materials Article
Poly(vinylidene Fluoride-Hexafluoropropylene) Porous Membrane with Controllable Structure and Applications in Efficient Oil/Water Separation Xinya Wang
ID
, Changfa Xiao *, Hailiang Liu, Qinglin Huang, Junqiang Hao and Hao Fu
State Key Laboratory of Separation Membranes and Membrane Processes, National Center for International Joint Research on Separation Membranes, Tianjin Polytechnic University, No. 399, Binshui Road, Xiqing District, Tianjin 300387, China;
[email protected] (X.W.);
[email protected] (H.L.);
[email protected] (Q.H.);
[email protected] (J.H.);
[email protected] (H.F.) * Correspondence:
[email protected]; Tel.: +86-022-83955299 Received: 1 February 2018; Accepted: 18 March 2018; Published: 18 March 2018
Abstract: Poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) porous membranes are fabricated via thermally induced phase separation (TIPS) with mixed diluent (dibutyl phthalate (DBP)/dioctyl phthalate (DOP)). The effects of mixed diluent are discussed in detail in term of morphology, mean pore size, selective wettability, etc. The results show that the membrane structure changes from spherulitic to bicontinuous with the change of DBP/DOP ratio. It is also found that the degree of crystallization decreases with the decrease of DBP/DOP ratio in mixed diluent. When liquid–liquid (L-L) phase separation precedes solid–liquid (S-L) phase separation, the obtained membranes have outstanding hydrophobicity and lipophilicity, excellent mechanical property. Additionally, the PVDF-HFP hybrid membranes are prepared with silica (SiO2 ) particles and the effect of SiO2 content on structure and properties is discussed. It is found that the PVDF-HFP hybrid membrane with 2 wt % SiO2 (M3-S2) has better properties and higher filtration rate and separation efficiency for surfactant-stabilized water-in-oil emulsion separation. Moreover, the membrane M3-S2 also exhibits excellent antifouling performance for long-running. Keywords: PVDF-HFP; thermally induced phase separation; mixed diluent; hybrid membrane; water-in-oil emulsion separation
1. Introduction Recently, an emerging fluoro-copolymer, PVDF-HFP, is attracting more and more attention. As shown in Figure 1, PVDF-HFP can be obtained via emulsion polymerization of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) [1]. Compared with polyvinylidene fluoride (PVDF), PVDF-HFP has better properties, such as higher solubility, higher hydrophobicity and better mechanical strength, owing to the combination of HFP [2–4]. To date, most studies focused on the fabrication of PVDF-HFP membranes via non-solvent induced phase separation (NIPS) method and the obtained membranes were applied in processes of membrane distillation (MD), polymer battery and gas absorption, etc. Khayet et al. [5] prepared PVDF-HFP hollow fiber membranes via NIPS method and studied the effects of polymer concentrations on MD process. Hemmat et al. [6] studied the effects of three salt additives (CaCO3 , LiCl, and CaCl2 ) on MD performance of PVDF-HFP membrane. Wang et al. [7] prepared PVDF-HFP flat-sheet asymmetric membranes and investigated the effects of molecular weight of PEG on performance for MD process. Khayet et al. [8] investigated the effects of solvent compositions (N,N-dimethylformamide (DMF), DMAC and trimethylphosphate (TMP)) on the PVDF-HFP hollow fiber membrane structure and MD performance. Stephan et al. [9], Hwang et al. [10] and Li et al. [11] prepared PVDF-HFP flat-sheet membranes and applied them in Materials 2018, 11, 443; doi:10.3390/ma11030443
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al. [10] and Li et al. [11] prepared PVDF-HFP flat-sheet membranes and applied them in polymer polymer battery technology. Additionally, Jiraratananon et fabricated al. [12] fabricated modified PVDF-HFP battery technology. Additionally, Jiraratananon et al. [12] modified PVDF-HFP hollow hollow fiber membrane by using the method of dehydrofluorination and applied it in a membrane fiber membrane by using the method of dehydrofluorination and applied it in a membrane gas gas absorption process. However, there studies on fabrication of PVDF-HFP membrane absorption process. However, there are are veryvery fewfew studies on fabrication of PVDF-HFP membrane via via TIPS method. Cui et al. [13] prepared PVDF-HFP membranes with sulfolane via TIPS method TIPS method. Cui et al. [13] prepared PVDF-HFP membranes with sulfolane via TIPS method for for lithium lithium ion ion batteries. batteries. Cao Cao et et al. al. [14] [14] investigated investigated the theCO CO22 extraction extraction of ofDBP DBPfrom fromPVDF-HFP/DBP PVDF-HFP/DBP former former films films prepared prepared via via TIPS. TIPS.
Figure Figure 1. 1. polymerization Polymerization equation equation of of PVDF-HFP. PVDF-HFP.
