Polyacrylonitrile

Article pubs.acs.org/IECR

Ionic Cross-Linked Poly(acrylonitrile-co-acrylic acid)/Polyacrylonitrile Thin Film Nanofibrous Composite Membrane with High Ultrafiltration Performance Yin Yang,† Xiong Li,† Lingdi Shen,‡ Xuefen Wang,*,† and Benjamin S. Hsiao*,§ †

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Material Science Engineering, Donghua University, Shanghai, 201620, P. R. China ‡ School of Chemistry and Chemical Engineering, Jiangsu Normal University, Xuzhou, 221116, China § Department of Chemistry, Stony Brook University, Stony Brook, New York 11794, United States S Supporting Information *

ABSTRACT: A new method for fabrication of thin film nanofibrous composite (TFNC) ultrafiltration (UF) membrane consisting of an ultrathin poly(acrylonitrile-co-acrylic acid) (PANAA) barrier layer based on a polyacrylonitrile (PAN) nanofibrous support layer was proposed in this study. First, a thin PAN-AA nanofibrous layer was electrospun and deposited on a thicker PAN nanofibrous substrate. Then, the as-prepared PAN-AA nanofibers were swollen in the alkaline buffer solution and merged imperceptibly as an integrated nonporous hydrogel layer on the PAN substrate. The PAN-AA hydrogel layer was cross-linked with different bivalent metal cations (Ca2+, Mg2+) to form an ultrathin barrier layer, of which the thickness and porosity were optimized by controlling the depositing time of PAN-AA nanofibers and pH value of buffer solution. Proteins with different molecular weights were used to evaluate the ultrafiltration performance of the resultant composite membranes. Due to its hydrophilic and negative charged barrier layer, the PAN-AA-Mg and PAN-AA-Ca TFNC UF composite membranes exhibited excellent permeate flux (221.2 and 219.2 L/m2 h) and rejection efficiency (97.8% and 95.6%) for bovine serum albumin (BSA) aqueous solution (1 g/L) at 0.3 MPa. The PAN-AA TFNC UF membranes could be used to retain solutes, of which the radius was larger than 4.6 nm.

1. INTRODUCTION

purification, biomacromolecular material concentration, and blood dialysis. In recent decades, thin film composite (TFC) membranes are well researched for ultrafiltration and nanofiltration processes, which have been proven to be an effective media for the production of high quality water.14−16 In general, the structure of TFC membrane includes a mechanical support layer, a porous matrix, and a functional barrier layer. This threetiered structure of composite membrane could provide enough mechanical property and filtration efficiency, which can be applied in different separating process by using different materials and fabrication methods of each layer. It is noteworthy that the conventional reinforced porous matrix is normally a dense asymmetric membrane made by phase inversion methods which contains a sponge-or fingerlike disconnected pore structure. Significantly, electrospun nanofibrous membrane has been researched as supporting substrate

Low fresh water supply has become a big problem all over the world since the last century. To solve this problem, many kinds of liquid filtration and separation processes have been adopted and spread rapidly throughout the word. For all these separating processes driven by concentration,1,2 temperature,3,4 osmotic pressure,5,6 vacuum degree,7,8 and electric charge,9,10 various functional membranes were developed as the most important media for water treatment.11 These membranes are usually distinguished with their operating pressure and filtration accuracy which include microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. Among these pressure-driven membranes, an ultrafiltration (UF) membrane is one kind of semipermeable membrane which allows water and solutes of low molecular weight permeate, and the suspended solids and solutes of high molecular weight are retained.12 The molecular weight of mean retained matter is in a range of 1−1000 kDa, such as polypeptide, polysaccharide, protein, enzyme, virus, etc.13 In the ultrafiltration process, hollow fiber and flat-sheet membranes are widely applied in food industry, biological pharmacy, and medical treatment research for potable water © XXXX American Chemical Society

Received: January 18, 2017 Revised: February 28, 2017 Accepted: February 28, 2017

A

DOI: 10.1021/acs.iecr.7b00244 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

azobis(isobutyronitrile) (AIBN), methanol, ethanol, N,N′dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) was kindly supplied by Shanghai Chemical Reagent Plant. Several kinds of proteins such as bovine serum albumin (BSA, Mw = 67 000 Da), egg albumin (Mw = 45 000 Da), pepsin (Mw = 35 000 Da), α-chymotrypsin (Mw = 24 500 Da), trypsinogen (Mw = 24 000 Da), cytochrome C (Mw = 13 000 Da) were all purchased from Sigma-Aldrich. Sodium carbonate (Na2CO3), sodium hydroxide (NaOH), calcium chloride (CaCl2), magnesium chloride (MgCl2), acetic acid, and phosphate buffered saline, borax salt, were purchased from Sinopharm Group Chemical Reagent Co., Ltd., without further purification before use. Ultrapure water (resistance ≥18.0 MΩ/cm) was obtained from a Thermo Fisher Ultrapure water purification system (D7401 EASY pure II UV, USA) 2.2. Synthesis of Poly(acrylonitrile-co-acrylic acid). Poly(acrylonitrile-co-acrylic acid) (PAN-AA) with a viscosityaveraged molecular weight (Mη) of 2.4 × 105 g/mol was synthesized by radical polymerization according to the method reported by Kobayashi et al.37 AN was dried over molecular sieves and distilled under atmospheric pressure at 78 °C to remove the inhibitor. AA was distilled under reduced pressure (16 mmHg, 50 °C). AIBN was recrystallized in supersaturated ethanol solution. All other reagent grade chemicals were used without further purification. Polymerization was carried out in DMSO solution as follows. In a reaction vessel with 500 mL capacity, 30.4 g (566 mmol) of purified AN, 7.51 g (104 mmol) of AA, 110.5 g of DMSO, and 0.22 g of AIBN were introduced. Polymerization was carried out at 60 °C for 6 h in nitrogen flow. The reaction was terminated and the mixture was poured into a large quantity of water to precipitate the crude copolymer. After washing with hot deionized water and methanol alternatively several times, the copolymer was then dried under vacuum at 40 °C for at least 24 h. The intrinsic viscosity of copolymer was determined by Ubbelodhe-type viscometer in DMF at 30 °C. Molecular weight (Mη = 2.4 × 105) of poly(acrylonitrile-co-acrylic acid) was calculated by the following Mark−Houwink equation:

