sulfonated poly(ether ether ketone) blend membranes

Journal of Membrane Science 250 (2005) 1–10

Manufacture and characterisation of polyetherimide/sulfonated poly(ether ether ketone) blend membranes W. Richard Bowen ∗ , Shu Ying Cheng, Teodora A. Doneva, Darren L. Oatley Centre for Complex Fluids Processing, School of Engineering, University of Wales Swansea, Singleton Park, Swansea SA28PP, UK Received 29 July 2003; received in revised form 21 June 2004; accepted 1 July 2004

Abstract Charged UF/NF membranes have been developed by phase inversion. The membranes were prepared with polyetherimide (PEI) and sulfonated poly(ether ether ketone) (SPEEK). SPEEK was used to improve the hydrophilic properties and permeability of the PEI membrane, as well as to provide surface charges. The membrane properties were reproducible when the proportions of SPEEK were ∼3.0% and ∼6.0% in the total blend of polymers. With the increase of SPEEK from 3% to 6% in the casting solution, water permeability increased from 24.0 ± 2.1 × 10−11 to 36.6 ± 3.0 × 10−11 m3 s−1 N−1 , rejection of sodium chloride and PEG 1500 increased from 0.399 to 0.599, and 0.600 to 0.931, respectively (as measured at 100 kN m−2 ). SEM cross-sectional images showed a top layer with a sponge-type structure and a support layer with a finger-like structure. With the addition of tetrahydrofuran (THF) and 1,4-dioxane to the casting solution, water permeability decreased, rejection of PEG 1500 decreased and rejection of NaCl was generally little changed. © 2004 Elsevier B.V. All rights reserved. Keywords: Charged membrane; Polyetherimide; Sulfonated poly(ether ether ketone); Atomic force microscopy; Scanning electron microscopy

1. Introduction Polyetherimide (PEI) is of particular interest in the fabrication of both UF and NF membranes [1–3]. Kim and Lee [2] prepared integrally skinned uncharged PEI asymmetric nanofiltration membranes by the dry/wet phase inversion method. The aromatic imide units of PEI provide high performance properties such as considerable mechanical strength, thermal stability and chemical resistance, while the flexible ether linkages provide good processability. PEI has been successfully used in preparation of asymmetric membranes for gas separation and pervaporation [4,5]. PEI exhibits impressively high selecivities for all important gas pairs [6]. PEI has also been used to reduce membrane swelling at sulfonated poly(ether ether ketone) (SPEEK) ion-exchange membranes in order to reduce/increase hydrogen bond formation [7].

∗

Corresponding author. Tel.: +44 1792 295862; fax: +44 1792 296862. E-mail address: [email protected] (W.R. Bowen).

0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.07.004

Pure sieving by the pore structure is not the only effective factor on the retention of membranes in filtration processes [8]. For example, Noordman et al. [9] investigated the retention of salts with ultrafiltration membranes with a cut-off value of 15 kD, no steric effect of the pores on the ionic substances being expected. Thus, the only force that could achieve a retention of salt ions was the electrochemical interaction between the charged pore surfaces and the different ions. The rejection observed for phosphate ions was surprisingly high (up to 80%) considering the cut-off value of the membrane used. With decreasing pore dimensions electrochemical interactions between solutes in the feed and the surface charges of the pore walls become important. For charged molecules transport phenomena like back-diffusion or charge repulsion are enhanced in nanofiltration compared to ultrafiltration and microfiltration [10]. Many studies have been conducted concerning the addition of hydrophilic polymers such as polyethylene glycol (PEG) or polyvinylpyrrolidone (PVP) in the membrane cast-

