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Atmospheric Pressure Plasma Functionalized Polymer Mesh: an Environment-friendly and Efficient Tool for Oil/Water Separation Faze Chen, Jinlong Song, Ziai Liu, Jiyu Liu, Huanxi Zheng, Shuai Huang, Jing Sun, Wenji Xu, and Xin Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01770 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 26, 2016

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ACS Sustainable Chemistry & Engineering

Atmospheric Pressure Plasma Functionalized Polymer Mesh: an Environment-friendly and Efficient Tool for Oil/Water Separation Faze Chen1, Jinlong Song1, 2, Ziai Liu1, Jiyu Liu1, Huanxi Zheng1, Shuai Huang1, Jing Sun1, Wenji Xu1, Xin Liu1* 1

Key Laboratory for Precision and Non-Traditional Machining Technology of Ministry of Education, Dalian University of Technology, No.2

Linggong Road, Ganjingzi District, Dalian 116024, China 2

Center of Smart Interfaces, Technische Universität Darmstadt, Alarich-Weiss-Straße 10, Darmstadt 64287, Germany

ABSTRACT Oil/water separation has been addressed by various materials characterized with super-wettability, but most of the methods involve corrosive or toxic chemicals which will cause environmental concerns. Proposed herein is an environment-friendly method to realize oil/water separation. Nylon mesh is exposed to atmospheric pressure plasma for surface modification, by which micro/nano structures and oxygen-containing groups are created on nylon fibers. Consequently, the functionalized mesh possesses superhydrophilicity in air and thus superoleophobicity underwater. The water pre-wetted mesh is then used to separate oil/water mixtures with the separation efficiency above 97.5% for various oil/water mixtures. Results also demonstrate that the functionalized nylon mesh has excellent recyclability and durability in terms of oil/water separation. Additionally, polyurethane sponge slice and polyester fabric are also functionalized and employed to separate oil/water mixtures efficiently, demonstrating the wide suitability of this method. This simple, green and highly efficient method overcomes a nontrivial hurdle for environmentally-safe separation of oil/water mixtures, and offers insights into the design of advanced materials for practical oil/water separation. Keywords: Superhydrophilic, underwater superoleophobic, atmospheric pressure plasma, surface modification, oil/water separation, nylon mesh

INTRODUCTION Oil pollution caused by the oily industrial wastewater and the frequent oil spill accidents has become one of the most urgent global environmental problems. Therefore, developing methods for the collection and removal of oil spills from water has attracted keen attentions. Traditional methods, such as oil containment fences1, oil skimmers collection, absorption by sorbents or dispersants2,3, in situ combustion etc., are commonly used but suffer from the limits of low efficiency, low selectivity, high operation cost and poor recyclability etc. Consequently, advanced oil/water separation materials that can address the above-mentioned problems are highly desirable. Recently, materials with extreme wettability, e.g., superhydrophobicity-superoleophilicity, superhydrophilicity-underwater-superoleophobicity, have attracted increased attention for oil/water separation 4-6. Oil absorbent materials, such as superhydrophobic-superoleophilic textiles and sponges/foams (polymer, cellulose and metal-based)7-13, can absorb oil while simultaneously repel water. Though these materials exhibit high separation selectivity, recyclability and separation efficiency remain as challenges since *

Corresponding author. Tel.: 86-411-84708422; Fax: 86-411-84708422. E-mail address: [email protected] (Dr. Xin Liu)

