Interfacial polymerization on hydrophobic PVDF UF membranes surface: Membrane wetting through pressurization

Applied Surface Science 356 (2015) 1207–1213

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Interfacial polymerization on hydrophobic PVDF UF membranes surface: Membrane wetting through pressurization Ju Sung Lee, Hyun Ho Lee, Jin Ah Seo, Hyun Sic Park, Jinwon Park, Byoung Ryul Min ∗ Department of Chemical and Biomolecular Engineering, Yonsei University, 120-749, 50 Yonsei-ro, Seodaemun-gu, Seoul, South Korea

a r t i c l e

i n f o

Article history: Received 26 May 2015 Received in revised form 22 August 2015 Accepted 25 August 2015 Available online 28 August 2015 Keywords: Interfacial polymerization PVDF Penetration pressure Hydrophobic surface Nanofiltration

a b s t r a c t PVDF is widely used in water treatment membranes because of it high chemical resistance and thermal stability levels, and desirable mechanical properties. On the other hand, it is seldom used as support membrane for RO membranes, as it is difficult to undertake interfacial polymerization by traditional methods due to characteristic of hydrophobic surface. However, if the MPD solution is applied at pressures which exceed the pressure at which the PVDF membrane pushes water away, then it can be wetted within the membrane and PA/PVDF composite membrane can be prepared through the reaction of the wetted MPD and TMC. The theoretical penetration pressure needed to wet MPD solution in PVDF with pore size of 10 nm, calculated using Jurin’s Law, is 8.8 bar. In this study, PVDF membrane was immersed in MPD solution for 4 h at pressures higher than theoretical penetration pressure using N2 gas at 25 ◦ C. Interfacial polymerization with TMC was undertaken with surface of the PVDF membrane wetted in MPD solution in this manner to form a thin but consistent PA layer, which was verified through FT-IR and SEM. Salt rejection and permeation flux measurements for NaCl 5000 ppm was conducted for the PA/PVDF membranes prepared in this manner at 25 ◦ C, 30 bar using cross-flow water permeation system. PA/PVDF composite membrane wetted with MPD solution and interfacial polymerization undertaken at 10, 16 and 20 bar with N2 gas displayed salt rejection ratio of 37.94, 41.79 and 51.03%, and permeation flux of 7.38, 5.26 and 7.94LMH, respectively. The salt rejection ratio for membrane wetted with MPD at 16 bar with CO2 gas displayed salt rejection ratio of 78.26% and permeation flux of 4.91LMH. The results confirmed the possibility of using PVDF UF membrane of superior properties as support membrane for NF and RO. © 2015 Elsevier B.V. All rights reserved.

1. Introduction In the past century and a half, the world population has increased four-fold while the demand for water has increased seven-fold. The water problem will become only more serious as populations continue to increase and economies expand. According to the World Health Organization, 1.1 billion people lack potable water and 2.6 billion have inadequate sanitation facilities. These facts clearly point to the need to protect existing fresh water resources, while developing new water resources to satisfy the increasing demand. Such endeavors will require more advanced water treatment technology. In this regard, membranes are preferred over other water treatment technologies such as sanitization, distillation, or filtering systems [1]. Research on membranes has advanced rapidly since industrial membranes were first

∗ Corresponding author. Tel.: +82 2 2123 2757; fax: +82 2 312 6401. E-mail address: [email protected] (B.R. Min). http://dx.doi.org/10.1016/j.apsusc.2015.08.226 0169-4332/© 2015 Elsevier B.V. All rights reserved.

