Design and Modification of Janus Polyvinylidene Fluoride/Deacetylated Cellulose Acetate Nanofiber Membrane and its Multifunctionality

A novel type of Janus polyvinylidene fluoride/deacetylated cellulose acetate (PVDF/D‐CA) nanofiber membrane with multifunctional application is successfully fabricated by sequential electrospinning. The morphology, chemical composition, surface wettability, oil–water separation performance, and unidirectional liquid transmission capability have been systematically analyzed. It has asymmetric wettability property in air and in liquid systems for its bottom and top sides. It can be used as a water‐removing, or oil‐removing substance for oil–water mixture and emulsion under a harsh environment only under gravity driving force (DF) by simply switching the different sides. It possesses on‐demand water and oil collection ability, and moisture‐wicking performance due to its switchable permeation and unidirectional liquid transmission capability. Its multifunction application can be easy to control by adjusting the thickness of the top and bottom sides through controlling the electrospinning time. Its excellent mechanical property, chemical stability, durability under harsh environment and outstanding anti‐pollution ability endow the membrane with powerful cyclic stability and reusability.


Introduction
Recently, oil emissions caused by oil transport accidents and industrial wastewater have been a concerning global problem, resulting in devastating destruction of the marine environment. [1] In particular, today's rapidly developing economy drawbacks of the single-layer membrane by just shifting the directional permeability system. [7] For the Janus nanofiber membranes, besides the oil-water separation application, to development a new function also has become a hot topic and is attracting increasing attention from researchers. [8][9][10] For example, Fu et al. [11] designed a novel Janus membrane by blinding the cycle self-assembly of FeIII and phytic acid with a one-sided coating poly(dimethylsiloxane) (PDMS) layer for multifunctional applications. The Janus membrane has unidirectional liquid transport ability, that is, the water droplets can quickly penetrate the hydrophobicity surface into hydrophilicity surface and spread over on the hydrophilicity surface of the Janus membrane when it has suitable hydrophilic and hydrophobic layer thickness, which is beneficial to achieve the function of moisture-wicking. It can be applied in water collection under oil, moisture-wicking, demulsification, and oil-water separation. Nevertheless, although the Janus membranes could be applied in various fields due to its multifunctional, to our knowledge, they maybe have the drawbacks of high implementation cost, complicated production process, generating second pollution, poor adaptability for extreme environmental and low chemical or mechanical stability. Therefore, the key purpose of this study is to fabricate the Janus membrane with a simple production process, low fabrication cost, good reusability, excellent chemical stability, and multifunctional applications.
In particular, cellulose acetate (CA) as a natural carbohydrate polymer has gained more and more interest in various fields based on its advantages of rich in natural resources, low cost, good environmentally friendly. Especially, it has been widely applied in the oil-water separation fields due to its excellent wettability. For example, the CA membrane was prepared using the phase inversion technique with excellent separation and antipollution properties, environmental suitability, and recyclability. [12,13] In our previous study, [14] the deacetylated cellulose acetate (D-CA) membrane was fabricated by using electrospinning technique and deacetylation treatment. It shows high separation flux and separation efficiency to remove water and oil from oil-water mixture, respectively, and also has excellent antipollution property and recyclability. Nevertheless, natural carbohydrate polymers and their derivation (such as cellulose, CA, chitosan, and starch) generally have poor mechanical property, which limits its practical application. In order to overcome the shortcoming, their composite materials have widely prepared by introducing other materials with excellent mechanical property. [15,16] Ma et al. proposed polyimide (PI)/CA electrospun fibers with a core-sheath structure, [17] its tensile strength is more than 200 MPa and higher than the single CA membrane. Wang et al. [18] fabricated CA/polyurathane (PU) composite nanofiber with highly efficient oil-water separation, which also possess higher mechanical property than CA nanofiber membrane. Moreover, the thermoplastic polymer of polyvinylidene fluoride (PVDF) has widely applied in the oil-water separation field based on its advantages of remarkable chemical stability, prominent corrosion resistance, outstanding mechanical property, superoleophilicity, and low surface energy. [19] Therefore, in this study, the PVDF and CA materials were chosen to fabricate the top and bottom sides of the Janus nanofiber membrane due to their superoleophilicity and superhydrophilicity, respectively.
Herein, the Janus nanofiber membrane with the top surface of PVDF and bottom surface of CA was prepared by electrospinning technique. Then the prepared Janus PVDF/ CA nanofiber membrane was deacetylated to obtain the Janus PVDF/D-CA membrane. The PVDF/D-CA nanofiber membrane has not only high separation efficiency and separation flux for removing water and oil from oil-water mixture and emulsion solution, but also has on-demand water and oil collection, moisture-wicking abilities. The multifunction of the Janus nanofiber membrane can be easy to control by simply adjusting the thickness of the top and bottom sides through adjusting the electrospinning time.