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mechanical squeezing after each absorption cycle is indispensable to retrieve the absorbed oils. Oil/water filtration materials, such as superhydrophobic-superoleophilic or superhydrophilic-underwater-superoleophobic meshes and membranes14-21, enable oil or water to penetrate through selectively to realize oil/water mixtures separation. Song et al.15 realized self-driven one-step oil/water separation by superhydrophobic-superoleophilic stainless steel mesh fabricated by chemical deposition and low surface energy modification. Lu et al.16 created superhydrophobic mild steel mesh by two-step chemical immersion method and used it to separate oil/water. Xue et al.17 reported a hydrogel-coated superhydrophilic and underwater superoleophobic mesh for oil/water separation. Li et al.18 fabricated superhydrophilic and underwater-superoleophobic copper mesh by spray coating method and successfully separated oil/water mixtures. However, most of the reported methods to impart these materials super-wettability involve corrosive or toxic chemicals, which means that other environmental pollutions will be caused, thus severely limiting their applications. It is therefore of great importance to develop environment-friendly and efficient methods to fabricate materials with super-wettability for the separation of oil/water mixtures. Fish scales22 inspire researchers that the superhydrophilic surfaces with micro/nano structured textures in air are generally superoleophobic unederwater17-20. As an environment-friendly method, plasmas are widely employed for surface (super)hydrophilization23-25. Especially atmospheric pressure plasmas (APPs), who eliminate the use of expensive vacuum equipment required by traditional low-pressure plasma, have attracted much attention for surface (super)hydrophilization.25-28 However, to the best of our knowledge, no works about oil/water separation by APP functionalized materials have been reported to date. Synthetic fibers are often selected for oil/water separation study because of their relatively low cost, nontoxicity, light weight, good mechanical strength and excellent corrosion resistance. Several methods, including hydrogel-coating29, sol-gel30, hydrothermal process31, self-assembly32, solvent swelling33, sonochemistry irradiation34 and salt-induced phase-inversion35, have been developed to fabricate fibers with super-wettability, which were subsequently used to separate oil/water mixtures. However, most of the above-mentioned methods have drawbacks with regards to their environmental consequence and fabrication efficiency, as they require either chemicals or complex fabrication processes. In this paper, we proposed a facial and environment-friendly method to fabricate super-wettability materials for efficient oil/water separation. APP treatment was used to impart nylon mesh superhydrophilicity and underwater-superoleophobicity. The functionalized nylon mesh possessed high oil/water separation efficiency, good recyclability and durability. APP functionalized polyurethane sponge and polyester fabric were also successfully used to separate oil/water mixtures, demonstrating the versatility of this technique.

EXPERIMENTAL Materials

Nylon (polyamide-6.6) meshes with a mesh number of 500 were bought from Tangzheng Wire Mesh Co., Ltd. (Anping, China). Polyurethane sponge and polyester fabric were bought from Wal-Mart Stores in Dalian, China and used after cutting. Hexane, hexadecane and 2 ACS Paragon Plus Environment

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dichloromethane were purchased from Sigma-Aldrich (USA). Diesel was bought from a local gas station and peanut oil was purchased from Luhua Co. (China). Plasma Treatment A low temperature helium (He) APP that could generate various reactive species (e.g. excited He and O atoms) was used as plasma source (see Supporting Information, Fig. S1), similar to the one previously reported by Chen et al36,37. A stainless steel tube with inner diameter (ID) of 2.2 mm and outer diameter (OD) of 2.8 mm was inserted in a quartz tube (ID: 3.0 mm, OD: 5.0 mm) and the distance between the stainless steel tube top and the quartz tube outlet was 1.0 cm. The stainless steel tube was connected to an AC power supply (Nanjing Suman Electronics, CTP-2000 K) and was used as high voltage electrode. The driving frequency of the power supply was fixed at 60 kHz. The stainless steel tube also served as working gas channel, through which high purity He at a constant flow rate of 3.0 slm (standard liters per minute) flushed. When a high voltage (Vrms≈4.0 kV) was applied to the stainless steel tube, He APP generated and propagated in the surrounding air. The distance between the nylon mesh and the quartz tube outlet was about 3.0 mm. Oil/Water Separation For oil/water separation, nylon mesh (5.0 cm × 5.0 cm) was scanned by He APP at a scanning speed of 2 mm/s and step size of 5 mm. Then the plasma functionalized nylon mesh was fixed between two plastic tubes with similar inner diameter of 3.0 cm with a clamping device (see Supporting Information, Fig. S2). Before the separation process, a small amount of water was poured into the upper tube to pre-wet the APP-treated nylon mesh. Then the mixtures of oil and water (50% v/v) were poured into the upper tube. The gravity-driven separation was achieved and the separated water was collected to evaluate the separation efficiency. Hexane, hexadecane, diesel, and peanut oil were used as exemplars of floating oil, and were dyed with different colors for better visualization. The basic parameters of these oils are list in Table I. Table I Basic parameters of oil used in the experiments. Oil 3