During TIPS process, the diluent is a key factor for controlling the membrane structure. The During TIPS process, the diluent is a key factor for controlling the membrane structure. The diluent compositions influenced the interaction between the diluent and polymer which have a diluent compositions influenced the interaction between the diluent and polymer which have a significant effect on membrane structure [15]. Li et al. [16], Song et al. [17] and Wang et al. [18] significant effect on membrane structure [15]. Li et al. [16], Song et al. [17] and Wang et al. [18] prepared PVDF flat/hollow fiber membranes with mixed diluents via TIPS method and studied the prepared PVDF flat/hollow fiber membranes with mixed diluents via TIPS method and studied effects of diluent compositions on membrane structure and performance. It was found that desirable the effects of diluent compositions on membrane structure and performance. It was found that structure and outstanding performance can be obtained by changing diluent compositions. desirable structure and outstanding performance can be obtained by changing diluent compositions. Nevertheless, the membrane prepared via TIPS method shows unsatisfactory properties. Thus, the Nevertheless, the membrane prepared via TIPS method shows unsatisfactory properties. Thus, modification method is becoming more important for membrane properties. Sun et al. [19] fabricated the modification method is becoming more important for membrane properties. Sun et al. [19] PVDF hollow fibers and flat sheet membranes with hydrophilic surface via amine treatment. Zhang fabricated PVDF hollow fibers and flat sheet membranes with hydrophilic surface via amine treatment. et al. [20] modified PVDF membrane surface by doping ammonia water into polymer solution and Zhang et al. [20] modified PVDF membrane surface by doping ammonia water into polymer solution formed micro/nanostructure via the modified phase-inversion process. Wang et al. [21] attempted to and formed micro/nanostructure via the modified phase-inversion process. Wang et al. [21] attempted fabricate PVDF membranes modified with Au nanoparticles which provided sufficient capacity for to fabricate PVDF membranes modified with Au nanoparticles which provided sufficient capacity continuous reduction of p-nitrophenol (4-NP) and methylene blue (MB) in chloroform-in-water for continuous reduction of p-nitrophenol (4-NP) and methylene blue (MB) in chloroform-in-water emulsion. Hou et al. [22] investigated the preparation of polytetrafluoroethylene (PTFE) composite emulsion. Hou et al. [22] investigated the preparation of polytetrafluoroethylene (PTFE) composite membrane with a hydrophilic poly(vinyl alcohol)/silica nanoparticles (PVA-Si) hybrid fibrous membrane with a hydrophilic poly(vinyl alcohol)/silica nanoparticles (PVA-Si) hybrid fibrous coating coating and the obtained membrane has a hydrophobic substrate and an in-air hydrophilic and and the obtained membrane has a hydrophobic substrate and an in-air hydrophilic and underwater underwater superoleophobic top surface. Graphene membranes covalently modified with superoleophobic top surface. Graphene membranes covalently modified with polydimethylsiloxane polydimethylsiloxane were prepared by Lei et al. [23] and presented the potential for water/oil were prepared by Lei et al. [23] and presented the potential for water/oil separation. However, these separation. However, these methods are difficult to scale up owing low maneuverability and poor methods are difficult to scale up owing low maneuverability and poor stability. Compared with stability. Compared with various modification methods, direct blending inorganic materials with various modification methods, direct blending inorganic materials with polymers is easy and effective. polymers is easy and effective. In this study, PVDF-HFP and mixed diluent (DBP)/(DOP) are used for fabricating porous In this study, PVDF-HFP and mixed diluent (DBP)/(DOP) are used for fabricating porous membranes via TIPS method. The effect of the interaction between mixed diluent and PVDF-HFP on membranes via TIPS method. The effect of the interaction between mixed diluent and PVDF-HFP on the structure of obtained membranes is investigated and the structure is controlled through changing the structure of obtained membranes is investigated and the structure is controlled through changing the composition of mixed diluent. Additionally, PVDF-HFP hybrid membranes are fabricated by the composition of mixed diluent. Additionally, PVDF-HFP hybrid membranes are fabricated by blending hydrophobic SiO2 particles and the effects of SiO2 contents on structure and performance are blending hydrophobic SiO2 particles and the effects of SiO2 contents on structure and performance discussed in detail. are discussed in detail.
2. Experimental 2. Experimental 2.1. Materials 2.1. Materials PVDF-HFP (Kynar2500, ≈18 mol % HFP), semi-crystalline copolymer, was purchased from ◦ C) % PVDF-HFP (Kynar2500, copolymer, from Arkema (Singapore). DBP (b.p.≈18 340mol andHFP), DOP semi-crystalline (b.p. 386 ◦ C) were purchasedwas frompurchased Tianjin Kermel Arkema (Singapore). 340 °C) and DOP 386 °C) were purchased from Tianjin Kermel Chemical Reagent Co.DBP Ltd.(b.p. (Tianjin, China) Silica(b.p. particles (SiO 2 average particle size 40 nm were Chemical Reagent Co. Ltd. (Tianjin, China) Silica particles (SiO 2 average particle size 40 nm were purchased from Guangzhou GBS High-tech & Industry Co., Ltd. (Guangzhou, China) Graphene purchased was frompurchased Guangzhou GBSXiamen High-tech & Industry Ltd. (Guangzhou, Graphene (KNG-G5) from Knano GrapheneCo., Technology Co., Ltd. China) (Xiamen, China) (KNG-G5) was purchased from Xiamen Knano Graphene Technology Co., Ltd. (Xiamen, China) Alcohol, used as an extractant, was provided by AR grade. Diesel was purchased from Tianjin Fengchuan Chemical Reagent Technologies Co. Ltd. (Tianjin, China).
