- Email: [email protected]
toluene plasma at low pressure and its wettability
Current Applied Physics 9 (2009) 1223–1226
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Surface modification of polyimide films, filter papers, and cotton clothes by HMDSO/toluene plasma at low pressure and its wettability Soon Cheon Cho a, Yong Cheol Hong c, Soon Gook Cho b, Young Yeon Ji c, Chang Soo Han a, Han Sup Uhm c,* a
Nano-Mechanical Systems Research Center Korea Institute of Machinery and Materials (KIMM), 171, Jang-Dong, Yousung, Daejeon 305-343, Republic of Korea Department of Electrical, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea c Department of Molecular Science and Technology, Ajou University, San 5, Wonchon-Dong, Youngtong-Gu, Suwon 443-749, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 13 March 2008 Received in revised form 29 January 2009 Accepted 29 January 2009 Available online 5 February 2009 PACS: 52.80.Vp 52.75.d 81.90.+c 81.60.Jw
a b s t r a c t This study demonstrates the hydrophobic and super-hydrophobic coating of various substrates by 3:1 HMDSO/toluene glow plasma discharge at low pressure and investigates the hydrophobic and superhydrophobic behavior of the polyimide films, filter papers, and cotton clothes. The effect of 3:1 HMDSO/toluene plasma treatments on the surface of polyimide films, filter papers, and cotton clothes were investigated in terms of total surface free energy by measuring the contact angles with probe liquids. In representation, the total surface free energies of the polyimide films before and after the 3:1 HMDSO/toluene plasma modification estimated from the Owens–Wendt equation decreased from 44.5 mN/m to 13.94 mN/m, showing the significant improvement of hydrophobicity of all sample surfaces treated by 3:1 HMDSO/toluene plasma. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Super-hydrophobic coating PI films, filter papers, and cotton clothes HMDSO/toluene plasma
1. Introduction Hydrophobic and super-hydrophobic surface treatments of various substrates are of great interest in recent years [1], which are potentially suitable for various applications, such as dust-free and self-cleaning surfaces for solar cells, satellite dishes, roofing and building glasses and corrosion protection [2]. In general, hydrophobic and super-hydrophobic surfaces have been produced mainly in two ways. (1) One is to create a rough structure on a hydrophobic surface (contact angle > 90°). (2) The other is to modify a rough surface by materials with low surface free energy. In addition, the contact angles hysteresis is very low, producing a surface off which water droplets simply roll [3]. Conventionally, water repellence is accomplished using solvents and organic reagents, mostly wax emulsions, quaternary ammonium salts and hydrophobic resin finishes, which require discarding and can cause environmental problems as well as the health concerns of workers because of the disposal of harmful waste in the treatment baths [4]. Plasma treatment is an environmentally friendly process used to create good quality depositions on substrates. Cold plasma pro* Corresponding author. E-mail address: [email protected] (H.S. Uhm). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.01.020
cessing is a well-known and widely used technique for surface modification by etching, corrosion or deposition. Plasmas create extremely reactive species such as ions, free radicals and metastable species, which allow reactions to occur at much lower temperatures than in conventional methods, and even reactions unique to plasma conditions. Low quantities of reagents are used and disposed in plasma processing owing to the short treatment times and low pressures, and low energies are used in most processes making them economically attractive [5]. Recently, artificial hydrophobic and super-hydrophobic surfaces of various substrates are made through Organic silicon compounds. One of their great advantages is the ease of manipulation, since they are liquids of low toxicity. For example, Tan et al. reported the creation of hydrophobic filter papers with the water contact angle of 120° by modifying the surface of HMDS coating [6]. In this sense, we developed a simple method for plasma modification of polyimide film (PI film), filter papers, and cotton clothes by making use of alternating-current (AC) glow discharge plasma in 3:1 HMDSO/toluene. Also, we present the formation of hydrophobic coating on the MWCNTs, and wetting properties of the modified MWCNTs are investigated by measuring the contact angles and calculating the total surface energies from the equation of Owens–Wendt [7].
