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Polar polymer membranes via thermally induced phase separation using a universal crystallizable diluent
Journal of Membrane Science 446 (2013) 482–491
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Polar polymer membranes via thermally induced phase separation using a universal crystallizable diluent Hong-Qing Liang, Qing-Yun Wu, Ling-Shu Wan, Xiao-Jun Huang, Zhi-Kang Xu n MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 February 2013 Received in revised form 21 June 2013 Accepted 7 July 2013 Available online 12 July 2013
Dimethyl sulfone (DMSO2) was used as a universal crystallizable diluent to prepare polar polymer membranes via thermally induced phase separation (TIPS). The polar polymers adopted herein include poly(vinylidene fluoride) (PVDF), polyacrylonitrile (PAN) and cellulose acetate (CA). Intensive investigation was carried out to study the phase separation behaviors and the membrane performances. Equilibrium phase diagrams and polarized optical microscope results indicate a solid solid phase separation mechanism for all the three polar polymer/diluent systems. Scanning electron microscopy observations show that tubular-like pores are irregularly distributed in the PVDF and PAN membranes, whereas a compacted structure can be found in the CA membranes. The pore size, surface porosity, water flux and overall porosity become large when the membranes are prepared with low polymer concentration or at small cooling rate. Results of tensile tests confirm that the mechanical strength of the membranes can be enhanced by increasing the polymer concentration or cooling rate. Moreover, DMSO2 has been efficiently recovered by recrystallization and sublimation. In conclusion, this work may provide a green preparation method to produce polar polymeric membranes via TIPS. & 2013 Elsevier B.V. All rights reserved.
Keywords: Thermally induced phase separation Universal diluent Dimethyl sulfone Polar polymer Poly(vinylidene fluoride)
1. Introduction Thermally induced phase separation (TIPS) is a well-known method to fabricate porous polymer membranes due to its advantages including easy control, low tendency for defects formation, and diverse microstructures that are desirable for various applications of membrane. Since it had been introduced by Castro in 1980s, this method has attracted wide investigations in respect of conducting process [1,2], phase separation mechanism [3], structure controlling [4,5], and properties enhancement [6,7]. Nevertheless, great efforts have been still made to drive the further development of TIPS both in theoretical and practical aspects. Hanks et al. [8] reported a deterministic model for matrix solidification in liquid liquid (L L) TIPS. They chose isotactic polypropylene–diphenyl ether as a representative system to confirm the simulation and found that the simple deterministic approach can provide accurate prediction for the final cell size diameter and distribution of isotropic L L TIPS membranes. Roh et al. [9]
Abbreviations: TIPS, thermally induced phase separation; L L, liquid liquid; S L, solid liquid; L S, liquid solid; S S, solid solid; PVDF, poly(vinylidene fluoride); PAN, polyacrylonitrile; CA, cellulose acetate; DMSO2, dimethyl sulfone; DSC, differential scanning calorimetry; FESEM, field emission scanning electron microscopy n Corresponding author. Tel.: +86 571 8795 2605; fax: +86 571 8795 1773. E-mail address: [email protected] (Z.-K. Xu). 0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.07.008
described a novel diluent mixture containing paraffin and poly (tetramethylene glycol) for the preparation of polyethylene membranes with controlled pore size and porosity. Tanaka et al. [10] combined the nonsolvent and thermally induced phase separation to prepare asymmetric poly(L-lactic acid) membrane. Their membranes were suggested to be served as efficient and rapid depth filters to deal with bacterial cells. Generally, a typical TIPS process is defined as follows: a polymer is dissolved into a high-boiling and low molecular weight diluent at an elevated temperature, then the homogeneous solution is cooled to induce phase separation, and a microporous structure can be created after removing the diluent. Diluent is one of the dominant factors that determine specific phase separation process (e.g. L L, solid liquid (S L), liquid solid (L S), and solid solid (S S) demixing) and subsequently control the final microstructures of membranes. These membranes usually have various porous structures including bicontinuous [11,12], cellular [13,14], spherulitic [15,16], needle- or sheet-like pores [17,18]. Furthermore, TIPS by far has been adopted to prepare membranes from a number of polymers, including polyethylene [19–22], polypropylene [23–26], polystyrene [27,28], poly(vinylidene fluoride) (PVDF) [29–31], poly(ethylene-co-vinyl alcohol) [32,33], poly (methyl methacrylate) [34,35], and poly(lactic acid) [10,36,37]. Nevertheless, few reports have ever proposed the idea of using a universal diluent to prepare porous membranes from polar polymers (such as polyacrylonitrile (PAN) and cellulose acetate (CA))
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Table 1 Physical properties of the polymers and diluent. Properties
Unit
DMSO2
PVDF
PAN
CA
Molecular weight Degree of polymerization Molar volume Density Heat of fusion Melting temperaturea Crystallization temperature Solubility parametera Dipole moment Dielectric constant
g/mol
94 N/A 75 1.16 18300a 382 365 29.9 4.4d 47.4a
110,000 (Mn) 277,000 (Mw) 1719 36.4 1.76 6700c 443 409 23.2 1.9e 8.4
80,000 (Mn) 190,000 (Mw) 1509 45.6 1.14 5021 593 N/A 26.0 3.9e 6.5
92,000 (Mn) 210,000 (Mw) 384 189 1.3 12600b 508 N/A 27.8 N/A 7.0
cm3/mol g/cm3 J/mol K K MPa1/2 D
a
Ref. [50]. Represented by the data of cellulose butyrate, Ref. [51]. c Ref. [52]. d Ref. [38]. e Ref. [53]. b
via TIPS. One of the most important challenges is to search an appropriate diluent due to the crystallinity and high melting point of polar polymers. Such a searching step is time and cost consuming that limits the application broadening of TIPS. Furthermore, the interactions between a diluent and different polymers may differ from each other significantly, which will lead to quite different behaviors of solubility and phase separation. In this study, a series of porous membranes were prepared from different polar polymers including PVDF, PAN and CA via TIPS, in which dimethyl sulfone (DMSO2) was successfully introduced as a universal crystallizable diluent. DMSO2 is a widely used polar solvent with high boiling point, especially valuable in high temperature condition [38]. Theoretically, DMSO2 has a solubility parameter close to those of PVDF, PAN and CA (Table 1), indicating that homogeneous solutions are reasonable to be obtained at elevated temperature. On the other hand, DMSO2 will crystallize and precipitate from the solutions as temperature falls to 365 K [18], and phase separation can be expected. Therefore, it is feasible to use DMSO2 as a universal diluent for these polar polymer membranes via TIPS. Based on this consideration, we studied the phase separation mechanisms of PVDF/DMSO2, PAN/DMSO2, CA/ DMSO2 binary systems from the aspects of thermodynamic theory and experiments. We also focused on the effects of polymer concentration and cooling bath on the pore size, surface porosity, water flux, overall porosity, and mechanical properties of the resultant membranes. Furthermore, DMSO2 as a non-toxic diluent was effectively recovered by recrystallization or sublimation herein, which potentially cuts the cost of membrane preparation and reduces environmental pollution from diluent. The application of crystallizable diluent may provide a novel solution to membrane structure control and a green strategy for the fabrication of polar polymer membranes.
2. Experimental 2.1. Materials DMSO2 (99%) was purchased from Dakang Chemicals Co., China. PVDF (Mn ¼110,000 g/mol, Solefs 6010) was a commercial product of Solvay Solexis, Belgium. PAN (Mn ¼80,000 g/mol) was supplied by Anqing Petroleum Chemical Co., China. CA (Mn ¼ 92,000 g/mol) was obtained from Sinopharm Chemical Reagent Co., Ltd. All the polymer powders were dried at 333 K under vacuum for 4 h before use. Deionized water was used as the extractant. Other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. and used without further purification.
2.2. Preparation of membranes One polymer/DMSO2 mixture was heated at 433 K to form a homogeneous solution. After degassing air bubbles, the solution was quickly poured onto a stainless steel mold (thickness 200 μm), which was preheated in an oven at 433 K. The mold was then quenched in a cooling bath (water bath at 277 K and 303 K or air bath at 303 K) to induce the solution to phase separation. Afterwards, the obtained nascent membrane was taken out of the mold and immersed in deionized water. A wet membrane was formed as soon as the diluent was completely extracted. To gain a dry membrane, the wet one was washed with an ethanol–hexane sequence, and then dried in vacuum for 24 h at 333 K.
2.3. Determination of phase behavior Differential scanning calorimetry (DSC, Q1000, TA instruments, USA) was used to determine the melting and crystallization temperatures of the mixture. Sample was prepared by directly quenching the polymer/diluent solution into liquid nitrogen, which prevented the macroscopic phase separation. Solid sample of 6–8 mg was hermetically sealed in an aluminum DSC pan, heated from 25 to 473 K at 20 K/min, and maintained at 473 K for 3 min to eliminate thermal history. It was then cooled to 0 K at 10 K/min and reheated immediately to 473 K at 10 K/min. Peak maxima in the cooling and reheating sequences were regarded as the crystallization temperature and the melting point, respectively. The phase separation process was visualized by an optical microscope (Nikon Eclipse E600POL, Japan). A small section of polymer/diluent solid sample was placed between a pair of microscope slides, and put on a hot stage (Linkam TMS-93) with a temperature controller (Linkam THMS-600). It was heated at 30 K/min to 473 K, maintained for 3 min, and then cooled to 273 K at 10 K/min. The field of vision was recorded at the moment of solidification.
2.4. Wide-angle X-ray diffraction (WAXD) WAXD was carried out on a Rigaku D/Max-2550PC X-ray diffractometer (Panalytical, Netherlands). The radial scans at a voltage of 40 kV and a current of 40 mA using a Cu:Ka radiation were employed on the samples. Data were collected at 0.01671 interval with counting for 10 s at each step.
