b. Key Laboratory of Marine Materials and Related Technologies, Ningbo Institute of Materials Tech-nology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China
Superhydrophobic surfaces with water contact angles larger than 150° and sliding angles less than 10° have drawn a great deal of attention due to their potential applications in self-cleaning [1, 2], anti-icing , oil-water separation , corrosion resistant surfaces [5, 6], marine antifouling , etc. It is known that an appropriate surface roughness and a low surface energy material are two key factors for superhydrophobicity . In recent decades, many technologies such as template , etching , chemical vapor deposition , electrospinning , sol-gel process , layer-by-layer deposition  have been reported to prepare superhydrophobic surfaces . However, the fine-scale rough structures constructed by the above-mentioned technologies are easily destroyed by the mechanical force such as finger contact, abrasive, and washing, resulting in dramatically reducing contact angles . Moreover, regeneration of superhydrophobic surfaces is difficult to achieve . Therefore, a simple method to prepare a renewable superhydrophobic surface is highly desirable.
Thermally induced phase separation (TIPS) is one of the most useful methods to fabricate porous polymer membranes, which is suitable for large-scale industrialization . It has several advantages such as easiness of control, low tendency for defects formation, and diverse microstructures that are desirable for engineering applications . Moreover, the isotropic and anisotropic structures can be generated, and the pore size can be effectively controlled . TIPS process has been adopted to prepare porous polyvinylidene fluoride (PVDF) membranes . PVDF is usually used to construct superhydrophobic surfaces due to its low surface energy . However, until now, a superhydrophobic surface of PVDF has never been reported to be prepared via TIPS process.
In this work, a renewable superhydrophobic surface was constructed via TIPS and mechanical peeling. A series of porous PVDF membranes with hierarchical structures were prepared by TIPS process using dimethyl sulfone and polyethylene glycol as a crystallizable diluent and additive, respectively. The pore morphology and size of the PVDF membranes can be controlled by polymer concentration and cooling condition. After peeling off the top layer, rough structures with hundreds of nanometers to several microns were obtained. The peeled membrane surfaces displayed superhydrophobicity with a high water contact angle (CA=152°) and a low sliding angle (SA=7.2°). Moreover, the superhydrophobicity can be easily recovered for many times by a simple mechanical peeling. This simple method for fabricating a renewable superhydrobic surface is low cost, and suitable for large-scale industrialization, which may offer more opportunities for practical applications.Ⅱ. EXPERIMENTS A. Materials
Polyvinylidene fluoride (PVDF, Mw=11, 000) was purchased from Solvay and dried at 50 ℃ under vacuum for 24 h before use. Dimethyl sulfone (DMSO2, 99% purity) was provided by Dakang Chemicals Co., China. Polyethylene glycol (PEG400, Mw=380-430), ethanol and hexane were analytically pure and obtained from Sinopharm Chemical Reagent Co. Ltd. DI water was used as the extraction agent.B. Preparation of PVDF membranes
PVDF, DMSO2 and PEG400 were weighed in a certain mass ratio according to Table Ⅰ, and mixed in a glass vessel. The mixture was heated at 160 ℃ with vigorous stirring to form a homogenous solution. When the air bubbles were all degassed, the solution was quickly transferred onto a stainless steel mold (thickness ~200 μm) preheated in an oven at 160 ℃. Then, the mold was quenched into a cooling bath (water bath at 30 ℃, or air bath at 30 ℃) to induce a phase separation and form a solidified nascent membrane. Subsequently, the obtained nascent membrane was immersed in DI water to completely extract the diluent, and then a wet membrane can be obtained. To avoid the collapse of pores in drying, wet membrane was washed with an ethanol-hexane sequence, and dried in vacuum for 12 h at 30 ℃.C. Peeling off the top layer of PVDF membranes
Rough, microstructured surfaces were prepared by peeling off the top layers of PVDF membranes, as shown in FIG. 1. A piece of PVDF membrane with a size of 1 cm×3 cm was first fixed on a slide glass by using double adhesive tapes. Then, the single-sided adhesive tape was adhered firmly to the membrane surface under certain pressure. Subsequently, the tape with the adhering top layer was removed quickly from the membrane surface, resulting in the superhydrophobic peeled surfaces.D. Characterization