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S.C. Cho et al. / Current Applied Physics 9 (2009) 1223–1226
operating the discharge, the device is evacuated up to a base pressure of one Torr. Afterwards, mixed HMDSO/Toluene was injected through the reactor chamber, bubbled by 0.01 lpm argon gas. Monomers were injected by pressure gradient at room temperature, with the working pressure controlled by changing the aperture of the pumping valve. The treatment of the PI film, filter papers, and cotton clothes were carried out at 60 W and at a pressure of 3.0 Torr for 30 s. The experiments were performed varying the HMDSO (98%, Aldrich)/toluene (99.8%, Samchun chemical) mixing rate ratio. Three types of depositions were made: (1) toluene alone; (2) 75:25 HMDSO/toluene; (3) HMDSO alone. PI films are prepared from DuPont Company. Also, cotton fiber (Pointfours Corporation in Korea) and Filter paper was chosen substrate since it is the most hydrophilic type of paper. The various substrates sample are analyzed by employing a Fourier transform infrared (FT-IR), spectroscopy, X-ray photoelectron spectroscopy (XPS), contact angle analyzer. To investigate the change in the surface free energy of the samples, water, polyethylene-glycol (PEG), and glycerol as probe liquids were used. The total surface free energy is calculated from the Owens–Wendt equation by measuring the contact angles with probe liquids [9]. 3. Results and discussion Fig. 1. Schematic of HMDSO/toluene glow plasma system at low pressure for preparing hydrophobic/super-hydrophobic various materials.
2. Experiment The glow discharge plasma system for PI film, filter papers, and cotton clothes surface treatment is shown in Fig. 1, similar to the application of multi-needle injection plasma at atmospheric pressure stabilized by the flowing channel of a working gas through a hot electrode [8]. A plane as a hot electrode was used in the present experiment, for uniform and stable plasma generation within reaction chamber of the AC power supply used for the experiment. The AC power supply is a commercial high-voltage transformer for neon light and is operated at 15 kV, 20 kHz. Two electrodes operated by AC voltages with a sinusoidal wave comprise the hot electrode and the ground electrode floated in air. Plasma treatments were carried out in a cylindrical reactor (outer diameter 60.5 mm, thickness 3 mm, and length 550 mm) connected to a rotary pump and a hot electrode. The reactor chamber was made of fused quartz. The plane as a hot electrode is diameter of 40 mm and made of aluminum (Al). The sample holder (50 mm diameter) was 5 mm away from the hot electrode and made of Al. Before
Fig. 2 shows the shapes and contact angles of water droplets placed on PI film samples before and after plasma treatment for different HMDSO/toluene mixing rate ratio compositions. The contact angles with water were measured for a drop of water (20 ul) from a micro syringe. Before the treatment of the cotton fiber and Filter paper, the contact angles are exactly 0°. Water is absorbed by cotton fiber and Filter paper. After treatment, however, the contact angles of the cotton fiber and Filter paper in Fig. 2 is about 174°, 157° revealing contact angles close to super-hydrophobic property, respectively. The contact angles of the raw sample in Fig. 2a are about 56°, revealing a contact angles close to hydrophilic property. Figs. 2b–d show the contact angles of PI film surfaces treated by only toluene, only HMDSO and 3:1 toluene/HMDSO plasmas, corresponding to the contact angles of 72°, 85°, and 104°, respectively. The result exhibits that the hydrophobicity of PI film sample surfaces after exposure to the plasmas increases markedly. The hydrophobicity increased is due to the increase in the roughness, in addition to the change in the chemical composition on the surface. The increased roughness is due to the plasma composition and hence the chemical of the deposited film from the glow plasma.
Fig. 2. (a)–(c) are the photograph of Shapes and contact angles of water drops placed on cotton clothes, PI films and filter paper samples of before and after 3:1 HMDSO/ toluene plasmas, respectively. (d) Shows plots of the contact angles versus HMDSO/toluene mixing rate ratio composition after the plasma treatment. The circles and stars correspond to the filter paper and cotton clothes samples treated by 3:1 HMDSO/toluene plasmas, respectively.
S.C. Cho et al. / Current Applied Physics 9 (2009) 1223–1226
qffiffiffiffiffi c ð1 þ cos hÞ qffiffiffiffiffip Y ¼ l qffiffiffiffiffi ¼ cs X þ cds 2 cdl
1225
ð1Þ
pffiffiffipffi c where X = pffiffiffildffi : cl
Fig. 3. Owens–Wendt plot for the PI films according to Eq. (1). Each of data points is an averaged value of five repeated-measurements.
