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water emulsion separation
Accepted Manuscript Title: Thermo-responsive PVDF/PSMA composite membranes with micro/nanoscale hierarchical structures for oil/water emulsion separation Author: Xiaoyu Yuan Wei Li Zhenguo Zhu Na Han Xingxiang Zhang PII: DOI: Reference:
S0927-7757(16)31089-5 http://dx.doi.org/doi:10.1016/j.colsurfa.2016.12.047 COLSUA 21254
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
23-7-2016 22-12-2016 28-12-2016
Please cite this article as: Xiaoyu Yuan, Wei Li, Zhenguo Zhu, Na Han, Xingxiang Zhang, Thermo-responsive PVDF/PSMA composite membranes with micro/nanoscale hierarchical structures for oil/water emulsion separation, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2016.12.047 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights ●Polyvinylidene fluoride/poly (stearyl methacrylate) composite membranes could separate water-in-oil emulsions easily. ●Thermo-responsive membrane with “open-closed’’ membrane pores was fabricated by comb-like polymer. ●The orthogonal experiment design was applied to determine the optimum casting conditions.
Graphical abstract
Schematic description of thermo-responsive characteristics of the membrane and separation for water/oil emulsions
Thermo-responsive PVDF/PSMA composite membranes with micro/nanoscale hierarchical structures for oil/water emulsion separation Xiaoyu Yuan, Wei Li*, Zhenguo Zhu, Na Han, Xingxiang Zhang* State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Municipal Key Laboratory of Advanced Energy Storage Material and Devices, School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China *Corresponding author: School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China TEL: +86 02283955429; FAX: +86 022 83955282. E-mail address: [email protected](Wei Li); [email protected] (Xingxiang Zhang).
Abstract A series of thermo-responsive polyvinylidene fluoride (PVDF)/poly(stearyl methacrylate) (PSMA) composite membranes with a highly hydrophobic surface were fabricated using a non solvent induced phase separation method. The introduction of comb-like polymer-PSMA endowed the composite membranes with hydrophobicity and thermoresponsive characteristics, and the transition response to temperature could be adjusted by the PSMA content in the composite membranes. The effects of PSMA on membrane morphology, hydrophobicity, and oil-water separation performance were studied. Furthermore, the thermo-responsive and crystallographic characteristics
of the membranes were investigated. An orthogonal experiment design (OED) was applied in this study to evaluate the separation performance of membranes for waterin-oil emulsions with different solution compositions and casting conditions. Together with their excellent thermoresponsivity, the high flux and separation efficiency support these membranes’ promise for separation of oil-water mixtures and solutes with small core size differences.
Keywords: Oil/water emulsion separation; Hydrophobicity; Thermo-responsive; Polyvinylidene fluoride
Introduction With increases in industrial oily wastewater and polluted oceanic water, oil/water separation has become a worldwide issue
[1]
. Environmental and economic demands
require functional materials that can selectively and effectively filter or absorb oil or water from oil-water mixtures [2]. Several conventional techniques have been reported for separation of free oil/water mixtures, such as centrifugation, flotation, adsorption and de-emulsification
[3-6]
. However, these methods suffer from certain drawbacks,
such as low separation efficiencies, high operating costs and even secondary pollution. In addition, these methods are not applicable to water/oil emulsions, especially for surfactant-stabilized emulsions with droplet sizes of less than 20 μm. Additionally, real oil-water mixtures are not always well layered but exist in the form of stable emulsions[7]. Thus, it is crucial to develop facile energy- and cost-effective processes for effective separation of water/oil emulsions[8]. Compared with the above methods, membrane separation technology offers many advantages for treatment of oil-water mixtures, including no secondary pollution, no complex equipment and low costs[9]. Polyvinylidene fluoride (PVDF) is a commercially available fluoropolymer with low surface free energy (25 dynes·cm-1), high mechanical strength, good chemical resistance and thermal stability, and excellent aging resistance[10-12] and has been widely investigated for application as a separation membrane[9,13]. It is well known that the wettability of a solid surface is controlled by the chemical composition and the surface roughness[14]. The presence of the proper multi-scale roughness can render a pristine hydrophobic surface more hydrophobic due to the air trapped underneath the liquid droplets. Many reports are available on superhydrophobic functional materials fabricated using a combination of low surface energy and proper micro/nanoscale roughness structures
[15]
. L. Jiang et al [9] used ammonia as an inert
solvent additive to fabricate a superhydrophobic/superoleophilic PVDF membrane via an inert solvent induced phase-inversion process. However, liquid-liquid demixing and formation of a dense skin layer occurred readily in a water bath due to coagulation. Kandasubramanian et al [16] reported a feasible method for production of a superhydrophobic PVDF/camphor soot particle and PVDF/candle soot particle composite using a combination of solvent exchange and a slow gelation process. A composite coating-like hierarchical microstructure with a micro/nano-scaled rough surface was obtained. Stimulus-responsive materials can display controlled changes in response to external stimuli such as light, temperature, pH, electricity and magnetism physical or chemical changes
[17-21]
. Due to their potential, fabrication of intelligent controllable
materials has attracted increasing attention and has been widely studied in recent years. L. Feng et al
[17]
fabricated thermo and pH dual-controllable oil/water
separation materials via photo-initiated free radical polymerization of dimethylamino ethyl methacrylate (DMAEMA). The PDMAEMA hydrogel-coated mesh could selectively separate water from oil/water mixtures and facilitate water and oil permeation through the mesh in an orderly manner, and the mixture components could be collected separately by adjusting the temperature or pH. However, to the best of our knowledge, little information is available on fabrication of such thermoresponsive membrane by introducing a comb-like polymer to adjust the pose size of membrane. The comb-like polymer
[22]
is a special macromolecule in which the polymeric
backbone is referred to as the main chain and the grafted molecular chain is referred to as the side chain. Poly(stearyl methacrylate) (PSMA) is a typical comb-like polymer that exhibits obvious phase change behaviors with Tm at 40.6 °C and a melting enthalpy (△Hm) of 75.1 J·g-1 (see supplementary Figure S1). The side chain
of PSMA is a long alkyl chain, which not only could increase the hydrophobicity of the composite membranes, but also endow the membrane with temperature responsivity by the form of macromolecular chains at various temperatures when used as additive in membranes. In other words, the pore size of the composite membranes can be adjusted by melting and crystallization of the PSMA at various temperatures. Compared with other materials, intelligent controllable separation materials offer additional advantages in simplification of separation devices, enhanced separation efficiency and reduced energy use. The orthogonal experiment design (OED) method
[23,24]
is well accepted as a
modern approach to characterization and optimization of system performance in many research areas. The OED method is simple and efficient and greatly simplifies multifactorial experiments that are cumbersome and expensive. In this work, we report a novel thermoresponsive and hydrophobic PVDF composite membrane fabricated by blending PSMA and PVDF, and the OED method is used to investigate the effect of solution compositions and casting conditions on the separation performances of membranes. The pore size of the thermoresponsive membrane can be adjusted according to the form of the macromolecular chains at various temperatures, which successfully conquers the restriction of the initially formed membrane structure. The as-fabricated PVDF composite membranes with special wettability can easily separate both micrometer- and nanometer-sized surfactant-free and surfactant-stabilized waterin-oil emulsions at the room temperature and general pressure conditions, and the oil purity in the filtrate after separation reaches 99.98%. More importantly, compared with the fixed-structure membrane, the membranes with temperature-sensitive “openclosed’’ pores are more suitable for separation of solutes with small size differences.
