- Email: [email protected]
Surface properties of HMDSO plasma treated polyethylene terephthalate
Surface & Coatings Technology 200 (2005) 953 – 957 www.elsevier.com/locate/surfcoat
Surface properties of HMDSO plasma treated polyethylene terephthalate S. Zaninia,T, C. Riccardia, M. Orlandib, P. Esenaa, M. Tontinia, M. Milanic, V. Cassiod a
Dipartimento di Fisica Occhialini, Universita` di Milano-Bicocca e INFM, p.za della Scienza, 3 I-20126 Milano, Italy Dipartimento di Scienza dell’Ambiente e del Territorio, Universita` di Milano-Bicocca, p.za della Scienza, 1 I-20126 Milano, Italy c Dipartimento di Scienza dei Materiali, Universita` degli Studi di Milano-Bicocca, Via R. Cozzi 53, 20125 Milano, Italy d NEOGRAF-R&D, Via Padre Pietro Calandri 4 I-12033 Moretta (CN), Italy
b
Available online 10 March 2005
Abstract HMDSO plasma polymer films are known to reduce the resulting surface free energy and the permeation rate of water vapour and oxygen through the treated polymer substrates. In this work, plasma polymer films were deposited from hexamethyldisiloxane (HMDSO) on polyethylene terephthalate surfaces under different operating conditions and the resulting surface properties were investigated. The structure and bondings in the deposited films were studied by means of IR spectroscopy. The thickness of the deposited films was deduced observing images acquired with a focused ion beam (FIB). Hydrophobic performances were investigated by means of water droplet contact angle measurements. Correlation between operating deposition conditions and resulting structure and surface properties of the films were also discussed. D 2005 Elsevier B.V. All rights reserved. Keywords: HMDSO; Plasma polymerisation; Polyethylene terephthalate
1. Introduction Polymers represent materials of the future, due to their low density, flexibility, softness, inertness and low cost of production. Employment as electrical wire insulation, several kinds of packaging, water tubes, window profiles and lenses for glasses is very frequent. The possibility of selective modification of the surface, while keeping bulk characteristics unchanged, has greatly increased the applicability of polymers. For instance, it is well known that the permeation of gases and vapours through polymers represents an important problem in their use as packaging materials [1,2]. Several methods have been proposed and utilised in order to reduce the permeation rate of gases and vapours through polymers. One of the most important methods is the deposition of silicon oxide coatings, due to the high barrier and hydrophobic properties of these films
T Corresponding author. Tel.: +390264482327; fax: +390264482367. E-mail address: [email protected] (S. Zanini). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.01.093
[3–7]. PECVD of silicon oxides is one of the techniques allowing industrial-scale deposition of high-quality barrier coatings. Many organosilicon compounds such as hexamethyldisiloxane (HMDSO) or tetraethoxysilane (TEOS) mixed with oxidants (O2) or noble gases (Ar, He) are used as SiO2 precursors. In particular, hexamethyldisiloxane is one of the most common monomer described in the literature concerning PECVD deposition of silicon oxide thin films. However, the composition and structure of deposited films, hence their performance, change with deposition conditions. Reactor type and geometry, pressure, gas composition and flow, power input and temperature together influence the plasma polymerisation process. Soft coatings of SiOx with high content of methylene and methyl groups are obtained by using pure HMDSO in plasma process yields. Increasing the power input per monomer flow (stronger fragmentation of the HMDSO molecules) or adding oxygen to the plasma process (formation of volatile CO2 and H2O molecules) results in a reduced carbon content [6,8]. However, a systematic control of all the operating conditions is necessary to obtain the growth of films with the desired composition
954
S. Zanini et al. / Surface & Coatings Technology 200 (2005) 953–957
and structure and for the reproducibility of the process. In this work, we have conducted an experimental study of pure HMDSO discharges in order to understand how operating parameters influence the composition of the deposited films and how the composition is related with their hydrophobic properties. We treated polyethylene terephthalate films in order to analyse the wetting behaviour and the gas barrier characteristics of the deposited films on the substrate. The composition of the coatings, deposited under different conditions, has been investigated by Fourier-transform infrared absorption spectroscopy (FT-IRAS) and the results have been analysed and compared with contact angle measurements. The thickness of the deposited films was deduced observing images acquired with a focused ion beam (FIB). The effect of deposition conditions on film composition and properties has been investigated and discussed.
