Formation of plasma-polymerized superhydrophobic coating using an atmospheric-pressure plasma jet

Thin Solid Films 675 (2019) 34–42

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Formation of plasma-polymerized superhydrophobic coating using an atmospheric-pressure plasma jet

T

Md. Mokter Hossaina, Quang Hung Trinhb, Duc Ba Nguyena, M.S.P. Sudhakarana, ⁎ Young Sun Moka, a b

Department of Chemical and Biological Engineering, Jeju National University, Jeju 63243, Republic of Korea Center for Advanced Chemistry, Institute of Research and Development, Duy Tan University, 03 Quang Trung, Da Nang, Viet Nam

A R T I C LE I N FO

A B S T R A C T

Keywords: Superhydrophobic coatings Tetramethylsilane 3-aminopropyl(diethoxy)methylsilane, plasma jet Thin film

The conditions for the deposition of stable superhydrophobic coatings on glass substrates using a dielectric barrier discharge plasma jet with tetramethylsilane (TMS) and 3-aminopropyl(diethoxy)methylsilane (APDMES) as precursors were investigated. The coatings, which were formed under different plasma conditions, were characterized by various methods including atomic force microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, static water contact angle (WCA) and sliding angle measurement, and a scratch test. The results presented that superhydrophobic and mechanically stable plasma coatings could be obtained by optimizing the treatment time, applied voltage, gas flow rate into the plasma chamber, and APDMES/TMS ratio. The results indicated that the use of TMS on its own as the coating precursor led to the formation of unstable coatings. However, the mechanical stability increased significantly when APDMES and TMS were used in combination. As a result, a durable coating layer with a WCA of 163° and sliding angle of 3° was achieved for an APDMES/TMS ratio of 1.7.

1. Introduction Non-thermal plasma polymerization processes are of considerable interest because they can be used to effectively produce protective or hydrophobic coating layers on the surfaces of various materials such as glass, fabric, powder, and polymers [1–9] at low cost. The surface wettability is determined by the measurement of the water contact angle (WCA) and sliding angle. Surfaces of which the WCA exceeds 150° and the sliding angle is < 10° are known to be superhydrophobic surfaces that are highly repellent to water [10–13]. Surfaces with these properties have found use in many applications such as self-cleaning windshields [14–18], anti-contamination surfaces [19,20], snow repelling surfaces for windows and antennas [21,22], anti-biofouling paints for boats [23–26], and anti-icing [27,28] and anti-corrosion surfaces [29–32]. The wettability of a solid surface is a property that depends on both the surface roughness and surface chemistry [33–35], and is directly related to the surface free energy. Basically, materials with low surface energies are used to prepare superhydrophobic surfaces. Many research groups have attempted to fabricate durable superhydrophobic surfaces on metals, organic, or inorganic substrates

⁎

[36–43]. Long-lasting hydrophobic coatings are not easily achievable by using only materials with low surface energy, mainly because of their poor adhesion to solid surfaces. Materials with a low surface energy such as hexamethyldisiloxane (O[Si(CH3)3]2), pentamethyldisiloxane (C5H16OSi2), tetramethyldisiloxane ([(CH3)2SiH]2O), tetramethylsilane (TMS, Si(CH33)4), trimethylsilane (HSi(CH3)3), and tetraethyl orthosilicate (Si(OC2H5)4) are well-known organosilicon precursors capable of forming hydrophobic layers. These chemicals are characterized by their poor adhesion to solid surfaces, and additionally, the coating layers created do not function well after several scratches owing to their chemical nature [44–46]. For industrial applications, wear resistance and adhesive properties are critical factors. The poor adhesive properties may be improved by incorporating aminopropylethoxysilanes, which are silanating agents, to modify the surface of silica-based materials. The aminosilanes anchor to the surface, forming Si-O-Si covalent bonding and hydrogen bonding with the amino group. Among the aminosilanes, (3-aminopropyl)triethoxysilane (H2N(CH2)3Si (OC2H5)3) and 3-aminopropyl(diethoxy)methylsilane (APDMES, CH3Si (OC2H5)2(CH2)3NH2) have widely been used along with low surface energy materials [47–54]. The present work was an attempt to produce stable

