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Gas barrier property of silica-based films on PET synthesized by atmospheric pressure plasma enhanced CVD
SCT-21317; No of Pages 4 Surface & Coatings Technology xxx (2016) xxx–xxx
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Gas barrier property of silica-based films on PET synthesized by atmospheric pressure plasma enhanced CVD Mai Moritoki ⁎, Takanori Mori, Akira Shirakura, Tetsuya Suzuki Center for Science of Environment, Resources and Energy, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan
a r t i c l e
i n f o
Article history: Received 31 October 2015 Revised 13 June 2016 Accepted in revised form 26 June 2016 Available online xxxx Keywords: Atmospheric pressure plasma enhanced chemical vapor deposition Gas barrier property Silica-based film Polyethylene terephthalate Trimethylsilane
a b s t r a c t In this study, silica-based films were synthesized on polyethylene terephthalate (PET) using the mixture gas of trimethylsilane (TrMS), N2, and O2 by the atmospheric pressure plasma enhanced CVD (AP-PECVD) method. We investigated the effect of the oxygen flow rate on the gas barrier property of silica-based films with various O2 flow rates ranging from 0 mL/min to 1000 mL/min. As the O2 flow rate increased to 500 mL/min, the oxygen transmission rate (OTR) of the films decreased to 8.2 cm3/m2/24 h/atm, which is approximately one third of that of the uncoated PET. On the other hand, the OTR increased when the O2 flow rate exceeded 500 mL/min. From the fourier-transform infrared (FT-IR) spectra, the intensity of Si-O-Si peaks increased and the shape of Si-O-Si peaks changed to be sharp with increasing the O2 flow rate, indicating that the structure of the films became dense. Oxygen molecules could not permeate through the highly dense films, so that the gas barrier property was improved. Using the atomic force microscope (AFM), many pinholes with a diameter of approximately 30 nm were observed on the film surface at the O2 flow rate over 500 mL/min. From the observation of the uncoated PET exposed to N2/O2 plasma, the surface texture hardly changed and kept its smooth surface at less O2 flow rate up to 500 mL/min, while the rugged texture were obviously observed at the O2 flow rate over 500 mL/min because of the etching effect in plasma. The holes were formed because the films were synthesized at the PET surface in the rugged shape. Therefore, the formation of the pinholes on the film surface could allow oxygen molecules to diffuse into the films. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Polymeric materials are currently used for engineering materials such as eyewear lenses and liquid crystal displays for cell-phones [1,2]. Polyethylene terephthalate (PET) is one of the most widely used materials for food packaging because of its lightweight, flexibility, and high transparency. PET has been studied to improve its gas barrier property aimed at protecting contents from degradation, particularly that resulting from oxidative processes. Coating with gas barrier films on PET is an effective approach to improving gas barrier property. Silica-based films for polymers have been studied because they have their excellent gas barrier property and heat resistance, maintaining their transparency [3]. In general, silica-based films are synthesized by plasma enhanced chemical vapor deposition (PECVD) method under low pressure. However, that synthesis process costs much because it needs chamber, vacuum system, and long processing time. One of the effective ways to overcoming the problems is an atmospheric pressure PECVD (APPECVD) system because vacuum system is not required in the synthesis under atmospheric pressure. Okazaki et al. reported on the mechanism
⁎ Corresponding author. E-mail address: [email protected] (M. Moritoki).
