Hydrophobic perfluoropolyether (PFPE) coated thin film composite

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Hydrophobic perfluoropolyether (PFPE) coated thin film composite (TFC) membranes for organic solvent nanofiltration (OSN) Bofan Li, Yue Cui, and Tai-Shung Chung ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.8b00171 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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ACS Applied Polymer Materials

Hydrophobic Perfluoropolyether (PFPE) Coated Thin Film Composite (TFC) Membranes for Organic Solvent Nanofiltration (OSN)

Bofan Li, Yue Cui, Tai-Shung Chung*,

Department of Chemical & Biomolecular Engineering National University of Singapore, 4 Engineering Drive 4 Singapore 117585

*Corresponding author

*Email: [email protected]

1 ACS Paragon Plus Environment

ACS Applied Polymer Materials 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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Abstract Fluoropolymer, with its high chemical stability and hydrophobicity, has great potential in applications involving organic solvents, e.g. organic solvent nanofiltration (OSN). A UV-curable fluorinated polymer, perfluoropolyether (PFPE, Fluorolink® AD1700), was coated and polymerized onto a cross-linked polyimide substrate to modify its surface properties for OSN. The PFPE coating can convert the hydrophilic substrate to a hydrophobic membrane and narrow the pore size of the membrane to make it suitable for OSN applications. Different coating concentrations, ranging from 1 wt% to 10 wt%, were applied and the composite membranes were characterized in various organic solvents (acetonitrile, ethanol, isopropanol and hexane). The permeances of these solvents were found to be inversely proportional to the products of their viscosities and molar volumes. The separation performance of the composite membrane coated by 10 wt% PFPE exhibited rejections of >90% to orange II sodium salt (MW= 350.32 g·mol-1) and remazol brilliant blue (MW= 626.54 g·mol-1) in isopropanol. 7-day tests were also conducted to (1) separate tetracycline from ethanol/tetracycline solutions and (2) recover hexane from hexane/𝛽 -carotene solutions. The fluxes and rejections of both membranes do not fluctuate significantly during 7-day continuous tests with performance comparable with and superior to most literature data. Therefore, the PFPE coated membranes may have great potential to be next-generation OSN membranes for industrial applications.

Keywords: Fluoropolymer; perfluoropolyether; organic solvent nanofiltration; pharmaceuticals; food additives


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1. Introduction Fluoropolymer is a class of polymers containing mostly carbon (C) and fluorine (F) atoms. Due to high F electronegativity and strong C-F bonds, the presence of F atoms confers the fluoropolymer family with superior properties, such as high chemical resistance, thermal resistance and hydrophobicity. Hence, it is widely applied in many fields, e.g. fuel cells, optical fibers and biomedical devices

1-4.

Recently, a new type of UV-curable fluoropolymer, perfluoropolyether

(PFPE), was developed by Solvay Specialty Polymers with the trademark of Fluorolink®. It was specially designed for the use of surface modifications, as its fluoro-elements might migrate towards the air interface and formed a unique film on the substrate.

Basically, PFPE consists of some repetitive units, such as –CF2O-, -CF2CF2O-, -CF2CF2CF2Oand CF(CF3)CF2O-, in the backbone. To achieve the UV-curable functionality, its terminal groups comprises photoreactive groups, either acrylic or methacrylic groups

5-6.

The polymerization

reaction is usually triggered by UV irradiation of photo initiators, which are dissociated into free radicals in the initiation step. In the subsequent propagation step, the free radicals attack the C=C bonds of undissociated molecules to form ·C-C free radicals, which can then attack the adjacent C=C bonds to generate C-C bonds. As a result, the polymerization is initiated and the oligomer grows into a polymer to form a dense film 5. Despite its capability of altering the surface properties and easy operation, there are limited applications of PFPE in the membrane field. One example was the incorporation of Fluorolink® MD700 into an interpenetrating polymer network so that the chemical stability of an anion exchange membrane was enhanced to protect its air electrodes under extreme conditions 7. Another example was to coat Fluorolink® AD1700 and MD700 onto



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membranes for membrane distillation applications by taking the advantage of its hydrophobic nature 8.

