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Preparation and surface properties of transparent UV-resistant “petal effect” superhydrophobic surface based on polybenzoxazine
Applied Surface Science 353 (2015) 1137–1142
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation and surface properties of transparent UV-resistant “petal effect” superhydrophobic surface based on polybenzoxazine Juan Liu, Xin Lu, Zhong Xin ∗ , Changlu Zhou Shanghai Kay Lab of Chemical Engineering for Multiphase structure materials, State key laboratory of chemistry engineering, School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China
a r t i c l e
i n f o
Article history: Received 24 March 2015 Received in revised form 22 June 2015 Accepted 28 June 2015 Available online 10 July 2015
a b s t r a c t The rose petal shows super water repellence together with high adhesion of water, which was named as “pinned effect” or “petal effect”. In this work, the “petal effect” superhydrophobic surface based on polybenzoxazine was prepared by one-step thermal curing method. The highest water CA was 161.5◦ . The droplets could be pinned to the surface and could not roll off, even when it was turned upside down. The morphological characterization of the samples was characterized by SEM, which exhibited that the surface had micro- and nano- structures. Meanwhile, it also possessed excellent surface properties, such as good UV-resistance, transparent property, thermal stability and strong adhesion to substrate which made it possible to apply in academic research and industrial applications. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Roses are not only the symbol of romantic love, but also contribute to the fabrication of smart artificial materials. Rose petals show super water repellence with a contact angle (CA) of about 152.4◦ . Moreover, these droplets become pinned to the surface and cannot roll off, even when the petal is turned upside down. This phenomenon is known as “pinned effect” or “petal effect” [1–4], which has attracted great interest in recent years [5,6]. Jiang and coworkers [2] noted that there were hierarchical micropapillae and nanofolds existed on the petals’ surfaces of red roses. This “petal effect” surface could find its potential application in transferring a droplet from a hydrophobic to a hydrophilic surface [7]. Some additional properties would facilitate an excellent performance as well as wider applications for this unique surface. For instance, superhydrophobic surfaces with good UV-resistance could flourish their potential application in aviation aircraft windows for antifog or anti-icing under intense UV irradiation, improving the safety of aviation. Furthermore, transparent and scratch resistant superhydrophobic surfaces are good candidates for the satellite’s solar panels coating, which play the significant role of dustproof outside the earth atmosphere. Generally, the “petal effect” surface was prepared by adding TiO2 [8,9], SiO2 [10,11], or other nanoparticles into some low surface
∗ Corresponding author. Tel.: +86 21 64251005; fax: +86 21 64251772. E-mail address: [email protected] (Z. Xin). http://dx.doi.org/10.1016/j.apsusc.2015.06.177 0169-4332/© 2015 Elsevier B.V. All rights reserved.
free energy materials, such as PVC, PDMS and PS. Polybenzoxazine, with a wide range of interesting features [12–16] including nearzero shrinkage upon curing, low water absorption, high char yield, excellent electrical properties, high thermally stable performance, etc. [10,17], is one of the common low surface free energy materials [18–22]. Most superhydrophobic surfaces [11,23,24,10] based on polybenzoxazine exhibited the “lotus effect” in the past papers, yet the “petal effect” surfaces based on polybenzoxazine aroused the increasing number of interest recently [25–28]. It has been reported one kind of superhydrophobic polybenzoxazine hybrid surface by B-ala PBZ and SiO2 NPs [24,10]. It was performed through a twostep process. First, the B-ala benzoxazine and nanoparticles mixed in THF was spin-coated onto a glass, then cured for 1 h. Subsequently, the surface was modified with 0.1% (w/v) B-ala PBZ film. Upon 5 min UV irradiation, this superhydrophobic surface had high adhesion to water droplet. And the contact angle of water for this surface decreased rapidly upon increasing the UV exposure time, from 161.1◦ to 0◦ after 60 min of UV exposure [24]. Raza et al. [27] had fabricated superhydrophobic films with robust adhesion to the water droplets utilizing in situ polymerized fluorinated polybenzoxazine (F-PBZ) and SiO2 nanoparticles. All these pioneering works open the new chapter of the application of polybenzoxazine in “petal effect” surfaces. In our previous study [29], a series of silane-functional polybenzoxazine films were synthesized using a facile one-step thermal curing method. These films had low surface free energy and high adhesive force to the substrates simultaneously. In this work, we used one of these benzoxazine monomers named MPtmos to prepare the superhydrophobic surface with “petal effect”.
