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Preparation and characterization of superhydrophobic silica-based surfaces by using polypropylene glycol and tetraethoxysilane precursors
Surface & Coatings Technology 201 (2007) 9579 – 9586 www.elsevier.com/locate/surfcoat
Preparation and characterization of superhydrophobic silica-based surfaces by using polypropylene glycol and tetraethoxysilane precursors Kuei-Chien Chang, Yu-Kai Chen, Hui Chen ⁎ Department of Chemical and Materials Engineering, National Central University, Jhongli, Taiwan, R.O.C. Received 25 December 2006; accepted 10 April 2007 Available online 18 April 2007
Abstract Preparation of superhydrophobic silica-based surfaces via sol–gel process by adding polypropylene glycol (PPG) polymer into the precursor solution has been developed. Surface roughness of the films was obtained by removing the organic polymer at 500 °C and then the hydrophobic groups bonded onto the films were obtained by chemical reaction with hexamethyldisilazane (HMDS). Physical properties of the as-prepared films were analyzed by contact angle measurements, scanning electron microscopy (SEM), UV–VIS scanning spectrophotometer and Fourier transform infrared (FT-IR) spectrophotometer. The experimental parameters were varied by the type of silane species, the weight ratio of PPG solution to precursor solution, the hydrolysis time of the precursor solution, the molecular weight of PPG, the casting temperature and the evaporation temperature. The phase separation of the PPG polymer rich domain occurred on the substrates at a lower temperature. The result showed that the contact angles of the films prepared at 5 °C were greater than 150° when the weight ratio of PPG solution to precursor solution was 5. In addition, the transmittance of the films was greater than 80% simultaneously. © 2007 Elsevier B.V. All rights reserved. Keywords: Polypropylene glycol(PPG); Superhydrophobic surface; Hybrid; Lotus effect
1. Introduction Hydrophilic and hydrophobic surfaces are governed by both surface roughness and chemical composition. One of the famous phenomena was the lotus leaf. The “Lotus Effect” has its marvelous properties that possess superhydrophobic surfaces, with a water contact angle (CA) greater than 150°, and selfcleaning phenomenon. In general, the contact angles can be enhanced in two dominated elements from the lotus leaf structure: one is low surface energy materials (chemical method) and the other is roughness (geometrical method) [1–5]. Conventionally, for the hydrophilic materials, in order to form the superhydrophobic films, modification of surface chemistry is always in conjunction with enhancement of the surface ⁎ Corresponding author. Postal: No. 300, Jhongda Rd., Jhongli City, Taoyuan County 32001, Taiwan, R.O.C. Tel.: +886 3 4227151x34216; fax: +886 3 4273643. E-mail address: [email protected] (H. Chen). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.04.021
roughness. However, superhydrophobic surfaces have been prepared by controlling the surface topography by various processing methods, such as: sol–gel method [5,6], organic/ inorganic hybrid method [7–9], CVD method [10], electrochemical method [11], embossing method [12], plasma method [13], phase separation method [3,14,15–20], template method [21–23] and other methods [24–27]. The practical application of superhydrophobic surfaces was limited by some preparation conditions, multi-step processes, expensive low surface energy materials, etc. Most of the barefluorinated materials have the expensive price and often are vulnerable to environment attacks while causing ozone shield to crack more seriously. Hence, the other hydrophobic group must be replaced for good use. Because the surface free energy of the substituted group which was –CF3 b –CH3 [28], modification of CH3-terminated surfaces was chosen to reduce the cost and increased the practicality. It is usually introduced a supplementary end-capping by means of trimethylchlorosilane (TMCS) and hexamethyldisilazane (HMDS) [29] for modifying the
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hydrophilic groups to enhance hydrophobic properties of the asprepared surfaces. The TMCS and HMDS reacted with Si–OH as shown below: ≡Si–OH þ ðCH3 Þ3 SiCl → ≡Si–O–SiðCH3 Þ3 þ HCl 2≡Si–OH þ ðCH3 Þ3 SiNHSiðCH3 Þ3 → 2≡Si–O–SiðCH3 Þ3 þ NH3 The as-prepared films were acquired by the hybridization of silica and organic polymeric species [30–33]. The structure of organic–inorganic polypropylene glycol (PPG)–silica hybrid was presented as covalent bonds between the siloxane phase and polymer chains. The structural features deduced from SAXS results indicate the existence of a phase separation process at a nanometric scale [34–37]. It was considered that PPG-silica nanocomposite was dispersed completely over the films. In this research, superhydrophobic silica-based films were prepared by adding PPG polymer into the precursor solution via sol–gel process firstly. After casting the PPG-silica network to form the thin films, the PPG phase can be removed at high temperature and rough surfaces with hydrophilic property can be generated. Finally, HMDS was introduced to react with asprepared films during the coating process to form a covalent bond of CH3 group onto the surfaces [21]. The methyls, meanwhile, will bond with the surfaces of the substrates, that the superhydrophobic property of the films can be obtained. In order to investigate the preparation parameters in this study, the type of silane species, the hydrolysis time of the precursor solution, molecular weight of PPG polymer, the casting temperature of the mixing solution, and the effect of evaporation temperature for the casting thin films were discussed. 2. Experimental procedures 2.1. Materials Tetraethoxysilane (TEOS, reagent grade), methyltriethoxysilane (MTEOS, reagent grade) and dimethyldiethoxysilane (DMDEOS, reagent grade) were all purchased from Shin-Etsu Chemical Co. Ltd., Japan. Polypropylene glycols (molecular weight was 1000 and 4000, reagent grade) were obtained from ACROS. 1,1,1,3,3,3-Hexamethyldisilazane (HMDS) was purchased from Lancaster. Other reagents were as follows: Hydrochloric acid (HCl) (Merck, reagent grade), methanol (MeOH) (ECHO, reagent grade), ethanol (EtOH) (ECHO, reagent grade), toluene (TEDIA, reagent grade) and n-hexane (NASA, certified grade). 2.2. Sample preparation There were two types of solutions prepared in advance. One was a precursor solution prepared from mixtures with the molar composition, 1 TEOS: 4 H2O (pH = 2): 4 EtOH and then reacted at 60 °C for 4 h. Another was PPG solution with two different molecular weights (1000 and 4000), prepared from mixing 10% to 100% of PPG in ethanol solution. On the other hand, the