of stabilization of glow plasma under atmospheric pressure and succeeded in preventing a transition from glow to arc discharge [4–6]. In the synthesis of the silica-based films by the AP-PECVD method, liquid sources such as tetraethoxysilane (TEOS) and hexamethyldisiloxane (HMDSO) are mainly used with the equipment such as vaporizers [7,8]. Trimethylsilane ((CH3)3HSi, TrMS) is gaseous at room temperature under atmospheric pressure and can be used in PECVD process to produce dielectric films [9]. Using TrMS enable the deposition of silica-based films without such equipment. However, silica-based films from TrMS have not been investigated regarding their gas barrier property. Szalowski et al. reported that the action of oxygen or some other oxidant accelerates to transform the process gas into the films which contain less organic fragments [10]. In this study, therefore, silica-based films were synthesized using TrMS with varying O2 flow rate on PET substrate. We investigated the effect of the O2 flow rate on the gas barrier property. 2. Experimental methods 2.1. Film preparation A schematic diagram of AP-PECVD equipment is illustrated in Fig. 1. The plasma was evenly produced and sustained between the upper and
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lower electrodes, whose sizes were 10 × 100 mm2 and 300 × 120 mm2, respectively. The gap length of gas flow way is 1 mm. The upper electrode was connected to the electric power supply and the other was connected to the earth. The upper electrode was covered with a 1 mm thick dielectric plate (Al2O3) whose size was 30 × 120 mm2 to prevent the transition from glow to arc discharge. As the substrates, silicon wafers with the thickness of 0.38 mm and PET with that of 50 μm were set on the lower electrode moving at speeds ranging from 0.1 mm/s to 10 mm/s in parallel to the upper one. The distance between the electrodes was 1 mm. The silica-based films were synthesized from a mixture gas of TrMS, N2, and O2. TrMS flow rate and a total gas flow rate were fixed at 0.5 mL/min and 20 L/min, respectively. O2 flow rate was varied with 0, 10, 100, 300, 500, 750, and 1000 mL/min. The voltage was fixed at 18 kV and the pulsed frequency was fixed at 20 kHz with a pulse width of 5 μs. 2.2. Film characterization The film thickness was measured by a contact-type surface profiler (Dektak 3030, Veeco Instruments Inc.). The measurements were performed at 10 different locations. The chemical bonding structure of the films was analyzed by a fourier transform infrared (FT-IR) spectroscopy (ALPHA-T, Bruker Corp.) in absorbance mode. The chemical concentration ratio of silica-based films was analyzed by a X-ray photoelectron spectroscopy (XPS: JPS-9000MC, JEOL Ltd.). For elemental analysis, Ar ion sputtering was applied before measurement to remove surface contaminants and oxide. The surface morphology of the films and PET surface was observed by an atomic force microscope (AFM: SPM-9700, Shimadzu Corp.). The scanning area was 10 × 10 μm2. OTR was determined by an oxygen transmission tester (Ox-Tran model 2/21, Mocon Inc.). Tests were carried out at 23 °C and 90%RH. 3. Results and discussion 3.1. Characteristics of silica-based films Fig. 2 shows the deposition rate of the silica-based films as a function of the O2 flow rate. As the O2 flow rate increased from 0 to 100 mL/min, the deposition rate decreased from 62.7 to 12.3 nm/s. Little variation was found when the O2 flow rate exceeded 100 mL/min. Fig. 3 shows the elemental composition of the silica-based films as a function of the O2 flow rate. As the O2 flow rate increased, the elemental compositions of Si and O increased, and those of C and N decreased. The elemental composition of the films hardly changed when the films were synthesized beyond 100 mL/min. Noborisaka et al. reported that the size of the particles was bigger and the surface roughness increased owing to an increase in the amount of N incorporation [11]. At less O2 flow rate, the films with high N content were composed of particles, thus the
Fig. 1. Schematic diagram of the AP-PECVD method.
Fig. 2. Deposition rate of the silica-based films as a function of the O2 flow rate.
deposition rate was much higher than that of the silica-based films synthesized with high O2 flow rate. Fig. 4 shows the OTR of the silica-based films as a function of the O2 flow rate. The thickness of each film was unified at 100 nm by changing the deposition time based on the deposition rate because the OTR depends on the film thickness. When the O2 flow rate was 0 and 10 mL/min, the OTR was 21.3 and 21.7 cm3/m2/24 h/atm. As the O2 flow rate increased to 500 mL/min, the OTR decreased to 8.2 cm3/m2/24 h/atm which was approximately one third of that of the uncoated PET. On the other hand, the OTR increased to 18.1 cm3/m2/ 24 h/atm with increasing the O2 flow rate from 500 to 1000 mL/min. The OTR value of about 8 cm3/m2/24 h/atm is not enough for practical use. It is said that the OTR of under 100 cm3/m2/24 h/atm is desirable for practical use for food packaging. It is difficult to compare the gas barrier property because the OTR value depends on the substrate polymers. Asakawa et al. reported the OTR of diamond like carbon (DLC) film synthesized on PET whose OTR was 0.07 cm3/m2/24 h/atm under low pressure was 0.035 cm3/m2/24 h/atm [12]. This value is good enough for practical use for food packaging. However, the cracks appear in the DLC film synthesized under low pressure, which causes to deteriorate the gas barrier property. Fig. 5 shows FT-IR spectra of the silica-based films at the range of 500 to 4000 cm−1. In this study, we could compare the amount of chemical bonds by the peak intensity with other samples because the thickness of the silica-based films was unified at 100 nm by changing the deposition time based on the deposition rate. Each spectrum has the strong peak attributed to Si-O-Si in the range from 1000 to 1200 cm−1. Absorption peaks corresponding to Si-(CH3)2 at 800 cm−1, Si-(CH3)x (x = 1, 2, 3) at 1270 cm−1, and C\\H bonds at 2970 cm−1 were observed. The peaks around at 1580 cm−1 were attributed to C\\N bonds, the peaks
Fig. 3. Elemental composition rate of the films as a function of the O2 flow rate.