Corresponding author. E-mail address: [email protected] (Y.S. Mok).

https://doi.org/10.1016/j.tsf.2019.02.017 Received 23 August 2018; Received in revised form 8 February 2019; Accepted 11 February 2019 Available online 12 February 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. (a) Schematic diagram of the plasma jet reactor system and (b) homemade sliding angle measuring tool.

polymerization applications because their concentrations in the discharge gas are usually very low (a few hundred ppm) as compared to traditional wet chemical methods. Because they are used in low concentrations, most of these precursors are consumed during the plasma reactions rather than being released into the environment. The primary goal of this work is to determine the optimal conditions under which a superhydrophobic coating layer can be formed with good stability. 2. Experimental 2.1. Materials The two precursors, TMS (≥99.0 (GC)) and APDMES (97%), were purchased from Sigma-Aldrich (USA), and were used without further purification. TMS, APDMES, and the APDMES/TMS mixture are abbreviated to T, A, and A/T, respectively. Ar (99.99%) was used to generate plasma discharge, and N2 (99.99%) was used as the shielding gas to protect the plasma jet from interference by oxygen and water vapor diffused from ambient air.

Fig. 2. Schematic diagram of the homemade scratch tester.

superhydrophobic coating films on glass substrates with an atmospheric-pressure plasma jet generated by dielectric barrier discharge (DBD). The DBD plasma jet was generated using a multi-needle-to-plate reactor. The precursors for the coating were TMS and APDMES. The inclusion of TMS as a precursor has the function of enabling the plasma to alter the nature of glass surfaces from hydrophilic to hydrophobic, which is achieved because of the four methyl groups of the TMS molecule. The high vapor pressure of TMS (718 mmHg at 25 °C) prevents the deposition of TMS inside the reactor under normal conditions without plasma activation. APDMES is useful because it imparts robustness owing to the formation of Si-O-Si covalent bonds and participation in hydrogen bonding with the amino group. However, both of these precursors have known disadvantages, namely, high cost and explosiveness. In addition, the precursors are toxic to aquatic life with long-lasting effects. Inhalation or contact with TMS may irritate or burn the skin, eyes, and its vapor may cause dizziness or suffocation, whereas APDMES could cause skin burns and eye damage. Despite these drawbacks, TMS and APDMES are considered suitable for plasma

2.2. Preparation of the plasma reactor and scratch tester A schematic diagram of the plasma polymerization system is depicted in Fig. 1. The plasma jet was generated in a DBD reactor consisting of a glass tube, shown in Fig. 1(a), in which three stainless steel needles arranged at 120° intervals to one another were inserted. The needles served as high voltage (HV) electrode. The plasma reactor was operated with a high-voltage alternating current power source (Korea Switching Co., Korea) of which the frequency output was set to 11.5 kHz. The outer and inner diameter of the glass tube was 8.0 and 5.0 mm, respectively. The distance between the tip of the HV electrode and the exit nozzle of the glass tube was maintained constant at 75 mm. The gap between the exit nozzle of the tube and the glass substrate was 2 mm. The substrate sample was placed on a Teflon plate and 35

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Fig. 3. Effect of plasma parameters on WCA and sliding angle: effect of (a) the treatment time, (b) applied voltage, (c) concentration of A, and (d) total gas flow rate. Unless they were varied in one of the reported experiments, the deposition conditions were set as treatment time of 40 s, applied voltage of 7.5 kV, concentration of A and T of 135 ppm, and total flow of 5 L/min.

shielding gas; which were separately fed into the system by mass flow controllers (see Fig. 1(a)). The APDMES and TMS were contained in Pyrex flasks and were delivered to the plasma reactor by bubbling with Ar. The concentration of APDMES was changed from 135 to 324 ppm (parts per million, volumetric) by varying the temperature to vary its vapor pressure, while the concentration of TMS remained constant at 135 ppm. The effects of the addition of APDMES on the stability and hydrophobicity of the coating were examined at various A/T (APDMES/ TMS) ratios of 1, 1.4, 1.7, and 2.4. The area of the coating was estimated to approximate 1500 mm2.