In order to understand the difference in the effects of the different HMDSO/toluene mixing rate ratio compositions were investigated by contact angles and surface free energy. Water adsorption is related not only to the hydrophobic character of the surface, as is water contact angles, but also to the surface free energy of the various substrates, which could be affected by the plasma deposition. Fig. 3 shows contact angles for different HMDSO/toluene mixing rate ratio compositions. To examine the effects of HMDSO/toluene mixing rate ratio glow plasma on the surface free energy and contact angles of various substrates were measured with water, glycerol, and polyethylene-glycol (PEG). The components of the total surface free energy for the PI films sample were determined from the Owens and Wendt equation [9]. The equation is a linear equation, Y = mX + b, in which the slope m and intercept b are given by the square root of polar and dispersive components of the solid surface free energy [(cps )1/2 and (cds )1/2, respectively], such as Table 1 Surface energy parameters of the probe liquids used in this experiment and the contact angles measured experimentally. Probe liquid
c1
PEG Glycerol Water
48.3 64 72.8
cdl
29.3 34 21.8
cpl
19 30 51
PI films contact angle (°) Before treatment
Only toluene
Only HMDSO
HMDSO/ toluene
37 49 56
55 62 76
80 86 90
86 95 104
The subscripts s and l represent solid and liquid, respectively. The surface free energy components are shown in Table 1. Namely, the X values are given by X = 0.805 for polyethylene-glycol (PEG), 0.94 for glycerol, and 1.53 for water. According to the Eq. (1), the surface free energy components of the PI films were calculated from the contact angles measured from the three probe liquids in Table 1. Fig. 3 shows the plots according to (1), using the contact angles of the three probe liquids measured in this paper. The boxes, circles and triangles in Fig. 3 represent the data points of raw, 100% HMDSO, 100% toluene, and 3:1 HMDSO/toluene plasma-treated the polyimide films. Each data point in Fig. 3, for the contact angles, is an average of more than five repeat measurements. We determined the polar and dispersive components of the total surface free energy of the PI film samples from the slope and the intercept of the linear fit in Fig. 3 The total surface energy is defined as: cds þ cps . Hence, the surface free energy components of untreated PI films were determined to be 44.5 mN/m, (cds = 30.86 mN/ m, and cps = 13.64 mN/m), respectively. However, the surface free energy for the surface of toluene or HMDSO deposited on the surface of the PI films decreased. The effect of decreasing surface free energy on the film composition is shown in Table 1 for films deposited from toluene, HMDSO, 3:1 HMDSO/toluene plasmas, respectively. As shown Table 1, the total surface free energy after plasma treatment decreased to 32.63 mN/m (cps = 17.01 mN/m, and cds = 15.62 mN/m), 22.65 mN/m (cps = 19.97 mN/m, and cds = 2.68 mN/m), for toluene and HMDSO plasma-treated PI films. However, the total surface free energy after plasma treatment decreased drastically to 13.94 mN/m (cps = 2.77 mN/m, and cds = 11.17 mN/m) for 3:1 HMDSO/toluene plasma-treated PI films. Such increased surface free energy could explain why 3:1 HMDSO/toluene treated PI films are more hydrophobic than the untreated PI films. Fourier transform infrared (FT-IR) spectroscopy was applied to characterize the surface of PI film surface. Fig. 4 shows the FT-IR spectra of the PI film surface untreated and treated PI films in 3:1 HMDSO/toluene plasma systems in the range 750– 3990 cm1. The FT-IR were recorded with a Bruker IFS-66/S spectrometer in the 400–4000 cm1 wavenumber range using pressed KBr pellets. As shown in Fig. 4a, there are some absorption peaks at about 1720 cm1 due to C@O peak, at about 1650 cm1 due to amide coupling, at about 1200 and 1380 cm1 due to N–H peak, at about 1500 cm1 due to the C@C stretching vibration and stretching vibration at 1770 cm1. The FT-IR of PI film deposited from 3:1
Fig. 4. FT-IR spectra of PI film before and after 3:1 HMDSO/toluene plasma treatment.
