Inhibition of Heterogeneous Ice Nucleation by Bioinspired Coatings

Then, the polycondensation was carried out at 230 °C, and the pressure of the reaction device was gradually ... PBS-C, 1/1.8/0, 50000, 1.8, 110.6, 72...
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Inhibition of Heterogeneous Ice Nucleation by Bioinspired Coatings of Polyampholytes Zhiyuan He,†,§ Liuchun Zheng,‡,§ Zhenqi Liu,†,§ Shenglin Jin,†,§ Chuncheng Li,*,‡,§ and Jianjun Wang*,†,§ Key Laboratory of Green Printing, Institute of Chemistry and ‡Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China § University of Chinese Academy of Sciences, Beijing 100049, PR China Downloaded via UNIV PIERRE ET MARIE CURIE on August 21, 2018 at 05:04:25 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

ABSTRACT: Control of heterogeneous ice nucleation (HIN) on foreign surfaces is of great importance for anti-ice-nucleation material design. In this work, we studied the HIN behaviors on various ion-modified poly(butylene succinate) (PBS) surfaces via chain-extension reaction. Inspired by antifreeze proteins (AFPs), the PBS-based polyampholytes, containing both negative and positive charge groups on a single chain, show excellent performance of ice nucleation inhibition and freezing delay. Unlike the extremely high price and low availability of AFPs, these PBS-based polyampholytes can be commercially synthesized under mild reaction conditions. Through water freezing tests on a wide range of substrates at different temperatures, these PBS-based polyampholytes have shown application value of tuning ice nucleation via a simple spin-coating method. KEYWORDS: heterogeneous ice nucleation, polyampholyte, poly(butylene succinate), bioinspired coatings, interface introduce AFPs onto solid substrates to control HIN.28−30 Francis et al. attached AFPs to polymer chains in order to form polymer−protein conjugate coatings, which were able to delay ice freezing time as well as inhibit ice nucleation.28 Jin et al. also coated Chaetoceros neogracile (Cn-AFP) on an aluminum material. Compared to bare aluminum and other conventional hydrophilic aluminum coatings, the aluminum surfaces with surface-bound AFPs inhibited ice formation more efficiently.29 More recently, Liu et al. discovered the Janus effect of AFPs on ice nucleation via selectively binding AFPs to solid substrates. Most interestingly, they found that the water molecules on the non-ice-binding face of AFPs exhibit disordered structure due to the existence of bulky hydrophobic groups and charged groups. Therefore, ice nucleation is energetically inhibited on top of the nonice-binding face of AFPs.30 This has inspired the synthesis of AFP mimics with charged groups for HIN inhibition because of the extremely high price and low

1. INTRODUCTION The undesired formation of ice always causes serious problems such as building damages, energy losses, and safety risks of human beings.1−4 Current de-icing strategies are mainly based on conventional thermal, electrical, and mechanical approach to remove ice, which are costly, inefficient, and environmental harmful.5,6 To solve these problems, many passive anti-icenucleation surfaces have been developed to delay heterogeneous ice nucleation (HIN), because nucleation is the first and rate-limiting step of the transition from water to ice.7−12 It has been reported that HIN can be controlled via changing the features of surface, such as the surface wettability, roughness, lattice parameters, and local electric field.2,8,10,13−21 Although a number of excellent experimental and theoretical works have been successfully carried out in pinning down the critical parameters for tuning HIN, only a few up-scalable coating materials are available for delaying ice nucleation.13,20−25 Nature has developed its own approaches to control ice formation, i.e., antifreeze proteins (AFPs) can protect organisms from freezing damage by controlling ice formation effectively.26,27 Some recent attempts have been undertaken to © 2017 American Chemical Society

Received: July 10, 2017 Accepted: August 16, 2017 Published: August 16, 2017 30092

DOI: 10.1021/acsami.7b10014 ACS Appl. Mater. Interfaces 2017, 9, 30092−30099

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthsis of (a) PBS-diol and (b) PBS-Based Derivatives with Variable Functional Groups