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2.5. Field emission scanning electron microscopy (FESEM) observation Membrane morphologies of cross-section and surfaces were examined by FESEM (Hitachi S4800, Japan) with an accelerating voltage of 10 kV. To obtain a tidy cross-section, the membrane sample was freeze-fractured in liquid nitrogen. All samples were gold sputtered using a sputter coating instrument (EDT2000) before imaged by FESEM. 2.6. Water flux measurement Water flux of membranes (surface area is 4.9 cm2) was measured in a pressure driven filtration cell. Membrane samples were carefully wetted with ethanol before measurement to allow water impregnating the membrane pores. The membrane was first installed in the permeation cell and compacted by filtering ultrapure water at 0.12 MPa and 295 K for 30 min. Then the pressure was lowered to 0.10 MPa for operation. Each sample was repeated at least three parallel experiments. Pure water flux (Jw) was calculated by the following equation: Jw ¼
V AΔt
ð1Þ
where V, A and Δt denote the volume of permeated water (L), sample area (m2), and the permeation time (h), respectively. 2.7. Pore size and porosity analyses Average pore size was determined by analyzing FESEM images of the cross-section of membranes using an Image tool [39]. As for the tubular pores, their width was defined as the pore size in this work. Surface porosity of each membrane was measured by an image analysis software of Image-Pro Plus Version 6.0 [40]. The overall porosity is defined as the volume of pores divided by the total volume of membrane. A wet membrane was first weighed as soon as the superficial water on the membrane surface was removed by dry filter paper. Afterward, the wet membrane was dried through the process as mentioned in Section 2.2. The weight of dry membrane was measured again. Porosity of the membrane was denoted as P, which was calculated with the gravimetric method P¼
ðw0 w1 Þ=ρwater 100 ðw0 w1 Þ=ρwater þ w1 =ρ
ð2Þ
where w0 is the weight of the wet membrane, w1 is the weight of the dry membrane, and ρwater or ρ is the density of water or polymer, respectively. 2.8. Tensile test Tensile strength and elongation for the membranes were evaluated in a stress–strain test using a tensile test instrument (RGM-4000, Shenzhen REGER Instrument Co., Ltd., China). The test was carried out at a strain rate of 5 mm/min at 293 K and a relative humidity of 74%. The membrane samples were prepared in rectangle shape with a gauge length of 20 mm and a width of 10 mm. The thickness was around 200 μm and exactly examined by a thickness gauge (CH-1-S/ST, Shanghai Liuling Instrument Plant, China). Each data was the average of at least three parallel experiments. 2.9. Recovery of crystallizable diluent DMSO2 was recovered from the nascent membrane by recrystallization or sublimation. The nascent membrane was dissolved in
a smallest amount of solvent at 343 K to form a saturated solution when using the recrystallization method. The solvents adopted here include ethanol, methanol and water. After filtration, the solution was cooled at 255 K for 24 h, while the water solution was cooled at 273 K. Needle-like crystals of DMSO2 were recovered by filtration. Another recovery method was sublimation. The nascent membrane was placed in a sublimation apparatus and then heated to 353 K under vacuum. Solid DMSO2 inside the membrane was gradually sublimated and condensed on a cooled surface. At last, purified DMSO2 can be collected from the cooled surface.
3. Results and discussion 3.1. Phase diagrams of the polymer/diluent systems Generally, phase separation mechanism is the dominant factor to the final membrane morphology and properties. Accordingly, it is necessary to predict the possible phase separation behavior. Equilibrium phase diagrams can be calculated according to the Flory–Huggins solution thermodynamics. Theoretically, the melting point of polymer would be depressed in the presence of diluent [41]. The melting point depression of polymer can be calculated using Eq. (3) [21,42] 1 1 Rβ V u ¼ 1þ ð1ϕ2 Þ2 Tm ΔH u V 1 " 1 R Vu 1 lnðϕ2 Þ ð1ϕ2 Þ ð3Þ 1 0 þ N N T m ΔH u V 1 where Tm is the melting point of polymer in a solution, T 0m is the melting point of neat polymer, ΔHu is the enthalpy of fusion per repeat unit of polymer, Vu is the molar volume of polymer, N is the degree of polymerization, ϕ2 is the volume fraction of polymer, R is the gas constant, and β is a constant which is calculated from the solubility parameters of polymer (δ2) and diluent (δ1) along with the molar volume of diluent (V1), using the equation (δ1 δ2)2V1/R. Similarly, Eq. (4) can be used to calculate the melting point depression of a crystallizable diluent [42] " #1 " # 1 Rβϕ22 1 R 1 ϕ ¼ 1þ 1 þ lnð1ϕ Þ ð4Þ 2 T m;1 N 2 ΔH 1 T 0m;1 ΔH 1 where Tm,l is the melting point of diluent in a polymer solution, T 0m;1 is the melting point of neat diluent, ΔH1 is the enthalpy of fusion of neat diluent. Table 1 lists the parameters used for calculation. Fig. 1(a–c) shows typical DSC heating thermograms for the PVDF/DMSO2, PAN/DMSO2, and CA/DMSO2 binary systems. In each curve, a peak around 383 K indicates the melting point of DMSO2 in the mixture. These experimental melting points of DMSO2 are in good agreement with those calculated results (Fig. 1(d–f)). It is clear that another peak at relatively low temperature exists in the PAN/DMSO2 and CA/DMSO2 mixtures, which may result from the interaction between polymers and diluent. Interestingly, almost no signal is found for the melting of polymers, though they are all semi-crystalline polymers. Theoretically, the melting points of PVDF and PAN are above those of DMSO2 in the mixtures as indicated by the dash lines in Fig. 1(d, e). Actually, the melted DMSO2 is a good solvent for all the polymers. PVDF and PAN would not preferentially crystallize from the solution unless DMSO2 crystallizes when the temperature reach the melting temperature of DMSO2. Meanwhile, the polymer immediately solidifies because of a large supercooling degree. Thus, the equilibrium phase diagram suggests that both PVDF/DMSO2 and PAN/DMSO2 binary systems undergo solid–solid phase separation. In contrast, an eutectic point
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Fig. 1. Thermal analysis results of the polymer/diluent mixtures: (a, d) PVDF/DMSO2, (b, e) PAN/DMSO2, and (c, f) CA/DMSO2. (a–c) DSC thermograms and (d–f) melting points (■, ▲) plotted on the calculated equilibrium phase diagram. The dash lines in (d–f) represent the theoretical melting curves of polymers, while the solid lines represent the theoretical melting curves of DMSO2.
Fig. 2. FESEM images of PVDF, PAN, CA membranes: (a) cross-section (250 ), (b) cross-section (2000 ), and (c) top-surface. The membranes were prepared with 20 wt% polymer in water bath at 303 K.
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is calculated in CA/DMSO2 system with 50 wt% polymer, which consists with the invariable temperature of its second melting point as mentioned above. It indicates that CA/DMSO2 system thermodynamically goes through liquid–solid phase separation. Real-time POM was used to observe the cooling process of the polymer/ DMSO2 mixtures at a rate of 10 1C/min (Figs. S1 and S2 in Supporting Information). Neither crystallization nor liquid–liquid phase separation takes place at the initial stage upon cooling. When the temperature reaches the crystallization temperature of DMSO2, the samples solidify in a few seconds and giant spherulites are formed, in which no preferential crystallization has been observed [43,44]. This result is in well consistent with the crystalline behaviors of the three systems, which show only one crystalline peak upon cooling (Fig. S3 in Supporting Information). WAXD was then used to confirm the crystallization of both components in the three binary systems (Fig. S4 in Supporting Information). Combining with the results of DSC, POM, and WAXD, we can confirm a solid–solid phase separation mechanism for all the three systems. Among others, this is a result of dynamics for CA/DMSO2 system which may overcome the narrow liquid–solid area at large cooling rate. 3.2. Morphologies of PVDF, PAN and CA membranes Fig. 2 shows the cross-section and surface morphologies of PVDF, PAN and CA membranes prepared with a polymer concentration of 20 wt%. Irregular tubular pores are observed in PVDF and PAN membranes due to the crystallization of DMSO2. As mentioned above, both PVDF/DMSO2 and PAN/DMSO2 binary systems undergo
solid–solid phase separation. The membrane pore morphology will mainly depend on the crystal structure of diluent. Upon cooling, needle-like crystals of DMSO2 will form. Thus tubular pores will be obtained after subsequent extraction of diluent. Furthermore, the tubular pores are irregularly distributed in PVDF and PAN membranes, as the nucleation of diluent crystals is quite random without any applied temperature gradient. However, this kind of morphology cannot be found in the CA membrane. It has been reported that the sulfur-oxygen linkages in DMSO2 are coordinate covalent single S+O bonds, with both of the shared electrons coming from the sulfur [38]. The molecular surface electrostatic potentials confirm the highly negative characters of the oxygens, which can form strong hydrogen bonds with residual hydroxyl groups of CA. Nevertheless, PVDF or PAN may interact with DMSO2 through dipole–dipole interaction [44,45], which is much weaker than the hydrogen bonding interaction. Given the dipole moments and dielectric constants of the polymers (Table 1), we can conclude that the polymer DMSO2 interaction is in the order of CA4PAN4PVDF. This result is in line with the melting point depression as mentioned above. Since the PVDF DMSO2 interaction is weak, DMSO2 easily crystallizes and separates from the polymer solution. Accordingly, the average pore size of PVDF membranes (2.1 μm) is larger than that of PAN membranes (0.3 μm). There are small pores on the wall of the tubular pores in PVDF membrane, suggesting high interconnection between pores. Otherwise, owing to the strong CA DMSO2 interaction, the crystallization of DMSO2 is mostly inhibited, and a weak phase separation leads to a compacted structure. On the other hand, the surface of PVDF membrane is full of nanopores ( 21.5 nm), while dense surfaces can be seen for PAN and
Fig. 3. FESEM images of PVDF membranes prepared from 10–30 wt% PVDF: (a) cross-section (250 ), (b) cross-section (2000 ), and (c) top-surface. The membranes were prepared in water bath at 303 K.
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CA membranes (Fig. 2(c)). Besides, the PAN membrane surface has regular “stripes” with a certain orientation, which may also be caused by the crystallization of DMSO2 [18].
small as a consequence [18]. PAN and CA membranes show a similar tendency on their morphology (Figs. S5 and S6 in Supporting Information).
3.3. Effects of polymer concentration on the membrane structures
3.4. Effects of cooling rate on the membrane structures
Fig. 3 presents the morphologies of PVDF membranes prepared with different polymer concentrations. The pore size reduces from 3.5 μm to 1.5 μm with an increase of PVDF concentration from 10 wt% to 30 wt% (Fig. 4(a)). The surface porosity also dramatically decreases with the PVDF concentration. It may be caused by the increasing viscosity of the PVDF/DMSO2 solution, which will prevent the crystallization of DMSO2 and then the pores become
Except for polymer concentration, cooling rate also has great impact on the membrane morphology since TIPS is a dynamiccontrolled process. Fig. 5 indicates that the tubular pores of PVDF membranes become large as the temperature of water bath increases. PVDF membrane prepared in 30 1C water has pore size of approximately 2.1 μm and surface porosity of about 6.2%, which are larger than those prepared in 277 K water ( 1.3 μm and 4.8%)
Fig. 4. Surface porosity and pore size of PVDF, PAN and CA membranes varied with polymer concentration (a) and cooling bath (b). All the membranes in (a) were prepared in water bath at 303 K, and those in (b) were prepared with polymer concentration of 20 wt%.