Fabrication of Janus PVDF/D-CA Nanofiber Membranes
The PVDF powder of 3 g was put into 37 mL of mixed solvent with acetone/DMF (v/v, 1/4), and the CA powder of 6.8 g was placed into 40 mL of mixture solvent with DMF/acetone (v/v, 1/2). And then, they were oscillated at 60 °C for 3 h to obtain the uniform CA (17 wt%) and PVDF (10 wt%) electrospinning solution, respectively. An electrospinning device including a syringe attached to a needle with an inner diameter of 0.52 mm, a collector plate of aluminum sheet, and a high voltage supply was used to fabricate the Janus PVDF/CA nanofiber membranes. Firstly, the CA nanofiber was prepared by electrospinning. Secondly, the PVDF nanofiber was prepared by electrospinning and directly collected on the CA nanofiber membrane surface to obtain the Janus PVDF/CA nanofiber membrane. Figure 1a provides the preparation processing of the Janus PVDF/CA nanofiber membranes. The solution injection rate, electrospinning voltage, collection distance are 1 mL h −1 , 15 kV, and 15 cm for all samples, respectively. The electrospinning temperature is room temperature (about 25 °C), and the ambient humidity is 40%. Figure 1b provides the schematic diagram of the preparation process for the Janus PVDF/D-CA nanofiber membranes. Firstly, the above-prepared Janus PVDF/CA nanofiber membranes were immersed into 0.03 m NaOH aqueous solution at ambient temperature for 3 h. Secondly, it was removed out, cleaned several times by water, and dried 12 h in a drying cabinet. During the deacetylation treatment process, only CA bottom side is immersed into the weak alkaline solution, while the PVDF top side remains upon without touching the weak alkaline solution. After deacetylation, partial of the ester groups (CH 3 −C=O) from the CA nanofibers changed into hydroxyl group (−OH) in D-CA molecule. Therefore, the D-CA bottom side will show superhydrophilic and lipophilic properties in the air because the lipophilic group of ester group and hydrophilic group of hydroxyl group were coexisting in the D-CA molecule. [14] A series of PVDF/D-CA nanofiber membranes with various thicknesses was fabricated by controlling the electrospinning time of D-CA and PVDF nanofiber to evaluate their eligibility for a Janus membrane. When the electrospinning time of D-CA bottom side is 1 h, the electrospinning time of PVDF top side is set as 0.5, 1, 2, and 3 h, the corresponding Janus PVDF/D-CA are named as J-P0.5C1, J-P1C1, J-P2C1, J-P3C1, respectively. When the electrospinning time of PVDF top side is 1 h, the electrospinning time of D-CA bottom side is set as 0.5, 1, 2, 3, and 4 h, the corresponding Janus PVDF/D-CA are named as J-P1C0.5, J-P1C1, J-P1C2, J-P1C3, J-P1C4, respectively. The detailed electrospinning time for different Janus PVDF/D-CA nanofiber membranes is listed in Table 1.

Characterization
The morphology of the Janus nanofiber membranes was observed with a scanning electron microscope (FESEM, Nova NanoSEM 450, FEI Company, USA) at an accelerating voltage of 15.0 kV. The chemical composition of Janus nanofiber membranes was assessed by using an FT-IR spectrometer (Nicolet 6700) in the range of 4000-600 cm −1 with a resolution of 4 cm −1 and a number of scans of 64. The surface chemical performance of the Janus nanofiber membranes was observed by ESCALAB 250 XPS equipment (Thermo Electron Corporation, USA). The water contact angle (WCA) and oil contact angle (OCA) under the air and liquid were observed by using contact  angle analyzer Krüss DSA25 machine (Germany), respectively, to determine the wettability of Janus nanofiber membranes. Each sample was measured with distilled water or oil droplets (2 µL) and repeated eight times. The chemical oxygen demand (COD) meter (DR900) and COD digestion apparatus (DRB200) were used to measure the separation efficiency. The mechanical properties of samples were investigated using the universal testing machine (UTM2102 Suns Technology Stock Co. Ltd., China). All samples were cut into strips of 1 × 3 mm. Five samples for each type were measured at room temperature with tensile speed of 2 mm min −1 .

Separation Experiment of Oil-Water Mixture
A homemade separation device was applied to measure the separation performances for oil-water mixtures, and its effective filtration area was 0.4 cm 2 . Firstly, 10 mL of water dyed by methylene blue was put into 10 mL of oil dyed by methylene blue to obtain 20 mL of the oil-water mixture. Then, the PVDF/D-CA nanofiber membrane was fastened between two glass fixtures. The hydrophilic side (D-CA) faces upward for the waterremoving process, in turn, the lipophilic side (PVDF) faces upward for the oil-removing process due to its Janus performance. The separation flux and separation efficiency of Janus nanofiber membrane were evaluated by using petroleum ether, n-hexane, chloroform, carbon tetrachloride, kerosene, peanut oil, and toluene for this study. All the filtration processes were performed only under the gravitational force (GF).