Table 1 Thickness and RMS roughness of each sample. Sample

Thickness (nm)

Standard deviation of the average thickness

RMS roughness (nm)

A T A/T = 1 A/T = 1.4 A/T = 1.7 A/T = 2.4

207 385 422 451 470 494

9 11 11 13 12 14

51 78 81 83 87 79

2.4. Characterizations

reciprocated below the stationary plasma jet at a speed of 90 mm/s.

The surface morphology, roughness and roughness of the coatings were analyzed by field emission scanning electron microscopy (FESEM, JSM-6700F, JEOL, Japan) at an operating voltage of 15 kV and a 3D nano-profiling system (3D Optical Surface Profiler, NV-2400, Nanosystem, Korea). The coating layer was partially removed carefully using a razor blade to make a step from the substrate to the coating surface, and the step height was measured by the 3D nano-profiling

2.3. Preparation of coating The plasma-induced polymerization of TMS and APDMES was performed on soda-lime glass substrates with dimensions of 75 mm × 27 mm × 1.2 mm. The total gas flow rate was fixed at 5 L/ min, including 2.5 L/min plasma gas of Ar and 2.5 L/min of N2 as 36

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Fig. 4. 2D nano-profiler and FESEM images, shown side by side, for the three selected samples of A100 (APDMES: 135 ppm; TMS: 0 ppm), T100 (APDMES: 0 ppm; TMS: 135 ppm) and A/T = 1.7 (APDMES: 230 ppm; TMS: 135 ppm). Treatment time: 40 s; applied voltage: 7.5 kV; total flow: 5 L/min.

increasing load [55]. Usually, the normal force, tangential force, friction coefficient, acoustic emission, and penetration depth are measured continuously during the scratch test [55]. The main component of the scratch tester in Fig. 2 was a DC motor, which drove the needle (tip radius: 0.25 mm) across the surface of the sample to create a scratch with predetermined vertical loads. The needle holder, which was connected to the driving shaft of the motor via a yarn, was guided by a sliding guide that traveled back and forth in the horizontal direction. The sample for testing was positioned below the tip of the needle and attached using double-sided tape to prevent the samples from moving during scratching. Before and after a scratch test, the needle tip was cleaned with ethanol and a tissue wiper. The moving speed of the tip was set to 37 mm/s and the length of the scratch was approximately 10 mm. The breakdown force of the coating layer was measured by varying the load applied to the sample. The wear tracks were observed by an optical microscope (2MP 1000 × 8 LED USB Digital Microscope Endoscope Zoom Camera, A4Tech, Taiwan).

system utilizing white light interferometry. The static WCAs and sliding angles of the coatings were measured by a goniometer (Phonix 300, Surface & Electro-Optics Co., Ltd., Korea) and homemade measuring tool (Fig. 1(b)), respectively, using the sessile drop technique by dropping about 5 μL of distilled water on the surface. The composition of the deposited thin film and its chemical properties were investigated by Fourier transform infrared spectroscopy (FTIR, FTIR-7600, Lambda Scientific, Australia) and X-ray photoelectron spectroscopy (XPS, Theta Probe AR-XPS System, Thermo Fisher Scientific). The total X-ray acquisition time was 2 min and the spot size was 400 μm. A monochromated AlKα X-ray source at 150 W (10 mA, 15 kV) was used as the excitation source. The pass energy of the X-ray microprobe was selected to be 300 eV. XPS measurements were performed without any previous ionic bombardment to prevent the real structures of the atomic layers from being destroyed. The binding energies of the XPS peaks are given with a precision of ± 0.2 eV. A homemade scratch tester was employed to examine the mechanical stability of the coating layer, as shown in Fig. 2. In the case of commercially available scratch testers, a diamond tip moves across the surface to create a scratch with either constant or progressively 37

