PVDF membranes based on electrospun nanofibers with controlled crosslinking induced by solvent vapor

Journal of Membrane Science 512 (2016) 1–12

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Microporous CA/PVDF membranes based on electrospun nanofibers with controlled crosslinking induced by solvent vapor Chunling Liu a, Xianfeng Li a,n, Tao Liu a, Zhen Liu a, Nana Li a, Yufeng Zhang a, Changfa Xiao a, Xianshe Feng a,b,n a State Key Laboratory of Separation Membranes and Membrane Processes, School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China b Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

art ic l e i nf o

a b s t r a c t

Article history: Received 3 February 2016 Received in revised form 30 March 2016 Accepted 31 March 2016 Available online 4 April 2016

In this study, a small amount of polyvinylidenefluoride (PVDF) was incorporated into cellulose acetate (CA) to electrospin CA/PVDF composite nanofiber membranes, followed by exposure to solvent vapor for crosslinking. Under controlled solvent vapor environment, the surfaces of the nanofibers were partially “melt,” and the contact points among the nanofibers formed physical crosslinks. It was demonstrated that the solvent vapor induced crosslinking was simple yet effective to enhance the mechanical strength of the resulting membranes. The degree of crosslinking could be controlled by adjusting the solvent vapor composition and the exposure time of the membrane in solvent vapor. To further reinforce the membrane, the CA/PVDF composite nanofiber membrane were electrospun onto a nonwoven substrate layer. The resulting membranes had pore sizes in the range of nm to mm. The membranes with a mean pore size of 0.75 mm and 0.59 mm exhibited a high pure water flux and good rejection to emulsion lattices when tested for emulsion separations. These results have revealed the potential of the membranes for separation and filtration applications. & 2016 Elsevier B.V. All rights reserved.

Keywords: Porous membrane Electrospinning Nanofiber Cross-linking Cellulose acetate

1. Introduction Cellulose acetate (CA) membranes are most widely used for various separation applications due to such advantageous characteristics as low prices, moderate chlorine resistance, good biocompatibility and hydrophilicity. In the past three decades, microporous CA membranes have been used in a broad spectrum of applications, including bioengineering, pharmaceutical and food industries [1–6], pretreatment prior to nanofiltration or reverseosmosis processes [2] and water treatment [7–9]. CA membranes in form of flat sheets or hollow fibers are traditionally fabricated method by the phase inversion technique [1–12]. Electrospinning is an emerging cost-effective technique for producing nanofibers with fiber diameters in the range of few microns down to few nanometers, which can be used to fabricate membranes in the mat form. Electrospun nanofiber membranes have the advantages of high specific surface areas, surface to volume ratios and porosities, as well as interconnected pore structures [13–15]. Being one of the most abundant renewable n

Corresponding author. E-mail addresses: [email protected] (X. Li), [email protected] (X. Feng). http://dx.doi.org/10.1016/j.memsci.2016.03.062 0376-7388/& 2016 Elsevier B.V. All rights reserved.

polymeric material, CA has attracted a great deal of attention as a material for electrospun nanofiber membranes [16–18]. Moreover, due to its low toxicity and high biocompatibility, electrospun CA nanofiber membranes are particularly suitable for uses in cell culture [19,20], transdermal drug delivery [20], tissue engineering scaffolds [20,21], removal of toxins from solutions [22] and bioprocessing and medical filtration [19,20,23,24]. However, electrospun CA nanofiber membranes typically have the drawbacks of weak bonding among the nanofibers that compromises the structural stability of the membrane, and large pore sizes and wide pore size distributions that affect the sharpness of separations. Several approaches have been attempted to enhance the bonding of nanofibers at the junction points in the nanofiber mats by “welding,” either chemically or physically. Similar to chemical cross-linking which can be allowed to occur during or after electrospinning of the nanofibers [25–27], physical cross-linking of the nanofibers can be achieved by heat-press treatment [28,29], using a solvent vapor [15] or by controlling the solvent composition of the electrospinning solution [30]. However, it has been found that electrospinning of nanofibers from CA alone was difficult to control especially when the CA electrospinning solution was prepared using such good volatile solvent as acetone because the as formed nanofibers could be partially melted by the solvent vapor in the spinning chamber,

