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Surface modification of PE film by DBD plasma in air
Applied Surface Science 255 (2008) 3421–3425
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface modification of PE film by DBD plasma in air C.-S. Ren *, K. Wang, Q.-Y. Nie, D.-Z. Wang, S.-H. Guo State Key Laboratory of Material Modification by Electron, Ion and Laser Beams, Dalian University of Technology, Dalian 116023, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 April 2008 Received in revised form 22 September 2008 Accepted 24 September 2008 Available online 7 October 2008
In this paper, surface modification of polyethylene (PE) films is studied by dielectric barrier discharge plasma treatment in air. The treated samples were examined by water contact angle measurement, calculation of surface free energy, Fourier transform infrared attenuated total reflection spectroscopy (FTIR-ATR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The water contact angle changes from the original value of 93.28 to the minimum value of 53.38 and surface free energy increases from 27.3 to 51.89 J/m2 after treatment time of 50 s. Both ATR and XPS show some oxidized species are introduced into the sample surface by the plasma treatment and that the change tendencies of the water contact angle and surface free energy with the treatment time are the same as that of the oxygen concentration on the treated sample surface. Cu films were deposited on the treated and untreated PE surfaces. The peel adhesive strength between the Cu film and the treated sample is 1.5 MPa, whereas the value is only 0.8 MPa between the Cu film and the untreated PE. SEM pictures show that the Cu film deposited on the plasma treated PE surface is smooth and the crystal grain is smaller, contrarily the Cu film on the untreated PE surface is rough and the crystal grain is larger. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Dielectric barrier discharge Polyethylene Surface modification Adhesive strength
1. Introduction Polymers, which play an increasing role as structural materials, are applied more and more widely in industry in recent years. Some well-known properties of polymers, such as low surface energy, thermal stability, low friction coefficient and hydrophobic characteristic, are advantages in some applications, whereas they are disadvantages for other applications and have to be overcome. Recently, modification of the surface properties of polymers becomes a subject, which many researchers are interested in. The usual two means, referred to as physical and chemical methods, are adopted to modify the surface properties of polymers. However, the chemical method is water-wasting and easily causes environment pollution. In contrast, the physical method is paid more and more attention in recent years due to its cleanness and high efficiency. Polymer surface modification by discharge plasma is of great and increasing industrial application value [1] because it can uniformly modify the surface of the treated samples without changing the matrix properties. Lots of experimental and theoretical studies about the subject had been performed in recent years. For example, Liu et al. [2] had reported that, the examination of pre- and post-plasma treated polymer surfaces by
* Corresponding author. Tel.: +86 411 84709795; fax: +86 411 84707161. E-mail address: [email protected] (C.-S. Ren). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.09.064
atom force microscopy revealed that only the outermost layer was affected by dielectric barrier discharge (DBD) plasma treatment. Guruvenket et al. [3] had modified polystyrene and polyethylene (PE) using microwave electron cyclotron resonance plasma to improve the surface wettability. Bhowmik et al. [5] had studied the wettability and physicochemical characteristics when PP film was exposed to a DC glow discharge on the condition of airflow by different electrode structure. Coen et al. [4] had modified polypropylene (PP), polymethylmethacrylate, polytetrafluoroethylene (PTFE) and polyethyletetraphalate under high vacuum conditions. The results suggested that plasma treatment could cause topographical modification on the treated polymer surface. DBD, which is characterized by the configuration of at least one of the electrodes is covered with an insulating layer, is proved to be a promising technology in the modification of surface properties of polymers in recent years. The dielectric serves two key functions in the discharge operation: (i) it limits the amount of charge transported from one electrode surface to another via a single microdischarge and (ii) it distributes these microdicharges over the entire electrode area. Usually, voltages of a few kV and frequencies ranging from 5 to 500 kHz are used [2]. The mean electron energy in DBD plasma is in the range of 0–10 eV [6], while the chemical binding energy of polymers is less than 10 eV [7]. Therefore, energetic particles in DBD can break the chemical bonds of polymers. In addition, to modify the surface properties of polymers without expensive vacuum system is another advantage of DBD. Nowadays, DBD has been used widely
