Facile construction of stable hydrophobic surface via covalent self-assembly of silane-terminated fluorinated polymer

Applied Surface Science 507 (2020) 145138

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Facile construction of stable hydrophobic surface via covalent self-assembly of silane-terminated fluorinated polymer

T

Mingliang Peia, Lixia Huob, Xiaomei Zhaoa, Shuzhen Chena, Jiaxin Lia, Zixin Penga, ⁎ ⁎ ⁎ Kaifeng Zhangb, , Hui Zhoub, , Peng Liua, a

State Key Laboratory of Applied Organic Chemistry and Laboratory of Special Function Materials and Structure Design of the Ministry of Education, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China b Key Laboratory of Science and Technology on Vacuum Technology and Physics, Lanzhou Institute of Physics, Lanzhou, Gansu 730010, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Stable hydrophobic surface Covalent self-assembly Silane-terminated fluorinated polymer Surface-attached polymer Chain transfer free radical polymerization

A facile strategy was developed to construct stable hydrophobic surface via the covalent self-assembly of silaneterminated fluorinated polymer as surface-attached polymer, which was synthesized by the free radical polymerization of 2-(perfluorohexyl)ethyl methacrylate (FOL) with γ-mercaptopropyltrimethoxysilane (MPS) as chain transfer agent. The synthesis condition for the silane-terminated fluorinated polymer (f-PFOL) and coating condition for the covalent hydrophobic coating on the Silicon (Si100) wafer were optimized with the static contact angle analysis, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) measurements. The surface-free energies (γs) was stable at around 9.5 mJ/m2 after washing off the non-functionalized poly(2-(perfluorohexyl)ethyl methacrylate) (PFOL), with the f-PFOL/PFOL mixture solution concentration higher than 0.045 g/mL. The proposed approach provides a universally applicable coating technique for the stable hydrophobic surface on various substrates.

1. Introduction Hydrophobic surfaces, which are defined with a static water contact angle above 90°, show many promising potentials, such as anti-wetting, self-cleaning, antifogging and anti-icing applications [1]. The surfacecoating technique has been recognized as the facile and efficient method to construct the hydrophobic surfaces [2], by coating the low surface free-energy materials (fluoropolymers [3] or organosilicon compounds [4]) on various solid substrates. However, also because of the low surface free-energy, such materials were coated onto the solid surfaces with very weak interfacial interaction, which usually declines the durability of the resultant hydrophobic coatings [5–7]. Covalent binding might be a potential method to solve the above problem, in which the low surface free-energy materials are expected to be immobilized onto the solid substrates via stable covalent bond. The fluoroalkylsilanes are the typical sample for the durable hydrophobic surface via surface covalent self-assembly [8–12]. However, a much lower surface fluorine concentration could be achieved due to their short fluorocarbon chains, limiting their application. “Grafting from” strategy has been developed for the durable fluorocarbon hydrophobic coatings, by surface-initiated polymerization (SIP) of fluorinated monomers from the initiator-immobilized surfaces. ⁎

Jeyaprakash et al fabricated the fluoropolymer brushes on the SiO2 surfaces via the photo-surface-initiated radical polymerization of heptadecafluorodecyl acrylate from the benzophenone silane-modified surface [13], in which the thickness of the surface-attached fluorocarbon polymer film could be controlled by adjusting the polymerizing time. The fluorinated copolymer brush-immobilized nano-imprinted surfaces were also fabricated via the surface-initiated atom transfer radical polymerization (SI-ATRP) technique [14,15]. Therefore, a synergetic effect of the surface geometry and the low surface free energy was achieved, due to the maintained surface geometry with a sufficiently thin polymer layer. Compared with the complex procedure and limited application of the SIP method, the surface coating method is easily handled method to coating the coupling agent-containing fluorocarbon polymers onto the solid surfaces. In the approach, the synthesis of the coupling agentcontaining fluorocarbon polymers is much simple and facile. Furthermore, the durable fluorocarbon hydrophobic coatings could be constructed in a large area or patterned region. For the purpose, Chen and Kim designed poly(3-(trimethoxysilyl) propyl methacrylate) grafted poly(vinylidene fluoride) (PVDF) by graft polymerization of with PVDF as macroinitiator [16]. After solution coating, small particles were found because of the polycondensation of the massive silane