2. Experiment 2.1. Materials PVDF (Solef 6010) was purchased from Solvay (France) (Mn=320,000). The PVDF powder was dried at 100 oC under vacuum for 24 h to remove moisture before use in doping preparation. PSMA (Mn=108,124) was synthesized via suspension polymerization. N, N-dimethylacetamide (DMAc), acetone, Span 80, toluene and chloroform were all of analytical grade and used as received. Ethanol was used as soft coagulant in the solvent-exchange method.
2.2. Membrane preparation The desired amount of PSMA was weighed and poured into a tank containing a DMAc/acetone mixed solvent. A certain quantity of the dried PVDF powder was poured into the PSMA solution, and the polymer-doped mixture was subjected to continuous stirring at 50 ºC for 6 h for homogenization. After the entrapped air bubbles were removed, the de-aerated solutions were used to cast membranes at 20 oC and 50% relative humidity using a casting knife with a gap of 250 μm. The nascent membrane was immersed in ethanol coagulant and subsequently immersed in a distilled water coagulation bath for 24 h to precipitate the polymer and remove the residual solvent. Finally, the wet membranes were dried in air at room temperature to obtain dry composite membranes. The orthogonal array is described as La(bc), where L is the symbol of the orthogonal design, a is the number of experimental runs, b is the number of levels and c is the number of factors. The orthogonal array L9(34) was selected for this investigation and is shown in Table 1. Compositions of dope solution and casting conditions for flat-sheet composite membrane preparation are also presented. Table1 Compositions of dope solution and casting conditions for composite membrane fabrication
A
B
PVDF/PSMA
Time (s)
No
C
D
Dope solution
Coagulation bath
temperature (oC)
temperature (oC)
1
1(9/0)
1(5)
1(30)
1(20)
2
1(9/0)
2(300)
2(40)
2(30)
3
1(9/0)
3(600)
3(50)
3(40)
4
2(8/1)
1(5)
2(40)
3(40)
5
2(8/1)
2(300)
3(50)
1(20)
6
2(8/1)
3(600)
1(30)
2(30)
7
3(7/2)
1(5)
3(50)
2(30)
8
3(7/2)
2(300)
1(30)
3(40)
9
3(7/2)
3(600)
2(40)
1(20)
2.3. Fabrication of water-in-oil emulsions Surfactant-free water-in-oil emulsions (labeled as SFE) were fabricated by mixing water and oil at a 1:9 volume ratio and sonicating the mixture at a power of 2 kW for 1.5 h to produce a white and milky solution. The emulsions of toluene and chloroform were referred to as SFE-1 and SFE-2, respectively. Surfactant-stabilized water-in-oil emulsions (labeled as SSE) were fabricated using the following steps. For SSE-1, span 80 (0.5 g) was added into toluene (114 mL), and water (1 mL) was subsequently added. The mixture was stirred for 3 h. For SSE-2, span 80 (0.5 g) was added into chloroform (114 mL), and water (1 mL) was subsequently added. The mixture was stirred for 3 h. All surfactant-stabilized waterin-oil emulsions were stable for more than 30 days, and no demulsification or precipitation was observed.
2.4. Separation of oil-in-water and water-in-oil emulsions The separation experiments were performed at room temperature and general pressure conditions. The effective diameter of the filter was 1.5 mm.
2.5. Temperature-sensitive permeation The temperature sensitivity of the PVDF composite membrane was measured by the change in pure water flux with various temperatures. The pure water flux of the membrane was measured using a method described previously[25]. All membranes were initially pre-pressurized at a pressure of 0.12 MPa for 30 min to obtain a steady state. Before the flux of pure water at various temperatures was measured, the membrane was pre-pressurized at the corresponding temperature for 20 min, and the flux was calculated using the following equation as described elsewhere:
Jw
M A △t
(1)
where M, A and △t represent the permeate quality (M) of water solution, the effective of membrane area (m2) and the permeation time (h), respectively.
2.6. Membrane characterization The morphologies of the membranes were characterized by field-emission scanning electron microscopy (FE-SEM; S4800, Hitachi, Japan) at 10 kV. Prior to the observations, all samples were coated with gold under vacuum. The membrane hydrophobicity was measured with a contact angle meter (DSA100, Krüss, Germany) to obtain the static contact angle (CA). The water droplets used in the measurements were 2 μL in volume. For each membrane sample, five measurements were collected to obtain the average CA value. The roughness parameters and three-dimensional surface images of the PVDF membranes were investigated by TCCM (Axio CSM 700, ZEISS, Germany). The effectiveness of oil-water separation was examined by optical microscopy, and the purity of the oil in the collected filtrates was determined using a Karl Fischer moisture titrator (Mettler Toledo C20). Dynamic light scattering (DLS) measurements were performed on a Zetasizer Nano ZS. The degree of crystallization
and thermoresponsivity of the dry membranes were investigated using differential scanning calorimeter (Model DSC200F3, Netzsch, Germany), and the determination temperature was varied from -20 to 200 oC at a heating or cooling rate of 10 oC/min under a nitrogen atmosphere. The temperature sensitivity of the composite membranes was also assessed by the change in pure water flux with various temperatures. The compositions of the composite membranes were analyzed using attenuated total reflection infrared Fourier transform spectroscopy (ATR-FTIR) with a Nicolet iS50 FT-IR spectrometer, which was equipped with an ATR sampling accessory. The IR spectra were accumulated at a resolution of 4 cm-1 over the range of 650-4000 cm-1 with a total of 16 scans. The crystalline structures of the membranes were analyzed using an American D8A A25 X-ray diffractometer with Cu Kα radiation (k=0.154 nm) and operated at the range of 5-40°.
3. Results and discussion 3.1. Morphology and wettability of membranes
Figure 1. Morphology and microstructure of membranes with various solution compositions and casting conditions
Figure 1. (Continued)
Figure 1. (Continued)
The effects of various solution compositions and casting conditions on the morphology and microstructure of the membranes can be observed from the SEM micrographs shown in Figure 1. No skin layers were found on the surfaces of M-1, M2 and M-3, whereas obvious porous structures can be observed on the top of the membranes, with interconnected holes in the networks constructed by PVDF small petal-like or globule structures connected with each other. The imperfections in the crystalline structure on the surface of M-1 create porosity, which might be caused by
the shorter time in the ethanol coagulation bath might have been insufficient to complete the crystallization. In contrast, M-4, M-5 and M-6 show a porous microstructure of uniformly packed microspheres with diameters of 0.5-1.0 μm, which could be interpreted as the formation of PVDF spherical particles due to crystallization during precipitation, which is a solid-liquid de-mixing process. As demonstrated in the cross-section of the membranes, the membranes are uniformly skinless and are composed of equal-sized spherical particles. Such a structure suggests that the crystallites are nucleated simultaneously and subsequently grow in all directions until they join with adjacent particles. Compared with M-5 and M-6, the surface of M-4 is rather porous, which could suggest that the conditions of the casting membrane are different. When the ratio of PVDF/PSMA reaches 7/2, i.e., M-7, M-8, M-9, the surfaces of the membranes consist of PVDF small broccoli-like hierarchical microstructures that connect with each other. The aggregation of the micro/nano microstructure enhances the roughness of the surface to tailor the hydrophobic property. It is worth noting that the membranes are composed of irregular spherical particles that can be observed in the cross-sections of M-7, M-8, M-9, which are similar to the surface of a cactus, and this structure of the porous channel creates a higher flux and separation efficiency for oil-water separation.