Separation Experiment of Oil-Water Emulsion
The separation performances for Janus PVDF/D-CA nanofiber membranes were evaluated by using various emulsions. The homogeneous and stable water-in-oil (W/O, 1/100, v/v) or oil-inwater (O/W, 1:100, v/v ) emulsions with 0.1 mg mL −1 span 80 were obtained by stirring for 1 h, then sonicated for 3 h at room temperature. The obtained W/O and O/W emulsions stood for 24 h, and no demulsification occurred. The emulsion separation also was carried out by a homemade separation device only under gravity driving force (DF), and its effective separation area is 0.4 cm 2 .
In this study, the following Formula (1) was used to calculate the separation flux (F) for the Janus PVDF/D-CA nanofiber membranes.
where S (0.4 cm 2 ) is the active area of the nanofiber membrane, V is the liquid volume of water or oil passing through the nanofiber membranes, T is the time of the water or oil passing through the nanofiber membrane.
The following Formula (2) was used to calculate the separation efficiency (η) for the Janus PVDF/D-CA nanofiber membrane.
C 0 is the COD values of the original oil-water mixture and emulsion. C 1 is the COD value of filtrate after separation.

Moisture-Wicking Performance of Janus PVDF/D-CA Nanofiber Membranes
The water evaporate and water vapor transmission performances for the Janus PVDF/D-CA membrane were tested and used to evaluate its moisture-wicking ability. The electrospinning time of the D-CA and PVDF sides are 3 h and 10 min, respectively. For water evaporation test, firstly, 200 µL water was dropped into PVDF/D-CA membrane with a dimension of 2.5 × 2.5 cm from the PVDF side into the D-CA side. Secondly, the wetted sample was weighted as M 0 and immediately put into an oven with 40 °C. Then the weight of the sample was measured every 5 min and recorded as M 1 . Finally, the sample's weight loss (M 0 − M 1 ) was obtained with different water evaporate times. The curve of weight loss with water evaporate time was plotted, and the slope of the curve could be applied to evaluate the water evaporation rate. For the water vapor transmission test, firstly, a container with a diameter of 2 cm and filled 20 mL water was covered by PVDF/D-CA nanofiber membranes, then the PVDF side is facing to the water. The initial 20 mL water weigh was recorded as W 0 , and the effective water vapor transmission area was 3.14 cm 2 and recorded as S. Secondly, the wrapped container was put in an oven with relative humidity of 40% and temperature of 35 °C. The residual water in the container was observed every 12 h and recorded as W 1. Finally, the water vapor transmission weight of each area ((W 0 − W 1 )/3.14) of the sample was obtained with different water vapor transmission times. For comparison purpose, the D-CA nanofiber membrane with electrospinning time of 3 h also was used to perform the above-mentioned test.

Chemical Stability
The corrosion resistance and chemical structure stability capabilities for Janus PVDF/D-CA membrane were assessed by using an oil-water corrosive solution. An amount of 50 mL of oils (n-hexane or carbon tetrachloride according to the separation type) were mixed with 50 mL of NaOH, HCl, and NaCl with concentration of 0.5 mol L −1 to obtain 100 mL corrosive solution, respectively. A home-made cross-flow filtration equipment also was used to separate the corrosive solution only gravity as a DF.

Wear-Resisting Property
An abrasion test was executed to evaluate the mechanical stability of Janus PVDF/D-CA nanofiber membrane. A 100 g weight was placed upon both sides of the Janus membrane surface, respectively. And then, it was pushed on sandpaper with 2500 mesh. The nanofiber membrane was drawn at a distance of 10 cm as one effective abrasion, and the test was repeated 10 times for all samples. After abrasion, the WCA of the nanofiber membrane was measured.

