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Breakthrough the upperbond of permeability vs. tensile strength of TIPS-prepared PVDF membranes
Journal of Membrane Science 604 (2020) 118089
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Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
Breakthrough the upperbond of permeability vs. tensile strength of TIPS-prepared PVDF membranes Ji-Hao Zuo , Chao Wei , Peng Cheng , Xi Yan , Yan Chen , Wan-Zhong Lang * The Education Ministry Key Laboratory of Resource Chemistry, Shanghai Key Laboratory of Rare Earth Functional Materials, Department of Chemistry and Chemical Engineering, Shanghai Normal University, 100 Guilin Road, Shanghai, 200234, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Poly(vinylidene fluoride) (PVDF) Thermally induced phase separation (TIPS) Air bath Permeability Tensile strength
Permeability and mechanical strength normally present an opposite trend for separation membranes. There is a trade-off line of permeability vs. tensile strength for the prepared poly(vinylidene fluoride) (PVDF) membranes via thermally induced phase separation (TIPS) method. In this work, TIPS-prepared PVDF membranes were fabricated with water (PVDFT-water) and air (PVDFT-air) as quenching bath for comparatively studying. PVDFT-air membranes present larger average pore size, higher permeability and better mechanical strength than those of PVDFT-water membranes due to the lower cooling rate of air bath. It is wonderful that all PVDFT-air membranes can break though the trade-off line of permeability vs. tensile strength of TIPS-prepared PVDF membranes. The optimal membrane (M-A30) shows superior perfor mance, such as ultrahigh water flux of 3275 L m 2 h 1⋅bar 1 and tensile strength of 5.7 MPa. Additionally, this research provides a feasible method to regulate the morphology, pore size and permeability of TIPS-prepared PVDF membranes.
1. Introduction As a semi-crystalline polymer, PVDF has drawn widespread attention in membrane field because of a variety of outstanding properties, such as heat-resistance, weatherability, chemical stability and oxidation resis tance [1–3]. Thus it is constantly employed to prepare microfiltration (MF) [4] and ultrafiltration (UF) [5] membranes. PVDF membranes are typically prepared via the nonsolvent induced phase separation (NIPS) [6] and thermally induced phase separation (TIPS) [7,8]. NIPS generally involves a lot of process parameters and makes the membrane prepa ration be complicated and uncontrollable [9,10]. TIPS has been widely used to prepare PVDF membranes, apart from the advantages of struc ture controllability and continuous production. One of the vital advan tages is the ability to produce membranes from semi-crystalline polymers (e.g., polyethylene (PE), polypropylene (PP), polyacrylonitrile (PAN), and poly(ethylene chlorotrifluoroethylene) (ECTFE) [11–14]) that are normally insoluble in solvents at ambient temperature [15–17]. PVDF membranes prepared by TIPS method can be widely applied in many fields, for example wastewater treatment, drinking water pro duction, gas dehydration, desalination process and oil/water separation [18–22]. In general, one superior separation membrane should have
both high permeability and high mechanical strength. For the tradi tional TIPS method, cold water is commonly used as quenching bath. The rate of rapid heat exchanging may bring about the formation of dense skin layers and closed pores [23], which is prone to suffering from low permeation flux. To overcome the shortcoming of low permeation flux, the pore size of the TIPS-prepared membranes can be well regu lated by adjusting the quenching temperature to enhance its perme ability. For example, Saljoughi et al. [24] studied the effects of coagulation bath temperature (CBT) on morphology, pure water permeation flux and thermal/chemical stability of the asymmetric cel lulose acetate (CA) membranes. The results showed that the water flux increased with the increase of CBT. Similarly, Xu et al. [25] systemati cally investigated the effects of CBT and composition on morphology, wettability, water permeability, separation performance and antifouling property of the polysulfone (PSf) membranes. The results demonstrated that the PSf membranes were more permeable when the CBT increased from 8 � C to 60 � C. Ghasem et al. [26] also reported the effects of quenching temperature on the characteristics and gas absorption per formance of PVDF microporous hollow fiber membranes fabricated via TIPS method. They found that the pore size and water permeability increased with quenching temperature. However, enlarged pore size
* Corresponding author. E-mail address: [email protected] (W.-Z. Lang). https://doi.org/10.1016/j.memsci.2020.118089 Received 18 January 2020; Received in revised form 23 March 2020; Accepted 23 March 2020 Available online 26 March 2020 0376-7388/© 2020 Elsevier B.V. All rights reserved.
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Journal of Membrane Science 604 (2020) 118089
would form loose membrane structure. The membrane pore size increased and solute rejection decreased; meanwhile the mechanical strength was considerably weakened. Poor mechanical strength would greatly limit the applications of membranes. Kim et al. [27] summarized the relationship between the perme ability and mechanical strength of the PVDF membranes prepared by TIPS method in the past ten years. They plotted a trade-off line between permeability and mechanical strength according to Eq. (1). It is noted that the intersections between permeability and mechanical strength of most PVDF membranes fabricated by TIPS were below the trade-off line [9,28–37], as shown in Fig. 1. This arouses great enthusiasm for us to further modify TIPS-prepared PVDF membranes against poor mechani cal strength, and eventually improve durability during practical applications. Herein, robust PVDF membranes were fabricated using air as quenching bath via TIPS method, which can easily break though the trade-off line between permeability and mechanical strength proposed by Kim [27]. This method is easily operated, eco-friendly and energy-saving. The control membranes with water as quenching bath were also prepared and compared. The effects of quenching medium (air and water) and quenching temperature on the membrane morphology, crystallization behaviors, pore size, wettability, permeability and me chanical strength of the resulted membranes were investigated in detail.