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to modify the surface of polyimide, PP, PTFE, poly (methylpentene), and so on [8,9]. In this paper, PE films are modified using air DBD, the surface energy and adhesive strength with metal films of which are improved. And our investigation will contribute to the application of air plasma and enrich the progress made in technology recently, which has shown that air plasmas can dramatically improve the adhesion and wettability of polymer surfaces [10,11]. 2. Experiment Fig. 1 presents the schematic show of the experimental setup. The discharge room has the dimension of 20 cm 20 cm 20 cm. The two parallel electrodes are stainless steel. The top electrode is covered by thin quartz foil with the thickness of 1 mm and diameter of 4.5 cm, and the grounded electrode, which can be adjusted up and down to change the discharge gap, is covered by PE sample. The discharge is generated by a high voltage power supply with frequency range of 1–20 kHz and effective voltage range of 0–20 kV. The sample derived from a commercial PE film (100 mm thick) was cut into the dimension of 5.5 cm 5.5 cm square, and then was directly put into the discharge system. The experiment was operated with the condition of discharge voltage of 16 kV, frequency of 4 kHz and the electrode gap of 3 mm. The discharge treatment was performed in flowing air. Cu film was deposited with an unbalanced magnetron sputtering system which is enhanced by radio frequency inductively coupled plasma. The substrate was applied a pulse bias voltage with the amplitude of 100 V and 50% duty cycle. The deposition time was 1 h and the film’s thickness was about 600 nm. Scanning electron microscopy (SEM) (S-366, Cambridge, England), Fourier transform infrared attenuated total reflection spectroscopy (FTIR-ATR) (560ESP, Nicolet Instrument Co., USA) and X-ray photoelectron spectroscopy (XPS) (Amicus, Shimadzu Co., Japan) were used to analyze the changes of the surface morphology and surface chemical compositions of the PE films. Water contact angle values were obtained with 5-point average of the measurements obtained from the same sample with the dimension of 5.5 cm 5.5 cm. Surface free energy was calculated by the measurement values of water contact angle. The peel adhesive strength of Cu film with the PE film was measured with a tensile testing machine (LYS-50000) with digital display at a pull velocity of 2 mm/min at room temperature. A commercially available two-component polyurethane resin had been used as adhesive. Each measured value was the average of the measurements of three bonded samples.
Fig. 1. Schematic show of the experimental setup.
Fig. 2. The variation curve of water contact angle and O atomic concentration versus DBD treatment time.
3. Results and discussion 3.1. Water contact angle and surface free energy Fig. 2 shows the changes of the water contact angle and oxygen atomic concentration of PE sample versus air-DBD treatment time. The water contact angle reduces with the treatment time extending. The value of the untreated sample is 93.28, and it dramatically decreases to less than 708 after treatment time of 1 s. It continues to decrease with the increase of treatment time but the change velocity becomes slow, which suggests that the changes on the surface of the treated PE samples mainly happen within the first second treatment time. When the treatment time increases beyond 20 s or more, the contact angle has hardly changed and this shows that the chemical etching, which tends to deplete the oxidized species introduced onto the sample surface by DBD treatment, becomes obvious. Lastly, a dynamic equilibrium is formed between surface modification and surface chemical etching. Cui and Brown [12] had observed that effective plasmainduced chemical etching appears to equilibrate after 25% of the surface carbon being oxidized. Water contact angle measurements on the treated PE surfaces show the hydrophilic characteristic. The introduction of polar molecules on the treated PE surface will cause the decrease of water contact angle. Besides, the change of surface morphology can also lead to the decrease of the water contact angle of the treated PE films. Fig. 3 shows the change of the surface free energy of PE with the plasma treatment time. It can be found that the surface free energy
Fig. 3. The variation curve of surface free energy versus DBD treatment time.