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J. Liu et al. / Applied Surface Science 353 (2015) 1137–1142
Scheme 1. Preparation of the superhydrophobic coating by MP-tmos benzoxazine and silica nanoparticles.
This surface was prepared by one-step thermal curing method, which also possesses several merits, such as high adhesion, good UV-resistance, transparent property and thermal stability. The simple fabrication and excellent surface properties make this surface possible to apply in academic research and industrial applications. 2. Material and methods 2.1. Materials and chemical reagents 3-(trimethoxysilyl)-n-propyl-3,4-dihydro-6-methyl-2H-1,3benzoxazine (MP-tmos) was synthesized by 4-methylphenol (Sinopharm Chemical Reagent Co., Ltd.), 3-Aminopropyltrimethoxysilane (3-APTMOS) (Shanghai Haiqu Reagent Corp.) and Paraformaldehyde (Shanghai Lingfeng Chemical Corp.). The synthetic method of MP-tmos benzoxazine monomer was same
as our previous study [29]. All chemicals were of analytically pure grade. Silica nanoparticles were 15 nm precipitated hydrated silica particles. 2.2. Preparation of nanocomposite coating Superhydrophobic coating on a glass slide was performed through a one-step process. MP-tmos benzoxazine (0.5 g) was mixed with silica nanoparticles (0.05–0.6 g) in terahydrofuran (THF)(10 mL). After keeping the solutions in an ultrasound bath for 2 h, the mixture was spin-coated onto a glass slide (10 × 10 × 1 mm3 ) at 1500 rpm for 45 s and then cured in an oven at a stationary temperature for 1 h. It’s worth noticing that the glass slides were first cleaned by water, ethanol, and acetone through ultrasonic treatment and then purged with piranha solution (a mixture 7:3 (v/v) of 98% H2 SO4 and 30% H2 O2 ) for 1 h at 120 ◦ C (Piranha solution had a strong oxidizing property, which should
J. Liu et al. / Applied Surface Science 353 (2015) 1137–1142
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Table 1 The water contact angle of the silica nanoparticles-containing polybenzoxazine surfaces. Samplea
MP-tmos (g)
SiO2 (g)
WCA (◦ )
MS-0.5/0.6 MS-0.5/0.5 MS-0.5/0.3 MS-0.5/0.15 MS-0.5/0.05
0.5 0.5 0.5 0.5 0.5
0.6 0.5 0.3 0.15 0.05
154.3 ± 0.9 149.2 ± 1.1 148.2 ± 1.2 145.8 ± 0.5 139.0 ± 0.8
a MS-a/b: b g silica nano-particles were added in a g MP-tmos benzoxazine monomer to prepared the films.
be handled carefully) followed by ultrasonic treatment for 5 min to remove the spare piranha solution. These glass slides were then blown dry with nitrogen gas to form a layer of Si-OH groups on the surface. The water contact angles of these substrates were less than 5◦ .
Fig. 1. The water contact angle of the polybenzoxazine-silica surfaces cured at different temperature.
3. Results and discussion 2.3. Properties of nanocomposite coatings The surface morphology and the microstructure of the polymer surface were characterized using a FEI-Nova-450 scanning electron microscope (SEM) with an acceleration voltage of 10 kV. And the elements on the surface were characterized by energy dispersive X-ray spectroscopy (EDX) with an acceleration voltage of 10 kV. The water contact angles of the surfaces of the substrates were determined by contact angle goniometry at 25 ◦ C using a DataPhysics OCA20 optical goniometer interfaced with image-capture software by injecting a 4 L liquid drop. A SDT Q600 V8.3 Build 101 thermal gravimetric analyzer was used to investigate the thermal stability of the polymer. The SiO2 -polybenzoxazine sample (about 10 mg in weight) was heated under a N2 atmosphere from ambient temperature to 800 ◦ C at the heating rate of 10 ◦ C/min. The thermal degradation temperature was taken as the onset temperature at which 5 wt% of weight loss occurs.