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Alcohol, used as an extractant, was provided by AR grade. Diesel was purchased from Tianjin Fengchuan Chemical Reagent Technologies Co. Ltd. (Tianjin, China). 2.2. Determination of Phase Diagram Mixed diluent (DBP and DOP) was mixed with 30 wt % PVDF-HFP at an appropriate temperature (150 ◦ C) in a clean glass vessel and stirred for approximately 3 h to get homogeneous solution. Then, the glass vessel was directly quenched in liquid nitrogen to obtain solid polymer–diluent mixtures. To measure cloud points, a small amount of solid polymer–diluent mixtures was heated to 180 ◦ C on a universal heat-cool stage (THMS 600, Linkam, Surrey, UK) using 20 ◦ C·min−1 heating rate and held for 1 min, then cooled to room temperature using the cooling rate of 5 ◦ C·min−1 . The cloud points were the temperature at which the appearance of turbidity was observed by optical microscopy (5050zoom, Olympus Co., Tokyo, Japan). DSC (DSC204F1, Netzsch, Selb, Germany) was used for measuring crystallization temperature (Tc ). A small amount of solid mixtures was sealed in an aluminum DSC pan, heated to 180 ◦ C and held for 5 min to erase thermal history and then cooled to room temperature at 10 ◦ C·min−1 . During the cooling process, the onset of exothermic peak was identified as Tc . 2.3. Fabrication of PVDF-HFP Membrane First, 30 wt % PVDF-HFP and 70 wt % mixed diluent were fed into a clean and dried glass vessel and stirred at 150 ◦ C for approximately 4 h to obtain a homogeneous solution. After standing for almost 3 h with atmospheric pressure, the bubble in the solution could be removed. The homogeneous solution was cast onto the pre-clean glass substrate with a thickness of 200 µm at 170 ◦ C and then the glass substrate was directly quenched in the water bath (5 ◦ C). The remaining diluent in films was extracted by absolute ethanol which was then removed by immersing in pure water. Finally, the membranes were dried using a drier (FD-1A-50, Shanghai, China). The composition of the obtained PVDF-HFP membranes is exhibited in Table 1. Table 1. The composition of the obtained PVDF-HFP membranes. Membrane
PVDF-HFP/wt %
DBP/wt %
DOP/wt %
SiO2 /wt %
M0 M1 M2 M3/M3-S0 M4 M3-S1 M3-S2 M3-S3
30 30 30 30 30 29 28 27
70 63 56 49 42 49 49 49
7 14 21 38 21 21 21
1 2 3
2.4. Characterization The dried membranes were fractured in liquid nitrogen. The morphologies of membrane surface and cross-section were observed with SEM (TM3030, Hitachi, Tokyo, Japan) after being coated with gold. An atomic force microscope (AFM, Agilent-S5500, Agilent, Palo Alto, CA, USA) was employed to analyze the surface states of the prepared membranes. The hydrophobicity and lipophilicity of membranes were measured by contact angle goniometer (DSA-100, Kruss, Hamburg, Germany) at room temperature. To obtain average value, the contact angle was measured five times at different locations. The mechanical strength tests of obtained membranes were performed with an electron instrument (YG061F Laizhou, Laizhou, China). Each membrane was cut into 5 mm (width) × 30 mm (length) specimen and the tensile rate was 20 mm·min−1 . Five measurements were performed for every sample. The mean pore size of PVDF-HFP porous membranes was investigated using automatic
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mercury porosimeter (AutoPore IV-9500, Tektronix, Beaverton, OR, USA) and the porosity of membrane was measured by a gravimetric method and calculated as follows:
ε(%) =
(W1 −W2 ) ρA (W1 −W2 ) 2 +W ρA ρB
(1)
where ε(%) is the porosity of the membrane; W1 and W2 are dry and wet membrane weights, respectively; and ρA and ρB are the density of polymer and butanol, respectively. The enthalpy of fusion and degree of crystallization of obtained membranes were measured using DSC (German Netzsch, DSC204F1, Netzsch, Selb, Germany) and calculated as follows: Xc =
∆H f ∆H 0f
× 100%
(2)
where Xc is the crystallinity of PVDF-HFP membrane, ∆Hf is the fusion enthalpy of PVDF-HFP membrane, and ∆H 0f is the fusion enthalpy of PVDF-HFP with 100% crystallinity, i.e., 104.7 J·g−1 [24]. 2.5. Preparation of Surfactant-Free and Surfactant-Stabilized Water-in-Oil Emulsion To obtain a homogeneous turbid solution, 1 mL pure water was added dropwise into 100 mL diesel and vigorously stirred for at least 6 h. After standing for 1 day, the surfactant-free water-in-oil emulsion was prepared. For surfactant-stabilized water-in-oil emulsion, 0.2 g span80 with a hydrophile–lipophile balance (HLB) value of 4.3 was added into 100 mL diesel and vigorously stirred for 0.5 h. Then, 1 mL pure water was added dropwise into the mixture and stirred vigorously for 6 h to obtain surfactant-stabilized water-in-oil emulsion. After standing for more than 7 days, no obvious agglomeration and precipitation was observed. 2.6. Surfactant-Free and Surfactant-Stabilized Emulsion Separation Experiments The prepared PVDF-HFP membrane was placed on the porous support between feeding bottle and filter tip and sealed carefully by Teflon tape. The separation experiment was carried out at a negative pressure (−0.09 MPa). The filtration rate was measured on the permeated volume of surfactant-free or surfactant-stabilized water-in-oil emulsion through the membrane and determined as follows: V (3) J= S×t where J is the filtration rate (L·m−2 ·h−1 ), V is the permeate quantity (L), S is the effective area (m2 ) and t is the testing time (h). The separation efficiency of the surfactant-free or surfactant-stabilized water-in-oil emulsion was calculated as follows: C f − Cp R (%) = × 100% (4) Cf where Cf and Cp are water content of feed and permeate water-in-oil emulsion, respectively; and R is the separation efficiency. To get the average value, every membrane was tested at least five times. The water content in the filtrate was detected by using the Karl Fischer titrator (C20, Mettler Toledo, Greifensee, Switzerland) and then the purity was analyzed to obtain the filter fineness. Photographs of the emulsion and filtrate were taken with a Nikon optical microscope (E200, Nikon, Tokyo, Japan). 2.7. Antifouling Performance To evaluate the antifouling performance of the obtained membrane, the filtration rate of the membrane M3-S2 for treatment of emulsions separation in 15 cycles was performed. After each cycle, the membrane was carefully taken out and simply washed with ethanol, and then dried in air.
