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Nonsolvent assisted electrospraying is utilized to fabricate hierarchically porous microspheres for superhydrophobic coating.
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Facile preparation of hierarchically porous polymer microspheres for superhydrophobic coating Jiefeng Gao, Julia Shuk-Ping Wong, Mingjun Hu, Wan Li, Robert. K. Y. Li*
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Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X DOI: 10.1039/b000000x A facile method, i.e., nonsolvent assisted electrospraying, is proposed to fabricate hierarchically porous microspheres. The pore size on the mircorsphere surface ranges from a few tens to several hundred nanometers. Thermally and nonsolvent induced phase separations as well as breath figure are responsible for the formation of the hierarchical structures with different nano- sized pores. The nonsolvent could not only induce phase separation, but also stabilize the interface between the droplet and air, which can prevent the droplet from strong deformation, and is therefore beneficial to the formation of regular and uniform microspheres. On the other hand, solvent evaporation, polymer diffusion and Coulomb fission during electrospraying influence the morphology of finally obtained products. In this paper, the influence of polymer concentration, the weight ratio between nonsolvent and polymer and the flowing rate on the morphology of the porous microsphere are carefully studied. The hierarchically porous microsphere significantly increases the surface roughness and thus the hydrophobicity, and the contact angle can reach as high as 152.2 ±1.2. This nonsolvent assisted electrospraying opens a new way to fabricate superhydrophobic coating materials.
Introduction 20
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Polymer micro/nano particles have attracted much attention recently because of their potential applications in many fields such as controlled release, catalyst, scaffold, selective separation, chemical sensors.1-6 It is quite desirable if pores are introduced into the micro/nano spheres, because porous structure possesses a higher specific surface area, a lower density as well as a better permeability, which are required in many demanding applications.7-10 Generally, polymer particles could be fabricated by heterogeneous polymerization using the immiscibility of two or more liquids, including suspension, precipitation, multistage, membrane/microchannel emulsification and microfluidic polymerization.11 A porogen is often required in the solution, in order to obtain porous polymer particles. A good solvent, a nonsolvent, liner polymers, water and solid particles could serve as the porogen.12-20 Generally, the porogen that is initially dissolved or swollen in the monomer phase separates with oligomers or macromolecules during the polymerization, leading to the pore formation. Pores are also generated after the removal of solid particles embedded on polymer beads via washing or etching. Pore size and density could be controlled by choosing different porogens. It was reported that small pores (below 10 nm) and hence very high surface area values (800 m2/g) were obtained if a good solvent was chosen as the porogen; however, a poor solvent corresponded to larger pores and thus a lower surface area.21 Although, heterogeneous polymerization is popular for preparation of porous microspheres, a large number of emulsifiers or stabilizers are usually required for the This journal is © The Royal Society of Chemistry [year]
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polymerization route, which inevitably bring in impurities and thus influence the performance of the materials. On the other hand, polymerization usually needs delicate manipulation, and the whole fabrication process is complicated. For example, the interfacial tension should be carefully controlled, because it can affect the geometry of the monomer droplets. Therefore, it is high of importance to explore a simple but effective approach to obtain porous polymer microspheres. Electrospraying, which is a modified version of electrospinning, has been developed to prepare microspheres with porous structures. Jiang and coworkers fabricated microspheres by electrospraying polystyrene (PS)/dimethyformamide (DMF) solution. Vapor induced phase separation was responsible for the formation of the porous structure. During the electrospraying, the polymer solution droplet was cooled down, due to the solvent rapid evaporation. As a result, the water vapor in air condensed on these cold PS/DMF droplets. Because of the immiscibility between the PS and water, micro-phase separation occurred on the watercondensed sites, and eventually led to the generation of a macroporous structure.22 Porous polymer particles could also be obtained by electrospraying polymer solution into a nonsolvent bath, at the bottom of which the collector is immersed. It is crucial that the solvent does not evaporate completely before the spheres deposit on the collector, and mass exchange between the residual solvent in the droplet and the nonsolvent would occur, giving rise to the phase separation. The nonsolvent induced phase separation was regarded as the main mechanism for the generation of the porous microstructure.23, 24 However, as reported in the literatures the electrosprayed microspheres are [journal], [year], [vol], 00–00 | 1
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usually irregular and nonuniform. In addition, it is also difficult to control the porous structure. Here, in this article, nonsolvent assissted electrospraying, is first proposed to prepare hierarchically porous microspheres. The employed nonsolvent, hexanol, facilitates the formation of hierarchical pores with various sizes on the sphere surface. Furthermore, the hexanol can stabilize the droplet interface, resulting in the formation of regularly spherical microspheres. In addition to its simplicity in comparison with other techniques, this method can realize a large fabrication of the hierarchically porous materials by only extending the electrspraying time. It has also been demonstrated that the micro- and nano-scale hierarchically porous structures significantly improve the surface roughness, and thus enhances the hydrophobicity of the sphere coated substrate. This nonsolvent assisted electrospraying opens a new way to fabricate superhydrophobic coating materials.
Experimental Materials
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Polymethylmethacrylate (PMMA), a commercial product from Chi Mei company (Mw=112000). Dichloromethane (DCM) and hexanol with boiling points of 39.8 and 156 °C respectively were purchased from Sigma-Aldrich Corporation, and used as received.
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Fig. 1 Schematic illustration of (a) a typical electrospraying setup and (b) formation of hierarchically porous microspheres. (c) SEM and (d) TEM images of porous microspheres prepared from hexanol assisted electrospraying. Polymer concentration is 3 wt%. (e) Size of hierarchical pores for the electrosprayed microspheres shown in (c). Scale bars: (c and d) 1 m.
Morphology and formation mechanism of the hierarchically porous microspheres.
