Polysulfone Electrospun

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Mixed Matrix Polytetrafluoroethylene/Polysulfone Electrospun Nanofibrous Membranes for Water Desalination by Membrane Distillation Mohamed Khayet, and Rong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06792 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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ACS Applied Materials & Interfaces

Mixed Matrix Polytetrafluoroethylene/Polysulfone Electrospun Nanofibrous Membranes for Water Desalination by Membrane Distillation

Mohamed Khayet †,‡,*, Rong Wang §,£ †

Department of Structure of Matter, Thermal Physics and Electronics, Faculty of Physics,

University Complutense of Madrid, Avda. Complutense s/n 28040 Madrid (Spain) ‡ Madrid

Institute of Advances Studies of Water (IMDEA Water Institute), Calle Punto Com n°

2, 28805 Alcalá de Henares, Madrid (Spain) §

Singapore Membrane Technology Centre, Nanyang Environment and Water Research Institute,

Nanyang Technological University, 1 Cleantech Loop, Singapore 637141, Singapore £

School of Civil and Environmental Engineering, Nanyang Technological University, 50

Nanyang Avenue, Singapore 639798, Singapore

* Corresponding authors: [email protected] (M. Khayet); [email protected] (R. Wang)

KEYWORDS: polytetrafluoroethylene nanoparticle, polysulfone; electrospinning; mixed matrix membrane; superhydrophobicity; membrane distillation.

1 ACS Paragon Plus Environment

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ABSTRACT Electrospinning technique was used successfully to fabricate nanofibers of polysulfone (PSF) in which polytetrafuoroethylene nanoparticles (PTFE NPs) were embedded. The size of the PTFE NPs is only 1.7 to 3.6 times smaller than the nanofiber diameter. The transition from hydrophobic to superhydrophobic character of the bead-free PSF electrospun nanofiber mats occurred with a PTFE NPs loading in the range 12 – 18 % of the PSF weight. Transmission electron microscopy images revealed protruding nanosized asperities on the fiber surface due to the embedded PTFE NPs in the PSF matrix. For low PTFE NPs content in PSF matrix (< 6 % of the polymer weight), the PTFE NPs were arranged one by one in a single file along the PSF nanofiber axis. The structural characteristics of the nanofibers and electrospun nanofibrous membranes (ENMs) were studied by means of different techniques and their relationship with the PTFE NPs loading in PSF were discussed. The PSF/PTFE ENMs were tested in desalination by direct contact membrane distillation (DCMD) and the obtained performance was discussed in terms of the ENMs structural characteristics. Competitive permeate fluxes, as high as 39.5 kg/m2h, with stable low permeate electrical conductivities (< 7.145 μS/cm) for 30 g/L NaCl aqueous solution and transmembrane temperature of 60°C were achieved without detecting any inter-fiber space wetting.


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INTRODUCTION Electrospinning is one of the versatile techniques commonly considered for the fabrication of nanofibers for different potential applications (i.e. textiles, filtration, biomedicine, etc.).1-4 Basically, the nanofibers can be electrospun into a specific structure with void volume fractions greater than 90%, very large surface-area-to-volume ratio, high water contract angles reaching values up to or beyond 160º being able to biomimetic superhydrophobic surfaces with “petal effect”, “gecko effect” or “lotus effect”5,6, high surface roughness, low tortuosity factors, controlled thickness, well interconnected structure, etc.7-9 During last ten years, this technique has been widely used for the preparation of not only hydrophobic membranes for membrane distillation (MD) process, but also superhydrophobic ones characterized by water contact angles greater than 150º.9−13 In fact, in the field of desalination, the advantages of MD over other membrane separation processes such as reverse osmosis (RO) and electrodialysis (ED) include its high quality produced water (i.e. not only distilled water is produced by MD but also ultra-pure water) and its capability to treat very high saline aqueous solutions up to their saturation as well as its low membrane fouling propensity.7,8 The barriers for commercial implementation of MD technology are membrane wetting over time, low permeability and fouling/scaling issues.14-16 To overcome these challenges, MD membranes must be specially designed in order to exhibit high liquid entry pressure (LEP), high hydrophobic and omniphobic characters, high void volume fraction or porosity, low thermal conductivity coefficient, good thermal stability, excellent chemical resistance and adequate mechanical properties to guarantee