steel plate with the casting solution was quenched into a coagulation � bath (water bath at 0, 30, 60 � C and air bath at 30, 40, 60, 80, 100 C) for 0.5 h to induce phase separation. At last, the solidified nascent mem branes were peeled off from the stainless steel plate, and then succes sively immersed in absolute ethanol, deionized water for 12 h at least two times to extract the residual diluent. Detailed preparation condi tions including dope composition, preparation temperature and quenching medium/temperature of the PVDF membranes are listed in Table 1. The PVDF membranes prepared with water as quenching bath were recorded as PVDFT-water; while the ones prepared with air as quenching bath were recorded as PVDFT-air membranes. The specific name of membrane was named by its quenching bath and temperature. For example, M-W0 represented the sample was quenched in water bath � � at 0 C; M-A30 indicated the sample was quenched in air bath at 30 C. The detailed sample names and their preparation parameters were summarized in Table 1. 2.3. Characterizations of PVDF membranes 2.3.1. Morphologies of PVDF membranes The surface and cross-section morphologies of the membranes were observed by a field emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan) with an accelerating voltage of 5.0 kV. A membrane sheet was quickly fractured in liquid nitrogen to obtain clear cross-section. All samples were placed onto a copper holder and sput tered with gold before observing the FESEM images. The AFM images of external surface were examined by an atomicforce microscopy (AFM, Agilent Technologies-5500, USA) operated in tapping mode. The membrane samples were placed in a glass substrate and tested at room temperature with the scanning size of 20 μm�20 μm. Scanning was performed at a speed of 2 Hz. The surface parameters including average roughness (Ra) and roughness (Rq), surface skewness (Rsk) and surface kurtosis (Rku) were calculated from Eq. (2) to Eq. (4), which can be referred in the works of Stawikowska and Livingston [38]. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n 1X Rq ¼ (2) Z2 n i¼1 i
2. Experimental 2.1. Materials Poly(vinylidene fluoride) (PVDF, FR904) in powder was obtained � from Shanghai 3F New Material Co. Ltd (China) and dried at 70 C for 24 h before used. Dibutyl phthalate (DBP) used as a diluent was purchased from Shanghai Aladdin Chemistry Co. Ltd (China). Absolute ethanol (�99.7%) was purchased from Shanghai Chemical Agent Co. Ltd. (China). Deionized water (DI) was self-made by a reverse osmosis (RO) system. None of the chemicals were further purified before used. 2.2. Preparation of PVDF membranes First, casting solutions were prepared by mixing PVDF with DBP in a certain proportion, followed by continuous mechanical stirring at 200 � C until the solution became homogeneous. Then, the prepared dopes were placed in an oven at 200 � C overnight to release air bubbles. Next, the homogeneous casting solutions were cast uniformly onto a stainless steel plate, which was pre-heated in an oven at 200 � C. The stainless
Rsk ¼
n 1 X Z3 3 nRq i¼1 i
(3)
Rku ¼
n 1 X Z4 4 nRq i¼1 i
(4)
2.3.2. Crystallization properties of PVDF membranes The crystallization behaviors of the membranes were evaluated by a wide-angle X-ray diffraction (WAXD, D/max-II B, Japan) in the 2θ range of 5–60� and the attenuated total reflectance Fourier transform infrared spectra (ATR-FTIR, Lambda Scientific Pty Ltd, Australia). The thermal properties of the PVDF membranes were measured by DSC (DSCQ100, TA Instruments, USA), which were carried out under nitrogen Table 1 The detailed preparation conditions of PVDF membranes. Membrane no.
M-W0 M-W30 M-W60 M-A30 M-A60 M-A80 M-A100
Fig. 1. Current permeability upperbound vs. tensile strength for TIPS-prepared PVDF membranes (The original picture comes from Kim’s paper [27]). � � Permeability L ⋅ m 2 ⋅ h 1 ⋅ bar 1 ¼ 11219e 0:575�Tensile Strength½Mpa� (1)
2
Mass fractions (wt %) PVDF
DBP
25 25 25 25 25 25 25
75 75 75 75 75 75 75
Membrane preparation � temperature( C)
Quenching bath medium/ temperature
200 200 200 200 200 200 200
Water/0 � C Water/30 � C Water/60 � C Air/30 � C Air/60 � C Air/80 � C Air/100 � C
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Journal of Membrane Science 604 (2020) 118089
atmosphere at a heating rate of 10 C/min from 25 C to 280 C. The melting heat (ΔHf) of each sample was calculated according to the measured DSC results. The crystallinity (Xc) of the PVDF membranes was calculated on the basis of the following Eq. (5): �
Xc ð%Þ ¼
ΔHf � 100% ΔHf*
�
3. Results and discussion
�
3.1. Morphologies, average pore size and porosity of PVDFT-water and PVDFT-air membranes
(5)
Figs. 2–4 show the FESEM images of PVDFT-water and PVDFT-air membranes prepared with different quenching bath temperatures. From Figs. 2–4, it can be seen that all membranes display bi-continuous fibrous structure on the whole. This structure is the result of liquidliquid (L-L) separation [39]. The pore structure and size presents a strong dependence on the crystal growth during the phase separation for the membranes undergoing L-L phase separation [40–42]. Table 2 shows the average pore size and porosity of PVDF membranes in terms of different quenching mediums and temperatures. For the PVDFT-water membranes, the pores on the surface are fairly uniform from Fig. 2. However, the pore structure of PVDFT-air membranes becomes irregular and intertwine, and the pore size is much bigger than those of the PVDFT-water membranes. It is well known that the heat transfer rate of air bath is much lower than that of water bath. The slower cooling rate gives adequate chance of growth for the PVDF fibrils. Therefore, the fibrils of PVDFT-air membranes grow larger than those of PVDFT-water membranes. Scheme 1 illustrates the phase separation mechanism of PVDFT-water and PVDFT-air membrane in the TIPS process. For the PVDFT-water mem branes, fast cooling rate leads to fast phase separation rate of the cast solution. The PVDF chains have no abundant time to gather together before solidification. Thus, the smaller polymer aggregates and smaller pore size of the resultant membranes were formed. By contrary, for the PVDFT-air membranes, polymer chains have abundant time to gather together before solidification, and thus bigger aggregates and larger pore size were formed due to slow cooling rate and low phase separation rate. Additionally, the pore size increases with the increase of quenching temperature for both PVDFT-water and PVDFT-air membranes. From Table 2, for the PVDFT-water membranes, the average pore size gradually increases from 0.187 μm for M-W0 to 0.391 μm for M-W60. The average pore size of PVDFT-air membranes increases from 0.522 to 0.801 μm as � the quenching increases from 30 to 100 C. Meanwhile, the porosities of the membranes have the same variation tendency with the pore size, and increase with increasing quenching temperature. The smaller tempera ture difference between the dope and the quenching bath leads to the lower cooling rate in TIPS process [43,44]. The slower cooling rate causes the system to go through the L–L phase separation region (below the monotectic point) slowly and arrive at the crystallization line, implying the polymer solution spends more time to go through the L–L phase separation region. Pore size is determined by the time delay within the L-L phase separation region. The longer period in L-L phase separation region yields bigger pores for the TIPS-prepared membranes [45,46]. To put it another way, when the quenching temperature is lower than the spinodal decomposition point, the super-cooling depth decreases substantially with the increase of quenching temperature. Thus, larger pores would be formed due to a slower solidification rate of the polymer-rich phase, which leave a longer time for the polymer-lean phase to grow [30,47,48]. Overall, both slower cooling rate and longer periods in the L-L phase separation region lead to larger pore size for the PVDFT-air membrane with higher quenching temperature. On the whole, adjusting quenching medium and temperature is an effective approach to tailor the pore size and structure of the TIPS-prepared PVDF membranes. The FESEM images of the bottom surfaces of PVDFT-water and PVDFTair membranes are shown in Fig. 3. Different from the top surfaces, the bottom surfaces present a sheaf-like structure. The relationship between pore size and quenching medium/temperature is the same as the top surfaces. The cross-sectional images of PVDFT-water and PVDFT-air membranes are shown in Fig. 4. It is clear that the cross-sections of PVDFT-air membranes are more porous than those of PVDFT-water membranes. Meanwhile, the cross-sections of both PVDFT-water and PVDFT-air
where ΔH*f is the melting enthalpy of pure PVDF (104.7 J/g) and ΔHf is the melting enthalpy of the tested membrane.