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S.C. Cho et al. / Current Applied Physics 9 (2009) 1223–1226 Table 2 Elemental composition of PI films deposition from HMDSO/toluene plasma. HMDSO/ toluene mixing rate ratio (%)
C
O
75:25 100:0
57.17 59.4
21.62 24.32
Binding energies (eV)
Atomic ratio
Si
Si 2p
O 1s
C 1s
C/O
C/Si
O/Si
21.21 16.29
103.3 102.4
533.7 532.7
285.9 285
0.93 0.86
3.24 4.39
3.49 5.1
Table 2 reports the summary of the elemental analysis. The main components in the coating are carbon, silicon, and oxygen. The Si 2p, C 1s and O 1s peaks are symmetrical and positioned at about 102.4, 285 and 532.7 eV with 100% HMDSO, respectively. However, the Si, C and O peaks were shifted to position increases 103.3, 285.9, and 533.7 eV 3:1 HMDSO/toluene, respectively. The highest intensity peak at 285.9 eV was due to C–Si bonding which is as expected for methyl legends present in partially decomposed HMDSO and toluene. Shown in Table 2 are the binding energies and atomic percentages of the silicon, oxygen and carbon present in the PI film coatings prepared using 100% HMDSO and 3:1 HMDSO/toluene. The coatings were comprised of 16.29% to 21.21% silicon, 24.32% to 21.62% oxygen and 59.4% to 57.17% carbons. As shown Table 2, atomic ratio shows C/O, C/Si and O/Si for 100% HMDSO and 3:1 HMDSO/toluene, the surface of PI films after 3:1 HMDSO/toluene is richer in Si and poorer in O. This indicates a decrease in the oxygen content for high values of the contact angles. 4. Conclusion
Fig. 5. X-ray diffraction patterns of PI films. High-resolution XPS spectra of the astreatment PI films: (a) Si 2p (b) C 1s region and O 1s.
HMDSO/toluene plasma is shown in Fig. 4b. The stronger absorption band in the range 1000–1150 cm1 can be assigned to the Si–O–Si asymmetric stretching mode. Other typical absorption bands can be assigned: the Si–CH3 rocking vibration at 840– 850 cm1, the CH3 symmetric bending in Si–CH3 at 1260 cm1, the CHx symmetric and asymmetric stretching at 2960 cm1. By comparing HMDSO and 3:1 HMDSO/toluene samples, it is observed that the characteristic peaks of 3:1 HMDSO/toluene treated PI film were present in the range of 700–650 cm1 (1.3.5-substd benzene, 1.3-substd benzene) and 700 cm1 (–(CH2)n–, –CH@CH–(CIS)). These characteristic peaks can evaluate the hydrophobic properties of the coatings effectively. The PI film surface composition and bonding of the samples were investigated by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 5, a spectrum of 3:1 HMDSO/toluene on PI film and
We reported the surface modification of various materials using glow discharge plasma with different HMDSO/toluene mixing rate ratio compositions. The contact angles measurement indicated that the 3:1 HMDSO/toluene plasma was more effective for the modification of the samples than the 100% HMDSO plasmas, showing the highest contact angle (104°) after plasma treatment. Also, we studied the wettability properties of the PI surfaces by estimating total surface free energies from the Owens–Wendt equation. The total surface free energy of the PI sample treated by 3:1 HMDSO/toluene plasma increased markedly from 44.5 mN/m to 13.94 mN/m. The samples with a hydrophobic surface may improve the self-cleaning surfaces. Thus, the 3:1 HMDSO/toluene plasma at low pressure may be applicable to the treatment and desorption promotion of other various materials surfaces. Acknowledgements This work was supported financially by the Nuclear Fusion Research Center (NFRC) and BK 21 program in part. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
R. Blossey, Nat. Mater. 2 (2003) 301. M.H. Kim, K.H. Kim, N.Y. Lee, K.S. Shin, Y.S. Kim, Chem. Commun. (2007) 2237. D. Oner, T.J. McCarthy, Langmuir 16 (2000) 7777. Y. Liu, X. Chen, J.H. Xin, Nanotechnology 17 (2006) 3259. S.C. Cho, Y.C. Hong, H.S. Uhm, J. Mater. Chem. 17 (2007) 232. I.H. Tan, M.L.P. Da silvab, N.R. Demarquette, J. Mater. Chem. 11 (2001) 1019. D.K. Owens, R.C. Wendt, J. Appl. Polym. Sci. 13 (1969) 1741. Y.C. Hong, H.S. Uhm, Appl. Phys. Lett. 88 (2006) 244101. D.K. Owens, R.C. Wendt, J. Appl. Polym. Sci. 13 (1969) 1741.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Surface modification of polyimide films, filter papers, and cotton clothes by HMDSO/toluene plasma at low pressure and its wettability Soon Cheon Cho a, Yong Cheol Hong c, Soon Gook Cho b, Young Yeon Ji c, Chang Soo Han a, Han Sup Uhm c,* a
