Functionalization of polyacrylonitrile with tetrazole groups for ultrafiltration membranes

Abstract

A series of tetrazole-functionalized polyacrylonitrile (TZ-PAN) copolymers were synthesized via a post-modification cycloaddition reaction of nitriles with azide for ultrafiltration (UF) membrane application. The as-obtained TZ-PAN copolymers were further modified by N-arylation with methyl iodide to get methylated tetrazole polyacrylonitrile (MTZ-PAN). The chemical structures of TZ-PAN and MTZ-PAN copolymers were confirmed by ¹H NMR spectra. The degree of functionalization (DF) of PAN copolymers ranged from 5% to 16%. Subsequently, the ultrafiltration membranes based on the TZ-PAN and MTZ-PAN copolymers were prepared by a non-solvent phase inversion method. The morphological structure, hydrophilicity and mechanical properties of the UF membranes were thoroughly investigated. The TZ-PAN membranes showed a lower water contact angle than that of the PAN UF membrane (about 63°) due to the high hydrophilicity of the tetrazole groups. Thus, a high water flux was observed for TZ-PAN membranes. Moreover, the hydrophilicity of PAN copolymers with tetrazole copolymers could be tuned by N-arylation of the tetrazole groups. The highest water flux of 715.41 L per (m² h bar) was achieved for the TZ-PAN-16 UF membrane. The investigation of water recovery ratio indicated the TZ-PAN-16 showed excellent anti-fouling properties. Although the hydrophilicity of MTZ-PAN decreased after N-arylation, the water flux was still as high as 242.1 L per (m² h bar) with a BSA rejection of 80.1%. These results indicated that tetrazole modification of PAN is an effective approach to tune PAN UF membrane properties.

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Introduction

There is considerable interest in the development of new polymeric materials for membrane applications during separation processes. Particularly, the demand of developing new materials with desirable properties such as high permeability, high selectivity and chemical resistance is enormous for applications to water purification areas.^1–5 Polyacrylonitrile (PAN) has been widely used for ultrafiltration (UF) membranes because of its low cost, excellent chemical resistance and thermal stability. However, the major challenge to PAN ultrafiltration operations is membrane fouling caused by adsorption and deposition of hydrophobic compounds on the membrane surface in the feed stream, which results in flux decline and poor mechanical properties during the long time use.^6–8 To solve this problem, membranes are generally cleaned by aggressive chemicals to remove the foulants. However, the membranes still need to be replaced when the cleaning is ineffective. Moreover, both the cleaning and replacement processes increases operating costs, which may cost 30–50% and 10–30% of the operating fee of a typical UP plant, respectively. Thus, the investigation of anti-fouling UF membrane has received a growing interest in recent years. Generally, the fouling behaviour is highly dependent on the nature of membrane materials. Increasing the hydrophilicity of membranes has been confirmed to be an effective approach to improve the fouling resistance.^9–13

Recently, it has been reported that PAN membranes have been surface modified by graft copolymerization of various hydrophilic monomers, or by coupling of hydrophilic polymers or biomolecules. For example, Dai and co-workers reported that a surface grafting membrane by ring-opening polymerization of glycomonomer D-gluconamidoethyl methacrylate (GAMA) using ultraviolet (UV)-initiated technique.¹⁴ Abedi et al. had prepared antifouling PAN membranes by adding PEG-modified PAN copolymer as additive.¹⁵ However, these approaches are limited by the fact that the hydrophilic modification occurs only on the membrane surface, while internal pores remain susceptible to fouling. The alternative approach to prepare anti-fouling PAN UF membrane has been developed by adding amphiphilic copolymer to membrane casting solution along with PAN material. For example, Jung et al. had added poly(acrylonitrile-ran-potassium 3-sulfopropyl acrylate) into PAN casting solution to prepare hydrophilic and protein fouling resistance PAN UF membranes.¹⁶ Mayes and coworkers further employed the amphiphilic comb-shaped copolymer polyacrylonitrile-graft-poly(ethylene oxide) (PAN-g-PEO) with a hydrophilic PEO side chains as a macromolecular additive for PAN UF membranes.¹⁷ The improved porosity, water flux and fouling resistance to proteins were observed in all cases. Therefore, the direct hydrophilic modification of PAN polymer was considered as a better way for fouling resist UF membranes.