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TMS copolymer on the surface, leading to a thicker coating layer with more roughness and surface coverage. However, extending the period of treatment beyond a certain limit did not lead to a further increase in the WCA. This is because, even though the thickness continued to increase, the surface roughness and coverage did not change significantly. Samples of which the WCA exceeded 150° were selected for sliding angle measurement. From the viewpoint of the sliding angle in Fig. 3, almost all the samples with a WCA of > 150° had a sliding angle of 5 to 3°. It should also be noted that the thicker the thin film, the lower the transparency of the coated sample. In consideration of both WCA and transparency, 40 s was chosen as the optimal treatment time, at which the WCA was 163°. In the next step, the appropriate applied voltage was determined by fixing the treatment time at 40 s. The effect of the applied voltage on the WCA was examined at voltages up to 8 kV by maintaining the A/T ratio, treatment time, and flow rate constant at 1.7, 40 s and 5 L/min, respectively (Fig. 3(b)). Basically, the higher the applied voltage, the more intense the plasma jet was, leading to enhancement of the deposition rate and the formation of particulates in the gas phase. The formation of particulates of APDMES and TMS, known as dusty plasma, is prerequisite to obtaining rough surfaces. The WCA increased as the voltage was increased from 6 to 7.5 kV, but there was no significant change in the WCA when the voltage was further increased to 8 kV. Thus, 7.5 kV was selected as the appropriate applied voltage. Using a treatment time of 40 s and voltage of 7.5 kV, as described above, a set of experiments was carried out to determine the appropriate A/T ratio. The dependence of the WCA on the A/T ratio is presented in Fig. 3(c) with a total gas flow rate of 5 L/min (Ar: 2.5 L/min; N2: 2.5 L/min). For the purpose of comparison, the WCA obtained with APDMES alone (A100) and TMS alone (T100) are also plotted in this figure. The hydrophilic nature of APDMES resulted in the A100 sample exhibiting a low WCA of 60°. On the other hand, the T100 sample showed superhydrophobicity with a WCA of 152°. Despite the hydrophilic nature of APDMES, however, the coatings prepared with a mixture of these two precursors exhibited increases in the WCA from 154 to 163° as the A/T ratio was increased from 1 to 1.7. However, increasing the A/T ratio further to 2.4 led to a decrease in the WCA, suggesting that conversion of the coating layer from hydrophobic toward hydrophilic commences near this A/T ratio. Based on the results in Fig. 3(c), the appropriate A/T ratio was determined to be 1.7. Finally, the total gas flow rate of Ar and N2 was varied from 3 to 7 L/ min by increasing the Ar flow rate (i.e., the N2 flow was set at 2.5 L/ min) to understand its effect on the WCA. The treatment time, applied voltage, and A/T ratio were maintained constant at 40 s, 7.5 kV, and 1.7, respectively. Fig. 3(d) shows the effect of the gas flow rate on the WCA. In the range 3 to 5 L/min, the plasma jet was observed to be more intense, and corresponded to an increase in the WCA from 124 to 160°. However, above 5 L/min, the WCA started to decrease, which was attributed to the decrease in the residence time and precursor concentrations. As indicated by the results in Fig. 3(d), the best result was obtained at an overall flow rate of 5 L/min.

Fig. 5. Effect of (a) aging time and (b) annealing temperature on the WCA. Si-O-Si Si-CH3 Si-C

NH2

CO2

Si-CH3

A100

Absorbance (arb. units)

A/T=2.4

A/T=1.7

A/T=1.4

A/T=1.0

T100 500

1000

1500

2000

2500

3000

3500

-1

Wavenumber (cm ) Fig. 6. FTIR spectra of samples prepared at different A/T ratios.