Nano-Mechanical Systems Research Center Korea Institute of Machinery and Materials (KIMM), 171, Jang-Dong, Yousung, Daejeon 305-343, Republic of Korea Department of Electrical, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea c Department of Molecular Science and Technology, Ajou University, San 5, Wonchon-Dong, Youngtong-Gu, Suwon 443-749, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 13 March 2008 Received in revised form 29 January 2009 Accepted 29 January 2009 Available online 5 February 2009 PACS: 52.80.Vp 52.75.d 81.90.+c 81.60.Jw
a b s t r a c t This study demonstrates the hydrophobic and super-hydrophobic coating of various substrates by 3:1 HMDSO/toluene glow plasma discharge at low pressure and investigates the hydrophobic and superhydrophobic behavior of the polyimide films, filter papers, and cotton clothes. The effect of 3:1 HMDSO/toluene plasma treatments on the surface of polyimide films, filter papers, and cotton clothes were investigated in terms of total surface free energy by measuring the contact angles with probe liquids. In representation, the total surface free energies of the polyimide films before and after the 3:1 HMDSO/toluene plasma modification estimated from the Owens–Wendt equation decreased from 44.5 mN/m to 13.94 mN/m, showing the significant improvement of hydrophobicity of all sample surfaces treated by 3:1 HMDSO/toluene plasma. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Super-hydrophobic coating PI films, filter papers, and cotton clothes HMDSO/toluene plasma
1. Introduction Hydrophobic and super-hydrophobic surface treatments of various substrates are of great interest in recent years [1], which are potentially suitable for various applications, such as dust-free and self-cleaning surfaces for solar cells, satellite dishes, roofing and building glasses and corrosion protection [2]. In general, hydrophobic and super-hydrophobic surfaces have been produced mainly in two ways. (1) One is to create a rough structure on a hydrophobic surface (contact angle > 90°). (2) The other is to modify a rough surface by materials with low surface free energy. In addition, the contact angles hysteresis is very low, producing a surface off which water droplets simply roll [3]. Conventionally, water repellence is accomplished using solvents and organic reagents, mostly wax emulsions, quaternary ammonium salts and hydrophobic resin finishes, which require discarding and can cause environmental problems as well as the health concerns of workers because of the disposal of harmful waste in the treatment baths [4]. Plasma treatment is an environmentally friendly process used to create good quality depositions on substrates. Cold plasma pro* Corresponding author. E-mail address: [email protected] (H.S. Uhm). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.01.020
cessing is a well-known and widely used technique for surface modification by etching, corrosion or deposition. Plasmas create extremely reactive species such as ions, free radicals and metastable species, which allow reactions to occur at much lower temperatures than in conventional methods, and even reactions unique to plasma conditions. Low quantities of reagents are used and disposed in plasma processing owing to the short treatment times and low pressures, and low energies are used in most processes making them economically attractive [5]. Recently, artificial hydrophobic and super-hydrophobic surfaces of various substrates are made through Organic silicon compounds. One of their great advantages is the ease of manipulation, since they are liquids of low toxicity. For example, Tan et al. reported the creation of hydrophobic filter papers with the water contact angle of 120° by modifying the surface of HMDS coating [6]. In this sense, we developed a simple method for plasma modification of polyimide film (PI film), filter papers, and cotton clothes by making use of alternating-current (AC) glow discharge plasma in 3:1 HMDSO/toluene. Also, we present the formation of hydrophobic coating on the MWCNTs, and wetting properties of the modified MWCNTs are investigated by measuring the contact angles and calculating the total surface energies from the equation of Owens–Wendt [7].
