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Nanofiber Composite Membrane with Intrinsic Janus Surface for Reversed-Protein-Fouling Ultrafiltration Anbharasi Vanangamudi, Ludovic F Dumée, Mikel C. Duke, and Xing Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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ACS Applied Materials & Interfaces

Nanofiber Composite Membrane with Intrinsic Janus Surface for Reversed-Protein-Fouling Ultrafiltration Anbharasi Vanangamudi,*,1,2 Ludovic F. Dumée,2 Mikel C. Duke,1 Xing Yang,*,1 1

Institute for Sustainability and Innovation (ISI), College of Engineering and Science, Victoria University, Melbourne, Victoria 8001, Australia 2

Deakin University, Waurn Ponds, Institute for Frontier Materials, Victoria 3216, Australia

KEYWORDS: ultrafiltration, composite membranes, nanofibers, hydrophilic modification, antifouling

ACS Paragon Plus Environment

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ABSTRACT: Janus nanofiber based composite ultrafiltration (UF) membranes were fabricated via a two-step method, i.e., consecutive electrospinning of hydrophilic nylon-6,6/chitosan nanofiber blend and conventional casting of hydrophobic poly(vinylidene difluoride) (PVDF) dope solution. The as-developed PVDF/nylon-6,6/chitosan membranes were investigated for its morphology using Scanning Electron Microscopy (SEM) by which 18 wt% PVDF was chosen as the optimum base polymer concentration due to optimal degree of integration of cast and nanofiber layers. This membrane was benchmarked against the pure PVDF and PVDF/nylon-6,6 membranes in terms of surface properties, permeability and its ability to reverse protein fouling. The improved hydrophilicity of the PVDF/nylon-6,6/chitosan membrane was revealed from the 72% reduction in the initial water contact angle compared to the pure PVDF benchmark, due to the incorporation of intrinsic hydrophilic hydroxyl and amine functional groups on the membrane surface confirmed by FTIR. The integration of the nanofiber and cast layers has led to altered pore arrangements offering about 93% rejection of bovine serum albumin (BSA) proteins with a permeance of 393 L.m-2.h-1.bar-1 in cross-flow filtration experiments; while the PVDF benchmark only had a BSA rejection of 67% and a permeance of 288 L.m-2.h-1.bar-1. The PVDF/nylon-6,6/chitosan membrane exhibited high fouling propensity with 2.2 times higher reversible fouling and 78% decrease in the irreversible fouling compared to the PVDF benchmark after 4 h of filtration with BSA foulants.


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INTRODUCTION Ultrafiltration (UF) membranes are widely used in separation applications including water treatment, food and bio-processing, chemical and medical applications.1-4 These membranes have the ability to capture compounds or remove contaminants of size ranging from 0.01 to 0.1 micron at low transmembrane pressure (0.5 to 1 bar). However, it is essential for the membranes to survive high fouling environment caused mainly by the hydrophobic proteins and other organic components to have better performance and lesser cleaning frequencies.3,

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Several

approaches have been considered to overcome membrane fouling such as (1) pre-treatment, (2) chemical cleaning and backwashing which may accelerate the wear or damage the membrane structure and (3) altering membrane hydrophilicity by incorporating hydrophilic materials in/on to the hydrophobic membranes which can reduce protein-membrane interactions for increased flux recovery and easy cleaning.8-11 Poly(vinylidene difluoride) (PVDF) is one of the most commonly used membrane materials due to good mechanical, chemical and thermal stabilities.12-14 However, PVDF membranes are naturally hydrophobic and therefore exhibit irreversible fouling behaviour from proteins and natural organic matter (NOMs).15-17 Towards improving the hydrophilicity of the PVDF membranes, hydrophilic modification methods have been explored to tailor the surface engineering of the membranes.18 Blending has been widely used to prepare hydrophilically modified PVDF membranes by mixing hydrophilic polymer additives copolymers

23-24

and inorganic nanoparticles

25-27

19-22

, amphiphilic

into the PVDF dope solution. Blending is a

common means for manufacturers to build in the essential hydrophilic property to the commercial membranes.18 However, owing to the incompatibility of these hydrophilic additives