Density at 25 °C [g/cm ] Surface tension at 20 °C [mN/m] Kinematic viscosity at 40 °C [cSt]

Hexane

Hexadecane

Diesel

0.65 17.9 0.42

0.77 27.5 2.93

0.84 28.3 4.33

Peanut oil Dichloromethane 0.92 34.5 39.6

1.325 23.1 0.33

Characterization Contact angles (CAs) and sliding angles (SAs) of water and various oils were measured by an optical contact angle meter (Krüss, DSA100, Germany) at room temperature employing ∼5 μL water and oil droplets. The surface morphologies of the samples were observed by scanning electron microscope (SEM, JSM-6360LV, Japan) after being coated by a thin layer of gold to avoid charge deposition. The surface chemistries were characterized by fourier transform infrared spectrum (FTIR, ThermoFisher 6700, American) with high resolution of 0.09 cm−1 and X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi, American) with a monochromatic Al Kα (1486.6 eV) X-ray beam, and the C 1s peak at 284.6 eV was used as reference. High-resolution C 1s spectra were deconvoluted into different chemical groups with mixed Lorentzian–Gaussian components by XPS Peak v4.1 fitting software. The tensile test of single 3 ACS Paragon Plus Environment


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nylon fiber was conducted on a micro-force tester (Instron 3345R2773, America). The length of the fiber was fixed to be 20 cm, and the tensile rate was 5 mm/min. The oil intrusion pressures were obtained by measuring the maximum height of oil on the pre-wetted nylon mesh in a tube. The separated oil was characterized by FTIR to verify whether the oil contained water. Additionally, the oil content in the collected water was calculated from the absorbance peak intensity of FTIR. The residual oil was extracted by CCl 4 from the permeated water, and then the absorbance intensity values around 2850 cm−1, 2930 cm−1 and 2960 cm−1 were used to calculate the oil content.

RESULTS AND DISCUSSION Fig. 1(a) shows the photograph of water droplet deposited on APP functionalized nylon mesh in air. It clearly shows that the water spread completely on the mesh, and the water CA was about 3 °(Fig. 1(b)). Additionally, the functionalized mesh exhibited superoleophilicity in air (e.g., the oil CAs for dichloromethane and hexadecane were ~0 °). By contrast, when oil droplets (dichloromethane) were deposited on the APP functionalized nylon mesh that was immersed in water, they could be supported as spherical shapes, as shown in Fig. 1(c). The measured CA for dichloromethane and hexadecane droplets were respectively ~159 ° and ~162 °, exhibiting excellent underwater superoleophobicity. Moreover, the two sides of the mesh, one directly exposed to the plasma (top side) and the other one closely contacted with the sample holder (bottom side), exhibited similar wettability. This could be attributed to the plasma penetration resulted from the efficient transportation of active species by the gas flow.

Fig. 1 Photograph of (a, b) water on the plasma functionalized nylon mesh in air and (c, d) oil droplets on the plasma functionalized nylon mesh under water.

Fig. 2(a) shows the time-lapsed images of a water droplet contacting with the plasma functionalized nylon mesh in air. The droplet could be absorbed by the mesh, and then quickly 4 ACS Paragon Plus Environment