1224
S.C. Cho et al. / Current Applied Physics 9 (2009) 1223–1226
operating the discharge, the device is evacuated up to a base pressure of one Torr. Afterwards, mixed HMDSO/Toluene was injected through the reactor chamber, bubbled by 0.01 lpm argon gas. Monomers were injected by pressure gradient at room temperature, with the working pressure controlled by changing the aperture of the pumping valve. The treatment of the PI film, filter papers, and cotton clothes were carried out at 60 W and at a pressure of 3.0 Torr for 30 s. The experiments were performed varying the HMDSO (98%, Aldrich)/toluene (99.8%, Samchun chemical) mixing rate ratio. Three types of depositions were made: (1) toluene alone; (2) 75:25 HMDSO/toluene; (3) HMDSO alone. PI films are prepared from DuPont Company. Also, cotton fiber (Pointfours Corporation in Korea) and Filter paper was chosen substrate since it is the most hydrophilic type of paper. The various substrates sample are analyzed by employing a Fourier transform infrared (FT-IR), spectroscopy, X-ray photoelectron spectroscopy (XPS), contact angle analyzer. To investigate the change in the surface free energy of the samples, water, polyethylene-glycol (PEG), and glycerol as probe liquids were used. The total surface free energy is calculated from the Owens–Wendt equation by measuring the contact angles with probe liquids [9]. 3. Results and discussion Fig. 1. Schematic of HMDSO/toluene glow plasma system at low pressure for preparing hydrophobic/super-hydrophobic various materials.
2. Experiment The glow discharge plasma system for PI film, filter papers, and cotton clothes surface treatment is shown in Fig. 1, similar to the application of multi-needle injection plasma at atmospheric pressure stabilized by the flowing channel of a working gas through a hot electrode [8]. A plane as a hot electrode was used in the present experiment, for uniform and stable plasma generation within reaction chamber of the AC power supply used for the experiment. The AC power supply is a commercial high-voltage transformer for neon light and is operated at 15 kV, 20 kHz. Two electrodes operated by AC voltages with a sinusoidal wave comprise the hot electrode and the ground electrode floated in air. Plasma treatments were carried out in a cylindrical reactor (outer diameter 60.5 mm, thickness 3 mm, and length 550 mm) connected to a rotary pump and a hot electrode. The reactor chamber was made of fused quartz. The plane as a hot electrode is diameter of 40 mm and made of aluminum (Al). The sample holder (50 mm diameter) was 5 mm away from the hot electrode and made of Al. Before
Fig. 2 shows the shapes and contact angles of water droplets placed on PI film samples before and after plasma treatment for different HMDSO/toluene mixing rate ratio compositions. The contact angles with water were measured for a drop of water (20 ul) from a micro syringe. Before the treatment of the cotton fiber and Filter paper, the contact angles are exactly 0°. Water is absorbed by cotton fiber and Filter paper. After treatment, however, the contact angles of the cotton fiber and Filter paper in Fig. 2 is about 174°, 157° revealing contact angles close to super-hydrophobic property, respectively. The contact angles of the raw sample in Fig. 2a are about 56°, revealing a contact angles close to hydrophilic property. Figs. 2b–d show the contact angles of PI film surfaces treated by only toluene, only HMDSO and 3:1 toluene/HMDSO plasmas, corresponding to the contact angles of 72°, 85°, and 104°, respectively. The result exhibits that the hydrophobicity of PI film sample surfaces after exposure to the plasmas increases markedly. The hydrophobicity increased is due to the increase in the roughness, in addition to the change in the chemical composition on the surface. The increased roughness is due to the plasma composition and hence the chemical of the deposited film from the glow plasma.
Fig. 2. (a)–(c) are the photograph of Shapes and contact angles of water drops placed on cotton clothes, PI films and filter paper samples of before and after 3:1 HMDSO/ toluene plasmas, respectively. (d) Shows plots of the contact angles versus HMDSO/toluene mixing rate ratio composition after the plasma treatment. The circles and stars correspond to the filter paper and cotton clothes samples treated by 3:1 HMDSO/toluene plasmas, respectively.
