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Crystallization induced phase separation: a unique tool to design microfiltration membranes with high flux and sustainable antibacterial surface Maya Sharma, Sanjay Remanan, Giridhar Madras, and Suryasarathi Bose Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04064 • Publication Date (Web): 08 Feb 2017 Downloaded from http://pubs.acs.org on February 15, 2017

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Industrial & Engineering Chemistry Research

Crystallization induced phase separation: a unique tool to design microfiltration membranes with high flux and sustainable antibacterial surface

Maya Sharma1, Sanjay Remanan2, Giridhar Madras3 and Suryasarathi Bose2* 1

Center for Nano Science and Engineering,

2

Department of Materials Engineering,

3

Department of Chemical Engineering,

Indian Institute of Science, Bangalore-560012, India.

*

Corresponding author: S. Bose. Phone: 91-80-2293 3407; Email: [email protected]

ACS Paragon Plus Environment


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Abstract In the present study, a strategy has been used to fabricate microfiltration membranes through phase separation induced by crystallization in PVDF/PMMA (polyvinylidene fluoride/polymethyl methacrylate) blends. Nanoporous channels in the membranes were designed by selective etching of PMMA and tuned by varying the PMMA concentration in the blend. The interconnectivity of the designed membranes was tuned by changing the concentration of PMMA in the blend. Scanning electron microscopy (SEM) studies showed that the spherulites appeared more compact in the 90/10 blend as compared to 70/30 and 60/40 PVDF/PMMA blends. The obtained flux was higher compared to membranes that are commercial available. Biofouling of membranes is a major concern and in order to address this concern, silver was sputtered on the token membranes and leaching of Ag+ was monitored using inductively coupled plasma atomic emission spectroscopy (ICP). This strategy is ‘scalable’ and is an industrially viable route to design antibiofouling token membranes in large scale. Keywords: PVDF nanoporous membranes; high water flux; anti-biofouling surfaces.


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Introduction PVDF/PMMA blend has an upper critical solution temperature that forms a miscible phase at high temperatures but phase separates upon cooling, either by liquid-liquid phase separation (LLPS) or by solid-liquid separation through crystallization of PVDF1-3. There is always a competition between LLPS and crystalline-amorphous melt phase transition during cooling. The morphology follows a droplet-matrix system when LLPS dominates over crystallization. However, the morphology appears like stacks of interconnecting spherical particles when crystallization is the dominant factor4. The blending of PVDF with PMMA (≤ 40% concentration) leads to phase separation driven by fast crystallization of PVDF. During crystallization, the interlamellar and interspherulitic regions of PVDF are occupied by PMMA and a boundary between the crystalline region and liquid-like amorphous region is clearly observed. Many studies have reported the formation and origin of interphase using small angle X-ray scattering (SAXS) and broadband dielectric spectroscopy5,6. This clearly indicates that the PMMA concentration in the PVDF/PMMA blend strongly influences the structural properties in the blend. The control of the crystallization kinetics and spherulite size of PVDF is imperative in controlling the overall crystalline morphology. Hence, by selective etching of the amorphous PMMA from the blends, porous structures can be derived and can be further explored for separation technologies. One such aspect is the focus of this study. An ‘industrially scalable’ technology of fabricating membranes is in great demand and the existing technologies often are cumbersome processes or involve longer time scales in designing suitable membranes. Recently, different methods have been used to impregnate Ag on porous polymeric membranes for antibacterial properties and to prevent biofouling7, 8,9,

10

. Zhang et al.11

prepared polyethersulfone (PES) based ultrafiltration membranes with Ag NPs by phase inversion and obtained more than 99.8% inactivation of bacteria. In other study, Ag