Figure 2. Three-dimensional surface micrographs and line roughness of the membranes
Figure 2. (Continued)
Figure 2. (Continued)
Table 2. Roughness parameters and contact angles of the membranes Surface roughness parameter (μm) Sample
Contact angle (°) Ra (μm)
Rq (μm)
Rmax (μm)
M-1
0.93±0.04
5.11±0.66
10.29±1.56
132.2±1.6
M-2
1.87±0.04
6.28±0.24
14.56±0.45
133.2±1.0
M-3
1.94±0.03
6.56±0.23
16.18±1.34
131.7±1.6
M-4
1.29±0.01
8.46±0.2
13.02±0.72
137.5±1.1
M-5
1.14±0.01
5.94±0.24
10.11±0.15
126.2±1.6
M-6
0.87±0.05
3.62±0.45
8.28±1.25
127.4±1.5
M-7
1.39±0.09
10.16±1.3
14.39±1.88
139.5±3.7
M-8
0.71±0.11
2.74±0.53
5.98±1.11
143.3±2.4
M-9
0.85±0.03
3.40±0.37
6.33±0.38
138.4±2.8
According to the models developed by Thomas Young, Wenzel and Cassie-Baxter [15]
, the hydrophobic surface chemistry and hierarchical micro/nanostructure are of
equal importance in achieving super-hydrophobicity. The surface morphology and microstructure of the membrane have a great influence on its hydrophobicity, and the surface roughness enhances the surface wettability. The surface properties of the PVDF membranes were investigated using TCCM. Figure 2 shows the threedimensional surface images. The roughness parameters and contact angles of the membranes are presented in Table 2. The average roughness (Ra), the root mean square roughness (Rq) and the maximum roughness (Rmax) were analyzed for the images. Ra and Rq appear to be the most helpful and consistent in characterization of the surface topography of the membranes. M-1, M-2 and M-3 exhibit similar contact angles at 131.7°-132.2° and present relatively rough surfaces covered with a multitude of small peaks (Ra=0.93±0.04, Ra=1.87±0.04 and Ra=1.94±0.03, respectively). However, M-7, M-8 and M-9 exhibit relatively smooth surfaces with Ra=1.39±0.09, Ra=0.71±0.11 and Ra=0.85±0.03, respectively, and the high hydrophobicity of the membranes could be attributed to the addition of PSMA. It can be clearly observed that the contact angle of the surface is closely related to both chemical composition and surface roughness.
Figure 3. (a) Water contact angles of M-8 with various drop ages; (b) Wettability of M-8 toward oil
In general, the hydrophobicity is evaluated according to the water contact angle, and membranes with higher hydrophobicities exhibit larger contact angles. The contact angle of water on M-8 is 143°, and no obvious decline of the WCA is observed with increasing drop age, as shown in Figure 3(a). This result might be attributed to the nano/microscale hierarchical roughness combined with the low surface free energy of PVDF. However, the hydrophobicity of the membranes is also improved by introduction of PSMA into the polymer matrix. As described in Figure 3(b), when an oil drop contacts the superoleophilicity surface, it spreads along the membrane surface and permeates through the cross-section of the membrane within 15 s.
3.2. Separation performance of membranes for water/oil emulsions
Figure 4. (a) Photograph of separation for water/oil emulsions; (b) photographs and optical microscope images of the feed emulsion and (c) filtrate emulsion
Figure 5. Schematic description of membrane separation for water/oil emulsions
Figure 6. DLS data of the feed emulsions and their filtrate correspondingly for SEE-1 and SEE-2
Figure 7. Flux and oil purity in the filtrate after permeation through the composite membranes: (a) SEE-1; (b) SEE-2
The as-fabricated membranes with special wettability can be applied in water/oil
separation. A diagrammatic sketch of separation for water-in-oil emulsions is depicted in Figure 4. Due to the multi-level micro/nanostructure of the composite membranes, the emulsion droplets are demulsified and immediately permeate through the membranes, whereas water is retained when the emulsions are poured onto the membranes. Figure 5 shows a schematic of the membrane separation process for water/oil emulsions. During water-in-oil emulsion separation, the membrane was placed in a continuous oil environment, and its oleophilicity and under-oil hydrophobicity guarantee the penetration of oil and effective interception of water droplets. Optical microscopy was applied to examine the separation effectiveness by comparing the feed with the collected filtrate. A significant difference in phase composition could be noted between the feed and the corresponding filtrate. The collected filtrate turns obviously transparent compared with the original milky-white feed emulsion, which indicates the effectiveness of the PVDF composite membranes for separation of various water-in-oil emulsions. So from the comparison of opticl micrograph, the emulsions could be successfully separated in one step, driven only by gravity and without any external force (see supplementary Figure S2). To investigate the emulsion droplets distribution (beyond the resolution of optical microscope, ~200 nm) furthermore, the emulsion droplets before and after filtration were also monitored by DLS, and the measured diameter distribution was shown as shown in Figure 6. The droplet size of the SEE-1 and SEE-2 ranged from 100 nm to 1.5 μm approximately. After filtration, no droplets around these ranges are observed, indicating the effective separation. The purity of the purified oil was investigated by measuring the water weight percentage in the filtrate. The flux and oil purities in the filtrate after permeation through the as-fabricated PVDF membranes are presented in Figure 7. The separation
results for SEE-1 and SEE-2 effectively illustrate the high separation efficiency of our membranes. Similar effective separations were also achieved for the SFE-1 and SFE-2 systems (see supplementary Figure S3). M-3 presents the highest flux for all emulsions, which could be due to the larger pore size and porosity generated in the membrane formation process. However, this structure inevitably reduced the separation efficiency. As shown in Figure 7(b), with increasing addition of PSMA, the flux of M-9 reaches 230 kg·m-2 h-1, and the purity of purified oil remains at a relatively high level of 99.98%, which is in accordance with the cactus-like spherical particles in the cross-section. Thus, both the cactus-like spherical particle structure and the low surface free energy of PSMA are beneficial to increase the flux and the separation efficiency. Table 3. L9(34) standard orthogonal array in terms of coded factor levels (values 1-3): physical values and experimental results No.
A
B
C
D
Purity of oil（%）
1
1
1
1
1
99.948
2
1
2
2
2
99.952
3
1
3
3
3
99.955
4
2
1
2
3
99.972
5
2
2
3
1
99.971
6
2
3
1
2
99.965
7
3
1
3
2
99.955
8
3
2
1
3
99.963
9
3
3
2
1
99.967
K1
99.952
99.958
99.959
99.962
K2
99.969
99.962
99.964
99.957
K3
99.962
99.962
99.960
99.963
Q
4.62*10-4
0.870*10-4
4.20*10-4
1.98*10-4
Y
1 9 Y 99.961 9 i1 i
QT nSY2 1.167*103
Factors ordered by significance (from most to least) ACDB Optimal combination
A2B2C2D3 or A2B3C2D3
Table 4. Analysis of variance for separation efficiency
Sum of squares
Degrees of
Mean square of
of deviations
freedom
deviation
Parameters
F ratio
F0.05
Significance
Factor A
QA=4.62*10-4
2
S A2 =2.31*10-4
5.25
19
*
Factor C
QC=4.20*10-4
2
SC2 =2.10*10-4
4.77
19
*
Factor D
QD=1.98*10-4
2
S D2 =0.99*10-4
2.25
19
*
Error E
QE=0.87*10-4
2
SE2 =0.44*10-4
Sum T
QT=0.290
8
The OED was selected to analyze the separation performance of the membranes with different compositions of dope solution and casting conditions and to determine the level of influence of each factor on the separation efficiency for SEE-2 driven only by gravity. The details of the calculation processes for the mean and range can be found in reference [26]. The results of the OED calculations are summarized in Table 3 and Table 4. The analysis of variance table contains the sum of squares, degrees of freedom, F variance ratio, F standard value and the significance. It can be clearly observed that the sum squares of deviations of Factor E is the least, and thus it can be
used as the error in the OED calculations. The F variance ratio is the ratio of variance due to the effect of a particular factor and the variance due to the error. The F standard value is determined from an F-table as the statistical level of significance
[27]
. In this
study, if the F-test value is greater than F0.05, the process parameter is considered significant. As shown in Table 4, Factor A (the ratio of PVDF/PSMA), Factor C (dope solution temperature), and Factor D (coagulation bath temperature) are insignificant for the membrane separation performance, but Factor A (F ratio=5.25), i.e., the ratio of PVDF/PSMA, has a greater impact compared with the others, which is in good accordance with the morphology and microstructure of the membranes (Figure 1).