mixing solution was prepared by mixing above two solutions (the weight ratio of PPG solution to precursor solution was from 0.2 to 5) at 30 °C. The other precursor solutions with MTEOS and DMDEOS were also prepared by using the same method as above condition. 2.3. Preparation of thin films The substrates, glass slides, were vacuum-locked during the spin-coating process. A chemical substrate cleaning procedure was ultrasonicated for 30 min with HCl (0.1 N), NaOH (0.1 N), deionized water and iso-propylalcohol, respectively. Uniform films were prepared at a spinning rate of 4500 rpm for 15 s while spraying the mixing solution onto the clean substrates. In all cases, hybrid films were allowed to react at 250 °C for 2 h and then the temperature was raised to 500 °C for 2 h. The surface chemistry of all the films was modified with selfassembly monolayer by using HMDS solution prepared by dissolving HMDS in toluene and the concentration of it was 10 wt.%. The self-assembly time and temperature of the solution was 6 h and 110 °C, respectively. The whole procedure of preparation of superhydrophobic thin films was sketched in Scheme 1. 2.4. Instrumentation Surface morphologies of the films were observed under scanning electron microscopy (SEM, Hitachi S-4200). Water contact angles of prepared films were measured using manual contact angle goniometer (Kyowa interface sciences CA-D) as follows: water droplet (the size was 5 μL) was gently placed onto the films and the average value measured over five different locations for each sample were taken. By applying the θ/2 method and regulating the droplet size to about 20 scale and observed through the eyepiece. The composition of the treated surfaces was studied by Fourier transform infrared spectrometer (FT-IR, JASCO FT/IR-410). The optical transmittance was determined by UV–VIS scanning spectrophotometer (JASCO V-530). 3. Results and discussion In this study, the silica–PPG hybrid mixing solution was prepared by mixing the precursor solution (inorganic part) and the PPG solution (organic part). In order to prepare the films, the above mixing solution was coated onto the glass substrates. The covalent bonds between the inorganic (siloxane) and organic (polymer) phase were presented in the silica–PPG (SiO2–PPG) nanocomposites [34–37], so the PPG polymer was able to be dispersed homogeneously over the films. Then, the porous and rough structures of the films can be obtained by decomposition of the PPG polymer at high temperature. The TGA results of PPG polymers were shown in Fig. 1. The curve of PPG4000 was shifted to a higher temperature region than that of PPG1000. It was indicated that the rate of decomposition of PPG1000 polymer was faster than that of PPG4000. It was also indicated that weight loss of PPG1000 and PPG4000 have been
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Scheme 1. Schematic of the procedure on the preparation of superhydrophobic films.
decomposed completely when the temperature reached 450 °C. The temperature of removing PPG polymer from the films was set to 500 °C that made sure only inorganic materials were in the presence of the surfaces and the structures of the network on the films were not destroyed. In order to investigate the preparation conditions of the films, the type of silane species, the hydrolysis time of the precursor solution, the molecular weight of PPG, the casting temperature and the evaporation temperature were discussed. 3.1. Silane species There were three kinds of silanes for preparation in this research. The effect of the contact angles of the films prepared by different silanes on the PPG weight fraction of PPG solution was shown in Fig. 2. It was indicated that the contact angles of the films were increased with increasing the PPG weight fraction of the PPG solution when the films were prepared by TEOS. There were four functional groups in the TEOS structures to become
Fig. 1. TGA result of PPG polymers.
hydroxyl groups and then to form network in the hybrid materials after polycondensation reaction, so the dense and rough structures can be acquired after the decomposition of PPG polymer at high temperature. It was considered that increasing the PPG weight fraction in the films, the roughness of the films was increased. On the other hand, when the films were prepared with MTEOS or DMDEOS, the contact angles showed almost no obvious variation, even increasing the PPG weight fraction of the PPG solution. If hydrolysis reaction of precursor solutions occurred completely, there were three hydroxyl and one methyl groups in the MTEOS and two hydroxyl and two methyl groups in the DMDEOS, respectively. The crosslinking density of the hybrid material prepared by MTEOS was less than that by TEOS [38]. In addition, the methyl groups of MTEOS and DMDEOS in the films were decomposed to become a hydroxyl group and the PPG polymer was also almost completely decomposed when the temperature was increased to 400 °C. The rough surfaces of the films were easily destroyed to become the flat surfaces by
Fig. 2. Effect of contact angles of the films prepared by different silane precursor solutions on PPG4000 weight percentage of PPG solution.