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nanoparticles were incorporated onto polysulfone membranes by both ex situ synthesis and in situ reduction of silver ions12. Park et al13., described the AgNP-poly(vinylidene fluoride) (Ag-PVDF) membrane nanocomposites that are anti-biofouling by the incorporation of Ag NPs onto thiol modified polyethylene glycol (PEG) on PVDF membrane. Polyethylenimine coated silver nanoparticles were electrostatically assembled over negatively charged plasma treated polysulfone membrane14. In another work15, hybrid membranes were prepared by blending the Ag and halloysite nanotubes into the PES membrane matrix for improved membrane antifouling performance. In this work, membranes were derived using phase separation in PVDF/PMMA blends and the porous channels were tailored by etching out the amorphous phase (here PMMA) and tuned by changing the composition of the latter in the blends. The influence of different antibacterial agents like CNTs, Ag/TiO2 nanoparticles has been reported in our previous study16 and herein we have incorporated Ag to render the membranes antibacterial and address the concerns related to the biofouling of the membranes. The latter is a serious concern and deteriorates the membrane performance significantly. The leaching of silver ions was monitored using ICP. The pure water flux was evaluated and to our surprise, the flux was different when the active surface was reversed. This motivated us to study these membranes in greater detail with respect to spherulitic distribution on both the sides of the token membranes. This along with variation in rheology of the phases led to differential distribution of the pores. The thermal properties of the blends were analysed by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The surface and cross sectional morphology was established through scanning electron microscopy (SEM).

Experimental


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Materials and methods Both PVDF with Mw 440 000 g/mol (Kynar 761) and PMMA with Mw 95 000 g/mol (Atuglas v825) were procured from Arkema. Glacial acetic acid and hexane was obtained from commercial sources. Preparation of blends PVDF/PMMA blends at different compositions ranging from 90/10 to 50/50 (wt/wt) were prepared by melt extrusion using a twin screw extruder (Polylab, Thermo Haake Minilab II) at 220°C and 60 rpm for 20 min under an inert N2 atmosphere. A detailed experimental setup has been reported elsewhere16. Prior to mixing, samples were pre-dried under vacuum for 12 h to remove the moisture. Preparation of membranes The melt blended samples were compression molded at 220°C and several breathing steps were followed while pressing to avoid formation of air bubbles in the film. The obtained films (c.a. 150 µm) were then immersed in glacial acetic acid for seven days to remove the PMMA phase from the blends17. Modification of membranes: silver sputtering 70/30 PVDF/PMMA etched membranes were silver sputtered using Anelva RF sputtering (SPF-332H). The conditions used for sputtering are, deposition pressure: 2 x 10-5 mbar, Deposition time: 3 minutes, Substrate voltage: 1.5 kV, Substrate to target distance: 7 cm. The thickness was estimated to be 50 nm in 3 min and the morphology of these membranes was characterized by scanning electron microscopy. It is important to note that the thickness was estimated to be 50 nm for 3 min sputtering on a silicon wafer. Keeping all the parameters constant, silicon substrate was replaced with a porous membrane. As the RF is independent on substrate properties, we expect similar coating thickness on polymeric membranes. We



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made an attempt to measure the thickness of the sputtered silver on the polymer membrane but as the roughness of etched PVDF membranes was very high, it is very difficult to find the exact Ag thickness using AFM or ellipsometer. We made an attempt to measure the thickness using the profilometer or cross section SEM and then estimated the rate of deposition for one set of conditions: constant Ar pressure, constant target power or constant substrate to target distance. We then tried to vary the time of deposition and continued for at least five depositions to be more accurate. We then plotted thickness versus the time of deposition. The thickness was estimated to be 50 nm in 3 min on a silicon wafer. Keeping all the parameters constant, silicon substrate is replaced with porous membranes. As the RF is independent on substrate properties, we expect similar coating thickness on polymeric membranes. Ag+ leaching experiments Inductively coupled plasma-optical emission spectroscopy (ICP-OES, Thermo-iCAP 6000) was used to quantify the depletion of Ag+ ions. Leaching of the silver from the sputtered membrane surface was studied as a function of time. The membranes were incubated at 37 oC for 1, 7 and 14 days in 10 ml of DI water in an orbital shaker at 100 rpm. For statistical measurement, we used three replicates for each sample. The solution containing silver ions was used for Ag+ ions quantification. Using a set of single-element external calibration standards, the final concentration of Ag+ ions was evaluated in the range of 0.01-100 ppm. Antibacterial efficiency of silver sputtered membranes Escherichia coli (Gram-negative bacteria, strain 25922 ATCC) was harvested in the log phase by growing in 100 ml of sterile Luria Broth (LB) for 6 h at 37 °C. E. coli concentration of 107 CFU/ml was achieved by re-suspending bacteria culture in phosphate buffer saline (PBS) solution. 1ml of the prepared E. coli culture was added to the unmodified