3.3. Thermoresponsivity of membranes
Figure 8. DSC thermograms of the membranes
Table 5. Thermal analysis of the membranes Tmpb
△Hmd
Xmc
Tcpf
△Hch
X cc
(oC)
(J/g)
(%)
(oC)
(J/g)
(%)
M-1
174.0
61.9
59.1
139.1
50.0
47.8
M-4
172.2
58.9
61.1
135.6
54.7
58.8
M-7
172.5
51.4
63.1
142.5
51.9
63.7
Sample
Table 6. Thermal analysis of the membranes and PSMA
Tmoa
Tmpb
Tmec
△Hmd
Tcoe
Tcpf
Tceg
△Hch
(oC)
(oC)
(oC)
(J/g)
(oC)
(oC)
(oC)
(J/g)
M-4
6.5
15.8
21.8
6.0
2.6
18.1
25.4
9.7
M-7
22.7
29.9
34.5
24.0
2.7
19.7
27.4
23.5
PSMA
36.4
40.6
44.0
75.1
21.4
25.5
28.8
68.0
Sample
a
Onset temperature on DSC heating curve; b Peak temperature on DSC heating curve;
c
Endset temperature on DSC heating curve; d Enthalpy on DSC heating curve;
e
Onset temperature on DSC cooling curve; f Peak temperature on DSC cooling curve;
g
Endset temperature on DSC cooling curve; h Enthalpy on DSC cooling curve.
DSC analysis was conducted on the fabricated flat-sheet composite membranes to examine the effect of PSMA on the crystallinity degree (Xc) of the membranes. The heating and cooling thermograms of the composite membranes with various ratios of PVDF to PSMA are presented in Figure 8, and the results are summarized in Table 5. The Xc of the fabricated membranes is based on either the melting (Xmc) or crystallization (Xcc) obtained from the following equation [28]:
Xc
△Hf
W *100% △Hf*
(2)
where ΔHf is the experimental heat of fusion (ΔHm) or the experimental heat of crystallization (ΔHc), ΔHf*=105 J ·g-1 (the heat of fusion of the 100% crystalline PVDF), and W is the weight fraction of the polymer in the mixture. From Table 5, which verified that PSMA has a significant influence on the thermal properties of PVDF, it can be found that Xc increased from 47.8% to 63.7%, whereas the PSMA is incorporated in PVDF matrix, which can be explained by the fact that the PSMA promotes nucleation of the PVDF crystalline phase. The DSC curves also demonstrate
that M-4 and M-7 exhibit obvious phase change behaviors, but M-1 does not. For M-7, the peak temperature on the DSC heating curve (Tmpb) and the melting enthalpy (△Hmd) of the PSMA in the composite membranes are 29.9 oC and 24.0 J · g-1, respectively. The Tmpb and △Hmd values of the PSMA in the composite membranes decrease from 29.9 to 15.8 oC and from 24.0 to 6.0 J · g-1, respectively, as the PVDF/PSMA ratio varies from 7/2 to 8/1 (Table 6). These results could be interpreted as due to the movement of PSMA molecular chains restricted by many PVDF segments in membranes. A similar trend is noted in the DSC cooling curve. Thus, the pore size of the composite membranes could be regulated by crystallization and melting of PSMA in the membranes, and the transition response to temperature could also be adjusted using the content of PSMA in the composite membranes.
Figure 9. Variation of the flux of the membranes with temperature
Figure 10. Response mechanism of the composite membrane under various temperatures
To investigate the temperature responsiveness of the composite membranes, the variation in the membrane flux of the membranes with temperature was studied. Figure 9 describes the effect of temperature on the water flux. Theoretically, the pure water flux of the membranes (pristine PVDF, M-1) increases with increasing temperature. This result could be explained by the increased mobility of water with increasing temperature. The composite membranes show different flux values as the temperature changes because PSMA is a thermo-sensitive polymer. For M-4, the pure water flux decreases with increasing temperature in the range of 0 to 20 oC due to the reduced membrane pore size with melted PSMA, whereas the flux increases with increasing temperature after 20 oC due to the increased mobility of the water. In the process of cooling, with the crystallization of PSMA, the large pore size and high porosity are generally beneficial to water passing through the membranes, and thus the pure water flux of the composite membranes increases. Figure 10 schematically illustrates the thermoresponsive mechanism and the microstructure of the composite membrane surface. The water flux data can be used to estimate the change in the membrane pore size. A similar trend occurs in M-7, and the only difference is the transition response to temperature. The change of temperature influences the pore size
of the membrane, and the transition remarkably changes with the content of PSMA in the composite membranes, which indicates the thermoresponsive “open-closed’’ pores with the crystallization and melting of PSMA in the membranes, which is more suitable for separation of solutes with small size differences. The response to temperature of the membrane is consistent with the DSC thermograms.
3.4. Crystallographic studies
Figure 11. ATR-FTIR spectra of the membranes
The ATR-FTIR spectra of membranes with various ratios of PVDF and PSMA are presented in Figure 11. In the case of M-4 and M-7, three strong peaks can be observed near 2920, 2850 and 1724 cm-1. The former two peaks are due to C-H stretching vibration, and the latter peak is due to the -O-C=O stretching bond of PSMA. The intensity of the peaks increases with the increasing content of PSMA, but these peaks are not present in the M-1 spectrum. As a result, it could be concluded that the composite formation occurred. The main absorption peaks at 760, 795, 840,
880, 970, 1070, 1180 and 1400 cm-1 are present in all of the membrane spectra. The absorption peaks at 760, 795, 970, 1070 and 1400 cm-1 are attributed to formation of the α-phase in the crystallization zones. The absorption peaks at 840 and 1180 cm-1 are characteristic of the PVDF β-phase crystal structure [29]. It should be noted that the addition of PSMA causes an increase in the absorption spectrum intensity at 795 cm-1 but a decrease in the absorption spectrum intensity at 840 cm-1, indicating that the variety of PSMA content has a significant effect on the crystalline form of PVDF in the composite membranes. In other words, PSMA addition affects the polymorphism of the PVDF in favor of α-phase.
Figure 12. XRD patterns of the membranes
Both the crystallization behaviors and structure are essential to the morphology and microstructure of the membrane during membrane formation. WXRD measurements were performed to examine the crystalline structures of the overall membranes. As described in Figure 12, the increases in the intensity of the diffraction peaks at 18.3° (100) and 26.6° (021) is due to enhancement of the crystallization zones in
PVDF/PSMA. The diffraction peaks near 18.3° and 26.6° are characteristic of the PVDF α-phase crystal structure. Thus, it can be observed that the addition of PSMA is conducive to α-phase
[30]
, which is in good agreement with the result of ATR-FTIR
spectra.
Conclusion PVDF/PSMA composite membranes with thermoresponsivity and highly hydrophobic surfaces were fabricated using the phase separation method. From the result of the OED, the ratio of PVDF/PSMA had a greater impact on the separation performance for water in oil emulsions than the casting conditions. The flux of M-9 reached 230 kg·m-2 h-1, and the purity of the purified oil remained at a relatively high level of 99.98%. More importantly, the addition of PSMA endowed the composite membranes with thermoresponsive characteristics. The composite membranes M-4 and M-7 showed a decreased water flux near 6.5 and 22.7 oC, respectively, which also demonstrated that the transition response to temperature could be adjusted by the content of PSMA in the composite membranes. Thus, membranes with temperaturesensitive “open-closed’’ pores might find widespread application in separation of solutes with small size differences.