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resulting in excessive physical crosslinking and even collapse of the three-dimensional structure. On the other hand, if the solvent is much less volatile the nanofiber jets may still be in wet state when they reach the collector due to limited solvent evaporation during the short travel distance from the needle tip to the collector, which will cause the fibers to stick together on the collector. In worse cases, the fiber jets may even break up and retract into beads before they reach the collector.To address this issue, we chose to incorporate a small amount of a polyvinylidene fluoride (PVDF) homopolymer into the CA electrospinning solution to improve the electrospninnability of the nanofibers, followed by posttreatment of the nanofiber membranes with a solvent vapor under controlled conditions (composition, temperature).The rationale for this approach was the following: (1) PVDF was much more stable than CA against the solvent vapor, and incorporating PVDF into CA enhanced the fabricability of the nanofiber membranes, (2) the presence of PVDF in the nanofibers acted as a frame to maintain the highly porous three-dimensional structure, and (3) the resulting nanofibers became more flexible because of the much lower glass transition temperature of PVDF than that of CA. In this study, a small amount of PVDF was blended with CA to improve electrospinnability of the nanofibrous membranes, followed by controlled crosslinking in a solvent vapor environment. A mixed solvent of acetone and N,N-dimethylacetamide (DMAc) was used in the electrospinning as well as the subsequent vapor treatment. The modified electrospinning allowed the nanofibers to crosslink in a short period of time and the porous structure of the resulting mat membrane was easily controlled. As a reinforcement to increase the mechanical strength of the membrane, the CA/ PVDF nanofibers were electrospun onto a PET nonwoven substrate. The resulting membranes were tested for water flux and emulsion rejection. Their morphology, porosity, hydrophilicity, surface elements and thermal properties were also investigated.

experimental setup of electrospinning. The electrospinning was carried out under a high voltage of 16 kV applied to the two needle spinnerets with an inner diameter of 0.40 mm, and the solution extrusion rate was 0.36 ml/h. The fibers were collected using a rotating drum collector of 6 cm in diameter, which was located 20 cm away from the spinnerets. The rotating speed of the fiber collector roller was set at 200 rpm. The electrospinning was operated at 25 °C and 30% relative humidity. For comparison purposes, at the same conditions, the CA and PVDF nanofiber membranes were also prepared separately from CA solution with a polymer concentration of 19 wt% in acetone and DMAc (volume ratio 2:1), and PVDF solution with a polymer concentration of 16 wt% in acetone and DMAc (volume ratio 1:2), respectively. 2.2.2. Solvent vapor-treatment The as-prepared CA/PVDF nanofibrous membranes were subjected to solvent vapor treatment. Liquid acetone and DMAc at various ratios (i.e., acetone/DMAc¼ 1/0, 2/1, 1/2, 0/1) were placed in glass bottles, and then the bottle mouths were covered with the as-prepared CA/PVDF nanofiber membranes with the nanofibers facing the solvent, as shown schematically in Fig. 2. This provided a direct contact between the nanofibers and the solvent vapor in the overhead space of the solvent containers. A cling wrap was placed on top of the membrane as a seal to prevent evaporative loss of the solvents. The bottle/membrane assembly was placed in a 50 °C environmental chamber for a given period of time 5– 90 min) to induce physical crosslinking of nanofibers.

2. Experimental 2.1. Materials Cellulose acetate (CA-398-30,acetyl content 39.8%) and PVDF (Solef 1015) were supplied by Eastman Chemical Company and Solvay Specialty Polymers, respectively. The polyester nonwoven fabric (PET, 20 g/m2) was provided from Rongwei Nonwovens Co. Ltd. (Yangzhou, China). Polyethylene (PE) cling wrap was obtained from the Top Group (Jiangsu, China). Acetone ( Z99.5 wt%) and DMAc (Z99.5 wt%) were purchased from Rionlon Chemical Co. and Kermel Chemical Reagent Co., respectively. All the above materials were used without further purification.Polytetrafluoroethylene (PTFE) emulsion with a mean lattice diameter of 200 nm (0.6 wt%) was supplied by Shanghai 3 F New Materials Co. Ltd.

Fig. 1. Schematic diagram of experimental set up for electrospinning.