2.2. Chain-Extension Reaction. Dihydroxyl-teminated PBS oligomer (PBS-diol) was prepared by esterification and polycondensation of succinic acid and 1,4-butanediol according to procedures proposed by Zheng et al.39,40 The esterification of 1,4-butanediol (780.32 g) and succinic acid (619.68 g) was carried out at 180 °C with catalyst of Ti(OBt)4 (0.28 mL) under nitrogen atmosphere. Then, the polycondensation was carried out at 230 °C, and the pressure of the reaction device was gradually reduced to 5−15 Pa. The polymerization time lasted for 5 h to obtain the PBS-diol. PBS-diols were purified by repeated reprecipitation from their chloroform solutions by methanol and dried for 24 h at 80 °C. The PBS-based derivatives were synthesized in bulk using HDI as a chain extender at 130 °C under nitrogen atmosphere for 0.5 h. For all the samples, molar ratio of −NCO to −OH was fixed at 1/1. The molar ratio of RC/PBS-diols for PBS-C and RN/PBS-diols for PBS-N were both set at 1/1. The molar ratios of RC/RN/PBS-diols were set to be 0.5/0.5/1, 1/1/1, and 2/2/1 for PBS-CN9, PBS-CN15 and PBSCN28, respectively, as shown in Scheme 1 and Table 1. When the molar ratio of RC (or RN)/PBS-diols was more than 2/1, the coating was unstable during the multiple cycle freezing test.

availability of AFPs as well as the labor-intensive surface modification method employed.30−36 The polyampholytes, containing both negative and positive charge groups on a single chain, have been identified as good AFP analogues and show high cryopreservation efficiency in the supercooled solutions.31,37,38 Matsumura et al. found that the combined effect of amino and carboxyl groups endowed polyampholytes with AFP activities, which contributed to successful cryopreservation by membrane protection.38 They also suggested that polyampholytes could be used as new materials for cryoprotective agents in various preserving functions. Mitchell et al. also synthesized an AFP inspired polyampholyte and found that the polyampholyte with the ratio of cationic to anionic groups (1:1) exhibited maximum ice recrystallization inhibiting efficiency.31,37 In this work, we prepared a series of polyampholytes under mild synthetic condition, which exhibited excellent performance of ice nucleation inhibition and freezing delay. In order to improve the coating ability, thermal stability, and processing properties of polyampholytes, the poly(butylene succinate) (PBS) prepolymer segments were incorporated into the polymer chains according to a chain-extension strategy.39,40 On the basis of simple surface-casting method, these polyampholytes will become promising candidates as anti-ice-nucleation materials into environmentally friendly applications.

Table 1. Characterization Data of PBS-Based Derivatives sample PBS PBS-C PBS-N PBS-CN9 PBSCN15 PBSCN28

2. EXPERIMENTAL SECTION 2.1. Materials and Film Preparation. Succinic acid and 1,4butanediol were purchased from Beijing Chemical Reagents Corp. The catalyst of Titanium(IV) butoxide was distilled before use. Hexamethylene diisocyanate (HDI) were purchased from Sigma-Aldrich. 2,2Bis(hydroxymethyl)butyric acid (RC) and N-methyldiethanolamine (RN) were purchased from Aladdin. The PBS-based derivative thin films were spin-coated with a chloroform solution of 10 mg/mL onto silicon substrates at 3500 rpm for 60 s. Silicon wafers were sequentially cleaned by sonication in ethanol, acetone, and ultrapure water before use. All films were dried at vacuum oven at 30 °C for 12 h to remove the residual solvent. The thicknesses of all different PBS-based derivative thin films were about 140−150 nm, which were measured by ellipsometry (Woolam Co., Inc.).

