Colloids and Surfaces A: Physicochemical and Engineering Aspects
Fabrication of superhydrophobic hybrids from multiwalled carbon nanotubes and poly(vinylidene fluoride)
Graphical abstract
This study demonstrated a facile method to fabricate superhydrophobic porous coatings of multiwalled carbon nanotube (MWCNT)/polyvinylidene fluoride (PVDF) via phase-separation method.
Highlights
► Multiwalled carbon nanotubes (MWCNTs) and poly(vinylidene fluoride) (PVDF) form superhydrophobic hybrids. ► Water contact angle up to 166°. ► Effect of hybrid composition on water contact angle and surface microstructure. ► Potential application for large-area superhydrophobic coating.
Introduction
Surfaces with extreme water repellent properties are commonly observable on plant leaves in nature, which can cause water to roll off leaving little or no residue and carry away any resting surface contamination [1], [2]. This non-wetting property of plant leaves is known as the lotus-leaf effect or self-cleaning effect. The hydrophobicity is governed by both the chemical composition and the geometric microstructure of the surface [3], [4], [5], [6]. For lotus leaves, the superhydrophobicity is found to be a result of the hierarchical rough structure, as well as the wax layer present on the leaf surface [1], [3], [7]. Thus, superhydrophobicity (defined as a contact angle (CA) > 150° and a sliding angle (SA) < 10°) can be obtained by the enhancement of surface roughness. Therefore, to fabricate a superhydrophobic surface, two key factors must be considered: surface roughness and surface energy of materials, which have been demonstrated by Wenzel and Cassie-Baxter models [5], [6]. An initially slightly hydrophobic solid surface (CA > 90°) may become highly hydrophobic (CA approaching 180°) after roughening.
Inspired by the unique property of lotus leaves, many artificial superhydrophobic surfaces have been fabricated by tailoring the geometric microstructure of surface using techniques such as anodic oxidation [8], [9], electrodeposition and chemical etching [10], [11], plasma etching [12], [13], laser treatment [14], [15], electrospinning [16], [17], chemical vapour deposition [18], [19], sol–gel processing [20], [21], lithographic patterning [22], [23], vertical alignment of nanotubes or nanofibers [24], [25], phase separation [26], [27], assembly [28], [29], [30], [31], [32], [33], glancing angle deposition [34], and solution-immersion methods [35], [36]. Rough and superhydrophobic surfaces formed by phase separation are one important type of superhydrophobic surfaces. For example, spongy polypropylene aggregates with diameters about 1.0 μm have shown a contact angle of 160° [37]. Recent publications in this area have included polyvinyl chloride [38], polycarbonate [27] and polystyrene [39] as well as some fluoropolymers [40]. Block copolymers have also been used, which can be deposited on a substrate from their solutions and result in roughly structured surfaces through phase separation [41]. The phase-separation method has the simplicity compared to most of the others described above and the ease in forming a conformal coating.