2.3.3. Average pore sizes and porosities of PVDF membranes The average pore size of each membrane was investigated by a Membrane Pore Size Analyzer (3H–2000PB, Beishide Instrument-S&T. (Beijing) Co. Ltd. China). The membrane sample was dried before testing. Then, one circular membrane sheet was completely wetted by the Porofil standard wetting solution with a surface tension of 16 dyn/ cm. The porosity (ε) of each membrane was evaluated with the gravi metric method, which was calculated according to Eq. (6): ðm1
ε¼
ðm1
m2 Þ=ρ
m2 Þ=ρ
water
water � � 100 þ m2 ρ
(6)
p
where m1(g) and m2(g) are the weight of the wet membrane and dry membrane, respectively; ρp (g/cm3) and ρwater (g/cm3) are the PVDF � density (ρp ¼ 1.77–1.80 g/cm3) and the water density at 25 C (ρwater ¼ 3 0.997 g/cm ), respectively. 2.3.4. Dynamic contact angles of PVDF membranes The surface wettability of PVDF membranes was measured by a water contact angle analyzer (KRÜSS DSA30, German) equipped with video camera at room temperature. The dry membrane samples attached to the slide glass with the top surface facing up, and a drop of DI water (3.0 μL) was dripped onto the top surface of membranes. All tests were recorded in movies for 10 min in air and repeated at least 3 times to minimize experimental error. 2.3.5. Mechanical properties of PVDF membranes The mechanical properties including tensile strength, breaking elongation and Young’s modulus of PVDF membranes were evaluated by a testing instrument (QJ210A, Shanghai Qingji Instrumentation Sci. & Tech. Co., Ltd, Shanghai, China) at room temperature. The samples were prepared in strip shape with a length of 8 cm and a width of 3 mm, followed by fixed vertically between two pairs of tweezers with a length of 50 mm. Then, the sample was extended with the stretching rate of 50 mm/min until it was broken. Each sample was measured at least five times and then averaged to confirm the reliability. 2.3.6. Permeation performance measurement The permeation fluxes for pure water were measured with a deadend filtration system. The newly prepared membranes were prepressured until the flux was stable, and then tested for pure water flux. The pure water flux (Jw, L⋅m 2⋅h 1⋅bar 1) of the membranes was calculated according to the following Eq. (7): Jw ¼
V A⋅t⋅ΔP
(7)
where V (L) is the volume of permeated water, A (m2) is the effective membrane area, ΔP (bar) is the transmembrane pressure and t (h) is the permeation time.
3
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Journal of Membrane Science 604 (2020) 118089
Fig. 2. SEM images of top surface of PVDF membranes prepared at different quenching mediums and temperatures.
Fig. 3. SEM images of bottom surface of PVDF membranes prepared at different quenching mediums and temperatures.
Fig. 4. SEM images of overall cross-section and corresponding inner morphology of PVDF membranes prepared at different quenching mediums and temperatures.
4
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Journal of Membrane Science 604 (2020) 118089
roughness changes the surface microstructure, which has a direct impact on the wettability and permeability of the resulted membranes. The negative Rsk values are found for all membranes, which suggests that the surfaces of membranes are occupied by valleys. Rku value is the indicator of height distribution. It can be seen that the Rku values of PVDFT-air membranes are evidently higher than those of PVDFT-water membranes, which suggests that the former have a sharper height distribution in comparison with the latter.
Table 2 Average pore size and porosity of the as-prepared PVDF membranes. Membrane no.
Mean pore size(nm)
Porosity (%)
M-W0 M-W30 M-W60 M-A30 M-A60 M-A80 M-A100
187.4 281.8 390.8 522.4 626.2 744.2 801.3
77.2 � 78.5 � 79.9 � 77.3 � 77.6 � 79.8 � 80.5 �
� 18.2 � 16.0 � 34.4 � 13.3 � 35.2 � 10.7 � 25.9
0.2 0.6 0.1 0.5 0.2 0.1 0.4
3.2. Crystallization properties of PVDFT-water and PVDFT-air membranes
membranes exhibit more porous tendency with the increase of quenching temperature. The surface roughness of membranes was tested by AFM technique with tapping mode. The AFM images and roughness parameters of PVDFT-water and PVDFT-air membranes are shown in Fig. 5 and Table 3, respectively. From Fig. 5, the nodules on the surfaces of PVDFT-water membranes are markedly smaller than those on the PVDFT-air mem branes. The results are attributed to the cooling rate difference in TIPS process between two types of membranes. For the PVDFT-air membranes, a lower cooling rate gives much longer time for the nodules to grow, and thus the larger nodules are formed compared with the PVDFT-water membranes. From Fig. 5 and Table 3, it also can be observed that the increase of quenching temperature can evidently enhance the surface roughness (Ra and Rq values) for both PVDFT-water and PVDFT-air mem branes. The increase of quenching bath temperature leads to the decrease of cooling rate and contributes to the prolongation of the growth time of the PVDF fibrils in TIPS process. The increase of
To detect the polymorphisms of PVDFT-water and PVDFT-air mem branes prepared by various quenching temperatures, XRD character izations were performed to investigate the effects of quenching medium Table 3 Top surface parameters of the as-prepared PVDF membranes obtained from AFM measurement. \Membrane no.
Ra(nm)
Rq(nm)
M-W0 M-W30 M-W60 M-A30 M-A60 M-A80 M-A100
38.2 53.2 62.4 10.5 43.4 81.1 112.7
45.1 66.7 76.5 13.6 54.5 100.6 149.0
Rsk 0.39 0.37 0.02 0.17 0.33 0.23 0.76
Scheme 1. Schematic illustration of the phase separation mechanism of PVDFT-water and PVDFT-air membrane during phase inversion.
Fig. 5. AFM images of top surfaces of PVDF membranes prepared at different quenching mediums and temperatures. 5
Rku 2.33 2.68 2.37 2.83 3.30 3.20 4.52
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Journal of Membrane Science 604 (2020) 118089
and temperature on the crystalline regions of PVDF, and the results are shown in Fig. 6. From Fig. 6, all the membranes exhibit similar diffraction patterns. The positions of the three peaks are 18.5� , 20.1� and 26.7� , corresponding to the reflections of (020), (110) and (021) planes of α phase. The ATR-FTIR spectra of PVDF membranes in Fig. 7 show that the peaks at 763, 795, 855, 879 and 973 cm 1 are found for all PVDF membranes that are indicative of the α-phase PVDF. The peaks at 855 and 879 cm 1 could be attributed to the vibration of amorphous components and CH2 vibration in PVDF, respectively. The results are consistent with the XRD results. It suggests that the crystal forms of PVDF polymer are mainly α phase and remain unchanged whatever the membranes quenched in water or air, or whatever quenching tempera ture are. The thermal properties of the PVDF membranes were. The melting temperature(Tm) and crystallinity (Xc) of the PVDF membranes were measured by DSC technique, and the results are presented in Fig. 8 and Table 4. The main peaks in Fig. 8 are attributed to the melting of crystallized PVDF chains. It can be seen that the Tm and Xc decrease with the increase of quenching temperature for both PVDFT-water and PVDFTair membranes. When the quenching temperature rises and the temper ature difference between the quenching bath and dope declines, the driving power for the crystallization of PVDF is declined, and thus the crystallinity (Xc) is decreased [49].
Fig. 7. ATR–FTIR spectra of PVDF membranes prepared at different quenching medium and temperature.