The “petal effect” surface based on polybenzoxazine was fabricated via one-step thermal curing method by adding the SiO2 nanoparticles into one non-fluorine low surface free energy polybenzoxazine 3-(trimethoxysilyl)-n-propyl-3,4-dihydro-6-methyl2H-1,3-benzoxazine (MP-tmos), according to our previous work [29] (Scheme 1). These nanocomposite films, named as MS-a/b (a means the weight of MP-tmos benzoxazine monomer, and b means the weight of the silica nano-particles) were cured at 200 ◦ C for 1 h to investigate the wetting properties by CA measurement. The results were shown in Table 1. With the increasing of the nanoparticle proportion, the water contact angles of the films increased continuously. And the contact angles of MS-0.5/0.6 film could reach up to 154.3 ± 0.9◦ , exhibiting a good hydrophobicity. Further, the curing temperatures of MS-0.5/0.6, MS-0.5/0.5 and MS-0.5/0.3 were optimized to improve the superhydrophobic
Fig. 2. The water contact angle of (a) benzoxazine-silica surface; (b) polybenzoxazine-silica surface cured at 180 ◦ C for 1 h; Shapes of 8 L water droplets on the polybenzoxazine-silica surface with different tilt angles: (c) 90◦ and (d) 180◦ .
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J. Liu et al. / Applied Surface Science 353 (2015) 1137–1142 Table 2 The EDX results of the microstructure in the polybenzoxazine-silica surface.
Fig. 3. SEM image of (a) polybenzoxazine-silica surface and (b) enlarged view of polybenzoxazine-silica surface image.
properties. As shown in Fig. 1, the CA of MS-0.5/0.3 and MS-0.5/0.5 were stable at ca. 150◦ with the increasing of the curing temperature. However, the CA of MS-0.5/0.6 film cured at 180 ◦ C for 1 h could reach up to 161.5 ± 0.7◦ from 40.5◦ before cured. Therefore, we focused our research on MS-0.5/0.6, since it possesses the best hydrophobic performance after curing. Simultaneously, the sliding angle was measured. The droplet became pinned to the surface and could not roll off, even when the surface was turned upside down, showing the “pinned effect” or “petal effect” phenomenon. This indicates that the polybenzoxazine-silica surface prepared by simple one-step thermal curing method was not a traditional superhydrophobic surface, but a “petal effect” surface. It is expected that a surface with a sufficiently high adhesive force to a liquid will have many potential applications, such as liquid transportation without loss and the analysis of tiny volumes of liquid samples [24]. The “petal effect” superhydrophobic polybenzoxazine-silica surface can be used as a “mechanical hand” to transfer small water droplets from superhydrophobic surface to a hydrophilic one (Fig. 2) [30]. To further investigate the mechanism for the formation of this “petal effect” surface, the polybenzoxazine-silica surface morphology was measured by SEM (Fig. 3). Since the film was prepared through the reaction between polybenzoxazine and silica particles, and the size of polybenzoxazine was much smaller than
Element
Weight (%)
Atomic (%)
Net Int. Error
C O Si
14.73 38.00 47.27
23.21 44.95 31.85
0.03 0.01 0.01
the silica particle, the surface morphology of the film was dominated by the silica particles. Therefore, there were many separate microislands (40–50 m) on the “petal effect” superhydrophobic surface (Fig. 3a). Each microisland was covered with nanospheres (30–50 nm; Fig. 3b). These micro- and nano-scale binary structures dramatically increased the surface roughness, which led to composite interfaces [31] and gave an appropriate volume of air sealing on the surface. When the surface flipped even upside down, the air pockets would have a negative pressure and have an attraction to the water droplet, thereby strengthening the adhesion between the droplets and the surface [32]. In order to confirm the composition of the micro- and nanostructures of this superhydrophobic surface, the elements were characterized by EDX. The results (Table 2) showed that there were 3 elements (C, O and Si) existed in the microstructure. Since the polymer was the only source of carbon, we could confirm that the microstructure consisted of both polymer and silica particles. The similar test was also measured in the nanosphere area, and the result was the same as the micro-structure. In our previous study, it was proved that only by curing at a fixed temperature for a short time, the ring-opening reaction of the benzoxazine and the cross-linkage of Si O Si would occur simultaneously [29]. The trimethoxysilane groups of the MP-tmos benzoxazine monomer reacted with silanol groups on the surface to produce Si O Si linkage. As a result, the silica nano-particles were strongly adhered to the substrates through MP-tmos benzoxazine (as shown in Scheme 1). The adhesion of the polybenzoxazinesilica coating on the substrates was investigated by the cross-cut method [33]. The SEM image of the polybenzoxazine-silica surface after cross-cut test was shown in Fig. 4, which demonstrated that there was a strong adhesive force between the SiO2 -containing polybenzoxazine film and the substrates. To achieve commercialization, performances of UV-resistance, transparency and thermal stability are of high importance. Therefore, MS-0.5/0.6 surface was irradiated under 254 nm UV light
Fig. 4. SEM image of MS superhydrophobic surface after adhesion test.
