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Synthesis and properties control of fluorinated organic–inorganic hybrid films
Applied Surface Science 258 (2011) 1412–1416
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis and properties control of fluorinated organic–inorganic hybrid films Qingjie Yu ∗ , Jianming Xu, Yuanyuan Han College of Chemical Engineering, Huaqiao University, Xiamen, 361021, PR China
a r t i c l e
i n f o
Article history: Received 5 August 2011 Received in revised form 15 September 2011 Accepted 20 September 2011 Available online 28 September 2011 Keywords: Fluorinated hybrid films Tetramethoxysilane Water repellency
a b s t r a c t Fluorinated organic–inorganic hybrid films were prepared by free-radical random copolymerization and sol–gel process through dodecafluoroheptyl methacrylate (DFMA), vinyltriethoxysilane (VTES), and tetramethoxysilane (TMOS). It was found that the prepared fluorinated organic–inorganic hybrid film was very hydrophobic and exhibits excellent water repellency. Hydrophobic fluorocarbon side chains were preferentially enriched to the outermost layer at the interface of coating film–air, and three layers probably exist in the coating films. The fluorinated hybrid films possessed fluorocarbon side chains orient toward the air originating from DFMA as the top layer, hydrocarbon backbone chain originating from vinyl polymerization as the middle layer, and silica network originating from the hydrolysis and condensation of siloxane as the bottom layer. It demonstrated that most of TMOS added might be arranged to the bottom layer of the fluorinated hybrid films, and had a slight impact on the enrichment of fluorocarbon side chains of the outermost layer. However, the useful properties of the fluorinated organic–inorganic hybrid films such as thickness and corrosion resistant can be significantly improved by the increase of TMOS content. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Materials with low surface free energy have gained much interest for water and oil-repellent nonstick applications together with microelectronics in the fields of biomaterials and coating engineering. The surface free energy of materials is mainly determined by the chemical structure and composition at the surface layer. The reported fluorine containing polymers show high surface segregation of the perfluoroalkyl side chains, which causes very low free energy on their surfaces [1–3]. The wetting behavior is determined by the nature of the pendent chain [4–6], the length of the pendent chain [7–11], the concentration of the functional surface groups [12], synthesis processing [13], the reaction time of sol–gel reaction [9], coating technique and film-forming conditions [1,14–17]. In addition, fluoroalkylsilane have been widely used as surface modification agents because of their high hydrophobic properties originating from fluorine as well as good adhesion between hybrids and substrates originating from silicone. They have important applications as additives for modifying surface properties in the fields of coatings, adhesives, films, fibers and moldings [18,19]. Hybrid fluorosilicone materials containing both fluorine and silicone by adding the fluorocarbon side-chain entities to a preformed siloxane polymer or by polymerization of suitable monomers, present a variety of interesting properties, and have
∗ Corresponding author. E-mail address: [email protected] (Q. Yu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.09.093
attracted considerable attention [20–27]. However, the major drawbacks of such fluoroalkylsilane or fluorinated silicon oligomer materials applying as coatings and films are its thickness and wearing resistance, which will limit its further application. In this study, three layered fluorinated organic–inorganic hybrid films with low surface free energy were synthesized by free-radical random copolymerization and sol–gel method from DFMA, VTES, and TMOS. An important role of TMOS added was to increase the density of the reaction sites of −OH groups with the surface area and the thickness of fluorinated organic–inorganic hybrid films. In addition, the effect of TMOS amounts on the surface properties and morphology of fluorinated organic–inorganic hybrid films was also investigated. We hope that the durability of the fluorinated organic–inorganic hybrid films can be improved by the addition of TMOS; meanwhile, the wetting behavior of the fluorinated hybrid films remains stable by reasonable controlling the structure of coating films. 2. Experimental 2.1. Reagents Dodecafluoroheptyl methacrylate [CH2 C(CH3 )COOCH2 CF(CF3 ) CFHCF(CF3 )CF3 ] (DFMA, 95%) was purchased from XeoGia Fluorine-Silicon Chemical Co. (China). Vinyltriethoxysilane [CH2 CHSi(OC2 H5 )3 ] (VTES, 97%) obtained from Sigma–Aldrich. Other reagents such as tetramethoxysilane (TMOS, 97%), benzoyl peroxide (BPO), tetrahydrofuran (THF) and hydrochloric acid
Q. Yu et al. / Applied Surface Science 258 (2011) 1412–1416
Fig. 1. Reaction scheme of fluorinated organic–inorganic hybrid films.