Organic solvent nanofiltration (OSN) also known as solvent resistant nanofiltration (SRNF), is a separation technology aiming to recover organic solvents or concentrate valuable products in pharmaceutical, petrochemical and food industries. Comparing with the traditional distillation and evaporation processes, OSN is superior in energy consumption, operational costs and minimization of greenhouse gas emission 9-18. Moreover, it can be operated at mild temperatures, in which most active pharmaceutical ingredients (APIs) and food additives are stable. To improve the efficiency of OSN, the membrane should be fabricated with good separation performance and stability in organic solvents. This motivates researchers to design various OSN membranes via different methods 9-14, 19-25. Thin film composite (TFC) membranes are the most studied one since the thin dense selective layer can provide a high rejection to solutes with a low transport resistance to solvents. Some commonly used techniques for fabricating TFC membranes are interfacial polymerization, coating and plasma treatment 26-32. Among them, coating is the simplest and most popular method to fabricate TFC membranes for OSN. Usually, the polymer or monomer solutions are applied on the substrate surface via dip or spin coating, followed by solvent evaporation and polymerization, if required. Polydimethylsiloxane (PDMS) coated membranes have been widely investigated, but the swelling of PDMS limits their applications in non-polar solvents

33-34.

Polypyrrole (PPy), being a polymer with high chemical resistance, has been studied by a few researchers for OSN. However, the polymerization of PPy requires the use of strong oxidizing agents, such as FeCl3 and H2O2, which are not environmentally-friendly

28, 35.

Other researchers

have attempted to use rigid polymers, e.g. poly(1-(trimethylsilyl)-1-propyne) (PTMSP) and


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polymers of intrinsic microporosity (PIMs), to form the thin selective layer on the porous substrate. One of the challenges for using rigid polymers is to minimize the defects induced by the rigidity 29, 36-37.

In this work, the fluoropolymer PFPE is explored as a potential coating polymer for OSN membranes. The Fluorolink® AD1700 is deposited onto a lab-fabricated Matrimid® membrane crosslinked with 1,6-hexanediamine (HDA) and then polymerized under UV irradiation. Different concentrations of PFPE solutions, ranging from 1 wt% to 10 wt%, are prepared to tune the separation performance of the newly developed PFPE coated membranes. Several organic solvents, i.e., ethanol, isopropanol (IPA), hexane, and acetonitrile, containing various dyes, APIs and food additives are tested for OSN applications using the as-prepared PFPE coated membranes. To the best of our knowledge, this is the first study of fabricating PFPE membranes for OSN. The high solvent fluxes and good rejections as well as good chemical stability and hydrophobicity of the membranes make them very promising as next-generation OSN membranes.

2. Experimental methods 2.1 Materials Fluorolink® AD1700 was obtained from Solvay Specialty Polymers Italy S.p.A. It contains perfluoropolyether-urethane acrylate oligomer (MW = 4000 g·mol-1), which has the chemical structure of RH–CF2O–(CF2CF2O)m–(CF2O)n–CF2–RH, as described elsewhere 8. RH is the urethane (meth)acrylate block that can be crosslinked under UV irradiation. Benzoin (98%, Sigma Aldrich) was chosen as the photo-initiator as it has the maximum absorbance at wavelengths around 250 nm

38,

which falls within the emission range of the UV lamp used in this study. A



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commercial polyimide polymer, Matrimid® 5218, was procured from Vantico Inc. and used to fabricate membrane substrates. N-methyl-2-pyrrolidinone (NMP, Merck) and polyethylene glycol 400 (PEG 400, Acros Organics) were used as the solvent and pore former, respectively. 1,6Hexanediamine (HDA, 98%, Sigma Aldrich) was employed to crosslink the substrate. Various solvents, i.e., ethyl acetate, acetonitrile, ethanol, isopropanol, and hexane were supplied from VWR Inc. Orange II sodium salt (85%, MW= 350.32 g·mol-1), remazol brilliant blue (50%, MW= 626.54 g·mol-1), tetracycline (≥98%, MW= 444.43 g·mol-1) and 𝛽-carotene (≥ 97%, MW= 536.87 g·mol-1) were procured from Sigma-Aldrich for rejection and application tests.

2.2 Preparation of crosslinked Matrimid® membrane substrates The fabrication and crosslinking procedures were similar to our previous work

21, 39.

