- Email: [email protected]
Influence of environmental humidity on plasma etching polyamide 6 films
Applied Surface Science 258 (2012) 5574–5578
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Influence of environmental humidity on plasma etching polyamide 6 films Zhiqiang Gao ∗ College of Textile and Clothing Engineering, Dezhou University, Shandong, 253023, China
a r t i c l e
i n f o
Article history: Received 27 February 2011 Received in revised form 2 February 2012 Accepted 3 February 2012 Available online 24 February 2012 Keywords: Plasma Polyamide Environmental humidity (EH) XPS
a b s t r a c t The environmental humidity (EH) may have potential influence on atmospheric pressure plasma treatment. In order to investigate how the environmental humidity affects atmospheric pressure plasma treatment, polyamide 6 (PA 6) films were treated by helium/oxygen (He/O2 ) plasmas using atmospheric pressure plasma jet (APPJ) at different environmental humidity. The plasma treated samples had lower contact angles than the control. Atomic force microscopy (AFM) showed increased surface roughness, while X-ray photoelectron spectroscopy (XPS) revealed increased oxygen contents after the plasma treatments. The plasma treated films had higher T-peel strength than that of the control as revealed by T-peel strength tests. It was shown that the addition of environmental humidity increased effectiveness of the plasma in polymer surface modification after the treatment. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Plasma treatments can induce not only physical modifications such as etching, and chain scission [1,2] but also chemical effects such as cross-linking [3], grafting and polymerization [4]. It is an effective way to modify the properties of the polymer surface without affecting the bulk properties. In recent years, an atmospheric pressure plasma jet (APPJ) is invented to produce the homogeneous plasma at low temperature [5]. Water vapor in ambient air has various influences on operation and durability of plasma treatment through surface interactions such as surface force and friction [7–9] or surface chemical reactions with it [10]. However, little work has focus on the effects of environmental humidity (EH) on plasma treatment. This study is designed to investigate how environmental humidity affects the plasma treatment using an atmospheric pressure plasma jet. The wettabilities of polyamide 6 films were evaluated by measuring the water contact angles using the sessile drop method. The surface morphology and chemical changes were examined by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The weight loss was also studied after plasma treatment and the T-peel strength of the plasma treated and control films were carried out to show the change of surface adhesion properties of the films.
2.1. Material
∗ Tel.: +86 534 8985530. E-mail addresses: [email protected], [email protected] 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.02.004
The sample material studied here was commercial low density polyamide 6 film with a density of 1.134 g/cm3 . The samples were cut to 2 cm × 5 cm. The samples were soaked in acetone for 10 min and then dried in a desiccator for 24 h at room temperature to remove finishes.
2.2. Plasma treatment The samples were carried out on an APPJ apparatus (Atomflo250, Surfx Company, USA) with He/O2 . Helium (≥99.99% pure) was introduced into the plasma jet as the carrier gas at a flow rate 20 L/min and 0.2 L/min of O2 was added as the reactive gas. The temperature of plasma treatment was at room temperature. Plasma treatment time was 30 s. The APPJ employs a capacitively coupled electrode design and produces a stable discharge at atmospheric pressure with 13.56 MHz radio frequency power. More detailed information about the plasma machine was given in Ref. [4,11]. Small environment was a key condition in this experiment that a plastic bag was used to control it. All treatments were done orderly to promise consistent effects. Sequences of three conditions were arranged from dry to normal to wet (dry condition: EH 10%, normal condition: EH 65%, wet condition: EH 100%). After plasma treatment, the samples were immediately placed into clean plastic bags to minimize potential contamination.
Z. Gao / Applied Surface Science 258 (2012) 5574–5578
2.3. Contact angle measurement The contact angle was measured to determine the wettability of polyamide 6 films surface using sessile drop method. A 2 L drop of distilled water was put on the surface with a microliter syringe and observed through a microscope. The value of contact angle was an average of at least five readings at different places of the same sample surface.
