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Methylsilicone-functionalized superhydrophobic polyurethane porous membranes as antifouling oil absorbents
Accepted Manuscript Title: Methylsilicone-functionalized superhydrophobic polyurethane porous membranes as anti-fouling oil absorbents Authors: Hong-Mo Kim, Joonbum Lee, Jiae Seo, Ji-Hun Seo PII: DOI: Reference:
S0927-7757(19)30273-0 https://doi.org/10.1016/j.colsurfa.2019.03.073 COLSUA 23320
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
21 January 2019 20 March 2019 21 March 2019
Please cite this article as: Kim H-Mo, Lee J, Seo J, Seo J-Hun, Methylsiliconefunctionalized superhydrophobic polyurethane porous membranes as anti-fouling oil absorbents, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), https://doi.org/10.1016/j.colsurfa.2019.03.073 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Elsevier Editorial System(tm) for Colloids and Surfaces A Manuscript Draft Manuscript Number:
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Title: Methylsilicone-functionalized superhydrophobic polyurethane porous membranes as antifouling oil absorbents Article Type: Full Length Article
Keywords: polyurethane, superhydrophobic, superoleophilic, self-cleaning, oil removal Corresponding Authors: Prof. Ji-Hun Seo, Email: [email protected] Tel: +82-2-3290-3263, Fax: +82-2-928-3584
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Corresponding Author's Institutions: Department of Materials Science and Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841
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First Author: Hong-Mo Kim
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Order of Authors: Hong-Mo Kim, Joonbum Lee, Jiae Seo, Ji-Hun Seo
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Manuscript Region of Origin: Republic of Korea
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Suggested Reviewers:
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Graphical abstract
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Methylsilicone-functionalized superhydrophobic
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polyurethane porous membranes as anti-fouling oil absorbents Hong-Mo Kim#, Joonbum Lee#, Jiae Seo, Ji-Hun Seo*
Department of Materials Science and Engineering, Korea University, 145 Anam-ro, Seongbukgu, Seoul 02841, Republic of Korea
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# these authors contribute equally to this work
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* E-mail: [email protected]
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Abstract:
The purpose of this study is to fabricate porous polyurethane membranes that maintain oil absorption performance even when the surface is contaminated. The segmented polyurethane
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(SPU) films were prepared by the solvent casting method and the porous SPU membranes were prepared by electrospinning. A superhydrophobic methylsilicone network was formed on the hydroxylated SPU (SPU-OH) film and
membrane surfaces
by condensation with
methyltrichlorosilane. The morphological changes to the methylsilicone-network-coated SPU
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(SPU-OH-Si) were observed by field emission scanning electron microscopy. The changes in the
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chemical components on the surface by the introduction of the methylsilicone network were
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confirmed by attenuated total reflection Fourier transform infrared spectroscopy, and the type of
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wettability of the surface was confirmed to be superhydrophobicity and superoleophilicity on the SPU-OH-Si film or membrane by contact angle measurements. The self-cleaning performances
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of the prepared surfaces were confirmed using methylene blue, Oil Red O, and SiC adsorption
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tests. As a result, the SPU-OH-Si surface displayed excellent self-cleaning properties, as well as oil removal performance, in contaminated water. Furthermore, the oil absorption property was
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preserved even when the membrane was pre-contaminated with SiC particles.
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Keywords: polyurethane, superhydrophobic, superoleophilic, self-cleaning, oil removal
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1. Introduction Segmented polyurethane (SPU) has many attractive properties such as excellent mechanical properties, chemical stability, and a relatively high biocompatibility [1,2]. For these reasons,
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SPU has been utilized for a variety of engineering applications, including textiles and automobile interiors, as well as biomedical applications such as artificial blood vessels and heart valves [3– 5]. By using the phase inversion or electrospinning processes, these SPU can also be fabricated as porous membrane structures [6]. Porous SPU membranes have been widely used as wound dressings, scaffolds, and separation membranes for gas or organic molecules because of their
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as their large surface area and porous structure [7–9].
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excellent mechanical properties, which result from their segmented molecular structure, as well
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As interest in environmental protection has increased, demand for polymer materials with
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excellent oil adsorption properties that can be used to treat the water pollution caused by oil
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spills has increased [10]. Porous membranes made of SPUs that have suitable hydrophobicity
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and oleophilicity are known to have excellent oil adsorption properties and are attracting much attention as oil removing materials [11–13]. For these reasons, many researchers have focused on
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increasing porosity of these membranes to maximize oil absorption [14]. However, SPU surfaces display surface energies of 40–50 mJ/m2, and this range falls within the “fouling zone” based on
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the Baier curve [15,16]. Thus, surface fouling always occurs when polyurethane-based materials
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are used, and this is a significant problem in the use of SPU membranes. In particular, the Abbreviation:
SPU, segmented polyurethane; SPU-OH, hydroxylated SPU; SPU-OH-Si, methylsilicone-network-coated SPU; KPS, Potassium persulfate; MTCS, methyltrichlorosilane; PTFE, polytetrafluoroethylene; SEM, scanning electron microscopy * Corresponding author. E-mail address: [email protected] (J.-H. Seo)
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functionality of the porous SPU membrane is drastically decreased when the contamination occurs over a large area of the porous membrane surface [17]. Because surface contamination is an inevitable problem in many engineered environments, it is necessary to develop a porous SPU
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membrane with self-cleaning ability so that effective oil absorption can be achieved even in contaminated conditions.
In recent decades, many investigations into the fabrication of nanotopologies mimicking lotus leaves, which have self-cleaning characteristics, have been conducted [18]. Among these, the nanofilament-structured methylsilicone network formed by a chlorosilane reaction is known to
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impart superhydrophobic and superoleophilic characteristics to a material surface bearing
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hydroxyl groups, and this reaction can be achieved by a simple immersion process [19]. For this
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reason, this technique has been mainly applied to rigid material surfaces such as glass and silicon
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wafers, whose surfaces are easily functionalized with hydroxyl groups [20]. If this technique could be applied to SPU surfaces, an oil removal membrane that is able to absorb oil even in
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contaminating conditions could be developed. Thus, we have developed an optimized process for
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carrying out the trichlorosilane reaction on the SPU surface, yielding superhydrophobic surfaces.
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The SPU surface was hydroxylated to initiate the reaction of trichlorosilane for the formation of a superhydrophobic methylsilicone network, and the self-cleaning property was imparted on the
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SPU surface. The final aim of the present study is to develop an oil removal membrane with self-
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cleaning ability that allows oil absorption, even in contaminating conditions.
2. Material and methods 2-1. Materials
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Pellethane 2363-80AE® (pellet) was purchased from Lubrizol (Wickliffe, Ohio, USA). Potassium persulfate (KPS), methylene blue solution, and methyltrichlorosilane (MTCS) were purchased from Sigma–Aldrich (St. Louis, MO, USA). Oil Red O was purchased from Alfa
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Aesar (Ward Hill, MA, USA). Petroleum ether, dimethylformamide (DMF), and ethanol were purchased from SAMCHUN Chemical (Gyeonggi-Do, Korea).
2.2.Preparation of the SPU film
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The segmented polyurethane (SPU) film was prepared by solvent casting. The pellets of SPU
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were immersed in ethanol for 3 days to remove the processing agents, impurities, and low-
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molecular-weight components used in processing. The film was dried in an oven at 60 °C for 48
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h and dissolved in DMF, yielding a 12.5% (w/v) solution. For the removal of the gel particles in solution, the solution was filtered with a polytetrafluoroethylene (PTFE) membrane under low
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to obtain the SPU films.
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pressure through a pump. A certain amount of the filtered solution was cast in a glass Petri dish
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2.3.Preparation of the porous SPU membrane The porous SPU membrane was prepared by electrospinning. The pellets of SPU were
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dissolved at a concentration of 40 wt.% in dimethylformamide. The electrospinning apparatus was composed of a syringe, a needle (21 gauge), a collector, and high-voltage power supply (ESR100D; NanoNC, Korea). A high voltage (20 kV) was applied to the SPU solution, and the flow rate of the solution was set at 2 mL/h. The tip to collector distance was 30 cm.
