Durable Anti-Icing Coatings Based on Self-Sustainable Lubricating

May 12, 2017 - A versatile, convenient, and cost-effective method that can be used for grafting anti-icing materials onto different surfaces is highly...
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Durable Anti-Icing Coatings Based on Self-Sustainable Lubricating Layer Jing Chen,*,† Kaiyong Li,*,‡ Shuwang Wu,§ Jie Liu,§ Kai Liu,§ and Qingrui Fan§ †

Department of Chemistry, School of Science, Tianjin University of Science & Technology, Tianjin Economic and Technological Development Area Campus, No. 29, 13th Avenue, Tianjin Economic and Technological Development Area, Tianjin 300457, P. R. China ‡ School of Materials Science and Engineering, Luoyang Institute of Science and Technology, Wangcheng Road, Luoyang 471023, P. R. China § Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun North First Street 2, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: A versatile, convenient, and cost-effective method that can be used for grafting anti-icing materials onto different surfaces is highly desirable. Based on musselinspired chemistry, the anti-icing coating with extremely low ice adhesion is enabled by constructing a self-sustainable lubricating layer, achieved via modifying solid substrates with a highly hydrophilic conjugate of poly(acrylic acid)−dopamine. Both unfreezable and freezable water remain liquidlike at subzero conditions and synergistically fulfill the role of lubrication for reducing the ice adhesion. The anti-icing coatings show excellent stability in harsh environments and durability after the cross-linking. More importantly, this coating can be applied to various substrates and is of great promise for practical applications.

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demonstrated that alkyl- and fluoroalkyl-grafted surfaces gradually deteriorated and lose their ice repellency after repeated icing/deicing cycles.26 To overcome the above limitations, the continuous lubricating layer is explored as a new concept to reduce ice adhesion. Kim and co-workers reported that slippery liquid-infused porous surfaces have superior antifrosting and anti-icing properties as compared to those of superhydrophobic surfaces.27 Dou et al. used polyurethane as the main component for preparing anti-icing coating. The polymer chain consists of hydrophobic and hydrophilic components, and the hydrophilic hard component can absorb water, forming a lubricating layer.28 Chen designed self-lubricating icephobic coatings by blends of poly(dimethylsiloxane)−poly(ethylene glycol) amphiphilic copolymer and poly(dimethylsiloxane), and low ice adhesion was realized on smooth, hydrophobic surfaces.29 However, Ozbay suggested that porous surfaces infused with the hydrophilic lubricants would be a promising candidate for anti-icing.30 Recently, a proof-of-concept has been proposed to demonstrate the feasibility of introducing an aqueous lubricating layer between ice and the solid substrate for reducing ice adhesion,

INTRODUCTION Ice formation and accretion on the surfaces pose severe consequences in a wide spectrum of fields including airfoils, power transmission lines, and wind turbines.1−4 Over the past decades, many strategies have been proposed to alleviate and/ or eliminate the adverse impact of icing.5−7 The active strategies commonly used for combating icing, such as chemical, thermal, and mechanical methods, suffer from high cost, and detrimental environmental consequences, and are usually energy-intensive.8 It is therefore desirable to engineer surfaces for minimizing ice formation and/or reducing ice adhesion, which is referred to as a passive anti-icing method, bearing the advantages of low-energy consumption and limited environmental impact.9−13 The surfaces with low ice adhesion preferably allow the formed ice to be easily shed off by human assistance or natural forces, such as wind and gravity.14 During the past decade, lowering ice adhesion via superhydrophobic surfaces has been of particular interest in the academic and commercial communities and has commonly been proposed to mitigate icing problems.15−20 However, a large number of investigations have shown that frost can build up within the micro/nanostructured features of superhydrophobic surfaces under subzero conditions,21 leading to the anchoring of ice,22−24 which in turn results in the increase of ice adhesion during icing/deicing cycles.25 Kulinich et al. also © 2017 American Chemical Society