Morphology of Janus PVDF/D-CA Nanofiber Membrane
The diameter distributions (right) and SEM images (left) of Janus PVDF/CA and PVDF/D-CA membranes before and after deacetylation are exhibited in Figure 2. Taking the J-P1C3 sample as an example, the electrospinning time for the PVDF top sides and CA bottom side is 1 and 3 h, respectively. It can be seen that, for PVDF/CA nanofiber membrane, both bottom (CA) side and top (PVDF) side have good morphology with smooth and uniform nanofibers. Moreover, the PVDF/D-CA nanofiber membrane remains good nanofiber morphology for the bottom and top sides after deacetylation. Compared with PVDF/CA nanofiber membrane, the average diameter of PVDF/D-CA membrane is slightly reduced for both the top and bottom layers. It can be attributed to the fact that the nanofibers were swelled when they were immersed into the weak alkaline solution during the deacetylation process. From Figure S1 (Supporting Information), it also can be concluded that all samples have good nanofiber morphology under different electrospinning times before and after deacetylation. The average diameter of the CA side is in the range of 265 to 290 nm, and the average diameter of the PVDF side is in the range of 189 to 204 nm. There was no significant influence of electrospinning time on the morphology and diameter distribution of nanofibers. The cross-section of the Janus PVDF/D-CA nanofiber membrane is provided in Figure 2c. After deacetylation, there was no obvious boundary between D-CA and PVDF layers, meaningfully excellent adhesion between different nanofiber layers. Figure 2d provides FTIR spectra for pure CA and PVDF membranes before and after alkaline treatment. For the CA layer, the characteristic bands of asymmetric stretching of the C-O and stretching vibration of C=O were observed at peaks of 1237 and 1747 cm −1 , respectively. [20] After deacetylation, the peaks at 1747 and 1237 cm −1 were significantly decreased and even disappeared for D-CA compared to that of CA. Moreover, an additional peak at 3459 cm −1 attached to the −OH group was observed in D-CA, suggesting the ester groups (C=O) in the CA molecule changed into hydroxyl group (−OH) in D-CA molecule. [14,21] For PVDF, the symmetric and asymmetric stretching vibrations of the CH 2 group and C-F band are represented at 2974, 3017, and 1193 cm −1 , respectively. [22][23][24] After alkaline treatment, there is no obvious change for the FTIR spectrum of PVDF nanofibers because of its outstanding corrosion resistance under harsh environment. Moreover, the XPS spectrum of D-CA and PVDF/D-CA nanofiber membranes were provided in Figure 2e-h. For D-CA nanofiber, there are two signal peaks attributed to C and O elements, however, for PVDF/D-CA nanofiber, a new signal peak attributed to F element at 687.6 eV is appeared due to the introduction of PVDF (Figure 2e). There are three signal peaks at 531.9, 532.6, and 533.5 eV attributed to C=O, C−O−C, and C−OH, respectively, for both D-CA and PVDF/D-CA nanofiber (Figure 2f,g). It is worth noting that the F···OH bond is formed between the hydroxyl bond (−OH) of D-CA and the fluorine bond (C−F) of PVDF because the fluorine bond can be used as proton acceptors to accept the hydroxyl bond of D-CA (Figure 2h), suggesting the chemical interaction is occurred between D-CA and PVDF, which further confirmed there is strong interfacial bonding force between top and bottom sides of PVDF/D-CA nanofiber membrane.

Permeability of Janus PVDF/D-CA Membrane
The thickness for both sides of the Janus PVDF/D-CA nanofiber membrane is very important factor in their permeability for liquid (water or oil). In this study, the nanofiber membrane thickness can be designed simply by controlling the electrospinning time, as shown in Figure 3a Figure 3c, it can be found that when the electrospinning time of the CA side is constant at 1 h, the separation properties were controlled by adjusting the electrospinning time of the PVDF side. The PVDF/D-CA nanofiber membranes of J-P0.5C1, J-P1C1, and J-P2C1 can separate both water and oil by simply shifting the different side facing up only under gravity force, which indicated that they all can be used as the Janus membranes. However, when the electrospinning time of PVDF increased to 3 h, the J-P3C1 nanofiber membrane only can be used to separate oil. It also can be found that the separation flux of water reduced, and the separation flux of oil firstly enhanced and then reduced with the increasing of PVDF electrospinning time. The highest separation flux for oil was achieved when the PVDF electrospinning time was 1 h. Therefore, the  as the Janus membranes only under GF. However, when the electrospinning time of CA is 0.5 h, the J-P1C0.5 nanofiber membrane only can be applied to separate oil. The separation flux of water and oil firstly enhanced and then reduced, with the increase in electrospinning time of the CA side. Therefore, the J-P1C3 nanofiber membrane was chosen for further investigation due to its excellent permeability for both water and oil in the following section, if no special statement. All the water and oil separation efficiency for PVDF/D-CA nanofiber membranes were up to 98.5% (Figure 3d,f). Those results showed that the Janus nanofiber membranes can be fabricated simply by adjusting the thickness of the CA and PVDF sides.
To further illustrate how the Janus membrane works, the working principle diagram for Janus PVDF/D-CA nanofiber membranes was shown in Figure 3g. The water removing process was used as an example. In this example, the thickness of the PVDF side is a constant, and the thickness of the CA side changed with the changing of electrospinning time. When the water droplet expose to the nanofiber membrane, the hydrophilic force (HF), the transmembrane resistance force (RF), and the capillary force (CF) will be produced between the nanofiber membrane and water droplet. And the HF, RF, and CF values all depend on the thickness of the membrane, surface wettability, and its micropore structure and morphology. [25] The surface wettability and micropore structure of nanofiber are the major factors that affect CF. The membrane thickness is the major factor that affects HF and RF. In this study, the CF can be assumed as constant because the nanofiber membrane's wettability and micro pore structure almost have no significant change with the increasing of electrospinning time. The HF is a positive force for water through the membrane, while the RF is negative force for resisting water through the membrane. Therefore, if the difference of ∆F = HF -RF > 0, it is beneficial to infiltrate the membrane, on the contrary, if the difference of ∆F = HF -RF < 0, it is disadvantage to pass through the membrane. In additionally, the GF is another positive force for permeability of the water. Therefore, the finally DF can be considered as DF = ∆F + GF. Based on above-mentioned assumption, the working principle of Janus membrane maybe have the following conditions. (1) when the CA side is thin, the HF come from CA is smaller than the RF come from PVDF, resulting in ∆F < 0, which makes the water separation flux smaller (if |∆F| < GF), or even not through (|∆F| > GF) ( Figure 3g1); (2) with the increase of CA side thickness, the HF come from CA increased, if the HF = RF, resulting in ∆F = 0, which makes the water separation flux slightly increased, and the water separation was performed only under the GF (DF = GF) ( Figure 3g2); (3) with the further increase of CA side thickness, the HF come from CA further increased, resulting in ∆F > 0, meaning the water separation was performed under DF = ∆F + GF, which makes the water separation flux significantly increased ( Figure 3g3); (4) if the thickness of CA side further increased, the RF come from CA and PVDF will significantly increase, meaning the ∆F < 0, if the |∆F| > GF (DF < 0), the water will not can go through the membrane (Figure 3g4). Likewise, by inversion of the membrane side, the separation property for oil permeating will have the same varying tendency with the changing of the PVDF side thickness. As a result, based on above assumption, the appropriate thickness ratio between the hydrophobic (PVDF) layer and hydrophilic (CA) layer was needed to adjust the separation property of Janus PVDF/D-CA membrane, and it can be achieved by easily controlling the electrospinning time.