with 37% concentration were purchased from Tianjin Kermel Chemical Reagent Co. (China) and used as received without further purification. Deionized water was used throughout the experiments. 2.2. Film formation Fluorinated organic–inorganic hybrid films were synthesized by free-radical random copolymerization using DFMA as fluorinecontaining monomer, VTES as silicone-containing monomers and TMOS. DFMA and VTES were added into a three neckrounds bottom flask connecting to a condenser and thermometer with stirring. After DFMA and VTES were dissolved in THF solvent, BPO as initiator was added and reacted at 60 ◦ C for 3 h. The molar ratio of DFMA:VTES:THF:BPO was 1:1:60:0.05. And then, the as-prepared copolymer was co-hydrolyzed and co-condensed with TMOS using HCl as acidic catalyst, while molar ratios of TMOS:DFMA changed from 1:1 to 12:1. After stirred at 60 ◦ C for 3 h under magnetic stirring, the homogenous gel solution was obtained. The whole synthetic procedures are given in Fig. 1. 1060 Aluminum alloy panels (Dalian Aluminum Manufacture Co., China) were cut to a size of 20 mm × 20 mm × 1 mm and used as the substrates for coating. The samples were cleaned with 800 meshes silicon carbide paper, washed with a detergent and deionized water, rinsed with acetone in an ultrasonic bath, and then dried by exposure to air. Film deposition was carried out at room temperature by the vertical dipping method using a draw speed in the range of 10–12 cm/min. The coated film was dried in the air for 30 min at 25 ◦ C to evaporate the solvents and then heated to 140 ◦ C at a rate of 0.5 ◦ C/min in an oven under N2 atmosphere. 2.3. Characterization of coating films The chemical transformations of fluorinated organic–inorganic hybrid films were identified by attenuated total reflectance infrared spectroscopy (EQUINOX55, KBr tablet). FT-IR spectra were recorded by using ATR objective of a BRUKER IFS 484 microscope at room temperature. The spectra were recorded in the 4000–400 cm−1 range with a resolution of 2 cm−1 . The gel content measurements were carried out according to ASTM D2765. Weighed samples in a copper net were put into boiling xylene for 12 h. The extracted samples were washed using acetone, and then dried to a constant weight in a vacuum oven. The gel content is expressed in terms of the percentage of the weight remaining. The values of gel fraction deviated from the average by less than ±2%.
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Fig. 2. FT-IR spectra of fluorinated hybrid films with different TMOS/DFMA molar ratios: (a) 1:1, (b) 2:1, (c) 4:1, (d) 8:1 and (e) 12:1.
Surface and cross-sectional micrographs of the coated samples were inspected using a scanning electron microscopy (SEM, Hitachi S-500). The contact angles of fluorinated hybrid coatings were measured by sessile-drop method with a Data Physics OCA 20 apparatus at 25 ◦ C. Wetting liquids used for contact angle measurements were water and methylene iodide suggested by Owens and Wendt. In order to avoid any surface contamination, all specimens were washed with acetone, ethanol and deionized water in sequence and accurately dried just before measurement. Static contact angle determinations were repeatedly carried out with the same sample and an average value of the contact angle was determined on the basis of 20 measurements at least. The surface of fluorinated hybrid films was examined by Xray photoelectron spectroscopy (XPS). The X-ray source provided monochromatic Al K␣ radiation (1486.6 eV). Typical operating conditions include the following: test chamber pressure 2 × 10−9 Torr; 15 kV, 150 W for the Al K␣ source. Elemental survey scans from 0 to 1000 eV were acquired with pass energy of 50 eV, while highresolution scan of the C 1s, O 1s, F 1s and Si 2p regions were acquired with pass energy of 20 eV. The XPS peaks were decomposed using a peak fitting routine. The lines used in the fitting of a peak envelope are defined according to their centered position, half-width, shape (Gaussian or Lorentzian distribution) as well as their intensity. The corrosion protection of the coating films was evaluated by potentiodynamic polarization in 0.5 mol/L sodium chloride aqueous solutions exposing to air. The potentiodynamic polarization was performed in a conventional electrochemical cell by using a saturated calomel electrode (SCE) as reference and a platinum wire as counter-electrode. The exposed area of the working electrode was 1.0 cm2 . All potentials were measured at 25 ◦ C. The potential was varied from −1.0 V to positive potentials at a scanning rate of 5 mV/s. And prior to the above measurements, the samples are kept in sodium chloride aqueous solution for 10 min to attain a steady state. 3. Results and discussion 3.1. FT-IR and XPS studies FT-IR spectra of the fluorinated hybrid films are shown in Fig. 2. The plain band centered at 3440 cm−1 and the sharp band at 1640 cm−1 are respectively assigned to the stretching and bending absorption of OH groups. The characteristic stretching bands of the C–H group (CH2 ), C O group, C–F groups (CF2 and CF3 ),–O–C(O)–C
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Q. Yu et al. / Applied Surface Science 258 (2011) 1412–1416 Table 1 Elemental atomic composition by XPS analysis and in bulk (in bracket, calculated from stoichiometry) for fluorinated hybrid films with different molar ratios of TMOS/DFMA. TMOS/DFMA (molar ratio)
C 1s
Si 2p
O 1s
F 1s
1:1 4:1 8:1
34.2 (28.5) 33.4 (20.3) 32.3 (15.8)
4.2 (10.3) 5.3 (22.0) 7.5 (28.4)
16.3 (17.5) 16.6 (26.6) 17.0 (31.6)
45.3 (41.7) 44.8 (29.7) 43.2 (23.1)
Analysis of quantitative surface can be derived from photoionization peak areas of C, O, Si and F through empirical atomic sensitivity factors. The atomic surface composition (C, O, Si and F) of fluorinated hybrid films compared to the bulk composition (calculated from stoichiometry) is reported in Table 1. The surface content of C and F atoms are significantly higher than that of the corresponding bulk concentrations, indicating a strong surface enrichment of fluorocarbon with respect to silica network (Si and O atoms). In addition, it can be found that there is a slow drop in the surface content of C and F atoms with increasing TMOS/DFMA mole ratio. However, the difference of the atomic composition measured by XPS and calculated from stoichiometry increase dramatically with increasing TMOS content. These results suggest that, most of TMOS added might be arranged to the bottom layer of fluorinated hybrid films to form silica network. Based on FT-IR and XPS results, it can be deduced that three layers probably existed in the fluorinated organic–inorganic hybrid films: the top layer oriented toward the air was mainly composed of fluorocarbon side chains originating from DFMA; the middle layer was composed of hydrocarbon backbone chains originating from vinyl polymerization; the bottom layer was mainly composed of –O–Si–O– network originating from hydrolysis and condensation of siloxane.