Corresponding authors. E-mail addresses: [email protected] (K. Zhang), [email protected] (H. Zhou), [email protected] (P. Liu).

https://doi.org/10.1016/j.apsusc.2019.145138 Received 23 August 2019; Received in revised form 13 December 2019; Accepted 19 December 2019 Available online 23 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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2. Experimental section

groups in the grafted side chains. Wehbi et al synthesized PVDF-based copolymer with pendant trialkoxysilane groups via a radical copolymerization of vinylidene fluoride with functional 2-trifluoromethyl acrylate cyclic carbonate, followed by introduction of silane pendant groups [17]. A satisfactory adhesion onto steel was achieved due to the formation of the strong Si–O–metal bonds. Yu et al synthesized a series of polymeric silane-based water repellent agents by the radical copolymerization of various fluorinated monomers with 3-trimethoxysilylpropyl methacrylate (TSMA), for the durable superhydrophobic polyester fabrics [18]. Different from the reported fluorocarbon polymers with more silane coupling side groups [16–18], the end silane-functionalized polymer is expected to favor the orientation of the self-assembled fluorocarbon polymer brushes, as the fluoroalkylsilanes [8]. Furthermore, the polycondensation of silane groups could also be avoided due to the lower silane group content. Chain transfer free radical polymerization is a well-established technique to design the end-functionalized polymer [19] or solid surface modification [20–22], mainly due to the weak S–H bond and the high reactivity of the thiyl radicals [23]. In the present work, silane-terminated fluorinated polymer (f-PFOL) was synthesized via the free radical polymerization of 2-(perfluorohexyl)ethyl methacrylate (FOL) with γ-mercaptopropyltrimethoxysilane (MPS) as chain transfer agent (Scheme 1(a)). After coating the solution of the resultant product (containing silane-terminated fluorinated polymer (f-PFOL) and non-functionalized poly(2(perfluorohexyl)ethyl methacrylate) (PFOL)) on the solid substrate such as hydroxylated single-crystal silicon (Si-100) wafer, the f-PFOL, as attachable polymer, could covalently self-assembled onto the Si-100 surface during heating (Scheme 1(b)). The surface-free energies (γs) was stable at around 9.5 mJ/m2 after washing off the non-functionalized PFOL with CFC-113. The proposed approach, combining the facile chain transfer free radical polymerization and surface covalent selfassembly, provided a universally applicable coating technique for the durable hydrophobic surface on various substrates.

2.1. Materials and reagents Polished single-crystal silicon (Si-100) wafers were obtained from the 46th Research Institute of China Electronics Technology Group Corporation (CETC), Tianjin, China) about 0.28 mm thick and Sailing Plate Glass Company. Trifluorooctyl methacrylate (FOL, 98%) was obtained from ShangFu Science and Technology Ltd and washed 1% NaOH solution. 3Mercaptopropyltrimethoxysilane (MPS, 97%) was purchased from J&K Scientific Co. Ltd., Beijing, China. Azobisisobutyronitrile (AIBN) was bought from Tianjin Chemicals Ltd. Co. Tianjin, China, and re-crystallized in ethanol. Benzotrifluoride (AR, ˃99%) and 1,1,2-trifluorotrichloroethane (CFC-113, 99.5%) were purchased from Aladdin reagent company. Column chromatography silica gel (200–300 meshes) was used as received from Qingdao Marine Chemical Plant Branch. All other reagents were analytical reagent grade from Tianjin Chemical Reagent Co. Ltd., Tianjin, China.

2.2. Synthesis of fluorinated polymers The fluorinated polymers were prepared by the facile free radical chain-transfer polymerization of FOL with MPS as chain transfer agent, as following: 5.0 mL of FOL (7.75 g, 0.018 mol), AIBN (0.075 g, 0.1% w/w ratio of FOL), and different amounts of MPS were dissolved into 20 mL of benzotrifluoride (Table 1). The mixture solution was freezethawed in nitrogen for three times and then heated at 70 ℃ for 24 h. The product was collected via precipitation in methanol and dried in vaccum. The PFOL was separated from the f-PFOL by chemical adsorption of f-PFOL onto the silica gel. Generally, the polymerization products (2.0 g) were dissolved into 30 mL of benzotrifluoride and then added into 200 mg of column chromatography silica gel (200–300 meshes), stirring at 80 °C for 8 h. After removal the silica gel by filtration, the filtrate, which contained only PFOL, was precipitated in methanol. The separated PFOL was dried in vacuum at 40 °C.

