Polyamide thin film composite membrane using mixed amines of thiourea and m-phenylenediamine

Abstract

Polyamide thin film composite nanofiltration membranes were synthesized on the polyethersulfone (PES) support via the interfacial polymerization between trimesoyl chloride (TMC) and the amine mixture of m-phenylenediamine (MPD) and thiourea (TU). It is indicated that the composite membrane (PES-M5T5) prepared at the MPD/TU ratio of 5 : 5 shows higher flux and glucose rejection than the membranes prepared using the individual amine. Further the optimal membrane PES-M5T5-TEA, prepared in the presence of triethylamine (TEA) additive, shows the largest flux of 39.7 L m⁻² h⁻¹ at an operation pressure of 1.6 MPa and the glucose rejection higher than 97.0%. In combination with characterizations of ATR-IR, Raman, SEM, XPS, AFM and contact angle measurements, it is illustrated that the TEA additive can promote the IP between TMC and MPD + TU, resulting in more amount of thioamide structures generated in the top layers of PES-M5T5-TEA, which can form more free volume and consequently improve the membrane flux. In addition, the PES-M5T5-TEA membrane shows excellent chlorine resistance against immersion of 200 ppm NaClO solution more than 100 h.

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1. Introduction

Thin-film composite (TFC) membranes synthesized on porous supports through the interfacial polymerization (IP) have been widely applied in the nanofiltration (NF) and reverse osmosis processes.^1–5 TFC membranes are generally polymerized using monomers of diamine and acyl chloride. Roh⁶ compared the separation performance of TFC membranes prepared through IP between TMC and aromatic diamines or aliphatic diamines, and concluded that TFC membranes with aliphatic diamine exhibited high water flux but low NaCl rejection while those with aromatic diamines showed high NaCl rejection but a relative low flux. Jegal et al.⁷ and Kim et al.⁸ suggested that the aliphatic alkyl chains can facilitate the formation of free volume in polyamide layers and then enhance the membrane flux, comparing with aromatic chains. It is still a challenge to select proper monomers for the manufacture of TFC membrane with high flux and good rejection towards small organic molecules and salts.

In order to enhance the membrane flux, certain amine mixtures have been adopted to prepare TFC membranes, instead of single diamine. For instance, Mohan et al.⁹ used a mixture of 2,3-diaminopyridine (DAP) and m-phenylenediamine (MPD) to react with trimesoyl chloride (TMC), and reported that the water flux increased to 67.4 L m⁻² h⁻¹ at 1 MPa but the rejection towards NaCl decreased to 88.0% for the membrane prepared under the optimal DAP/MPD ratio of 2/8. Zhou et al.¹⁰ prepared polyamide TFC membrane through IP between TMC and the mixture of m-phenylenediamine (MPD) and m-phenylene diamine-5-sulfonic acid (SMPD), and reported that the flux increased to 45 L m⁻² h⁻¹ at 1.5 MPa and the NaCl rejection maintained at 90% for the membrane prepared at the optimal SMPD/MPD ratio of 0.3. In addition, Zhang et al.¹¹ synthesized polyamide TFC NF membranes via IP between 1,2,4,5-benzene tetracarbonyl chloride and MPD, and found that through introducing the methylene group into the chemical structure chain of polyamide the membrane flux can be increased greatly while the rejection towards glucose maintained at 90%. Inspired by these reports, we are intriguing to synthesize TFC NF membranes using simultaneously the aromatic and aliphatic diamines, so as to explore a pathway to manufacture TFC membranes with both high flux and good rejection towards small organic molecules.

Thiourea (TU) is a cheap diamine. To synthesize an aromatic azo-polymer, poly(thiourea-azo-naphtyl), Kauser and Hussain¹² reported that the thiourea group was susceptible to transform into the thiocyanate group under acidic solutions, suggesting that thiourea is probably easy to be covalently linked with aromatic polymers. In this article, we adopted the amine mixture of MPD and TU to react with TMC via the IP in the absence and the presence of triethylamine additive, so as to synthesize a new polyamide TFC membrane on a commercial polyethersulfone ultrafiltration membrane. It is indicated that the polyamide membrane prepared at the optimal MPD/TU ratio of 5 : 5 in the presence of triethylamine shows high flux and good rejections towards glucose and salts. The mechanism that triethylamine increases the membrane flux was investigated by characterizations of attenuated total reflection infrared spectroscopy (ATR-IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), contact angle measurements, etc.

