UV-cured Fluoride-free Polyurethane Functionalized Textile with pH

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UV-cured Fluoride-free Polyurethane Functionalized Textile with pH-induced Switchable Superhydrophobicity and Underwater Superoleophobicity for Controllable Oil/Water Separation Kunlin Chen, Weiwei Gou, Xuemei Wang, Chanjuan Zeng, Fangqing Ge, Zhenjiang Dong, and Chaoxia Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03851 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 29, 2018

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ACS Sustainable Chemistry & Engineering

UV-cured Fluoride-free Polyurethane Functionalized Textile with pHinduced Switchable Superhydrophobicity and Underwater Superoleophobicity for Controllable Oil/Water Separation Kunlin Chen*, Weiwei Gou, Xuemei Wang, Chanjuan Zeng, Fangqing Ge, Zhenjiang Dong, Chaoxia Wang*

Key Laboratory of Eco-Textiles, Ministry of Education, School of Textiles and Clothing, Jiangnan University, 1800 Lihu Road, Wuxi, Jiangsu 214122, P. R. China

*Corresponding author: Tel: +86 510 85912009 Mailing address: [email protected] (Kunlin Chen) [email protected] (Chaoxia Wang)

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ABSTRACT: Design and fabrication of smart surfaces for separation of oily water have been developed rapidly. Nevertheless, construction of these smart surface materials by using green and sustainable paths for intelligent separation of oily water still faces huge challenges. The aim of this study was to develop a facile, sustainable and energy-efficient approach for the fabrication of smart coatings with pH-responsive switchable wettability. The coating was formed on the textiles from UV-cured fluoridefree pH-responsive polyurethane (pH-PU). The wettability of obtained textiles could switch reversibly between superhydrophobicity and underwater superoleophobicity due to the protonation and deprotonation of the pH-PU under different pH conditions, which was used to successfully separate either oil or water from oily water with high separation efficiencies. More importantly, oil-in-water and water-in-oil emulsions could all be selectively separated with high water flux and oil purity by as-fabricated textiles. And these pH-PU functionalized textile or sponge performed effectively for oil collection of oil spilled on water. Our work provided a fast, economical and sustainable pathway to prepare smart coatings with switchable wettability for both environmental protection and energy-saving merits. KEYWORDS: Polyurethane, UV-cured, Textiles, pH-responsive, Oil/water separation

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INTRODUCTION With the increase of oil spill accidents in marine and industrial waste water with oil, which have resulted in serious ecological environment destruction, many developing novel technologies have been reported to fabricate various materials for effectively separating oil and water mixtures,1-5 such as porous polymer membranes,6-8 polymernanoparticle composites,9-13 carbon nanotube network films,14,15 kinds of aerogels.16-18 These

approaches

are

generally

categorized

as

two

strategies:

i)

creating

micro/nanostructures on the special wetting substrates; ii) chemically modifying the micro/nano-structured surface with other hydrophobic or hydrophilic materials.4,1925Although

these oil/water-separating materials may well solve the separable problems,

most of these materials possess only single extreme surface-wetting performances, such as

superhydrophobicity/superoleophilicity

superoleophobicity.26-28 If

a

surface

can

or switch

superhydrophilicity/underwater its

superoleophobicity

and

superoleophilicity in response to external stimulation, it would provide a great flexibility for advanced applications, such as controllable, filtration remote operation and ondemand automation of oil/water separation process. For instance, Minko group reported pH-responsive superhydrophobic surfaces via grafting the block-copolymer poly(styrene-block-4-vinylpyridine) P(S-b-4VP) brush onto the nanoparticles.29 The formation of hierarchically structured aggregates could be adjusted by the pH, and the wettability of surface was reversibly tuned. We also reported a transparent surface with reversibly tunable wettability based on the assembly polystyrene spheres monolayer.30 However, these materials did not have the oil/water separation performance. In addition, due to the low surface energy of fluoride, many 3



