Journal of Membrane Science 446 (2013) 482–491

Contents lists available at ScienceDirect

Journal of Membrane Science

0376-73http://d

AbbreS�L, sofluorideDSC, difmicrosc

n CorrE-m

journal homepage: www.elsevier.com/locate/memsci

Polar polymer membranes via thermally induced phase separationusing 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

a r t i c l e i n f o

Article history:Received 26 February 2013Received in revised form21 June 2013Accepted 7 July 2013Available online 12 July 2013

Keywords:Thermally induced phase separationUniversal diluentDimethyl sulfonePolar polymerPoly(vinylidene fluoride)

88/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.memsci.2013.07.008

viations: TIPS, thermally induced phase seplid� liquid; L�S, liquid�solid; S�S, solid�so); PAN, polyacrylonitrile; CA, cellulose acetateferential scanning calorimetry; FESEM, field eopyesponding author. Tel.: +86 571 8795 2605; fail address: xuzk@zju.edu.cn (Z.-K. Xu).

a b s t r a c t

Dimethyl sulfone (DMSO2) was used as a universal crystallizable diluent to prepare polar polymermembranes via thermally induced phase separation (TIPS). The polar polymers adopted herein includepoly(vinylidene fluoride) (PVDF), polyacrylonitrile (PAN) and cellulose acetate (CA). Intensive investiga-tion 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 phaseseparation mechanism for all the three polar polymer/diluent systems. Scanning electron microscopyobservations 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, waterflux and overall porosity become large when the membranes are prepared with low polymerconcentration or at small cooling rate. Results of tensile tests confirm that the mechanical strength ofthe 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 mayprovide a green preparation method to produce polar polymeric membranes via TIPS.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Thermally induced phase separation (TIPS) is a well-knownmethod to fabricate porous polymer membranes due to itsadvantages including easy control, low tendency for defectsformation, and diverse microstructures that are desirable forvarious applications of membrane. Since it had been introducedby Castro in 1980s, this method has attracted wide investigationsin respect of conducting process [1,2], phase separation mechan-ism [3], structure controlling [4,5], and properties enhancement[6,7]. Nevertheless, great efforts have been still made to drive thefurther development of TIPS both in theoretical and practicalaspects. Hanks et al. [8] reported a deterministic model for matrixsolidification in liquid� liquid (L�L) TIPS. They chose isotacticpolypropylene–diphenyl ether as a representative system to confirmthe simulation and found that the simple deterministic approach canprovide accurate prediction for the final cell size diameter anddistribution of isotropic L�L TIPS membranes. Roh et al. [9]

ll rights reserved.

aration; L�L, liquid� liquid;lid; PVDF, poly(vinylidene; DMSO2, dimethyl sulfone;mission scanning electron

ax: +86 571 8795 1773.

described a novel diluent mixture containing paraffin and poly(tetramethylene glycol) for the preparation of polyethylene mem-branes with controlled pore size and porosity. Tanaka et al. [10]combined the nonsolvent and thermally induced phase separation toprepare asymmetric poly(L-lactic acid) membrane. Their membraneswere suggested to be served as efficient and rapid depth filters todeal with bacterial cells.

Generally, a typical TIPS process is defined as follows: apolymer is dissolved into a high-boiling and low molecular weightdiluent at an elevated temperature, then the homogeneous solu-tion is cooled to induce phase separation, and a microporousstructure can be created after removing the diluent. Diluent is oneof the dominant factors that determine specific phase separationprocess (e.g. L�L, solid� liquid (S�L), liquid�solid (L�S), andsolid�solid (S�S) demixing) and subsequently control the finalmicrostructures of membranes. These membranes usually havevarious 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 membranesfrom a number of polymers, including polyethylene [19–22],polypropylene [23–26], polystyrene [27,28], poly(vinylidene fluor-ide) (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 auniversal diluent to prepare porous membranes from polar poly-mers (such as polyacrylonitrile (PAN) and cellulose acetate (CA))

Table 1Physical properties of the polymers and diluent.

Properties Unit DMSO2 PVDF PAN CA

Molecular weight g/mol 94 110,000 (Mn) 277,000 (Mw) 80,000 (Mn) 190,000 (Mw) 92,000 (Mn) 210,000 (Mw)Degree of polymerization N/A 1719 1509 384Molar volume cm3/mol 75 36.4 45.6 189Density g/cm3 1.16 1.76 1.14 1.3Heat of fusion J/mol 18300a 6700c 5021 12600b

Melting temperaturea K 382 443 593 508Crystallization temperature K 365 409 N/A N/ASolubility parametera MPa1/2 29.9 23.2 26.0 27.8Dipole moment D 4.4d 1.9e 3.9e N/ADielectric constant 47.4a 8.4 6.5 7.0

a Ref. [50].b Represented by the data of cellulose butyrate, Ref. [51].c Ref. [52].d Ref. [38].e Ref. [53].