developed in the 1960s [2]. Among the various membrane technologies, RO has become prominent in the field of water treatment because of its advantages of low cost, low energy consumption, and simple equipment and operation [3]. NF is a pressure-driven process which is conducted as pressures which are 0.5–2 Mpa lower than for RO. Accordingly, NF is recognized as one of the most superior processed for production of portable water and water treatment [4]. Polyamide (PA) thin-film composite (TFC) membrane is widely used for NF and RO [5,6]. Many studies are being conducted in order to improve key aspects of PA composite membranes such as antifouling and durability. In particular, the hydrophilicity of membrane surface is closely related to these aspects, and numerous studies on hydrophilicity are being conducted such as hydrophilic surface modifying macromolecule and polyurethane end-capped with polyethylene glycol (LSMM) [7,8]. Meanwhile, durability of PA composite membrane is related with used materials. TFC membrane, which has asymmetric structure, is composed of nonwoven fabric, porous support layer, and

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dense thin layer [9]. In past studies, polymers such as polysulfone, polyethersulfone, polycarbonate, polyphenylene oxides, poly(styrene-co-acrylonitrile), poly(phthalazinone ether sulfone ketone), polyacrylonitrile, polyetherimide, polypropylene have been mainly used as material for porous support membrane [10]. PVDF, another polymer material, has mechanical properties, thermal stability, and chemical resistance superior to the aforementioned polymer materials [2,11]. Despite such superior characteristics, it is seldom used as support membranes for PA composite membranes in the application of NF and RO processes due to difficulties in forming PA thin layer on the PVDF support in interfacial polymerization due to its hydrophobicity [12–15]. Several studies have been conducted to attempt coating of consistent and thin layer on PVDF support in order to overcome this limitation, but they have faced problems such as minimal salt rejection ratio or excessively complicated procedure [16–18]. D. Correia [19] introduced a method to modify surface wettability of PVDF fiber through oxygen plasma treatment. This method endows the surface with hydrophilicity by replacing hydrophobic C F groups on the membrane surface with hydrophilic groups such as COOH, C O, and O H. This is a good method to enhance hydrophilicity, but may not be so beneficial for the membrane surface. While the mass of the surface comprises a relatively very small portion of the surface, it is a factor which is directly related to membrane performance. This method has the drawback of weakening thermal and mechanical stability, and chemical resistance, which are characteristics of PVDF. The method introduced in this paper has the advantage of avoiding this problem by allowing for wetting in aqueous phases without structural changes to PVDF. This study introduces a new pressurization method in order to apply PVDF UF membrane as support membrane for RO or NF. Wetting of PVDF with MPD solution was successfully conducted by applying pressures higher than theoretical penetration pressures calculated using Jurin’s Law, and then consistent PA/PVDF composite membranes prepared through reaction with TMC.

2. Theory 2.1. Penetration pressure

2 cos() rp g

(1)

Where h is the depth of water (m) in the container,  is the surface tension (mN/m) of the solution,  is the contact angle (◦ ) between the solution and membrane, rp is the radius (m) of the container,  is the density (g/m3 ) of the solution, and g is the gravitational acceleration (m/s2 ). By multiplying the density and gravitational acceleration to both sides of the equation above, Eq. (2) is obtained. hg =

2 cos() = Pr rp

Pore size (nm)

Contact angle (◦ )

Surface tension (mN/m)

Penetration pressure (bar)

10 20 30

91.77 91.77 91.77

70.98 70.98 70.98

8.8 4.4 2.9

expresses the penetration pressure needed for wetting. This may be expressed as Eq. (3) below. Pp = −Pr = −

2 cos() rp

(3)

