Microsphere-Fiber Interpenetrated Superhydrophobic PVDF

Subscriber access provided by Grand Valley State | University

Surfaces, Interfaces, and Applications

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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08191 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 29, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

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. KEYWORDS: Superhydrophobic microporous membranes, waterproof and breathable membrane, perfluorinated-free, electrospinning method

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 post-modifying base membranes with perfluorinated

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

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 environmental friendly requirement from the viewpoint of practical application.

INTRODUCTION Waterproof and breathable membranes are able to prevent the penetration of liquid water droplets yet allow water vapor pass through. Therefore, they have shown varieties of potential applications, including protective clothing, building materials, chemical and aerospace equipments.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 fibrillation, template methods, phase separation, melt blown and electrospinning.8-10 Among them, electrospinning has proven to be an effective method to


2

Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fabricate microporous membranes with 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 polyurethane (PU)-based microporous membrane exhibited good breathability with 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 post-coating on the membrane or modification in the polymer chains.21-27 Ge et al. fabricated the superhydrophobic microporous PU membranes by introducing long perfluoroalkyl chain.28 The as-prepared membrane with water contact angle (WCA) of 156° exhibited improved waterproofness with hydrostatic pressure of 39.3 kPa while maintained good breathability with 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 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 evidences have confirmed that perfluorinated agents especially with long perfluoroalkyl chains bring about serious contamination which is harmful to both 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 environmental


3


Page 4 of 31

friendly and perfluorinated-free strategy to design superhydrophobic electrospun membrane with good water resistance and vapor transmission is still highly required. Bioinspired construction of hierarchical micro/nanostructure is a well-known strategy to design superhydrophobic surface.34,35 Following this concept, superhydrophobic membranes could be achieved in an environmental friendly way in the absence of perfluorinated moieties. Introducing hydrophobic nanoparticles is a widely used method to construct 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 blending or coating method always suffer from inhomogeneous dispersity of the nanoparticles on the membrane which limits the improvement of hydrophobicity.41 In addition, direct incorporation method also confronts the problem of poor stability of the nanoparticles because of the weak interaction between nanoparticles and membranes. Despite 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 make the superhydrophobic modification process more complicated and would also bring about secondary pollution to the environment.42-47 Hence, to develop a facile and environmental friendly methodology to fabricate superhydrophobic electrospun membrane and avoid complex posttreatment procedure is necessary. In this work, we report a facile and perfluorinated-free strategy to fabricate superhydrophobic

microporous

membrane.

A

novel

microsphere-fiber

interpenetrated

mircoporous superhydrophobic PVDF membrane is achieved through a high-humidity induced one-step electrospinning process. The microsphere/fiber structure of the electrospun membrane


4

Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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.

RESULTS AND DISCUSSION The microsphere-fiber interpenetrated superhydrophobic PVDF microporous membranes were fabricated via a one-step electrospinning process in a high humidity environment. Figure 1a illustrates the fabrication process of the microsphere-fiber interpenetrated membrane. The solution is ejected from the high-voltage syringe needle and deforms into a conical shape called Taylor cone under electrostatic field between the syringe needle and the metallic rotating roller collector. During the following jets flight in the air under high relative humidity (RH) of 90%, part of the fibers gradually transforms to microspheres forming a microsphere-fiber interlinked structure. This is because as an organic solvent with 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 dyne/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 thus further induces the formation of microspheres. With increasing the electrospinning time, the microspheres interlinks with fibers accumulated layer by layer on the collector and finally formed a microsphere-fiber interpenetrated membrane. Furthermore, to stabilize the microsphere-fiber interpenetrated


5


Page 6 of 31

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 structure 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-1d. 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. Micro-protrusions 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 penetrate 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 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 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 and 2d display the