oil/water-separating materials contain the fluorides inside that can be hardly decomposed, and they would pollute the environment.31,32 Therefore, developing scalable oil/waterseparating materials with fluorine-free substances draws great interests from people in both academic and industrial areas. Recently, researchers have done some important work in smart surface materials with wettability transition in response to external stimulation,33,34 such as temperature,35-37 light exposure,38-41 electrical field42 and pH.43,44 Lei et al. reported a smart melamine sponge with grafting hydrophobic octadecyltrichlorosilane and thermo-responsive poly(N-isopropylacrylamide) (PNIPAAm) into the sponge skeletons.45 Due to temperature-sensibility of PNIPAAm, the modified sponge could reversibly switch the surface wettability between superhydrophilicity and superhydrophobicity. This sponge was able to be applied to absorb and release the oil from oily water under different temperature environments. Tian et al. developed a micro/nanoscale mesh film coated with an aligned ZnO nanorod array, which has phototunable underwater oil adhering performance.38 It can switch the contact mode between UV illuminating and storing in the dark because of the hierarchical structured ZnO, which can act as an oil/water separated material in responsive to UV light. Shi and coworkers used electroless metal deposition combined with surface modification to obtain a pH-responsive smart device.46 The obtained smart device was able to switch the wettability from superhydrophobicity to superhydrophilicity by altering the pH of aqueous solution, which was used to separate the ternary oil/water/oil mixture. Although intelligent materials can controllably separate oil/water mixtures, the preparation approaches of these materials usually required complicated synthesis procedures and fluoride materials. It is an 4


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increasing demand for the development of more facile and efficient approach to realize the intelligent separation of oily water. PU coatings have excellent wearing durability and are safe to human body, and are well suited for fabrics or textiles.47,48 Moreover, UV-curing PU coatings may possess many other advantages, such as energy and time savings, low environmental impact, and very fast and efficient polymerization.49,50 In addition, due to the nanoscale cage structure of polyhedral oligomeric silsesquioxane (POSS), the introduction of POSS into the polymer main chains at the molecular level for the construction of organic/inorganic hybrid materials is a good strategy to acquire high performance materials,51 which can improve the hybrid material performance, such as surface performances, mechanical properties, thermal stability, and biological function. In this work, we reported a facile and environmentally friendly method for preparing smart textiles using UV-cured fluorine-free pH-responsive polyurethane (pH-PU). The detail schematic diagram of the fabrication process is depicted in the Scheme 1. The pristine textile was firstly dip-coated the POSS-containing pH-responsive PU resins, and then cured by UV light irradiation. The as-obtained textile shows excellent superhydrophobicity in air, and also exhibits the superoleophobicity in water after treated by the acid aqueous solution due to the protonation of LEDA chains within PU. This special wettability endowed the textile with efficient oil/water separation capability, and could intelligently separate the light or heavy oil from the oily water by adjusting the pH value of solution. Furthermore, this functionalized textile could selectively separate oil in oil-in-water or water-in-oil emulsions with high oil purity and water flux. Simultaneously, pH-PU functionalized textiles and sponges were able to effectively absorb the oil from 5



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oily water. The present study offers a simple and effective strategy for large-scale fabrication of smart surface materials that would potentially be used in a wide range of applications.

Scheme 1. Schematic sketch of the fabrication process of fluorine-free pH-responsive polyurethane functionalized textile.

EXPERIMENTAL SECTION Materials Mercaptoethanol (ME, 98%) was supplied from Guangdong Wengjiang Chemical Reagent Co., Ltd. (China). Octavinyl polyhedral oligomeric silsesquioxane (OV-POSS) was obtained from Shanghai Rong Bai Biological Technology Co., Ltd. (China). Hydroxyl-terminated

polydimethylsiloxane

(PDMS-OH,

Mw=400

g·mol-1)

were

Shanghai Dow Corning Co., Ltd. (USA). Polypropylene glycol (PPG, Mw=1000g·mol-1) was purchased from Jinin Huakai Resin Co., Ltd. (China). 2- Isophorone diisocyanate (IPDI, 99%), pentaerythritol triacrylate (PETA, technical grade), ditin butyl dilaurate (DBTDL, 95%), 2-hydroxy-2-methylpropiophenone (HMPP, 97%) and hydroquinone 6