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3. Results and Discussion 3. Results and Discussion 3.1. Phase Diagram 3.1. Phase Diagram The phase diagram of polymer/diluent system has important data for preparing membranes The phase diagram of polymer/diluent important data preparing membranes via via TIPS. The phase diagram for this systemsystem (30 wt has % PVDF-HFP and for 70 wt % mixed diluents with TIPS. The phase diagram this system (30 wt 70 wt % mixed diluents with varied varied DBP/DOP ratios) for is shown in Figure 2. % In PVDF-HFP Figure 2, it and is seen clearly that T c decreased slowly DBP/DOP ratios) is shown in Figure 2. In Figure 2, it is seen clearly that T c decreased slowly with the with the increase of DBP/DOP ratio, while Tcloud decreased rapidly. When the DBP/DOP ratio was increase of 7/3, DBP/DOP while decreased rapidly. When the DBP/DOP ratio was larger than larger than Tc wasratio, higher thanTTcloud cloud and the S-L phase separation preceded L-L phase separation. 7/3, Tthe c was higher than Tcloud and the S-L phase separation preceded L-L phase separation. With the With further increase of DOP content, the L-L phase separation preceded the S-L phase separation. further increase of DOPbetween content,DBP the L-L phase preceded the S-L phase separation. In addition, the relation content in separation mixed diluent was shown in Supplementary Materials. 140 Crystalization temperature(Tc)
Temperature(℃)
130
Cloud point (Tcloud)
120 110 100 90 80 70 30
40
50
60
70
80
90
100
Composition(wt%,DBP/DBP+DOP)
Figure 2. 2. Phase with mixed mixed diluent of different Figure Phase diagram diagram of of 30 30 wt wt % % PVDF-HFP PVDF-HFP membranes membranes prepared prepared with diluent of different DBP contents. contents. DBP
3.2. The Mixed Diluent Diluent Compositions Compositions on on PVDF-HFP PVDF-HFP Membranes Membranes 3.2. The Effect Effect of of Mixed During the the membrane the kinetic kinetic factors During membrane preparation, preparation, the factors and and thermodynamic thermodynamic factors factors are are important important for membrane structure [25]. The membrane structure can be controlled by the preparing for membrane structure [25]. The membrane structure can be controlled by the preparing condition, condition, such as polymer concentration, cooling rate and interaction between polymer and diluent [26]. The The such as polymer concentration, cooling rate and interaction between polymer and diluent [26]. ratio of DBP/DOP had an obvious effect on the structure of PVDF-HFP membranes. In Figure 3(A-1), it is ratio of DBP/DOP had an obvious effect on the structure of PVDF-HFP membranes. In Figure 3(A-1), seen clearly that the cross section entirely presented spherulitic structure when the diluent was only it is seen clearly that the cross section entirely presented spherulitic structure when the diluent was DBP.DBP. The reason for the of spherulites depended on the on strong interaction between PVDFonly The reason forformation the formation of spherulites depended the strong interaction between HFP and DBP which caused the S-L phase separation [27]. As the DBP/DOP ratio decreased (but PVDF-HFP and DBP which caused the S-L phase separation [27]. As the DBP/DOP ratio decreased larger than 7/3), the spherulite size became smaller and the section structure denser. It can be (but larger than 7/3), the spherulite size became smaller and the section structure denser. It can be explained as as the the higher higher DBP/DOP DBP/DOP ratio stronger interaction interaction between between PVDF-HFP PVDF-HFP and and explained ratio gave gave rise rise to to stronger mixed diluent. The strong interaction facilitated occurrence of S-L phase separation before L-L phase mixed diluent. The strong interaction facilitated occurrence of S-L phase separation before L-L phase separation and Additionally, thethe stronger interaction made the separation and spherultic spherulticstructure structurecan canbebeobtained. obtained. Additionally, stronger interaction made system present stronger nucleation activity and less primary nuclei formed. When the polymer the system present stronger nucleation activity and less primary nuclei formed. When the polymer started to tocrystallize, crystallize,diluent diluent was excluded from the spherulites The less primary nuclei started was excluded from the spherulites easily.easily. The less primary nuclei formed formedspherulites larger spherulites and led to bigger pores. the ratio decreased further, thepresented structure larger and led to bigger pores. When theWhen ratio decreased further, the structure presented bicontinuous structure gradually. In addition, the bicontinuous structure got much looser bicontinuous structure gradually. In addition, the bicontinuous structure got much looser with the with the decrease of DBP/DOP ratio. The reason is that, when the ratio of DBP/DOP was low enough, decrease of DBP/DOP ratio. The reason is that, when the ratio of DBP/DOP was low enough, L-L L-L phase separation can occur before S-L phase separation and the L-L phase separation was driven phase separation can occur before S-L phase separation and the L-L phase separation was driven by the by the nucleation and growth of the lean-phase. In addition, the lower DBP/DOP ratio made the nucleation and growth of the lean-phase. In addition, the lower DBP/DOP ratio made the interaction interaction between PVDF-HFP and mixed diluent get weakerled interaction led toformore between PVDF-HFP and mixed diluent get weaker. Theweaker. weaker The interaction to more time L-L time for L-L phase separation, leading to looser structure with larger pores. phase separation, leading to looser structure with larger pores. From the surface SEM and AFM images of PVDF-HFP membranes, it can be found clearly that the surface of M0 was composed of spherulites and surface roughness was considerably higher than that of other membranes. As the DBP/DOP ratio decreased (but larger than 7/3), the membrane surface became denser due to the smaller space between spherulites. In addition, the surface became
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smoother with the decrease of DBP/DOP ratio, which was verified by the AFM images. With further Materials 2018, 11, 443 6 of 12 increase of DOP content, the surface became more porous and roughness parameters increased.
Figure 3. The The SEM SEM and and AFM AFM morphology morphology of PVDF-HFP membrane: Figure 3. of PVDF-HFP membrane: (A–E) (A–E) M0–M4 M0–M4 (1: (1: cross cross section; section; 2: surface). 2: surface).