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2.4.Formation of the methylsilicone network on SPU The SPU film was immersed in degassed distilled water solution of 2.5% (w/v) KPS and the reaction was carried out at 80 °C for 24 h. After the reaction, the SPU film was washed several
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times with hot water and vacuum dried at room temperature for 24 h. The hydroxylated SPU film was then immersed in a petroleum ether solution of 0.05 M MTCS and reacted at 25 °C for 24 h. After the reaction, the SPU film was sequentially washed with petroleum ether, ethanol, and water and dried at 120 °C for 10 min. A similar protocol was applied to the porous SPU membrane to prepare a superhydrophobic SPU membrane, except for the use of different
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concentrations of KPS (20%, w/v) and MTCS (0.1 M).
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2.5.Micro-tensile test of the SPU film and membrane
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The specimens for micro-tensile test were prepared using the same procedure described above, except for the dimension (ASTM D1708-13638). The specimens were then subjected to the
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E3000LT dynamic test instrument (Instron corp., UK) at the speed of 1 mm/s.
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2.6.Characterization of the surface
Scanning electron microscopy (SEM) images were obtained using S-4300 apparatus (Hitachi,
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Japan) and used to determine the morphology of the SPU surface. The 3D surface profiles were collected by VK-8710 confocal laser scanning microscope
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(CLSM; Keyence, Japan). The focal planes with high laser intensity due to reflection by sample surfaces were collected and the height of the planes were profiled on the 3D spaces.
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The chemical composition of surfaces was investigated via X-ray photoelectron spectroscopy using X-tool X-ray photoelectron spectrometer (Ulvac-Phi, Inc.; Japan). Both survey and narrow scan spectra were obtained for detailed quantitative analysis.
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Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) analysis was performed using an Agilent Cary 630 FTIR spectrometer (Agilent Technologies Inc., Santa Clara, CA, USA) equipped with a diamond ATR module to confirm the introduction of functional groups.
The static contact angle of the SPU surface with untreated, hydroxylated, and methylsilicone
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network was measured by sessile drop or captive bubble method using contact angle goniometer
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(Phoenix 150, Surface Electro Optics, Ltd., Gyeonggi-do, Korea). The contact angle was
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measured using distilled water at three points on each sample.
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2.7.Water repelling and self-cleaning tests
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The water-repelling effect of the treated SPU was confirmed by immersing the sample in the distilled water solution in which methylene blue had been dissolved. The self-cleaning effect was
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examined by spraying the surface with SiC, which was used as a contaminant. After contamination, the contaminated SPU surface was exposed to water droplets to confirm the self-
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cleaning effect.
2.8.Oil removal tests These tests were carried out to confirm whether oil can be removed in an underwater environment when SiC is deposited on the porous SPU membrane. In the test, distilled water
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dyed with methylene blue was partially contaminated with hexadecane droplets dyed with Oil Red O. To this solution, the SiC-contaminated SPU membrane was immersed into the water side and slowly removed, in the process coming into contact with the floating hexadecane droplets.
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The similar experiment was conducted for chloroform removal test. In this case, transparent water was used and chloroform was dyed by Oil Red O.
3. Results and discussion
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Superhydrophobic surfaces formed by mimicking the surface structure of lotus leaves have
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been actively researched in recent decades because of its self-cleaning properties. Because water
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droplets are positioned in the Cassie–Baxter state on these surfaces, the superhydrophobic
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properties could be derived from the large number of air pockets inside the nanostructured surface morphology [21]. The formation of the methylsilicone network is known as an effective
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method to prepare nanostructured hydrophobic surface morphologies. Because glassy siloxane
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bonding is the main backbone of the methylsilicone network, this method has been mainly
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applied to the surface modification of the inorganic materials such as glass, silicon wafers, and titanium oxide [22,23]. To apply this technique to the SPU surface successfully, a two-step
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chemical reaction was used. As shown in Scheme 1, free hydroxyl groups were first introduced on the SPU surface through the reaction between SPU and hydroxyl radicals generated by the
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thermal decomposition of KPS. Then, the free hydroxyl groups on the SPU surface reacted with MTCS to form a networked methylsilicone structure.
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Scheme 1. Schematic illustration and mechanism of methylsilicone network formation on the
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segmented polyurethane (SPU) surface.
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3.1. Optimization of methylsilicone formation on SPU film To optimize the formation of the methylsilicone network on the SPU surface, the
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morphologies of the SPU samples at each step of methylsilicone formation on the SPU surface were investigated by SEM. First, the SPU surface was hydroxylated by reacting the bare SPU
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surface with KPS. When KPS molecules are heated, persulfate molecule generates radical species and these radical species take protons from water molecules, leaving hydroxyl radicals [24]. The hydroxyl radicals then take protons from the SPU surface, and the radical species are transferred onto the SPU surface. The surface radicals then react with hydroxyl radicals in the solution, and, finally, hydroxyl group is formed on the SPU surface [25]. Compared with the
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untreated SPU film (Fig. 1a), there is no significant morphological change on the surface of the hydroxylated SPU film (SPU-OH) (Fig. 1b). Because no macromolecular synthetic step was
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introduced, only slight changes to the SPU-OH surface were expected.
Fig. 1. SEM images of (a) the untreated SPU film, (b) SPU-OH film, and (c) SPU-Si film; (d–f)
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the trial and error procedure used to optimize the formation of the methylsilicone network; and
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(g–i) the SPU-OH-Si film.
The formation of the methylsilicone network on the SPU-OH surface is initiated by the attachment of MTCS molecules to the SPU-OH surface. Then, silylchloride groups are reacted with water molecules in solution, and a large number of silanol groups are formed on the SPU
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surface. The silanol groups then undergo a condensation reaction to form an inorganic siloxane network on the SPU surface [26]. To optimize the reaction process for methylsilicone formation on SPU-OH, the surface was reacted with MTCS solutions of different concentrations, for
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different reaction times, and with different solvents. As a result, methylsilicone network was stably formed when reacted at room temperature for 24 h. As shown in Fig. 1d, a particle-like morphology was formed at a concentration of 1.2 M and for 1.5 h reaction time. When the reaction time was increased to 24 h, larger aggregates were observed on the surface (Fig. 1e). When the reaction solvent was changed to petroleum ether, even larger aggregates were observed.
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The ideal uniform methylsilicone network was obtained when the SPU-OH surface was reacted
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with MTCS at 0.05 M concentration for 24 h in petroleum ether.
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As shown in Fig. 1g-i, a methylsilicone network with a cylindrical shape and a diameter of
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about 38 nm was formed. The formation of a three-dimensional nanofilament structure is known
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to be induced by the vertical polymerization of MTCS via hydrolysis and polycondensation
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reactions, as described above. To confirm whether hydroxylation is an essential step for the stable formation of the methylsilicone network on the SPU surface, the MTCS condensation
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reaction was also conducted on the SPU surface without pre-hydroxylation (SPU-Si). As shown in Fig. 1c, no significant morphological changes were observed when the SPU surface was not
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hydroxylated by KPS. Because the initial immobilization of MTCS molecules on SPU is crucial
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for the stable coating of the nanofilament network on the SPU surface, non-hydroxylated SPU was not coated with the 3-D methylsilicone network. This indicates that the formation of the 3-D methylsilicone network was successfully induced on the SPU surface, and the hydroxylation of the SPU surface is essential for surface modification.
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The changes to the chemical components induced by the introduction of free hydroxyl groups and MTCS on the SPU surfaces were confirmed by ATR-FTIR analysis. The absorption peak corresponding to free hydroxyl groups introduced by KPS overlap with that of the -NH- groups
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in the urethane units in SPU near 3330 cm-1 [27]. However, when the free hydroxyl groups were introduced on the SPU surface by KPS, the total area of the absorption peak near 3330 cm-1 might be slightly increased because the hydroxylation process does not accompany the consumption of–NH- groups. For this reason, we calculated the change in peak area near 3330 cm-1 by normalizing the peak area with that of the absorption peak of the benzene ring near
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1600 cm-1, which does not participate in the reaction in the SPU [28]. As a result, a slight
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increase in the peak area near 3330 cm-1 (A3330/A1600) was confirmed after the hydroxylation
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process (Fig. 2b). A more drastic change in the surface properties after hydroxylation was
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confirmed from the contact angle measurement. As shown in Fig. 2b, the non-treated SPU surface showed a contact angle value of around at 80°, which is similar to those of the bare SPU
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surfaces reported in previous studies [29]. In contrast, the contact angle was decreased to around
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45° after the hydroxylation process, which indicates that the surface polarity changed from the
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hydrophobic state to the hydrophilic state. Although direct evidence of hydroxylation was hard to obtain, the slight increase in the IR peak area corresponding to hydroxyl groups and the drastic
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decrease in contact angle could be indirect evidence of the successful introduction of the free hydroxyl groups on the SPU surface by KPS. ATR-FTIR and contact angle analysis were further
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conducted after the methylsilicone network had formed on the SPU-OH surface. The introduction of MTCS was confirmed by the presence of peaks at around 770, 1015, and 1270 cm-1, which are attributed to the Si-O-Si vibrations in the methylsilicone network (Fig. 2a) [30].
