Antifouling Thermoplastic Composites with Maleimide Encapsulated in

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Antifouling Thermoplastic Composites with Maleimide Encapsulated in Clay Nanotubes Ye Fu,†,‡,§ Congcong Gong,†,‡ Wencai Wang,*,†,‡ Liqun Zhang,*,†,‡,§ Evgenii Ivanov,⊥ and Yuri Lvov*,∥,⊥

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†

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, ‡State Key Laboratory of Organic−Inorganic Composites, and §Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ∥ Institute for Micromanufacturing, Louisiana Tech University, Ruston, Louisiana 71272, United States ⊥ I. Gubkin Russian State University of Oil and Gas, Moscow 119296, Russia S Supporting Information *

ABSTRACT: An antifouling ethylene-vinyl acetate copolymer (EVA) coating with halloysite clay nanotubes loaded with maleimide (TCPM) is prepared. Such antifoulant encapsulation allowed for extended release of TCPM and a long-lasting, efficient protection of the coated surface against marine microorganisms proliferation. Halloysite also induces the composite’s anisotropy due to parallel alignment of the nanotubes. The maleimide loaded halloysite incorporated into the polymer matrix allowed for 12-month release of the bacterial inhibitor preventing fouling; it is much longer than the 2−3 month protection when TCPM is directly admixed into EVA. The antifouling properties of the EVA-halloysite nanocomposites were tested by monitoring surface adhesion and proliferation of marine V. natriegens bacteria with SEM. As compared to the composite directly doped with TCPM-antifoulant, there were much less bacteria accumulated on the EVA-halloysite-TCPM coating after a 2-month exposure to seawater. Field tests at South China Sea marine station further confirmed the formulation efficiency. The doping of 28 wt % TCPM loaded halloysite drastically enhanced material antifouling property, which promises wide applications for protective marine coating. KEYWORDS: marine antifouling, thermoplastic, sustained release, nanotubes, halloysite losses.3 Therefore, it is necessary to use antifouling coating in all structures immersed in a marine environment. A major breakthrough in antifouling technology occurred when paint companies started using highly effective tributyltin added directly to paint coatings.9,10 However, the tributyltin additive was prohibited in 2008 for its toxicity and persistence in the environment.11 The current less toxic substitutes (copper, zinc, and titanium oxide) can be found in almost all marine paints, but there are still environmental concerns with

1. INTRODUCTION Biofouling is the undesirable accumulation of microorganisms, plants, and animals in water circulation systems (aquaculture facilities, ship hulls, and submarines).1−5 Biofouling causes problems for aquaculture; the farmers must clean their nets four times during the season, accounting for 50% of their operating costs.6,7 The biological fouling has a great influence on ships and submarines causing higher frictional resistance and shortened service life.4 If the vessel is not protected by antifouling coatings, its bottom may gather up to 150 kg of fouling per square meter in less than 6 months in sea, decreasing the vessel speed and increasing fuel consumption by 40%.8 Ship hull cleaning, paint removal, repainting, and associated environmental compliance result in large economic © 2017 American Chemical Society

Received: July 5, 2017 Accepted: August 16, 2017 Published: August 16, 2017 30083

DOI: 10.1021/acsami.7b09677 ACS Appl. Mater. Interfaces 2017, 9, 30083−30091

Research Article

ACS Applied Materials & Interfaces

and it is widely represented in sea and river estuaries. These microorganisms first adsorb on structures immersed in sea and form a biofilm on the surface, which provides conditions for other sea creatures adhesion. The antifouling property was further characterized with the antifouling panel test in shallow submergence at the Sanya Bay, China. The longer antifouling protection with halloysite formulation demonstrated an advantage of a clay nanotube maleimide encapsulation and offers a strategy for effective, nontoxic, and durable antifouling composite materials.

