the side chain.
75
76
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
These changes in flux may be caused by the solubility and tortuousity changes for the PAS ionomer membrane with different alkyl side chain. The transport of liquid through a dense, non-porous membrane is greatly influenced by the solubility and diffusivity of the organic solute. For the PAS membranes, as already shown in Fig. 4, the amount of water molecules participating in the hydration shell surrounding the ionic aggregates is decreased with increasing the side chain length of the ionomer, which results in the reduced flux for the ionomer membrane with longer side chain of the matrix polymer. The lower flux for the ionomer membrane made of the matrix polymer with short side chain may result from the morphology and the tortuousity of the hydration shell. More studies on the morphology and tortuousity of the ionomer membrane with flexible alkyl side chain are needed for better understanding of the flux behavior for the ionomer membrane with short side chain of the matrix polymer. It was observed in Fig. 9(b) that the rejection of PEG 200 and PEG 1000 showed the maximum values when the number of carbon atoms in the alkyl side chain of the ionomer was around 8. The changes in rejection of PEG 200 and PEG 1000 results from the amount of hydration shell and the flexibility of the matrix polymer surrounding the hydration shell. The lower rejection for the membrane made of the matrix polymer with shorter alkyl side chain is mainly attributed to the large amount of the hydration shell. As the side chain length increases, the amount of hydration shell decreases, which is already indicated by the swelling ratio and non-freezing water content. Even though small amount of hydration shell for the ionomer membrane made of the matrix polymer with longer side chain is expected to cause higher rejection, the rejection for the ionomer membrane whose matrix polymer having the number of carbon atoms in the alkyl side chain greater than 8 is found experimentally to decrease with increase of the side chain length. This decrease of rejection for the ionomer membrane made of the matrix polymer with longer side chain resulted from the flexibility of matrix polymer surrounding the hydration shell. As the side chain length of the ionomer increases, the flexibility of the matrix polymer increases. When the larger solute passes through the hydration shell surrounded by the flexible polymer chain, with increase of the
flexibility of the ionomer membrane, the solute repelled at the entrance of percolation path or entrapped in the isolated small channels could come to pass through the size changeable hydration shell. Thus, the flexibility of the ionomer membrane made of the matrix polymer with longer side chain reduces the rejection of PEG solute. This consideration is depicted in Fig. 10. The polar solutes whose size is smaller than that of the hydration shell, like PEG 200, can pass through the path in the ionomer membrane irrespective of pressure. However, the polar solutes greater than the hydration shell in size, like PEG 1000, would be repelled at the entrance or entrapped in the hydration shell of the ionomer membrane. When the matrix polymer surrounding the hydration shell is rigid at room temperature, the transport of solute is hindered by hydration shell irrespective of pressure. For the ionomer membrane whose matrix is quite flexible, the solute repelled at the entrance of percolation path or entrapped in the isolated small channels at low pressure could pass through the size changeable hydration shell at high pressure.
Fig. 10. Schematic illustration of separation mechanism of PEG solution for the PAS ionomer membrane.
J.-W. Lee et al. / Journal of Membrane Science 167 (2000) 67–77
4. Conclusions The flux and rejection of PEG aqueous solution for poly(alkyl methacrylate-co-sodium methacrylate) ionomer membrane showed maximum values at a certain side chain length of the ionomer. As the side chain length of the ionomer increases, the amount of hydration shell decreases while the flexibility of the matrix polymer increases. The amount of hydration shell and the flexibility of the matrix polymer chain may affect the performance of the ionomer membrane.
Acknowledgements This work was supported by the Korean Ministry of Science and Technology in 1997. References [1] M. Nystrom, L. Kaipia, S. Luque, Fouling and retention of nanofiltration membranes, J. Membr. Sci. 98 (1995) 249–262. [2] K. Scott, Handbook of Industrial Membranes, Elsevier, Oxford, 1995. [3] R. Levenstein, D. Hasson, R. Semiat, Utilization of the Donnan effect for improving electrolyte separation with nanofiltration membranes, J. Membr. Sci. 116 (1996) 77–92. [4] S. Wadley, C.J. Brouckaert, L.A.D. Baddock, C.A. Buckley, Modelling of nanofiltration applied to the recovery of salt from waste brine at a sugar decolourisation plant, J. Membr. Sci. 102 (1995) 163–175.
77
[5] J.M.K. Timmer, H.C. van der Horst, T. Robbertsen, Transport of lactic acid through reverse osmosis and nanofiltration membranes, J. Membr. Sci. 85 (1993) 205–216. [6] A.E. Childress, M. Elimelech, Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes, J. Membr. Sci. 119 (1996) 253– 268. [7] R.J. Petersen, Composite reverse osmosis and nanofiltration membranes, J. Membr. Sci. 83 (1993) 81–150. [8] P. Eriksson, Water and salt transport through two types of polyamide compositr membranes, J. Membr. Sci. 36 (1988) 297–313. [9] H.T. Kim, J.K. Park, K.H. Lee, Preparation, characterization, and performance of poly(acrylamidocaproic acid) partially neutralized with calcium for use in nanofiltration, J. Appl. Polym. Sci. 60 (1996) 1811–1819. [10] M.R. Tant, K.A. Mauritz, G.L. Wilkes, Ionomer, Blackie, New York, 1997. [11] A.A. Gronowski, M. Jiang, H.L. Yeager, G. Wu, A. Eisenberg, Structure and properties of hydrocarbon ionomer membranes 1. Poly(methyl methacrylate-co-methacrylic acid), J. Membr. Sci. 82 (1993) 83–97. [12] C.D. Ihm, S.K. Ihm, Pervaporation of water-ethanol mixtures through sulfonated polystyrene membranes prepared by plasma graft-polymerization, J. Membr. Sci. 98 (1995) 89–96. [13] H. Matsuyama, M. Teramoto, M. Tsuchiya, Percolation permeability in styrene-methacrylic acid ionomers, J. Membr. Sci. 118 (1996) 177–184. [14] A. Eisenberg, H.L. Yeager (Eds.), Perfluorinated Ionomer Membranes, ACS Symp. Ser. No. 180, American Chemical Society, Washington, DC, 1982. [15] J. Ramkumar, B. Maiti, T.S. Krishnamoorthy, Transport of some nitrogen heterocyclic and aromatic compounds through metal ion containing Nafion ionomer membrane, J. Membr. Sci. 125 (1997) 269–279. [16] T. Hatakeyama, Thermal Analysis: Fundamentals and Applications to Polymer Science, Wiley, West Sussex, 1994.