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(≥99.5%) were purchased from Shanghai Aladdin Chemical Reagent Co., Ltd. (China). Lauryl diethanolamine (LDEA, 97%) was purchased from Nantong Chenrun Chemical Co., Ltd. (China). Diphenylmethyl phosphine (DMPP, 98%) was obtained from Shanghai Linen Technology Development Co., Ltd. (China). Photoinitiator 1173 (HMPP) was obtained from BASF (Germany). Acetic acid (≥99%), ammonia solution (25%) and tetrahydrofuran (THF, ≥99.5%) were supplied from Sinopharm Chemical Reagent Co., Ltd. (China). Commercially available cotton fabrics were obtained from a local fabric store. Synthesis of Fluorine-Free pH-Responsive Polyurethane Firstly, OV-POSS (6.34 g, 0.01mol) was dissolved into 50 mL glass reactor with THF (30 mL) and a magnetic stirrer to form homogeneous solution. Subsequently, ME (1.56 g, 0.02 mol) and DMPP (5 mg) were added into the solution and meanwhile purged with pure nitrogen. In the following, the stirring was kept constant at 20 oC for 3 h. The synthesis route is shown in Scheme 2.

Scheme 2. Synthetic route of ME-POSS.

A proper amount of IPDI, PDMS400, PPG1000 and DBTDL were firstly put into a 250 ml three-necked flask equipped with a mechanical stirrer using acetone as the solvent, and meanwhile pure nitrogen was inlet. Subsequently, the mixture was heated up to 70 oC 7



for 2 h. Then ME-POSS and LDEA THF solution was added into the mixture, and continued to react at 70 oC to obtain NCO terminated prepolymer until the isocyanate (NCO) content of system reached to the theoretical NCO value. Subsequently, the reaction system was cooled down to 40 oC and a proper amount of PETA and hydroquinone were placed into the reaction mixture, and continued to react at 40 oC for 4 h. The detailed synthesis process is shown in Scheme 3.

Scheme 3. Synthetic route of pH-responsive PU based on the POSS.

Preparation of Functionalized Textile The pristine textile was immersed in the mixture of pH-PU resins using acetone as solvent and HMPP (pH-PU: 40 wt%, HMPP: 1.5 wt %) for 5 min. Then the immersed textile was irradiated under UV light for 30 s to form functionalized textile. The intensity 8


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of the UV light was 300 Wm-2 at 365 nm. The preparation of functionalized sponge could refer to the above method. Characterizations Fourier transform infrared (FT-IR) measurements were carried out with a Nicolet Nexus 470 spectrometer (ThermoFisher, USA), with a resolution of 0.5 cm-1 and an accumulation of 32 scans. Crystallographic information of PU and POSS was tested through powder XRD (Bruker, D8 Advance X-ray diffractometer) with Cu Kα radiation. Molecular weight and distribution of PU was determined by gel permeation chromatography (GPC, Waters THF, USA) in tetrahydrofuran at 40 ºC with an elution rate of 0.5 ml min−1. The morphologies of as-prepared samples was observed by a scanning electron microscope (SEM, SU1510, HITACHI, Japan) at an accelerating voltage of 5 KV, and their surface elements were measured by energy-dispersive X-ray spectroscopy (EDS) attached to SEM. The surface wettability of samples was evaluated by testing their water contact angle (WCA), oil contact angle (OCA) and sliding angle (SA), which are determined by an OCA15 contact angle analyzer (Dataphysics, Germany), using 3 μL deionized water and n-Hexadecane (HD) droplets, respectively. The average value was obtained by calculating five parallel measurements. X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 5000C ECSA) measurement was used to investigate the surface atomic compositions of samples using a monochromatized Al Kα X-ray source. The abrasion stability of samples was measured by a method according to the AATCC Test Method 8-2001. After washing and drying, their contact angles were tested. The washing stability of samples was tested according to the normal washing process of 9



AATCC 61-2003. The samples were rinsed each time after washing, and subsequently contact angles were tested. The prepared textile sample was fixed between two glass vessels to serve as a membrane for the oil/water separation test. The dichloromethane/water mixture and hexane/water mixture were used as a representative to inspect the oil/water separation efficiency of the superhydrophobic/superoleophilic and superhydrophilic/underwater superoleophobic separation, respectively. The volume ratio of water and oil was 50/50%. After each separation process, the textile was washing with ethanol and drying for the next reuse. The separation efficiency can be calculated according to the following formula: η= (M2/M1) ×100%, where M1 and M2 are the weight of the water before and after the separation process, respectively.

