Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 28210−28218
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Microsphere-Fiber Interpenetrated Superhydrophobic PVDF Microporous Membranes with Improved Waterproof and Breathable Performance Gaoshuo Jiang,†,§ Liqiang Luo,† Lu Tan,‡ Jinliang Wang,§ Shenxiang Zhang,§ Feng Zhang,*,‡ and Jian Jin*,§ †
Department of Chemistry, Shanghai University, Shanghai 200444, P. R. China College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, P. R. China § i-Lab, CAS Key Laboratory of Nano-Bio Interface, CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, 215123, P. R. China
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‡
S Supporting Information *
ABSTRACT: Superhydrophobic membranes with extreme liquid water repellency property are good candidates for waterproof and breathable application. Different from the mostly used strategies through either mixing or postmodifying base membranes with perfluorinated compounds, we report in this work a facile methodology to fabricate superhydrophobic microporous membranes made up of pure poly(vinylidene fluoride) (PVDF) via a high-humidity induced electrospinning process. The superhydrophobic property of the PVDF microporous membrane is contributed by its special microsphere-fiber interpenetrated rough structure. The effective pore size and porosity of the PVDF membranes could be well tuned by simply adjusting the PVDF concentrations in polymer solutions. The membrane with optimized superhydrophobicity and porous structure exhibits improved waterproof and breathable performance with hydrostatic pressure up to 62 kPa, water vapor transmission rate (WVT rate) of 10.6 kg m−2 d−1, and simultaneously outstanding windproof performance with air permeability up to 1.3 mm s−1. Our work represents a rather simple and perfluorinated-free strategy for fabricating superhydrophobic microporous membranes, which matches well with the environmentally friendly requirement from the viewpoint of practical application. KEYWORDS: superhydrophobic microporous membranes, waterproof and breathable membrane, perfluorinated-free, electrospinning method
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fibrillation, template methods, phase separation, melt blown, and electrospinning.8−10 Among them, electrospinning has proven to be an effective method to fabricate microporous membranes with an interconnected porous structure by accumulation of fibers. The porous structure of the membrane could be controlled by simply tuning the fiber diameter and packing density.11−15 Up to now, great efforts have been made on the fabrication of electrospun microporous membranes with waterproofness and breathability.16−19 The waterproof and breathable membrane was first reported in 2007, where the
INTRODUCTION Waterproof and breathable membranes are able to prevent the penetration of liquid water droplets yet allow water vapor to pass through. Therefore, they have shown varieties of potential applications, including protective clothing, building materials, chemical and aerospace equipment.1−3 Ideally, waterproof and breathable membranes should possess a porous structure with proper pore size and high porosity to meet the requirement of high water vapor transmission (WVT) rate. Furthermore, the membranes need outstanding hydrophobicity with high hydrostatic pressure to prevent liquid water from penetrating through the membrane.4−7 To construct waterproof and breathable membranes, a variety of approaches have been developed, such as mechanical © 2018 American Chemical Society
Received: May 17, 2018 Accepted: July 27, 2018 Published: July 27, 2018 28210
DOI: 10.1021/acsami.8b08191 ACS Appl. Mater. Interfaces 2018, 10, 28210−28218
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic illustrating the fabrication process of the superhydrophobic PVDF membrane. (b and c) Top-view and (d) cross-section SEM images of the membrane.
suffers from inhomogeneous dispersity of the nanoparticles on the membrane which limits the improvement of hydrophobicity.41 In addition, the direct incorporation method also confronts the problem of poor stability of the nanoparticles because of the weak interaction between nanoparticles and membranes. Despite that many efforts have been made to improve the dispersity and stability by introducing chemical bonds between nanoparticles and membranes by grafting, etching, and depositing, etc., these methods required complex chemical modification which makes the superhydrophobic modification process more complicated and would also bring about secondary pollution to the environment.42−47 Hence, to develop a facile and environmentally friendly methodology to fabricate a superhydrophobic electrospun membrane and avoid a complex post-treatment procedure is necessary. In this work, we report a facile and perfluorinated-free strategy to fabricate a superhydrophobic microporous membrane. A novel microsphere-fiber interpenetrated microporous superhydrophobic PVDF membrane is achieved through a high-humidity induced one-step electrospinning process. The microsphere/fiber structure of the electrospun membrane could be controlled by tuning the solvent constituent of the PVDF solution. In addition, the effective pore size and porosity of the PVDF membranes could be well tuned by simply adjusting the PVDF concentrations in polymer solutions. The as-prepared superhydrophobic microporous membrane exhibits good waterproofness, breathability, and outstanding windproof performance. The facile strategy for fabricating superhydrophobic membranes from the pure polymer matches well with the environmental protection, and the membranes could also meet the potential requirements in various applications.