2. Experimental part 2.1. Materials Hexamethyldisiloxane (HMDSO) was obtained from Fluka (z99.5%) and used as received. Samples of PET were provided by Neograf s.r.l. and consist of transparent films with grammature of 16.8 g/m2 and thickness of 12 Am.
On the contrary, since discharges are operated in non-flux conditions, pressure inside the chamber slowly changes during the treatment time due to film deposition and monomer dissociation. Samples were mounted onto a copper ring (diameter 17 cm) and held parallel and at a fixed distance of 3 cm from the antenna. Plasma depositions were effectuated at a fixed starting pressure of 1 mbar, varying the power input from 20 W to 100 W and the treatment time from 1 min to 10 min. 2.3. Characterisation methods Treated and untreated sample’s surface hydrophobicity has been characterised by the water contact angle, measuring the advancing contact angle of a water droplet deposited on the surface and observed through a telescope equipped with a goniometer (Rame`-Hart 100 Goniometer System). Films structures were determined by means of a Fourier transform infrared (FT-IR) spectrometer (Nicolet Avatar 360) suitable for the collection of spectra in the range between 400 and 4000 cm 1. For each spectrum 32 scans, with a spectral resolution of 4 cm 1, were recorded. The thickness of the deposited films was deduced analysing images acquired by means of a dFocused Ion BeamT apparatus (FEI Strata DB 235 M dual beam), with an imaging resolution of 5 nm.
2.2. Plasma deposition apparatus 3. Results and discussion In order to understand how operating conditions can influence the composition and the hydrophobic performance of the deposited films, plasma treatment of PET samples under different conditions has been effectuated. All samples have been treated with pure HMDSO plasma,
Transmittance
HMDSO discharges have been generated in a low pressure, capacitively-coupled radiofrequency (RF) plasma reactor. The plasma is produced inside a cylindrical stainless steel vacuum chamber (diameter 20 cm) with several ports for gas inlet and diagnostics. Before operating the discharge, the device is evacuated up to a residual pressure of 10 5 mbar by means of a turbo-molecular pump (Turbovac 150 V) in turn evacuated by a rotary pump (Varian SD300). Afterwards, HMDSO is injected through a micrometer valve. After reaching the desired initial pressure, the micrometer valve is closed. It has been verified that, without discharge, pressure remains constant for at least 1 h, since wall outgassing is limited after the evacuation process. A Pirani Gauge (Varian SenTorr) records pressure, given in N2 equivalent, the absolute calibration for HMDSO being not precisely known. Radiofrequency power is delivered to the discharge by an RF antenna consisting in a stainless steel circular plate (18 cm-diameter). The antenna is externally connected, through a semi-automated matching network (Huttinger PFM 1500), to a 13.56 MHz-RF power supplier (Huttinger PFR-300) which provides an RF voltage with respect to the grounded chamber. After the electrical breakdown, when a stable plasma state is formed, the discharge is fed at a constant power level for a determined treatment time.
4000
3000
2000
Wavenumber
1000
cm-1
Fig. 1. Typical FT-IR spectrum of the coating. Plasma parameters: power input=60 W, treatment time=10V, pressure=1 mbar. See text for bands assignment.
S. Zanini et al. / Surface & Coatings Technology 200 (2005) 953–957
955
10 9 8
SiO/SiCH3
7 6 5 4 3 2 1 0 0
2
4
6
8
10
12
Treatment time (min) Fig. 2. Evolution of the absorbance ratio of the band assigned to Si–O–Si (1000–1150 cm 1) with Si–CH3 (1260 cm 1) as a function of the exposure time. Plasma parameters: power input=60 W, pressure=1 mbar.