S.C. Cho et al. / Current Applied Physics 9 (2009) 1223–1226
qffiffiffiffiffi c ð1 þ cos hÞ qffiffiffiffiffip Y ¼ l qffiffiffiffiffi ¼ cs X þ cds 2 cdl
1225
ð1Þ
pffiffiffipffi c where X = pffiffiffildffi : cl
Fig. 3. Owens–Wendt plot for the PI films according to Eq. (1). Each of data points is an averaged value of five repeated-measurements.
In order to understand the difference in the effects of the different HMDSO/toluene mixing rate ratio compositions were investigated by contact angles and surface free energy. Water adsorption is related not only to the hydrophobic character of the surface, as is water contact angles, but also to the surface free energy of the various substrates, which could be affected by the plasma deposition. Fig. 3 shows contact angles for different HMDSO/toluene mixing rate ratio compositions. To examine the effects of HMDSO/toluene mixing rate ratio glow plasma on the surface free energy and contact angles of various substrates were measured with water, glycerol, and polyethylene-glycol (PEG). The components of the total surface free energy for the PI films sample were determined from the Owens and Wendt equation [9]. The equation is a linear equation, Y = mX + b, in which the slope m and intercept b are given by the square root of polar and dispersive components of the solid surface free energy [(cps )1/2 and (cds )1/2, respectively], such as Table 1 Surface energy parameters of the probe liquids used in this experiment and the contact angles measured experimentally. Probe liquid
c1
PEG Glycerol Water
48.3 64 72.8
cdl
29.3 34 21.8
cpl
19 30 51
PI films contact angle (°) Before treatment
Only toluene
Only HMDSO
HMDSO/ toluene
37 49 56
55 62 76
80 86 90
86 95 104
The subscripts s and l represent solid and liquid, respectively. The surface free energy components are shown in Table 1. Namely, the X values are given by X = 0.805 for polyethylene-glycol (PEG), 0.94 for glycerol, and 1.53 for water. According to the Eq. (1), the surface free energy components of the PI films were calculated from the contact angles measured from the three probe liquids in Table 1. Fig. 3 shows the plots according to (1), using the contact angles of the three probe liquids measured in this paper. The boxes, circles and triangles in Fig. 3 represent the data points of raw, 100% HMDSO, 100% toluene, and 3:1 HMDSO/toluene plasma-treated the polyimide films. Each data point in Fig. 3, for the contact angles, is an average of more than five repeat measurements. We determined the polar and dispersive components of the total surface free energy of the PI film samples from the slope and the intercept of the linear fit in Fig. 3 The total surface energy is defined as: cds þ cps . Hence, the surface free energy components of untreated PI films were determined to be 44.5 mN/m, (cds = 30.86 mN/ m, and cps = 13.64 mN/m), respectively. However, the surface free energy for the surface of toluene or HMDSO deposited on the surface of the PI films decreased. The effect of decreasing surface free energy on the film composition is shown in Table 1 for films deposited from toluene, HMDSO, 3:1 HMDSO/toluene plasmas, respectively. As shown Table 1, the total surface free energy after plasma treatment decreased to 32.63 mN/m (cps = 17.01 mN/m, and cds = 15.62 mN/m), 22.65 mN/m (cps = 19.97 mN/m, and cds = 2.68 mN/m), for toluene and HMDSO plasma-treated PI films. However, the total surface free energy after plasma treatment decreased drastically to 13.94 mN/m (cps = 2.77 mN/m, and cds = 11.17 mN/m) for 3:1 HMDSO/toluene plasma-treated PI films. Such increased surface free energy could explain why 3:1 HMDSO/toluene treated PI films are more hydrophobic than the untreated PI films. Fourier transform infrared (FT-IR) spectroscopy was applied to characterize the surface of PI film surface. Fig. 4 shows the FT-IR spectra of the PI film surface untreated and treated PI films in 3:1 HMDSO/toluene plasma systems in the range 750– 3990 cm1. The FT-IR were recorded with a Bruker IFS-66/S spectrometer in the 400–4000 cm1 wavenumber range using pressed KBr pellets. As shown in Fig. 4a, there are some absorption peaks at about 1720 cm1 due to C@O peak, at about 1650 cm1 due to amide coupling, at about 1200 and 1380 cm1 due to N–H peak, at about 1500 cm1 due to the C@C stretching vibration and stretching vibration at 1770 cm1. The FT-IR of PI film deposited from 3:1
Fig. 4. FT-IR spectra of PI film before and after 3:1 HMDSO/toluene plasma treatment.
