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Self-Healing Underwater Superoleophobic and Anti-Biofouling Coatings Based on the Assembly of Hierarchical Microgel Spheres Kunlin Chen, Shuxue Zhou, and Limin Wu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b06816 • Publication Date (Web): 21 Dec 2015 Downloaded from http://pubs.acs.org on December 21, 2015
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Self-Healing Underwater Superoleophobic and Anti-Biofouling Coatings Based on the Assembly of Hierarchical Microgel Spheres Kunlin Chen, Shuxue Zhou*, Limin Wu* Department of Materials Science and State Key Laboratory of Molecular Engineering of Polymers, Advanced Coatings Research Center of Ministry of Education of China Fudan University, Shanghai 200433, China
*Corresponding author, email:
[email protected]
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ABSTRACT: Marine biofouling has been plaguing people for thousands of years. While
various
strategies
have
been
developed
to
antifouling
(including
superoleophobic) coatings, none of these exhibits self-healing property because the bestowal of a zoetic self-repairing function to lifeless artificial water/solid interfacial materials is usually confronted with tremendous challenges. Here we present a self-repairing underwater superoleophobic and anti-biofouling coating through the self-assembly of hydrophilic polymeric chain modified hierarchical microgel spheres. The obtained surface material not only has excellent underwater superoleophobicity but also owns very good subaqueous anti-biofouling property. More importantly, this surface material can recover the oil- and biofouling- resistant properties once its surface is mechanically damaged, similar to the skins of some marine organisms such as sharks or whales. This approach is feasible and easily mass-produced, and could open a pathway and possibility for the fabrication of other self-healing functional water/solid interfacial materials. KEYWORDS: microgel spheres, underwater superoleophobic, anti-biofouling, self-healing, coatings, surface materials
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Marine biofouling on shipping and leisure vessels, heat exchangers, oceanographic sensors and aquaculture systems, has been plaguing people for thousands of years. 1-11 And the increasing oil leakage accidents in ocean in the past years make the antifouling of marine vessels more complex.12-25 Although hydrophilic polymers, e.g., poly (ethylene glycol), zwitterionic biomaterials and polysaccharide, have demonstrated very good biofouling-resistant properties against some foulants in ocean, such as proteins and bacteria,26-28 but pure hydrophilic polymer-based coatings inevitably weaken some properties of coatings, e.g., anti-corrosion, mechnical property, and their surface superoleophobicity is not enough to prevent the adhesion of oily pollutants in seawater. 29-33 Recently, combination of the hierarchical micro/nano structured surfaces with the high surface energy is considered as one of the most efficient strategies to achieve subaqueous assemblies,
superoleophobic 33,34,39,40
surfaces.33-35
Hydrogels,29,36-38
polyelectrolyte
calcium alginate41 and photocatalytic nanoparticles42-44 have been
successfully used to fabricate underwater superoleophobic surfaces. For instance, Xu et al. fabricated a polyelectrolyte/clay hybrid coating with robust and stable underwater superoleophobicity via layer-by-layer (LbL) assembly.32 Because of the nacre-like microstructure, ion-induced roughness increment and high surface energy of
employed
building
blocks,
the
obtained
(chitosan-montmorillonite
clay/poly(diallyl-dimethylammonium chloride))100-salt film not only displays fine mechanical properties, but also possesses stable superoleophobicity under seawater. Manna et al. reported a thin polymer coating with robust underwater superoleophobicity based on the covalent LbL assembly of polymer multilayers using branched
poly(ethyleneimine)
and
the
amine-reactive
polymer
poly(vinyl-4,4-dimethylazlactone).34 The as-obtained coatings not only exhibit 3 ACS Paragon Plus Environment
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functional tolerance to severe physical abrasion but also can retain their oil-resistant property upon a broad range of physical, chemical, and environmental challenges, owing to their porous structures and the chemical stability of their amide/amide based crosslinks. Nonetheless, fabrication of these surface materials is perceptibly time-consuming, and little data of these subaqueous superoleophobic materials reported above involved their biofouling-resistant property against halobios such as proteins, bacteria, diatoms, spores of macroalgae, etc. More importantly, none of these surfaces has self-healing ability and will certainly sacrifice their superoleophobic or possible anti-biofouling properties once the surfaces of these materials are mechanically damaged on the land or in underwater environment. Indeed, designing lifeless artificial interfacial materials with similar superoleophobic and anti-biofouling properties, and self-healing ability to the surfaces of alive marine organisms (e.g., sharks, whales), has always been the biggest dream of scientists and engineers in this field, and certainly the most challenging, especially in cruel and various marine environments. In this study, we have successfully fabricated for the first time a self-healing underwater oil-repellent and biofouling-resistant coating. In contrast to the previously reported underwater superoleophobic or antifouling surface materials, this surface material we demonstrate here not only has prominent underwater superoleophobicity but also owns exceptional subaqueous anti-biofouling property simultaneously due to its three-dimensionally (3D) ordered structure via the self-assembly of hierarchical hybrid microgel spheres and the hydrophilic copolymeric chains grafted on the microgel spheres. Of particular interest is the fact that this new class of surface material can recover its original oil-repellent and biofouling-resistant properties in water once mechanically damaged on its surface. Accordingly, we would believe this 4 ACS Paragon Plus Environment
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study opens a new pathway and possibility for the fabrication of self-repairing superoleophobic and antifouling coatings even other responsive surface materials.