3. Results and discussion 3.1. Water contact angle measurements

3.2. Surface morphology and roughness The quality of the thin film produced by the plasma jet depends on parameters such as the treatment time, the applied voltage (or plasma intensity), the A/T ratio, and the gas flow rate. Fig. 3 shows the dependence of the WCA and sliding angle on these parameters. First, the treatment time was varied from 0 to 60 s at a constant A/T ratio, applied voltage, and gas flow rate of 1.7, 7.5 kV, and 5 L/min (Ar: 2.5 L/ min; N2: 2.5 L/min), respectively. As seen in Fig. 3(a), the WCA was changed from 23 to 162° by increasing the treatment time from 0 to 60 s. The WCA is well known to be closely related to the surface roughness and coverage of substrate. With a short treatment time of 0 to 20 s, the surface roughness and coverage were relatively poor. Longer treatment time allowed the deposition of additional APDMES-

In addition, 3D nano-profiler and FESEM analyses were conducted to investigate the surface morphology, roughness, and the coating thickness. The thickness of the coating was measured for all samples in Fig. 3(c). The coating layers were partially removed by carefully using a razor blade to create a step from the substrate to the coating surface and the step height was measured using a 3D optical surface profiler (NV2200, NanoSystem, Korea). The root-mean-square (RMS) roughness and thickness of the coating are summarized in Table 1. The 3D nano-profiler images of selected samples (A100, T100, and A/T = 1.7) are presented in Fig. 4(a), (c) and (e), where it can be seen that the number of needle-like peaks corresponding to the surface roughness differs among the samples, depending on the precursor composition. The 3D nano38

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60

O1s

(a)

(b)

C1s N1s A/T=1.7

Intensity (counts/s)

O1s

Si2p

C1s

N1s A100

C1s Si2p

C1s N1s O1s Si2p

50 Atomic percentage (%)

Si2p

40

30

20

O1s N1s

10 T100 0

0

200

400 600 Binding Energy (eV)

T100

800

A100

T/A=1.7

Fig. 7. Surface elemental composition of the selected samples.

Fig. 8. (a) UV-visible spectra of the samples prepared at different A/T ratios, and (b) top and side view of the selected sample (A/T = 1.7).

thin coating layer. The FESEM images of the selected samples (A100, T100, and A/ T = 1.7) are shown in Fig. 4(b), (d), and (f), where nano-structure particles of different sizes ranging from 70 to 200 nm can be observed. The structure of the thin films was found to differ in response to varying the ratio between the two precursors. In the case of the A100 sample (Fig. 4(b)), the nanoparticles deposited on the surface formed smooth circular islands of different sizes but in the case of the T100 sample (Fig. 4(d)), the appearance of the deposited layer resembled that of cauliflower as a result of the aggregation of particles smaller than those of the A100 sample (Fig. 4(b)). When an APDMES/TMS mixture was fed into the reactor, the coating morphology was still cauliflower-like but with smaller aggregates. As the ratio increased from A/T = 1 to A/ T = 1.7, the surface roughness and thickness increased with increasing WCA. Increasing the ratio further by increasing the concentration of APDMES led to the formation of larger nanoparticles with a smoother surface and eventually decreased the WCA. The mechanism whereby APDMES is anchored to the surface via silanol groups can be explained

Table 2 Scratch test results for each sample. Sample

Applied force (N)

A100 T100 A/T = 1 A/T = 1.4 A/T = 1.7 A/T = 2.4

0.17 0.03 0.04 0.05 0.06 0.08

profiler images showed that the A100 (Fig. 4(a)) sample had the lowest surface roughness. When a mixture of APDMES and TMS was used as precursor, the roughness increased substantially. This led to an increase in the WCA, which is in good agreement with the results presented in Fig. 3. The formation of rough surfaces with nano-scale topographic features is the result of the generation of particulates through gas-phase condensation reactions of the precursor fragments produced by the plasma jet. The deposition of these particulates on the surface forms a 39