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Fig. 4. FTIR-ATR spectra for (a) untreated, (b) 1 s treated, (c) 10 s treated, (d) 20 s treated, (e) 30 s treated and (f) 40 s treated PE film.
increases from 27.3 to 51.89 J/m2 with the treatment time changing from 0 to 50 s and the change tendency is similar to that of oxygen atomic concentration. This indicates that the change of surface free energy is mainly determined by the introduction of polar molecules on the treated PE surface. Surface free energy is higher, the water contact angle is lower and the adhesive strength between PE and metal films is higher. 3.2. FTIR-ATR analysis Fig. 4 shows the FTIR-ATR spectra for pre- and post-treated PE films, (a) for the untreated PE film, (b)–(f) for 1, 10, 20, 30 and 40 s DBD-treated PE film, respectively. Comparing (a) with (b), there obviously appear two new peaks at 1733 and 1646 cm 1 for the 1 s treated sample. While the 20 s treated PE sample shows two new peaks at 1242 and 3419 cm 1 further. The peak at 1733 cm 1 corresponds to C O stretching vibration and 1646 cm 1 to COO– asymmetrical stretching vibration. The peak at 1242 cm 1 should corresponds to COO– symmetrical stretching vibration and 3419 cm 1 to –OH group vibration. Lehocky et al. [13] had also found similar groups on the surface of the PE treated with oxygen plasma. An oxygen plasma can react with the polymer surface to produce a variety of atomic oxygen functional groups, such as C–O, C O, O–C O and CO3 at the surface [3]. Besides, plasma treatment can also lead to the production of some surface radical sites [14,15]. These radical sites will take oxidation reaction with oxygen molecules very easily in air and produce peroxides [16]. Therefore, the oxidized species are introduced onto the surface of PE when it is treated by air DBD, which can further lead to the decrease of water contact angle on the treated sample surface and the increase of surface free energy. Generally speaking, when polymers are exposed to plasma, two processes simultaneously happen, one is etching of polymer surface through the reaction of atomic oxygen with the surface carbon atom, giving volatile reaction products, the other is the formation of oxygen functional groups on the polymer surface through the interaction between the active species from the plasma and the surface atom [17]. There not appears new peak for the 30 and 40 s treated samples compared with the 20 s treated sample and the amplitude also hardly changes, which suggests the etching and oxidation come to equilibrium when the treatment time goes beyond 30 s. This phenomenon accords with the change tendency of water contact angle and surface free energy with the exposed time.
Fig. 5. Carbon (1 s) XPS spectra for (a) an untreated PE film and (b) a 20 s treated PE film.
3.3. XPS analysis Fig. 5 shows carbon (1 s) XPS spectra for (a) an untreated PE film and (b) a 20 s treated PE film. The XPS spectrum for the untreated sample can be fitted with two peaks, one is the peak at binding energy Eb = 285.0 eV and the other at binding energy Eb = 286.3 eV. The former is the hydrocarbon signal peak whereas the latter should be the peak of the certain intrinsic low-level oxidized carbon group [12]. Oxygen atomic concentration is measured at 3.80% whereas carbon atomic concentration at 18.62% in the second peak. From Fig. 5(b), it can be found that additional oxidized species appear and oxygen atomic concentration increases. Two new peaks emerge on the high-energy side, one is at Eb = 288.0 eV and the other at Eb = 289.2 eV. The former represents ketone [–(C O)–] and/or acetal [–(O–C–O)–] carbons, the latter represents carboxyl [–(C–O)–O–] carbon [18]. The results further confirm the analysis results of FTIR-ATR spectra. From Figs. 2 and 3, it can be seen that the change tendency of oxygen atomic concentration rigidly consists with that of water contact angle and surface free energy with the exposed time. The processing of introducing oxidized groups onto the surface of PE films mainly happens in the first second when the surface of the PE sample is exposed to the air DBD, so the water contact angle decreases quickly. Oxygen atomic concentration increases 24.17% after 20 s treatment time, while it does 24.21 and 24.27% after 30 and 40 s treatment time, respectively, which suggests that the etching and oxidization behavior on the surface of the PE film reach equilibrium, much more oxygen atomic being introduced onto the surface of the PE film could not be realized even if the treatment time is further extended.