RESULTS AND DISCUSSION As shown in Figure 1a, microgel spheres (MS) were synthesized via precipitation polymerization of N-isopropylacrylamide (NIPAM), methacrylic acid (MAA) and poly (ethylene glycol) diacrylate (PEGDA). Then, SiO2 nanoparticles derived from the sol-gel process of tetraethoxysilane (TEOS) were coated on the surfaces of MS via complexing interaction between the silanol groups of SiO2 and the carboxylic groups of MAA to obtain the hierarchical microgel spheres (HMS).45 Hydrophilic block copolymer,
poly
(methacryloxy
propyl
trimethoxyl
silane)-block-(poly
(hydroxyethyl methylacrylate)-co-poly (2-methacryloyloxyethyl phosphorylcholine)) (PMPS-b-(PHEMA-co-PMPC))
was
synthesized
by
atom
transfer
radical
polymerization and both the gel permeation chromatography (GPC) and 1H NMR measurements suggest that the PMPS-b-(PHEMA-co-PMPC) copolymer has been successfully synthesized (see Supplementary Figure S1 and Figure S2). Then this copolymer grafted to the surfaces of HMS to obtain modified hierarchical microgel spheres (MHMS). SiO2 nanoparticles provide not only the multiactivated grafting-points for hydrophilic polymeric chains, but also the indispensable hierarchical structure of the coming surface material. These MHMS were then self-assembled on the glass substrate, and spin-coated by a layer of thermoset acrylic resin. When the solution of thermoset acrylic resin and crosslinking agent, 1, 6-hexamethylene diisocyanate trimer (t-HDI), was fully penetrated into the voids of MHMS and the superfluous upper resin was washed off using ethyl acetate, this
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system was cured at 80 ºC to obtain the self-repairing underwater superoleophobic and anti-biofouling coating (denoted as MHMS-based coating here). Figure 1b shows the as-obtained MS are monodisperse and spherical. After coated by SiO2 nanoparticles and modified by hydrophilic copolymers, the rugged surface morphology clearly indicates that the microgels have been covered with a dense layer of nanoparticles (Figure 1c), and an increasing mean diameter from 356 to 425 nm measured by dynamical light scattering (DLS, Figure 1d). The peak at 1246 cm-1 assigned to P=O stretching vibration appeared in the FTIR spectrum of the MHMS (see
Supplementary
Figure
S3),
indicating
that
hydrophilic
copolymers
PMPS-b-(PHEMA-co-PMPC) have been successfully grafted onto the surfaces of HMS. Additionally, the DLS diameter decreasing with temperatures confirms the reversibly thermosensitive behavior of the microgels (see Supplementary Figure S4). The self-assembly of MHMS produces a regular surface structure (Figure 1e and f), and this film still possesses an acceptable 3D ordered structure with a thickness of about 7 µm (Figure 1g), compared to the physically blended film of MHMS and thermoset resin (denoted as B-coating), which shows an irregular structure (see Supplementary Figure S5). In air, the surface of MHMS-based coating is both hydrophilic with the water contact angle (WCA) of 31.2° (Figure 2a) and oleophilic with the oil contact angle (OCA) of 6.8° (see Supplementary Figure S6). Interestingly, however, when this MHMS-based coating is submerged in water, the surface is extremely non-wetting to hexadecane (HD) with the OCA of about 160.8° (Figure 2b), while the surfaces of pure microgel spheres derived coating (denoted as MS-based coating) and B-coating are only oleophobic in water, with OCA of 146.5o and 135.8o, respectively (see Supplementary Figure S6). As for the HMS derived coating (denoted as HMS-based 6 ACS Paragon Plus Environment
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coating), although it shows superoleophobic OCA in aqueous medium, the suspending oil droplet is adhering onto the surface of coating (Figure 2c). On the contrary, the surface of MHMS-based coating in water presents not only the superoleophobicity but also oil-nonadhesion (Figure 2d). These results indicate that the MHMS-based coating has an excellent oil-repellent property. This is because the unique hierarchical structure of microgel spheres and their surfaces grafted by rich hydrophilic polymeric chains causes a layer of water molecules on the surface of MHMS-based film with a regular surface structure to reduce the contact between the oil and the material surface. Besides HD, this MHMS-based coating also exhibits excellent underwater superoleophobicity against other oily liquids, including hexane, heptane, decane, liquid paraffin, petroeum ether and trifluoroethyl methacrylate (Figure 2e). Moreover, this surface is highly transparent, being less than 10% of reduction within visible transmittance wavelength compared with the bare glass (Figure 2f), suggesting that this coating may find more potentials than the previously reported underwater superoleophobic coatings. Next, we further investigated the oil-repellent behavior of this surface material in highly acidic, alkaline, and salty environments since one of the biggest challenges is the chemical stability of oil-repellent materials under high ionic strength and high salinity for marine application.34,41 As shown in Figure 2g, the MHMS-based coating remains highly superoleophobic after submerged in acidic or alkaline solutions (pH: from 1 to 13) for 30 min, and in a series of NaCl concentrations ranging from 1 mol l−1 to fully saturated water (see Supplementary Figure S7). These results imply a high stability of MHMS-based coating against high acidic, alkaline, and salty environments. In addition, it can even keep its underwater superoleophobicity and high hydrophilicity after immersed in the artificial seawater for 30 days (Figure 2h), 7 ACS Paragon Plus Environment