Received: March 25, 2017 Accepted: May 5, 2017 Published: May 12, 2017 2047

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Figure 1. Synthesis of the PAA−DA conjugate and schematic representation of the mussel-inspired construction of the anti-icing coating with an aqueous lubricating layer. PAA−DA conjugate was first synthesized, and dissolved in deionized water. Then, equimolar sodium periodate (NaIO4) was added to cross-link the conjugate. Cleaned substrates were incubated in the above solution to obtain the self-sustainable lubricating-layer-coated substrates. When the temperature decreased or under humid conditions, the cross-linked PAA−DA begins to swell as they collect water in their vicinity. Thus, an aqueous lubricating layer forms naturally as the icing occurs. Ice on the coated surface with the SSLL can be easily removed due to the low ice adhesion strength.

first conjugated with dopamine (DA) via carbodiimide coupling chemistry (Figure 1) (1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride/hydrochloric acid, EDC·HCl), and then the PAA−DA conjugates were deposited onto solid substrates by mussel-inspired surface chemistry.35 We propose that both unfreezable and freezable water remain liquidlike at subzero conditions and synergistically fulfill the role of lubrication for reducing ice adhesion. By tailoring the conjugation efficiency of DA to PAA, the ice adhesion strength on the substrates can be greatly reduced compared to that of the as-received surfaces. Moreover, the stability of PAA−DA film can be greatly improved through NaIO4-induced crosslinking. The outstanding stability under harsh conditions and the facile construction of the PAA−DA film make it attractive for fighting icing problems. The mussel-inspired surface chemistry is used to fabricate anti-icing coatings, which has been suggested as a facile and universal substrate-unspecific strategy for modifying virtually all types of material surfaces.35 PAA, possessing a high density of carboxyl along its backbone, was exploited as the matrix for binding a large number of water molecules. The carboxyl activated by EDC·HCl was reacted with DA in phosphatebuffered saline (PBS) (pH 6.0) for 24 h under a N2 atmosphere (Figure 1), followed by dialysis. The content of DA conjugated to PAA was tailored by the molar feed ratio between PAA and DA and quantified by 1H NMR (Table 1 and Figure S1 in the Supporting Information). As shown in Figure 1, the anti-icing coating was constructed on the solid substrate by the dipcoating method. The solid substrate was immersed in a solution of the as-prepared PAA−DA conjugate, followed by adding sodium periodate (NaIO4), in which catechol served as the

which is formed by hygroscopic polymers due to water absorption or condensation.31 However, it is not practical to prepare micropores and to graft cross-linked hygroscopic polymers in the micropores on different types of solid substrates. Moreover, the ice adhesion strength, τice, has not been greatly reduced to such an extent that the accreted ice can be passively removed in the practical scenario without the help of external energy, for which extremely low values of τice are required (about 20 kPa).14 Herein, we design anti-icing coatings with extremely low ice adhesion by constructing a self-sustainable lubricating layer (SSLL), which is achieved via modifying solid surfaces with highly water-absorbing, hydrophilic poly(acrylic acid) (PAA). The strong association of water molecules with the PAA chain leads to an overlayer of hydration water that remains liquidlike down to low temperatures, serving as a lubricating layer for reducing ice adhesion, which is termed as an SSLL that fulfills its full potentiality through the uptake of water upon encountering humid conditions. Compared with conceptually different types of anti-icing coatings that function through maintaining a lubricating layer of low surface-energy oil infused into micro/nanostructured substrates,27 which are susceptible to malfunction due to various possibilities, including the displacement and removal of oil by water or frost,32−34 SSLL retains its potentiality as long as water is present and thus maintains long-lasting anti-icing properties.