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0.18

3.3. Stability tests

0.16

The stability of the coating was examined by performing natural and thermal aging. All the samples were stored in centrifuge tubes at room temperature. The aging time may have a significant effect on the WCA in the case of a powder coating. Trinh et al. [56] reported that the WCA could increase by 15° to 40° within 30 days of natural aging after a hydrophobic coating was deposited on phosphor powder substrate. Unlike the coating on the powder substrate, the WCA of the glass surface was hardly affected by aging (15 days, 30 days, and 90 days) (Fig. 5(a)), indicating that the coating formed on the glass surface had good physicochemical stability. The thermal stability of the coating was examined by conducting annealing tests at 200 and 300 °C for 1 h. As can be seen in Fig. 5(b), annealing hardly affected the WCA of all the samples.

0.14

Applied force (N)

0.12 0.10 0.08 0.06 0.04 0.02 0.00 T100

A/T=1

A/T=1.4

A/T=1.7

A/T=2.4

3.4. Spectroscopic analyses

A100

Ratio of A/T

FTIR analyses were conducted to chemically characterize the coating layers, and the results are presented in Fig. 6. Because soda-lime glass is not transparent to infrared radiation, the polymer thin films were deposited on potassium bromide (KBr) disks under the same experimental conditions. The infrared spectra were collected by taking the average of 10 scans at 1 cm−1 resolution in the wavenumber range of 500 to 3500 cm−1 using the absorbance mode. The backbone of the coating, Si-O-Si, which is often related to an improvement in wear resistance, absorbs at 1047 cm−1 [57–60]. SieC stretching appears at 800 cm−1 [58], and SieO stretching in Si-OH is detected at 825 to 940 cm−1 [58,61,62]. The Si-CH3 symmetric deformation is related to the absorption bands at 1260 cm−1, 1350 cm−1, 1366 cm−1, and 2966 cm−1, the former three of which are of medium intensity, whereas that at 2966 cm−1 has weak intensity [57,63]. These peaks grew in

Fig. 9. Breakdown forces of the samples obtained by the scratch tests.

in terms of the formation of bonds between the amino group and the hydroxyl group [44,45]. Each APDMES molecule occupies two silanol sites: one of the hydroxyl groups of silanol forms a Si-O-Si bridge and the other participates in H-bonding with the amine group of APDMES. Contrary to the surface morphology of the A100 sample, the T100 and A/T = 1.7 samples exhibited rougher surface structures, which are consistent with the 3D nano-profiler images.

Fig. 10. Optical microscopy images of selected samples (1000-times magnification). 40

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by using APDMES as the second precursor in combination with TMS. Under conditions appropriate to the formation of a suitable coating, namely, treatment time of 40 s, applied voltage of 7 kV, A/T ratio of 1.7, and overall gas flow rate of 5 L/min, the maximum WCA that was achieved was 163° with a sliding angle of 3°. The superhydrophobicity mainly resulted from the high surface roughness generated by the plasma jet, which was confirmed by the FESEM and 3D nano-profiler results. The aging and annealing test confirmed that the coating film prepared by the plasma jet had good stability, and additionally, the UV–vis spectroscopic analysis showed that there was no significant loss of transparency in the visible region. These results suggest that a nonthermal atmospheric-pressure plasma jet could potentially be used to prepare superhydrophobic surfaces on various solid materials.