6

Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


instantaneous sliding behavior of a water droplet on the membrane under the condition that the surface is tilted to about 5° (see Movie S1). Water droplet quickly slides along the tilted face. The formation of superhydrophobic microsphere-fiber interpenetrated structure depends greatly on the RH of surrounding environment during electrospinning process. Figure 3a1-3c1 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 3a2-3c2. The PVDF membrane shows relatively smooth morphology without the formation of any microspheres or fibers at low RH of 30% (Figure 3a2). This is because that NMP, as a high boiling solvent, can not evaporate quickly from the jet solution during the electrospinning process. PVDF gradually re-dissolves in the residual NMP and forms a smooth amorphous membrane without pores. Increasing the RH to 60%, a flat-cake type structure appeares on the fibers. It is assumed that at 60% RH, there is no sufficient water vapor to induce the phase separation between jet solution and moist air when the jet solution fly through the air. As a result, the spheres in the jet solution turn to be 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 jet solution in the form of microsphere-fiber interpenetrated structure (Figure 3c2). These results indicate that the higher RH in air, the much quicker phase separation and finally the more microshperes 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-4d. The PVDF membrane composed of clean and smooth fibers is obtained when


7


Page 8 of 31

using DMF as solvent. DMF has lower boiling point (153°C) with surface tension of 25.7 dyne/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-4c). Further increasing the NMP content in the solvent, the spindles gradually transform to microshperes as shown in Figure 4c in the case of DMF/NMP (v:v=1:2). When solely using NMP as solvent, the membrane exhibits microsphere-fiber 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. Combination of high RH and the solvent with high surface tension could induce the formation of such a microsphere-fiber interpenetrated structure. 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-5e. It shows that all the membranes display microsphere-fiber interpenetrated structures, but with different microsphere size. 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-5e). 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 thus larger microsphere


8

Page 9 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 membranesprepared 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 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 are also investigated. As shown in Figure 6a, the pore size distributions of these PVDF membranes varies 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 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.


9


Page 10 of 31

Waterproof performance which represents the minimum entry pressure for liquid water in hydrophobic membrane is evaluated by measuring the hydrostatic pressure. The hydrostatic pressure is elucidated by the Young-Laplace equation:

= − ∙

4 ∙

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 non-cylindrical

pores

among

the

adjacent

microspheres.

Benefiting

from

the

superhydrophobicity, the PVDF membrane prepared from NMP solution with 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 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 to 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 behaves the best windproof performance with air permeability up to 1.3 mm s-1. The windproof performance of our PVDF membranes is superior to most of reported fibrous membranes. Two mathematical models regarding the overall mass


10

Page 11 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 the theoretical and experimental values behave 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 Fickian diffusion model in which expounded diffusion flux is positively related to diffusion coefficient that is determined by porosity (see Figure S5 and detail 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 minutes (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.


11


Page 12 of 31

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 152.7° and SA slightly increases from the initial 4.8° to 6.2°, indicating a good flexibility.

CONCLUSION In summary, a novel mircoporous 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 superhydrophobic fibrous membrane from pure PVDF polymer matches well with the environmental friend criterion. The facile preparation


12

Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


process and excellent perfromance of our membrane shows great potential for the application as waterproof and breathable membrane.

EXPERIMENTAL SECTION Materials. PVDF powder (Solef1015, Mn = 238000) 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. De-ionized 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 12h to sufficiently eliminate bubbles. The concentrations of PVDF/NMP solution 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 a 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 asprepared membranes were deposited on the grounded metallic rotating roller collector with a tipto-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


13


Page 14 of 31

dried in oven for 2 h at 60°C, and they were finally stored at room temperature. The thickness 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 S-4800, 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 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:

=

− × 100%

Where ρ0 and ρ1 are density of raw PVDF materials and PVDF membranes, respectively. Measurement of membrane performances. The waterproofness of the membranes were evaluated by measuring hydrostatic pressure at a hydrostatic pressure test equipment (YG812C, Ningbo Textile Instruments Co., Ltd.) according to the AATCC 127 standard with 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:

WVT rate =

% − %& × 24 '


14

Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 were 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). ASSOCIATED CONTENT Supporting Information. 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) Water droplets sliding on a slope (Movie S1). (Movie) A demonstration of the breathable performance (Movie S2). (Movie) These materials are available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail (F. Zhang): [email protected]; *E-mail (J. Jin): [email protected]. ORCID Feng Zhang: 0000-0001-7614-3129;


15


Page 16 of 31

Jian Jin: 0000-0003-0429-300X. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT 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 Natural Science Foundation of Jiangsu Province (Grant No. BE2015072). REFERENCES (1) Bohácek, J.; Singh, C. Self-sealing and Puncture Resistant Breathable Membranes for Water-evaporation Applications. Adv. Mater. 2015, 27, 6620-6624. (2) Cao, W.; Cudney, H. H.; Waser, R. Smart Materials and Structures. Proc. Natl. Acad. Sci. 1999, 96, 8330-8331. (3) Fornasiero, F. Water Vapor Transport in Carbon Nanotube Membranes and Application in Breathable and Protective Fabrics. Curr. Opin. Chem. Eng. 2017, 16, 1-8. (4) Ikem, V. O.; Menner, A.; Horozov, T. S.; Bismarck, A. Highly Permeable Macroporous Polymers Synthesized from Pickering Medium and High Internal Phase Emulsion Templates. Adv. Mater. 2010, 22, 3588-3592.


16

Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(5) Li, Y.; Yang, F.; Yu, J.; Ding, B. Hydrophobic Fibrous Membranes with Tunable Porous Structure for Equilibrium of Breathable and Waterproof Performance. Adv. Mater. Interfaces 2016, 3, 1600516. (6) Gibson, P.; Schreuder, G. H.; Rivin, D. Transport Properties of Porous Membranes Based on Electrospun Nanofibers. Colloids Surf. A 2001, 187, 469-481. (7) Yang, F.; Li, Y.; Yu, X.; Wu, G.; Yin, X.; Yu, J.; Ding, B. Hydrophobic Polyvinylidene Fluoride Fibrous Membranes with Simultaneously Water/Windproof and

Breathable

Performance. RSC Adv. 2016, 6, 87820-87827. (8) Yu, B.; Han, J.; He, X. B.; Xu, G. P.; Ding, X. B. Effects of Tourmaline Particles on Structure and Properties of Polypropylene Filtration Melt-blown Nonwoven Electrets. J. Macromol. Sci. Part B : Phys. 2012, 51, 619-629. (9) Nejati, S.; Boo, C.; Osuji, C. O.; Elimelech, M. Engineering Flat Sheet Microporous PVDF Films for Membrane Distillation. J. Membr. Sci. 2015, 492, 355-363. (10) Tabatabaei, S. H.; Carreau, P. J.; Ajji, A. Microporous Membranes Obtained from Polypropylene Blend Films by Stretching. J. Membr. Sci. 2008, 325, 772-782. (11) Wang, X.; Ding, B.; Sun, G.; Wang, M.; Yu, J. Electro-spinning/netting: A Strategy for the Fabrication of Three-dimensional Polymer Nano-fiber/nets. Prog. Mater. Sci. 2013, 58, 11731243. (12) Greiner, A.; Wendorff, J. H. Electrospinning: A Fascinating Method for the Preparation of Ultrathin Fibers. Angew. Chem. Int. Ed. 2007, 46, 5670-5703. (13) Si, Y.; Yu, J.; Tang, X.; Ge, J.; Ding, B. Ultralight Nanofibre-assembled Cellular Aerogels with Superelasticity and Multifunctionality. Nat. Commun. 2014, 5, 5802.


