pH-Induced Switchable Superwettability of Efficient ... - ACS Publications

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pH-Induced Switchable Superwettability of Efficient Antibacterial Fabrics for Durable Selective Oil/Water Separation Yuchen Fu, Biyu Jin, Qinghua Zhang, Xiaoli Zhan, and Fengqiu Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09159 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 15, 2017

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ACS Applied Materials & Interfaces

pH-Induced Switchable Superwettability of Efficient Antibacterial Fabrics for Durable Selective Oil/Water Separation Yuchen Fu,† Biyu Jin,† Qinghua Zhang,*† Xiaoli Zhan, † and Fengqiu Chen† †Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, P. R. China

ABSTRACT: The superhydrophobic antibacterial fabrics with intelligent switchable wettability were fabricated by the crosslink reaction among pH-responsive antibacterial copolymer tethered hydroxyl groups, methylol-contained poly(ureaformaldehyde) nanoparticles (PUF NPs) and hexamethylene diisocyanate. It was found that the surface concentration of N+ were heavily influenced by acid solutions, resulting in the rapid wettability conversion from superhydrophobicity/superoleophilicity to superhydrophilicity/underwater superoleophobicity in remarkably short time. Above responsiveness feature of coated cotton fabric contribute prominent selective oil/water separation property, and the separation efficiency invariably remained more than 98.2% even after 8 reuse cycles, which exhibited brilliant durability. More importantly, the coated cotton fabric possessed excellent self-cleaning performance after contamination by oil and held high bactericidal rate (more than 80%) regardless of pH treatment thus could abate the surface biological pollution caused by bacteria proliferation. The attractive properties of the prepared smart superwetting materials shows great promise for potential application in oil/water separation from environmental-protection perspective.

KEYWORDS: pH-responsibility, superwetting, antibacterial, oil/water separation, durability

1. INTRODUCTION Oil/water separation has always been a critical and urgent issue because of the frequent discharge of the oily wastewater from factories and marine oil spill accidents.1-3 Due to the serious environmental pollution and energy waste caused by oily wastewater and oil leakage,4,5 the oil/water separation materials owning special wettability have been widely studied to solve this problem in recent years.6-10 Although there are many methods such as gravity, centrifugation, ultrasonic, adsorption, and filtration utilized to separate oil/water mixtures, lower efficiency and stability have become the biggest drawbacks of these conventional oil/water separation methods compared with superwetting materials.11-13 On the basis of wettability differences, the separation materials can be divided into three types in general: “oil-removing” type materials14,15 owing superhydrophobicity/ superoleophilicity, “water-removing” type materials16,17 owing superhydrophilicity/under-water super-oleophobicity, and smart controlled separation materials.18,19 As the development of technology and the higher requirements of people on materials, the smart

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separation materials with switchable wetting behaviors have received much attention because of the simple process, high separation efficiency, and low energy consumption.20,21 When the environment stimuli such as temperature,22,23 light,24,25 pH,26,27,28 voltage,29 solvent,30 magnetism,31 etc, changed, the smart materials are able to achieve the switch of wettability. As for a pH-responsive material, plenty of compounds can be used to realize

its

responsibility,

like

(dimethylamino)

ethyl

methacrylate

(DMAEMA),

HS(CH2)11NH2,

HS(CH2)10COOH, and any other containing groups of amine or carboxyl.32-36 Poly(DMAEMA) and its derivatives interest researchers since they can response to the changes of pH due to the protonation/deprotonation of the tertiary amine groups,37 which have wide applications in various fields, such as biomedical areas38 and oil/water separation.39 For instance, Liu et al40 developed pH-responsive coatings by a

solution-processable

dip

coating

poly(dodeyl

methacrylate-co-3-trimethoxysilylpropyl

methacrylate-co-2-dimethylaminoethyl methacry-late) (PDMA-co-PTMSPMA-co-PDMAEMA) on different substrates, which displayed superhydrophobic and superhydrophilic transition via ex situ and in situ pH control. Wang et al41 fabricated a self-cleaning cotton fabric with smart-control and reusable functions for oil/water separation by grafting PDMAEMA via a surface-initiated atom transfer radical polymerization (ATRP) technique. However, when oils permeates through the separation material, the material is prone to be polluted by these oils especially high viscosity oils.42 More importantly, there exist incalculable bacteria and other contaminants in reality, so the biofilm is easily formed on the surface of the separation material, leading to spillage of materials and low separation efficiency. Nevertheless, few efforts have been put into the work focusing on oil/water separation materials with antibacterial activities, to our best knowledge. Actually, we have successfully prepared superhydrophobic materials with antibacterial performances using poly(ureaformaldehyde) nanoparticles (PUF NPs) in our previous work,43 while they were not able to response to environment changes. Herein, the quaternary ammonium salts (QAS)-functionalized fluorinated copolymer containing PDMAEMA segments was synthesized via free radical polymerization. Then cotton fabric was chosen as the substrate on account of its ecofriendly, low price, durability and permeability, and was coated with crosslinked networks obtained from the reaction of the as-prepared QAS-functionalized fluorinated copolymer with hydroxyl groups, reactive PUF NPs and hexamethylene diisocyanate (HDI) by a simple spray method. The chemical structure and surface topography of the fabricated pH-responsive superhydrophobic cotton fabric were measured by Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The wettability of pH-treated coated cotton fabrics was assessed by water contact angle (WCA) measurements, and chemical compositions of the coated fabric before and after pH treatment were studied by X-ray photoelectron spectroscopy (XPS). Moreover, the antibacterial, self-cleaning, and pH-responsive oil/water separation performances of the coated cotton fabric were systematically investigated by a train of experiments.