intensity as the amount of APDMES increased. The noisy peaks at around 1600 cm−1 are attributed to the moisture attached to the KBr substrate. The peak at 1630 cm−1 is due to presence of -NH2 [64] and its intensity is decreased when the concentration of APDMES decreased. The low-intensity -NH2 peak found on the TMS-alone (T100) coated surface is probably due to the use of N2 as the shielding gas. The peaks of CO2 in the range 2340–2360 cm−1 are due to the presence of CO2 in the ambient environment [65]. XPS analyses were performed on three representative samples, namely, T100, A100, and A/T = 1.7 (Fig. 7). The compositions of all three of these samples contain silicon (Si), carbon (C), oxygen (O), and nitrogen (N) in different atomic ratios (Fig. 7(b)). The T100 sample contained 38% of carbon, 32.5% of oxygen, 28.1% of silicon, and 1.4% of nitrogen. The small amount of nitrogen found in the T100 sample, is due to the use of N2 as the shielding gas. On the other hand, the percentage of atomic carbon is higher than that of oxygen in sample T100, which suggests that the surface is hydrophobic in nature. In comparison, sample A100 contained 49.7% of oxygen, 21.3% of carbon, 24.5% of silicon, and 4.5% of nitrogen, whereas the atomic composition of the sample with A/T = 1.7 is 37.2% of oxygen, 40.8% of carbon, 18.8% of silicon, and 3.2% of nitrogen. These atomic percentages of the samples suggest that the sample coated with APDMES alone (A100) is hydrophilic in nature as it contains more oxygen (49.7%) than carbon (21.3). The transparency of samples before and after coating was examined by UV–visible spectroscopy in the range from 390 to 700 nm. Generally, hydrophobicity and transmittance have an inverse relationship because of their dependence on the surface thickness and roughness. As the coating thickness and roughness increase, the hydrophobicity increases but the transparency decreases [66]. The UV–vis transmission spectra of the prepared samples are presented in Fig. 8(a). For information, a photographic image of the side and top view of the sample (A/T = 1.7) is displayed in Fig. 8(b). The transmittance of the bare glass itself is < 80% in the visible region. Fig. 8(a) shows that the coating film did not significantly decrease the transparency of the glass. Only the sample prepared at A/T = 2.4 slightly decreased the visibility in the range of 390 to 450 nm. The WCA of this sample (A/T = 2.4) was not good either, compared to the samples prepared at other A/T ratios (Fig. 3).