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Fig. 2. (a) ATR-FTIR spectra of the untreated SPU film, SPU-OH film, and SPU-OH-Si film. (b)
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Change in the peak area after the introduction of free hydroxyl groups, and the air bubble contact
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angle on the surface of the untreated SPU film and SPU-OH film. Here, *** indicates p < 0.001.
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The wettability of the SPU-OH-Si to the water and hexadecane was measured by the sessile
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drop method. Fig. 3 shows the resulting contact angle of the various surfaces to the water and
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hexadecane droplets. The untreated SPU surface has contact angles of 80° for water and 20° for hexadecane. In the case of the SPU-Si surface whose surface was treated with MTCS without
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hydroxylation, the water contact angle was increased to around 130°, which indicates that partial formation of methylsiloxane particles on the SPU imparted limited hydrophobicity to the SPU
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surface. In contrast, the water droplet was completely repelled from the SPU-OH-Si surface; that is, a non-wetting surface was formed on the SPU-OH-Si membrane. Fig. 3b shows the real-time response of the water droplet to the SPU-OH-Si surface. As clearly shown, the SPU-OH-Si surface completely repelled the water droplet.
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Fig. 3. (a) Contact angles of the untreated SPU film, SPU-Si film, and SPU-OH-Si film surfaces
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using the sessile drop method. The white bars are the contact angle with water and the gray bars
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are the contact angle with hexadecane. Images of the (b) repellence of a water droplet and (c)
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attraction of a hexadecane droplet on the SPU-OH-Si film.
Because the hydrophobic methylsilicone groups form a nanowire network, water droplets on
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the surface are thought to be in the Cassie–Baxter state, which means that large amount of air is
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in direct contact with water droplet, increasing the hydrophobicity of the surface [31]. When hexadecane was placed in contact with the SPU-OH-Si surface, the hexadecane droplet was
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immediately absorbed by the surface and the contact angle could not be measured, as shown in Fig. 3c. This superoleophilic property of the SPU-OH-Si surface is consistent with previous results, which have indicated that hydrophobic methylsilicone network can be successfully introduced onto the SPU surface by simple immersion [32].
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3.2. Anti-fouling and self-cleaning test on the modified SPU surface To confirm the self-cleaning effect of the methylsilicone network on the SPU surface, each SPU surface was immersed into a methylene-blue-containing aqueous solution. As shown in Fig.
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4, the untreated SPU film adsorbed a large amount of methylene blue immediately after immersion and even after immersion in pure water. This indicates that significant amount of polar foulants can be adsorbed on the untreated SPU surface. In contrast, the SPU-OH-Si surface almost completely repelled the methylene blue solution, and the small amount of residual foulant on the surface was completely eliminated when the surface was immersed in pure water. Thus,
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the methylsilicone network is superhydrophobic, preventing the initial fouling and yielding a
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self-cleaning effect [33].
Fig. 4. Sample immersed in distilled water stained with methylene blue and then removed: (a) untreated SPU film and (b) SPU-OH-Si film. After self-cleaning using clean distilled water: (c) untreated SPU film and (d) SPU-OH-Si film.
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The self-cleaning performance against dust fouling was confirmed using silicon carbide (SiC). SiC particles are known to have a similar size and shape to the dust that occurs naturally, and SiC is known to be easily adsorbed on various material surfaces by strong van der Waals
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interactions [34]. For these reasons, SiC has been widely used as a model foulant in the field of environmental engineering [35]. As shown in Fig. 5, untreated SPU and SPU-OH-Si samples were contaminated with a significant quantity of SiC particles (1.25 mg/mm2). When each sample was tilted about 50o, the untreated SPU film surface was still fully covered with the SiC particles. In contrast, most of the SiC particles on SPU-OH-Si film were immediately removed
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from the surface when the contaminated surface was tilted about 50o. The SPU surface normally
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has a surface energy of 40–50 mJ/m2, and this surface is known to adsorb organic and inorganic
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foulants only weakly [36]. However, the SPU-OH-Si surface displayed weak interactions with
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SiC because of the densely formed methylsilicone network. To confirm the self-cleaning effect, water droplets were slowly dropped onto the contaminated surfaces after the surfaces had been
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tilted. As a result, water droplets accumulated on the tilted SPU surface, and the SiC particles
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remained on the SPU surface, even after the passage of the water droplets. This result indicates
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that the SiC particles interact strongly with the SPU surface and are not easily washed away by water droplets. In contrast, the SPU-OH-Si surface repelled the water droplets, and, at the same
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time, the adsorbed SiC particles were removed by the repelled droplets, which is representative of the self-cleaning properties of superhydrophobic surfaces [37]. The adsorbed SiC particles
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were almost eliminated after several water droplets came into contact with the surface. This shows that the surface of the SPU with the methylsilicone network has excellent anti-fouling properties with respect to SiC and even the partially adsorbed particles can be easily removed
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using distilled water; this is strong evidence for the self-cleaning property of the SPU-OH-Si
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surface.
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Fig. 5. Tests for anti-fouling and self-cleaning effects using silicon carbide: (a) untreated SPU
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film and (f) treated SPU film. (b,g) Samples contaminated with SiC, (c,h) samples tilted, (d,i) samples after dropping water on the surface, and (e,j) samples after dropping 30 drops of water
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on the surface.
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3.3. Preparation of a porous SPU membrane coated with methylsilicone network The final purpose of the present study is to develop an oil removal membrane with self-
cleaning properties. To this end, an SPU membrane was prepared using the electrospinning method, and the prepared membrane was functionalized with a methylsilicone network. Fig. 6 shows the SEM images of the prepared membranes. As shown in Fig. 6a, a well-networked
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porous SPU membrane (diameter of 1.4 μm) was formed using the optimized conditions. One of the important reason for using polyurethane is that its good mechanical flexibility prevents the material from breaking easily. Although the tensile strength of the SPU fibrous membrane was
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decreased to about 40% of the bulk SPU film, the SPU membrane still maintain the good flexibility and did not easily broken even though the tensile strain reached to 600%. (Fig. S1). When the SPU membrane was hydroxylated by KPS, a slightly etched morphology was observed on the SPU-OH membrane surface. Because the hydroxylation step was conducted in an aqueous medium at elevated temperatures, the SPU surface hydrophilized by hydroxylation might be
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partially dissolved into the aqueous solution, and this is presumably the cause of the observed
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morphological changes to the membrane. Although partial morphological changes were observed
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to the SPU-OH membrane, the overall networked membrane structure did not collapse. Based on
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this result, the SPU-OH membrane was allowed to react with an MTCS solution, thus forming the methylsilicone network. Fig. 6c shows the morphology of the SPU-OH-Si membrane. As
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clearly shown, the outermost surface of the fibrous membrane was covered with wire-like
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methylsilicone, which is similar result with the SPU-OH-Si film. This indicates that the
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methylsilicone network could be also formed on the fibrous SPU membrane if the membrane is hydroxylated. Unfortunately, the methylsilicone network could not be formed on the fiber
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surface at the inner-side of the membrane. This is probably due to the limited diffusion of the MTCS molecules to the inner-side of the membrane or because of the closely packed fibers,
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which limit free space and disturb the formation of the methylsilicone network inside the membrane. In any case, the purpose of the methylsilicone network on the SPU membrane is to impart a self-cleaning property on the outermost surface of the membrane, and we determined that the SPU membrane had been successfully coated with the methylsilicone network. The
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superhydrophobic and superoleophilic properties of the SPU-OH-Si membrane were confirmed by sessile drop measurements with water and hexadecane, respectively. As shown in Fig. 6d, the untreated SPU membrane shows hydrophobic and oleophilic properties to sessile drops of water
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and hexadecane, respectively. On the SPU-OH-Si membrane, the water droplets had a roundshape and were not easily positioned on the surface, which indicates that superhydrophobic surface property was imparted by the formation of the methylsilicone network. Similar to the
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untreated SPU membrane, the SPU-OH-Si membrane also displayed a superoleophilic property.