17


Page 18 of 31

(14) Nuraje, N.; Khan, W. S.; Lei, Y.; Ceylan, M.; Asmatulu, R. Superhydrophobic Electrospun Nanofibers. J. Mater. Chem. A 2013, 1, 1929-1946. (15) Ning, J.; Yang, M.; Yang, H.; Xu, Z. Tailoring the Morphologies of PVDF Nanofibers by Interfacial Diffusion during Coaxial Electrospinning. Mater. Des. 2016, 109, 264-269. (16) Ge, J.; Si, Y.; Fu, F.; Wang, J.; Yang, J.; Cui, L.; Ding, B.; Yu, J.; Sun, G. Amphiphobic Fluorinated Polyurethane Composite Microfibrous Membranes with Robust Waterproof and Breathable Performances. RSC Adv. 2013, 3, 2248-2255. (17) Li, Y.; Zhu, Z.; Yu, J.; Ding, B. Carbon Nanotubes Enhanced Fluorinated Polyurethane Macroporous Membranes for Waterproof and Breathable Application. ACS Appl. Mater. Interfaces 2015, 7, 13538-13546. (18) Mao, X.; Chen, Y.; Si, Y.; Li, Y.; Wan, H.; Yu, J.; Sun, G.; Ding, B. Novel Fluorinated Polyurethane Decorated Electrospun Silica Nanofibrous Membranes Exhibiting Robust Waterproof and Breathable Performances. RSC Adv. 2013, 3, 7562-7569. (19) Wang, J.; Raza, A.; Si, Y.; Cui, L.; Ge, J.; Ding, B.; Yu, J. Synthesis of Superamphiphobic Breathable Membranes Utilizing SiO2 Nanoparticles Decorated Fluorinated Polyurethane Nanofibers. Nanoscale 2012, 4, 7549-7556. (20) Kang, Y. K.; Park, C. H.; Kim, J.; Kang, T. J. Application of Electrospun Polyurethane Web to Breathable Water-proof Fabrics. Fibers Polym. 2007, 8, 564-570. (21) Peng, C.; Chen, Z.; Tiwari, M. K. All-organic Superhydrophobic Coatings with Mechanochemical Robustness and Liquid Impalement Resistance. Nat. Mater. 2018, 17, 355360.


18

Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22) Yamanaka, M.; Sada, K.; Miyata, M.; Hanabusa, K.; Nakano, K. Construction of Superhydrophobic Surfaces by Fibrous Aggregation of Perfluoroalkyl Chain-containing Organogelators. Chem. Commun. 2006, 2248-2250. (23) Grignard, B.; Vaillant, A.; de Coninck, J.; Piens, M.; Jonas, A. M.; Detrembleur, C.; Jerome, C. Electrospinning of a Functional Perfluorinated Block Copolymer as a Powerful Route for Imparting Superhydrophobicity and Corrosion Resistance to Aluminum Substrates. Langmuir 2011, 27, 335-342. (24) Lee, E. J.; An, A. K.; He, T.; Woo, Y. C.; Shon, H. K. Electrospun Nanofiber Membranes Incorporating Fluorosilane-coated TiO2 Nanocomposite for Direct Contact Membrane Distillation. J. Membr. Sci. 2016, 520, 145-154. (25) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y.; Wang, Z. Superhydrophobic Surfaces: from Structural Control to Functional Application. J. Mater. Chem. 2008, 18, 621-633. (26) Li, X. M.; Reinhoudt, D.; Crego, C. M. What do We Need for a Superhydrophobic Surface? A Review on the Recent Progress in the Preparation of Superhydrophobic Surfaces. Chem. Soc. Rev. 2007, 36, 1350-1368. (27) Ren, L. F.; Xia, F.; Chen, V.; Shao, J.; Chen, R.; He, Y. TiO2-FTCS Modified Superhydrophobic PVDF Electrospun Nanofibrous Membrane for Desalination by Direct Contact Membrane Distillation. Desalination 2017, 423, 1-11. (28) Ge, J.; Si, Y.; Fu, F.; Wang, J.; Yang, J.; Cui, L.; Ding, B.; Yu, J.; Sun, G. Amphiphobic Fluorinated Polyurethane Composite Microfibrous Membranes with Robust Waterproof and Breathable Performances. RSC Adv. 2013, 3, 2248-2255.