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long-term MD performance.7-9 Among the few designed superhydrophobic membranes for MD, one can find unmodified electrospun nanofibrous membranes (ENMs),17-18 surface or integrally modified ENMs10,19-26 and mixed matrix ENMs.11,27-33 These last ENMs consist of a polymeric matrix in which functional particles or nano-additives are embedded in order to improve one or most of the following parameters (permeability, rejection, fouling resistance, mechanical properties, thermal and chemical stability). Superhydrophobic unmodified electrospun polystyrene (PS) ENMs supported on polyethylene terephthalate (PET) backing material (water contact angle, θ = 150.2 ± 1.2º) was fabricated with different thicknesses for MD and showed competitive permeation fluxes over 10 h of testing.17 A higher θ value 162º was achieved for the unmodified ENMs prepared with aromatic fluorinated polyoxadiazoles and polytriazoles.18 On the other hand, as modified hydrophilic ENMs proposed for MD, polyvinyl alcohol (PVA) ENM was transformed to superhydrophobic by grafting a low surface energy fluoroalkylsilane (FAS) achieving a θ value of 158º;24 poly(trimethyl hexamethylene terephthalamide) (PA6(3)T) ENM was coated with poly (1 H,1 H,2 H,2H-perfluorodecyl acrylate) (PPFDA) using initiated chemical vapour deposition (iCVD) to render it superhydrophobic (θ = 151º);34 and polydimethylsiloxane (PDMS) coated polysulfone (PSF) ENM followed by cold-press post-treatment showed an improved θ value from 113.2º for PSF to 151.7º for PDMS/PSF.25 In general, superhydrophobic modified ENMs involved only the two fluorinated hydrophobic polymers polyvinylidene fluoride (PVDF)10,19,20,23 and polyvinylidene fluoride hexafluoropropylene (PVDF-HFP)21,22,26 while as


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additives hydrophobic silica (SiO2) nanoparticles,19,20,23 silver nanoparticle (Ag) and poly-dopamine (PDA),10 tetrafluoroethylene oligomers (OTFE) particles,26 carbon nanotube (CNT),21 and polyacrylonitrile (PAN)22 were used for modification procedures. It must be pointed out that PVDF11,17,27-29 and PVDF-HFP30,31,33 are the only two polymers that have been used so far for the preparation of superhydrophobic mixed matrix ENMs. Probably, to ensure the high hydrophobic character of this type of ENMs. As fillers clay (Cloisite 20A),11 silica (SiO2),27,28,32,33 titanium dioxide (TiO2)30,31 and polytetrafluoroethylene (PTFE)29 nanoparticles were employed and θ values as high as 163º were reached.32 In other words, hydrophilic host polymers have been discarded to produce superhydrophobic mixed matrix ENMs. An attempt is made in this study to prepare superhydrophobic mixed matrix ENMs using a hydrophilic polymer matrix. In general, superhydrophobic membranes proved to be promising for MD as they exhibit more stable MD performance than other types of membranes with permeate fluxes as high as 104.8 ± 4.9 kg/m2.h17 and permeate electrical conductivities below 5 µS/cm.10,17,19-21,23-25,27,30,32 However, few research studies have been focused on superhydrophobic MD membrane engineering and some related issues appeared and should be sorted out. In the case of superhydrophobic modified ENMs, there is a concern about the duration and the adhesion strength of the surface modification layer as a result of weak bonding on the rough surface of the nanofibers, and the environmental concerns about the toxic fluorinated chemicals; while for the unmodified ENMs, the stability of the superhydrophobic surface character induced by the single polymer chains



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need to be further considered. In this work, attempts have been made for the first time to prepare superhydrophobic self-sustained mixed matrix PSF ENMs using PTFE nanoparticles for MD. The effects of PTFE loading in PSF matrix on nanofiber structure, hydrophobicity, wettability, morphological characteristics and MD desalination performance of the ENMs were investigated. The size of the PTFE NPs is only 1.7 to 3.6 times smaller than the electrospun nanofiber diameter. PSF polymer is an attractive membrane material commonly used in filtration, hemodialysis, pervaporation and bio-artificial organs due to its excellent chemical resistance, good thermal stability and suitable mechanical properties.35-37 PSF membranes were traditionally prepared by the phase inversion technique and recently by electrospinning.37,38 Because of its low hydrophobic character (i.e. water contact angle, θ = 82.3 ± 2.1º) 37 and low LEP, the application of PSF in MD separation is limited and surface modification of PSF phase inversion membranes by surface modifying macromolecules (SMMs) or CF4 plasma treatment process were necessary to improve their top surface hydrophobicity and render them adequate for desalination by direct contact membrane distillation (DCMD).39,40 Recently, superhydrophobic and self-cleaning PDMS modified PSF ENMs has been proposed for DCMD desalination.25 Good anti-wetting properties and competitive MD permeate fluxes were achieved compared to the membranes prepared with hydrophobic polymers. It is to be noted that PTFE is known as fluorocarbon polymer with unique characteristics that include outstanding chemical resistance, strong hydrophobicity, high fracture toughness, low


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surface friction and good thermal stability.41,42 θ values on a smooth PTFE surface are between 98 and 112°.43 These exceptional characteristics made PTFE an ideal membrane material for MD. Various types of PTFE membranes have been prepared with different techniques and procedures (i.e. uniaxial and biaxial stretching,44,45 matrix spinning followed by sintering and removal of the matrix material,46 cold pressing,47 membrane-splitting and paste-extrusion followed by sintering).46,48 However, many of these current methods are rather cumbersome involving tedious and multiple steps because PTFE is difficult to melt, it is largely insoluble in solution and large amounts of lubricants are necessary in the fabrication process inducing a considerable environmental pollution.45 It must be mentioned that few research studies have been carried out on the design and preparation of PTFE ENMs for MD46,49 and all PTFE ENMs involved PVA matrix subjected to a sintering treatment for its decomposition leaving only PTFE nanofibrous membrane.49-51 θ achieved values up to 162.1º depending on the sintering temperature.49 To solve the problem of the weak mechanical properties faced by these PTFE ENMs, the precursor PTFE/PVA composite was reinforced applying co-electrospinning of both PTFE/PVA and PAN.46 A superhydrophobic (θ = 159.3º) and an excellent lipophilic characters were observed for this ENM with good vacuum membrane distillation (VMD) stability in a harsh aqueous solution (35 g/L NaCl and 10 g/L NaOH) being the permeate flux 14.5 kg/m2.h and the salt rejection factor above 99.8% during 35 h at 80ºC and 0.08 MPa low pressure.