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with hydrophobic PVDF, they can be easily washed out during membrane preparation and operation processes which led to decrease in water permeability.28-29 As an alternative to blending, surface modification via coating is another simple way to improve PVDF membrane hydrophilicity. When coated with poly(vinyl alcohol) (PVA), the surface hydrophilicity of the PVDF membrane was enhanced by 16% increase in water contact angle with 14% higher flux recovery from bovine serum albumin (BSA) solution.30 Also, chitosan coating on to PVDF membrane enhances the hydrophilicity by a 40% increase in water contact angle with 47% higher flux recovery from BSA solution compared to control PVDF membrane.31 These can also be defined as Janus membranes having hydrophobic PVDF on one side and hydrophilic polymer on the other side. Although coating is highly effective at reducing protein fouling, more work is needed to improve the stability of the coating to the surface of the PVDF, while also having a high functional surface area to ensure increased hydrophilicity and decreased membrane-protein interactions as a result of the coating.18 Hence, future progress to fouling resistant hydrophilic PVDF membranes appears to require incorporation of hydrophilic functional groups that are intrinsic to the membrane material. An ideal hydrophilic material that may be coated on to hydrophobic PVDF is polymeric nanofibers. Polymeric nanofibers are an advanced material with unique properties such as high surface area, high porosity, interconnected pore structure and versatile surface functionalities. Electrospinning was found to be the most versatile approach to fabricate nanofiber mats.32 The high surface area of electrospun nanofibers made of hydrophilic polymer facilitates the incorporation of highly dense hydrophilic functional groups. Selectivity, surface charge, mechanical strength and porosity of the polymer network can be modified using the nanofibers to improve their performances towards water purification. Recent studies indicate that nanofiber


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based UF membranes offer enhanced fouling resistance and water permeability due to hydrophilic coatings on the nanofiber layer.33-36 Low fouling composite membranes having 10 times higher flux than the commercial NF filter (Down NF270) were prepared by coating chitosan on the electrospun poly(acrylonitrile) (PAN) nanofiber scaffold electrospun on to the non-woven PET micro-filter substrate.34 Only limited studies of coating hydrophilic polymer on the PVDF membrane have been reported mainly due to the high hydrophobicity of PVDF which makes them difficult to surface adhere hydrophilic additives.31 Hence, it is desirable to develop a method that can effectively integrate the hydrophobic support and hydrophilic layer nanofiber layer and PVDF substrate to overcome the material incompatibility. Recently, an alternate method has been used to strengthen the integration between the hydrophobic PES nanofiber substrate and hydrophilic polysulfone cast layer by sequential electrospinning and conventional casting techniques.37. In this work, Janus nanofibrous composite UF membranes were prepared by consecutive electrospinning of hydrophilic nylon-6,6/chitosan blend and conventional casting of hydrophobic PVDF polymer. Here, nylon-6,6 and chitosan was chosen as the hydrophilic blend due to their good miscibility and the presence of intrinsic hydrophilic hydroxyl and amine functional groups.38 The critical anchoring of an open hydrophilic nylon-6,6/chitosan nanofiber mat to the hydrophobic PVDF supporting layer was first demonstrated by varying the PVDF/solvent ratio to differ the dope viscosity enabling it to partially infiltrate into the nanofibers. Pure PVDF and PVDF/nylon-6,6 membranes were fabricated as benchmarks. The well formed PVDF/nylon6,6/chitosan membranes were then studied in terms of morphology, surface chemistry and filtration performance. The fouling resistance and flux recovery of the PVDF/nylon-6,6/chitosan


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membranes were investigated with BSA as the model organic contaminant, compared against the benchmark membranes. EXPERIMENTAL: Materials. PVDF Kynar® 761 grade with a melting point of 165–172 °C was purchased from Arkema Pte. Ltd., Singapore. The following chemicals were purchased from sigma Aldrich and used as received: Polyamide-6,6 (nylon-6,6), chitosan (190-310 kDa molecular weight), poly(vinyl pyrrolidone) (PVP-K-40), poly(ethylene glycol) (PEG), bovine serum albumin (BSA), analytical grade N,N′-dimethyl acetamide (DMAC), >95% formic acid, 75% ethanol, sodium chloride (NaCl) and glycerol. Deionized water (DI) used in all experiments was obtained from the Milli-Q plus system (Millipore, Bedford, MA, USA). Fabrication of Nanofiber Composite membrane. The schematic of the preparation of bi-layer composite membrane is shown in Figure 1. Firstly, the functional nylon-6,6/chitosan nanofiber layer was prepared by electrospinning a dope solution containing 5 wt% nylon-6/6 and 1 wt% chitosan polymers dissolved in formic acid. The polymer ratio used was similar to the optimized ratio from previous work used to obtain uniform nanofiber mat.38 The nylon-6,6/chitosan solution was stirred continuously overnight at 40⁰C until it becomes homogenous. A 5 mL volume of the prepared dope solution was electrospun at a rate of 0.2 mL/h by applying a voltage of 16 kV between the tip of the spinneret and the metal plate (collector). The tip to collector distance was maintained at approximately 150 mm. The electrospun nanofiber mat was then heat pressed at 90⁰C at an applied pressure of 100 kPa for 5 min to improve membrane compactness.