polyurethane (PU)-based microporous membrane exhibited good breathability with a WVT rate of 9 kg m−2 d−1.20 However, it featured poor hydrophobicity and thus limited waterproofness with a hydrostatic pressure of only 3.7 kPa. To improve the hydrophobicity of the membrane, low surface energy materials (i.e., perfluorinated segments) were often introduced into the membrane by either postcoating on the membrane or modification in the polymer chains.21−27 Ge et al. fabricated the superhydrophobic microporous PU membranes by introducing a long perfluoroalkyl chain.28 The asprepared membrane with water contact angle (WCA) of 156° exhibited improved waterproofness with hydrostatic pressure of 39.3 kPa while it maintained good breathability with a WVT rate of 9.2 kg m−2 d−1. Later, Wang et al. presented the superhydrophobic membrane by employing the waterborne perfluorinated polyurethane (WFPU) segment with a perfluorohexyl chain in the polyacrylonitrile (PAN) membrane. The membrane exhibited good waterproofness with hydrostatic pressure as high as 83.4 kPa. 29 Although the perfluorinated modification could endow the membrane with superhydrophobicity as well as good waterproofness and breathability, more and more evidence has confirmed that perfluorinated agents especially with long perfluoroalkyl chains bring about serious contamination which is harmful to both the environment and human beings.30−32 In many countries, the production and use of organic perfluorinated compounds have been strictly limited.33 Therefore, developing an environmentally friendly and perfluorinated-free strategy to design a superhydrophobic electrospun membrane with good water resistance and vapor transmission is still highly required. Bioinspired construction of hierarchical micro/nanostructures is a well-known strategy to design a superhydrophobic surface.34,35 Following this concept, superhydrophobic membranes could be achieved in an environmentally friendly way in the absence of perfluorinated moieties. Introducing hydrophobic nanoparticles is a widely used method to construct a rough structure on the membrane surface.36−40 As we know, a uniform and continuous micro/nanostructure on the membrane surface has a great impact on its hydrophobicity. However, direct incorporation of nanoparticles into electrospun membranes using a blending or coating method always
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RESULTS AND DISCUSSION The microsphere-fiber interpenetrated superhydrophobic PVDF microporous membranes were fabricated via a onestep electrospinning process in a high humidity environment. Figure 1a illustrates the fabrication process of the microspherefiber interpenetrated membrane. The solution is ejected from the high-voltage syringe needle and deforms into a conical shape called a Taylor cone under electrostatic field between the syringe needle and the metallic rotating roller collector. 28211
DOI: 10.1021/acsami.8b08191 ACS Appl. Mater. Interfaces 2018, 10, 28210−28218
Research Article
ACS Applied Materials & Interfaces During the following jets flight in the air under high relative humidity (RH) of 90%, part of the fibers gradually transform to microspheres, forming a microsphere-fiber interlinked structure. This is because, as an organic solvent with a high boiling point of 203 °C, NMP is hard to evaporate in air. During the electrospinning process, on the one hand, the high surface tension (33.7 dyn/cm) of NMP tries to make the surface area per unit mass smaller by transforming the jets into microspheres;48 on the other hand, a vapor-induced phase separation could happen on the interface between the microsphere and the high humidity air that thus further induces the formation of microspheres. With increasing the electrospinning time, the microspheres interlink with fibers accumulated layer by layer on the collector and finally formed a microsphere-fiber interpenetrated membrane. Furthermore, to stabilize the microsphere-fiber interpenetrated structure, the obtained electrospun membrane is immersed into water to exchange the residual solvent with water before being dried at 60 °C, which would avoid the structural damage at high temperature. The microsphere-fiber interpenetrated structure induces interconnected pores and rough surface structure, which serves as interconnected routes for