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Table 1 The preparation conditions and the contact angles of the films Precursor solutiona b
PPGc solution
TEOS:H2O :EtOH (molar ratio)
Hydrolysis time (h)
PPG wt.% in EtOH
1:4:4
2 4 6 8
80%
Contact angle (°)
128.9 128.8 128.8 127.4
a
The precursor solutions were reacted at 60 °C. The pH value of H2O was 2. c The molecular weight of PPG polymer was 4000 and the weight ratio of PPG solution to precursor solution was 1. b
further condensation of these hydroxyl groups. As mentioned above, the TEOS silane was chosen as an inorganic part to prepare the films. 3.2. The hydrolysis time of the precursor solution In a typical sol–gel process, the precursor is accompanied with a series of hydrolysis and polycondensation reactions to form a colloidal suspension, or a “sol”. It was considered that with increasing the reaction time of the precursor solution, the phase separation of organic and inorganic part was increased. In order to investigate this effect, the contact angles of the films prepared by the precursor solution during different hydrolysis time were shown in Table 1. With increasing the hydrolysis time of precursor solutions from 2 to 8 h, the values of contact angles were almost the same values. Therefore, 4 h was chosen as the hydrolysis time to prepare precursor solutions in order to make sure the hydrolysis reaction completely occurred.
Fig. 4. Effect of contact angles of the films prepared by the mixing solution at different casting temperatures.
discussed. The effect of contact angles of the films on these factors were shown in Fig. 3. With increasing the weight fraction of PPG in the PPG solution, the contact angles of the films were increased when the films were prepared by PPG1000 and PPG4000, respectively. On the other hands, the contact angles of the films prepared by PPG4000 were higher than that by PPG1000. It was indicated that the surface roughness can be easily obtained by the PPG4000 polymer because the microphase separation was easily to occur when the films were prepared by using high molecular weight of PPG. 3.4. The casting temperature
The roughness of the films was obtained by the decomposition of PPG at high temperature. Therefore, the weight percentage of PPG solution and molecular weight of PPG polymer in the sol–gel process were the two main factors to be
From the previous results of preparation of silica–polyethylene glycol (PEG) hybrid in our laboratory, the mixing time of the mixing solution could affect the formation of gel. And, the formation of gel easily appeared in that system. On the contrary, for preparation of the silica–PPG hybrid, the rate of condensation reaction in the silica–PPG hybrid casting solution was slow, so the gel phase was not easily obtained. Therefore, the contact angles of the films prepared by different mixing
Fig. 3. Effect of contact angle of the film prepared by different PPG polymers on PPG weight percentage of PPG solution at 30 °C.
Fig. 5. Effect of contact angles of the films prepared at different evaporation temperatures on the ratio of PPG solution to precursor solution.
3.3. Molecular weight of PPG polymer
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Fig. 6. Image of water drop on the surfaces of the films prepared with the different ratio of PPG solution to precursor solution was (a) 0.2 (b) 0.6 (c) 1 (d) 3 (e) 5 at 5 °C. The value of contact angles of the films are shown in the insert of the diagram.
times (2–30 min) of casting solution was almost kept at the same value, 135°. But the roughness of the films could be affected by the casting temperature and the results were shown in Fig. 4. With decreasing the casting temperature, the contact angles of the films were increased. It was considered that the micro-phase separation of PPG polymer easily occurred at lower temperature so that surface roughness was enhanced. PPG polymer had the lower melting point, so the lower mobility of polymer chain existed at a lower casting temperature. And chemically bonded materials of silica–PPG hybrid had the higher chemical stability than non-chemically bonded ones [2]. For the reason mentioned above, there was no obvious phenomenon of phase separation for silica–PPG hybrid. But a nanometric scale of micro-phase separation of that was observed from SAXS analysis [34,37]. Therefore, the more excellent data of contact angle can be acquired at a lower casting temperature.
