Article pubs.acs.org/Langmuir
Cite This: Langmuir XXXX, XXX, XXX−XXX
Superhydrophilic Anti-Icing Coatings Based on Polyzwitterion Brushes Bang Liang,†,‡ Guangyu Zhang,† Zhenxing Zhong,†,‡ Yan Huang,† and Zhaohui Su*,† †
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of Science and Technology of China, Hefei, Anhui 230026, P. R. China S Supporting Information *
ABSTRACT: In this work, an anti-icing coating based on superhydrophilic polyzwitterion brushes is reported. Poly(sulfobetaine methacrylate) (PSBMA) brushes were synthesized by surface-initiated atom-transferred radical polymerization to create superhydrophilic films on silicon wafers. The thickness of the PSBMA brushes film increased linearly with the polymerization time, and the film remained superhydrophilic in a nonassociated state when the thickness was less than ∼100 nm. DSC and FTIR analyses revealed that PSBMA contains more nonfreezable bound water than typical polyelectrolytes such as poly(sodium styrenesulfonate) and poly(2-(methacryloyloxy)ethyltrimethylammonium chloride), leading to lower ice adhesion strength than the latter two. At −20 °C, the PSBMA brushes coating demonstrated low ice adhesion strength of 60 kPa, showing a significant reduction in ice adhesion by up to ∼75% compared to uncoated silicon wafer. The optimum PSBMA layer thickness for low ice adhesion was ∼100 nm. These findings suggest that polyzwitterions are excellent candidates for anti-icing coating application.
1. INTRODUCTION Ice formation and accumulation on various surfaces often causes catastrophic consequences or enormous economic losses in many fields such as aviation, ground transportation, power transmission, communication, and refrigeration.1−3 To mitigate the icing issue, design and fabrication of anti-icing surfaces has become an important task. In addition, anti-icing surfaces have broad practical implications for vehicles, equipment, and buildings operating in Arctic conditions. Therefore, anti-icing surfaces have aroused intense interest in the past several years. Generally, active methods and passive methods are the two main strategies for preventing ice formation on the surfaces and removing ice from the surfaces after its accretion.4 The conventional active methods, including electrothermal, chemical, and mechanical approaches, are often expensive, inefficient, time and energy consuming, or detrimental to the environment. On the other hand, passive anti-icing coatings, on which icing can be delayed and suppressed or the accreted ice can be easily cleaned up by wind or gravity, have become the method of choice. Superhydrophobic coatings have been explored for this purpose.5−10 It has been demonstrated that superhydrophobic surfaces can alleviate or even eliminate ice formation by repelling overcooled water drops before freezing, which can be ascribed to relatively low adhesion force between the superhydrophobic surfaces and the water drops.5 Moreover, on superhydrophobic surfaces freezing of water drops can be significantly delayed by the air cushion located at the surface due to its insulating properties.6 However, superhydrophobicity © XXXX American Chemical Society
of the surfaces would be destroyed easily under high humidity or at high pressure.11−13 Recently, it has been demonstrated that liquid lubricants infused into porous/nonporous coatings can endow them with excellent anti-icing properties because the continuous lubricant layer at the surface can minimize ice adhesion dramatically.14−18 Unfortunately, long-term performance of this kind of coatings is an issue due to depletion of the lubricant with the removal of the ice.19 Inspired by ice skating, Wang and co-workers demonstrated that water, when paired with a hydrophilic polymer coating, is a good lubricant for fighting the icing problem, because the aqueous lubricating layer is naturally replenished from the ambient conditions,20−22 which opened a new avenue in design and fabrication of icephobic surfaces, and anti-icing coatings based on various hydrophilic materials have been explored. It has been shown that poly(ethylene glycol) can substantially reduce the adhesion strength between ice and the substrate due to its ability to hydrogen-bond water molecules that do not freeze and serve as a self-lubricating interfacial layer.23 Polyelectrolytes have been explored as icephobic materials because they are hygroscopic and can support a self-lubricating liquid water layer that does Special Issue: Zwitterionic Interfaces: Concepts and Emerging Applications Received: March 27, 2018 Revised: May 9, 2018
A
DOI: 10.1021/acs.langmuir.8b01009 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir not freeze at relatively low temperatures.21 It was further demonstrated that ice adhesion strength24 and ice propagation behavior25,26 on polyelectrolyte coatings are directly related to the ability of the counterions to coordinate water. Unlike traditional polyelectrolytes, polyzwitterions are a class of charged polymers that carry a positive and a negative charge in each repeating unit, which form a strong dipole.27 As a result, polyzwitterions are capable of forming a dense and thick quasi liquid water layer at the surface because they can interact with water molecules by not only electrostatic interactions but also dipole−dipole interactions.28 Due to this unique character, zwitterionic materials such as polymers of sulfobetaine methacrylate (SBMA)29 and carboxybetaine methacrylate (CBMA)30 have demonstrated outstanding resistance to protein adsorption and biofilm formation. However, application of polyzwitterions for anti-icing coatings has yet to be explored. In the present work, we synthesized SBMA-based zwitterionic polymer brushes on silicon wafer and investigated their antiicing performance, and demonstrated that the zwitterionic side groups are beneficial to binding nonfreezable water, which can lead to better anti-icing performance than typical polyelectrolytes.