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Figure 1. Schematic of preparation of bi-layer PVDF/nylon-6,6/chitosan membrane Secondly, the PVDF dope solutions of varying concentrations of 12, 15, 18, 21 and 24 wt% (as given in Table 1) were prepared in DMAC as the base solvent along with 8 wt% PVP (pore forming agent) and allowed to stir overnight at 50⁰C until becoming homogenous. The dope solutions were cast onto the as-obtained electrospun nylon-6,6/chitosan nanofiber mat using MSK-AFA-III automatic thick film coater at a casting speed of 52 mm/s with 50 micron thickness set in the film applicator. After 10 seconds of evaporation time, the cast films was immersed into a coagulation tank of DI water at 25°C to obtain the nascent PVDF/nylon6,6/chitosan bi-layer composite membranes which were then transferred into another bath with flowing water to remove the residual DMAC solvent. Finally, the composite membranes were post treated by immersing into a mixture of 20 vol% ethanol, 40 vol% glycerol and 40 vol% DI water, after which the membranes were air dried at ambient temperature (22⁰C) before characterization.


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Table 1. Dope composition for PVDF/nylon-6,6/chitosan membranes Casting dope PVDF

Electrospinning dope PVP-K40

DMAC

Nylon-6,6

Chitosan

Sample (wt%)

(wt%)

(wt%)

12

8

80

15

8

77

18

8

74

21

8

71

24

8

68

(wt%)

(wt%)

5

1

PVDF/nylon6,6/chitosan

Similarly, the benchmark PVDF/nylon-6,6 membrane was prepared by obtaining a nanofiber mat via electrospinning a dope solution containing only 5 wt% nylon-6,6, followed by casting 18 wt% PVDF dope solution onto the nanofiber mat. The pure PVDF membrane was fabricated by casting 18 wt% PVDF dope solution. The dried membranes were stored in a desiccator at room temperature before use. Membrane characterization. The surface morphology of the composite membranes was observed using scanning electron microscopy (SEM) (ZEISS SUPRA 55VP, Germany) with an accelerating voltage of 5 kV and working distance of 10 mm. The membrane samples were sputter coated with a 5 nm layer of gold in high vacuum, using a Leica EM ACE600 prior to imaging using SEM. The average fibre diameters of the membranes were obtained from the surface SEM images using Image J software based on scans at six different spots of each sample. The cross section of the membranes was also analysed to demonstrate the integration of


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nanofiber and PVDF support layer and their resulting interfaces. The surface functional groups of the membrane samples were investigated by Fourier transform infrared (FTIR) spectroscopy with a single reflection attenuated total reflection (ATR) plate (FTIR Universal ATR-9, PerkinElmer, United States). The spectra were measured in the transmittance mode from the wavenumber range 4000–450 cm−1 integrated for 45 scans. The dynamic water contact angles (CAw) of the membrane samples were determined using an optical contact angle meter CAM101 (KSV instruments, Finland) to investigate the surface hydrophilicity. Each membrane sample was cut into thin strips and the two edges of the membrane were pasted by using sticky tape on to a glass slide for the contact angle measurement. A glass syringe filled with DI water was used to dispense about 4 µL droplet through a needle onto the membrane surface. Each measurement was recorded every 10 s over the duration of 120 s. The pore size and pore size distribution of the membranes were measured using a Porometer 3Gzh from Quantachrome. The membrane samples of 25 mm diameter each were completely wetted in the Porofil™ liquid before analysis and then placed in the sample holder. The pore size and pore size distribution curves of the membranes were obtained using the 3GWin Control software based on the Washburn and Hagen-Poiseuille equations.39 The pore size was measured three times for each membrane to obtain the average pore size and confirm reproducibility. The thickness of the control and composite membranes were measured from the SEM cross sectional images. The zeta potential of membranes were determined based on streaming potential measurements by SurPASS electro kinetic analyser (Anton Paar Corporation, Austria) with an adjustable gap cell. The membrane sample with dimension of 35 mm x 15 mm was placed in the clamping cell with two spacers. 500 mL of 1 mM aqueous sodium chloride solution was supplied to cell as the background electrolyte and 0.1 M HCl (acid) and 0.1 M


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NaOH (base) were used for titration. The zeta potential was measured at a pH range of 3-11. For each sample, the zeta potential measurement was repeated three times and the average was taken. The mechanical properties of the membranes were studied using an Instron universal materials testing machine (Model 3360) with a load cell of 50N and a loading velocity of 25 mm/min. Each membrane sample with dimension of 10 mm x 60 mm was tested three times to obtain average values of tensile stress and strain. Membrane performance tests. The clean water permeance (L.m-2.h-1.bar-1) was determined using a dead end filtration cell with an effective membrane area of 8.55x10-4 m2. The membrane was placed in the dead end cell and 50 mg/L NaCl solution was used as the feed. The cell was pressurized to 100 kPa using compressed nitrogen and the amount of permeate collected per minute was recorded at room temperature. The measurement was repeated three times using three different membranes prepared under the same conditions and was averaged to provide a mean permeance value. The membrane performance was evaluated in terms of contaminant rejection and fouling behaviour under 100 kPa of pressure. The membrane samples were tested individually in a cross flow filtration setup