2. Experimental

2.1. Materials

Polyethersulfone (PES) ultrafiltration membranes with a molecular weight cutoff (MWCO) about 30 kDa were purchased from the Shengwanquan Nova Membrane Technology Co., Beijing, China. N-hexane, m-phenylene diamine (MPD), thiourea (TU), triethylamine (TEA), trimesoyl chloride (TMC) and glucose (D+, anhydrous) were purchased from Sinopharm Chemical Reagent Co. Ltd. All the chemicals were of analytical grade and used without further purification. Deionized water was used in all the experiments.

2.2. Membrane preparation

The polyamide (PA) composite NF membranes were prepared on the surface of PES support by interfacial polymerization between TMC and the mixture of thiourea and m-phenylenediamine with different weight ratios (MPD/TU), as shown in Fig. 1. The following procedure was adopted to prepare the composite membranes in the absence of TEA. An aqueous solution containing the mixed amine MPD + TU (with the total amine concentration of 2 wt%) was poured on the top of the commercial PES ultrafiltration membrane at room temperature, after 2 min the excess solution was removed. Next, a 0.1 wt% TMC hexane solution was poured on the impregnated PES support for 1 min to synthesize the PA thin film composite NF membranes, followed by the curing at 70 °C for 6 min. The obtained composite membrane was washed by water and then characterized. In order to make the discussion clearly, the composite NF membrane was denoted in terms of the MPD/TU ratio, i.e., PES-M7T3, PES-M5T5 and PES-M3T7 indicate respectively the PA composite membrane prepared with the MPD/TU ratio of 7 : 3, 5 : 5, and 3 : 7. As a control, the PA composite membrane was also prepared using the individual amine. PES-M and PES-T indicate the PA composite membrane prepared via the IP between TMC and MPD or TU.

Fig. 1 Schematic structure of polyamide TFC membrane prepared via the IP between TMC and the amine mixture of MPD + TU.

The PA composite membrane with the MPD/TU ratio of 5 : 5 was also prepared in the presence of TEA, adopting the similar procedure as above except of the first immersion step of the amine solution. The first step is to use an aqueous solution containing the mixed amine MPD + TU (with the MPD/TU ratio of 5 : 5) and TEA (with the concentration ranged from 20 to 60 g L⁻¹) to immerse on the top of the commercial PES ultrafiltration membrane at room temperature, after 2 min the excess solution was removed. The composite NF membrane was cured at 70 °C for certain period ranged from 6 min to 15 min. The final obtained membrane was denoted as PES-M5T5-TEA.

Further, the unsupported thin films were prepared via IP between TMC and the amine mixture with the MPD/TU ratio of 5 : 5 in the presence and in the absence of TEA respectively, in order to discern the structural difference in the top layer of PES-M5T5-TEA, comparing with PES-M5T5. The typical procedure was as follows: a 0.1 wt% TMC hexane solution was poured into the mixed amine MPD + TU aqueous solution (with the total amine concentration of 2 wt% and the MPD/TU ratio of 5 : 5) in the presence or the absence of TEA. The thin films generated at the interface between the organic and the aqueous phase were sampled, followed by the curing at 70 °C for 6 min, and then characterized by Raman spectroscopy.

2.3. Characterization

Attenuated total reflectance infrared spectroscopy (ATR-IR, MAGNA-560 Nicolet Co., USA) with a zinc selenide crystal at a 45° angle of incidence was used to detect the chemical structure of the composite membranes with a penetration depth about 600 nm. At least 16 scans were recorded for each spectrum at a resolution of 8.0 cm⁻¹.

Raman spectroscopy (DXR Microscope, Thermo Electron Co., USA) was used to distinguish the chemical groups in the unsupported thin film of PES-M5T5-TEA and PES-M5T5 with a He–Ne 632.8 nm laser.

Scanning electron microscope (SEM, NanoSEM430 FEI Co., USA) was used to observe the surface and cross-section morphology of the prepared composite membranes. The membrane samples were sputter-coated with gold before SEM analysis.

Contact angle goniometer (JC2000C Contact Angle Meter, Powereach Co., Shanghai, China) was used to measure the hydrophilicity of the composite membranes. The water was taken as a probe liquid. Every membrane was tested in six different locations of its surface.