and ethanol were easily migrated onto the surfaces so that the serious phase separation of the PPG polymer can be taken place under the lower temperature. It was also attributed to have a partial crystallization and phase separation below a critical temperature [35]. Therefore, the surface roughness can be obtained after removing the organic part of the films at high
Table 2 The preparation conditions and characteristics of the films Evaporation temperature
PPG solution /precursor solution (wt. ratio)
Contact angle (°)
−10 °C
0.2 0.4 0.6 0.8 1 2 3 4 5 0.2 0.4 0.6 0.8 1 2 3 4 5 0.2 0.4 0.6 0.8 1 2 3 4 5
116.2 118.5 127.1 135.1 159.9 159.7 159.3 159.3 160.7 115.4 118.5 131.9 145.1 150.9 160.6 159.9 160.3 159.7 117.3 122.6 124.4 127.5 130.1 130.8 134.6 132.8 131.9
3.5. The effect of the evaporation temperature for the casting thin films After casting mixing solution onto the substrates, hybrid films were obtained when the solvent was evaporated. It was expected that the micro-phase separation of the hybrid films could be influenced by the evaporation temperature. The effect of the contact angles of the films on the different evaporation temperatures was shown in Fig. 5. The contact angles of the films were increased when the weight ratio of the PPG solution to the precursor solution was increased at all different evaporation temperatures. It was considered that the roughness of the surface easily occurred at high weight ratio of PPG. On the other hand, the contact angles of the films prepared by the evaporation temperature of − 10 °C and 5 °C were greater than that of 30 °C. It was considered that the chain of the PPG polymer was not easily moved and the micro-phase separation easily occurred at a lower temperature. At the same time, water
5 °C
30 °C
Transmittance (T%) 400 (nm)
600 (nm)
800 (nm)
31.8 37.6 43.9 25.4 30.0 33.2 54.2 63.6 62.3 97.8 81.8 48.5 31.9 8.8 48.3 35.1 41.8 50.2 98.7 94.8 100.9 100 95.7 97.7 94.4 96 94.8
56.8 62.1 56.3 40.4 43.4 44.3 65.8 74.3 73.5 99.1 96.4 72.6 49.5 32.3 71.3 57.3 65.1 71.5 102.0 98.3 102.6 100.6 97.1 100.5 99.2 99.2 98.5
71.3 74.4 63.8 53.8 53.7 53 72.1 80.2 78.9 102.3 100.3 85.2 64.5 55.9 83.8 72.8 78.4 83.5 103.5 99.5 102.7 102.3 98.4 101.4 100 101.6 101.4
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Fig. 7. Effect of contact angles and transmittance of the films on the weight ratio of PPG solution to precursor solution prepared at (a) −10 °C and (b) 5 °C.
temperature. The pictures of water drop on the films with the different ratio of PPG solution to precursor solution were shown in Fig. 6. The superhydrophobic thin films can be obtained
when the weight ratio of PPG solution to precursor solution was greater than 1. The data of transmittance (T%) of visible light of films with the wavelength range from 400 nm to 800 nm was shown in Table 2 and the relationship between contact angle and transmittance with the wavelength of 600 nm prepared at − 10 °C and 5 °C were shown in Fig. 7(a) and (b), respectively. It was indicated that transmittance of the as-prepared films were decreased with increasing the weight ratio of PPG solution to precursor solution from 0.2 to 1, but that increased with increasing the weight ratio of PPG polymer solution to precursor solution from 1 to 5. In other words, the higher and lower ratio of PPG solution to precursor solution showed the better result of transmittance of substrates, but the less one can be obtained when the weight ratio was between 1 and 4. It can be explained from the SEM images as shown in Fig. 8. When the weight ratio of PPG solution to precursor solution was high or low, the continuous matrix was easily obtained and the transmittance of substrates was high. It was considered that when the concentration of PPG polymer or precusor was high, the rich domain was easy to appear and the continuous surface can be easily observed as shown in Fig. 8(a) and (e). However, the discontinuous surface can be obtained from Fig. 8(c) because the macro phase separation of PPG polymer occurred effectively. It was also the reason why the transmittance of the film was low when the weight of PPG solution to precursor solution was 1. The superhydrophobic films with high transmittance (N80%) were prepared by this method. Fig. 9 showed that FT-IR spectra of as-prepared hybrid films and rough films with and without HMDS treatment, respectively. For the hybrid films as shown in Fig. 9(a), the PPG polymer still existed onto the surfaces so that the absorption band of methyl groups around 2951 cm− 1 was appeared obviously. After the surfaces with heat treatment and without
Fig. 8. SEM images on the surfaces of the films prepared with the different ratio of PPG solution to precursor solution (a) 0.2 (b) 0.6 (c) 1 (d) 3 (e) 5 at 5 °C.
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Fig. 9. FT-IR spectra of the organic–inorganic hybrid films (a) without any treatment, (b) with heat treatment at 500 °C and (c) with heat treatment at 500 °C and modification by HMDS.
modification by HMDS as shown in Fig. 9(b), methyl groups around 2951 cm− 1 disappeared and the absorption band of OH around 3500 cm− 1 has been obviously increased at this stage. At the same time, the siloxane network grew, as indicated by Si–O–Si bands at around 750 and 1050 cm− 1. When the films were modified with HMDS as shown in Fig. 9(c), it was indicated that –OH compound decreased. The absorption band of OH around 3500 cm− 1 has been obviously dropped, meanwhile, the absorption band of water adsorbed on silica gel around 1500–1800 cm− 1 was decreased. The intensity of bands relating to methyl groups around 2951 cm− 1, which slightly increased with modification of HMDS. It was revealed that hydrophobic groups could bond onto the surfaces. The infrared spectra information has been interpreted in detail in a previous paper [39]. From the above result of analysis, it can be supported that modification had successfully introduced hydrophobic groups onto the rough surfaces.
(methyl) groups of the surfaces were modified by HMDS. It was showed that the contact angles of the films can be controlled by the type of silane species, the hydrolysis time of the precursor solution, the molecular weight of PPG, the casting temperature and the evaporation temperature. Nevertheless, preparation of the films at lower evaporation temperature resulted in the high contact angles of the as-prepared films. The films prepared at 5 °C evaporation temperature exhibited the excellent contact angles which were greater than 160° and the transmittance of the films were greater than 80% at the same time.
4. Conclusions
References
Superhydrophobic silica-based surfaces have been prepared by a hybridization of TEOS and PPG polymer with suitable conditions. The surface roughness was promoted by removing PPG polymer from the films. In addition, the hydrophobic
Acknowledgements The authors thank the National Central University of Taiwan for their kind help, especially from the Department of Chemical and Materials Engineering for the use of contact angle goniometer and AFM analyzer.