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spread on and infiltrated the mesh within 0.25 s. After the plasma functionalized nylon mesh was immersed in water, an 8 μL dichloromethane droplet deposited on the mesh could be easily picked up by a microsyringe, and then the suspended droplet could make contact with and then lost contact with the mesh for many cycles (Fig. 2(b) and Video S1 in Supporting Information). The oil droplet did not leave the needle after being picked up, and remained almost spherical during the up-down processes, indicating an ultralow adhesion between the oil droplet and the mesh underwater. To further demonstrate the ultralow oil adhesion of the functionalized mesh underwater, the dynamic properties of oil droplets on the mesh were recorded. As shown in Fig. 2(c) and Video S2 in Supporting Information, dichloromethane and hexadecane droplets could easily rolled off from the ~5 °tilted mesh surface, therefore the SAs of the oil droplets on the mesh were ~5 °. Fig. 2(d) and (e) depict the CAs and SAs of nylon mesh as a function of plasma treatment time. For the original nylon mesh, the CA for water, dichloromethane and hexadecane in air was respectively about 85.5±5.8 °, 0 °and 0 °, and the underwater oil CAs of dichloromethane and hexadecane were respectively 115.0±13.2 ° and 114.2±3.4 °. After 5 s plasma treatment, the mesh became superhydrophilic (water CA~8.5±2.3 °) and superoleophilic (CAs of dichloromethane and hexadecane were both 0 °) in air and quasi-superoleophobic underwater (CAs of dichloromethane and hexadecane were respectively 149.4±3.9 ° and 149.0±3.6 °), which could also be demonstrated by the advancing/receding oil contact angles (see Supporting Information, Fig. S3). When the plasma treatment was prolonged to 30 s, the CAs for dichloromethane and hexadecane respectively increased to 156.1±2.7 °and 158.5±5.0 °and the oil SAs were all less than 10 °, showing superior underwater oil repellence (also see Supporting Information, Fig. S3).



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Fig. 2 (a) Time series of a water droplet contacting with the plasma functionalized nylon mesh in air. (b) Dichloromethane droplet making contact with and losing contact with a plasma functionalized nylon mesh in water. (c) Time-lapsed snapshots of dichloromethane and hexadecane droplets rolling on a tilted plasma functionalized nylon mesh in water. CAs (d) and SAs (e) of nylon mesh as a function of plasma treatment time.

To explore the mechanism of plasma functionalization of the nylon mesh, SEM, AFM, FTIR and XPS measurements were conducted and analyzed. Fig. 3(a)-(c) show the SEM images of the original nylon mesh with different magnifications. The average fiber diameter and pore size of the nylon mesh was about 50 and 25 μm, respectively. The untreated mesh surface was relatively smooth and only some trenches and bumps that might form during manufacturing existed on the nylon fiber. After plasma exposure, the surface morphology of the nylon mesh changed obviously (Fig. 3(d)-(f)). The plasma treated mesh surface became rougher and was composed of 6 ACS Paragon Plus Environment

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sub-micron scale pits and flocculent particles. The superhydrophilicity and micro/nano structures were crucial for the functionalized mesh to absorb and lock water to form a repellent barrier to oils, thus creating underwater superoleophobicity (see Supporting Information, Fig. S4). The increased roughness of nylon mesh after plasma treatment was also demonstrated by AFM measurements. The AFM images of the original and plasma functionalized nylon mesh were shown in Fig. 3(g) and (h), respectively. After plasma exposure, the roughness of the nylon mesh increased from 24.26 nm to 31.21 nm. Fig. 3(i) shows the loading-displacement curves of the original and the plasma functionalized nylon fiber. It could be seen that both the two fibers fractured after being stretched by ~35% of its length, showing excellent tensibility. The fracture loading of the two fibers were ~1.4 N, demonstrating that the tensile strength of them was similar. Therefore, APP treatment here had little effect on the nylon’s tensile strength, which ensured the robustness of the APP functionalized nylon meshes for practical applications.

Fig. 3 SEM images of original (a-c) and plasma functionalized (d-f) nylon mesh; AFM images of original (g) and plasma functionalized (h) nylon mesh; (i) Loading-displacement curves of the original and the plasma functionalized nylon fiber.



