Stable Superhydrophobic Porous Coatings from Hybrid ABC Triblock

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Stable Superhydrophobic Porous Coatings from Hybrid ABC Triblock Copolymers and Their Anti-Corrosive Performance Xin Zhou, Junhua Kong, Jiao-Tong Sun, Hui Li, and Chaobin He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08482 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 13, 2017

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ACS Applied Materials & Interfaces

Stable Superhydrophobic Porous Coatings from Hybrid ABC Triblock Copolymers and Their Anti-Corrosive Performance Xin Zhou,† Junhua Kong, ‡ Jiaotong Sun,† Hui Li,† Chaobin He*,†,‡ †Department of Materials Science and Engineering, National University of Singapore, Singapore 117574, Singapore ‡Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, Singapore 138634, Singapore

Abstract Superhydrophobic porous surfaces with ultra-low water adhesion were successfully fabricated via micelle fusion-aggregation assembly of newly designed linear hybrid ABC triblock copolymers, where A, B, and C denote poly(dimethylsiloxane) (PDMS), polystyrene (PS), and poly(methacrylolsobutyl polyhedral oligomeric silsesquioxane) (PiBuPOSSMA), respectively. It was found that aggregation behavior in diluted solution and subsequent formation of nano/microscale hierarchical surfaces in condensed state were affected by the molar mass of the triblock copolymers, which were evidenced by dynamic light scattering (DLS), SEM and TEM studies. Increasing of PiBuPOSSMA content can significantly increase roughness of the resulting coatings, leading to an increase of apparent water contact angles from 145.7 ± 1° to 157.3 ± 1.1°. The optimized PDMS-PS-PiBuPOSSMA surface 1

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possesses unique nano/microscale hierarchical morphology, large apparent water contact angle (157.3 ± 1.1°), small roll-off angle (~3°), low contact angle hysteresis (~0.9°), long-term stability and good chemical and thermal resistance. Moreover, it exhibits superior performance in preventing corrosive species such as ions and water in contact with the underlying metallic substrate (stainless steel) in 3.5 wt% NaCl aqueous solution with high inhibition efficiency and long-term preservability, which could be attributed to the synergistic effect of superhydrophobic surface and capillary action arising from the underlying porous structure. Keywords: hybrid block copolymer, POSS, self-assembly, superhydrophobic, anticorrosion

Introduction Corrosion of metals is an economic and environmental issue that can lead to economic losses, a waste of resources and jeopardize the safety of users.1 Recent researches have demonstrated the promising utilization of coatings with superhydrophobic (SH) surfaces for retardation of metal corrosion.2-24 Truly SH surfaces are identified by large apparent contact angles (>150°), small roll-off angles (97%) was purchased from Gelest, Inc., and used without further purification. Methacrylolsobutyl POSS (MA0702) was bought from hybrid plastics. Styrene (Alfa Aesar, 99%) was passed through a short column with basic alumina oxide to remove the inhibitors. CuBr (Sigma-Aldrich, 98%) was stirred overnight in acetic acid and then washed with methanol, followed by drying under 4


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vacuum. Other chemicals were purchased from Sigma-Aldrich and used as received. Millipore water (18 MΩ) was used throughout the experiments. Preparation of Micellar Solutions. All the solvents were filtered through 0.2 µm PTFE filters before use. Typically, PDMS-PS-PiBuPOSSMA was first dissolved in DCM, which is a good solvent for all blocks. Then, a certain amount of selective solvent DMF was added dropwise to the vials and stirred (600 rpm). Finally, the resultant bluish transparent solutions (10 mg/mL) were centrifuged at 800 rpm for 10 min to eliminate dust and air bubbles which may be introduced by the polymer samples or during the preparation process. Lower concentration samples were obtained by dilution of the 10 mg/mL polymer solutions prepared by the aforesaid method. Other polymeric solutions based on different polymers were prepared in the same manner. Deposition of the superhydrophobic/porous coatings. The superhydrophobic/porous coatings were prepared by using the solution-casting method. Type 304 stainless steel (SS) cylinder substrates with a diameter of 25 mm and thickness of 6 mm were sealed with epoxy resin and then abraded with emery papers up to 1200 grade to expose only top surface for further deposition. The substrates were washed with deionized water, ethanol, and acetone in an ultrasonic bath for 10 min, and blow dried thoroughly before use. Then, the polymer solutions with different concentrations were sonicated to eliminate air bubbles and uniformly spread on the substrate surfaces. The ratio of solution volume/substrate surface area is fixed at 0.9 µL/mm2. After that, the samples were allowed to cast in an ambient atmosphere (relative humidity: 60 – 65%, 23 ± 2 ºC) overnight and dried under vacuum at for another 24 h until reaching completed dry. 5