To prepare

the dope, Matrimid® polymer was vacuum dried at 70°C for one day. NMP and PEG 400 were employed as the solvent and pore former. The dope mixture contained 18 wt% Matrimid, 16 wt% PEG 400 and the rest is NMP. It was further stirred at 70°C for 24 h to form a homogeneous solution, which was then degassed for at least one day before casting. Afterwards, a casting knife with a height of 100 µm and a glass plate were used to fabricate the membrane substrate, followed by phase inversion inside a deionized (DI) water bath. The as-cast membrane should be stored in DI water for at least 24 h to fully induce the phase inversion. For crosslinking the membrane, the as-cast substrate was dipped into a 5 wt% HDA solution in IPA/water (50/50). After one day, the membrane substrate was taken out and immersed in DI water for further modification. The asprepared substrate is a UF membrane with a pure water permeance of 203 LMH/bar and an MWCO of 97,700 Da tested in water 21.


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2.3 Fabrication of PFPE coated membranes To minimize the penetration of coating solutions and pore collapse caused by the subsequent drying, the membrane substrates were first pre-wetted and heat-treated with a 50/50 wt% glycerol/water mixture at 80°C for 30 min. Afterwards, glycerol/water residues on the surface of the pretreated substrate were wiped away using filter papers and a rubber roller. The pretreated substrate was then clipped into a customized frame so that the coating solution could only contact the top surface of the substrate.

The coating solutions were prepared with different oligomer concentrations, ranging from 1-10 wt%, as tabulated in Table 1. Ethyl acetate and benzoin were chosen as the solvent and photoinitiator, respectively. The weight ratio of photo-initiator to the oligomer was kept at 1:10 to ensure successful polymerization. The solution was stirred for 2 h at room temperature and degassed for 30 min before coating. Subsequently, a coating solution of around 10 ml was deposited onto the top surface of the substrate and left for 10 min, followed by drying at ambient temperature overnight. The coated membrane was then irradiated for 10 min by a UV lamp (40 W) with the wavelength of 254 nm to initiate the polymerization of PFPE oligomers. To wash away the unpolymerized oligomers, the coated membrane was immersed into fresh ethyl acetate for 10 min and then dried at ambient temperature overnight. Subsequently, the as-prepared membrane was soaked and stored in isopropanol for further tests. Solvent exchange was performed if the testing solvent was not isopropanol. A schematic diagram is displayed in Figure 1 to illustrate this process.



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Figure 1. Schematic diagram for the fabrication process of PFPE coated membranes.


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Table 1. Coating concentration and surface atomic composition of the PFPE membranes. Coating composition Membrane ID

®

(Fluorolink AD1700: ethyl acetate: benzoin)

C 1s

N 1s

O 1s

F 1s

(At%)

(At%)

(At%)

(At%)*

F/C

Crosslinked substrate

-

75.61

7.91

16.67

-

-

1 wt%

1: 98.9: 0.1

56.53

4.38

18.85

20.24

0.36

3 wt%

3: 96.7 : 0.3

53.94

4.77

15.82

25.47

0.47

5 wt%

5: 94.5 : 0.5

51.30

4.08

15.80

28.81

0.56

10 wt%

10: 89 :1

43.54

3.60

15.35

37.52

0.86

*AT: atomic concentration 2.4 Membrane characterizations A Bruker Fourier Transform Infrared Spectrometer (FTIR) was utilized to confirm the polymerization of PFPE oligomers. The FTIR spectra were generated under the attenuated total reflectance (ATR) mode. To minimize the membrane substrate from interfering with the results, the powder of PFPE was irradiated by UV light and used for the ATR-FTIR characterization. A JEOL JSM-6700F field emission scanning electron microscopy (FESEM) was employed to characterize the morphology of the top surface and cross-section of the prepared membranes. FESEM samples were prepared by freeze-drying and fracturing in liquid nitrogen. A layer of platinum was coated on the samples using a JEOL JFC-1300 platinum coater. X-ray photoelectron spectroscopy (XPS) was used to analyze the surface atomic concentrations on a Kratos AXIS UltraDLD spectrometer. The X-ray source was produced by a monochromatized Al Kα X-ray 9 ACS Paragon Plus Environment


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source (1486.71 eV, 5 mA, 15 kV). The hydrophobicity was studied by measuring the static water contact angles of freeze-dried membranes using a Rame Hart contact angle goniometer. The average contact angle of at least 10 points on different locations of each sample was reported.