H.-Q. Liang et al. / Journal of Membrane Science 446 (2013) 482–491 483

via TIPS. One of the most important challenges is to search anappropriate diluent due to the crystallinity and high melting pointof polar polymers. Such a searching step is time and costconsuming that limits the application broadening of TIPS. Further-more, the interactions between a diluent and different polymersmay differ from each other significantly, which will lead to quitedifferent behaviors of solubility and phase separation.

In this study, a series of porous membranes were preparedfrom different polar polymers including PVDF, PAN and CA viaTIPS, in which dimethyl sulfone (DMSO2) was successfully intro-duced as a universal crystallizable diluent. DMSO2 is a widely usedpolar solvent with high boiling point, especially valuable in hightemperature condition [38]. Theoretically, DMSO2 has a solubilityparameter close to those of PVDF, PAN and CA (Table 1), indicatingthat homogeneous solutions are reasonable to be obtained atelevated temperature. On the other hand, DMSO2 will crystallizeand precipitate from the solutions as temperature falls to 365 K[18], and phase separation can be expected. Therefore, it is feasibleto use DMSO2 as a universal diluent for these polar polymermembranes via TIPS. Based on this consideration, we studied thephase separation mechanisms of PVDF/DMSO2, PAN/DMSO2, CA/DMSO2 binary systems from the aspects of thermodynamic theoryand experiments. We also focused on the effects of polymerconcentration and cooling bath on the pore size, surface porosity,water flux, overall porosity, and mechanical properties of theresultant membranes. Furthermore, DMSO2 as a non-toxic diluentwas effectively recovered by recrystallization or sublimationherein, which potentially cuts the cost of membrane preparationand reduces environmental pollution from diluent. The applicationof crystallizable diluent may provide a novel solution to mem-brane structure control and a green strategy for the fabrication ofpolar polymer membranes.

2. Experimental

2.1. Materials

DMSO2 (99%) was purchased from Dakang Chemicals Co., China.PVDF (Mn¼110,000 g/mol, Solefs 6010) was a commercial productof Solvay Solexis, Belgium. PAN (Mn¼80,000 g/mol) was suppliedby Anqing Petroleum Chemical Co., China. CA (Mn¼92,000 g/mol)was obtained from Sinopharm Chemical Reagent Co., Ltd. All thepolymer powders were dried at 333 K under vacuum for 4 h beforeuse. Deionized water was used as the extractant. Other reagentswere purchased from Sinopharm Chemical Reagent Co., Ltd. andused without further purification.

2.2. Preparation of membranes

One polymer/DMSO2 mixture was heated at 433 K to form ahomogeneous solution. After degassing air bubbles, the solutionwas quickly poured onto a stainless steel mold (thickness�200 μm), which was preheated in an oven at 433 K. The moldwas then quenched in a cooling bath (water bath at 277 K and303 K or air bath at 303 K) to induce the solution to phaseseparation. Afterwards, the obtained nascent membrane was takenout of the mold and immersed in deionized water. A wetmembrane was formed as soon as the diluent was completelyextracted. To gain a dry membrane, the wet one was washed withan ethanol–hexane sequence, and then dried in vacuum for 24 h at333 K.

2.3. Determination of phase behavior

Differential scanning calorimetry (DSC, Q1000, TA instruments,USA) was used to determine the melting and crystallization tem-peratures of the mixture. Sample was prepared by directly quenchingthe polymer/diluent solution into liquid nitrogen, which preventedthe macroscopic phase separation. Solid sample of 6–8 mg washermetically sealed in an aluminum DSC pan, heated from 25 to473 K at 20 K/min, and maintained at 473 K for 3 min to eliminatethermal history. It was then cooled to 0 K at 10 K/min and reheatedimmediately to 473 K at 10 K/min. Peak maxima in the cooling andreheating sequences were regarded as the crystallization tempera-ture and the melting point, respectively.

The phase separation process was visualized by an opticalmicroscope (Nikon Eclipse E600POL, Japan). A small section ofpolymer/diluent solid sample was placed between a pair ofmicroscope slides, and put on a hot stage (Linkam TMS-93) witha temperature controller (Linkam THMS-600). It was heated at30 K/min to 473 K, maintained for 3 min, and then cooled to 273 Kat 10 K/min. The field of vision was recorded at the moment ofsolidification.