in TFC membrane to enhance the permeability because of its high porosity (up to over 80%), interconnected open pore structure, microscale interstitial space, and large surface-tovolume ratio.17,18 Generally, the mean pore size of nanofibrous substrate has a relationship of 3 times the average fiber diameter.19 Therefore, the electrospun nanofibrous substrate with average fiber diameter of about 100 nm will be suitable for removing matter with particle sizes larger than 0.22 μm on the scale of microfiltration.20 However, the relatively large surface pore size and high porosity17 will cause the easy penetration of casting solution during the dip-coating process. Thus, various approaches have also been developed to construct a functional barrier layer on the nanofibrous substrate, such as soaking in a coagulating bath,21,22 introducing a hydrophilic dopamine transition layer,23,24 swelling the top electrospun nanofiber by solvent vapor25,26 or suitable mixture of good and poor solvent,27 remelting electrosprayed nanofiber or nanoparticles by a hot-pressing treatment,28,29 and interfacial polymerization.23,30 Meanwhile, different characteristics of membrane materials need to be considered for fabricating the functional barrier layer of the TFC membranes, such as solubility,21,22 hydrophilicity,23,24 swelling properties,25−27 glass-transition temperature,28,29 and functional groups for cross-linking21 and polymerization.17,23,30 In this work, a double-layer nanofibrous composite membrane was produced by electrospinning technology, swelling process, and ionic cross-linking. First, a relatively thicker (several ten microns) PAN nanofibrous support layer was fabricated as a porous matrix by an electrospinning technique. Then, an ultrathin poly(acrylonitrile-co-acrylic acid) (PAN-AA) nanofibrous layer was directly electrospun onto the polyacrylonitrile (PAN) nanofibrous substrate. To cover the relatively large pores on the surface of the PAN nanofibrous substrate, the PAN-AA nanofibrous layer was modified with alkaline buffer solution, in which the PAN-AA nanofibrous layer could be swollen into a nonpore hydrogel layer due to the abundant carboxyl groups on PAN-AA nanofibers.31−34 On the other hand, the carboxyl groups on PAN-AA hydrogel could form chelation with many kinds of multivalent metal cations according to the previous studies.35,36 Thus, some bivalent cations could be used to cross-link the PAN-AA hydrogel layer, i.e., magnesium ions (Mg2+) and calcium ions (Ca2+), but some kind of multivalent heavy metal ions would not be suitable to cross-link the PAN-AA barrier layer used in ultrafiltration process because of their toxicity. Meanwhile, calcium ions and magnesium ions are widely distributed in the natural environment and organisms which can participate in biochemical reactions. So the ionic cross-linked PAN-AA composite membranes could be used in preparation of ultrafiltration processes without further influencing the permeate solution. The swollen process and cations for cross-linking will affect the pore structure of the PAN-AA barrier layer, and then further influence the filtration property of the nanofibrous composite membrane. The resultant thin film composite membranes could be widely used in ultrafiltration systems such as separating proteins with different molecular weights, concentrating biomacromolecular material solution, and producing ultrapure water for biopharmaceuticals and food industry.

[η] = 2.78 × 10−4M 0.76

(1)

The copolymerization was also checked by measuring Fourier transform infrared (FTIR) spectra and 1H NMR spectra in d6-DMSO, as the absorbance peaks of FTIR νmax/ cm−1: 3470 (hydroxyl group, −OH), 1732 (carbonyl group, CO), 1627 (carboxyl group, −COOH), and chemical shifts of 1H NMR δH/ppm: 12.87 (1H, −COOH of AA), 2.05 (2H, −CH2− of AN), 1.85 (2H, −CH2− of AA). The result were in accordance with the reports in the literature.38 The degree of acrylic acid (DAA) was about 9.95%, which was calculated by following equation: DAA (%) =

2A COOH × 100 A CH2(AA) + A CH2(AN)

(2)

Where, ACOOH is the peak area of protons from carboxyl group (−COOH) attached to AA, ACH2(AA) is the peak area of protons from methylene (−CH2−) attached to AA, and ACH2(AN) is the peak area of protons from methylene (−CH2−) attached to AN. And, the degree of acrylonitrile (DAN) was about 90.05%, which was calculated as the following equation:

2. EXPERIMENT 2.1. Materials. PAN (Mw = 150 000) was purchased from J&K Scientific Ltd., which was dried in vacuum drying oven for 24 h before use at 50 °C. Acrylonitrile (AN), acrylic acid (AA),

DAN(%) = (1 − DAA ) × 100 B

(3) DOI: 10.1021/acs.iecr.7b00244 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Scheme 1. Fabrication of Ionic Cross-Linked PAN-AA Nanofibrous Ultrafiltration Composite Membrane

Scheme 2. Reactions for (1) Swelling PAN-AA Copolymer with Alkaline Buff Solution and (2) Cross-Linking PAN-AA Swollen Hydrogel Layer with Different Bivalent Metal Cations: Mg2+, Ca2+

2.3. Fabrication of PAN-AA/PAN Double-Layer Nanofibrous Substrates. Dried PAN and PAN-AA powder were dissolved in DMF with gentle stirring around 50 °C in water bath for at least 12 h to obtain homogeneous solutions, respectively. The above polymer solutions were subject to a customer-built electrospinning setup described in our previous work.39 The fabrication process of PAN-AA/PAN double-layer nanofibrous membranes was similar to our early research.26 First, 10 mL PAN solution (10 wt %) was electrospun on a grounded rotating metal drum covered with aluminum foil (the applied electric voltage was 18 kV and the solution feed rate was 16.6 μL/min) until the thickness of the PAN nanofibrous substrate reached around 40 μm.17 Then, an ultrathin layer of PAN-AA nanofibers was electrospun from 8 wt % PAN-AA solution and deposited directly above the PAN nanofibrous substrate for a period of time. During the PAN-AA nanofibers depositing process, the applied electric voltage was 24 kV, the solution feed rate was 8.0 μL/min, and the thickness of PAN-

AA layer could be easily adjusted by controlling the depositing time (20−80 min). The spinneret with diameter of 0.7 mm made a translational oscillatory motion perpendicular to the drum rotation direction (the oscillation distance was about 30 cm and the distance between the spinneret and the grounded drum was 15 cm) driven by a step motor to ensure the production of uniform electrospun membrane with sufficient area (i.e., larger than 30 × 31.4 cm2) for measurement. The surrounding temperature was ∼40 °C, and the environmental humidity was controlled at ∼40%. The as-prepared PAN-AA/ PAN double-layer nanofibrous membranes were dried at 50 °C in vacuum for at least 24 h after electrospinning for further use, and the resultant samples were denoted as PAN-AAx/PAN, where x stands for the depositing time of PAN-AA nanofibers. 2.4. Fabrication of TFNC Ultrafiltration Membranes by Swelling Technique and Ionic Cross-Linking. For preparation of the TFNC ultrafiltration membranes containing a PAN-AA barrier layer and a PAN nanofiber substrate, a new C