Fig. 5. FESEM images of PVDF membranes cooled in different cooling baths: (a) cross-section (300 ), (b) cross section (2000 ), and (c) top-surface. The membranes were prepared from 20 wt% PVDF.
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(Fig. 4(b)). The membranes even have pore size of 4.1 μm and surface porosity of 7.8% when prepared in 303 K air bath. Air has smaller thermal capacity than water, so the cooling of solution in air is slower than that in water. As the cooling rate decreases, DMSO2 crystals have long time to grow, leading to larger pore sizes and surface porosity in the as-prepared membranes. Similar results are found in PAN and CA membranes, especially for those CA membranes (Figs. S7 and S8 in Supporting Information). CA membranes show compacted structure and have almost no pores when quenched in 277 K water bath. If 303 K air bath is used, the membranes have large pores ( 2.3 μm) and high surface porosity (15.8%). Moreover, an orientated texture can be visualized on the surface of PVDF and PAN membranes when quenched in 303 K air bath. 3.5. Water flux and mechanical properties of PVDF, PAN and CA membranes We have studied the effects of polymer concentration and cooling rate on the water flux and overall porosity of PVDF, PAN, and CA membranes (Figs. 6 and 7). The results demonstrate that water flux and overall porosity of all the membranes increase with
the decrease of polymer concentration or cooling rate. It is due to the large pore size and surface porosity at low polymer concentration or cooling rate (Fig. 4). Among others, PVDF membranes possess the highest water flux (1491 L/m2 h) and overall porosity (91.1%) at the optimum polymer concentration (10 wt%) and cooling condition (303 K water bath). However, the water flux of PAN or CA membranes is quite small though their porosities are still quite high. Firstly, low surface porosity may account for the low water flux of these two kinds of membranes. Secondly, the pore structures in the cross-section have great effect: the pores are almost parallel to the surface of PAN membranes with small pore size, and CA membranes are too compacted. Mechanical properties are crucial to the application of membranes. Fig. 8(a) shows the stress strain curves of PVDF membranes prepared from different polymer concentrations. The stress strongly increases with the strain at the initial stage. Then a nontypical yielding occurs when the strain reaches the yield point. At the last stage of deformation, the samples fail by tearing quickly at the end of the clamps, characterized by downward tails at the end of the respective stress strain curve. Fig. 8(b) shows the corresponding tensile strength and elongation results, indicating that the membranes prepared from higher polymer concentration have
Fig. 6. Porosity (a) and water flux (b) of PVDF, PAN and CA membranes in different polymer concentrations. The membranes were prepared in water bath at 303 K. CA membrane from 10 wt% CA is too brittle to test the water flux.
Fig. 7. Porosity (a) and water flux (b) of PVDF, PAN and CA membranes in different cooling conditions. The membranes were prepared from 20 wt% polymer.
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Fig. 8. Stress–strain curve (a) and the corresponding tensile strength and elongation (b) for PVDF membrane prepared with different polymer concentrations: (1) 10 wt%, (2) 20 wt%, and (3) 30 wt%. The membranes were prepared in water bath at 303 K.
Fig. 9. Sress–strain curve (a) and the corresponding tensile strength and elongation (b) for PVDF membrane prepared at different cooling baths: (1) 277 K water, (2) 303 K water, and (3 and 3′) 303 K air. The membranes were prepared with polymer concentration of 20 wt%. The stress directions applied to samples of (3) and (3′) were perpendicular and parallel to the direction of orientation observed on the membrane surface, respectively.
Table 2 Mechanical properties of the membranes prepared in different conditions. Membrane
Polymer concentration (wt%)
Cooling bath (K)
Thickness (μm)
Elongation (%)
Tensile strength (MPa)
PVDF
10 20 30 20 20c 20d
303 303 303 277 303b 303b
188.8 7 9.0 284.0 7 8.7 270.378.4 174.7 7 28.6 227.7 7 27.3 231.0 731.1
16.0 7 1.4 22.8 7 5.3 33.77 7.9 43.4 7 19.2 62.5 7 12.5 42.5 7 12.1
0.9 7 0.3 2.8 7 0.1 7.5 71.3 3.0 7 0.2 2.5 7 0.3 3.8 7 1.1
PAN
10c 10d 20 30 20 20c 20d
303 303 303 303 277 303b 303b
145.3 7 17.7 143.0 7 7.2 182.7 7 3.2 252.0 7 37.2 179.7 7 35.5 179.7 7 5.9 197.7 7 12.0
4.17 0.4 3.9 7 0.4 4.4 7 1.7 4.9 7 2.2 4.8 7 0.8 8.3 7 2.4 0.7 7 0.1
7.6 70.8 3.7 7 0.4 7.7 72.0 16.5 7 4.3 11.7 7 3.6 15.8 7 0.7 3.0 7 0.5
CA
10a 20 30 20 20
303 303 303 277 303b
N/A 164.37 15.8 157.7 75.1 95.7 7 4.7 185.0 7 10.6
N/A 10.0 7 2.2 11.2 7 4.1 6.2 7 1.8 1.2 70.2
N/A 46.9 7 3.4 39.0 7 2.0 41.8 7 9.4 7.5 70.6
a
The membrane from 10 wt% CA is too brittle to prepare a sample for tensile test. The cooling medium is air, while others are water. The sample has oriented pattern that is parallel to the stress directions. d The sample has oriented pattern that is perpendicular to the stress directions. b c
better tensile strength and elongation. We also studied the effects of cooling rate on the tensile strength and elongation of PVDF membranes (Fig. 9). It shows that the tensile strength and
elongation of the membranes quenched in 303 K water bath is lower than those obtained in 277 K water bath. This result is due to the large pores induced by lower cooling rate. Particularly, the
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Table 3 Recovery results of DMSO2 from the nascent PVDF membranes. No. Recovery procedure
Extraction efficiencya (E, %)
Recovery yieldb (Y, %)
1
1) Extracted by 30 mL ethanol at 338 K for 6 h. 2) Recrystallized at 255 K for 24 h.
91.8
17.9
2
1) Extraction by 25 mL methanol at 323 K for 6 h. 2) Recrystallized at 255 K for 24 h.
90.5
43.1
3
1) Extracted by 20 mL water at 91.8 338 K for 6 h. 2) Concentrated at 348 K. 3) Recrystallized at 277 K for 24 h.
21.8
Sublimated at 353 K and 0.1 MPa for 91.2 7h
88.9
4
and PAN membranes, whereas no such pores exist in the CA membrane. The pore size and surface porosity of the three membranes become larger when lower polymer concentration or cooling rate is submitted to the experiment. The water flux and porosity of all the membranes increase with the decline of polymer concentration or cooling rate. Among them, PVDF membrane possesses the highest porosity and water flux at the optimum polymer concentration (10 wt%) and cooling condition (303 K water bath). We can also improve the tensile strength and elongation of membranes by increasing the polymer concentration or cooling rate. Moreover, DMSO2 can be recovered by recrystallization and sublimation. This work may provide a green preparation method to produce microporous polymer membranes via TIPS.
Acknowledgments
a Extraction yield is calculated by equation of E¼ (1 m1/m0) 100, where m0 is the weight of nascent membrane, and m1 is the weight of membrane after extraction. b Recovery yield is calculated by equation of Y ¼ (m3/m2) 100, where m2 is the theoretical weight of DMSO2 in membrane, and m3 is the weight of recovered DMSO2 crystals.
membranes also exhibit an anisotropic mechanical property because of the orientated texture, which has been reported in previous work [46,47]. Table 2 summarizes the specific values of the mechanical properties for PVDF, PAN, and CA membranes prepared under different conditions. The tensile strength and elongation for PAN or CA membranes increase with polymer concentration or cooling bath. Similarly, PAN membranes show the anisotropic mechanical property for the case prepared in 10 wt% polymer concentration or 303 K air bath. In contrast, the elongations of PAN and CA membranes are much smaller than that of PVDF membrane prepared in the same condition, while the former ones show the higher tensile strength and modulus than the latter one. It indicates that PVDF membranes are tough, while PAN or CA membranes are hard and brittle. Among all the membranes, the membranes prepared from 30 wt% PVDF in 303 K water bath have exhibited the most excellent mechanical properties. The tensile strength and elongation have reached as high as 7.5 71.3 MPa and 33.7 77.9%, which are also much higher than those reported in other issues [48,49]. 3.6. Recovery of the crystallizable diluent DMSO2 DMSO2 is a crystallizable diluent, so it can be recovered from the nascent membrane by recrystallization or sublimation. Herein, we choose methanol, ethanol and water as the recrystallization solvents. The recovery efficiencies of DMSO2 by recrystallization from ethanol, methanol and water are 17.9%, 43.1% and 21.8% respectively, which are lower than that by sublimation at 353 K (88.9%) (Table 3). It means that we can reuse the diluent by various recovery paths. The recovery of diluent may cut the cost of membrane preparation and reduce the pollution of diluent.
4. Conclusion DMSO2 has been proved to be a universal crystallizable diluent for polar polymer membranes via TIPS. The applicable polymers include PVDF, PAN and CA. Tubular-like pores are obtained in PVDF
The research is financially supported by the National Natural Science Foundation of China (Grant no. 21174124). The authors also thank Dr. Mao Peng in Zhejiang University for helping the experiments of tensile test.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2013.07.008.