Please cite this article as: M. Moritoki, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.06.074
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Fig. 4. OTR of the silica-based films as a function of the O2 flow rate.
around at 1650 cm−1 were attributed to C_O bonds, the band at approximately 930 and 3350 cm−1 were also attributed to Si-OH group [13,14]. From the FT-IR spectra, the peaks of Si-(CH3)x, C\\H bonds, C\\N bonds, and C_O bonds decreased with the increase of the O2 flow rate. The main difference was the shape of the Si-O-Si stretching mode. The highest peak around at 1070 cm−1 was assigned to a Si-O-Si network which has a three-dimensional siloxane structure [14,15]. At less O2 flow rate, the peak had a shoulder at a higher wavenumber, which could produce a micro-porous structure called cage structure and a lower film density [16]. As the O2 flow rate increased, the shoulder peak at a higher wavelength decreased. This indicated that the Si-O-Si chains connected each other with increasing the O2 flow rate, and that less cage structure was incorporated in the film at higher O2 flow rate, which may lead to increase the film density. Table 2 shows the bond strengths of the considered precursor [17]. This suggests that Si\\H and Si\\C bonds tend to be broken easily and that Si\\O bonds are difficult to be broken. Therefore, the precursors were efficiently decomposed and the Si\\O bonds which have relatively high bond energy were formed by reactive plasma. For these reasons, it is considered that the Si-O-Si network structure increased as the oxygen flow rate increased, which led to the improvement of the gas barrier property of the silica-based films. However, the intensity of a shoulder peak of Si-O-Si hardly changed when the O2 flow rate was over 500 mL/min. Park et al. reported that metastable oxygen atom decreased with increasing the O2 flow rate [18]. Electrons dissipated most of the absorbed energy through collision reactions with oxygen molecules rather than nitrogen atoms. Therefore, the peak intensity reached a constant value with a further increase of the O2 flow rate.
Fig. 6. AFM images of the silica-based films with the O2 flow rate of (a) 0 mL/min, (b) 500 mL/min, (c) 750 mL/min, and (d) 1000 mL/min.
Fig. 6 shows the AFM images of the silica-based films with various O2 flow rates. Granular appearances were observed at the film surface with less O2 flow rate. The size of the particles on the surface decreased as the O2 flow rate increased to 500 mL/min. When the O2 flow rate was 750 Table 1 OTR of the PET exposed to N2/O2 plasma with various O2 flow rates. O2 flow rate (mL/min)
OTR (cm2/m2/24 h/atm)
100 500 750 1000
21.0 21.9 21.8 22.0
Table 2 Strengths of the bonds in the precursor.
Fig. 5. FT-IR spectra of the silica-based films synthesized with various O2 flow rates.
Bond
Bond energy (eV)
Si\ \H Si\ \N C\ \H Si\ \C Si\ \O
3.1 3.4 4.3 4.6 8.3
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Fig. 7. AFM images of the PET exposed to N2/O2 plasma with the O2 flow rate of (a) 100, (b) 500 mL/min, (c) 750 mL/min, and (d) 1000 mL/min.