at a starting pressure of 1 mbar, in non-flux conditions. Treatment time (from 1 min to 10 min) and power input (from 20 W to 100 W) has been chosen as varying parameters. For each sample, hydrophobic characteristics have been investigated by means of contact angle analysis. Untreated PET shows a contact angle of 608. KBr tablets were exposed to plasma together with PET samples and FT-IRAS spectra of the deposited films were recorded. A typical spectrum of the deposit is shown in Fig. 1. According to literature [9–12], the stronger absorption band in the range 1000–1150 cm 1 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 cm 1, the CH3 symmetric bending in Si– CH3 at 1260 cm 1, the CHx symmetric and asymmetric stretching at 2900–2960 cm 1. The absorption band at 800 cm 1 can be assigned, following the literature, to Si–O–Si bending mode [9,10] and to Si–C stretching and CH3
rocking in Si–(CH3)2 [12]. We have focused our attention on the absorption bands at 1000–1150 cm 1, 1260 cm 1 and 2900–2960 cm 1. Fig. 2 shows absorbance ratio of the band assigned to Si–O–Si (1000–1150 cm 1) with Si–CH3 (1260 cm 1), for several times of exposure. It can be noticed that, increasing the time of exposure in non-flux conditions, there is an increase of Si–O–Si bonds relatively to Si–CH3 bonds. This indicates a decrease in the carbon content for high values of the treatment time, resulting in a more SiO2-like film [12]. In particular, increasing the treatment time there is a decrease of the CH3 groups directly bonded to the Si atoms. As shown in Fig. 3, by increasing the time of exposure, there is a decrease of the Si–CH3 bonds relatively to the total CHx bonds (2900– 2960 cm 1). This indicates a stronger fragmentation of the monomer (breaking of Si–CH3 bonds) and a successively recombination that leads to the formation of C–C bonds. In fact, in non-flux conditions, the power per mole HMDSO
2 1,8 1,6
SiCH3/CHx
1,4 1,2 1 0,8 0,6 0,4 0,2 0 0
2
4
6
8
10
12
Treatment time (min) Fig. 3. Evolution of the absorbance ratio of the band assigned to Si–CH3 (1260 cm 1) with CHx (2900–2960 cm 1) as a function of the exposure time. Plasma parameters: power input=60 W, pressure=1 mbar.
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S. Zanini et al. / Surface & Coatings Technology 200 (2005) 953–957
120
Contact angle (o)
100
80
60
40 0
2
4
6
8
10
12
Treatment time (min) Fig. 4. Water droplet contact angles measured on PET plasma treated samples as a function of the exposure time. Plasma parameters: power input=60 W, pressure=1 mbar. Untreated PET shows a contact angle of 608.
increases during treatment time owing to the film deposition. Consequently, the plasma composition and hence the chemical composition of the deposited film changes during the treatment time. For long treatment time, we can then suppose the film having a gradient structure, with carbon content and Si–CH3 bonds diminishing from the first deposited layer to the film surface. Diminishing of Si–CH3 bonds is in agreement with the contact angle measurements. Fig. 4 shows the water droplet contact angles measured on PET samples treated with different exposure time. A continuous decrease of the hydrophobic characteristics of the treated sample by increasing the treatment time can be noticed. Since terminal CH3 groups are responsible for the low wetting properties [6], there is a good correlation between the result of FT-IR analysis and the water droplet contact angle measurements. Fig. 5 shows absorbance ratio of the band assigned to Si–CH3 (1260 cm 1) with the band assigned to
the CHx groups (2900–2960 cm 1), for several power inputs. There is an increase of the CHx bonds relatively to Si–CH3 bonds by increasing the power input. In agreement with literature [6,8], this indicates a stronger fragmentation of the monomer in the plasma phase. The decrease in Si– CH3 bonds relatively to total CHx bonds results in lower hydrophobic properties for the samples treated at higher power input. In fact, contact angle values measured on samples treated at 20 W, 60 W and 100 W are 1048, 928 and 898, respectively. Thickness of the films, which can influence the barrier properties of the coatings, was deduced observing images acquired by means of a dFocused Ion BeamT apparatus (FIB). For samples treated with an exposure time of 10 min and with a power input of 60 W, the coating thickness appears to be 120F5 nm. More analyses are in progress in order to evaluate the barrier properties of the coatings. Correlation between
0,6
SiCH3/CHx
0,5 0,4 0,3 0,2 0,1 0 0
20
40
60
80
100
120
Power input (W) Fig. 5. Evolution of the absorbance ratio of the band assigned to Si–CH3 (1260 cm 1) with CHx (2900–2960 cm 1) as a function of the power input. Plasma parameters: treatment time=3V, pressure=1 mbar.
S. Zanini et al. / Surface & Coatings Technology 200 (2005) 953–957
barrier properties and structure and thickness of the coatings will be discussed in a future work.