2.2. Membrane preparation 2.2.1. Electrospinning Cellulose acetate and PVDF in a mass ratio of 9:1 was dissolved in a mixed solvent of acetone and DMAc (volume ratio 2:1) at room temperature to form a solution with an overall polymer concentration of 16 wt%. The as-prepared polymer solution was charged into two 5-ml syringes, which were placed side by side in the electrospinning setup. The nanofibers were electrospun using the dual-syringe system under identical conditions onto a PET nonwoven substrate, thereby forming CA/PVDF composite nanofiber membrane. As a comparison, nanofiber membranes were also spun in the absence of the PET substrate under the same electrospinning conditions. Fig. 1 shows a schematic diagram of the

Fig. 2. Schematic diagram of solvent vapor-treatment.

C. Liu et al. / Journal of Membrane Science 512 (2016) 1–12

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Fig. 3. Schematic set up to determine water flux and emulsion rejection.

2.3. Membrane characterization 2.3.1. Surface and cross-sectional morphology The surface morphology and cross-sections of the porous CA/ PVDF membranes were examined under a scanning electron microscopy (SEM) (Quanta200, FEI Company, USA). When examining membrane cross sections, membrane samples with the nonwoven support were prepared with a sharp razor blade, and the membrane samples without the nonwoven support were prepared with the freeze and snap method using liquid nitrogen. Prior to SEM examination, the samples were dried for 12 h before being sputtercoated with a thin layer of gold. The average diameter of nanofibers was determined with 50 nanofibers using Image-Pro Plus 6.0. 2.3.2. Water flux and emulsion rejection Fig. 3 shows aschematic diagram for measuring water flux and emulsion rejection in cross-flow mode.The effective membrane area in the test cell was 1.9 cm2. Water flux and rejection rate were determined by pure water and polytetrafluoroethylene (PTFE) emulsion, respectively. The turbidity of the feed PTFE emulsion was 372 NTU, and the amount of permeate water removed was negligible as compared to the feed volume so as to maintain a constant emulsion concentration during each filtration run. All the permeation tests were conducted at 207 0.5 °C under a hydraulic pressure of 0.1 MPa gauge. Water flux was calculated from

JW =

V A⋅t

(1)

where JW was the water flux, V was the volume of permeate water, A was the effective membrane area for permeation, and t was the time. The turbidity of the permeate solution was measured using a Hach turbidimeter (2100Q) to evaluate the polytetrafluoroethylene emulsion rejection, which was calculated by

⎛ Cp ⎞ ⎟ × 100% R = ⎜1 − Cf ⎠ ⎝

(2)

where R was the emulsion rejection, and Cp and Cf were the concentrations of PTFE latex in the permeate and feed, respectively. All measurements were conducted with at least three membrane samples, and the average values were reported. 2.3.3. Porosity The porosity of the membranes was measured with a capillary flow porometer (CFP-1100-A, Porous Materials Inc. Ithaca, NY), which was able to detect pore sizes in the range of 0.013–500 mm. Prior to the porometric measurements, the membranes were completely wetted with a wetting liquid Galwick, which was provided by Porous Materials Inc. and had a surface tension of 15.9 dynes/cm. 2.3.4. Water contact angle The hydrophilicity of the membrane was measured using a contact angle meter (DSA100, Krüss, Germany) with the sessile

Fig. 4. SEM images of CA/PVDF membrane surfaces.(a) Pristine nanofiber membrane without solvent vapor treatment, and (b–f) Nanofiber membranes after exposure to saturated acetone vapor at 50 °C for 5, 10, 30, 60 and 90 min, respectively.

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Fig. 5. SEM images of CA/PVDF membrane surfaces after exposure to an acetone/DMAc vapor mixture at 50 °C for (a) 5, (b) 10, (c) 30, (d) 60 and (e) 90 min, respectively. The solvent vapor was in equilibrium with liquid acetone/DMAc in a mass ratio of 2:1.

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Fig. 6. SEM images of CA/PVDF membrane surfaces after exposure to an acetone/DMAc vapor mixture at 50 °C for (a) 10, (b) 30, (c) 60 and (d) 90 min, respectively. The solvent vapor was in equilibrium with liquid acetone/DMAc in a mass ratio of 1:2.

drop method. The water droplet volume was 5 ml, and the contact angle values reported were an average of eight measurements on different spots on the membrane sample.