and 1000 mL/min, many pinholes with a diameter of approximately 30 nm were observed on the surface. Harvey et al. reported that the silicon oxide coating acted as simple defective blocks to oxygen transportation and that the dominant transport mechanism was permeation through the polymer substrate, followed by flux through available defects in the coatings [19]. It is considered that oxygen molecules permeated readily through the films with pinholes. Therefore, the OTR increased when the pinholes were observed at the film surfaces. 3.2. Surface morphology of PET substrates exposed to N2/O2 plasma To clarify the mechanism of the formation of the pinholes, the PET substrates were exposed to N2/O2 plasma at the following gas flow rates: the O2 flow rate = 100, 500, 750, and 1000 mL/min, with the total gas flow rate of 20 L/min. The other parameters were used in reference to the condition of the film deposition. Table 1 shows the OTR of the uncoated PET substrates after plasma treatment. The OTR hardly changed with each O2 flow rate. It considered that the O2 flow rate in N2/O2 plasma had no effect on the OTR of the PET substrates. Fig. 7 shows the surface morphology of the PET exposed to N2/O2 plasma. The surface became smooth with the O2 flow rate ranging from 100 to 500 mL/min. The rugged surfaces were obviously observed when the O2 flow rate was 750 and 1000 mL/min. These results indicated the etching mechanism of polymer: the reactive particles such as ions, atoms, and excited molecules in plasma collide with the surface of the PET, which progress chemical reactions at the surface. As the O2 flow rate in the N2/O2 plasma increased, the reactive particles from oxygen increased, thus the surface became rough. Leterrier [20] and Matthews et al. [21] reported that PET is characterized by crystalline and amorphous regions, and silicate particles. In the early stage of the deposition process, it is assumed that the crystalline regions were hardly etched due to their high bond energy. The activated plasma would preferentially etch the amorphous region, which led to the formation of the interfacial region. Therefore, the silica-based films were synthesized with defects and the pinholes were formed. 4. Conclusions In this paper, silica-based films were synthesized from a mixture gas of TrMS, N2, and O2 using AP-PECVD method and we investigated the
effect of the oxygen flow rate on the gas barrier property. As the O2 flow rate increased, the gas barrier property improved. The minimum OTR of 8.2 cm3/m2/24 h/atm was obtained at the O2 flow rate of 500 mL/min, which is approximately one third of the OTR of the uncoated PET. By increasing the O2 flow rate, the composition ratio of N and C decreased, and Si-O-Si chains were promoted to connect each other. The gas barrier property deteriorated when the O2 flow rate was 750 mL/min and above. The surface analysis of the uncoated PET with plasma treatment revealed that many pinholes were formed on the surface of the PET with high O2 flow rate in the N2/O2 plasma, which led to the low gas barrier property.
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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Gas barrier property of silica-based films on PET synthesized by atmospheric pressure plasma enhanced CVD Mai Moritoki ⁎, Takanori Mori, Akira Shirakura, Tetsuya Suzuki Center for Science of Environment, Resources and Energy, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan
a r t i c l e
i n f o
Article history: Received 31 October 2015 Revised 13 June 2016 Accepted in revised form 26 June 2016 Available online xxxx Keywords: Atmospheric pressure plasma enhanced chemical vapor deposition Gas barrier property Silica-based film Polyethylene terephthalate Trimethylsilane
a b s t r a c t In this study, silica-based films were synthesized on polyethylene terephthalate (PET) using the mixture gas of trimethylsilane (TrMS), N2, and O2 by the atmospheric pressure plasma enhanced CVD (AP-PECVD) method. We investigated the effect of the oxygen flow rate on the gas barrier property of silica-based films with various O2 flow rates ranging from 0 mL/min to 1000 mL/min. As the O2 flow rate increased to 500 mL/min, the oxygen transmission rate (OTR) of the films decreased to 8.2 cm3/m2/24 h/atm, which is approximately one third of that of the uncoated PET. On the other hand, the OTR increased when the O2 flow rate exceeded 500 mL/min. From the fourier-transform infrared (FT-IR) spectra, the intensity of Si-O-Si peaks increased and the shape of Si-O-Si peaks changed to be sharp with increasing the O2 flow rate, indicating that the structure of the films became dense. Oxygen molecules could not permeate through the highly dense films, so that the gas barrier property was improved. Using the atomic force microscope (AFM), many pinholes with a diameter of approximately 30 nm were observed on the film surface at the O2 flow rate over 500 mL/min. From the observation of the uncoated PET exposed to N2/O2 plasma, the surface texture hardly changed and kept its smooth surface at less O2 flow rate up to 500 mL/min, while the rugged texture were obviously observed at the O2 flow rate over 500 mL/min because of the etching effect in plasma. The holes were formed because the films were synthesized at the PET surface in the rugged shape. Therefore, the formation of the pinholes on the film surface could allow oxygen molecules to diffuse into the films. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Polymeric materials are currently used for engineering materials such as eyewear lenses and liquid crystal displays for cell-phones [1,2]. Polyethylene terephthalate (PET) is one of the most widely used materials for food packaging because of its lightweight, flexibility, and high transparency. PET has been studied to improve its gas barrier property aimed at protecting contents from degradation, particularly that resulting from oxidative processes. Coating with gas barrier films on PET is an effective approach to improving gas barrier property. Silica-based films for polymers have been studied because they have their excellent gas barrier property and heat resistance, maintaining their transparency [3]. In general, silica-based films are synthesized by plasma enhanced chemical vapor deposition (PECVD) method under low pressure. However, that synthesis process costs much because it needs chamber, vacuum system, and long processing time. One of the effective ways to overcoming the problems is an atmospheric pressure PECVD (APPECVD) system because vacuum system is not required in the synthesis under atmospheric pressure. Okazaki et al. reported on the mechanism
⁎ Corresponding author. E-mail address: [email protected] (M. Moritoki).