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after calibration with standards. All data were expressed as mean ± standard deviation (SD). 2.5.2. X-ray Photoelectron Spectroscopy (XPS). The surface chemical groups of PAN-AA nanofibrous substrate, PAN-AANa swollen membrane, PAN-AA-Ca UF membrane, and PANAA-Mg UF membrane were characterized by X-ray photoelectron spectroscopy (XPS) measurements using a Kratos Axis UltraDLD spectrometer (Kratos Analytical-A Shimadzu Group Company, Japan) with monochromatic Al Kα radiation as the excitation and an X-ray power of 75 W. Survey scans were taken from 0 to 1200 eV binding energies, using both electrostatic and magnetic time of 100 ms. Elemental peaks were fitted using Casa XPS software (Casa Software Ltd., Teignmouth, Devon, UK). Component peak shapes were fitted using a Gaussian−Lorentzian model. 2.5.3. Fourier Transform Infrared (FTIR) Spectroscopy. The chemical structure of PAN nanofibrous substrates, PAN-AA nanofibrous substrates, PAN-AA-Na swollen midlayer, PANAA-Ca, and PAN-AA-Mg UF composite membranes were investigated by Fourier transform infrared (FTIR) spectroscopy using a FTIR spectrometer (Nicolet 8700, Thermo Scientific, USA) with resolution of 4 cm−1 with an OMNIC Sampler in attenuated total reflectance (ATR) mode. 2.5.4. Mechanical Properties of PAN-AA/PAN Nanofibrous Membranes. The mechanical properties of PAN nanofibrous substrates, PAN-AA/PAN nanofibrous substrates, PAN-AA-Na swollen membranes, PAN-AA-Ca UF membranes, and PANAA-Mg UF membrane were measured by using a tensile testing machine (Model WDW3020, Changchun Kexin, China). All of measurements were carried out at room temperature. For the tensile test, the gauge length was 40−50 mm, and the narrow width at center was 10 mm. The thickness of specimen was in the range of 40−60 μm. The chosen cross-head speed was 10 mm/min. 2.6. Ultrafiltration Performance Evaluation. A dead-end stirred cell (Amicon 8050; Millipore) connected with a solution reservoir and a nitrogen gas cylinder was used to characterize the ultrafiltration permeability of the PAN-AA-Ca and PANAA-Mg UF composite membranes at constant pressure of 0.3 MPa, an effective filtration area of 13.4 cm2, and a constant temperature of 25 °C. Each membrane was precompacted with pure water at 0.2 MPa until the flux reached a plateau (for 0.5− 1.0 h), and the molecular weight cutoffs (MWCOs) of the developed composite membranes were investigated using several kinds of protein solution (100 mg/L) such as bovine serum albumin (BSA), egg albumin, pepsin, α-chymotrypsin, trypsinogen, and cytochrome C as the feeding solution. The rejection for different proteins of the developed membranes was calculated using the following equation:

and simple fabrication route was developed, the fabrication process is shown in Scheme 1. First, the as-prepared PANAAx/PAN double-layer nanofibrous mats were treated with aqueous alkaline buffer solution for a period of time. For the process of swelling, the PAN-AA/PAN nanofibrous substrates were clamped with a Teflon holder and, then, soaked and saturated in the buffer solution for a period of time at 40 °C. The membrane surface was rolled with a glass rod to eliminate any little bubbles forming in the process of soaking. And the pH value of buffer solution was adjusted with NaOH dissolved in ultrapure water (1.0 mol/L). When NaOH in buffer solution reacted with carboxyl group on PAN-AA nanofibrous layer and form carboxylic acid sodium salt (PAN-AA-Na) as formula 1 in Scheme 2, the PAN-AA nanofibers would be induced to swell into a very thin PAN-AA hydrogel layer on the surface of the PAN nanofibrous mat as PAN-AA is hydrophilic and pH sensitive.31,40,41 Then, the PAN-AAx-Na swollen membranes (i.e., PAN-AAx/PAN nanofibrous substrate swollen by alkaline buffer solution) were taken out of the buffer solution carefully and washed with ultrapure water gently to avoid washing away the PAN-AA-Na hydrogel layer from the surface of PAN nanofibrous substrate until they were totally neutralized. Third, the as-prepared PAN-AA-Na swollen membranes were dried in the shade at room temperature for more than 6 h and then immersed into bivalent metal salt aqueous solution (CaCl2 or MgCl2, 0.1 mol/L) for 1 h for the cross-linking of PAN-AA hydrogel film. The bivalent cation in the solution replaced sodium ions on PAN-AA-Na hydrogel layer and worked as cross-linker as shown in formula 2 of Scheme 2. During the ionic cross-linking process, if the concentration of bivalent cations aqueous solution was too low (for example 0.01 mol/ L), it would take much longer time for ionic exchanging, and it would be difficult for the cations to chelate with carboxyl groups on different PAN-AA copolymer chains. Then, the asprepared PAN-AA hydrogel would not be cross-linked enough to form compact polymer networks for further filtration tests, such as reject biomacromolecular with different molecular weights. On the other hand, if the concentration of bivalent ions was too higher (for example 1.0 mol/L), the process of ionic exchanging and cross-linking reacted too fast to be controlled and the PAN-AA barrier layer would shrink in excess and damage monolithic structure of composite membrane. Thus, the divalent cations solution (0.1 mol/L) was selected as the appropriate cross-linking bath. The resultant PAN-AA-Mg and PAN-AA-Ca composite membranes (i.e., PAN-AA barrier layer cross-linked with Ca2+ and Mg2+, respectively) were held in an oven for 0.5 h at 60 °C to make a denser barrier layer. Finally, the resulting composite membranes were successively washed and stored in ultrapure water for further testing. 2.5. Characterizations of Resultant Membranes. 2.5.1. Scanning Electron Microscopy (SEM) Measurements. The morphology of the electrospun PAN nanofibers, PAN-AA nanofibers, PAN-AA/PAN double-layer nanofibrous substrate, PAN-AA-Na swollen membranes, PAN-AA-Ca, and PAN-AAMg nanofibrous ultrafiltration composite membrane were examined by scanning electron microscopy (SEM) system (Phenom G2 pro, FEI, USA) and field emission scanning electron microscopy (FESEM) system (Hitachi, S4800). All specimens received 30 s of gold or platinum coating to minimize the charging effect. For cross-sectional views, all samples were prepared by fracturing the sample in liquid nitrogen. The fiber diameter and pore area were measured by I using the ImageJ analysis program (http://rsb.info.nih.gov/ij/)

⎛ Cp ⎞ R = ⎜1 − ⎟ × 100% Cf ⎠ ⎝

(4)

Where Cp and Cf are the protein concentrations of permeate and feed solutions, respectively, which were detected by using an ultraviolet (UV) spectroscopy (TU1950, Purkinje, China) at a wavelength of 280 nm. Standard UV absorption curves of proteins were plotted by varying concentrations of the initial protein feed solutions, by which the concentration of filtrate solution was determined.11 The stability of the composite membrane was measured by using 4 types of BSA solutions (1 g/L) with different pH values. The BSA feed solutions were prepared by dissolving D


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Figure 1. SEM images of different electrospun nanofibers: (A) PAN, (B) PAN-AA.