References [1] P.M. Atkinson, D.R. Lloyd, Anisotropic flat sheet membrane formation via TIPS: thermal effects, J. Membr. Sci. 171 (2000) 1–18. [2] H. Matsuyama, S. Berghmans, M.T. Batarseh, D.R. Lloyd, Effects of thermal history on anisotropic and asymmetric membranes formed by thermally induced phase separation, J. Membr. Sci. 142 (1998) 27–42. [3] P. van de Witte, P.J. Dijkstra, J.W.A. van den Berg, J. Feijen, Phase separation processes in polymer solutions in relation to membrane formation, J. Membr. Sci. 117 (1996) 1–31. [4] Z. Song, M. Xing, J. Zhang, B. Li, S. Wang, Determination of phase diagram of a ternary PVDF/γ-BL/DOP system in TIPS process and its application in preparing hollow fiber membranes for membrane distillation, Sep. Purif. Technol. 90 (2012) 221–230. [5] S. Liu, C. Zhou, W. Yu, Phase separation and structure control in ultra-high molecular weight polyethylene microporous membrane, J. Membr. Sci. 379 (2011) 268–278. [6] A. Cui, Z. Liu, C. Xiao, Y. Zhang, Effect of micro-sized SiO2-particle on the performance of PVDF blend membranes via TIPS, J. Membr. Sci. 360 (2010) 259–264. [7] J.J. Blaker, V. Maquet, R. Jérôme, A.R. Boccaccini, S.N. Nazhat, Mechanical properties of highly porous PDLLA/Bioglasss composite foams as scaffolds for bone tissue engineering, Acta Biomater. 1 (2005) 643–652. [8] P.L. Hanks, D.R. Lloyd, Deterministic model for matrix solidification in liquid– liquid thermally induced phase separation, J. Membr. Sci. 306 (2007) 125–133. [9] S.C. Roh, M.J. Park, S.H. Yoo, C.K. Kim, Changes in microporous structure of polyethylene membrane fabricated from PE/PTMG/paraffin ternary mixtures, J. Membr. Sci. 411–412 (2012) 201–210. [10] T. Tanaka, T. Nishimoto, K. Tsukamoto, M. Yoshida, T. Kouya, M. Taniguchi, D. R. Lloyd, Formation of depth filter microfiltration membranes of poly(L-lactic acid) via phase separation, J. Membr. Sci. 396 (2012) 101–109. [11] J. Yang, D.W. Li, Y.K. Lin, X.L. Wang, F. Tian, Z. Wang, Formation of a bicontinuous structure membrane of polyvinylidene fluoride in diphenyl ketone diluent via thermally induced phase separation, J. Appl. Polym. Sci. 110 (2008) 341–347. [12] Y. Lin, Y. Tang, H. Ma, J. Yang, Y. Tian, W. Ma, X. Wang, Formation of a bicontinuous structure membrane of polyvinylidene fluoride in diphenyl carbonate diluent via thermally induced phase separation, J. Appl. Polym. Sci. 114 (2009) 1523–1528. [13] W. Yave, R. Quijada, D. Serafini, D.R. Lloyd, Effect of the polypropylene type on polymer–diluent phase diagrams and membrane structure in membranes formed via the TIPS process: Part I. Metallocene and Ziegler–Natta polypropylenes, J. Membr. Sci. 263 (2005) 146–153. [14] R. Lv, J. Zhou, Q. Du, H. Wang, W. Zhong, Preparation and characterization of EVOH/PVP membranes via thermally induced phase separation, J. Membr. Sci. 281 (2006) 700–706.
H.-Q. Liang et al. / Journal of Membrane Science 446 (2013) 482–491
[15] M. Gu, J. Zhang, X. Wang, W. Ma, Crystallization behavior of PVDF in PVDFDMP system via thermally induced phase separation, J. Appl. Polym. Sci. 102 (2006) 3714–3719. [16] I.J. Roh, S. Ramaswamy, W.B. Krantz, A.R. Greenberg, Poly(ethylene chlorotrifluoroethylene) membrane formation via thermally induced phase separation (TIPS), J. Membr. Sci. 362 (2010) 211–220. [17] C. Funk, P. Hanks, K. Kaczorowski, D. Lloyd, Diluent crystal alignment in the formation of membranes via liquid–solid thermally induced phase separation, J. Porous Mater. 16 (2009) 453–458. [18] Q.-Y. Wu, L.-S. Wan, Z.-K. Xu, Structure and performance of polyacrylonitrile membranes prepared via thermally induced phase separation, J. Membr. Sci. 409–410 (2012) 355–364. [19] H. Matsuyama, M.-M. Kim, D.R. Lloyd, Effect of extraction and drying on the structure of microporous polyethylene membranes prepared via thermally induced phase separation, J. Membr. Sci. 204 (2002) 413–419. [20] H. Matsuyama, H. Okafuji, T. Maki, M. Teramoto, N. Kubota, Preparation of polyethylene hollow fiber membrane via thermally induced phase separation, J. Membr. Sci. 223 (2003) 119–126. [21] J. Yoon, A.J. Lesser, T.J. McCarthy, Locally anisotropic porous materials from polyethylene and crystallizable diluents, Macromolecules 42 (2009) 8827–8834. [22] C. Zhang, Y. Bai, Y. Sun, J. Gu, Y. Xu, Preparation of hydrophilic HDPE porous membranes via thermally induced phase separation by blending of amphiphilic PE-b-PEG copolymer, J. Membr. Sci. 365 (2010) 216–224. [23] H. Matsuyama, M. Yuasa, Y. Kitamura, M. Teramoto, D.R. Lloyd, Structure control of anisotropic and asymmetric polypropylene membrane prepared by thermally induced phase separation, J. Membr. Sci. 179 (2000) 91–100. [24] H. Matsuyama, T. Maki, M. Teramoto, K. Asano, Effect of polypropylene molecular weight on porous membrane formation by thermally induced phase separation, J. Membr. Sci. 204 (2002) 323–328. [25] B. Luo, Z. Li, J. Zhang, X. Wang, Formation of anisotropic microporous isotactic polypropylene (iPP) membrane via thermally induced phase separation, Desalination 233 (2008) 19–31. [26] W. Yave, R. Quijada, M. Ulbricht, R. Benavente, Syndiotactic polypropylene as potential material for the preparation of porous membranes via thermally induced phase separation (TIPS) process, Polymer 46 (2005) 11582–11590. [27] S.-W. Song, J.M. Torkelson, Coarsening effects on the formation of microporous membranes produced via thermally induced phase separation of polystyrenecyclohexanol solutions, J. Membr. Sci. 98 (1995) 209–222. [28] C. Gao, A. Li, X. Yi, J. Shen, Construction of cell-compatible layer and culture of human umbilical vascular endothelial cells on porous polystyrene membranes, J. Appl. Polym. Sci. 81 (2001) 3523–3529. [29] S. Rajabzadeh, T. Maruyama, T. Sotani, H. Matsuyama, Preparation of PVDF hollow fiber membrane from a ternary polymer/solvent/nonsolvent system via thermally induced phase separation (TIPS) method, Sep. Purif. Technol. 63 (2008) 415–423. [30] X. Li, G. Xu, X. Lu, C. Xiao, Effects of mixed diluent compositions on poly (vinylidene fluoride) membrane morphology in a thermally induced phaseseparation process, J. Appl. Polym. Sci. 107 (2008) 3630–3637. [31] G.-L. Ji, C.-H. Du, B.-K. Zhu, Y.-Y. Xu, Preparation of porous PVDF membrane via thermally induced phase separation with diluent mixture of DBP and DEHP, J. Appl. Polym. Sci. 105 (2007) 1496–1502. [32] M. Shang, H. Matsuyama, M. Teramoto, D.R. Lloyd, N. Kubota, Preparation and membrane performance of poly(ethylene-co-vinyl alcohol) hollow fiber
[33]
[34] [35]
[36] [37]
[38] [39]
[40]
[41] [42]
[43]
[44]
[45]
[46]
[47] [48]
[49]
[50] [51] [52] [53]
491
membrane via thermally induced phase separation, Polymer 44 (2003) 7441–7447. J. Zhou, H. Zhang, H. Wang, Q. Du, Effect of cooling baths on EVOH microporous membrane structures in thermally induced phase separation, J. Membr. Sci. 343 (2009) 104–109. P.D. Graham, A.J. Pervan, A.J. McHugh, The dynamics of thermal-induced phase separation in PMMA solutions, Macromolecules 30 (1997) 1651–1655. H. Matsuyama, Y. Takida, T. Maki, M. Teramoto, Preparation of porous membrane by combined use of thermally induced phase separation and immersion precipitation, Polymer 43 (2002) 5243–5248. T. Tanaka, D.R. Lloyd, Formation of poly(L-lactic acid) microfiltration membranes via thermally induced phase separation, J. Membr. Sci. 238 (2004) 65–73. K.C. Shin, B.S. Kim, J.H. Kim, T.G. Park, J.D. Nam, D.S. Lee, A facile preparation of highly interconnected macroporous PLGA scaffolds by liquid–liquid phase separation II, Polymer 46 (2005) 3801–3808. T. Clark, J. Murray, P. Lane, P. Politzer, Why are dimethyl sulfoxide and dimethyl sulfone such good solvents? J. Mol. Model. 14 (2008) 689–697. M.E. Vanegas, R. Quijada, D. Serafini, Microporous membranes prepared via thermally induced phase separation from metallocenic syndiotactic polypropylenes, Polymer 50 (2009) 2081–2086. C.V. Funk, B.L. Beavers, D.R. Lloyd, Effect of particulate filler on cell size in membranes formed via liquid–liquid thermally induced phase separation, J. Membr. Sci. 325 (2008) 1–5. P.J. Flory, Principle of Polymer Chemistry, Cornell University Press, Ithaca, New York, 1953. W.R. Burghardt, Phase diagrams for binary polymer systems exhibiting both crystallization and limited liquid–liquid miscibility, Macromolecules 22 (1989) 2482–2486. M.H. Rahman, A.K. Nandi, Supramolecular organization of poly(vinylidene fluoride)–camphor sulfonic acid blends:giant spherulites, Langmuir 19 (2003) 7056–7060. D. Dasgupta, S. Manna, S. Malik, C. Rochas, J.M. Guenet, A.K. Nandi, Thermodynamic structural and morphological investigation of poly(vinylidene fluoride)–camphor systems, preparing porous gels from a solid solvent, Macromolecules 38 (2005) 5602–5608. Q.-Y. Wu, X.-N. Chen, L.-S. Wan, Z.-K. Xu, Interactions between polyacrylonitrile and solvents: density functional theory study and two-dimensional infrared correlation analysis, J. Phys. Chem. B 116 (2012) 8321–8330. H. Zhou, G.L. Wilkes, Orientation-dependent mechanical properties and deformation morphologies for uniaxially melt-extruded high-density polyethylene films having an initial stacked lamellar texture, J. Mater. Sci. 33 (1998) 287–303. A. Galeski, Strength and toughness of crystalline polymer systems, Prog. Polym. Sci. 28 (2003) 1643–1699. G.-L. Ji, B.-K. Zhu, Z.-Y. Cui, C.-F. Zhang, Y.-Y. Xu, PVDF porous matrix with controlled microstructure prepared by TIPS process as polymer electrolyte for lithium ion battery, Polymer 48 (2007) 6415–6425. Y. Tang, Y. Lin, W. Ma, Y. Tian, J. Yang, X. Wang, Preparation of microporous PVDF membrane via tips method using binary diluent of DPK and PG, J. Appl. Polym. Sci. 118 (2010) 3518–3523. W.M. Haynes, CRC Handbook of Chemistry and Physics, 91st ed., CRC press/ Tayler and Francis, Boca Laton, FL. E.M. James, Polymer Data Handbook, Oxford University Press, Oxford, 1999. D.W.V. Krevelen, Properties of Polymers, 4th ed., Elsevier, B.V., Amsterdam, 2009. A.J. Lovinger, Ferroelectric polymers, Science 220 (1983) 1115–1121.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Polar polymer membranes via thermally induced phase separation using a universal crystallizable diluent Hong-Qing Liang, Qing-Yun Wu, Ling-Shu Wan, Xiao-Jun Huang, Zhi-Kang Xu n MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 February 2013 Received in revised form 21 June 2013 Accepted 7 July 2013 Available online 12 July 2013
Dimethyl sulfone (DMSO2) was used as a universal crystallizable diluent to prepare polar polymer membranes via thermally induced phase separation (TIPS). The polar polymers adopted herein include poly(vinylidene fluoride) (PVDF), polyacrylonitrile (PAN) and cellulose acetate (CA). Intensive investigation was carried out to study the phase separation behaviors and the membrane performances. Equilibrium phase diagrams and polarized optical microscope results indicate a solid solid phase separation mechanism for all the three polar polymer/diluent systems. Scanning electron microscopy observations show that tubular-like pores are irregularly distributed in the PVDF and PAN membranes, whereas a compacted structure can be found in the CA membranes. The pore size, surface porosity, water flux and overall porosity become large when the membranes are prepared with low polymer concentration or at small cooling rate. Results of tensile tests confirm that the mechanical strength of the membranes can be enhanced by increasing the polymer concentration or cooling rate. Moreover, DMSO2 has been efficiently recovered by recrystallization and sublimation. In conclusion, this work may provide a green preparation method to produce polar polymeric membranes via TIPS. & 2013 Elsevier B.V. All rights reserved.