RESULTS AND DISCUSSION Synthesis of Fluoride-Free UV-Cured pH-PU The FT-IR spectrum was firstly used to analyze the LDEA, ME-POSS and synthesized pH-PU. As Figure 1a shows, an absorption peak at 3400 cm-1 from -OH stretching mode can be readily observed in the spectrum of ME-POSS, indicating ME has been grafted into the POSS molecules. It is clearly found that the absorption peak at 3400cm-1 and 2280 cm-1 derived from -OH and -NCO stretching mode respectively have disappeared in the pH-PU, and two new peaks at 1730 cm-1 and 1100 cm-1 corresponding to the C=O and Si-O-Si stretching vibrations appeared in the FT-IR spectrum of pH-PU. In addition, the pH-PU was further characterized by XRD analysis. From Figure 1b, the PU was overall amorphous with one broad diffraction band at 18. When increasing the POSS 10


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content into the PU, the peak at 8.1 for the (101) plane of POSS cages was found, suggesting

that

POSS

copolymerized

into

PU.

Besides,

the

51

molecular

weight of synthesized pH-PU was about 30000 g mol-1 (Figure S1, Table S1). All these evidences illustrated that POSS and LDEA have been introduced into the PU, and the pHPU has been successfully synthesized.

Figure 1. (a) FT-IR analysis of LDEA, ME-POSS and pH-PU; (b) XRD analysis of the pH-PU hybrids (PU-POSS0: no POSS in the PU; PU-POSS10: POSS content in the PU is 10%; PU-POSS20: POSS content in the PU is 20%).

Preparation of pH-Responsive Polyurethane Functionalized Textile The representative SEM images of uncoated and coated textiles were shown in Figure 2a-f. A low-magnification SEM image of the uncoated textiles revealed a smooth surface, which was made of fibers with an average diameter of 10-20 μm (Figure 2a). From the high-magnification SEM image, the fibers clearly showed the cylindrical-shape that were separated from each other (Figure 2b, c), which could help the water or oil to easily pass through the textiles. After coated with a layer of fluorine-free pH-responsive PU resins, it is clearly shown that there were some continuous membranes on the fibers and the gaps between fibers (Figure 2d, e). Owing to the introduction of POSS into the PU resins, numerous nano-protuberancese immobilized on the surface were found (Figure 2f). The 11



surface roughness can contribute to the realization of special wettability of coated textiles. The EDS analysis of the coated textile surface was further investigated. From Figure 2g, the chemical composition of surface was composed of four elements: C, O, N, and Si, which were evenly distributed across all the surface of textile. This result further proved that the surface of textile was coated a layer of PU resins.

Figure 2. SEM images of as-prepared textiles: (a) the pristine textile with a smooth surface, (b,c) the magnified SEM image of (a); (d) the coated textiles with PU resins, (e,f) the magnified SEM image of (d); (g) EDS analysis and SEMEDS mappings of the coated textiles.

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The incorporation of POSS into PU resins could afford the formed films with the surface performances and mechanical properties due to the nanoscale cage structure of POSS.52 Thus, the content of POSS in the PU resins is very important. As shown in Figure 3a, the WCA of surface was increased along with increasing the content of POSS in the resins. When the content of POSS reached to 20 wt%, the obtained surface achieved the superhydrophobicity (155o). Nevertheless, when the content of POSS continued to increase, the WCA of film did not advance. It is because incorporating too much POSS into PU would influence the film-forming ability. In addition, LDEA chains can be protonated, so the content of LDEA monomer would influence the wettability of film after treated by acid aqueous solution. From Figure 3b, it is found that the WCA of film was declining as a function of the increase of content of LDEA in the PU resins after the protonation (treated with pH 2 acid aqueous solution). When the content of LDEA was 10 wt%, the WCA of film reached the minimum. This is because when LDEA chains were protonated, its water binding capacity was improved. The more LDEA chains the PU resins involved, the stronger the hydrophilicity of film was.