Figure 2. SEM images of PAN-AA nanofibers electrospun on PAN nanofibrous substrate with different depositing time: (A) 20, (B) 40, (C) 60, and (D) 80 min.

ultrathin PAN-AA nanofibrous top layer and a thick PAN nanofibrous support layer (the thickness is about 40 μm). Figure 1 shows the typical SEM image of PAN and PAN-AA nanofiber mat. As can be seen from Figure 1A, the average PAN fiber diameter was about 347 nm. For the electrospinning of PAN-AA, concentration of 8 wt % PAN-AA/DMF solution was used to obtain nanofibers within a narrow diameter range (123 ± 38 nm) and without beads (Figure 1B). For preparation of the TFNC ultrafiltration membranes containing a PAN-AA barrier layer and a PAN nanofiber substrate, a new and simple fabrication route was developed, the schematic illustration of the fabrication process was shown in Scheme 1. First, the as-prepared PAN-AAx/PAN doublelayer nanofibrous mats were treated with aqueous alkaline buffer solution for a period of time. For the process of swelling, the PAN-AA/PAN nanofibrous substrates were clamped with a Teflon holder, then soaked and saturated in the buffer solution for a period of time at 40 °C. The membrane surface was rolled with a glass rod to eliminate any little bubbles formed in the process of soaking. And the pH value of buffer solution was

BSA in different types of solvents, including ultrapure water, and three different buffers of pH 3.0, 7.0, and 9.0 (1% acetate acid, phosphate buffered saline, borax salt dissolved in ultrapure water). Pure water as blank test was operated as well. Meanwhile, the permeate flux (J) was determined by direct measurement of the flux in terms of liter per square meter per hour (L/(m2 h)) as calculated using the following equation: J=

V A ·Δt

(5)

Where V is the volume of permeate solution during the test time, A is the effective membrane area, Δt is the test time.

3. RESULTS AND DISCUSSION 3.1. Fabrication of PAN-AA/PAN Nanofibrous Mats. For the preparation of TFNC ultrafiltration membrane which combined a PAN nanofibrous substrate with an ultrathin PANAA functional barrier layer, the double-layer nanofibrous mats were fabricated via electrospinning technique containing an E


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Figure 3. SEM images of PAN-AA nanofibers deposited on PAN nanofibrous substrate for 60 min and immersed in different pH buffer solutions for 1 h: (A) 8.0, (B) 10.0, (C) 12.0. (D) Corresponding surface porosity of PAN-AA nanofibrous swollen layer.

adjusted with NaOH dissolved in ultrapure water (1.0 mol/L). When NaOH in buffer solution reacted with carboxyl group on PAN-AA nanofibrous layer and form carboxylic acid sodium salt (PAN-AA-Na) as formula 1 in Scheme 2, the PAN-AA nanofibers would be induced to swell into a very thin PAN-AA hydrogel layer on the surface of the PAN nanofibrous mat as PAN-AA is hydrophilic and pH sensitive.31,40,41 Then, the PAN-AAx-Na swollen membranes (i.e., PAN-AAx/PAN nanofibrous substrate swollen by alkaline buffer solution) were taken out of the buffer solution carefully and washed with ultrapure water gently to avoid washing away the PAN-AA-Na hydrogel layer from the surface of PAN nanofibrous substrate until they were totally neutralized. Third, the as-prepared PAN-AA-Na swollen membranes were dried in the shade at room temperature for more than 6 h and then immersed into bivalent metal salt aqueous solution (CaCl2 or MgCl2, 0.1 mol/ L) for 1 h for the cross-linking of PAN-AA hydrogel film. The bivalent cation in the solution replaced sodium ions on PANAA-Na hydrogel layer and worked as cross-linker as shown in formula 2 of Scheme 2. During the ionic cross-linking process, if the concentration of bivalent cations aqueous solution was too low (for example 0.01 mol/L), it would take much longer time for ionic exchanging, and it would be difficult for the cations to chelate with carboxyl groups on different PAN-AA copolymer chains. Then, the as-prepared PAN-AA hydrogel would not be cross-linked enough to form compact polymer networks for further filtration tests, such as reject biomacromolecular with different molecular weights. On the other hand, if the concentration of bivalent ions was too higher (for example 1.0 mol/L), the process of ionic exchanging and crosslinking reacted too fast to be controlled and the PAN-AA barrier layer would shrink in excess and damage monolithic structure of composite membrane. Thus, the divalent cations solution (0.1 mol/L) was selected as the appropriate cross-

linking bath. The resultant PAN-AA-Mg and PAN-AA-Ca composite membranes (i.e., PAN-AA barrier layer cross-linked with Ca2+ and Mg2+ respectively) were held in an oven for 0.5 h at 60 °C to make a denser barrier layer. Finally, the resulting composite membranes were successively washed and stored in ultrapure water for further testing. The thickness of PAN-AA nanofibrous top layer was controlled depending on the depositing time of electrospinning PAN-AA nanofibers, as shown in Figure 2, the PAN-AA electrospun nanofibers first deposited along the PAN nanofibers, and then the PAN-AA nanofibers began to across and fill the pores on the surface of PAN nanofibrous substrate. The bottom PAN nanofibrous layer was totally covered by PAN-AA nanofibers when the depositing time is larger than 40 min. Though the PAN nanofibrous substrates could not been seen, it was still important to adjust the thickness of PAN-AA nanofibrous layer which would affect the integrity and micro pores on barrier layer of PAN-AA/PAN composite membranes that will be discussed in the following sections. 3.2. Optimization of PAN-AA Barrier Layer of TFNC Membranes by Swelling and Ionic Cross-Linking. Although PAN nanofibers are difficult to swell and dissolve in water, hydrophilic PAN-AA nanofibers are easy to swell in buffer solution by adjusting the pH value. Since PAN-AA is one kind of pH sensitive copolymers as the acrylic acid groups on PAN-AA could undergo dramatic charge at a specific pH called pKa, which could cause an alternation of the hydrodynamic volume of the polymer chains. The solid state of PAN-AA will transform to a swollen hydrogel state31 in basic solution. To find the suitable pKa for swelling PAN-AA nanofibrous layer, the PAN-AA60/PAN double-layer nanofibrous substrates were immersed into buffer solutions with different pH values for 1 h. As shown in Figure 3, when the pH value of buffer solution increased from 8.0 to 10.0, the diameter of PAN-AA nanofibers F


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Figure 4. SEM images of PAN-AA nanofibers electrospun on PAN nanofibrous substrate with different depositing time and immersed into buffer solution (pH = 12.0) for 4 h: (A) 20, (B) 40, (C) 60, and (D) 80 min.