Electrospraying 25
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For nonsolvent assisted electrospraying, the solvent and nonsolvent were first mixed with a certain ratio, and then a desired amount of polymer was dissolved in the mixture. The prepared ternary solution was subsequently subject to magnetic stirring for three hours at room temperature. The stable and transparent polymer solution was then loaded in a syringe connected with a metallic needle for electrospraying. The electrosprayed products were collected in a grounded aluminum foil, which was 12 cm far from the metallic needle. Unless specified, the parameters for electrospraying were chosen as follows: polymer concentration: 3 wt%; the weight ratio between polymer and nonsolvent: 2:1; the voltage: 12 kv; the flowing rate: 1ml/h.
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Morphology Characterization
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The morphologies of the electrosprayed microspheres were characterized by a field emission scanning electron microscope (FESEM JSM6335) operated at 5kv. The electrosprayed microspheres were also deposited on copper grids covered with a carbon film for Transmission Electron Microscopy (TEM) observation, and the accelerating voltage was 20 kv.
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Contact angle measurement
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Contact angle (CA) measurements were carried out on a threeaxis horizontal tilt stage on an optical table. Deionized droplets were carefully dropped onto the surface of the sample (spin coated PMMA film and the electrosprayed PMMA film). The CA values were determined by averaging at least four separate runs.
Results and discussion
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Fig. 1a gives a schematic illustration for a typical electrospraying setup, which essentially consists of a syringe connected with a metallic needle, a high voltage power supply, a grounded collector, etc. A digital pump is used to control the flow rate of the polymer solution (stored in the syringe) to high precision. It is obvious that the setup is essentially the same as employed for electrospinning. Whether fibres or particles will form from the process depends mainly on the viscosity of the polymer solution.. When a high voltage is applied to the metallic needle, the pendent polymer solution at the nozzle will be subjected to the electrical field, and the induced charges are evenly distributed over the surface. Under the electrostatic repulsion and the coulombic force caused by the electrical field, the polymer solution droplet is stretched into a conical jet and further breaks into small droplets provided that the applied electric field is sufficiently high, and polymer particles are finally obtained when the solvent is completely evaporated during the process.25-27 On the other hand, the electrified jet is elongated and whipped to form fibers if the viscosity of the solution increases to a critical value. In this case, electrospinning other than electrospraying occurs.28, 29 Fig. 1b shows a schematic diagram for the evolution of the solution droplet during the electrospraying. The original solution, namely Polymethylmethacrylate (PMMA)-Dichloromethane (DCM)hexanol ternary system, is first prepared. PMMA macromolecular chain is disentangled and separated from each other in DCM. Hexanol can't dissolve PMMA(hence called the non-solvent) but is miscible with DCM. During electrospraying, DCM evaporates first, due to its much lower boiling point compared with hexanol. Naturally, the hexanol following DCM also migrates to the surface region, because the hexanol is only miscible with DCM. In addition, the hexanol can be regarded as a kind of surfactant This journal is © The Royal Society of Chemistry [year]
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hand, chain entanglements and Coulomb fission can also affect the microstructure of the electrosprayed microsphere. Coulomb fission occurs when a charged droplet reaches the Rayleigh limit,35 at which the drop cannot hold more charges than a limiting value and break into many small but highly charged offspring droplets. The competing mechanism of chain entanglements and Coulomb fission can lead to various morphologies for the electrosprayed particles. The competing result could be described by φRay, φent and φov, which refer to the polymer volume fraction in a droplet at the Rayleigh limit, the critical entanglement polymer volume fraction, and critical chain overlap polymer volume fraction, respectively. If φRay < φov, which means polymer chain entanglements are absent when the Rayleigh limit is reached, the droplet behaving like a pure liquid experiences the Coulomb fission, generating many offsprings. When φov < φRay < φent, the droplet can keep some integrity, but is not strong enough to prevent the particle from deforming via stretching during the electrospraying, leading to the formation of tailed particles and ultrathin fibers. After φRay > φent, fiber-free and spherical particles could be obtained, because sufficient chain entanglements can prevent the droplet from deforming. φRay could be described using Equation (1) listed below: 36
Ray
288 air
(1)
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I 3 d Q
Where φ, γ, εair, I, Q and d refer to the polymer volumn fraction, the surface tension of solution, the permittivity of air, the current, the liquid flow rate and the initial droplet diameter, respectively. 85
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Fig. 2 shows the SEM and TEM images of electrosprayed microspheres at a very low polymer concentration. It can be found that at a polymer concentration of 0.75 wt%, the microspheres (with the diameter of about 1.5 μm) did not have a perfect spherical shape. Most of the microspheres collapsed to form several large voids (see Fig. 2a, a'), which were much larger than that formed from a higher polymer solution concentration (shown in Fig. 1c). The diameter of some of the large voids was even comparable with that of the whole microsphere. When the
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Factors influencing the hierarchically porous microspheres 55
As discussed before, electrospraying is a very rapid and complex process, and the solvent evaporation and polymer diffusion during the process are two important factors influencing the morphology of finally obtained products.31-34 On the other This journal is © The Royal Society of Chemistry [year]
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Fig. 2 SEM images of the hierarchically porous microspheres. Polymer solution concentration is (a) 0.75 wt% and (c) 1.5 wt%. (a'), (c') are the magnified SEM images for (a) and (c). (b), (d) are the TEM images of the corresponding microspheres. Scale bars: (a and c) 10 m; (a' and c') 1 m; (b) 200 nm; (d) 500 nm.