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S0927-7757(16)31089-5 http://dx.doi.org/doi:10.1016/j.colsurfa.2016.12.047 COLSUA 21254
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
23-7-2016 22-12-2016 28-12-2016
Please cite this article as: Xiaoyu Yuan, Wei Li, Zhenguo Zhu, Na Han, Xingxiang Zhang, Thermo-responsive PVDF/PSMA composite membranes with micro/nanoscale hierarchical structures for oil/water emulsion separation, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2016.12.047 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights ●Polyvinylidene fluoride/poly (stearyl methacrylate) composite membranes could separate water-in-oil emulsions easily. ●Thermo-responsive membrane with “open-closed’’ membrane pores was fabricated by comb-like polymer. ●The orthogonal experiment design was applied to determine the optimum casting conditions.
Graphical abstract
Schematic description of thermo-responsive characteristics of the membrane and separation for water/oil emulsions
Thermo-responsive PVDF/PSMA composite membranes with micro/nanoscale hierarchical structures for oil/water emulsion separation Xiaoyu Yuan, Wei Li*, Zhenguo Zhu, Na Han, Xingxiang Zhang* State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Municipal Key Laboratory of Advanced Energy Storage Material and Devices, School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China *Corresponding author: School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China TEL: +86 02283955429; FAX: +86 022 83955282. E-mail address: [email protected](Wei Li); [email protected] (Xingxiang Zhang).
Abstract A series of thermo-responsive polyvinylidene fluoride (PVDF)/poly(stearyl methacrylate) (PSMA) composite membranes with a highly hydrophobic surface were fabricated using a non solvent induced phase separation method. The introduction of comb-like polymer-PSMA endowed the composite membranes with hydrophobicity and thermoresponsive characteristics, and the transition response to temperature could be adjusted by the PSMA content in the composite membranes. The effects of PSMA on membrane morphology, hydrophobicity, and oil-water separation performance were studied. Furthermore, the thermo-responsive and crystallographic characteristics
of the membranes were investigated. An orthogonal experiment design (OED) was applied in this study to evaluate the separation performance of membranes for waterin-oil emulsions with different solution compositions and casting conditions. Together with their excellent thermoresponsivity, the high flux and separation efficiency support these membranes’ promise for separation of oil-water mixtures and solutes with small core size differences.
Keywords: Oil/water emulsion separation; Hydrophobicity; Thermo-responsive; Polyvinylidene fluoride
Introduction With increases in industrial oily wastewater and polluted oceanic water, oil/water separation has become a worldwide issue
[1]
. Environmental and economic demands
require functional materials that can selectively and effectively filter or absorb oil or water from oil-water mixtures [2]. Several conventional techniques have been reported for separation of free oil/water mixtures, such as centrifugation, flotation, adsorption and de-emulsification
[3-6]
. However, these methods suffer from certain drawbacks,
such as low separation efficiencies, high operating costs and even secondary pollution. In addition, these methods are not applicable to water/oil emulsions, especially for surfactant-stabilized emulsions with droplet sizes of less than 20 μm. Additionally, real oil-water mixtures are not always well layered but exist in the form of stable emulsions[7]. Thus, it is crucial to develop facile energy- and cost-effective processes for effective separation of water/oil emulsions[8]. Compared with the above methods, membrane separation technology offers many advantages for treatment of oil-water mixtures, including no secondary pollution, no complex equipment and low costs[9]. Polyvinylidene fluoride (PVDF) is a commercially available fluoropolymer with low surface free energy (25 dynes·cm-1), high mechanical strength, good chemical resistance and thermal stability, and excellent aging resistance[10-12] and has been widely investigated for application as a separation membrane[9,13]. It is well known that the wettability of a solid surface is controlled by the chemical composition and the surface roughness[14]. The presence of the proper multi-scale roughness can render a pristine hydrophobic surface more hydrophobic due to the air trapped underneath the liquid droplets. Many reports are available on superhydrophobic functional materials fabricated using a combination of low surface energy and proper micro/nanoscale roughness structures
[15]
. L. Jiang et al [9] used ammonia as an inert
solvent additive to fabricate a superhydrophobic/superoleophilic PVDF membrane via an inert solvent induced phase-inversion process. However, liquid-liquid demixing and formation of a dense skin layer occurred readily in a water bath due to coagulation. Kandasubramanian et al [16] reported a feasible method for production of a superhydrophobic PVDF/camphor soot particle and PVDF/candle soot particle composite using a combination of solvent exchange and a slow gelation process. A composite coating-like hierarchical microstructure with a micro/nano-scaled rough surface was obtained. Stimulus-responsive materials can display controlled changes in response to external stimuli such as light, temperature, pH, electricity and magnetism physical or chemical changes
[17-21]
. Due to their potential, fabrication of intelligent controllable
materials has attracted increasing attention and has been widely studied in recent years. L. Feng et al
[17]
fabricated thermo and pH dual-controllable oil/water
separation materials via photo-initiated free radical polymerization of dimethylamino ethyl methacrylate (DMAEMA). The PDMAEMA hydrogel-coated mesh could selectively separate water from oil/water mixtures and facilitate water and oil permeation through the mesh in an orderly manner, and the mixture components could be collected separately by adjusting the temperature or pH. However, to the best of our knowledge, little information is available on fabrication of such thermoresponsive membrane by introducing a comb-like polymer to adjust the pose size of membrane. The comb-like polymer
[22]
is a special macromolecule in which the polymeric
backbone is referred to as the main chain and the grafted molecular chain is referred to as the side chain. Poly(stearyl methacrylate) (PSMA) is a typical comb-like polymer that exhibits obvious phase change behaviors with Tm at 40.6 °C and a melting enthalpy (△Hm) of 75.1 J·g-1 (see supplementary Figure S1). The side chain
of PSMA is a long alkyl chain, which not only could increase the hydrophobicity of the composite membranes, but also endow the membrane with temperature responsivity by the form of macromolecular chains at various temperatures when used as additive in membranes. In other words, the pore size of the composite membranes can be adjusted by melting and crystallization of the PSMA at various temperatures. Compared with other materials, intelligent controllable separation materials offer additional advantages in simplification of separation devices, enhanced separation efficiency and reduced energy use. The orthogonal experiment design (OED) method
[23,24]
is well accepted as a
modern approach to characterization and optimization of system performance in many research areas. The OED method is simple and efficient and greatly simplifies multifactorial experiments that are cumbersome and expensive. In this work, we report a novel thermoresponsive and hydrophobic PVDF composite membrane fabricated by blending PSMA and PVDF, and the OED method is used to investigate the effect of solution compositions and casting conditions on the separation performances of membranes. The pore size of the thermoresponsive membrane can be adjusted according to the form of the macromolecular chains at various temperatures, which successfully conquers the restriction of the initially formed membrane structure. The as-fabricated PVDF composite membranes with special wettability can easily separate both micrometer- and nanometer-sized surfactant-free and surfactant-stabilized waterin-oil emulsions at the room temperature and general pressure conditions, and the oil purity in the filtrate after separation reaches 99.98%. More importantly, compared with the fixed-structure membrane, the membranes with temperature-sensitive “openclosed’’ pores are more suitable for separation of solutes with small size differences.
2. Experiment 2.1. Materials PVDF (Solef 6010) was purchased from Solvay (France) (Mn=320,000). The PVDF powder was dried at 100 oC under vacuum for 24 h to remove moisture before use in doping preparation. PSMA (Mn=108,124) was synthesized via suspension polymerization. N, N-dimethylacetamide (DMAc), acetone, Span 80, toluene and chloroform were all of analytical grade and used as received. Ethanol was used as soft coagulant in the solvent-exchange method.