Figure 5. SEM images of PAN-AA nanofibers deposited on PAN nanofibrous substrate for 60 min and immersed into buffer solution (pH = 12.0) for different times: (A) 1, (B) 2, (C) 3, and (D) 4 h.

higher pH value. When the pH increased to 12.0, PAN-AA nanofibers could be swollen into hydrogel and spread along the bottom PAN nanofibers. The surface porosity of PAN-AA nanofibrous layer decreased from 81.02% to 12.15%, as shown in Figure 3D, which were measured with ImageJ analysis program.42,43 Due to the short swelling time, some regions

just slightly increased. Since the swelling behavior of PAN-AA nanofibers was mostly influenced by acrylic acid (AA) content of PAN-AA copolymer synthesized by radical polymerization,37 which was about 9.95% according to 1H NMR analysis (much lower than that of traditional PAA hydrogel), the PAN-AA nanofibrous layer would be swollen in buffer solution with G


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Figure 6. SEM images of surface (left column) and cross section (right column) of PAN-AA/PAN nanofibrous composite membranes cross-linked by different bivalent metal cations: (A, B) PAN-AA60-Ca, (C, D) PAN-AA60-Mg, (E, F) PAN-AA80-Ca, and (G, H) PAN-AA80-Mg.

solution with pH value of 12.0 for longer than 4 h to be swollen sufficiently. As the PAN content of PAN-AA copolymer was higher than 90% according to 1H NMR analysis, the parts of PAN within PAN-AA nanofibers could maintain the framework of PAN-AA nanofibrous layer and the parts of PAA could be swollen into hydrogel to fill the interconnected pores within PAN-AA nanofibrous layer. As shown in Figure 4, it could be seen that the PAN-AA nanofibers were gradually swollen into an ultrathin PAN-AA hydrogel layer and covered the surface pores of the bottom PAN nanofibrous surface layer at the pKa of PAN-AA copolymer. Due to the short depositing time (20− 40 min), the thickness of electrospun PAN-AA nanofibrous

were not covered by PAN-AA layer. If the pH value of buffer solution was higher than 12.0, the strongly alkaline surrounding condition would hydrolyze and resolve the PAN nanofibrous substrate. Thus, the pH value of 12.0 was chosen as the pKa of PAN-AA nanofibrous layer. In order to obtain nonporous barrier layer, the top PAN-AA nanofibrous layer should be thick enough to be swollen into a PAN-AA hydrogel layer and to cover the microscale pores (0.3−1.0 μm) on the surface of PAN substrate. The doublelayer PAN-AAx/PAN nanofibrous membranes with different depositing time of PAN-AA nanofibers were swollen into PANAA-Na hydrogel layer by immersing them into the buffer H


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noteworthy that the PAN-AA-Na swollen hydrogel layer and the ionic cross-linked PAN-AA barrier layer would not penetrate into the interconnected PAN nanofibrous substrates as shown in Figure S1. Thus, the ionic cross-linked PAN-AA hydrogel layer became a denser functional barrier layer and the filtration performance of the result composite membranes were affected by different divalent metal ions, which will be discussed in the following sections. 3.3. Characterization of PAN-AA/PAN Nanofibrous TFNC Membrane Surface. 3.3.1. FTIR Spectroscopy. The surface chemical composition of each layer in PAN-AA composite membranes was provided by FTIR spectroscopy. Figure 7 shows the FTIR spectra for PAN nanofibrous

layer was not enough to cover the whole surface of the PAN nanofibrous substrate as shown in Figure 4A and B. When the depositing time was longer than 60 min, the PAN-AA60 and PAN-AA80 nanofibrous layer were thick enough to be swollen into a nonporous PAN-AA hydrogel layer, which showed a weave like surface due to the PAN-block framework within PAN-AA layer. And the PAN-AAx-Na swollen layer could firmly combine with the surface nanofibers of PAN bottom layer, which appeared to have better mechanical properties that will be discussed in the following sections. But the thickness of PAN-AA80-Na swollen layer (∼490 nm) was much thicker than that of PAN-AA60-Na swollen layer (∼360 nm) as shown in Figure S1C−D, which might influence the permeate flux of the resultant composite membrane. Thus, PAN-AA60/PAN nanofibrous membranes were used to be swollen into the base membrane for preparing composite membranes. Since the PAN-AA nanofibrous layer could be swollen at certain pKa of buffer solution, the thickness of PAN-AA nanofibrous layer could be controlled by depositing time, which should be longer than 40 min. On the other hand, the appropriate swelling time for PAN-AA nanofibers was also important. The PAN-AA nanofibers would not be swollen and fill the holes within PAN-AA nanofibrous layer if the immersing time of buffer solution was not long enough. To cover the surface pores on PAN nanofibrous substrate, PAN-AA nanofibrous layer with same thickness (depositing time: 60 min) were immersed into buffer solution at pH value of 12.0 for different period of time and then washed with ultrapure water. SEM photos of these PAN-AA60/PAN swollen membranes were showed in Figure 5. The surface pore size and porosity of PAN-AA-Na swollen layer was obviously decreased and finally disappeared as the immersing time increased from 1 to 4 h. Therefore, the PAN-AA/PAN nanofibrous substrate should be immersed in buffer solution for at least 4 h to be sufficiently swollen into an integrated hydrogel layer which could cover the surface larger pores (∼0.6 μm) on the PAN nanofibrous substrate. Although there are no obvious big pores (diameter larger than 20 nm) on PAN-AA-Na hydrogel layer which could be observed by SEM analysis, the PAN-AA-Na swollen hydrogel layer was neither firm nor compact enough for further filtration tests. To obtain an integrated PAN-AA barrier layer, the PANAA60/PAN swollen membranes were cross-linked in bivalent metal salt solution (MgCl2 or CaCl2, 100 mmol/L) for 1 h. Figure 6 (left column) shows the surface SEM images of PAN mats covered by different bivalent ionic cross-linked PAN-AA barrier layer, both the PAN-AA-Ca and PAN-AA-Mg (i.e., PAN-AA hydrogel layer cross-linked with calcium ions and magnesium ions, respectively) ultrafiltration membranes maintain the structure of PAN-AA-Na swollen layer without obvious difference in the surface roughness. According to the cross-sectional SEM images of PAN-AA/PAN composite membranes (Figure 6 right column) and PAN-AA-Na swollen membranes (Figure S1), the cross-linked PAN-AA barrier layer was a little thinner (200−330 nm) than PAN-AA-Na swollen hydrogel layer (360−490 nm), because of the increased crosslinking points within PAN-AA layer would decrease the distance between freedom carboxyl groups inside the PANAA-Na swollen hydrogel. The thickness of ionic cross-linked PAN-AA60 barrier layers (Figure 6B ∼200 nm and Figure 6D ∼220 nm) was a little thinner than that of PAN-AA80 barrier layers (Figure 6F ∼310 nm and Figure 6H ∼330) due to the increase of depositing time for PAN-AA nanofibrous layer. It is