Wettability Behavior and Switchable Oil-Water Mixture Separation for Janus PVDF/D-CA Membranes
For oil-water separation membranes, its wettability behavior plays a vital role in the separation performance. The Janus nanofiber membrane has attracted extensive attention because of its distinct wetting selectivity for different surfaces.  Figure 4a,c, it can be seen that the WCA and OCA sharply decreased to 0° when the water or oil droplets dropped onto the bottom side (D-CA), showing its superoleophilicity and superhydrophilicity properties (superamphiphilic) in air. Moreover, the WCA also quickly decreased to 0° under various oils, suggesting superhydrophilicity property under oil. And the OCA for different oils ranges from 120° to 140°, indicating the oleophobicity under water. This result showed that the bottom side (D-CA) can be used as the waterremoving material. From Figure 4b,d, it can be seen that it exhibits hydrophobicity (140.5°) and superlipophilicity (0°) in the air for the top side (PVDF). And the OCA quickly decreased to 0° under various oils, indicating the superlipophilicity under water. The WCA for different oils ranges from 120° to 150°, indicating the hydrophobicity under oil. This result showed that the top side (PVDF) can be used as the oil removing material. Figure S2 (Supporting Information) provides the wettability behavior of Janus PVDF/D-CA nanofiber membranes with different electrospinning times. It showed that the electrospinning time has no significant effect on the wettability behavior for the top and bottom sides of Janus nanofiber membrane. It indicated that the prepared Janus PVDF/D-CA nanofiber possesses good wetting selectivity by simply changing the different sides to achieve the different separation requirements.
As for switchable separation, the oil-water mixture separation performance of the J-P1C3 nanofiber membrane was performed using a homemade separation device shown in Figure 4g only under the gravity force. When the bottom (CA) surface is facing up, the water is removed from the light oil-water mixture. When the top (PVDF) surface is facing up, the oil is removed from the heavy oil-water mixture. The separation fluxes of water removing are 8817 ± 291, 7556 ± 271, 7205 ± 245, 8501 ± 250, 6964 ± 280 Lm −2 h −1 for petroleum ether/water, n-hexane/water, toluene-water, peanut oil-water, and kerosene-water mixture, respectively (Figure 4e). The separation fluxes of oil removing are 17 477 ± 294 and 20 774 ± 301 Lm −2 h −1 for chloroform-water and carbon tetrachloride-water mixture, respectively. The separation efficiency of water and oil are reached up to 99.9% and 99.6%, respectively (Figure 4f). It indicates that the Janus PVDF/D-CA nanofiber membrane has multifunction and switchable ability due to its Janus wettability property and low transmembrane resistance. Figure S3 (Supporting Information) displays the cyclic separation efficiency and separation flux of Janus PVDF/D-CA membrane for kerosene-water and carbon tetrachloride-water mixtures before and after water washing. It can be found that the separation efficiency and separation flux of membrane have no significant change, and still retain the original high value after 11 cycles, suggesting its great reutilization ability.   Adv. Mater. Interfaces 2023, 10, 2201550