Table 1 Water contact angles of polyamide 6 films before and after plasma treatment. Sample
Control EH 10% EH 65% EH 100%
Contact angle (◦ ) Mean
Standard deviation
76 48 29 35
5.1 2.7 1.8 1.9
EH = environmental humidity.
2.4. Weight loss After plasma treatment, the samples were immediately weighed to estimate the etching effects on the layers of polyamide 6 films. The plasma etching effect described by weight loss ratio was calculated as following equation [5]. weight loss (%) =
5575
(W0 − W1 ) × 100% W0
where W0 and W1 are the masses of polyamide 6 films before and after plasma treatment, respectively 2.5. Morphological study The surface morphology of polyamide 6 films before and after plasma treatment was scanned with an Atomic Force Microscopy (Multimode Nanoscope IIIa, Digital Instrument, USA). Changes in surface roughness of the plasma treated polyamide 6 films were expressed as difference in the root mean square (RMS). Each surface roughness value was calculated as the mean of at least 10 measurements in the different region of the film surfaces. 2.6. XPS analysis The change in the chemical composition on polyamide 6 film surfaces and chemical binding state were examined using XPS. The X-ray source was Mg K␣ (1253.6 eV), operating at 300 W. The analysis was carried out under ultra high vacuum conditions of 10−9 –10−10 torr. Photo emitted electrons were collected at a takeoff angle of 90◦ and curve fitting was performed in the C1s peaks.
Table 2 Weight loss of polyamide 6 films before and after plasma treatment. Sample
Weight loss (%)
EH 10% EH 65% EH 100%
0.41 0.30 0.11
EH = environmental humidity.
Table 3 Surface roughness of polyamide 6 films before and after plasma treatment. Sample
Control EH 10% EH 65% EH 100%
RMS (nm) Average
Standard deviation
12.06 18.63 17.21 14.82
0.85 1.68 1.53 1.24
EH = environmental humidity.
More water molecules in environment could react with the ions and active species of plasma, so that most of them could not reach with the surface and weaken the effect of plasma treatment. The advisable water molecules in the treating environment of APPJ are in favor of the physical and chemical reactions between active species and the surface of the materials. 3.2. Weight loss
To study the effect of plasma treatment on adhesion, a standard T-peel test was carried out using universal testing machine at a rate of 100 mm/min at room temperature. For this study a scotch tape of width 2.0 cm was stuck over a length of 4.0 cm on the sample of polyamide6 film. Be carefully that there were no air gaps or wrinkles and kept under a pressure of 1.0 kg for 10 min. T-peel test was carried out after fixing one end of the sample in one jaw and the scotch tape end with a piece of paper adhered to it in another jaw. T-peel strengths are reported as force of peel per centimeter of sample width. For every treatment, five samples were prepared for T-peel strength measurement and the mean value was taken.
Plasma etching is due to the physical removal of molecules or fragments or the breaking up of bonds, chain scission, and degradation processes. Table 2 shows that the weight loss after plasma treatment, which decreases with increasing the EH. It can be ascribed to the competition of water molecules with plasma molecules for adsorption on the active sites. The higher coverage of the active sites by the water molecules lead to a reduction in the number of active sites available for the reaction of plasma molecules, and thus resulted in the reduction of plasma molecules activity with increasing environmental humidity. With the EH increasing, it seemed that more water molecules absorbed into the film surfaces which means the thin water layers plays an important role on quenching the etching effect of the films in plasma treatment. So the weight loss decreased with EH increasing.