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Characterizations and Measurement. 1H nuclear magnetic resonance (1H NMR) spectra were recorded on a Bruker 400 MHz Ultrashield NMR instrument at room temperature using CDCl3 containing 0.03% (v/v) TMS (tetramethylsilane) as a solvent. The GPC analysis was performed on a Waters 2690 Alliance system with three phenomenox linear 5 mm styragel columns (500, 104 and 106 Å) and Waters 2420 ELS detector by using THF as eluent at 25 o

C with the flow rate of 1 mL/min. Calibration with poly(methyl methacrylate) standards was

used. Dynamic light scattering (DLS) measurements were performed at a scattering angle of 90° on a BI-200SM-3 DLS system with a He-Ne laser operating at a wavelength of 632.8 nm. The non-negative least squares (NNLS) plots were applied to analyze the intensity autocorrelation functions (the Siegert relationship) to obtain the particle size distribution.44 The refractive index of solvents mixture was calculated through ݊ଶ = ∅஺ ݊஺ ଶ + ∅஻ ݊஻ ଶ relation, where ∅ is the volume fraction of each solvent. While, the viscosity was estimated by equation ln ߟ = ܺ஺ ln ߟ஺ + ܺ஻ ln ߟ஻ , where X denotes the molar fraction of each component. Scanning electron microscopy (SEM) images were obtained with a field-emission scanning electron microscope (Zeiss Supra 40 FE-SEM) at an operating voltage of 5 kV. The specimens were sputter-coated with Au (about 8 nm) to ensure image quality. Transmission electron microscopy (TEM) measurements were conducted on a JEOL 2010F microscope operated at 200 kV. The samples were prepared by depositing 20 µL of the polymer solutions (0.2 mg/mL) onto carbon-coated copper grids without any selective staining. The coating thickness was estimated by a KLA Tencor Alpha-Step IQ surface profiler. Apparent water contact angles were measured by a video contact angle system (VCA 6


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Optima). Static apparent contact angles were measured from 5 µL droplets by averaging the values of five independent tests from different positions. The tilted plane method was used to determine the sliding angles and contact angle hysteresis. Specifically, a droplet (7 µL) was placed on an inclined plane and its contact angles (advancing and receding angles) were measured when it started sliding down. The droplet profiles were captured by a high-speed camera with a frame rate 60 fps and analyzed by a gray-scale analysis imaging system which implemented a circumference curve fit and baseline determination to measure the contact angles. The underwater durability of the SH coating was evaluated by immersion tests. The samples (5.5 cm2) were respectively immersed 100 mL 3.5 wt% NaCl aqueous solutions with different pH at 22 °C up to 7 days. pH values of the aqueous solutions were adjusted by 1 M HCl and 1 M NaOH and tested on a Traceable pH Meter. The contact angles of each sample were measured at different time points. Electrochemical corrosion characterization was carried out on an electrochemical workstation (PGSTAT302, Autolab) within the voltage window of -0.5 V ~ 0.5 V based on the determined open circuit potential (OCP) of the system. Three-electrode system was used to characterize the anti-corrosion properties of the coatings. The coating-on-SS, Pt and Ag/AgCl standard electrode were used as working, counter and reference electrodes, respectively.

