silica superhydrophobic coating on poly(chloro-p-xylylene) film by introducing a polydimethylsiloxane adhesive layer

Surface & Coatings Technology 350 (2018) 201–210

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Building a mechanically stable polydimethylsiloxane/silica superhydrophobic coating on poly(chloro-p-xylylene) film by introducing a polydimethylsiloxane adhesive layer ⁎

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Hong Shaoa, Yonglian Yua,b, Yongsheng Lia,c, Maobing Shuaic, Zhoukun Hea, , Changyu Tanga, , Xiuyun Lib, Jun Meia, Qiang Fud a

Chengdu Green Energy and Green Manufacturing Technology R&D Center, Chengdu Development Center of Science and Technology, China Academy of Engineering Physics, Chengdu 610207, China State Key Laboratory of Environment-friendly Energy Materials, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China c Science and Technology on Surface Physics and Chemistry Laboratory, Mianyang 621907, China d College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Poly(chloro-p-xylylene) Superhydrophobic coating Adhesive layer Mechanical stability Polydimethylsiloxane Silica

Building a superhydrophobic coating on polymer films is an important way to obtain water-proof surfaces. However, due to the poor mechanical strength and weak adhesion between coating and substrate, the superhydrophobicity of the coating is easily lost under mechanical loads. In this study, a facile approach was proposed to build a mechanically stable superhydrophobic poly(chloro-p-xylylene) (PPXC) film by pre-coating a polydimethylsiloxane adhesive layer (PDMS AL) on the original PPXC film and coating a superhydrophobic PDMS and silica (PDMS/SiO2) layer on the PDMS AL layer. As demonstrated by Fourier transform infrared (FTIR) and scanning electron microscopy (SEM) measurements, the thermal cross-linking reaction between the PDMS/SiO2 coating and the PDMS AL layer could dramatically promote the interaction between the PDMS/SiO2 coating and the PPXC film. By tuning pre-curing time and the content of PDMS AL, the mechanical stability of the PDMS/ SiO2 coating on PPXC film could be adjusted. Under the optimal conditions of pre-curing time and the content of PDMS AL, the PPXC film showed robust superhydrophobicity and self-cleaning ability against various mechanical damages, such as 3M tape peeling and cyclic abrasion. The superhydrophobic coating on PPXC film with PDMS AL showed a higher peeling resistance than that on PPXC film without PDMS AL. Moreover, the superhydrophobic coating on PPXC film with PDMS AL maintained its original superhydrophobicity even after 4000 cycles of abrasion.

1. Introduction Electronic devices such as cell phones and telecommunication cabinets have been widely used outdoors. Therefore, it is necessary to improve the protection of the devices from electrochemical corrosion or electrical short circuits caused by moisture (water vapor) and liquid water. Poly(chloro-p-xylylene) (PPXC) film as a high moisture barrier material can be coated on various substrates by chemical vapor deposition (CVD) polymerization and has been used to protect semiconductor chips, sensors, and printing circuit boards [1–3]. Although the dense PPXC film can block moisture, it cannot effectively protect electronic devices from water-induced electrical short circuit due to its weak water repellency with a water contact angle (WCA) about 84°.

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Besides, under high humidity, weak water repellency of the PPXC film also leads to its high adhesion with condensed water droplets, thus deteriorating the moisture barrier property of the PPXC film [4]. Droplets can easily roll away from a superantiwetting surface such as superoleophobic or superhydrophobic surface. Such surfaces usually have a high WCA greater than 150° and a low roll-off angle (RA) less than 10° [5–14]. The fabrication process of superhydrophobic surface is much easier and cheaper than that of superoleophobic surface because it is not required to use fluoride materials or special “Re-entrant” or “Overhang” structures to obtain the superhydrophobicity [15–17]. Therefore, endowing the PPXC film with the superhydrophobicity might be an effective strategy to improve the water repellency and solve the abovementioned problems [18]. Superhydrophobicity

Corresponding authors. E-mail addresses: [email protected] (Z. He), [email protected] (C. Tang).

https://doi.org/10.1016/j.surfcoat.2018.07.022 Received 29 January 2018; Received in revised form 12 June 2018; Accepted 6 July 2018 Available online 09 July 2018 0257-8972/ © 2018 Elsevier B.V. All rights reserved.