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and tends to find its place on the surface of the droplet with the hydrophilic group (-OH) heading outside while the hydrophobic component (-CH2) chain staying inside. It is worth noting that the hexanol gathering on the surface could stabilize the interface between the droplet and air, which can prevent the droplet from strong deformation, and is therefore beneficial to the formation of regular microspheres. With continuous loss of the solvent, phase separation occurs, forming the polymer rich phase and polymer poor phase. The polymer rich phase consisting of concentrated PMMA, begins to entangle and shrink, occupying the interior part. However, the polymer poor phase which contains most of DCM and hexanol escapes to the exterior surface of the droplet, and subsequently the hexanol starts to nucleate and grow on the surface of the droplet, leading to the formation of the relatively large pores. On the other hand, thermally induced phase separation (TIPS), due to the cooling effect caused by the evaporation of the volatile DCM, may be responsible for the formation of tiny pores inside the walls of the large voids. At the same time, the breath figure phenomenon may also occur as the water vapor condenses on the surface. Finally, the hierarchically porous microspheres can be obtained when both the DCM and the hexanol are completely evaporated. It was found that most of electrosprayed PMMA microspheres collapsed, displaying irregular shape (Fig. S1†), if nonsolvent was not added into the polymer solution. It was also worth noting that many tiny pores with a few tens of nanometers could be observed on the sphere surface, which was probably caused by TIPS. However, for our hexanol-assisted (where hexanol is served as the nonsolvent) electrospraying, it can be clearly seen that highly regularly spherical microspheres with hierarchical pores through the whole surface are produced (Fig. 1c). Interestingly, hierarchical pores with different sizes were observed on the microsphere surface. Here, three kinds of hierarchically porous structures with different diameters can be defined: (1) large and deep voids (>350 nm); (2) small and shallow pores ( φent, and Coulomb fission could be avoided due to the fact that entanglements are present before the Rayleigh limit is reached. With further increasing the Q to 3ml/h, many round microspheres tend to be transformed to elliptic microrods (See Fig. 5d). The current I on the surface of microsphers increase with Q, generating a larger electrostatic force and thereby driving the microspheres to deform to form the elliptic microrods. Following the microspheres deforming, the surface pores also became elongated, exhibiting the elliptic shape.
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Surface wettability is a very important aspect of material properties, which can affect the practical application of materials in many fields. Surface with a water contact angle (CA) higher than 150 is called superhydrophobic surface. Water droplets on a superhydrophobic surface easily roll off even at inclinations of only a few degrees, and as a result, surface contaminants is removed easily. The unique water repellent properties have attracted considerable interest from both industry and academy, due to their promising applications such as self-cleaning, antifogging and oil-water separation.40-43 Generally, the surface wettability is mainly determined by the chemical composition and the geometrical architecture of the surface. Materials with a low surface free energy tend to exhibit hydrophobic performance. However, the maximum contact angle for a fluorinated smooth surface is reported only about 120.44 Inspired by the nature, scientists mimick plant leaves such as lotus and silver ragwort to obtain hierarchical surface with a large roughness, in order to achieve higher contact angles.45-48 In fact, the apparent contact angle could be described by the Cassie-Baxter model, in which liquids are assumed to only contact the solid through the top of the asperities, and air pockets are assumed to be trapped
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underneath the liquid. It is well known that the air trapped in the surface is very important to the hydrophobicity, because the water CA of air is regarded to be 180. The Cassie-Baxter equation is listed below: 49
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cosCB f1 cos0 f 2
(2)
Where θCB is the CB contact angle, θ0 is the contact angle on a flat surface, and f1, f2 are the fractions of the solid surface and air in contact with the liquid, respectively. Note that f1 + f2 =1, and it could be concluded that a larger f2 (more air pockets) will induce a larger θCB. In our experimental, the hierarchically porous microstructure of the electrosprayed microsphere could significantly improve the surface roughness, and was therefore beneficial to the increase of the surface hydrophobicity. The flat PMMA film obtained by a spinning coating possesses a CA of 86.6 ± 0.8 (Fig. 6a); however, the CA significantly increases to 135 ± 1.4 for the porous microsphres elctrosprayed without nonsolvent assistance (Fig. 6b), and it further increases to 152.6 ± 1.2 for the hierarchically porous microsphere (Fig. 6c). As such a contact angle, the surface becomes superhydrophobic. For the flat film, the surface is smooth without roughness, corresponding to a low CA; however, a high CA can be achieved for the electrosprayed microsphere coated substrate, because particles randomly stacked onto a substrate could create a hydrophobic surface. On the other hand, pores on the microsphere surface are capable of capturing more air, thus increasing the hydrophobicity. There are only tiny pores on the microsphere surface if nonsolvent is not used. However, the microspheres based on hexanol assisted electrospraying have deep voids, shallow pores and tiny pores inside the voids, and the miro/nano hierarchical structure created a larger surface roughness, so that more air could be trapped in the surface pores, leading to a higher CA. Table 1 lists the CAs and f2 for the porous microsphere coated substrate with different electrospraying parameters (the polymer solution concentration, weight ratios between nonsolvent and polymer and the flowing rate). It is found the CAs for all the hierarchically porous electrosprayed microspheres is higher than 135 , i.e., the CA for the porous microsphres prepared without nonsolvent assistance. Especially, as mentioned, the CA of the porous microsphere coated surface reaches as high as 152.6 ± 1.2 when the polymer concentration is 3 wt%, and f2 is calculated to be 0.894, which indicates the air trapped in the rough surface containing hierarchical pores is the main reason for the achievement of superhydrophobicity. The tailed microspheres obtained from electrospraying polymer solution with concentration of 3.75 wt% displays a decreased CA, i.e., 144.5 ± 2.2. On the other hand, the very large voids on the surface of the porous sphere (see Fig. 2a') decreased the surface roughness, leading to a relatively lower CA, namely 138.6 ± 1.5 for polymer concentration of 0.75 wt%. As mentioned above, WR could influence the morphology of the porous microsphere, and thus the CA. It is found that the CAs are 141.5 ± 1.4, 148.6 ± 2.1, 150. 5 ± 1.1 and 143. 7 ± 1.2 when WR are set as 1:4, 1:1, 4:1, and 10:1, respectively. The appearance of thin fibers attached on the microsphere (Fig. 4a) and coalescence of the surface pores (Fig. 4d) decrease the surface roughness of the substrate, causing the decrease of the CAs. Ultrathin fibers and tails are also observed for porous microspheres with the flowing rate of 0.33, and 0.67 ml/h, which correspond to the relatively low CAs, 6 | Journal Name, [year], [vol], 00–00
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Fig. 6 Images of water droplets (a) on a spin-coated PMMA film (b) and (c) on PMMA microsphere coated film electrosprayed without and with nonsolvent assistance.