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suggesting that the MHMS-based coating can serve for a long time in marine environment, which originates from the good chemical and structural stability of MHMS-based coating in seawater. Furthermore, we still evaluated the effect of mechanical abrasion on the surface property of the MHMS-based coating with a piece of sandpaper. The result shows that the surface can still hold its underwater superoleophobicity (160.8°→150.3°), but increasing oil-adhesion after abrasion under 10 kPa pressure. However, because of the crosslinking effect between -OH of MHMS and t-HDI, this coating shows the remarkable mechanical durability and good adhesion to the substrate even after cycling between abrading and immersing (Figure 3a, b). Very interestingly, this marred surface gradually recovers its original underwater oil-nonadhesion after immersed in water for 12 h (Figure 3c). Checked from surface morphologies by SEM, the micro-/nano-protuberances disappear and a relatively smooth surface appears after abrasion (Figure 3d). After immersed in water, the surface of MHMS-based coating morphs from its smoothness back to the micro/nano structure (Figure 3e). Atomic force microscopy (AFM) measurements also showed that the roughness of the abraded surface increased from 30.3 to 45.8 nm after immersion in water (see Supplementary Figure S8), being close to the original roughness (42.8 nm). These facts clearly indicate the MHMS-based coating presented here has very good self-healing function which is attributed to the unique 3D hierarchical micro/nano structure and the rich hydrophilic macromolecule chains of MHMS-based coating. Another usually occurring physical damage is to crush the coating surface on the land or in the water. As shown in Figure 3f, after pressed by a glass slide, instead of the hierarchical structure, the coating surface is significantly smooth. Fortunately, due to the swelling of MHMS in the coating, this crushed surface can restore its original 8 ACS Paragon Plus Environment
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topographic feature after immersed in water at 20 oC for 25 min (Figure 3g). Even after the cycles of press and immersion for 6 times, this coating still keeps its superoleophobicity (Figure 3h). Again, this crushed coating can acquire oil-repellent property due to the regeneration of topographic features after immersed in water (Figure 3i vs j). Moreover, owing to the thermosensitivity of MHMS, this self-healing ability of MHMS-based coating is temperature responsive: the lower the temperature, the faster the self-repairing process is (see Supplementary Figure S9). On the other hand, compared with HMS-based coating, B-coating and widely used polyurethane (PU)-coating which was prepared by thermoset acrylic resin crosslinked by t-HDI here, the MHMS-based coating displays the lowest WCA, and has no obvious change after abraded and immersed in water (WCA: 35.1°) due to its inherent hydrophilicity of the 3D-structure with abundant hydrophilic polymer chains, as shown in Figure 4a. This can be further confirmed by X-ray photoelectron spectroscopy (XPS) analysis as shown in Figure 4b. After immersed in water for 12 h, the P atomic concentration of the worn MHMS-based coating nearly has no variation (0.11%→0.10%) and Si increases from 1.11% to 5.96%. All these results further verify that the as-obtained MHMS-based coating has very good self-repairing ability and this property is resulted from the swelling MHMS and the self-replenishing hydrophilic polymer chains in underwater environment. Generally, the biofouling of marine organisms immediately starts by the adhesions of organic particles (e.g., protein, 1 min) and then bacteria and diatoms (1-24 h) when a clean surface is immersed in natural seawater.3 Herein, we examined the biofouling-resistant ability of the as-obtained MHMS-based coating using bovine serum albumin (BSA) protein as a model foulant in PBS buffer solution. In contrast to the protein adsorptions on PU coating, HMS-based coating and B-coating, the 9 ACS Paragon Plus Environment
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MHMS-based coating exhibits significantly decreased BSA adsorption (Figure 4c), being about 3 µg cm-2, which is usually considered to be extremely low fouling value.18 Moreover, the proteins adsorbed on the surfaces of the abrasion-immersed and the as long as 30 days incubated MHMS-based coatings are still low (