Fig. 3. XPS C 1s core level spectra of fluorinated hybrid films with various TMOS/DFMA molar ratios: (a) 1:1, (b) 4:1 and (c) 8:1.
group and–C–O group are strongly shown at wavenumbers of 2800–3000, 1700, 1210–1250, 1160, and 977 cm−1 respectively [21,27]. The present of these bands assigned to fluorocarbon side chains at low TMOS/DFMA molar ratio indicate that fluorocarbon side chains in the films were more enriched to the film surface. The Si–C stretching motion is located at 695 cm−1 . Moreover, the present of absorption bands at 745 cm−1 assigned to –(CH2 )4 – vibrations formed by free-radical random copolymerization is based on the principal chains (hydrocarbon chains), suggesting that the fluorinated hybrid films with both fluorine and siliconecontaining groups were well synthesized. With the increase of the TMOS content, FT-IR spectra of the fluorinated hybrid films show more absorption bands originated from siloxane monomer. For the samples with the highest molar ratio of TMOS/DFMA (Fig. 2e), there is a very pronounced band appearing at 1080 cm−1 together with a band at 795 cm−1 , corresponding to the vibration absorption of Si–O–Si groups originating from TMOS and VTES. Aluminum alloy as substrate can form networks with silicates, thus enhancing the cross-link density of the film further because some Si–O–Si bonds are replaced by the Si–O–Al group at 936 cm−1 [28,29]. In order to obtain more quantitative information on the relative concentration of DFMA segments at the surface with respect to the bulk nominal concentration, and hence to derive information on the fluorinated segments segregation at the surface, high-resolution XPS C 1s core-level spectra of fluorinated hybrid materials are shown in Fig. 3. The spectra were curve resolved into six Gaussian peaks: –CF3 around 293.6 eV, –CF2 around 292.8 eV, –C O around 289.2 eV, –C–O–C O around 287.9 eV, –C–C O around 285.4 eV, and hydrocarbon (–CHn , n = 0–3) around 284.4 eV [3,30]. The spectra of the fluorinated hybrid materials with various TMOS/DFMA molar ratios were different from each other. A high segregation of fluorocarbon side chains at the surface was revealed from the spectrum of fluorinated hybrid materials at low TMOS content, which is consistent with the results of FT-IR spectra.
3.2. Surface properties of fluorinated hybrid films XPS analysis has confirmed the surface enrichment of fluorocarbon side chains in the water-repellent film. In Table 2, the surface free energies of fluorinated organic–inorganic hybrid films are compared as a function of TMOS/DFMA molar ratios. It can be seen that the fluorinated organic–inorganic hybrid films exhibit very low surface free energies 11.5 mJ/m2 while TMOS/DFMA mole ratio is 1:1. With the increase of TEOS/DFMA mole ratio, the surface free energy increases slowly for all the films. Therefore, the increase of TMOS content has a slight influence on the enrichment of fluorocarbon side chains of the top layer, and the surface wettability of fluorinated hybrid films, because most of TMOS added might be arranged to the bottom layer of fluorinated hybrid films. An important role of TMOS is to increase the density of the reaction sites of the –OH group and the thickness of organic–inorganic hybrid coatings. As is shown in Table 2, it can be seen that the increase of TMOS amounts can enhance hydrolysis and condensation of alkoxide precursors, and increase the amount of wet materials deposited onto the substrate, which may increase the thickness and durability of fluorinated hybrid films.
Table 2 Surface free energy and thickness of fluorinated hybrid films as a function of TEOS/DFMA molar ratio. TEOS/DFMA (molar ratio)
1:1 2:1 4:1 8:1 12:1
Contact angle (◦ ) H2 O
CH2 I2
120.3 116.6 113.9 111.1 110.2
92.7 89.7 85.3 83.3 81.9
Surface free energy (mJ/m2 )
Standard deviation
Gel content (wt%)
Viscosity (mPa s)
Thickness (m)
11.5 12.8 14.9 15.8 16.5
1.5 1.2 1.4 1.6 1.3
63.6 66.4 68.9 70.7 71.8
0.9 1.1 1.3 1.4 1.5
1.5 2.0 2.5 2.9 3.4
Q. Yu et al. / Applied Surface Science 258 (2011) 1412–1416
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Fig. 4. SEM images of fluorinated hybrid films with various TMOS/DFMA molar ratios: (a) 1:1, (b) 4:1, (c) 8:1, (d) 12:1 and cross section (e) of fluorinated hybrid films while TMOS/DFMA molar ratio is 12:1.