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RESULTS AND DISCUSSION Synthesis of Self-Sustainable Lubricating Anti-Icing Layer. To explore this conception, the inexpensive PAA was 2048

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existence of noncovalently self-assembled DA on the covalent coupling PAA−DA. The effect of cross-linking on the morphology of PAA−DA film was investigated by atomic force microscopy (AFM). Figure 2a,e shows that the crosslinking exerts an indistinguishable effect on surface morphology, both presenting abundant nanoprotuberances on the surfaces. The cross-linked film exhibits a root-mean-square roughness of 5.60 nm, almost the same as that of the un-crosslinked counterpart (5.19 nm). The virtually unchangeable morphologies before and after cross-linking, presented to advantage, demonstrate the gentleness characteristic of catechol chemistry, which is superior to thermally induced cross-linking, usually leading to microscale wavelike profiles, and which is consolidated by the cross-sectional SEM images of PD25coated silicon wafer before and after cross-linking. To illustrate the outstanding stability of the cross-linked PAA−DA film, we conducted soak testing in three types of etching solutions. The un-cross-linked films are obviously disintegrated after soaking in the solutions (Figure 2b−d), whereas the cross-linked films maintain their morphologies intact after treatment with the solutions (Figure 2f−h). Moreover, the values of τice for the cross-linked films after soaking in the solutions indistinguishably vary, and are nearly same as that of the counterpart without soaking. However, τice for the un-cross-linked films increases significantly after soaking in the solutions, further substantiating the remarkable stability of the cross-linked PAA− DA film (Figure S6). These results demonstrate that the stability of the PAA−DA film can be significantly enhanced by NaIO4-induced cross-linking, and the cross-linked films remain intact even in a harsh environment. Anti-Icing Performances. It has been generally proposed that water in the matrix of hydrophilic polymer exists in two distinct states, that is, nonfreezable water associated with the hydrophilic groups along the polymer chain to the extent that the well-oriented hydrogen bonding is in a locally favorable configuration to sufficiently prevent this fractional water from freezing, and freezable water residing within macromolecular interstices and/or with proximal occupancy around the

Table 1. Molar Feed Ratio between PAA and DA and Conjugation Efficiency of the As-Prepared PAA−DA Conjugate conjugate

PAA (mmol)

DA (mmol)

PAA−DAa

fb (%)

PD3 PD6 PD13 PD25 PD40

3 3 3 3 3

0.3 0.6 0.9 1.2 1.5

100:10 100:20 100:30 100:40 100:50

3 6 13 25 40

a Molar feed ratio between PAA and DA. bf is conjugation efficiency of DA to PAA calculated from 1H NMR spectra.

anchoring group for depositing the conjugate onto the substrate and simultaneously as the site to cross-link the conjugate via NaIO4-induced dismutation for the purpose of improving the durability of the film. The role of cross-linking played by NaIO4 can be corroborated by performing NaIO4mediated oxidation of a solution of PAA−DA conjugate (Figure S2). Stability of the Anti-Icing Coating. The silicon wafer was selected as a model substrate for preparing the PAA−DA film, and the variations in surface chemistry upon surface treatment monitored by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy (Figures S3 and S4) verify the formation of the PAA−DA film and the cross-linking between PAA−DA conjugates. It should be noted that the ratio of nitrogen-to-carbon calculated by XPS for the PD25-coated surface is 0.05, which is almost the same as that of 1 H NMR (0.055). The existence of a nitrogen peak (N 1s, 399.9 eV) on the coated silicon wafer in Figure S3a compared with the absence of a nitrogen peak on the uncoated silicon wafer also confirms the formation of PAA−DA coating. The two components of the high-resolution spectrum of N 1s (Figure S3d) correspond to amine groups (R-NH2, 401.7 eV) and substituted amines (R-NH-R, 399.8 eV), indicating the cross-linking between PAA−DA conjugates. The presence of a minor amount of primary amine is because of the concurrent

Figure 2. AFM height images (2 μm × 2 μm) of PAA−DA films (PD25) deposited on silicon wafers before (top) and after (bottom) cross-linking in the air (a, e), immersed in (b, f) 0.1 M NaOH, (c, g) 0.1 M HCl, and (d, h) 5 M NaCl for 2 h. 2049