J. Liu et al. / Applied Surface Science 353 (2015) 1137–1142
Fig. 5. Contact angle of polybenzoxazine-silica surface measured after irradiation by 254 nm UV-light for different times.
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Fig. 7. Thermal stability of polybenzoxazine-silica composite materials.
Table 3 Thermal stability of polybenzoxazine-silica composite material. Resin sample
Temp. at 5 wt% loss (◦ C)
Temp. at 10 wt% loss (◦ C)
Char yield (%) 800 ◦ C
MS-0.5/0.3 MS-0.5/0.5 MS-0.5/0.6
354 365 376
422 454 472
72.91 80.79 83.10
Even heated to 800 ◦ C, the char yields of these films were 72.91%, 80.79% and 83.10% (Table 3), respectively, which revealed that all the composite materials had good thermal stability. The superhydrophobic films were also treated at high temperature, and then their water contact angles were measured. The results showed that the polybenzoxazine-silica surface still maintained the superhydrophobic properties after 300 ◦ C treatment. Fig. 6. Light transmitting property of polybenzoxazine-silica surface.
4. Conclusions irradiation for different times, and then the water CA were measured. As shown in Fig. 5, no obvious changes were observed in the surfaces even irradiated for 60 min, which indicated that the “petal effect” superhydrophobic surface exhibited good UVresistance performances. Moreover, the water CA of the MP-tmos polybenzoxazine was also measured after exposed in 254 nm UV light for 60 min, and the results showed that the precursor of the MS-0.5/0.6 surface was UV-resistant. Due to the other component of the MS-0.5/0.6 surface, the silica particle, was also stable under UV light irradiation. That was why the superhydrophobic surface had good UV resistance property. Then, the transparency of this smart surface was tested by covering it on the university badge (Fig. 6). The clear vision of the badge indicated a good transparency of this “petal effect” superhydrophobic surface. This property was also related to the micro- and nanostructure on the surface. The microstructure islands were composed of nanospheres, and the size of nanospheres was 30–50 nm, which is much smaller than the visible light wavelength. Also, there were many silica nanoparticles and Si-O-Si bonds in the film, which was close to the glass’s composition, so the film’s refractive index was similar to the glass. For those two reasons, the film showed transparent property. Finally, the thermal stabilities of these polybenzoxazine-silica composite materials were evaluated by TGA. MS-0.5/0.3, MS0.5/0.5 and MS-0.5/0.6 were cured at 180 ◦ C for 1 h, and then heated under N2 atmosphere from ambient temperature to 800 ◦ C at heating rate of 10 ◦ C/min. As shown in Fig. 7, no obvious losses of the weight were found when the films were heated to 300 ◦ C.
In summary, the superhydrophobic surface was fabricated by adding SiO2 nanoparticles into the polybenzoxazine via one-step thermal curing method. The highest water CA of the superhydrophobic surface could reach up to 161.5◦ , and this film exhibited “petal effect”. The morphology of the surface was investigated by SEM, which showed that each microisland on the surface was covered with nanospheres. It should be highlighted that the surface possessed several excellent properties. Upon cross-cut test, there was little loss of the film, which indicated strong adhesion of the surface to substrates. Moreover, this surface demonstrated brilliant UV-resistance property, because no obvious change was observed on the surface even irradiated for 60 min. The surface also displayed outstanding transparency and thermal stability, and the maximum char yields could reach up to 83.10% even heated to 800 ◦ C. All of these properties endowed this surface with great potential in the further application of the aviation and aerospace field.