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Fig. 6. SEM pictures for (a) the untreated and (b) 20 s treated PE films.
Fig. 7. SEM pictures of Cu films deposited on (a) the untreated and (b) 20 s treated PE films.
3.4. SEM
4. Conclusion
Fig. 6 is the SEM pictures for (a) the untreated and (b) treated PE film of 20 s. Compare (a) with (b), it can be found that the surface becomes rough for the DBD-treated sample, there having more caves and protuberances on its surface. The increase of surface roughness is benefit to the increase of surface energy and also the decrease of water contact angle, and the reason should be attribute to the chemical etching when the sample is exposed to air DBD.
The surface of PE film was modified by DBD in air. The modification effectively changes the surface morphology and the surface chemical compositions. The surface becomes rough and some new functional groups consisting with oxygen have been introduced onto the DBD-treated PE surface, and this leads to the decrease of water contact angle and the increase of surface free energy for the DBD-treated PE film. During the treatment time of 1 s, the water contact angle, surface free energy and oxygen atomic concentration change fast, after that the changes become slow and finally reach saturation. Cu film had been deposited onto the DBDtreated PE sample and the film has smaller grain size and denser structure. The Cu film deposited on the DBD-treated PE surface has stronger adhesive strength compared with that on the untreated PE surface. So it is an effective method to improve the adhesive strength between PE and metal films by air DBD treatment on PE surface.
3.5. Cu film deposited on the PE surface Fig. 7 shows the SEM pictures of the deposited Cu film of (a) on the untreated and (b) on the 20 s air DBD-treated PE surfaces. It could be seen that the Cu film has smaller crystal grain size and denser film structure for the DBD-treated PE film compared with the untreated. For the DBD-treated PE film, there are more active sites on its surface which can offer more nucleuses for the crystal of the deposited Cu ions or atoms, so the deposited film has smaller crystal grain size. The peel adhesive strength was also measured for the samples of (a) and (b) and the values are 0.8 and 1.5 MPa, respectively. The adhesive strength is increased greatly due to DBD treatment which mainly attribute to the following two aspects: one is the increase of surface roughness and the other is the oxidized characteristic on the DBD-treated PE surface [19,20].
Acknowledgment The work is supported by ‘‘National Natural Science Foundation of China’’ under the grant nos. 50537020 and 10775027. References [1] K.L. Mittal, A. Pizzi, Adhesion Promotion Techniques, Marcel Dekker Press, New York, 1999. [2] C.Z. Liu, N.Y. Cui, N.M.D. Brown, B.J. Meenan, Surf. Coat. Technol. 185 (2004) 311.
C.-S. Ren et al. / Applied Surface Science 255 (2008) 3421–3425 [3] S. Guruvenket, G.M. Rao, M. Komath, A.M. Raichur, Appl. Surf. Sci. 236 (2004) 278. [4] M.C. Coen, R. Lehmann, P. Groening, L. Schlapbach, Appl. Surf. Sci. 9729 (2003) 1. [5] S. Bhowmik, P. Jana, T.K. Chaki, S. Ray, Surf. Coat. Technol. 185 (2004) 81. [6] R.J. Carman, R.P. Mildren, J. Phys. D: Appl. Phys. 33 (2000) L99. [7] J.R. Chen, X.Y. Wang, T. Wakida, J. Appl. Polym. Sci. 72 (1998) 1327. [8] R. Seebock, H. Esrom, M. Charbonnier, M. Romand, U. Kogelschatz, Surf. Coat. Technol. 142–144 (2001) 455. [9] G. Borcia, C.A. Anderson, N.M.D. Brown, Appl. Surf. Sci. 221 (2004) 203. [10] S. Ishikawa, K. Yukimura, K. Matsunaga, T. Maruyama, Surf. Coat. Technol. 130 (2002) 52. [11] S. Meiners, J.G.H. Salge, E. Prinz, F. Forster, Surf. Coat. Technol. 98 (1998) 1121. [12] N.Y. Cui, N.M.D. Brown, Appl. Surf. Sci. 189 (2002) 31.