2.3.5. Crystallization and surface elements Crystallization of the nanofibers was characterized by differential scanning calorimeter (DSC) (DSC200, Netzsch, Germany)

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Fig. 7. SEM images of CA/PVDF membrane surfaces after exposure to saturated DMAc vapor at 50 °C for (a) 10, (b) 30, (c) 60 and (d) 90 min, respectively.

and x-ray diffraction (XRD) (D8 Discover, Bruker, Germany). Ascending DSC thermograms were obtained with 8 mg of the membrane sample sealed in an aluminum pan at a heating rate of 10 °C/min.The XRD analysis was conducted with Cu Kα radiation

(λ ¼ 1.5418 Å). The elemental compositions of the membrane surfaces were analyzed using x-ray photoelectron spectroscopy (XPS) (K-alpha, ThermoFisher, UK). In all cases, the PET nonwovensupports were removed from the membranes before the DSC, XRD,

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Table 1 Pressure and composition of solvent vapor used for the membrane crosslinking. Liquid acetone/DMAc mass ratio

1:0 2:1 1:2 0:1

Solvent vapor used for membrane crosslinking Vapor pressure of acetone (kPa)

Vapor pressure of DMAc (kPa)

81.7 61.3 35.0 0

0 0.29 0.66 1.16

and XPS tests.

3. Results and discussion 3.1. Characterization of membrane 3.1.1. Surface and cross-sectional morphology The surface morphologies of the CA/PVDF nanofiber membranes were shown in Figs. 4–7 after treatment with solvent vapor generated with liquid solvents at an acetone/DMAc mass ratio of 1/0, 2/1, 1/2, 0/1, respectively. The membranes were exposed to the solvent vapor for a different period of time. The original pristine nanofiber membranes showed a uniform and smooth surface (Fig. 4a) and the average fiber diameter was ca. 400 nm. As shown in Figs. 4–6, the nanofibers were physically crosslinked after the membranes were subjected to exposure to the solvent vapor. The degree of crosslinking was more significant when pure acetone was used to treat the nanofiber membranes, resulting in a drastic decrease in the membrane porosity. The crosslinking was induced by surface “fusion” of the fibers in the solvent vapor to form physical bonds at the contacting points of the nanofibers. As expected, excessive crosslinking would occur when the membrane was exposed to acetone vapor for a sufficiently long period of time. This was found indeed the case as shown in Fig. 4c–f, where the highly porous pristine nanofiber membrane showed a dense and smooth surface after acetone vapor treatment. Since acetone vapor can hardly melt PVDF, to better control the cross-linking of the nanofibers, we chose to blend DMAc with acetone to adjust the vapor-induced crosslinking of the membrane. It was shown that using the mixed solvent system for membrane crosslinking, nanofiber membranes with a broad range of porous structures were obtained readily even at a low DMAc content in the vapor phase (Figs. 5 and 6). Interestingly, when the membranes were exposed to pure DMAc vapor, there was little membrane crosslinking even after 90 min. Clearly, for a given vapor treatment time, the degree of crosslinking of the CA/PVDF nanofibers induced by the solvent vapor was lowered with a decrease in acetone content in the solvent. This can be attributed to the much higher volatility of acetone than DMAc. At 50 °C, the saturated vapor pressure of acetone is 81.7 kPa, while the saturated vapor pressure of DMAc is much lower, although different values (e.g., 0.47–2.16 kPa) in the DMAc vapor pressure have been reported or predicted on the basis of Antoine equation (see, for example, [31–34]). For illustration purpose, let us take a DMAc vapor pressure of 1.16 kPa at 50 °C, which was experimentally determined by Nasirzadeh et al. [34]. Then, the pressure and composition of the vapor phase in the overhead space used to crosslink the membranes can be estimated, as shown in Table 1. By adding the less volatile DMAc to acetone, the acetone vapor

Fig. 8. Cross sections of membranes M1 and M3. In the figure, samples for M1and M3-1 were prepared with a sharp razor blade, and sample M3-2 was prepared by fracturing in liquid nitrogen.