SEM micrographs of the fluorinated organic–inorganic hybrid films with different TMOS/DFMA molar ratios are shown in Fig. 4. It clearly shows a strong effect of the TMOS/DFMA molar ratio on the structural evolution of fluorinated hybrid films. It is apparent from Fig. 4a–c that the fluorinated hybrid films with low TMOS/DFMA molar ratio have a smooth surface without detectable cracks. However, the fluorinated hybrid films tend to exhibit brittleness and micro-cracks when TMOS/DFMA mole ratio increases to 12:1 (Fig. 4d). The present of micro–cracks formed during heat treated process is as a result of the accumulation of shear stress caused by the dehydration of gel and the evaporation of solvent. This micro-structural characteristic can be explained by the decline of the flexible organic compositions, which could relax the stress and reduce the micro-cracks caused by the volume shrinkage of silicon network to produce smooth surface because of the increase of inorganic composition. Potentiodynamic polarization curves of the fluorinated hybrid films with different molar ratios of TMOS/DFMA are shown in Fig. 5. It is obvious that the increase of the TMOS content may result in a better corrosion resistance as it was indicated by a gradually enhancing corrosion potential and corrosion current. The improvement of corrosion resistance ability may be due to the increase of the film’s thickness. However, the films derived from the highest TMOS/DFMA molar ratio (Fig. 5e) exhibited poor
corrosion resistance. The deterioration of corrosion resistance ability will be mainly attributed to the present of micro-cracks. The corrosive solutions were more easily attacked these micro-cracks and permeated into the coatings.
Fig. 5. Potentiodynamic polarization curves of fluorinated hybrid films with different molar ratios of TMOS/DFMA: (a) 1:1, (b) 2:1, (c) 4:1, (d) 8:1 and (e) 12:1.
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Q. Yu et al. / Applied Surface Science 258 (2011) 1412–1416
4. Conclusion In this paper, we present a method to synthesize and control relevant properties of fluorinated organic–inorganic hybrid films by controlling the structure of the coating film. Three layered fluorinated organic–inorganic hybrid films were synthesized, which had very low surface energy and could be used as water repellent functional coatings to apply onto metal substrates. It was demonstrated that most of TMOS added might be arranged to the bottom layer of the films, and had a slight influence on the enrichment of fluorocarbon side chains of the top layer. On contrary, the useful properties of the fluorinated organic–inorganic hybrid films as thickness and corrosion resistance can be significantly improved by the increase of TMOS content. Acknowledgement This work was financially supported by Natural Science Foundation of Fujian Province of China (Grant No. 2011J01051) and Foundation of Huaqiao University (09BS505). References [1] Y. Urushihara, T. Nishino, Langmuir 21 (2005) 2614. [2] T. Nishino, M. Meguro, K. Nakamae, M. Matsushita, Y. Ueda, Langmuir 15 (1999) 4321. [3] J. Tsibouklis, P. Graham, P.J. Eaton, J.R. Smith, T.G. Nevell, J.D. Smart, R.J. Ewen, Macromolecules 33 (2000) 8460. [4] Y. Katano, H. Tomono, T. Nakajima, Macromolecules 27 (1994) 2342. [5] S. Borkar, K. Jankova, H.W. Siesler, S. Hvilsted, Macromolecules 37 (2004) 788. [6] M.A. Mchugh, A.G. Domech, I.H. Park, D. Li, E. Barbu, P. Graham, J. Tsibouklis, Macromolecules 35 (2002) 6479.
[7] C.C. Chang, W.C. Chen, Chem. Mater. 14 (2002) 4242. [8] Y. Furukawa, T. Yoneda, Polym. Chem. 41 (2003) 2704. [9] S. Perutz, J. Wang, E.J. Kramer, C.K. Ober, K. Ellis, Macromolecules 31 (1998) 4272. [10] H. Sawada, K. Ikeno, T. Kawase, Macromolecules 35 (2002) 4306. [11] H. Sawada, D. Tamada, T. Kawase, Y. Hayakawa, M. Baba, Macromolecules 30 (1997) 6706. [12] I.J. Park, S.B. Lee, C.K. Choi, Macromolecules 31 (1998) 7555. [13] M. Krupers, P.J. Slangen, M. Möller, Macromolecules 31 (1998) 2552. [14] I.J. Park, S.B. Lee, C.K. Choi, Polymer 38 (1997) 2523. [15] J. Tsibouklis, M. Stone, A.A. Thorpe, P. Graham, V. Peters, R. Heerlien, J.R. Smith, K.L. Green, T.G. Nevell, Biomaterials 20 (1999) 1229. [16] H. Ito, T. Imae, T. Nakamura, M. Sugiura, Y. Oshibe, Colloid Interface Sci. 276 (2004) 290. [17] T. Imae, H. Tabuchi, K. Funayama, A. Sato, T. Nakamura, N. Amaya, Colloid Surf. A 167 (2000) 73. [18] B.T. Qian, Z.Q. Shen, Longmuir 21 (2005) 9007. [19] K. Tadanaga, J. Morinaga, A. Matsuda, T. Minami, Chem. Mater. 12 (2000) 590. [20] D.K. Kim, S.B. Lee, K.S. Doh, J. Colloid Interface Sci. 05 (1998) 417. [21] D.K. Kim, S.B. Lee, J. Colloid Interface Sci. 247 (2002) 490. [22] H. Sawada, N. Itoh, T. Kawase, M. Mitani, H. Nakajima, M. Nishida, Y. Moriya, Langmuir 10 (1994) 994. [23] H. Sawada, A. Ohashi, M. Oue, M. Baba, M. Abe, M. Mitani, H. Nakajima, J. Fluorine Chem. 75 (1995) 121. [24] J. Nakagawa, K. Kamogawa, N. Momozawa, H. Sakai, T. Kawase, H. Sawada, Y. Sano, M. Abe, Langmuir 14 (1998) 2061. [25] J. Nakagawa, K. Kamogawa, H. Sakai, T. Kawase, H. Sawada, J. Manosroi, A. Manosroi, M. Abe, Langmuir 14 (1998) 2055. [26] P. Fabbri, M. Messori, M. Montecchi, S. Nannarone, L. Pasquali, F. Pilati, C. Tonelli, M. Toselli, Polymer 47 (2006) 1055. [27] H.J. Jeong, D.K. Kim, S.B. Lee, S.H. Kwon, K. Kadonoz, J. Colloid Interface Sci. 235 (2001) 130. [28] R.Z. Zand, A.E. Langroudi, A. Rahimi, J. Non-Cryst. Solids 351 (2005) 1307. [29] R.Z. Zand, A.E. Langroudi, A. Rahimi, Prog. Org. Coat. 53 (2005) 286. [30] C.M. Kassis, J.K. Steehler, D.E. Betts, Macromolecules 29 (1996) 3247.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis and properties control of fluorinated organic–inorganic hybrid films Qingjie Yu ∗ , Jianming Xu, Yuanyuan Han College of Chemical Engineering, Huaqiao University, Xiamen, 361021, PR China