Acknowledgment This work was supported by the Korea Research Institute of Chemical Technology (KRICT), Daejeon, Korea, and the Basic Science Research Program through the National Research Foundation, South Korea funded by the Korean government (MSIT) (2016R1A2A2A05920703 & 2018R1A4A1025998). References [1] E.M. Liston, L. Martinu, M.R. Wertheimer, Plasma surface modification of polymers for improved adhesion: a critical review, J. Adhes. Sci. Technol. 7 (1993) 1091–1127, https://doi.org/10.1163/156856193X00600. [2] P. Hamerli, T. Weigel, T. Groth, D. Paul, Surface properties of and cell adhesion onto allylamine-plasma-coated polyethylenterephtalat membranes, Biomaterials 24 (2003) 3989–3999, https://doi.org/10.1016/S0142-9612(03)00312-0. [3] P.K. Chu, J.Y. Chen, L.P. Wang, N. Huang, Plasma-surface modification of biomaterials, Mater. Sci. Eng. R. Rep. 36 (2002) 143–206, https://doi.org/10.1016/ S0927-796X(02)00004-9. [4] B.D. Ratner, Surface modification of polymers: chemical, biological and surface analytical challenges, Biosens. Bioelectron. 10 (1995) 797–804, https://doi.org/10. 1016/0956-5663(95)99218-A. [5] J. Ryu, T. Wakida, T. Takagishi, Effect of corona discharge on the surface of wool and its application to printing, Text. Res. J. 61 (1991) 595–601, https://doi.org/10. 1177/004051759106101006. [6] A. Pavlath, K.S. Lee, Glow discharge induced graft polymerization of nonvolatile monomers on wool, J. Macromol. Sci. Part A – Chem. 10 (1976) 619–630, https:// doi.org/10.1080/00222337608061204. [7] W.J. Thorsen, R.Y. Kodani, A corona discharge method of producing shrink-resistant wool and mohair, Text. Res. J. 36 (1966) 651–661, https://doi.org/10. 1177/004051756603600710. [8] W.J. Thorsen, Improvement of cotton spinnability, strength, and abrasion resistance by corona treatment, Text. Res. J. 41 (1971) 455–458, https://doi.org/10.1177/ 004051757104100512. [9] T.L. Ward, R.R. Benerito, Modification of cotton by radiofrequency plasma of ammonia, Text. Res. J. 52 (1982) 256–263, https://doi.org/10.1177/ 004051758205200405. [10] T.S. Wong, T. Sun, L. Feng, J. Aizenberg, Interfacial materials with special wettability, MRS Bull. 38 (2013) 366–371, https://doi.org/10.1557/mrs.2013.99. [11] P. Dimitrakellis, E. Gogolides, Hydrophobic and superhydrophobic surfaces fabricated using atmospheric pressure cold plasma technology: a review, Adv. Colloid Interf. Sci. 254 (2018) 1–21, https://doi.org/10.1016/j.cis.2018.03.009. [12] J. Genzer, K. Efimenko, Recent developments in superhydrophobic surfaces and their relevance to marine fouling: a review, Biofouling 22 (2006) 339–360, https:// doi.org/10.1080/08927010600980223. [13] S. Subhash Latthe, A. Basavraj Gurav, C. Shridhar Maruti, R. Shrikant Vhatkar, Recent progress in preparation of superhydrophobic surfaces: a review, J. Surf. Eng. Mater. Adv. Technol. 2 (2012) 76–94, https://doi.org/10.4236/jsemat.2012. 22014. [14] S.S. Latthe, C. Terashima, K. Nakata, M. Sakai, A. Fujishima, Development of sol–gel processed semi-transparent and self-cleaning superhydrophobic coatings, J. Mater. Chem. A 2 (2014) 5548–5553, https://doi.org/10.1039/C3TA15017H. [15] T. Kamegawa, Y. Shimizu, H. Yamashita, Superhydrophobic surfaces with photocatalytic self-cleaning properties by nanocomposite coating of TiO 2 and polytetrafluoroethylene, Adv. Mater. 24 (2012) 3697–3700, https://doi.org/10.1002/ adma.201201037. [16] Y. Liu, J. Chen, D. Guo, M. Cao, L. Jiang, Floatable, self-cleaning, and carbon-blackbased superhydrophobic gauze for the solar evaporation enhancement at the air–water interface, ACS Appl. Mater. Interfaces 7 (2015) 13645–13652, https://doi. org/10.1021/acsami.5b03435. [17] C.-H. Xue, Y.-R. Li, P. Zhang, J.-Z. Ma, S.-T. Jia, Washable and Wear-resistant superhydrophobic surfaces with self-cleaning property by chemical etching of fibers and hydrophobization, ACS Appl. Mater. Interfaces 6 (2014) 10153–10161, https:// doi.org/10.1021/am501371b.

3.5. Scratch test The mechanical strength of the coating was analyzed using the homemade scratch tester after annealing the samples at 300 °C for 1 h. The conditions under which the scratch tests were conducted are summarized in Table 2, and the test results are shown in Fig. 9. The weight of the needle and holder was 3.5 g, and thus the minimum force applied to the sample was 0.03 N. The sample prepared with TMS alone (T100) did not sustain the minimum force. For the other samples, the force was gradually increased until the breakdown force was reached. Fig. 10 shows images of the result of the scratch test that were captured using the digital microscope. As can be seen, increasing the A/T ratio resulted in good mechanical strength. This result is the consequence of APDMES forming an Si-O-Si bridge and participating in H-bonding via interaction with the silanol group of glass, as discussed above. Even though the mechanical strength of the A/T = 2.4 sample was superior to those prepared at lower A/T ratios, the best condition under which both the superhydrophobicity and strength are satisfactory is A/ T = 1.7, considering all the other factors discussed above. 4. Conclusions A simple and convenient plasma-based method suitable for fabricating a stable superhydrophobic coating was demonstrated. The precursors TMS and APDMES were effectively copolymerized to form a thin superhydrophobic film on the glass surface by the atmosphericpressure plasma jet. The poor mechanical strength, which is a problem associated with TMS-alone coatings, could be improved to some extent 41

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