2.2. Membrane preparation The desired amount of PSMA was weighed and poured into a tank containing a DMAc/acetone mixed solvent. A certain quantity of the dried PVDF powder was poured into the PSMA solution, and the polymer-doped mixture was subjected to continuous stirring at 50 ºC for 6 h for homogenization. After the entrapped air bubbles were removed, the de-aerated solutions were used to cast membranes at 20 oC and 50% relative humidity using a casting knife with a gap of 250 μm. The nascent membrane was immersed in ethanol coagulant and subsequently immersed in a distilled water coagulation bath for 24 h to precipitate the polymer and remove the residual solvent. Finally, the wet membranes were dried in air at room temperature to obtain dry composite membranes. The orthogonal array is described as La(bc), where L is the symbol of the orthogonal design, a is the number of experimental runs, b is the number of levels and c is the number of factors. The orthogonal array L9(34) was selected for this investigation and is shown in Table 1. Compositions of dope solution and casting conditions for flat-sheet composite membrane preparation are also presented. Table1 Compositions of dope solution and casting conditions for composite membrane fabrication
A
B
PVDF/PSMA
Time (s)
No
C
D
Dope solution
Coagulation bath
temperature (oC)
temperature (oC)
1
1(9/0)
1(5)
1(30)
1(20)
2
1(9/0)
2(300)
2(40)
2(30)
3
1(9/0)
3(600)
3(50)
3(40)
4
2(8/1)
1(5)
2(40)
3(40)
5
2(8/1)
2(300)
3(50)
1(20)
6
2(8/1)
3(600)
1(30)
2(30)
7
3(7/2)
1(5)
3(50)
2(30)
8
3(7/2)
2(300)
1(30)
3(40)
9
3(7/2)
3(600)
2(40)
1(20)
2.3. Fabrication of water-in-oil emulsions Surfactant-free water-in-oil emulsions (labeled as SFE) were fabricated by mixing water and oil at a 1:9 volume ratio and sonicating the mixture at a power of 2 kW for 1.5 h to produce a white and milky solution. The emulsions of toluene and chloroform were referred to as SFE-1 and SFE-2, respectively. Surfactant-stabilized water-in-oil emulsions (labeled as SSE) were fabricated using the following steps. For SSE-1, span 80 (0.5 g) was added into toluene (114 mL), and water (1 mL) was subsequently added. The mixture was stirred for 3 h. For SSE-2, span 80 (0.5 g) was added into chloroform (114 mL), and water (1 mL) was subsequently added. The mixture was stirred for 3 h. All surfactant-stabilized waterin-oil emulsions were stable for more than 30 days, and no demulsification or precipitation was observed.
2.4. Separation of oil-in-water and water-in-oil emulsions The separation experiments were performed at room temperature and general pressure conditions. The effective diameter of the filter was 1.5 mm.
2.5. Temperature-sensitive permeation The temperature sensitivity of the PVDF composite membrane was measured by the change in pure water flux with various temperatures. The pure water flux of the membrane was measured using a method described previously[25]. All membranes were initially pre-pressurized at a pressure of 0.12 MPa for 30 min to obtain a steady state. Before the flux of pure water at various temperatures was measured, the membrane was pre-pressurized at the corresponding temperature for 20 min, and the flux was calculated using the following equation as described elsewhere:
Jw
M A △t
(1)
where M, A and △t represent the permeate quality (M) of water solution, the effective of membrane area (m2) and the permeation time (h), respectively.
2.6. Membrane characterization The morphologies of the membranes were characterized by field-emission scanning electron microscopy (FE-SEM; S4800, Hitachi, Japan) at 10 kV. Prior to the observations, all samples were coated with gold under vacuum. The membrane hydrophobicity was measured with a contact angle meter (DSA100, Krüss, Germany) to obtain the static contact angle (CA). The water droplets used in the measurements were 2 μL in volume. For each membrane sample, five measurements were collected to obtain the average CA value. The roughness parameters and three-dimensional surface images of the PVDF membranes were investigated by TCCM (Axio CSM 700, ZEISS, Germany). The effectiveness of oil-water separation was examined by optical microscopy, and the purity of the oil in the collected filtrates was determined using a Karl Fischer moisture titrator (Mettler Toledo C20). Dynamic light scattering (DLS) measurements were performed on a Zetasizer Nano ZS. The degree of crystallization
and thermoresponsivity of the dry membranes were investigated using differential scanning calorimeter (Model DSC200F3, Netzsch, Germany), and the determination temperature was varied from -20 to 200 oC at a heating or cooling rate of 10 oC/min under a nitrogen atmosphere. The temperature sensitivity of the composite membranes was also assessed by the change in pure water flux with various temperatures. The compositions of the composite membranes were analyzed using attenuated total reflection infrared Fourier transform spectroscopy (ATR-FTIR) with a Nicolet iS50 FT-IR spectrometer, which was equipped with an ATR sampling accessory. The IR spectra were accumulated at a resolution of 4 cm-1 over the range of 650-4000 cm-1 with a total of 16 scans. The crystalline structures of the membranes were analyzed using an American D8A A25 X-ray diffractometer with Cu Kα radiation (k=0.154 nm) and operated at the range of 5-40°.
3. Results and discussion 3.1. Morphology and wettability of membranes
Figure 1. Morphology and microstructure of membranes with various solution compositions and casting conditions
Figure 1. (Continued)
Figure 1. (Continued)
The effects of various solution compositions and casting conditions on the morphology and microstructure of the membranes can be observed from the SEM micrographs shown in Figure 1. No skin layers were found on the surfaces of M-1, M2 and M-3, whereas obvious porous structures can be observed on the top of the membranes, with interconnected holes in the networks constructed by PVDF small petal-like or globule structures connected with each other. The imperfections in the crystalline structure on the surface of M-1 create porosity, which might be caused by
the shorter time in the ethanol coagulation bath might have been insufficient to complete the crystallization. In contrast, M-4, M-5 and M-6 show a porous microstructure of uniformly packed microspheres with diameters of 0.5-1.0 μm, which could be interpreted as the formation of PVDF spherical particles due to crystallization during precipitation, which is a solid-liquid de-mixing process. As demonstrated in the cross-section of the membranes, the membranes are uniformly skinless and are composed of equal-sized spherical particles. Such a structure suggests that the crystallites are nucleated simultaneously and subsequently grow in all directions until they join with adjacent particles. Compared with M-5 and M-6, the surface of M-4 is rather porous, which could suggest that the conditions of the casting membrane are different. When the ratio of PVDF/PSMA reaches 7/2, i.e., M-7, M-8, M-9, the surfaces of the membranes consist of PVDF small broccoli-like hierarchical microstructures that connect with each other. The aggregation of the micro/nano microstructure enhances the roughness of the surface to tailor the hydrophobic property. It is worth noting that the membranes are composed of irregular spherical particles that can be observed in the cross-sections of M-7, M-8, M-9, which are similar to the surface of a cactus, and this structure of the porous channel creates a higher flux and separation efficiency for oil-water separation.