2.4. Ice Adhesion Strength Measurement. The device for measuring the ice adhesion strength consists of a force transducer (Imada ZP-500N), a home-built cold stage and a syringe pump (LSP02−2A). Cuvettes (1 × 1 × 4 cm3) were hydrophobized by oxygen plasma treatment for 10 min followed by gas phase reaction with PFDTS at 170 °C for 48 h in a desiccator under vacuum. The cuvette was placed on the substrate on the cold stage in a closed box purged with nitrogen to avoid frost formation, and then deionized water (0.5 mL) was introduced with a syringe and the temperature was maintained at −20 °C for 1 h to freeze the water into an ice column on the substrate. The thrust meter, mounted on a syringe pump stage, can move forward and backward at a rate of 0.8 mm/s. The height between the thrust point and the ice−substrate interface was keep at ∼1 mm to avoid additional torque.23 The maximum force was recorded as the value to separate the ice column from the substrate surface.16,20−22 Ice adhesion strength for each coating was measured three times. 2.5. Characterizations. Dried samples were obtained by heating the polymers in a vacuum oven at 100 °C for 24 h. Contents of different types of water were calculated as mass percentages with respect to the dried polymer. Freezing temperatures and contents of different water in polymer films were characterized by differential scanning calorimetry (DSC) performed on a TA DSC Q100. In this process, a dried sample was placed in a hermetic aluminum crucible, to which a certain amount of water was added with a microsyringe. Then, the sample was tightly sealed to prevent the water from escaping. Each sample was maintained at 29 °C for 3 h to attain an equilibrium state of absorption, and the water content in the sample is simply
2. EXPERIMENTAL SECTION 2.1. Materials. Sulfobetaine methacrylate (SBMA), copper(I) bromide, 2,2′-bipyridine (BPY), 2-bromoisobutyryl bromide, and (3aminopropyl)trimethoxysilane were purchased from J&K Chemical Co., Ltd. Ethyl-2-bromoisobutyrate and 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFDTS) were obtained from Aladdin Chemical Reagent Co., Ltd. Ethanol, tetrahydrofuran (THF), and triethylamine were purchased from Beijing Chemical Reagents Company. Copper(I) bromide was stirred in acetic acid at room temperature for 10 h, suction-filtered, rinsed with absolute ethanol, and dried under vacuum at 100 °C for 4 h. Triethylamine was stored with 4 Å molecular sieves for 24 h to remove residual water. The initiator, 2bromo-2-methyl-N-[3-(trimethoxysilyl)propyl]propanamide (BrTMOS), was synthesized following a literature procedure31 (SI, Figures S1 and S2). Water used in all experiments was produced by a PGeneral GWA-UN4 purification system (18.2 MΩ·cm resistivity). 2.2. Immobilization of BrTMOS on Silicon Substrate. Silicon wafers (ca. 2 × 2 cm2) were immersed in a boiling piranha solution (7:3, 98% H2SO4:30% H2O2 mixture) for 1 h, and then removed and rinsed with a copious amount of water and blown dry with nitrogen. Cleaned silicon substrates were immersed in a THF solution of the BrTMOS (40 mmol/L) for 24 h without stirring at room temperature, followed by rinsing with THF, ethanol, and water thoroughly, and were dried in a nitrogen stream. 2.3. Surface-Initiated Atom Transfer Radical Polymerization (SI-ATRP) of SBMA. A silicon substrate with the initiator immobilized was placed in a 100 mL Schlenk flask, which was sealed with a rubber septum. Then the flask was purged with nitrogen for three times and maintained under a nitrogen atmosphere. SBMA (5.431 g) was added to a 100 mL Schlenk flask containing 20 mL of water/methanol mixture (1/3, v/v) and a magnetic stirrer, and the solution was degassed by purging nitrogen for 15 min. BPY (242.89 mg), CuBr (111.54 mg) was dissolved in 16 mL of water/methanol mixture (1/3, v/v) in another 100 mL Schlenk flask, and the solution was degassed by purging nitrogen for 15 min, and then transferred to the flask containing SBMA to make a solution with a SBMA/CuBr/BPY molar ratio of 25/1/2 and a SBMA concentration of 0.54 M. The mixture was sealed with a rubber septum and bubbled with nitrogen for 15 min. Finally, the solution was transferred to the flask containing the silicon substrate via a cannula, and the polymerization was allowed to proceed at 60 °C for a given time, and terminated by opening the rubber septum. Then the silicon substrate was rinsed again with the water/methanol mixture at 60 °C, and dried with nitrogen.
Ca =
Wa × 100% WP
(1)
where Wa and WP are the mass of the added water and the dried polymer, respectively. For each sample, the cooling and heating cycle was repeated three times to ensure the reproducibility of the DSC measurements. After the runs, each crucible with sample was weighed again to make sure that the weight loss was less than 0.1% in the entire process. The content of the nonfreezable bound water in the sample is calculated by
Cnf =
Wa − ΔH /ΔH0 × 100% WP
(2)
where ΔH is the measured melting endotherm of water in the sample, and ΔH0 the melting enthalpy of water, which is 333.5 J/g.32 FTIR spectra were acquired on a Nicolet 6700 spectrophotometer at 4 cm−1 resolution averaged over 32 scans. A gauged amount of water was added to the dried polymer followed by equilibrating for 3 h at room temperature in a sealed Petri dish, and then excess water was carefully removed from the sample. The water content in the polymer film was calculated by eq 1 before the sample was analyzed by FTIR. NMR spectra were recorded on an AVANCE III HD500 spectrometer equipped with a 4 mm MAS probe. Water contact angles of all samples were measured on a Ramé-Hart 200-F1 standard goniometer at room temperature with ultrapure water (5 μL) as the probe fluid. The thickness of the brush on silicon wafer was measured by ellipsometry (MM-16, France, HORIBA Jobin Yvon). The incident angle of the ellipsometry was 70° and the wavelength range used for fitting was from 430 to 825 nm. The morphologies of the films were characterized with atomic force microscopy (AFM) on a Bruker Nanoscope IIIA scanning probe microscope at tapping mode using Si tips (radii