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Experimental Materials Polysulfone (PSF, UDEL P-3500 LCD, Solvay Specialty Polymers; Mw = 79,000 g/mol; ρ = 1.24 g/mL) was used as a host polymer for the preparation of the mixed matrix ENMs. Before its use, PSF was dried at 120 °C in a vacuum desiccator placed in a heating mantle (Selecta) and connected to a vacuum pump (Vacuubrand brand, model MZ2C). The morphology and size distribution of the polytetrafluoroethylene nanoparticles (PTFE NPs, ρ = 2.15 g/mL, Tm = 321 ºC, Sigma-Aldrich) are presented in Figure S1. The dope solutions were prepared by a mixture of solvents, acetone and N,N-dimethyl acetamide (DMAC) both purchased from Sigma-Aldrich. Isopropyl alcohol (IPA, Sigma-Aldrich) and POREFIL® (Porometer) were employed as wetting liquids for the measurements of the void volume fraction (ε) and the inter-fiber space (di), respectively. Sodium chloride (NaCl, Panreac) was employed to prepare the salt aqueous feed solutions for the liquid entry pressure (LEP) measurements and DCMD tests. All chemicals were of analytical grade and were used as received without further purification.

Preparation of PSF nanofibers doped with PTFE NPs PTFE NPs were first dispersed in 80/20 wt% DMAC/acetone mixture under a vigorous and mild magnetic stirring (Heidolph Model 2021, Schwabach, Germany) and ultrasonic (Fisherbrand, model FB15050) for 30 min. PSF polymer was added to each dispersion, kept at 55 ºC in an orbital shaker thermostatic bath (Stuart SBS40) during 24 h and then subjected to


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magnetic stirring agitation of 120 rpm for about 4 h until the complete dissolution of the polymer. The concentration of PSF in the suspension was maintained fixed at 25 wt%, whereas the amount of PTFE NPs was varied from 0 to 6 wt% (i.e 0 – 24 % of the PSF polymer weight). Table 1 summarizes the measured electrical conductivity (χ), viscosity (μ) and surface tension (σ) at 23 ± 1ºC of the prepared PSF and PSF/PTFE NPs suspensions. The surface tension was measured by the pendant drop shape analysis using an Optical Contact AngleMeter (CAM200). The electrical conductivity was measured by the conductivity meter (Cyber Scan con11 Conductivity/TDS/1C, Eutech Instruments) and the viscosity by a digital Viscosimeter (Brookfield, DV-Iþ) connected to a thermostat (Model HETO21-DT-1, Rego S.A). Details of these measurements are described elsewhere.52 Prior to characterization and electrospinning, the polymer suspensions were degassed overnight at room temperature.

Table 1. Surface tension (σp), electrical conductivity (χp) and viscosity (μp) of the polymer suspensions measured at 23 ºC and diameter (df) of the prepared nanofibers. CPTFE

CPTFE

σp

χp

μp

df

(wt%)

(%) *

(mN/m)

(μS/cm)

(Pa.s)

(nm)

PSF

0

0

41.8 ± 0.5

2.17 ± 0.01

2.01 ± 0.01

491.0 ± 96.2

PSF/PTFE-0.5

0.5

2

42.7 ± 0.4

2.58 ± 0.01

2.23 ± 0.01

535.9 ± 141.1

PSF/PTFE-1.5

1.5

6

43.5 ± 0.4

2.87 ± 0.01

2.80 ± 0.01

609.3 ± 214.5

PSF/PTFE-3

3

12

44.5 ± 0.1

3.06 ± 0.02

3.54 ± 0.01

840.5 ± 286.4

PSF/PTFE-4.5

4.5

18

44.8 ± 0.1

3.17 ± 0.02

4.16 ± 0.02

927.9 ± 275.3

PSF/PTFE-6

6

24

46.1 ± 0.2

3.23 ± 0.02

4.84 ± 0.03

1003.2 ± 261.2

ENMs

* Amounts based on the PSF polymer weight 9 ACS Paragon Plus Environment


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The polymer suspension was first sucked into a glass syringe clamped to the circulation pump (KDS-200, Scientific) and connected to a metallic needle (0.6/0.9 mm inner/outer diameters). Subsequently, the dispersion was extruded through the needle with a circulation flow rate of 1.23 mL/h. A positive DC voltage of 24.1 kV was simultaneously applied near the tip of the needle, giving rise to the formation of a stable electrified jet flow. The obtained ENMs were assembled on a grounded metallic collector placed 20 cm from the needle tip. The ambient temperature and relative humidity were 23 ± 2 °C and 37 ± 2%, respectively. The obtained fibrous membranes during 2 h electrospinning time were dried in a vacuum oven at 120 °C for 2 h to remove completely the residual solvent. The resultant ENMs containing PTFE NPs contents of 0, 0.5, 1.5, 3, 4.5 and 6 wt % (0, 2, 6, 12, 18 and 24 % based on PSF polymer weight) were denoted hereafter as PSF, PSF/PTFE-0.5, PSF/PTFE-1.5, PSF/PTFE-3, PSF/PTFE-4.5 and PSF/PTFE-6, respectively.