Scheme 1. Synthesis of the fluorinated polymer via the chain transfer free radical polymerization (a) and the surface covalent self-assembly of f-PFOL on the hydroxylated silicon wafers (b). 2

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Table 1 Synthesis condition and some key parameters of the fluorinated polymers. Samples

PFOL f-PFOL/PFOL-1 f-PFOL/PFOL-2 f-PFOL/PFOL-3

Polymerization condition

Product parameters

FOL (mL)

MPS (mL)

f-PFOL content (%)

ƞ of PFOL

5.0 5.0 5.0 5.0

0 0.058 0.116 0.232

0 48.5 43.5 41.0

0.130 0.115 0.109 0.108

υC-H 2970

υC-F

υC=O 2848

2.3. Construction of stable fluorocarbon hydrophobic coatings

PFOL f-PFOL/PFOL

The surface hydroxylated Si-100 wafers were used as the substrate for the stable hydrophobic fluorocarbon coatings. The Si-100 wafers were washed successively with acetone, ethanol and water for 30 min under ultrasonic condition respectively and rinsed with ethanol and dried in a stream of N2 gas. Then, they were heated in piranha solution (containing H2SO4:H2O2 (30%) = 3:1v/v) at 80 ℃ until no bubbling [24]. Finally, the hydroxylated Si-100 wafers was obtained by washing with water and ethanol. The stable fluorocarbon hydrophobic coatings were then constructed by the surface covalent self-assembly of the f-PFOL as attachable polymer onto the Si-100 surface during heating. The fluorinated polymers with different viscosities was dissolved into 10 mL of mixture solution (perfluorohexane:ethyl pentafluoropropionate = 3:1) to obtain a mixture solution containing f-PFOL and PFOL. The solution was coated onto the surface hydroxylated Si-100 wafers with an amount of 0.020 µL/mm2, which was the threshold value for fully coating the wafers. The coated wafers were heated at 40 ℃ for 10 min. The static contact angle was measured before and after washing the PFOL off with excessive amount of CFC-113 and dried at room temperature. To verify the applicability of the proposed stable hydrophobic coating on the surface of the other substrate, the stainless steel plate (9Cr18, Ra < 0.2 nm) was used as an example, via the surface covalent self-assembly of the f-PFOL as abovementioned.

4000

3200

1738 1234

2400 1600 Wavenumber (cm-1)

1144

710

800

Fig. 1. FTIR spectra of PFOL and f-PFOL/PFOL-1 mixture.

3. Results and discussion 3.1. Synthesis and characterization of fluorinated polymers After the free radical polymerization of FOL with MPS as chain transfer agent, a mixed product was obtained, containing f-PFOL and PFOL (Scheme 1(a)). Its FT-IR spectrum was shown in Fig. 1. The absorbance peaks at 2970 and 2848 cm−1 of stretching vibration of C-H group, 1738 cm−1 of stretching vibration of C = O group, 1234, 1144, and 710 cm−1 of the rocking and wagging vibrations of C-F group could be seen [27], like the PFOL synthesized without MPS. The characteristic peak of MPS could no be found because of the low content and weak absorbance. To adjust the molecular weight of the fluorinated polymer as well as the f-PFOL content in the products, MPS was added with different amounts, as summarized in Table 1. The molecular weight of the fluorinated polymer was evaluated with the inherent viscosity of the PFOL. The f-PFOL content in the products was calculated from the PFOL content by the gravimetric method, after separating PFOL from f-PFOL by the chemical adsorption on silica gel. In the free radical polymerization of FOL without MPS, only PFOL was obtained with inherent viscosity (ƞ) of 0.130 dL/g. With the amount of MPS increasing from 0.058 mL to 0.232 mL, the f-PFOL content in the f-PFOL/PFOL mixture decreased from 48.5% to 41.0%, and the inherent viscosity of the separated PFOL also decreased from 0.115 to 0.108 dL/g, all values were lower than the PFOL synthesized without MPS (Table 1). The results demonstrated that the molecular weight of the fluorinated polymers decreased with increasing the MPS amount in the polymerization.