3.3. Wettability of PVDFT-water and PVDFT-air membranes The water contact angles of PVDFT-water and PVDFT-air membranes are presented in Fig. 9. It can be seen that the water contact angles of PVDFT-water and PVDFT-air membranes both slightly increase with the increase of quenching temperature. This should be attributed to the variation of surface roughness of the membranes. According to the Wenzel model [50], the rough surface increases the contact area of the “solid-liquid” and presents higher intuitive geometric area, and thus the surface hydrophobicity is enhanced. From Table 3, the surface rough ness increases for both PVDFT-water and PVDFT-air membranes, which should be the reason for the increase of the water contact angles of PVDFT-water and PVDFT-air membranes. 3.4. Water permeability and mechanical properties of PVDFT-water and PVDFT-air membranes
Fig. 8. DSC curves of PVDF membranes prepared at different quenching me dium and temperature.
Fig. 10 displays the relationship between water flux and quenching temperature in different quenching mediums. From Fig. 10, the pure water flux evidently increases with the increase of quenching tempera ture for both PVDFT-water and PVDFT-air membranes. M-W0 membrane shows the lowest pure water flux of 1435.0 L m 2 h 1⋅bar 1 in all
Table 4 The melting temperature(Tm) and crystallinity (Xc) of PVDF membranes. Membrane no.
Tm (� C)
Xc(%)
M-W0 M-W30 M-W60 M-A30 M-A60 M-A80 M-A100
162.92 161.89 161.72 166.69 165.05 164.83 162.72
45.1 43.3 42.6 43.5 42.7 42.0 40.4
PVDFT-water membranes. The pure water fluxes of PVDFT-air membranes are much higher than those of PVDFT-water membranes, which is attributed to the much higher pore size of the PVDFT-air membranes (Table 2). The pure water flux of M-A100 achieves the maximum value of 10097.4 L m 2 h 1⋅bar 1, which is quite high for microporous membranes. A good microfiltration membrane used in practical separation pro cesses must have excellent mechanical strength [51,52]. The mechanical properties (tensile strength, elongation and Young modulus) of PVDFT-water and PVDFT-air membranes were measured and listed in Table 5. The mechanical strength of the PVDFT-air membranes is much greater than that of the PVDFT-water membranes, which is a prominent
Fig. 6. XRD patterns of PVDF membranes prepared at different quenching medium and temperature. 6
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Journal of Membrane Science 604 (2020) 118089
opposite trend upon a crowd of experiments. For the same membrane, the mechanical strength correspondingly decreases as the permeability increases. Conversely, the permeability will reduce when the mechani cal strength is improved. It is difficult to perform a large-scale separation operation when the tensile strength of membrane is less than 2 MPa [27]. It has been mentioned that Kim et al. summarized the relationship between the permeability and mechanical strength of the TIPS-prepared PVDF membranes, and they draw the trade-off line between perme ability and tensile strength according to Eq. (1). It was found that most TIPS-prepared PVDF membranes cannot overcome the upper bound. Fig. 11 shows the relationships between the permeability and tensile strength of PVDFT-water and PVDFT-air membranes in this work (M-W0, M-W30, M-W60, M-A30, M-A60, M-A80 and M-A100) and previous reports in Kim’ review. It is wonderfully found that all PVDFT-air mem branes (M-A30, M-A60, M-A80 and M-A100) locate above the trade-off line and also are superior to the previous reports in Kim’s review. However, PVDFT-water membranes are dwarfed by comparison and they all locate below the trade-off line. This shows that a TIPS-prepared PVDF membrane with both high permeability and high tensile strength could be simultaneously achieved by optimizing the cooling rate of mem branes via properly tuning the quenching medium (air and water) and quenching temperature.
Fig. 9. Water contact angles of PVDFT-water and PVDFT-air membranes.
4. Conclusions PVDFT-air and PVDFT-water membranes were prepared via TIPS method with air and water as quenching bath. The PVDFT-air membranes show much bigger pore size than that of the PVDFT-water membranes due to the much lower heat transfer rate of air bath. Also, the pore size in creases with the increase of quenching temperature for both PVDFT-water and PVDFT-air membranes. Thus, the pure water fluxes of PVDFT-air membranes are much higher than those of PVDFT-water membranes. MA100 achieves the maximum pure water flux of 10097.4 L m 2 h 1⋅bar 1, which is quite high for the microporous membranes. The PVDFT-air membranes present much greater mechanical strength than the PVDFT-water membranes, which is a prominent advantage in practical applications. It is surprising that all PVDFT-air membranes can easy overcome the upper bound between the permeability and tensile strength proposed by Kim [27]; while PVDFT-water membranes cannot do it. This research provides a feasible way to regulate the PVDF membrane structures and pore size with promoted performance for other applications.
Fig. 10. Pure water flux as a function of quenching temperature for PVDFT-water and PVDFT-air membranes. Table 5 The mechanical properties of as-prepared PVDF membranes. Membrane no.
Tensile strength (MPa)
Breaking elongation (%)
Young modulus (MPa)
M-W0 M-W30 M-W60 M-A30 M-A60 M-A80 M-A100
1.83 � 0.17 1.76 � 0.08 1.59 � 0.05 5.67 � 0.24 3.22 � 0.20 2.37 � 0.11 1.81 � 0.09
17.71 � 2.45 10.02 � 2.01 8.02 � 1.62 15.25 � 1.98 13.36 � 2.37 12.69 � 1.63 11.02 � 1.34
46.84 � 1.79 50.91 � 5.66 49.12 � 4.31 120.83 � 8.74 75.80 � 7.15 61.56 � 6.59 50.23 � 4.72
advantage in practical applications. It has been mentioned in our pre vious work [53]. PVDF fibers are randomly “stretched” during devel oping when quenching in air, which results in an irregular and intertwined pore structures. This structure is closely connected with each other, contributed to extraordinary mechanical strength [32]. The increase of quenching temperature will result in the slightly decrease of mechanical strength, which is due to the loose structure caused by the enlarged pore size [26]. Therefore, it is very important to choose a suitable quenching medium and quenching temperature to prepare PVDF membranes with excellent mechanical properties. Permeability and mechanical strength are two key indicators for separation membranes in practical applications. However, many re searchers found that these two indicators normally presented an
Fig. 11. Permeability upperbound vs. tensile strength for PVDFT-water and PVDFT-air membranes. (The upperbound equation is given in Eq. (1)). 7
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Journal of Membrane Science 604 (2020) 118089
Declaration of competing interest [16]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[17]
CRediT authorship contribution statement [18]
Ji-Hao Zuo: Conceptualization, Investigation, Writing - original draft. Chao Wei: Validation. Peng Cheng: Investigation. Xi Yan: Re sources. Yan Chen: Methodology. Wan-Zhong Lang: Supervision, Writing - review & editing.
[19] [20]
Acknowledgments
[21]
The research is supported by Science and Technology Commission of Shanghai Municipality (14520502900), Shanghai Engineering Research Center of Green Energy Chemical Engineering (18DZ2254200) and In ternational Joint Laboratory on Resource Chemistry (IJLRC).
[22]
[23]
Appendix A. Supplementary data
[24]
Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2020.118089.
