Static and Dynamic Hydrophobic Properties of Honeycomb

This work reported the static and dynamic hydrophobic properties of the honeycomb porous and peeled films with pincushion structures by changing the ...
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Static and Dynamic Hydrophobic Properties of Honeycomb Structured Films via Breath Figure Method Zhiguang Li,*,† Xiaoyan Ma,*,‡ Que Kong,‡ Duyang Zang,‡ Xinghua Guan,‡ and Xuehong Ren† †

Key Laboratory of Eco-Textiles of Ministry of Education, College of Textiles and Clothing, Jiangnan University, Wuxi 214122, China Key Laboratory of Space Applied Physics and Chemistry and Key Laboratory of Polymer Science and Technology, Ministry of Education, Shaanxi Province, School of Natural and Applied Sciences, Northwestern Polytechnical University, Xi’an 710129, China

‡

ABSTRACT: This work reported the static and dynamic hydrophobic properties of the honeycomb porous and peeled films with pincushion structures by changing the concentrations of the copolymer solution. Honeycomb porous films were fabricated via breath figure technology employing a pentablock copolymer containing poly(ethylene glycol)/PEG, poly(methyl methacrylate)/PMMA, and poly(trifluoroethyl methacrylate)/PTFEMA. The relationship between the copolymer concentrations and the pore size (D) and rim width (W) was examined. Moreover, we studied the hydrophobic properties from different surfaces of flat, porous, and pincushion structures, and the contact angles, W/D ratios, and fraction of air on the porous and pincushion films were obtained. It was noteworthy that the W/D ratios and surface hydrophobic properties had a notable correlation. Finally, the dynamic behaviors of water droplets impacting on these porous and pincushion films were examined using a high speed camera. It examined the influence of the fraction of air on the measurement of the dynamic morphologies, contact angle in equilibrium state, maximum spreading diameter and maximum height of the droplet, and adhesive property of porous and pincushion films. The films obtained in this paper may have some potential applications as hydrophobic, self-cleaning, and antibacterial surfaces.

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INTRODUCTION Superhydrophobic surfaces have attracted significant attention owing to various potential applications such as self-cleaning surfaces, anti-icing surfaces, and impermeable textiles.1−3 As we all know, these superhydrophobic properties are strongly dependent on the surface chemical composition and surface topology.4−9 Superhydrophobic surfaces have been prepared using several methods to lower the surface energy and enhance roughness. To our knowledge, the breath figure (BF) can facilely control surface chemistry and topology of the film.10−14 It is an efficient, inexpensive, and versatile method to prepare films with highly ordered honeycomb structures.15,16 This method utilizes water as a nontoxic and easily available template for the fabrication of ordered porous structures, which can dynamically control the formation process and surface morphology.17−19 The honeycomb porous structure with increased surface roughness obtained from the BF method can enhance the hydrophobic property. Furthermore, the surface of this film is stripped to prepare a pincushion structure which exhibits a superhydrophobic property. The water contact angles (CAs) on the honeycomb porous and pincushion structures are, accordingly, 20−30° and 50−60° higher than that on the flat film in previous investigations.6,20 Such a dramatic increase in CA is often described as Cassie−Baxter-type wetting, where air is trapped between the copolymer surface and water.21,22 © 2016 American Chemical Society

However, the dynamic superhydrophobic property must be maintained in practical situations. It is critical that the dynamic behavior of the water droplet rebounds off the superhydrophobic interface. Hence, the impact dynamic properties have aroused increasing interest which relates to industrial applications, such as inkjet printing, spray coating, spray cooling, and microfabrication.3,23 Moreover, the water impact behavior on a solid surface provides a model system to obtain the surface properties. To our knowledge, the water dynamic behavior is affected by the surface wettability and roughness, while the relationship between the dynamic behavior and surface property remains poorly understood.23,24 On the other hand, various investigations of the relation between the CA and surface structure or chemical composition have been conducted. Nevertheless, understanding of the correlation between the surface structure and dynamic hydrophobic property remains limited, especially for the honeycomb porous and pincushion structure surface states.25 In our previous work, the static and dynamic hydrophobic properties of three different pentablock copolymers of PTFEMA-b-PMMA-b-PEG-b-PMMA-b-PTFEMA have been Received: June 19, 2016 Revised: July 25, 2016 Published: August 2, 2016 18659

DOI: 10.1021/acs.jpcc.6b06186 J. Phys. Chem. C 2016, 120, 18659−18664

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The Journal of Physical Chemistry C