Results and Discussion Two dissimilar organic/inorganic hybrid triblock terpolymers with different POSS content, PDMS58-PS214-PiBuPOSSMA35 and PDMS58-PS214-PiBuPOSSMA70 were synthesized and 7



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studied, where the subscripts correspond to the number-average degrees of polymerization of corresponding

block

as

determined

by

1

H

NMR.

The

synthetic

route

of

PDMS-PS-PiBuPOSSMA terpolymers is shown in Scheme 1. The block copolymerization was accomplished via the modification of monocarbinol-terminated PDMS with ATRP initiator (2-bromoisobutyryl bromide), followed by ATRP of styrene and iBuPOSSMA monomers respectively. The detailed synthesis and characterization of the terpolymers in this study can be found in the supporting information.

Scheme 1. Schematic illustration of the synthetic route to the hybrid ABC triblock copolymer.

Aggregation behaviors in solution. To better understand the aggregation behavior of PDMS-PS-PiBuPOSSMA triblock copolymers in solution, the diblock copolymer PDMS-PS is used as a comparison. It is found that for PDMS58-PS214, the intensity autocorrelation function (Figure 1a) in DMF exhibits a single exponential as expected for monodisperse particles. By modeling the DLS curves using NNLS method as shown in Figure 1a, the 8


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experimental scattering curves fits in well with the theoretical curves throughout the entire correlation time τ. The NNLS plots (Figure 1b) deduced from the intensity autocorrelation function demonstrates the formation of narrow distributed spherical micelles with a mean hydrodynamic diameter of 42.8 nm. The small peak located at 11.7 nm is the result of unimers (single polymer chain) which could be attributed to chain exchange between micelles accessible due to the extremely high flexibility of the siloxane chain of PDMS. These results indicate that dimethylformamide (DMF) is a non-solvent for PDMS and PDMS58-PS214 can aggregate to form spherical micelles with PDMS core surrounded by PS corona in DMF. It was found that the scattering intensity was too low to afford an accurate fitting when PDMS58-PS214 was dissolved in in 1:1 (v/v) dichloromethane/dimethylformamide (DCM/DMF) solvent, suggesting that there is no aggregation of PDMS58-PS214 in 1:1 (v/v) DCM/DMF, and indicating that the hydrophobic interaction of POSS segment is weakened by the addition of DCM.

Figure 1. (a) Intensity autocorrelation functions and NNLS fits of PDMS58-PS214 in 1:1 (v/v) DCM/DMF and DMF; (b) DLS NNLS plots of PDMS58-PS214 micelles in DMF; DLS NNLS plots of (c) PDMS58-PS214-PiBuPOSSMA35 and (d) PDMS58-PS214-PiBuPOSSMA70 self-assemblies in 1:1 (v/v) 9



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DCM/DMF. The insets show schematic drawings of simple spherical core-corona micelles in solutions, where red, blue and black colors represent PDMS, PS and PiBuPOSSMA blocks, respectively. All the sample concentrations are 0.5 mg/mL

DLS studies of PDMS-PS-PiBuPOSSMA in 1:1 (v/v) DCM/DMF (Figure 1c and d) show formation of nanostructured aggregates. Considering that the PDMS-PS could not forms aggregates in 1:1 (v/v) DCM/DMF, the aggregation of PDMS-PS-PiBuPOSSMA could be attributed to the strong solvophobic PiBuPOSSMA blocks, which hold the micelles together. Furthermore, the NNLS plot of PDMS58-PS214-PiBuPOSSMA35 self-assemblies in 1:1 (v/v) DCM/DMF (Figure 1c) reveals a monomodal distribution of the micelles with a mean hydrodynamic diameter of 105.3 nm, whereas the PDMS58-PS214-PiBuPOSSMA70 self-assemblies (Figure 1d) clearly show a bimodal size distribution with average hydrodynamic diameters of 68.6 nm and 214.8 nm, respectively. The transformation from monomodal to bimodal aggregations could be explained by the kinetics of BCPs micellization. The micellization of BCPs is a three-step process including unimer consumption, micelle fusion/fission, and micelle fusion/chain exchange.45 The second stage, micelle fusion/fission, gives rise to a bimodal aggregation at intermediate times. In our polymer system, with increasing length of the solvophobic chain (PiBuPOSSMA), the unimer exchange in the final stage can be significantly suppressed due to the high activation energy. Consequently, the system is kinetically frozen at the bimodal stage, resulting in the bimodal size distributions observed in our experiment.