The chemical composition of the top layer of PA composite membranes was analyzed by X-ray photoelectron spectroscopy (XPS, PHI5000VersaProbe, ULVAC-PHI Inc., Osaka, Japan) using AlKα as the radiation source and the spectra were taken with the electron emission angle at 45°.

Tapping mode AFM was performed for analyzing surface roughness analysis (Agilent 5500, America) in air. Three different regions on the membrane sample were measured to obtain the average value. The scanning area of each membrane was 5 μm × 5 μm.

The permeation tests were carried out in a cross-flow nanofiltration system for the membrane samples with the diameter of 8 cm. All membrane separation experiments were performed at least three times under the operation pressure of 1.6 MPa and room temperature. The initial glucose concentration was 10 g L⁻¹ in the feed solution, and the initial salt concentration was 1000 ppm in the feed. The permeate solutions were collected in a measuring cylinder to calculate the flux, F, using the following equation.

where V (L) is the total volume of permeate in the experimental time, t (h) is the operation time, and A (m²) is the effective membrane area.

The glucose (MW = 180 g mol⁻¹) concentration was measured with an Agilent Technologies 1200 series HPLC equipped with a 4.6 mm ID × 150 mm column maintained at 30 °C, using the mobile phase of 75/25 acetonitrile/water at the flow rate of 1.4 mL min⁻¹. The salt concentration in aqueous solutions was measured by electrical conductivity (DDS-307, Shanghai Shengci Instrument Co., Ltd. China). The rejection towards glucose or salt was calculated as below.

where R is the retention rate of the glucose or salt, C_p is the glucose or salt concentration in permeate while C_f is the glucose or salt concentration in the feed.

The chlorine resistance of PES-M5T5-TEA composite membrane was measured with 200 ppm NaClO solution, to keep the hypochlorite concentration constant during the experiment, the sodium hypochlorite solution was placed in darkness and replaced with the freshly prepared solution after two days. After a certain time of chemical oxidation, the composite membrane was washed completely with water, and water flux and rejection were evaluated again. An average of times was 22.4 h and the duration of the cycles of immersion was 5.

3. Results and discussion

3.1. Polyamide composite membranes prepared with different MPD/TU weight ratios

Fig. 2 shows the ATR-IR spectra of PA composite membranes prepared through the IP between TMC and the amine mixture TU + MPD, as well as those prepared via IP between TMC and individual amine of MPD or TU. For the PES-M, there are bands at 1665 cm⁻¹, 1612 cm⁻¹ and 1539 cm⁻¹, which are respectively due to the C=O group of amide, the aromatic ring breathing,¹³ and the C–N stretching of amide.¹⁴ Whereas for the PES-T, there are a small band at 1045 cm⁻¹ corresponding to the S=C group, besides the bands of aromatic ring at 1612 cm⁻¹ and the C–N of amide at 1539 cm⁻¹. For the PA membranes prepared with amine mixture, i.e., PES-M3T7, PES-M5T5 and PES-M7T3, there are four bands at 1665 cm⁻¹, 1612 cm⁻¹, 1539 cm⁻¹ and 1045 cm⁻¹, illustrating that TMC reacts with both MPD and TU so as to generate the structural groups of aromatic polyamide and the S=C group, as displayed in Fig. 1.

Fig. 2 ATR-IR spectra of the polyamide membranes.

Fig. 3 displays the morphology of these PA composite membranes. It is clear that there are grainy structures on the surface of PES-M membrane (Fig. 3a) and the thickness of the top IP layer is about 327 nm; whereas the PES-T membrane (Fig. 3e) shows a relative smooth surface with certain crevices and the thickness of the top layer about 110 nm. Previous literature has also reported that during the IP between TMC and MPD it is easy to form the ridge and valley structures on the membrane surface,¹⁵ and the membrane surface becomes uneven due to the shrinkage during the drying period.¹⁶ In contrast, PES-T membrane shows smooth surface because the aliphatic amine TU reacts with TMC to generate small molecular weight polyamide with the chemical structures, as shown in Fig. 1b. For the PA composite membranes prepared with the amine mixture MPD + TU, the thickness of the top layer is 173 nm, 137 nm and 158 nm respectively for PES-M7T3, PES-M5T5 and PES-M3T7. PES-M5T5 shows the thinnest top layer, which results in the optimal NF separation performance, as shown in Fig. 4 and 5.