Characterization of PSF and PSF/PTFE mixed matrix ENMs The morphology of the ENMs was studied by field emission scanning electron microscopy (FE-SEM, JEOL Model JSM-6335F). The size distribution of the nanofibers was determined and the arithmetic weighted mean of the fiber diameters together with the corresponding deviation were estimated. The structure of the nanofibers was further analyzed by transmission electron microscopy (TEM, JEM 3000 F, JEOL). The surface elements of ENMs were analyzed by X-ray photoelectron spectroscopy (XPS)


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using a Physical Electronics Spectrometer (PHI 5700) with X-ray Mg Kα radiation (300 W, 15 kV, 1253.6 eV) as excitation source and a multi-channel hemispherical electroanalyzer. High-resolution spectra of C1s, O1s, F1s and S2p in the binding energy range 0-1130 eV were recorded at two take-off angles (15º and 75º corresponding to 2.5 and 9.3 nm from the top surface of the sample, respectively). The characteristic peaks of oxygen (O1s) and sulphur (S2p) atoms of PSF were detected at 531 and 167.8 eV, respectively, while the peak of the binding energy at 685.7 eV corresponds to fluorine (F1s) atom associated to PTFE NPs (a typical XPS diagram of PSF/PTFE mixed matrix ENMs is shown in Figure S2). Therefore, the elemental composition of the different layers of the nanofibers could be determined by contrasting the fluorine, oxygen and sulphur elements. Based on the peak intensity of each element, the atomic percentage change of each element was evaluated. PHI ACCESS ESCA-V6.0F software package was used for acquisition and data analysis. The water contact angle (θ) was measured at 23ºC using a computerized optical system CAM100, equipped with a CCD camera, frame grabber and image analysis software CAM200usb. More information can be found elsewhere.7,52,53 The void volume fraction, which is defined as the volume of the inter-fiber space related to the total volume of the ENM sample, was determined as reported elsewhere.7 Basically for this measurement the densities of each sample were measured using isopropyl alcohol (IPA), which penetrates inside the inter-fiber space of the ENM and distilled water, which does not enter into the inter-fiber space.



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The mean size of the inter-fiber space (“pore size”) and its distribution were obtained using a capillary flow porometer (POROLUXTM 100). The followed procedure was described elsewhere.52 The liquid entry pressure of water (LEP) measurements of distilled water and 30 g/L NaCl aqueous solution was carried out using the experimental system and the procedure detailed in53. The thickness (δ) of the ENMs was measured by the micrometer Millitron Phywe (Mahr Feinprüf, type TYP1202IC).

MD performance test Direct contact membrane distillation (DCMD) tests were carried out using Lewis cell shown schematically in 54. In this study, the feed and permeate temperatures were set at 80ºC and 20ºC, respectively; and both the feed and permeate circulation rates were maintained at 500 rpm. Distilled water and 30 g/L NaCl aqueous solution were used as feed.

Results and discussions Morphological structure of the nanofibers and the PSF/PTFE mixed matrix ENMs All prepared electrospinning suspensions had good spinability and produced electrospun PSF/PTFE fibers with good fibrous structures. Figure 1 displays some representative SEM images of both PSF and PSF/PTFE webs, revealing that the nanofibers were oriented randomly forming integrated and bead-free networks, and indicating that the PTFE NPs were uniformly


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dispersed and efficiently bounded together by the PSF matrix. Although at a first glance the SEM images of both PSF and PSF/PTFE nanofibers would appear to be qualitatively similar, on closer inspection it is clear that significant and consistent differences exist. High magnification SEM and TEM images present more details on the fiber morphology. The TEM images show the internal structure of the PSF and PSF/PTFE nanofibers. While the PSF nanofiber exhibits a uniform and homogenous surface with fine holes and micro-grooves within a sponge-like structure (Figure 1a), the PSF/PTFE fibers show rougher surfaces with warts uniformly distributed on their surfaces due to the assembly of PTFE NPs and their encapsulation by PSF (Figure 1c,d,f). From TEM images (Figure 1-b), protruded nanosized asperities due to the embedded PTFE NPs were observed on the PSF/PTFE nanofiber surface (Figure 1c). For low PTFE NPs content in PSF matrix ( 150º).

θ (º)

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


160 155 150 145 140 135 130 0

0.5

1.5 CPTFE

3 (wt.%)

4.5

6

Figure 2. Water contact angle (θ) of PSF and PSF/PTFE ENMs.