Fig. 6. SEM images of (a,a') the untreated SPU membrane, (b,b') SPU-OH membrane, and (c,c') SPU-OH-Si membrane. Images of the membrane wettability to water and oil: (d) untreated SPU membrane and (e) SPU-OH-Si membrane.
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The increased hydrophobicity with the methylsilicone functionalization was probably due to the increased surface roughness which is the main mechanism of the superhydrophobicity. To confirm the increased surface roughness, 3-D surface profile was analyzed by means of the
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confocal laser microscopic analysis. As shown in Fig. 7, surface roughness was increased from 19.5 to 82.5, and 303 mm as the forms of substrate was changed from SPU film to SPU membrane, and methylsilicone-functionalized membrane, respectively. This indicate that the increased surface roughness due to the formation of the hydrophobic methylsilicone network is
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one of the main cause of the superhydrophobicity of the SPU-OH-Si membrane.
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Fig. 7. 3-D surface topology analyzed by confocal laser scanning microscope. (a) SPU film, (b)
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SPU membrane, and (c) SPU-OH-Si membrane.
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To confirm the changes in surface element by formation of the methylsilicone network, XPS
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analysis was conducted on the SPU-OH-Si surfaces. As clearly shown in Fig. 8, characteristic Si2p peak was clearly observed from the SPU-OH-Si surface. In case of SPU-OH surface, only increased O1s peak was characteristically observed due to the hydroxylated surface. This indicates that the networked structure observed from the SEM was composed of siloxane compound formed from the MTCS condensation reactions.
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Fig. 8. Characteristic XPS peaks of (a) the untreated SPU membrane, (b) SPU-OH membrane,
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and (c) SPU-OH-Si membrane.
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To confirm the absorption capacity of the fibrous membrane, the changes in the weights of the
SPU samples before and after immersion into water and hexadecane were analyzed. As shown in
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Fig. 9, the weights of the film-shaped SPU samples did not change after immersion in water and hexadecane. Although the SPU-OH-Si film was superoleophilic, the film-like sample could not absorb the hexadecane. This is probably due to the densely packed SPU chains inside the bulk film samples. Because the SPU chains tightly interact each other through strong hydrogen bonds
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formed by urethane [38], there is no space for the absorption of hexadecane molecules. In contrast, the untreated SPU fibrous membrane showed some water absorption (ca. 30 wt.% change) and significant (ca. 80 wt.% change) hexadecane absorption. Because the fibrous
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membrane has a significant free volume, the absorption capacity of the water and hexadecane was increased significantly. In the case of the SPU-OH-Si fibrous membrane, a more significant increase in the absorption capacity for hexadecane was observed. As shown in Fig. 9, a 100% weight change was confirmed when the SPU-OH-Si membrane was immersed in hexadecane. Because the methylsilicone network is known to be superoleophilic [39], the present result is
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thought to be due to the increased oleophilic nature of the SPU-OH-Si. In contrast, the amount of
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water absorption was significantly decreased (3–5 wt.%) compared to that of the untreated SPU
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membrane. This is probably due to the strong water repellency of the SPU-OH-Si membrane
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induced by the superhydrophobic methylsilicone network.
Fig. 9. Increasing ratio of water and oil absorption in the untreated SPU film, SPU-OH-Si film, untreated SPU membrane, and SPU-OH-Si membrane. Here, * indicates p < 0.05 and ** indicates p < 0.01.
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Fig. 10. Tests for anti-fouling and self-cleaning effects using silicon carbide: (a) untreated SPU
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membrane and (g) SPU-OH-Si membrane, (b,h) samples contaminated with SiC, (c,i) samples
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tilted, (d,e,j,k) removal of silicon carbide via self-cleaning with water, and (f,l) after cleaning
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with water.
3.4. Anti-fouling and self-cleaning tests on the porous SPU membrane
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The effect of the methylsilicone network on the anti-fouling and self-cleaning properties of the
porous SPU membrane was investigated in the same manner as for the SPU film samples. Fig. 10
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shows the results of the fouling test conducted using SiC particles. Similar to the results for the SPU film, a large number of SiC particles remained on the untreated SPU membrane when the contaminated membrane was tilted slightly. Even after immersion in water, the SPU membrane remained significantly contaminated with a significant quantity of SiC. However, no significant
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adsorption occurred on the SPU-OH-Si membrane when it was slightly tilted after contamination with a large amount of SiC. Moreover, when the membrane was immersed in the water, even a small amount of residual SiC was completely removed from the surface. This indicates that the
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SPU-OH-Si membrane displays excellent initial anti-fouling and self-cleaning ability against SiC because of the superhydrophobicity induced by the methylsilicone network.
3.5. Oil absorption performance in contaminated environment
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To confirm the oil absorption in a contaminated state, the prepared membranes were
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contaminated with SiC and then used to remove hexadecane droplets floating on water. As
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shown in Fig. 11, the uncontaminated SPU membranes (both untreated and treated) displayed
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good oil absorption properties. However, the oil absorption property was not maintained on the untreated SPU membrane when it was contaminated by SiC (Fig. 11b). This is because of the
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blocking effect of the surface SiC on the oil absorption process. Because the untreated SPU
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surface strongly interacts with SiC, the oleophilic properties of the SPU surface were completely
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blocked by the SiC particles, even after immersion in water. In the case of the SPU-OH-Si membrane, the amount of initially adsorbed SiC was not significant, as described above, and,
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when the membrane was immersed in water, the residual small amount of SiC was completely eliminated. As a result, the oil absorption property was not affected, and a similar oil removal
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effect to that of the original SPU-OH-Si membrane was achieved.
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Fig. 11. A test for oil absorption after the self-cleaning of the surface contaminated with silicon
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carbide. The degree of oil absorption of the (a) untreated SPU membrane and (b) SPU-OH-Si
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membrane without SiC contamination and (c) untreated SPU membrane and (d) SPU-OH-Si
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membrane with SiC contamination.
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A similar effect was also observed when water was contaminated with an organic solvent having a higher density than water. Fig. 12 shows chloroform absorption performance of the
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prepared membrane. As shown in Fig. 12a and 12c, the untreated and treated SPU membranes
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showed good chloroform absorption property when the organic solvent below the water. However, the chloroform droplets were not eliminated when the untreated SPU membrane was
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contaminated with SiC. In contrast, the SPU-OH-Si membrane was hardly contaminated by SiC, and even the small amount of SiC adsorbed on the membrane was removed immediately after immersion in water, and the membrane removed the sunken chloroform droplets from the water. This indicates that the SPU-OH-Si membrane display excellent anti-fouling and self-cleaning properties and could be used to remove organic solvent from water, regardless of their density.
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Fig. 12. A test for chloroform absorption after the self-cleaning of the surface contaminated with
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silicon carbide. The degree of chloroform absorption in the (a) untreated SPU membrane and (b)
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SPU-OH-Si membrane without SiC contamination and (c) untreated SPU membrane and (d)
4. Conclusions
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SPU-OH-Si membrane with SiC contamination.
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For environmental protection, oil contamination was always remained as an important issue.
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Therefore, it is important to synthesize polymeric materials having excellent oil adsorption properties for use as oil absorbents. However, in most cases, oil absorbent materials reveal
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excellent oil removal performance only when the surface is kept in clean state. Because oleophilic materials are easily contaminated by various organic/inorganic foulants, the development of an oil absorbent material showing excellent oil removal performance regardless of surface fouling is essential. In this study, the optimal process for preparing a porous SPU membrane bearing superhydrophobic and self-cleaning property has been suggested. Despite
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surface contamination, excellent underwater oil absorption behavior was observed for the prepared membrane. Therefore, the results of this study are expected to make a great contribution
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to the development of novel oil absorbent materials.
Acknowledgements
This research was partially supported by Korea University-Future Research Unit (K1822141). This research is also supported by the X-Project (NRF-2018R1E1A2A02086660) and Basic
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Science Research Program (NRF-2015R1C1A1A01054022) funded by the National Research
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Foundation, Republic of Korea.