Tetrazoles, which could be synthesized by cycloaddition reaction between azide and nitriles in the presence of Lewis acid catalyst, are a class of synthetic organic heterocyclic compound. The tetrazoles ring containing four nitrogen atoms could lose a proton to act as a weak acid and form a hydrogen-bonding network, and thus display an excellent hydrophilicity. Actually, it has been reported that the tetrazole has a pK_a value of 4.89 which is very close to the pK_a value of CH₃COOH (4.75). Thus, it has been employed as a bioisostere for the carboxylate group.^18–20 Most recently, the tetrazole has also been investigated in proton exchange membranes and gas separation membrane to promote proton and CO₂ transport.^21–23 To our best knowledge, tetrazoles have not been used as a hydrophilic function group for fouling resistance in ultrafiltration membrane.

Herein, we firstly introduced the hydrophilic tetrazole groups into PAN polymer for improvement of anti-fouling properties of PAN UF membrane. Subsequently, the methylation of tetrazole groups has been prepared for tuning the hydrophilicity of PAN UF membranes. A detailed investigation on the properties of UF membranes with different tetrazole content was performed, such as hydrophilicity, water flux, BSA rejection and fouling resistance, and compared to those of pristine PAN membrane.

Experimental

Materials

Polyacrylonitrile (PAN) with a molecular weight (M_n) of 50 000 was purchased from Nanjing Stable Company. Sodium azide (NaN₃, 99.5%; Sigma Aldrich), ammonium chloride (NH₄Cl, 99%; Sigma Aldrich), methyl iodide (CH₃I, 99%; Sigma Aldrich), potassium carbonate (K₂CO₃, 99%, Sigma Aldrich), N,N-dimethylformamide (DMF, 98%; Sigma Aldrich) and N-methylpyrrolidinone (NMP, 99.5%; Sigma Aldrich) were used as received. Bovine serum albumin (BSA, M_w = 68 kDa) and polyethylene glycol (PEG₆₀₀) was bought from Aladdin.

Synthesis of tetrazole functionalized polyacrylonitrile (TZ-PAN-x)

Tetrazole functionalized polyacrylonitriles (TZ-PAN) were prepared according to the previous reports.^24,25 Generally, in a 1000 mL flask equipped with a mechanical stirrer and a reflux condenser, PAN (30 g, 566.0 mmol) was dissolved in 150 mL DMF. The solution was stirred at room temperature for 30 min until the polymer completely dissolved. Then, sodium azide (7.8 g, 113.2 mmol), ammonium chloride (7.2 g, 113.2 mmol) were slowly added in the flask, and the mixture was continuously stirred at 120 °C for 8, 15 and 120 min, respectively, in order to control the degree of functionalization (DF). After cooling to 50 °C, the product was obtained by precipitation of the solution into a mixture of hydrochloric acid and water (1 : 50, v : v). The fiber-like polymer was washed three times with water to remove the excess of sodium azide and ammonium chloride, and then dried under vacuum at 60 °C for 24 h to obtained copolymers TZ-PAN-x, where x is the degree of functionalization which was determined by ¹H NMR.

Synthesis of methylated tetrazole functionalized PAN (MTZ-PAN-x)

Potassium carbonate (3.15 g, 22.8 mmol) was added into the TZ-PAN-16 (10 g, 151.7 mmol; DF was 16%) solution with 100 mL NMP. The reaction was carried out at 60 °C for 2 h for dehydrogenation of the N–H of tetrazole group. After cooling to room temperature, methyl iodide (3.36 mL, 54.6 mmol) was added by dropwise into the mixture. The reaction was carried out overnight at 60 °C. The obtained product MTZ-PAN-16 was subsequently precipitated in a mixture of hydrochloric acid and water (1 : 50, v : v) and washed three times by water to remove the excess of potassium carbonate, and then dried under vacuum at 60 °C for 24 h.

Ultrafiltration (UF) membrane preparation

All the UF membranes were prepared via a typical phase inversion method according to the previous reports.^26–28 Firstly, the polymer–PEG₆₀₀–NMP with the mass ratio of 18/10/72 was stirred for 4 h to obtain a transparent solution. After filtration and degas, the viscous solution was casted on a clean glass plate. The plate was then immersed into the coagulation bath at 30 °C. The obtained membrane with the thickness of about 120 μm was washed by immersion of the membrane into deionized water for 48 h for properties characterization.