[25]
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Matsuyama, Effect of additives on the morphology and properties of poly(vinylidene fluoride) blend hollow fiber membrane prepared by the thermally induced phase separation method, J. Membr. Sci. 423–424 (2012) 189–194. X. Li, H. Liu, C. Xiao, S. Ma, X. Zhao, Effect of take-up speed on polyvinylidene fluoride hollow fiber membrane in a thermally induced phase separation process, J. Appl. Polym. Sci. 128 (2013) 1054–1060. Z. Cui, N.T. Hassankiadeh, S.Y. Lee, J.M. Lee, K.T. Woo, A. Sanguineti, V. Arcella, Y.M. Lee, E. Drioli, Poly(vinylidene fluoride) membrane preparation with an environmental diluent via thermally induced phase separation, J. Membr. Sci. 444 (2013) 223–236. N.T. Hassankiadeh, Z. Cui, J.H. Kim, D.W. Shin, A. Sanguineti, V. Arcella, Y.M. Lee, E. Drioli, PVDF hollow fiber membranes prepared from green diluent via thermally induced phase separation: effect of PVDF molecular weight, J. Membr. Sci. 471 (2014) 237–246. N.T. Hassankiadeh, Z. Cui, J.H. Kim, D.W. Shin, S.Y. 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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
Breakthrough the upperbond of permeability vs. tensile strength of TIPS-prepared PVDF membranes Ji-Hao Zuo , Chao Wei , Peng Cheng , Xi Yan , Yan Chen , Wan-Zhong Lang * The Education Ministry Key Laboratory of Resource Chemistry, Shanghai Key Laboratory of Rare Earth Functional Materials, Department of Chemistry and Chemical Engineering, Shanghai Normal University, 100 Guilin Road, Shanghai, 200234, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Poly(vinylidene fluoride) (PVDF) Thermally induced phase separation (TIPS) Air bath Permeability Tensile strength
Permeability and mechanical strength normally present an opposite trend for separation membranes. There is a trade-off line of permeability vs. tensile strength for the prepared poly(vinylidene fluoride) (PVDF) membranes via thermally induced phase separation (TIPS) method. In this work, TIPS-prepared PVDF membranes were fabricated with water (PVDFT-water) and air (PVDFT-air) as quenching bath for comparatively studying. PVDFT-air membranes present larger average pore size, higher permeability and better mechanical strength than those of PVDFT-water membranes due to the lower cooling rate of air bath. It is wonderful that all PVDFT-air membranes can break though the trade-off line of permeability vs. tensile strength of TIPS-prepared PVDF membranes. The optimal membrane (M-A30) shows superior perfor mance, such as ultrahigh water flux of 3275 L m 2 h 1⋅bar 1 and tensile strength of 5.7 MPa. Additionally, this research provides a feasible method to regulate the morphology, pore size and permeability of TIPS-prepared PVDF membranes.
1. Introduction As a semi-crystalline polymer, PVDF has drawn widespread attention in membrane field because of a variety of outstanding properties, such as heat-resistance, weatherability, chemical stability and oxidation resis tance [1–3]. Thus it is constantly employed to prepare microfiltration (MF) [4] and ultrafiltration (UF) [5] membranes. PVDF membranes are typically prepared via the nonsolvent induced phase separation (NIPS) [6] and thermally induced phase separation (TIPS) [7,8]. NIPS generally involves a lot of process parameters and makes the membrane prepa ration be complicated and uncontrollable [9,10]. TIPS has been widely used to prepare PVDF membranes, apart from the advantages of struc ture controllability and continuous production. One of the vital advan tages is the ability to produce membranes from semi-crystalline polymers (e.g., polyethylene (PE), polypropylene (PP), polyacrylonitrile (PAN), and poly(ethylene chlorotrifluoroethylene) (ECTFE) [11–14]) that are normally insoluble in solvents at ambient temperature [15–17]. PVDF membranes prepared by TIPS method can be widely applied in many fields, for example wastewater treatment, drinking water pro duction, gas dehydration, desalination process and oil/water separation [18–22]. In general, one superior separation membrane should have
both high permeability and high mechanical strength. For the tradi tional TIPS method, cold water is commonly used as quenching bath. The rate of rapid heat exchanging may bring about the formation of dense skin layers and closed pores [23], which is prone to suffering from low permeation flux. To overcome the shortcoming of low permeation flux, the pore size of the TIPS-prepared membranes can be well regu lated by adjusting the quenching temperature to enhance its perme ability. For example, Saljoughi et al. [24] studied the effects of coagulation bath temperature (CBT) on morphology, pure water permeation flux and thermal/chemical stability of the asymmetric cel lulose acetate (CA) membranes. The results showed that the water flux increased with the increase of CBT. Similarly, Xu et al. [25] systemati cally investigated the effects of CBT and composition on morphology, wettability, water permeability, separation performance and antifouling property of the polysulfone (PSf) membranes. The results demonstrated that the PSf membranes were more permeable when the CBT increased from 8 � C to 60 � C. Ghasem et al. [26] also reported the effects of quenching temperature on the characteristics and gas absorption per formance of PVDF microporous hollow fiber membranes fabricated via TIPS method. They found that the pore size and water permeability increased with quenching temperature. However, enlarged pore size
* Corresponding author. E-mail address: [email protected] (W.-Z. Lang). https://doi.org/10.1016/j.memsci.2020.118089 Received 18 January 2020; Received in revised form 23 March 2020; Accepted 23 March 2020 Available online 26 March 2020 0376-7388/© 2020 Elsevier B.V. All rights reserved.
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would form loose membrane structure. The membrane pore size increased and solute rejection decreased; meanwhile the mechanical strength was considerably weakened. Poor mechanical strength would greatly limit the applications of membranes. Kim et al. [27] summarized the relationship between the perme ability and mechanical strength of the PVDF membranes prepared by TIPS method in the past ten years. They plotted a trade-off line between permeability and mechanical strength according to Eq. (1). It is noted that the intersections between permeability and mechanical strength of most PVDF membranes fabricated by TIPS were below the trade-off line [9,28–37], as shown in Fig. 1. This arouses great enthusiasm for us to further modify TIPS-prepared PVDF membranes against poor mechani cal strength, and eventually improve durability during practical applications. Herein, robust PVDF membranes were fabricated using air as quenching bath via TIPS method, which can easily break though the trade-off line between permeability and mechanical strength proposed by Kim [27]. This method is easily operated, eco-friendly and energy-saving. The control membranes with water as quenching bath were also prepared and compared. The effects of quenching medium (air and water) and quenching temperature on the membrane morphology, crystallization behaviors, pore size, wettability, permeability and me chanical strength of the resulted membranes were investigated in detail.