a r t i c l e
i n f o
Article history: Received 5 August 2011 Received in revised form 15 September 2011 Accepted 20 September 2011 Available online 28 September 2011 Keywords: Fluorinated hybrid films Tetramethoxysilane Water repellency
a b s t r a c t Fluorinated organic–inorganic hybrid films were prepared by free-radical random copolymerization and sol–gel process through dodecafluoroheptyl methacrylate (DFMA), vinyltriethoxysilane (VTES), and tetramethoxysilane (TMOS). It was found that the prepared fluorinated organic–inorganic hybrid film was very hydrophobic and exhibits excellent water repellency. Hydrophobic fluorocarbon side chains were preferentially enriched to the outermost layer at the interface of coating film–air, and three layers probably exist in the coating films. The fluorinated hybrid films possessed fluorocarbon side chains orient toward the air originating from DFMA as the top layer, hydrocarbon backbone chain originating from vinyl polymerization as the middle layer, and silica network originating from the hydrolysis and condensation of siloxane as the bottom layer. It demonstrated that most of TMOS added might be arranged to the bottom layer of the fluorinated hybrid films, and had a slight impact on the enrichment of fluorocarbon side chains of the outermost layer. However, the useful properties of the fluorinated organic–inorganic hybrid films such as thickness and corrosion resistant can be significantly improved by the increase of TMOS content. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Materials with low surface free energy have gained much interest for water and oil-repellent nonstick applications together with microelectronics in the fields of biomaterials and coating engineering. The surface free energy of materials is mainly determined by the chemical structure and composition at the surface layer. The reported fluorine containing polymers show high surface segregation of the perfluoroalkyl side chains, which causes very low free energy on their surfaces [1–3]. The wetting behavior is determined by the nature of the pendent chain [4–6], the length of the pendent chain [7–11], the concentration of the functional surface groups [12], synthesis processing [13], the reaction time of sol–gel reaction [9], coating technique and film-forming conditions [1,14–17]. In addition, fluoroalkylsilane have been widely used as surface modification agents because of their high hydrophobic properties originating from fluorine as well as good adhesion between hybrids and substrates originating from silicone. They have important applications as additives for modifying surface properties in the fields of coatings, adhesives, films, fibers and moldings [18,19]. Hybrid fluorosilicone materials containing both fluorine and silicone by adding the fluorocarbon side-chain entities to a preformed siloxane polymer or by polymerization of suitable monomers, present a variety of interesting properties, and have
∗ Corresponding author. E-mail address: [email protected] (Q. Yu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.09.093
attracted considerable attention [20–27]. However, the major drawbacks of such fluoroalkylsilane or fluorinated silicon oligomer materials applying as coatings and films are its thickness and wearing resistance, which will limit its further application. In this study, three layered fluorinated organic–inorganic hybrid films with low surface free energy were synthesized by free-radical random copolymerization and sol–gel method from DFMA, VTES, and TMOS. An important role of TMOS added was to increase the density of the reaction sites of −OH groups with the surface area and the thickness of fluorinated organic–inorganic hybrid films. In addition, the effect of TMOS amounts on the surface properties and morphology of fluorinated organic–inorganic hybrid films was also investigated. We hope that the durability of the fluorinated organic–inorganic hybrid films can be improved by the addition of TMOS; meanwhile, the wetting behavior of the fluorinated hybrid films remains stable by reasonable controlling the structure of coating films. 2. Experimental 2.1. Reagents Dodecafluoroheptyl methacrylate [CH2 C(CH3 )COOCH2 CF(CF3 ) CFHCF(CF3 )CF3 ] (DFMA, 95%) was purchased from XeoGia Fluorine-Silicon Chemical Co. (China). Vinyltriethoxysilane [CH2 CHSi(OC2 H5 )3 ] (VTES, 97%) obtained from Sigma–Aldrich. Other reagents such as tetramethoxysilane (TMOS, 97%), benzoyl peroxide (BPO), tetrahydrofuran (THF) and hydrochloric acid
Q. Yu et al. / Applied Surface Science 258 (2011) 1412–1416
Fig. 1. Reaction scheme of fluorinated organic–inorganic hybrid films.