Acknowledgements This research was financially supported by the Nanotech Foundation of Science and Technology Commission of Shanghai Municipality (Grant 0652nm001), by the National Natural Science Funds of China (Grant NO. u1162110), the Fundamental Research Funds for the Central Universities (WA1514015), and China Postdoctoral Science Foundation (2015M571509).
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation and surface properties of transparent UV-resistant “petal effect” superhydrophobic surface based on polybenzoxazine Juan Liu, Xin Lu, Zhong Xin ∗ , Changlu Zhou Shanghai Kay Lab of Chemical Engineering for Multiphase structure materials, State key laboratory of chemistry engineering, School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China
a r t i c l e
i n f o
Article history: Received 24 March 2015 Received in revised form 22 June 2015 Accepted 28 June 2015 Available online 10 July 2015
a b s t r a c t The rose petal shows super water repellence together with high adhesion of water, which was named as “pinned effect” or “petal effect”. In this work, the “petal effect” superhydrophobic surface based on polybenzoxazine was prepared by one-step thermal curing method. The highest water CA was 161.5◦ . The droplets could be pinned to the surface and could not roll off, even when it was turned upside down. The morphological characterization of the samples was characterized by SEM, which exhibited that the surface had micro- and nano- structures. Meanwhile, it also possessed excellent surface properties, such as good UV-resistance, transparent property, thermal stability and strong adhesion to substrate which made it possible to apply in academic research and industrial applications. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Roses are not only the symbol of romantic love, but also contribute to the fabrication of smart artificial materials. Rose petals show super water repellence with a contact angle (CA) of about 152.4◦ . Moreover, these droplets become pinned to the surface and cannot roll off, even when the petal is turned upside down. This phenomenon is known as “pinned effect” or “petal effect” [1–4], which has attracted great interest in recent years [5,6]. Jiang and coworkers [2] noted that there were hierarchical micropapillae and nanofolds existed on the petals’ surfaces of red roses. This “petal effect” surface could find its potential application in transferring a droplet from a hydrophobic to a hydrophilic surface [7]. Some additional properties would facilitate an excellent performance as well as wider applications for this unique surface. For instance, superhydrophobic surfaces with good UV-resistance could flourish their potential application in aviation aircraft windows for antifog or anti-icing under intense UV irradiation, improving the safety of aviation. Furthermore, transparent and scratch resistant superhydrophobic surfaces are good candidates for the satellite’s solar panels coating, which play the significant role of dustproof outside the earth atmosphere. Generally, the “petal effect” surface was prepared by adding TiO2 [8,9], SiO2 [10,11], or other nanoparticles into some low surface
∗ Corresponding author. Tel.: +86 21 64251005; fax: +86 21 64251772. E-mail address: [email protected] (Z. Xin). http://dx.doi.org/10.1016/j.apsusc.2015.06.177 0169-4332/© 2015 Elsevier B.V. All rights reserved.
free energy materials, such as PVC, PDMS and PS. Polybenzoxazine, with a wide range of interesting features [12–16] including nearzero shrinkage upon curing, low water absorption, high char yield, excellent electrical properties, high thermally stable performance, etc. [10,17], is one of the common low surface free energy materials [18–22]. Most superhydrophobic surfaces [11,23,24,10] based on polybenzoxazine exhibited the “lotus effect” in the past papers, yet the “petal effect” surfaces based on polybenzoxazine aroused the increasing number of interest recently [25–28]. It has been reported one kind of superhydrophobic polybenzoxazine hybrid surface by B-ala PBZ and SiO2 NPs [24,10]. It was performed through a twostep process. First, the B-ala benzoxazine and nanoparticles mixed in THF was spin-coated onto a glass, then cured for 1 h. Subsequently, the surface was modified with 0.1% (w/v) B-ala PBZ film. Upon 5 min UV irradiation, this superhydrophobic surface had high adhesion to water droplet. And the contact angle of water for this surface decreased rapidly upon increasing the UV exposure time, from 161.1◦ to 0◦ after 60 min of UV exposure [24]. Raza et al. [27] had fabricated superhydrophobic films with robust adhesion to the water droplets utilizing in situ polymerized fluorinated polybenzoxazine (F-PBZ) and SiO2 nanoparticles. All these pioneering works open the new chapter of the application of polybenzoxazine in “petal effect” surfaces. In our previous study [29], a series of silane-functional polybenzoxazine films were synthesized using a facile one-step thermal curing method. These films had low surface free energy and high adhesive force to the substrates simultaneously. In this work, we used one of these benzoxazine monomers named MPtmos to prepare the superhydrophobic surface with “petal effect”.