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pressure was lowered significantly, which reduced the capability of the solvent vapor to partially melt the nanofiber surface for membrane crosslinking. In the extreme case where acetone was absent, the DMAc vapor was found to be incapable of crosslinking the CA/PVDF nanofibers under the experimental conditions tested. As one may expect, with an increase in the vapor-induced crosslinking, the degree of cross-linking of the nanofibers increased, while the porosity of crosslinked membranes would be compromised. An ideal circumstance would be partial dissolution of nanofiber surfaces that are just sufficient to form crosslinks at the contacting points of the nanofiber mats. Preferably, the surface fusion of the nanofibers should be completed within a short period of time one exposed to the solvent vapor. This way, the nanofiber membranes will be reinforced mechanically by the crosslinks without affecting the membrane porosity and specific surface area. To further investigate other properties, five nanofiber membranes prepared under different conditions were chosen, including the original pristine nanofiber membrane (shown in Fig. 4a), a membrane treated with mixed solvent vapor at 50 °C for 5 min (at a liquid acetone/DMAc mass ratio of 2:1) (shown in Fig. 5a), a membrane treated with mixed solvent vapor at 50 °C for 90 min (at a liquid acetone/DMAc mass ratio of 1:2) (shown in Fig. 6d), a membrane treated with mixed solvent vapor at 50 °C for 90 min (at a liquid acetone/DMAc mass ratio of 2:1) (shown in Fig. 5e), and a membrane treated with acetone vapor at 50 °C for 10 min (shown in Fig. 4c). These membranes are designated as membranes M1, M2, M3, M4, and M5, respectively. Fig. 8 shows the cross sections of membranes M1 and M3. For M1, the dual layer structure of electrospun nanofibers on PET nonwoven support was significantly deformed by the razor blade during sample preparation because of the very loosely stacked nanofibers without any physical crosslinks. The sample preparation with the cryo-snap method was also found challenging. On the other hand, the dual layer structure of membrane M3, which was crosslinked with the aid of the solvent vapor, is clearly shown; however, the porous structure of the nanofibers was destroyed by the razor blade used in sample preparation (see M3-1). When membrane M3 sample was fractured in liquid nitrogen after the PET support was removed, the cross section of the membrane was examined (see M3-2), where interconnected porous structure was clearly observed and there were still some nanofibers in the interior of the membrane due to an incomplete “weld” of the contacting points in the nanofiber mats. In all cases, solvent vapor treatment could not only cross-link the nanofibers, but also

control the pore size of membranes. 3.1.2. Water flux and emulsion rejection The water permeation flux and the turbidity of the permeate solution for membranes M1 to M5 were measured in cross-flow filtration mode, and the results are presented in Figs. 9 and 10, respectively. The corresponding rejection of these membranes to PTFE emulsion is shown in Fig. 11. As expected, the water flux decreased while the emulsion rejection increased from M1 to M5. This is mainly related to the pore size and porosity of the membrane, which was in an agreement with observations under SEM. Among the five membranes tested, the original pristine CA/PVDF membrane (M1), which had the biggest pore size, showed the highest water flux (17.4 m3/m2  h) and the lowest rejection (44.4%) to the emulsion with a permeate turbidity of 293 NTU. On the other hand, membrane M5, which had a relatively dense surface, showed a high rejection to the PTFE emulsion (99.99%) but at a low water flux of 0.12 m3/m2  h. Membranes M2 and M3 had a high porosity and interconnected pores, and they showed a high water flux good rejection to the emulsion. It should be noted that the primary function of PET nonwoven substrate in the membranes was to provide a mechanical support to the nanofibrous

Fig. 10. Turbidity of permeate solution obtained with the five membranes. Feed PTFE emulsion 372 NTU, operating pressure 0.1 MPa, and temperature 20 °C.

100 18000

17400 90

16000

80

Rejection (%)

Water Flux (L/m2.h)

14000 12000 10000

6910

8000

70

60

6000

50 4000

2110

40

2000

329

121

M4

M5

0 M1

M2

M3

Membrane Fig. 9. Water flux of the membranes. Operating pressure 0.1 MPa and temperature 20 °C.

M1

M2

M3

M4

M5

Membrane Fig. 11. Rejection of PTFE emulsion by the five membranes. Feed PTFE emulsion 372 NTU, operating pressure 0.1 MPa, and temperature 20 °C.

C. Liu et al. / Journal of Membrane Science 512 (2016) 1–12

“mat” membrane, and it had very little effect on the filtration performance. This is confirmed by a separate experiment of emulsion filtration with the nonwoven fabric alone, where an emulsion rejection of 5.4% (permeate turbidity 352 NTU) and a much higher water flux (39.8m3/m2  h) were obtained. All these

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results demonstrate that the functional nanofiber membranes were successfully fabricated by electrospinning, followed by controlled exposure to solvent vapor for physical crosslinking of the nanofibers. The vapor-induced crosslinking of the nanofibers allows the membrane integrity to be enhanced significantly without

Fig. 12. SEM images of membrane surfaces after filtration with PTFE emulsion for membranes (a) M1, (b) M2, (c) M3, (d) M4, and (e) M5.