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and sputtered 70/30 membranes and kept for incubation at 37 oC for 1 h. The unmodified membranes were taken as control samples. For statistical measurement, we used three replicates for each sample. To investigate the attached bacterial cells (biofouling) on membranes, the membrane surfaces was gently rinsed with PBS solution and stained with the LIVE/DEAD® BacLight™ Bacterial Viability kit (Molecular Probes, Invitrogen)18. Adherent cells were stained using 3.3 mM SYTO 9 and 20 mM propidium iodide for 20-30 min and further imaged with an inverted fluorescence microscope (Leica) in both the green and red channels. The viability of bacterial cells was estimated by counting cells which are stained green and red from three independent replicates. Further to study the membranes under scanning electron microscope, membranes were fixed with 3.7% formaldehyde solution for 20 min. To remove the excess formaldehyde from membrane surfaces, membranes were gently rinsed with PBS solution. Prior to SEM imaging, samples were sputtered with gold while the membranes were dried using serial ethanol dehydration. To understand the outcome of microbial biofouling on the flux of the permeate, E. coli suspension solution was passed through the membranes in a flux setup7. The prepared E. coli suspension solution (ca. 105 CFU/mL) was the feed solution that was passed through the setup for continuously for 1 h. After completion of test, the membranes were removed and gently sonicated in DI water for 2 min. For SEM imaging, the membranes were subsequently fixed with 3.7% formaldehyde solution for 20 min. To remove the excess formaldehyde from membrane surfaces, membranes were gently rinsed with PBS solution. To examine the presence of E. coli in permeate, the permeate water was collected in sterilized sealed tube. This was cultured in nutrient agar, incubated at 37 °C for 18 h and the viable E. coli colonies were counted by the standard plate count.



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Characterization of blends Scanning electron microscope (SEM ULTRA 55, FESEM (Carl Zeiss)) was used to establish the morphology of the blends. Thermogravimetric analysis (TGA) was carried out using NETZSCH instrument at the heating rate of 10 °C min−1 in N2 atmosphere. Thermal analyses were performed using differential scanning calorimeter (DSC-Q200, TA Instrument). All the samples were scanned from −50 oC to 220 °C with a temperature ramp of 10 °C min−1. The samples were subjected to a heat−cool−heat temperature cycles in order to erase any changes in thermal history due to processing. The contact angles of the membranes were evaluated using water drop method (OCA, 15EC, Data physics). DI water (1 µl) was used for the testing using micro syringe by sessile drop approach. Triplicates were done for each membrane. The contact angles were determined by the software. The surface topography and roughness of the nanoporous membranes was investigated using AFM (Dimension Icon ScanAsyst, Bruker). Transmembrane flux Scheme 1 illustrates the cross-flow setup to determine pure water flux16,

19

. The

porous membrane was fixed in the test cell and water was passed through the membrane at different pressures with a booster pump. The retentate was discarded while permeate was continuously collected and used for flux calculations. All the measurements were performed at ambient conditions. Prior to measurements, all membranes were stabilized for 1 h at 30 psi. In all cases, the experiments were repeated thrice and the average of three samples was measured. The transmembrane flux (Jw) was obtained, = ⁄ (1) In eq. (1), V is the volume of collected permeate, A is the effective area of the membrane under measurement and t is the time taken to fill the volume (V).


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Scheme 1: Schematic of in-house cross flow set up to measure pure water flux. Dynamic antifouling performance of membranes was evaluated using protein adsorption (Bovine Serum Albumin, BSA) tests. For the dynamic adsorption test, after the membrane stabilisation, pure water permeability J0 was measured at 20 psi. Afterwards, the prepared BSA solution (0.3 g L-1) was circulated through the setup for 2 h and the final permeation flux was recorded as Jp. After completion of test, the membranes were taken out and gently sonicated in PBS buffer for 30 min followed by rinsing with deionized water. Then, the pure water flux of the cleaned membranes JR was again measured. Flux recovery ratio (FRR) is calculated as follows15: FRR = (JR/ J0) X 100%



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Results and Discussions Selective etching of PMMA from PVDF/PMMA blend: assessed using TGA and DSC Scheme 2 depicts the method of preparation of porous PVDF/PMMA membranes.

Scheme 2: Cartoon illustrating the mechanism of membrane formation of porous PVDF membranes. Thermogravimetric analysis was carried out before and after etching PMMA to ensure the complete removal of PMMA from the blends. The TGA profile of the blends before etching shows two-step degradation. PMMA starts to degrade around 320 oC and this step ends at 400 o

C. The degradation of PVDF starts at 420 oC and ends at 480 oC, as shown in Figure 1.

However, etched out samples show degradation in a single step corresponding to the PVDF phase. Thus, the TGA profile of the blends after etching out PMMA is similar to that of neat PVDF indicating the complete removal of PMMA phase from the blends.