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Preparation and characterization of superhydrophobic silica-based surfaces by using polypropylene glycol and tetraethoxysilane precursors Kuei-Chien Chang, Yu-Kai Chen, Hui Chen ⁎ Department of Chemical and Materials Engineering, National Central University, Jhongli, Taiwan, R.O.C. Received 25 December 2006; accepted 10 April 2007 Available online 18 April 2007
Abstract Preparation of superhydrophobic silica-based surfaces via sol–gel process by adding polypropylene glycol (PPG) polymer into the precursor solution has been developed. Surface roughness of the films was obtained by removing the organic polymer at 500 °C and then the hydrophobic groups bonded onto the films were obtained by chemical reaction with hexamethyldisilazane (HMDS). Physical properties of the as-prepared films were analyzed by contact angle measurements, scanning electron microscopy (SEM), UV–VIS scanning spectrophotometer and Fourier transform infrared (FT-IR) spectrophotometer. The experimental parameters were varied by the type of silane species, the weight ratio of PPG solution to precursor solution, the hydrolysis time of the precursor solution, the molecular weight of PPG, the casting temperature and the evaporation temperature. The phase separation of the PPG polymer rich domain occurred on the substrates at a lower temperature. The result showed that the contact angles of the films prepared at 5 °C were greater than 150° when the weight ratio of PPG solution to precursor solution was 5. In addition, the transmittance of the films was greater than 80% simultaneously. © 2007 Elsevier B.V. All rights reserved. Keywords: Polypropylene glycol(PPG); Superhydrophobic surface; Hybrid; Lotus effect
1. Introduction Hydrophilic and hydrophobic surfaces are governed by both surface roughness and chemical composition. One of the famous phenomena was the lotus leaf. The “Lotus Effect” has its marvelous properties that possess superhydrophobic surfaces, with a water contact angle (CA) greater than 150°, and selfcleaning phenomenon. In general, the contact angles can be enhanced in two dominated elements from the lotus leaf structure: one is low surface energy materials (chemical method) and the other is roughness (geometrical method) [1–5]. Conventionally, for the hydrophilic materials, in order to form the superhydrophobic films, modification of surface chemistry is always in conjunction with enhancement of the surface ⁎ Corresponding author. Postal: No. 300, Jhongda Rd., Jhongli City, Taoyuan County 32001, Taiwan, R.O.C. Tel.: +886 3 4227151x34216; fax: +886 3 4273643. E-mail address: [email protected] (H. Chen). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.04.021
roughness. However, superhydrophobic surfaces have been prepared by controlling the surface topography by various processing methods, such as: sol–gel method [5,6], organic/ inorganic hybrid method [7–9], CVD method [10], electrochemical method [11], embossing method [12], plasma method [13], phase separation method [3,14,15–20], template method [21–23] and other methods [24–27]. The practical application of superhydrophobic surfaces was limited by some preparation conditions, multi-step processes, expensive low surface energy materials, etc. Most of the barefluorinated materials have the expensive price and often are vulnerable to environment attacks while causing ozone shield to crack more seriously. Hence, the other hydrophobic group must be replaced for good use. Because the surface free energy of the substituted group which was –CF3 b –CH3 [28], modification of CH3-terminated surfaces was chosen to reduce the cost and increased the practicality. It is usually introduced a supplementary end-capping by means of trimethylchlorosilane (TMCS) and hexamethyldisilazane (HMDS) [29] for modifying the
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hydrophilic groups to enhance hydrophobic properties of the asprepared surfaces. The TMCS and HMDS reacted with Si–OH as shown below: ≡Si–OH þ ðCH3 Þ3 SiCl → ≡Si–O–SiðCH3 Þ3 þ HCl 2≡Si–OH þ ðCH3 Þ3 SiNHSiðCH3 Þ3 → 2≡Si–O–SiðCH3 Þ3 þ NH3 The as-prepared films were acquired by the hybridization of silica and organic polymeric species [30–33]. The structure of organic–inorganic polypropylene glycol (PPG)–silica hybrid was presented as covalent bonds between the siloxane phase and polymer chains. The structural features deduced from SAXS results indicate the existence of a phase separation process at a nanometric scale [34–37]. It was considered that PPG-silica nanocomposite was dispersed completely over the films. In this research, superhydrophobic silica-based films were prepared by adding PPG polymer into the precursor solution via sol–gel process firstly. After casting the PPG-silica network to form the thin films, the PPG phase can be removed at high temperature and rough surfaces with hydrophilic property can be generated. Finally, HMDS was introduced to react with asprepared films during the coating process to form a covalent bond of CH3 group onto the surfaces [21]. The methyls, meanwhile, will bond with the surfaces of the substrates, that the superhydrophobic property of the films can be obtained. In order to investigate the preparation parameters in this study, the type of silane species, the hydrolysis time of the precursor solution, molecular weight of PPG polymer, the casting temperature of the mixing solution, and the effect of evaporation temperature for the casting thin films were discussed. 2. Experimental procedures 2.1. Materials Tetraethoxysilane (TEOS, reagent grade), methyltriethoxysilane (MTEOS, reagent grade) and dimethyldiethoxysilane (DMDEOS, reagent grade) were all purchased from Shin-Etsu Chemical Co. Ltd., Japan. Polypropylene glycols (molecular weight was 1000 and 4000, reagent grade) were obtained from ACROS. 1,1,1,3,3,3-Hexamethyldisilazane (HMDS) was purchased from Lancaster. Other reagents were as follows: Hydrochloric acid (HCl) (Merck, reagent grade), methanol (MeOH) (ECHO, reagent grade), ethanol (EtOH) (ECHO, reagent grade), toluene (TEDIA, reagent grade) and n-hexane (NASA, certified grade). 2.2. Sample preparation There were two types of solutions prepared in advance. One was a precursor solution prepared from mixtures with the molar composition, 1 TEOS: 4 H2O (pH = 2): 4 EtOH and then reacted at 60 °C for 4 h. Another was PPG solution with two different molecular weights (1000 and 4000), prepared from mixing 10% to 100% of PPG in ethanol solution. On the other hand, the