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with an effective membrane area of 42x10-4 m2 and flow velocity of 12.6

cm/s. The feed solution was prepared by dissolving 1,000 mg/L BSA and 50 mg/L NaCl into DI water. The rejection of BSA in percentage (%), R was defined as, R = [(CF – CP) / CF] x 100 %

(1)

Where CF and CP are the concentrations of feed and permeate, respectively, in mg/L, which were measured by UV-Visible spectrophotometer at 280 nm wavelength.31 To evaluate the fouling behaviour of the membranes, the membrane permeance was measured over time for 5 h with


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regular cleaning after every filtration cycle, i.e., 1 h. The membranes were cleaned by soaking in DI water for 15 mins with mild stirring at 100 rpm in at room temperature. To study the fouling behaviour, the relative permeance in percentage after each filtration cycle was defined by, Relative Permeance (%) = Pt / P0 * 100

(2)

Where P0 is the initial permeance at time “0” and Pt is the permeance at time “t”, in L.m-2.h-1.bar1

. Membrane fouling can be classified into reversible and irreversible fouling depending on the

strength of attachment of contaminants on to the membrane surface. The reversible fouling on the membrane can be removed by means of physical cleaning whereas the irreversible fouling cannot be removed by physical cleaning. The reversible fouling (RF), irreversible fouling (IF) and total/specific fouling (TF) after 4 filtration and cleaning cycles were calculated by: IF = (P0 – Pn) / P0

(3)

TF = (P0 – Pe) / P0

(4)

RF = TF – IF

(5)

Where Pn is the initial permeance of 5th filtration cycle, and Pe is the permeance at the end of 4th filtration cycle.40 RESULTS and DISCUSSION The surface morphology of the as-prepared membranes with varying PVDF concentrations were examined using SEM and the images are shown in Figure 2. Figures 2a to 2e indicate the PVDF/nylon-6,6/chitosan composite membranes with five PVDF concentrations from 12 to 24 wt% prepared in DMAC solvent with 8 wt% of PVP; while Figure 2f presents the surface of the PVDF/nylon-6,6 benchmark membrane at 18 wt% PVDF concentration. The SEM images


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illustrate that, the nylon-6,6/chitosan nanofibers on the membrane surface at various PVDF concentrations are homogenously distributed and have the same average nanofiber diameter of 196 nm; while the PVDF/nylon-6,6 benchmark membrane has a nanofiber diameter of 149 nm. The nanofiber sizes were found to fall within the diameter range reported for nylon-6/chitosan nanofibers in literature.38 In Figure 2a, the composite membrane with 12 wt% PVDF shows that the nanofiber layer was infiltrated by the cast PVDF layer, which blocked the active surface of the nanofibers. As the PVDF concentration increased to 15 and 18 wt%, the infiltration of the cast PVDF was less dominant and only partially intrudes into the nanofiber layer, as observed in Figure 2b and 2c. In contrast, no PVDF infiltration could be observed from Figure 2d and 2e, the images of membranes with 21 and 24 wt% PVDF concentrations, leaving the nanofiber layer separately on the top surface. Overall, 18 wt% PVDF is considered as a good compromise in terms of layer infiltration.


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Figure 2. SEM images of PVDF/nylon-6,6/chitosan membrane surfaces with (a) 12 wt% PVDF, (b) 15 wt% PVDF, (c) 18 wt% PVDF, (d) 21 wt% PVDF, (e) 24 wt% PVDF and (f) PVDF/nylon-6,6 membrane surface with 18 wt% PVDF The integration of PVDF and nanofiber layers was further analysed through the crosssectional SEM images of the as-prepared PVDF/nylon-6,6/chitosan membranes as shown in Figure 3, in which the layer interface was identified. The extent of integration of cast and


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nanofiber layers was found to decrease with the increase in PVDF concentration from 12 to 24 wt% due to the increase in solution viscosity respectively. Accordingly, the interface between two layers became more distinct as the PVDF concentration increased. For example, at PVDF concentration above 18 wt%, the cast and nanofiber layers were completely separated leading to poor integration; while at PVDF lower than 18% has caused serious infiltration of PVDF dope solution into blocking the nanofiber mat (Figure 2). Thus, 18 wt% PVDF was chosen as the optimal composition for the support layer in the following investigations to form PVDF/nylon6,6/chitosan membranes. The PVDF/nylon-6,6 and pure PVDF membranes with 18 wt% PVDF concentration were selected as benchmarks. The average thickness of the PVDF/nylon6,6/chitosan membrane is increased to 430 µm compared to the pure PVDF and PVDF/nylon-6,6 membranes having 300 µm and 340 µm, respectively. This is due to the addition of nylon6,6/chitosan nanofibers into the PVDF layer to form PVDF/nylon-6,6/chitosan composite membrane. Here, the thickness of the composite membrane depends on the nanofiber mat which has an increased fiber diameter compared to PVDF membrane as discussed previously.