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Figure 3. (a) Ice adhesion strength measured at −15 °C on a series of substrates coated by the cross-linked PAA−DA conjugates (PD3 to PD40), and the substrates coated by PAA and DA alone. (b) The temperature dependence of ice adhesion strength on the cross-linked PAA−DA film (PD25). The solid line is provided as a guide to the eye. (c) The distribution of the freezing temperature of macroscopic droplets located on the PD25-modified surface. (d) Differential scanning calorimetry (DSC) curve of the PD25 conjugate during the cooling process at a rate of 3 °C min−1. The numbered arrows indicate the cooling and subsequent heating runs. Error bars in (a) and (b) indicate the standard deviation (SD).

attached water molecules.36−38 Moreover, the fractions of the two types of water can be tailored via cross-linking the hydrophilic polymer.36 To explore this strategy, the conjugation efficiency (f) of DA to PAA was varied by changing the molar feed ratio of PAA to DA (Table 1) to investigate the effect of cross-linking on the ice adhesion. Compared to the bare silicon wafer (τice = 189 ± 20 kPa), the presence of the PAA−DA film can greatly reduce the ice adhesion, and the increase in f gradually lowers the ice adhesion to values as low as τice = 25 ± 7 kPa for PD25 (Figure 3a). However, the further increasing f significantly increases τice to 170 ± 39 kPa (PD40). The crosslinked network between the PAA−DA conjugates swells upon hydration to such an extent that the threshold of the degree of swelling is attained, that is, the critical concentration of water, to open up the network, thus allowing more water molecules to penetrate into the network up to the state of maximum water uptake (MWU). Therefore, the increasing f leads to an increase in the cross-linking which, in turn, reduces the swellability of the network and consequently results in the decrease in its MWU from PD3 to PD40 (Table S1). Okoroafor et al. reported that under the condition of full hydration, the quantity

of unfreezable water (Cunf) increased with the increase of crosslinking, whereas the increase of cross-linking led to the decrease in the quantity of freezable water (Cf).36 We propose that both the unfreezable and freezable water remain liquidlike at subzero conditions, as discussed later, and synergistically fulfill the role of the lubricating layer for reducing the ice adhesion. The increasing f increases the fraction of Cunf in the network from PD3 to PD25, rendering the unfreezable water to realize the potentiality of lubrication more sufficiently. Simultaneously, Cf progressively decreases with the increasing f from PD3 to PD25, but the freezing temperature of the freezable water is gradually depressed from PD3 to PD25, as discussed below. Then, we can expect that under conditions of measuring ice adhesion (−15 °C, Figure 3a), the fraction of the freezable water that would freeze gradually decreases from PD3 to PD25. Therefore, the synergistic effect between the unfreezable and freezable water lowers gradually the ice adhesion from PD3 to PD25. However, the increase of cross-linking up to a critical degree significantly reduces MWU to such an extent that the network cannot accommodate sufficient water to play the role of lubrication, thus resulting in the significant increase of ice 2050

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Figure 4. (a) Variation in the ice adhesion strength measured at −15 °C on the PD25-coated substrate with the number of icing/deicing cycles. (b) Long-term stability of the PD25 film soaked in PBS solution. (c) Comparison of the ice adhesion strength measured at −15 °C on a wide range of substrates before and after the application of the cross-linked PAA−DA film (PD25). PE and PDMS denote polyethylene and poly(dimethylsiloxane), respectively. Error bars indicate the SD.