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[13] M. Lehocky, H. Drnovski, B. Lapcikova, A.M.B. Timmons, T. Trindade, M. Zembala, L. Lapcik Jr., Colloids Surf. A 22 (2003) 125. [14] R. Wilken, A. Hollander, J. Behnisch, Surf. Coat. Technol. 116–119 (1999) 991. [15] M. Kuzuya, K. Ito, S.I. Kondo, Y. Yamauchi, Thin Solid Films 345 (1999) 85. [16] Y.M. Chung, M.J. Jung, J.G. Han, M.W. Lee, Y.M. Kim, Thin Solid Films 447–448 (2004) 354. [17] M. Morra, E. Occhiello, F. Garbassi, Surf. Interf. Anal. 16 (1990) 412. [18] G. Beamson, D. Briggs, High Resolution XPS of Organic Polymers, Wiley Press, New York, 1992. [19] M. Noeske, J. Degenhardt, S. Strudthoff, U. Lommatzsch, Int. J. Adhes. Adhes. 24 (2004) 171. [20] S. Kisin, F. Scaltro, P. Malanowski, P.G.T. Van der Varst, G. de With, Polym. Degrad. Stabil. 92 (2007) 605.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface modification of PE film by DBD plasma in air C.-S. Ren *, K. Wang, Q.-Y. Nie, D.-Z. Wang, S.-H. Guo State Key Laboratory of Material Modification by Electron, Ion and Laser Beams, Dalian University of Technology, Dalian 116023, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 April 2008 Received in revised form 22 September 2008 Accepted 24 September 2008 Available online 7 October 2008
In this paper, surface modification of polyethylene (PE) films is studied by dielectric barrier discharge plasma treatment in air. The treated samples were examined by water contact angle measurement, calculation of surface free energy, Fourier transform infrared attenuated total reflection spectroscopy (FTIR-ATR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The water contact angle changes from the original value of 93.28 to the minimum value of 53.38 and surface free energy increases from 27.3 to 51.89 J/m2 after treatment time of 50 s. Both ATR and XPS show some oxidized species are introduced into the sample surface by the plasma treatment and that the change tendencies of the water contact angle and surface free energy with the treatment time are the same as that of the oxygen concentration on the treated sample surface. Cu films were deposited on the treated and untreated PE surfaces. The peel adhesive strength between the Cu film and the treated sample is 1.5 MPa, whereas the value is only 0.8 MPa between the Cu film and the untreated PE. SEM pictures show that the Cu film deposited on the plasma treated PE surface is smooth and the crystal grain is smaller, contrarily the Cu film on the untreated PE surface is rough and the crystal grain is larger. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Dielectric barrier discharge Polyethylene Surface modification Adhesive strength
1. Introduction Polymers, which play an increasing role as structural materials, are applied more and more widely in industry in recent years. Some well-known properties of polymers, such as low surface energy, thermal stability, low friction coefficient and hydrophobic characteristic, are advantages in some applications, whereas they are disadvantages for other applications and have to be overcome. Recently, modification of the surface properties of polymers becomes a subject, which many researchers are interested in. The usual two means, referred to as physical and chemical methods, are adopted to modify the surface properties of polymers. However, the chemical method is water-wasting and easily causes environment pollution. In contrast, the physical method is paid more and more attention in recent years due to its cleanness and high efficiency. Polymer surface modification by discharge plasma is of great and increasing industrial application value [1] because it can uniformly modify the surface of the treated samples without changing the matrix properties. Lots of experimental and theoretical studies about the subject had been performed in recent years. For example, Liu et al. [2] had reported that, the examination of pre- and post-plasma treated polymer surfaces by
* Corresponding author. Tel.: +86 411 84709795; fax: +86 411 84707161. E-mail address: [email protected] (C.-S. Ren). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.09.064