mixing solution was prepared by mixing above two solutions (the weight ratio of PPG solution to precursor solution was from 0.2 to 5) at 30 °C. The other precursor solutions with MTEOS and DMDEOS were also prepared by using the same method as above condition. 2.3. Preparation of thin films The substrates, glass slides, were vacuum-locked during the spin-coating process. A chemical substrate cleaning procedure was ultrasonicated for 30 min with HCl (0.1 N), NaOH (0.1 N), deionized water and iso-propylalcohol, respectively. Uniform films were prepared at a spinning rate of 4500 rpm for 15 s while spraying the mixing solution onto the clean substrates. In all cases, hybrid films were allowed to react at 250 °C for 2 h and then the temperature was raised to 500 °C for 2 h. The surface chemistry of all the films was modified with selfassembly monolayer by using HMDS solution prepared by dissolving HMDS in toluene and the concentration of it was 10 wt.%. The self-assembly time and temperature of the solution was 6 h and 110 °C, respectively. The whole procedure of preparation of superhydrophobic thin films was sketched in Scheme 1. 2.4. Instrumentation Surface morphologies of the films were observed under scanning electron microscopy (SEM, Hitachi S-4200). Water contact angles of prepared films were measured using manual contact angle goniometer (Kyowa interface sciences CA-D) as follows: water droplet (the size was 5 μL) was gently placed onto the films and the average value measured over five different locations for each sample were taken. By applying the θ/2 method and regulating the droplet size to about 20 scale and observed through the eyepiece. The composition of the treated surfaces was studied by Fourier transform infrared spectrometer (FT-IR, JASCO FT/IR-410). The optical transmittance was determined by UV–VIS scanning spectrophotometer (JASCO V-530). 3. Results and discussion In this study, the silica–PPG hybrid mixing solution was prepared by mixing the precursor solution (inorganic part) and the PPG solution (organic part). In order to prepare the films, the above mixing solution was coated onto the glass substrates. The covalent bonds between the inorganic (siloxane) and organic (polymer) phase were presented in the silica–PPG (SiO2–PPG) nanocomposites [34–37], so the PPG polymer was able to be dispersed homogeneously over the films. Then, the porous and rough structures of the films can be obtained by decomposition of the PPG polymer at high temperature. The TGA results of PPG polymers were shown in Fig. 1. The curve of PPG4000 was shifted to a higher temperature region than that of PPG1000. It was indicated that the rate of decomposition of PPG1000 polymer was faster than that of PPG4000. It was also indicated that weight loss of PPG1000 and PPG4000 have been
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Scheme 1. Schematic of the procedure on the preparation of superhydrophobic films.
decomposed completely when the temperature reached 450 °C. The temperature of removing PPG polymer from the films was set to 500 °C that made sure only inorganic materials were in the presence of the surfaces and the structures of the network on the films were not destroyed. In order to investigate the preparation conditions of the films, the type of silane species, the hydrolysis time of the precursor solution, the molecular weight of PPG, the casting temperature and the evaporation temperature were discussed. 3.1. Silane species There were three kinds of silanes for preparation in this research. The effect of the contact angles of the films prepared by different silanes on the PPG weight fraction of PPG solution was shown in Fig. 2. It was indicated that the contact angles of the films were increased with increasing the PPG weight fraction of the PPG solution when the films were prepared by TEOS. There were four functional groups in the TEOS structures to become
Fig. 1. TGA result of PPG polymers.
hydroxyl groups and then to form network in the hybrid materials after polycondensation reaction, so the dense and rough structures can be acquired after the decomposition of PPG polymer at high temperature. It was considered that increasing the PPG weight fraction in the films, the roughness of the films was increased. On the other hand, when the films were prepared with MTEOS or DMDEOS, the contact angles showed almost no obvious variation, even increasing the PPG weight fraction of the PPG solution. If hydrolysis reaction of precursor solutions occurred completely, there were three hydroxyl and one methyl groups in the MTEOS and two hydroxyl and two methyl groups in the DMDEOS, respectively. The crosslinking density of the hybrid material prepared by MTEOS was less than that by TEOS [38]. In addition, the methyl groups of MTEOS and DMDEOS in the films were decomposed to become a hydroxyl group and the PPG polymer was also almost completely decomposed when the temperature was increased to 400 °C. The rough surfaces of the films were easily destroyed to become the flat surfaces by
Fig. 2. Effect of contact angles of the films prepared by different silane precursor solutions on PPG4000 weight percentage of PPG solution.