As discussed above, the ratio of DBP/DOP had an effect on the phase separation process. From the surface SEM and AFM images of PVDF-HFP membranes, it can be found clearly that the Therefore, it also influenced the degree of crystallization of PVDF-HFP membrane. The crystallinity surface of M0 was composed of spherulites and surface roughness was considerably higher than that was determined by DSC (Figure 4) and the fusion enthalpy and crystallinity are summarized in Table of other membranes. As the DBP/DOP ratio decreased (but larger than 7/3), the membrane surface 2. It was noticed clearly that the crystallinity of the obtained membranes decreased with the decrease became denser due to the smaller space between spherulites. In addition, the surface became smoother of DBP/DOP ratio. When the diluent was only DBP, the growth of spherulites gave the high with the decrease of DBP/DOP ratio, which was verified by the AFM images. With further increase of crystallinity during the S-L phase separation process. With the decrease of DBP/DOP ratio, L-L phase DOP content, the surface became more porous and roughness parameters increased. separation was observed and led to the formation of polymer-lean phase during L-L phase As discussed above, the ratio of DBP/DOP had an effect on the phase separation process. separation, which facilitated the formation of amorphous phase of the membrane. Therefore, it also influenced the degree of crystallization of PVDF-HFP membrane. The crystallinity was determined by DSC (Figure 4) and the fusion enthalpy and crystallinity are summarized in Table 2. It was noticed clearly that the crystallinity of the obtained membranes decreased with the decrease of DBP/DOP ratio. When the diluent was only DBP, the growth of spherulites gave the high crystallinity during the S-L phase separation process. With the decrease of DBP/DOP ratio, L-L phase separation was observed and led to the formation of polymer-lean phase during L-L phase separation, which facilitated the formation of amorphous phase of the membrane.
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M0 M2 M4
60
80
100
120
140
Temperature(℃)
M1 M3
160
180
Figure 4. diagram of the melting curves of theofPVDF-HFP membranes prepared with mixed 4. DSC DSC diagram of the melting curves the PVDF-HFP membranes prepared with diluents. mixed diluents. Table of of fusion andand crystallinity of the membranes prepared with mixed Theenthalpy enthalpy fusion crystallinity ofPVDF-HFP the PVDF-HFP membranes prepared with Table 2. 2. The diluent. mixed diluent.
Membrane M0 M0 M1 M1 M2 M2 M3 M3 M4 M4
Membrane
−1) Fusion Enthalpy (J·g− Fusion Enthalpy (J·g 1 ) 29.00 29.00 28.69 28.69 27.80 27.80 27.02 27.02 26.08 26.08
Crystallinity (%) Crystallinity (%) 27.70 27.40 27.70 27.40 26.56 26.56 25.81 25.81 24.91 24.91
Lipophilic-hydrophobic Lipophilic-hydrophobic surface surface of of membrane membrane was was aa key key factor factor for for the the water water in in oil oil emulsion emulsion separation effectively [28]. To investigate the selective wettability of the prepared membranes, a separation effectively [28]. To investigate the selective wettability of the prepared membranes, a water water or diesel droplet was used to measure the water or oil contact angle. In Figure 5, it can been or diesel droplet was used to measure the water or oil contact angle. In Figure 5, it can been seen that seen that contact the water contact anglefirstly decreased firstly withofthe increase of When DOP the content. Whenratio the the water angle decreased with the increase DOP content. DBP/DOP DBP/DOP ratio was larger than 7/3, the water contact angle increased with the increase of DOP was larger than 7/3, the water contact angle increased with the increase of DOP content. As an example content. As an example to the(M3), PVDF-HFP membrane (M3), as shown Figure 6, the was to the PVDF-HFP membrane as shown in Figure 6, the oil droplet in was dropped onoil thedroplet membrane dropped on spread the membrane surface spread out oil rapidly. the samethe time, the oil and droplet surface and out rapidly. At theand same time, the dropletAtpenetrated membrane the penetrated the membrane and the oil contact angled decreased to almost 0° within 5 s, which ◦ oil contact angled decreased to almost 0 within 5 s, which illustrated the excellent oil wettability of illustrated excellentInoil wettability of PVDF-HFP membranes. In and addition, the surface PVDF-HFP the membranes. addition, the surface morphology such as micronanostructure had morphology suchonasmembrane micro- and nanostructure hadAccording an obvious effect on morphology membrane selective an obvious effect selective wettability. to membrane and the wettability. According to membrane morphology and the results in Figures 5 and 6, it is summarized results in Figures 5 and 6, it is summarized that the PVDF-HFP membrane had excellent lipophilicity that PVDF-HFP membrane had excellent lipophilicity and hydrophobicity. and the hydrophobicity.
Contact Angle(°)
In consideration of lipophilicity and hydrophobicity of membrane, it was promising to separate 140 surfactant-free water-in-oil emulsion. In the surfactant-free water-in-oil emulsion separation experiments, these membranes can repeatedly separate the immiscible surfactant-free water-in-oil 120 emulsion with filtration rate and separation efficiency, as shown in Table 3. It was reported that the membrane M0 exhibited higher 100 filtration rate (894.41 L·m−2 ·h−1 ) than other membranes. The reason is that surfactant-free water-in-oil emulsion passed through the membrane with large pores easily. With 80 the decrease of DBP/DOP ratio, the filtration rate first decreased and then increased. This changing 60 membrane structure, which first became denser and then looser trend has a direct relation with the with decrease of DBP/DOP ratio. Although the filtration rate of membrane M0 was higher than other 40 membranes, the separation efficiency was just 75.51% owing to its larger pores. With the decrease of 20 denser and then more porous, which resulted in the changing trend DBP/DOP ratio, the surface became of filtration rate. From a general view, the highest water rejection in the membrane M3 was acceptable 0 when considering the better hydrophobicity and more M0 M1 M2applicable M3 pore M4structure for surfactant-free Figure 5. Influence of DBP/DOP ratio on the static water contact angle.