2.4. Analysis and characterization The inherent viscosity of PFOL was determined with an Ubbelohde viscometer (size 0.47 mm) with concertation of 0.5 g/dL in trifluoroacetic acid solution at 25 ± 0.1 °C (n = 5, RSD < 3%) [25]. X-ray photoelectron spectroscopy (XPS) analysis was performed on a VG Scientific ESCALAB 250Xi-XPS photoelectron spectrometer (Thermo Fisher-VG Scientific, USA) with Al Kα as the sputtering source at 12 kV and 6 mA. During analysis, the pressure in analyzer chamber, total spectral energy and step size were set at 10−8 Pa, 100 eV, and 1 eV, respectively. For calibration, the binding energy of C1 s was set at 284.7 eV. Curve fittings and quantification of spectra were measured by using Thermo Scientific Avantage software. All the measurements were conducted at a room temperature (25 ± 1 °C). Static contact angle analysis was carried out on a Tantec CAM-Micro Contact Angle Meter equipped with microinjectors and flat needles, with 2.0 μL of water or diiodomethane (DIM), (n = 5, RSD < 3%). The surface free-energy (γs) was calculated from the contact angles of the two solvents (water and DIM) using the Owens-Wendt-Rabel-Kaelble method [26]. AFM images were acquired with an MFP-3D Origin (Asylum Research, USA) working in contact mode. The scans were performed on areas of 2.5 × 2.5 nm2 collecting images with 256 × 256 pixels, and 12 × 12 nm2 collecting images with 512 × 512 pixels. The mean roughness (Ra) and root mean square roughness (Rq) were calculated from different sets of AFM images (n = 5, RSD < 3%).

3.2. Effect of f-PFOL/PFOL mixture on stable fluorocarbon hydrophobic coatings on Si-100 wafers The fluorocarbon hydrophobic coatings were constructed with the hydroxylated Si-100 wafers as substrate, by coating the solution (0.010 g/mL) of the f-PFOL/PFOL mixtures prepared with different MPS amounts. The static contact angle and surface free-energy (γs) of the coatings with the four fluorinated polymers were compared in Fig. 2. The as-coated Si-f-PFOL/PFOL films showed the static contact angles of about 113° and 98° for water and DIM for all the four samples (Fig. 2 a and b). After heating at 100 °C for 10 min, the static contact angles remained unchanged. However, the static contact angles of the PFOL coatings distinctly decreased to 30° and 45° after washing with 3

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150

a

before heating after heating after washing with CFC-113

before heating after heating after washing with CFC-113

100

90 60

80 60 40

30 0

b

120

CA ( o )

CA ( o )

120

140

20

L/ f- P FO

-3 -2 -1 PF O L O L / PF O L O L /P FO L P F O L f-P F f-P F

γs (mJ/m 2)

60

s)

0

L f-P F O

L -1 L -3 O L L -2 / P F O F O L /P F O F O L /P F O PF P P ff-

c before heating after heating after washing with CFC-113

45 30 15 0

L f-P F O

L -1 O L -2 O L -3 F O L /PFO L/ P F P F O L / P F P O F P ff-

Fig. 2. The static contact angles of the coated silicon wafers at a concentration of 0.010 g/mL, measured by sessile drop method with water (a) and DIM (b) and the calculated surface free-energy (c), respectively (mean ± S.D. (n = 3)).