with 37% concentration were purchased from Tianjin Kermel Chemical Reagent Co. (China) and used as received without further purification. Deionized water was used throughout the experiments. 2.2. Film formation Fluorinated organic–inorganic hybrid films were synthesized by free-radical random copolymerization using DFMA as fluorinecontaining monomer, VTES as silicone-containing monomers and TMOS. DFMA and VTES were added into a three neckrounds bottom flask connecting to a condenser and thermometer with stirring. After DFMA and VTES were dissolved in THF solvent, BPO as initiator was added and reacted at 60 ◦ C for 3 h. The molar ratio of DFMA:VTES:THF:BPO was 1:1:60:0.05. And then, the as-prepared copolymer was co-hydrolyzed and co-condensed with TMOS using HCl as acidic catalyst, while molar ratios of TMOS:DFMA changed from 1:1 to 12:1. After stirred at 60 ◦ C for 3 h under magnetic stirring, the homogenous gel solution was obtained. The whole synthetic procedures are given in Fig. 1. 1060 Aluminum alloy panels (Dalian Aluminum Manufacture Co., China) were cut to a size of 20 mm × 20 mm × 1 mm and used as the substrates for coating. The samples were cleaned with 800 meshes silicon carbide paper, washed with a detergent and deionized water, rinsed with acetone in an ultrasonic bath, and then dried by exposure to air. Film deposition was carried out at room temperature by the vertical dipping method using a draw speed in the range of 10–12 cm/min. The coated film was dried in the air for 30 min at 25 ◦ C to evaporate the solvents and then heated to 140 ◦ C at a rate of 0.5 ◦ C/min in an oven under N2 atmosphere. 2.3. Characterization of coating films The chemical transformations of fluorinated organic–inorganic hybrid films were identified by attenuated total reflectance infrared spectroscopy (EQUINOX55, KBr tablet). FT-IR spectra were recorded by using ATR objective of a BRUKER IFS 484 microscope at room temperature. The spectra were recorded in the 4000–400 cm−1 range with a resolution of 2 cm−1 . The gel content measurements were carried out according to ASTM D2765. Weighed samples in a copper net were put into boiling xylene for 12 h. The extracted samples were washed using acetone, and then dried to a constant weight in a vacuum oven. The gel content is expressed in terms of the percentage of the weight remaining. The values of gel fraction deviated from the average by less than ±2%.
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Fig. 2. FT-IR spectra of fluorinated hybrid films with different TMOS/DFMA molar ratios: (a) 1:1, (b) 2:1, (c) 4:1, (d) 8:1 and (e) 12:1.
Surface and cross-sectional micrographs of the coated samples were inspected using a scanning electron microscopy (SEM, Hitachi S-500). The contact angles of fluorinated hybrid coatings were measured by sessile-drop method with a Data Physics OCA 20 apparatus at 25 ◦ C. Wetting liquids used for contact angle measurements were water and methylene iodide suggested by Owens and Wendt. In order to avoid any surface contamination, all specimens were washed with acetone, ethanol and deionized water in sequence and accurately dried just before measurement. Static contact angle determinations were repeatedly carried out with the same sample and an average value of the contact angle was determined on the basis of 20 measurements at least. The surface of fluorinated hybrid films was examined by Xray photoelectron spectroscopy (XPS). The X-ray source provided monochromatic Al K␣ radiation (1486.6 eV). Typical operating conditions include the following: test chamber pressure 2 × 10−9 Torr; 15 kV, 150 W for the Al K␣ source. Elemental survey scans from 0 to 1000 eV were acquired with pass energy of 50 eV, while highresolution scan of the C 1s, O 1s, F 1s and Si 2p regions were acquired with pass energy of 20 eV. The XPS peaks were decomposed using a peak fitting routine. The lines used in the fitting of a peak envelope are defined according to their centered position, half-width, shape (Gaussian or Lorentzian distribution) as well as their intensity. The corrosion protection of the coating films was evaluated by potentiodynamic polarization in 0.5 mol/L sodium chloride aqueous solutions exposing to air. The potentiodynamic polarization was performed in a conventional electrochemical cell by using a saturated calomel electrode (SCE) as reference and a platinum wire as counter-electrode. The exposed area of the working electrode was 1.0 cm2 . All potentials were measured at 25 ◦ C. The potential was varied from −1.0 V to positive potentials at a scanning rate of 5 mV/s. And prior to the above measurements, the samples are kept in sodium chloride aqueous solution for 10 min to attain a steady state. 3. Results and discussion 3.1. FT-IR and XPS studies FT-IR spectra of the fluorinated hybrid films are shown in Fig. 2. The plain band centered at 3440 cm−1 and the sharp band at 1640 cm−1 are respectively assigned to the stretching and bending absorption of OH groups. The characteristic stretching bands of the C–H group (CH2 ), C O group, C–F groups (CF2 and CF3 ),–O–C(O)–C