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Fig. 4(a) shows the FTIR spectra of the original and the plasma functionalized nylon meshes. For the untreated mesh, the band at 3293 cm−1 attributed to the inherent N−H stretching vibrations of nylon. The peaks at 2930 and 2859 cm−1 were respectively related to the CH2 asymmetric and symmetric stretching vibrations. The absorption band at 1630 cm−1 was assigned to the amide carbonyl C=O stretching vibration of the secondary amide band (amide I), while the amide II band at 1532 cm−1 was attributed to the N−H bending motion38. The peaks between 1000 and 1260 cm−1 could be attributed to the C−O stretching vibration. The bending of O=C−N group could be observed at 674 cm−1.39 After plasma functionalization, the intensity of the C=O stretching band and the O=C−N bending band were significantly enhanced, which clearly indicated that the plasma functionalization introduced oxygen onto the nylon fiber surface. The increased intensities for the N−H bands and CH2 bands could be attributed to the low molecular-weight materials formed by plasma etching.40 To further study the surface chemistry of the original and plasma treated nylon meshes, XPS spectra were recorded and depicted in Fig. 4(b), and the relative contents of the elements (at.%) were list in Table II. It could be clearly seen that the two mesh surfaces were all composed of C, O and N. The original nylon mesh surface contained 76.83 at.% C, 14.96 at.% O and 8.21 at.% N. After plasma treatment, the relative content of C decreased to 66.56 at.%, while O and N increased respectively to 20.56 and 12.88 at.%. High-resolution C 1s peaks fitting were carried out to study the surface functional groups change. As shown in Fig. 4(c) and (d), the C 1s peaks were deconvoluted into five peaks: C−C at 284.6 eV, C−N at 285.35±0.05 eV, C−O at 286.5 eV, −CONH at 287.75±0.05 eV and O−C=O at 288.5 eV.41 The analysis results shown in Table II demonstrated that after plasma treatment, the content of C−C group decreased while that of oxygen-containing groups (such as C−O, −CONH and O−C=O) increased. As previously studies demonstrated, 41-45 during plasma treatment of polymers, the highly reactive and energetic plasma species promoted fiber surface etching, and the difference of etching rates of amorphous and crystalline regions 46,47 resulted in the increase of surface roughness. Besides the etching process, the energetic electrons and particles could lead to scission of polymer molecular chains, which led to the formation of free radicals that interacted with other plasma generated reactive species (such as O, OH and N), creating new functional groups on the sample surface. Therefore, the significantly increased oxygen-containing polar groups after plasma treatment were responsible for the improved hydrophilicity41,48.


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Fig. 4 FTIR (a) and XPS spectra (b) and peak-fitted high-resolution C 1s spectra (c, d) of nylon meshes. Table II. Relative contents of the elements and the moieties of C 1s peak-fitting of the original and the plasma functionalized nylon meshes.

Original nylon mesh Plasma functionalized nylon mesh

Moieties (%)

C 1s (at.%)

O 1s (at.%)

N 1s (at.%)

C−C

C−N

C−O

76.83 66.56

14.96 20.56

8.21 12.88

69.07 56.61

19.38 19.37

3.14 5.24

−CONH O−C=O 8.41 13.01

0.00 5.77

For the oil/water separation, we employed hexane, hexadecane, diesel and peanut oil as oil exemplars. Fig. 5(a) shows the underwater static CAs and SAs for the four oils, and the CA hysteresis was lower than 10 °, demonstrating excellent underwater oil repellence of the plasma functionalized nylon mesh. The intrusion pressure (Pin) of oils indicated the maximum oil column height (hmax) that the plasma functionalized nylon mesh could support. The intrusion pressure could be obtained by the following equation:

Pin   ghmax

(1)

where ρ was the density of the oil, g was the acceleration of gravity. According to the experimentally measured hmax, the intrusion pressure of the oils were calculated and present in Fig. 5(b). The intrusion pressures for hexane and hexadecane respectively reached up to about 3.2 and 3.6 kPa, and those of diesel and peanut oil were also larger than 2.2 kPa, showing excellent oil supporting capacity when the plasma functionalized nylon mesh was immersed in water. 9 ACS Paragon Plus Environment


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Video S3 (see Supporting Information) presents the oil/water separations and Fig. 5(c) illustrates the digital photographs of hexadecane/water separation process. We could see that when the mixtures were poured into the upper tube, the oils were stopped by the pre-wetted nylon mesh while the water could flow through easily. The separation efficiency (R) of oil/water mixtures was calculated by the ratio between the collected water mass (Ms) and the water mass of the original oil/water mixtures (M0):