commonly encountered ρe.36,40 The predicted Tc of the freezable water in the network is presented in Figure S6 as a function of φw with χ of PAA with water, χ = 0.50.41,42 It can be seen that the water remains liquidlike down to subzero conditions, and the depression of the freezing temperature progressively increases with the degree of cross-linking. To confirm the existence of an aqueous lubricating layer, we investigated the temperature dependence of τice for the crosslinked PAA−DA film (PD25). Figure 3b shows that τice remains constant down to about −25 °C, indicating that the fractional freezable water is in a liquidlike state to serve as the lubrication layer, as qualitatively reconciled with the results in Figure S7. The progressively decreasing temperature leads to the gradual increase in τice from −25 to −42 °C, which is caused by the gradual freezing of the fractional freezable water (Figure S7) to the extent that the lubrication attributed to the freezable water progressively malfunctions, and unfreezable water is concurrently deficient to fulfill the lubrication. When temperature is further lowered down to −42 °C, the fractional freezable water completely freezes resulting in the complete malfunction of lubrication and the ensuing higher τice, which again remains constant down to the lower temperature where the effect of mechanical interlocking dominates. The measurements of ice nucleation temperature of single macroscopic droplets located on the PD25-modified surface were performed in a home-built setup.43 Three to four droplets of 0.1 μL ultrapure water were separately placed onto the substrate, and each droplet froze independently without

adhesion for PD40 and the substrate coated by highly crosslinked DA alone. For the substrate coated by PAA alone, τice is higher than that of the least-cross-linked PD3. This can be explained by the fact that the high hygroscopicity characteristic of PAA results in nearly complete freezing of its hydration water, making it deficient to serve as a lubricating layer. Flory had presented a theory correlating the variation of solvent’s crystallization temperature with its concentration within the solvent−polymer network. The depression of the freezing temperature is given by36,39 1 1 R ⎧ ⎨[−ln(1 − φp) − φp − χφp2] − 0 = ΔHM ⎩ Tc Tc ⎪

⎡ ⎛ φp ⎞⎤⎫ + ⎢ρe ⎜ − φp1/3⎟⎥⎬ ⎠⎦⎭ ⎣ ⎝2

⎪

(1)

where Tc is the freezing temperature of water in the polymeric network, T0c is the freezing temperature of pure water, ΔHM is the molar enthalpy of crystallization, R is the gas constant, φp is the volume fraction of the polymer within the network, χ is the interaction parameter of polymer−water, ρe is a structure factor, accounting for the effect of cross-linking. From eq 1, one would expect that the increasing φp depresses Tc in the swollen network (the first term in the brackets), whereas the second term prefers to increase Tc, suggested to be associated with the tendency of the swollen network to expel water during crystallization. However, the effect of the second term is considerably minute due to the order of 0.01−0.02 of 2051

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and freezable water remain liquidlike at subzero conditions and synergistically fulfill the role of lubrication for reducing the ice adhesion. By controlling conjugation efficiency, low ice adhesion strength can be obtained, which can be maintained at 25 kPa. The robustness and durability of the anti-icing coating are verified by icing/deicing cycles and soaking tests. This coating can be applied to various substrates, indicating that this anti-icing coating is of great promise for practical applications.