4. Conclusions Plasma polymer films were deposited from hexamethyldisiloxane (HMDSO) on polyethylene terephthalate surfaces under different operating conditions and the resulting structure and properties of the coatings were investigated by means of FT-IR spectroscopy, contact angle analysis and FIB analysis. In non-flux conditions, deposition parameters such as exposure time and power input together influence the structure and composition of the films, hence their hydrophobic performance. We have shown that higher treatment times result in a stronger fragmentation of the monomer in the plasma phase, with a decrease of the Si–CH3 bonds relatively to the Si–O–Si bonds and to the total CHx bonds. The same results are shown for treatment at higher power input. For treatment at high exposure time (10 min) and high power input (100 W), we obtained coatings with a Si–O–Si matrix and a CHx polymer resulting from the fragmentation and successively recombination of the monomer and not necessarily directly bonded to the Si atoms. Instead, for shorter treatment time and lower power input, we obtained coatings with a higher degree of linearization of the precursor (Si–O–Si chains and high retention of Si–CH3 bonds). Structure and composition of the films are correlated with their hydrophobic performance. In fact, samples treated with lower exposure time and power input (hence with high concentration of Si–CH3 bonds) are more hydrophobic. Moreover, structure and composition of the films influence their barrier properties. Generally, dense SiOx coatings (deposited, for example, from HMDSO/O2
957
plasmas) exhibit high barrier properties. However, pure HMDSO-derived films might be of importance for their selective permeation. From this point of view, the capability of controlling the film composition by varying the operating conditions opens interesting perspectives. The influence of deposition conditions on barrier properties of the films will be the object of further investigations.
Acknowledgments The authors are pleased to acknowledge the help of their laboratory technician M. Piselli during the experimental activity of this research.
References [1] F.A. Philips, Spec. Rep.-N. Y. State Agric. Exp. Stn. 58 (1985) 33. [2] Matthew D. Steven, Joseph H. Hotchkiss, Packag. Technol. Sci. 15 (2002) 17. [3] G. Czeremuszkin, M. Latreche, M.R. Wertheimer, A.S. da Silva Sobrinho, Plasmas Polym. 6 (2001) 107. [4] M.S. Hedenqvist, K.S. Johansson, Surf. Coat. Technol. 172 (2003) 7. [5] M. Creatore, F. Palumbo, R. D’Agostino, Pure Appl. Chem. 74 (2002) 407. [6] D. Hegemann, H. Brunner, C. Oehr, Plasmas Polym. 6 (2001) 221. [7] J. Behnisch, J. Tyczkowski, M. Gazicki, I. Pela, A. Hollander, R. Ledzion, Surf. Coat. Technol. 98 (1998) 872. [8] D. Hegemann, U. Vohrer, C. Oehr, R. Riedel, Surf. Coat. Technol. 116 (1999) 1033. [9] M. Walker, K.-M. Baumgartner, J. Feichtinger, M. Kaiser, A. Schulz, E. Rauchle, Vacuum 57 (2000) 387. [10] Min Tae Kim, Thin Solid Films 311 (1997) 157. [11] N. Shirtcliffe, P. Thiemann, M. Stratmann, G. Grunmeier, Surf. Coat. Technol. 142–144 (2001) 1121. [12] C. Vautrin-Ul, C. Boisse-Laporte, N. Benissad, A. Chausse, P. Leprince, R. Messina, Prog. Org. Coat. 38 (2000) 9.