2.4. Wide-angle X-ray diffraction (WAXD)

WAXD was carried out on a Rigaku D/Max-2550PC X-raydiffractometer (Panalytical, Netherlands). The radial scans at avoltage of 40 kV and a current of 40 mA using a Cu:Ka radiationwere employed on the samples. Data were collected at 0.01671interval with counting for 10 s at each step.

H.-Q. Liang et al. / Journal of Membrane Science 446 (2013) 482–491484

2.5. Field emission scanning electron microscopy (FESEM)observation

Membrane morphologies of cross-section and surfaces wereexamined by FESEM (Hitachi S4800, Japan) with an acceleratingvoltage of 10 kV. To obtain a tidy cross-section, the membranesample was freeze-fractured in liquid nitrogen. All samples weregold 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 mea-sured in a pressure driven filtration cell. Membrane samples werecarefully wetted with ethanol before measurement to allow waterimpregnating the membrane pores. The membrane was firstinstalled in the permeation cell and compacted by filtering ultra-pure water at 0.12 MPa and 295 K for 30 min. Then the pressurewas lowered to 0.10 MPa for operation. Each sample was repeatedat least three parallel experiments. Pure water flux (Jw) wascalculated by the following equation:

Jw ¼ VAΔt

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 imagesof the cross-section of membranes using an Image tool [39]. As forthe tubular pores, their width was defined as the pore size in thiswork. Surface porosity of each membrane was measured by animage analysis software of Image-Pro Plus Version 6.0 [40]. Theoverall porosity is defined as the volume of pores divided by thetotal volume of membrane. A wet membrane was first weighed assoon as the superficial water on the membrane surface wasremoved by dry filter paper. Afterward, the wet membrane wasdried through the process as mentioned in Section 2.2. The weightof dry membrane was measured again. Porosity of the membranewas denoted as P, which was calculated with the gravimetricmethod

P ¼ ðw0�w1Þ=ρwater

ðw0�w1Þ=ρwater þw1=ρ100 ð2Þ

where w0 is the weight of the wet membrane, w1 is the weight ofthe dry membrane, and ρwater or ρ is the density of water orpolymer, respectively.

2.8. Tensile test

Tensile strength and elongation for the membranes wereevaluated in a stress–strain test using a tensile test instrument(RGM-4000, Shenzhen REGER Instrument Co., Ltd., China). The testwas carried out at a strain rate of 5 mm/min at 293 K and a relativehumidity of �74%. The membrane samples were prepared inrectangle shape with a gauge length of 20 mm and a width of10 mm. The thickness was around 200 μm and exactly examinedby a thickness gauge (CH-1-S/ST, Shanghai Liuling InstrumentPlant, China). Each data was the average of at least three parallelexperiments.

2.9. Recovery of crystallizable diluent

DMSO2 was recovered from the nascent membrane by recrys-tallization or sublimation. The nascent membrane was dissolved in

a smallest amount of solvent at 343 K to form a saturated solutionwhen using the recrystallization method. The solvents adoptedhere include ethanol, methanol and water. After filtration, thesolution was cooled at �255 K for 24 h, while the water solutionwas cooled at 273 K. Needle-like crystals of DMSO2 were recov-ered by filtration. Another recovery method was sublimation. Thenascent membrane was placed in a sublimation apparatus andthen heated to 353 K under vacuum. Solid DMSO2 inside themembrane was gradually sublimated and condensed on a cooledsurface. At last, purified DMSO2 can be collected from the cooledsurface.

3. Results and discussion

3.1. Phase diagrams of the polymer/diluent systems

Generally, phase separation mechanism is the dominant factorto the final membrane morphology and properties. Accordingly, itis necessary to predict the possible phase separation behavior.Equilibrium phase diagrams can be calculated according to theFlory–Huggins solution thermodynamics. Theoretically, the melt-ing point of polymer would be depressed in the presence ofdiluent [41]. The melting point depression of polymer can becalculated using Eq. (3) [21,42]

1Tm

¼ 1þ RβΔHu

Vu

V1

� �ð1�ϕ2Þ2

� ��1

� 1

T0m

þ RΔHu

Vu

V1

� �1� 1

N

� �ð1�ϕ2Þ�

lnðϕ2ÞN

� ��"ð3Þ

where Tm is the melting point of polymer in a solution, T0m is the

melting point of neat polymer, ΔHu is the enthalpy of fusion perrepeat unit of polymer, Vu is the molar volume of polymer, N is thedegree of polymerization, ϕ2 is the volume fraction of polymer, R isthe gas constant, and β is a constant which is calculated from thesolubility parameters of polymer (δ2) and diluent (δ1) along withthe molar volume of diluent (V1), using the equation (δ1�δ2)2V1/R.