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Scheme 1. Preparation of the superhydrophobic coating by MP-tmos benzoxazine and silica nanoparticles.
This surface was prepared by one-step thermal curing method, which also possesses several merits, such as high adhesion, good UV-resistance, transparent property and thermal stability. The simple fabrication and excellent surface properties make this surface possible to apply in academic research and industrial applications. 2. Material and methods 2.1. Materials and chemical reagents 3-(trimethoxysilyl)-n-propyl-3,4-dihydro-6-methyl-2H-1,3benzoxazine (MP-tmos) was synthesized by 4-methylphenol (Sinopharm Chemical Reagent Co., Ltd.), 3-Aminopropyltrimethoxysilane (3-APTMOS) (Shanghai Haiqu Reagent Corp.) and Paraformaldehyde (Shanghai Lingfeng Chemical Corp.). The synthetic method of MP-tmos benzoxazine monomer was same
as our previous study [29]. All chemicals were of analytically pure grade. Silica nanoparticles were 15 nm precipitated hydrated silica particles. 2.2. Preparation of nanocomposite coating Superhydrophobic coating on a glass slide was performed through a one-step process. MP-tmos benzoxazine (0.5 g) was mixed with silica nanoparticles (0.05–0.6 g) in terahydrofuran (THF)(10 mL). After keeping the solutions in an ultrasound bath for 2 h, the mixture was spin-coated onto a glass slide (10 × 10 × 1 mm3 ) at 1500 rpm for 45 s and then cured in an oven at a stationary temperature for 1 h. It’s worth noticing that the glass slides were first cleaned by water, ethanol, and acetone through ultrasonic treatment and then purged with piranha solution (a mixture 7:3 (v/v) of 98% H2 SO4 and 30% H2 O2 ) for 1 h at 120 ◦ C (Piranha solution had a strong oxidizing property, which should
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Table 1 The water contact angle of the silica nanoparticles-containing polybenzoxazine surfaces. Samplea
MP-tmos (g)
SiO2 (g)
WCA (◦ )
MS-0.5/0.6 MS-0.5/0.5 MS-0.5/0.3 MS-0.5/0.15 MS-0.5/0.05
0.5 0.5 0.5 0.5 0.5
0.6 0.5 0.3 0.15 0.05
154.3 ± 0.9 149.2 ± 1.1 148.2 ± 1.2 145.8 ± 0.5 139.0 ± 0.8
a MS-a/b: b g silica nano-particles were added in a g MP-tmos benzoxazine monomer to prepared the films.
be handled carefully) followed by ultrasonic treatment for 5 min to remove the spare piranha solution. These glass slides were then blown dry with nitrogen gas to form a layer of Si-OH groups on the surface. The water contact angles of these substrates were less than 5◦ .
Fig. 1. The water contact angle of the polybenzoxazine-silica surfaces cured at different temperature.
3. Results and discussion 2.3. Properties of nanocomposite coatings The surface morphology and the microstructure of the polymer surface were characterized using a FEI-Nova-450 scanning electron microscope (SEM) with an acceleration voltage of 10 kV. And the elements on the surface were characterized by energy dispersive X-ray spectroscopy (EDX) with an acceleration voltage of 10 kV. The water contact angles of the surfaces of the substrates were determined by contact angle goniometry at 25 ◦ C using a DataPhysics OCA20 optical goniometer interfaced with image-capture software by injecting a 4 L liquid drop. A SDT Q600 V8.3 Build 101 thermal gravimetric analyzer was used to investigate the thermal stability of the polymer. The SiO2 -polybenzoxazine sample (about 10 mg in weight) was heated under a N2 atmosphere from ambient temperature to 800 ◦ C at the heating rate of 10 ◦ C/min. The thermal degradation temperature was taken as the onset temperature at which 5 wt% of weight loss occurs.