Fig. 3 SEM images of surface (the top panel) and cross-section (the bottom panel) of membranes (a) PES-M, (b) PES-M7T3, (c) PES-M5T5, (d) PES-M3T7 and (e) PES-T membranes.

Fig. 4 Flux and rejection of the PA composite membranes for 10.0 g L⁻¹ glucose aqueous solution.

Fig. 5 The flux (a) and the rejection (b) of PA composite membranes for the salt aqueous solution.

Water contact angles were measured to assess the hydrophilicity of these PA composite membranes. As listed in Table 1, the average static contact angle is 85.2 ± 2.4° on PES-M, 81.8 ± 3.0° on PES-T, however the PA membranes prepared with the amine mixture show lower contact angles. The PES-M5T5 membrane shows the smallest contact angle as 68.7 ± 0.5°. The smaller the static contact angle, the more hydrophilic the membrane surface.¹⁷ It is indicted that the hydrophilicity of PA composite membrane surfaces can be modulated by the MPD/TU ratio in the amine mixture.

Table 1 The water contact angles of the PES support membrane and the composite membranes

Membrane	Water contact angles (deg)
PES-T	81.8 ± 3.0
PES-M3T7	73.5 ± 3.9
PES-M5T5	68.7 ± 0.5
PES-M7T3	71.5 ± 1.0
PES-M	85.2 ± 2.4

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Fig. 4 displays the flux and rejection of the PA composite NF membranes for 10.0 g L⁻¹ glucose aqueous solution. It is clear that PES-T membrane has a flux about 474.9 L m⁻² h⁻¹ but a low glucose rejection about 16.6%, whereas the PES-M membrane shows a low flux of 12.3 L m⁻² h⁻¹ but high glucose rejection of 99.4%. Over the PA membranes prepared with the amine mixtures, the flux and the glucose rejection are respectively 14.5 L m⁻² h⁻¹ and 97.7% for PES-M3T7, 19.4 L m⁻² h⁻¹ and 97.6% for PES-M5T5, 14.7 L m⁻² h⁻¹ and 99.1% for PES-M7T3. The flux of PES-M5T5 is higher than that of PES-M by a fraction of 60.0%, while the glucose rejection of PES-M5T5 (97.6%) is close to that of PES-M (99.4%).

The performance of the PA membranes prepared with the amine mixture was further assessed using aqueous solutions of inorganic salts including NaCl, MgCl₂, MgSO₄, and Na₂SO₄ respectively.

As shown in Fig. 5a, the PES-T membrane has the flux of 353.8 L m⁻² h⁻¹ for Na₂SO₄ solution, 452.3 L m⁻² h⁻¹ for MgSO₄, 578.9 L m⁻² h⁻¹ for NaCl and 618.7 L m⁻² h⁻¹ for MgCl₂ solution, whereas the corresponding rejection is respectively 80.1% for Na₂SO₄, 28.9% for MgSO4, 13.6% for NaCl and 4.6% for MgCl₂ (Fig. 5b). In the case of PES-M membrane, the flux and the rejection are also dependent on the salt species, i.e., 10.1 L m⁻² h⁻¹ and 94.9% for Na₂SO₄, 10.8 L m⁻² h⁻¹ and 91.9% for MgSO₄, 13.4 L m⁻² h⁻¹ and 92.8% for NaCl and 12.1 L m⁻² h⁻¹ and 95.0% for MgCl₂ solution. Similar with the separation performance of glucose aqueous solution, the PA membranes prepared with the amine mixture show the flux a little higher than that of PES-M, while the salt rejection greatly higher than that of PES-T. In particular, the PES-M5T5 membrane shows the rejection higher than 90% for these four kinds of salts.

Previously, Duan et al.¹⁸ reported that adding triethylamine (TEA) in the MPD aqueous solution can enhance the water flux of the IP composite membrane without compromising the salt rejection. Thus, we synthesized the PES-M5T5 membrane in the presence of TEA in order to improve the separation performance further.

3.2. Improved PES-M5T5 membrane with TEA

Adopting TEA as the additive in the amine mixture of MPD + TU, we synthesized PA composite membranes with the MPD/TU ratio of 5 : 5, denoted as PES-M5T5-TEA membrane, and studied the effect of TEA concentration on the separation performance. As shown in Fig. 6a, in the presence of TEA, the flux of PES-M5T5-TEA membrane is higher than that of PES-M5T5. Moreover the flux of PES-M5T5-TEA membrane is also dependent on the curing time (Fig. 6b). It is indicated that with the TEA additive the separation performance of PES-M5T5-TEA membrane can be improved greatly, with the largest flux of 39.7 L m⁻² h⁻¹ and the glucose rejection higher than 97.0% at the optimal TEA concentration of 40 g L⁻¹ and the curing time of 10 min, much better than the performance of PES-M5T5 (19.4 L m⁻² h⁻¹ and 97.6%).