The microstructural variation of each membrane at different positron penetration depths was investigated using a Doppler broadening energy spectroscopy (DBES). A variable mono-energy slow positron beam was generated in a position annihilation spectroscopy (PAS) to control the positron penetration depth. 22Na was utilized as the source of positrons. A total of 30 spectra were generated with one million counts in each spectrum. The detailed procedures can be found elsewhere 40-41. The R-parameter of DBES was recorded and analyzed to study the depth profile of voids or large pores (nm-µm).

2.5 OSN performance The OSN performances of the PFPE coated membranes were measured using dead-end filtration cells at room temperature. The fabricated membranes were cut into 2cm circles for tests while a feed solution of around 300ml was filled into the cells. A stirrer was placed above the membrane, which was stirred at 500 rpm to minimize the concentration polarization effect. The detailed design of the permeation cell has been reported elsewhere 42-43. A pre-stabilization period of 5 hours was conducted before taking measurements. The pure solvent permeance was expressed in Eq. (1) 𝑃𝑢𝑟𝑒 𝑠𝑜𝑙𝑣𝑒𝑛𝑡 𝑝𝑒𝑟𝑚𝑒𝑎𝑛𝑐𝑒 =

𝑄

(1)

𝐴 ∙ ∆𝑃

where Q (L/h) is the flux of the permeate, A (m2) is the testing area of the membrane and ΔP is the trans-membrane pressure of 5 bar.


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The rejections to organic solutes (i.e., dyes, APIs and food additives) were measured using 50 ppm feed solutions under a trans-membrane pressure of 5 bar with pre-stabilization of 5 hours. A UVVis spectrometer (Pharo 300, Merck) was employed to measure the concentration according to the Beer-Lambert law. The rejections were computed according to Eq. (2).

(

)

𝐶𝑝

(2)

𝑅 = 1 ― 𝐶𝑓 × 100% where Cf and Cp are the solute concentrations of the feed and permeate, respectively.

For each test, the average result of at least three membrane samples were reported. Furthermore, some membranes were tested 7 days after 5-hour stabilization to ensure the stability and reproducibility of the membranes. During the tests, samples were collected every 12 hours and the feed solution was refilled every day.

3. Results and discussion 3.1 Membrane characterizations 3.1.1 Photopolymerization reaction of PFPE oligomers To confirm the photo-induced polymerization, ATR-FTIR is utilized to reveal the changes of bonds before and after the UV irradiation. As displayed in Figure 2a, the peak at 1640 cm-1 represents the stretching of C=C bonds, while the peaks at 665-700 cm-1, 810 cm-1 and 970 cm-1 belong to the bending of =C-H bonds. After the UV irradiation, the stretching peak at 1640 cm-1 disappears and the bending peaks at 665-700 cm-1, 810 cm-1 and 970 cm-1 are weakened, indicating the disappearance of C=C bonds

6, 44.

All of them suggest the successful photo induced

polymerization under the designed experimental conditions.



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Figure 2. (a) Comparison of ATR-FTIR spectra between PFPE powder before and after UV irradiation and (b) Water contact angles of the crosslinked substrate and PFPE membranes with an increase in PFPE coating concentration. 3.1.2 Surface compositions The surface atomic compositions of the crosslinked substrate and PFPE coated membranes are determined using XPS and the results are tabulated in Table 1. The successful coating can also be verified by comparing the crosslinked substrate and PFPE coated membranes. For the crosslinked substrate, fluorine can hardly be detected, whereas the atomic concentration (AT) of fluorine content jumps to 20 % upon coating with 1 wt% PFPE solutions. This confirms that the fluorine elements on the modified surface mainly come from the CF3 or CF2 groups of the PFPE polymer. As the PFPE concentration in the coating solution increases, the fluorine content on the surface of the fabricated membranes increases. Since the polymerization would not change the elemental composition, a higher PFPE concentration (containing more oligomer) tends to form a larger polymer which can remain on the surface even after being washed with ethyl acetate. Thus, the increased fluorine content indicates that the PFPE oligomer has undergone a higher degree of polymerization.