10


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Figure

2.

Typical

TEM

image

of

(a)

PDMS58-PS214-PiBuPOSSMA35

and

(b)

PDMS58-PS214-PiBuPOSSMA70 micelles. The inset presents a schematic drawing of a core-shell dry micelle.

TEM

images

in

Figure

2a

and

b

confirm

the

spherical

morphology

of

PDMS-PS-PiBuPOSSMA self-assemblies. In Figure 2a, the relatively dark core and gray shell can be clearly resolved from the image, and the average diameter is 58.1 nm. By considering the intrinsic difference in electron density of the PS and POSS-containing blocks, we ascribe the dark core as POSS-rich domain and the gray shell as collapsed PDMS-PS corona. Notably, the fusion of micelles hardly reaches to the POSS-rich core, which indicates its good stability. Upon increasing the length of PiBuPOSSMA, the diameter increases and a bumpy gray shell was observed. The PiBuPOSSMA is considered to be highly rigid, and thus it could be the elastic stretching of core blocks that gives rise to a bumpy core-corona interface with constrained chain configurations. This clearly demonstrates that the aggregation behavior of the ABC triblock copolymers markedly depends on the POSS-containing block with varying molar mass. Comparing the TEM and DLS results, one can identify that the micellar sizes measured by TEM are much smaller than that of DLS. This discrepancy occurred because: first, the micelle corona (polymer chains) are more stretched in the solution. Second, the hydrodynamic size includes not only the micelle 11



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diameter but also whatever is attached to, and moves with the outer surface. However, to prepare TEM samples, the dispersing solvents were stripped, which leads to the collapse and aggregation of individual micelles. Third, the calculations of the refractive index and viscosity of solvent mixture are only approximations in the DLS test and could also contribute to the discrepancy.

Figure 3. SEM images of polymer coating surfaces prepared from 1:1 (v/v) DCM/DMF copolymer solutions at 5 mg/mL, (a) and (d) PDMS58-PS214, (b) and (e) PDMS58-PS214-PiBuPOSSMA35, (c) and (f) PDMS58-PS214-PiBuPOSSMA70. The inserts present typical apparent water contact angle measurements of corresponding surfaces.

SH surface and formation mechanism. After solution casting PDMS-PS-PiBuPOSSMA micellar solutions onto a substrate, it is found that the resulting coating is capable of achieving superhydrophobic state with ultra-low water adhesion. Figure 3 a and d show a resulting PDMS58-PS214 coating surface. Submicron-sized protrusions in connected together were observed. In addition, due to the excessive fusion of polymer chains, the top surface was covered by smooth sheet-like aggregates, resulting in a moderate water repellency with a CA 12