It is assumed for ENMs that the air pockets generated on the inter-fiber space become finer and more numerous per unit area as the size of the fiber decreases resulting in a greater apparent contact angle (i.e. following Cassie and Baxter model, the larger the fraction of air pockets, the greater the apparent contact angle).55 Nevertheless, an opposite trend could be plotted between θ and df (see in Figure S3). This indicates that not only the observed unique structure of the PSF/PTFE fiber surface (i.e. protruded nanosized asperities on the fiber surface, Figure 1c,d) may



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provide extra scale roughness improving its hydrophobic character, but also it may be expected that the chemical structure of the nanofiber surface (e.g. the CF2 functional group of the PTFE NPs) may improve the hydrophobic character of the nanofiber. It is clear that the addition of even the smallest quantity of PTFE NPs (0.5 wt %) resulted in a notable increase of the surface hydrophobicity (i.e. θ increased from 133.6 ± 2.3º for PSF to 145.6 ± 2.4º for PSF/PTFE-0.5). All these results suggest that the PTFE NPs are able to change the physical and/or chemical fiber surface characteristics during very short processing time of electrospinning. However, no clear trend could be plotted between the water contact angle and the amount of PTFE NPs in PSF matrix. Note that by increasing the PTFE concentration from 4.5 to 6 wt% the contact angle was reduced slightly, but it was maintained greater than 150º. This decrease although it is within the error limits may be attributed partly to the decrease of the surface roughness due to the appeared elliptical beads on the surface of the ENMs prepared with the highest PTFE NPs loading (i.e. 6 wt%). However, there is a clear improvement of the superhydrophobicity (i.e. from 148.8 ± 2.1º to 153.1 ± 2.0º) when the amount of PTFE was increased from 3 to 4.5 wt%. This may be the result of a series of interrelated phenomena associated to both the physical structure as indicated previously and the surface chemistry of these nanofibers that is expected to be different from the composition of their interior. In order to gain some understanding of the surface elemental composition of the PSF/PTFE fibers, XPS analysis was carried out on PSF, PSF/PTFE-1.5, PSF/PTFE-3, and PSF/PTFE-6 at two take-off angles 15º and 75º. The evaluated atomic percentage of oxygen (O1s) and sulphur


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(S2p) atoms associated to PSF and fluorine (F1s) atom of PTFE NPs are summarized in Table 2.

Table 2. Atomic percentage of the elements found on the surface of the ENMs at different XPS take-off angles. Take-off ENMs

C1s

F1s

O1s

S2p

(%)

(%)

(%)

(%)

15

84.7

0

11.4

3.9

75

86.3

0

10.4

3.3

15

85.3

4.2

8.2

2.3

75

84.7

4.1

8.4

2.8

15

84.2

5.1

8.1

2.6

75

84.0

5.3

8.3

2.4

15

83.9

5.0

8.3

2.8

75

83.7

5.4

8.2

2.7

angle (º)

PSF

PSF/PTFE-1.5

PSF/PTFE-3

PSF/PTFE-6

With the increase of the probe depth from 2.5 to 9.3 nm of the surface, the amounts of oxygen and sulfur in the PSF nanofiber were dropped by 8 % and 15.4 %, respectively. This may suggest a reorientation of PSF polymer chain by the simple application of electrospinning. Oxygen is an electronegative group in the nonpolar PSF polymer chain. Provided that electrospinning was performed applying a positive polarity voltage, electronegative groups will be attracted by the positive charges accumulated at the surface of the electrified liquid jet. Subsequently, after solvent evaporation throughout the induced electric field between the needle tip and the metallic collector, a nanofiber with electronegative dipoles will be formed.56-59 In this case, oxygen 19 ACS Paragon Plus Environment


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double-bonded with sulphur would be more prone to be aligned towards the fiber surface, whereas the hydrophobic isopropylidene group was left to be positioned away from the surface. More depth research studies should be carried out in this interesting area in order to elucidate PSF molecular reorientation applying both positive and negative polarity voltages. With the addition of PTFE NPs to PSF matrix, the content of both oxygen and sulphur was reduced in all samples and no significant changes were detected for the two take-off angles. This indicated that the PSF polymer chains did not have the chance to be reoriented, but instead they were aggregates forming protruded asperities on the fiber surface due to the presence of PTFE NPs as discussed previously (Figure 1c,d). Besides, no clear variation of the atomic percentage of fluorine was detected at the two take-off angles (i.e. fluorine to carbon ratio was maintained practically the same for all PSF/PTFE nanofibers). It has been generally accepted that low surface free energy additives having surface active segments or end-groups, such as fluoroalkyl end-functionalized polymers or fluorinated segments,54,60-62 tended to undergo spontaneous migration to the outer surface during electrospinning because of their low surface free energy. However, in some cases abnormal fluorine aggregations in the core of the electrospun fibers has been observed (e.g. electrospun fibrous membranes of polyurethane elastomers containing perfluoropolyether segments.63 This was attributed to the formation of FPU chains aggregations in the concentrated solution and to their subsequent fast coagulation due to the rapid solvent evaporation and fast formation of nanofibers during electrospinning process. In our work, no surface segregation of PTFE occurred although its surface free energy was smaller than that of