Permeation and antifouling properties of membranes

The prepared membranes were tested using a stirred dead-end filtration cell (MSC300, Shanghai Mosu Science Equipment Co. Ltd.) with a effective area of 19.625 cm². Firstly, the membrane was pre-compacted by deionized water at 0.2 MPa for 30 min. Subsequently, the pressure was lowered to 0.1 MPa for performance testing. All the ultrafiltration experiments were carried out under the same testing condition. The pure water flux was obtained using the following equation:where the V and t and are the volume of permeate and the filtration time respectively; S refers to the effective membrane area and the ΔP is the transmembrane pressure.

Subsequently, the bovine serum albumin solution in PBS solution having a concentration of 500 mg L⁻¹ at a pH value of 7.4 was used as feed solution for membrane rejection testing. The permeability of protein filtration was donated as J_P. The rejection ratio R was calculated according to the following equation:

where C_P and C_b are the concentrations of the permeation and the feed respectively. These concentrations were determined using UV spectrophotometer (HITACHI, U-3900) at 278 nm. Then, the membrane was vigorously flushed by deionized water for 30 min. The flux recovery was obtained from the filtration by deionized water for 15 min under the same condition. All cycles were performed three times to further research each membrane's antifouling property.

Characterizations

¹H NMR spectra were recorded with a Bruker AVANCE-III spectrometer at a frequency of 400 MHz. The surface and cross-section morphologies of the membranes were characterized by SEM (FEI Quanta 200 FEG). The samples were sputter-coated with Au/Pd for 20 s at 20 mA. Contact angle (CA) measurement was performed on JGW-360A equipment in static mode at room temperature. Each contact angle was reported as the average value from five times measurement. The mechanical properties were measured by an YG028 tensile tester (Wenzhou taiyuan instrument Co. Ltd., China). All the tensile tests were carried out at a stepper motor speed of 10 mm min⁻¹ at room temperature.

Membrane porosity was determined by the gravimetric method:

where ω₁ and ω₂ are the weight of the wet membrane and the dry membrane, respectively. A is the membrane effective area (m²), ρ refers to the water density (0.998 g cm⁻³) and l is the membrane thickness (m). The Guerout–Elford–Ferry equation (eqn (4)) was employed to determine the membrane mean pore size (r_m) based on the pure water flux and porosity data:where the γ is the water viscosity (8.9 × 10⁻⁴ Pa s), Q refers to the volume of the permeate pure water per unit time (m³ s⁻¹) and ΔP is the operation pressure (0.1 MPa).

Results and discussion

Synthesis of the tetrazole and methylated tetrazole functionalized polyacrylonitriles (TZ-PANs and MTZ-PANs)

The tetrazole-functionalized polyacrylonitrile (TZ-PAN) copolymers were synthesized by the cycloaddition of sodium azide and nitriles groups of PAN using Lewis acid (NH₄Cl) as catalyst, as shown in Scheme 1. It has been observed that the maximal rate of cycloaddition reaction was observed at 120 °C according to previous report.²⁵ Thus, the reaction was carried out at 120 °C for various times in order to control the degree of functionalization. The target maximum degree of functionalization (DF) of TZ-PAN was designed to be 20% for ultrafiltration membrane application. Although the degree of function seemed expedient to use the method of basic-acidic titration because the tetrazole is an N–H acid of moderate strength (pK_a of tetrazole amounts to 4.10), the DFs were determined by ¹H NMR spectra after transferring the tetrazole to methylated tetrazole groups. As shown in Fig. 1, a new peak at 4.25 ppm was observed in the ¹H NMR spectrum of MTZ-PAN-5 which was attributed to the methyl groups in tetrazole groups, and the DF was calculated based on. The conversion degree was increased gradually and reached 80% when the reaction time is 120 min. The DF was calculated to be in the range of 5–16% from the integral ratio of methyl protons H₁ and methylene protons H₂ (Table 1).

Scheme 1 The synthesis of TZ-PAN and MTZ-PAN copolymer.

Fig. 1 ¹H NMR spectra of PAN and MTZ-PAN-5 recorded in DMSO-d6.