Keywords: Thermally induced phase separation Universal diluent Dimethyl sulfone Polar polymer Poly(vinylidene fluoride)
1. Introduction Thermally induced phase separation (TIPS) is a well-known method to fabricate porous polymer membranes due to its advantages including easy control, low tendency for defects formation, and diverse microstructures that are desirable for various applications of membrane. Since it had been introduced by Castro in 1980s, this method has attracted wide investigations in respect of conducting process [1,2], phase separation mechanism [3], structure controlling [4,5], and properties enhancement [6,7]. Nevertheless, great efforts have been still made to drive the further development of TIPS both in theoretical and practical aspects. Hanks et al. [8] reported a deterministic model for matrix solidification in liquid liquid (L L) TIPS. They chose isotactic polypropylene–diphenyl ether as a representative system to confirm the simulation and found that the simple deterministic approach can provide accurate prediction for the final cell size diameter and distribution of isotropic L L TIPS membranes. Roh et al. [9]
Abbreviations: TIPS, thermally induced phase separation; L L, liquid liquid; S L, solid liquid; L S, liquid solid; S S, solid solid; PVDF, poly(vinylidene fluoride); PAN, polyacrylonitrile; CA, cellulose acetate; DMSO2, dimethyl sulfone; DSC, differential scanning calorimetry; FESEM, field emission scanning electron microscopy n Corresponding author. Tel.: +86 571 8795 2605; fax: +86 571 8795 1773. E-mail address: [email protected] (Z.-K. Xu). 0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.07.008
described a novel diluent mixture containing paraffin and poly (tetramethylene glycol) for the preparation of polyethylene membranes with controlled pore size and porosity. Tanaka et al. [10] combined the nonsolvent and thermally induced phase separation to prepare asymmetric poly(L-lactic acid) membrane. Their membranes were suggested to be served as efficient and rapid depth filters to deal with bacterial cells. Generally, a typical TIPS process is defined as follows: a polymer is dissolved into a high-boiling and low molecular weight diluent at an elevated temperature, then the homogeneous solution is cooled to induce phase separation, and a microporous structure can be created after removing the diluent. Diluent is one of the dominant factors that determine specific phase separation process (e.g. L L, solid liquid (S L), liquid solid (L S), and solid solid (S S) demixing) and subsequently control the final microstructures of membranes. These membranes usually have various porous structures including bicontinuous [11,12], cellular [13,14], spherulitic [15,16], needle- or sheet-like pores [17,18]. Furthermore, TIPS by far has been adopted to prepare membranes from a number of polymers, including polyethylene [19–22], polypropylene [23–26], polystyrene [27,28], poly(vinylidene fluoride) (PVDF) [29–31], poly(ethylene-co-vinyl alcohol) [32,33], poly (methyl methacrylate) [34,35], and poly(lactic acid) [10,36,37]. Nevertheless, few reports have ever proposed the idea of using a universal diluent to prepare porous membranes from polar polymers (such as polyacrylonitrile (PAN) and cellulose acetate (CA))
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Table 1 Physical properties of the polymers and diluent. Properties
Unit
DMSO2
PVDF
PAN
CA
Molecular weight Degree of polymerization Molar volume Density Heat of fusion Melting temperaturea Crystallization temperature Solubility parametera Dipole moment Dielectric constant
g/mol
94 N/A 75 1.16 18300a 382 365 29.9 4.4d 47.4a
110,000 (Mn) 277,000 (Mw) 1719 36.4 1.76 6700c 443 409 23.2 1.9e 8.4
80,000 (Mn) 190,000 (Mw) 1509 45.6 1.14 5021 593 N/A 26.0 3.9e 6.5
92,000 (Mn) 210,000 (Mw) 384 189 1.3 12600b 508 N/A 27.8 N/A 7.0
cm3/mol g/cm3 J/mol K K MPa1/2 D
a
Ref. [50]. Represented by the data of cellulose butyrate, Ref. [51]. c Ref. [52]. d Ref. [38]. e Ref. [53]. b
via TIPS. One of the most important challenges is to search an appropriate diluent due to the crystallinity and high melting point of polar polymers. Such a searching step is time and cost consuming that limits the application broadening of TIPS. Furthermore, the interactions between a diluent and different polymers may differ from each other significantly, which will lead to quite different behaviors of solubility and phase separation. In this study, a series of porous membranes were prepared from different polar polymers including PVDF, PAN and CA via TIPS, in which dimethyl sulfone (DMSO2) was successfully introduced as a universal crystallizable diluent. DMSO2 is a widely used polar solvent with high boiling point, especially valuable in high temperature condition [38]. Theoretically, DMSO2 has a solubility parameter close to those of PVDF, PAN and CA (Table 1), indicating that homogeneous solutions are reasonable to be obtained at elevated temperature. On the other hand, DMSO2 will crystallize and precipitate from the solutions as temperature falls to 365 K [18], and phase separation can be expected. Therefore, it is feasible to use DMSO2 as a universal diluent for these polar polymer membranes via TIPS. Based on this consideration, we studied the phase separation mechanisms of PVDF/DMSO2, PAN/DMSO2, CA/ DMSO2 binary systems from the aspects of thermodynamic theory and experiments. We also focused on the effects of polymer concentration and cooling bath on the pore size, surface porosity, water flux, overall porosity, and mechanical properties of the resultant membranes. Furthermore, DMSO2 as a non-toxic diluent was effectively recovered by recrystallization or sublimation herein, which potentially cuts the cost of membrane preparation and reduces environmental pollution from diluent. The application of crystallizable diluent may provide a novel solution to membrane structure control and a green strategy for the fabrication of polar polymer membranes.
2. Experimental 2.1. Materials DMSO2 (99%) was purchased from Dakang Chemicals Co., China. PVDF (Mn ¼110,000 g/mol, Solefs 6010) was a commercial product of Solvay Solexis, Belgium. PAN (Mn ¼80,000 g/mol) was supplied by Anqing Petroleum Chemical Co., China. CA (Mn ¼ 92,000 g/mol) was obtained from Sinopharm Chemical Reagent Co., Ltd. All the polymer powders were dried at 333 K under vacuum for 4 h before use. Deionized water was used as the extractant. Other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. and used without further purification.
2.2. Preparation of membranes One polymer/DMSO2 mixture was heated at 433 K to form a homogeneous solution. After degassing air bubbles, the solution was quickly poured onto a stainless steel mold (thickness 200 μm), which was preheated in an oven at 433 K. The mold was then quenched in a cooling bath (water bath at 277 K and 303 K or air bath at 303 K) to induce the solution to phase separation. Afterwards, the obtained nascent membrane was taken out of the mold and immersed in deionized water. A wet membrane was formed as soon as the diluent was completely extracted. To gain a dry membrane, the wet one was washed with an ethanol–hexane sequence, and then dried in vacuum for 24 h at 333 K.
2.3. Determination of phase behavior Differential scanning calorimetry (DSC, Q1000, TA instruments, USA) was used to determine the melting and crystallization temperatures of the mixture. Sample was prepared by directly quenching the polymer/diluent solution into liquid nitrogen, which prevented the macroscopic phase separation. Solid sample of 6–8 mg was hermetically sealed in an aluminum DSC pan, heated from 25 to 473 K at 20 K/min, and maintained at 473 K for 3 min to eliminate thermal history. It was then cooled to 0 K at 10 K/min and reheated immediately to 473 K at 10 K/min. Peak maxima in the cooling and reheating sequences were regarded as the crystallization temperature and the melting point, respectively. The phase separation process was visualized by an optical microscope (Nikon Eclipse E600POL, Japan). A small section of polymer/diluent solid sample was placed between a pair of microscope slides, and put on a hot stage (Linkam TMS-93) with a temperature controller (Linkam THMS-600). It was heated at 30 K/min to 473 K, maintained for 3 min, and then cooled to 273 K at 10 K/min. The field of vision was recorded at the moment of solidification.
2.4. Wide-angle X-ray diffraction (WAXD) WAXD was carried out on a Rigaku D/Max-2550PC X-ray diffractometer (Panalytical, Netherlands). The radial scans at a voltage of 40 kV and a current of 40 mA using a Cu:Ka radiation were employed on the samples. Data were collected at 0.01671 interval with counting for 10 s at each step.