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Q. Yu et al. / Applied Surface Science 258 (2011) 1412–1416 Table 1 Elemental atomic composition by XPS analysis and in bulk (in bracket, calculated from stoichiometry) for fluorinated hybrid films with different molar ratios of TMOS/DFMA. TMOS/DFMA (molar ratio)
C 1s
Si 2p
O 1s
F 1s
1:1 4:1 8:1
34.2 (28.5) 33.4 (20.3) 32.3 (15.8)
4.2 (10.3) 5.3 (22.0) 7.5 (28.4)
16.3 (17.5) 16.6 (26.6) 17.0 (31.6)
45.3 (41.7) 44.8 (29.7) 43.2 (23.1)
Analysis of quantitative surface can be derived from photoionization peak areas of C, O, Si and F through empirical atomic sensitivity factors. The atomic surface composition (C, O, Si and F) of fluorinated hybrid films compared to the bulk composition (calculated from stoichiometry) is reported in Table 1. The surface content of C and F atoms are significantly higher than that of the corresponding bulk concentrations, indicating a strong surface enrichment of fluorocarbon with respect to silica network (Si and O atoms). In addition, it can be found that there is a slow drop in the surface content of C and F atoms with increasing TMOS/DFMA mole ratio. However, the difference of the atomic composition measured by XPS and calculated from stoichiometry increase dramatically with increasing TMOS content. These results suggest that, most of TMOS added might be arranged to the bottom layer of fluorinated hybrid films to form silica network. Based on FT-IR and XPS results, it can be deduced that three layers probably existed in the fluorinated organic–inorganic hybrid films: the top layer oriented toward the air was mainly composed of fluorocarbon side chains originating from DFMA; the middle layer was composed of hydrocarbon backbone chains originating from vinyl polymerization; the bottom layer was mainly composed of –O–Si–O– network originating from hydrolysis and condensation of siloxane.
Fig. 3. XPS C 1s core level spectra of fluorinated hybrid films with various TMOS/DFMA molar ratios: (a) 1:1, (b) 4:1 and (c) 8:1.
group and–C–O group are strongly shown at wavenumbers of 2800–3000, 1700, 1210–1250, 1160, and 977 cm−1 respectively [21,27]. The present of these bands assigned to fluorocarbon side chains at low TMOS/DFMA molar ratio indicate that fluorocarbon side chains in the films were more enriched to the film surface. The Si–C stretching motion is located at 695 cm−1 . Moreover, the present of absorption bands at 745 cm−1 assigned to –(CH2 )4 – vibrations formed by free-radical random copolymerization is based on the principal chains (hydrocarbon chains), suggesting that the fluorinated hybrid films with both fluorine and siliconecontaining groups were well synthesized. With the increase of the TMOS content, FT-IR spectra of the fluorinated hybrid films show more absorption bands originated from siloxane monomer. For the samples with the highest molar ratio of TMOS/DFMA (Fig. 2e), there is a very pronounced band appearing at 1080 cm−1 together with a band at 795 cm−1 , corresponding to the vibration absorption of Si–O–Si groups originating from TMOS and VTES. Aluminum alloy as substrate can form networks with silicates, thus enhancing the cross-link density of the film further because some Si–O–Si bonds are replaced by the Si–O–Al group at 936 cm−1 [28,29]. In order to obtain more quantitative information on the relative concentration of DFMA segments at the surface with respect to the bulk nominal concentration, and hence to derive information on the fluorinated segments segregation at the surface, high-resolution XPS C 1s core-level spectra of fluorinated hybrid materials are shown in Fig. 3. The spectra were curve resolved into six Gaussian peaks: –CF3 around 293.6 eV, –CF2 around 292.8 eV, –C O around 289.2 eV, –C–O–C O around 287.9 eV, –C–C O around 285.4 eV, and hydrocarbon (–CHn , n = 0–3) around 284.4 eV [3,30]. The spectra of the fluorinated hybrid materials with various TMOS/DFMA molar ratios were different from each other. A high segregation of fluorocarbon side chains at the surface was revealed from the spectrum of fluorinated hybrid materials at low TMOS content, which is consistent with the results of FT-IR spectra.