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3.1.3 Surface hydrophobicity The surface hydrophobicity of the crosslinked substrates and PFPE coated membranes is determined by measuring their water contact angles (Figure 2b). The water contact angle of the crosslinked substrate is around 57°. It dramatically increases to 97° when an oligomer solution of 1 wt% is coated on its surface. The water contact angle increases further as the oligomer concentration in the coating solution increases. A similar trend has also been reported previously by other researchers 8. This trend is consistent with the XPS results; namely, the more F containing groups remain on the membrane surface, the more hydrophobic the PFPE coated membrane is. Thus, the hydrophilic crosslinked substrates are converted to hydrophobic membranes even when a low coating concentration of 1 wt% is applied. The water contact angle of the PFPE coated membrane with a 10 wt% oligomer solution can achieve 124°. This makes the PFPE coated membranes suitable to treat most organic solvents since they are much more hydrophobic than water.

3.1.4 Surface morphologies The top surfaces and upper parts of the cross-sections of PFPE coated membranes are observed using FESEM, as displayed in Figure 3. Different from the polymer nodules appearing on the crosslinked substrates as reported in our previous work

21,

the top surfaces of PFPE coated

membranes are quite smooth. However, small cracks can be observed on their top surfaces. This cracking morphology may originate from the freeze-drying process used for sample preparation. From the cross-section images of PFPE coated membranes, an ultra-thin dense-selective film can be observed on each top surface, but the interface between the coating and substrate is still hard to differentiate. This may be ascribed to the diffusion or intrusion of the PFPE oligomers into the



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voids of substrates, as discussed in the following section. The interpenetration of PFPE into the porous PI substrate and chain entanglement ensure that the PFPE is firmly coated onto the substrate. Since it is difficult to determine its thickness from the FESEM images, an advanced technique, PAS, is employed to investigate its thickness as a function of coating concentration.

Figure 3. FESEM images of the top and cross-section morphologies of PFPE coated membranes. 3.1.5 Membrane microstructures PAS is employed to investigate the depth profile of microstructure and its changes across the membrane thickness of the crosslinked and coated substrates. The measured R parameter is plotted as a function of (1) positron incident energy (i.e., bottom X-axis), (2) mean depth (i.e., top X-axis) and (3) PFPE coating concentration, as shown in Figure 4. The R-parameter reveals the pore size evolution along the depth, where a larger R-parameter stands for a higher intensity of voids in the range of nm-µm. The R parameters of all membranes first decrease to a certain value and then increase. The change from the initial decrease to the subsequent increase implies a transition from the dense-selective layer to the porous substrate. Thus, one can estimate (1) the dense-layer 14 ACS Paragon Plus Environment

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thickness from the location of the minimum and (2) the intensity of voids in the dense-selective layer from the minimal value of R parameter.

Figure 4. (a) R-parameter of the crosslinked substrate and PFPE membranes as a function of positron incident energy (mean depth) and (b) the enlarged 0-5 KeV region. Figure 4b is the magnified plot in the range of 0-5 KeV where the width of the valley increases and the minimal value of the valley decreases with an increase in coating concentration. Clearly, an increase in PFPE coating concentration not only leads to a decrease in pore size in the denseselective layer but also increases its thickness. A similar trend has been observed in other’s work as well 8. In addition, the coating reduces the porosity of the underneath substrates because PFPE oligomers may diffuse or intrude into the voids of substrates during the coating process and then polymerized during the UV radiation. Consequently, the intensity of voids and pore size of the substrates decreases with an increase in PFPE oligomer concentration in the coating solution. Since a higher PFPE coating concentration may result in a larger polymer with a higher degree of chain entanglement during the UV radiation, the polymerized PFPE becomes difficult to be washed away from the membrane surface.