Similarly, Eq. (4) can be used to calculate the melting pointdepression of a crystallizable diluent [42]

1Tm;1

¼ 1þ Rβϕ22

ΔH1

" #�11

T0m;1

� RΔH1

1� 1N

� �ϕ2 þ lnð1�ϕ2Þ

� �" #ð4Þ

where Tm,l is the melting point of diluent in a polymer solution,T0m;1 is the melting point of neat diluent, ΔH1 is the enthalpy of

fusion of neat diluent. Table 1 lists the parameters used forcalculation.

Fig. 1(a–c) shows typical DSC heating thermograms for thePVDF/DMSO2, PAN/DMSO2, and CA/DMSO2 binary systems.In each curve, a peak around 383 K indicates the melting point ofDMSO2 in the mixture. These experimental melting points ofDMSO2 are in good agreement with those calculated results(Fig. 1(d–f)). It is clear that another peak at relatively low tempera-ture exists in the PAN/DMSO2 and CA/DMSO2 mixtures, which mayresult from the interaction between polymers and diluent. Interest-ingly, almost no signal is found for the melting of polymers, thoughthey are all semi-crystalline polymers. Theoretically, the meltingpoints of PVDF and PAN are above those of DMSO2 in the mixturesas indicated by the dash lines in Fig. 1(d, e). Actually, the meltedDMSO2 is a good solvent for all the polymers. PVDF and PAN wouldnot preferentially crystallize from the solution unless DMSO2crystallizes when the temperature reach the melting temperatureof DMSO2. Meanwhile, the polymer immediately solidifies becauseof a large supercooling degree. Thus, the equilibrium phase diagramsuggests that both PVDF/DMSO2 and PAN/DMSO2 binary systemsundergo solid–solid phase separation. In contrast, an eutectic point

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) meltingpoints (■, ▲) plotted on the calculated equilibrium phase diagram. The dash lines in (d–f) represent the theoretical melting curves of polymers, while the solid lines representthe 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.

H.-Q. Liang et al. / Journal of Membrane Science 446 (2013) 482–491 485

H.-Q. Liang et al. / Journal of Membrane Science 446 (2013) 482–491486

is calculated in CA/DMSO2 system with 50 wt% polymer, whichconsists with the invariable temperature of its second melting pointas mentioned above. It indicates that CA/DMSO2 system thermo-dynamically goes through liquid–solid phase separation. Real-timePOM was used to observe the cooling process of the polymer/DMSO2 mixtures at a rate of 10 1C/min (Figs. S1 and S2 inSupporting Information). Neither crystallization nor liquid–liquidphase separation takes place at the initial stage upon cooling. Whenthe temperature reaches the crystallization temperature of DMSO2,the samples solidify in a few seconds and giant spherulites areformed, in which no preferential crystallization has been observed[43,44]. This result is in well consistent with the crystalline behaviorsof the three systems, which show only one crystalline peak uponcooling (Fig. S3 in Supporting Information). WAXD was then used toconfirm the crystallization of both components in the three binarysystems (Fig. S4 in Supporting Information). Combining with theresults of DSC, POM, and WAXD, we can confirm a solid–solid phaseseparation mechanism for all the three systems. Among others, this isa result of dynamics for CA/DMSO2 systemwhich may overcome thenarrow 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 of20 wt%. Irregular tubular pores are observed in PVDF and PANmembranes due to the crystallization of DMSO2. As mentionedabove, both PVDF/DMSO2 and PAN/DMSO2 binary systems undergo

Fig. 3. FESEM images of PVDF membranes prepared from 10–30 wt% PVDF: (a) cross-secprepared in water bath at 303 K.

solid–solid phase separation. The membrane pore morphology willmainly depend on the crystal structure of diluent. Upon cooling,needle-like crystals of DMSO2 will form. Thus tubular pores will beobtained after subsequent extraction of diluent. Furthermore, thetubular pores are irregularly distributed in PVDF and PAN membranes,as the nucleation of diluent crystals is quite random without anyapplied temperature gradient. However, this kind of morphologycannot be found in the CA membrane. It has been reported that thesulfur-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 highlynegative characters of the oxygens, which can form strong hydrogenbonds with residual hydroxyl groups of CA. Nevertheless, PVDF or PANmay interact with DMSO2 through dipole–dipole interaction [44,45],which is much weaker than the hydrogen bonding interaction. Giventhe dipole moments and dielectric constants of the polymers (Table 1),we can conclude that the polymer�DMSO2 interaction is in the orderof CA4PAN4PVDF. This result is in line with the melting pointdepression as mentioned above. Since the PVDF�DMSO2 interactionis weak, DMSO2 easily crystallizes and separates from the polymersolution. Accordingly, the average pore size of PVDF membranes(2.1 μm) is larger than that of PAN membranes (0.3 μm). There aresmall pores on the wall of the tubular pores in PVDF membrane,suggesting high interconnection between pores. Otherwise, owing tothe strong CA�DMSO2 interaction, the crystallization of DMSO2 ismostly inhibited, and a weak phase separation leads to a compactedstructure. On the other hand, the surface of PVDF membrane is full ofnanopores (�21.5 nm), while dense surfaces can be seen for PAN and

tion (250� ), (b) cross-section (2000� ), and (c) top-surface. The membranes were