Surface properties of HMDSO plasma treated polyethylene terephthalate S. Zaninia,T, C. Riccardia, M. Orlandib, P. Esenaa, M. Tontinia, M. Milanic, V. Cassiod a
Dipartimento di Fisica Occhialini, Universita` di Milano-Bicocca e INFM, p.za della Scienza, 3 I-20126 Milano, Italy Dipartimento di Scienza dell’Ambiente e del Territorio, Universita` di Milano-Bicocca, p.za della Scienza, 1 I-20126 Milano, Italy c Dipartimento di Scienza dei Materiali, Universita` degli Studi di Milano-Bicocca, Via R. Cozzi 53, 20125 Milano, Italy d NEOGRAF-R&D, Via Padre Pietro Calandri 4 I-12033 Moretta (CN), Italy
b
Available online 10 March 2005
Abstract HMDSO plasma polymer films are known to reduce the resulting surface free energy and the permeation rate of water vapour and oxygen through the treated polymer substrates. In this work, plasma polymer films were deposited from hexamethyldisiloxane (HMDSO) on polyethylene terephthalate surfaces under different operating conditions and the resulting surface properties were investigated. The structure and bondings in the deposited films were studied by means of IR spectroscopy. The thickness of the deposited films was deduced observing images acquired with a focused ion beam (FIB). Hydrophobic performances were investigated by means of water droplet contact angle measurements. Correlation between operating deposition conditions and resulting structure and surface properties of the films were also discussed. D 2005 Elsevier B.V. All rights reserved. Keywords: HMDSO; Plasma polymerisation; Polyethylene terephthalate
1. Introduction Polymers represent materials of the future, due to their low density, flexibility, softness, inertness and low cost of production. Employment as electrical wire insulation, several kinds of packaging, water tubes, window profiles and lenses for glasses is very frequent. The possibility of selective modification of the surface, while keeping bulk characteristics unchanged, has greatly increased the applicability of polymers. For instance, it is well known that the permeation of gases and vapours through polymers represents an important problem in their use as packaging materials [1,2]. Several methods have been proposed and utilised in order to reduce the permeation rate of gases and vapours through polymers. One of the most important methods is the deposition of silicon oxide coatings, due to the high barrier and hydrophobic properties of these films
T Corresponding author. Tel.: +390264482327; fax: +390264482367. E-mail address: [email protected] (S. Zanini). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.01.093
[3–7]. PECVD of silicon oxides is one of the techniques allowing industrial-scale deposition of high-quality barrier coatings. Many organosilicon compounds such as hexamethyldisiloxane (HMDSO) or tetraethoxysilane (TEOS) mixed with oxidants (O2) or noble gases (Ar, He) are used as SiO2 precursors. In particular, hexamethyldisiloxane is one of the most common monomer described in the literature concerning PECVD deposition of silicon oxide thin films. However, the composition and structure of deposited films, hence their performance, change with deposition conditions. Reactor type and geometry, pressure, gas composition and flow, power input and temperature together influence the plasma polymerisation process. Soft coatings of SiOx with high content of methylene and methyl groups are obtained by using pure HMDSO in plasma process yields. Increasing the power input per monomer flow (stronger fragmentation of the HMDSO molecules) or adding oxygen to the plasma process (formation of volatile CO2 and H2O molecules) results in a reduced carbon content [6,8]. However, a systematic control of all the operating conditions is necessary to obtain the growth of films with the desired composition
954
S. Zanini et al. / Surface & Coatings Technology 200 (2005) 953–957
and structure and for the reproducibility of the process. In this work, we have conducted an experimental study of pure HMDSO discharges in order to understand how operating parameters influence the composition of the deposited films and how the composition is related with their hydrophobic properties. We treated polyethylene terephthalate films in order to analyse the wetting behaviour and the gas barrier characteristics of the deposited films on the substrate. The composition of the coatings, deposited under different conditions, has been investigated by Fourier-transform infrared absorption spectroscopy (FT-IRAS) and the results have been analysed and compared with contact angle measurements. The thickness of the deposited films was deduced observing images acquired with a focused ion beam (FIB). The effect of deposition conditions on film composition and properties has been investigated and discussed.
2. Experimental part 2.1. Materials Hexamethyldisiloxane (HMDSO) was obtained from Fluka (z99.5%) and used as received. Samples of PET were provided by Neograf s.r.l. and consist of transparent films with grammature of 16.8 g/m2 and thickness of 12 Am.
On the contrary, since discharges are operated in non-flux conditions, pressure inside the chamber slowly changes during the treatment time due to film deposition and monomer dissociation. Samples were mounted onto a copper ring (diameter 17 cm) and held parallel and at a fixed distance of 3 cm from the antenna. Plasma depositions were effectuated at a fixed starting pressure of 1 mbar, varying the power input from 20 W to 100 W and the treatment time from 1 min to 10 min. 2.3. Characterisation methods Treated and untreated sample’s surface hydrophobicity has been characterised by the water contact angle, measuring the advancing contact angle of a water droplet deposited on the surface and observed through a telescope equipped with a goniometer (Rame`-Hart 100 Goniometer System). Films structures were determined by means of a Fourier transform infrared (FT-IR) spectrometer (Nicolet Avatar 360) suitable for the collection of spectra in the range between 400 and 4000 cm 1. For each spectrum 32 scans, with a spectral resolution of 4 cm 1, were recorded. The thickness of the deposited films was deduced analysing images acquired by means of a dFocused Ion BeamT apparatus (FEI Strata DB 235 M dual beam), with an imaging resolution of 5 nm.