Fig. 6 The flux and the glucose rejection of PES-M5T5 membranes (a) prepared with the curing time of 10 min but different TEA concentration, and (b) prepared with TEA concentration of 40 g L⁻¹ but different curing time. (c) The flux and salt rejection of the PES-M5T5-TEA membrane prepared at TEA concentration of 40 g L⁻¹ and the curing time of 10 min.

In addition, the performance of PES-M5T5-TEA membrane was further assessed for the salt aqueous solutions. As shown in Fig. 6c, the PES-M5T5-TEA membrane shows respectively the flux of 39.3 L m⁻² h⁻¹ for Na₂SO₄ solution with the salt rejection of 97.1%, the flux of 38.6 L m⁻² h⁻¹ for MgSO₄ solution with the salt rejection of 93.9%, the flux of 36.9 L m⁻² h⁻¹ for NaCl solution with the salt rejection of 95.8%, and the flux of 38.3 L m⁻² h⁻¹ for MgCl₂ solution with the salt rejection of 91.2%. Comparing with Fig. 5, it is indicated that the flux of PES-M5T5-TEA membrane is increased more than 70% meanwhile maintain the high salt rejection.

In order to understand deeply the effect of TEA on the PA NF membrane, we measured the ATR-IR and Raman spectra of the membrane PES-M5T5-TEA, which has the optimal separation performance with the flux of 39.7 L m⁻² h⁻¹ and the glucose rejection higher than 97.0% (Fig. 6). Fig. 7a shows ATR-IR spectra of the optimal membrane PES-M5T5-TEA, comparing with PES-M5T5. For the membrane PES-M5T5-TEA, there are bands at 1045 cm⁻¹, 1539 cm⁻¹ and 1666 cm⁻¹, which are attributed to the C=S vibration of thioamide,¹⁹ the C–N stretching of amide and the C=O group of amide,¹⁴ respectively, besides the band at 1612 cm⁻¹ due to the aromatic ring breathing. In addition, the bands at 1169 cm⁻¹ and 1500 cm⁻¹, due to the C–N and the aromatic ring of unreacted amines in the PES-M5T5 membrane, are disappeared in the PES-M5T5-TEA membrane. The band intensity ratios of I₁₀₄₅/I₁₆₁₂, I₁₅₃₉/I₁₆₁₂ and I₁₆₆₆/I₁₆₁₂ are calculated as 1.03, 0.79 and 0.57 for PES-M5T5-TEA, whereas these ratios for the PES-M5T5 membrane equal respectively to 0.73, 0.83 and 0.56. The variation of I₁₀₄₅/I₁₆₁₂ and I₁₅₃₉/I₁₆₁₂ ratios indicates that in the presence of TEA there are more amount of C=S groups but a relative less C–N of amide, suggesting that TEA additives can facilitate the formation of thioamide structures in PES-M5T5-TEA, as displayed in Fig. 1b.

Fig. 7 (a) ATR-IR and (b) Raman spectra of the optimal membrane PES-M5T5-TEA, comparing with PES-M5T5.

Fig. 7b shows Raman spectra of the optimal membrane PES-M5T5-TEA, comparing with PES-M5T5. For the membrane PES-M5T5-TEA, the band at 1662 cm⁻¹ is attributed to C=O group of amide, the band at 1600 cm⁻¹ due to the aromatic ring breathing, the two bands at 1550 cm⁻¹ and 1342 cm⁻¹ can be assigned to thioamide group I [ v (C–N) + δ(NH)] and II [v (C–N) + v (C=S)],²⁰ respectively, while the band at 1003 cm⁻¹ is due to the C=S vibration of thioamide.²¹ In the case of PES-M5T5, the major two bands correspond to the aromatic rings (1600 cm⁻¹) and the C=S of thioamide (1003 cm⁻¹). Thus, it is clear that the TEA additive can promote the formation of thioamide in the top layers of membranes.