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Fig. 1. Schematic diagram of the fabrication procedure of mechanically stable superhydrophobic coating on PPXC film controlled by introducing PDMS AL.

fabricated superhydrophobic surface exhibited the excellent mechanical stability and could withstand both 3M tape peeling and more than 4000 cycles of abrasion tests without losing its superhydrophobicity and self-cleaning ability.

depends on chemical compositions with a low surface energy and proper micro- and nano-scale roughness [19–24]. Plasma etching is an important strategy to rough the materials for the purpose of obtaining superhydrophobicity [15,16,25]. For example, Bi and co-workers created a rough surface on PPXC film by plasma etching and adding the components with low surface energy [26,27]. The approach produced a superhydrophobic PPXC film, but it unavoidably damaged of the PPXC surface and probably degraded the water vapor barrier property of the film. Building a superhydrophobic coating consisting of a polymer binder and nanoparticles on the substrates was an alternative way to fabricate a superhydrophobic surface [28,29], which would not damage the underlying film [20,30,31]. In previous studies, superhydrophobic surfaces were obtained by spin coating, dip coating, spray coating or casting of silica (SiO2) nanoparticles [32–34], but the superhydrophobicity of the coating prepared with these methods were easily destroyed under mechanical loads (e.g., peeling and abrasion tests) [35]. After complicated post-treatments, the mechanical stability of the coating could be greatly improved [36–40]. However, these multi-step treatments were not applicable to industrial fields. Recently, we fabricated stable superhydrophobic polydimethylsiloxane (PDMS) by onestep 3D printing [41]. However, this approach was characterized by the expensive cost and limited application scope. The weak stability of superhydrophobic coating with nanoparticles might be caused by its poor mechanical stability and weak adhesion strength between coating and substrates. In this case, pre-treatments might more efficiently improve the mechanical stability of superhydrophobicity than post-treatments. To enhance the adhesion between the superhydrophobic coating and the substrates, polar or reactive groups were inserted onto rigid substrates (e.g., glass and magnesium alloy) by pre-treatments [42–45]. Particularly, introducing an adhesive layer (AL) between substrate and coating could significantly improve the mechanical stability of the superhydrophobic coating by strong chemical bonding [46, 47]. To the best of our knowledge, fabricating robust superhydrophobic coating on the anti-stick plastic substrates such as PPXC film by introducing an AL was seldom reported. Besides, the effects of processing parameters of AL on the mechanical stability of the superhydrophobic coating were neglected. Herein, we reported a facile and low-cost approach to fabricate a mechanically stable superhydrophobic coating based on PDMS and SiO2 nanoparticles on PPXC film by inserting PDMS AL between the air plasma-treated PPXC film and the coating. The PDMS-wrapped SiO2 nanoparticles were partially embedded in uncured PDMS AL in the cross-linking reaction during thermal curing. As a result, the superhydrophobic coating was robustly fixed onto the PPXC film by PDMS AL. By optimizing pre-curing time and the content of PDMS AL, the

2. Experimental section 2.1. Materials Poly(chloro-p-xylylene) (PPXC) film with a thickness of 110 μm was provided by China Academy of Engineering Physics. Polydimethylsiloxane adhesive (PDMS, SE1700) and its curing agent (10:1 in weight ratio) were purchased from Dow Corning (USA). Hydrophobic SiO2 nanoparticles (JT-SQ, 10–30 nm) were purchased from Chengdu Today Company (China). Alcohol and hexane were obtained from Kelong Chemical Company (Chengdu, China). 2.2. Building a PDMS AL on PPXC film PDMS adhesive and its curing agent (10:1 in weight ratio) were dissolved in hexane to form an adhesive solution with various PDMS concentrations (0.0–6.0 wt%). Before dipping into the adhesive solution, the PPXC film was cleaned with alcohol and distilled water for three times (Fig. 1a) and treated with air plasma (Plasma generator, CTP-2000K, China) for 60 s under 150 V (Fig. 1b) to generate polar functional groups on its surface. Then, air plasma-treated PPXC film (denoted as PPXC-A) was dip-coated in the PDMS adhesive solution for 2 min with a dip coating machine (SYDC-100, Shanghai SAN-YAN Technology Co., Ltd., Shanghai, China) at a lowering speed of 6000 μm/ s and a lifting speed of 1000 μm/s for 5 times. Finally, PDMS adhesivecoated PPXC film (denoted as PPXC-A/AL) was thermally pre-cured at 80 °C for 0–60 min (Fig. 1c). 2.3. Preparation of the superhydrophobic coating on PPXC-A/AL film The PDMS adhesive with a curing agent (10:1 in weight ratio) was dissolved in hexane to form a homogeneous solution. Then, SiO2 nanoparticles stabilized by PDMS were dispersed into the resultant solution and sonicated (100 W) for 30 min to obtain a homogeneous suspension. The weight ratio of PDMS to SiO2 nanoparticles was set as 2:1 and the concentration of the final solution was 5.0 wt%. PPXC-A/AL film was dip-coated into the mixture solution of PDMS and SiO2 nanoparticles, and then thermally cured at 80 °C for 60 min to form a robust superhydrophobic coating on PPXC film (denoted as PPXC-A/ AL/HL, Fig. 1d). 202