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100 (a)

weight loss %

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Before etching 70/30 50/50 90/10 PVDF

0

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After etching 70/30 50/50 90/10 PVDF

0

100

Temperature (oC)

Figure 1: TGA profile of blends before and after etching of PMMA from PVDF/PMMA blends, (a) before etching, (b) after etching. Further confirmation of PMMA removal can be obtained from DSC measurements, as shown in supplementary information (See Figure S1). In DSC thermographs of 50/50 blends, before etching, two glass transition (Tg) temperatures corresponding to PVDF and PMMA were obtained whereas a single Tg was observed after etching. This clearly shows that PMMA phase was etched out from the blends. This was confirmed in all the blend compositions.



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Nanoporous channels derived using crystallization induced phase separation The obtained membranes were fractured in liquid nitrogen to assess the cross-section and surface morphologies using FESEM. Figure 2 shows the etched morphologies (top, bottom and cross sectional) of token 50/50 PVDF/PMMA blend. It is clearly visible that 50/50 PVDF/PMMA blend is amorphous as the concentration of PMMA is ≤ 50 wt%3. After etching out the PMMA phase from the 50/50 blends, the membranes became highly porous. Cryofractured cross-section micrographs, as shown in the Figure 2c, indicates the uniform distribution of pores throughout the membranes. The surface and cross-sectional morphologies of these membranes are completely different from morphologies derived from liquid-liquid demixing (or immersion precipitation) method20. Generally, in liquid-liquid demixing the cross-section of the membrane can be divided in three distinct regions. First, a thin dense layer (‘skin’); second, a region composed of parallel columnar macrovoids that extend upto the center of the membrane and third, a cellular morphology, in which closed pores are enveloped in a polymer mat20.

These characteristics are absent in the token

membranes of 50/50 PVDF/PMMA blends. In another study, PVDF-HFP porous membranes were designed using gelation induced PVDF−HFP nanocrystal particle formation; and subsequent leaching of PMMA followed by further crystallization of PVDF−HFP4. As PVDF exhibits higher crystallinity than PVDF-HFP, the SEM morphologies are quite different from etched PVDF-HFP membranes. SEM micrographs of 60/40 PVDF/PMMA blends are shown in Figure 3. PVDF spherulites are clearly visible in these micrographs and more interestingly, inter and intraspherulitic pores are evident in the cross-sectional morphology. As 60/40 PVDF/PMMA blends are at the verge of crystallinity, spherulites are coarser due to the exclusion of PMMA from the lamellae bundles. Fibrillar morphology was obtained in these samples. The pore size progressively decreases as the concentration of PMMA decreases indicating that these pores


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can be attributed to the amorphous pockets of PMMA. These coarser spherulites often result in diffuse Maltese cross pattern that is ascribed to the randomness of the orientation of lamellar bundles21. The top and bottom surface of the token membranes is similar to 60/40 blends. Interestingly, we observed that the pore density increases towards the outer edge of the spherulites. Okabe et al.22 reported that the amount of excluded PMMA increases as the distance from the spherulite center increases. Interfibrillar region is clearly visible from Figure 3d. From cross-sectional morphologies, we can conclude that pores are asymmetrical (or more in form of channels) in these blends and the average pore size is in submicron range. Similar to 60/40 blends, 70/30 PVDF/PMMA blends also showed spherulitic morphology as shown in Figure 4. Spherulite size increases with increasing PVDF concentration as obtained in our earlier studies16. Top and bottom surface morphologies are similar with average sizes of 25-50 nm pores (not shown here). Interestingly, pores appear smaller on the surface than in the cross section. Similar to 60/40 blends, nanochannels are also observed in 70/30 blends. As expected, average pore size in 70/30 is less than 60/40 PVDF/PMMA blends. Porous morphologies for etched 90/10 PVDF/PMMA blends are shown in Figure 5. Spherulites appeared more compact in 90/10 as compared to 70/30 and 60/40 PVDF/PMMA blends. As spherulites are more compact in 90/10 blends, the average pore size is minimum in these set of blends. In all the blends, except 50/50, interphase regions are clearly marked between lamellar bundles (more clearer in 90/10 blends), suggesting that the lamellar bundles are separated by the amorphous PVDF/PMMA blends that is excluded from the spherulites (interspherulitic regions). This indicates that as the concentration of PMMA decreases from 60/40 to 90/10 blends, spherulites become compact and grows as the regularity of the lamellar bundles decreases22. The pore sizes and morphologies of different membranes have a direct effect on water flux, as described in later sections.