Table 1 Reaction characteristic for the synthesis of TZ-PAN copolymers

Sample	Reaction time	DF	Conversion
TZ-PAN-5	8 min	5%	25%
TZ-PAN-10	15 min	10%	50%
TZ-PAN-16	120 min	16%	80%

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Thermal stability and hydrophilicity

The thermal properties of membranes were assessed with TGA under a dry N₂ atmosphere.²⁹ The weight loss of TZ-PAN-10 up to ca. 150 °C was ascribed to the loss of water molecules, absorbed by the highly hygroscopic tetrazole groups. The last weight loss, which started around 300 °C, was attributed to the decomposition of the tetrazole groups and polymer backbone (Fig. 2). Compared to PAN membrane, the introduction of tetrazole or methylated tetrazole groups was found to result in the increasing the velocities of the degradation. As shown in Fig. 3, the TZ-PAN-10 showed a high degradation velocity. It is assumed that the tetrazole without methylation could be decomposed into nitrogen very quickly.

Fig. 2 TGA profiles of PAN, TZ-PAN-10 and MTZ-PAN-10 membranes.

Fig. 3 DTA profiles of PAN, TZ-PAN-10 and MTZ-PAN-10 membranes.

The water contact angles of the TZ-PAN-x and MTZ-PAN-x membranes were measured and showed in Fig. 4. Obviously, the water contact angle decreased with the increasing of tetrazole group content for TZ-PAN membranes. However, the water contact angels of MTZ-PAN membranes with methylated tetrazole groups increased with increasing DF. It is believed that the methylated tetrazole group tend to absorb less water than tetrazole group due to the hydrophobicity of methyl groups. For example, the TZ-PAN-16 with 16 mol% tetrazole groups showed a contact angle of ∼51° which is lower than that of PAN membrane (∼63°). However, when the content of tetrazole groups were more than 25 mol%, the polymer become too hydrophilic to be used to prepare a free-standing membrane by phase inversion method. After the methylation, the MTZ-PAN-16 membrane had a water contact angel of 71°.

Fig. 4 The dependence of water contact angle of PAN, TZ-PAN and MTZ-PAN ultrafiltration membrane on the content of tetrazole groups.

Morphological structures of ultrafiltration membranes

The morphological structures of PAN, TZ-PAN and MTZ-PAN ultrafiltration membranes were investigated by SEM. As shown in Fig. 5, there was no apparent crack and agglomeration on membranes surface, indicating that the phase inversion process worked well. There were no significant differences in surface morphology between the TZ-PAN and MTZ-PAN membranes. A typical closely packed nodules surface morphological structure was observed for all membranes. The cross-section of asymmetric membranes showed a fingerlike structure which consisted of a dense layer and a porous sublayer. The fingerlike pore increased with increasing tetrazole content, which may induce an increasing water flux during the UF tests.

Fig. 5 The surface and cross-section morphologies of TZ-PAN and MTZ-PAN membranes.

As well as known, the porosity and pore size of UF membranes have significant effects on the water permeability and protein rejection. Thus, the porosity and mean pore size of as-obtained UF membranes were measured by gravimetric method and the results were listed in Table 2. The TZ-PAN membranes displayed higher porosity in the range of 80.3–91.2% than that of PAN membrane (60.2%). The high porosity of TZ-PAN membranes probably resulted from the hydrophilic property of tetrazole, which increased the diffusion and exchanging rate between water and NMP, and thus the rate of phase inversion.³⁰ Thus, both the porosity and mean pore size of TZ-PAN membranes increased with the increasing in the content of tetrazole. As shown in Table 2, the TZ-PAN UF membranes showed the average pore size in the range of 29–53 nm. These values were higher than that of PAN membrane (25 nm). After methylation of tetrazole group, the MTZ-PAN displayed higher porosity and mean pore size than that of PAN membrane. It is believed that the methylated tetrazole group still had a strong hydrogen bonding interaction with water which could increase the diffusion and exchanging rate between water and NMP, and thus the rate of phase inversion. However, the porosity of MTZ-PAN decreased with the increasing in content methylated tetrazole groups when the mean pore size of MTZ-PAN increased. A possible explanation for this behaviour is that the hydrophilic additive of PEG and membrane fabrication conditions of hydrophobic MTZ-PAN membranes had significant effect on the morphological structures of membranes though a more complete study and optimization of membrane fabrication conditions would be needed to explain this phenomenon.