3. Results and discussion
3.3. Morphological study
3.1. Contact angle measurement
The surface modification as well as the change in morphology of polyamide 6 films was measured by AFM analysis. Fig. 1 shows the surface of the control which is relatively smooth and the surfaces of polyamide 6 film show rough morphology after plasma treatment. The root mean square (RMS) roughness of the control and plasma treated polyamide 6 film was reported in Table 3. The RMS roughness of the control is 12.06 nm, while after plasma treatment of EH 10%, 65% and 100% the values are 18.63 nm, 17.21 nm and 14.82 nm, respectively. It is seen that the values of RMS gradually increase after plasma treatment. The roughness increased due to
2.7. T-peel strength
The mean water contact angle of the control was 76◦ and it decreased significantly after plasma treatment. Table 1 shows that the contact angles for plasma treated films decreased with increasing the EH from 10% to 100%. The contact angle of EH 65% is the lowest, which shows that the treatment effect was the best in this EH. It seemed that the thin water layer formed on the films when they were in the sealed system. It meant that the present of water has a significant negative effect on the plasma treatment.
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Z. Gao / Applied Surface Science 258 (2012) 5574–5578
Fig. 1. AFM images of polyamide 6 film surfaces (a – control, b – EH 10%, c – EH 65%, d – EH 100%).
the plasma species impacted on the surface and removed the top few layers of polyamide 6 films [6]. And the surface treated with EH 10% was largely modified compared to the control surface by etching effect. It means that more water molecules in environment could react with the ions and active species of plasma, so that most of them could not reach with the surface and weaken the effect of plasma treatment. This is the result from the etching effect of the plasma treatments. The samples treated with EH 10% had the largest roughness, which is in accordance with the results of the weight loss.
3.4. Surface chemical composition analysis XPS was applied to analyze the surface chemical composition of polyamide 6 films before and after plasma treatment. The photoelectron peaks at 532 eV, 400 eV and 285 eV correspond to the
O1s, N1s and C1s, respectively. Compared with the control, the O1s intensities after plasma treatment were raised at the expense of the C1s intensities. The concentration of these elements is given in Table 4. The results of the element composition of the control samples are in accordance with those described in literature [12,13]. For the plasma treated samples, the oxygen content increased from Table 4 XPS elemental analysis of control and plasma treated polyamide 6 films. Sample
Control EH 10% EH 65% EH 100%
Chemical composition (%)
Atomic ratio (%)
C1s
O1s
N1s
O/C
N/C
77.39 65.48 63.46 53.24
14.53 25.30 30.23 37.32
8.08 9.22 6.31 9.44
18.78 38.64 47.64 70.09
10.44 14.08 9.9 17.73
EH = environmental humidity.
Table 5 Results of deconvolution analysis of C1s peaks of the control and plasma treated polyamide 6 films. Sample
Relative area corresponding to different chemical bonds (%) C C (284.6 eV)
Control EH 10% EH 65% EH 100%
58.41 49.62 45.36 31.86
EH = environmental humidity.
C N (285.4 eV) 31.82 23.51 26.62 27.79
C O (286.5 eV) – 13.11 18.20 10.58
CONH (287.9 eV) 9.77 13.76 9.82 17.41
COO (288.5 eV) – – – 12.36
Z. Gao / Applied Surface Science 258 (2012) 5574–5578
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Fig. 2. Deconvoluted XPS C1s peaks for polyamide 6 film surfaces (a – control, b – EH 10%, c – EH 65%, d – EH 100%).
14.53% to 25.30%, 30.23% and 37.32% for EH 10%, 65% and 100% groups, respectively. This phenomenon was probably due to a result of the presence of atomic oxygen during the process, resulting from the plasma gas including O2 and the dissociation of atmospheric O2 [14]. In order to investigate the change of functional groups, deconvolution analysis of the C1s peaks was performed as shown in Fig. 2 and Table 5. Deconvolution analysis of C1s peaks showed an increase of oxygen functional groups in surface such as C O bonds (286.5 eV) and carboxyl groups ( COO ) (288.5 eV) after plasma treatment [15,16]. From the results shown in Table 5, for the treated samples, the amount of carbon atoms bonded to oxygen C O and ( COO ) in the films for EH 65% and 100% groups were larger than that for EH 10% group. This means that probably more containing oxygen groups are present in these two groups. The higher concentration of containing oxygen groups for EH 65% and 100% groups may be attributed to more water molecules interaction with the plasma active particles. More containing oxygen species were produced on polyamide 6 surfaces when EH increased. The intensity of the O1s peaks increases with increasing the EH. This suggests that the amount of active species such as O, O+ , H3 O+ and O* increases with increasing EH, and the active species can easily form reactive oxygen-containing functional groups on the surface.