Here, Pp is penetration pressure or wetting pressure (mPa).  is the surface tension of the liquid.  is the contact angle formed between the membrane and the liquid. rp is the radius of membrane pore [20]. Table 1 shows the penetration pressure calculated for different pore sizes according to Eq. (3). 3. Experimental 3.1. Materials PVDF with average molecular amount of 700,000 (g/mol), named Solef® 6020, was purchased from Solvay Korea (Korea). Solvent N,N-dimethylacetamide (DMAc) for the dope solution for the preparation of the membrane was purchased from Sigma Aldrich (USA). M-phenylenediamine (MPD), Trimesoyl Chloride purchased from TCI (Japan) and n-hexane, NaOH purchased from Samchun Chemical (Korea) were used as reagents for the interfacial polymerization. For the feed solution, NaCl was purchased from Duksan Pure Chemical (Korea). The gases used for the pressurization, N2 and CO2 gas were purchased from Samheung Gas (Korea). The PET Non-woven used as support was purchased from Philos (Korea). The DI water used for the preparation of the variable solution and testing of the membranes was produced using aqua MAXTM . All the materials were used without further purification. 3.2. Preparation of PVDF ultrafiltration membrane

Hydrophobic materials tend to push water away as they have low affinity for water. However, wetting solution can penetrate the material if greater pressure is applied, which is how MPD solution can be wetted inside PVDF membrane. Reaction with TMC during this process enables the formation of PA layer on the PVDF membrane surface. The pressure needed for wetting can be calculated using Jurin’s Law as shown in Eq. (1). h=

Table 1 Penetration pressure for different pore sizes calculated using contact angle between MPD solution and PVDF membrane, surface tension of MPD solution, and Eq. (3).

(2)

Here, Pr is the pressure (mPa) of the hydrophobic membrane in pushing away the wetting solution, which when multiplied by −1

PVDF dope solution was prepared at 25 ◦ C by mixing PVDF and DMAc at a 13 to 87% ratio. The PVDF dope solution was stirred at 80 rpm while it was being dissolved. Non-solvent induced phase separation (NIPS) method was used to prepare the PVDF ultrafiltration membrane. The prepared PVDF dope solution was thinly coated on the PET non-woven with a 150 ␮m casting knife, and immersed immediately in a coagulation bath of 25 ◦ C with non-solvent (DI water) without any evaporation time. Thereafter, the membrane underwent phase inversion for 30 min in the coagulation bath. Then the membranes were rinsed in DI water bath for about 1day and then rinsed at room temperature for 24 h in order to remove and remnant solvents. 3.3. Wetting of MPD solution into PVDF ultrafiltration membrane In order to wet the prepared PVDF ultrafiltration membrane with MPD solution, the membranes were immersed in a batch with 2% MPD solution containing 0.05% NaOH as shown in Fig. 1. (NaOH was added to promote reaction between MPD and TMC [15,21].) Then the sealed batch was pressurized with gas with a pressure higher than the theoretical penetration pressure. In order to confirm the influence of the salt rejection and permeation flux from the pressure for the wetting, the membranes were pressurized with N2 gas at 10, 16 and 20 bar for 4 h respectively. In

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[4,22].) Prior to the reaction, excess MPD solution on the surface of the membrane was evenly removed with tissue, (MPD solution forms small bubbles on the membrane surface due to the PVDF’s hydrophobic characteristic, making it difficult to use the existing methods of using a roller or shaking [3,23].) and reaction was induced by pouring TMC solution onto the membrane for 20 s. PA/PVDF composite membrane for which reaction was completed was generously rinsed with Hexane, (Hexane was used to remove the TMC in reaction to terminate the interfacial polymerization reaction.) and then again with DI water. 3.5. Membrane characterization 3.5.1. Chemical characterization of membrane The formation of the PA layer on the PVDF ultrafiltration membrane was confirmed with a Fourier transform infra-red spectrophotometer (FT-IR, SPECTRUM 100, Perkin Elmer, USA). Each membrane was measured by cumulating 32 scans per sample at a wavenumber range of 4000–650 cm−1 . Fig. 1. Illustration of simple pressurization process to apply wetting solution to hydrophobic membrane.