water vapor transportation and creates a superhydrophobic surface, respectively. The SEM images of the microsphere-fiber interpenetrated membrane are shown in Figure 1b−d. The membrane prepared from PVDF/NMP (0.15 g mL−1) solution was chosen as a typical example. Figure 1b shows the rough structure of the membrane composed of interpenetrated microspheres and fibers. These microspheres are not isolated but interlink each other through polymer fibers. The detailed structure in Figure 1c shows that the average diameter of microspheres is 3.9 μm. Microprotrusions are also observed on the surfaces of these microspheres, which is attributed to the vapor-induced phase separation (Figure S1). Figure 1d is the cross-sectional SEM image of the membrane. It shows that the microsphere-fiber interpenetrated structure penetrates the whole membrane from top to bottom. The thickness of the membrane is about 55 μm. SEM images of the obtained electrospun membrane without being immersed in water confirm the formation of a microsphere-fiber interpenetrated structure during the electrospinning process (Figure S2). Figure 2a shows the optical photograph of the as-prepared PVDF membrane. The white PVDF membrane exhibits uniform superhydrophobicity all over the whole membrane (20 × 18 cm2) with a water contact angle (WCA) of 152.5°. The membrane superhydrophobicity is also evaluated by dropping salty water, colored water (stained with methyl orange (MO) and brilliant blue (BB)), coffee, and milk on the membrane surface. As shown in Figure 2b, these liquids all exhibit sphere-like droplets on the membrane surface without spreading, indicating the excellent repellency properties of the membrane to these liquids. Figure 2c,d displays the instantaneous sliding behavior of a water droplet on the membrane under the condition that the surface is tilted to about 5° (see Movie S1). The water droplet quickly slides along the tilted face. The formation of a superhydrophobic microsphere-fiber interpenetrated structure depends greatly on the RH of the surrounding environment during the electrospinning process. Figure 3a1−c1 schematically illustrates the transformation of microspheres in jet solutions at RH of 30%, 60%, and 90%, respectively, and the corresponding SEM images of the resulting membranes are shown in Figure 3a2−c2. The
Figure 2. (a) Photograph of an as-prepared PVDF membrane with water droplets (about 3 μL) on it, showing a contact angle of 152.5°. (b) Photograph of the PVDF membrane with various liquid droplets on it. (c and d) Photographs of a water droplet sliding on a slope.
Figure 3. Schematic illustration of the transformation of microspheres in jet solutions at RH of (a1) 30%, (b1) 60%, and (c1) 90%. Topview SEM images of the PVDF membranes prepared at RH of (a2) 30%, (b2) 60%, and (c2) 90%.
PVDF membrane shows relatively smooth morphology without the formation of any microspheres or fibers at low RH of 30% (Figure 3a2). This is because NMP, as a high boiling solvent, cannot evaporate quickly from the jet solution during the electrospinning process. PVDF gradually redissolves in the residual NMP and forms a smooth amorphous membrane without pores. Increasing the RH to 60%, a flatcake type structure appeares on the fibers. It is assumed that, at 60% RH, there is not sufficient water vapor to induce the phase separation between jet solution and moist air when the jet solution flies through the air. As a result, the spheres in the jet solution turn to be a flat-cake structure because of solution sprawled on the collector (Figure 3b2). At high RH up to 90%, the phase separation takes place quickly and PVDF precipitates from the jet solution in the form of a microsphere-fiber interpenetrated structure (Figure 3c2). These results indicate that the higher RH in air, the much quicker phase separation and finally the more microspheres structure in the membrane. The solvent constituent of the PVDF solution has also a great influence on the membrane morphology. Four kinds of solvents with different surface tension, namely, DMF, DMF/ NMP (v:v = 2:1), DMF/NMP (v:v = 1:2), and NMP, are employed to prepare PVDF solutions. The corresponding PVDF membranes prepared from the four solvents are characterized as shown in Figure 4a−d. The PVDF membrane 28212
DOI: 10.1021/acsami.8b08191 ACS Appl. Mater. Interfaces 2018, 10, 28210−28218