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Figure 2: Scanning electron microscopy of 50/50 PVDF/PMMA membrane with etched morphology (a) Top surface, (b) Bottom surface, (c) Cross sectional morphology.


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Figure 3: Scanning electron microscopy of 60/40 PVDF/PMMA membrane with etched morphology (a) Top surface, (b) Bottom surface, (c) Cross sectional morphology, (d) enlarged section of (c).



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Figure 4: Scanning electron microscopy of 70/30 PVDF/PMMA membrane with etched morphology (a) Top surface, (b) Bottom surface, (c) Cross sectional morphology (d) enlarged section of (c).


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Figure 5: Scanning electron microscopy of 90/10 PVDF/PMMA membrane with etched morphology (a) Top surface, (b) Bottom surface, (c) & (d) Cross sectional morphology, magnified portion of cross-section respectively. The three-dimensional AFM height images of the different nanoporous membranes (60/40, 70/30 and 90/10) show similar results. For instance, the 60/40 blend, the surface appears rough with small, circular spherulite morphology (in accordance with SEM, see supplementary information S2). 3D height image reveals clear lamellar structures of PVDF with distinct spherulitic boundaries. Interestingly, as the PMMA content increases in PVDF/PMMA blend, the surface roughness has substantially amplified (see Figure S2). The observed roughness of 90/10 blend is lower compared for 70/30 and 60/40 membranes,



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respectively. The variation of the static water contact angle of the etched and unetched token PVDF/PMMA blends is also illustrated in supplementary information (See Figure S3). The unetched membranes exhibited similar water contact angles of 75°± 2°, which is higher than that of etched membranes. The maximum difference in contact angle before and after etching is observed in 60/40 PVDF/PMMA blends. These results are in harmony with AFM results where 60/40 nanoporous membranes showed maximum surface roughness (148 nm). As the roughness of the membrane increases, the hydrophilicity of the membranes also increases and thereby decreases the water contact angle. Nunes et al.23 observed similar results for PVDF/PMMA phase inversion membranes in which membrane hydrophilicity increases with the PMMA concentration. Water permeability and compaction of membranes Transmembrane flux of the membranes was determined using vertical cross flow setup shown in Scheme 1. A 40 mm disc shaped token membrane was placed in the test cell and water was pressurised through the system from 10 to 30 psi. All membranes were stabilised for 1 h at 30 psi prior to measurements. Mural et al.24 reported a direct relationship between applied pressure and permeate flux in PE/PEO blend membranes. Similar results were obtained in this study where permeate flux of PVDF nanoporous membranes increases non-linearly with applied transmembrane pressure (as shown in later sections) indicating the presence of asymmetric pores.


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2500 PVDF/PMMA etched membrane Surface-1 flux @ 30 psi 2000

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-1

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0

Membranes

Figure 6: Transmembrane Flux of all membranes at 30 psi.

The PVDF membranes blended with 50 wt% PMMA has higher permeate flux (J) than PVDF membranes blended with 10 wt% PMMA. The flux decreases with decrease in PMMA concentration. 50/50 token membranes have highly non-uniform surface as well as high porosity (due to the removal of 50% PMMA phase). Thus these are not suitable for evaluating pure water flux under the current operating conditions. The reason why the permeate fluxes of 90/10 membranes are lower than those of 70/30 and 60/40 membranes (as shown in Figure 6) is because of the pore size and its distribution. PVDF membranes blended with 40 wt% PMMA, showed higher permeate flux due to better connectivity of pores. The water permeability depends on the pore size distribution over the membrane. The lower pore size will lead to lower permeability. Yan et al.25 also found that increasing driving force across the membrane increases the water permeability through the membrane. This can be a characteristic of asymmetric nature of membranes. Finally, membranes such as 60/40 and 70/30 compositions can be considered as industrially viable membranes. Further, to observe



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the effect of compositional difference on both sides, flux was evaluated by reversing the sides, as illustrated in Figure S4. There is a small difference in transmembrane flux on both sides. This difference in flux is possibly due to the difference in composition and the crystalline morphology, as explained by concentration gradient formed in membranes. Presumably, during compression molding, the less viscous PMMA phase spreads more, while the higher viscous PVDF spreads less during molding. This leads to a gradient in the morphology of the sample and the concentration of PVDF is different on both sides as confirmed by XPS (see Figure S5). This leads to different surface concentrations, morphology and flux differences in the PVDF/PMMA blends.