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S0927-7757(19)30273-0 https://doi.org/10.1016/j.colsurfa.2019.03.073 COLSUA 23320
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
21 January 2019 20 March 2019 21 March 2019
Please cite this article as: Kim H-Mo, Lee J, Seo J, Seo J-Hun, Methylsiliconefunctionalized superhydrophobic polyurethane porous membranes as anti-fouling oil absorbents, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), https://doi.org/10.1016/j.colsurfa.2019.03.073 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Elsevier Editorial System(tm) for Colloids and Surfaces A Manuscript Draft Manuscript Number:
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Title: Methylsilicone-functionalized superhydrophobic polyurethane porous membranes as antifouling oil absorbents Article Type: Full Length Article
Keywords: polyurethane, superhydrophobic, superoleophilic, self-cleaning, oil removal Corresponding Authors: Prof. Ji-Hun Seo, Email: [email protected] Tel: +82-2-3290-3263, Fax: +82-2-928-3584
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Corresponding Author's Institutions: Department of Materials Science and Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841
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First Author: Hong-Mo Kim
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Order of Authors: Hong-Mo Kim, Joonbum Lee, Jiae Seo, Ji-Hun Seo
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Manuscript Region of Origin: Republic of Korea
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Graphical abstract
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Methylsilicone-functionalized superhydrophobic
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polyurethane porous membranes as anti-fouling oil absorbents Hong-Mo Kim#, Joonbum Lee#, Jiae Seo, Ji-Hun Seo*
Department of Materials Science and Engineering, Korea University, 145 Anam-ro, Seongbukgu, Seoul 02841, Republic of Korea
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# these authors contribute equally to this work
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* E-mail: [email protected]
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Abstract:
The purpose of this study is to fabricate porous polyurethane membranes that maintain oil absorption performance even when the surface is contaminated. The segmented polyurethane
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(SPU) films were prepared by the solvent casting method and the porous SPU membranes were prepared by electrospinning. A superhydrophobic methylsilicone network was formed on the hydroxylated SPU (SPU-OH) film and
membrane surfaces
by condensation with
methyltrichlorosilane. The morphological changes to the methylsilicone-network-coated SPU
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(SPU-OH-Si) were observed by field emission scanning electron microscopy. The changes in the
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chemical components on the surface by the introduction of the methylsilicone network were
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confirmed by attenuated total reflection Fourier transform infrared spectroscopy, and the type of
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wettability of the surface was confirmed to be superhydrophobicity and superoleophilicity on the SPU-OH-Si film or membrane by contact angle measurements. The self-cleaning performances
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of the prepared surfaces were confirmed using methylene blue, Oil Red O, and SiC adsorption
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tests. As a result, the SPU-OH-Si surface displayed excellent self-cleaning properties, as well as oil removal performance, in contaminated water. Furthermore, the oil absorption property was
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preserved even when the membrane was pre-contaminated with SiC particles.
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Keywords: polyurethane, superhydrophobic, superoleophilic, self-cleaning, oil removal
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1. Introduction Segmented polyurethane (SPU) has many attractive properties such as excellent mechanical properties, chemical stability, and a relatively high biocompatibility [1,2]. For these reasons,
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SPU has been utilized for a variety of engineering applications, including textiles and automobile interiors, as well as biomedical applications such as artificial blood vessels and heart valves [3– 5]. By using the phase inversion or electrospinning processes, these SPU can also be fabricated as porous membrane structures [6]. Porous SPU membranes have been widely used as wound dressings, scaffolds, and separation membranes for gas or organic molecules because of their
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as their large surface area and porous structure [7–9].
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excellent mechanical properties, which result from their segmented molecular structure, as well
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As interest in environmental protection has increased, demand for polymer materials with
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excellent oil adsorption properties that can be used to treat the water pollution caused by oil
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spills has increased [10]. Porous membranes made of SPUs that have suitable hydrophobicity
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and oleophilicity are known to have excellent oil adsorption properties and are attracting much attention as oil removing materials [11–13]. For these reasons, many researchers have focused on
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increasing porosity of these membranes to maximize oil absorption [14]. However, SPU surfaces display surface energies of 40–50 mJ/m2, and this range falls within the “fouling zone” based on
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the Baier curve [15,16]. Thus, surface fouling always occurs when polyurethane-based materials
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are used, and this is a significant problem in the use of SPU membranes. In particular, the Abbreviation:
SPU, segmented polyurethane; SPU-OH, hydroxylated SPU; SPU-OH-Si, methylsilicone-network-coated SPU; KPS, Potassium persulfate; MTCS, methyltrichlorosilane; PTFE, polytetrafluoroethylene; SEM, scanning electron microscopy * Corresponding author. E-mail address: [email protected] (J.-H. Seo)
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functionality of the porous SPU membrane is drastically decreased when the contamination occurs over a large area of the porous membrane surface [17]. Because surface contamination is an inevitable problem in many engineered environments, it is necessary to develop a porous SPU
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membrane with self-cleaning ability so that effective oil absorption can be achieved even in contaminated conditions.
In recent decades, many investigations into the fabrication of nanotopologies mimicking lotus leaves, which have self-cleaning characteristics, have been conducted [18]. Among these, the nanofilament-structured methylsilicone network formed by a chlorosilane reaction is known to
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impart superhydrophobic and superoleophilic characteristics to a material surface bearing
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hydroxyl groups, and this reaction can be achieved by a simple immersion process [19]. For this
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reason, this technique has been mainly applied to rigid material surfaces such as glass and silicon
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wafers, whose surfaces are easily functionalized with hydroxyl groups [20]. If this technique could be applied to SPU surfaces, an oil removal membrane that is able to absorb oil even in
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contaminating conditions could be developed. Thus, we have developed an optimized process for
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carrying out the trichlorosilane reaction on the SPU surface, yielding superhydrophobic surfaces.
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The SPU surface was hydroxylated to initiate the reaction of trichlorosilane for the formation of a superhydrophobic methylsilicone network, and the self-cleaning property was imparted on the
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SPU surface. The final aim of the present study is to develop an oil removal membrane with self-
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cleaning ability that allows oil absorption, even in contaminating conditions.
2. Material and methods 2-1. Materials
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Pellethane 2363-80AE® (pellet) was purchased from Lubrizol (Wickliffe, Ohio, USA). Potassium persulfate (KPS), methylene blue solution, and methyltrichlorosilane (MTCS) were purchased from Sigma–Aldrich (St. Louis, MO, USA). Oil Red O was purchased from Alfa
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Aesar (Ward Hill, MA, USA). Petroleum ether, dimethylformamide (DMF), and ethanol were purchased from SAMCHUN Chemical (Gyeonggi-Do, Korea).
2.2.Preparation of the SPU film
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The segmented polyurethane (SPU) film was prepared by solvent casting. The pellets of SPU
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were immersed in ethanol for 3 days to remove the processing agents, impurities, and low-
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molecular-weight components used in processing. The film was dried in an oven at 60 °C for 48
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h and dissolved in DMF, yielding a 12.5% (w/v) solution. For the removal of the gel particles in solution, the solution was filtered with a polytetrafluoroethylene (PTFE) membrane under low
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to obtain the SPU films.
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pressure through a pump. A certain amount of the filtered solution was cast in a glass Petri dish
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2.3.Preparation of the porous SPU membrane The porous SPU membrane was prepared by electrospinning. The pellets of SPU were
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dissolved at a concentration of 40 wt.% in dimethylformamide. The electrospinning apparatus was composed of a syringe, a needle (21 gauge), a collector, and high-voltage power supply (ESR100D; NanoNC, Korea). A high voltage (20 kV) was applied to the SPU solution, and the flow rate of the solution was set at 2 mL/h. The tip to collector distance was 30 cm.
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2.4.Formation of the methylsilicone network on SPU The SPU film was immersed in degassed distilled water solution of 2.5% (w/v) KPS and the reaction was carried out at 80 °C for 24 h. After the reaction, the SPU film was washed several
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times with hot water and vacuum dried at room temperature for 24 h. The hydroxylated SPU film was then immersed in a petroleum ether solution of 0.05 M MTCS and reacted at 25 °C for 24 h. After the reaction, the SPU film was sequentially washed with petroleum ether, ethanol, and water and dried at 120 °C for 10 min. A similar protocol was applied to the porous SPU membrane to prepare a superhydrophobic SPU membrane, except for the use of different
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concentrations of KPS (20%, w/v) and MTCS (0.1 M).