namely 142. 8 ± 1.5 and 143. 9 ± 1.4, respectively. The CA rises to 150.3± 1.7 for the flowing rate of 2 ml/h, and it slightly decreases to 147. 9 ±1.2, due to the elongated porous structure. Table 1 Surface properties of the porous microsphere prepared under different electrospraying parameters. CA ( )
f2
138.6 ±1.5 141.7 ±1.6 147.3 ±1.8 152.6 ±1.2 144.5 ±2.2
0.764 0.797 0.850 0.894 0.824
1:4 1:1 4:1 10:1 Flowing rate (ml/h) 0.33 0.67
141.5 ±1.4 148.6 ±2.1 150.1 ±1.1 143.7 ±1.2
0.795 0.862 0.874 0.817
142.8 ±1.5 143.9 ±1.4
0.808 0.819
2 3
150.3 ±1.7 147.9 ±1.2
0.876 0.856
Electrospraying parameters Polymer concentration (w%) 0.75 1.5 2.65 3 3.75 Weight ratio between nonsolvent and polymer
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In summary, hierarchically porous microspheres were prepared based on nonsolvent assisted electrospraying. Thermally and nonsolvent induced phase separations as well as the breath figure were responsible for the formation of the hierarchical structures with different sized surface pores. The porous microspheres was mechanically weak and tend to collapse at a very low polymer concentrations, and gradually became more integrated with the increase of the polymer concentration. It was found the surface pore became smaller and less apparent when the ratio between the nonsolvent and polymer (WR) increased, and the porous structure was even eliminated when the WR was raised to 10:1, due presumably to the coalescence of the excessive nonsolvent. The flowing rate could also influence the morphology of the porous microspheres, and the microspheres tailed with ultrathin nanofiber were observed at a low flowing rate. The hierarchically porous microsphere significantly increased the surface roughness and thus the hydrophobicity of the substrate, and the contact angle could reach as high as 152.6 ± 1.2. This nonsolvent assisted electrospraying opens a new way to fabricate superhydrophobic coating materials.
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This work is supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project Number: CityU 118313). 5
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Notes and references Department of Physics and Materials Science,City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong. E-mail: [email protected] † Electronic Supplementary Information (ESI) available: [SEM images of microspheres prepared from electrospraying PMMA solution without nonsolvent]. See DOI: 10.1039/b000000x/ 1 X. T. Shi, S. Chen, J. H. Zhou, H. J. Yu, L. Li and H. K. Wu, Adv. Functional. Mater., 2012, 22, 3799-3807. 2 C. Y. Gao, H. Zhang, M. Wu, Y. Liu, Y. P. Wu, X. L. Yang and X. Z. Feng, Polym. Chem., 2012, 3, 1168-1173. 3 S. S. Hwang, C. G. Cho, and H. Kim, Electrochim. Acta., 2010, 55, 3236-3239. 4 A. Salinas-Castillo, M. Camprubi-Robles and R. Mallavia, Chem. Commun., 2010, 46, 1263-1265. 5 H. J. Lee, Y. J Cho, W. Cho and M. Oh, ACS Nano 2013, 7, 491-499. 6 M. Ye, S. Kim and K. Park, J. Control. Release., 2010, 146, 241-260. 7 X. Zhang and K. Tu, J. Am. Chem. Soc. 2006, 128, 15036-15037. 8 J. Han, G. Song and R. Guo, Adv. Mater., 2006, 18, 3140-3144. 9 B. Q. Xie, H. F. Shi, G. M. Liu, Y. Zhou, Y. Wang, Y. Zhao and D. J. Wang, Chem. Mater., 2008, 20, 3099-3104. 10 J. R. Xu, G. J. Chen, R. Yan, D. Wang, M. C. Zhang, W. Q. Zhang and P. C. Sun, Macromolecules, 2011, 44, 3730-3738. 11 M. T. Gokmen and F. E. Du Prez, Prog. Polym. Sci., 2012, 37, 365405. 12 J. H. Ahn, J. E. Jang, C. G. Oh, S. K. Ihm, J. Cortez and D. C. Sherrington, Macromolecules 2006, 39, 627-632. 13 C. Garcia-Diego and J. Cuellar, Ind. Eng. Chem. Res., 2005, 44, 82378247. 14 D. Horak, J. Labsky, J. Pilar, M. Bleha, Z. Pelzbauer and F. Svec, Polymer, 1993, 34, 3481-3489. 15 F. S. Macintyre and D. C. Sherrington, Macromolecules, 2004, 37, 7628-7636. 16 W. Q. Zhou, T. Y. Gu, Z. G. Su and G. H. Ma, Polymer 2007, 48, 1981-1988. 17 F. Gao, Z. G. Su, P. Wang and G. H. Ma, Langmuir 2009, 25, 38323838. 18 D. Stefanec and P. Krajnc, React. Funct. Polym., 2005, 65, 37-45. 19 X. Hou, X. K. K. Wang, B. Gao and J. Yang, Carbohydr. Polym., 2008, 72, 248-254. 20 X. Ge, M. Wang, H. Wang, Q. Yuan, X. Ge, H. Liu and T. Tang, Langmuir, 2010, 26, 1635-1641. 21 W. H. Li and H. D. H. Stöver, J. Polym. Sci. Part A: Polym Chem, 1998, 36, 1543-1551. 22 L. Jiang, Y. Zhao and J. Zhai, Angew. Chem. Int. Ed., 2004, 43, 43384341. 23 Y. Q. Wu and R. L. Clark, J. Colloid. Interface. Sci., 2007, 310, 529535. 24 Q. C. Zhang, J. Liu, X. J. Wang, M. X. Li and J. Yang, Colloid. Polym. Sci., 2010, 288, 1385-1391. 25 R. P. A. Hartman, J. P. Borra, D. J. Brunner, J. C. M. Marijnissen and B. Scarlett, J. Electrostat., 1999, 47,143-170. 26 A. Jaworek and A. Krupa, Exp. Fluids., 1999, 27, 43-52. 27 A. Jaworek and A. Krupa, J. Aerosol Sci., 1999, 30, 873-893. 28 M. M. Munir, A. B. Suryama, F. Iskandar and K. Okuyama, Polymer, 2009, 50, 4935-4943. 29 C. Wang, C. H. Hsu and J. H. Lin, Macromolecules, 2006, 39, 575583. 30 Y. Wang, Z. M. Liu, Y. Huang, B. X. Han and G. Y. Yang, Langmuir, 2006, 22, 1928. 31 J. Yao, L. K. Lim, J. Xie, J. Hua and C. Wang, J. Aerosol. Sci., 2008, 39, 987-1002.