Fig. 3. Dependence of T-peel strength on different environmental humidity.
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Z. Gao / Applied Surface Science 258 (2012) 5574–5578
3.5. T-peel strength Plasma treated polyamide 6 films were subjected to T-peel strength measurement as a function of different EH to understand the effect of functional groups on bonding strength. The values of peel strength are plotted against different EH in Fig. 3. It was seen that the T-peel strength increased after plasma treatment. It could be due to the etching and surface modification effects of the plasma treatments, which may roughen the surfaces and increase the wettability of the films. These are corresponding to the XPS and AFM results, the plasma treatment created more hydrophilic polar groups and more roughness surface. 4. Conclusions The atmospheric pressure plasma treated samples had lower contact angles than the control. Atomic force microscopy results showed increased surface roughness, while X-ray photoelectron spectroscopy revealed increased oxygen contents after the plasma treatments. The plasma treated films had higher T-peel strength than that of the control as revealed by T-peel strength tests. It was shown that higher EH could form a thin water layer on the polymer surface. Then, the thin water layer on the polymer surface could not be favorable to the plasma modification effect. Acknowledgments This work was supported by the National High Technology Research and Development Program of China (No. 2007AA03Z101),
Natural Science Foundation for the Youth (No. 50803010), the Talent Introduction of Dezhou University (No. 11RC04). References [1] K.N. Pandiyaraj, V. Selvarajan, R.R. Deshmukh, M. Bousmina, Surf. Coat. Technol. 202 (2008) 4218. [2] V. Svorcik, K. Kolarova, P. Slepicka, A. Mackova, M. Novotna, V. Hnatowicz, Polym. Degrad. Stab. 91 (2006) 1219. [3] I. Banik, K.S. Kim, Y.I. Yun, D.H. Kim, C.M. Ryu, C.E. Park, J. Adhes. Sci. Technol. 16 (2002) 1155. [4] Y. Shin, K. Son, D.I. Yoo, J. Appl. Polym. Sci. 103 (2007) 3655. [5] J.Y. Jeong, S.E. Babayan, V.J. Tu, Plasma Sources Sci. Technol. 7 (1998) 282. [6] J. Park, I. Henins, H.W. Herrmann, Appl. Phys. Lett. 76 (2000) 288. [7] P.R. Scheeper, J.A. Voorthuyzen, W. Olthuis, P. Bergveld, Sens. Actuators 30 (1992) 231. [8] C.H. Mastrangelo, C. Hsu, J. Microelectromech. Syst. 2 (1993) 44. [9] B. Bhushan, Tribology Issues and Opportunities in MEMS, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1998, p. 443. [10] T. Tsuchiya, A. Inoue, J. Sakata, Digest of technical papers of the 10th International Conference on Solid-State Sensors and Actuators, vol. 1, 1999, p. 488. [11] W.J.V. Ooij, S. Luo, S. Datta, Plasma Polym. 4 (1999) 33. [12] D. Lee, J.M. Park, S.H. Hong, Y. Kim, IEEE Trans. Plasma Sci. 33 (2005) 949. [13] L. Zhu, C. Wang, Y. Qiu, Surf. Coat. Technol. 201 (2007) 7453. [14] T. Kondo, H. Ito, K. Kusakabe, K. Ohkawa, K. Honda, Y. Einaga, A. Fujishima, T. Kawai, Diamond Relat. Mater. 17 (2008) 48. [15] Z. Gao, S. Peng, J. Sun, L. Yao, Y. Qiu, Appl. Surf. Sci. 255 (2009) 7683. [16] C. Canal, F. Gaboriau, S. Villeger, U. Cvelbar, A. Ricard, Int. J. Pharm. 367 (2009) 155.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Influence of environmental humidity on plasma etching polyamide 6 films Zhiqiang Gao ∗ College of Textile and Clothing Engineering, Dezhou University, Shandong, 253023, China
a r t i c l e
i n f o
Article history: Received 27 February 2011 Received in revised form 2 February 2012 Accepted 3 February 2012 Available online 24 February 2012 Keywords: Plasma Polyamide Environmental humidity (EH) XPS