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2.5.Micro-tensile test of the SPU film and membrane
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The specimens for micro-tensile test were prepared using the same procedure described above, except for the dimension (ASTM D1708-13638). The specimens were then subjected to the
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E3000LT dynamic test instrument (Instron corp., UK) at the speed of 1 mm/s.
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2.6.Characterization of the surface
Scanning electron microscopy (SEM) images were obtained using S-4300 apparatus (Hitachi,
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Japan) and used to determine the morphology of the SPU surface. The 3D surface profiles were collected by VK-8710 confocal laser scanning microscope
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(CLSM; Keyence, Japan). The focal planes with high laser intensity due to reflection by sample surfaces were collected and the height of the planes were profiled on the 3D spaces.
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The chemical composition of surfaces was investigated via X-ray photoelectron spectroscopy using X-tool X-ray photoelectron spectrometer (Ulvac-Phi, Inc.; Japan). Both survey and narrow scan spectra were obtained for detailed quantitative analysis.
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Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) analysis was performed using an Agilent Cary 630 FTIR spectrometer (Agilent Technologies Inc., Santa Clara, CA, USA) equipped with a diamond ATR module to confirm the introduction of functional groups.
The static contact angle of the SPU surface with untreated, hydroxylated, and methylsilicone
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network was measured by sessile drop or captive bubble method using contact angle goniometer
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(Phoenix 150, Surface Electro Optics, Ltd., Gyeonggi-do, Korea). The contact angle was
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measured using distilled water at three points on each sample.
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2.7.Water repelling and self-cleaning tests
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The water-repelling effect of the treated SPU was confirmed by immersing the sample in the distilled water solution in which methylene blue had been dissolved. The self-cleaning effect was
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examined by spraying the surface with SiC, which was used as a contaminant. After contamination, the contaminated SPU surface was exposed to water droplets to confirm the self-
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cleaning effect.
2.8.Oil removal tests These tests were carried out to confirm whether oil can be removed in an underwater environment when SiC is deposited on the porous SPU membrane. In the test, distilled water
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dyed with methylene blue was partially contaminated with hexadecane droplets dyed with Oil Red O. To this solution, the SiC-contaminated SPU membrane was immersed into the water side and slowly removed, in the process coming into contact with the floating hexadecane droplets.
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The similar experiment was conducted for chloroform removal test. In this case, transparent water was used and chloroform was dyed by Oil Red O.
3. Results and discussion
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Superhydrophobic surfaces formed by mimicking the surface structure of lotus leaves have
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been actively researched in recent decades because of its self-cleaning properties. Because water
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droplets are positioned in the Cassie–Baxter state on these surfaces, the superhydrophobic
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properties could be derived from the large number of air pockets inside the nanostructured surface morphology [21]. The formation of the methylsilicone network is known as an effective
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method to prepare nanostructured hydrophobic surface morphologies. Because glassy siloxane
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bonding is the main backbone of the methylsilicone network, this method has been mainly
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applied to the surface modification of the inorganic materials such as glass, silicon wafers, and titanium oxide [22,23]. To apply this technique to the SPU surface successfully, a two-step
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chemical reaction was used. As shown in Scheme 1, free hydroxyl groups were first introduced on the SPU surface through the reaction between SPU and hydroxyl radicals generated by the
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thermal decomposition of KPS. Then, the free hydroxyl groups on the SPU surface reacted with MTCS to form a networked methylsilicone structure.
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Scheme 1. Schematic illustration and mechanism of methylsilicone network formation on the
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segmented polyurethane (SPU) surface.
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3.1. Optimization of methylsilicone formation on SPU film To optimize the formation of the methylsilicone network on the SPU surface, the
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morphologies of the SPU samples at each step of methylsilicone formation on the SPU surface were investigated by SEM. First, the SPU surface was hydroxylated by reacting the bare SPU
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surface with KPS. When KPS molecules are heated, persulfate molecule generates radical species and these radical species take protons from water molecules, leaving hydroxyl radicals [24]. The hydroxyl radicals then take protons from the SPU surface, and the radical species are transferred onto the SPU surface. The surface radicals then react with hydroxyl radicals in the solution, and, finally, hydroxyl group is formed on the SPU surface [25]. Compared with the
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untreated SPU film (Fig. 1a), there is no significant morphological change on the surface of the hydroxylated SPU film (SPU-OH) (Fig. 1b). Because no macromolecular synthetic step was
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D
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A
N
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introduced, only slight changes to the SPU-OH surface were expected.
Fig. 1. SEM images of (a) the untreated SPU film, (b) SPU-OH film, and (c) SPU-Si film; (d–f)
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the trial and error procedure used to optimize the formation of the methylsilicone network; and
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(g–i) the SPU-OH-Si film.
The formation of the methylsilicone network on the SPU-OH surface is initiated by the attachment of MTCS molecules to the SPU-OH surface. Then, silylchloride groups are reacted with water molecules in solution, and a large number of silanol groups are formed on the SPU
12
surface. The silanol groups then undergo a condensation reaction to form an inorganic siloxane network on the SPU surface [26]. To optimize the reaction process for methylsilicone formation on SPU-OH, the surface was reacted with MTCS solutions of different concentrations, for
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different reaction times, and with different solvents. As a result, methylsilicone network was stably formed when reacted at room temperature for 24 h. As shown in Fig. 1d, a particle-like morphology was formed at a concentration of 1.2 M and for 1.5 h reaction time. When the reaction time was increased to 24 h, larger aggregates were observed on the surface (Fig. 1e). When the reaction solvent was changed to petroleum ether, even larger aggregates were observed.
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The ideal uniform methylsilicone network was obtained when the SPU-OH surface was reacted
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with MTCS at 0.05 M concentration for 24 h in petroleum ether.
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As shown in Fig. 1g-i, a methylsilicone network with a cylindrical shape and a diameter of
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about 38 nm was formed. The formation of a three-dimensional nanofilament structure is known
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to be induced by the vertical polymerization of MTCS via hydrolysis and polycondensation
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reactions, as described above. To confirm whether hydroxylation is an essential step for the stable formation of the methylsilicone network on the SPU surface, the MTCS condensation
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reaction was also conducted on the SPU surface without pre-hydroxylation (SPU-Si). As shown in Fig. 1c, no significant morphological changes were observed when the SPU surface was not
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hydroxylated by KPS. Because the initial immobilization of MTCS molecules on SPU is crucial
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for the stable coating of the nanofilament network on the SPU surface, non-hydroxylated SPU was not coated with the 3-D methylsilicone network. This indicates that the formation of the 3-D methylsilicone network was successfully induced on the SPU surface, and the hydroxylation of the SPU surface is essential for surface modification.
13
The changes to the chemical components induced by the introduction of free hydroxyl groups and MTCS on the SPU surfaces were confirmed by ATR-FTIR analysis. The absorption peak corresponding to free hydroxyl groups introduced by KPS overlap with that of the -NH- groups
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in the urethane units in SPU near 3330 cm-1 [27]. However, when the free hydroxyl groups were introduced on the SPU surface by KPS, the total area of the absorption peak near 3330 cm-1 might be slightly increased because the hydroxylation process does not accompany the consumption of–NH- groups. For this reason, we calculated the change in peak area near 3330 cm-1 by normalizing the peak area with that of the absorption peak of the benzene ring near
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1600 cm-1, which does not participate in the reaction in the SPU [28]. As a result, a slight
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increase in the peak area near 3330 cm-1 (A3330/A1600) was confirmed after the hydroxylation
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process (Fig. 2b). A more drastic change in the surface properties after hydroxylation was
M
confirmed from the contact angle measurement. As shown in Fig. 2b, the non-treated SPU surface showed a contact angle value of around at 80°, which is similar to those of the bare SPU
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surfaces reported in previous studies [29]. In contrast, the contact angle was decreased to around
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45° after the hydroxylation process, which indicates that the surface polarity changed from the
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hydrophobic state to the hydrophilic state. Although direct evidence of hydroxylation was hard to obtain, the slight increase in the IR peak area corresponding to hydroxyl groups and the drastic
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decrease in contact angle could be indirect evidence of the successful introduction of the free hydroxyl groups on the SPU surface by KPS. ATR-FTIR and contact angle analysis were further
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conducted after the methylsilicone network had formed on the SPU-OH surface. The introduction of MTCS was confirmed by the presence of peaks at around 770, 1015, and 1270 cm-1, which are attributed to the Si-O-Si vibrations in the methylsilicone network (Fig. 2a) [30].