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bonded to any coating (Fig. 2a). To improve the adhesion between PPXC film and coating layer, air plasma treatment on the PPXC film surface was performed to generate hydrophilic functionalities (Fig. S2) [26,27,50], thus resulting in a decreased WCA of 17.5 ± 2.3° (Fig. 2b). After dip-coating PPXC film in PDMS adhesive solution, WCA on its surface (PPXC-A/AL) increased to 118.6 ± 1.7°. This result indicated that PDMS adhesive solution could easily spread over the PPXC film with enhanced surface wettability and form a desirable PDMS AL (Fig. 2c). Furthermore, PDMS/SiO2 coating was successfully built on PPXC-A/AL and formed a superhydrophobic surface (PPXC-A/AL/HL) with a high WCA of 167.5 ± 2.5° and low RA close to zero (Fig. 2d). Water droplets could rapidly roll off the surface, which showed a selfcleaning ability [51,52]. Physical morphology is one of the important factors determining the surface wettability. Thus, the effects of PDMS AL on the cross-sectional and surface morphologies were characterized by SEM (Fig. 3). In the cross-sectional image of PPXC film (Fig. 3a), many filaments perpendicular to its surface were found and they might be formed in the CVD fabrication process of PPXC film. This structure was also beneficial to the immersion of the PDMS AL and the enhancement of the adhesion strength between PDMS AL and PPXC film. PDMS AL easily spread over the air plasma-treated PPXC film and formed a uniform PDMS AL (Fig. 3b and c) with a thickness of about 2.1 μm. PDMS AL was firmly adhered to PPXC film and no obvious void was observed at the interface. Although a new superhydrophobic coating based on PDMS/SiO2 was further built on PDMS AL, there was no obvious void at the interface between them (Fig. 3c). This result suggested that the superhydrophobic coating was partly embedded in the underlying PDMS AL to form a robust bonding due to the good compatibility between the coating and the PDMS AL. In contrast, there were visible voids at the interface between the coating and the PPXC film when PDMS AL was absent (Fig. 3d, denoted as PPXC-A/HL), indicating a weak adhesion between them. Furthermore, the effects of the PDMS AL on the surface morphologies of superhydrophobic coatings are shown in Fig. 3e and f. The formation of pores on the surface could be explained by the “Breath Figures” method [53], and it also demonstrated that the relative humidity had an obvious effect on the surface morphologies in our previous publication [54]. The dimension of pores showed a light difference due to the fluctuation of relative humidity (about 70% RH) from the oven. However, the coatings with similar superhydrophobicity (with a WCA about 167° and RA about 0° shown in Fig. 2d) were obtained on the PPXC film samples with/without PDMS AL. Therefore, the introduction of PDMS AL had no obvious effect on the surface superhydrophobicity, which was mainly determined by the multi-scale roughness from the micro-scale pores, SiO2 nanoparticle aggregations and nano-scale SiO2 nanoparticles. To reveal the relationship between surface wettability and surface roughness, the surface roughness was measured by AFM (Fig. 4). For PPXC film (Fig. 4a), its surface RMS roughness was about 13.8 nm, indicating that the surface was very smooth. Thus, its surface wettability was mainly determined by the chemical compositions [55]. However, the surface roughness after air plasma treatment (Fig. 4b, 60.9 nm) showed a slight increase [50], which might be another reason for the improved adhesion between PDMS AL and PPXC film in addition to the generated hydrophilic OeC]O and C]O groups demonstrated by the results of high-resolution C1s X-ray photoelectron spectroscopy of PPXC-A (Fig. S2). Therefore, WCA on PPXC-A was decreased to about 17.5° due to the slight increase in the roughness and the generated hydrophilic groups (Fig. 2b). When PPXC-A was coated with PDMS AL (Fig. 4c), the surface roughness with a RMS of about 91.2 nm showed a slight change. This result might be attributed to the improved roughness of PPXC film after air plasma treatment. The WCA on PDMS AL (Fig. 2c) was about 10° higher than that of pure PDMS film with a WCA of about 107° [41,56]. When the superhydrophobic coating was built on the PPXC film, the obvious micro- and nano-scale structures based on