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2.5. Field emission scanning electron microscopy (FESEM) observation Membrane morphologies of cross-section and surfaces were examined by FESEM (Hitachi S4800, Japan) with an accelerating voltage of 10 kV. To obtain a tidy cross-section, the membrane sample was freeze-fractured in liquid nitrogen. All samples were gold sputtered using a sputter coating instrument (EDT2000) before imaged by FESEM. 2.6. Water flux measurement Water flux of membranes (surface area is 4.9 cm2) was measured in a pressure driven filtration cell. Membrane samples were carefully wetted with ethanol before measurement to allow water impregnating the membrane pores. The membrane was first installed in the permeation cell and compacted by filtering ultrapure water at 0.12 MPa and 295 K for 30 min. Then the pressure was lowered to 0.10 MPa for operation. Each sample was repeated at least three parallel experiments. Pure water flux (Jw) was calculated by the following equation: Jw ¼
V AΔt
ð1Þ
where V, A and Δt denote the volume of permeated water (L), sample area (m2), and the permeation time (h), respectively. 2.7. Pore size and porosity analyses Average pore size was determined by analyzing FESEM images of the cross-section of membranes using an Image tool [39]. As for the tubular pores, their width was defined as the pore size in this work. Surface porosity of each membrane was measured by an image analysis software of Image-Pro Plus Version 6.0 [40]. The overall porosity is defined as the volume of pores divided by the total volume of membrane. A wet membrane was first weighed as soon as the superficial water on the membrane surface was removed by dry filter paper. Afterward, the wet membrane was dried through the process as mentioned in Section 2.2. The weight of dry membrane was measured again. Porosity of the membrane was denoted as P, which was calculated with the gravimetric method P¼
ðw0 w1 Þ=ρwater 100 ðw0 w1 Þ=ρwater þ w1 =ρ
ð2Þ
where w0 is the weight of the wet membrane, w1 is the weight of the dry membrane, and ρwater or ρ is the density of water or polymer, respectively. 2.8. Tensile test Tensile strength and elongation for the membranes were evaluated in a stress–strain test using a tensile test instrument (RGM-4000, Shenzhen REGER Instrument Co., Ltd., China). The test was carried out at a strain rate of 5 mm/min at 293 K and a relative humidity of 74%. The membrane samples were prepared in rectangle shape with a gauge length of 20 mm and a width of 10 mm. The thickness was around 200 μm and exactly examined by a thickness gauge (CH-1-S/ST, Shanghai Liuling Instrument Plant, China). Each data was the average of at least three parallel experiments. 2.9. Recovery of crystallizable diluent DMSO2 was recovered from the nascent membrane by recrystallization or sublimation. The nascent membrane was dissolved in
a smallest amount of solvent at 343 K to form a saturated solution when using the recrystallization method. The solvents adopted here include ethanol, methanol and water. After filtration, the solution was cooled at 255 K for 24 h, while the water solution was cooled at 273 K. Needle-like crystals of DMSO2 were recovered by filtration. Another recovery method was sublimation. The nascent membrane was placed in a sublimation apparatus and then heated to 353 K under vacuum. Solid DMSO2 inside the membrane was gradually sublimated and condensed on a cooled surface. At last, purified DMSO2 can be collected from the cooled surface.
3. Results and discussion 3.1. Phase diagrams of the polymer/diluent systems Generally, phase separation mechanism is the dominant factor to the final membrane morphology and properties. Accordingly, it is necessary to predict the possible phase separation behavior. Equilibrium phase diagrams can be calculated according to the Flory–Huggins solution thermodynamics. Theoretically, the melting point of polymer would be depressed in the presence of diluent [41]. The melting point depression of polymer can be calculated using Eq. (3) [21,42] 1 1 Rβ V u ¼ 1þ ð1ϕ2 Þ2 Tm ΔH u V 1 " 1 R Vu 1 lnðϕ2 Þ ð1ϕ2 Þ ð3Þ 1 0 þ N N T m ΔH u V 1 where Tm is the melting point of polymer in a solution, T 0m is the melting point of neat polymer, ΔHu is the enthalpy of fusion per repeat unit of polymer, Vu is the molar volume of polymer, N is the degree of polymerization, ϕ2 is the volume fraction of polymer, R is the gas constant, and β is a constant which is calculated from the solubility parameters of polymer (δ2) and diluent (δ1) along with the molar volume of diluent (V1), using the equation (δ1 δ2)2V1/R. Similarly, Eq. (4) can be used to calculate the melting point depression of a crystallizable diluent [42] " #1 " # 1 Rβϕ22 1 R 1 ϕ ¼ 1þ 1 þ lnð1ϕ Þ ð4Þ 2 T m;1 N 2 ΔH 1 T 0m;1 ΔH 1 where Tm,l is the melting point of diluent in a polymer solution, T 0m;1 is the melting point of neat diluent, ΔH1 is the enthalpy of fusion of neat diluent. Table 1 lists the parameters used for calculation. Fig. 1(a–c) shows typical DSC heating thermograms for the PVDF/DMSO2, PAN/DMSO2, and CA/DMSO2 binary systems. In each curve, a peak around 383 K indicates the melting point of DMSO2 in the mixture. These experimental melting points of DMSO2 are in good agreement with those calculated results (Fig. 1(d–f)). It is clear that another peak at relatively low temperature exists in the PAN/DMSO2 and CA/DMSO2 mixtures, which may result from the interaction between polymers and diluent. Interestingly, almost no signal is found for the melting of polymers, though they are all semi-crystalline polymers. Theoretically, the melting points of PVDF and PAN are above those of DMSO2 in the mixtures as indicated by the dash lines in Fig. 1(d, e). Actually, the melted DMSO2 is a good solvent for all the polymers. PVDF and PAN would not preferentially crystallize from the solution unless DMSO2 crystallizes when the temperature reach the melting temperature of DMSO2. Meanwhile, the polymer immediately solidifies because of a large supercooling degree. Thus, the equilibrium phase diagram suggests that both PVDF/DMSO2 and PAN/DMSO2 binary systems undergo solid–solid phase separation. In contrast, an eutectic point
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Fig. 1. Thermal analysis results of the polymer/diluent mixtures: (a, d) PVDF/DMSO2, (b, e) PAN/DMSO2, and (c, f) CA/DMSO2. (a–c) DSC thermograms and (d–f) melting points (■, ▲) plotted on the calculated equilibrium phase diagram. The dash lines in (d–f) represent the theoretical melting curves of polymers, while the solid lines represent the theoretical melting curves of DMSO2.
Fig. 2. FESEM images of PVDF, PAN, CA membranes: (a) cross-section (250 ), (b) cross-section (2000 ), and (c) top-surface. The membranes were prepared with 20 wt% polymer in water bath at 303 K.
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is calculated in CA/DMSO2 system with 50 wt% polymer, which consists with the invariable temperature of its second melting point as mentioned above. It indicates that CA/DMSO2 system thermodynamically goes through liquid–solid phase separation. Real-time POM was used to observe the cooling process of the polymer/ DMSO2 mixtures at a rate of 10 1C/min (Figs. S1 and S2 in Supporting Information). Neither crystallization nor liquid–liquid phase separation takes place at the initial stage upon cooling. When the temperature reaches the crystallization temperature of DMSO2, the samples solidify in a few seconds and giant spherulites are formed, in which no preferential crystallization has been observed [43,44]. This result is in well consistent with the crystalline behaviors of the three systems, which show only one crystalline peak upon cooling (Fig. S3 in Supporting Information). WAXD was then used to confirm the crystallization of both components in the three binary systems (Fig. S4 in Supporting Information). Combining with the results of DSC, POM, and WAXD, we can confirm a solid–solid phase separation mechanism for all the three systems. Among others, this is a result of dynamics for CA/DMSO2 system which may overcome the narrow liquid–solid area at large cooling rate. 3.2. Morphologies of PVDF, PAN and CA membranes Fig. 2 shows the cross-section and surface morphologies of PVDF, PAN and CA membranes prepared with a polymer concentration of 20 wt%. Irregular tubular pores are observed in PVDF and PAN membranes due to the crystallization of DMSO2. As mentioned above, both PVDF/DMSO2 and PAN/DMSO2 binary systems undergo
solid–solid phase separation. The membrane pore morphology will mainly depend on the crystal structure of diluent. Upon cooling, needle-like crystals of DMSO2 will form. Thus tubular pores will be obtained after subsequent extraction of diluent. Furthermore, the tubular pores are irregularly distributed in PVDF and PAN membranes, as the nucleation of diluent crystals is quite random without any applied temperature gradient. However, this kind of morphology cannot be found in the CA membrane. It has been reported that the sulfur-oxygen linkages in DMSO2 are coordinate covalent single S+O bonds, with both of the shared electrons coming from the sulfur [38]. The molecular surface electrostatic potentials confirm the highly negative characters of the oxygens, which can form strong hydrogen bonds with residual hydroxyl groups of CA. Nevertheless, PVDF or PAN may interact with DMSO2 through dipole–dipole interaction [44,45], which is much weaker than the hydrogen bonding interaction. Given the dipole moments and dielectric constants of the polymers (Table 1), we can conclude that the polymer DMSO2 interaction is in the order of CA4PAN4PVDF. This result is in line with the melting point depression as mentioned above. Since the PVDF DMSO2 interaction is weak, DMSO2 easily crystallizes and separates from the polymer solution. Accordingly, the average pore size of PVDF membranes (2.1 μm) is larger than that of PAN membranes (0.3 μm). There are small pores on the wall of the tubular pores in PVDF membrane, suggesting high interconnection between pores. Otherwise, owing to the strong CA DMSO2 interaction, the crystallization of DMSO2 is mostly inhibited, and a weak phase separation leads to a compacted structure. On the other hand, the surface of PVDF membrane is full of nanopores ( 21.5 nm), while dense surfaces can be seen for PAN and
Fig. 3. FESEM images of PVDF membranes prepared from 10–30 wt% PVDF: (a) cross-section (250 ), (b) cross-section (2000 ), and (c) top-surface. The membranes were prepared in water bath at 303 K.
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CA membranes (Fig. 2(c)). Besides, the PAN membrane surface has regular “stripes” with a certain orientation, which may also be caused by the crystallization of DMSO2 [18].
small as a consequence [18]. PAN and CA membranes show a similar tendency on their morphology (Figs. S5 and S6 in Supporting Information).
3.3. Effects of polymer concentration on the membrane structures
3.4. Effects of cooling rate on the membrane structures
Fig. 3 presents the morphologies of PVDF membranes prepared with different polymer concentrations. The pore size reduces from 3.5 μm to 1.5 μm with an increase of PVDF concentration from 10 wt% to 30 wt% (Fig. 4(a)). The surface porosity also dramatically decreases with the PVDF concentration. It may be caused by the increasing viscosity of the PVDF/DMSO2 solution, which will prevent the crystallization of DMSO2 and then the pores become
Except for polymer concentration, cooling rate also has great impact on the membrane morphology since TIPS is a dynamiccontrolled process. Fig. 5 indicates that the tubular pores of PVDF membranes become large as the temperature of water bath increases. PVDF membrane prepared in 30 1C water has pore size of approximately 2.1 μm and surface porosity of about 6.2%, which are larger than those prepared in 277 K water ( 1.3 μm and 4.8%)
Fig. 4. Surface porosity and pore size of PVDF, PAN and CA membranes varied with polymer concentration (a) and cooling bath (b). All the membranes in (a) were prepared in water bath at 303 K, and those in (b) were prepared with polymer concentration of 20 wt%.