triggering/affecting the freezing of the proximal one by maintaining enough distance between the droplets. The surface was cooled at a rate of 2 °C min−1 (the cooling rate is the same as that in the ice adhesion test for PD25) to induce ice nucleation and the temperatures of ice nucleation were recorded. The number of freezing events performed is 207, and the statistical distribution of the freezing temperatures on the PD25-modified surface are investigated. Figure 3c shows that the freezing temperature can be well fitted with a Gaussian function. The average freezing temperature of macroscopic droplets on the PD25-modified surface is −27.2 ± 1.7 °C. This is consistent with the change of ice adhesion with the temperature (Figure 3b). Moreover, the delayed ice nucleation in the droplets can also be observed and quantified using DSC for the PD25 sample. As shown in Figure 3d, as the sample is cooled below 0 °C, the water becomes supercooled until ice formation occurs at −27 °C, resulting in the rising of the temperature due to the release of the latent heat of freezing. Then, the sample is cooled again down to −70 °C. In the subsequent heating run, the formed ice melts at 0 °C. The results of delayed ice nucleation consolidate the effect of temperature on the ice adhesion and also provide the additional benefit of developing icephobic coatings, if supercooled water can be shed from the surface prior to freezing.29 The durability of the anti-icing coating holds important significance for its deployment under real-world scenarios to realize its full potentiality. To characterize the durability of antiicing coatings, we conducted two simple tests: repeated icing/ deicing and soaking in the solution as shown in Figure 4a,b. It can be seen that the values of τice are nearly constant over 40 icing/deicing cycles, which presents a noticeable advantage over the state-of-the-art anti-icing coatings based on superhydrophobic surfaces, whose icephobicity left much to be desired as embodied by the remarkable increase of τice over a few icing/ deicing cycles due to the damage to mechanically weak micro/ nanostructures.25 The availability of the adhesive property imparted by DA endows the PAA−DA coating with strong adhesion to the substrate, preventing the coating from delaminating from the substrate during icing/deicing cycles. Second, the aqueous lubricating layer is self-sustainable and stimulus-responsive to water, which can be reversibly formed during icing/deicing cycles. Over soaking in PBS for 30 days, the anti-icing coatings remain icephobic with low τice, indicating excellent stability and exhibiting a promising potential for realworld applications (Figure 4b). To illustrate the versatility of the fabrication of the anti-icing coating from a wide range of material systems, we investigated the ice adhesion strength on a series of substrates with a wide range of surface energies coated by the PAA−DA conjugate. As shown in Figure 4c, the values of τice on a variety of substrates are dramatically reduced upon the application of the cross-linked conjugate. The results clearly demonstrate that the strategy of constructing the anti-icing coating in this work can be extended to a wide range of material systems, irrespective of material chemistry, which is rooted in the universal substrate unspecificity exhibited by DA, of modifying virtually all types of material surfaces.

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EXPERIMENTAL SECTION Materials. PAA (Mw ≈ 100 000, 35 wt % aqueous solution; Aldrich), DA hydrochloride, EDC·HCl (J&K Scientific), sodium periodate (NaIO4; J&K Scientific) were used as received. Other reagents were of analytical grade and used as received, without further purification. For all experiments, deionized water was used. Synthesis of PAA−DA Conjugate. An aqueous solution of PAA (617.1 mg of a 35 wt % solution) was diluted with 0.1 M PBS (30 mL), and 173 mg EDC·HCl, and 171 mg DA hydrochloride were then added to the PAA solution for 24 h. The resultant mixture was dialyzed by a regenerated cellulose membrane (molecular weight cutoff = 3500) in deionized water to such an extent that DA in the washing solution was undetectable by ultraviolet−visible (UV−vis) spectroscopy. The final PAA−DA conjugate was lyophilized to give a fluffy white solid. Construction of Anti-Icing Coating on a Variety of Solid Substrates. The PAA−DA conjugate was dissolved in deionized water at a concentration of 10% (w/v). Then, equimolar sodium periodate (NaIO4) was added to cross-link the conjugate. The cleansed substrates were incubated in the above solution for 6 h. Then the solid substrates (including aluminum alloy, copper, glass, ceramic, polyethylene, poly(dimethylsiloxane), polypropylene, and epoxy resin) were removed, rinsed with ultrapure water, and dried under a stream of nitrogen gas. Characterization. The FT-IR spectroscopy was obtained using a Vertex 80 (Bruker, Germany) with an attenuated total reflectance (ATR) silicon crystal. The ATR silicon crystal was coated by the PAA−DA conjugate. XPS was performed on the Thermo Scientific ESCALab 250Xi using 200 W monochromatic Al Kα radiation. The base pressure in the analysis chamber was about 3 × 10−9 mbar. The pass energy is 30 eV. The number of scans used for the survey is 1. The number of scans for detailed XPS are 1, 9, 1, and 2 for C, N, O, and Si, respectively. Typically, the hydrocarbon C 1s line at 284.8 eV from adventitious carbon is used for energy referencing. The morphologies of the solid substrates coated by the PAA−DA conjugates were obtained by AFM (Multimode8; Bruker Nano Inc., Germany) in the ScanAsyst mode. Measurement of Ice Adhesion Strength. The ice adhesion strength was measured using a custom-made setup as described previously.22 All of the ice adhesion strengths were averaged over nine measurements. The setup for measuring the ice adhesion shear strength consists of an XY motion stage, a force transducer (Imada ZP-500N), a home-built cooling stage, and water-filled cuvettes that were frozen onto the test surfaces. The cooling stage could hold nine samples. Then, the cooling stage with the cuvettes atop of it was placed in a closed box, which was purged with nitrogen gas to minimize the frost formation outside the cuvettes. The test was carried out after the water (Milli-Q) in the cuvette was kept at −15 °C for 5 h,