Figure 3. (a) Changes in the WCA for film with different content of POSS in PU resins. (b) Changes in the WCA for film after protonated with different content of LDEA in PU resins.

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pH-Responsive Switchable Wettability of Functionalized Textiles As 20 wt% POSS and 10 wt% LDEA were incorporated into the PU resins, the corresponding

functionalized

textile

showed

the

superhydrophobicity

and

superoleophilicity (Figure 4 a, b). Moreover, the water droplet could not adhere to the surface of textiles and easily roll off the surface (SA=7.6o, Supplementary Movie S1), and while the oil droplet was quickly wetting the surface. However, the LDEA chains of PU resins would convert to the hydrophilicity from the hydrophobicity after treated with acid aqueous solution (pH=2), and thus the surface of textile showed the hydrophilicity and could be entirely wetted by the water droplet after 3.5s (Figure 4c). When the textile was submerged into the water simultaneously, the textile revealed the underwater superoleophobicity, which cannot stick to the surface (Figure 4d), and the oil droplets were easily rolling down the inclined surface in the water (SA=5.1o, Supplementary Movie S2), suggesting the low oil adhesion property of surface under water. What’s more, the pH-responsive switchable wettability of surface between the superhydrophobicity and underwater superoleophobicity was reversible, and the reversible cycles could be repeated for 10 times, implying that this pH-induced switchable wettability of functionalized textile is stable and durable during treatment (Figure 4e). Before the LDEA chains were protonated, the POSS and PDMS in the PU dominated on the surface prevented the textile from being wetted by the water but had high affinity for oil, thus the surface displayed superhydrophobicity and superoleophilicity. After treated with the acidic water (pH=2.0), the surface of textile became the hydrophilicity due to the protonated LDEA chains. The protonated LDEA chains were dominantly exposed the exterior of textile and trapped the water, which effectively blocked the access of oil 14


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(Figure 4f). Therefore, the hydrated layer combined with hierarchical structures of textile endowed the surface with the underwater superoleophobicity.2 The protonation and deprotonation of tertiary amine groups in LDEA chains were further investigated by XPS. From Figure 4g, the surface contained C, O, N, and Si chemical elements, and the N atomic concentration of the surface increased from 2.04 % to 4.65 %. Further, the N 1s spectrum was curve-fitted into two peaks by XPS differentiation imitation. As shown in Figure 4h, the fitting peaks centered at binding energies of 401.5 and 399.9 eV were originated from HR2N+- and R2N-,

53,54

respectively, which were from pH-PU resins.

After being treated with acid aqueous solution (pH=2), it is observed that the intensity of the HR2N+- peak obviously increases by comparison with the intensity of the R2N- peak resulted from the protonation of tertiary amine groups of LDEA chains (Figure 4i). The results manifested that the change of the surface special wettability was derived from the protonation and deprotonation of tertiary amine groups of LDEA chains within pH-PU resins.

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Figure 4. (a,b) Snapshots of water droplet (dyed by vat green) and n-hexane (dyed by oil red O) on the surface of polyurethane functionalized textile before protonation in air, and their adhesion to the surface and the process for wetting the surface. (c) Water on the surface of polyurethane functionalized textile after protonation in air, and the process for wetting the surface. (d) Oil droplet of dichloromethane (dyed by oil red O, heavier than water) on the surface of polyurethane functionalized textile after protonation in water, and the adhesion to the surface. (e) Variation of WCA and underwater OCA for the functionalized textile along with the number of the repeated protonation/deprotonation cycles. (f) The mechanism map of the reversible protonation/ deprotonation process of LDEA chains within pH-PU. (g) The chemical composites of the surface of functionalized textile before and after being treated with acid aqueous solution (pH=2) by XPS spectra analysis. XPS spectra of N1s signals of the pH-PU film before (h) and after (i) being treated with acid aqueous solution (pH=2).