2.4. Characterization 2.4.1. Contact angle and roll-off angle measurements The static water contact angles (WCA) of the samples were measured by a contact angle goniometer (DSA100, Kruss, Germany). Samples were firstly loaded onto the stage and then distilled water droplets (5 μL) were placed on the surface for WCA measurement. The reported WCA was calculated by averaging the WCAs from both the left and right sides of the droplets. For each sample, five data points were collected to calculate the average value. The dynamic water roll-off angles (RA) of the samples were also measured by a contact angle goniometer (DSA100, Kruss, Germany) with distilled water droplets (10 μL). RA was obtained when water droplets began to roll-off the surface which was tilted slowly to a certain angle. Five measurements for each sample were obtained and then averaged. 2.4.2. Fourier transform infrared (FTIR) measurements The curing reaction of PDMS on the PPXC film was explored by FTIR (Nicolet-iS10, Thermo Fisher Company, USA) in the range of 4000–400 cm−1 under an ATR mode. At least 3 data points on each sample were collected to confirm the FTIR results. 2.4.3. Scanning electron microscopy (SEM) observation The surface and cross-sectional morphologies of samples were examined by scanning electron microscope (SEM, XL-30-ESEM-FEG instrument, FEI PHILIPS) under an acceleration voltage of 20 kV. The prepared sample was sliced to obtain the cross-section after immersing into liquid nitrogen for half an hour. A thin layer of Au was sputtercoated on the surface and cross-section to increase its conductivity before testing. At least 3 positions on each sample were scanned to confirm the surface morphologies. 2.4.4. Atomic force microscopy (AFM) AFM images were obtained by an atomic force microscope (SPI4000, Seiko Instruments) in a tapping mode under ambient conditions. Commercial silicon nitride tips (Olympus, AC240TS-R3) with a tip radius of about 10 nm were used as received in the AFM experiments. Five positions on each sample were scanned to confirm the surface morphologies and the surface root-mean-square (RMS) roughness was calculated. 2.4.5. Mechanical stability tests The mechanical stability of the superhydrophobic coating on the PPXC film was examined by 3M tape peeling test and abrasion test under a pressure of 4 g/cm2 (Supporting information, Fig. S1). The adhesion of the superhydrophobic coating on PPXC film was evaluated by cross-cut tape tests according to GBT9286-98 standard, which was similar to American Standard Test Method (ASTM D3359-02) [48, 49]. A cross-cut was made through the coating to the PPXC film substrate, and then 3M tape was applied over the cut and then removed. In the abrasion test, three different kinds of friction materials (pure cotton fabric, polyester fabric with 90 wt% polyester and 10 wt% cotton, and three layers of dust-free paper) rubbed on the sample back and forth with a movement distance of 10 cm and each movement cycle was 5 s. The thickness of wrapping materials was kept at about 185 μm. The abrasion cycles were set by a computer program (Fig. S1c) to maintain the same abrasion conditions for each cycle. The reported data were the averaged results of three different tests of each sample. 3. Results and discussion 3.1. Building a superhydrophobic surface on PPXC film Fig. 2 shows the change in surface wettability on PPXC film at different surface treatment stages. The pristine PPXC film had a weak hydrophobicity with a WCA of 84.2 ± 1.4° and was difficult to be 203

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Fig. 2. Profiles of water droplets on different samples: (a) PPXC film, (b) PPXC-A, (c) PPXC-A/AL (4.0 wt% PDMS solution and 10 min pre-curing), and (d) PPXC-A/ AL/HL (5.0 wt% PDMS/SiO2 coating on PPXC-A/AL sample after 60 min thermal curing).

reaction of PDMS AL (Fig. 5a), residual reactive C]C (1610 cm−1) and SieH (2160 cm−1) groups could take part in the cross-linking reaction with the superhydrophobic coating during thermal curing. However, all the reactive groups C]C (1610 cm−1) and SieH (2160 cm−1) disappeared when PDMS AL layer and PDMS/SiO2 superhydrophobic coating layer were cured completely (Fig. 5b). Moreover, through the cross-over reaction of the groups at the interface of the two coating layers, the PDMS AL could effectively bond with the coating. Therefore, the covalent cross-linking reaction between PDMS AL and superhydrophobic coating could facilitate the improvement in the adhesion strength between the superhydrophobic coating and the PPXC film. According to the FTIR results, we predicted that pre-curing time of PDMS AL should be less than 20 min so that enough reactive groups were retained to enhance the adhesion strength between the PDMS AL and the superhydrophobic coating. To verify this prediction, WCAs and RAs on the samples of PPXC-A/AL/HL obtained after different precuring time were measured before and after 3M tape peeling test (Fig. 6a). In general, the WCAs and RAs of the coating with good mechanical stability should not change obviously after 3M tape peeling test. All the samples with superhydrophobic coating showed a high WCA of around 167° and a low RA less than 5° before 3M tape peeling test. However, after 3M tape peeling test, the WCA of superhydrophobic coating without pre-curing of PDMS AL (0 min) decreased sharply from 167° to 102° due to coating exfoliation by 3M tape. RA result showed that water droplets could not roll off the surface which had lost its superhydrophobicity. The weak adhesion strength of the coating should be ascribed to the fact that the uncured PDMS AL was damaged by the solvent when building the superhydrophobic coating via dip-coating (Fig. S3). Proper pre-curing time not only improved the solvent resistance of PDMS AL but also ensured enough reactive groups for the subsequent cross-linking reaction with the superhydrophobic coating. As a result, the superhydrophobic coating bonded by PDMS AL obtained after 5-min or 10-min pre-curing showed the improved stability of WCA higher than 150° and RA about 5° against tape peeling damage. Although WCA on the superhydrophobic PPXC film after 3M tape peeling test was still about 150° when pre-curing time of PDMS AL further increased to 20 min, SA result showed that water droplets could not roll off the surface. Moreover, WCA on the superhydrophobic PPXC film after 30-min or 60-min pre-curing was decreased and water droplets