19


Page 20 of 31

(29) Wang, J.; Li, Y.; Tian, H.; Sheng, J.; Yu, J.; Ding, B. Waterproof and Breathable Membranes of Waterborne Fluorinated Polyurethane Modified Electrospun Polyacrylonitrile Fibers. RSC Adv. 2014, 4, 61068-61076. (30) Ng, C. A.; Hungerbuhler, K. Bioaccumulation of Perfluorinated Alkyl Acids: Observations and Models. Environ. Sci. Technol. 2014, 48, 4637-4648. (31) Lee, H.; D'eon, J.; Mabury, S. A. Biodegradation of Polyfluoroalkyl Phosphates as a Source of Perfluorinated Acids to the Environment. Environ. Sci. Technol. 2010, 44, 3305-3310. (32) Nakata, H.; Kannan, K.; Nasu, T.; Cho, H. S.; Sinclair, E.; Takemura, A. Perfluorinated Contaminants in Sediments and Aquatic Organisms Collected from Shallow Water and Tidal Flat Areas of the Ariake Sea, Japan: Environmental Fate of Perfluorooctane Sulfonate in Aquatic Ecosystems. Environ. Sci. Technol. 2006, 40, 4916-4921. (33) Zhao, J.; Li, Y.; Sheng, J.; Wang, X.; Liu, L.; Yu, J.; Ding, B. Environmentally Friendly and Breathable Fluorinated Polyurethane Fibrous Membranes Exhibiting Robust Waterproof Performance. ACS Appl. Mater. Interfaces 2017, 9, 29302-29310. (34) Wang, S.; Liu, K.; Yao, X.; Jiang, L. Bioinspired Surfaces with Superwettability: New Insight on Theory, Design, and Applications. Chem. Rev. 2015, 115, 8230-8293. (35) Chu, Z. L.; Feng, Y. J.; Seeger, S. Oil/Water Separation with Selective Superantiwetting/Superwetting Surface Materials. Angew. Chem. Int. Ed. 2015, 54, 2328-2338. (36) Wang, X.; Ding, B.; Yu, J.; Wang, M. Engineering Biomimetic Superhydrophobic Surfaces of Electrospun Nanomaterials. Nano Today 2011, 6, 510-530. (37) Su, C.; Li, Y.; Dai, Y.; Gao, F.; Tang, K.; Cao, H. Fabrication of Three-dimensional Superhydrophobic Membranes with High Porosity via Simultaneous Electrospraying and Electrospinning. Mater. Lett. 2016, 170, 67-71.


20

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Gao, J.; Huang, X.; Xue, H.; Tang, L.; Li, R. K. Y. Facile Preparation of Hybrid Microspheres for Super-hydrophobic Coating and Oil-water Separation. Chem. Eng. J. 2017, 326, 443-453. (39) Li, Y.; Zou, C.; Shao, J.; Zhang, X.; Li, Y. N. Preparation of SiO2/PS Superhydrophobic Fibers with Bionic Controllable Micro-nano Structure via Centrifugal Spinning. RSC Adv. 2017, 7, 11041-11048. (40) Lai, Y.; Huang, J.; Cui, Z.; Ge, M.; Zhang, K. Q.; Chen, Z.; Chi, L. Recent Advances in TiO2-based Nanostructured Surfaces with Controllable Wettability and Adhesion. Small 2016, 12, 2203-2224. (41) Zou, H.; Wu, S.; Shen, J. Polymer/Silica Nanocomposites: Preparation, Characterization, Properties, and Applications. Chem. Rev. 2008, 108, 3893-3957. (42) Liao, Y.; Loh, C. H.; Wang, R.; Fane, A. G. Electrospun Superhydrophobic Membranes with Unique Structures for Membrane Distillation. ACS Appl. Mater. Interfaces 2014, 6, 1603516048. (43) Li, X.; Yu, X.; Cheng, C.; Deng, L.; Wang, M.; Wang, X. Electrospun Superhydrophobic Organic/Inorganic Composite Nanofibrous Membranes for Membrane Distillation. ACS Appl. Mater. Interfaces 2015, 7, 21919-21930. (44) Xue, C. H.; Li, M.; Guo, X. J.; Li, X.; An, Q. F.; Jia, S. T. Fabrication of Superhydrophobic Textiles with High Water Pressure Resistance. Surf. Coat. Technol. 2017, 310, 134-142. (45) Jin, S.; Park, Y.; Park, C. H. Preparation of Breathable and Superhydrophobic Polyurethane Electrospun Webs with Silica Nanoparticles. Text. Res. J. 2016, 86, 1816-1827.