of stabilization of glow plasma under atmospheric pressure and succeeded in preventing a transition from glow to arc discharge [4–6]. In the synthesis of the silica-based films by the AP-PECVD method, liquid sources such as tetraethoxysilane (TEOS) and hexamethyldisiloxane (HMDSO) are mainly used with the equipment such as vaporizers [7,8]. Trimethylsilane ((CH3)3HSi, TrMS) is gaseous at room temperature under atmospheric pressure and can be used in PECVD process to produce dielectric films [9]. Using TrMS enable the deposition of silica-based films without such equipment. However, silica-based films from TrMS have not been investigated regarding their gas barrier property. Szalowski et al. reported that the action of oxygen or some other oxidant accelerates to transform the process gas into the films which contain less organic fragments [10]. In this study, therefore, silica-based films were synthesized using TrMS with varying O2 flow rate on PET substrate. We investigated the effect of the O2 flow rate on the gas barrier property. 2. Experimental methods 2.1. Film preparation A schematic diagram of AP-PECVD equipment is illustrated in Fig. 1. The plasma was evenly produced and sustained between the upper and
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lower electrodes, whose sizes were 10 × 100 mm2 and 300 × 120 mm2, respectively. The gap length of gas flow way is 1 mm. The upper electrode was connected to the electric power supply and the other was connected to the earth. The upper electrode was covered with a 1 mm thick dielectric plate (Al2O3) whose size was 30 × 120 mm2 to prevent the transition from glow to arc discharge. As the substrates, silicon wafers with the thickness of 0.38 mm and PET with that of 50 μm were set on the lower electrode moving at speeds ranging from 0.1 mm/s to 10 mm/s in parallel to the upper one. The distance between the electrodes was 1 mm. The silica-based films were synthesized from a mixture gas of TrMS, N2, and O2. TrMS flow rate and a total gas flow rate were fixed at 0.5 mL/min and 20 L/min, respectively. O2 flow rate was varied with 0, 10, 100, 300, 500, 750, and 1000 mL/min. The voltage was fixed at 18 kV and the pulsed frequency was fixed at 20 kHz with a pulse width of 5 μs. 2.2. Film characterization The film thickness was measured by a contact-type surface profiler (Dektak 3030, Veeco Instruments Inc.). The measurements were performed at 10 different locations. The chemical bonding structure of the films was analyzed by a fourier transform infrared (FT-IR) spectroscopy (ALPHA-T, Bruker Corp.) in absorbance mode. The chemical concentration ratio of silica-based films was analyzed by a X-ray photoelectron spectroscopy (XPS: JPS-9000MC, JEOL Ltd.). For elemental analysis, Ar ion sputtering was applied before measurement to remove surface contaminants and oxide. The surface morphology of the films and PET surface was observed by an atomic force microscope (AFM: SPM-9700, Shimadzu Corp.). The scanning area was 10 × 10 μm2. OTR was determined by an oxygen transmission tester (Ox-Tran model 2/21, Mocon Inc.). Tests were carried out at 23 °C and 90%RH. 3. Results and discussion 3.1. Characteristics of silica-based films Fig. 2 shows the deposition rate of the silica-based films as a function of the O2 flow rate. As the O2 flow rate increased from 0 to 100 mL/min, the deposition rate decreased from 62.7 to 12.3 nm/s. Little variation was found when the O2 flow rate exceeded 100 mL/min. Fig. 3 shows the elemental composition of the silica-based films as a function of the O2 flow rate. As the O2 flow rate increased, the elemental compositions of Si and O increased, and those of C and N decreased. The elemental composition of the films hardly changed when the films were synthesized beyond 100 mL/min. Noborisaka et al. reported that the size of the particles was bigger and the surface roughness increased owing to an increase in the amount of N incorporation [11]. At less O2 flow rate, the films with high N content were composed of particles, thus the
Fig. 1. Schematic diagram of the AP-PECVD method.