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3.2 OSN performance 3.2.1 Pure solvent permeances To investigate the solvent transport properties across the PFPE coated membranes, various solvents, including water, acetonitrile, ethanol, isopropanol and hexane, are used for the filtration tests. Among them, water is the only inorganic (non-carbon containing) solvent, whereas the rest are all organic (carbon containing) solvents. Figure 5a shows their permeances at a transmembrane pressure of 5 bar. Although water has the smallest molecular weight and size, it has the lowest permeance among them. This may be ascribed to the high hydrophobicity of the membranes and high surface tension of water. As all PFPE coated membranes are hydrophobic with water contact angles larger than 90°, the resistance for water molecules to enter the membrane pores is very large. In addition, the high surface tension of water also increases its transport resistance from pores to pores across the membranes. In contrast, organic solvents are more hydrophobic and usually possess a much lower surface tension than water. Therefore, the designed PFPE coated membranes are very suitable for the transport and recovery of organic solvents.

The permeances of the selected organic solvents can be ranked in a descending order from acetonitrile, followed by hexane, ethanol to isopropanol. The pure solvent permeances of these four organic solvents are correlated with their physicochemical properties in order to investigate the transport behavior across the PFPE coated membranes. Table 2 lists some of their physiochemical properties, such as molar volume, viscosity and solubility parameter 45. Figure 5b shows their permeances inversely proportional to the products of viscosity and molar volume. Molar volume (Vm) is the volume of one molar solvent molecules, which can represent the solvent size. Viscosity (µ) is an intrinsic property of a fluid that indicates how easily one fluid can flow.


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Usually, organic solvents with a small size and low viscosity can pass through the membrane easily with a higher flux 46-50. Thus, the pure solvent permeance is correlated to the reciprocal product of its molar volume and viscosity. The correlations shown in Figure 5b are all well fitted for ethanol, IPA, hexane and acetonitrile with R2 values larger than 0.85 for all PFPE coated membranes. Especially, the R2 values for membranes coated with 1, 5 and 10 PFPE wt% are above 0.9, indicating the existence of strong correlations. Interestingly, the slope of the correlating line becomes smaller as the coating concentration increases. This implies that the solvent flux becomes less dependent on solvent properties once the coating concentration increases because of the decrease in porosity of the selective layer, as indicated by the R-parameter. In other words, once the selective layer becomes thicker and denser, the transport resistance for all solvents increases. As a result, the transport resistance in the selective layer plays a more important role in determining the flux than the solvent properties.

45

Table 2. Physicochemical properties of the selected solvents . Viscosity (@25°C cP)

Molar volume 3 (cm /mol)

Solubility parameter 1/2 (MPa )

Surface tension (@20°C -1 mNm )

Dielectric constant (@20°C)

Water

0.89

18.02

47.8

72.75

79.7

Ethanol

1.081

58.68

27.4

22.3

22.4

Acetonitrile

0.38

52.86

24.3

29.1

37.5

IPA

2.058

76.92

23.5

21.7

18.3

Hexane

0.31

130.5

14.1

18.4

1.9

Solvents



Page 18 of 34

Figure 5. (a) Pure solvent permeances (water, acetonitrile, ethanol, IPA and hexane) of PFPE coated membranes at a trans-membrane pressure of 5 bar and (b) Correlation between pure solvent permeances of PFPE coated membranes and reciprocal products of solvent viscosity and molar volumes. 3.2.2 Dye rejection tests In order to investigate the separation performance of the PFPE coated membranes, two dyes; namely, orange II sodium salt (MW= 350.32 g·mol-1) and remazol brilliant blue (MW= 626.54 g·mol-1), are dissolved in isopropanol and filtered through the membranes. They are specifically 18 ACS Paragon Plus Environment

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chosen because both are negatively charged and possess similar solubility parameters so that one can minimize the influence of solute’s physiochemical properties on rejection. In this way, the rejection of these two dyes is mainly determined by their sizes. Figure 6a shows the results in terms of pure solvent permeance and rejection, while Table 3 summarizes the physicochemical properties of the dyes, such as molecular weight, structure and solubility parameter 39, 51-52. Consistent with our expectation, the rejection of remazaol brilliant blue is higher than that of orange II sodium salt for all membranes because the former has a much larger molecular weight and size than the latter. Moreover, the rejections towards both dyes increase as the coating concentration increases. Specifically, the rejection of orange II sodium salt is only 28.7% but increases to 92.7% when the coating concentration increases from 1 wt% to 10 wt%. In contrast, the rejection of remazol brilliant blue increases from 77.7% to 96.8% because of its bigger size. These results are consistent with the data obtained from PAS and pure solvent permeance. Therefore, the separation capability of the PFPE coated membranes can be easily manipulated by coating a different amount of PFPE oligomers on the substrates. This makes the scale up and production of PFPE coated membranes simple and easy for various applications.