Controllable Oil/Water Mixtures Separation of Functionalized Textiles 16


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The superhydrophobic and underwater superoleophobic properties of pH-PU endow the functionalized cotton fabrics with enormous potential for gravity driven oil/water separation. In our study, the separation performance of functionalized cotton toward various oil and water mixtures was evaluated. As revealed in Figure 5a, a piece of functionalized cotton fabric was fixed between two glass funnels with an iron clamp, and a mixture of dichloromethane and water dyed by oil red O and vat green, respectively, was slowly poured onto the surface of functionalized cotton fabric. It can be seen that red oil quickly passed through the cotton fabric and dropped into the conical flask because of the superoleophilicity of the coating layer on the fabric. However, green water was repelled and remained above the fabric owing to the superhydrophobicity and low wateradhesion of the surface of functionalized cotton fabric (Supplementary Movie S3). Furthermore, when this functionalized fabric was treated with the acidic aqueous water (pH=2.0) and then directly used for the oil/water separation, as shown in Figure 5b, it was found that green water quickly permeated through the fabric and increasingly gathered in the conical flask. Meanwhile, red oil (n-hexane) was retained on the fabric, and no red oil was found in the conical flask (Supplementary Movie S4). These oil/water separations entirely relied on the gravity without other external force. For practical applications, much of the concern is focused on the durability and recyclability of the oil/water separation materials. Figure 5c shows the variation of WCAs of functionalized cotton fabric along with the abrading cycles. Although there was a slight decrease of the fabric, it still kept the superhydrophobicity after 300 cycles of abrasion, and the blue water droplets were able to stand very well on the surface, which was presented in the inserted photograph of Figure 5c. Moreover, it was found this functionalized cotton could 17



maintain its superhydrophobic property (151.3°) even after 30 cycles washing, as shown in Figure 5d. When these functionalized cotton fabrics after abraded or washed were used to separate the oil/water mixtures, their separation efficiency was slightly declined compared to functionalized cotton without abrasion or washing, while it was able to keep above 99% (Figure 5e), displaying quite stable and durable superwettability. All these good results are due to the high mechanical stability of cross-linked polymers. Additionally, the functionalized cotton was also able to be used in separation of other heavy oil, such as chloroform, and after treated with the acidic aqueous water (pH=2.0), the functionalized cotton could also separate other light oil, such as soybean oil, n-decane, octane, petroleum ether and raw oil. The separation efficiency of these oil/water mixtures was all above 99% (Figure 5f). As shown in Figure 5g, the WCA and OCA of the functionalized cotton decreased slightly after 30 times repeated separation, which may be due to the adhesion of a small amount of oil on the fabrics. However, the functionalized cotton fabrics still sustained the superhydrophobicity or underwater superoleophobicity, and the separation efficiency remained almost the same values after 30 times repeated separation (Figure 5h), exhibiting the excellent recycling performance of functionalized textile for oil/water separation.

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Figure 5. pH-controllable separation of the oil (dyed by oil red O)/water (dyed by vat green) using the pH-PU functionalized textile. (a) Oil (red) passed through the fabric, while water (green) retained above the upper glass tube. (b) After being treated by acid aqueous solution (pH=2), water can pass through the fabric, while oil left on the upper glass tube. Variation of WCA of functionalized cotton fabric with abrasion cycles (c) and washing cycles (d). The insets are the photographs of water droplets on the functionalized textiles after abrasion or washing, respectively. (e) Oil/water separation efficiency of different textiles (original functionalized cotton, functionalized cotton after abraded, and functionalized cotton after washed). (f) The separated efficiency for different oil/water mixtures. (g) The changes of WCAs and OCAs functionalized cotton as a function of oil/water separation cycles. (h) Variation of the separated efficiency of the functionalized cotton fabrics as a function of oil/water separation cycles.