Switchable Oil-Water Emulsion Separation for Janus PVDF/D-CA Membrane
The Janus PVDF/D-CA membrane can be switched to separate the W/O and O/W emulsions because of its different wettability properties for different sides. Figure 5a shows the schematic illustration of switching emulsion separation. When the top side (PVDF) facing up, the water will be trapped. At the same time, clean oil flows through the Janus nanofiber membrane because of its wettability performance of superoleophylic under water, hydrophobicity under oil. On the contrary, when the bottom side (D-CA) facing up, the oil will be trapped, while clean water flows through the Janus nanofiber membrane because of its wettability properties of oleophobic under water and superhydrophilic under oil. Figure 5b shows the optical images of chloroform-inwater and water-in-chloroform emulsions before and after separation, respectively. It can be seen that the emulsions show milky color, and the water droplets in chloroform and chloroform droplets in water have the size with nanometes to micrometers range. After filtering, the filtrates turned transparent clear, and the water droplets or oil droplets were eliminated. The result showed that the Janus PVDF/D-CA nanofiber membranes can successfully switchable separate the W/O and O/W emulsions.  Notice that all the emulsions separation process were performed only by GF. It also can be noted that the separation flux of emulsion increased with the decreasing of oils viscosity, this result is in agreement with the results of other research. [25,26]   For practical application, the nanofiber membrane not only need excellent separation properties, but also need prominent reutilizing. The cycle emulsion separation performance of Janus PVDF/D-CA nanofiber membrane was evaluated to assess its reusability, as shown in Figure 5g-j. The CCl 3 -in-water and waterin-CCl 3 emulsions are used as an example. Both for the bottom and top sides, the nanofiber membrane can be reused by washing in water. The cycle separation process was carried out by the following: firstly, the Janus PVDF/D-CA nanofiber membrane was applied to separate the W/O or O/W emulsions; secondly, the nanofiber membrane was washed several times by water and dried by using a vacuum oven; finally the washed membrane was used to separation emulsion again. The separation and washing process was repeated. When the top surface facing up, the separation efficiency and separation flux of CCl 3 -in-water emulsion have no significant change after 11 cycles, indicating excellent reutilization ability of the top side (Figure 5g,h). When the bottom surface facing up, the separation efficiency and separation flux of water-in-CCl 3 emulsion also have no significant change after 11 cycles, indicating excellent reutilization ability of the bottom side (Figure 5i,j). From Figure 5k, it can be seen that after deacetylation treatment, the white, dry Janus PVDF/D-CA nanofiber membrane was filled by red oil and change into red when it was immersed into the red oil bath, and then it was surprising to find that most of the red oil inside the membrane is floated out when it was put into the water bath, meaning that the nanofiber membrane has the self-cleaning ability due to the special wettabillity of D-CA, thus resulting in the excellent reutilization ability of the nanofiber membrane during oil-water separation processing. Table S2 (Supporting Information) displays W/O and O/W emulsions separation property of Janus PVDF/D-CA membrane compared to other results of various Janus membranes. This result indicates that the separation efficiency and flux of Janus PVDF/D-CA prepared in this study is higher than most of the other kinds of Janus membranes. Those results suggested that the Janus PVDF/D-CA membranes possess prominent separation performance and reusability, showing extensive application prospects in wastewater treatment domains. Figure 6a depicts the stress-strain curves of pure PVDF, CA, PVDF/CA, D-CA, and J-P1C3 nanofiber membranes. Their corresponding tensile strength, Young's modulus, and elongation at break are obtained from the stress-strain curves, as given in Table 2. The electrospinning time for PVDF and D-CA are 1 and 3 h, respectively. As shown in Table 2, the Young's modulus and ultimate tensile strength of PVDF/CA are larger than that of CA, but lower than that of PVDF. After deacetylated, the ultimate tensile strength of D-CA improved in comparison with that of CA nanofiber mambrane. Moreover, the Janus J-P1C3 nanofiber membrane has Young's modulus of 7.9 MPa, tensile strength value of 0.8 MPa, and an elongation at a break of 8.8%, which still has the good mechanical performance. Abrasion performance of J-P1C3 membranes is another important factor for practical application. As shown in Figure 6c, for the abrasion test, a weight of 100 g was put on the bottom or top surface of J-P1C3 membranes, and then they were together placed on the sandpaper (2500 mesh). The samples and the weight were together dragged to move back and forth on the surface of sandpaper mesh. The test was repeated 20 times for each sample. The WCA was measured after each cycle. The WCA of the bottom and top surface of the J-P1C3 nanofiber membrane after different abrasion cycles were shown in Figure 6b. The WCA of the top side only slightly decreased as the abrasion cycle increased, changing from 142 ± 0.3° to 138.4 ± 0.4° after abrasion 20 cycles. This result suggested there is no significant influence of abrasion on the wettability of both sides of J-P1C3 nanofiber membrane.