H.-Q. Liang et al. / Journal of Membrane Science 446 (2013) 482–491 487

CA membranes (Fig. 2(c)). Besides, the PAN membrane surface hasregular “stripes” with a certain orientation, which may also be causedby the crystallization of DMSO2 [18].

3.3. Effects of polymer concentration on the membrane structures

Fig. 3 presents the morphologies of PVDF membranes preparedwith different polymer concentrations. The pore size reduces from3.5 μm to 1.5 μm with an increase of PVDF concentration from10 wt% to 30 wt% (Fig. 4(a)). The surface porosity also dramaticallydecreases with the PVDF concentration. It may be caused by theincreasing viscosity of the PVDF/DMSO2 solution, which willprevent the crystallization of DMSO2 and then the pores become

Fig. 4. Surface porosity and pore size of PVDF, PAN and CA membranes varied with polymin water bath at 303 K, and those in (b) were prepared with polymer concentration of

Fig. 5. FESEM images of PVDF membranes cooled in different cooling baths: (a) cross-seprepared from 20 wt% PVDF.

small as a consequence [18]. PAN and CA membranes show asimilar tendency on their morphology (Figs. S5 and S6 in Support-ing Information).

3.4. Effects of cooling rate on the membrane structures

Except for polymer concentration, cooling rate also has greatimpact on the membrane morphology since TIPS is a dynamic-controlled process. Fig. 5 indicates that the tubular pores of PVDFmembranes become large as the temperature of water bathincreases. PVDF membrane prepared in 30 1C water has pore sizeof approximately 2.1 μm and surface porosity of about 6.2%, whichare larger than those prepared in 277 K water (�1.3 μm and 4.8%)

er concentration (a) and cooling bath (b). All the membranes in (a) were prepared20 wt%.

ction (300� ), (b) cross section (2000� ), and (c) top-surface. The membranes were

H.-Q. Liang et al. / Journal of Membrane Science 446 (2013) 482–491488

(Fig. 4(b)). The membranes even have pore size of �4.1 μm andsurface porosity of �7.8% when prepared in 303 K air bath. Air hassmaller thermal capacity than water, so the cooling of solution inair is slower than that in water. As the cooling rate decreases,DMSO2 crystals have long time to grow, leading to larger poresizes and surface porosity in the as-prepared membranes. Similarresults are found in PAN and CA membranes, especially for thoseCA membranes (Figs. S7 and S8 in Supporting Information). CAmembranes show compacted structure and have almost no poreswhen quenched in 277 K water bath. If 303 K air bath is used, themembranes have large pores (�2.3 μm) and high surface porosity(15.8%). Moreover, an orientated texture can be visualized on thesurface of PVDF and PAN membranes when quenched in 303 Kair bath.

3.5. Water flux and mechanical properties of PVDF, PAN and CAmembranes

We have studied the effects of polymer concentration andcooling rate on the water flux and overall porosity of PVDF, PAN,and CA membranes (Figs. 6 and 7). The results demonstrate thatwater flux and overall porosity of all the membranes increase with

Fig. 6. Porosity (a) and water flux (b) of PVDF, PAN and CA membranes in differentCA 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 differe

the decrease of polymer concentration or cooling rate. It is dueto the large pore size and surface porosity at low polymerconcentration or cooling rate (Fig. 4). Among others, PVDF mem-branes possess the highest water flux (1491 L/m2 �h) and overallporosity (91.1%) at the optimum polymer concentration (10 wt%)and cooling condition (303 K water bath). However, the water fluxof PAN or CA membranes is quite small though their porosities arestill quite high. Firstly, low surface porosity may account for thelow water flux of these two kinds of membranes. Secondly, thepore structures in the cross-section have great effect: the pores arealmost parallel to the surface of PAN membranes with small poresize, and CA membranes are too compacted.