PBS-diol/RC/ RNa

Mnb

PDIb

Tm (°C)c

ΔHm (J/g)c

Tg (°C)c

1/0/0 1/1.8/0 1/0/1.6 1/0.9/0.9 1/1.5/1.5

90000 50000 53000 50000 65000

1.3 1.8 1.7 1.8 1.5

110.2 110.6 107.4 110.4 109.7

54.3 72.5 46.5 63.9 63.4

−34.5 −31.5 −33.2 −31.5 −31.1

1/2.8/2.8

48000

1.6

110.4

63.2

−36.9

a

Determined by elemental analysis. bDetermined by GPC. cDetermined by DSC. 2.3. Synthesis of Other Anti-Ice-Nucleation Coatings Based on Different Surface Modification Strategies. 2.3.1. SelfAssembled Monomers (SAMs). Mercaptododecanoic acid (MDA), mercaptoundecanesulfonate (MUS), trimethylammoniumundecanethiol (TAUDT), mercapto-oligo(ethylene glycol) (MOEG), and mercaptoundecanol (MDC) were all purchased from ProChimia and used as received without further purification. Before preparing the SAMs on the gold surfaces, the silicon wafers evaporation deposited 30093

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coupled with a high-speed camera (Phantom v7.3) and a closed cell placed atop a cryostage (Linkam THMS 600) (see in Figure S2).9 The sample cell was composed of a rubber O-ring sandwiched between two cover glasses. Because the volume of the closed cell is small enough (with a volume of 0.283 cm3), the water droplet thermodynamically equilibrates with the water vapor. Before the HIN tests, the identically sized water droplet (with a fixed volume of 1.0 μL) was placed atop the PBS-based derivative surfaces, as such the effect of heterogeneous ice nucleation (HIN) on different PBS-based derivative surfaces could be compared. The whole sample preparation process was carried out in a biosafety cabinet to minimize possible contaminants. The asprepared samples were cooled down from room temperature at different cooling rates on a cryostage with a microscope coupled with a high-speed camera. The time resolution is 1 ms, i.e., 1000 frames per second, and the temperature resolution is 0.1 °C, respectively. The temperature at the onset of ice nucleation was identified through a video recorded by the high-speed camera, which is determined as TH. The tD is defined as the time interval between the time when the PBSbased derivative surface reaches the preset temperature and that when the ice nucleation occurs. The tD is measured at target temperature (−25.0 °C) when temperature is lowered from the room temperature at a cooling rate of 5.0 °C/min. All the results of the TH and tD are the mean of more than 200 independent freezing events for each PBSbased derivative surface. The error bars are the standard errors of the mean, and the distribution of TH and tD are consolidated by the Student’s t test.