The morphologies of the PVDF membranes were observed by scanning electron microscopy (SEM, TM3000, Hitachi, Japan) with an accelerating voltage of 15 kV. The cross-section and surfaces of samples were sputtered with gold before test. Mercury intrusion porosimetry (Auto Pore IV9500, Micromeritics, USA) was used to determine the average pore size, porosity and the pore size distribution of PVDF membranes. The surface roughness of PVDF membranes before or after peeling the surface layer was determined by a digital microscopy (KH-8700, HIROX, Japan). Static contact angles and sliding angles of water droplets on the membrane surface were determined from optical contact angle measurement system (Kruss DSA100, Germany). At least five replicated measurements were carried out in order to get the average value.Ⅲ. RESULTS AND DISCUSSION A. Morphology of PVDF membranes
A series of porous PVDF membranes were prepared via TIPS process using DMSO2 and PEG400 as the crystallizable diluent and additive, respectively. FIG. 2 shows the surface morphologies of the obtained PVDF membranes. It is clear that upper and down surface morphologies of PVDF membranes have a great difference under water cooling conditions, while are almost the same when air was used as the cooling medium. The upper surfaces of PVDF membranes obtained in water bath exhibit spongy structure, and the down surfaces have many dense and uniform pores, as shown in FIG. 2(a)-(c). In comparison, the pore distribution on membrane surfaces obtained in air is more uniform (FIG. 2(e)-(f)), due to the similar cooling rate of upper and down surface and slow crystallization rate of PVDF and DMSO2. Moreover, the PVDF membranes become denser as the polymer concentration increases. The dense surface offers better mechanical strength, while the loose surface is more easily peeled off.
FIG. 3(a1)-(f1) demonstrates cross-section morphologies of the obtained PVDF membranes, while FIG. 3(a2)-(f2) and (a3)-(f3) shows the pore structures near the upper surfaces and down surfaces, respectively. It is clear that the pores near both surfaces are smaller than those inside the cross-section. The membranes obtained in water cooling possess spherical crystals morphologies through the cross-section (FIG. 3(a)-(d)). Large spherulites arrang loosely near the upper surface, and the spherulites become small and dense downward from the upper surface, which can be called "Ooo" distribution of spherulites. Under air cooling, the obtained membranes also display spherulitic structure (FIG. 3(e)-(f)), but large spherulites arranged in the middle region of membranes, showing "oOo" distribution in the cross-section. Furthermore, all the membranes have multilayer structures, and the density of each layer is different, which provides a basis for constructing superhydrophobic surfaces by peeling off the top layer.
Furthermore, mercury intrusion porosimetry was used to determine the average pore size, porosity and the pore size distribution of PVDF membranes. As list in Table Ⅰ, the average pore size and porosity of PVDF membranes decrease with the increasing of PVDF concentration no matter prepared in water bath or air bath. On the other hand, with the same PVDF concentration, PVDF membranes prepared in air bath have larger pores and higher porosity than those prepared in water bath. This result is consistent with those shown in SEM images. FIG. 4 compares the pore size distribution of PVDF membranes prepared from different conditions. It is clear that PVDF membranes show relatively narrow pore size distribution except for the sample of 10wt% WC, which has large amount of macropores with diameters ranging from 0.1 μm to 10 μm.B. Surface morphologies of peeled PVDF membranes
FIG. 5 shows the surface morphologies of peeled PVDF membranes. The surfaces of peeled 10wt% WC and 15wt% WC membranes display spherulitic structure, and the spherulite diameter is 2-3 microns for 10wt% WC membrane and 1-2 microns for 15wt% WC membrane. When the polymer concentration increases to 20wt% and 25wt%, the membranes show finger-like pore structure (FIG. 5(c) and (d)). The 15wt% AC and 20wt% AC membranes also exhibit finger-like pore structure (FIG. 5(e) and (f)). What is more, it is clear that there are more bumps and depressions on the surface of the peeled membranes, forming more "v^v" peak-valley structure, which is more rough than the island structure of the original surface. This indicates peeled PVDF membrane surface shows high roughness, which plays a key role in constructing a superhydrophobic surface.