R

Ms 100% M0

(2)

The results shown in Fig. 5(d) demonstrated that the separation efficiency of the plasma functionalized nylon mesh was up to about 99% for hexadecane/water mixture and above 97.5% for other testing oil/water mixtures. Water flux during oil/water separation was another important parameter to characterize the separation ability of the mesh. The water flux (Fwater) of the plasma functionalized nylon mesh could be calculated by equation (3): V (3) Fwater  St where V was the water volume in the oil/water mixture, S was the cross sectional area of the mesh that was exposed to oil/water mixtures and t was the required time for the complete permeation of water. As shown in Fig. 5(e), the water flux for hexane/water mixture reached up to about 37000 L m-2 h-1, and that for diesel was larger than 22000 L m-2 h-1.


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Fig. 5 (a) Underwater CAs and SAs of different oils on the plasma functionalized nylon mesh. (b) Intrusion pressure of the pre-wetted functionalized nylon mesh for different oils. (c) Digital photographs of hexadecane/water separation. The separation efficiency (d) and water flux (e) of different oil/water mixtures.

The purity of the separated oils was investigated by FTIR and as shown in Fig. 6(a), all the absorption bands were owing to the characteristic vibrations of alkanes and esters 49. The bands from 2853 to 2957 cm−1 were assigned to the –C–H asymmetric and symmetric stretching vibration of –CH3 and –CH2– groups. The –C=O stretching vibration of ester groups was found at 1742 cm−1. The band around 1466 cm−1 corresponding to the –C–H scissoring vibration of –CH2– groups and the –C–H asymmetric stretching vibration of –CH3 groups. The bands at 1377 cm−1 originated from the –C–H symmetric bending vibration of –CH3 groups. The bands from 1158 to 1234 cm−1 were assigned to the –C–H bending vibration of –CH2– groups. The –C–O stretching vibration of ester groups could be observed at 1096 cm−1. The bands around 722 cm−1 were attributed to the –(CH2)n– rocking vibration and –HC=CH– bending vibration. No band assigned 11 ACS Paragon Plus Environment


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to water was observed, confirming the high purity of the separated oils. Additionally, the oil content in the collected water was calculated and depicted in Fig. 6(b). The result demonstrated that the oil content in the collected water was lower than 90 ppm for the four oil/water mixtures, especially for the hexane/water mixture, of which the oil content could be as low as 45.3±5.1 ppm. Therefore, the plasma functionalized nylon mesh can be a high efficient tool for oil/water separation.

Fig. 6 (a) The FTIR spectra of the separated oils. (b) The oil content in the collected water.

The durability of the functionalized mesh used for oil/water separation was an important issue for practical application. Recyclability of the mesh was tested by cycling the oil/water separation for more than 20 times. The mesh was firstly used to separate diesel/water mixture for 10 cycles, and then hexadecane/water for another 10 cycles. As depicted in Fig. 7(a), the separation efficiency during the 20 cycles maintained above 96% for both diesel/water mixture and hexadecane/water mixture, and the mesh maintained excellent underwater oil repellence after being reused for 20 times (Fig. 7(b)), indicating good recyclability of the plasma functionalized nylon mesh for oil/water separation. As discussed above, the plasma induced superhydrophilicity was crucial for the underwater superoleophobicity. But it is well known that after plasma exposure, the modified surface tends to reorganize in order to minimize its surface energy to maintain the equilibrium between the hydrophilic surface (high surface energy) and hydrophobic air (low surface energy medium), i.e. the aging behavior50-52. So the plasma induced 12 ACS Paragon Plus Environment

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(super)hydrophilicity always decreased and even recovered to the state before plasma treatment when the sample was stored in air. However, when the treated sample was stored in water (high surface energy medium), the interfacial energy was low, so the wettability recovery could be greatly limited51,53. When the plasma functionalized nylon mesh was used for oil/water separation here, it must be wetted by water and thus its stability could be well maintained. As shown in Fig. 7(c), the underwater CAs of dichloromethane changed a little during 4 days’ immersion in water, indicating a stable underwater superoleophobicity. And after immersing for 4 days, the separation efficiencies of the mesh for diesel/water mixture and peanut oil/water mixture kept above 96%, demonstrating a durable oil/water separation ability of the wetted mesh.