atom force microscopy revealed that only the outermost layer was affected by dielectric barrier discharge (DBD) plasma treatment. Guruvenket et al. [3] had modified polystyrene and polyethylene (PE) using microwave electron cyclotron resonance plasma to improve the surface wettability. Bhowmik et al. [5] had studied the wettability and physicochemical characteristics when PP film was exposed to a DC glow discharge on the condition of airflow by different electrode structure. Coen et al. [4] had modified polypropylene (PP), polymethylmethacrylate, polytetrafluoroethylene (PTFE) and polyethyletetraphalate under high vacuum conditions. The results suggested that plasma treatment could cause topographical modification on the treated polymer surface. DBD, which is characterized by the configuration of at least one of the electrodes is covered with an insulating layer, is proved to be a promising technology in the modification of surface properties of polymers in recent years. The dielectric serves two key functions in the discharge operation: (i) it limits the amount of charge transported from one electrode surface to another via a single microdischarge and (ii) it distributes these microdicharges over the entire electrode area. Usually, voltages of a few kV and frequencies ranging from 5 to 500 kHz are used [2]. The mean electron energy in DBD plasma is in the range of 0–10 eV [6], while the chemical binding energy of polymers is less than 10 eV [7]. Therefore, energetic particles in DBD can break the chemical bonds of polymers. In addition, to modify the surface properties of polymers without expensive vacuum system is another advantage of DBD. Nowadays, DBD has been used widely
C.-S. Ren et al. / Applied Surface Science 255 (2008) 3421–3425
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to modify the surface of polyimide, PP, PTFE, poly (methylpentene), and so on [8,9]. In this paper, PE films are modified using air DBD, the surface energy and adhesive strength with metal films of which are improved. And our investigation will contribute to the application of air plasma and enrich the progress made in technology recently, which has shown that air plasmas can dramatically improve the adhesion and wettability of polymer surfaces [10,11]. 2. Experiment Fig. 1 presents the schematic show of the experimental setup. The discharge room has the dimension of 20 cm 20 cm 20 cm. The two parallel electrodes are stainless steel. The top electrode is covered by thin quartz foil with the thickness of 1 mm and diameter of 4.5 cm, and the grounded electrode, which can be adjusted up and down to change the discharge gap, is covered by PE sample. The discharge is generated by a high voltage power supply with frequency range of 1–20 kHz and effective voltage range of 0–20 kV. The sample derived from a commercial PE film (100 mm thick) was cut into the dimension of 5.5 cm 5.5 cm square, and then was directly put into the discharge system. The experiment was operated with the condition of discharge voltage of 16 kV, frequency of 4 kHz and the electrode gap of 3 mm. The discharge treatment was performed in flowing air. Cu film was deposited with an unbalanced magnetron sputtering system which is enhanced by radio frequency inductively coupled plasma. The substrate was applied a pulse bias voltage with the amplitude of 100 V and 50% duty cycle. The deposition time was 1 h and the film’s thickness was about 600 nm. Scanning electron microscopy (SEM) (S-366, Cambridge, England), Fourier transform infrared attenuated total reflection spectroscopy (FTIR-ATR) (560ESP, Nicolet Instrument Co., USA) and X-ray photoelectron spectroscopy (XPS) (Amicus, Shimadzu Co., Japan) were used to analyze the changes of the surface morphology and surface chemical compositions of the PE films. Water contact angle values were obtained with 5-point average of the measurements obtained from the same sample with the dimension of 5.5 cm 5.5 cm. Surface free energy was calculated by the measurement values of water contact angle. The peel adhesive strength of Cu film with the PE film was measured with a tensile testing machine (LYS-50000) with digital display at a pull velocity of 2 mm/min at room temperature. A commercially available two-component polyurethane resin had been used as adhesive. Each measured value was the average of the measurements of three bonded samples.
Fig. 1. Schematic show of the experimental setup.
Fig. 2. The variation curve of water contact angle and O atomic concentration versus DBD treatment time.