separation effectively [28]. To investigate the selective wettability of the prepared membranes, a water or diesel droplet was used to measure the water or oil contact angle. In Figure 5, it can been seen that the water contact angle decreased firstly with the increase of DOP content. When the DBP/DOP ratio was larger than 7/3, the water contact angle increased with the increase of DOP MaterialsAs 2018, 443 8 of 12 content. an11, example to the PVDF-HFP membrane (M3), as shown in Figure 6, the oil droplet was dropped on the membrane surface and spread out rapidly. At the same time, the oil droplet penetrated the membrane and the oil contact angled decreased to almost 0° within 5 s, which water-in-oil emulsion. When high separation efficiency is maintained, higher filtration rates mean a illustrated the excellent oil wettability of PVDF-HFP membranes. In addition, the surface membrane is more applicable for surfactant-free water-in-oil emulsion separation. According to the morphology such as micro- and nanostructure had an obvious effect on membrane selective above results, membrane M3 is more applicable for surfactant-free water-in-oil emulsion separation. wettability. According to membrane morphology and the results in Figures 5 and 6, it is summarized In addition, some typical properties of the obtained membranes shown in Supplementary Materials that the PVDF-HFP membrane had excellent lipophilicity and hydrophobicity. also indicated that membrane M3 is better than others for this application. 140 120
Contact Angle(°)
100 80 60 40 20 0
M0
M1
M2
M3
M4
Figure Influence DBP/DOP ratio static water contact angle. Figure 5. 5. Influence of of DBP/DOP ratio on on thethe static water contact angle. Materials 2018, 11, 443
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Figure of oil oil (diesel) (diesel) contact contact angle angle depended depended on on time time for for upper upper surface surface of of M3. M3. Figure 6. 6. Variation Variation of
In consideration of lipophilicity and hydrophobicity of membrane, it was promising to separate Table 3. Filtration rate and separation efficiency for surfactant-free water-in-oil emulsion of PVDF-HFP surfactant-free water-in-oil emulsion. In the surfactant-free water-in-oil emulsion separation porous membranes.
experiments, these membranes can repeatedly separate the immiscible surfactant-free water-in-oil emulsion with filtration rate and as shown It was reported Membrane M0 separation efficiency, M1 M2 in Table 3.M3 M4that the −2 −1 membrane M0 rate (894.41 ·h ± ) than membranes. The reason 894.41 ±filtration 103.33 460.42 ± 25.15 L·m 277.55 17.63 other 497.41 ± 67.27 609.43 ± 41.21 is Filtration rate (L·exhibited m−2 ·h−1 ) higher Separation efficiency water-in-oil (%) 75.51 ± 6.72 89.21 ± 0.81 ± 0.41 99.41 ± 0.12 pores 97.56 ± 0.44 that surfactant-free emulsion passed through 95.45 the membrane with large easily. With the decrease of DBP/DOP ratio, the filtration rate first decreased and then increased. This changing 3.3. Effect SiO2 Content PVDF-HFP Hybrid Membranes trend has of a direct relationonwith the membrane structure, which first became denser and then looser with decrease of DBP/DOP ratio. Although the filtration rate of membrane M0 was higher than other According to Section 3.2, membrane M3-S0 is more applicable for water-in-oil emulsion separation. membranes, the separation efficiency was just 75.51% owing to its larger pores. With the decrease of Therefore, mixed diluent (DBP/DOP = 7/3) was used to prepare PVDF-HFP hybrid membrane with DBP/DOP ratio, the surface became denser and then more porous, which resulted in the changing varying SiO2 contents and the structure and properties of obtained hybrid membranes were shown trend of filtration rate. From a general view, the highest water rejection in the membrane M3 was in Supplementary Materials. The filtration rate and separation efficiency for surfactant-stabilized acceptable when considering the better hydrophobicity and more applicable pore structure for water-in-oil emulsion are presented in Figure 7. The optical microscope images, photographs and size surfactant-free water-in-oil emulsion. When high separation efficiency is maintained, higher filtration distribution of the emulsion before and after filtrated are shown in Figure 8. rates mean a membrane is more applicable for surfactant-free water-in-oil emulsion separation. In Figure 7, it can been seen clearly that membrane M3-S0 can separate surfactant-stabilized According to the above results, membrane M3 is more applicable for surfactant-free water-in-oil water-in-oil emulsion with filtration rate of 433.61 L·m−2 ·h−1 . The introduction of SiO2 particles emulsion separation. had a significant effect on filtration rate and the filtration rate decreased with the increase of SiO2 contents. The reason is that the introduction of SiO2 particles facilitated the formation of some Table 3. Filtration rate and separation efficiency for surfactant-free water-in-oil emulsion of PVDFmicro/nano-protrusions on membrane surfaces, and, with the increase of SiO2 contents, more HFP porous membranes. and more microspheres were formed, which blocked the membrane pores. The denser membrane M0 M1 M2 from passing M3through the membrane. M4 structureMembrane suppressed surfactant-stabilized water-in-oil emulsion Filtration rate (L·m−2·h−1) Separation efficiency (%)
894.41 ± 103.33 75.51 ± 6.72
460.42 ± 25.15 89.21 ± 0.81
277.55 ± 17.63 95.45 ± 0.41
497.41 ± 67.27 99.41 ± 0.12
609.43 ± 41.21 97.56 ± 0.44
3.3. Effect of SiO2 Content on PVDF-HFP Hybrid Membranes According to Section 3.2, membrane M3-S0 is more applicable for water-in-oil emulsion
Materials Materials2018, 2018,11, 11,443 443
99ofof12 12
Materials 2018, 11, 443 are also shown in Figure 8. Firstly, from the photographs of emulsions before9and 12 The results Theabove above results are also shown in Figure 8. Firstly, from the photographs of emulsions before of and after afterseparation separationprocess, process,ititisisclearly clearlyseen seenthat thatthe theemulsion emulsionbecame becamevery veryclear clearafter afterbeing beingfiltrated. filtrated. Secondly, from the optical microscope images, it is found that there were many emulsion droplets in Secondly, from the optical microscope images, it is found that there were many emulsion droplets in Compared with the result in Table 3, it was found that the filtration rate for surfactant-stabilized initial emulsion, whereas there were almost no emulsion droplets in filtrate after being filtrated by initial emulsion, whereas there were almost droplets in filtrate after beingbecause filtratedthe by water-in-oil emulsions was lower than that no foremulsion surfactant-free water-in-oil emulsion the membrane M3-S2. the membrane M3-S2. demulsification process weakened the filtration rate for surfactant-stabilized emulsions significantly.