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32 I. W. Lenggoro, T. Hata, F. Iskandar, M. M. Lunden and K. Okuyama, J. Mater. Res., 2000, 15, 733-743. 33 K. Okuyama and I. W. Lenggoro, Chem. Eng. Sci., 2003, 58, 537-547. 34 R. Vehring, W. R. Foss and D. Lechuga-Ballesteros, J. Aerosol. Sci., 2007, 38, 728-746. 35 L. Rayleigh, Philos. Mag., 1882, 14, 184-186. 36 B. Almería, W. W. Deng, T. M. Fahmy and A. Gomez, J. Colloid. Interface. Sci., 2010, 343, 125-133. 37 J. W. Xie, L. K .Y. Lim, Y. Phua, J. S. Hua and C. H. Wang, J. Colloid. Interface. Sci., 2006, 302, 103-112. 38 P. van de. Witte, P. J. Dijkstra, J. W. A. Van den. Berg and J. Feijen, J. Membrane. Sci., 1996, 117, 1-31. 39 N. Bock, M. A. Woodruff, D. W. Hutmacher, T. R. Dargaville, Polymers, 2011, 3, 131. 40 L. Feng, S. Li, Y. Li, H. Li, L. Zhang, J. Zhai, Y. Song, B. Liu, L. Jiang and D. Zhu, Adv. Mater., 2002, 14, 1857-1860. 41 R. Blossey, Nat. Mater., 2003, 2, 301-306. 42 X. Gao, X. Yan, X. Yao, L. Xu, K. Zhang, J. Zhang, B. Yang and L. Jiang, Adv. Mater., 2007, 19, 2213-2217. 43 L. Feng, Z. Zhang, Z. Mai, Y. Ma, B. Liu, L. Jiang and D. Zhu, Angew. Chem. Int. Ed., 2004, 43, 2012-2014. 44 D. Quere, Annu. Rev. Mater. Res., 2008, 38, 71-99. 45 X. M. Li, D. Reinhoudt and M. Crego-Calama, Chem. Soc. Rev., 2007, 36, 1350. 46 C. H. Xue and J. Z. Ma, J. Mater. Chem. A, 2013, 1, 4146-4161. 47 Z. G. Guo, W. M. Liu and B. L. Su, J. Colloid. Interface. Sci., 2011, 353, 335-355. 48 X. F. Wang, B. Ding, J. Y. YU and M. R. Wang, Nano today 2011, 6, 510-530. 49 A. B. D. Cassie and S. Baxter, Trans. Faraday Soc., 1944, 40, 546551.
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Nonsolvent assisted electrospraying is utilized to fabricate hierarchically porous microspheres for superhydrophobic coating.
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Facile preparation of hierarchically porous polymer microspheres for superhydrophobic coating Jiefeng Gao, Julia Shuk-Ping Wong, Mingjun Hu, Wan Li, Robert. K. Y. Li*
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Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X DOI: 10.1039/b000000x A facile method, i.e., nonsolvent assisted electrospraying, is proposed to fabricate hierarchically porous microspheres. The pore size on the mircorsphere surface ranges from a few tens to several hundred nanometers. Thermally and nonsolvent induced phase separations as well as breath figure are responsible for the formation of the hierarchical structures with different nano- sized pores. The nonsolvent could not only induce phase separation, but also stabilize the interface between the droplet and air, which can prevent the droplet from strong deformation, and is therefore beneficial to the formation of regular and uniform microspheres. On the other hand, solvent evaporation, polymer diffusion and Coulomb fission during electrospraying influence the morphology of finally obtained products. In this paper, the influence of polymer concentration, the weight ratio between nonsolvent and polymer and the flowing rate on the morphology of the porous microsphere are carefully studied. The hierarchically porous microsphere significantly increases the surface roughness and thus the hydrophobicity, and the contact angle can reach as high as 152.2 ±1.2. This nonsolvent assisted electrospraying opens a new way to fabricate superhydrophobic coating materials.