The “petal effect” surface based on polybenzoxazine was fabricated via one-step thermal curing method by adding the SiO2 nanoparticles into one non-fluorine low surface free energy polybenzoxazine 3-(trimethoxysilyl)-n-propyl-3,4-dihydro-6-methyl2H-1,3-benzoxazine (MP-tmos), according to our previous work [29] (Scheme 1). These nanocomposite films, named as MS-a/b (a means the weight of MP-tmos benzoxazine monomer, and b means the weight of the silica nano-particles) were cured at 200 ◦ C for 1 h to investigate the wetting properties by CA measurement. The results were shown in Table 1. With the increasing of the nanoparticle proportion, the water contact angles of the films increased continuously. And the contact angles of MS-0.5/0.6 film could reach up to 154.3 ± 0.9◦ , exhibiting a good hydrophobicity. Further, the curing temperatures of MS-0.5/0.6, MS-0.5/0.5 and MS-0.5/0.3 were optimized to improve the superhydrophobic
Fig. 2. The water contact angle of (a) benzoxazine-silica surface; (b) polybenzoxazine-silica surface cured at 180 ◦ C for 1 h; Shapes of 8 L water droplets on the polybenzoxazine-silica surface with different tilt angles: (c) 90◦ and (d) 180◦ .
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J. Liu et al. / Applied Surface Science 353 (2015) 1137–1142 Table 2 The EDX results of the microstructure in the polybenzoxazine-silica surface.
Fig. 3. SEM image of (a) polybenzoxazine-silica surface and (b) enlarged view of polybenzoxazine-silica surface image.
properties. As shown in Fig. 1, the CA of MS-0.5/0.3 and MS-0.5/0.5 were stable at ca. 150◦ with the increasing of the curing temperature. However, the CA of MS-0.5/0.6 film cured at 180 ◦ C for 1 h could reach up to 161.5 ± 0.7◦ from 40.5◦ before cured. Therefore, we focused our research on MS-0.5/0.6, since it possesses the best hydrophobic performance after curing. Simultaneously, the sliding angle was measured. The droplet became pinned to the surface and could not roll off, even when the surface was turned upside down, showing the “pinned effect” or “petal effect” phenomenon. This indicates that the polybenzoxazine-silica surface prepared by simple one-step thermal curing method was not a traditional superhydrophobic surface, but a “petal effect” surface. It is expected that a surface with a sufficiently high adhesive force to a liquid will have many potential applications, such as liquid transportation without loss and the analysis of tiny volumes of liquid samples [24]. The “petal effect” superhydrophobic polybenzoxazine-silica surface can be used as a “mechanical hand” to transfer small water droplets from superhydrophobic surface to a hydrophilic one (Fig. 2) [30]. To further investigate the mechanism for the formation of this “petal effect” surface, the polybenzoxazine-silica surface morphology was measured by SEM (Fig. 3). Since the film was prepared through the reaction between polybenzoxazine and silica particles, and the size of polybenzoxazine was much smaller than
Element
Weight (%)
Atomic (%)
Net Int. Error
C O Si
14.73 38.00 47.27
23.21 44.95 31.85
0.03 0.01 0.01
the silica particle, the surface morphology of the film was dominated by the silica particles. Therefore, there were many separate microislands (40–50 m) on the “petal effect” superhydrophobic surface (Fig. 3a). Each microisland was covered with nanospheres (30–50 nm; Fig. 3b). These micro- and nano-scale binary structures dramatically increased the surface roughness, which led to composite interfaces [31] and gave an appropriate volume of air sealing on the surface. When the surface flipped even upside down, the air pockets would have a negative pressure and have an attraction to the water droplet, thereby strengthening the adhesion between the droplets and the surface [32]. In order to confirm the composition of the micro- and nanostructures of this superhydrophobic surface, the elements were characterized by EDX. The results (Table 2) showed that there were 3 elements (C, O and Si) existed in the microstructure. Since the polymer was the only source of carbon, we could confirm that the microstructure consisted of both polymer and silica particles. The similar test was also measured in the nanosphere area, and the result was the same as the micro-structure. In our previous study, it was proved that only by curing at a fixed temperature for a short time, the ring-opening reaction of the benzoxazine and the cross-linkage of Si O Si would occur simultaneously [29]. The trimethoxysilane groups of the MP-tmos benzoxazine monomer reacted with silanol groups on the surface to produce Si O Si linkage. As a result, the silica nano-particles were strongly adhered to the substrates through MP-tmos benzoxazine (as shown in Scheme 1). The adhesion of the polybenzoxazinesilica coating on the substrates was investigated by the cross-cut method [33]. The SEM image of the polybenzoxazine-silica surface after cross-cut test was shown in Fig. 4, which demonstrated that there was a strong adhesive force between the SiO2 -containing polybenzoxazine film and the substrates. To achieve commercialization, performances of UV-resistance, transparency and thermal stability are of high importance. Therefore, MS-0.5/0.6 surface was irradiated under 254 nm UV light
Fig. 4. SEM image of MS superhydrophobic surface after adhesion test.