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Figure 3. Cross sectional SEM images of PVDF/nylon-6,6/chitosan membrane surfaces with (a) 12 wt% PVDF, (b) 15 wt% PVDF, (c) 18 wt% PVDF, (d) 21 wt% PVDF and (e) 24 wt% PVDF


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The nature of the functional groups on the membrane surface was studied using FTIR spectroscopy, as shown in Figure 4. The FTIR spectrum of the pure PVDF membrane (bottom curve in black) shows a short broad peak at 1170 cm-1 for the symmetrical stretching of –CF2 and at 870 cm-1 for the rocking vibration of –CH2, characterizing the substrate polymer. The spectrum of PVDF/nylon-6,6 membrane (middle curve in red) shows 2 sharp intense peaks at 1640 cm-1 and 1550 cm-1 for the C=O stretching vibration (Amide I) and CO-N-H bending (Amide II) vibrations, respectively, from the nylon-6,6 polymer. The PVDF/nylon-6,6/chitosan membrane (top curve in blue) has common peaks for nylon-66 with an additional characteristic broad peak at 1089 cm-1 for the –C-O-C stretching vibration, confirming the presence of chitosan. However, due to the low chitosan concentration of 1 wt%, the peak height of the –C-OC stretching peak was relatively small. For close comparison, the spectrum between the wavenumber ranging from 1140 cm-1 to 1000 cm-1 for both PVDF/nylon-6,6 and PVDF/nylon6,6/chitosan membranes have been magnified in the upper and lower right corners windows of Figure 4. It is clear that the characteristic –C-O-C stretching peak only appears on the PVDF/nylon-6,6/chitosan membrane surface. Thus, this analysis effectively confirmed the presence of surface functional hydroxyl and amine groups from nylon-6,6 and chitosan in the composite membrane.


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Figure 4. FTIR spectrum of PVDF, PVDF/nylon-6,6 and PVDF/nylon-6,6/chitosan membranes To evaluate the hydrophilicity of the as-prepared membranes, the dynamic water contact angles (CAw) were measured over 120 seconds and are given in Figure 5. It was found that the pure PVDF membrane showed the highest CAw, followed by the PVDF/nylon-66 and PVDF/nylon-6,6/chitosan membranes that exhibited the lowest CAw. Specifically, the initial CAw of the PVDF/nylon-6,6/chitosan membrane is 19º, which is much lower than that of the PVDF and PVDF/nylon-6,6 of 66º and 34º, respectively. This has indicated an improvement on the hydrophilicity by 70% and 40% compared to the PVDF and PVDF/nylon-6,6 membranes respectively. As shown in Figure 5, the dynamic CAw of the pure PVDF membrane remained relatively constant; while the curves of the PVDF/nylon-6,6 and PVDF/nylon-6,6/chitosan membranes decreased gradually over time. The CAw declining rate of the PVDF/nylon6,6/chitosan membrane was the highest compared to the benchmark membranes. This is


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attributed to the formation of stronger intermolecular hydrogen bond between intrinsic hydrophilic groups and water due to simultaneous presence of hydroxyl and amine functional groups (as confirmed in Figure 4) from the nylon-6,6/chitosan nanofibers on the surface. Similarly the PVDF/nylon-6,6 membrane showed higher declining rate than the pure PVDF due to the presence of amine functional groups from nylon-6,6 nanofibers. The surface hydrophilicity of the as-developed PVDF/nylon-6,6/chitosan membranes was found to be higher than the chitosan modified PVDF membranes reported in literature having an initial CAw of 61º.31 Also, it was found to be better than the PVDF membranes modified with amphiphilic copolymer containing polysiloxane with polyethylene oxide and polypropylene oxide side chains having an initial CAw of 48º.41 However, the declining rates of dynamic CAw differed for these membranes depending on the specific surface functionality.

Figure 5. Dynamic water contact angles (CAw) of the PVDF, PVDF/nylon-6.6 and PVDF/nylon-6,6/chitosan membranes with contact time for 120 s


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Figure 6 compares the differential pore distributions of the PVDF, PVDF/nylon-6,6 and PVDF/nylon-6,6/chitosan membranes in terms of pore diameters (Cumulative pore distributions for the membranes are given in Figure S1, Supporting Information). It can be seen that the PVDF/nylon-6,6/chitosan membrane has the narrowest distribution curve and smallest pore size, followed by the PVDF/nylon-6,6 membrane; while the pure PVDF has the widest distribution and overall larger pore size. The reduced pore size and narrower pore distribution of the PVDF/nylon-6,6 and PVDF/nylon-6,6/chitosan membranes is due to the pore integration of the nanofiber mat and the PVDF cast layer. This is consistent with the morphological observation via SEM (Figures 2 &3). The mean pore sizes of the PVDF/nylon-6,6/chitosan, PVDF/nylon-6,6 and pure PVDF membranes were 25 nm, 30 nm and 56 nm respectively.