these heavy metal-containing coatings (crabs and scallops are especially sensitive to this).12,13 We commit to the researching of “green” antifouling agents free of tributyltin and hazardous substances. The most prospective is N-(2,4,6-trichlorophenyl) maleimide (TCPM), which not only prevents shellfish, soft animals, and marine plants from becoming parasitic on the ship hulls, but also can degrade in seawater into harmless components. Its antifouling action inhibits the adhesion and interferes with the neurotransmitters of microorganisms, which can drive the marine creature out without causing bioaccumulation. According to reports published by the Ocean Policy Research Foundation of Tokyo, Japan (2009, 2010), TCPM has no effect on the environment, and its predicted environmental concentration (PEC)/predicted no-effect concentration (PNEC) ratio is less than 1.14,15 The second strategy is the clay nanotube encapsulation of TCPM, which allows for an extended supply and prevents the burst release in the initial period of marine exploitation. A highly effective and “green” antifouling system should be of low toxicity, no bioaccumulation in the food chain, not persistent in the environment, favorable in price/performance, and have an extended release to provide prolonged antifouling efficiency. For this, inexpensive and naturally occurring biocompatible halloysite clay nanotubes can be used with encapsulation of TCPM antifouling agent and then doped into paint or rubber vessel coatings. Halloysite occurs as a hydrated mineral consisting of rolled aluminosilicate sheets that has the formula of Al2Si2O5(OH)4· nH2O, n = 1−2, which is similar to kaolinite except for the presence of an additional water between the walls’ adjacent layers.16−19 Halloysite is a tube with 50−60 nm external diameter, 10−15 nm diameter lumen, and length of ca. 1 mkm. In addition to the hollow structure, halloysite possesses a negatively charged outer silica surface and a positively charged inner alumina surface promoting inner loading of negative molecules.16,19 Tubule halloysite clay is a natural material, which is not hazardous for the environment, and these clay nanotubes are abundantly available in thousands of tons. Halloysite nanotubes have been developed as an entrapment system for loading, storage, and controlled release of many chemicals (catalysts, anticorrosion agents, antioxidants, flame retardants, drugs, enzymes, and DNA).20−25 Loading functional chemicals into the tubes’ lumens extended their release time for days and months (or years) if loaded halloysite is embedded into polymers or paint.16 Halloysite is hydrophilic and allows for good dispersion in polar polymers such as epoxy, polyamide, polyethylene, poly(vinyl alcohol), polyacrylates, polysaccharides (chitosan, cellulose, and pectin), and in many commercial paints containing amphiphile additives.25−28 In this study, an antifouling thermoplastic composite was prepared by mixing the ethylene-vinyl acetate copolymer [EVA, (C2H4)x(C4H6O2)y] with trichlorophenyl maleimide (TCPM) loaded inside of halloysite clay nanotubes. The release kinetics of TCPM from halloysite in water, nonpolar solvent (cyclohexane), and EVA composites were analyzed to demonstrate its sustainability. The release time of TCPM from halloysite-EVA composite was extended to one year allowing for prolonged antifoulant action. The antifouling ability of the EVA-TCPMhalloysite was evaluated by monitoring the adhesion and proliferation of V. natriegens marine bacteria on the composite coating and was studied with scanning electron microscopy. V. natriegens, a Gram-negative marine azotobacter, forms biofilms,

2. EXPERIMENTAL SECTION 2.1. Materials. Halloysite (HNT) mined in Henan Province, China, was purified and supplied by Bing Zhang, Zhengzhou University. It contained ca. 98% of tubule halloysite clay with admixture of kaolinite. Ethylene-vinyl acetate copolymer EVA 630 (VA = 15%) was from Tosoh Finechem Corp., Japan. N-(2,4,6Trichlorophenyl) maleimide (TCPM) (99%) was purchased from Tokyo Chemical Industry Co., Ltd., Japan. Cyclohexane and ethanol were purchased from Beijing Chemical Plant, China. Vibrio natriegens (V. natriegens) ATCC 33899 were supplied by the Institute of Microbiology, Chinese Academy of Sciences, China. The 2216E liquid medium and solid medium were purchased from Qingdao Hopebio Technology Co., Ltd., China. Sodium chloride (AR) was purchased from Beijing Chemical Plant, China. Glycerol (AR) was obtained from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. All of these materials were used without further purification. 2.2. Preparation of Antifoulant TCPM Loaded with Halloysite Nanotubes. Halloysite (100 g L−1) was mixed thoroughly with TCPM deionized (DI) water solution (50 g L−1) and placed in a vacuum chamber at 0.1 MPa for 30 min and then at atmospheric pressure for 15 min. This vacuum cycle was repeated three times followed by samples washing with DI water to remove any unloaded TCPM. Air bubbles were removed with the vacuum, followed with high concentrated antifoulant pressured into the tube lumens capillary pressure. According to the Laplace equation, the capillary pressure in wettable 15 nm halloysite lumen is of about 180 atm, which is sufficiently high for pulling aqueous TCPM into the empty tube lumen. Resulted samples were placed in a vacuum oven at 60 °C for fast drying. The dried antifoulant loaded halloysite samples (HNTs/ TCPM) were milled to a fine powder. 2.3. Preparation of EVA-HNTs Composites. Halloysite, TCPM, and EVA were added into the mixing chamber of a Hakke rheometer RC 90 at 90 °C at a rotor speed of 80 rpm and mixed for 12 min. The compositions of the EVA compounds are tabulated in Table 1. The compound is press-shaped into a 2 mm-thickness sheet at 120 °C.