Fig. 5. FESEM images of PVDF membranes cooled in different cooling baths: (a) cross-section (300 ), (b) cross section (2000 ), and (c) top-surface. The membranes were prepared from 20 wt% PVDF.
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(Fig. 4(b)). The membranes even have pore size of 4.1 μm and surface porosity of 7.8% when prepared in 303 K air bath. Air has smaller thermal capacity than water, so the cooling of solution in air is slower than that in water. As the cooling rate decreases, DMSO2 crystals have long time to grow, leading to larger pore sizes and surface porosity in the as-prepared membranes. Similar results are found in PAN and CA membranes, especially for those CA membranes (Figs. S7 and S8 in Supporting Information). CA membranes show compacted structure and have almost no pores when quenched in 277 K water bath. If 303 K air bath is used, the membranes have large pores ( 2.3 μm) and high surface porosity (15.8%). Moreover, an orientated texture can be visualized on the surface of PVDF and PAN membranes when quenched in 303 K air bath. 3.5. Water flux and mechanical properties of PVDF, PAN and CA membranes We have studied the effects of polymer concentration and cooling rate on the water flux and overall porosity of PVDF, PAN, and CA membranes (Figs. 6 and 7). The results demonstrate that water flux and overall porosity of all the membranes increase with
the decrease of polymer concentration or cooling rate. It is due to the large pore size and surface porosity at low polymer concentration or cooling rate (Fig. 4). Among others, PVDF membranes possess the highest water flux (1491 L/m2 h) and overall porosity (91.1%) at the optimum polymer concentration (10 wt%) and cooling condition (303 K water bath). However, the water flux of PAN or CA membranes is quite small though their porosities are still quite high. Firstly, low surface porosity may account for the low water flux of these two kinds of membranes. Secondly, the pore structures in the cross-section have great effect: the pores are almost parallel to the surface of PAN membranes with small pore size, and CA membranes are too compacted. Mechanical properties are crucial to the application of membranes. Fig. 8(a) shows the stress strain curves of PVDF membranes prepared from different polymer concentrations. The stress strongly increases with the strain at the initial stage. Then a nontypical yielding occurs when the strain reaches the yield point. At the last stage of deformation, the samples fail by tearing quickly at the end of the clamps, characterized by downward tails at the end of the respective stress strain curve. Fig. 8(b) shows the corresponding tensile strength and elongation results, indicating that the membranes prepared from higher polymer concentration have
Fig. 6. Porosity (a) and water flux (b) of PVDF, PAN and CA membranes in different polymer concentrations. The membranes were prepared in water bath at 303 K. CA membrane from 10 wt% CA is too brittle to test the water flux.
Fig. 7. Porosity (a) and water flux (b) of PVDF, PAN and CA membranes in different cooling conditions. The membranes were prepared from 20 wt% polymer.
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Fig. 8. Stress–strain curve (a) and the corresponding tensile strength and elongation (b) for PVDF membrane prepared with different polymer concentrations: (1) 10 wt%, (2) 20 wt%, and (3) 30 wt%. The membranes were prepared in water bath at 303 K.
Fig. 9. Sress–strain curve (a) and the corresponding tensile strength and elongation (b) for PVDF membrane prepared at different cooling baths: (1) 277 K water, (2) 303 K water, and (3 and 3′) 303 K air. The membranes were prepared with polymer concentration of 20 wt%. The stress directions applied to samples of (3) and (3′) were perpendicular and parallel to the direction of orientation observed on the membrane surface, respectively.
Table 2 Mechanical properties of the membranes prepared in different conditions. Membrane
Polymer concentration (wt%)
Cooling bath (K)
Thickness (μm)
Elongation (%)
Tensile strength (MPa)
PVDF
10 20 30 20 20c 20d
303 303 303 277 303b 303b
188.8 7 9.0 284.0 7 8.7 270.378.4 174.7 7 28.6 227.7 7 27.3 231.0 731.1
16.0 7 1.4 22.8 7 5.3 33.77 7.9 43.4 7 19.2 62.5 7 12.5 42.5 7 12.1
0.9 7 0.3 2.8 7 0.1 7.5 71.3 3.0 7 0.2 2.5 7 0.3 3.8 7 1.1
PAN
10c 10d 20 30 20 20c 20d
303 303 303 303 277 303b 303b
145.3 7 17.7 143.0 7 7.2 182.7 7 3.2 252.0 7 37.2 179.7 7 35.5 179.7 7 5.9 197.7 7 12.0
4.17 0.4 3.9 7 0.4 4.4 7 1.7 4.9 7 2.2 4.8 7 0.8 8.3 7 2.4 0.7 7 0.1
7.6 70.8 3.7 7 0.4 7.7 72.0 16.5 7 4.3 11.7 7 3.6 15.8 7 0.7 3.0 7 0.5
CA
10a 20 30 20 20
303 303 303 277 303b
N/A 164.37 15.8 157.7 75.1 95.7 7 4.7 185.0 7 10.6
N/A 10.0 7 2.2 11.2 7 4.1 6.2 7 1.8 1.2 70.2
N/A 46.9 7 3.4 39.0 7 2.0 41.8 7 9.4 7.5 70.6
a
The membrane from 10 wt% CA is too brittle to prepare a sample for tensile test. The cooling medium is air, while others are water. The sample has oriented pattern that is parallel to the stress directions. d The sample has oriented pattern that is perpendicular to the stress directions. b c
better tensile strength and elongation. We also studied the effects of cooling rate on the tensile strength and elongation of PVDF membranes (Fig. 9). It shows that the tensile strength and
elongation of the membranes quenched in 303 K water bath is lower than those obtained in 277 K water bath. This result is due to the large pores induced by lower cooling rate. Particularly, the
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Table 3 Recovery results of DMSO2 from the nascent PVDF membranes. No. Recovery procedure
Extraction efficiencya (E, %)
Recovery yieldb (Y, %)
1
1) Extracted by 30 mL ethanol at 338 K for 6 h. 2) Recrystallized at 255 K for 24 h.
91.8
17.9
2
1) Extraction by 25 mL methanol at 323 K for 6 h. 2) Recrystallized at 255 K for 24 h.
90.5
43.1
3
1) Extracted by 20 mL water at 91.8 338 K for 6 h. 2) Concentrated at 348 K. 3) Recrystallized at 277 K for 24 h.
21.8
Sublimated at 353 K and 0.1 MPa for 91.2 7h
88.9
4
and PAN membranes, whereas no such pores exist in the CA membrane. The pore size and surface porosity of the three membranes become larger when lower polymer concentration or cooling rate is submitted to the experiment. The water flux and porosity of all the membranes increase with the decline of polymer concentration or cooling rate. Among them, PVDF membrane possesses the highest porosity and water flux at the optimum polymer concentration (10 wt%) and cooling condition (303 K water bath). We can also improve the tensile strength and elongation of membranes by increasing the polymer concentration or cooling rate. Moreover, DMSO2 can be recovered by recrystallization and sublimation. This work may provide a green preparation method to produce microporous polymer membranes via TIPS.
Acknowledgments
a Extraction yield is calculated by equation of E¼ (1 m1/m0) 100, where m0 is the weight of nascent membrane, and m1 is the weight of membrane after extraction. b Recovery yield is calculated by equation of Y ¼ (m3/m2) 100, where m2 is the theoretical weight of DMSO2 in membrane, and m3 is the weight of recovered DMSO2 crystals.
membranes also exhibit an anisotropic mechanical property because of the orientated texture, which has been reported in previous work [46,47]. Table 2 summarizes the specific values of the mechanical properties for PVDF, PAN, and CA membranes prepared under different conditions. The tensile strength and elongation for PAN or CA membranes increase with polymer concentration or cooling bath. Similarly, PAN membranes show the anisotropic mechanical property for the case prepared in 10 wt% polymer concentration or 303 K air bath. In contrast, the elongations of PAN and CA membranes are much smaller than that of PVDF membrane prepared in the same condition, while the former ones show the higher tensile strength and modulus than the latter one. It indicates that PVDF membranes are tough, while PAN or CA membranes are hard and brittle. Among all the membranes, the membranes prepared from 30 wt% PVDF in 303 K water bath have exhibited the most excellent mechanical properties. The tensile strength and elongation have reached as high as 7.5 71.3 MPa and 33.7 77.9%, which are also much higher than those reported in other issues [48,49]. 3.6. Recovery of the crystallizable diluent DMSO2 DMSO2 is a crystallizable diluent, so it can be recovered from the nascent membrane by recrystallization or sublimation. Herein, we choose methanol, ethanol and water as the recrystallization solvents. The recovery efficiencies of DMSO2 by recrystallization from ethanol, methanol and water are 17.9%, 43.1% and 21.8% respectively, which are lower than that by sublimation at 353 K (88.9%) (Table 3). It means that we can reuse the diluent by various recovery paths. The recovery of diluent may cut the cost of membrane preparation and reduce the pollution of diluent.
4. Conclusion DMSO2 has been proved to be a universal crystallizable diluent for polar polymer membranes via TIPS. The applicable polymers include PVDF, PAN and CA. Tubular-like pores are obtained in PVDF
The research is financially supported by the National Natural Science Foundation of China (Grant no. 21174124). The authors also thank Dr. Mao Peng in Zhejiang University for helping the experiments of tensile test.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2013.07.008.