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Figure 6. (a) Pure IPA permeances of PFPE membranes and their rejections towards 50ppm orange II sodium salt (closed square ■) and remazol brilliant blue (open square □) at a transmembrane pressure of 5 bar, 7-day stability tests using (b) 50 ppm tetracycline/ethanol (c) 50 ppm 𝛽-carotene/hexane feed solutions at 5 bar for 168 hours a The permeances are obtained from separating tetracycline/ethanol and 𝛽-carotene/hexane mixtures rather than pure ethanol and hexane

Table 3. Physicochemical properties of selected dyes, pharmaceuticals and food additives 52 . Dye name

Molecular formula

Molecular weight (gmol 1 ) and charge characteristics

Molecular structure

39, 51-

Solubility parameter 1/2 (MPa )


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Orange II C H N NaO S 16 11 2 4 sodium salt

Remazol brilliant C22H16N2Na2O11S3 blue

350.32 (negative)

29.2

626.54 (negative)

29.4

Tetracycline

C22H24N2O8

444.435 (neutral)

28.7

𝛽-carotene

C40H56

539.86 (neutral)

17.8

Table 4 compares the separation performance of the newly developed PFPE coated TFC membranes in solute/IPA mixtures with commercial OSN membranes and other polymeric membranes in the literature 28, 35, 53-56. Most of the literature works in Table 4 use rose bengal as the model solute (MW= 1017 g·mol-1) that is much larger than the dyes we have used. The former membranes may not be able to separate small MW dyes. In contrast, the 5 wt% PFPE coated membrane has a pure IPA permeance of 1.40 Lm-2h-1bar-1 with a rejection of 89.0% towards remazol brilliant blue (MW= 626 g·mol-1) and the 10 wt% PFPE coated membrane has a pure IPA permeance of 0.52 Lm-2h-1bar-1 and a rejection of 96.8% towards remazol brilliant blue. Therefore, the PFPE coated membranes exhibit higher permeances with satisfactory rejections compared with commercial membranes and other lab-made polymeric OSN membranes. Table 4. A comparison of separation performance between PFPE coated TFC membranes and other OSN membranes via coating for the separation of solute/IPA mixtures.



Pure IPA Membrane

Molecule

permeance -2 -1

Solute

-1

weight (g -1

(Lm h bar ) MPF-50

Page 22 of 34

Rejection Ref. (%)

mol )

0.72

Rose bengal

1017

98.1

28

122

1.13

Rose bengal

1017

99.9

28

Puramem

®

0.48

PEG600

600

~ 66

53

Polypyrrole/PAN-H

0.79

Rose bengal

1017

99.2

35

PDMS/PI

0.2

Rose bengal

1017

100

54

Segmented polymer networks/PAN-H

0.4

Rose bengal

1017

99

55

(SPEEK/PDDA)/PAN

0.2

Rose bengal

1017

99

56

350

68.8

626

89.0

TM

Starmem

Orange II sodium salt 5 wt% PFPE/PI

1.40 Remazol brilliant blue This work Orange II 350

92.7

626

96.8

sodium salt 10 wt% PFPE/PI

0.52 Remazol brilliant blue

3.3 7-day stability tests for other OSN applications In order to demonstrate the applicability of the PFPE coated membranes for broad applications in real life, one active pharmaceutical ingredient (i.e., tetracycline, MW=444.4 g/mole) and one food 22 ACS Paragon Plus Environment

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additive (i.e., 𝛽-carotene, MW=539.9 g/mole) are chosen as model molecules. Table 3 shows their molecular structures. Feed solutions are prepared by dissolving Tetracycline and 𝛽-carotene separately in ethanol and hexane at a concentration of 50 ppm. According to the relationship between solute size and rejection, a 5 wt% PFPE coating solution was chosen to prepare the PFPE coated membrane for the separation of tetracycline from ethanol/tetracycline solutions. Similarly, a 10 wt% PFPE solution was employed to coat the cross-linked polyimide substrate for the recovery of hexane from solutions containing 𝛽-carotene. Figure 6b and c show the evolution of permeance and rejection with time during 168-hour tests where the permeance and rejection were recorded for every 12 h after stabilization for 5 h. Interestingly, the permeance and rejection for both mixtures do not fluctuate significantly during the 7-day tests, suggesting that equilibrium stages have been maintained. In addition, the membranes exhibit good rejections with reasonable fluxes in both tetracycline/ethanol and 𝛽-carotene/hexane solutions, indicating their integrities are well preserved. The stability and testing duration are comparable with other reported membranes in OSN.