XPS spectra of PES-M5T5-TEA and PES-M5T5 membranes were analyzed to characterize the elemental compositions in the top layers of membranes. Both membranes have four elements including C, N, O and S, with the individual content equal 72%, 15%, 11% and 2% for PES-M5T5-TEA, 68%, 16%, 13% and 3% for PES-M5T5. Although the elemental composition of the membrane top layers shows no great variation in the presence of TEA additives, the deconvolution of XPS C1s and S2p spectra discloses the shift of functional groups in the membrane top layers resulted by TEA additives. As shown in Fig. 8, for the membrane PES-M5T5, there exist three carbon species including the O=C–N of amides at 288.1 eV and C–NH₂ of unreacted amines at 285.6 eV, whereas for PES-M5T5-TEA there are only two carbon species including C–C of aromatic rings (284.6 eV) and the O=C–N group of amides (287.8 eV); suggesting that TEA additives facilitate the IP between TMC and the mixed amines of MPD and TU so as to make the C–NH₂ of unreacted amines disappeared in the top layers of PES-M5T5-TEA. On the other hand, both PES-M5T5 and PES-M5T5-TEA membranes possess three sulphur species involving the C=S bond of thiourea (∼161.7 eV), the C–S bond (∼162.6 eV) and S–H bond of isothiourea (∼163.2 eV), however, the amount of the C=S species is much higher in the top layer of PES-M5T5-TEA.

Fig. 8 The deconvolution XPS C1s and S2p spectra of PES-M5T5-TEA and PES-M5T5 membranes.

We have added the measurement of AFM images of the optimal membrane PES-M5T5-TEA. Fig. 9 displays three-dimensional AFM images for PES-M5T5-TEA and PES-M5T5 membranes. The root mean square roughness (Rms) of the membrane surface is 18.8 nm for PES-M5T5-TEA, 13.7 nm for PES-M5T5 respectively. It is indicated that TEA additives makes the roughness of the membrane surface increased, resulting in the large flux of PES-M5T5-TEA. In combination with the ATR-IR, Raman and XPS spectra of PES-M5T5-TEA (Fig. 7 and 8), it is confirmed that the TEA additive can promote the IP between TMC and MPD + TU, resulting in more amounts of thioamide structures (Fig. 1b) generated in the top layers of PES-M5T5-TEA membranes, which can form more free volume and consequently improve the membrane flux.

Fig. 9 AFM images (including morphological statistics) of PES-M5T5-TEA and PES-M5T5 membranes.

Further the chlorine resistance of PES-M5T5-TEA composite NF membrane was measured after immersion in 200 ppm NaClO solution for various times. Fig. 10 shows the flux and the glucose rejection over PES-M5T5-TEA membrane after being experienced the NaClO exposure with different periods. It is indicated that with the long NaClO exposure time of 22 400 ppm h, the PES-M5T5-TEA membrane shows the flux of 36.2 L m⁻² h⁻¹ and the glucose rejection of 92.5%, suggesting that the PES-M5T5-TEA membrane has good chlorine resistance.

Fig. 10 The flux and rejection for glucose solution over PES-M5T5-TEA membrane with different exposure time of NaClO.

4. Conclusions

Polyamide TFC NF membranes were synthesized on porous polyethersulfone support via the interfacial polymerization between TMC and the amine mixture of MPD + TU. Characterized by ATR-IR, Raman, SEM and XPS, it is illustrated that the polyamide membranes (PES-M5T5) prepared at the optimal MPD/TU ratio of 5 : 5 have the thinnest top layer about 137 nm, and possess both aliphatic alkyl and aromatic chains structure. Adding 40 g L⁻¹ TEA in the amine mixture, the separation performance of PES-M5T5-TEA membrane can be improved greatly, with the largest flux of 39.7 L m⁻² h⁻¹ and the glucose rejection higher than 97.0%. It is illustrated that the TEA additive can promote the IP between TMC and MPD + TU, and make the roughness of the membrane surface increased, consequently resulting in more amounts of thioamide structures generated in the top layers of PES-M5T5-TEA, which can form more free volume and improve the membrane flux. In addition, such polyamide membranes show excellent chlorine resistance, with the stable glucose rejection and the flux after immersion in 200 ppm NaClO solution more than 100 h.

Acknowledgements

This work was supported by National High-tech R&D Program of China (2012AA03A609) and Program for Changjiang Scholars and Innovative Research Team in University (IRT1161).

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