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2. MATERIALS AND METHODS 2.1 Materials 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 8-Tridecafluorooctyl acrylate (TFOA), hexamethylene diisocyanate (HDI), 2-hydroxyethyl methacrylate (HEMA), and dibutyltin dilaurate (DBTDL) were purchased from Sigma-Aldrich. 2-(dimethylamino)ethyl methacrylate (DMAEMA) was purchased from Aladdin. The QAS monomer methacryloyloxyethyldimethyldodecyl ammonium bromide (QDEMA) and poly(urea-formaldehyde) (PUF)

nanoparticles

(denoted

as

PUF

NPs)

were

obtained

as

our

recent

work43

did.

2,

2’-Azobisisobutyronitrile (AIBN) was purchased from Aldrich and recrystallized from ethanol for further use. All the monomers, except QDEMA, were filtered to remove the inhibitor through a basic alumina column before use. Butyl acetate and acetonitrile were obtained from Sinopharm Chemical Reagent and used without further purification. Escherichia coli (E. coli) was obtained from the Microorganism Institute of Zhejiang University, and the culture medium was commercial.

2.2 Synthesis of pH-responsive copolymer RC The pH-responsive copolymer was synthesized by solvent polymerization. More specifically, TFOA (2.5 g), QDEMA (2.0 g), DMAEMA (5.0 g), and HEMA (0.5 g) were dissolved in mixed solvent of butyl acetate (10 g) and acetonitrile (10 g) and then poured into a three-necked round-bottom flask. The reactor was heated to 75 °C and purified with nitrogen gas for 15 min to remove the oxygen. Then the initiator AIBN (0.1 g) was added with agitation. The crude copolymer was obtained after 6 h of polymerization, sequently precipitated in n-hexane several times, and finally dried in vacuum oven at 50 °C. The synthesized pH-responsive copolymer was designated as RC.

2.3 Fabrication of pH-responsive superhydrophobic PUF-RC-coated cotton fabric The process of fabricating the pH-responsive superhydrophobic PUF-RC-coated fabric was illustrated in Scheme 1. The RC copolymer was first dissolved in the mixed solvent of butyl acetate and acetonitrile, with subsequent addition of HDI and PUF NPs. After vigorous agitation, the mixture was ultrasonicated for 10 min, and then sprayed onto both sides of cotton fabrics. The PUF-RC-coated cotton fabric was allowed to cure at room temperature for 2 h, and additional 12 h drying in an oven at 100 °C. The weight ratio of reactants RC copolymer, PUF NPs, and HDI was 7/3/0.28. Meanwhile, pure copolymer RC-coated cotton fabric was also



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prepared in a same way.

2.4 Self-cleaning and Antibacterial Tests The self-cleaning property of the oil-contaminated PUF-RC-coated fabric was examined by using water droplets to remove the copper (II) chloride dihydrate (CuCl2·2H2O) particles sprinkled on the fabric surface. The antibacterial performance of the coated cotton fabric was investigated against E. coli with the plate count method, as reported in another work.43

2.5 Selective oil/water Separation Experiments The as-prepared fabrics (PUF-RC-coated fabric and the coated fabrics treated by pH=1 and pH=13 solutions, respectively) were separately fixed between two glass vessels. And the oil/water mixtures (50% v/v) were poured onto the as-prepared fabrics. The selective oil/water absorption was tested by using PUF-RC-coated fabric (pH=1, 3 cm × 3 cm) to absorb 2~3 water droplets dyed by methylene blue (MB) added into 30 mL dichloromethane, and PUF-RC-coated fabric (pH=13, 3 cm × 3 cm) to absorb 2~3 toluene droplets dyed by Sudan I added into 30 mL water, respectively.