Figure 6. Differential pore number (in %) distributions of PVDF, PVDF/nylon-6,6 and PVDF/nylon-6,6/chitosan membranes


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The zeta potential of the membranes over a pH range of 3 to 11 was measured to indicate the surface charge of the as-prepared membranes. The results are given in Figure 7. Overall, the zeta potential of PVDF/nylon-6,6/chitosan membrane surface is least negative than that of the pure PVDF and PVDF/nylon-6,6 membranes, owing to the increase in the density of amine functional groups from nylon-6,6/chitosan nanofibers. The isoelectric point (IEP) of the PVDF/nylon-6,6/chitosan membrane was detected at pH 5.3, where there are equal number of oppositely charged groups resulting in neutral charge surface exhibiting zwitterionic characteristics and the ability to resist membrane-protein interactions.42-43 Although the PVDF/nylon-6,6/chitosan membrane was slightly negatively charged (-18 mV) at the operating pH of 6.7 (pH of BSA solution) of current filtration testing, it has greater potential for protein fouling resistance, as compared to the benchmark membranes.


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Figure 7. Zeta potential of PVDF, PVDF/nylon-6,6 and PVDF/nylon-6,6/chitosan membranes against pH The mechanical properties of the membranes indicate material strength and flexibility that are essential for a wide range of applications. The PVDF, PVDF/nylon-6,6 and PVDF/nylon-6,6/chitosan membranes were evaluated in terms of tensile stress, which is presented as a function of the strain at break (%) in Figure 8. The maximum tensile strength for PVDF, PVDF/nylon-6,6 and PVDF/nylon-6,6/chitosan was found to be 1, 2.5 and 2.6 MPa respectively. It is clear that compared to the pure PVDF membrane, the addition of nylon-6,6 and chitosan nanofibers has doubled the tensile stress of the composite membranes due to layer integration as indicated by Figure 3. Also, the higher strain at break (in %) above 39% indicates that the composite membranes are less brittle, compared to the pure PVDF (35.7%). This improvement can be attributed to the stable integration of the nanofiber mat and PVDF cast layer.


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Figure 8. Stress Vs Strain curves for PVDF, PVDF/nylon-6,6 and PVDF/nylon-6,6/chitosan membranes The as-prepared membranes were tested using a dead end filtration cell at a constant pressure of 100 kPa. Figure 9a shows the comparison of pure water permeance for PVDF, PVDF/nylon-6,6 and PVDF/nylon-6,6/chitosan membranes. The overall water permeance of the PVDF/nylon-6,6/chitosan membrane was the highest (542 L.m-2.h-1.bar-1), followed by the PVDF/nylon-6,6 membrane (498 L.m-2.h-1.bar-1) and pure PVDF exhibited the lowest permeance (493 L.m-2.h-1.bar-1). The permeance enhancement is mainly because of the addition of nylon-6,6 and chitosan nanofibers leading to improved hydrophilicity through the hydroxyl and amine functional groups. These functional groups may attract water towards the membrane surface by inter molecular hydrogen bonding which leads to an increase in permeance, as reported previously.8,

44

Interestingly, the permeance of the as-prepared PVDF/nylon-6,6/chitosan


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membrane was higher than the previously reported data in literature. For example, it was 3-fold higher than the zwitterionically modified PVDF membrane with a permeance of 160 L.m-2.h1

.bar-1; and 4-fold higher than the amphiphilic polyacryloylmorpholine (PACMO) grafted PVDF

membrane with a permeance of 130 L.m-2.h-1.bar-1.43, 45 The BSA is a serum albumin protein having a molecular weight of 66 kDa and an isoelectric point at pH 4.7. It was commonly used as model protein foulant in UF studies as it causes most rapid flux decline.46 Hence, in this work the water permeation and rejection properties of the fabricated membranes were evaluated by filtration experiments with 1,000 mg/L BSA solution in a cross flow filtration set-up. The results are shown Figure 9b. It is observed that the permeance increases due to the incorporation of nanofibers. The PVDF/nylon6,6/chitosan membrane showed the highest permeance of 393 L.m-2.h-1.bar-1, which is about 37% higher than the pure PVDF and 17% higher than the PVDF/nylon-6,6 membrane. The significant flux improvement over the benchmark membranes is associated with the enhanced surface hydrophilicity (Figure 5). A similar increasing trend was observed for the BSA rejection rates from 66.8% for pure PVDF, 88.9% for PVDF/nylon-6,6 to 92.8% for the