The surface roughness of PVDF membranes before and after peeling off the top layer has been determined by a digital microscopy. The surface roughness of the original PVDF membranes cannot be measured by this method, due to their relatively smooth surfaces before peeling (Ra < 0.1 μm). In contrast, the peeled PVDF membranes present a high surface roughness, which is higher than 0.15 μm (FIG. 6). Moreover, the Ra value of peeled PVDF membranes prepared in water bath increases with the PVDF concentration increases to 15wt%, and then drops as the PVDF concentration further increases. Additionally, the extremely high Ra value of 15wt% AC may be due to its large pores (Table Ⅰ) and uniform pore size distribution (FIG. 4). It is clear that the peeling of the top layer exactly constructs rough surface on PVDF membranes, which is important to obtain the superhydrophobicity.C. Superhydrophobicity of peeled PVDF membranes
Surface wettability is usually assessed by water CA and SA measurements. A superhydrophobic surface is defined as having a CA larger than 150° and a SA less than 10° . CA values of different PVDF membranes before and after peeling off the top layer are shown in FIG. 7. The CAs on the original surface of the PVDF membranes are 115°-121°, indicating the PVDF membranes are hydrophobic. After peeling off the top layer, the CAs increase dramatically and depend on pore size. Superhyophobic surface was obtained by mechanical peeling, with the maximum CA of 152.4° from 20wt% WC membrane. The superhydrophobic property is determined by surface roughness. As shown in FIG. 5, with the increase of the PVDF concentration, the PVDF crystal arrangements became more closely, and crystal sizes became smaller, which is beneficial to the increase of roughness. However, when the PVDF content reaches 25wt%, excessive dense surface layer was difficult to remove to construct a superhydrophobic surface.
In addition to CA, SA is commonly employed to characterize anti-wetting properties of a superhydrophobic surface . SAs on the peeled PVDF membrane surfaces are shown in FIG. 8. All the SAs are lower than 10°, indicating the peeled surfaces own the self-cleaning property . Among them, the peeled 10wt% WC surface has the minimum SA (7.2°).
The above results demonstrate that a superhydrophobic surface of PVDF with a CA larger than 150° and a SA less than 10° was successfully constructed via TIPS method and mechnical peeling.D. Regeneration of superhydrophobic surface
Superhydrophobicity depends on the hierarchical micro-and nano-structures of surfaces . The micro-/nano-structures are easily destroyed by the mechanical force such as finger contact, abrasive, and washing, resulting in dramatically reducing CA . Moreover, regeneration of superhydrophobic surfaces was difficult to achieve . As shown in FIG. 3 and FIG. 4, it has been found that the PVDF membranes prepared by TIPS method exhibit hierarchical structure, in which pore morphology and pore size distribution are very similar in a certain thickness. Therefore, it can be rendered with rough microstructured surface once more by mechanical peeling, which indicates that the superhydrophobicity of the peeled PVDF membrane surfaces can be regenerated. FIG. 9 displays the CAs of the 20wt% WC surface as a function of regeneration time. Once the rough surface is destroyed, the CA significantly decreased to 125°-130°, making the membrane surface lose superhydrophobicity. After repeating mechanical peeling, the damaged membrane surface is rendered with superhydrophobicity again, identical to the original superhydrophobicity. The CAs vary between 148° and 152° during 10 damage-regeneration cycles, indicated the regenerative surface also exhibited robust superhydrophobicity.Ⅳ. CONCLUSION
A renewable superhydrobic PVDF surface was fabricated via TIPS method and mechanical peeling. The PVDF membranes prepared from TIPS process exhibit multilayer structures, and the density of each layer is different, which provides a basis for constructing superhydrophobic surfaces by peeling off the top layer. The peeled membrane surface shows higher roughness, which plays a key role in constructing a superhydrophobic surface. The peeled PVDF membrane surfaces display superhydrophobicity with a high CA (152°) and a low SA (7.2°). Importantly, the superhydrophobicity can be easily recovered for many times by a simple mechnical peeling, identical to the original superhydrophobicity. This simple method for fabricating a renewable superhydrobic surface is low cost, and suitable for large-scale industrialization, which may offer more opportunities for practical applicationsⅤ. ACKNOWLEDGMENTS
This work is supported by the National Natural Science Foundation of China (No.51403107), the Natural Science Foundation of Ningbo (No.2015A610014), the Key Laboratory of Marine Materials and Related Technologies (No.2016K07), and K. C. Wong Magna Fund in Ningbo University.
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b. 中国科学院宁波材料技术与工程研究所, 中国科学院海洋新材料与应用技术重点实验室, 宁波 315201