3. Results and discussion 3.1. Water contact angle and surface free energy Fig. 2 shows the changes of the water contact angle and oxygen atomic concentration of PE sample versus air-DBD treatment time. The water contact angle reduces with the treatment time extending. The value of the untreated sample is 93.28, and it dramatically decreases to less than 708 after treatment time of 1 s. It continues to decrease with the increase of treatment time but the change velocity becomes slow, which suggests that the changes on the surface of the treated PE samples mainly happen within the first second treatment time. When the treatment time increases beyond 20 s or more, the contact angle has hardly changed and this shows that the chemical etching, which tends to deplete the oxidized species introduced onto the sample surface by DBD treatment, becomes obvious. Lastly, a dynamic equilibrium is formed between surface modification and surface chemical etching. Cui and Brown [12] had observed that effective plasmainduced chemical etching appears to equilibrate after 25% of the surface carbon being oxidized. Water contact angle measurements on the treated PE surfaces show the hydrophilic characteristic. The introduction of polar molecules on the treated PE surface will cause the decrease of water contact angle. Besides, the change of surface morphology can also lead to the decrease of the water contact angle of the treated PE films. Fig. 3 shows the change of the surface free energy of PE with the plasma treatment time. It can be found that the surface free energy
Fig. 3. The variation curve of surface free energy versus DBD treatment time.
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Fig. 4. FTIR-ATR spectra for (a) untreated, (b) 1 s treated, (c) 10 s treated, (d) 20 s treated, (e) 30 s treated and (f) 40 s treated PE film.
increases from 27.3 to 51.89 J/m2 with the treatment time changing from 0 to 50 s and the change tendency is similar to that of oxygen atomic concentration. This indicates that the change of surface free energy is mainly determined by the introduction of polar molecules on the treated PE surface. Surface free energy is higher, the water contact angle is lower and the adhesive strength between PE and metal films is higher. 3.2. FTIR-ATR analysis Fig. 4 shows the FTIR-ATR spectra for pre- and post-treated PE films, (a) for the untreated PE film, (b)–(f) for 1, 10, 20, 30 and 40 s DBD-treated PE film, respectively. Comparing (a) with (b), there obviously appear two new peaks at 1733 and 1646 cm 1 for the 1 s treated sample. While the 20 s treated PE sample shows two new peaks at 1242 and 3419 cm 1 further. The peak at 1733 cm 1 corresponds to C O stretching vibration and 1646 cm 1 to COO– asymmetrical stretching vibration. The peak at 1242 cm 1 should corresponds to COO– symmetrical stretching vibration and 3419 cm 1 to –OH group vibration. Lehocky et al. [13] had also found similar groups on the surface of the PE treated with oxygen plasma. An oxygen plasma can react with the polymer surface to produce a variety of atomic oxygen functional groups, such as C–O, C O, O–C O and CO3 at the surface [3]. Besides, plasma treatment can also lead to the production of some surface radical sites [14,15]. These radical sites will take oxidation reaction with oxygen molecules very easily in air and produce peroxides [16]. Therefore, the oxidized species are introduced onto the surface of PE when it is treated by air DBD, which can further lead to the decrease of water contact angle on the treated sample surface and the increase of surface free energy. Generally speaking, when polymers are exposed to plasma, two processes simultaneously happen, one is etching of polymer surface through the reaction of atomic oxygen with the surface carbon atom, giving volatile reaction products, the other is the formation of oxygen functional groups on the polymer surface through the interaction between the active species from the plasma and the surface atom [17]. There not appears new peak for the 30 and 40 s treated samples compared with the 20 s treated sample and the amplitude also hardly changes, which suggests the etching and oxidation come to equilibrium when the treatment time goes beyond 30 s. This phenomenon accords with the change tendency of water contact angle and surface free energy with the exposed time.
Fig. 5. Carbon (1 s) XPS spectra for (a) an untreated PE film and (b) a 20 s treated PE film.