PVDF/nylon-

6,6/chitosan membranes, corresponding to the decreasing order of the pore size of the three membranes (Figure 6) owing to good layer integration (Figure 3). It is worth noting that the BSA rejection of the PVDF/nylon-6,6/chitosan membranes is similar to previously reported surface modified hydrophilic PVDF membranes such as amphiphilic PACMO grafted PVDF membrane with 93% BSA rejection and zwitterionically modified PVDF membrane with 90% BSA rejection, while offering at least 3.5 times higher water permeance than the PACMO grafted (130 L.m-2.h-1.bar-1) and zwitterionically modified membranes (160 L.m-2.h-1.bar-1) respectively.43, 45


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Figure 9. Filtration performance of the PVDF, PVDF/nylon-6,6 and PVDF/nylon-6,6/chitosan membranes at 100 kPa pressure (a) Pure water permeance in dead end cell with 50 mg/L NaCl solution; (b) Water permeance and BSA rejection with 1,000 mg/L BSA solution in cross-flow filtration at 100 kPa pressure The surface-protein interaction of the as-prepared membranes was investigated in terms of permeance recovery and reversible and irreversible fouling throughout 5 filtration cycles. Figure 10a presents the permeance recovery of each membrane as relative permeance Pt/P0 (Equation 2) for every filtration cycle of 1 h over the course of 5-h operation. In general, the relative permeance for all membranes gradually decreased over time. However, compared to the benchmark PVDF and PVDF/nylon-6,6 membranes, PVDF/nylon-6,6/chitosan membrane still exhibited the highest relative permeance after each cleaning interval, indicating stronger ability to restore membrane performance in the presence of organic foulants. In particular, in the last filtration cycle (beginning of the 5th hour), the PVDF/nylon-6,6/chitosan membrane still maintained a high relative permeance of 92.9%, which is about 25% and 10% higher than the PVDF and PVDF/nylon-6,6 membranes, respectively. Based on the results shown in Figure 10a, specific fouling of the as-prepared membranes were calculated based on Equations 3-5. The


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results are presented in Figure 10b in terms of reversible fouling (RF, Equation 5, blue column) and irreversible fouling (IF, Equation 3, orange column) of BSA protein on to membrane surface after 4 filtration cycles. It was observed that the RF decreases in the order of PVDF, PVDF/nylon-6,6 and PVDF/nylon-6,6 membranes; while the IF increases in the same order, i.e., the PVDF/nylon-6,6/chitosan membrane has the highest RF of 0.054 and lowest IF of 0.071. In particular, the RF for the PVDF/nylon-6,6/chitosan membrane was enhanced by 2.1 and 1.7-fold compared to PVDF and PVDF/nylon-6,6 membranes respectively, indicating the weaker attachment of protein foulants on to the membrane surface and hence easy restoration of membrane performance by simple cleaning; while for the original PVDF membrane, high IF has led to a greater degree of permanent loss of the membrane performance. The acquired results highlight that the presence of intrinsic hydrophilic functional groups in the composite membrane can increase the intermolecular hydrogen bonding of membrane surface with the water molecules but will effectively weaken the hydrophobic interaction between BSA protein and the membrane surface. This is beneficial for preventing foulant build-up on the surface. Similarly, previous studies also reported that the increase in surface hydrophilicity of the membrane would lead to enhanced flux recovery with less fouling.30-31 The membrane-protein interactions are considered to play a key role in membrane fouling when using real solutions that have more complex mixtures of various components such as proteins, polysaccharides and humic substances. Although the current study focused on protein rich synthetic solution, future work studying the novel membrane’s ability to perform in such complex real water matrices for specific applications could be performed.


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Figure 10. Comparison of protein-fouling behaviour for PVDF, PVDF/nylon-6,6 and PVDF/nylon-6,6/chitosan membranes in cross flow filtration (a) relative permeance (permeance recovery) of each filtration cycle (b) Specific fouling - reversible fouling (RF) and irreversible fouling (IF) of BSA (feed: 1,000 mg/L BSA solution; flow velocity: 12.6 cm/s; 1 h for each filtration cycle with a 15 min cleaning interval) CONCLUSIONS This study developed a series of nanofiber based PVDF/nylon-6,6/chitosan UF membranes with intrinsic Janus surface via a 2-step electrospinning and conventional casting process. The SEM morphological analysis of PVDF/nylon-6,6/chitosan membranes demonstrated the wellintegrated layer structure between the hydrophilic nylon-6,6/chitosan nanofiber mat and hydrophobic PVDF substrate layer, eliminating the material compatibility issue. Hence the dope composition of the PVDF substrate was optimized at 18 wt%. Confirmed by the FTIR data, the presence of hydroxyl and amine functional groups on the membrane surface was contributed by nylon and chitosan nanofibers, offering high hydrophilicity to the PVDF/nylon-6,6/chitosan membranes leading to enhanced intermolecular hydrogen bonding between water and membrane