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of 145.7 ± 1°. Interestingly, in the presence of PiBuPOSSMA, surface roughness increased and the coating surface becomes superhydrophobic with a CA of 153 ± 1°. As can be seen from Figure 3 b and e, the PDMS58-PS214-PiBuPOSSMA35 surface comprised uniform interconnected particulate structure and the smooth sheet-like aggregates almost disappeared. The morphological change of the coating surfaces can be understood by considering the chain confinement effect of POSS molecules. It has been demonstrated that the PiBuPOSSMA generally shows a weak glass transition behavior due to a rigidity of the polymer backbone.46 The pendant massive POSS moieties are able to “bind” the methacrylate polymer chains, with the result of decreasing segment mobility, and thus ensuring the stability of polymeric micelles during solvent evaporation. Further increasing of PiBuPOSSMA leads to a high apparent contact angle up to 158.3°. As shown in Figure 3c and f. The PDMS58-PS214-PiBuPOSSMA70 surface exhibits a clearly particulate morphology with randomly distributed microvoids. Further scrutiny of the particulate reveals that it composed of micellar nanospheres with different sizes and slightly fused with each other in three dimensions. Notably, the surfaces of the micellar nanospheres display a bumpy morphology rather than smooth surfaces. These results coincide with the observation from TEM image and DLS bimodal size distributions. Compared with PDMS58-PS214-PiBuPOSSMA35 with monomodal aggregations, it is this twin-scale morphology which offers sufficiently trapped air pockets that allow water droplet stay partially on air cushions and partially on micellar nanospheres, and leads to a Cassie-Baxter wetting state. It is worth mention that the surface was unable to release the suspended droplet from the needle tip, indicating a rather weak interaction between a droplet and the surface. In 13



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a typically tilted plane test for a PDMS58-PS214-PiBuPOSSMA70 SH surface (Figure S3a), the sliding angle for a 7 µL water droplet was found to be as low as 3°. The advancing and receding angles were θadv/θrec=157.8°/156.9°, and the water contact angle hysteresis (θadv-θrec) was determined to be 0.9°. This corresponds to the extremely low resistance to depinning triple line across the surface. Still frames of a water drop impact test are shown in Figure S3b, it was observed that an impacting drop from 2 mm high was able to rebound into the air several times before it finally resided on the surface. Based

on

the

above

observations,

a

solvent

evaporation

induced

micelles

fusion-aggregation assembly is proposed for the formation of hierarchically structured surface. At the initial stage, the DCM evaporate more rapidly than DMF due to the high vapor pressure of DCM. As the amount of DCM decreases, the solubility of PDMS corona decreases gradually since DMF is a nonsolvent for the PDMS segments. Hence, the PDMS blocks will collapse to cover POSS-rich core or associate with the PDMS from other micelles via solvophobic interaction, resulting in fusion of micelles. Meanwhile, the evaporative cooling leads to the condensation of water vapor on the liquid surface penetrate into the solution (DMF is miscible with water). After that, the micelles/DMF system continue to absorb water vapor from the humid atmosphere and thus causing PS segments to undergo a non-solvent-induced phase separation before reaching the dried state. A bicontinuous structure with polymer-rich phase and solvent phase is formed before completely dry. At the final stage, the solvent phase can completely evaporate, leaving the hierarchical structure observed from the SEM images in Figure 3c and f.

14


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Figure 4. (a) apparent water contact angles and sliding angles of PDMS58-PS214-PiBuPOSSMA70 coating surfaces change with polymer concentration. (b) coating thickness as a function of the polymer concentration. The error bars represent the standard deviations.

Effect of Polymer Concentration on SH. The superhydrophobicity of the PDMS58-PS214-PiBuPOSSMA70 coating surfaces was found to be concentration dependent. Six different concentrations were used, 1, 2, 3.5, 5, 7.5 and 10 mg/mL. The resulting coatings are denoted as C1, C2, C3.5, C5, C7.5, and C10 for simplicity, where the numbers represent polymer concentrations of precursor solutions. As seen in Figure 4a, the apparent contact angles kept almost unchanged as the polymer concentration increases from 1.5 to 5 mg/mL, whereas, the sliding angles dropped from 11° to 3°. Further increasing of polymer concentration leads to a disappearance of superhydrophobicity, which is manifested by the decreasing of apparent contact angles accompanied with a dramatic upturn of sliding angles. To elucidate the above observations, the effect of polymer concentration on the aggregates morphology in solutions should be firstly considered. In the NNLS plots of all the concentrations (Figure S4), the micellar solutions exhibit two peaks which shift slightly toward smaller sizes as concentration increased. This could be attributed to the increasing contributions from interparticle interferences which can lead to a deviation from Stokes-Einstein-behaviour. Thus, the micelle structure is considered to be stable irrespective of their concentrations. During the solvent evaporation process, the micelles tend to 15