Mechanical properties are crucial to the application of mem-branes. Fig. 8(a) shows the stress�strain curves of PVDF mem-branes prepared from different polymer concentrations. The stressstrongly increases with the strain at the initial stage. Then a non-typical yielding occurs when the strain reaches the yield point. Atthe last stage of deformation, the samples fail by tearing quickly atthe end of the clamps, characterized by downward tails at the endof the respective stress�strain curve. Fig. 8(b) shows the corre-sponding tensile strength and elongation results, indicating that themembranes prepared from higher polymer concentration have

polymer concentrations. The membranes were prepared in water bath at 303 K.

nt cooling conditions. The membranes were prepared from 20 wt% polymer.

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 Kwater, 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′) wereperpendicular and parallel to the direction of orientation observed on the membrane surface, respectively.

Table 2Mechanical properties of the membranes prepared in different conditions.

Membrane Polymer concentration (wt%) Cooling bath (K) Thickness (μm) Elongation (%) Tensile strength (MPa)

PVDF 10 303 188.879.0 16.071.4 0.970.320 303 284.078.7 22.875.3 2.870.130 303 270.378.4 33.777.9 7.571.320 277 174.7728.6 43.4719.2 3.070.220c 303b 227.7727.3 62.5712.5 2.570.320d 303b 231.0731.1 42.5712.1 3.871.1

PAN 10c 303 145.3717.7 4.170.4 7.670.810d 303 143.077.2 3.970.4 3.770.420 303 182.773.2 4.471.7 7.772.030 303 252.0737.2 4.972.2 16.574.320 277 179.7735.5 4.870.8 11.773.620c 303b 179.775.9 8.372.4 15.870.720d 303b 197.7712.0 0.770.1 3.070.5

CA 10a 303 N/A N/A N/A20 303 164.3715.8 10.072.2 46.973.430 303 157.775.1 11.274.1 39.072.020 277 95.774.7 6.271.8 41.879.420 303b 185.0710.6 1.270.2 7.570.6

a The membrane from 10 wt% CA is too brittle to prepare a sample for tensile test.b The cooling medium is air, while others are water.c 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.

H.-Q. Liang et al. / Journal of Membrane Science 446 (2013) 482–491 489

better tensile strength and elongation. We also studied the effects ofcooling rate on the tensile strength and elongation of PVDFmembranes (Fig. 9). It shows that the tensile strength and

elongation of the membranes quenched in 303 K water bath islower than those obtained in 277 K water bath. This result is due tothe large pores induced by lower cooling rate. Particularly, the

Table 3Recovery results of DMSO2 from the nascent PVDF membranes.

No. Recovery procedure Extractionefficiencya (E, %)

Recoveryyieldb (Y, %)

1 1) Extracted by 30 mL ethanol at338 K for 6 h.

2) Recrystallized at �255 K for24 h.

91.8 17.9

2 1) Extraction by 25 mL methanolat 323 K for 6 h.

2) Recrystallized at �255 K for24 h.

90.5 43.1

3 1) Extracted by 20 mL water at338 K for 6 h.

2) Concentrated at 348 K.3) Recrystallized at 277 K for 24 h.

91.8 21.8

4 Sublimated at 353 K and 0.1 MPa for7 h

91.2 88.9

a Extraction yield is calculated by equation of E¼(1�m1/m0)�100, wherem0 isthe weight of nascent membrane, and m1 is the weight of membrane afterextraction.

b Recovery yield is calculated by equation of Y¼(m3/m2)�100, where m2 is thetheoretical weight of DMSO2 in membrane, and m3 is the weight of recoveredDMSO2 crystals.

H.-Q. Liang et al. / Journal of Membrane Science 446 (2013) 482–491490

membranes also exhibit an anisotropic mechanical propertybecause of the orientated texture, which has been reported inprevious work [46,47].

Table 2 summarizes the specific values of the mechanicalproperties for PVDF, PAN, and CA membranes prepared underdifferent conditions. The tensile strength and elongation for PANor CA membranes increase with polymer concentration or coolingbath. Similarly, PAN membranes show the anisotropic mechanicalproperty for the case prepared in 10 wt% polymer concentration or303 K air bath. In contrast, the elongations of PAN and CAmembranes are much smaller than that of PVDF membraneprepared in the same condition, while the former ones show thehigher tensile strength and modulus than the latter one.It indicates that PVDF membranes are tough, while PAN or CAmembranes are hard and brittle. Among all the membranes, themembranes prepared from 30 wt% PVDF in 303 K water bath haveexhibited the most excellent mechanical properties. The tensilestrength and elongation have reached as high as 7.571.3 MPa and33.777.9%, which are also much higher than those reported inother issues [48,49].

3.6. Recovery of the crystallizable diluent DMSO2

DMSO2 is a crystallizable diluent, so it can be recovered fromthe nascent membrane by recrystallization or sublimation. Herein,we choose methanol, ethanol and water as the recrystallizationsolvents. The recovery efficiencies of DMSO2 by recrystallizationfrom 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 variousrecovery paths. The recovery of diluent may cut the cost ofmembrane preparation and reduce the pollution of diluent.