Carbon nanotubes (CNTs) are one of candidates to create a surface with a roughness at mirco/nanometer level owing to their rigid cylindrical nanostructures with a diameter ranging from about 1 nm to dozens of nanometers and length ranging from hundreds of nanometers to micrometers. The research related to superhydrophobic materials based on CNTs has been attractive in the last decade [42]. Aligned carbon nanotube (ACNT) films prepared by chemical vapor deposition (CVD) methods and coated with fluoroalkylsilane showed a high water CA of about 163° [25], [43], [44]. CNT arrays on a monolayer of polystyrene colloidal crystals exhibited a remarkable superhydrophobicity with a CA of about 165° after treated with a low surface-energy material [45]. A stable and superhydrophobic surface was also prepared using ACNT forest and polytetrafluoroethylene as the low surface-energy material [18]. The most interesting feature of the superhydrophobic ACNT materials is their well defined structures, but the potential application of ACNT superhydrophobic materials in large-area coatings is limited due to the requirement of specific preparations [46]. In comparison with superhydrophobic materials based on ACNTs, superhydrophobic materials based on randomly laid CNTs have a greater prospect in fabrication of superhydrophobic coatings with large areas. A solution method is commonly used to fabricate superhydrophobic materials based on randomly laid CNTs, which is advantageous to subsequent coating. Multiwalled carbon nanotubes (MWCNTs) treated with cetyltrimethylammonium bromide and hydroxylic MWCNTs treated with perfluorooctanoic acid in water were found to exhibit a remarkable superhydrophobicity after dried [46], [47]. The polymers with conjugated structure, such as Nafion and regioregular poly(3-hexylthiophene)-blockpolystyrene, could be adsorbed onto the surface of CNTs through π–π interaction in solution and the resultant CNT/polymer films exhibited a superhydrophobicity [48], [49]. Transparent and conductive films could be fabricated from a silane sol mixture containing CNTs. The wettability of the obtained films could be varied from superhydrophobicity to superhydrophilicity by varying the chemical functionality of the silane sol [50].
As reported in our previous work, we developed a phase-separation technique of a sodium stearate-stabilized dispersion of multiwalled carbon nanotubes (MWCNTs) and prepared superhydrophobic hybrids from randomly laid MWCNTs and a biomaterial with low surface energy (stearic acid) [51]. In the present work, we continually focus on the preparation of superhydrophobic hybrids utilizing the phase-separation method and the unique structure of CNTs. The novel superhydrophobic porous hybrids of poly(vinylidene fluoride) (PVDF) with randomly laid MWCNTs have been successfully prepared by a precipitating process. The effect of MWCNT on water CA and phase structures of MWCNT/PVDF hybrids has been investigated. Compared with already established many other techniques such as anodic oxidation, electrodeposition and chemical etching, plasma etching, laser treatment, electrospinning, whose potential applications in large-area coatings are limited by the requirements of specific preparations, our method provides a feasible and economic way for bionic fabrication of superhydrophobic materials, and also a possibility to make large-area and strong superhydrophobic coatings.
Section snippets
Materials
MWCNTs were purchased from Iljin Nano Tech (Korea) with a diameter ranging from 10 to 15 nm and a length ranging from 10 to 20 μm and used as received. Polyvinylidene fluoride (PVDF) and N,N-dimethylformamide (DMF) were purchased from Alfa Aesar. Water was purified by a Millipore water purifying system.
Fabrication of superhydrophobic MWCNT/PVDF hybrids
A mixture of 0.5 g PVDF, a desired amount of MWCNTs and 20 ml DMF was stirred at 23 °C for 10 min and then ultrasonicated (Sonicator3000, Misonix) at 20 kHz and 24 W for 5 min. Then the mixture was poured
Preparation of MWCNT/PVDF hybrids
The addition of water as a non-solvent induced the phase separation in the MWCNT/PVDF/DMF mixture and precipitation of the polymer from the solution. Since MWCNTs preferred to precipitate with the PVDF from the solution, they were wrapped with PVDF and thus influenced the phase structure [52]. Fig. 1 shows the SEM images of the dry MWCNT/PVDF hybrid as a function of the MWCNT loading. From Fig. 1a, we can see the protrusion-like structures of neat PVDF within the 2 μm scale. With 1 wt. % MWCNTs,
Conclusions
Superhydrophobic multiwalled carbon nanotube (MWCNT)/polyvinylidene fluoride (PVDF) hybrids have been successfully prepared through a facile phase-separation method. MWCNTs precipitated with the PVDF together during the deposition of PVDF from its DMF solution, which was induced by water, and influenced the phase structure to result in porous structures. The rough surface with porous structures was revealed by scanning electron microscopy. The effect of MWCNT on water contact angle (CA) of
Acknowledgement
This work was supported by the A*STAR SERC Grant (0721010018), Singapore.
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