same concentration. The results indicated that heating treatment of the coatings favors the low surface free-energy, due to the reorganization of surface composition during the thermal treatment [28]. As for the DIM contact angle, the value increased from 93° and 95° to 99° and 100° for the as-prepared f-PFOL/PFOL-1 coatings and heated f-PFOL/PFOL-1 coatings, respectively (Fig. 3b). The DIM contact angle the f-PFOL-1 coatings exhibited an obvious increase with increasing the concentration of the f-PFOL/PFOL mixture to 0.045 g/mL, similar as the water contact angle. It was resulted from the removal of the free PFOL, which led to a low fluoropolymer amount coated with the low concentration solution. The calculated values of the surface free-energy (γs) were compared in Fig. 3c. The surface free-energy decreased obviously after heating at 100 °C for 10 min, due to the reorganization of surface composition [28]. Furthermore, with the low concentration, the as-coated f-PFOL/ PFOL-1 coatings and the f-PFOL-1 coatings after washing with CFC-113 showed high surface free-energy, both values decreased with increasing the concentration of the f-PFOL/PFOL-1 mixture until 0.045 g/mL. At a concentration ≥ 0.045 g/mL, the surface free-energy was stable at around 9.5 mJ/m2 for the coatings before and after heating and washing with CFC-113. The result indicated that the Si100 surface had been completely covered with the self-assembled fluoropolymer brushes, therefore a stable low free-energy surface could be obtained with the f-PFOL/PFOL-1 mixture at a high concentration of ≥ 0.045 g/mL. Further increasing the concentration of the f-PFOL/PFOL-1 mixture, the excess f-PFOL was washed off with the free PFOL and only the perfect self-assembled fluoropolymer film was remained, as a result, stable surface free-energy was achieved.

CFC-113, while the static contact angles of the three f-PFOL/PFOL coatings decreased slightly. The results indicated that the f-PFOL in the f-PFOL/PFOL mixtures had covalently self-assembled onto the Si-100 surface, which could not be washed off with the good solvent of PFOL, CFC-113. Therefore, higher values of the static contact angles were almost remained. The surface free-energy (γs) of the coating was compared in Fig. 2c. The value of the PFOL coating increased distinctly from 10 to 63 mJ/ m2, while the values of the three f-PFOL/PFOL coatings increased slightly. Furthermore, it was found that the amplification increased from f-PFOL/PFOL-1 to f-PFOL/PFOL-3. It should be resulted from the decreased f-PFOL content and their decreased molecular weight with increasing the amount of MPS as chain transfer agent in the free radical chain-transfer polymerization. So, the f-PFOL/PFOL-1 was selected for the further investigation. 3.3. Effect of f-PFOL/PFOL-1 concentration on stable fluorocarbon hydrophobic coatings on Si-100 wafers The static water and DIM contact angles of the f-PFOL/PFOL-1 coatings were recorded in Fig. 3 a and b, by coating the f-PFOL/PFOL-1 mixture with different concentrations but the same amount of 0.020 µL/mm2. The water contact angle of the f-PFOL/PFOL-1 coatings increased with increasing the concentration of the f-PFOL/PFOL mixture, from 0.005 to 0.035 g/mL and hold steady afterward (Fig. 3a). After heating at 100 °C for 10 min, a water contact angle of 111° was obtained, much higher than before heating of 85°, at a low concentration of 0.005 g/mL. The water contact angle increased slightly to 117° with increasing the concentration of the f-PFOL/PFOL mixture to 0.060 g/mL. After washing off the PFOL, the f-PFOL-1 coatings showed the similar tendency as the f-PFOL/PFOL-1 coatings before heating, but the water contact angle was slightly higher than the later ones at the

3.4. Characterization of stable f-PFOL-1 coatings on Si-100 wafers The stable f-PFOL-1 coatings were then characterized with the XPS 4

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140

a

120

120

100

60 40

CA (o)

CA ( o )

100 80

before heating after heating after washing with CFC-113

80 60 before heating after heating after washing with CFC-113

40 20

20 0

b

0

05 01 15 02 03 35 04 45 05 55 0 6 0.0 0. 0.0 0. 0. 0.0 0. 0.0 0. 0.0 0. Concentration (g/mL)

24

c

21

0 .0

05 0.01 .015 0.02 0.03 .035 0.04 .045 0.05 .055 0.06 0 0 0 0 Concentration (g/mL)

before heating after heating after washing with CFC-113

γs (mJ/m2)