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W.R. Bowen et al. / Journal of Membrane Science 250 (2005) 1–10

ing solution to improve the membrane performance [11–13]. For example, Cranford et al. [13] investigated the effects of PVP on PEI membranes and showed that PVP could leak out into the coagulation bath whilst phase separation occurred and that the formed membrane was a non-uniform PVP distribution membrane. In another approach, the addition of functional groups to the original membrane in order to increase the positive or negative fixed charges has reduced membrane fouling. Charged polymers not only change the hydrophilicity of membranes, but also provide charges on the surface. In previous studies [14–17], sulfonated poly(ether ether ketone) was introduced to modify polysulfone membranes in order to obtain polysulfone/SPEEK blend UF/NF membranes. SPEEK is only miscible with polysulfone in N-methyl-2-pyrrolidinone (NMP) over a small range. The role of SPEEK in the blend membrane was investigated and high flux, high salt rejection, and relatively low fouling was achieved. Humic acid separation from a model water was carried out by using polysulfone/SPEEK blend membranes [16]. Poly(ether ether ketone) (PEEK) is a relatively new polymer with high performance and high chemical stability. At 100% sulfonation, SPEEK can dissolve in water [18], implying its higher hydrophilicity. SPEEK and polyetherimide are miscible at all proportions [7]. Asymmetric membranes are generally prepared by the phase inversion method [19]. The interchange of solvent and nonsolvent coagulation happens due to diffusion whilst a casting solution consisting of polymer and solvent is immersed into a non-solvent coagulation bath, and the membrane is formed because the casting solution goes through a phase transition. The objective of this present study has been to investigate the effect of small amounts of SPEEK on the structure (including pore size and surface porosity) and electrical properties of polyetherimide ultrafiltration/nanofiltration membranes. The membranes synthesized have been characterized by filtration studies coupled with quantitative descriptions of rejection, by atomic force microscopy (AFM) and by scanning electron microscopy (SEM).

2. Experimental 2.1. Materials All the membranes characterized were prepared in the laboratory with polyetherimide supplied by Sigma–Aldrich, UK. SPEEK 223 was kindly supplied by PCI Membranes. SPEEK 223 has an ion exchange capacity (IEC) of 2.1 meq g−1 . The structural formula of polyetherimide and SPEEK are given in Fig. 1. PEI was dried for 8 h at 100 ◦ C and SPEEK for 2 h at 60 ◦ C before use. N-Methyl-2pyrrolidinone, 1,4-dioxane and tetrahydrofuran (THF) were from Aldrich of analytical grade. The solvents were used as received without further treatment. The solutes employed to characterize membranes were PEG 1500 with molecular weight of 1500 Da and sodium chloride supplied by FisherScientific, UK. 2.2. Preparation of membrane Asymmetric membranes were prepared by the phase inversion technique from casting solutions containing predetermined amounts of PEI and SPEEK polymers, NMP, and in some case 1,4-dioxane and THF. The compositions of the casting and coagulation conditions are presented in Table 1. Firstly, PEI was dissolved in solvent (NMP) with stirring at approximately 60 ◦ C to ensure the complete dissolution of the polymer. The obtained solutions were used to prepare pure polymer membranes. Then additives of SPEEK, THF and 1,4-dioxane were dissolved in the prepared PEI casting solutions with stirring at the room temperature for at least 4 h for preparation of negatively charged membranes. The casting solutions were maintained at room temperature for at least 1 day to release the bubbles and then cast onto a glass plate with a doctor blade (CAMAG) at a wet thickness of 0.250 mm. The casting films together with the glass plates were immersed in de-ionized water (DI) and maintained at room temperature of about 20 ◦ C for 2 h. The membranes fabricated from the casting solution were evaporated for 12 s prior to immersion into DI water except for those containing THF which were evaporated for 30 s. During gelation,

Fig. 1. The chemical structure of polyetherimide and SPEEK.

W.R. Bowen et al. / Journal of Membrane Science 250 (2005) 1–10

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Table 1 Composition and fabrication condition of membranes Membrane type

Mem-22

A3-22

A6-22

A3D

A6D

A3DT

A6DT

Composition (g) PEI SPEEK Dioxane THF NMP

17 – – – 60

16.5 0.5 – – 60

16 1 – –

16.5 0.5 20 – 40

16 1 20 – 40

16.5 0.5 1 10 49

16 1 1 10 49

Coagulation temperature (◦ C) Evaporation time (s)