3.3. XPS analysis Fig. 5 shows carbon (1 s) XPS spectra for (a) an untreated PE film and (b) a 20 s treated PE film. The XPS spectrum for the untreated sample can be fitted with two peaks, one is the peak at binding energy Eb = 285.0 eV and the other at binding energy Eb = 286.3 eV. The former is the hydrocarbon signal peak whereas the latter should be the peak of the certain intrinsic low-level oxidized carbon group [12]. Oxygen atomic concentration is measured at 3.80% whereas carbon atomic concentration at 18.62% in the second peak. From Fig. 5(b), it can be found that additional oxidized species appear and oxygen atomic concentration increases. Two new peaks emerge on the high-energy side, one is at Eb = 288.0 eV and the other at Eb = 289.2 eV. The former represents ketone [–(C O)–] and/or acetal [–(O–C–O)–] carbons, the latter represents carboxyl [–(C–O)–O–] carbon [18]. The results further confirm the analysis results of FTIR-ATR spectra. From Figs. 2 and 3, it can be seen that the change tendency of oxygen atomic concentration rigidly consists with that of water contact angle and surface free energy with the exposed time. The processing of introducing oxidized groups onto the surface of PE films mainly happens in the first second when the surface of the PE sample is exposed to the air DBD, so the water contact angle decreases quickly. Oxygen atomic concentration increases 24.17% after 20 s treatment time, while it does 24.21 and 24.27% after 30 and 40 s treatment time, respectively, which suggests that the etching and oxidization behavior on the surface of the PE film reach equilibrium, much more oxygen atomic being introduced onto the surface of the PE film could not be realized even if the treatment time is further extended.
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Fig. 6. SEM pictures for (a) the untreated and (b) 20 s treated PE films.
Fig. 7. SEM pictures of Cu films deposited on (a) the untreated and (b) 20 s treated PE films.
3.4. SEM
4. Conclusion
Fig. 6 is the SEM pictures for (a) the untreated and (b) treated PE film of 20 s. Compare (a) with (b), it can be found that the surface becomes rough for the DBD-treated sample, there having more caves and protuberances on its surface. The increase of surface roughness is benefit to the increase of surface energy and also the decrease of water contact angle, and the reason should be attribute to the chemical etching when the sample is exposed to air DBD.
The surface of PE film was modified by DBD in air. The modification effectively changes the surface morphology and the surface chemical compositions. The surface becomes rough and some new functional groups consisting with oxygen have been introduced onto the DBD-treated PE surface, and this leads to the decrease of water contact angle and the increase of surface free energy for the DBD-treated PE film. During the treatment time of 1 s, the water contact angle, surface free energy and oxygen atomic concentration change fast, after that the changes become slow and finally reach saturation. Cu film had been deposited onto the DBDtreated PE sample and the film has smaller grain size and denser structure. The Cu film deposited on the DBD-treated PE surface has stronger adhesive strength compared with that on the untreated PE surface. So it is an effective method to improve the adhesive strength between PE and metal films by air DBD treatment on PE surface.
3.5. Cu film deposited on the PE surface Fig. 7 shows the SEM pictures of the deposited Cu film of (a) on the untreated and (b) on the 20 s air DBD-treated PE surfaces. It could be seen that the Cu film has smaller crystal grain size and denser film structure for the DBD-treated PE film compared with the untreated. For the DBD-treated PE film, there are more active sites on its surface which can offer more nucleuses for the crystal of the deposited Cu ions or atoms, so the deposited film has smaller crystal grain size. The peel adhesive strength was also measured for the samples of (a) and (b) and the values are 0.8 and 1.5 MPa, respectively. The adhesive strength is increased greatly due to DBD treatment which mainly attribute to the following two aspects: one is the increase of surface roughness and the other is the oxidized characteristic on the DBD-treated PE surface [19,20].
Acknowledgment The work is supported by ‘‘National Natural Science Foundation of China’’ under the grant nos. 50537020 and 10775027. References [1] K.L. Mittal, A. Pizzi, Adhesion Promotion Techniques, Marcel Dekker Press, New York, 1999. [2] C.Z. Liu, N.Y. Cui, N.M.D. Brown, B.J. Meenan, Surf. Coat. Technol. 185 (2004) 311.
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