18 15 12 9 6 3 0 0.0

05 0.01 .015 0.02 0.03 .035 0.04 .045 0.05 .055 0.06 0 0 0 0 Concentration (g/mL)

Fig. 3. The static contact angles of the coated silicon wafers at different concentrations, measured by sessile drop method with water (a) and DIM (b) and the calculated surface free-energy (c), respectively (mean ± S.D. (n = 3)).

grafted on the Si100 surface were collapsed with a low grafting density, but oriented vertically at a high concentration of the f-PFOL/PFOL mixture owing to the high grafting density of the f-PFOL brushes, forcing the brushes to perpendicularly stretch away from the surface to avoid overlapping [28,29], as shown in Scheme 1(b). The 2D and 3D topography images of the hydroxylated silicon wafers before and after coated with the silane-terminated fluorocarbon polymer at various concentrations are shown in Fig. 5. The mean roughness (Ra) and root mean square roughness (Pq) were calculated and summarized in Table 3. The Ra and Rq of the Si-f-PFOL surface decreased over a scanning range of 2.5 μm × 2.5 μm after the selfassembly and increasing the f-PFOL/PFOL concentration, although both the Ra and Rq over a scanning range of 12 μm × 12 μm fluctuated around the values of the hydroxylated silicon wafers. Such results demonstrated a perfect self-assembled monolayer of f-PFOL could be obtained with a high f-PFOL/PFOL concentration of 0.045 g/mL [30].

and AFM technique. In the XPS analysis, the surface O atomic concentration increased while that of Si decreased, demonstrating the successful hydroxylation (Fig. 4a). The stable f-PFOL coatings prepared with different concentrations of the f-PFOL/PFOL mixture exhibited obvious F1s signal at 687 eV, S2p at 167 eV, and C1s (C-F) signal at 290 eV (Fig. 4b) [13]. With increasing the concentration of the f-PFOL/ PFOL mixture, the surface atomic concentration of F and C increased while those of O, Si, and S decreased (Table 2), revealing the silaneterminated fluoropolymer (f-PFOL) had been successful self-assembled onto the Si100 surface. The higher surface Si concentration in the stable f-PFOL-1 coatings prepared with the lower concentrations of the fPFOL/PFOL mixture demonstrated that the surface stable f-PFOL-1 coatings was < 10 nm in such cases, due to the collapsed fluoropolymer chains [28]. With a high f-PFOL/PFOL concentration of 0.045 g/mL, the surface Si concentration in the stable f-PFOL-1 coating was only 8.14%, meaning a thicker fluoropolymer film was resulted with a higher concentration of the f-PFOL/PFOL mixture, in which the fluoropolymer brushes in the self-assembled f-PFOL monomolecular layer should orient vertically (or at an angle) at the interface between the Si100 wafer and air with their functional silane end-groups covalently immobilized onto the Si100 wafer, owing to the high grafting density. The S atom, introduced by the chain transfer reagent MPS, was located at the functional silane end-groups in the f-PFOL. The surface S atomic concentration decreased with increasing the concentration of the f-PFOL/PFOL mixture, also indicating the covalent self-assembly of the f-PFOL on the Si100 wafer and the orientation of the fluoropolymer brushes. All the results demonstrated that the fluoropolymer brushes

3.5. Stable fluorocarbon hydrophobic coatings on another substrate To further reveal the applicability of the proposed strategy, the stable fluorocarbon hydrophobic coatings were fabricated on the stainless steel plates with the similar procedure as on the Si-100 wafer, except that the stainless steel sheets were used as received without any treatment. For the alloy sample, a widely used raw material for machine elements, the static water and DIM contact angles of the coatings were 117° and 95° after coating the f-PFOL/PFOL mixture (Fig. 6a), respectively. The surface free-energy was calculated to be about 10.5 mJ/m2. 5

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(a) O1s C1s

Si2p Si2s

Si Si-OH

1200

(b)

F1s

Si-f-PFOL0.02 F Auger

1000

800 600 400 Binding energy (eV)

O1s

C1s

S2p Si2s

200

0

Si-f-PFOL0.02

Si-f-PFOL0.03

Si-f-PFOL0.03

Si-f-PFOL0.045

Si-f-PFOL0.045

1200

1000

800 600 400 Binding energy (eV)