2.2. Plasma deposition apparatus 3. Results and discussion In order to understand how operating conditions can influence the composition and the hydrophobic performance of the deposited films, plasma treatment of PET samples under different conditions has been effectuated. All samples have been treated with pure HMDSO plasma,
Transmittance
HMDSO discharges have been generated in a low pressure, capacitively-coupled radiofrequency (RF) plasma reactor. The plasma is produced inside a cylindrical stainless steel vacuum chamber (diameter 20 cm) with several ports for gas inlet and diagnostics. Before operating the discharge, the device is evacuated up to a residual pressure of 10 5 mbar by means of a turbo-molecular pump (Turbovac 150 V) in turn evacuated by a rotary pump (Varian SD300). Afterwards, HMDSO is injected through a micrometer valve. After reaching the desired initial pressure, the micrometer valve is closed. It has been verified that, without discharge, pressure remains constant for at least 1 h, since wall outgassing is limited after the evacuation process. A Pirani Gauge (Varian SenTorr) records pressure, given in N2 equivalent, the absolute calibration for HMDSO being not precisely known. Radiofrequency power is delivered to the discharge by an RF antenna consisting in a stainless steel circular plate (18 cm-diameter). The antenna is externally connected, through a semi-automated matching network (Huttinger PFM 1500), to a 13.56 MHz-RF power supplier (Huttinger PFR-300) which provides an RF voltage with respect to the grounded chamber. After the electrical breakdown, when a stable plasma state is formed, the discharge is fed at a constant power level for a determined treatment time.
4000
3000
2000
Wavenumber
1000
cm-1
Fig. 1. Typical FT-IR spectrum of the coating. Plasma parameters: power input=60 W, treatment time=10V, pressure=1 mbar. See text for bands assignment.
S. Zanini et al. / Surface & Coatings Technology 200 (2005) 953–957
955
10 9 8
SiO/SiCH3
7 6 5 4 3 2 1 0 0
2
4
6
8
10
12
Treatment time (min) Fig. 2. Evolution of the absorbance ratio of the band assigned to Si–O–Si (1000–1150 cm 1) with Si–CH3 (1260 cm 1) as a function of the exposure time. Plasma parameters: power input=60 W, pressure=1 mbar.
at a starting pressure of 1 mbar, in non-flux conditions. Treatment time (from 1 min to 10 min) and power input (from 20 W to 100 W) has been chosen as varying parameters. For each sample, hydrophobic characteristics have been investigated by means of contact angle analysis. Untreated PET shows a contact angle of 608. KBr tablets were exposed to plasma together with PET samples and FT-IRAS spectra of the deposited films were recorded. A typical spectrum of the deposit is shown in Fig. 1. According to literature [9–12], the stronger absorption band in the range 1000–1150 cm 1 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 cm 1, the CH3 symmetric bending in Si– CH3 at 1260 cm 1, the CHx symmetric and asymmetric stretching at 2900–2960 cm 1. The absorption band at 800 cm 1 can be assigned, following the literature, to Si–O–Si bending mode [9,10] and to Si–C stretching and CH3
rocking in Si–(CH3)2 [12]. We have focused our attention on the absorption bands at 1000–1150 cm 1, 1260 cm 1 and 2900–2960 cm 1. Fig. 2 shows absorbance ratio of the band assigned to Si–O–Si (1000–1150 cm 1) with Si–CH3 (1260 cm 1), for several times of exposure. It can be noticed that, increasing the time of exposure in non-flux conditions, there is an increase of Si–O–Si bonds relatively to Si–CH3 bonds. This indicates a decrease in the carbon content for high values of the treatment time, resulting in a more SiO2-like film [12]. In particular, increasing the treatment time there is a decrease of the CH3 groups directly bonded to the Si atoms. As shown in Fig. 3, by increasing the time of exposure, there is a decrease of the Si–CH3 bonds relatively to the total CHx bonds (2900– 2960 cm 1). This indicates a stronger fragmentation of the monomer (breaking of Si–CH3 bonds) and a successively recombination that leads to the formation of C–C bonds. In fact, in non-flux conditions, the power per mole HMDSO
2 1,8 1,6
SiCH3/CHx
1,4 1,2 1 0,8 0,6 0,4 0,2 0 0
2
4
6
8
10
12
Treatment time (min) Fig. 3. Evolution of the absorbance ratio of the band assigned to Si–CH3 (1260 cm 1) with CHx (2900–2960 cm 1) as a function of the exposure time. Plasma parameters: power input=60 W, pressure=1 mbar.