20 12

20 12

20 12

20 12

20 12

20 30

20 30

the membrane peeled off from the glass plate spontaneously. The membranes were then moved to fresh DI water subject to solvent exchange for 1 day. The prepared membranes were stored in DI water. 2.3. Membrane characterization 2.3.1. Atomic force microscopy (AFM) Atomic force microscopy has an important advantage in the study of surfaces in quantifying both surface morphology and surface interactions in a single instrument [16]. An Autoprobe CP-100 (Park Scientific Instruments) was used to image the membranes. The surface morphology was measured in air with a scan rate of 1 Hz at 256 × 256 resolution in contact mode. 2.3.2. Scanning electron microscopy (SEM) A scanning electron microscope was used to image the cross-section of the asymmetric membranes. The membranes are cryogenically fractured in liquid nitrogen and then coated with Pt/Pd. The samples were measured with a Philips XL30 CP SEM. 2.3.3. Filtration experiments 2.3.3.1. Experimental set-up. Membrane rejection and membrane flux were measured for aqueous solutions of PEG 1500 (0.1 g/l) and sodium chloride (0.001 M). All experiments were carried out at room temperature (22 ± 2 ◦ C) and pH 5.8 ± 0.2, which is the pH of the pure water used in this study. The experimental device employed is a frontal filtration system composed of an Amicon 8050 cell with 50 ml feed solution and effective membrane area of 13.4 × 10−4 m2 . Each membrane was first subjected to pure water pressurization under a pressure of 500 kN m−2 for 2 h before use in order to obtain a stable permeability. Then water permeability of each membrane was measured from 100 to 400 or 500 kN m−2 . Subsequently, the solute rejection was measured with PEG 1500 and sodium chloride over the same pressure range. After each run, the membrane was washed using DI water to thoroughly remove the remains of the solute from the last running. Rejection was based on measurement of 15 ml of permeate after the first 4 ml permeate was removed. The concentration of sodium chloride was determined using a Russell RL 105 conductivity meter and probe at 25 ◦ C

and a conductivity versus concentration calibration curve. A simple calibration run with known concentration NaCl samples produces a calibration curve. The constants from the best fit curve were then fed into a Fortran program which accurately converts conductance into concentration. The concentration of PEG 1500 was analyzed through a HPLC system (ProStar 210, Varian Inc.) connected with a differential refractive index detector (ProStar 350, Varian Inc.). 2.3.3.2. Membrane characterization from rejection data. The rejection of solute was calculated using the following equation, Robs = 1 −

Cp Cf

(1)

where Cp and Cf are concentration of permeate solution and feed solution respectively. As a result of concentration polarization, the concentration of solute at the membrane wall is higher than that of the bulk. The real rejection is then defined as, Rreal = 1 −

Cp Cw

(2)

where Cw is the solute concentration at the membrane surface. The value of Cw may be calculated indirectly from the bulk feed solute concentration (Cf ) using a mass transfer correlation [20]. The real rejection of a membrane can be determined from the observed rejection.
ln

1 − Robs Robs




= ln

1 − Rreal Rreal

 +

Jv k

(3)

where k is the mass transfer coefficient. The result is equally applied to charged and uncharged solutes. A mass transfer correlation to predict k in a frontal filtration cell was developed by Opong and Zydney [21]. Bowen et al. [22] used the infinite rejection method proposed by Nakao and Kimura [23] to confirm their evaluation of the mass transfer coefficient within the Amicon cell as,
k = 0.23

r2 ν

0.567


ν Deff,∞

0.33

Deff,∞ 0.567 ω r

(4)

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W.R. Bowen et al. / Journal of Membrane Science 250 (2005) 1–10

The extended Nernst–Planck equation describes pore ion transport ji = −Di,p

zi ci Di,p dψ dci − F + Ki,c ci V dx RT dx

(5)

where Di,p = Ki,d Deff,∞ . Ki, c and Ki, d are hindrance factors of diffusion and convection of the ions inside the membrane [20]. Theoretical hindrance factors have been derived on a geometric basis and so should not be affected by changes in solvent viscosity. The following third-order polynomial equations based on the Donnan-steric-pore-model (DSPM) were used: Ki,c =

Ki,d

V = (2 − Φ)(1.0 + 0.054λ − 0.998λ2 + 0.441λ3 ) Vx (6)

Dp = = 1 − 2.3λ + 1.154λ2 + 0.224λ3 Deff,∞

zi Ci = 0,

i=1

n 

zi ci = −Xd

(7)