with gold were cleaned by plasma for 300 s before use. Then, the prepared gold wafers were immersed into the solution of the different functionalized thiols (MDA, MUS, TAUDT, MOEG, and MDC; 1 mg/mL) without stirring for 10 h and rinsed with ethanol and water. 2.3.2. Polymer Brushes (PBs). [2-(Methacryloyloxy)ethyl] trimethylammonium chloride (METAC, Sigma-Aldrich), 3-sulfopropyl methacrylate potassium (SPMA, Sigma-Aldrich), [2(methacryloyloxy)ethyl]-dimethyl(3-sulfopropyl)ammonium hydroxide (SBMA, Sigma-Aldrich), 2-(2-methoxyethoxy)ethyl methacrylate (OEGMA, Sigma-Aldrich), Copper bromide (CuBr, 99.999%, SigmaAldrich), 2,2′-bipyridine (bpy, 99%, Aladdin), and ethyl 2bromoisobutyrate (2-EBiB, Sigma-Aldrich) were used as received. The bromo-substituted thiol initiator was synthesized by Suzhou Institute of Nano-Tech. The polymer brushes were prepared by a typical surface-initiated atom transfer radical polymerization (SIATRP) method. Gold wafers were immersed into the solution of the thiol initiator (1 mg/mL) without stirring for 8 h and rinsed with ethanol and water. The polymer brushes of PMETA and PSPMA were synthesized with procedures similar to those in our previous work.9 The Cl− in PMETA brushes and K+in PSPMA brushes were exchanged with SO42− and Ca2+, respectively. The polymer brushes of PSBMA and POEGMA were synthesized according to the literature.41 2.3.3. Polyelectrolyte Multilayers (PEMs). Poly(ethylene imine) (PEI), poly(acrylic acid) (PAA), poly(diallyldimethylammonium chloride) (PDAD), poly(sodium 4-styrenesulfonate) (PSS), and poly(allylamine hydrochloride) (PAH) were all purchased from Sigma-Aldrich without further purification. PEMs were prepared according to the LBL deposition method. A layer of PEI was first deposited onto the substrate before the subsequent LBL deposition. Then, the as-prepared substrates were alternatively dipped in polyanion solutions (PAA and PSS, 1.0 g/L) for 30 min and polycation solutions (PDAD or PAH, 1.0 g/L). The substrates were rinsed by NaCl solutions (0.5 M) between each deposition process. Dipping and rinsing steps were alternated and repeated until the desired layers were obtained. 2.4. Characterization. 2.4.1. Proton Nuclear Magnetic Resonance Spectroscopy (1H NMR). The chemical structures of the PBSdiol were characterized by a Bruker Model DMX-400 NMR spectrometer using CDCl3 as the solvent and tetramethylsilane as the internal standard. The peaks at 1.67 (δH), 2.65 (δH), and 4.10 (δH) ppm corresponded to three different types of methylene protons in the repeating monomers of PBS-diol as shown in Figure S1. 2.4.2. Gel Permeation Chromatography (GPC). The numberaverage molecular weight (Mn) and polydispersity index (PDI) of different PBS-based derivatives were measured by a Waters Model 2695 GPC gel permeation chromatography using polystyrene as a standard. Chloroform was used as the fluent at a flowing rate of 1.0 mL/min. 2.4.3. Differential Scanning Calorimetry (DSC). The melting temperatures, glass transition temperatures, and melting enthalpy change (ΔHm) of different PBS-based derivatives were performed on a calorimeter (TA Q2000) under nitrogen atmosphere at a cooling/ heating rate of 10 °C/min. 2.4.4. Elemental Analysis. The atomic concentrations (C, N, and H) of different PBS-based derivatives were measured with a Vario EL elemental analyzer (Elementar, Germany). The molar ratios of RC and RN were estimated by the atomic concentrations (C, N, and H). 2.4.5. Atomic Force Microscope (AFM). The surface morphology and roughness were characterized by atomic force microscope (AFM). The surface morphology and roughness were conducted on AFM (Bruker Multimode 8) via “peak force tapping mode”. 2.4.6. Contact Angle (CA) Measurements. The static contact angle measurements were measured by a drop size analyzer (DSA-100, KRÜ SS, Germnay). The images were captured by an optical microscope precisely 60 s after the contact of a droplet with the different PBS-based derivatives. 2.5. Water Freezing Measurement. The HIN temperature (TH) and freezing delay time (tD) on PBS-based derivative surfaces were detected by the homemade experimental apparatus with a microscope

3. RESULTS AND DISCUSSION 3.1. Synthsis and Characterization of PBS-Based Polyampholytes. PBS-diol was prepared by esterification and polycondensation of succinic acid with 1,4-butanediol based on catalyst titanium(IV) butoxide (see Scheme 1a) similar to procedures proposed by Zheng et al.39,40 The molecular weight of PBS-diol was tuned to ∼5000 (detected by 1 H NMR) by controlling the polycondensation time. The chemical structure of PBS-diol was characterized by 1H NMR spectra (in Figure S1). The series of PBS-based derivative were designed and synthesized via melt chain-extension reaction in the presence of HDI as a chain extender at 130 °C. By regulating the feed molar ratios of PBS-diol, carboxyl-based group of RC, and amino-based group of RN, the chain-extented PBS-diol (PBS), the carboxylated PBS (PBS-C), the aminated PBS (PBS-N), and polyampholytes with random blocky carboxyl and amino groups (PBS-CN) were prepared (in Scheme 1b). Polyampholytes with different contents of RC (or RN) are designated as PBS-CNx, where x is the molar ratio between 10× RC (or 10× RN) and PBS-diol as determined by elemental analysis (in Tables 1 and S1). In order to obtain a net neutral charged state of polyampholytes, the content ratio of RC and RN in PBS-CN was controlled as 1:1 because the charge balance of polyampholytes is shown to be crucial to the antifreeze activity.37 The number-average molecular weight (Mn) and molecular weight distribution (PDI) of PBS-based derivatives were determined by GPC (in Table 1). The melting temperature (Tm) related to the crystallization of the PBS segment (Tm around 110 °C) was assessed by DSC (in Table 1 and Figure S3). The melting enthalpy values (ΔHm) of PBSCNx samples are almost the same, which are different from those of PBS, PBS-C, and PBS-N. It is suggested that the crystallinity of PBS-based derivatives are influenced significantly by the charge characteristic of polymer chain. Meantime, the glass transition temperatures (Tg) of PBS-based derivatives change when different charged groups are introduced into the PBS chains. The PBS-based derivative surfaces for ice nucleation tests were prepared via simple spin-coated method. 30094