J. Liu et al. / Applied Surface Science 353 (2015) 1137–1142
Fig. 5. Contact angle of polybenzoxazine-silica surface measured after irradiation by 254 nm UV-light for different times.
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Fig. 7. Thermal stability of polybenzoxazine-silica composite materials.
Table 3 Thermal stability of polybenzoxazine-silica composite material. Resin sample
Temp. at 5 wt% loss (◦ C)
Temp. at 10 wt% loss (◦ C)
Char yield (%) 800 ◦ C
MS-0.5/0.3 MS-0.5/0.5 MS-0.5/0.6
354 365 376
422 454 472
72.91 80.79 83.10
Even heated to 800 ◦ C, the char yields of these films were 72.91%, 80.79% and 83.10% (Table 3), respectively, which revealed that all the composite materials had good thermal stability. The superhydrophobic films were also treated at high temperature, and then their water contact angles were measured. The results showed that the polybenzoxazine-silica surface still maintained the superhydrophobic properties after 300 ◦ C treatment. Fig. 6. Light transmitting property of polybenzoxazine-silica surface.
4. Conclusions irradiation for different times, and then the water CA were measured. As shown in Fig. 5, no obvious changes were observed in the surfaces even irradiated for 60 min, which indicated that the “petal effect” superhydrophobic surface exhibited good UVresistance performances. Moreover, the water CA of the MP-tmos polybenzoxazine was also measured after exposed in 254 nm UV light for 60 min, and the results showed that the precursor of the MS-0.5/0.6 surface was UV-resistant. Due to the other component of the MS-0.5/0.6 surface, the silica particle, was also stable under UV light irradiation. That was why the superhydrophobic surface had good UV resistance property. Then, the transparency of this smart surface was tested by covering it on the university badge (Fig. 6). The clear vision of the badge indicated a good transparency of this “petal effect” superhydrophobic surface. This property was also related to the micro- and nanostructure on the surface. The microstructure islands were composed of nanospheres, and the size of nanospheres was 30–50 nm, which is much smaller than the visible light wavelength. Also, there were many silica nanoparticles and Si-O-Si bonds in the film, which was close to the glass’s composition, so the film’s refractive index was similar to the glass. For those two reasons, the film showed transparent property. Finally, the thermal stabilities of these polybenzoxazine-silica composite materials were evaluated by TGA. MS-0.5/0.3, MS0.5/0.5 and MS-0.5/0.6 were cured at 180 ◦ C for 1 h, and then heated under N2 atmosphere from ambient temperature to 800 ◦ C at heating rate of 10 ◦ C/min. As shown in Fig. 7, no obvious losses of the weight were found when the films were heated to 300 ◦ C.
In summary, the superhydrophobic surface was fabricated by adding SiO2 nanoparticles into the polybenzoxazine via one-step thermal curing method. The highest water CA of the superhydrophobic surface could reach up to 161.5◦ , and this film exhibited “petal effect”. The morphology of the surface was investigated by SEM, which showed that each microisland on the surface was covered with nanospheres. It should be highlighted that the surface possessed several excellent properties. Upon cross-cut test, there was little loss of the film, which indicated strong adhesion of the surface to substrates. Moreover, this surface demonstrated brilliant UV-resistance property, because no obvious change was observed on the surface even irradiated for 60 min. The surface also displayed outstanding transparency and thermal stability, and the maximum char yields could reach up to 83.10% even heated to 800 ◦ C. All of these properties endowed this surface with great potential in the further application of the aviation and aerospace field.
Acknowledgements This research was financially supported by the Nanotech Foundation of Science and Technology Commission of Shanghai Municipality (Grant 0652nm001), by the National Natural Science Funds of China (Grant NO. u1162110), the Fundamental Research Funds for the Central Universities (WA1514015), and China Postdoctoral Science Foundation (2015M571509).
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