Research Article
ACS Applied Materials & Interfaces
To further optimize the structure and hydrophobicity of the membrane, the concentration of PVDF solution is adjusted from 0.150 g mL−1 to 0.250 g mL−1. The representative SEM images of PVDF membranes fabricated from the solutions with different concentrations are presented in Figure 5a−e. It shows that all the membranes display microsphere-fiber interpenetrated structures, but with different microsphere sizes. In the membrane prepared from the solution with low concentration of 0.150 g mL−1, the average diameter of the obtained microspheres is 3.9 μm (Figure 5a). The average diameter of the microspheres increases gradually with increasing the concentration of PVDF solution, from 4.9, 6.6, 7.9, to 12.8 μm corresponding to the concentrations of 0.175, 0.200, 0.225, and 0.250 g mL−1, respectively (Figure 5b−e). This is ascribed to the stronger intermolecular interactions in the polymer solution with high PVDF concentration, which gives rise to the larger fiber diameter in jet solution and thus larger microsphere diameter.17 Moreover, the viscosity of solutions increases rapidly with increasing the concentration of polymer solution (Figure S3). As a result, more fibers are observed on the membranes prepared from higher polymer concentration with higher viscosity.48 Wettability is an important parameter as for waterproof and breathable membranes. The membrane surface structure has a great influence on the membrane wettability. The WCA and the advancing WCA (θadv) are employed to investigate the surface hydrophobicity of the PVDF membranes prepared from polymer solutions with different concentrations as shown in Figure 5f. PVDF membranes prepared from polymer solutions with lower concentrations from 0.150 to 0.200 g mL−1 all exhibit high WCAs more than 150°, indicating their superhydrophobic property. In the case of 0.225 and 0.250 g mL−1, the WCAs of the obtained PVDF membranes slightly decrease to 144.9° and 139.9°, respectively. Besides, θadv is also employed as a function of roughness to evaluate the true contact angles for the Young−Laplace equation (Young’s equiblibrium contact angles). θadv of the PVDF membranes prepared from the solutions with concentrations from 0.150 g mL−1 to 0.250 g mL−1 are 154.6°, 154.4°, 155.7°, 152.2°, and 147.5°, respectively, which agree well with WCAs. These
Figure 4. Top-view SEM images of the PVDF membranes prepared from polymer solutions with solvent of (a) DMF, (b) DMF/NMP (v:v = 2:1), (c) DMF/NMP (v:v = 1:2), and (d) NMP.
composed of clean and smooth fibers is obtained when using DMF as solvent. DMF has a lower boiling point (153 °C) with a surface tension of 25.7 dyn/cm. During the jet flight through air, DMF tends to evaporate quickly and the jet with low surface tension is stretched, elongated, and whipped to fibers.49 Using NMP and DMF as a mixed solvent, a few spindle structures are observed on the fibers because of the increased surface tension by introducing NMP in the solution (Figure 4b,c). Further increasing the NMP content in the solvent, the spindles gradually transform to microspheres as shown in Figure 4c in the case of DMF/NMP (v:v = 1:2). When solely using NMP as solvent, the membrane exhibits a microspherefiber interpenetrated structure which is quite different from the membrane prepared from PVDF solution using DMF as solvent. These results confirm that the surface tension of solvent has significant influence on the membrane morphology. The combination of high RH and the solvent with high surface tension could induce the formation of such a microsphere-fiber interpenetrated structure.
Figure 5. Top-view SEM images of PVDF membranes prepared from NMP with PVDF concentrations of (a) 0.150, (b) 0.175, (c) 0.200, (d) 0.225, and (e) 0.250 g mL−1. (f) WCA and θadv of the corresponding PVDF membranes. 28213
DOI: 10.1021/acsami.8b08191 ACS Appl. Mater. Interfaces 2018, 10, 28210−28218
Research Article
ACS Applied Materials & Interfaces
Figure 6. Microporous structure and waterproofness of PVDF membranes prepared with various PVDF concentrations. (a) Pore size distribution. (b) Maximum pore size and porosity. (c) Hydrostatic pressure. (d) Relationship among the hydrostatic pressure, dmax, and θadv of the PVDF membranes fitted by the Young−Laplace equation.