Figure 1. SEM images of the pentablock copolymer at different concentrations of (a, e) 20 mg/mL, (b, f) 30 mg/mL, (c, g) 40 mg/mL, and (d, h) 50 mg/mL. (a−d) Top images of the porous films; (e−h) side view at tilt angle 50° of the peeled films.

and peeled films are fabricated from the surface of the stripped porous films.29 The SEM images of the porous films (a−d) and peeled films (e−h) of 20, 30, 40, and 50 mg/mL are illustrated in Figure 1. It is shown that the ordered porous films are prepared from the four concentrations, and the pincushion structures are formed in the peeled films. Generally, the mass of the solvent is directly influenced by the concentration, which would have an impact on the duration of solvent volatilization. Moreover, the concentration can determine the viscosity and density of the copolymer solution, and further affect the deposition of water droplets, which would affect the pore size of honeycomb porous films.26 Figure 2 shows the concentration effects on D and W of the porous films. As mentioned, the D and W decrease as the concentration increases.

examined.26,27 Nevertheless, the rim width/pore size (W/D) and water impact behaviors with the water CAs are confused, as is also the adhesive property of the water impact behavior in the porous and peeled films. In this work, the films with porous and pincushion structures result from four concentrations (20, 30, 40, and 50 mg/mL) of the pentablock copolymer and their hydrophobic properties are studied. In addition, the interfacial static and dynamic hydrophobic properties of the films with different surface morphologies are investigated.

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EXPERIMENTAL SECTION

Materials. The pentablock copolymer of PTFEMA-bPMMA-b-PEG-b-PMMA-b-PTFEMA (Mn = 26790 g/mol, PDI = 1.49) was synthesized in our previous study.26 Deionized water was fabricated using an ELGA Lab water system in the experiment. Preparation of Honeycomb Films. The honeycomb film was fabricated via BF technology. The concentrations of 20, 30, 40, and 50 mg/mL of the pentablock copolymer were prepared in chloroform solution, and then cast onto a silicon wafer under a humid airflow. The ordered porous structure was prepared in the BF method. In addition, the flat film was cast from the copolymer dissolved in chloroform solution and volatilized under normal circumstances. Water Droplet Impact Dynamics Monitor. A water droplet of 10 μL was released at a certain height to obtain a uniform speed to have an impact on the porous and pincushion films. The dynamic impact behavior as a function of time was recorded using a camera of frame rate of 2000 fps (Trouble Shooter HR). Characterization. Surface morphologies of the films were characterized by a scanning electron microscope (SEM) carried on VEGA 3 LMH (Č esko TESCAN) with 10 kV accelerating voltage. CA was tested by the pendent drop method (JC2000D4 Powereach Tensionmeter).

Figure 2. Concentration effects on average pore size and rim width of porous films.

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The viscosity of the solution is increased by enhancing the concentration, which results in an increased efficiency of polymer precipitation of the water droplet. The water droplets are more slowly grown, and smaller D formed. Moreover, the precipitation rate increased with the enhancing concentration. The droplets are encapsulated and solidified at a short time result in small D. Meanwhile, the W has the same trend of becoming thicker with increasing concentration, which could be easily understood by the increasing compounds of the copolymer.

RESULTS AND DISCUSSION Copolymer Concentrations Contribute to the Honeycomb Porous Films. In previous BF studies, parameters such as solvent type, copolymer concentration, and humidity were used to achieve different surface morphologies of the pores.9,28 In this study, the copolymer concentration is controlled to regulate the morphology of the porous film. The porous films are obtained from four concentrations through the BF method, 18660

DOI: 10.1021/acs.jpcc.6b06186 J. Phys. Chem. C 2016, 120, 18659−18664

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The Journal of Physical Chemistry C Hydrophobic Properties from Various Surface Structure Films. The hydrophobic property of the film prepared from the BF method is affected by the surface morphology for the same copolymer. Tuning the surface morphology, such as pore size and rim width, may enhance the surface roughness leading to an increase of the hydrophobic property. The hydrophobic properties of the flat, porous, and peeled films prepared from the four concentrations are measured by the static CAs. The static CAs are exhibited in Figure 3.

Figure 4. Fractions of the air on the porous and peeled films of (a) 20 mg/mL, (b) 30 mg/mL, (c) 40 mg/mL, and (d) 50 mg/mL.

ratios is shown in Figure 5. It is found that a low W/D ratio generally indicates a high CA. Furthermore, a low W/D ratio

Figure 3. Concentration effect of water CAs on the flat, porous, and peeled films.