SiO2 nanoparticles were observed on the sample with PDMS AL (Fig. 4d) or without PDMS AL (Fig. 4e). These structures were well consistent with the SEM results (Fig. 3e and f). The formation of SiO2 nanoparticle networks contributed to the improvements in the surface roughness and surface hydrophobicity [56,57]. As shown in Fig. 4d and e, the increased RMS (110.7 and 118.1 nm) is responsible for the improved surface hydrophobicity. 3.2. Effects of PDMS AL on the mechanical stability of the superhydrophobic coating on PPXC film In the preparation process, the superhydrophobic coating was dipcoated onto the PDMS AL in hexane solvent. To avoid the dissolution of uncured PDMS AL by the solvent, the PDMS AL should be properly precured before building the next superhydrophobic coating. Meanwhile, its reactive groups should be partly retained for the cross-linking reaction with superhydrophobic coating. The above goals could be achieved by properly controlling pre-curing time of PDMS AL. Fig. 5a shows the FTIR spectra of the PDMS AL thermally cured for different time. The peak intensities of both C]C and SieH from PDMS AL decreased with the increase in curing time, suggesting that the crosslinking degree of PDMS also gradually increased with the addition reaction of C]C and SieH groups. When curing time was over 20 min, these peaks became so weak that they could not be observed. This result indicated that the cross-linking reaction of the PDMS reached an equilibrium state and that there were few residual reactive groups (C]C and SieH) in PDMS AL pre-cured for over 20 min. Therefore, the PDMS AL pre-cured for 10 min could avoid the dissolution of PDMS AL and retain enough reactive groups. Fig. 5b shows the FTIR spectra of PDMS AL and PDMS/SiO2 superhydrophobic coating before and after thermal cross-linking reaction. The peaks at 2964 cm−1 and 1410 cm−1 were assigned to the eCH3 groups and the peaks at 1009 cm−1 and 802 cm−1 were assigned to the characteristic groups of SieOeSi from PDMS. These peaks showed small change after thermal cross-linking reaction. This result indicated that the hydrophobicity of PDMS was retained during the thermal cross-linking reaction. Meanwhile, PDMS AL layer and PDMS/SiO2 superhydrophobic coating layer contained many reactive sites such as C]C (1610 cm−1) and SieH (2160 cm−1) groups before thermal cross-linking reaction. After 10-min cross-linking 204

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Fig. 3. Cross-sectional SEM images of (a) PPXC film (abbreviated to “P”), (b) PPXC-A/AL (4.0 wt% PDMS solution and 10-min pre-curing, PPXC-A was abbreviated to “PA” in the images), (c) PPXC-A/AL/HL (5.0 wt% PDMS/SiO2 coating on PPXC-A/AL sample after 60-min thermal curing), and (d) PPXC-A/HL (5.0 wt% PDMS/SiO2 coating on PPXC-A sample after 60-min thermal curing) and surface SEM images of (e) PPXC-A/AL/HL and (f) PPXC-A/HL. The insets show corresponding higher magnification images.

chemical compositions, but the destruction of surface physical structures. In addition to pre-curing time, the content of PDMS AL on PPXC film, which was controlled by the concentration of the PDMS adhesive solution (Fig. 7a), could greatly affect the mechanical stability of the superhydrophobic coating on PPXC film. The content of PDMS AL monotonously increased with the increase in the concentration of PDMS adhesive solution, thus increasing the thickness of PDMS AL. As shown in Fig. 7b–d, the averaged thickness of PDMS AL characterized by SEM was about 0.8 μm for 1.0 wt%, 1.7 μm for 3.0 wt%, 2.1 μm for 4.0 wt% (Fig. 3c), and 2.3 μm for 5.0 wt%. According to the above variations in the PDMS AL layer thickness with the content of PDMS AL, the increased proportion of the thickness of PDMS AL was less than the increased proportion of the content of PDMS AL, indicating that there was an obvious compaction effect of PDMS AL during curing under a higher content of PDMS AL. The compaction effect of PDMS AL was ascribed to the compressibility of flexible PDMS materials. Meanwhile, this compaction effect could also be found when the sample was cured with