steel plate with the casting solution was quenched into a coagulation � bath (water bath at 0, 30, 60 � C and air bath at 30, 40, 60, 80, 100 C) for 0.5 h to induce phase separation. At last, the solidified nascent mem branes were peeled off from the stainless steel plate, and then succes sively immersed in absolute ethanol, deionized water for 12 h at least two times to extract the residual diluent. Detailed preparation condi tions including dope composition, preparation temperature and quenching medium/temperature of the PVDF membranes are listed in Table 1. The PVDF membranes prepared with water as quenching bath were recorded as PVDFT-water; while the ones prepared with air as quenching bath were recorded as PVDFT-air membranes. The specific name of membrane was named by its quenching bath and temperature. For example, M-W0 represented the sample was quenched in water bath � � at 0 C; M-A30 indicated the sample was quenched in air bath at 30 C. The detailed sample names and their preparation parameters were summarized in Table 1. 2.3. Characterizations of PVDF membranes 2.3.1. Morphologies of PVDF membranes The surface and cross-section morphologies of the membranes were observed by a field emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan) with an accelerating voltage of 5.0 kV. A membrane sheet was quickly fractured in liquid nitrogen to obtain clear cross-section. All samples were placed onto a copper holder and sput tered with gold before observing the FESEM images. The AFM images of external surface were examined by an atomicforce microscopy (AFM, Agilent Technologies-5500, USA) operated in tapping mode. The membrane samples were placed in a glass substrate and tested at room temperature with the scanning size of 20 μm�20 μm. Scanning was performed at a speed of 2 Hz. The surface parameters including average roughness (Ra) and roughness (Rq), surface skewness (Rsk) and surface kurtosis (Rku) were calculated from Eq. (2) to Eq. (4), which can be referred in the works of Stawikowska and Livingston [38]. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n 1X Rq ¼ (2) Z2 n i¼1 i
2. Experimental 2.1. Materials Poly(vinylidene fluoride) (PVDF, FR904) in powder was obtained � from Shanghai 3F New Material Co. Ltd (China) and dried at 70 C for 24 h before used. Dibutyl phthalate (DBP) used as a diluent was purchased from Shanghai Aladdin Chemistry Co. Ltd (China). Absolute ethanol (�99.7%) was purchased from Shanghai Chemical Agent Co. Ltd. (China). Deionized water (DI) was self-made by a reverse osmosis (RO) system. None of the chemicals were further purified before used. 2.2. Preparation of PVDF membranes First, casting solutions were prepared by mixing PVDF with DBP in a certain proportion, followed by continuous mechanical stirring at 200 � C until the solution became homogeneous. Then, the prepared dopes were placed in an oven at 200 � C overnight to release air bubbles. Next, the homogeneous casting solutions were cast uniformly onto a stainless steel plate, which was pre-heated in an oven at 200 � C. The stainless
Rsk ¼
n 1 X Z3 3 nRq i¼1 i
(3)
Rku ¼
n 1 X Z4 4 nRq i¼1 i
(4)
2.3.2. Crystallization properties of PVDF membranes The crystallization behaviors of the membranes were evaluated by a wide-angle X-ray diffraction (WAXD, D/max-II B, Japan) in the 2θ range of 5–60� and the attenuated total reflectance Fourier transform infrared spectra (ATR-FTIR, Lambda Scientific Pty Ltd, Australia). The thermal properties of the PVDF membranes were measured by DSC (DSCQ100, TA Instruments, USA), which were carried out under nitrogen Table 1 The detailed preparation conditions of PVDF membranes. Membrane no.
M-W0 M-W30 M-W60 M-A30 M-A60 M-A80 M-A100
Fig. 1. Current permeability upperbound vs. tensile strength for TIPS-prepared PVDF membranes (The original picture comes from Kim’s paper [27]). � � Permeability L ⋅ m 2 ⋅ h 1 ⋅ bar 1 ¼ 11219e 0:575�Tensile Strength½Mpa� (1)
2
Mass fractions (wt %) PVDF
DBP
25 25 25 25 25 25 25
75 75 75 75 75 75 75
Membrane preparation � temperature( C)
Quenching bath medium/ temperature
200 200 200 200 200 200 200
Water/0 � C Water/30 � C Water/60 � C Air/30 � C Air/60 � C Air/80 � C Air/100 � C
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Journal of Membrane Science 604 (2020) 118089
atmosphere at a heating rate of 10 C/min from 25 C to 280 C. The melting heat (ΔHf) of each sample was calculated according to the measured DSC results. The crystallinity (Xc) of the PVDF membranes was calculated on the basis of the following Eq. (5): �
Xc ð%Þ ¼
ΔHf � 100% ΔHf*
�
3. Results and discussion
�
3.1. Morphologies, average pore size and porosity of PVDFT-water and PVDFT-air membranes
(5)
Figs. 2–4 show the FESEM images of PVDFT-water and PVDFT-air membranes prepared with different quenching bath temperatures. From Figs. 2–4, it can be seen that all membranes display bi-continuous fibrous structure on the whole. This structure is the result of liquidliquid (L-L) separation [39]. The pore structure and size presents a strong dependence on the crystal growth during the phase separation for the membranes undergoing L-L phase separation [40–42]. Table 2 shows the average pore size and porosity of PVDF membranes in terms of different quenching mediums and temperatures. For the PVDFT-water membranes, the pores on the surface are fairly uniform from Fig. 2. However, the pore structure of PVDFT-air membranes becomes irregular and intertwine, and the pore size is much bigger than those of the PVDFT-water membranes. It is well known that the heat transfer rate of air bath is much lower than that of water bath. The slower cooling rate gives adequate chance of growth for the PVDF fibrils. Therefore, the fibrils of PVDFT-air membranes grow larger than those of PVDFT-water membranes. Scheme 1 illustrates the phase separation mechanism of PVDFT-water and PVDFT-air membrane in the TIPS process. For the PVDFT-water mem branes, fast cooling rate leads to fast phase separation rate of the cast solution. The PVDF chains have no abundant time to gather together before solidification. Thus, the smaller polymer aggregates and smaller pore size of the resultant membranes were formed. By contrary, for the PVDFT-air membranes, polymer chains have abundant time to gather together before solidification, and thus bigger aggregates and larger pore size were formed due to slow cooling rate and low phase separation rate. Additionally, the pore size increases with the increase of quenching temperature for both PVDFT-water and PVDFT-air membranes. From Table 2, for the PVDFT-water membranes, the average pore size gradually increases from 0.187 μm for M-W0 to 0.391 μm for M-W60. The average pore size of PVDFT-air membranes increases from 0.522 to 0.801 μm as � the quenching increases from 30 to 100 C. Meanwhile, the porosities of the membranes have the same variation tendency with the pore size, and increase with increasing quenching temperature. The smaller tempera ture difference between the dope and the quenching bath leads to the lower cooling rate in TIPS process [43,44]. The slower cooling rate causes the system to go through the L–L phase separation region (below the monotectic point) slowly and arrive at the crystallization line, implying the polymer solution spends more time to go through the L–L phase separation region. Pore size is determined by the time delay within the L-L phase separation region. The longer period in L-L phase separation region yields bigger pores for the TIPS-prepared membranes [45,46]. To put it another way, when the quenching temperature is lower than the spinodal decomposition point, the super-cooling depth decreases substantially with the increase of quenching temperature. Thus, larger pores would be formed due to a slower solidification rate of the polymer-rich phase, which leave a longer time for the polymer-lean phase to grow [30,47,48]. Overall, both slower cooling rate and longer periods in the L-L phase separation region lead to larger pore size for the PVDFT-air membrane with higher quenching temperature. On the whole, adjusting quenching medium and temperature is an effective approach to tailor the pore size and structure of the TIPS-prepared PVDF membranes. The FESEM images of the bottom surfaces of PVDFT-water and PVDFTair membranes are shown in Fig. 3. Different from the top surfaces, the bottom surfaces present a sheaf-like structure. The relationship between pore size and quenching medium/temperature is the same as the top surfaces. The cross-sectional images of PVDFT-water and PVDFT-air membranes are shown in Fig. 4. It is clear that the cross-sections of PVDFT-air membranes are more porous than those of PVDFT-water membranes. Meanwhile, the cross-sections of both PVDFT-water and PVDFT-air
where ΔH*f is the melting enthalpy of pure PVDF (104.7 J/g) and ΔHf is the melting enthalpy of the tested membrane.