Table 1. Composition of EVA/HNTs Formulations (1, 2, and 3 are Reference Samples and 4 Is the Working Formulation)a

a

sample code

EVA (phr)

EVA#1 EVA#2 EVA#3 EVA#4

100 100 100 100

HNTs (phr)

HNTs-TCPM (phr)

TCPM (phr)

40 2.5 40 (28.6 wt %)

phr = mass part per hundred parts of polymer matrix.

2.4. Antifouling Assay. Vibrio natriegens (V. natriegens) ATCC 33899 were provided by the Institute of Microbiology, Chinese Academy of Sciences, China, and used as a model strain to investigate the antifouling ability of EVA/HNTs composites. V. natriegens was grown overnight in 2216E medium (tryptone 5 g L−1, yeast extract 1 g L−1, and FePO4 0.01 g L−1 with seawater) at 30 °C. The EVA/HNTs samples with the size of 10 × 10 × 2 mm were hung in the bacterial suspension (OD600 nm = 1.0) and incubated at 30 °C for a specific 30084


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Figure 1. (a,b) TEM and (c) SEM images, accomplished with EDS spectrum of halloysite elemental composition.

Scheme 1. Loading of Antifoulant TCPM into Halloysite Nanotubes

time. The surface was washed with the phosphate buffer saline (PBS) to remove the unattached bacteria for further characterization. 2.5. Characterization. Thermogravimetric analysis (TGA) of antifouling loaded halloysite was performed with TGA-60A (Shimadzu, Japan) from 30 to 600 °C at a rate of 10 °C min−1 with a steady nitrogen flow (50 mL min−1). Scanning electron microscopy (SEM) was performed on a Hitachi S-4800 (Hitachi, Japan). The surface of the sample was sputtered with a thin layer of gold prior to the measurements. The SEM measurements were performed at an accelerating voltage of 5 and 20 kV. Energy dispersive spectrometry (EDS) was performed on an HORIBA X-Max20 detector (Japan) attached to the scanning electron microscope. Transmission electron microscopy (TEM) images were recorded on FEI Taicnai G2 20 STWIN (FEI, U.S.) at 200 kV. Halloysite specimens were dropped onto carbon support films on a copper grid, and the solvent was allowed to evaporate before observation. The ultrathin section specimens of EVA/HNTs for TEM were cut with a diamond knife on Cro-Ultramicrotome (Leica EM UC6, Germany).

HR-TEM was performed on a JEM-3010 electron microscope (JEOL, Japan) with an acceleration voltage of 300 kV. The chemical structure of halloysite was characterized by Fourier transform infrared spectroscopy (FTIR) using a TENSOR 27 (Bruker, Germany) instrument. The test samples were prepared by dispersing in KBr uniformly and pressing the powder into pellets. Ultraviolet visible spectrophotometry (UV−vis) was carried out with a spectrophotometer Agilent 8453 (Agilent Technology, U.S.). UV−vis was used to analyze kinetics of antifoulant release both in water and in cyclohexane. Antifoulant release experiments were conducted by stirring 50 mg of TCPM loaded halloysite in 1 mL of DI water or cyclohexane. At each reading, the supernatant was removed by centrifugation, and fresh solvent was added. The collected supernatant was analyzed by UV−vis spectroscopy around 230 nm for the amount of TCPM. The strength of EVA/HNTs composites was analyzed using a tensile apparatus (CMT4104, Shenzhen SANS Testing Machine Co., Ltd., China) at 25 °C according to Chinese Standard GB/T1040-2006. 30085