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PSF. Because of the high dielectric constant of the used solvents DMAC and acetone, both would provide a high contribution to the surface net charge density, slowing down any possible surface migration of hydrophobic groups. It must be pointed out that this result may be also attributed to the larger size of the PTFE NPs (280.6 ± 50.6 nm) embedded in the PSF matrix than the XPS probe depth extent (i.e. less than 9.3 nm). However, as it was expected from the SEM and TEM images, the fluorine content was increased with the increase of the amount of PTFE NPs in PSF matrix up to 3 wt% and then reached a steady value (i.e. Surface F/C elemental ratio = 0.06) (Table 2). This value is far from the theoretical one of pure PTFE (3.16). Therefore, the observed decrease in contact angle at high concentrations of PTFE NPs from 4.5 to 6 wt% (Figure 2) could not be explained in terms of surface elemental composition, but it was more related with the surface physical morphology or surface roughness of the nanofibers. The elliptical beads detected on the surface of the PSF/PTFE-6 ENM reduced its water contact angle since both PSF/PTFE-6 and PSF/PTFE-4.5 webs exhibited practically similar fiber diameters taking into consideration their corresponding standard deviations.

Void volume fraction, Inter-fiber space and liquid entry pressure of PSF/PTFE mixed matrix ENMs With the increase of the PTFE NPs content in the PSF matrix, the void volume fraction (ε) was reduced (Table 3). This result may be attributed to the increase of the electrical conductivity of the PSF/PTFE dispersion with the increase of the PTFE NPs loading in PSF suspension



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(Table 1).

Table 3. Characteristics of the PSF and PSF/PTFE ENMs: void volume fraction (ε), mean inter-fiber space (di), liquid entry pressure (LEP) of distilled water and 30 g/L NaCl aqueous solution, and thickness (δ ). ε (%)

di (µm)

PSF

93.4 ± 0.9

PSF/PTFE-0.5

ENMs

δ

LEPH2O

LEP30 g/L

(104 Pa)

(104 Pa)

2.54 ± 0.13

2.4 ±0.7

2.8 ±0.4

472.4 ± 6.1

89.4 ± 1.9

1.87 ± 0.07

5.3 ± 0.6

6.7 ± 0.2

504.7 ± 5.4

PSF/PTFE-1.5

87.7 ± 1.1

1.74 ± 0.04

8.9 ± 0.3

9.6 ± 0.4

518.9 ± 5.0

PSF/PTFE-3

86.6 ± 1.4

1.42 ± 0.06

8.6 ± 0.2

9.9 ± 0.3

525.5 ± 3.8

PSF/PTFE-4.5

85.9 ± 0.6

1.19± 0.04

11.2 ± 0.2

12.3 ± 0.1

544.6 ± 2.6

PSF/PTFE-6

84.1 ± 0.9

0.98 ± 0.08

9.1 ± 0.8

10.3 ± 0.6

551.6 ± 3.6

(µm)

A higher electrical conductivity induces a faster and easier dissipation of the electric charges to the metallic collector during electrospinning resulting in less repulsive forces between nanofibers and in a more tightly compacted fibrous web. In other words, when the electrical conductivity is low, the electric charges accumulate on the formed nanofibers leading to more repulsive forces between them and, as a consequence, a more loosely packed fibrous web with a higher void volume fraction is formed. Besides, the inter-fiber space (di) was also reduced with the increase of PTFE NPs loading according to the void volume fraction results (Table 3). A linear relationship showing a decreasing trend could be plotted between di and the PTFE NPs content in the PSF matrix with reasonably high correlation factor. Compared to other 22 ACS Paragon Plus Environment

Page 23 of 51

superhydrophobic ENMs (Table 4), the PTFE/PSF mixed matrix ENMs and the F-TiO2/PVDF-HFP mixed matrix ENMs are the membranes exhibiting the highest ε values. Figure 3 shows the cumulative distribution of di of both PSF and PSF/PTFE ENMs. It can be seen that the size distribution curves of the PSF/PTFE mixed matrix ENMs shifted to the left in relation to PSF ENM and with the increase of the PTFE NPs content in PSF matrix. The maximum size of di was reduced more significantly than the minimum size of di with the incorporation of PTFE NPs in the PSF matrix (see Figure S4). Again, this was attributed to the formation of tightly packed structure of the mixed matrix ENMs. It must be pointed out that the detected elliptical beads at high PTFE NPs content in PSF (PSF/PTFE-6) could also reduce further both ε and di. As can be seen from Table 4, the di values of the PSF/PTFE mixed matrix ENMs are within the di range of other superphydrophobic nanofibrous membranes.