also could not roll off the surface, thus resulting in the loss of its original superhydrophobicity after 3M tape peeling test. The decrease in superhydrophobicity might be interpreted as follows. The over-cured PDMS AL had less reactive groups for the cross-linking reaction with the upper superhydrophobic coating. Therefore, though the surface physical structures of the sample obtained after 10-min pre-curing are almost the same before (Fig. 6b) and after (Fig. 6c) 3M tape peeling test, the surface physical structures of the sample obtained after 60-min precuring after 3M tape peeling test (Fig. 6e) were obviously damaged compared to that before 3M tape peeling test (Fig. 6d). Thus, WCA of the sample obtained after 10-min pre-curing showed small change, but it was obviously decreased compared to the sample obtained after 60min pre-curing. Meanwhile, SA result showed that water droplets could not roll off the surface. Furthermore, there was less chemical residues from the 3M tape and the samples obtained after different pre-curing time (0, 10 and 60 min) before and after 3M tape peeling test in Fig. 6f showed no obvious change in chemical compositions. Therefore, the change in the surface wettability should not be attributed to the surface

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Fig. 4. AFM images for surface morphologies of (a) PPXC film, (b) PPXC-A, (c) PPXC-A/AL (4.0 wt% PDMS solution and 10-min pre-curing), (d) PPXC-A/AL/HL (5.0 wt% PDMS/SiO2 coating on PPXC-A/AL sample after 60-min thermal curing), and (e) PPXC-A/HL (5.0 wt% PDMS/SiO2 coating on PPXC-A sample after 60-min thermal curing). The insets show averaged RMS results for each image.

induced by the lattice scraper after 3M tape peeling test was much narrower than that of the sample without PDMS AL (Fig. 9c). Therefore, the adhesion strength between the coating and PPXC film was gradually enhanced when the addition of PDMS adhesive increased up to 3.2 mg/ cm2. However, WCA decreased when the weight of PDMS AL increased to 4.5 mg/cm2 or 5.8 mg/cm2. Meanwhile, water droplets could not roll off the surface with a WCA lower than 150°. The decreased superhydrophobicity on the sample with a higher addition of PDMS AL after 3M tape peeling test was related to the undue thickness of PDMS AL because the optimum adhesive strength could only be realized with a proper PDMS AL thickness [58–60]. If the PDMS AL was too thin, the adhesive strength would not satisfy the requirement of mechanical stability. However, if it was too thick, the required delamination energy might drastically decrease [60], thus resulting in the decreased mechanical stability of the superhydrophobic coating. Therefore, the introduction of optimized PDMS AL by controlling its pre-curing time and content could greatly enhance the adhesion strength between the coating and the PPXC film. The proper introduction could obviously improve the mechanical stability of the superhydrophobic coating on PPXC film. PPXC film has been usually used to protect semiconductor chips, sensors, and printing circuit boards and the accurate structures in their application fields are easily destroyed when cleaning their surfaces. Therefore, we tried to simulate the actual cleaning process to protect the accurate structures of semiconductor chips, sensors, and printing circuit boards. Thus we chose pure cotton fabric, polyester fabric with 90 wt% polyester and 10 wt% cotton, and three layers of dust-free

solvent evaporation. The thickness of the coating with PDMS AL (4.0 wt %) and superhydrophobic coating (5.0 wt%) was measured before (Fig. 8a) and after (Fig. 8b) complete thermal curing. It decreased from 22 μm to 6 μm, displaying an obvious compaction effect after complete thermal curing of the sample due to solvent evaporation. The compaction effect decreased the number of defects (red circles in Fig. 8a) and increased the strength of coating. To reveal the relationship between the mechanical stability of superhydrophobic coating and the weight of PDMS AL, WCAs and RAs were measured before and after 3M tape peeling test on PPXC-A/AL/HL samples with different weights of PDMS AL (Fig. 9a). In the absence of PDMS AL (the weight of AL is zero), WCA on the superhydrophobic coating sharply decreased from 167.4° to 87.6° after 3M tape peeling test, and SA result showed that water droplets could not roll off the surface. This phenomenon should be mainly ascribed to the large-area damage of the coating (Fig. 9c) after 3M tape peeling test because the chemical compositions of samples showed less change after 3M tape peeling test (Fig. 9b). The superhydrophobic coatings (with 2.1 mg/cm2 and 3.2 mg/cm2 PDMS AL) still presented a WCA above 150° and a RA less than 6°, indicating that the excellent mechanical stability of the coating had been achieved by PDMS AL with the proper addition of PDMS AL. The phenomenon might be interpreted as follows. The higher content of PDMS AL led to a higher compaction degree, thus leading to fewer defects and more reactive groups at the interface between PDMS AL and superhydrophobic coating. When the addition of PDMS AL reached 3.2 mg/cm2, superhydrophobic coating showed the less damage after 3M tape peeling test (Fig. 9d) and the destroyed width