(8)

i=1

From the conditions of pore electroneutrality and zero current, Eq. (5) is rearranged to give an expression for potential gradient n (zi V/Di,p )(Ki,c ci − Ci,p ) dψ (9) = i=1  dx (F/RT ) ni=1 z2i ci The concentration gradient through the membrane pore is then related to the potential gradient as follows dci V F dψ = (Ki,c ci − Ci,p ) − zi ci dx Di,p RT dx

(10)

In order to solve the transport equation the solute concentrations at both pore interfaces, i.e. ci (0) and ci (x), must be known. These values are obtained from equilibrium partitioning, which relates the concentration in the bulk feed (or permeate) to that within the membrane pore. The expression used in this case is: 
ci −zi F (11) = Φi exp ψD Ci RT where Φi = (1 − λi )2

Ri = 1 −

Ci,p Ki,c Φi =1− Ci,f 1 − [1 − Ki,c Φi ] exp(−Pe )

(12)

The terms on the right hand side of Eq. (11) are the classic expressions for both steric and Donnan effects respectively [20,22,24]. Note that dielectric contributions to partitioning have been omitted in this study as a detailed charge characterisation was not required.

(13)

where Pe =

Ki,c rp2 Ki,c Vx = Pe Di,p 8ηDi,p

(14)

The rejection calculated using Eq. (13) for a given membrane and solute (i.e. known pore radius rp and solute radius rs ) is dependent only on the effective pressure driving force Pe . The viscosity term used in this expression is not that of the bulk viscosity, though it is replaced by the bulk viscosity if the value of Di, p is defined as Di,p = Ki,d Di,∞

where λ = rs /rp , the ratio of solute radius to pore radius. Electroneutrality in the bulk solution and in the membrane pore are described as n 

For the simple case of uncharged solutes, the transport equations simplify to give an algebraic result

η0 η

(15)

For the more complicated case of electrolytes, the transport equations must be solved numerically. Firstly, an approximation of the permeate concentration is required in order to solve Eq. (10). Then, using the value obtained for ci (x) and Eq. (11), the permeate concentration is calculated. Thus, the solution to the problem is the minimisation of the error function Error = fi = Ci,p(transport) − Ci,p(partitioning)

(16)

In the case of several electrolytes in solution, the numerical solution becomes a combined error minimisation where the overall error function becomes 2 fOverall = f12 + f22 + · · · + fn−1

(17)

with all the values for Ci, p known for n − 1 components and the nth component calculated from electroneutrality. Therefore, using the updated DSPM [24] model described above, salt rejection depends on only two parameters—effective pore radius rp and the ratio of effective membrane charge density to bulk electrolyte concentration ξ (Xd /Cb ). Thus, with a knowledge of rp and salt rejection data, Xd may be calculated.

3. Results and discussion The composition and fabrication conditions of the membranes are given in Table 1. All the membranes were made from the casting solutions with the same concentration of total polymers. Mem-22 represents a membrane with no additive in the casting solution, simply 22% PEI in NMP. The A-series represents the blend membranes of PEI/SPEEK. A3- and A6series represent the percent of SPEEK in the total polymer being close to 3% and 6%. The number after the dash represents the total polymer percent in the casting solution. D represents 1,4-dioxane, whilst T represents tetrahydrofuran with a volume of 10 ml in the casting solution.

W.R. Bowen et al. / Journal of Membrane Science 250 (2005) 1–10

Fig. 2. Water flux of different membranes with variation of applied pressure.

3.1. Water permeability Pure water flux was measured for the seven types of membranes using an applied pressure range of 100–400 kN m−2 and is illustrated in Fig. 2. Up to 10 samples of each type of membranes were measured to evaluate their reproducibility. The addition of SPEEK results in a remarkable increase in the water flux. Mem-22 had very low water permeability. When 3% SPEEK was introduced into the membrane, water permeability sharply increased by almost two orders of magnitude. When the percentage of SPEEK changed from 3% to 6%, the water permeability increased a further factor of 1.7. With the addition of 1,4-dioxane and THF the permeability decreased compared to PEI/SPEEK membranes produced from pure NMP. 3.2. Membrane characterization from solute transport data The rejection of PEG 1500 and NaCl for all types of membranes was measured in order to characterize the mem-