2.6 Characterization 1

H-NMR (Bruker Advance DMX500, 25 °C) and FT-IR (Nicolet 5700) characterizations were used to

analyze the chemical structure of copolymer RC and PUF-RC nanocomposite. The surface morphologies of the prepared cotton fabrics were investigated by SEM analysis (SIRISON, FEI Co., Ltd) with an accelerating voltage of 20 kV. XPS (Thermo Scientific, USA) with an Al Kα X-ray source was applied to study the surface composition of prepared cotton samples. The X-ray gun was operated at 350 mW and 14 kV, the pressure of analyzer chamber was 10-9~10-10 Pa. The surface wettability and pH-sensitivity of the PUF-RC-coated cotton fabric were measured by a CAM 200 optical contact-angle goniometer (KSV Co., Ltd, Helsinki, Finland) at ambient temperature. The coated fabric was separately immersed in varied pH aqueous solutions (pH=1, 3, 5, 7, 9, 11, 13) for 30 min, and then dried in an oven at 80 °C. The volume of the tested liquid droplet is 5 µL.


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Scheme 1. Synthesis procedure of pH-responsive PUF-RC-coated cotton fabric and its applications in oil/water separation under different pH treatments.

3. RESULTS AND DISCUSSION 3.1 Synthesis and characterization of pH-responsive copolymer RC and PUF-RC nanocomposite The pH-responsive copolymer RC was synthesized by free radical solution polymerization using AIBN as initiator in the mixed solvents of butyl acetate and acetonitrile. The chemical structure of copolymer RC was identified by 1HNMR spectra. As shown in Figure 1a, signals at 2.55 ppm and 2.27 ppm were assigned to – CH2 and –CH3 which were conjoint with N from PDMAEMA, respectively. The peak appeared at 2.8 ppm was ascribed to –CF2CH2– of PTFOA. The chemical shift at 4.05 ppm could be assigned to –CH2 separately linked with C=O in PDMAEMA and –OH in PHEMA. Signals at –CH2 and –CH3 conjoint with N+ moved to higher shifts due to the influence of quaternization, which were observed at 3.8 ppm and 3.45 ppm, respectively. The 1

HNMR results indicated that pH-responsive copolymer RC was successfully synthesized via simple free

radical polymerization. To further confirm the structure of products, FT-IR spectra of polymer nanoparticles PUF, pH-responsive copolymer RC, and nanocomposite PUF-RC were recorded after each step of preparation procedure as Figure 1b showed. The peaks at 1629 cm-1 and 1540 cm-1 were assigned to the –CONH– stretching vibration from PUF NPs. The same peaks were observed in the FT-IR spectrum of PUF-RC. The broad band at 3250~3500



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cm-1 appeared in all spectra was ascribed to the N-H and –OH groups, indicating the successful introduction of PUF NPs into RC copolymer. In the spectrum of RC, the peaks at 2950 cm-1 and 1727 cm-1 were corresponded to –CH3 vibration and C=O stretching vibration, respectively. The C–N+ stretching vibration from PQDEMA segments appeared at 1490 cm-1. And C–F vibration bands at 1250 cm-1 and 1140 cm-1 were separately assigned to –CF3 and –CF2 groups from PTFOA segments. All of these characteristic absorption peaks could be found at the same wavenumbers in the PUF-RC spectrum. Noticeably, the characteristic absorptions of – NCO groups (2272 cm-1) were not seen for copolymer RC and nanocomposite PUF-RC, showing that the pH-responsive PUF-RC nanocomposite was fabricated by complete crosslink reaction among RC, PUF NPs, and HDMI.

Figure 1. Chemical structure and 1HNMR spectra of pH-responsive copolymer RC (a), FT-IR spectra of PUF NPs, pH-responsive copolymer RC, and pH-responsive nanocomposite PUF-RC (b).

3.2 Surface morphology In order to study the influence of the introduction of pH-sensitive polymer RC and PUF NPs on the surface morphology of cotton fabrics, SEM analysis were applied to investigate the surface morphologies of the pristine cotton fabric, RC-coated cotton fabric, and PUF-RC-coated cotton fabric, respectively. As shown in Figure 2, the original cotton fabric (Figure 2a) was consisted of numerous microscale fibers with diameters ranging from 15 to 20 µm, contributing a macroscopically rough surface. It was obvious that the surface of each fiber was smooth at high magnification (inserted in Figure 2a). After sprayed by copolymer solutions, the fiber clusters of the cotton fabric were covered by a layer of film, and the film had a fairly smooth surface at high magnification due to the favorable film-forming property of RC (Figure 2b and inset), which indicated that RC was well crosslinked with HDMI on fibers of cotton fabrics.