As shown in Figure 3, with respect to the CA of about 96° on flat films, the CAs on these porous films are 129.52°, 122.43°, 118.98°, and 110.38°. The increase of the CA is due to the changes of the roughness on the surface. The hydrophobic property is reduced as a result from a less rough surface with an increase in the concentration.30−32 Moreover, it is obvious that the pincushion structure can improve the hydrophobic property and raise the CAs to 153.27°, 143.17°, 140.26°, and 134.91°, respectively. The relationship between the surface property and the CAs can be illustrated by the Cassie−Baxter law:33−38 cos θr = f1 cos θ − f2

Figure 5. W/D ratios as a function of water CAs.

increases the fraction of air on the surface, and thus the accessible surface area is reduced for the spread of the water droplets, while the air entrapped in pores supports the droplets. Both the increased fraction of air and surface roughness are favorable for enhancing the CA.31 With the decrease in the concentration, the hydrophobicity of the film is enhanced. To further investigate the relationship between the hydrophobic property and dynamic behavior of water droplets, the droplet impact behavior is investigated on different surfaces. Figure 6 shows the morphology variations of the water droplets as a function of time after impact on different surfaces. As illustrated in Figure 6, the droplet shows various impact behaviors such as spreading, retraction, and oscillation on these porous and peeled surfaces. At 0 ms, it is found that the water droplet is a spherical shape in its initial state. It changes into a pancake shape and stretches out, remaining completely fastened onto the surface at 5 ms. The droplet deforms and spreads rapidly, and after the maximum spread it moves back toward the center. It remains in contact with the surface and pulsates violently as the droplet kinetic energy is converted into vibrations.43 Finally, the equilibrium CAs of the droplets are decreased from part a to part d. In addition to the impact behavior, the maximum spreading diameter of the droplet, Dmax/D0, is significantly enhanced by the decreasing static CA as shown in Figure 7. For the first 2 ms, the Dmax/D0 value enhances rapidly regardless of the surface morphologies; this indicates that initial droplet spreading is dominated by inertial effects.44 There is no significant

(1)

Here f1 is the fraction of solid, and f 2 is the fraction of air on the surface (f1 + f 2 = 1). θr and θ are the CAs of the rough and flat surface, respectively. In this case, the air is trapped in the pores underneath by the water droplets. As a result, the water droplets are partly placed on the trapped air and are partly in contact with the solid surface. The air prevents water penetrating into the copolymer surface, thereby leading to a more hydrophobic surface.30 A higher f 2 results in larger CA and prevents the pores from being wetted.39 As a result, the CA can increase as the fraction of air increases. The fractions of the air on the porous and peeled films are shown in Figure 4. It is obvious that the pincushion film has higher f 2 than the honeycomb film. Moreover, the lower concentration of the film also has a higher f 2. In addition, variations in the surface morphologies allow the W and D to be adjusted from SEM images. The W/D ratio is correlated with the surface roughness which influences the hydrophobic property.4,40 Thus, the W/D ratio serves as an index to quantify the surface roughness of the porous film, with a low value representing a uniform distribution of evenly sized pores of the film.4,40−42 The relationship between CA and W/D 18661

DOI: 10.1021/acs.jpcc.6b06186 J. Phys. Chem. C 2016, 120, 18659−18664

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The Journal of Physical Chemistry C

Figure 6. Morphology variations of water droplets impacting different surfaces. (a, b) Peeled films obtained from 20 and 40 mg/mL, respectively. (c, d) Porous films obtained from 20 and 40 mg/mL, respectively.

Figure 8. Fractions of air effect on the Hmax/D0 of the water droplet impact behavior.

Here, E1 is the potential energy at a certain height, and E2 is the energy loss during water impact on the surface. E2 is mainly surface adsorption and transformation energy of the water droplet. The adsorption energy of a droplet on air is considerably lower than that on the copolymer surface. The fraction of the air promotes the droplet to rebound greatly, with less energy being lost. The capillary pressure is larger for small air pockets trapped on the surface which could assist the water droplets to rebound more.46 Hence, Hmax/D0 is increased by the enhancing fraction of air. The adhesive property of porous and peeled surfaces varies. Since the chemical compositions of the films are all the same, the differences of the surface structure are considered as the main reason for the adhesion distinctions. Adhesion, which could illuminate the bouncing effect quantitatively, is shown below:47

Figure 7. Contact diameter of the droplets measured by the initial diameter before the impact of water droplets as a function of time.