Figure 2. Three-dimensional surface micrographs and line roughness of the membranes
Figure 2. (Continued)
Figure 2. (Continued)
Table 2. Roughness parameters and contact angles of the membranes Surface roughness parameter (μm) Sample
Contact angle (°) Ra (μm)
Rq (μm)
Rmax (μm)
M-1
0.93±0.04
5.11±0.66
10.29±1.56
132.2±1.6
M-2
1.87±0.04
6.28±0.24
14.56±0.45
133.2±1.0
M-3
1.94±0.03
6.56±0.23
16.18±1.34
131.7±1.6
M-4
1.29±0.01
8.46±0.2
13.02±0.72
137.5±1.1
M-5
1.14±0.01
5.94±0.24
10.11±0.15
126.2±1.6
M-6
0.87±0.05
3.62±0.45
8.28±1.25
127.4±1.5
M-7
1.39±0.09
10.16±1.3
14.39±1.88
139.5±3.7
M-8
0.71±0.11
2.74±0.53
5.98±1.11
143.3±2.4
M-9
0.85±0.03
3.40±0.37
6.33±0.38
138.4±2.8
According to the models developed by Thomas Young, Wenzel and Cassie-Baxter [15]
, the hydrophobic surface chemistry and hierarchical micro/nanostructure are of
equal importance in achieving super-hydrophobicity. The surface morphology and microstructure of the membrane have a great influence on its hydrophobicity, and the surface roughness enhances the surface wettability. The surface properties of the PVDF membranes were investigated using TCCM. Figure 2 shows the threedimensional surface images. The roughness parameters and contact angles of the membranes are presented in Table 2. The average roughness (Ra), the root mean square roughness (Rq) and the maximum roughness (Rmax) were analyzed for the images. Ra and Rq appear to be the most helpful and consistent in characterization of the surface topography of the membranes. M-1, M-2 and M-3 exhibit similar contact angles at 131.7°-132.2° and present relatively rough surfaces covered with a multitude of small peaks (Ra=0.93±0.04, Ra=1.87±0.04 and Ra=1.94±0.03, respectively). However, M-7, M-8 and M-9 exhibit relatively smooth surfaces with Ra=1.39±0.09, Ra=0.71±0.11 and Ra=0.85±0.03, respectively, and the high hydrophobicity of the membranes could be attributed to the addition of PSMA. It can be clearly observed that the contact angle of the surface is closely related to both chemical composition and surface roughness.
Figure 3. (a) Water contact angles of M-8 with various drop ages; (b) Wettability of M-8 toward oil
In general, the hydrophobicity is evaluated according to the water contact angle, and membranes with higher hydrophobicities exhibit larger contact angles. The contact angle of water on M-8 is 143°, and no obvious decline of the WCA is observed with increasing drop age, as shown in Figure 3(a). This result might be attributed to the nano/microscale hierarchical roughness combined with the low surface free energy of PVDF. However, the hydrophobicity of the membranes is also improved by introduction of PSMA into the polymer matrix. As described in Figure 3(b), when an oil drop contacts the superoleophilicity surface, it spreads along the membrane surface and permeates through the cross-section of the membrane within 15 s.
3.2. Separation performance of membranes for water/oil emulsions
Figure 4. (a) Photograph of separation for water/oil emulsions; (b) photographs and optical microscope images of the feed emulsion and (c) filtrate emulsion
Figure 5. Schematic description of membrane separation for water/oil emulsions
Figure 6. DLS data of the feed emulsions and their filtrate correspondingly for SEE-1 and SEE-2
Figure 7. Flux and oil purity in the filtrate after permeation through the composite membranes: (a) SEE-1; (b) SEE-2
The as-fabricated membranes with special wettability can be applied in water/oil
separation. A diagrammatic sketch of separation for water-in-oil emulsions is depicted in Figure 4. Due to the multi-level micro/nanostructure of the composite membranes, the emulsion droplets are demulsified and immediately permeate through the membranes, whereas water is retained when the emulsions are poured onto the membranes. Figure 5 shows a schematic of the membrane separation process for water/oil emulsions. During water-in-oil emulsion separation, the membrane was placed in a continuous oil environment, and its oleophilicity and under-oil hydrophobicity guarantee the penetration of oil and effective interception of water droplets. Optical microscopy was applied to examine the separation effectiveness by comparing the feed with the collected filtrate. A significant difference in phase composition could be noted between the feed and the corresponding filtrate. The collected filtrate turns obviously transparent compared with the original milky-white feed emulsion, which indicates the effectiveness of the PVDF composite membranes for separation of various water-in-oil emulsions. So from the comparison of opticl micrograph, the emulsions could be successfully separated in one step, driven only by gravity and without any external force (see supplementary Figure S2). To investigate the emulsion droplets distribution (beyond the resolution of optical microscope, ~200 nm) furthermore, the emulsion droplets before and after filtration were also monitored by DLS, and the measured diameter distribution was shown as shown in Figure 6. The droplet size of the SEE-1 and SEE-2 ranged from 100 nm to 1.5 μm approximately. After filtration, no droplets around these ranges are observed, indicating the effective separation. The purity of the purified oil was investigated by measuring the water weight percentage in the filtrate. The flux and oil purities in the filtrate after permeation through the as-fabricated PVDF membranes are presented in Figure 7. The separation
results for SEE-1 and SEE-2 effectively illustrate the high separation efficiency of our membranes. Similar effective separations were also achieved for the SFE-1 and SFE-2 systems (see supplementary Figure S3). M-3 presents the highest flux for all emulsions, which could be due to the larger pore size and porosity generated in the membrane formation process. However, this structure inevitably reduced the separation efficiency. As shown in Figure 7(b), with increasing addition of PSMA, the flux of M-9 reaches 230 kg·m-2 h-1, and the purity of purified oil remains at a relatively high level of 99.98%, which is in accordance with the cactus-like spherical particles in the cross-section. Thus, both the cactus-like spherical particle structure and the low surface free energy of PSMA are beneficial to increase the flux and the separation efficiency. Table 3. L9(34) standard orthogonal array in terms of coded factor levels (values 1-3): physical values and experimental results No.
A
B
C
D
Purity of oil（%）
1
1
1
1
1
99.948
2
1
2
2
2
99.952
3
1
3
3
3
99.955
4
2
1
2
3
99.972
5
2
2
3
1
99.971
6
2
3
1
2
99.965
7
3
1
3
2
99.955
8
3
2
1
3
99.963
9
3
3
2
1
99.967
K1
99.952
99.958
99.959
99.962
K2
99.969
99.962
99.964
99.957
K3
99.962
99.962
99.960
99.963
Q
4.62*10-4
0.870*10-4
4.20*10-4
1.98*10-4
Y
1 9 Y 99.961 9 i1 i
QT nSY2 1.167*103
Factors ordered by significance (from most to least) ACDB Optimal combination
A2B2C2D3 or A2B3C2D3
Table 4. Analysis of variance for separation efficiency
Sum of squares
Degrees of
Mean square of
of deviations
freedom
deviation
Parameters
F ratio
F0.05
Significance
Factor A
QA=4.62*10-4
2
S A2 =2.31*10-4
5.25
19
*
Factor C
QC=4.20*10-4
2
SC2 =2.10*10-4
4.77
19
*
Factor D
QD=1.98*10-4
2
S D2 =0.99*10-4
2.25
19
*
Error E
QE=0.87*10-4
2
SE2 =0.44*10-4
Sum T
QT=0.290
8
The OED was selected to analyze the separation performance of the membranes with different compositions of dope solution and casting conditions and to determine the level of influence of each factor on the separation efficiency for SEE-2 driven only by gravity. The details of the calculation processes for the mean and range can be found in reference [26]. The results of the OED calculations are summarized in Table 3 and Table 4. The analysis of variance table contains the sum of squares, degrees of freedom, F variance ratio, F standard value and the significance. It can be clearly observed that the sum squares of deviations of Factor E is the least, and thus it can be
used as the error in the OED calculations. The F variance ratio is the ratio of variance due to the effect of a particular factor and the variance due to the error. The F standard value is determined from an F-table as the statistical level of significance
[27]
. In this
study, if the F-test value is greater than F0.05, the process parameter is considered significant. As shown in Table 4, Factor A (the ratio of PVDF/PSMA), Factor C (dope solution temperature), and Factor D (coagulation bath temperature) are insignificant for the membrane separation performance, but Factor A (F ratio=5.25), i.e., the ratio of PVDF/PSMA, has a greater impact compared with the others, which is in good accordance with the morphology and microstructure of the membranes (Figure 1).