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brane and study the effect of additives on membrane morphology and properties. Unfortunately, the rejection of PEG 1500 was close to zero for the Mem-22 membrane and so no meaningful pore size information could be obtained for this membrane. The rejection of PEG 1500 at all membranes increased with applied pressure. Addition of SPEEK resulted in increased PEG 1500 rejection. With the introduction of 1,4-dioxane and THF into the casting solution the rejection of PEG 1500 decreased. Thus, the A6-22 membrane showed the highest PEG 1500 rejection as well as having the highest water permeability. The effective pore radii calculated using Eq. (13) and the Stokes’ radius of PEG 1500 calculated from the empirical Lentsch equation [9] are also reported in Table 2. The effective pore radii decrease as the SPEEK content is increased. Addition of 1,4-dioxane and THF in the casting solution resulted in an increase in effective pore radius. With the introduction of SPEEK to the polymer blend, the membranes produced should gain in fixed charge. This increase in fixed charge will contribute to the rejection of charged solutes [9]. The rejection data from 0.001 M NaCl given in Table 2 clearly indicates that as the level of SPEEK is increased, the rejection of NaCl also increases. This confirms that addition of SPEEK increases the fixed charge density of the membrane. At the same percentage of SPEEK in the casting solution, the prepared membranes show similar rejection for NaCl, the addition of THF and dioxane causing only small changes in NaCl rejection. Values for the calculated effective membrane charge density, Xd , are also shown in Table 2. The A6-series membranes all have Xd values of greater magnitude with addition of 1,4-dioxame or THF decreasing the magnitude. The A3 series membranes have Xd values of lesser magnitude again with decrease of magnitude on addition of 1,4-dioxane or THF. To show the effect of variation of the applied pressure, flux and rejection for 0.001 M NaCl and 0.1 g/l PEG 1500 are shown in Figs. 3 and 4 for membranes A3-22 and A3DT. The linearity of flux data illustrates that the membrane com-

Table 2 Characteristics of prepared membranes Properties Filtration characterization Pm (m3 s−1 N−1 )1011 Rreal a , PEG1500 λ(rs /rp ) rp (nm) Rreal b , NaCl Xd (mol m−3 )b

Mem-22

A3-22

A6-22

A3D

A6D

A3DT

A6DT

0.35 ± 0.1 – – – 0.090 –

24.0 ± 2.1 0.60 0.508 2.21 0.399 −17.8

41.0 ± 4.9 0.931 0.767 1.46 0.599 −79.2

22.8 ± 2.2 0.343 0.362 3.11 0.401 −14.6

36.6 ± 3.0 0.839 0.659 1.71 0.600 −63.6

18.8 ± 2.0 0.336 0.376 2.99 0.444 −13.3

27.5 ± 1.9 0.692 0.545 2.07 0.579 −45.8

AFM characterization rAFM (nm) Roughnessc (nm) ε (%)

2.00 ± 0.39 2.6 10

1.68 ± 0.35 7.1 11

1.51 ± 0.22 6.0 12

1.27 ± 0.13 5.5 8

1.55 ± 0.1 5.8 13

1.16 ± 0.32 3.2 10

1.19 ± 0.21 4.2 10

a b c

From rejection of 0.1 g/l PEG1500 solutions. 400 kN m−2 for Mem-22, 100 kN m−2 for all other membranes. From rejection of 0.001 N NaCl solution. 400 kN m−2 for Mem-22, 100 kN m−2 for all other membranes. AFM measurements. Roughness is root-mean-square roughness over an area of 2 ␮m × 2 ␮m.

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W.R. Bowen et al. / Journal of Membrane Science 250 (2005) 1–10

Fig. 3. Effects of applied pressure on the rejection of NaCl solution at membranes A3DT and A3-22: (a) flux of 0.001 M NaCl and (b) rejection of 0.001 M NaCl.

pressibility and fouling were not significant and the pressure dependence is typical of that for porous membranes. The rejection of PEG 1500 increased with an increase in applied pressure. The rejections of sodium chloride at these two membranes containing the same proportion of SPEEK are close over the entire range of pressure even though the fluxes deviated significantly. 3.3. Membrane characterization by atomic force microscopy (AFM) and scanning electron microscopy (SEM) 3.3.1. Surface morphology All the membrane samples were imaged using Atomic Force Microscopy at 50 nm × 50 nm. The mean pore radii and deviation are reported in Table 2. The images of mem22, A3-22 and A6-22 are shown in Fig. 5. The dark regions in the images are depressions and pores. Different pore shapes, including circular, elliptical and slits were observed in the AFM images of the membrane surface. The mean pore radii obtained from AFM images are mostly smaller than those