addition, in order to confirm the influence of the salt rejection and flux by type of gas, the membranes were pressurized with CO2 gas at 16 bar for 4 h. After the pressurization process, the sealed batch was slowly vented for 1 min with vent valve. The PVDF ultrafiltration membranes wetted with MPD solution in this manner were used for the production of thin polyamide layer. 3.4. Fabrication of PA/PVDF composite membrane by interfacial polymerization The thin PA layer on top of the PVDF ultrafiltration membrane was produced using interfacial polymerization of membrane wetted with MPD solution (2%, w/w) containing NaOH (0.05%, w/w) and TMC solution (0.3%, w/w) on the membrane surface. (The concentrations of MPD, NaOH, and TMC were decided by referring to previous studies which determined optimal concentrations

3.5.2. Morphological characterization of membrane The morphology of the surface of membranes was observed at an accelerating voltage of 5 kV using a field emission scanning electron microscopy (FE-SEM, JEOL-6701F, JEOL, Japan). The membranes were coated with platinum (Pt) for 150 s before the SEM observation, due to their insulant property. 3.5.3. Salt rejection and permeation flux of PA/PVDF composite membrane A NF and RO filtration system (Fig. 2) using a cross-flow method with 0.000314 m2 of effective surface area was used for the salt rejection and feed solution permeation of the PA/PVDF composite membrane, and measured with NaCl solution (5000 mg/L) using driving force of N2 gas at 30 bar and 25 ◦ C. The permeation solution was analyzed using a conductivity meter (HI 8733, HANNA, USA). Salt rejection was calculated using Eq. (4) as follows.



R(%) =

Cp 1− Cf



× 100

Fig. 2. NF and RO filtration system using cross-flow method.

(4)

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Fig. 3. FT-IR peaks for (a) PVDF UF membrane and (b) PA/PVDF composite membrane.

where Cf and Cp are the salt concentration (mg/L) of the feed and permeation solution, respectively. The permeation flux of the feed solution was calculated using Eq. (5) as follows. J(LMH) =

V At

(5)

where V is the volume of the feed solution (L) permeated through the membrane, and A the effective surface area (m2 ) of the membrane. t represents the permeation time (h). 4. Results and discussion 4.1. Confirmation of PA formation by using FT-IR and SEM FT-IR was used along with SEM in order to confirm the formation of PA on the PVDF UF membrane. Unlike PVDF, PA produced by reaction of MPD with TMC has C O bond and N H bond. By confirming the IR peak of the two bonds in the PA/PVDF composite membrane, the formation of PA on the PVDF was confirmed. Fig. 3 is the FT-IR graph of the (a) PVDF UF membrane and (b) PA/PVDF composite membrane in the wavenumber range of 4000–650 cm−1 , respectively. Here, Fig. 3(b) shows C O stretch and N H bend of the amide group at 1660 cm−1 and 1547 cm−1 , respectively, confirming that the membrane was wetted with MPD by pressure and PA was formed well by reaction with TMC. The FT-IR also showed the phase of the PVDF UF membrane used in the study. PVDF is generally known to have non-poled ␣phase (855, 795 and 766 cm−1 ), electroactive ␤-phase (510, 840 and 1279 cm−1 ) and ␥-phase (431, 512, 776, 812, 833-838 and 1234 cm−1 ), depending on the structure of the polymer chain [19,24]. (This study did not track ␥-phase.) The ␣, ␤ and ␥-phases are related to the hydrophilicity of PVDF UF membrane surface, and so it is important to determine the phase. For PVDF UF membrane, which contains ␣ and ␤-phase, the ␤-phase quantity was calculated as in Eq. (6) below [19,24]. F(ˇ) =

Aˇ (Kˇ /K˛ )A˛ + Aˇ

(6)

where F(ˇ) represents ␤-phase content, and K˛ and Kˇ are the absorption coefficient for ␣ and ␤-phase, respectively. A˛ and Aˇ are the absorbance at 766 and 840 cm−1 , respectively. Here, the values for K˛ and Kˇ are 6.1 × 104 cm2 mol−1 and 7.7 × 104 cm2 mol−1 , respectively. Fig. 3(a) shows that the PVDF UF membrane used as support membrane for the PA contains both ␣ and ␤-phase. The ␤-phase content of the PVDF UF membrane was 80.09%, confirming that ␤-phase was superior to ␣-phase. Since ␤-phase is an electroactive phase, its hydrophilicity is relatively higher than non-poled