Introduction 20
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Polymer micro/nano particles have attracted much attention recently because of their potential applications in many fields such as controlled release, catalyst, scaffold, selective separation, chemical sensors.1-6 It is quite desirable if pores are introduced into the micro/nano spheres, because porous structure possesses a higher specific surface area, a lower density as well as a better permeability, which are required in many demanding applications.7-10 Generally, polymer particles could be fabricated by heterogeneous polymerization using the immiscibility of two or more liquids, including suspension, precipitation, multistage, membrane/microchannel emulsification and microfluidic polymerization.11 A porogen is often required in the solution, in order to obtain porous polymer particles. A good solvent, a nonsolvent, liner polymers, water and solid particles could serve as the porogen.12-20 Generally, the porogen that is initially dissolved or swollen in the monomer phase separates with oligomers or macromolecules during the polymerization, leading to the pore formation. Pores are also generated after the removal of solid particles embedded on polymer beads via washing or etching. Pore size and density could be controlled by choosing different porogens. It was reported that small pores (below 10 nm) and hence very high surface area values (800 m2/g) were obtained if a good solvent was chosen as the porogen; however, a poor solvent corresponded to larger pores and thus a lower surface area.21 Although, heterogeneous polymerization is popular for preparation of porous microspheres, a large number of emulsifiers or stabilizers are usually required for the This journal is © The Royal Society of Chemistry [year]
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polymerization route, which inevitably bring in impurities and thus influence the performance of the materials. On the other hand, polymerization usually needs delicate manipulation, and the whole fabrication process is complicated. For example, the interfacial tension should be carefully controlled, because it can affect the geometry of the monomer droplets. Therefore, it is high of importance to explore a simple but effective approach to obtain porous polymer microspheres. Electrospraying, which is a modified version of electrospinning, has been developed to prepare microspheres with porous structures. Jiang and coworkers fabricated microspheres by electrospraying polystyrene (PS)/dimethyformamide (DMF) solution. Vapor induced phase separation was responsible for the formation of the porous structure. During the electrospraying, the polymer solution droplet was cooled down, due to the solvent rapid evaporation. As a result, the water vapor in air condensed on these cold PS/DMF droplets. Because of the immiscibility between the PS and water, micro-phase separation occurred on the watercondensed sites, and eventually led to the generation of a macroporous structure.22 Porous polymer particles could also be obtained by electrospraying polymer solution into a nonsolvent bath, at the bottom of which the collector is immersed. It is crucial that the solvent does not evaporate completely before the spheres deposit on the collector, and mass exchange between the residual solvent in the droplet and the nonsolvent would occur, giving rise to the phase separation. The nonsolvent induced phase separation was regarded as the main mechanism for the generation of the porous microstructure.23, 24 However, as reported in the literatures the electrosprayed microspheres are [journal], [year], [vol], 00–00 | 1
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usually irregular and nonuniform. In addition, it is also difficult to control the porous structure. Here, in this article, nonsolvent assissted electrospraying, is first proposed to prepare hierarchically porous microspheres. The employed nonsolvent, hexanol, facilitates the formation of hierarchical pores with various sizes on the sphere surface. Furthermore, the hexanol can stabilize the droplet interface, resulting in the formation of regularly spherical microspheres. In addition to its simplicity in comparison with other techniques, this method can realize a large fabrication of the hierarchically porous materials by only extending the electrspraying time. It has also been demonstrated that the micro- and nano-scale hierarchically porous structures significantly improve the surface roughness, and thus enhances the hydrophobicity of the sphere coated substrate. This nonsolvent assisted electrospraying opens a new way to fabricate superhydrophobic coating materials.
Experimental Materials
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Polymethylmethacrylate (PMMA), a commercial product from Chi Mei company (Mw=112000). Dichloromethane (DCM) and hexanol with boiling points of 39.8 and 156 °C respectively were purchased from Sigma-Aldrich Corporation, and used as received.
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Fig. 1 Schematic illustration of (a) a typical electrospraying setup and (b) formation of hierarchically porous microspheres. (c) SEM and (d) TEM images of porous microspheres prepared from hexanol assisted electrospraying. Polymer concentration is 3 wt%. (e) Size of hierarchical pores for the electrosprayed microspheres shown in (c). Scale bars: (c and d) 1 m.
Morphology and formation mechanism of the hierarchically porous microspheres.
Electrospraying 25
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For nonsolvent assisted electrospraying, the solvent and nonsolvent were first mixed with a certain ratio, and then a desired amount of polymer was dissolved in the mixture. The prepared ternary solution was subsequently subject to magnetic stirring for three hours at room temperature. The stable and transparent polymer solution was then loaded in a syringe connected with a metallic needle for electrospraying. The electrosprayed products were collected in a grounded aluminum foil, which was 12 cm far from the metallic needle. Unless specified, the parameters for electrospraying were chosen as follows: polymer concentration: 3 wt%; the weight ratio between polymer and nonsolvent: 2:1; the voltage: 12 kv; the flowing rate: 1ml/h.
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Morphology Characterization
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The morphologies of the electrosprayed microspheres were characterized by a field emission scanning electron microscope (FESEM JSM6335) operated at 5kv. The electrosprayed microspheres were also deposited on copper grids covered with a carbon film for Transmission Electron Microscopy (TEM) observation, and the accelerating voltage was 20 kv.
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Contact angle measurement
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Contact angle (CA) measurements were carried out on a threeaxis horizontal tilt stage on an optical table. Deionized droplets were carefully dropped onto the surface of the sample (spin coated PMMA film and the electrosprayed PMMA film). The CA values were determined by averaging at least four separate runs.