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ACS Applied Materials & Interfaces The crosshead speed was 200 mm min−1. For each sample, five measurements from different dog-bone specimens were averaged. Differential scanning calorimetry (DSC) was performed with a Mettler-Toledo DSC instrument (Mettler-Toledo, Switzerland) under nitrogen atmosphere. Samples with a weight of approximately 5 mg were loaded in an aluminum crucible under dry conditions. The samples were heated from 30 to 250 °C at a heating rate of 10 °C min−1.

alumosilicate composition.16 Such structure of halloysite allows for the tube internal loading with fouling inhibitors at 10−15 vol %. The lumens of halloysite can be loaded by its exposing to concentrated antifoulant aqueous solution. As shown in Scheme 1, air was removed from the tube lumens and replaced with TCPM by cyclic application of vacuum and atmospheric pressure. To analyze the loading efficiency of antifoulant TCPM, the weight loss of the samples prepared with different halloysite and TCPM weight ratios was measured (Figure 2). The loading efficiency of TCPM loaded halloysite prepared with different weight ratios of raw material can be calculated with eq 1:

3. RESULTS AND DISCUSSION 3.1. Antifoulant Loading into Halloysite Lumens. The morphology of halloysite observed with scanning electron

C T = (1 − WH − T/WH) × 100%

where CT is the loading efficiency, WH−T is the weight of TCPM loaded halloysite at 600 °C, and WH is the weight of halloysite at 600 °C. The loading efficiency increases with the increasing antifoulant to halloysite ratio (Table 2). Typically, a successful halloysite lumen encapsulated drug reaches 10−12 wt %, which is close to theoretical estimation for the 12 nm inner and 50 nm outer diameter halloysite with a density of 2.53 g cm−3, while the organic antifoulant is close to 1.2 g cm−3. We assume that TCPM condensation could occur in the nanoconfined conditions during the loading from saturated solution. At the large excess of TCMP in the mixture (the third column), we have too much loading probably due to undesirable inhibitor adsorption at the outer tube’s surface. The optimal formation is presented in the first column when the halloysite/inhibitor ratio was 2:1. To confirm the mechanism of the antifoulant loading process, the FT-IR spectra of TCPM, pristine halloysite, and TCPM loaded halloysite were compared (Figure 3). In the FTIR spectrum, halloysite featured two distinct peaks at 3704 and 3625 cm−1, which are attributed to the stretching vibrations of inner and surface −OH groups. The band at 1094 cm−1 is assigned to the stretching vibration of apical Si−O, while the characteristic band of in-plane Si−O−Si stretching vibration is observed around 1038 cm−1. The peak of 1000 cm−1 is assigned to symmetric or perpendicular stretching vibrations of Si−O or Al−O groups. The vibrations ascribed to the Al−O−Si and Si− O−Si, respectively, at 537 and 467 cm−1 can further confirm the existence of corresponding group.17,18 In the spectrum of antifoulant TCPM, the absorption bands at 3104 and 3075 cm−1 are ascribed to the asymmetric and symmetric stretching vibrations of C−H, while the band at 1723 cm−1 is arise from the CO stretching. The obvious absorption peaks that emerged at 1475 cm−1 are in accordance with the in-plane stretching of CC. Characteristic band of inplane C−N stretching vibration was observed around 1144 cm−1, while the band at 686 cm−1 is assigned to the stretching of C−Cl. When the halloysite was loaded with TCPM, the vibrations of TCPM at 1723, 1475, and 686 cm−1 appeared, indicating loading of the inhibitor, while the band at 1038 cm−1 of the halloysite Si−O−Si stretching vibration was preserved. There are no new peaks, which indicates that the loading of TCPM is a physical process, and there are no chemical reactions between antifoulant and halloysite. 3.2. Release Kinetic of the Antifoulant. Ultraviolet− visible spectrophotometry was used to analyze kinetics of antifoulant release from halloysite both in water and in cyclohexane. In water, about 50 wt % of TCPM releases from

Figure 2. TGA curves of the samples prepared with different weight ratios of maleimide/halloysite (TCPM:HNT). The TCPM concentration was 5 wt %.