Figure 7. FTIR spectra of (a) PAN nanofibrous substrate, (b) PANAA nanofibrous layer, (c) PAN-AA-Na swollen layer, (d) PAN-AA-Ca composite membrane, and (e) PAN-AA-Mg composite membrane.

substrate (Figure 7a), PAN-AA nanofibrous layer (Figure 7b), PAN-AA-Na swollen hydrogel layer (Figure 7c), PAN-AA-Ca composite membrane (Figure 7d), and PAN-AA-Mg composite membrane (Figure 7e) (PAN-AA hydrogel layer cross-linked with magnesium ion). From the FTIR-ATR spectra of PAN, the characteristic absorption peaks appearing at 2243 and 1453 cm−1 were attributed to cyan groups (CN) in PAN and methyl groups mainly which come from the initiator remained in terminal group of PAN. After being covered with a thin PAN-AA nanofibrous layer, the PAN-AA/PAN double layer nanofibrous substrates showed small characteristic absorption peak at 1627 and 1732 cm−1 in Figure 7b, which were corresponding to the asymmetric stretching vibration of nonionized carboxyl group (−COOH) and stretching vibration of carbonyl groups (CO) from PAN-AA nanofibrous top layer. The characteristic absorption peaks of PAN-AA-Na swollen hydrogel layer at 1581 and 1359 cm−1 were detected and attributed to the asymmetric stretching vibration and symmetric stretching vibration of ionized carboxyl group (−COO−), which indicated that the carboxyl groups only formed monodentate ligands with sodium ions. Almost all the carboxyl acid group on the PAN-AA nanofibers reacted with the NaOH in buffer solution and changed into carboxyl sodium salt (−COO−Na+) as formula 1 in Scheme 2. These changes show a strong evidence of the PAN-AA-Na swollen hydrogel obtained by NaOH induced swelling process. Then the peak at 1581 cm−1 was enhanced and migrated to 1583 cm−1 as the sodium ions were replaced with calcium ions and magnesium ions and forming cross-linking points as formula 2 in Scheme 2, as shown in Figure 7d and e, respectively. And the peak at 1359 I


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Industrial & Engineering Chemistry Research cm−1 was migrated to 1409 cm−1, which is corresponding to symmetric stretching vibration of ionized carboxyl groups. It also indicated that the carboxyl groups could coordinate with bivalent ions in both monodentate and bidentate ligands. Besides the small peak appeared at 1721 cm−1 (Figure 7d and e) was corresponding to stretching vibration of carbonyl groups which meant that the amount of calcium ions or magnesium ions are not enough to cross-link all the carboxyl groups in PAN-AA hydrogel layer. Due to the bigger radius of Ca2+ ions, the chelation ability of Mg2+ with PAN-AA copolymer was stronger than that of Ca2+.44 All these results indicated that PAN-AA-Mg composite membrane has a denser barrier layer than PAN-AA-Ca composite membrane dose, which may influence the mechanical properties and filtration properties of these membranes. 3.3.2. XPS Analysis. In order to confirm the swelling process of PAN-AA nanofibrous layer and cross-linking process of PAN-AA hydrogel layer, XPS was employed to characterize the surface of PAN-AA nanofibrous membrane, PAN-AA-Na swollen membrane, PAN-AA-Ca composite membrane, and PAN-AA-Mg membrane. For all the XPS spectrum of PAN-AA series membranes, surface elemental compositions of all the fabricated membranes are shown in Table 1. The molar ratios

AA-Ca composite membrane, and PAN-AA-Mg composite membrane were investigated by tensile test. Figure 8 shows the

Figure 8. Stain−stress curve of different nanofibrous membranes: (a) PAN nanofibrous substrate, (b) PAN-AA/PAN nanofibrous membrane, (c) PAN-AA-Na swollen membrane, (d) PAN-AA-Ca composite membrane, and (e) PAN-AA-Mg composite membrane.

stress−strain curves of these membranes. The PAN single-layer nanofibrous substrate showed the lowest tensile strength (2.28 MPa) among all these membranes. After depositing PAN-AA nanofibers, the mechanical properties of PAN-AA/PAN nanofibrous substrates were significantly reinforced. The tensile strength of PAN-AA/PAN double-layer nanofibrous substrate increased to 8.51 MPa (Figure 8b) with the depositing time of 60 min, which was the suitable candidate for preparing the nonporous hydrogel layer as discussed in section 3.2. After being swollen into an integrity nonporous PAN-AA hydrogel, the PAN-AA layer was totally adhesive on the surface of PAN nanofibrous substrate. The PAN-AA hydrogel may combine more interconnection with the PAN nanofibers by forming more hydrogen bonds between carboxyl groups on PAN-AA layer and cyan groups on PAN. Thus, the tensile strength of PAN-AA-Na swollen membrane was further increased to 10.37 MPa (Figure 8c). Then, the PAN-AA barrier layer cross-linked with different bivalent ions showed the highest mechanical properties in Figure 8d and e. The PAN-AA-Ca composite membrane could obtain almost the same mechanical property as PAN-AA-Mg composite membrane, which exhibited better tensile strain (0.56) but lower tensile strength (12.75 MPa) than that of the PAN-AA-Mg composite membrane (tensile strain 0.44, tensile strength 13.60 MPa). 3.5. Filtration Properties of Nanofibrous TFNC Membrane. As the PAN-AA60/PAN and PAN-AA80/PAN double-layer nanofibrous membrane could be fully swollen into nonporous PAN-AA hydrogel by buffer solution, the asprepared surface nonporous PAN-AA60-Na and PAN-AA80Na swollen membranes were cross-linked with different bivalent cations (Ca or Mg) in aqueous chloride solution (0.10 mol/L) for 1 h. The separation performance of the asprepared PAN-AA/PAN ionic cross-linked composite membranes was tested by determining the molecular weight cut-offs (MWCOs), analyzing pH stability, and detecting long period operation. To determine the MWCOs of PAN-AA/PAN composite membranes, various solutes could be used, such as Dextran, proteins, polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), inulin, raffinose, sucrose, glucose, and so on. Among

Table 1. Surface Elemental Composition of PAN-AA, PANAA-Na, PAN-AA-Ca, and PAN-AA-Mg Membranes Characterized by XPS Measurementsa elemental compositions sample

C (%)

N (%)

O (%)

Na (%)

Ca (%)

PAN-AA PAN-AA-Na PAN-AA-Ca PAN-AA-Mg

79.18 77.58 78.51 78.33

16.20 15.68 13.65 13.44

4.62 5.99 6.77 6.84

0.75 0.07 0.09

0.99

Mg (%)