21


Page 22 of 31

(46) Sun, H.; Xu, Y.; Zhou, Y.; Gao, W.; Zhao, H.; Wang, W. Preparation of Superhydrophobic Nanocomposite Fiber Membranes by Electrospinning Poly (vinylidene fluoride)/Silane Coupling Agent Modified SiO2 Nanoparticles. J. Appl. Polym. Sci. 2017, 134, 44501. (47) Sheng, J.; Xu, Y.; Yu, J.; Ding, B. Robust Fluorine-free Superhydrophobic Amino-silicone Oil/SiO2 Modification of Electrospun Polyacrylonitrile Membranes for Waterproof-breathable Application. ACS Appl. Mater. Interfaces 2017, 9, 15139-15147. (48) Fong, H.; Chun, I.; Reneker, D. H. Beaded Nanofibers Formed during Electrospinning. Polymer 1999, 40, 4585-4592. (49) Seyed Shahabadi, S. M.; Rabiee, H.; Seyedi, S. M.; Mokhtare, A.; Brant, J. A. Superhydrophobic Dual Layer Functionalized Titanium Dioxide/Polyvinylidene Fluoride-cohexafluoropropylene (TiO2/PH) Nanofibrous Membrane for High Flux Membrane Distillation. J. Membr. Sci. 2017, 537, 140-150.


22

Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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.


23


Page 24 of 31

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.


24

Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Schematic illustration of the transformation of microspheres in jet solutions at RH of (a1) 30%, (b1) 60%, and (c1) 90%. Top-view SEM images of the PVDF membranes prepared at RH of (a2) 30%, (b2) 60% and (c2) 90%.


25


Page 26 of 31

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.


26

Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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.


27


Percentage (%)

90

60

30

(b)

0 1

2

3

4

5

8

90

6 60 4 30

2

0

0

6

Pore size (µm)

0.150

0.175

0.200

0.225

0.250

-1

Concentration (g mL )

(d) Hydrostatic pressure (kPa)

(c)

Maximum pore size Porosity

Porosity (%)

-1

0.150 g mL -1 0.175 g mL 0.200 g mL-1 0.225 g mL-1 0.250 g mL-1

Maximum pore size (µm)

(a)

Hydrostatic pressure (kPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

60

40

20

Experimental value Fitted curve

60

40

20

0

0 0.150

0.175

0.200

0.225

0.250

2

-1

Concentration (g mL )

4

6

8

-dmax/cosθadv

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 Young-Laplace equation.


28

Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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.


29


θadv SA (°)

160

25 20

120

1.0

15 80

SA (°)

1.5

(b)

0.150 g mL-1 0.175 g mL-1 0.200 g mL-1 0.225 g mL-1 0.250 g mL-1

θadv (°)

(a) Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 31

10

0.5 40 0.0 0%

5 0

0 50%

100%

Strain (%)

150%

0

10

20

50

Cycle number (times)

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.


30

Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC graphic


31

Microsphere-Fiber Interpenetrated Superhydrophobic PVDF

Recommend Documents