a b s t r a c t The environmental humidity (EH) may have potential influence on atmospheric pressure plasma treatment. In order to investigate how the environmental humidity affects atmospheric pressure plasma treatment, polyamide 6 (PA 6) films were treated by helium/oxygen (He/O2 ) plasmas using atmospheric pressure plasma jet (APPJ) at different environmental humidity. The plasma treated samples had lower contact angles than the control. Atomic force microscopy (AFM) showed increased surface roughness, while X-ray photoelectron spectroscopy (XPS) revealed increased oxygen contents after the plasma treatments. The plasma treated films had higher T-peel strength than that of the control as revealed by T-peel strength tests. It was shown that the addition of environmental humidity increased effectiveness of the plasma in polymer surface modification after the treatment. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Plasma treatments can induce not only physical modifications such as etching, and chain scission [1,2] but also chemical effects such as cross-linking [3], grafting and polymerization [4]. It is an effective way to modify the properties of the polymer surface without affecting the bulk properties. In recent years, an atmospheric pressure plasma jet (APPJ) is invented to produce the homogeneous plasma at low temperature [5]. Water vapor in ambient air has various influences on operation and durability of plasma treatment through surface interactions such as surface force and friction [7–9] or surface chemical reactions with it [10]. However, little work has focus on the effects of environmental humidity (EH) on plasma treatment. This study is designed to investigate how environmental humidity affects the plasma treatment using an atmospheric pressure plasma jet. The wettabilities of polyamide 6 films were evaluated by measuring the water contact angles using the sessile drop method. The surface morphology and chemical changes were examined by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The weight loss was also studied after plasma treatment and the T-peel strength of the plasma treated and control films were carried out to show the change of surface adhesion properties of the films.
2.1. Material
∗ Tel.: +86 534 8985530. E-mail addresses: [email protected], [email protected] 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.02.004
The sample material studied here was commercial low density polyamide 6 film with a density of 1.134 g/cm3 . The samples were cut to 2 cm × 5 cm. The samples were soaked in acetone for 10 min and then dried in a desiccator for 24 h at room temperature to remove finishes.
2.2. Plasma treatment The samples were carried out on an APPJ apparatus (Atomflo250, Surfx Company, USA) with He/O2 . Helium (≥99.99% pure) was introduced into the plasma jet as the carrier gas at a flow rate 20 L/min and 0.2 L/min of O2 was added as the reactive gas. The temperature of plasma treatment was at room temperature. Plasma treatment time was 30 s. The APPJ employs a capacitively coupled electrode design and produces a stable discharge at atmospheric pressure with 13.56 MHz radio frequency power. More detailed information about the plasma machine was given in Ref. [4,11]. Small environment was a key condition in this experiment that a plastic bag was used to control it. All treatments were done orderly to promise consistent effects. Sequences of three conditions were arranged from dry to normal to wet (dry condition: EH 10%, normal condition: EH 65%, wet condition: EH 100%). After plasma treatment, the samples were immediately placed into clean plastic bags to minimize potential contamination.
Z. Gao / Applied Surface Science 258 (2012) 5574–5578
2.3. Contact angle measurement The contact angle was measured to determine the wettability of polyamide 6 films surface using sessile drop method. A 2 L drop of distilled water was put on the surface with a microliter syringe and observed through a microscope. The value of contact angle was an average of at least five readings at different places of the same sample surface.