956
S. Zanini et al. / Surface & Coatings Technology 200 (2005) 953–957
120
Contact angle (o)
100
80
60
40 0
2
4
6
8
10
12
Treatment time (min) Fig. 4. Water droplet contact angles measured on PET plasma treated samples as a function of the exposure time. Plasma parameters: power input=60 W, pressure=1 mbar. Untreated PET shows a contact angle of 608.
increases during treatment time owing to the film deposition. Consequently, the plasma composition and hence the chemical composition of the deposited film changes during the treatment time. For long treatment time, we can then suppose the film having a gradient structure, with carbon content and Si–CH3 bonds diminishing from the first deposited layer to the film surface. Diminishing of Si–CH3 bonds is in agreement with the contact angle measurements. Fig. 4 shows the water droplet contact angles measured on PET samples treated with different exposure time. A continuous decrease of the hydrophobic characteristics of the treated sample by increasing the treatment time can be noticed. Since terminal CH3 groups are responsible for the low wetting properties [6], there is a good correlation between the result of FT-IR analysis and the water droplet contact angle measurements. Fig. 5 shows absorbance ratio of the band assigned to Si–CH3 (1260 cm 1) with the band assigned to
the CHx groups (2900–2960 cm 1), for several power inputs. There is an increase of the CHx bonds relatively to Si–CH3 bonds by increasing the power input. In agreement with literature [6,8], this indicates a stronger fragmentation of the monomer in the plasma phase. The decrease in Si– CH3 bonds relatively to total CHx bonds results in lower hydrophobic properties for the samples treated at higher power input. In fact, contact angle values measured on samples treated at 20 W, 60 W and 100 W are 1048, 928 and 898, respectively. Thickness of the films, which can influence the barrier properties of the coatings, was deduced observing images acquired by means of a dFocused Ion BeamT apparatus (FIB). For samples treated with an exposure time of 10 min and with a power input of 60 W, the coating thickness appears to be 120F5 nm. More analyses are in progress in order to evaluate the barrier properties of the coatings. Correlation between
0,6
SiCH3/CHx
0,5 0,4 0,3 0,2 0,1 0 0
20
40
60
80
100
120
Power input (W) Fig. 5. Evolution of the absorbance ratio of the band assigned to Si–CH3 (1260 cm 1) with CHx (2900–2960 cm 1) as a function of the power input. Plasma parameters: treatment time=3V, pressure=1 mbar.
S. Zanini et al. / Surface & Coatings Technology 200 (2005) 953–957
barrier properties and structure and thickness of the coatings will be discussed in a future work.
4. Conclusions Plasma polymer films were deposited from hexamethyldisiloxane (HMDSO) on polyethylene terephthalate surfaces under different operating conditions and the resulting structure and properties of the coatings were investigated by means of FT-IR spectroscopy, contact angle analysis and FIB analysis. In non-flux conditions, deposition parameters such as exposure time and power input together influence the structure and composition of the films, hence their hydrophobic performance. We have shown that higher treatment times result in a stronger fragmentation of the monomer in the plasma phase, with a decrease of the Si–CH3 bonds relatively to the Si–O–Si bonds and to the total CHx bonds. The same results are shown for treatment at higher power input. For treatment at high exposure time (10 min) and high power input (100 W), we obtained coatings with a Si–O–Si matrix and a CHx polymer resulting from the fragmentation and successively recombination of the monomer and not necessarily directly bonded to the Si atoms. Instead, for shorter treatment time and lower power input, we obtained coatings with a higher degree of linearization of the precursor (Si–O–Si chains and high retention of Si–CH3 bonds). Structure and composition of the films are correlated with their hydrophobic performance. In fact, samples treated with lower exposure time and power input (hence with high concentration of Si–CH3 bonds) are more hydrophobic. Moreover, structure and composition of the films influence their barrier properties. Generally, dense SiOx coatings (deposited, for example, from HMDSO/O2
957
plasmas) exhibit high barrier properties. However, pure HMDSO-derived films might be of importance for their selective permeation. From this point of view, the capability of controlling the film composition by varying the operating conditions opens interesting perspectives. The influence of deposition conditions on barrier properties of the films will be the object of further investigations.
Acknowledgments The authors are pleased to acknowledge the help of their laboratory technician M. Piselli during the experimental activity of this research.
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