DOI: 10.1021/acsami.7b10014 ACS Appl. Mater. Interfaces 2017, 9, 30092−30099

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Figure 1. (a) Distribution of heterogeneous ice nucleation temperature (TH) on different PBS-based derivative surfaces (PBS-C, PBS and PBSCN28). (b) TH of different PBS-based derivative surfaces. (c) Influence of cooling rate (from 1.0 °C/min to 10.0 °C/min) on the TH of different PBS-based derivative surfaces (PBS-C, PBS and PBS-CN28). The substrates are silicon wafers.

Figure S4 shows the surface morphology and average surface roughness (Ra) of PBS-based derivative surfaces detected by AFM. All the values of Ra are less than 7 nm, exhibiting relative smooth surfaces. We further studied the hydrophilicity of the PBS-based derivative surface as shown in Figure S5 and found that the static contact angle decreases as the content of RC (or RN) increases. 3.2. Ice Nucleation. Ice nucleation in the water drop started at the solid/water interface, followed by upward ice growth as shown in Figure S6. HIN of water droplets can be signified by a sudden change in opacity before and after freezing. In situ optical microscopic observation of water droplets shows that different PBS-based derivative surfaces have different effects on the HIN as shown in Figure S7. All the results on the TH were based on more than 200 independent freezing events for each PBS-based derivative surface. The distributions of TH on all PBS-based derivative surfaces approximately follow the normal distribution as shown in Figure 1a. The error bars are the standard errors of the mean, which is the standard deviation of the sample-mean’s estimate of a population mean. As shown in Figure 1b, it can be found that the TH on the PBS-C, PBS-N, PBS, PBS-CN9, PBS-CN15, and PBS-CN28 surfaces are −24.5 ± 0.6 °C, −24.8 ± 0.5 °C, −26.7 ± 0.7 °C, −27.3 ± 0.4 °C, −29.7 ± 0.4 °C, and −29.8 ± 0.5 °C, respectively, exhibiting a TH window up to 5.2 °C. The TH of water droplet on the polyampholyte surfaces is lower than that on other PBS-based derivative surfaces. Meanwhile, the TH decreases with the contents of RC or RN. The effect of the cooling rate (from 1.0 °C/min to 10.0 °C/min) on the HIN was also studied as shown in Figure 1c, showing the same trend of TH, i.e., TH,PBS‑C > TH,PBS > TH,PBS‑CN28. In order to consolidate the ability of different PBS-based derivative surfaces in tuning the HIN, we further studied the corresponding delay time (tD) at −25.0 °C. As shown in Figure 2a, the HIN induction time was significantly different on PBS-C and PBS-CN28 surfaces, e.g., water froze immediately (within 3 s) on PBS-C surface (upper row); in contrast, the water drop did not freeze until 9560 s later on PBS-CN28 surface (bottom row). The tD of water droplets on different PBS-based derivative surfaces shows the order of tD,PBS‑CN28 > tD,PBS‑CN15 > tD,PBS‑CN9 > tD,PBS > tD,PBS‑N > tD,PBS‑C as shown in Figure 2b. It is noted that the increase of the tD is consistent with the decrease of the TH (see in Figures 1b and 2b). Compared with other PBS-based derivative surfaces, the polyampholyte surfaces