3.2. Surface properties of fluorinated hybrid films XPS analysis has confirmed the surface enrichment of fluorocarbon side chains in the water-repellent film. In Table 2, the surface free energies of fluorinated organic–inorganic hybrid films are compared as a function of TMOS/DFMA molar ratios. It can be seen that the fluorinated organic–inorganic hybrid films exhibit very low surface free energies 11.5 mJ/m2 while TMOS/DFMA mole ratio is 1:1. With the increase of TEOS/DFMA mole ratio, the surface free energy increases slowly for all the films. Therefore, the increase of TMOS content has a slight influence on the enrichment of fluorocarbon side chains of the top layer, and the surface wettability of fluorinated hybrid films, because most of TMOS added might be arranged to the bottom layer of fluorinated hybrid films. An important role of TMOS is to increase the density of the reaction sites of the –OH group and the thickness of organic–inorganic hybrid coatings. As is shown in Table 2, it can be seen that the increase of TMOS amounts can enhance hydrolysis and condensation of alkoxide precursors, and increase the amount of wet materials deposited onto the substrate, which may increase the thickness and durability of fluorinated hybrid films.
Table 2 Surface free energy and thickness of fluorinated hybrid films as a function of TEOS/DFMA molar ratio. TEOS/DFMA (molar ratio)
1:1 2:1 4:1 8:1 12:1
Contact angle (◦ ) H2 O
CH2 I2
120.3 116.6 113.9 111.1 110.2
92.7 89.7 85.3 83.3 81.9
Surface free energy (mJ/m2 )
Standard deviation
Gel content (wt%)
Viscosity (mPa s)
Thickness (m)
11.5 12.8 14.9 15.8 16.5
1.5 1.2 1.4 1.6 1.3
63.6 66.4 68.9 70.7 71.8
0.9 1.1 1.3 1.4 1.5
1.5 2.0 2.5 2.9 3.4
Q. Yu et al. / Applied Surface Science 258 (2011) 1412–1416
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Fig. 4. SEM images of fluorinated hybrid films with various TMOS/DFMA molar ratios: (a) 1:1, (b) 4:1, (c) 8:1, (d) 12:1 and cross section (e) of fluorinated hybrid films while TMOS/DFMA molar ratio is 12:1.
SEM micrographs of the fluorinated organic–inorganic hybrid films with different TMOS/DFMA molar ratios are shown in Fig. 4. It clearly shows a strong effect of the TMOS/DFMA molar ratio on the structural evolution of fluorinated hybrid films. It is apparent from Fig. 4a–c that the fluorinated hybrid films with low TMOS/DFMA molar ratio have a smooth surface without detectable cracks. However, the fluorinated hybrid films tend to exhibit brittleness and micro-cracks when TMOS/DFMA mole ratio increases to 12:1 (Fig. 4d). The present of micro–cracks formed during heat treated process is as a result of the accumulation of shear stress caused by the dehydration of gel and the evaporation of solvent. This micro-structural characteristic can be explained by the decline of the flexible organic compositions, which could relax the stress and reduce the micro-cracks caused by the volume shrinkage of silicon network to produce smooth surface because of the increase of inorganic composition. Potentiodynamic polarization curves of the fluorinated hybrid films with different molar ratios of TMOS/DFMA are shown in Fig. 5. It is obvious that the increase of the TMOS content may result in a better corrosion resistance as it was indicated by a gradually enhancing corrosion potential and corrosion current. The improvement of corrosion resistance ability may be due to the increase of the film’s thickness. However, the films derived from the highest TMOS/DFMA molar ratio (Fig. 5e) exhibited poor
corrosion resistance. The deterioration of corrosion resistance ability will be mainly attributed to the present of micro-cracks. The corrosive solutions were more easily attacked these micro-cracks and permeated into the coatings.
Fig. 5. Potentiodynamic polarization curves of fluorinated hybrid films with different molar ratios of TMOS/DFMA: (a) 1:1, (b) 2:1, (c) 4:1, (d) 8:1 and (e) 12:1.
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4. Conclusion In this paper, we present a method to synthesize and control relevant properties of fluorinated organic–inorganic hybrid films by controlling the structure of the coating film. Three layered fluorinated organic–inorganic hybrid films were synthesized, which had very low surface energy and could be used as water repellent functional coatings to apply onto metal substrates. It was demonstrated that most of TMOS added might be arranged to the bottom layer of the films, and had a slight influence on the enrichment of fluorocarbon side chains of the top layer. On contrary, the useful properties of the fluorinated organic–inorganic hybrid films as thickness and corrosion resistance can be significantly improved by the increase of TMOS content. Acknowledgement This work was financially supported by Natural Science Foundation of Fujian Province of China (Grant No. 2011J01051) and Foundation of Huaqiao University (09BS505). References [1] Y. Urushihara, T. Nishino, Langmuir 21 (2005) 2614. [2] T. Nishino, M. Meguro, K. Nakamae, M. Matsushita, Y. Ueda, Langmuir 15 (1999) 4321. [3] J. Tsibouklis, P. Graham, P.J. Eaton, J.R. Smith, T.G. Nevell, J.D. Smart, R.J. Ewen, Macromolecules 33 (2000) 8460. [4] Y. Katano, H. Tomono, T. Nakajima, Macromolecules 27 (1994) 2342. [5] S. Borkar, K. Jankova, H.W. Siesler, S. Hvilsted, Macromolecules 37 (2004) 788. [6] M.A. Mchugh, A.G. Domech, I.H. Park, D. Li, E. Barbu, P. Graham, J. Tsibouklis, Macromolecules 35 (2002) 6479.
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