Antifouling Performance and Chemical Stability of PVDF/D-CA Nanofiber Membranes
The self-cleaning behavior of J-P1C3 nanofiber membrane was evaluated by spraying water or oil droplets onto the membrane bottom and top surfaces under oil or water, respectively,  as illustrated in Figure 7a. The water or oil droplets under the liquid system floated above the top and bottom surfaces readily and without sticking. Both surfaces of the Janus membrane display an utterly detached from sprayed water or oil droplets. It indicates that both surfaces of the Janus nanofiber membrane possess the low adhesion to water or oil droplets, resulting in an excellent antifouling capability for switchable removing oil or water from oil-water mixture and emulsion solution. By separating light oil (n-hexane)/water and water/heavy oil (carbon tetrachloride) mixtures with various corrosive solutions of NaCl, NaOH, and HCl with the concentration of 0.5 mol L −1 , as an example, the chemical stability of the Janus PVDF/D-CA nanofiber membrane has been assessed as shown in Figure 7b-e. The removing flux and efficiency of water for the bottom side and the oil removing flux and efficiency for the top side are no significant change under acid, alkaline, and saline solution compared to that of a neutral solution. Moreover, the fine nanofiber morphology still remains and diameter distribution has no apparent changes for both the bottom and top side nanofiber before and after separation under various harsh environments. Herein, the Janus PVDF/D-CA nanofiber membranes have excellent chemical stability and durability even under salts, alkaline, and acids corrosive solutions because of its intrinsically stable chemical structure, suggesting broad application prospects under harsh environmental circumstances.