1.31

Data are the mean values ±0.05, calculated from four separate experiments.

a

of N in cyan groups (CN) and O in carboxyl groups (−COOH) for PAN-AA nanofibrous membrane are around 16.20% and 4.62%, respectively. So the amount of carboxyl groups on surface of PAN-AA nanofibrous membrane is about 12.48% as calculated by eq 3. Since the XPS analysis method is a kind of semiquantitative spectrographic analysis, there is a little deviation between the amount of carboxyl groups calculated from XPS and the result calculated from 1H NMR (9.95%) for PAN-AA copolymer. The molar content of Na in carboxyl sodium salt (−COO−Na+) for PAN-AA-Na swollen hydrogel layer is highest (0.75%), which means only a few carbonyl acid groups became ionized carboxyl group (−COO−) during swelling process. After the bivalent ionic cross-linking process, the molar content of Na remained in PAN-AA-Ca barrier layer and PAN-AA-Mg barrier layer decreased to 0.07% and 0.09%, respectively. The remained sodium atoms indicated that the rate of bivalent ionic crosslinking was limited by concentration of bivalent ions and could not replace all the sodium ions within PAN-AA-Na swollen hydrogel. Therefore, the XPS results confirmed that the PANAA-Na swollen hydrogel has been cross-linked by bivalent ions (Ca2+ or Mg2+), which has also been proved by FTIR-ATR. 3.4. Mechanical Properties of Nanofibrous Substrates and TFNC Membrane. The mechanical properties of PAN single-layer nanofibrous substrates, PAN-AA/PAN double-layer nanofibrous membrane, PAN-AA-Na swollen membrane, PANJ


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3779.4 ± 38.2 to 1606.7 ± 14.6 L/(m2 h) (Table S2). Compared with the permeate water fluxes of ionic cross-linked PAN-AA composite membranes (444.6 L/(m2 h) for PANAA60-Ca membrane and 433.7 L/(m2 h) for PAN-AA60-Mg membrane, 267.9 L/(m2 h) for PAN-AA80-Ca membrane and 242.6 L/(m2 h) for PAN-AA80-Mg membrane, as listed in Table S2), these values were extremely lower than that of the PAN-AA-Na swollen membranes because of the freedom volume of PAN-AA-Na swollen layer would be shrunk by ionic cross-linking points (coordinate bonds between metallic ions and carboxyl groups), and thus the average thickness of PANAA ionic cross-linked barrier layers (ranged from 200 to 330 nm) were thinner than that of PAN-AA-Na swollen layer (ranged from 360 to 490 nm). The atom content of magnesium ions in PAN-AA cross-linked barrier layer is higher than that of calcium ions in PAN-AA cross-linked barrier layer according to XPS analysis in section 3.3.2, which might be ascribed to the ions radius of Mg2+ (86 pm) is smaller than that of Ca2+ (114 pm),47 and then the magnesium ions would be easier to penetrate into the PAN-AA hydrogel. According to FTIR spectroscopy in section 3.3.1, the carboxyl groups coordinated with bivalent ions in both monodentate and bidentate ligands. The coordination between Ca2+ and carboxyl groups was major in bidentate ligands, while the Mg2+ formed more monodentate ligands with carboxyl groups due to the stronger chelate ability of Ca2+ and the hydration of Mg2+.44 The calcium ions could grab more carboxyl groups on chains of PAN-AA around them. Several calcium cross-linking points within PAN-AA hydrogel may cause more gaps among the PAN-AA copolymer networks than that of magnesium ions induced. Thus, the structure of PAN-AA barrier layer cross-linked with Ca2+ may have large surface pore size than that cross-linked with Mg2+, which has been proved by the MWCO tests. According to the reports of ionic cross-linked polymer,33,36,48−50 different cross-linking ions and pH values of feed solution could influence the structure of PAN-AA barrier layer, which would further determine the ultrafiltration properties of PAN-AA composite membrane. Four types of BSA solution (1 g/L) with different pH surrounding were used to evaluate the ultrafiltration performance of different PAN-AA UF composite membranes, including PAN-AA60-Ca, PANAA60-Mg, PAN-AA80-Ca, and PAN-AA80-Mg composite membrane as shown in Table S2. The permeate flux and rejection are mainly determined by the surface barrier layer of the resultant composite membrane. Since the BSA may penetrate into micro pores of PAN-AA cross-linked barrier layer and the increase of BSA concentration during dead-end filtration process would aggravate the membrane fouling, the permeate flux of PAN-AA/PAN composite membranes for different BSA solutions exhibited almost half of the pure water flux as shown in Figure 10A. Compared with the PAN-AA composite membrane cross-linked with the same ions, the thicker the PAN-AA barrier layer, the higher BSA rejection and the lower water flux could be obtained, i.e., the filtration performances of PAN-AA60-Ca and PAN-AA80-Ca composite membrane was 221.2 L/(m2 h) with 96.8% and 93.1 L/(m2 h) with 99.0%, respectively. On the other hand, the types of metal ions used as cross-linker also influenced the filtration performance of these TFNC UF membranes. When the magnesium ions replaced the calcium ions as cross-linker, the permeate flux of PAN-AA60 series composite membranes for BSA solution (pure water) decreased from 221.2 to 219.2 L/ (m2 h), and the rejection increased from 96.8% to 97.8%.

of them, Dextran and polyethylene glycol (PEG) are widely used for measuring the MWCO of commercial ultrafiltration membranes. But, the molecular structure of Dextran and polyethylene glycol (PEG) are linear and tangled, which could be influenced by concentration and temperature of aqueous solution. Thus, the commercial ultrafiltration membranes are usually used to separate the solute with molecular weight ranged from one-third to one-half that of the MWCO. But the proteins have narrower molecular weight range and globoid structure, which are much better for determining actual radius of the micro pore on PAN-AA cross-linked barrier layer. The protein rejection rate for PAN-AA60-Ca and PAN-AA60-Mg composite membranes were obtained by standard ultrafiltration tests and the results were listed in Table S1. As the molecular weight of proteins increased from 13 000 Da (cytochrome C) to 67 000 Da (bovine serum albumin), the rejection rate of PAN-AA60-Ca membrane increased from 24.17% to 96.81%, meanwhile that of PAN-AA60-Mg membrane increased from 22.02% to 97.75%. Then, the filtration results of these proteins were described by drawing the logistic fitting curves as shown in Figure 9. There existed sharp changes on both curves

Figure 9. MWCO of PAN-AA ultrafiltration composite membranes cross-linked by different divalent cations.