Table 1 Water contact angles of polyamide 6 films before and after plasma treatment. Sample
Control EH 10% EH 65% EH 100%
Contact angle (◦ ) Mean
Standard deviation
76 48 29 35
5.1 2.7 1.8 1.9
EH = environmental humidity.
2.4. Weight loss After plasma treatment, the samples were immediately weighed to estimate the etching effects on the layers of polyamide 6 films. The plasma etching effect described by weight loss ratio was calculated as following equation [5]. weight loss (%) =
5575
(W0 − W1 ) × 100% W0
where W0 and W1 are the masses of polyamide 6 films before and after plasma treatment, respectively 2.5. Morphological study The surface morphology of polyamide 6 films before and after plasma treatment was scanned with an Atomic Force Microscopy (Multimode Nanoscope IIIa, Digital Instrument, USA). Changes in surface roughness of the plasma treated polyamide 6 films were expressed as difference in the root mean square (RMS). Each surface roughness value was calculated as the mean of at least 10 measurements in the different region of the film surfaces. 2.6. XPS analysis The change in the chemical composition on polyamide 6 film surfaces and chemical binding state were examined using XPS. The X-ray source was Mg K␣ (1253.6 eV), operating at 300 W. The analysis was carried out under ultra high vacuum conditions of 10−9 –10−10 torr. Photo emitted electrons were collected at a takeoff angle of 90◦ and curve fitting was performed in the C1s peaks.
Table 2 Weight loss of polyamide 6 films before and after plasma treatment. Sample
Weight loss (%)
EH 10% EH 65% EH 100%
0.41 0.30 0.11
EH = environmental humidity.
Table 3 Surface roughness of polyamide 6 films before and after plasma treatment. Sample
Control EH 10% EH 65% EH 100%
RMS (nm) Average
Standard deviation
12.06 18.63 17.21 14.82
0.85 1.68 1.53 1.24
EH = environmental humidity.
More water molecules in environment could react with the ions and active species of plasma, so that most of them could not reach with the surface and weaken the effect of plasma treatment. The advisable water molecules in the treating environment of APPJ are in favor of the physical and chemical reactions between active species and the surface of the materials. 3.2. Weight loss
To study the effect of plasma treatment on adhesion, a standard T-peel test was carried out using universal testing machine at a rate of 100 mm/min at room temperature. For this study a scotch tape of width 2.0 cm was stuck over a length of 4.0 cm on the sample of polyamide6 film. Be carefully that there were no air gaps or wrinkles and kept under a pressure of 1.0 kg for 10 min. T-peel test was carried out after fixing one end of the sample in one jaw and the scotch tape end with a piece of paper adhered to it in another jaw. T-peel strengths are reported as force of peel per centimeter of sample width. For every treatment, five samples were prepared for T-peel strength measurement and the mean value was taken.
Plasma etching is due to the physical removal of molecules or fragments or the breaking up of bonds, chain scission, and degradation processes. Table 2 shows that the weight loss after plasma treatment, which decreases with increasing the EH. It can be ascribed to the competition of water molecules with plasma molecules for adsorption on the active sites. The higher coverage of the active sites by the water molecules lead to a reduction in the number of active sites available for the reaction of plasma molecules, and thus resulted in the reduction of plasma molecules activity with increasing environmental humidity. With the EH increasing, it seemed that more water molecules absorbed into the film surfaces which means the thin water layers plays an important role on quenching the etching effect of the films in plasma treatment. So the weight loss decreased with EH increasing.
3. Results and discussion
3.3. Morphological study
3.1. Contact angle measurement
The surface modification as well as the change in morphology of polyamide 6 films was measured by AFM analysis. Fig. 1 shows the surface of the control which is relatively smooth and the surfaces of polyamide 6 film show rough morphology after plasma treatment. The root mean square (RMS) roughness of the control and plasma treated polyamide 6 film was reported in Table 3. The RMS roughness of the control is 12.06 nm, while after plasma treatment of EH 10%, 65% and 100% the values are 18.63 nm, 17.21 nm and 14.82 nm, respectively. It is seen that the values of RMS gradually increase after plasma treatment. The roughness increased due to
2.7. T-peel strength
The mean water contact angle of the control was 76◦ and it decreased significantly after plasma treatment. Table 1 shows that the contact angles for plasma treated films decreased with increasing the EH from 10% to 100%. The contact angle of EH 65% is the lowest, which shows that the treatment effect was the best in this EH. It seemed that the thin water layer formed on the films when they were in the sealed system. It meant that the present of water has a significant negative effect on the plasma treatment.