Extended Application of PVDF/D-CA Nanofiber Membranes
In addition to the oil-water mixture and emulsion solution separation application above, the extended application performance of PVDF/D-CA nanofiber membranes can be realized by adjusting the top and bottom sides thickness through simply controlling the electrospinning time. The electrospinning time of PVDF and D-CA sides is 10 min and 3 h, respectively, and the prepared nanofiber membrane is referred to as J-P10minC3. The bottom and top sides both showed uniform and smooth nanofiber morphology, and the nanofiber diameter also has no significant change before and after deacetylation treatment ( Figure S4, Supporting Information). For the bottom side (D-CA), it shows oleophilic and hydrophilic in air, hydrophilic under oil and oleophobic under water. The top side (PVDF) shows oleophilic and hydrophobic in air, oleophilic under water, and hydrophobic under oil ( Figure S5, Supporting Information). However, compared with the wettability performance of top side for J-P10minC3 and J-P1C3 nanofiber membranes (Figure 8a), it is worth noting that, for the top side (PVDF) of the J-P1C3 nanofiber membrane, the initial WCA is about 142° and it remains stable and no significant change with the increasing of contact time when the PVDF layer is thick, and this is because the PVDF layer is hydrophobic in air. Whereas, for the top side (PVDF) of J-P10minC3 membrane, the initial WCA is about 138°, the water droplet is not spread on the surface of nanofiber membrane, even kept its spherical shape without collapsing, but the water droplet volume is decreased with the increasing of contact time. This is because the water droplet gradually transmitted through the hydrophobic thin PVDF layer into the bottom hydrophilic CA layer with the increasing of contact time, resulting the volume of droplet decreased, but the WCA value remains no significant change because the water droplet still on the surface of hydrophobic PVDF layer. This result suggests that the top surfaces of the J-P10minC3 and J-P1C3 nanofiber membranes possess metastable and stable hydrophobicity, respectively. [25] Based on the above-mentioned result, the water droplets can penetrate through the hydrophobic top side (PVDF) and into the hydrophilic bottom side (D-CA) of J-P10minC3 (Figure 8a). The directional water transport behavior of the J-P10minC3 was observed as provided in Figure 8b,c and Figure S6 (Supporting Information). The n-Hexane/water mixture was prepared, and then the J-P10minC3 nanofiber membrane was put at the interface between oil and water. When the top side (PVDF) was facing upward, 0.2 mL of water droplets colored by methylene blue are dropped into the nanofiber membranes, then gradually penetrated through the membrane. It may be because when the water droplet expose to the hydrophobicity surface, the small transmembrane pressure will produce, but not enough to withstand the water. Therefore, it can still gradually permeate throughout the hydrophobic PVDF side due to its thin enough thickness. And then can easily contact the hydrophilic bottom surface, finally permeated throughout the whole membrane assisted by the hydrophilic interaction. [27] On the contrary, when the bottom side (D-CA) was facing upward, the water droplets gradually spread on the bottom side and can not go through the membrane. It may be because the large RF produced come from the top surface due to its hydrophobic. [28] Besides, Figure  S6 (Supporting Information) provides another method, which was used to confirm the directional water transport behavior of J-P10minC3. When 100 µL of water droplets colored with methylene blue were dripped into the bottom side (D-CA), it lay out on the surface of nanofibers and gradually soaked the whole hydrophilic surface, and without any water infiltration on the top side (PVDF), which remained unwetted, meaning the water droplet can not flow through the bottom side (D-CA) to the top side (PVDF) ( Figure S6a, Supporting Information). However, when the water droplet dripped on the top side (PVDF), it almost kept its shape without collapsing, and can swiftly gone through the PVDF side and be absorbed by the bottom side (D-CA) ( Figure S6b, Supporting Information). Those results indicated the J-P10minC3 nanofiber membrane possesses the unidirectional water transport performance. This function of the Janus membrane can be widely used in various fields, such as fog collection, on-demand water collection, moisture wicky, and demulsification. [29,30] Based on the above-mentioned unidirectional water transport behavior of the PVDF/D-CA nanofiber membrane, its ondemand water and oil collection performance were observed, as shown in Figure 8d,e. A laboratory-made device was prepared by wrapping one end of an open glass tube with J-P10minC3 nanofiber membrane. The top side (PVDF) faces outward the glass tube. For on-demand water collection, before collection, the hydrophilic side (D-CA) was prewetted by water and then put the self-made device into water-oil mixture, the water droplets (blue) under oil were successfully collected (Figure 8d). In addition, for on-demand oil collection, the self-made device was just put into water-oil mixture, the oil droplets (red) under water were successfully collected (Figure 8e). This result suggests that the Janus PVDF/D-CA nanofiber membranes possess on-demand water and oil collection performance to purposefully and selectively collect water and oil droplets from water-oil mixture, respectively. Based on the above-mentioned unidirectional water transport behavior of the J-P10minC3 nanofiber membrane, its water evaporate rate and water vapor transmission performance also were investigated to evaluate its moisture-wicking performance. The weight of wetted D-CA and PVDF/D-CA nanofiber membrane reduced with the increasing time due to the evaporation of water. The slope of weight loss versus time curve can be referred as nanofiber membrane water evaporation rate. The water evaporation rate of PVDF/D-CA membrane was larger than that of D-CA, suggesting the faster drying rate, which is beneficial to the moisture-wicking capacity (Figure 9a). Moreover, the water vapor transmission rate of PVDF/D-CA membrane is high compared with D-CA, suggesting better gas permeability, which is also beneficial to the moisturewicking capacity (Figure 9b). Based on the properties as mentioned above, the PVDF/D-CA nanofiber membrane is likely to possess an excellent moisture-wicking capability, allowing it to efficiently eliminate excess sweaty and provide fresh feeling to the skin without cooling you down too much. Moisturewicking is the process of transporting moisture away from the skin. The schematic diagram of moisture-wicking performance for Janus PVDF/D-CA nanofiber membrane is shown in Figure 9c. When the PVDF side is facing the skin, the sweat can swiftly through and transport into the D-CA side due to the unidirectional transport behavior. Meanwhile, the dry PVDF side (facing the skin) keeps the skin dry, and the sweat in the D-CA side will fastly evaporate due to its high water evaporating rate. Therefore, compared to pure D-CA nanofiber membrane (it may keep the skin sticky), the Janus PVDF/D-CA nanofiber membrane can keep the skin parched due to its unidirectional transport behavior, high water evaporates rate, and excellent gas permeability, suggesting excellent moisturewicking capability.

Conclusion
The Janus PVDF/D-CA nanofiber membranes with multifunctional applications was successfully fabricated by using a simple sequential electrospinning technique. Based on its outstanding asymmetric wettability of superhydrophilicity for the bottom (D-CA) side and superlipophilicity for the top (PVDF) side, it not only can be applied to remove oil, but also can be applied to remove water from oil-water mixtures, as well as the emulsion solutions only under the force of gravity by simple changing the different sides. The separation efficiency for water removing reached up to 99.9%, and for oil removing reached up to 99.8%. Moreover, it has excellent chemical stability and durability even under salts, alkaline, and acids corrosive solution and shows remarkable antipollution ability, endowing outstanding cyclic stability, and reusability. In particular, except for separating the oilwater mixture and emulsion solution, the extended application of on-demand liquid collection, the moisture-wicking ability also can be realized by adjusting the thickness of the top and bottom sides by simple controlling the electrospinning time. to endow the of PVDF/D-CA membrane. It demonstrated that Janus PVDF/D-CA membrane has great applications prospect in oil-water separation, on-demand liquid collection, and moisture-wicking fields, providing a new idea for fabricating Janus nanofiber membrane with multifunctionality.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.