Fig. 7 Durability of the plasma functionalized nylon mesh for oil/water separation. (a) Continuous separation of diesel/water mixture (10 cycles) and hexadecane/water mixture (10 cycles). (b) Underwater oil CAs of the mesh after being used for 20 times oil/water separations. (c) Stability of underwater superoleophobicity of the plasma functionalized nylon mesh.

Surface micro/nano texturing and hydrophilization are general for plasma treatment of polymeric materials, such as polyurethane54, polyester55, polypropylene56 and polyethyleneterephthalate57. To demonstrate the general applicability of plasma functionalization of polymeric materials for oil/water separation, we also treated polyurethane sponge slice (~2 mm in thickness) and polyester fabric with the APP under the same experiment conditions of nylon mesh. Original polyurethane sponge was hydrophobic in air (the CA for water was ~123.7±4.8 °) and superoleophilic underwater (the CA for dichloromethane was ~0 °), and its skeleton was relatively smooth (see Supporting Information, Fig. S5). By contrast, pit-like rough structures could be clearly seen on the APP treated polyurethane sponge slice, which could support 13 ACS Paragon Plus Environment


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dichloromethane droplets as spherical shapes indicating excellent underwater superoleophobicity (the CA for dichloromethane was ~158.5±3.7 °), as shown in Fig. 8(a)-(c). Polyester fabric was originally superhydrophilic in air (the CA for water was ~0 °) and thus highly oleophobic underwater (the CA for dichloromethane was ~141.7±4.2 °, see Supporting Information, Fig. S5). After APP exposure, as shown in Fig. 8(d)-(f), the polyester fabric surface became rougher and some groove-like sub-micro structures appeared, which turned the fabric into superoleophobic underwater (the CA for dichloromethane was ~162.3±2.6 °). The APP functionalized polyurethane sponge slice and polyester fabric with excellent underwater superoleophobicity were thus able to separated oil/water mixtures. Fig. 8(g) and (h) respectively depict the separation efficiency of the two functionalized polymeric materials, demonstrating their outstanding separation ability.

Fig. 8 The general applicability of plasma functionalization of polymeric materials for oil/water separation. The SEM images of the APP functionalized polyurethane sponge slice (a-c) and polyester fabric (d-f), and the inset images were the corresponding photographs of dichloromethane droplets on the samples; the separation efficiency of APP functionalized (g) polyurethane sponge slice and (h) polyester fabric for different oil/water mixtures.

CONCLUSIONS 14 ACS Paragon Plus Environment

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In summary, we reported the functionalization of nylon mesh by APP to create a tool for oil/water separation. SEM, AFM and XPS results demonstrated that APP treatment imparted the mesh superhydrophilicity in air by both roughing its surface and introducing oxygen-containing groups while had little influence on its mechanical properties. The as-prepared superhydrophilic mesh exhibited superoleophobicity while immersing in water and consequently realized high-efficient oil/water separation. The mesh could effectively separate oil/water mixtures for many cycles and even after long-term underwater storage, indicating that the APP functionalized nylon mesh possessed excellent recyclability and durability. Other polymer materials (e.g. polyurethane sponge and polyester fabric) could also be easily functionalized by APP and used for oil/water separation. The technical simplicity, environment-friendliness, effectiveness and versatility of plasma functionalization in terms of surface (super)hydrophilization well fits the principle of green and sustainable chemistry and engineering, and the reported method here could offer a new perspective on practically solving pollutions caused by oily industrial wastewater and oil spills.