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Table 1 The preparation conditions and the contact angles of the films Precursor solutiona b
PPGc solution
TEOS:H2O :EtOH (molar ratio)
Hydrolysis time (h)
PPG wt.% in EtOH
1:4:4
2 4 6 8
80%
Contact angle (°)
128.9 128.8 128.8 127.4
a
The precursor solutions were reacted at 60 °C. The pH value of H2O was 2. c The molecular weight of PPG polymer was 4000 and the weight ratio of PPG solution to precursor solution was 1. b
further condensation of these hydroxyl groups. As mentioned above, the TEOS silane was chosen as an inorganic part to prepare the films. 3.2. The hydrolysis time of the precursor solution In a typical sol–gel process, the precursor is accompanied with a series of hydrolysis and polycondensation reactions to form a colloidal suspension, or a “sol”. It was considered that with increasing the reaction time of the precursor solution, the phase separation of organic and inorganic part was increased. In order to investigate this effect, the contact angles of the films prepared by the precursor solution during different hydrolysis time were shown in Table 1. With increasing the hydrolysis time of precursor solutions from 2 to 8 h, the values of contact angles were almost the same values. Therefore, 4 h was chosen as the hydrolysis time to prepare precursor solutions in order to make sure the hydrolysis reaction completely occurred.
Fig. 4. Effect of contact angles of the films prepared by the mixing solution at different casting temperatures.
discussed. The effect of contact angles of the films on these factors were shown in Fig. 3. With increasing the weight fraction of PPG in the PPG solution, the contact angles of the films were increased when the films were prepared by PPG1000 and PPG4000, respectively. On the other hands, the contact angles of the films prepared by PPG4000 were higher than that by PPG1000. It was indicated that the surface roughness can be easily obtained by the PPG4000 polymer because the microphase separation was easily to occur when the films were prepared by using high molecular weight of PPG. 3.4. The casting temperature
The roughness of the films was obtained by the decomposition of PPG at high temperature. Therefore, the weight percentage of PPG solution and molecular weight of PPG polymer in the sol–gel process were the two main factors to be
From the previous results of preparation of silica–polyethylene glycol (PEG) hybrid in our laboratory, the mixing time of the mixing solution could affect the formation of gel. And, the formation of gel easily appeared in that system. On the contrary, for preparation of the silica–PPG hybrid, the rate of condensation reaction in the silica–PPG hybrid casting solution was slow, so the gel phase was not easily obtained. Therefore, the contact angles of the films prepared by different mixing
Fig. 3. Effect of contact angle of the film prepared by different PPG polymers on PPG weight percentage of PPG solution at 30 °C.
Fig. 5. Effect of contact angles of the films prepared at different evaporation temperatures on the ratio of PPG solution to precursor solution.
3.3. Molecular weight of PPG polymer
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Fig. 6. Image of water drop on the surfaces of the films prepared with the different ratio of PPG solution to precursor solution was (a) 0.2 (b) 0.6 (c) 1 (d) 3 (e) 5 at 5 °C. The value of contact angles of the films are shown in the insert of the diagram.
times (2–30 min) of casting solution was almost kept at the same value, 135°. But the roughness of the films could be affected by the casting temperature and the results were shown in Fig. 4. With decreasing the casting temperature, the contact angles of the films were increased. It was considered that the micro-phase separation of PPG polymer easily occurred at lower temperature so that surface roughness was enhanced. PPG polymer had the lower melting point, so the lower mobility of polymer chain existed at a lower casting temperature. And chemically bonded materials of silica–PPG hybrid had the higher chemical stability than non-chemically bonded ones [2]. For the reason mentioned above, there was no obvious phenomenon of phase separation for silica–PPG hybrid. But a nanometric scale of micro-phase separation of that was observed from SAXS analysis [34,37]. Therefore, the more excellent data of contact angle can be acquired at a lower casting temperature.
and ethanol were easily migrated onto the surfaces so that the serious phase separation of the PPG polymer can be taken place under the lower temperature. It was also attributed to have a partial crystallization and phase separation below a critical temperature [35]. Therefore, the surface roughness can be obtained after removing the organic part of the films at high
Table 2 The preparation conditions and characteristics of the films Evaporation temperature
PPG solution /precursor solution (wt. ratio)
Contact angle (°)
−10 °C
0.2 0.4 0.6 0.8 1 2 3 4 5 0.2 0.4 0.6 0.8 1 2 3 4 5 0.2 0.4 0.6 0.8 1 2 3 4 5
116.2 118.5 127.1 135.1 159.9 159.7 159.3 159.3 160.7 115.4 118.5 131.9 145.1 150.9 160.6 159.9 160.3 159.7 117.3 122.6 124.4 127.5 130.1 130.8 134.6 132.8 131.9
3.5. The effect of the evaporation temperature for the casting thin films After casting mixing solution onto the substrates, hybrid films were obtained when the solvent was evaporated. It was expected that the micro-phase separation of the hybrid films could be influenced by the evaporation temperature. The effect of the contact angles of the films on the different evaporation temperatures was shown in Fig. 5. The contact angles of the films were increased when the weight ratio of the PPG solution to the precursor solution was increased at all different evaporation temperatures. It was considered that the roughness of the surface easily occurred at high weight ratio of PPG. On the other hand, the contact angles of the films prepared by the evaporation temperature of − 10 °C and 5 °C were greater than that of 30 °C. It was considered that the chain of the PPG polymer was not easily moved and the micro-phase separation easily occurred at a lower temperature. At the same time, water
5 °C
30 °C
Transmittance (T%) 400 (nm)
600 (nm)
800 (nm)
31.8 37.6 43.9 25.4 30.0 33.2 54.2 63.6 62.3 97.8 81.8 48.5 31.9 8.8 48.3 35.1 41.8 50.2 98.7 94.8 100.9 100 95.7 97.7 94.4 96 94.8
56.8 62.1 56.3 40.4 43.4 44.3 65.8 74.3 73.5 99.1 96.4 72.6 49.5 32.3 71.3 57.3 65.1 71.5 102.0 98.3 102.6 100.6 97.1 100.5 99.2 99.2 98.5
71.3 74.4 63.8 53.8 53.7 53 72.1 80.2 78.9 102.3 100.3 85.2 64.5 55.9 83.8 72.8 78.4 83.5 103.5 99.5 102.7 102.3 98.4 101.4 100 101.6 101.4
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Fig. 7. Effect of contact angles and transmittance of the films on the weight ratio of PPG solution to precursor solution prepared at (a) −10 °C and (b) 5 °C.