4. Conclusion

DMSO2 has been proved to be a universal crystallizable diluentfor polar polymer membranes via TIPS. The applicable polymersinclude PVDF, PAN and CA. Tubular-like pores are obtained in PVDF

and PAN membranes, whereas no such pores exist in the CAmembrane. The pore size and surface porosity of the threemembranes become larger when lower polymer concentration orcooling rate is submitted to the experiment. The water flux andporosity of all the membranes increase with the decline ofpolymer concentration or cooling rate. Among them, PVDF mem-brane possesses the highest porosity and water flux at theoptimum polymer concentration (10 wt%) and cooling condition(303 K water bath). We can also improve the tensile strength andelongation of membranes by increasing the polymer concentrationor cooling rate. Moreover, DMSO2 can be recovered by recrystalli-zation and sublimation. This work may provide a green prepara-tion method to produce microporous polymer membranesvia TIPS.

Acknowledgments

The research is financially supported by the National NaturalScience Foundation of China (Grant no. 21174124). The authors alsothank Dr. Mao Peng in Zhejiang University for helping the experi-ments of tensile test.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe 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 thermalhistory on anisotropic and asymmetric membranes formed by thermallyinduced 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 separationprocesses 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 aternary PVDF/γ-BL/DOP system in TIPS process and its application in preparinghollow 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-highmolecular 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 theperformance 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, Mechanicalproperties of highly porous PDLLA/Bioglasss composite foams as scaffolds forbone 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 ofpolyethylene 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-lacticacid) 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 abicontinuous structure membrane of polyvinylidene fluoride in diphenylketone 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 abicontinuous structure membrane of polyvinylidene fluoride in diphenylcarbonate 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 onpolymer–diluent phase diagrams and membrane structure in membranesformed via the TIPS process: Part I. Metallocene and Ziegler–Natta polypro-pylenes, J. Membr. Sci. 263 (2005) 146–153.

[14] R. Lv, J. Zhou, Q. Du, H. Wang, W. Zhong, Preparation and characterization ofEVOH/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) 482–491 491

[15] M. Gu, J. Zhang, X. Wang, W. Ma, Crystallization behavior of PVDF in PVDF-DMP 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 chlorotri-fluoroethylene) 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 theformation 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 polyacrylonitrilemembranes 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 thestructure of microporous polyethylene membranes prepared via thermallyinduced phase separation, J. Membr. Sci. 204 (2002) 413–419.

[20] H. Matsuyama, H. Okafuji, T. Maki, M. Teramoto, N. Kubota, Preparation ofpolyethylene 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 frompolyethylene and crystallizable diluents, Macromolecules 42 (2009)8827–8834.

[22] C. Zhang, Y. Bai, Y. Sun, J. Gu, Y. Xu, Preparation of hydrophilic HDPE porousmembranes via thermally induced phase separation by blending of amphi-philic PE-b-PEG copolymer, J. Membr. Sci. 365 (2010) 216–224.

[23] H. Matsuyama, M. Yuasa, Y. Kitamura, M. Teramoto, D.R. Lloyd, Structurecontrol of anisotropic and asymmetric polypropylene membrane prepared bythermally induced phase separation, J. Membr. Sci. 179 (2000) 91–100.

[24] H. Matsuyama, T. Maki, M. Teramoto, K. Asano, Effect of polypropylenemolecular weight on porous membrane formation by thermally inducedphase separation, J. Membr. Sci. 204 (2002) 323–328.

[25] B. Luo, Z. Li, J. Zhang, X. Wang, Formation of anisotropic microporous isotacticpolypropylene (iPP) membrane via thermally induced phase separation,Desalination 233 (2008) 19–31.

[26] W. Yave, R. Quijada, M. Ulbricht, R. Benavente, Syndiotactic polypropylene aspotential material for the preparation of porous membranes via thermallyinduced phase separation (TIPS) process, Polymer 46 (2005) 11582–11590.

[27] S.-W. Song, J.M. Torkelson, Coarsening effects on the formation of microporousmembranes produced via thermally induced phase separation of polystyrene-cyclohexanol solutions, J. Membr. Sci. 98 (1995) 209–222.

[28] C. Gao, A. Li, X. Yi, J. Shen, Construction of cell-compatible layer and culture ofhuman umbilical vascular endothelial cells on porous polystyrene mem-branes, J. Appl. Polym. Sci. 81 (2001) 3523–3529.