Figure 2. (a) Optical microscopy images of HIN at −25.0 °C on the PBS-based derivative surfaces of PBS-C and PBS-CN28 (The scale bar is 200 μm). (b) Freezing delay time (tD) of water droplets on different PBS-based derivative surfaces. The substrates are silicon wafers.

exhibit high performance of ice nucleation inhibition and freezing delay. As stated above, TH on the PBS-CN28 surface can be wellcontrolled at relative low temperature of −29.8 ± 0.5 °C, and the water droplet is able to remain unfrozen for ∼9500 s at −25.0 °C, exhibiting good performance of inhibiting HIN. A comparison of TH on the PBS-CN28 surface with that of reported surfaces capable of tuning HIN based on different surface modifying strategies (SAMs, PBs and PEMs) is shown in Figure 3. Some works have shown that the HIN of water or aqueous NaCl droplets is strongly determined by the structure of coated monolayers.42−44 We prepared SAMs with typical 30095

DOI: 10.1021/acsami.7b10014 ACS Appl. Mater. Interfaces 2017, 9, 30092−30099

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Figure 3. Surface structures of (a) self-assembled monolayers (SAMs), (b) polymer brushes (PBs), and (c) polyelectrolyte multilayers (PEMs). (d) Comparison of TH of PBS-CN28 with different antifreeze proteins (AFPs) and other materials based on different surface modifying strategies.

Figure 4. (a) TH and (b) tD on a wide range of substrates before and after spin-coated with PBS-CN28.

(Figure 3a,d). Recently, we studied the ion-specific effect on HIN and found that the TH on polyelectrolyte brushes can be modulated by different counterions.9 By varying various counterions, the PMETA brush with SO42− has the lowest TH of −23.9 ± 0.7 °C, and PSPMA brush with Ca2+ has the

functional headgroups like carboxylic acid (MDA), sulfonic acid (MUS), quaternary ammonium (TAUDT), ethylene glycol (MOEG), and alcoholic hydroxyl (MDC) as shown in Figure 3a. The lowest TH of −25.6 ± 0.7 °C is observed on the surface of MOEG, which is higher than that on the PBS-CN28 surface 30096

DOI: 10.1021/acsami.7b10014 ACS Appl. Mater. Interfaces 2017, 9, 30092−30099

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ACS Applied Materials & Interfaces

Figure 5. Freezing of condensed water on (a) PBS-C and (b) PSS-CN28 surfaces at −16 °C for (1) 30 s, (2) 300 s, (3) 2400 s, and (4) 3100 s at humidity of 20 ± 2%. The scale bars are 50 μm and 2 mm in the main and insert figures, respectively. (c) Freezing delay times on the PBS-C and PSS-CN28 surfaces at different temperatures when the humidity is 20% ± 2%. (d) Freezing delay times on the PBS-C and PBS-CN28 coatings on different substrates at −16 °C when the humidity is 20 ± 2%.

lowest TH of −24.5 ± 0.5 °C. TH on polymer brushes of PSBMA and POEGMA are −21.5 ± 0.7 °C and −23.1 ± 0.6 °C, respectively (Figure 3b,d). Due to the labor-intensive surface-anchoring modification of SAMs and PBs, we further studied HIN on PEMs based on the simple layer-by-layer (LBL) deposition process. For PEMs of PAA/PDAD and PSS/ PAH with different outermost layers, the values of TH are all above −25.0 °C as shown in Figure 3c,d. Last, we compared the value of TH on PBS-CN28 surface with that on various AFP tethered surfaces (TH on various AFP tethered surfaces were measured in our previous study)30 and found that the TH of PBS-CN28 was higher than the TH of beetle Microdera punctipennis dzungarica (MpdAFP) and Marinomonas primoryensis (MpAFP) but lower than that of a moderately active fish AFP (type III AFP). Obviously, the PBS-CN28 surface has higher ice nucleation inhibition efficiency than the coatings of SAMs, PBs, and PEMs, exhibiting performance close to that of AFPs in inhibiting ice nucleation. More importantly, the polyampholyte of PBS-CN28 can be coated on almost all the substrates via a simple spin-coating method, forming stable HIN inhibition coatings. In fact, an effective anti-ice-nucleation coating should be able to adapt to different substrates. Therefore, we studied the HIN inhibition efficiency of PBS-CN28 coated on different substrates. Figure 4a shows the TH on various substrates such