2.3.3. Average pore sizes and porosities of PVDF membranes The average pore size of each membrane was investigated by a Membrane Pore Size Analyzer (3H–2000PB, Beishide Instrument-S&T. (Beijing) Co. Ltd. China). The membrane sample was dried before testing. Then, one circular membrane sheet was completely wetted by the Porofil standard wetting solution with a surface tension of 16 dyn/ cm. The porosity (ε) of each membrane was evaluated with the gravi metric method, which was calculated according to Eq. (6): ðm1
ε¼
ðm1
m2 Þ=ρ
m2 Þ=ρ
water
water � � 100 þ m2 ρ
(6)
p
where m1(g) and m2(g) are the weight of the wet membrane and dry membrane, respectively; ρp (g/cm3) and ρwater (g/cm3) are the PVDF � density (ρp ¼ 1.77–1.80 g/cm3) and the water density at 25 C (ρwater ¼ 3 0.997 g/cm ), respectively. 2.3.4. Dynamic contact angles of PVDF membranes The surface wettability of PVDF membranes was measured by a water contact angle analyzer (KRÜSS DSA30, German) equipped with video camera at room temperature. The dry membrane samples attached to the slide glass with the top surface facing up, and a drop of DI water (3.0 μL) was dripped onto the top surface of membranes. All tests were recorded in movies for 10 min in air and repeated at least 3 times to minimize experimental error. 2.3.5. Mechanical properties of PVDF membranes The mechanical properties including tensile strength, breaking elongation and Young’s modulus of PVDF membranes were evaluated by a testing instrument (QJ210A, Shanghai Qingji Instrumentation Sci. & Tech. Co., Ltd, Shanghai, China) at room temperature. The samples were prepared in strip shape with a length of 8 cm and a width of 3 mm, followed by fixed vertically between two pairs of tweezers with a length of 50 mm. Then, the sample was extended with the stretching rate of 50 mm/min until it was broken. Each sample was measured at least five times and then averaged to confirm the reliability. 2.3.6. Permeation performance measurement The permeation fluxes for pure water were measured with a deadend filtration system. The newly prepared membranes were prepressured until the flux was stable, and then tested for pure water flux. The pure water flux (Jw, L⋅m 2⋅h 1⋅bar 1) of the membranes was calculated according to the following Eq. (7): Jw ¼
V A⋅t⋅ΔP
(7)
where V (L) is the volume of permeated water, A (m2) is the effective membrane area, ΔP (bar) is the transmembrane pressure and t (h) is the permeation time.
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Journal of Membrane Science 604 (2020) 118089
Fig. 2. SEM images of top surface of PVDF membranes prepared at different quenching mediums and temperatures.
Fig. 3. SEM images of bottom surface of PVDF membranes prepared at different quenching mediums and temperatures.
Fig. 4. SEM images of overall cross-section and corresponding inner morphology of PVDF membranes prepared at different quenching mediums and temperatures.
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roughness changes the surface microstructure, which has a direct impact on the wettability and permeability of the resulted membranes. The negative Rsk values are found for all membranes, which suggests that the surfaces of membranes are occupied by valleys. Rku value is the indicator of height distribution. It can be seen that the Rku values of PVDFT-air membranes are evidently higher than those of PVDFT-water membranes, which suggests that the former have a sharper height distribution in comparison with the latter.
Table 2 Average pore size and porosity of the as-prepared PVDF membranes. Membrane no.
Mean pore size(nm)
Porosity (%)
M-W0 M-W30 M-W60 M-A30 M-A60 M-A80 M-A100
187.4 281.8 390.8 522.4 626.2 744.2 801.3
77.2 � 78.5 � 79.9 � 77.3 � 77.6 � 79.8 � 80.5 �
� 18.2 � 16.0 � 34.4 � 13.3 � 35.2 � 10.7 � 25.9
0.2 0.6 0.1 0.5 0.2 0.1 0.4
3.2. Crystallization properties of PVDFT-water and PVDFT-air membranes
membranes exhibit more porous tendency with the increase of quenching temperature. The surface roughness of membranes was tested by AFM technique with tapping mode. The AFM images and roughness parameters of PVDFT-water and PVDFT-air membranes are shown in Fig. 5 and Table 3, respectively. From Fig. 5, the nodules on the surfaces of PVDFT-water membranes are markedly smaller than those on the PVDFT-air mem branes. The results are attributed to the cooling rate difference in TIPS process between two types of membranes. For the PVDFT-air membranes, a lower cooling rate gives much longer time for the nodules to grow, and thus the larger nodules are formed compared with the PVDFT-water membranes. From Fig. 5 and Table 3, it also can be observed that the increase of quenching temperature can evidently enhance the surface roughness (Ra and Rq values) for both PVDFT-water and PVDFT-air mem branes. The increase of quenching bath temperature leads to the decrease of cooling rate and contributes to the prolongation of the growth time of the PVDF fibrils in TIPS process. The increase of
To detect the polymorphisms of PVDFT-water and PVDFT-air mem branes prepared by various quenching temperatures, XRD character izations were performed to investigate the effects of quenching medium Table 3 Top surface parameters of the as-prepared PVDF membranes obtained from AFM measurement. \Membrane no.
Ra(nm)
Rq(nm)
M-W0 M-W30 M-W60 M-A30 M-A60 M-A80 M-A100
38.2 53.2 62.4 10.5 43.4 81.1 112.7
45.1 66.7 76.5 13.6 54.5 100.6 149.0
Rsk 0.39 0.37 0.02 0.17 0.33 0.23 0.76
Scheme 1. Schematic illustration of the phase separation mechanism of PVDFT-water and PVDFT-air membrane during phase inversion.
Fig. 5. AFM images of top surfaces of PVDF membranes prepared at different quenching mediums and temperatures. 5
Rku 2.33 2.68 2.37 2.83 3.30 3.20 4.52
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and temperature on the crystalline regions of PVDF, and the results are shown in Fig. 6. From Fig. 6, all the membranes exhibit similar diffraction patterns. The positions of the three peaks are 18.5� , 20.1� and 26.7� , corresponding to the reflections of (020), (110) and (021) planes of α phase. The ATR-FTIR spectra of PVDF membranes in Fig. 7 show that the peaks at 763, 795, 855, 879 and 973 cm 1 are found for all PVDF membranes that are indicative of the α-phase PVDF. The peaks at 855 and 879 cm 1 could be attributed to the vibration of amorphous components and CH2 vibration in PVDF, respectively. The results are consistent with the XRD results. It suggests that the crystal forms of PVDF polymer are mainly α phase and remain unchanged whatever the membranes quenched in water or air, or whatever quenching tempera ture are. The thermal properties of the PVDF membranes were. The melting temperature(Tm) and crystallinity (Xc) of the PVDF membranes were measured by DSC technique, and the results are presented in Fig. 8 and Table 4. The main peaks in Fig. 8 are attributed to the melting of crystallized PVDF chains. It can be seen that the Tm and Xc decrease with the increase of quenching temperature for both PVDFT-water and PVDFTair membranes. When the quenching temperature rises and the temper ature difference between the quenching bath and dope declines, the driving power for the crystallization of PVDF is declined, and thus the crystallinity (Xc) is decreased [49].