Figure 7. (a) WVT rate and air permeability of PVDF membranes prepared with various PVDF concentrations. (b) A typical test demonstrating the breathable performance. (c) Schematic illustrating the process of the breathable behavior through the membrane.
results indicate that the size of microspheres affects the wettability. A proper microsphere size is requested to achieve superhydrophobicity. Apart from the wettability, the effect of PVDF concentration on the membrane pore size and distribution is also
investigated. As shown in Figure 6a, the pore size distributions of these PVDF membranes vary in the range of 0.92−6.21 μm. The average pore size increases with increasing the PVDF concentration, which is in well accordance with the diameter of the microspheres. The maximum pore size (dmax) and porosity 28214
DOI: 10.1021/acsami.8b08191 ACS Appl. Mater. Interfaces 2018, 10, 28210−28218
Research Article
ACS Applied Materials & Interfaces
Figure 8. (a) Stress−strain curves of PVDF membranes prepared with various PVDF concentrations. (b) Change of θadv and SA of the PVDF membrane with increasing cycle number. The PVDF membrane prepared from NMP with concentration of 0.15 g mL−1 is chosen for the experiment.
the theoretical and experimental values exhibit a similar increasing trend with the increase of polymer concentration. Another consideration is to theoretically fit the curve of WVT rate vs membrane porosity obtained based on experimental results. As shown in Figure S4, when the membrane porosity is smaller than 77%, there is no significant difference about the WVT rate. When the membrane porosity increases to more than 80%, the WVT rate increases greatly. A higher porosity leads to a higher WTV rate. The proportional relationship between WVT rate and membrane porosity can be explained by the Fickian diffusion model in which expounded diffusion flux is positively related to diffusion coefficient that is determined by porosity (see Figure S5 and detailed discussion therein). It demonstrates that the microsphere-fiber interpenetrated structure in our membrane is rather stable and can anti-deform under the air pressure drop of 100 Pa (Figure S6).7 To further demonstrate the breathability of our membrane, a typical experiment is carried out where allochroic silica gel particles are used as humidity indicator to confirm the water vapor transmitting through the membrane. As presented in Figure 7b, the PVDF membrane is covered on a beaker and boiling water is filled in it. After boiling to 100 °C, large quantities of steam are clearly observed passing through the membrane. The blue allochroic silica gel particles change to pink within 8 min (see Movie S2. The speed of this video is 2000 times the original video.). The microsphere-fiber interpenetrated feature of our membrane provides more interconnected channels and endows the membrane with good breathability. A simulation of the process of water vapor transmission through the as-prepared PVDF membrane is shown in Figure 7c. In addition, mechanical performance of the waterproof and breathable membranes is evaluated by tensile tests. As shown in Figure 8a, with increasing the PVDF concentration from 0.150 to 0.250 g mL−1, the breaking elongation of the membranes increases from 50% to 170% and the tensile strength increases from 0.78 to 1.42 MPa. It is because the proportion of fiber structure in the membrane increases with increasing polymer concentration. To further demonstrate the durability and flexibility for long time use, a self-made bending test is conducted where a PVDF membrane is bent to above 90° continuously for more than 50 cycles and the corresponding θadv and SA are measured every time (Figure S7). As shown in Figure 8b, during the whole 50 bending cycles, the θadv slightly decreases from the initial 154.6° to
are shown in Figure 6b. Similarly, with increasing the PVDF concentration from 0.150 to 0.250 g mL−1, dmax increases from 2.32 to 6.21 μm and the porosity increases from 75% to 84%, respectively. Waterproof performance which represents the minimum entry pressure for liquid water in a hydrophobic membrane is evaluated by measuring the hydrostatic pressure. The hydrostatic pressure is elucidated by the Young−Laplace equation P = −A·
4γ ·cos θadv dmax