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3.1.3. Porosity Table 2 shows the effective pore sizes of the membranes. The original pristine membrane (M1) showed a wide distribution in pore diameter from 0.80 to 1.93 mm, with a mean pore size of 1.03 mm. After crosslinking, the membranes (M2 – M5) showed narrower pore size distributions and smaller mean pore sizes. This indicates that the solvent vapor-treatment of the nanofibrous membranes will crosslink the CA/PVDF nanofibers and improve the membrane structure. The decreasing trend in the mean effective pore sizes of the membranes from membrane M1 to M5 is in agreement with SEM images of the membrane surfaces. The smallest pore size was observed for membrane M5, which corresponded to the rather dense surface of the membrane due to over crosslinking. It may be pointed out that the porosity data determined with the capillary flow porometer are based on the whole three-dimensional network structure of the membrane, and they do not always match with porosity data based on two-dimensional surface morphology of the membrane. 3.1.4. Water contact angle The contact angles of water on the five membranes are shown in Fig. 13. In general, the primary water contact angle (that is, the contact angle at the moment of water droplet contacting the membrane) was shown to decrease from membrane M1 to M5. In addition, membranes M1, M2 and M3 showed a slight hydrophobicity although the main constituent was hydrophilic CA, with PVDF accounting for only 10% in the nanofibers. This may be attributed to the nanostructure and the high porosity of the nanofiberous membranes, which tend to increase the roughness of the membrane surface and trap air pockets [14,35,36]. However, after 1 min, the water contact angles on the membrane surfaces all decreased, and the decrease in water contact angle is especially drastic for membranes M2, M3 and M4. Because of the intrinsic hydrophilicity of CA, the air pockets initially trapped on the membrane surface were unstable, and thus there was a gradual decrease in the water contact angle with the contact time. Membranes M2, M3 and M4 were found to be wetted completely after a few minutes, except for the un-crosslinked pristine membrane M1 which took a longer time to get fully wetted. In addition to the “loose” nanofiberous structure of membrane M1, this membrane also showed a slightly higher F element content than other solvent

vapor-treated membranes (which will be discussed later). Presumably, the partial melt of CA on the surfaces of the CA/PVDF nanofibers by acetone vapor during the vapor-induced

Fig. 13. Water contact angles of the membranes.

CA M5 Endo

compromising the pore size and porosity of the resulting membranes, a feature that is desirable for filtration applications. After the emulsion filtration experiments, the surface morphologies of five membranes (i.e., M1 to M5)were examined again, and they are shown in Fig. 12. It is obvious that some PTFE latices were deposited on the surfaces of the membranes. For the pristine membrane (M1) without solvent vapor induced crosslinking, more PTFE latices were deposited among the nanofibers as well. With a decrease in the pore size and porosity, there were a smaller number of PTFE latices deposited on the membrane. This is not surprising considering that surface fouling can occur more easily on membranes with larger pore sizes which tend to result in rougher surfaces [31].

M4 M3 M2 M1 PVDF 120

140

160

180

200

220

240

260

300

Fig. 14. DSC thermograms of membranes.

M5 M4 M3 M2

Table 2 Pore size of the five membranes.

M1

Membrane Largest pore diameter (mm)

Mean pore diameter (mm)

Smallest pore diameter (mm)

M1 M2 M3 M4 M5

1.03 0.75 0.59 0.30 0.08

0.80 0.60 0.34 0.08 0.07

1.93 0.97 1.05 1.04 0.74

280

Temperature(ο C)

Intensity (a.u.)

10

PVDF CA 10

15

20

25

30

35

2θ (°) Fig. 15. XRD patterns of membranes.

40

45

C. Liu et al. / Journal of Membrane Science 512 (2016) 1–12

O1s

C1s

F1s M5

Counts/s

M4 M3

M2 M1 CA 0

200

400

600

800

1000

1200

1400

Binding Energy (eV) Fig. 16. XPS spectra of membranes.