Controllable Oil/Water Emulsions Separation of Functionalized Textiles The separation of oil/water emulsions is always challenging. In this study, the separating performance of oil-in-water (O/W) and water-in-oil (W/O) emulsions of functionalized textile was further investigated. The whole separating process was driven only by the gravity of the emulsions. Figure 6a-e exhibits the separating processes of O/W emulsion (oil: n-hexane) stabilized with surfactant, and the functionalized cotton pre-wetted by the acid aqueous solution (pH 2), which was superhydrophilic and 20


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underwater superoleophobic, was used to separate O/W emulsion. It is found that the turbid feed emulsion became transparent after separation (Figure 6a), and numerous droplets were conspicuous in the optical microscopic image of the emulsion, while droplets disappeared in the image of filtrates (Figure 6b, c). This is because the water easily spread and permeated into the superhydrophilic textile to form a water layer to prevent oil droplets infiltrating. Meanwhile, many micro-scaled oil droplets were combined into larger droplets due to the coalescence effect, which would leave on the surface of the functionalized cotton.55,56 The UV/vis absorption spectroscopy was further used to monitor the content of n-hexane (dyed by the oil red O) in the water before and after the separating process. As revealed by the Figure 6d, the typical absorption intensity of red n-hexane at 517 nm was almost non-existent after one cycle of separation. As exhibited in Figure 6e. It is found that the functionalized cotton could effectively block the oil drops with relatively high oil rejection (99.8%) and water permeation flux (1456 Lm-2h-1) under driven by gravity only, indicating the high separated efficiency of functionalized textile for the O/W emulsion. Besides n-hexane/water emulsion, the functionalized cotton also displayed good separating performances for other oil emulsions, such as octane/water emulsion, dichloromethane/water emulsion and toluene/water emulsion. As for the W/O emulsion, the original functionalized textile can be directly used for the separation due to its superhydrophobicity and superoleophilicity. As shown in Figure 6f-h, the turbid emulsion became clear and transparent after separation, and the numerous micro-scale water droplets in oil were completely removed after separation from the optical microscopic image. In addition, the typical absorption intensity of green water at 620 nm disappeared in the filtrate after separation (Figure 6i), 21



revealing the separating ability of W/O emulsion. When the W/O emulsion contacted the functionalized cotton, the oil could quickly wet the surface and permeate into the functionalized cotton, while water (dyed by vat green) droplets were prevented due to the superhydrophobicity of surface. The small water droplets coalesced into larger droplets under gravity simultaneously, which were easy to be prevented on the surface of textile. As shown in Figure 6j, the purity of oil in filtrates for the water/n-hexane emulsions was above 99.9%, and the flux was up to 2068 Lm-2h-1 under driven by gravity only, which is higher than the water flux of n-hexane/water emulsions due to the high oil permeability of the superhydrophobic and superoleophilic textile. Further, the functionalized cotton could be also use to separate other W/O emulsions with high oil purity and oil flux, including water/octane emulsion, water/chloroform and water/toluene emulsion. Due to the pH-responsive special wettability of functionalized cotton, it can successfully accomplish the separations of oil/water and water/oil emulsions stabilized by surfactant.

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Figure 6. (a) A photograph of separation of n-hexane/water emulsion (n-hexane dyed by oil red O); photographs and optical micro-graphs of the feed emulsion (b) and filtrate (c); (d) the UV/vis absorption spectra of the feed emulsion and filtrate; (e) the oil rejection and fluxes of functionalized

cotton fabric for various emulsions. (f) A photograph of separation of water /n-hexane emulsion (water dyed with vat green); photographs and optical micro-graphs of the feed emulsion (g) and filtrate (h); (i) the UV/vis absorption spectra of the feed emulsion and filtrate; (j) the oil purity and fluxes of functionalized textile for various emulsions.