temperature. The pictures of water drop on the films with the different ratio of PPG solution to precursor solution were shown in Fig. 6. The superhydrophobic thin films can be obtained
when the weight ratio of PPG solution to precursor solution was greater than 1. The data of transmittance (T%) of visible light of films with the wavelength range from 400 nm to 800 nm was shown in Table 2 and the relationship between contact angle and transmittance with the wavelength of 600 nm prepared at − 10 °C and 5 °C were shown in Fig. 7(a) and (b), respectively. It was indicated that transmittance of the as-prepared films were decreased with increasing the weight ratio of PPG solution to precursor solution from 0.2 to 1, but that increased with increasing the weight ratio of PPG polymer solution to precursor solution from 1 to 5. In other words, the higher and lower ratio of PPG solution to precursor solution showed the better result of transmittance of substrates, but the less one can be obtained when the weight ratio was between 1 and 4. It can be explained from the SEM images as shown in Fig. 8. When the weight ratio of PPG solution to precursor solution was high or low, the continuous matrix was easily obtained and the transmittance of substrates was high. It was considered that when the concentration of PPG polymer or precusor was high, the rich domain was easy to appear and the continuous surface can be easily observed as shown in Fig. 8(a) and (e). However, the discontinuous surface can be obtained from Fig. 8(c) because the macro phase separation of PPG polymer occurred effectively. It was also the reason why the transmittance of the film was low when the weight of PPG solution to precursor solution was 1. The superhydrophobic films with high transmittance (N80%) were prepared by this method. Fig. 9 showed that FT-IR spectra of as-prepared hybrid films and rough films with and without HMDS treatment, respectively. For the hybrid films as shown in Fig. 9(a), the PPG polymer still existed onto the surfaces so that the absorption band of methyl groups around 2951 cm− 1 was appeared obviously. After the surfaces with heat treatment and without
Fig. 8. SEM images on the surfaces of the films prepared with the different ratio of PPG solution to precursor solution (a) 0.2 (b) 0.6 (c) 1 (d) 3 (e) 5 at 5 °C.
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Fig. 9. FT-IR spectra of the organic–inorganic hybrid films (a) without any treatment, (b) with heat treatment at 500 °C and (c) with heat treatment at 500 °C and modification by HMDS.
modification by HMDS as shown in Fig. 9(b), methyl groups around 2951 cm− 1 disappeared and the absorption band of OH around 3500 cm− 1 has been obviously increased at this stage. At the same time, the siloxane network grew, as indicated by Si–O–Si bands at around 750 and 1050 cm− 1. When the films were modified with HMDS as shown in Fig. 9(c), it was indicated that –OH compound decreased. The absorption band of OH around 3500 cm− 1 has been obviously dropped, meanwhile, the absorption band of water adsorbed on silica gel around 1500–1800 cm− 1 was decreased. The intensity of bands relating to methyl groups around 2951 cm− 1, which slightly increased with modification of HMDS. It was revealed that hydrophobic groups could bond onto the surfaces. The infrared spectra information has been interpreted in detail in a previous paper [39]. From the above result of analysis, it can be supported that modification had successfully introduced hydrophobic groups onto the rough surfaces.
(methyl) groups of the surfaces were modified by HMDS. It was showed that the contact angles of the films can be controlled by the type of silane species, the hydrolysis time of the precursor solution, the molecular weight of PPG, the casting temperature and the evaporation temperature. Nevertheless, preparation of the films at lower evaporation temperature resulted in the high contact angles of the as-prepared films. The films prepared at 5 °C evaporation temperature exhibited the excellent contact angles which were greater than 160° and the transmittance of the films were greater than 80% at the same time.
4. Conclusions
References
Superhydrophobic silica-based surfaces have been prepared by a hybridization of TEOS and PPG polymer with suitable conditions. The surface roughness was promoted by removing PPG polymer from the films. In addition, the hydrophobic
Acknowledgements The authors thank the National Central University of Taiwan for their kind help, especially from the Department of Chemical and Materials Engineering for the use of contact angle goniometer and AFM analyzer.
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