Fig. 4. Effects of applied pressure on rejection of PEG1500 at membranes A3-22 and A3DT: (a) flux of 0.1 g/l PEG 1500 and (b) rejection of 0.1 g/l PEG 1500.

obtained from solute rejection data due to pore-tip convolution [25]. Also, pore sizes obtained from a solute rejection correspond to a minimal size of the pore constriction experienced by the solute while passing through the pore, but pore sizes measured by AFM correspond to the pore entrances on the surface. Most importantly, the AFM measurements confirm that the variation in pore dimensions across the range of membranes is not very great. Analysis of the images in conjunction with digitally stored line profiles allows quantitative determination of surface pore size distributions, reported in Fig. 6. The membranes showed relatively narrow pore size distributions, even though the bigger or smaller pores existed but only in small quantities. Surface roughness and surface porosity were measured by AFM and are reported in Table 2. Surface porosity is defined as the ratio of the area of the pores to the total membrane surface area. Several AFM images of different parts of the same membrane were analyzed to obtain this data. Addition of 1,4-dioxane and especially THF resulted in smoother membranes. The surface porosity does not vary greatly across

W.R. Bowen et al. / Journal of Membrane Science 250 (2005) 1–10

Fig. 5. Surface images for (a) Mem-22; (b) A3-22 and (c) A6-22 by AFM.

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Fig. 6. Surface pore size distribution for (a) Mem-22; (b) A3-22 and (c) A6-22 from AFM images.

the range of membranes and is only weakly correlated with membrane permeability. 3.3.2. Cross-section morphology The membrane structure was observed by scanning electron microscopy after fracturing in liquid nitrogen. The cross section was coated with a thin gold layer. Structures of membrane cross-section and the top of crosssection for membranes of pure PEI and PEI/SPEEK blends

are shown in Fig. 7. All the membranes have similar structures—asymmetric structure, top layer with sponge-type structure and support layer with a finger-like structure. Mem22 has a thinner and denser top layer with a sponge-like structure. With the addition of SPEEK the membranes become more porous, in good agreement with experimental measurement of water permeability.

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W.R. Bowen et al. / Journal of Membrane Science 250 (2005) 1–10

Fig. 7. SEM images of membrane cross-section and top layer: (a) Mem-22 cross-section and top layer; (b) A3-22 cross-section and top layer and (c) A6-22 cross-section and top layer.

3.4. Effects of SPEEK on membrane performance and morphology Due to its high hydrophilicity and negative charges, SPEEK was used as an additive to change the PEI hydrophilicity and improve water permeability. As polyetherimide is a relatively hydrophobic polymer its water permeability is poor. In contrast, SPEEK is a highly hydrophilic polymer and favors water ingression, which will favor water diffusion into the casting solution and phase separation. In addition, SPEEK is a highly charged polymer. Due to elec-

trostatic repulsion between SPEEK molecules, the addition of SPEEK in the casting solution will impede the bundling of polymers which could give rise to larger pore size or interconnected pores. Therefore, the systematic addition of SPEEK might be expected to greatly increase permeability but not pore size. Experimental findings for the membranes studied are reported in Table 2. On addition of SPEEK to the casting solution the pure water permeability significantly improves and the pore size of the membrane was reduced. The surface porosity of the membranes showed only slight improve-