Table 2. Loading Efficiency of Antifoulant-TCPM Loaded Halloysite weight ratio (HNTs:TCPM) loading efficiency (wt %) ± 0.2

2:1 10.4

1:1 19.5

(1)

1:2 35.6

Figure 3. FT-IR spectra of antifoulant (TCPM), halloysite (HNTs), and TCPM loaded halloysite (HNTs-TCPM).

microscopy (SEM) and transmission electron microscopy (TEM) shows its tubular structure with the external diameter of 40−60 nm, the empty lumen of 15−20 nm, and length 800 ± 200 nm (Figure 1). The presence of silicon, aluminum, and oxygen atoms in halloysite was revealed by energy dispersive spectrometry (EDS), which coincides with the theoretical 30086


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ACS Applied Materials & Interfaces Scheme 2. Release of Antifoulant TCPM from Halloysite in the Matrix of EVA

Figure 4. Antifoulant TCPM release from (a) halloysite in DI water and cyclohexane; and (b) EVA composite doped with antifoulant TCPM loaded halloysite in DI water as compared to the control sample EVA #3, containing no halloysite (upper curve).

halloysite lumens within 24 h, while more than 90 wt % releases in cyclohexane at the same time. The release in cyclohexane is much faster, which is attributed to its higher solubility in cyclohexane. This release from the halloysite dispersed in both bulk liquids is rather fast, but it was drastically slowed when the nanotubes were included in the EVA matrix. It is due to clogging the tube’s ends, which decreases pore-controlled leakage of TCPM. In the actual antifouling composite formulation, halloysite will be inside the matrix, and thus released very slowly (shown in Scheme 2). The release kinetic of antifoulant TCPM from EVA was studied with two EVA samples: one is EVA admixed with 2.4 wt % antifoulant (EVA/TCPM), and the other is EVA doped with halloysite encapsulating equal amount of TCPM (EVA/HNTsTCPM), which allowed for maleimide releasing into the polymer matrix and its further migration to the composite surface. The release amount was calculated with the standard curve of absorbance versus concentration (Figure S1). The release rate of the control sample having direct admixture of the inhibitor was almost 5 times faster than that for halloysite formulation, EVA/HNTs-TCPM (Figure 4b). Analytical extrapolation allows estimating that 50% release in the EVA/HNTs-TCPM composite will take ca. 12 months, while no-encapsulation EVA/TCPM formulation (direct maleimide admixing) allowed for only 3 months’ supply. The clogging of halloysite nanotubes with polymeric EVA dramatically decreased the release rate of antifoulant, thus resulting in more than a year-lasting protection potential. 3.3. Morphology and Properties of the EVA/HNTs Nanocomposites. The dispersion of the clay nanotubes in the polymer matrix has a decisive impact on the final properties of

the composite. We obtained the composites with uniformly distributed halloysite as shown in Figure 5. The addition of antifoulant TCPM has no adverse influence on the dispersion of halloysite tubes. The TEM images of the composite doped with pristine halloysite EVA#2 (Figure 5e) and TCPM loaded halloysite EVA#4 (Figure 5f) show uniform dispersion of halloysite without aggregation. Such an excellent dispersion of single clay particles is obtained without any additional exfoliation processing of this tubular clay. One can estimate from the images that the amount of the tubes in the polymer matrix corresponds to the formulation of 28.6 wt % = 12.4 vol % added halloysite. The orientation of halloysite nanotubes in EVA matrix observed in TEM images reveals the anisotropic properties of EVA/HNTs nanocomposites. We assume that the tube orientation was induced by share-force during the composite film formation. The alignment of halloysite in the melted EVA during squeezing for the pate formation produces the tubes orientation. A similar halloysite orientation assisted by the Margoni flow was observed at the edges of drying halloysite droplets.29,30 This nanotube’s orientation phenomenon opens possibilities for formation of the plates with anisotropic strength and better coating smoothness. The orientation of halloysite nanotubes in EVA matrix resulted in the anisotropic tensile property of EVA/HNTs composite sheets (Figure 6a). The tensile strength and elongation at break along the halloysite alignment direction are much higher than those in the perpendicular direction. The elastic modulus along the alignment direction is 0.79 ± 0.02 MPa, and it is 1.5 times larger than the perpendicular value of 0.54 ± 0.02 MPa. The higher yield strength and more 30087