Antifouling Performance

To investigate the FRR and antifouling performance of membranes, BSA solution was filtered through the 70/30 PVDF/PMMA blends as shown in Figure 7. The first zone is the pure water flux with time and the second zone is the flux obtained by passing BSA solution through the membrane. Figure 7 indicates that, during the BSA solution experiment, the membrane flux decreased drastically. Flux recovered after first washing (buffer washing) was c.a. 50%. After running again with BSA solution, the flux further decreased to c.a. 35%. This FRR indicates that the protein fouling is partially reversible in these membranes.


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washed with Buffer solution

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600 450 300

Pure water

protein filtration flux cycle 1

Recovery Flux protein filtration flux cycle 2

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Operation time (min) Figure 7: Flux profiles of the 70/30 blend membranes with BSA solution (0.3 gL-1) filtration at 20 psi. The application of hydrodynamic pressure results in the reduction of the pore size26, 27

. Hence, we performed experiments to understand the compaction in the designed token

membranes. Experiments were performed by running the same membrane in a cross flow cell for eight hours (at 30 psi) for 3 days, as discussed in supplementary information (see Figure S6). The permeate flux decreases and it remained constant during the rest of the test reaching a pseudo-steady state28. As polymers show viscoelastic behaviour, the initial flux values were not obtained after relaxation times in the time scale of hours27. Flux of the membranes decreased with respect to time indicating that pores are compacted during the experiment. Among all the membranes, 60/40 membranes had higher flux during initial hours of experiment. 70/30 token membranes also showed similar trend as that of 60/40 membrane. However, in the case of 90/10 membranes, an irregular pattern is observed at the end of third



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day and the permeate flux decreased. As described by Persson et al.27, the permeate flux decreased due to the membrane porosity and viscoelastic properties of polymeric membranes. Kenneth et al.27 reported the compaction of the membrane under different pressures and found that increasing pressure reduces the thickness and reduces the water permeability of the membrane. Compaction can happen with decrease in the spatial distribution of water content with respect to time over the membrane and, therefore, permeability also decreased. Goossens et al.29 also suggested that membrane compaction can occur because of the plastic creep in the upper active layer of the asymmetric membranes.

In summary, from our observations, we can comment that membranes can undergo lesser compaction with increased pore size distribution. Interestingly, our membranes showed much higher flux than the membranes that are reported in the literature. Table 1 summarizes the flux obtained for different ultrafiltration membranes prepared using different preparation methods. The designed membrane with tuneable pores and higher flux can open new avenues in water filtration research.

Table 1: Comparison of transmembrane flux of membrane from literature. Membrane system

Preparation methodology

PVDF/Brij S10/PEI PVDF/PVDF-g-PAA PVDF/PMMA/Pd PVDF/OMWCNT/GO PVDF γ-Al2O3 PES-TiO2 PVDF-TiO2-LiCl.H2O PVDF/PVDF-g-PHEMA PVDF/GO PVDF/L2MM PVDF/LiCl/SiO2/SiO2– COOH/DMAc

NIPS NIPS NIPS NIPS NIPS NIPS Dry-jet wet spinning NIPS NIPS NIPS NIPS

Highest flux of modified membranes (Lm-2h-1) 1654±19 lmh/bar ~150 ~200 ~>200 ~150 470±10 82.50 115±3 26.49 114.8 368.3±29.4


Ref

30 31 32 33 34 35 36 37 38 39 40

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PVDF/AgNaY PVDF/EPTB PVDF/SPES/TiO2 PVDF/PES PVDF/TiO2 PVDF-g-PSBMA/PVDF