W.R. Bowen et al. / Journal of Membrane Science 250 (2005) 1–10

ment on addition of SPEEK. This result indicates that as the pore size is successively decreased, more pores are formed to maintain the similar level surface porosity. This decrease in pore size would normally result in an increase in hydrodynamic resistance to flow and a lower permeability. However, the permeability of the membranes significantly increased, which illustrates the extreme change from a hydrophobic to a hydrophilic membrane caused by the addition of SPEEK. Introduction of SPEEK to the polymer blend also has a significant influence on the rejection properties of the membrane. Firstly, as the SPEEK content is increased the pore size decreases, which leads to higher rejection of neutral species. Secondly, as SPEEK also introduces fixed charge to the membrane, the combination of smaller pores and increased membrane charge increases the rejection of charged species. 3.5. Effects of 1,4-dioxane and THF on membrane performance and morphology Membranes were fabricated using the phase inversion method. During this process, two distinct demixing phenomena are expected. Instantaneous liquid–liquid demixing causes the formation of a finger-like structure and delayed demixing causes the formation of a sponge-like structure within the membrane, respectively. 1,4-Dioxane is a cosolvent for PEI [3]. Adding 1,4-dioxane to the casting solution is expected to cause the formation of a dense layer of PEI on the membrane surface which inhibits the influx of water into the polymer solution. This phenomenon is due to the lower affinity of 1,4-dioxane than NMP for water. THF is a volatile solvent for PEI. During the membrane formation, THF is expected to evaporate easily and a denser and less porous surface will be formed. These expectations are in agreement with the water permeability data. The volatility of co-solvent has important effect on the membrane roughness. Due to volatility of THF, membrane cast using this co-solvent exhibited a smoother surface. However, addition of THF and 1,4-dioxane did not give increased uncharged solute rejection which may suggest the presence of defects in the membrane structure.

4. Conclusions The phase inversion process was employed to prepare asymmetrically skinned membranes. The effects of additives were studied on membrane morphology and performance. SPEEK was used to improve the hydrophilic property and permeability of the PEI membrane. The membrane properties were very reproducible when the proportion of SPEEK was 3% and 6% in the total polymer content. With an increase of SPEEK, the PEI/SPEEK blend membranes achieved higher water permeability and higher performance for rejection of uncharged PEG 1500 and salt. Experimental analysis revealed that addition of SPEEK contributes to high surface porosity but low pore size. The membranes were at the ultra-

9

filtration/nanofiltration boundary in terms of effective pore radius. THF and 1,4-dioxane are co-solvents for polyetherimide in NMP solution. Addition of these two materials into the casting solution produced the desired effect of forming a denser top layer through the high volatility of THF and the low affinity of dioxane for water, which is in a good agreement with water permeability. However, the data suggests that defects may be easily formed on addition of THF and dioxane causing a reduction in membrane performance. SEM studies showed the cross-section morphology of the membranes having a top layer with sponge-type structure and a support layer with finger-type structure. AFM analysis of surface pore radii showed that the membranes had a relatively narrow pore size distribution. Overall, these studies show that the membranes have a very promising combination of permeability and pore size, especially the A6-22 membrane. The effect of charge introduced by SPEEK is especially noteworthy. The membranes are promising for scale-up of production and resting on real process streams.

Acknowledgement We thank Mr. Martin Peer of PCI Membranes for supplying SPEEK 223.

Nomenclature List of symbols ci concentration inside the membrane (mol m−3 ) Ci, f bulk feed Concentration (mol m−3 ) Ci, p permeate Concentration (mol m−3 ) Cw concentration at membrane wall (mol m−3 ) D 1,4-dioxane Deff,∞ effective diffusion coefficient in infinite dilution (m2 s−1 ) Dp pore diffusion coefficient (m2 s−1 ) Jv volumetric flux per unit area of membrane (m3 m−2 s−1 ) k mass transfer coefficient (m s−1 ) Kc hindrance factor for convection Kd hindrance factor for diffusion Pe

Peclet number Pm water permeability (m s−1 ) P applied pressure (N m−2 ) effective pressure driving force (N m−2 ) Pe Robs observed rejection real rejection Rreal r radius of the stirrer (m) rp effective membrane pore radius (m) rs solute spherical (Stokes) radius (m) T THF

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V Vx zi Xd

solvent velocity (m s−1 ) maximum pore solvent velocity (m s−1 ) valence of ion i effective membrane charge density (mol m−3 )

[10]

[11] [12]

Greek symbols ψD Donnan potential (V) δ thickness of boundary layer (m) η viscosity (N s m−2 ) λ ratio of solute to pore radius ξ ratio of effective membrane charge density to bulk feed concentration Φ steric partitioning coefficient ν kinematic viscosity (m2 s−1 ) ω angular velocity (rad s−1 )

[13]

[14]

[15]

[16]

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