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CONCLUSIONS In conclusion, we developed a strategy of constructing antiicing coatings, which can be extended to a wide range of material systems. The anti-icing coatings show excellent stability after cross-linking the PAA−DA conjugate even under harsh conditions. We propose that both unfreezable 2052

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ORCID

which ensured that the water froze completely. Then, we drive the probe of the force transducer to the ice columns at a speed of 0.5 mm s−1, and the peak force required to detach each ice column was recorded to give the ice adhesion strength by dividing the peak force by the area between the ice and a solid substrate. This method has been widely used in the literature and gives a reproducible value of ice adhesion strength on various surfaces. Stability of PAA−DA Films. To investigate the stability of the PAA−DA films, the solid substrates coated by PAA−DA film were immersed in three types of solutions including 0.1 M NaOH, 0.1 M HCl, and 5 M aqueous solution of NaCl for 2 h. Note that in the following study, the un-cross-linked and crosslinked films were prepared using the conjugate PD25, unless otherwise stated. Furthermore, to test the long-term stability of the solid substrates coated by PAA−DA films, the substrates were immersed in PBS for a variety of periods (from 0 to 30 days), and washed with deionized water, and finally dried under a stream of nitrogen gas. The changes in the morphology and anti-icing performance of the solid substrates were characterized by using AFM and the ice adhesion test, respectively. DSC Measurement. DSC was performed using a DSC Q2000 (TA Instruments). An accurately weighed (10 mg) water microdrop was placed into an aluminum cup coated with a thin PD25 film and weighed quickly to avoid evaporation. Next, the aluminum lid was put on the pan and sealed hermetically. An empty pan was weighed and used as a reference. The experiment consisted of two runs: the first cooling step from 30 to −70 °C to freeze the water and the second step from −70 to 30 °C to melt the ice. The experiments were run at a scanning rate of 3 °C/min. Measurement of Ice Nucleation Temperature. The measurements of ice nucleation temperature of single macroscopic droplets located on the PD25-modified surface were performed in a home-built setup. The procedure for the deposition of macroscopic droplets and the fabrication of the sample cell were conducted in a biosafety cabinet. Then, three to four droplets of 0.1 μL ultrapure water were separately placed onto the substrate, and each droplet froze independently without triggering/affecting the freezing of the proximal one by maintaining enough distance between the droplets. The surface was cooled at a rate of 2 °C min−1 (the cooling rate is the same as that in the ice adhesion test for PD25) to induce ice nucleation and the temperatures of ice nucleation were recorded. The number of freezing events performed is 207, and the statistical distribution of the freezing temperatures on the PD25-modified surface is investigated.

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Jing Chen: 0000-0002-7228-0625 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support from the National Natural Science Foundation of China (Grant No. 21404081) and the Beijing National Laboratory for Molecular Sciences (Grant No. 20140149).

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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/acsomega.7b00359. 1 H NMR for different conjugates; UV−vis, XPS, FT-IR, SEM of PD25 before and after cross-linking; the freezing temperature of freezable water in the cross-linked PAA− DA system; MWU of the dry PAA−DA conjugate (PDF)

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*E-mail: [email protected] (J.C.). *E-mail: [email protected] (K.L.). 2053

DOI: 10.1021/acsomega.7b00359 ACS Omega 2017, 2, 2047−2054

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DOI: 10.1021/acsomega.7b00359 ACS Omega 2017, 2, 2047−2054