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precipitate out due to the decreasing solubility toward the mixture solvent. For the precursor solution with higher polymer concentration, the micelle corona could entangle with the neighbors more easily and early, leading to excessive fusions between micelles with increasing polymer concentration. We also measured thickness for the coatings obtained from different concentrations (Figure 4b). The coating thickness increases linearly with polymer concentration.

Figure 5. (a) SEM cross-sectional view of PDMS58-b-PS214-b-PiBuPOSSMA70 C5 coating, insert shows water droplets (10 µL) residing on the coating deposited on a glass substrate; (b) corresponding high magnification view.

Chemical and thermal stabilities of Coating. The PDMS58-b-PS214-b-PiBuPOSSMA70 C5 coating shows the best performance and is selected for further investigation. The cross-sectional images in Figure 5a and b indicate that the hierarchical structure for PDMS58-b-PS214-PiBuPOSSMA70 C5 coating exists throughout the entire volume, i.e. the coating is highly porous. In addition, the coating is scalable. The insert in Figure 5a shows that the coating area is uniform with a surface area of 12.5 cm2 and all the water droplets on the resulting coating surface take a sphere shape.

16


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Figure

6.

Effects

of

(a)

salinity

and

(b)

pH on

the

apparent

contact

angles on

a

PDMS58-b-PS214-b-PiBuPOSSMA70 C5 coating surface. Changes of apparent water contact angle of the coating surfaces as a function of (c) immersion time and (d) heating time. The error bars represent the standard deviations.

As shown in Figure 6a, liquid droplets regardless of NaCl concentrations from 0 to 5 mol/L have an apparent contact angle above 150°, while the resultant coating is more sensitive to pH change. An increasing and subsequent decreasing trend in apparent contact angles as pH values increases from 0 to 14 was observed (Figure 6b). However, when the pH exceeded 11, the coating tended to lose its superhydrophobicity. Therefore, the coating surface shows a good repellency towards corrosive liquids such as acidic and salty solutions, but a relatively poor performance towards strong alkaline liquids. Figure 6c depicts the underwater performances of the C5 coatings in salty solutions with different pH values. With increasing immersing time, the CA gradually decreases. The coating immersed in an alkaline salt solution (3.5wt% NaCl, pH: 9.9) lost its superhydrophobicity after 48h, while, the coating in acidic salt solution (3.5wt% NaCl, pH: 3.8) exhibited CA above 150° after 7 days. Surface chemistry of the SH coating is determined by both PDMS and PS. The PDMS component 17



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tends to be decomposed under alkaline condition, leading to a deterioration of the SH coatings. The heat resistance of the coating is interpreted in Figure 6d. The CA kept almost unchanged after 24h heating at 70°C. Notably, there is a gentle increase of CA from 157.7 ± 0.5° to 158.9 ± 1° after 1h heat treatment. This could be rationalized by the migration of flexible PDMS segments towards the polymer/air interface during the heating process which could reduce the surface free energy.

Figure 7. Potentiodynamic polarization curves of bare SS, SS/dense coat, SS/C10 coating, and SS/C5 coating in 3.5wt% NaCl aqueous solution for (a) day 1 and (b) day 7 at the room temperature.

Corrosion

Resistance

Performance.