NIPS NIPS NIPS NIPS NIPS NIPS

675±2 760 800 Kgm-2h-1 330 300 ~250

41

rGO/TiO2/PVDF

NIPS

~235 Kgm-2h-1

47

PVDF/TiO2/pDA

NIPS

~225

48

PVDF/PMMA membrane

Crystallization Induced Phase Separation

1900

This work

42 43 44 45 46

Modified Membranes: morphology, flux and anti-biofouling properties after silver sputtering The silver sputtered membrane surface morphology was observed by SEM, as shown in Figure 8a. It is evident that during sputtering, silver nanoparticles are well distributed on the membrane surface in form of nanoparticles. XPS was performed to investigate the elemental state of Ag onto Ag modified membranes. Figure 8b shows the XPS spectra of Ag 3d elemental scan with wide scan. The peak centered at 374.4 and at 368.2 eV was attributed to 3Agd3/2 and 3Agd5/2 band, which corresponds to elemental Ag. Water permeability of modified membrane was measured using the above mentioned in-house vertical flux setup. 70/30 silver sputtered token membranes showed a considerable reduction in flux over an hour, as seen in the Figure 8c &d. This reduction of initial flux indicates that some pores are partially occupied by silver nano particles. Rahimpour et al.49 observed that pure water permeability was reduced after modification of the PVDF membrane with acrylic acid and 2hydroxyethylmethacrylate (HEMA). Razmjou et al.35 also reported the similar decrease in flux after the modification of the PES ultrafiltration membrane with more than 2 wt% TiO2 nanoparticle on the modified membrane.



(a)

Ag 3d

3d 5/2

(b)

3d 3/2

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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366

368

370

372

374

376

Binding Energy, eV

O1s 0

200

400

600

800

Binding energy, eV


1000

1200

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1600 (c)

Flux @ 30 psi

1400

1000 70/30

-2

-1

Flux (Lm h )

1200

800 600

70/30-Ag

400 200 0

Blend Membranes

70/30 Ag

600

500 -2

-1

Flux (Lm h )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400

300

200

(d) 0

10

20

30

40

50

Time (min) Figure 8: Morphology of silver sputtered 70/30 PVDF/PMMA membranes (a), XPS



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profile of Ag-3d scan of modified membranes (b), Comparison of water permeability at 30 psi before and after modification (c), Permeate flux of Ag-PVDF membrane for one hour (d). Anti-bacterial performance of the membrane Biocidal activity of silver as a water disinfectant is well known50. The incorporation of silver in membranes and its biocidal activities are current research topics in the field of water purification16, 19, 51-53. Escherichia coli (E.coli) was chosen for testing because of its presence in drinking water and because it tends to form biofilms on hydrated surfaces54 including polymeric membranes. The biofilm formation (biofouling) on membranes cause declination of flux due to the formation of lesser permeable layer on the membrane surface55. Regardless of hydrophobicity of PVDF surface, it is known to form biofilm due to interactions between the bacterial and PVDF membrane surface56. To test the bactericidal property of the sputtered membranes, live and dead adhered cells were calculated using the fluorescent microscopy images. Of the total number of the E.coli cells attached on the surface of membranes, 99.9% were found to be non-viable after 1 h of incubation. In contrast, on the control membranes, all the attached bacteria were viable as shown in Figure 9 (a-b). The exceptional killing rate demonstrates the bactericidal nature of the sputtered silver on the leaching of silver ions from the sputtered membranes. This clearly indicates that these modified membranes can work as bactericidal as well as prevent biofouling on surfaces and more importantly are industrially scalable way to design antibacterial membranes with controlled release of silver ions.

To understand biofilm formation, SEM imaging was performed (as shown in Figure 9 (c-d)) on token membranes that were incubated for 1 h in E coli suspension solution. The biofilm was visible on the PVDF membrane surface that suggests bacterial cell adheres on the membrane surface (Figure 9c). Interestingly, no such biofilm was seen on sputtered


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membranes (Figure 9d). In the enlarged microscopic view, the dead bacterial cells adhered onto the sputtered membrane surface can be identified. This is in agreement with the results of fluorescence microscopy that confirms our hypothesis that sputtered membranes showed exceptional bactericidal activity. The antifouling property of membranes can be due to the depletion of Ag+ from the membrane surface which was characterized by ICP-OES analysis. The concentration of the silver ions released is 70 ppb (after 7 and 14 days). Release of silver in the solution can be quite effective from bactericidal activity point of view. Antimicrobial activities of the silver ions were well discussed by Feng et al.57 and Kim et al.58. Feng et al. showed that Ag+ ions prevent DNA replication and influences the permeability of the cell membrane. We also performed the leaching analysis of permeate from the flux experiment and no silver was detected. This shows that the depletion of silver from the membrane surface is different for the mobile phase and continuous phase and is similar to recently reported results59.