References [1] P.M. Atkinson, D.R. Lloyd, Anisotropic flat sheet membrane formation via TIPS: thermal effects, J. Membr. Sci. 171 (2000) 1–18. [2] H. Matsuyama, S. Berghmans, M.T. Batarseh, D.R. Lloyd, Effects of thermal history on anisotropic and asymmetric membranes formed by thermally induced phase separation, J. Membr. Sci. 142 (1998) 27–42. [3] P. van de Witte, P.J. Dijkstra, J.W.A. van den Berg, J. Feijen, Phase separation processes in polymer solutions in relation to membrane formation, J. Membr. Sci. 117 (1996) 1–31. [4] Z. Song, M. Xing, J. Zhang, B. Li, S. Wang, Determination of phase diagram of a ternary PVDF/γ-BL/DOP system in TIPS process and its application in preparing hollow fiber membranes for membrane distillation, Sep. Purif. Technol. 90 (2012) 221–230. [5] S. Liu, C. Zhou, W. Yu, Phase separation and structure control in ultra-high molecular weight polyethylene microporous membrane, J. Membr. Sci. 379 (2011) 268–278. [6] A. Cui, Z. Liu, C. Xiao, Y. Zhang, Effect of micro-sized SiO2-particle on the performance of PVDF blend membranes via TIPS, J. Membr. Sci. 360 (2010) 259–264. [7] J.J. Blaker, V. Maquet, R. Jérôme, A.R. Boccaccini, S.N. Nazhat, Mechanical properties of highly porous PDLLA/Bioglasss composite foams as scaffolds for bone tissue engineering, Acta Biomater. 1 (2005) 643–652. [8] P.L. Hanks, D.R. Lloyd, Deterministic model for matrix solidification in liquid– liquid thermally induced phase separation, J. Membr. Sci. 306 (2007) 125–133. [9] S.C. Roh, M.J. Park, S.H. Yoo, C.K. Kim, Changes in microporous structure of polyethylene membrane fabricated from PE/PTMG/paraffin ternary mixtures, J. Membr. Sci. 411–412 (2012) 201–210. [10] T. Tanaka, T. Nishimoto, K. Tsukamoto, M. Yoshida, T. Kouya, M. Taniguchi, D. R. Lloyd, Formation of depth filter microfiltration membranes of poly(L-lactic acid) via phase separation, J. Membr. Sci. 396 (2012) 101–109. [11] J. Yang, D.W. Li, Y.K. Lin, X.L. Wang, F. Tian, Z. Wang, Formation of a bicontinuous structure membrane of polyvinylidene fluoride in diphenyl ketone diluent via thermally induced phase separation, J. Appl. Polym. Sci. 110 (2008) 341–347. [12] Y. Lin, Y. Tang, H. Ma, J. Yang, Y. Tian, W. Ma, X. Wang, Formation of a bicontinuous structure membrane of polyvinylidene fluoride in diphenyl carbonate diluent via thermally induced phase separation, J. Appl. Polym. Sci. 114 (2009) 1523–1528. [13] W. Yave, R. Quijada, D. Serafini, D.R. Lloyd, Effect of the polypropylene type on polymer–diluent phase diagrams and membrane structure in membranes formed via the TIPS process: Part I. Metallocene and Ziegler–Natta polypropylenes, J. Membr. Sci. 263 (2005) 146–153. [14] R. Lv, J. Zhou, Q. Du, H. Wang, W. Zhong, Preparation and characterization of EVOH/PVP membranes via thermally induced phase separation, J. Membr. Sci. 281 (2006) 700–706.
H.-Q. Liang et al. / Journal of Membrane Science 446 (2013) 482–491
[15] M. Gu, J. Zhang, X. Wang, W. Ma, Crystallization behavior of PVDF in PVDFDMP system via thermally induced phase separation, J. Appl. Polym. Sci. 102 (2006) 3714–3719. [16] I.J. Roh, S. Ramaswamy, W.B. Krantz, A.R. Greenberg, Poly(ethylene chlorotrifluoroethylene) membrane formation via thermally induced phase separation (TIPS), J. Membr. Sci. 362 (2010) 211–220. [17] C. Funk, P. Hanks, K. Kaczorowski, D. Lloyd, Diluent crystal alignment in the formation of membranes via liquid–solid thermally induced phase separation, J. Porous Mater. 16 (2009) 453–458. [18] Q.-Y. Wu, L.-S. Wan, Z.-K. Xu, Structure and performance of polyacrylonitrile membranes prepared via thermally induced phase separation, J. Membr. Sci. 409–410 (2012) 355–364. [19] H. Matsuyama, M.-M. Kim, D.R. Lloyd, Effect of extraction and drying on the structure of microporous polyethylene membranes prepared via thermally induced phase separation, J. Membr. Sci. 204 (2002) 413–419. [20] H. Matsuyama, H. Okafuji, T. Maki, M. Teramoto, N. Kubota, Preparation of polyethylene hollow fiber membrane via thermally induced phase separation, J. Membr. Sci. 223 (2003) 119–126. [21] J. Yoon, A.J. Lesser, T.J. McCarthy, Locally anisotropic porous materials from polyethylene and crystallizable diluents, Macromolecules 42 (2009) 8827–8834. [22] C. Zhang, Y. Bai, Y. Sun, J. Gu, Y. Xu, Preparation of hydrophilic HDPE porous membranes via thermally induced phase separation by blending of amphiphilic PE-b-PEG copolymer, J. Membr. Sci. 365 (2010) 216–224. [23] H. Matsuyama, M. Yuasa, Y. Kitamura, M. Teramoto, D.R. Lloyd, Structure control of anisotropic and asymmetric polypropylene membrane prepared by thermally induced phase separation, J. Membr. Sci. 179 (2000) 91–100. [24] H. Matsuyama, T. Maki, M. Teramoto, K. Asano, Effect of polypropylene molecular weight on porous membrane formation by thermally induced phase separation, J. Membr. Sci. 204 (2002) 323–328. [25] B. Luo, Z. Li, J. Zhang, X. Wang, Formation of anisotropic microporous isotactic polypropylene (iPP) membrane via thermally induced phase separation, Desalination 233 (2008) 19–31. [26] W. Yave, R. Quijada, M. Ulbricht, R. Benavente, Syndiotactic polypropylene as potential material for the preparation of porous membranes via thermally induced phase separation (TIPS) process, Polymer 46 (2005) 11582–11590. [27] S.-W. Song, J.M. Torkelson, Coarsening effects on the formation of microporous membranes produced via thermally induced phase separation of polystyrenecyclohexanol solutions, J. Membr. Sci. 98 (1995) 209–222. [28] C. Gao, A. Li, X. Yi, J. Shen, Construction of cell-compatible layer and culture of human umbilical vascular endothelial cells on porous polystyrene membranes, J. Appl. Polym. Sci. 81 (2001) 3523–3529. [29] S. Rajabzadeh, T. Maruyama, T. Sotani, H. Matsuyama, Preparation of PVDF hollow fiber membrane from a ternary polymer/solvent/nonsolvent system via thermally induced phase separation (TIPS) method, Sep. Purif. Technol. 63 (2008) 415–423. [30] X. Li, G. Xu, X. Lu, C. Xiao, Effects of mixed diluent compositions on poly (vinylidene fluoride) membrane morphology in a thermally induced phaseseparation process, J. Appl. Polym. Sci. 107 (2008) 3630–3637. [31] G.-L. Ji, C.-H. Du, B.-K. Zhu, Y.-Y. Xu, Preparation of porous PVDF membrane via thermally induced phase separation with diluent mixture of DBP and DEHP, J. Appl. Polym. Sci. 105 (2007) 1496–1502. [32] M. Shang, H. Matsuyama, M. Teramoto, D.R. Lloyd, N. Kubota, Preparation and membrane performance of poly(ethylene-co-vinyl alcohol) hollow fiber
[33]
[34] [35]
[36] [37]
[38] [39]
[40]
[41] [42]
[43]
[44]
[45]
[46]
[47] [48]
[49]
[50] [51] [52] [53]
491
membrane via thermally induced phase separation, Polymer 44 (2003) 7441–7447. J. Zhou, H. Zhang, H. Wang, Q. Du, Effect of cooling baths on EVOH microporous membrane structures in thermally induced phase separation, J. Membr. Sci. 343 (2009) 104–109. P.D. Graham, A.J. Pervan, A.J. McHugh, The dynamics of thermal-induced phase separation in PMMA solutions, Macromolecules 30 (1997) 1651–1655. H. Matsuyama, Y. Takida, T. Maki, M. Teramoto, Preparation of porous membrane by combined use of thermally induced phase separation and immersion precipitation, Polymer 43 (2002) 5243–5248. T. Tanaka, D.R. Lloyd, Formation of poly(L-lactic acid) microfiltration membranes via thermally induced phase separation, J. Membr. Sci. 238 (2004) 65–73. K.C. Shin, B.S. Kim, J.H. Kim, T.G. Park, J.D. Nam, D.S. Lee, A facile preparation of highly interconnected macroporous PLGA scaffolds by liquid–liquid phase separation II, Polymer 46 (2005) 3801–3808. T. Clark, J. Murray, P. Lane, P. Politzer, Why are dimethyl sulfoxide and dimethyl sulfone such good solvents? J. Mol. Model. 14 (2008) 689–697. M.E. Vanegas, R. Quijada, D. Serafini, Microporous membranes prepared via thermally induced phase separation from metallocenic syndiotactic polypropylenes, Polymer 50 (2009) 2081–2086. C.V. Funk, B.L. Beavers, D.R. Lloyd, Effect of particulate filler on cell size in membranes formed via liquid–liquid thermally induced phase separation, J. Membr. Sci. 325 (2008) 1–5. P.J. Flory, Principle of Polymer Chemistry, Cornell University Press, Ithaca, New York, 1953. W.R. Burghardt, Phase diagrams for binary polymer systems exhibiting both crystallization and limited liquid–liquid miscibility, Macromolecules 22 (1989) 2482–2486. M.H. Rahman, A.K. Nandi, Supramolecular organization of poly(vinylidene fluoride)–camphor sulfonic acid blends:giant spherulites, Langmuir 19 (2003) 7056–7060. D. Dasgupta, S. Manna, S. Malik, C. Rochas, J.M. Guenet, A.K. Nandi, Thermodynamic structural and morphological investigation of poly(vinylidene fluoride)–camphor systems, preparing porous gels from a solid solvent, Macromolecules 38 (2005) 5602–5608. Q.-Y. Wu, X.-N. Chen, L.-S. Wan, Z.-K. Xu, Interactions between polyacrylonitrile and solvents: density functional theory study and two-dimensional infrared correlation analysis, J. Phys. Chem. B 116 (2012) 8321–8330. H. Zhou, G.L. Wilkes, Orientation-dependent mechanical properties and deformation morphologies for uniaxially melt-extruded high-density polyethylene films having an initial stacked lamellar texture, J. Mater. Sci. 33 (1998) 287–303. A. Galeski, Strength and toughness of crystalline polymer systems, Prog. Polym. Sci. 28 (2003) 1643–1699. G.-L. Ji, B.-K. Zhu, Z.-Y. Cui, C.-F. Zhang, Y.-Y. Xu, PVDF porous matrix with controlled microstructure prepared by TIPS process as polymer electrolyte for lithium ion battery, Polymer 48 (2007) 6415–6425. Y. Tang, Y. Lin, W. Ma, Y. Tian, J. Yang, X. Wang, Preparation of microporous PVDF membrane via tips method using binary diluent of DPK and PG, J. Appl. Polym. Sci. 118 (2010) 3518–3523. W.M. Haynes, CRC Handbook of Chemistry and Physics, 91st ed., CRC press/ Tayler and Francis, Boca Laton, FL. E.M. James, Polymer Data Handbook, Oxford University Press, Oxford, 1999. D.W.V. Krevelen, Properties of Polymers, 4th ed., Elsevier, B.V., Amsterdam, 2009. A.J. Lovinger, Ferroelectric polymers, Science 220 (1983) 1115–1121.