WSL = γLV(1 + cos θ )

difference in the value response of the four hydrophobic surfaces. However, after 2 ms, the values present fluctuate in the following and do not decrease to zero. Moreover, the maximum Dmax/D0 values are decreased from 1.22 to 1.04, while the minimum values are decreased from 0.93 to 0.43. This result demonstrates that the water droplet becomes pinned to the surface. This pinning appearance suggests that the initial impact of water enables it to penetrate into the pores on the surface and effectively pushes out the entrapped air. The enhanced Dmax/D0 value significantly increases the adhesion of the droplet to the copolymer surface and prevents the contact line from retracting. Therefore, the water droplet is unable to recover sufficient energy to rebound off, and as a consequence, it simply vibrates.43 The porous and peeled surfaces cannot be fully wetted due to the droplet not having sufficient kinetic energy. The water droplet would recede or shrink to form the equilibrium state. Richard et al. found that the transfer of the kinetic energy into droplet vibrations led to a limit of the energy recovery ratio of the impact.45 Figure 8 shows the fractions of air effect on the Hmax (the maximum height of the droplet)/D0 of the droplet impact on the honeycomb and pincushion surfaces. It is found that Hmax/D0 is significantly weakened by the increased CA. The Hmax/D0 is decided depending on the quantity of the residual energy (E), where

E = E1 − E2

(3)

Here WSL is the adhesion, and γLV is the surface tension of the liquid. The adhesion increases with decreasing CA. As a result, surfaces with low CAs have a better wettability result as more energy is dissipated from the inherent kinetic energy during collision with the copolymer surface.48 The spreading of the droplet significantly increases the adhesion and prevents the contact area from retracting. Therefore, the droplet is unable to recover sufficient energy to rebound off, and it simply vibrates on the surface. It has been demonstrated that the large adhesive force of honeycomb porous surfaces is induced by the volume change of sealed air pockets (vacuum effect) and Van der Waal forces.49 Figure 9 shows the schematic illustration of the adhesive state for the porous structure (left) and pincushion structure (right). For the peeled films, the water droplets can penetrate into the large pores of the pincushion surface, leading to high adhesion force and capillary force.50 Large amounts of air pockets on the surface can greatly reduce the contact area between the surface and droplet and therefore make the droplet move off the surface easily. On the other hand, the contribution of the vacuum effect of the porous surface is obviously larger than the pincushion surface. The porous films would lead to stronger capillary force, which can promote the droplet to enter into the porous structures.8 The droplet impregnates into the porous structure, therefore allowing the droplet to stick to the surface.

(2) 18662

DOI: 10.1021/acs.jpcc.6b06186 J. Phys. Chem. C 2016, 120, 18659−18664

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The Journal of Physical Chemistry C

Figure 9. Schematic illustration of the adhesive state for the porous structure (left) and pincushion structure (right).

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CONCLUSIONS In this paper, honeycomb ordered porous films from different concentrations of pentablock copolymer are successfully prepared through the breath figure. The static and dynamic hydrophobic properties of the porous and peeled films are investigated. The pore size and rim width of the honeycomb porous films are decreased as the enhancement of the concentration. It is found that a low W/D ratio generally indicates a high CA. Furthermore, the dynamic hydrophobic property is studied by water droplet impact on the honeycomb and pincushion strucrues. The rebounding tendency of the droplet becomes more obvious with increasing CA. Additionally, the droplet is unable to recover sufficient energy to rebound off, and as a result, it simply vibrates. The maximum height of the droplet is decided according to the amount of the residual energy. The volume change of sealed air pockets (vacuum effect) influences the maximum spreading diameter and the adhesive property. For the peeled films, the pincushion structure can achieve weak water adhesion. From another point of view, the droplet impregnates into the porous structure which allows the droplet to stick to the surface.

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Innovation Project of Science and Technology of Shaanxi Province (Grant 2013KTCG01-14).

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +86-510-85912007. E-mail: [email protected]. *Phone: +86-29-88431676. E-mail: [email protected]. cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant 51301139), Natural Science Foundation of Shaanxi Province (Grant 2012JQ1016), and 18663

DOI: 10.1021/acs.jpcc.6b06186 J. Phys. Chem. C 2016, 120, 18659−18664

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DOI: 10.1021/acs.jpcc.6b06186 J. Phys. Chem. C 2016, 120, 18659−18664