Fig. 2. Deposition rate of the silica-based films as a function of the O2 flow rate.
deposition rate was much higher than that of the silica-based films synthesized with high O2 flow rate. Fig. 4 shows the OTR of the silica-based films as a function of the O2 flow rate. The thickness of each film was unified at 100 nm by changing the deposition time based on the deposition rate because the OTR depends on the film thickness. When the O2 flow rate was 0 and 10 mL/min, the OTR was 21.3 and 21.7 cm3/m2/24 h/atm. As the O2 flow rate increased to 500 mL/min, the OTR decreased to 8.2 cm3/m2/24 h/atm which was approximately one third of that of the uncoated PET. On the other hand, the OTR increased to 18.1 cm3/m2/ 24 h/atm with increasing the O2 flow rate from 500 to 1000 mL/min. The OTR value of about 8 cm3/m2/24 h/atm is not enough for practical use. It is said that the OTR of under 100 cm3/m2/24 h/atm is desirable for practical use for food packaging. It is difficult to compare the gas barrier property because the OTR value depends on the substrate polymers. Asakawa et al. reported the OTR of diamond like carbon (DLC) film synthesized on PET whose OTR was 0.07 cm3/m2/24 h/atm under low pressure was 0.035 cm3/m2/24 h/atm [12]. This value is good enough for practical use for food packaging. However, the cracks appear in the DLC film synthesized under low pressure, which causes to deteriorate the gas barrier property. Fig. 5 shows FT-IR spectra of the silica-based films at the range of 500 to 4000 cm−1. In this study, we could compare the amount of chemical bonds by the peak intensity with other samples because the thickness of the silica-based films was unified at 100 nm by changing the deposition time based on the deposition rate. Each spectrum has the strong peak attributed to Si-O-Si in the range from 1000 to 1200 cm−1. Absorption peaks corresponding to Si-(CH3)2 at 800 cm−1, Si-(CH3)x (x = 1, 2, 3) at 1270 cm−1, and C\\H bonds at 2970 cm−1 were observed. The peaks around at 1580 cm−1 were attributed to C\\N bonds, the peaks
Fig. 3. Elemental composition rate of the films as a function of the O2 flow rate.
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Fig. 4. OTR of the silica-based films as a function of the O2 flow rate.
around at 1650 cm−1 were attributed to C_O bonds, the band at approximately 930 and 3350 cm−1 were also attributed to Si-OH group [13,14]. From the FT-IR spectra, the peaks of Si-(CH3)x, C\\H bonds, C\\N bonds, and C_O bonds decreased with the increase of the O2 flow rate. The main difference was the shape of the Si-O-Si stretching mode. The highest peak around at 1070 cm−1 was assigned to a Si-O-Si network which has a three-dimensional siloxane structure [14,15]. At less O2 flow rate, the peak had a shoulder at a higher wavenumber, which could produce a micro-porous structure called cage structure and a lower film density [16]. As the O2 flow rate increased, the shoulder peak at a higher wavelength decreased. This indicated that the Si-O-Si chains connected each other with increasing the O2 flow rate, and that less cage structure was incorporated in the film at higher O2 flow rate, which may lead to increase the film density. Table 2 shows the bond strengths of the considered precursor [17]. This suggests that Si\\H and Si\\C bonds tend to be broken easily and that Si\\O bonds are difficult to be broken. Therefore, the precursors were efficiently decomposed and the Si\\O bonds which have relatively high bond energy were formed by reactive plasma. For these reasons, it is considered that the Si-O-Si network structure increased as the oxygen flow rate increased, which led to the improvement of the gas barrier property of the silica-based films. However, the intensity of a shoulder peak of Si-O-Si hardly changed when the O2 flow rate was over 500 mL/min. Park et al. reported that metastable oxygen atom decreased with increasing the O2 flow rate [18]. Electrons dissipated most of the absorbed energy through collision reactions with oxygen molecules rather than nitrogen atoms. Therefore, the peak intensity reached a constant value with a further increase of the O2 flow rate.