Table 2 Porosity and surface mean pore size for PAN, TZ-PAN and MTZ-PAN membranes

Membranes	Porosity	Average diameter (nm)	Water capacity (g per g membrane)	Surface area rate of change
PAN	60.2%	24.61	2.03	95.8%
TZ-PAN-5	80.3%	28.61	2.82	106.7%
TZ-PAN-10	82.6%	38.88	3.20	122.9%
TZ-PAN-16	91.2%	52.68	4.30	166.6%
MTZ-PAN-5	76.4%	19.90	2.76	100.8%
MTZ-PAN-10	68.6%	26.40	2.74	103.5%
MTZ-PAN-16	66.8%	41.96	2.68	100.0%

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Mechanical properties

The TZ-PAN membranes are too brittle after drying for mechanical properties measurement probably due to the excessive swelling in water which resulted from the high hydrophilicity of tetrazole. However, the MTZ-PAN membranes displayed good mechanical properties after methylation of the tetrazole groups. As shown in Table 3, the MTZ-PAN membranes displayed the elongation-at-break in the range of 10.9–21.0%, and the tensile strength ranging from 2.5–6.2 MPa. These results indicate the MTZ-PAN membranes are more flexible probably because of the hydrophobicity of methylated tetrazole groups, which could prevent the membrane from swelling under the humidified condition. Generally, the excessive swelling of membrane under humidified condition would destroy the mechanical stability of membranes. Thus, the highly hydrophilic TZ-PAN-x membranes displayed very poor mechanical properties. The high water content results in the excessive swelling of TZ-PAN-x membranes in water, and thus the membranes broke into a piece when the membranes are dried from liquid water. The water capacity measurement further confirms this excessive water absorption behavior. As listed in Table 2, the TZ-PAN membranes had the water capacity ranging from 282% to 430%. These values are much higher than that of PAN membrane of 203% because of the high hydrophilicity of TZ-PAN membranes. However, the lower water capacity of about 276% is observed for MTZ-PAN membranes. Thus, the MTZ-PAN membranes showed better mechanical properties than that of TZ-PAN membranes.

Table 3 Tensile strength and elongation-at-break of PAN, MTZ-PAN membranes

Membranes	Tensile strength	Elongation-at-break
PAN	4.5 MPa	14.9%
MTZ-PAN-5	3.5 MPa	10.9%
MTZ-PAN-10	6.2 MPa	19.4%
MTZ-PAN-16	2.5 MPa	21.0%

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Pure water flux and protein retention

As expected, the pure water flux of membranes increased with increasing the content of tetrazole or methylated tetrazole groups (Fig. 6). The TZ-PAN-16 UF membrane showed the highest water flux of 715.4 L per (m² h bar) which could be contributed to its excellent hydrophilicity and high porosity as discussed above. This value was almost six times higher than that of PAN membrane (110.1 L per (m² h bar)) under the same testing condition. Although the methylated MTZ-PAN membranes showed higher hydrophobicity than that of PAN membrane which had been confirmed by CA measurement, higher pure water flux of MTZ-PAN membranes were observed. It is believed that the high average pore size of MTZ-PAN resulted in the high pure water flux, as shown in Table 2. Additionally, the BSA rejection decreased with the increasing in water flux of UF membranes probably owing to the big mean pore size. For example, the TZ-PAN-5 and TZ-PAN-10 membranes showed the pure water flux of 180 L per (m² h bar) and 360 L per (m² h bar), respectively. However, the BSA rejection decreased from 93.5% for TZ-PAN-5 to 88.2% for TZ-PAN-10 membrane. As shown in Fig. 6a, the MTZ-PAN-10 and MTZ-PAN-16 membranes showed higher water flux of than that of PAN membrane (110.1 L per (m² h bar)) in spite of their hydrophobicity probably due to their high mean pore size. The TZ-PAN-5 membrane displayed higher both of the water flux and BSA tetrazole groups than that of PAN membrane. It is assumed that the small amount of tetrazole groups not only increased the hydrophilicity of membrane, but also increased the interaction of repulsion between BSA and tetrazole groups, and thus the water flux and BSA rejection.

Fig. 6 Pure water flux (a) and retention rate (b) of TZ-PAN and MTZ-PAN membranes.