Fig. 5. (a) FTIR spectra of AL (4.0 wt%) on PPXC-A surface with different pre-curing time. (b) FTIR spectra of AL (4.0 wt%) and HL (4.0 wt% AL and 5.0 wt% HL) before and after thermal curing (60 min for AL and 60 min for HL). 206

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Fig. 6. (a) WCAs (black) and RAs (blue) on PPXC-A/AL/HL (4.0 wt% AL and 5.0 wt% HL after 60-min complete thermal curing reaction) before (solid) and after (open) 3M tape peeling test with different pre-curing time of PDMS AL. indicates that water droplets could not roll off the surface with a RA lower than 10° for certain pre-curing time. SEM images of PPXC-A/AL/HL (4.0 wt% AL and 5.0 wt% HL after 60-min complete thermal curing reaction) with 10-min pre-curing time before (b) and after (c) 3M tape peeling test and PPXC-A/AL/HL (4.0 wt% AL and 5.0 wt% HL after 60-min complete thermal curing reaction) with 60-min pre-curing time before (d) and after (e) 3M tape peeling test. FTIR results (f) for the samples with different pre-curing time (0, 10 and 60 min) before and after 3M tape peeling test. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

paper under a pressure condition of 4 g/cm2 to conduct the mechanical abrasion test in the study. The mechanical stability of superhydrophobic coating on PPXC film with/without PDMS AL was evaluated by a homemade abrasion test system (Figs. S1 and 10). Based on the above discussion, the coating without PDMS AL rapidly lost its superhydrophobicity after 1500 cycles of abrasion (with a WCA lower than 150° and a RA higher than 10°) in the mechanical abrasion tests performed with all the three different kinds of wrapping materials. The loss of superhydrophobicity was ascribed to the large-area destruction of surface physical structures shown in Fig. 10c. However, the coating with optimized PDMS AL lost its superhydrophobicity after 5000 cycles of abrasion (Fig. 10a). The loss could be attributed to the small defects of the physical structures shown in the red arrows in Fig. 10d. However, the retained coating on PPXC-A/AL/HL after 5000 cycles of abrasion was still much more than that on PPXC-A/HL even after 1500 cycles of abrasion. Therefore, the mechanical stability of the coating could be greatly improved by PDMS AL. After 6000 cycles, the WCA on PPXC-A/ AL/HL (10-min pre-curing of 4.0 wt% AL and 5.0 wt% HL after 60-min complete thermal curing reaction) dramatically decreased to near 130°, which was still greater than that on PPXC-A/HL without PDMS AL (about 95°, 5.0 wt% HL after 60-min complete thermal curing reaction) and pure PPXC film (about 84°). Therefore, the stability of surface hydrophobicity of PPXC film could be significantly enhanced, thus reducing the cleaning times and extending the lifetime of semiconductor chips, sensors, and printing circuit boards. A superhydrophobic coating with a WCA greater than 150° might be at a “Cassie-Baxter” state or a “Wenzel” state, as distinguished by the

dynamic behaviors of water droplets on the surface [61,62]. Only a superhydrophobic coating with a “Cassie-Baxter” state (with a WCA higher than 150° and a RA lower than 10°) showed a good self-cleaning ability. Therefore, we further tested the behaviors of water droplets on the sample after 4000 cycles of abrasion with pure cotton fabric, and the self-cleaning ability of the superhydrophobic coating with the optimized PDMS AL could be retained (Fig. 10b). The mechanical stability between the superhydrophobic coating and the PPXC film was significantly improved by the reactive PDMS AL, which could induce the cross-linking reaction between superhydrophobic coating and PDMS AL and enhance the strong adhesion of PDMS AL on PPXC film. Therefore, PDMS/SiO2 superhydrophobic coating on PPXC film could be successfully achieved by the introduction of PDMS AL between the coating and substrate, and it showed obvious improvement in the mechanical stability against 3M tape peeling test and abrasion test with different kinds of wrappings.