200

0

310

300

C1s(C-F) C1s(C-C/C-H)

290 280 Binding energy (eV)

270

Fig. 4. XPS survey spectra of the pristine Si-100 wafer and Si-OH (a), Si-f-PFOL0.02, Si-f-PFOL0.03 and Si-f-PFOL0.045 coatings prepared with the f-PFOL/PFOL concentration of 0.020, 0.030 and 0.045 g/mL, respectively (b).

fabrication of stable hydrophobic surface. After the optimization of the polymerization condition and the coating amount, a stable low surfacefree energies around 9.5 mJ/m2 was achieved for the covalent hydrophobic coating on the Silicon (Si100) wafer after washing off the nonfunctionalized poly(2-(perfluorohexyl)ethyl methacrylate) (PFOL) and the excess silane-terminated fluorinated polymer (f-PFOL), with the fPFOL/PFOL mixture solution concentration higher than 0.045 g/mL. The self-assembled f-PFOL monolayers with different f-PFOL/PFOL solution concentrations were characterized with XPS and AFM techniques, and the possible mechanism was also proposed as following: low concentration of the f-PFOL/PFOL mixture resulted the collapsed fluoropolymer brushes with a low grafting density, while a high concentration of the f-PFOL/PFOL mixture led to the stretched fluoropolymer brushes with a high grafting density. The developed method is expected as a facile and efficient approach for the fabrication of the stable hydrophobic surface on various substrates.

Table 2 Surface atomic concentrations of the Si-f-PFOL coatings prepared with different f-PFOL/PFOL concentrations. Samples

Si Si-OH Si-f-PFOL0.02 Si-f-PFOL0.03 Si-f-PFOL0.045

Surface atomic concentration (%) O

Si

F

C

S

24.48 27.39 20.33 13.52 7.22

40.46 30.97 43.08 26.34 8.14

– – 26.53 35.62 49.82

35.06 41.63 2.82 20.06 33.12

– – 7.23 4.45 1.71

After heating and even washing with CFC-113, no obvious variation was found in the contact angles. After washing off the free PFOL molecules, the hydrophobic surface was well maintained. It demonstrated the successful surface covalent self-assembly of the f-PFOL on the stainless steel plates [31]. On the contrary, there was no hydrophobic coatings on the PFOL coated stainless steel plates after washing with CFC-113 (Fig. 6b).

Author contributions PL conceived and designed the study. MLP, LXH, XMZ, SZC, JXL, and ZXP performed the experiments. PL, MLP, KFZ and HZ analyzed the data. PL wrote the paper. All authors read and approved the manuscript.

4. Conclusions In summary, silane-terminated fluorinated polymer was synthesized via the facile free radical polymerization of 2-(perfluorohexyl)ethyl methacrylate (FOL) with γ-mercaptopropyltrimethoxysilane (MPS) as chain transfer agent and used as surface-attached polymer for the 6

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Fig. 5. 2D and 3D topography images of the hydroxylated silicon wafers before and after coated with fluorocarbon polymer at various concentrations.

Table 3 Mean roughness (Ra) and root mean square roughness (Rq) of the hydroxylated silicon wafers before and after coated with fluorocarbon polymer at various concentrations. Samples

Si-OH Si-f-PFOL0.020 Si -f-PFOL0.030 Si -f-PFOL0.045

Mean roughness (Ra, nm)

Root mean square roughness (Rq, nm)

Scansize:2.5 nm

Scansize:12 nm

Scansize:2.5 nm

Scansize:12 nm

0.596 0.527 0.421 0.300

0.873 1.023 0.859 0.867

0.732 0.668 0.514 0.380

1.236 1.277 1.573 1.143

7

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Fig. 6. The static contact angles and calculated surface free-energy of the f-PFOL/PFOL-1(a) and PFOL (b) coated stainless steel plates at a concentration of 0.045 g/ mL, measured by sessile drop method with water and DIM, respectively (mean ± S.D. (n = 3)). Note: 1, 2 and 3 represents the as-prepared coating, after treating at 100℃ for 10 min and after washing with CFC-113 solution, respectively.

Declaration of Competing Interest

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