Flux recovery and antifouling performance

Generally, the membrane fouling causes a significant decrease in water flux as mentioned above, and thus leads to the increasing of production costs, energy consumption and the cleaning frequencies. The antifouling properties of UF membranes testing were carried out using 500 mg L⁻¹ BSA as feed solution. Three cycles of BSA solution filtration was performed. For each cycle, BSA solution was passed through the compacted membrane for 2 h at first. Then the ultrafiltration membrane was washed thoroughly and passed through deionized water for 1 h. The changing of flux recovery as a function of time was evaluated during the filtration runs. As shown in Fig. 7a, the TZ-PAN-16 UF membrane could achieve a pure water flux recovery ratio of 73.8% after three cycles of filtration, which is much higher than that of PAN UF membrane (∼48.6%). Moreover, the TZ-PAN membrane showed comparable anti-fouling properties to other hydrophilic modificated PAN even at high concentration of feed solution of 500 mg L⁻¹ BSA, as shown in Table 4. Unfortunately, the TZ-PAN-5 and TZ-PAN-10 with lower tetrazole content displayed lower pure water flux recovery ratio. It is assumed that the big mean pore size of the TZ-PAN-5 and TZ-PAN-10 membranes resulted in their poor pure water flux of. The UF membranes were not able to be backwashed in lab-scale but only the surface wash. Thus, the BSA molecules absorbed in the pore wall could not be removed. Thus the low pure water flux recovery ratio was observed. The further optimization of the membrane fabrication condition is ongoing for the optimized morphological structures of membranes. After methylation of tetrazole, the MTZ-PAN-5 membrane showed better anti-fouling properties due to its hydrophilicity than that of PAN membrane. However, the MTZ-PAN-10 and MTZ-PAN-16 with high methyl groups showed high hydrophobicity and thus displayed a poor fouling resistance. These results indicated that the fouling resistance of PAN UF membrane could be tuned by the controlling of the tetrazole or methylated tetrazole content in PAN polymer.

Fig. 7 Time-dependent flux recovery ratios of TZ-PAN (a) and MTZ-PAN (b) ultrafiltration membranes during the BSA solution filtration process.

Table 4 Comparison of tetrazole modificated polyacrylonitrile membrane with other copolymers in antifouling process

Copolymer	Membrane solution	Feed solution concentration	Cycle number	Flux recovery (%)
TZ-PAN-16	Polymer	500 mg L^−1 BSA	1	93
TZ-PAN-16	Polymer	500 mg L^−1 BSA	2	90
PAN-PEO^17	Additive	100 mg L^−1 BSA	1	100
PAN-r-SAPS^16	Additive	5 mg L^−1 BSA	1	80

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In order to investigate the effect of pH value on the anti-fouling properties of as-obtained membranes. The flux recovery ratios of TZ-PAN-10 and MTZ-PAN-10 membranes have been measured at different pH values. As shown in Fig. 8, both the TZ-PAN and MTZ-PAN membranes displayed increasing in flux recovery ratios, e.g. anti-fouling properties with the pH value increased. It is assumed that the membranes surface was ionized when the pH value is higher than the pKa value of tetrazole (pK_a = 4.89) which could increase the anti-fouling properties of membranes.

Fig. 8 Time-dependent flux recovery ratios of TZ-PAN-10 (a) and MTZ-PAN-10 (b) ultrafiltration membranes during the BSA solution filtration process at different pH.

Conclusions

In this study, tetrazole and methylated tetrazole functionalized PAN copolymers were prepared by the Lewis acid-catalyzed 2 + 3 cycloaddition for PAN UF membranes. The tetrazole content was controlled by the controlling of reactive time, and determined to be in the range of 5–16% according to the ¹H NMR results. The prepared UF membranes showed an improved hydrophilicity comparing with the PAN membrane. The hydrophilicity could be controlled by the content of tetrazole groups, and the N-methylation of tetrazole groups to get methylated tetrazole polyacrylonitrile (MTZ-PAN). Lower contact angle, higher mean pore size and porosity were observed for TZ-PAN UF membrane due to the hydrophilicity of tetrazole groups than those of PAN membrane. Thus, the water flux increased with the increasing of tetrazole content. The highest water flux of 715.41 L per (m² h bar) was achieved for TZ-PAN-16 UF membrane. Although the hydrophilicity of MTZ-PAN decreased after N-methylation, the water flux was still as high as 242.1 L per (m² h bar) with a BSA rejection of 80.1%. These results indicated that tetrazole modification of PAN could tune the PAN UF membrane properties effectively. Further investigation into tetrazole and its derivatives, especially blend with PAN, is expected to have an impact on UF membrane material science.

Acknowledgements

Financial support for this work was provided by the State Key Laboratory of Separation Membranes and Membrane Processes (Tianjin Polytechnic University, No. M2-201508), the Hundred Talents Program of the Chinese Academy of Sciences, the National Natural Science Fund of China (51138008, 51478314, 51308391) and Tianjin Technology Support Program (14ZCDGSF00128).

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