␣-phase. The superiority of ␤-phase in PVDF UF membrane lowers water contact angle value, leading to decrease in penetration pressure. This characteristic acts as an advantage in wetting the PVDF UF membrane in MPD solution. If the PA has been coated well on the PVDF support membrane, its distinct morphology could be observed on the surface of the PA composite membrane. On this basis, PA formation was confirmed using SEM. Fig. 4 shows the SEM images of PVDF UF membrane and PA/PVDF composite membrane. Fig. 4(a) shows the images magnified at 50,000 times for the PVDF UF membrane, while Fig. 4(b), (c), (d) and (e) shows the same magnifications for the PA/PVDF composite membrane. Here, Fig. 4(a) displays the typical flat surface and very small pores of PVDF. In contrast, the surface of PA/PVDF composite membrane (Fig. 4(b), (c), (d) and (e)) displays the typical rough surface of PA, which was confirmed to be evenly cover the support membrane. Other characteristics could also be confirmed in detail. The PA/PVDF composite membranes displayed different surface morphology depending on the gas used during the wetting process. The surface of the PA/PVDF composite membrane prepared by wetting with CO2 (Fig. 4(b)) displayed a much rougher and dimensional morphology than the surfaces of PA/PVDF composite membrane prepared by wetting with N2 (Fig. 4(c), (d) and (e)). It is presumed that these protrusions are formed as the CO2 , which dissolves in greater amounts in MPD solution compared with N2 , creates thrust in the direction of polyamide during the interfacial polymerization process conducted at room temperature and atmospheric pressure. The surface of the membranes prepared by wetting with N2 showed a tendency to become more threedimensional rougher as wetting pressure increased. This would support the aforementioned presumption. However, additional research is needed in order to verify it. In this manner, it was confirmed using FT-IR and SEM that PA not only formed well through the reaction of MPD and TMC, but was evenly coated on the PVDF UF membrane. 4.2. Salt rejection and permeation flux of PA/PVDF composite membrane Fig. 5(a) shows the graph the salt rejection and permeation flux for the PA/PVDF composite membrane produced by wetting PVDF UF membrane with MPD solution using N2 gas for 4 h at 10, 16 and 20 bar, respectively, and conducting interfacial polymerization with TMC. Fig. 5(b) shows the salt rejection graph for the PA/PVDF composite membrane produced by wetting PVDF UF membrane using N2 and CO2 gas, respectively, for 4 h at 16 bar and conducting interfacial polymerization in order to confirm salt rejection by type of gas. Fig. 5(a) shows that as the pressure applied for wetting the MPD solution increases, the salt rejection ratio for the PA/PVDF composite membrane also increases at NaCl 5000 ppm and 30 bar. This tendency is attributable to the relationship between PVDF pore size and penetration pressure. Eq. (3) derived from Jurin’s law shows this relationship very clearly. Penetration pressure increases as pore size decreases, which means that even wetting is possible for even smaller pore sizes as wetting pressure rises. According to values calculated with Eq. (3), pore sizes of up to 8.8, 5.5, 4.4 nm can be wetted at wetting pressures of 10, 16, and 20 bar, respectively. Ultimately, the rise of wetting pressure increases the number of pores that can be wetted. It may be presumed that PA is coated more evenly on the PVDF for PA/PVDF composite membrane prepared at wetting pressure of 20 bar than the one prepared under 10 bar condition, leading to an increase in salt rejection. On the other hand, permeation flux does not increase with increase in wetting pressure, but displays its lowest value at 16 bar before rising again at wetting pressure of 20 bar. At wetting pressure of 20 bar, optimal performance was observed with NaCl salt rejection ratio of 51.03% and permeate flux of 7.94 LMH. Fig. 5(b) shows that