14
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Fig. 2. (a) ATR-FTIR spectra of the untreated SPU film, SPU-OH film, and SPU-OH-Si film. (b)
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Change in the peak area after the introduction of free hydroxyl groups, and the air bubble contact
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A
angle on the surface of the untreated SPU film and SPU-OH film. Here, *** indicates p < 0.001.
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The wettability of the SPU-OH-Si to the water and hexadecane was measured by the sessile
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drop method. Fig. 3 shows the resulting contact angle of the various surfaces to the water and
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hexadecane droplets. The untreated SPU surface has contact angles of 80° for water and 20° for hexadecane. In the case of the SPU-Si surface whose surface was treated with MTCS without
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hydroxylation, the water contact angle was increased to around 130°, which indicates that partial formation of methylsiloxane particles on the SPU imparted limited hydrophobicity to the SPU
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surface. In contrast, the water droplet was completely repelled from the SPU-OH-Si surface; that is, a non-wetting surface was formed on the SPU-OH-Si membrane. Fig. 3b shows the real-time response of the water droplet to the SPU-OH-Si surface. As clearly shown, the SPU-OH-Si surface completely repelled the water droplet.
15
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Fig. 3. (a) Contact angles of the untreated SPU film, SPU-Si film, and SPU-OH-Si film surfaces
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using the sessile drop method. The white bars are the contact angle with water and the gray bars
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are the contact angle with hexadecane. Images of the (b) repellence of a water droplet and (c)
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attraction of a hexadecane droplet on the SPU-OH-Si film.
Because the hydrophobic methylsilicone groups form a nanowire network, water droplets on
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the surface are thought to be in the Cassie–Baxter state, which means that large amount of air is
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in direct contact with water droplet, increasing the hydrophobicity of the surface [31]. When hexadecane was placed in contact with the SPU-OH-Si surface, the hexadecane droplet was
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immediately absorbed by the surface and the contact angle could not be measured, as shown in Fig. 3c. This superoleophilic property of the SPU-OH-Si surface is consistent with previous results, which have indicated that hydrophobic methylsilicone network can be successfully introduced onto the SPU surface by simple immersion [32].
16
3.2. Anti-fouling and self-cleaning test on the modified SPU surface To confirm the self-cleaning effect of the methylsilicone network on the SPU surface, each SPU surface was immersed into a methylene-blue-containing aqueous solution. As shown in Fig.
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4, the untreated SPU film adsorbed a large amount of methylene blue immediately after immersion and even after immersion in pure water. This indicates that significant amount of polar foulants can be adsorbed on the untreated SPU surface. In contrast, the SPU-OH-Si surface almost completely repelled the methylene blue solution, and the small amount of residual foulant on the surface was completely eliminated when the surface was immersed in pure water. Thus,
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the methylsilicone network is superhydrophobic, preventing the initial fouling and yielding a
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A
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self-cleaning effect [33].
Fig. 4. Sample immersed in distilled water stained with methylene blue and then removed: (a) untreated SPU film and (b) SPU-OH-Si film. After self-cleaning using clean distilled water: (c) untreated SPU film and (d) SPU-OH-Si film.
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The self-cleaning performance against dust fouling was confirmed using silicon carbide (SiC). SiC particles are known to have a similar size and shape to the dust that occurs naturally, and SiC is known to be easily adsorbed on various material surfaces by strong van der Waals
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interactions [34]. For these reasons, SiC has been widely used as a model foulant in the field of environmental engineering [35]. As shown in Fig. 5, untreated SPU and SPU-OH-Si samples were contaminated with a significant quantity of SiC particles (1.25 mg/mm2). When each sample was tilted about 50o, the untreated SPU film surface was still fully covered with the SiC particles. In contrast, most of the SiC particles on SPU-OH-Si film were immediately removed
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from the surface when the contaminated surface was tilted about 50o. The SPU surface normally
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has a surface energy of 40–50 mJ/m2, and this surface is known to adsorb organic and inorganic
A
foulants only weakly [36]. However, the SPU-OH-Si surface displayed weak interactions with
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SiC because of the densely formed methylsilicone network. To confirm the self-cleaning effect, water droplets were slowly dropped onto the contaminated surfaces after the surfaces had been
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tilted. As a result, water droplets accumulated on the tilted SPU surface, and the SiC particles
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remained on the SPU surface, even after the passage of the water droplets. This result indicates
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that the SiC particles interact strongly with the SPU surface and are not easily washed away by water droplets. In contrast, the SPU-OH-Si surface repelled the water droplets, and, at the same
CC
time, the adsorbed SiC particles were removed by the repelled droplets, which is representative of the self-cleaning properties of superhydrophobic surfaces [37]. The adsorbed SiC particles
A
were almost eliminated after several water droplets came into contact with the surface. This shows that the surface of the SPU with the methylsilicone network has excellent anti-fouling properties with respect to SiC and even the partially adsorbed particles can be easily removed
18
using distilled water; this is strong evidence for the self-cleaning property of the SPU-OH-Si
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M
A
N
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surface.
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Fig. 5. Tests for anti-fouling and self-cleaning effects using silicon carbide: (a) untreated SPU
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film and (f) treated SPU film. (b,g) Samples contaminated with SiC, (c,h) samples tilted, (d,i) samples after dropping water on the surface, and (e,j) samples after dropping 30 drops of water
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on the surface.
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3.3. Preparation of a porous SPU membrane coated with methylsilicone network The final purpose of the present study is to develop an oil removal membrane with self-
cleaning properties. To this end, an SPU membrane was prepared using the electrospinning method, and the prepared membrane was functionalized with a methylsilicone network. Fig. 6 shows the SEM images of the prepared membranes. As shown in Fig. 6a, a well-networked
19
porous SPU membrane (diameter of 1.4 μm) was formed using the optimized conditions. One of the important reason for using polyurethane is that its good mechanical flexibility prevents the material from breaking easily. Although the tensile strength of the SPU fibrous membrane was
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decreased to about 40% of the bulk SPU film, the SPU membrane still maintain the good flexibility and did not easily broken even though the tensile strain reached to 600%. (Fig. S1). When the SPU membrane was hydroxylated by KPS, a slightly etched morphology was observed on the SPU-OH membrane surface. Because the hydroxylation step was conducted in an aqueous medium at elevated temperatures, the SPU surface hydrophilized by hydroxylation might be
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partially dissolved into the aqueous solution, and this is presumably the cause of the observed
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morphological changes to the membrane. Although partial morphological changes were observed
A
to the SPU-OH membrane, the overall networked membrane structure did not collapse. Based on
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this result, the SPU-OH membrane was allowed to react with an MTCS solution, thus forming the methylsilicone network. Fig. 6c shows the morphology of the SPU-OH-Si membrane. As
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clearly shown, the outermost surface of the fibrous membrane was covered with wire-like
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methylsilicone, which is similar result with the SPU-OH-Si film. This indicates that the
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methylsilicone network could be also formed on the fibrous SPU membrane if the membrane is hydroxylated. Unfortunately, the methylsilicone network could not be formed on the fiber
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surface at the inner-side of the membrane. This is probably due to the limited diffusion of the MTCS molecules to the inner-side of the membrane or because of the closely packed fibers,
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which limit free space and disturb the formation of the methylsilicone network inside the membrane. In any case, the purpose of the methylsilicone network on the SPU membrane is to impart a self-cleaning property on the outermost surface of the membrane, and we determined that the SPU membrane had been successfully coated with the methylsilicone network. The
20
superhydrophobic and superoleophilic properties of the SPU-OH-Si membrane were confirmed by sessile drop measurements with water and hexadecane, respectively. As shown in Fig. 6d, the untreated SPU membrane shows hydrophobic and oleophilic properties to sessile drops of water
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and hexadecane, respectively. On the SPU-OH-Si membrane, the water droplets had a roundshape and were not easily positioned on the surface, which indicates that superhydrophobic surface property was imparted by the formation of the methylsilicone network. Similar to the
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M
A
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untreated SPU membrane, the SPU-OH-Si membrane also displayed a superoleophilic property.