Fig. 6. AFM images of the silica-based films with the O2 flow rate of (a) 0 mL/min, (b) 500 mL/min, (c) 750 mL/min, and (d) 1000 mL/min.
Fig. 6 shows the AFM images of the silica-based films with various O2 flow rates. Granular appearances were observed at the film surface with less O2 flow rate. The size of the particles on the surface decreased as the O2 flow rate increased to 500 mL/min. When the O2 flow rate was 750 Table 1 OTR of the PET exposed to N2/O2 plasma with various O2 flow rates. O2 flow rate (mL/min)
OTR (cm2/m2/24 h/atm)
100 500 750 1000
21.0 21.9 21.8 22.0
Table 2 Strengths of the bonds in the precursor.
Fig. 5. FT-IR spectra of the silica-based films synthesized with various O2 flow rates.
Bond
Bond energy (eV)
Si\ \H Si\ \N C\ \H Si\ \C Si\ \O
3.1 3.4 4.3 4.6 8.3
Please cite this article as: M. Moritoki, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.06.074
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M. Moritoki et al. / Surface & Coatings Technology xxx (2016) xxx–xxx
Fig. 7. AFM images of the PET exposed to N2/O2 plasma with the O2 flow rate of (a) 100, (b) 500 mL/min, (c) 750 mL/min, and (d) 1000 mL/min.
and 1000 mL/min, many pinholes with a diameter of approximately 30 nm were observed on the surface. Harvey et al. reported that the silicon oxide coating acted as simple defective blocks to oxygen transportation and that the dominant transport mechanism was permeation through the polymer substrate, followed by flux through available defects in the coatings [19]. It is considered that oxygen molecules permeated readily through the films with pinholes. Therefore, the OTR increased when the pinholes were observed at the film surfaces. 3.2. Surface morphology of PET substrates exposed to N2/O2 plasma To clarify the mechanism of the formation of the pinholes, the PET substrates were exposed to N2/O2 plasma at the following gas flow rates: the O2 flow rate = 100, 500, 750, and 1000 mL/min, with the total gas flow rate of 20 L/min. The other parameters were used in reference to the condition of the film deposition. Table 1 shows the OTR of the uncoated PET substrates after plasma treatment. The OTR hardly changed with each O2 flow rate. It considered that the O2 flow rate in N2/O2 plasma had no effect on the OTR of the PET substrates. Fig. 7 shows the surface morphology of the PET exposed to N2/O2 plasma. The surface became smooth with the O2 flow rate ranging from 100 to 500 mL/min. The rugged surfaces were obviously observed when the O2 flow rate was 750 and 1000 mL/min. These results indicated the etching mechanism of polymer: the reactive particles such as ions, atoms, and excited molecules in plasma collide with the surface of the PET, which progress chemical reactions at the surface. As the O2 flow rate in the N2/O2 plasma increased, the reactive particles from oxygen increased, thus the surface became rough. Leterrier [20] and Matthews et al. [21] reported that PET is characterized by crystalline and amorphous regions, and silicate particles. In the early stage of the deposition process, it is assumed that the crystalline regions were hardly etched due to their high bond energy. The activated plasma would preferentially etch the amorphous region, which led to the formation of the interfacial region. Therefore, the silica-based films were synthesized with defects and the pinholes were formed. 4. Conclusions In this paper, silica-based films were synthesized from a mixture gas of TrMS, N2, and O2 using AP-PECVD method and we investigated the
effect of the oxygen flow rate on the gas barrier property. As the O2 flow rate increased, the gas barrier property improved. The minimum OTR of 8.2 cm3/m2/24 h/atm was obtained at the O2 flow rate of 500 mL/min, which is approximately one third of the OTR of the uncoated PET. By increasing the O2 flow rate, the composition ratio of N and C decreased, and Si-O-Si chains were promoted to connect each other. The gas barrier property deteriorated when the O2 flow rate was 750 mL/min and above. The surface analysis of the uncoated PET with plasma treatment revealed that many pinholes were formed on the surface of the PET with high O2 flow rate in the N2/O2 plasma, which led to the low gas barrier property.
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Please cite this article as: M. Moritoki, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.06.074