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Fig. 4. SEM images of PVDF UF membrane (a), PA/PVDF composite membrane prepared by pressurization with CO2 at 16 bar (b) and PA/PVDF composite membrane prepared by pressurization with N2 at 10, 16, 20 bar ((c), (d), (e)).

producing PA/PVDF composite membrane by wetting hydrophobic support membrane with MPD solution using CO2 results in higher salt rejection ratio than when using N2 . PA/PVDF composite membrane produced using CO2 showed a salt rejection ratio of 78.26% at

NaCl 5000 ppm and 30 bar, while that produced using N2 showed a salt rejection ratio of 41.79%. The reason CO2 shows better results than N2 is that increasing water solubility of gas is larger than N2 due to the pressure

Fig. 5. Salt rejection ratio and permeation flux of PA/PVDF composite membrane at 25 ◦ C, 30 bar (operating pressure). (a) Changes in salt rejection and permeation flux due to changes in wetting pressure when using N2 , (b) Salt rejection ratio and permeation flux following type of gas used during wetting (16 bar).

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Fig. 6. (a) Change in water solubility of N2 and CO2 gas with pressure increase, (b) Change in surface tension of water with pressure increase in N2 and CO2 gas.

increment. The more increasing water solubility of gas based on pressure, the more decreasing surface tension of MPD solution. Reduction of surface tension leads to reduction in penetration pressure according to Eq. (3), which results in better wetting of MPD on the hydrophobic support. Fig. 6(a) shows the changes in water solubility according to N2 and CO2 pressure [25–27]. Fig. 6(b) indicates the changes surface tension of water according to increase in pressure when N2 and CO2 are applied to water [28–30]. Fig. 6(a) and (b) shows the same tendency that helps to explain the result in Fig. 5(b). Permeation flux showed little difference between the two gases with 4.91 LMH when wetting with CO2 and 5.26 LMH when wetting with N2 . This means that the effect that wetting pressure has on permeation flux is greater than that of gas type. In other words, in the pressurization method, rejection ratio is greatly affected by type of gas used for wetting, but permeation flux is more greatly affected by other factors such as wetting pressure. PA/PVDF composite membrane prepared by pressurization method showed nanofiltration performance. This indicates that the physical adhesion between PA and PVDF is stable, and the composite membrane is reusable. The adhesion of PA with respect to PVDF is a very important factor directly related to membrane performance. However, this study was conducted on a lab scale. Accordingly, there is the need for studies on key aspects such PA-PVDF as adhesion, PA degradation, and reusability of PA/PVDF composite membrane on a large scale, expanding out this study conducted on a lab scale.

5. Conclusions Using FT-IT to confirm PA formation in PA/PVDF composite membrane produced by wetting PVDF UF membrane with 2% MPD solution with gas and conducting interfacial polymerization with 0.3% TMC, IR peaks of C O stretch and N H bend for amide group were confirmed at 1660 cm−1 and 1547 cm−1 , confirming formation of PA on the PVDF. SEM confirmed that the PA formed in this manner was evenly coated on the PVDF. PA/PVDF composite membrane produced in this manner showed changes in salt rejection and permeation flux according to changes in gas pressure during wetting and by type of gas. Among PA/PVDF composite membrane produced at pressures of 10, 16 and 20 bar, the one produced at 20 bar displayed optimal performance with salt rejection ratio of 51.03% and permeation flux of 7.94 LMH. By type of gas applied, PA/PVDF composite membrane produced using CO2 displayed salt rejection ratio of 78.26%, significantly higher than when using N2 . Based on these results, hydrophobic PVDF UF membrane was successfully wetted with MPD solution using penetration pressure calculated from Jurin’s Law, confirming possibilities for using PVDF

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