The

performance

of

the

PDMS58-PS214-PiBuPOSSMA70 C5 superhydrophobic/porous coating for anticorrosive protection in 3.5wt% NaCl at room temperature was investigated. Additionally, a dense PDMS58-PS214-PiBuPOSSMA70 coating without any structural features cast from THF and C10 coating were also used as comparisons. Figure 7 shows anodic and cathodic polarization curves for different samples in 3.5wt% NaCl after 1 day and 7 days’ immersion. Tafel behavior was observed for the anodic and cathodic branches of the polarization curves. The results of the Tafel extrapolation of the polarization curves in Figure 7 are detailed in Table S2. Ecorr for bare stainless steel (SS) and SS/dense coating were determined to be -234.8 mV 18


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and -188.4 mV. The positive shift in Ecorr indicates a good protection of SS by the dense polymer coating. Corrosion protection can be further improved by constructing the polymer coating porous. Compared to SS/dense coating, Ecorr of SS/C10 coating is more positive (-173.8 mV) and its Jcorr is also lower (0.3 nA/cm2). Moreover, significant enhancement of corrosion protection was observed when the coating is not only porous but also superhydrophobic. In the case of SS/C5 coating, Ecorr is -158.2 mV and Jcorr is only 0.067 nA/cm2, although the thickness of C5 (17.5 ± 2.2 µm) is thinner than that of the C10 (33.2 ± 0.5µm). The corrosion rate of SS/C5 system is about four orders of magnitude less than that of bare SS. The improvement is caused by the effective prevention of water from infiltrating through the hierarchically structured coating. The Cassie-Baxter wetting state at the outer surface of C5 coating can provide an air cushion at the water/coating interface, which serves as a robust barrier of water/ions transportation. On the other hand, when water contacts with the porous structure, convex meniscus forms in the pores because of low surface tension of the coating surfaces and the liquid/air interface is subject to a pressure pointed towards the liquid phase, therefore, further imbibition is unfavorable. These two phenomena collectively give rise to the good anticorrosive performance of the SH/porous coating. The long-term anticorrosive protection capability of SH/porous coating was also examined after 7 days’ immersion. Continued immersion of bare SS causes the Jcorr to increase to 279.6 nA/cm2. As for the SS/dense coating and SS/C10 coating, Jcorr increases to 3.98 and 2.5 nA/cm2, respectively. The increasing of Jcorr could relate to the breakdown of the coatings and the initiation of pitting corrosion. Contrastively, C5 coating with both SH and porous features displayed minimal deterioration in performance as evidenced by lowest Jcorr (0.133 nA/cm2) 19



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and the highest Ecorr (-155.3 mV). This clearly demonstrates that the combination of SH and porous features could markedly enhance the reliability of anticorrosive coatings. Conclusions A solvent evaporation induced micelles fusion-aggregation assembly accompanied by nonsolvent vapor-induced phase separation approach has been demonstrated for the construction of superhydrophobic/porous coatings. Nano/micro scale morphology and superhydrophobicity could be achieved with the aid of the hybrid PiBuPOSSMA block. The optimized coating processes a unique twin-scale structure where the bumpy nanospheres with different sizes serve as nanoroughness while the nanosphere assemblies induce microroughness. The coating surface is superhydrophobic with a good repellency towards acidic/salty liquids and a moderate heat resistance. The uniform thin coating exhibits an excellent anticorrosive performance with a high inhibition efficiency and little deterioration after 7 days. Owing to the synergistic effect of superhydrophobic surface and underlying porous morphology, the superhydrophobic/porous structure in this study shows a great promise of applications in anticorrosive paints for metal protection.

ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Details of the hybrid triblock copolymer synthesis and characterization, including 1H NMR spectra and GPC traces; representative dynamic apparent water contact angle test; NNLS

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plots of solutions with different polymer concentrations; results of the Tafel extrapolation of the polarization curves. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail [email protected] (C.H.).

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge the financial support of this work from the Science and Engineering Research Council under the Agency for Science, Technology and Research of Singapore. Zhou Xin also acknowledges the research scholarship supported from the National University of Singapore.

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Stable Superhydrophobic Porous Coatings from Hybrid ABC Triblock

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