1400 70/30 control 70/30 Ag-PVDF

1200 1000

-2

-1

Flux (Lm h )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

800 600 400 200

(e) 10

20

30

40

50

Time (min)

Figure 9: (a-b) fluorescence microscopy of membranes after bacterial attack; control membrane and Ag-PVDF membrane, respectively. Scanning electron microscopy of (c) neat PVDF membranes showing E. coli cells after 1 h of incubation, (d) sputtered membranes with no biofilm after 1 h incubation. (inset showing dead E. coli bacteria). (e) Bacterial permeate Flux after 1 h with Control and Ag- PVDF membrane

Transmembrane permeate flux for both membranes (sputtered and unmodified) with time (for 1 h) was measured at 30 psi, as shown in Figure 9e. The higher flux for PVDF membranes can be attributed to higher porosity. Recently, Fan et al.60 reported that at higher applied pressure more bacteria will come onto the membrane surface leading to gradual


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reduction of permeate flux by clogging. E. coli with standard size of about 2 µm is larger when compared to the pore dimension of the token membrane. These two possibilities can be attributed to the reduction of permeate flux. But one can also think that flexibility of cell wall can leads to the passage of bacteria across the pores61. Fan et al.60 reported that at higher concentration of E. coli there is a chance of mutual collision of bacteria leading to bonding and forming bigger cells. Park et al.13 reported the effect of E. coli fouling on thiolated silver modified PVDF membrane by an in-line experiment. Our results showed that normalised flux was decreased when E. coli-LB solution is passed across the membrane. However, their silver modified membrane showed improved permeability compared to control indicates the reduced biofouling in modified membranes. Recently, Zeng et al.62 developed a PVDF membrane and observed that bacterial flux of the membrane was gradually reduced over a period of 20 h and graphene oxide quantum dots modified membrane showed less biofouling as compared to control membrane. The membranes obtained after the flux measurements were monitored using SEM (not shown here). No biofilm was present on the sputtered membrane but biofilm was observed on the neat PVDF membrane surface. There were also no viable bacterial colonies obtained in permeate. As the pore size of membranes is less than E. coli size, bacterial cells were not able to pass through the token membrane. This novel approach of membrane fabrication (crystallization induced phase separation) with improved flux will find applications in water purification. Conclusions The morphology of PVDF microfiltration membranes was controlled using crystallization induced phase separation in PVDF/PMMA blends. As PVDF/PMMA is a melt miscible blend, the samples were allowed to crystallize and the amorphous PMMA phase, was etched out to obtain nanoporous structures. The porosity can be tuned by altering the PMMA concentration in the blends. We observed that 60/40 PVDF/PMMA blends showed



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

larger pores as compared to 90/10 PVDF/PMMA blends. Interestingly, we also found that pure water flux obtained was different when the sides of the token membranes were reversed. This can be attributed to gradient in morphology during hot pressing and was confirmed using XPS. The obtained water flux from this strategy was higher than the available reported flux values. We further modified PVDF nanoporous membranes by sputtering silver on the surface. The bacterial cell viability was distinctly suppressed (99 %) in silver sputtered membranes. The ICP analysis suggests that slow Ag+ ions release from the sputtered membrane surface assisted in developing antibacterial surface. Our findings open new avenues in designing water filtration membranes and understanding the crystal kinetics for tuning pore size in microfiltration membranes. Supporting information

Selective etching of PMMA from PVDF/PMMA blend: assessed using DSC, roughness of membranes using AFM, water contact angle, Process governed gradient in morphology in PVDF/PMMA films prepared using compression molding assessed by XPS, membrane compaction.

Acknowledgements Authors would like to acknowledge Department of Science and Technology (DST) for financial assistance. Giridhar Madras thanks DST for the JC Bose fellowship. The corresponding author thanks the Indian National Science Academy (INSA) for financial support. Authors would also like to acknowledge MNCF, Center for Nanoscience and Engineering for characterization facilities.


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Graphical Abstract Crystallization induced phase separation: a unique tool to design microfiltration membranes with high flux and sustainable antibacterial surface Maya Sharma, Sanjay Remanan, Giridhar Madras and Suryasarathi Bose


Crystallization Induced Phase Separation: Unique ... - ACS Publications

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