The Oil Absorption of Functionalized Textile and Sponge As for plenty of oily water coming from oil spills or other organic pollutants, using the adsorbent in the mixture to intelligently adsorb oil or organic pollutants may be the best strategy to solve this issue. Textile is a good candidate for fabricating adsorbing material because of its particularity including low cost, low density, mechanical stability and good flexibility. As shown in Figure 7a, a piece of functionalized cotton fabric with superhydrophobicity and superoleophilicity in air quickly adsorbed the oil only when it contacted with the oil layer of water, leaving a cleaning surface of water. Furthermore, the absorbed oil of the fabric could be easily released from the fabric with immersing it 23



into acid aqueous solution (pH 2.0), and the functionalized fabric could be used again for adsorbing oil from water after treating with alkali aqueous solution and drying. However, the practical application of the textile for the oil adsorption from oily water was restricted with the inherent poor uptake capacity of textile. In order to circumvent this issue, a pristine melamine sponge with superhydrophilicity was dip-coated with the pH-PU and UV-cured to form the functionalized sponge (Figure S2), and it became superhydrophobic and an effective adsorbing material for collecting oil from oily water (Figure S3). The oil-absorbing process of the functionalized sponge was shown in Figure 7b. It was observed that the oil in the oily water was quickly sucked out by the sponge, leaving a clean water surface without oil, and then the adsorbed oil in the sponge was able to be gathered in the other container by a manual extruding process. More importantly, this sponge could be reused for the oil absorption, and the number of absorption/squeezing recycle can achieve 30 times (Figure 7c). Moreover, this functionalized sponge was able to absorb other oils, including petroleum ether, toluene, n-decane, n-octane, and dichloromethane from water with all above 1400% weight gains (Figure 7d), indicating a good oil absorption performance of the fabric. These absorbing materials would become highly cost-effective and sustainable materials for oil spilling cleanup and has big potential in the field of oil removal.

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Figure 7. Photographs of the oil adsorbing and recycling process for the functionalized textile (a) and functionalized sponge (b). (c) The recycle number of adsorbing/squeezing oil (n-hexane) from water by the functionalized sponge. (d) Absorbency of the functionalized sponge for various oil from water, such as dimethyl silicone, olive oil, n-hexadecane, n-decane. Oils were dyed by oil red O.

CONLUSION In summary, we demonstrated a facile and effective approach to fabricate smart textiles by using UV curable fluorine-free pH-responsive polyurethane. The as-prepared textiles exhibited the characteristics of excellent superhydrophobicity in air, underwater superoleophobicity after treated by the acid water due to the protonation and 25



deprotonation of pH-PU resins in response to the pH of the aqueous solution, and can intelligently separate either oil or water from oily water through altering the pH value of solution. Most importantly, this functionalized textile could selectively separate oil from oil/water or water/oil emulsions with high oil purity and flux. Moreover, pH-PU functionalized sponges could effectively absorb the oil from oily water and were able to reuse for many times. Our work provided a fast, economical and sustainable approach to achieve smart surface materials with reversibly switchable wettability, which would have great potentials in practical applications, including the oil spill cleanup, wastewater purification, fuel purification and ion exchange.

ASSOCIATED CONTENT Supporting Information Preparation of surfactant-stabilized emulsions; oil absorption; pH-PU molecular weight with different content POSS; the photographs of a piece of sponge and sponge absorbed water in the water; the images of water droplets on the surfaces of functionalized sponge; the image of functionalized sponge floating on the water. (PDF) Video of water droplets easily rolling off from a surface in air; video of dichloromethane droplets easily rolling off from a surface in water; video of the separation of dichloromethane/water mixture; video of the separation of n-hexane/water mixture. (AVI)

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is financially supported by the National Natural Science Foundation of China (No. 51703082), the Natural Science Foundation of Jiangsu Province (BK20160186), National First-Class Discipline Program of Light Industry Technology and Engineering (LITE2018-21) and International Joint Research Laboratory for Advanced Functional Textile Materials of Jiangnan University.

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A smart textile based on UV-cured fluorine-free pH-responsive polyurethane is elaborately designed and successfully fabricated by the facile, sustainable and environmentally friendly approach. 20x10mm (600 x 600 DPI)


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UV-cured Fluoride-free Polyurethane Functionalized Textile with pH

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