[29] S. Rajabzadeh, T. Maruyama, T. Sotani, H. Matsuyama, Preparation of PVDFhollow fiber membrane from a ternary polymer/solvent/nonsolvent systemvia 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 phase-separation 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 viathermally 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 andmembrane performance of poly(ethylene-co-vinyl alcohol) hollow fiber

membrane via thermally induced phase separation, Polymer 44 (2003)7441–7447.

[33] J. Zhou, H. Zhang, H. Wang, Q. Du, Effect of cooling baths on EVOHmicroporous membrane structures in thermally induced phase separation, J.Membr. Sci. 343 (2009) 104–109.

[34] P.D. Graham, A.J. Pervan, A.J. McHugh, The dynamics of thermal-inducedphase separation in PMMA solutions, Macromolecules 30 (1997) 1651–1655.

[35] H. Matsuyama, Y. Takida, T. Maki, M. Teramoto, Preparation of porousmembrane by combined use of thermally induced phase separation andimmersion precipitation, Polymer 43 (2002) 5243–5248.

[36] T. Tanaka, D.R. Lloyd, Formation of poly(L-lactic acid) microfiltration membranesvia thermally induced phase separation, J. Membr. Sci. 238 (2004) 65–73.

[37] K.C. Shin, B.S. Kim, J.H. Kim, T.G. Park, J.D. Nam, D.S. Lee, A facile preparation ofhighly interconnected macroporous PLGA scaffolds by liquid–liquid phaseseparation II, Polymer 46 (2005) 3801–3808.

[38] T. Clark, J. Murray, P. Lane, P. Politzer, Why are dimethyl sulfoxide anddimethyl sulfone such good solvents? J. Mol. Model. 14 (2008) 689–697.

[39] M.E. Vanegas, R. Quijada, D. Serafini, Microporous membranes prepared viathermally induced phase separation from metallocenic syndiotactic polypro-pylenes, Polymer 50 (2009) 2081–2086.

[40] C.V. Funk, B.L. Beavers, D.R. Lloyd, Effect of particulate filler on cell size inmembranes formed via liquid–liquid thermally induced phase separation, J.Membr. Sci. 325 (2008) 1–5.

[41] P.J. Flory, Principle of Polymer Chemistry, Cornell University Press, Ithaca, NewYork, 1953.

[42] W.R. Burghardt, Phase diagrams for binary polymer systems exhibiting bothcrystallization and limited liquid–liquid miscibility, Macromolecules 22 (1989)2482–2486.

[43] M.H. Rahman, A.K. Nandi, Supramolecular organization of poly(vinylidenefluoride)–camphor sulfonic acid blends:giant spherulites, Langmuir 19 (2003)7056–7060.

[44] D. Dasgupta, S. Manna, S. Malik, C. Rochas, J.M. Guenet, A.K. Nandi, Thermo-dynamic structural and morphological investigation of poly(vinylidene fluor-ide)–camphor systems, preparing porous gels from a solid solvent,Macromolecules 38 (2005) 5602–5608.

[45] Q.-Y. Wu, X.-N. Chen, L.-S. Wan, Z.-K. Xu, Interactions between polyacryloni-trile and solvents: density functional theory study and two-dimensionalinfrared correlation analysis, J. Phys. Chem. B 116 (2012) 8321–8330.

[46] H. Zhou, G.L. Wilkes, Orientation-dependent mechanical properties anddeformation morphologies for uniaxially melt-extruded high-density poly-ethylene films having an initial stacked lamellar texture, J. Mater. Sci. 33(1998) 287–303.

[47] A. Galeski, Strength and toughness of crystalline polymer systems, Prog.Polym. Sci. 28 (2003) 1643–1699.

[48] G.-L. Ji, B.-K. Zhu, Z.-Y. Cui, C.-F. Zhang, Y.-Y. Xu, PVDF porous matrix withcontrolled microstructure prepared by TIPS process as polymer electrolyte forlithium ion battery, Polymer 48 (2007) 6415–6425.

[49] Y. Tang, Y. Lin, W. Ma, Y. Tian, J. Yang, X. Wang, Preparation of microporousPVDF membrane via tips method using binary diluent of DPK and PG, J. Appl.Polym. Sci. 118 (2010) 3518–3523.

[50] W.M. Haynes, CRC Handbook of Chemistry and Physics, 91st ed., CRC press/Tayler and Francis, Boca Laton, FL.

[51] E.M. James, Polymer Data Handbook, Oxford University Press, Oxford, 1999.[52] D.W.V. Krevelen, Properties of Polymers, 4th ed., Elsevier, B.V., Amsterdam, 2009.[53] A.J. Lovinger, Ferroelectric polymers, Science 220 (1983) 1115–1121.