5576
Z. Gao / Applied Surface Science 258 (2012) 5574–5578
Fig. 1. AFM images of polyamide 6 film surfaces (a – control, b – EH 10%, c – EH 65%, d – EH 100%).
the plasma species impacted on the surface and removed the top few layers of polyamide 6 films [6]. And the surface treated with EH 10% was largely modified compared to the control surface by etching effect. It means that more water molecules in environment could react with the ions and active species of plasma, so that most of them could not reach with the surface and weaken the effect of plasma treatment. This is the result from the etching effect of the plasma treatments. The samples treated with EH 10% had the largest roughness, which is in accordance with the results of the weight loss.
3.4. Surface chemical composition analysis XPS was applied to analyze the surface chemical composition of polyamide 6 films before and after plasma treatment. The photoelectron peaks at 532 eV, 400 eV and 285 eV correspond to the
O1s, N1s and C1s, respectively. Compared with the control, the O1s intensities after plasma treatment were raised at the expense of the C1s intensities. The concentration of these elements is given in Table 4. The results of the element composition of the control samples are in accordance with those described in literature [12,13]. For the plasma treated samples, the oxygen content increased from Table 4 XPS elemental analysis of control and plasma treated polyamide 6 films. Sample
Control EH 10% EH 65% EH 100%
Chemical composition (%)
Atomic ratio (%)
C1s
O1s
N1s
O/C
N/C
77.39 65.48 63.46 53.24
14.53 25.30 30.23 37.32
8.08 9.22 6.31 9.44
18.78 38.64 47.64 70.09
10.44 14.08 9.9 17.73
EH = environmental humidity.
Table 5 Results of deconvolution analysis of C1s peaks of the control and plasma treated polyamide 6 films. Sample
Relative area corresponding to different chemical bonds (%) C C (284.6 eV)
Control EH 10% EH 65% EH 100%
58.41 49.62 45.36 31.86
EH = environmental humidity.
C N (285.4 eV) 31.82 23.51 26.62 27.79
C O (286.5 eV) – 13.11 18.20 10.58
CONH (287.9 eV) 9.77 13.76 9.82 17.41
COO (288.5 eV) – – – 12.36
Z. Gao / Applied Surface Science 258 (2012) 5574–5578
5577
Fig. 2. Deconvoluted XPS C1s peaks for polyamide 6 film surfaces (a – control, b – EH 10%, c – EH 65%, d – EH 100%).
14.53% to 25.30%, 30.23% and 37.32% for EH 10%, 65% and 100% groups, respectively. This phenomenon was probably due to a result of the presence of atomic oxygen during the process, resulting from the plasma gas including O2 and the dissociation of atmospheric O2 [14]. In order to investigate the change of functional groups, deconvolution analysis of the C1s peaks was performed as shown in Fig. 2 and Table 5. Deconvolution analysis of C1s peaks showed an increase of oxygen functional groups in surface such as C O bonds (286.5 eV) and carboxyl groups ( COO ) (288.5 eV) after plasma treatment [15,16]. From the results shown in Table 5, for the treated samples, the amount of carbon atoms bonded to oxygen C O and ( COO ) in the films for EH 65% and 100% groups were larger than that for EH 10% group. This means that probably more containing oxygen groups are present in these two groups. The higher concentration of containing oxygen groups for EH 65% and 100% groups may be attributed to more water molecules interaction with the plasma active particles. More containing oxygen species were produced on polyamide 6 surfaces when EH increased. The intensity of the O1s peaks increases with increasing the EH. This suggests that the amount of active species such as O, O+ , H3 O+ and O* increases with increasing EH, and the active species can easily form reactive oxygen-containing functional groups on the surface.