1226
S.C. Cho et al. / Current Applied Physics 9 (2009) 1223–1226 Table 2 Elemental composition of PI films deposition from HMDSO/toluene plasma. HMDSO/ toluene mixing rate ratio (%)
C
O
75:25 100:0
57.17 59.4
21.62 24.32
Binding energies (eV)
Atomic ratio
Si
Si 2p
O 1s
C 1s
C/O
C/Si
O/Si
21.21 16.29
103.3 102.4
533.7 532.7
285.9 285
0.93 0.86
3.24 4.39
3.49 5.1
Table 2 reports the summary of the elemental analysis. The main components in the coating are carbon, silicon, and oxygen. The Si 2p, C 1s and O 1s peaks are symmetrical and positioned at about 102.4, 285 and 532.7 eV with 100% HMDSO, respectively. However, the Si, C and O peaks were shifted to position increases 103.3, 285.9, and 533.7 eV 3:1 HMDSO/toluene, respectively. The highest intensity peak at 285.9 eV was due to C–Si bonding which is as expected for methyl legends present in partially decomposed HMDSO and toluene. Shown in Table 2 are the binding energies and atomic percentages of the silicon, oxygen and carbon present in the PI film coatings prepared using 100% HMDSO and 3:1 HMDSO/toluene. The coatings were comprised of 16.29% to 21.21% silicon, 24.32% to 21.62% oxygen and 59.4% to 57.17% carbons. As shown Table 2, atomic ratio shows C/O, C/Si and O/Si for 100% HMDSO and 3:1 HMDSO/toluene, the surface of PI films after 3:1 HMDSO/toluene is richer in Si and poorer in O. This indicates a decrease in the oxygen content for high values of the contact angles. 4. Conclusion
Fig. 5. X-ray diffraction patterns of PI films. High-resolution XPS spectra of the astreatment PI films: (a) Si 2p (b) C 1s region and O 1s.
HMDSO/toluene plasma is shown in Fig. 4b. The stronger absorption band in the range 1000–1150 cm1 can be assigned to the Si–O–Si asymmetric stretching mode. Other typical absorption bands can be assigned: the Si–CH3 rocking vibration at 840– 850 cm1, the CH3 symmetric bending in Si–CH3 at 1260 cm1, the CHx symmetric and asymmetric stretching at 2960 cm1. By comparing HMDSO and 3:1 HMDSO/toluene samples, it is observed that the characteristic peaks of 3:1 HMDSO/toluene treated PI film were present in the range of 700–650 cm1 (1.3.5-substd benzene, 1.3-substd benzene) and 700 cm1 (–(CH2)n–, –CH@CH–(CIS)). These characteristic peaks can evaluate the hydrophobic properties of the coatings effectively. The PI film surface composition and bonding of the samples were investigated by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 5, a spectrum of 3:1 HMDSO/toluene on PI film and
We reported the surface modification of various materials using glow discharge plasma with different HMDSO/toluene mixing rate ratio compositions. The contact angles measurement indicated that the 3:1 HMDSO/toluene plasma was more effective for the modification of the samples than the 100% HMDSO plasmas, showing the highest contact angle (104°) after plasma treatment. Also, we studied the wettability properties of the PI surfaces by estimating total surface free energies from the Owens–Wendt equation. The total surface free energy of the PI sample treated by 3:1 HMDSO/toluene plasma increased markedly from 44.5 mN/m to 13.94 mN/m. The samples with a hydrophobic surface may improve the self-cleaning surfaces. Thus, the 3:1 HMDSO/toluene plasma at low pressure may be applicable to the treatment and desorption promotion of other various materials surfaces. Acknowledgements This work was supported financially by the Nuclear Fusion Research Center (NFRC) and BK 21 program in part. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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