4. Conclusion In summary, a mechanically stable superhydrophobic coating on PPXC film was successfully fabricated by inserting PDMS AL between the air-plasma-treated PPXC film and the superhydrophobic coating. The improved mechanical stability of the superhydrophobic coating against various mechanical damages such as 3M tape peeling and cyclic abrasion would be attributed to the following reasons. Firstly, PDMS AL was firmly adhered to PPXC film and no obvious void existed at their interfaces. Secondly, the superhydrophobic coating was partly 207

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Fig. 7. (a) Effects of the concentration of PDMS AL on the weight of PDMS AL and cross-sectional SEM images of samples (red arrows in a) with different PDMS solutions: (b) 1.0 wt%, (c) 3.0 wt%, and (d) 5.0 wt%. The insets show the high-magnification images for each SEM image. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 8. Cross-sectional SEM images of PPXC-A/AL/HL sample (10-min pre-curing of 4.0 wt% AL and 5.0 wt% HL) before (a) and after (b) second thermal curing. The thickness is the total thickness of PDMS AL and superhydrophobic coating due to the undistinguished interface between these two layers before solvent evaporation.

Acknowledgements

embedded into the underlying PDMS AL and participated in the thermal cross-linking reaction, thus forming a robust bonding and promoting the adhesion between the coating and the PDMS AL. Thirdly, appropriate pre-curing time and the proper content of AL could improve the solvent resistance of PDMS AL and provide enough reactive groups for subsequent cross-linking reaction of PDMS AL with the superhydrophobic coating. The study can provide an efficient strategy to improve the hydrophobicity of PPXC film and satisfy the industrial applications of PPXC film in semiconductor chips, sensors, and printing circuit boards.

The authors would like to acknowledge the financial support from the National Natural Science Foundation of China (grant numbers 21504106, 51573217) and the program on rubber sealant with high barrier properties.

Author contributions Hong Shao, Zhoukun He and Changyu Tang conceived and designed the experiment; Yonglian Yu, Yongsheng Li and Maobing Shuai did the experimental work of the fabrication of smooth and free-standing PPXC 208

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Fig. 9. (a) Variations of WCAs (black) and RAs (blue) on PPXC-A/AL/HL (10-min pre-curing of AL and 5.0 wt% HL after 60-min thermal curing) with different additions of PDMS AL before (solid) and after (open) 3M tape peeling test. indicates that water droplets could not roll off the surface with a RA lower than 10° for certain additions of PDMS AL. FTIR results (b) for samples with different weights of PDMS AL (0, 3.2 and 5.8 mg/cm2) before and after 3M tape peeling test. SEM images of PPXC-A/HL without PDMS AL after (c) 3M tape peeling test and PPXC-A/AL/HL with optimized PDMS AL (10min pre-curing time and 3.2 mg/cm2 AL) after (d) 3M tape peeling test. Red arrow (c) indicates the peeled area by 3M tape, and red dash lines (c and d) indicate the destroyed width induced by the lattice scraper after 3M tape peeling test. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 10. (a) Variation of WCAs (black, red and green) on PPXC-A/HL (5.0 wt% HL after 60-min complete thermal curing reaction) and PPXC-A/ AL/HL (10-min pre-curing of 4.0 wt% AL and 5.0 wt% HL after 60-min complete thermal curing reaction) with cycles of abrasion for different materials (black for pure cotton fabric, red for polyester fabric, and green for dust-free paper), and RAs (blue) on PPXC-A/HL (5.0 wt% HL after 60-min complete thermal curing reaction) and PPXC-A/AL/HL (10-min pre-curing of 4.0 wt% AL and 5.0 wt% HL after 60-min complete thermal curing reaction) with cycles of abrasion for pure cotton fabric. and indicate that water droplets could not roll off the surface with a RA lower than 10° for certain cycles of abrasion. (b) Self-cleaning ability of PPXC-A/ AL/HL after 4000 cycles of abrasion with pure cotton fabric. SEM images of PPXC-A/HL (c, 5.0 wt% HL after 60-min complete thermal curing reaction) and PPXC-A/AL/HL (d, 10-min pre-curing of 4.0 wt% AL and 5.0 wt% HL after 60-min complete thermal curing reaction) with 1500 and 5000 cycles of abrasion with pure cotton fabric, respectively. Red arrows indicate the damaged area and the blue arrows indicate the abrasion direction. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

film; Hong Shao, Yonglian Yu and Zhoukun He performed the other experiments; Xiuyun Li, Jun Mei and Qiang Fu did work about data interpretation with AFM and FTIR analysis; and Hong Shao wrote the paper.

Appendix A. Supplementary data The photographs of the abrasion test system (Fig. S1), high-resolution C1s XPS spectra of PPXC-A (Fig. S2) and SEM cross-sectional image of uncured PDMS AL with dip-coated superhydrophobic coating on PPXC-A film surface (Fig. S3). Supplementary data to this article can be found online at https://doi.org/10.1016/j.surfcoat.2018.07.022.

Conflicts of interest

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The authors declare no conflict of interest.

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