as glass, ceramic, silicon, plastic, rubber, and metal surfaces before and after spin-coating with PBS-CN28. It is obvious that the TH decreased substantially when PBS-CN28 was coated on different substrates. To further consolidate the ice nucleation inhibition effect of PBS-CN28, we further studied the corresponding tD at −25.0 °C as shown in Figure 4b. The water drop atop almost all the substrates, except the silicon substrate, froze immediately before the temperature reached −25.0 °C as the temperature was lowered from the room temperature at a cooling rate of 5.0 °C/min. The value of tD on the silicon substrate is about 1600 s. In strong contrast, all the values of tD on the PBS-CN28 coated surfaces are more than 8000 s. It demonstrates clearly that the coating of PBS-CN28 has potential application value of inhibiting ice nucleation on a wide range of substrates. In the real world, when the temperature is lower than the dew point, plenty of freezing events of water microdroplets will form on the chilled surface, causing great difficulty in surface HIN inhibition strategy. Therefore, the test of freezing delay time on surfaces in a real environment is critical to many antiice-nucleation applications. We studied the freezing delay times on various PBS-C and PBS-CN28 coated substrates at different temperatures when the humidity was fixed at 20 ± 2%. As shown in Figure 5a, all the condensed water droplets froze in a short period (within 300 s) on the PBS-C surface at −16 °C. In 30097

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strong contrast, no ice crystals formed on the PBS-CN28 surface until water condensed continuously for almost 2400 s as shown in Figure 5b. We further measured the freezing delay times as the function of temperatures (from −12 to −20 °C) as shown in Figure 5c. The freezing delay time on PBS-CN28 surfaces increased from 400 ± 150 s to 7800 ± 2600 s with the increase of temperature from −20 to −12 °C. Meanwhile, all the freezing delay times on PBS-CN28 surfaces were longer than those on PBS-C surfaces at any temperature. The freezing delay times on PBS-C and PBS-CN28 coated glass, metal, and polymer surfaces were also investigated as shown in Figure 5d. The freezing delay times on PBS-CN28 coated surfaces were observed to be almost 10 times longer than those on the PBS-C coated surfaces. It is noted that the coating of PBS-CN28 has potential application value of tuning ice nucleation under different conditions. In future, more chemical modifications and coating processing methods will be utilized to improve the long-term stability and surface-bound property of PBS-based polyampholytes.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10014. 1 H NMR spectrum of PBS-diol; experimental apparatus; melting behaviors, surface morphology, roughness, contact angle, and elemental analysis of PBS-based derivative surfaces; optical microscopic observation of freezing of a water droplet on PBS-based derivative surfaces (PDF)

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REFERENCES

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4. CONCLUSION Inspired by AFPs, we have synthesized various PBS-based derivatives, especially the PBS-based polyampholytes with the ratio of amino to carboxyl groups (1:1). We found that the PBS-based polyampholyte coatings have the potential application value of inhibiting ice nucleation on a wide range of substrates. Compared with other labor-intensive AFP-attached surface modifications, the PBS-based polyampholytes can be coated on almost all the substrates via a simple spin-coating method. It is estimated that these PBS-based polyampholytes will become promising candidates as anti-ice-nucleation materials.

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Research Article

AUTHOR INFORMATION

Corresponding Authors

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

Jianjun Wang: 0000-0002-1704-9922 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the 973 Program (2012CB933801) and the National Natural Science Foundation of China (51436004, 21421061, 21503240, and 21574137). 30098

DOI: 10.1021/acsami.7b10014 ACS Appl. Mater. Interfaces 2017, 9, 30092−30099

Research Article

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DOI: 10.1021/acsami.7b10014 ACS Appl. Mater. Interfaces 2017, 9, 30092−30099