AA
BB
M3-S3 M3-S3
M3-S3 M3-S3
99.34 99.34
M3-S2 M3-S2
M3-S2 M3-S2
99.84 99.84
M3-S1 M3-S1
M3-S1 M3-S1
99.32 99.32
M3-S0 M3-S0
M3-S0 M3-S0
98.87 98.87
0 0
100 100
200 200
300 300
400 400
Filtration mm-2-2·h·h-1-1) ) FiltrationRate Rate(L· (L·
500 500
0 0
20 20
40 40
60 60
80 80
Separation SeparationEfficiency(%) Efficiency(%)
100 100
Figure 7.7. Filtration rate (A); and separation efficiency (B) for water-in-oil Figure 7. Filtration rate separation efficiency for surfactant-stabilized surfactant-stabilized water-in-oil Figure Filtration rate (A);(A); and and separation efficiency (B) for (B) surfactant-stabilized water-in-oil emulsion emulsion of the PVDF-HFP hybrid membranes. emulsion of the PVDF-HFP hybrid membranes. of the PVDF-HFP hybrid membranes.
Figure 8. Optical microscope images, photographs and size distribution of surfactant-stabilized Figure microscope images, photographs and size distribution Figure 8.8. Optical Optical microscope photographs sizefiltrate. distribution ofof surfactant-stabilized surfactant-stabilized water-in-oil emulsion: (A) feed; images, (B) M3 filtrate; and (C)and M3-S2 water-in-oil emulsion: (A) feed; (B) M3 filtrate; and (C) M3-S2 filtrate. water-in-oil emulsion: (A) feed; (B) M3 filtrate; and (C) M3-S2 filtrate.
Although the filtration of membrane M3-S0 was higherfouling than others, thewhich separation efficiency During During the the long long running running time, time, unavoidable unavoidable membrane membrane fouling occurs, occurs, which leads leads to to lower lower was just rate 98.87% owing to itsefficiency. looser surface and relatively poorseparation hydrophobicity. With is the addition filtration and separation In this work, a repeating experiment applied filtration rate and separation efficiency. In this work, a repeating separation experiment is appliedto to of SiO2 particles, the surface becameof denser, which resulted inAslower filtration rate. However, the evaluate the antifouling performance the membrane M3-S2. shown in Figure 9, the change of evaluate the antifouling performance of the membrane M3-S2. As shown in Figure 9, the change of separation efficiency was larger than 99% owing to its better hydrophobicity and denser structure. filtration filtrationflux fluxwith withcycle cyclenumber numberisissmall. small.In Inaddition, addition,the theseparation separationefficiency efficiencyhad hadno nodiscernable discernable The above results are also shown in Figure 8. Firstly, from the photographs of emulsions before and difference differenceduring duringthe therepeating repeatingexperiment. experiment.These Theseresults resultsindicated indicatedan anexcellent excellentantifouling antifoulingproperty property after separation process, it is clearly seen that the emulsion became very clear after being filtrated. of of the the membrane membrane M3-S2. M3-S2. In In addition, addition, aa comparison comparison of of the the mechanical mechanical property property and and separation separation Secondly, fromfor thesurfactant-stabilized optical microscope images, it is found that there were many emulsion droplets in performance water-in-oil emulsion between the PVDF-HFP performance for surfactant-stabilized water-in-oil emulsion between the PVDF-HFP hybrid hybrid initial emulsion, whereas there werereported almost no emulsion droplets in filtrate after being filtrated by the membranes and PVDF membranes in the literature has been made, as shown in Table membranes and PVDF membranes reported in the literature has been made, as shown in TableS5. S5. membrane M3-S2. PVDF-HFP PVDF-HFPmembrane membranepossessed possessedbetter bettermechanical mechanicalproperties propertiesdue dueto toits itsinherent inherentand andoutstanding outstanding During the long running time, unavoidable membrane fouling occurs, whichof leads to lower flexibility. flexibility. Furthermore, Furthermore, the the excellent excellent hydrophobicity hydrophobicity and and desirable desirable pore pore size size of PVDF-HFP PVDF-HFP filtration rate and separation efficiency. In this work, a repeating separation experiment is applied to membrane resulted in higher filtration rate and separation efficiency for surfactant-stabilized membrane resulted in higher filtration rate and separation efficiency for surfactant-stabilizedwaterwaterevaluate the antifouling performance of the membrane M3-S2. As shown in Figure 9, the change of in-oil in-oilemulsion. emulsion. filtration flux with cycle number is small. In addition, the separation efficiency had no discernable difference during the repeating experiment. These results indicated an excellent antifouling property of the membrane M3-S2. In addition, a comparison of the mechanical property and separation
Materials 2018, 11, 443
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performance for surfactant-stabilized water-in-oil emulsion between the PVDF-HFP hybrid membranes and PVDF membranes reported in the literature has been made, as shown in Table S5. PVDF-HFP membrane possessed better mechanical properties due to its inherent and outstanding flexibility. Furthermore, the excellent hydrophobicity and desirable pore size of PVDF-HFP membrane resulted Materials 2018, 11, 443 rate and separation efficiency for surfactant-stabilized water-in-oil emulsion. 10 of 12 in higher filtration 100 80 200 60 40
100
20 0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Separation efficiency(%)
Filtration rate(Lm-2h-1)
300
0
Cycle Number(times)