Results and discussion
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Fig. 1a gives a schematic illustration for a typical electrospraying setup, which essentially consists of a syringe connected with a metallic needle, a high voltage power supply, a grounded collector, etc. A digital pump is used to control the flow rate of the polymer solution (stored in the syringe) to high precision. It is obvious that the setup is essentially the same as employed for electrospinning. Whether fibres or particles will form from the process depends mainly on the viscosity of the polymer solution.. When a high voltage is applied to the metallic needle, the pendent polymer solution at the nozzle will be subjected to the electrical field, and the induced charges are evenly distributed over the surface. Under the electrostatic repulsion and the coulombic force caused by the electrical field, the polymer solution droplet is stretched into a conical jet and further breaks into small droplets provided that the applied electric field is sufficiently high, and polymer particles are finally obtained when the solvent is completely evaporated during the process.25-27 On the other hand, the electrified jet is elongated and whipped to form fibers if the viscosity of the solution increases to a critical value. In this case, electrospinning other than electrospraying occurs.28, 29 Fig. 1b shows a schematic diagram for the evolution of the solution droplet during the electrospraying. The original solution, namely Polymethylmethacrylate (PMMA)-Dichloromethane (DCM)hexanol ternary system, is first prepared. PMMA macromolecular chain is disentangled and separated from each other in DCM. Hexanol can't dissolve PMMA(hence called the non-solvent) but is miscible with DCM. During electrospraying, DCM evaporates first, due to its much lower boiling point compared with hexanol. Naturally, the hexanol following DCM also migrates to the surface region, because the hexanol is only miscible with DCM. In addition, the hexanol can be regarded as a kind of surfactant This journal is © The Royal Society of Chemistry [year]
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hand, chain entanglements and Coulomb fission can also affect the microstructure of the electrosprayed microsphere. Coulomb fission occurs when a charged droplet reaches the Rayleigh limit,35 at which the drop cannot hold more charges than a limiting value and break into many small but highly charged offspring droplets. The competing mechanism of chain entanglements and Coulomb fission can lead to various morphologies for the electrosprayed particles. The competing result could be described by φRay, φent and φov, which refer to the polymer volume fraction in a droplet at the Rayleigh limit, the critical entanglement polymer volume fraction, and critical chain overlap polymer volume fraction, respectively. If φRay < φov, which means polymer chain entanglements are absent when the Rayleigh limit is reached, the droplet behaving like a pure liquid experiences the Coulomb fission, generating many offsprings. When φov < φRay < φent, the droplet can keep some integrity, but is not strong enough to prevent the particle from deforming via stretching during the electrospraying, leading to the formation of tailed particles and ultrathin fibers. After φRay > φent, fiber-free and spherical particles could be obtained, because sufficient chain entanglements can prevent the droplet from deforming. φRay could be described using Equation (1) listed below: 36
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I 3 d Q
Where φ, γ, εair, I, Q and d refer to the polymer volumn fraction, the surface tension of solution, the permittivity of air, the current, the liquid flow rate and the initial droplet diameter, respectively. 85
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Fig. 2 shows the SEM and TEM images of electrosprayed microspheres at a very low polymer concentration. It can be found that at a polymer concentration of 0.75 wt%, the microspheres (with the diameter of about 1.5 μm) did not have a perfect spherical shape. Most of the microspheres collapsed to form several large voids (see Fig. 2a, a'), which were much larger than that formed from a higher polymer solution concentration (shown in Fig. 1c). The diameter of some of the large voids was even comparable with that of the whole microsphere. When the
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As discussed before, electrospraying is a very rapid and complex process, and the solvent evaporation and polymer diffusion during the process are two important factors influencing the morphology of finally obtained products.31-34 On the other This journal is © The Royal Society of Chemistry [year]
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Fig. 2 SEM images of the hierarchically porous microspheres. Polymer solution concentration is (a) 0.75 wt% and (c) 1.5 wt%. (a'), (c') are the magnified SEM images for (a) and (c). (b), (d) are the TEM images of the corresponding microspheres. Scale bars: (a and c) 10 m; (a' and c') 1 m; (b) 200 nm; (d) 500 nm.
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and tends to find its place on the surface of the droplet with the hydrophilic group (-OH) heading outside while the hydrophobic component (-CH2) chain staying inside. It is worth noting that the hexanol gathering on the surface could stabilize the interface between the droplet and air, which can prevent the droplet from strong deformation, and is therefore beneficial to the formation of regular microspheres. With continuous loss of the solvent, phase separation occurs, forming the polymer rich phase and polymer poor phase. The polymer rich phase consisting of concentrated PMMA, begins to entangle and shrink, occupying the interior part. However, the polymer poor phase which contains most of DCM and hexanol escapes to the exterior surface of the droplet, and subsequently the hexanol starts to nucleate and grow on the surface of the droplet, leading to the formation of the relatively large pores. On the other hand, thermally induced phase separation (TIPS), due to the cooling effect caused by the evaporation of the volatile DCM, may be responsible for the formation of tiny pores inside the walls of the large voids. At the same time, the breath figure phenomenon may also occur as the water vapor condenses on the surface. Finally, the hierarchically porous microspheres can be obtained when both the DCM and the hexanol are completely evaporated. It was found that most of electrosprayed PMMA microspheres collapsed, displaying irregular shape (Fig. S1†), if nonsolvent was not added into the polymer solution. It was also worth noting that many tiny pores with a few tens of nanometers could be observed on the sphere surface, which was probably caused by TIPS. However, for our hexanol-assisted (where hexanol is served as the nonsolvent) electrospraying, it can be clearly seen that highly regularly spherical microspheres with hierarchical pores through the whole surface are produced (Fig. 1c). Interestingly, hierarchical pores with different sizes were observed on the microsphere surface. Here, three kinds of hierarchically porous structures with different diameters can be defined: (1) large and deep voids (>350 nm); (2) small and shallow pores ( φent, and Coulomb fission could be avoided due to the fact that entanglements are present before the Rayleigh limit is reached. With further increasing the Q to 3ml/h, many round microspheres tend to be transformed to elliptic microrods (See Fig. 5d). The current I on the surface of microsphers increase with Q, generating a larger electrostatic force and thereby driving the microspheres to deform to form the elliptic microrods. Following the microspheres deforming, the surface pores also became elongated, exhibiting the elliptic shape.