where γ represents the surface tension of water, θadv is the advancing water contact angle, and dmax is the maximum pore size; A as geometry coefficient is used to describe the irregular noncylindrical pores among the adjacent microspheres. Benefiting from the superhydrophobicity, the PVDF membrane prepared from NMP solution with a concentration of 0.150 g mL−1 exhibits the highest hydrostatic pressure up to 62 kPa (Figure 6c). With increasing the concentration of polymer solutions, the hydrostatic pressure decreases gradually to 55, 35, 26, and 22 kPa, corresponding to the concentration of 0.175, 0.200, 0.225, and 0.250 g mL−1, respectively, due to the increase of dmax. The relationship among hydrostatic pressure, dmax, and θadv fits well the Young−Laplace equation and A is calculated to be 0.54 (Figure 6d). The breathable performance is evaluated by measuring the WTV rate of the membranes. As shown in Figure 7a, all the membranes exhibit similar WVT rate in the range of 10.6−11.1 kg m−2 d−1, indicating our PVDF membranes possessed good breathable performance. The windproof performances of the PVDF membranes is evaluated by measuring their air permeability. As shown in Figure 7a, the PVDF membranes display outstanding windproofness with air permeability ranging from 1.3 to 4.7 mm s−1. The PVDF membrane prepared from the polymer solution with the concentration of 0.150 g mL−1 exhibits the best windproof performance with air permeability up to 1.3 mm s−1. The windproof performance of our PVDF membranes is superior to that of most of the reported fibrous membranes. Two mathematical models regarding the overall mass transfer coefficient for the water vapor transmission process have been considered to validate the experimental observations on membranes performance. One is the comparison of the calculated air permeability of the membranes with the experimental values based on Poiseuille’s law (see Figure S4 and detail discussion therein). It shows that 28215
DOI: 10.1021/acsami.8b08191 ACS Appl. Mater. Interfaces 2018, 10, 28210−28218
Research Article
ACS Applied Materials & Interfaces 152.7° and SA slightly increases from the initial 4.8° to 6.2°, indicating a good flexibility.
Porosity =
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ρ0
× 100%
where ρ0 and ρ1 are the density of raw PVDF materials and PVDF membranes, respectively. Measurement of Membrane Performances. The waterproofness of the membranes was evaluated by measuring hydrostatic pressure at a hydrostatic pressure test equipment (YG812C, Ningbo Textile Instruments Co., Ltd.) according to the AATCC 127 standard with a pressurization rate of 6.0 kPa min−1. The WVT rate that represented the breathable performance was measured according to the ASTM E96 inverse cup standards by using a water vapor transmission test equipment (YG 601H, Ningbo Textile Instruments Co., Ltd.), under constant temperature of 38 °C and relative humidity of 50% . The WVT rate values were calculated as follows m − m2 WVT rate = 1 × 24 S
CONCLUSION In summary, novel microporous PVDF waterproof and breathable membranes were fabricated through a facile and perfluorinated-free high-humidity induced electrospinning process. The unique microsphere-fiber interpenetrated hierarchical structure endowed the PVDF microporous membrane with superhydrophobicity. The effective pore size could be tuned in the range of 0.92−6.21 μm by adjusting the concentration of the PVDF solution. The relationship between porous structure and waterproof/breathable performance was investigated in detail. The membrane with optimized wettability and pore structure showed improved waterproof and breathable performance with hydrostatic pressure of 62 kPa, water vapor transmission rate (WVT rate) of 10.6 kg m−2 d−1, and outstanding windproof performance with air permeability up to 1.3 mm s−1. The perfluorinated-free fabrication of a superhydrophobic fibrous membrane from pure PVDF polymer matches well with the environmentally friendly criterion. The facile preparation process and excellent performance of our membrane show great potential for the application as a waterproof and breathable membrane.
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ρ0 − ρ1
where the unit of WVT rate is kilograms per square meter per day, m1 is the mass of the test cup before the test, m2 is the mass of the test cup after the test, and S is the test area. The air permeability of the membranes was measured under a differential pressure of 100 Pa according to the ASTM D 737 criterion. The results of hydrostatic pressure, WVT rate, and air permeability were obtained from three measurements per sample. The mechanical performances were measured on a universal material testing machine (Instron 3365, America).