3.3. Thermoresponsivity of membranes
Figure 8. DSC thermograms of the membranes
Table 5. Thermal analysis of the membranes Tmpb
△Hmd
Xmc
Tcpf
△Hch
X cc
(oC)
(J/g)
(%)
(oC)
(J/g)
(%)
M-1
174.0
61.9
59.1
139.1
50.0
47.8
M-4
172.2
58.9
61.1
135.6
54.7
58.8
M-7
172.5
51.4
63.1
142.5
51.9
63.7
Sample
Table 6. Thermal analysis of the membranes and PSMA
Tmoa
Tmpb
Tmec
△Hmd
Tcoe
Tcpf
Tceg
△Hch
(oC)
(oC)
(oC)
(J/g)
(oC)
(oC)
(oC)
(J/g)
M-4
6.5
15.8
21.8
6.0
2.6
18.1
25.4
9.7
M-7
22.7
29.9
34.5
24.0
2.7
19.7
27.4
23.5
PSMA
36.4
40.6
44.0
75.1
21.4
25.5
28.8
68.0
Sample
a
Onset temperature on DSC heating curve; b Peak temperature on DSC heating curve;
c
Endset temperature on DSC heating curve; d Enthalpy on DSC heating curve;
e
Onset temperature on DSC cooling curve; f Peak temperature on DSC cooling curve;
g
Endset temperature on DSC cooling curve; h Enthalpy on DSC cooling curve.
DSC analysis was conducted on the fabricated flat-sheet composite membranes to examine the effect of PSMA on the crystallinity degree (Xc) of the membranes. The heating and cooling thermograms of the composite membranes with various ratios of PVDF to PSMA are presented in Figure 8, and the results are summarized in Table 5. The Xc of the fabricated membranes is based on either the melting (Xmc) or crystallization (Xcc) obtained from the following equation [28]:
Xc
△Hf
W *100% △Hf*
(2)
where ΔHf is the experimental heat of fusion (ΔHm) or the experimental heat of crystallization (ΔHc), ΔHf*=105 J ·g-1 (the heat of fusion of the 100% crystalline PVDF), and W is the weight fraction of the polymer in the mixture. From Table 5, which verified that PSMA has a significant influence on the thermal properties of PVDF, it can be found that Xc increased from 47.8% to 63.7%, whereas the PSMA is incorporated in PVDF matrix, which can be explained by the fact that the PSMA promotes nucleation of the PVDF crystalline phase. The DSC curves also demonstrate
that M-4 and M-7 exhibit obvious phase change behaviors, but M-1 does not. For M-7, the peak temperature on the DSC heating curve (Tmpb) and the melting enthalpy (△Hmd) of the PSMA in the composite membranes are 29.9 oC and 24.0 J · g-1, respectively. The Tmpb and △Hmd values of the PSMA in the composite membranes decrease from 29.9 to 15.8 oC and from 24.0 to 6.0 J · g-1, respectively, as the PVDF/PSMA ratio varies from 7/2 to 8/1 (Table 6). These results could be interpreted as due to the movement of PSMA molecular chains restricted by many PVDF segments in membranes. A similar trend is noted in the DSC cooling curve. Thus, the pore size of the composite membranes could be regulated by crystallization and melting of PSMA in the membranes, and the transition response to temperature could also be adjusted using the content of PSMA in the composite membranes.
Figure 9. Variation of the flux of the membranes with temperature
Figure 10. Response mechanism of the composite membrane under various temperatures
To investigate the temperature responsiveness of the composite membranes, the variation in the membrane flux of the membranes with temperature was studied. Figure 9 describes the effect of temperature on the water flux. Theoretically, the pure water flux of the membranes (pristine PVDF, M-1) increases with increasing temperature. This result could be explained by the increased mobility of water with increasing temperature. The composite membranes show different flux values as the temperature changes because PSMA is a thermo-sensitive polymer. For M-4, the pure water flux decreases with increasing temperature in the range of 0 to 20 oC due to the reduced membrane pore size with melted PSMA, whereas the flux increases with increasing temperature after 20 oC due to the increased mobility of the water. In the process of cooling, with the crystallization of PSMA, the large pore size and high porosity are generally beneficial to water passing through the membranes, and thus the pure water flux of the composite membranes increases. Figure 10 schematically illustrates the thermoresponsive mechanism and the microstructure of the composite membrane surface. The water flux data can be used to estimate the change in the membrane pore size. A similar trend occurs in M-7, and the only difference is the transition response to temperature. The change of temperature influences the pore size
of the membrane, and the transition remarkably changes with the content of PSMA in the composite membranes, which indicates the thermoresponsive “open-closed’’ pores with the crystallization and melting of PSMA in the membranes, which is more suitable for separation of solutes with small size differences. The response to temperature of the membrane is consistent with the DSC thermograms.
3.4. Crystallographic studies
Figure 11. ATR-FTIR spectra of the membranes
The ATR-FTIR spectra of membranes with various ratios of PVDF and PSMA are presented in Figure 11. In the case of M-4 and M-7, three strong peaks can be observed near 2920, 2850 and 1724 cm-1. The former two peaks are due to C-H stretching vibration, and the latter peak is due to the -O-C=O stretching bond of PSMA. The intensity of the peaks increases with the increasing content of PSMA, but these peaks are not present in the M-1 spectrum. As a result, it could be concluded that the composite formation occurred. The main absorption peaks at 760, 795, 840,
880, 970, 1070, 1180 and 1400 cm-1 are present in all of the membrane spectra. The absorption peaks at 760, 795, 970, 1070 and 1400 cm-1 are attributed to formation of the α-phase in the crystallization zones. The absorption peaks at 840 and 1180 cm-1 are characteristic of the PVDF β-phase crystal structure [29]. It should be noted that the addition of PSMA causes an increase in the absorption spectrum intensity at 795 cm-1 but a decrease in the absorption spectrum intensity at 840 cm-1, indicating that the variety of PSMA content has a significant effect on the crystalline form of PVDF in the composite membranes. In other words, PSMA addition affects the polymorphism of the PVDF in favor of α-phase.
Figure 12. XRD patterns of the membranes
Both the crystallization behaviors and structure are essential to the morphology and microstructure of the membrane during membrane formation. WXRD measurements were performed to examine the crystalline structures of the overall membranes. As described in Figure 12, the increases in the intensity of the diffraction peaks at 18.3° (100) and 26.6° (021) is due to enhancement of the crystallization zones in
PVDF/PSMA. The diffraction peaks near 18.3° and 26.6° are characteristic of the PVDF α-phase crystal structure. Thus, it can be observed that the addition of PSMA is conducive to α-phase
[30]
, which is in good agreement with the result of ATR-FTIR
spectra.
Conclusion PVDF/PSMA composite membranes with thermoresponsivity and highly hydrophobic surfaces were fabricated using the phase separation method. From the result of the OED, the ratio of PVDF/PSMA had a greater impact on the separation performance for water in oil emulsions than the casting conditions. The flux of M-9 reached 230 kg·m-2 h-1, and the purity of the purified oil remained at a relatively high level of 99.98%. More importantly, the addition of PSMA endowed the composite membranes with thermoresponsive characteristics. The composite membranes M-4 and M-7 showed a decreased water flux near 6.5 and 22.7 oC, respectively, which also demonstrated that the transition response to temperature could be adjusted by the content of PSMA in the composite membranes. Thus, membranes with temperaturesensitive “open-closed’’ pores might find widespread application in separation of solutes with small size differences.
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