Table 3 Surface elemental composition of membranes. Membrane

CA M1 M2 M3 M4 M5

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were observed at 20.8°, 24.9° and 28.8°, whereas the characteristic peaks of CA nanofiber membrane were observed at 24.9° and 29°. All the membranes (M1 – M5) showed sharp peaks at 24.9° and 28.9°, due to the presence of CA in the CA/PVDF nanofibers. The weak peak shown at 20.8° is caused by the small content of PVDF (10 wt%) in the membranes. The XPS spectrum was used to determine the elemental composition of the membrane surfaces for the five membranes,which is shown in Fig. 16. Based on the XPS spectra, the elemental composition of the membrane surface was calculated, and results are presented in Table 3. As expected, C and O are the main elements on the membrane surface for all the CA/PVDF and CA nanofiber membranes. The characteristic binding energy of F element appeared on the spectra of five CA/PVDF membranes (M1M5). It is interesting to note that the F element content of the pristine un-crosslinked CA/PVDF nanofiber membrane (M1) tends to be slightly higher than that of crosslinked nanofiber membranes (M2-M5). This is not unexpected since the partial melt of CA on the surfaces of the CA/PVDF nanofibers when exposed to acetone vapor during the vapor-induced crosslinking would cover the PVDF constituent in the nanofiber.

4. Conclusions

Surface elemental composition (wt%) C

O

F

62.7 60.3 60.4 60.9 62.0 61.8

37.3 32.7 33.6 32.5 31.2 31.9

0 7.0 6.0 6.6 6.7 6.3

crosslinking would cover the PVDF constituent in the nanofiber. The un-crosslinked structure with a slightly high F element on the fiber surfaces in Membrane M1 may be attributed to the longer time required to reach thermodynamic equilibrium on the membrane surface when the water droplet was placed on the membrane for contact angle measurement. 3.1.5. Crystallization and surface elements The DSC patterns of the nanofibrous membranes are shown in Fig. 14. The CA nanofiber membrane showed a low endothermic peak between 215 and 242 °C due to the melting of the nanofibers, while the PVDF nanofiber membrane showed a high endothermic peak between 145 and 181 °C. The CA/PVDF nanofiber membranes (M1-M4) showed a strong endothermic peak between 217 and 241 °C and a weak endothermic peak between 144 °C and 175 °C, which corresponded to the melting of CA and PVDF, respectively. Interestingly, the DSC of membrane M5 showed an obvious difference from the rest of the membranes, and a low and broad endothermic peak with a bimodal shape located between 201 and 240 °C. The bimodal shape of the endothermic peak may be attributed to the change in crystallization of the CA nanofiber due to a long exposure to the acetone vapor. It corresponds to the significant densification of the membrane surface and disappearance of the nanofibers. Since CA and PVDF are immiscible, as one may expect, all the membranes M1 to M5 have separate melting peaks of PVDF and CA. Meanwhile, the melt temperature of PVDF in CA/ PVDF nanofiber membranes was slightly lower than that of the pure PVDF nanofiber membrane, which is believed to be a result of the influence of CA in the CA/PVDF nanofibers. The XRD patterns of nanofiber membranes are displayed in Fig. 15. The characteristic peaks of PVDF nanofiber membrane

CA/PVDF composite membranes were prepared by physical crosslinking of electrospun CA/PVDF nanofiber membranes with solvent vapor. It was shown that the pore size and degree of crosslinking of the nanofiber membranes could be controlled easily by adjusting the composition of acetone/DMAc solvent and the solvent vapor exposure time. For a given solvent vapor composition, the cross-linking degree of the membrane increased and the pore size decreased with an increase in the vapor treatment duration. The membranes had pore sizes in the range of nm to mm. Especially, the porous CA/PVDF nanofiber membranes with a mean pore size of 0.75 and 0.59 mm yielded a high water flux and high rejection rate to emulsion lattices when tested for PTFE emulsion separations. The nanofiber membranes prepared by electrospinning followed by solvent vapor induced crosslinking have significant potential for separation and filtration applications (e.g., water treatment, food processing and other separations).

Acknowledgements The authors thank the financial support by the National Natural Science Foundation of China (No. 51273147, 51503144), the Scientific Research Foundation for the Returned Overseas Chinese Scholars and State Key Laboratory of Separation Membranes and Membrane Processes (Z2-201528).

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