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EXPERIMENTAL SECTION
ASSOCIATED CONTENT
S Supporting Information *
Materials. PVDF powder (Solef1015, Mn = 238 000) was purchased from the Solvay Chemicals Company and used as received. N-methyl-pyrrolidone (NMP), N,N-dimethylformamide (DMF), methyl orange, and brilliant blue were purchased from Shanghai Chemical Reagent Co., Ltd. Allochroic silica gel particles were purchased from Aladdin Chemical Reagent Co., China. All the reagents were used as received without further purification. Deionized water (18 MΩ) was prepared using a Millipore Direct-Q system. Fabrication of PVDF Membranes. Briefly, PVDF solutions were prepared through dissolving PVDF powder in NMP, DMF, DMF/ NMP, respectively, and stirred at 80 °C for 12 h to form transparent solutions; then the solutions were kept at 25 °C for 12 h to sufficiently eliminate bubbles. The concentrations of PVDF/NMP solutions were 0.150, 0.175, 0.200, 0.225, and 0.250 g mL−1. PVDF in DMF and PVDF in DMF/NMP solutions were also prepared according to the above procedure. The electrospinning process was performed using an SS-2535H spinning equipment (Beijing Ucalery Co., Ltd.). Typically, the solution was loaded into a syringe and ejected through a metallic needle with a constant feed rate of 1.28 mL h−1. The syringe was fixed in the horizontal direction. A high voltage of 12 kV was applied to the needle tip. The as-prepared membranes were deposited on the grounded metallic rotating roller collector with a tip-to-collector distance of 12 cm. The temperature was kept at 25 ± 2 °C, and relative humidity was kept at 30 ± 5%, 60 ± 5%, and 90 ± 5%, respectively. The corresponding membranes were immersed in deionized water for 2 h to remove the residual solvent. Then, the membranes were dried in an oven for 2 h at 60 °C, and they were finally stored at room temperature. The thicknesses of the membranes were 50−60 μm. Characterization of Polymer Solutions and Membranes. Viscosity measurements of dope solutions were done on a rotational rheometer (Haake RS6000). The microstructures of the membranes were characterized using scanning electron microscopy (Hitachi S4800, Japan). Optical images and videos were captured using a Canon Camera (Nikon, Japan). Water contact angle measurements were conducted on a Data-Physics OCA 20 at room temperature using a water droplet. The average value of three measurements performed at different positions on the same membrane was adopted as the contact angle value. The pore sizes were measured via a bubble point method using a pore size analyzer (3H-2000PB). Porosity was calculated using the following equation
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b08191. Water droplets sliding on a slope (Movie S1) (MPG) A demonstration of the breathable performance (Movie S2) (MPG) The enlarged SEM image of the microsphere (Figure S1). SEM images of PVDF membranes without being immersed in water (Figure S2). Viscosities of polymer solutions with various PVDF concentrations (Figure S3). The theoretical and experimental curves of the air permeability (Figure S4). The relationship between WVT rate and porosity (Figure S5). A schematic illustration of the windproof performances (Figure S6). Photographs of a self-made bending test for the PVDF membrane (Figure S7) (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (F.Z.). *E-mail:
[email protected] (J.J.). ORCID
Jian Jin: 0000-0003-0429-300X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to thank Prof. Bin Ding from the College of Textiles, Donghua University, for help with measurements of WVT rate and hydrostatic pressure. This work was supported by the National Natural Science Funds for Distinguished Young Scholar (Grant No. 51625306), the Key Project of National Natural Science Foundation of China (Grant No. 21433012), the National Natural Science Foundation of China (Grant Nos. 21406258, 51603229, and 51403231), and the 28216
DOI: 10.1021/acsami.8b08191 ACS Appl. Mater. Interfaces 2018, 10, 28210−28218
Research Article
ACS Applied Materials & Interfaces
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Natural Science Foundation of Jiangsu Province (Grant No. BE2015072).
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DOI: 10.1021/acsami.8b08191 ACS Appl. Mater. Interfaces 2018, 10, 28210−28218