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Figure 5. SEM images of EVA/HNTs composites with different formulas: (a) pure EVA#1, (b) EVA#2 composited with pristine halloysite, (c) EVA#3 doped with antifoulant TCPM, and (d) EVA#4 composited with TCPM loaded halloysite. TEM images of (e) EVA#2 and (f) EVA#4. Halloysite consists of 12.4 vol % of the composite.

Figure 6. (a) The tensile profile of EVA with the inclusion of halloysite nanotubes in two perpendicular directions, and (b) DSC curves of EVA composite: pure EVA, EVA composited with pristine halloysite (EVA/HNTs), EVA doped with antifoulant TCPM (EVA/TCPM), and EVA composited with TCPM loaded halloysite (EVA/HNTs-TCPM).

The DSC curves of the nanocomposites are shown in Figure 6b. The inclusion of halloysite and antifoulant TCPM makes the full width at half-maximum of fusion peaks get wider. The crystallinity can be calculated with the equation:

significant hardening shown in the stress−strain curve along the alignment direction of halloysite may be attributed to the macromolecules’ orientation along the nanotubes. Another strengthening mechanism may be related to the clay nanotubes bridging the polymer breaks/gaps appearing during a sample elongation. These two mechanisms are also discussed in refs 31 and 32.

wc = 30088

ΔHm ΔHm0

×

1 wE

(2) DOI: 10.1021/acsami.7b09677 ACS Appl. Mater. Interfaces 2017, 9, 30083−30091

Research Article


EVA. Halloysite nanotubes may act as crystallization nucleation sites inducing the polymer crystallization along their orientation direction. Orientation of clay nanotubes in polymeric matrix improves the composite film surface strength and smoothness as it was demonstrated for sand-resistant halloysite-epoxy solar battery coatings.33 Such nanotubes’ alignment along the water flow direction in marine coating may be also helpful. In FT-IR spectra of the EVA composites (Figure 7), the characteristic peaks of EVA appear at 2920 cm−1 attributed to the −CH2− stretching vibration, 1740 cm−1 ascribed to the CO stretching vibration, and 1240 cm−1 assigned to the C− O stretching vibration. The appearance of characteristic peaks of HNTs in EVA-HNTs, EVA-HNTs/TCPM proves an incorporation of halloysite into the EVA. The emergence of TCPM characteristic peaks in the FT-IR spectra of EVA/ TCPM and EVA/HNTs-TCPM confirms that TCPM maintains the original chemical structure without any decomposition during the fabrication process. 3.4. Antifouling Test of EVA/Halloysite Nanocomposites. To analyze the antifouling property of EVA/HNTs, the cultured marine V. natriegens on the EVA/HNTs composite surfaces were fixed with ethanol followed by SEM monitoring. The images of EVA/HNTs were incubated in the bacterial suspension (OD600 nm = 1.0) at 30 °C for 3 days before and after shaking at 150 r min−1 in 100 mL of artificial seawater (replaced every 5 days) for 60 days. Few bacteria can be found on the surface of EVA (pure or loaded with antifoulant), Figure 8a,e. On the surface of EVA with empty halloysite were bacteria

Figure 7. FT-IR spectra of halloysite, antifoulant TCPM, pure EVA, EVA composited with halloysite, EVA doped with TCPM, and EVA composited with TCPM loaded halloysite.