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However, the surface merely possessed microscale roughness, since there was no nano-scale structure observed. Figure 2c was the surface morphology of the PUF-RC-coated cotton fabric. The fabric surface became very rough from an overall perspective, and each cotton fiber was covered by a layer of rugged coating, on which plenty of nanoscale protrusions was found after further magnification (Image with red border in Figure 2c). This phenomenon demonstrated that a micro-nano dual structure was formed on the surface of cotton fabrics and surface roughness was greatly improved by the addition of PUF NPs into the copolymer. Besides, the uniform hierarchical structure that covered on each fiber surface suggested the applicability of spraying method. In addition, the surface morphologies of cotton fabrics treated by different pH solutions were also analyzed. Figure 2d-f displayed the SEM images of PUF-RC-coated cotton fabrics after treatment by pH=1, pH=7, and pH=13 solutions, respectively. Compared with the coated fabric without pH treatment, the surfaces morphology of pH-treated coated cotton fabrics did not change much. According to magnified figures inserted in Figure 2d-f, the nanoscale protrusions covering on the cotton fibers withstood strong acid or alkali, thus the surfaces still owned hierarchical structures. It turned out that the PUF-RC-coated cotton fabric surface with dual structure could not be changed by different pH solutions, which laid the foundation of realizing the superwetting behaviors of PUF-RC-coated fabrics.

Figure 2. SEM images with 500 magnification of pristine cotton (a), RC-coated cotton (b), PUF-RC-coated cotton (c), PUF-RC-coated cotton treated by pH=1 (d), PUF-RC-coated cotton treated by pH=7 (e), PUF-RC-coated cotton treated by pH=13 (f), respectively. Rectangular region with red border was corresponding images under 1000 magnification.



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3.3 XPS analysis of the surface chemical composition The chemical composition of cotton fabrics at different states was measured by XPS to further confirm the successful copolymer and nanocomposite modification of the cotton fabrics. As shown in Figure 3, the cotton fabric is mainly composed of C, O elements, which appeared at 284.8 eV and 545.2 eV on the XPS spectrum of the original cotton fabric. Compared to the original fabric, two new signals at 399.3 eV and 685.5 eV were emerged on the surface of RC-coated and PUF-RC-coated cotton fabrics, which were assigned to N 1s and F 1s. Meanwhile, the intensity of F 1s signal was much greater than that of other elements’ signals, indicating the enrichment effect of fluorinated segments on the fabric surface. The comparison of elements appeared on the surfaces of these three cotton fabrics confirmed that RC copolymer and PUF-RC crosslinked networks were successfully coated on the fabric substrate, which agreed well with the result of FT-IR.

Figure 3. Survey XPS spectra of pristine, RC-coated, and PUF-RC-coated cotton fabrics. As N element incorporated in RC and PUF-RC can be divided into N+ (401.6 eV) and N (399 eV) two types, hence, the high resolution XPS spectra of the N 1s was applied to quantitatively measure the N+ content on pH-responsive fabric surface and also reflect the degree of protonation of modified fabric surface after treatment by different pH solutions. Figure 4 presents the N 1s core-level spectra of original and three different pH solutions treated PUF-RC-coated fabrics. The appearance of peak at 401.6 eV (C-N+) on the spectra of four samples verified the existence of N+. The contents of main elements on the fabric surfaces were also calculated and listed in Table S1. As shown in Table S1, the surface concentration of C, O, N and F slightly changed after pH treatment. However, the N+ content varied with different pH solutions obviously when N 1s was highly resolved. The N+ concentration on the surface of PUF-RC-coated fabric almost had no


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change after treatment by pH=7 or pH=13 solutions, while, N+ content increased from 0.58% to 1.46% after being treated by pH=1 solution. These results could be explained that tertiary amine from PDMAEMA segments can react with H+ and produce protonated N+ (HN+), leading to substantial increase of N+ percentage. The increased concentration of hydrophilic protonated N+ will have a great influence on the wettability of the fabric surface which will be discussed minutely later.

Figure 4. N 1s core-level spectra of (a) PUF-RC-coated cotton fabric, (b) PUF-RC-coated cotton fabric treated by pH=1, (c) PUF-RC-coated cotton fabric treated by pH=7, and (d) PUF-RC-coated cotton fabric treated by pH=13, respectively.

3.4 Wettability measurements and pH-responsibility In order to study the dynamic transition of wetting behaviors on the surface of PUF-RC-modified cotton fabric, water contact angles (WCAs) of the surface treated by different pH solutions were measured by CAM 200 optical contact-angle goniometer. As illustrated in Figure S1, the coated fabric surfaces remained superhydrophobicity with WCAs over 150° even undergoing treatment by pH≥7 solutions. Once the coated



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fabric was treated by solutions with pH

pH-Induced Switchable Superwettability of Efficient ... - ACS Publications

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