On average, the rejection of the 5wt% PFPE coated membrane towards tetracycline is 91.8 ± 1.7 % with an ethanol permeance of 1.65 ± 0.20 Lm-2h-1bar-1. Compared with the pure ethanol permeance as reported in Figure 5a, there is a ~50% reduction of permeance possibly due to membrane compaction, concentration polarization, pore blocking and fouling phenomena. Similarly, the hexane permeance and rejection towards 𝛽-carotene for the 10 wt% PFPE coated membrane are 0.77 ± 0.08 Lm-2h-1bar-1 and 94.2 ± 3.4 %, respectively. It also has a ~50% reduction of permeance compared to the pure hexane permeance. These results confirm the excellent stability of PFPE



Page 24 of 34

coated membranes in ethanol and hexane under a pressure of 5 bar for a reasonable duration and assure their potential for industrial applications.

4. Conclusions In this study, a new type of UV-curable PFPE, Fluorolink® AD1700, was coated and polymerized on the HDA crosslinked Matrimid® substrate for OSN applications. A range of coating concentrations, i.e., 1 wt%, 3 wt%, 5 wt% and 10 wt%, was applied to manipulate the membrane performance. The water contact angles of the PFPE coated membranes were all greater than 90°, suggesting the successful conversion from hydrophilic substrates to hydrophobic membranes. PAS results revealed that the intensity of voids and pore size of the PFPE coated membranes decreased as the coating concentration increased. Various solvents were used to determine the pure solvent permeances through the PFPE coated membranes. It was found that the pure solvent permeance was inversely proportional to its viscosity and molar volume. In addition, two dyes, orange sodium II salt and remazol brilliant blue, were dissolved in IPA to determine the separation performance of the PFPE coated membranes. The membrane coated with a 10 wt% PFPE solution could retain both dyes with rejections greater than 90%. The PFPE coated membranes were further employed to recover ethanol and hexane from tetracycline and 𝛽-carotene solutions, respectively. The 5wt% PFPE coated membrane has a rejection of 91.8 ± 1.7 % towards tetracycline and an ethanol permeance of 1.65 ± 0.19 Lm-2h-1bar-1. Similarly, the 10 wt% PFPE coated membrane has a rejection of 94.2± 3.4 % towards 𝛽-carotene and a hexane permeance of 0.77 ± 0.08 Lm-2h-1bar-1. The flux and rejection of the membranes in tetracycline/ethanol and 𝛽-carotene/ hexane remained stable with little fluctuations for 168 hours (7 days), thereby demonstrating their reliability over long-term operations. Therefore, the novel PFPE coated membranes have great potential for


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applications to recover and purify organic solvents in pharmaceutical, food and petrochemical industries. In the future, other possibilities of utilizing the PFPE to improve OSN performance will be explored as its chemical resistance can be improved simply by UV irradiation to suit OSN applications.

Acknowledgment The authors would like to acknowledge the National Research Foundation, Prime Minister's Office, Singapore for funding this research under its Competitive Research Program for the project entitled, “Development of solvent resistant nanofiltration membranes for sustainable pharmaceutical and petrochemical manufacture”; (CRP Award no. NRF-CRP14-2014-01 (NUS grant number: R-279000-466-281)). The authors would like to give thanks to Dr. Susilo Japip and Mr. Yuqi Gao for their valuable suggestions and assistance in this work.



Page 26 of 34

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Multilayered Polyelectrolytes: Study of Preparation Conditions. J. Membr. Sci. 2010, 358, 150157.



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For Table of Contents Only


Hydrophobic perfluoropolyether (PFPE) coated thin film composite

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