Figure 9. 9. Filtration Filtration rate rate and and separation separation efficiency filtration filtration flux flux of of the the membrane membrane M3-S M3-S for for repeating repeating Figure separation experiment. experiment. separation
4. Conclusions Conclusions 4. In this thispaper, paper, novel PVDF-HFP porous membranes are fabricated viaoil/water TIPS forseparation. oil/water In novel PVDF-HFP porous membranes are fabricated via TIPS for separation. The effects of mixed andofadditives of the structure andare properties studied. The effects of mixed diluent and diluent additives the structure and properties studied.are The phase The phase diagram of the ternary system shows that the T cloud of the system increases quickly, while diagram of the ternary system shows that the Tcloud of the system increases quickly, while Tc increases Tc increases slowly with decrease ofratio. DBP/DOP ratio. In theofmain type of phasecan separation slowly with decrease of DBP/DOP In addition, theaddition, main type phase separation change can change from S-L phase separation to L-L phase separation with decrease of DBP/DOP ratio. from S-L phase separation to L-L phase separation with decrease of DBP/DOP ratio. Therefore, Therefore, a bicontinuous structure could be obtained by increasing DOP content. In addition, the a bicontinuous structure could be obtained by increasing DOP content. In addition, the crystallization crystallization membranes with the decrease ofratio. DBP/DOP results of membranes of decreases withdecreases the decrease of DBP/DOP The ratio. resultsThe also showalso thatshow the that the membranes prepared with mixed diluent have excellent mechanical properties and membranes prepared with mixed diluent have excellent mechanical properties and hydrophobicity. hydrophobicity. Whenratio the DBP/DOP is 7/3,membrane the obtained membrane reaches optimalInproperties. When the DBP/DOP is 7/3, theratio obtained reaches optimal properties. addition, In addition,hybrid PVDF-HFP hybridare membranes are successfully with different SiO 2 content. PVDF-HFP membranes successfully fabricated withfabricated different SiO content. The PVDF-HFP 2 The PVDF-HFP hybrid 2 wt % SiO2 particles shows higher separation efficiency of hybrid membrane with membrane 2 wt % SiOwith 2 particles shows higher separation efficiency of 99.84% for the 99.84% for the surfactant-stabilized water-in-oil emulsion separation. More the hybrid surfactant-stabilized water-in-oil emulsion separation. More importantly, theimportantly, hybrid membrane also membrane also exhibits excellent antifouling performance. exhibits excellent antifouling performance. Acknowledgments: The authors thank the financial online support of the National Natural Science Foundation of Supplementary Materials: The following are available at http://www.mdpi.com/1996-1944/11/3/443/s1, Figure Effect of mixed diluent σ-γ curve (A) and of η-γTianjin curve (B); Figure S2. Relation between DBP content in China S1. (51673149), Science and on Technology Plans (14JCZDJC37300) and the industrial chain mixed diluentinnovation and the difference of solubility (∆δ)Administration between PVDF-HFP and mixed diluent; Figure S3. collaborative major projects of the parameter State Oceanic (BHSF2017-01). The SEM morphology of PVDF-HFP hybrid membranes. A–D: M3-S0~M3-S3, 1: cross section, 2: surface; Table S1.Contributions: The non-Newtonian different polymer solutions; Table S2. Somethe properties for DBP, DOP, and Author Xinyaindex Wangofand Changfa Xiao conceived and designed experiments, Xinya Wang PVDF-HFP; Table S3. The typical properties of prepared PVDF-HFP membranes; Table S4. Characterization of analyzed the data and wrote the paper; Hailiang Liu and Qinglin Huang checked and revised the paper; PVDF-HFP hybrid membranes; Table S5. Comparison of the mechanical property and separation performance for Junqiang Hao and Hao Fu completed the literature search and helped the study design. surfactant-stabilized water-in-oil emulsion. Conflicts of Interest:The Theauthors authors declare conflict of interest. thank the no financial support of the National Natural Science Foundation of China Acknowledgments: (51673149), Science and Technology Plans of Tianjin (14JCZDJC37300) and the industrial chain collaborative innovation major projects of the State Oceanic Administration (BHSF2017-01). References Author Contributions: Xinya Wang and Changfa Xiao conceived and designed the experiments, Xinya Wang 1. Wypych, G. Handbook of Polymers; Elsevier: Toronto, ON, Canada, 2012. analyzed the data and wrote the paper; Hailiang Liu and Qinglin Huang checked and revised the paper; 2. Lalia, B.S.; E.; Arafat,the H.A.; Hashaikeh, R. Nanocrystalline Junqiang Hao andGuillen, Hao Fu completed literature search and helped the studycellulose design. reinforced PVDF-HFP membranes for membrane distillation application. Desalination 2014, 332, 134–141, Conflicts of Interest: The authors declare no conflict of interest. doi:10.1016/j.desal.2013.10.030. 3. Shi, L.; Wang, R.; Cao, Y.M. Effect of the rheology of poly(vinylidene fluoride-co hexafluropropylene) (PVDF–HFP) dope solutions on the formation of microporous hollow fibers used as membrane contactors. J. Membr. Sci. 2009, 344, 112–122, doi:10.1016/j.memsci.2009.07.041. 4. Tian, X.Z.; Jiang, X. Poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP) membranes for ethyl acetate removal from water. J. Hazard. Mater. 2008, 153, 128–135, doi:10.1016/j.jhazmat.2007.08.029. 5. Garcia-Payo, M.C.; Essalhi, M.; Khayet, M. Effects of PVDF-HFP concentration on membrane distillation
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