ASSOCIATED CONTENT Supporting Information Schematic diagram of the He APP generator; the typical optical emission spectrum of He APPJ; schematic diagram of oil/water separation device; The static and dynamic (advancing/receding) oil contact angles of the APP functionalized nylon mesh; the schematic diagrams of the wettability of nylon mesh surface in air and water; photograph of water (in air) and dichloromethane (in water) droplets on original polyurethane sponge slice and polyester fabric; the SEM images of original polyurethane sponge slice and polyester fabric (PDF). Process of a dichloromethane droplet contacting with and then leaving away the plasma functionalized, submerged nylon mesh (AVI). Process of dichloromethane and hexadecane droplets rolling off from a ~5° tilted plasma functionalized nylon mesh (AVI). Oil (hexane, hexadecane, diesel and peanut oil)/water separation by plasma functionalized, pre-wetted nylon mesh (AVI).

ACKNOWLEDGEMENT The authors thank Prof. Zhenkun Lei and Mr. Weikang Li from Department of Engineering Mechanics, Dalian University of Technology for his help in nylon fiber’s mechanical properties measurement. This work was financially supported by National Natural Science Foundation of China (NSFC, Grant No. 51305060 and 51275072), the National Basic Research Program of China (973 Program, Grant No. 2015CB057304), the Fundamental Research Funds for the Central Universities (DUT15RC(4)21 and DUT15ZD241).

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For Table of Contents Use Only.

Title: Atmospheric Pressure Plasma Functionalized Polymer Mesh: an Environment-friendly and Efficient Tool for Oil/Water Separation Authors: Faze Chen, Jinlong Song, Ziai Liu, Jiyu Liu, Huanxi Zheng, Shuai Huang, Jing Sun, Wenji Xu, Xin Liu Synopsis: Atmospheric pressure plasma functionalized polymer mesh was employed as an environment-friendly and efficient tool to separate oil/water mixtures


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Fig. 1 Photograph of (a, b) water on the plasma functionalized nylon mesh in air and (c, d) oil droplets on the plasma functionalized nylon mesh under water. 150x95mm (300 x 300 DPI)

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Fig. 2 (a) Time series of a water droplet contacting with the plasma functionalized nylon mesh in air. (b) Dichloromethane droplet making contact with and losing contact with a plasma functionalized nylon mesh in water. (c) Time-lapsed snapshots of dichloromethane and hexadecane droplets rolling on a tilted plasma functionalized nylon mesh in water. CAs (d) and SAs (e) of nylon mesh as a function of plasma treatment time. 150x171mm (300 x 300 DPI)


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Fig. 3 SEM images of original (a-c) and plasma functionalized (d-f) nylon mesh; AFM images of original (g) and plasma functionalized (h) nylon mesh; (i) Loading-displacement curves of the original and the plasma functionalized nylon fiber. 160x140mm (300 x 300 DPI)



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Fig. 4 FTIR (a) and XPS spectra (b) and peak-fitted high-resolution C 1s spectra (c, d) of nylon meshes. 140x109mm (300 x 300 DPI)


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Fig. 5 (a) Underwater CAs and SAs of different oils on the plasma functionalized nylon mesh. (b) Intrusion pressure of the pre-wetted functionalized nylon mesh for different oils. (c) Digital photographs of hexadecane/water separation. The separation efficiency (d) and water flux (e) of different oil/water mixtures. 150x149mm (300 x 300 DPI)



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Fig. 6 (a) The FTIR spectra of the separated oils. (b) The oil content in the collected water. 80x119mm (300 x 300 DPI)


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Fig. 7 Durability of the plasma functionalized nylon mesh for oil/water separation. (a) Continuous separation of diesel/water mixture (10 cycles) and hexadecane/water mixture (10 cycles). (b) Underwater oil CAs of the mesh after being used for 20 times oil/water separations. (c) Stability of underwater superoleophobicity of the plasma functionalized nylon mesh. 140x102mm (300 x 300 DPI)



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Fig. 8 The general applicability of plasma functionalization of polymeric materials for oil/water separation. The SEM images of the APP functionalized polyurethane sponge slice (a-c) and polyester fabric (d-f), and the inset images were the corresponding photographs of dichloromethane droplets on the samples; the separation efficiency of APP functionalized (g) polyurethane sponge slice and (h) polyester fabric for different oil/water mixtures. 160x127mm (300 x 300 DPI)


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