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Surface wettability is a very important aspect of material properties, which can affect the practical application of materials in many fields. Surface with a water contact angle (CA) higher than 150 is called superhydrophobic surface. Water droplets on a superhydrophobic surface easily roll off even at inclinations of only a few degrees, and as a result, surface contaminants is removed easily. The unique water repellent properties have attracted considerable interest from both industry and academy, due to their promising applications such as self-cleaning, antifogging and oil-water separation.40-43 Generally, the surface wettability is mainly determined by the chemical composition and the geometrical architecture of the surface. Materials with a low surface free energy tend to exhibit hydrophobic performance. However, the maximum contact angle for a fluorinated smooth surface is reported only about 120.44 Inspired by the nature, scientists mimick plant leaves such as lotus and silver ragwort to obtain hierarchical surface with a large roughness, in order to achieve higher contact angles.45-48 In fact, the apparent contact angle could be described by the Cassie-Baxter model, in which liquids are assumed to only contact the solid through the top of the asperities, and air pockets are assumed to be trapped
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underneath the liquid. It is well known that the air trapped in the surface is very important to the hydrophobicity, because the water CA of air is regarded to be 180. The Cassie-Baxter equation is listed below: 49
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cosCB f1 cos0 f 2
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Where θCB is the CB contact angle, θ0 is the contact angle on a flat surface, and f1, f2 are the fractions of the solid surface and air in contact with the liquid, respectively. Note that f1 + f2 =1, and it could be concluded that a larger f2 (more air pockets) will induce a larger θCB. In our experimental, the hierarchically porous microstructure of the electrosprayed microsphere could significantly improve the surface roughness, and was therefore beneficial to the increase of the surface hydrophobicity. The flat PMMA film obtained by a spinning coating possesses a CA of 86.6 ± 0.8 (Fig. 6a); however, the CA significantly increases to 135 ± 1.4 for the porous microsphres elctrosprayed without nonsolvent assistance (Fig. 6b), and it further increases to 152.6 ± 1.2 for the hierarchically porous microsphere (Fig. 6c). As such a contact angle, the surface becomes superhydrophobic. For the flat film, the surface is smooth without roughness, corresponding to a low CA; however, a high CA can be achieved for the electrosprayed microsphere coated substrate, because particles randomly stacked onto a substrate could create a hydrophobic surface. On the other hand, pores on the microsphere surface are capable of capturing more air, thus increasing the hydrophobicity. There are only tiny pores on the microsphere surface if nonsolvent is not used. However, the microspheres based on hexanol assisted electrospraying have deep voids, shallow pores and tiny pores inside the voids, and the miro/nano hierarchical structure created a larger surface roughness, so that more air could be trapped in the surface pores, leading to a higher CA. Table 1 lists the CAs and f2 for the porous microsphere coated substrate with different electrospraying parameters (the polymer solution concentration, weight ratios between nonsolvent and polymer and the flowing rate). It is found the CAs for all the hierarchically porous electrosprayed microspheres is higher than 135 , i.e., the CA for the porous microsphres prepared without nonsolvent assistance. Especially, as mentioned, the CA of the porous microsphere coated surface reaches as high as 152.6 ± 1.2 when the polymer concentration is 3 wt%, and f2 is calculated to be 0.894, which indicates the air trapped in the rough surface containing hierarchical pores is the main reason for the achievement of superhydrophobicity. The tailed microspheres obtained from electrospraying polymer solution with concentration of 3.75 wt% displays a decreased CA, i.e., 144.5 ± 2.2. On the other hand, the very large voids on the surface of the porous sphere (see Fig. 2a') decreased the surface roughness, leading to a relatively lower CA, namely 138.6 ± 1.5 for polymer concentration of 0.75 wt%. As mentioned above, WR could influence the morphology of the porous microsphere, and thus the CA. It is found that the CAs are 141.5 ± 1.4, 148.6 ± 2.1, 150. 5 ± 1.1 and 143. 7 ± 1.2 when WR are set as 1:4, 1:1, 4:1, and 10:1, respectively. The appearance of thin fibers attached on the microsphere (Fig. 4a) and coalescence of the surface pores (Fig. 4d) decrease the surface roughness of the substrate, causing the decrease of the CAs. Ultrathin fibers and tails are also observed for porous microspheres with the flowing rate of 0.33, and 0.67 ml/h, which correspond to the relatively low CAs, 6 | Journal Name, [year], [vol], 00–00
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Fig. 6 Images of water droplets (a) on a spin-coated PMMA film (b) and (c) on PMMA microsphere coated film electrosprayed without and with nonsolvent assistance.
namely 142. 8 ± 1.5 and 143. 9 ± 1.4, respectively. The CA rises to 150.3± 1.7 for the flowing rate of 2 ml/h, and it slightly decreases to 147. 9 ±1.2, due to the elongated porous structure. Table 1 Surface properties of the porous microsphere prepared under different electrospraying parameters. CA ( )
f2
138.6 ±1.5 141.7 ±1.6 147.3 ±1.8 152.6 ±1.2 144.5 ±2.2
0.764 0.797 0.850 0.894 0.824
1:4 1:1 4:1 10:1 Flowing rate (ml/h) 0.33 0.67
141.5 ±1.4 148.6 ±2.1 150.1 ±1.1 143.7 ±1.2
0.795 0.862 0.874 0.817
142.8 ±1.5 143.9 ±1.4
0.808 0.819
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0.876 0.856
Electrospraying parameters Polymer concentration (w%) 0.75 1.5 2.65 3 3.75 Weight ratio between nonsolvent and polymer
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In summary, hierarchically porous microspheres were prepared based on nonsolvent assisted electrospraying. Thermally and nonsolvent induced phase separations as well as the breath figure were responsible for the formation of the hierarchical structures with different sized surface pores. The porous microspheres was mechanically weak and tend to collapse at a very low polymer concentrations, and gradually became more integrated with the increase of the polymer concentration. It was found the surface pore became smaller and less apparent when the ratio between the nonsolvent and polymer (WR) increased, and the porous structure was even eliminated when the WR was raised to 10:1, due presumably to the coalescence of the excessive nonsolvent. The flowing rate could also influence the morphology of the porous microspheres, and the microspheres tailed with ultrathin nanofiber were observed at a low flowing rate. The hierarchically porous microsphere significantly increased the surface roughness and thus the hydrophobicity of the substrate, and the contact angle could reach as high as 152.6 ± 1.2. This nonsolvent assisted electrospraying opens a new way to fabricate superhydrophobic coating materials.
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This work is supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project Number: CityU 118313). 5
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DOI: 10.1039/C3NR05281H