Fig. 3. Dependence of T-peel strength on different environmental humidity.
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Z. Gao / Applied Surface Science 258 (2012) 5574–5578
3.5. T-peel strength Plasma treated polyamide 6 films were subjected to T-peel strength measurement as a function of different EH to understand the effect of functional groups on bonding strength. The values of peel strength are plotted against different EH in Fig. 3. It was seen that the T-peel strength increased after plasma treatment. It could be due to the etching and surface modification effects of the plasma treatments, which may roughen the surfaces and increase the wettability of the films. These are corresponding to the XPS and AFM results, the plasma treatment created more hydrophilic polar groups and more roughness surface. 4. Conclusions The atmospheric pressure plasma treated samples had lower contact angles than the control. Atomic force microscopy results showed increased surface roughness, while X-ray photoelectron spectroscopy revealed increased oxygen contents after the plasma treatments. The plasma treated films had higher T-peel strength than that of the control as revealed by T-peel strength tests. It was shown that higher EH could form a thin water layer on the polymer surface. Then, the thin water layer on the polymer surface could not be favorable to the plasma modification effect. Acknowledgments This work was supported by the National High Technology Research and Development Program of China (No. 2007AA03Z101),
Natural Science Foundation for the Youth (No. 50803010), the Talent Introduction of Dezhou University (No. 11RC04). References [1] K.N. Pandiyaraj, V. Selvarajan, R.R. Deshmukh, M. Bousmina, Surf. Coat. Technol. 202 (2008) 4218. [2] V. Svorcik, K. Kolarova, P. Slepicka, A. Mackova, M. Novotna, V. Hnatowicz, Polym. Degrad. Stab. 91 (2006) 1219. [3] I. Banik, K.S. Kim, Y.I. Yun, D.H. Kim, C.M. Ryu, C.E. Park, J. Adhes. Sci. Technol. 16 (2002) 1155. [4] Y. Shin, K. Son, D.I. Yoo, J. Appl. Polym. Sci. 103 (2007) 3655. [5] J.Y. Jeong, S.E. Babayan, V.J. Tu, Plasma Sources Sci. Technol. 7 (1998) 282. [6] J. Park, I. Henins, H.W. Herrmann, Appl. Phys. Lett. 76 (2000) 288. [7] P.R. Scheeper, J.A. Voorthuyzen, W. Olthuis, P. Bergveld, Sens. Actuators 30 (1992) 231. [8] C.H. Mastrangelo, C. Hsu, J. Microelectromech. Syst. 2 (1993) 44. [9] B. Bhushan, Tribology Issues and Opportunities in MEMS, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1998, p. 443. [10] T. Tsuchiya, A. Inoue, J. Sakata, Digest of technical papers of the 10th International Conference on Solid-State Sensors and Actuators, vol. 1, 1999, p. 488. [11] W.J.V. Ooij, S. Luo, S. Datta, Plasma Polym. 4 (1999) 33. [12] D. Lee, J.M. Park, S.H. Hong, Y. Kim, IEEE Trans. Plasma Sci. 33 (2005) 949. [13] L. Zhu, C. Wang, Y. Qiu, Surf. Coat. Technol. 201 (2007) 7453. [14] T. Kondo, H. Ito, K. Kusakabe, K. Ohkawa, K. Honda, Y. Einaga, A. Fujishima, T. Kawai, Diamond Relat. Mater. 17 (2008) 48. [15] Z. Gao, S. Peng, J. Sun, L. Yao, Y. Qiu, Appl. Surf. Sci. 255 (2009) 7683. [16] C. Canal, F. Gaboriau, S. Villeger, U. Cvelbar, A. Ricard, Int. J. Pharm. 367 (2009) 155.