between 35 000 and 45 000 Da. The interception of the curvefitting for the rejection rate versus molecular weight of proteins with the line of 90% rejection resulted in an MWCO of ∼43 500 Da for PAN-AA60-Ca composite membrane and a MWCO of ∼47 300 Da for PAN-AA60-Mg composite membrane. Therefore, the MWCOs of PAN-AA60-Ca and PAN-AA60-Mg composite membranes were almost equal to the molecular weight of Egg albumin (45 000 Da) in the range of 45400 ± 1900 Da. According to the relationship between solute radius (i.e., Stok’s radius) and average pore size (radius) on ultrafiltration membrane developed by Sarbolouki,45 the average pore size on the ionic cross-linked PAN-AA barrier layer should be smaller than 4.6 nm. Due to the bigger average solute radius of BSA (7.5 nm),46 the rejection to BSA for both kinds of ionic cross-linked PAN-AA composite membranes are higher than 95%. The pure water fluxes of PAN-AA-Na swollen membranes and PAN-AA cross-linked composite membranes were tested. As the average thickness of PAN-AA60-Na and PAN-AA80-Na swollen hydrogel layer increased from ∼360 to ∼490 nm (Figure S1 C−D), the pure water flux of PAN-AA60-Na and PAN-AA80-Na swollen membranes decreased sharply from K


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different BSA solutions could maintain higher than 93%. The permeate flux of PAN-AA60/PAN composite membrane were higher than that of PAN-AA80/PAN composite membrane, which indicated that the PAN-AA/PAN series composite membrane can provide good efficiency in BSA separation and concentration. Long period performance of the antifouling property is also evaluated by taking two types of TFNC UF membranes, i.e., PAN-AA60-Ca and PAN-AA60-Mg, of which the BSA rejection efficiency are both higher than 96% as shown in Figure 11.

Figure 11. Pure water permeate performance of different ionic crosslinked PAN-AA TFNC UF membranes for long period of operation.

During the first hour for prepressing at same operating pressure (0.3 MPa), the permeate flux of both composite membranes dropped fast from ∼440 to ∼220 L/(m2 h) due to the penetration and fouling of BSA. Then, the permeate flux decreased slowly for the next 11 h, and permeate flux of PANAA60-Mg composite membrane was higher than that of PANAA60-Ca composite membrane. It seems that the hydration of Mg2+ would provide more penetrating water channels in PANAA barrier layer. After 12 h continuous operation, the permeate flux of PAN-AA-Ca and PAN-AA-Mg composite membrane can still be higher than 178.7 and 174.7 L/(m2 h), respectively. The results revealed that the as-prepared TFNC UF membranes remain high permeate fluxes for long period protein concentration and separation.

Figure 10. Ultrafiltration performance of different ionic cross-linked PAN-AA TFNC UF membranes at 0.3 MPa: (A) flux and (B) rejection.

Additionally, the thickness of PAN-AA80-Mg barrier layer was the largest among those different ionic cross-linked PAN-AA barrier layers, thus, the permeate flux (91.9 L/(m2 h)) was the lowest but with the highest rejection (99.9%). Compared with the commercial UF membrane (Millipore, PLGC04710), which only has a permeate flux of 56.1 L/(m2 h) and a BSA rejection of 97.6% at 0.3 MPa, the permeate flux of PAN-AA60-Mg composite membranes was 3 times higher with an approximate BSA rejection rate. As the pH value of BSA solution decreased from 9.0 to 3.0, the permeate fluxes of PAN-AA/PAN composite were decreased first and then increased, and the rejections of PAN-AA/PAN composite were increased first and then decreased. In detail, the permeate flux of PAN-AA60-Ca composite membrane for BSA buffer solution (1 g/L, pH: 3.0− 9.0) was decreased from 230.7 to 223.0 L/(m2 h) and then increased to 236.7 L/(m2 h) as shown in Figure 10A. Meanwhile, the rejection of PAN-AA60-Ca composite membranes was increased from 93.5% to 95.6% and then decreased to 93.0% as shown in Figure 10B. As the PAN-AA copolymer is a kind of pH-sensitive material, the ionic cross-linked PAN-AA barrier layer would still exhibit different swollen degree in different pH surrounding. When the pH value of BSA buffer solution was 7.0, the PAN-AA barrier layer would be in suitable swelling condition. So the PAN-AA/PAN TFNC UF membranes could obtain optimized filtration performance than BSA buffer solutions with higher or lower pH values, and the rejection of PAN-AA/PAN composite membranes for

4. CONCLUSION The PAN-AA/PAN nanofibrous composite membranes were successfully fabricated by double-layer electrospinning technique, swelling process in buff solution and cross-linking with bivalent ions (Ca2+, Mg2+). With two steps modification, the top PAN-AA ionic cross-linked barrier layer based on PAN nanofibrous substrate obtained a MWCO around 45 000 Da and provided a good ultrafiltration performance in different BSA aqueous solution. The PAN-AA60-Ca composite membrane exhibited a result of 444.6 L/(m2 h) in pure water flux, 220 L/(m2 h) in permeate flux and 96.8% in BSA (pure water solution) rejection at 0.3 MPa, while the PAN-AA60-Mg composite membrane presented a result of 433.7 L/(m2 h) in pure water flux, 219.2 L/(m2 h) in permeate flux and 97.8% in BSA (pure water solution) rejection. The thicker PAN-AA80 series composite membrane improved the BSA rejection rate over 99.0%, but the lower the pure water flux (PAN-AA80-Ca L


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Industrial & Engineering Chemistry Research 267.9 L/(m2 h), PAN-AA80-Mg 242.6 L/(m2 h)) and the permeate flux (PAN-AA80-Ca 93.1 L/(m2 h), PAN-AA80-Mg 91.9 L/(m2 h)). Compared with commercial UF membrane (Millipore, PLGC04710), the permeate flux efficiency was enhanced about 300%, and the long period filtration tests demonstrated that PAN-AA60 series composite membrane could maintain high permeate flux (>170 L/(m2 h)) after 12 h continuous operation. Consequently, this work may provide a new method for preparing nanofibrous composite membrane for different grades of accuracy in filtration applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00244. SEM images of cross sections of PAN-AA/PAN swollen membranes with different depositing time of PAN-AA nanofibers; rejection rates of proteins with different molecular weight used to determine the MWCO of PAN-AA60-Ca and PAN-AA60-Mg composite membrane; pure water fluxes for PAN-AA-Na swollen membranes and ionic cross-linked PAN-AA composite membranes; stability of ionic cross-linked PAN-AA series composite membrane tested with four kinds of BSA solutions with different solvent and pH values (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-21-67792860. Fax: +86-21-67792855. E-mail: [email protected] (X.W.). *E-mail address: [email protected] (B.S.H.). ORCID

Xuefen Wang: 0000-0002-7045-0328 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Program of Shanghai Science and Technology Innovation International Exchange and Cooperation (15230724700), National Natural Science Foundation of China (51273042, 21174028), Program for New Century Excellent Talents in University (NCET-13-0725), and Program of Changjiang Scholars and Innovative Research Team in University (IRT1221).

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