Fig. 6. SEM images of (a,a') the untreated SPU membrane, (b,b') SPU-OH membrane, and (c,c') SPU-OH-Si membrane. Images of the membrane wettability to water and oil: (d) untreated SPU membrane and (e) SPU-OH-Si membrane.
21
The increased hydrophobicity with the methylsilicone functionalization was probably due to the increased surface roughness which is the main mechanism of the superhydrophobicity. To confirm the increased surface roughness, 3-D surface profile was analyzed by means of the
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confocal laser microscopic analysis. As shown in Fig. 7, surface roughness was increased from 19.5 to 82.5, and 303 mm as the forms of substrate was changed from SPU film to SPU membrane, and methylsilicone-functionalized membrane, respectively. This indicate that the increased surface roughness due to the formation of the hydrophobic methylsilicone network is
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M
A
N
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one of the main cause of the superhydrophobicity of the SPU-OH-Si membrane.
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Fig. 7. 3-D surface topology analyzed by confocal laser scanning microscope. (a) SPU film, (b)
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SPU membrane, and (c) SPU-OH-Si membrane.
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To confirm the changes in surface element by formation of the methylsilicone network, XPS
A
analysis was conducted on the SPU-OH-Si surfaces. As clearly shown in Fig. 8, characteristic Si2p peak was clearly observed from the SPU-OH-Si surface. In case of SPU-OH surface, only increased O1s peak was characteristically observed due to the hydroxylated surface. This indicates that the networked structure observed from the SEM was composed of siloxane compound formed from the MTCS condensation reactions.
22
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Fig. 8. Characteristic XPS peaks of (a) the untreated SPU membrane, (b) SPU-OH membrane,
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and (c) SPU-OH-Si membrane.
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To confirm the absorption capacity of the fibrous membrane, the changes in the weights of the
SPU samples before and after immersion into water and hexadecane were analyzed. As shown in
A
Fig. 9, the weights of the film-shaped SPU samples did not change after immersion in water and hexadecane. Although the SPU-OH-Si film was superoleophilic, the film-like sample could not absorb the hexadecane. This is probably due to the densely packed SPU chains inside the bulk film samples. Because the SPU chains tightly interact each other through strong hydrogen bonds
23
formed by urethane [38], there is no space for the absorption of hexadecane molecules. In contrast, the untreated SPU fibrous membrane showed some water absorption (ca. 30 wt.% change) and significant (ca. 80 wt.% change) hexadecane absorption. Because the fibrous
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membrane has a significant free volume, the absorption capacity of the water and hexadecane was increased significantly. In the case of the SPU-OH-Si fibrous membrane, a more significant increase in the absorption capacity for hexadecane was observed. As shown in Fig. 9, a 100% weight change was confirmed when the SPU-OH-Si membrane was immersed in hexadecane. Because the methylsilicone network is known to be superoleophilic [39], the present result is
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thought to be due to the increased oleophilic nature of the SPU-OH-Si. In contrast, the amount of
N
water absorption was significantly decreased (3–5 wt.%) compared to that of the untreated SPU
A
membrane. This is probably due to the strong water repellency of the SPU-OH-Si membrane
A
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induced by the superhydrophobic methylsilicone network.
Fig. 9. Increasing ratio of water and oil absorption in the untreated SPU film, SPU-OH-Si film, untreated SPU membrane, and SPU-OH-Si membrane. Here, * indicates p < 0.05 and ** indicates p < 0.01.
24
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Fig. 10. Tests for anti-fouling and self-cleaning effects using silicon carbide: (a) untreated SPU
M
membrane and (g) SPU-OH-Si membrane, (b,h) samples contaminated with SiC, (c,i) samples
D
tilted, (d,e,j,k) removal of silicon carbide via self-cleaning with water, and (f,l) after cleaning
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with water.
3.4. Anti-fouling and self-cleaning tests on the porous SPU membrane
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The effect of the methylsilicone network on the anti-fouling and self-cleaning properties of the
porous SPU membrane was investigated in the same manner as for the SPU film samples. Fig. 10
A
shows the results of the fouling test conducted using SiC particles. Similar to the results for the SPU film, a large number of SiC particles remained on the untreated SPU membrane when the contaminated membrane was tilted slightly. Even after immersion in water, the SPU membrane remained significantly contaminated with a significant quantity of SiC. However, no significant
25
adsorption occurred on the SPU-OH-Si membrane when it was slightly tilted after contamination with a large amount of SiC. Moreover, when the membrane was immersed in the water, even a small amount of residual SiC was completely removed from the surface. This indicates that the
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SPU-OH-Si membrane displays excellent initial anti-fouling and self-cleaning ability against SiC because of the superhydrophobicity induced by the methylsilicone network.
3.5. Oil absorption performance in contaminated environment
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To confirm the oil absorption in a contaminated state, the prepared membranes were
N
contaminated with SiC and then used to remove hexadecane droplets floating on water. As
A
shown in Fig. 11, the uncontaminated SPU membranes (both untreated and treated) displayed
M
good oil absorption properties. However, the oil absorption property was not maintained on the untreated SPU membrane when it was contaminated by SiC (Fig. 11b). This is because of the
D
blocking effect of the surface SiC on the oil absorption process. Because the untreated SPU
TE
surface strongly interacts with SiC, the oleophilic properties of the SPU surface were completely
EP
blocked by the SiC particles, even after immersion in water. In the case of the SPU-OH-Si membrane, the amount of initially adsorbed SiC was not significant, as described above, and,
CC
when the membrane was immersed in water, the residual small amount of SiC was completely eliminated. As a result, the oil absorption property was not affected, and a similar oil removal
A
effect to that of the original SPU-OH-Si membrane was achieved.
26
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Fig. 11. A test for oil absorption after the self-cleaning of the surface contaminated with silicon
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carbide. The degree of oil absorption of the (a) untreated SPU membrane and (b) SPU-OH-Si
A
membrane without SiC contamination and (c) untreated SPU membrane and (d) SPU-OH-Si
D
M
membrane with SiC contamination.
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A similar effect was also observed when water was contaminated with an organic solvent having a higher density than water. Fig. 12 shows chloroform absorption performance of the
EP
prepared membrane. As shown in Fig. 12a and 12c, the untreated and treated SPU membranes
CC
showed good chloroform absorption property when the organic solvent below the water. However, the chloroform droplets were not eliminated when the untreated SPU membrane was
A
contaminated with SiC. In contrast, the SPU-OH-Si membrane was hardly contaminated by SiC, and even the small amount of SiC adsorbed on the membrane was removed immediately after immersion in water, and the membrane removed the sunken chloroform droplets from the water. This indicates that the SPU-OH-Si membrane display excellent anti-fouling and self-cleaning properties and could be used to remove organic solvent from water, regardless of their density.
27
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Fig. 12. A test for chloroform absorption after the self-cleaning of the surface contaminated with
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silicon carbide. The degree of chloroform absorption in the (a) untreated SPU membrane and (b)
A
SPU-OH-Si membrane without SiC contamination and (c) untreated SPU membrane and (d)
4. Conclusions
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D
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SPU-OH-Si membrane with SiC contamination.
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For environmental protection, oil contamination was always remained as an important issue.
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Therefore, it is important to synthesize polymeric materials having excellent oil adsorption properties for use as oil absorbents. However, in most cases, oil absorbent materials reveal
A
excellent oil removal performance only when the surface is kept in clean state. Because oleophilic materials are easily contaminated by various organic/inorganic foulants, the development of an oil absorbent material showing excellent oil removal performance regardless of surface fouling is essential. In this study, the optimal process for preparing a porous SPU membrane bearing superhydrophobic and self-cleaning property has been suggested. Despite
28
surface contamination, excellent underwater oil absorption behavior was observed for the prepared membrane. Therefore, the results of this study are expected to make a great contribution
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to the development of novel oil absorbent materials.
Acknowledgements
This research was partially supported by Korea University-Future Research Unit (K1822141). This research is also supported by the X-Project (NRF-2018R1E1A2A02086660) and Basic
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Science Research Program (NRF-2015R1C1A1A01054022) funded by the National Research
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Foundation, Republic of Korea.
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