CFF (%)

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


100 90 80 70 60 50 40 30 20 10 0

PSF PSF/PTFE-0.5 PSF/PTFE-1.5 PSF/PTFE-3 PSF/PTFE-4.5 PSF/PTFE-6

0

1

2

3

4

5 di (µm)

6

7

8

Figure 3. Cumulative distribution of the inter-fiber space (cumulative filter flow, CFF) of the PSF and PSF/PTFE ENMs with different PTFE NPs loadings in the PSF matrix. 23 ACS Paragon Plus Environment


Page 24 of 51

Taking into account the observed reduction not only of di but also its maximum size and the increase of θ beyond 150º with the addition of the PTFE NPs in PSF matrix, it is expected high values for the liquid entry pressure (LEP) into the inter-fiber space of the PSF/PTFE mixed matrix ENMs. From the measured LEP values with distilled water and 30 g/L of NaCl aqueous solutions summarized in Table 3, it can be seen an enhancement of the LEP of the PSF/PTFE ENMs in relation to PSF ENM and the LEP values of saline solution are greater than those of distilled water for each ENM. This is due to the higher surface tension of the NaCl solution compared to that of distilled water.64,65 The highest LEP value (1.23 105 Pa) was obtained for the PSF/PTFE-4.5 membrane due to its superhydrophobic character and lowest maximum inter-fiber space. The PSF/PTFE-6 ENM showed lower LEP values than those of the PSF/PTFE-4.5 although it is also superhydrophobic. This result is justified by its slightly greater maximum inter-fiber space compared to PSF/PTFE-4.5 and to the presence of elliptical beads in its web structure. The obtained values for the LEP of the PSF/PTFE mixed matrix ENMs are not high enough due mainly to their high maximum inter-fiber space. Nevertheless, these values are similar to most of superhydrophobic ENMs reviewed in Table 4. In general, it can be concluded that the PSF/PTFE mixed matrix ENMs provided the basic structural prerequisites for MD applications.


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DCMD performance of PSF/PTFE mixed matrix ENMs Both PSF and PSF/PTFE ENMs were tested for desalination by DCMD. Figure 4 shows the obtained DCMD permeate fluxes (J) together with the NaCl rejection factors of the PSF/PTFE mixed matrix ENMs. Because of the low LEP values of the PSF membrane (Table 3), the NaCl concentration of the permeate showed a considerable increase after only 2 h of DCMD operation (i.e. the permeate electrical conductivity increased from 4.16 to 325.1 μS/cm) and the registered permeate flux was greater than that of the PSF/PTFE ENMs (i.e. 53.78 and 51.33 kg/m2.h for distilled water and 30 g/L NaCl solution, respectively). Therefore, PSF ENM is not suitable for MD desalination although its θ value is quite high (133.6 ± 2.3º). In general, the obtained NaCl rejection factors of all PSF/PTFE mixed matrix ENMs were greater than 99.99 % and their permeate fluxes were in the range 32.7 – 44.8 kg/m2.h. A very slight increase of the electrical conductivity (Xp) of the permeate was detected during DCMD testing (Figure 4b, e.g. for the membrane PSF/PTFE-4.5 Xp increased from 3.9 ± 0.7 to 6.1 ± 1.0 µS/cm in a permeate volume around 0.25 L). Because of the reduction of water vapor pressure with the NaCl concentration and the effect of the concentration polarization, the permeate flux of all PSF/PTFE ENMs was lower when using NaCl aqueous solution as feed than that of distilled water.7 In contrast to what it was expected, an enhancement of the permeate flux was observed with the increase of PTFE NPs content in PSF matrix up to 4.5 wt%, then it was reduced for the PSF/PTFE-6 ENM. This observed decrease of the permeate flux of the PSF/PTFE-6 compared to that of PSF/PTFE-4.5 (5.5-8%) may be justified by the predominant effect of its thickness (Table 3) and the presence



of agglomerated elliptical beads in its web structure.

Distilled water 30 g/L

50

0

2

4

6

8

25

100.0

45 40

Initial Xp

20

99.9

Xp (µS/cm)

30 25 20

α

15

99.8

10

99.7

5

99.6

α (%)

Final Xp

35

J (kg/m2.h)

15 10 5 0

0

1.50

3.00 CPTFE NPs (wt%)

4.50

6.00

99.5 0.5

1.5

3

4.5

6

CPTFE NPs (wt%)

(a)

(b)

50

20

40

15 J

30

10

Xp

20

Xp (µS/cm)

0.50

J (kg/m2.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

Page 26 of 51

5

10 0

0 0

2

4

6

8

10

12

14

16

18

20

Time (h) (c) Figure 4. Permeate flux (a) and NaCl rejection factor (α=(1−Cb,p/Cb,f) 100, where Cb,f and Cb,p are the concentrations of the bulk feed and permeate solutions, respectively) together with the initial and final electrical conductivity (Xp) of the permeate (b) of the PSF/PTFE mixed matrix ENMs, and permeate flux of the PSF/PTFE-4.5 mixed matrix ENM with its electrical conductivity as a function of DCMD operation time (c) (distilled water and 30 g/L NaCl aqueous solution as feed, Tf = 80ºC, Tp = 20°C, stirring rate 500 rpm).