Fig. 7. ATR–FTIR spectra of PVDF membranes prepared at different quenching medium and temperature.
3.3. Wettability of PVDFT-water and PVDFT-air membranes The water contact angles of PVDFT-water and PVDFT-air membranes are presented in Fig. 9. It can be seen that the water contact angles of PVDFT-water and PVDFT-air membranes both slightly increase with the increase of quenching temperature. This should be attributed to the variation of surface roughness of the membranes. According to the Wenzel model [50], the rough surface increases the contact area of the “solid-liquid” and presents higher intuitive geometric area, and thus the surface hydrophobicity is enhanced. From Table 3, the surface rough ness increases for both PVDFT-water and PVDFT-air membranes, which should be the reason for the increase of the water contact angles of PVDFT-water and PVDFT-air membranes. 3.4. Water permeability and mechanical properties of PVDFT-water and PVDFT-air membranes
Fig. 8. DSC curves of PVDF membranes prepared at different quenching me dium and temperature.
Fig. 10 displays the relationship between water flux and quenching temperature in different quenching mediums. From Fig. 10, the pure water flux evidently increases with the increase of quenching tempera ture for both PVDFT-water and PVDFT-air membranes. M-W0 membrane shows the lowest pure water flux of 1435.0 L m 2 h 1⋅bar 1 in all
Table 4 The melting temperature(Tm) and crystallinity (Xc) of PVDF membranes. Membrane no.
Tm (� C)
Xc(%)
M-W0 M-W30 M-W60 M-A30 M-A60 M-A80 M-A100
162.92 161.89 161.72 166.69 165.05 164.83 162.72
45.1 43.3 42.6 43.5 42.7 42.0 40.4
PVDFT-water membranes. The pure water fluxes of PVDFT-air membranes are much higher than those of PVDFT-water membranes, which is attributed to the much higher pore size of the PVDFT-air membranes (Table 2). The pure water flux of M-A100 achieves the maximum value of 10097.4 L m 2 h 1⋅bar 1, which is quite high for microporous membranes. A good microfiltration membrane used in practical separation pro cesses must have excellent mechanical strength [51,52]. The mechanical properties (tensile strength, elongation and Young modulus) of PVDFT-water and PVDFT-air membranes were measured and listed in Table 5. The mechanical strength of the PVDFT-air membranes is much greater than that of the PVDFT-water membranes, which is a prominent
Fig. 6. XRD patterns of PVDF membranes prepared at different quenching medium and temperature. 6
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opposite trend upon a crowd of experiments. For the same membrane, the mechanical strength correspondingly decreases as the permeability increases. Conversely, the permeability will reduce when the mechani cal strength is improved. It is difficult to perform a large-scale separation operation when the tensile strength of membrane is less than 2 MPa [27]. It has been mentioned that Kim et al. summarized the relationship between the permeability and mechanical strength of the TIPS-prepared PVDF membranes, and they draw the trade-off line between perme ability and tensile strength according to Eq. (1). It was found that most TIPS-prepared PVDF membranes cannot overcome the upper bound. Fig. 11 shows the relationships between the permeability and tensile strength of PVDFT-water and PVDFT-air membranes in this work (M-W0, M-W30, M-W60, M-A30, M-A60, M-A80 and M-A100) and previous reports in Kim’ review. It is wonderfully found that all PVDFT-air mem branes (M-A30, M-A60, M-A80 and M-A100) locate above the trade-off line and also are superior to the previous reports in Kim’s review. However, PVDFT-water membranes are dwarfed by comparison and they all locate below the trade-off line. This shows that a TIPS-prepared PVDF membrane with both high permeability and high tensile strength could be simultaneously achieved by optimizing the cooling rate of mem branes via properly tuning the quenching medium (air and water) and quenching temperature.
Fig. 9. Water contact angles of PVDFT-water and PVDFT-air membranes.
4. Conclusions PVDFT-air and PVDFT-water membranes were prepared via TIPS method with air and water as quenching bath. The PVDFT-air membranes show much bigger pore size than that of the PVDFT-water membranes due to the much lower heat transfer rate of air bath. Also, the pore size in creases with the increase of quenching temperature for both PVDFT-water and PVDFT-air membranes. Thus, the pure water fluxes of PVDFT-air membranes are much higher than those of PVDFT-water membranes. MA100 achieves the maximum pure water flux of 10097.4 L m 2 h 1⋅bar 1, which is quite high for the microporous membranes. The PVDFT-air membranes present much greater mechanical strength than the PVDFT-water membranes, which is a prominent advantage in practical applications. It is surprising that all PVDFT-air membranes can easy overcome the upper bound between the permeability and tensile strength proposed by Kim [27]; while PVDFT-water membranes cannot do it. This research provides a feasible way to regulate the PVDF membrane structures and pore size with promoted performance for other applications.
Fig. 10. Pure water flux as a function of quenching temperature for PVDFT-water and PVDFT-air membranes. Table 5 The mechanical properties of as-prepared PVDF membranes. Membrane no.
Tensile strength (MPa)
Breaking elongation (%)
Young modulus (MPa)
M-W0 M-W30 M-W60 M-A30 M-A60 M-A80 M-A100
1.83 � 0.17 1.76 � 0.08 1.59 � 0.05 5.67 � 0.24 3.22 � 0.20 2.37 � 0.11 1.81 � 0.09
17.71 � 2.45 10.02 � 2.01 8.02 � 1.62 15.25 � 1.98 13.36 � 2.37 12.69 � 1.63 11.02 � 1.34
46.84 � 1.79 50.91 � 5.66 49.12 � 4.31 120.83 � 8.74 75.80 � 7.15 61.56 � 6.59 50.23 � 4.72
advantage in practical applications. It has been mentioned in our pre vious work [53]. PVDF fibers are randomly “stretched” during devel oping when quenching in air, which results in an irregular and intertwined pore structures. This structure is closely connected with each other, contributed to extraordinary mechanical strength [32]. The increase of quenching temperature will result in the slightly decrease of mechanical strength, which is due to the loose structure caused by the enlarged pore size [26]. Therefore, it is very important to choose a suitable quenching medium and quenching temperature to prepare PVDF membranes with excellent mechanical properties. Permeability and mechanical strength are two key indicators for separation membranes in practical applications. However, many re searchers found that these two indicators normally presented an
Fig. 11. Permeability upperbound vs. tensile strength for PVDFT-water and PVDFT-air membranes. (The upperbound equation is given in Eq. (1)). 7
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Declaration of competing interest [16]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[17]
CRediT authorship contribution statement [18]
Ji-Hao Zuo: Conceptualization, Investigation, Writing - original draft. Chao Wei: Validation. Peng Cheng: Investigation. Xi Yan: Re sources. Yan Chen: Methodology. Wan-Zhong Lang: Supervision, Writing - review & editing.
[19] [20]
Acknowledgments
[21]
The research is supported by Science and Technology Commission of Shanghai Municipality (14520502900), Shanghai Engineering Research Center of Green Energy Chemical Engineering (18DZ2254200) and In ternational Joint Laboratory on Resource Chemistry (IJLRC).
[22]
[23]
Appendix A. Supplementary data
[24]
Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2020.118089.
[25]
References
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