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surface. Subsequently, the water permeance of the new Janus membranes was found to be higher than the benchmark membranes. Also, the integration of nanofiber mat and the PVDF cast layers resulted in reduced pore size that greatly improved the BSA rejection. Through a dedicated filtration-cleaning study, the new Janus membrane was found to effectively reverse protein attachment onto the surface with a reversible fouling increased by 2.1-fold than the pure PVDF membrane. Although PVDF/Nylon-6,6/Chitosan membrane showed minor improvements in terms of tensile strength and BSA rejection compared to the PVDF/Nylon-6,6 membrane, they offered 17% higher permeance of BSA solution with 1.7-fold increase in reversible fouling indicating its superior properties for waste water treatment. Therefore, the as-developed PVDF/nylon-6,6/chitosan composite membrane has greater potential in treating effluents with hydrophobic protein foulants assisted with minimal or facile post-cleaning. ASSOCIATED CONTENT Supporting Information. Differential and Cumulative pore number percent of PVDF, PVDF/nylon-6,6 and PVDF/nylon6,6/chitosan membranes measured using the porometer (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Ms. Anbharasi Vanangamudi, E-mail: [email protected], Tel.: +61 399 197 640. *Dr. Xing Yang, E-mail: [email protected], Tel.: +61 399 197 690. Author Contributions


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The experimental work was designed and carried out by Ms. Anbharasi Vanangamudi under the guidance of Dr. Ludovic Dumee, Dr. Xing Yang and Prof. Mikel Duke. The manuscript was written and reviewed by all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by Victoria India Institute via Victoria India Doctoral Scholarship. Dr. L. DUMEE acknowledges Deakin University for his Alfred Deakin Post-Doctoral Fellowship. Dr Xing Yang would like to acknowledge Victoria University for the Industry Postdoctoral Fellowship. REFERENCES (1) Prince, J. A.; Bhuvana, S.; Boodhoo, K. V. K.; Anbharasi, V.; Singh, G. Synthesis and Characterization of PEG-Ag Immobilized PES Hollow Fiber Ultrafiltration Membranes with Long Lasting Antifouling Properties. J. Membr. Sci 2014, 454, 538-548. (2) El-Gazzar, F. E.; Marth, E. H. Ultrafiltration and Reverse Osmosis in Dairy Technology: A Review. J. Food Prot. 1991, 54 (10), 801-809. (3) Shi, X.; Tal, G.; Hankins, N. P.; Gitis, V. Fouling and Cleaning of Ultrafiltration Membranes: A review. J. Water Process Eng. 2014, 1, 121-138. (4) Krause, B.; Storr, M.; Ertl, T.; Buck, R.; Hildwein, H.; Deppisch, R.; Göhl, H. Polymeric Membranes for Medical Applications. Chem. Ing. Tech. 2003, 75 (11), 1725-1732. (5) Daraei, P.; Madaeni, S. S.; Ghaemi, N.; Khadivi, M. A.; Astinchap, B.; Moradian, R. Enhancing Antifouling Capability of PES Membrane via Mixing with Various Types of Polymer Modified Multi-walled Carbon Nanotube. J. Membr. Sci 2013, 444, 184-191. (6) Zhang, J.; Xu, Z.; Shan, M.; Zhou, B.; Li, Y.; Li, B.; Niu, J.; Qian, X. Synergetic Effects of Oxidized Carbon Nanotubes and Graphene Oxide on Fouling Control and Anti-Fouling Mechanism of Polyvinylidene Fluoride Ultrafiltration Membranes. J. Membr. Sci. 2013, 448, 8192. (7) Rana, D.; Matsuura, T. Surface Modifications for Antifouling Membranes. Chem. Rev. 2010, 110 (4), 2448-2471. (8) Prince, J. A.; Bhuvana, S.; Anbharasi, V.; Ayyanar, N.; Boodhoo, K. V. K.; Singh, G. Self-Cleaning Metal Organic Framework (MOF) based Ultra Filtration Membranes - A Solution


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(43) Zhao, X.; He, C. Efficient Preparation of Super Antifouling PVDF Ultrafiltration Membrane with One Step Fabricated Zwitterionic Surface. ACS Appl. Mater. Interfaces 2015, 7 (32), 17947-17953. (44) Martín, A.; Arsuaga, J. M.; Roldán, N.; de Abajo, J.; Martínez, A.; Sotto, A. Enhanced Ultrafiltration PES Membranes Doped with Mesostructured Functionalized Silica Particles. Desalination 2015, 357, 16-25. (45) Liu, J.; Shen, X.; Zhao, Y.; Chen, L. Acryloylmorpholine-Grafted PVDF Membrane with Improved Protein Fouling Resistance. Indus. Eng. Chem. Res. 2013, 52 (51), 18392-18400. (46) Miao, R.; Wang, L.; Feng, L.; Liu, Z.-W.; Lv, Y.-T. Understanding PVDF Ultrafiltration Membrane Fouling Behaviour Through Model Solutions and Secondary Wastewater Effluent. Desal. Water Treatment 2014, 52 (25-27), 5061-5067.

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