in which wc is the crystallinity, ΔHm is the fusion enthalpy of EVA/HNTs nanocomposites, ΔH0m is the fusion enthalpy of 100% crystalline polyethylene, H0m = 293 mJ mg−1, and wE is the EVA content in the nanocomposites. The crystallinity of pure EVA is 24.7 ± 0.4 wt %, while the inclusion of halloysite improves it to 31.1 ± 0.4 wt %, which indicates that clay nanotubes have a significant influence on the crystallization of

Figure 8. SEM images of the bacteria on the surface (first column EVA and the second column EVA-halloysite formulations): (a) pure EVA, (c,e) EVA directly doped with TCPM, and (b) EVA composited with halloysite, (d,f) EVA with TCPM loaded halloysite. The samples were incubated in the bacterial suspension for 3 days (c,d) directly and (e,f) after shaking in seawater for 60 days. 30089



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attached first, and V. natriegens even formed a biofilm on the surface (Figure 8b). The bacteria and halloysite can be well differentiated with their sizes: the clay tube diameter is 50 nm and V. natriegens is more than 700 nm; therefore, these large ellipsoidal particles on the surface are bacteria. The halloysite inclusion increases the surface roughness, which leads to better proliferation of bacteria.34 After exposing/shaking in artificial seawater for 60 days, the amount of bacteria attached on the surface of EVA directly doped with TCPM increases significantly, while there were no bacteria on the surface of EVA composites with TCPM encapsulated in halloysite (Figure 8e,f). The amount of V. natriegens adsorbed on the plate of EVA doped with TCPM after exposing in artificial seawater for 60 days was estimated as ca. 1.7 × 108 (with 20 SEM images analyzed), while only 20−50 bacteria were detected on the EVA with TCPM-halloysite surface. The bacteria adsorbed on the sample surface were observed with SEM, and the amount of these bacteria was calculated. The total amount of the EVA loaded TCPM was the same (3.0 ± 0.1 wt %) in the both formulations. This result clearly demonstrates excellent antibacterial protection with the halloysite formulation. The sustained release of antifoulant from halloysite nanotubes slowed the TCPM diffusion into the external environment and prolonged its action time, significantly improving the antifouling property of the composite. This result was further confirmed by the marine antifouling panels tests (blank plate and coated with halloysite formulation) exposed in shallow sea submergence. After soaking in the Sanya Bay, South China Sea, for 3 months, 35 × 25 cm2 flat panel coated with halloysite encapsulated TCPM remained clean, while on the control uncoated panel many marine microorganisms colonies (spots) were observed (Figure S2). Longer marine test results with 6 months exposure became available recently and also confirmed an excellent antifouling protection.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b09677. Figure of the standard curve of absorbance versus concentration for TPCM water solution, and the optical images of test plates after the shallow submergence in the South China Sea for 3 months (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wencai Wang: 0000-0001-6204-9684 Liqun Zhang: 0000-0002-8106-4721 Yuri Lvov: 0000-0003-0722-5643 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant numbers 51673013, 51373010, 51221002, and 51320105012). Y.L. and E.I. thank the Ministry of Education and Science of the Russian Federation (grant 14.Z50.31.0035) for support of halloysite formulation technique elaboration.

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REFERENCES

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4. CONCLUSIONS A long-acting antifouling ethylene-vinyl acetate (EVA) copolymer composite was prepared with the N-(2,4,6trichlorophenyl) maleimide (TCPM) using halloysite clay as tubule nanocontainers to encapsulate TCPM and to extend its release time. Encapsulating the antifoulant agent in the clay nanotubes at a concentration of 3 wt % allowed for its extended release over 12 months, resulting in much longer protection as compared to TCPM directly admixed into this EVA. The adhesion of marine bacteria V. natriegens onto the nanocomposite surface displayed a good antifouling property, thus preventing formation of the primary biofilm layer. As compared to the EVA directly doped with TCPM, very few bacteria were attached on the composite with halloysite encapsulated TCPM after exposure to seawater for 60 days. The inclusion of halloysite provided the polymer matrix with anisotropic properties reinforced parallel to the share-strength direction and allowing for further surface smoothing. Such clay nanotube-antifoulant-EVA formulations endow antimicrobial protection and can be applied as coating materials with good marine fouling prevention. Halloysite clay nanotubes are a natural abundantly available material, which allows for scaling up of our technology. 30090


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