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As discussed previously, with the increase of the PTFE NPs content in PSF matrix, both di and ε were reduced whereas δ was increased. Therefore, the permeate flux should have been reduced with the increase of PTFE NPs from 0.5 to 4.5 wt% in PSF matrix. This unexpected trend in Figure 4a may be attributed partly to the apparent increase of the ratio of the mean electrospun nanofiber diameter to the inter-fiber space (df/di) inducing more contribution of Knudsen mass transport compared to Brownian molecular diffusion and greater permeate fluxes.66 In DCMD configuration, generally mass transport through the pores takes place in three regions depending on Knudsen number,7 namely, Knudsen region, continuum region (or ordinary-diffusion region) and transition region (or combined Knudsen/ordinary-diffusion region). For the DCMD configuration used in the present study, viscous (Poiseuille) type of flow is negligible because the hydrostatic pressures of the feed and permeate are maintained at atmospheric pressure. During mass transport through a web structure, collisions occur between water vapor molecules and nanofibers, between water vapor molecules and each other’s and between water vapor molecules and air entrapped inside the void volume space of the web structure. Essalhi and Khayet,66 by means of a theoretical model and experimental studies, demonstrated that Knudsen contribution in ENMs was high for small size of inter-fiber space and large diameter of nanofibers because of the high probability of collisions between water vapor molecules and nanofibers. In the present study, the ratio df/di showed an enhancement with the increase of PTFE NPs content in PSF matrix from 0.3 to 0.8 for the PSF/PTFE mixed matrix membranes. This would suppose a Knudsen contribution of 35.1, 41.1, 64.0 and 86.6% for the



Page 28 of 51

PSF/PTFE-0.5, PSF/PTFE-1.5, PSF/PTFE-4.5 and PSF/PTFE-6 ENMs, respectively.66 In addition, it must not be ignored a possible slight mass transport through the cavities between PTFE NPs and PSF polymeric chains inducing a further enhancement of the permeability with the increase of the PTFE NPs in the PSF matrix. A through research study should be carried out to elucidate this supposition using different types of MD membranes. The membrane PSF/PTFE-4.5 was selected for long-term DCMD desalination because of its better MD performance compared to the other mixed matrix membranes prepared in this study. The results are presented in Figure 4c for a period of 19 h DCMD operation (i.e. three consecutive days stopping the test at nights). Both the permeate flux and the permeate concentration were maintained stable over the tested DCMD operation (i.e. a permeate flux of 39.5 ± 0.6 kg/m2.h and an electrical conductivity in the range 6.085 – 7.145 µS/cm with a mean value of 6.6 ± 0.4 µS/cm, corresponding to a permeate concentration in the range 1.6 – 2.0 mg/L with a mean value of 1.8 ± 0.1 mg/L and a salt rejection factor greater than 99.99%). No sign of membrane inter-fiber space wetting was detected as a very slight variation of the permeate electrical conductivity was registered around the mentioned mean vale (i.e. no continuous increase of the electrical conductivity of the permeate was recorded). Compared to other proposed superhydrophobic ENMs used in MD desalination and reviewed in Table 4, the PSF/PTFE-4.5 is one of the best mixed matrix ENM proposed for MD desalination although it is the thickest ENM. This is attributed to its unique nanofiber surface and to its reasonably adequate MD characteristics. The highest permeate fluxes (greater than 50 kg/m2.h) were


Page 29 of 51 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


exhibited by the unmodified superhydrophobic ENMs (i.e. supported polysterene (PS) on polyethylene terephthalate (PET) backing material with very low permeate electrical conductivities, about 2.5 µS/cm; and fluorinated polyxadiazole (F-POD) and fluorinated polytriazole (F-PT) but with smaller NaCl rejection factors as low as 99.95%).



Unmodified

ENMs

Table 4. Characteristics and MD performance of superhydrophobic ENMs proposed for desalination.

θ

Material a

(º)

25 wt% PS in DMF (PET support) 17

150.2 ±1.2 150.2 ±1.2

18 wt% F-POD in NMP 18

153

18 wt% F-PT in NMP

18

200% FS-SiO2/PVDF 3 wt% PVDF 19

Surface modified

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

Page 30 of 51

ε

δ

LEP (105 Pa)

di (μm)

(%)

(μm)

0.8

1.15

77.5

120

0.6

0.76

69

60

0.9

1.7

-

-

J (kg/m2.h) 51±4.5 (DCMD, 10 h, 70/20ºC, 35 g/L NaCl) 104.8±4.9 (DCMD, 10 h, 70/20ºC, 20 g/L NaCl) 78 (DCMD, No long-term test, 80/22ºC, Red sea) 85 (DCMD, No long-term test, 80/22ºC, Red sea)

Xp (μS/cm) &/or α (%) ∼2.4 ∼2.5 > 99.95%

162

0.76

2.9

-

-

154.0

1.5 *

0.32

78.5 *

72

25 (DCMD, 25 h, 60/20ºC, 3.5 g/L NaCl)

99.99%)

76

21 (DCMD, 40 h, 60/20ºC, 3.5 g/L NaCl)

99.99%)

18.9 (DCMD, 50 h, 60/20ºC, 3.5 g/L NaCl) 25 (DCMD, 100 h, 60/15ºC, 35 g/L NaCl)

Polysulfone Electrospun

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