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CF4 plasma using atmospheric pressure plasma jet
Applied Surface Science 256 (2009) 1496–1501
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface modification of a polyamide 6 film by He/CF4 plasma using atmospheric pressure plasma jet Zhiqiang Gao a,c, Jie Sun b, Shujing Peng a,c, Lan Yao a,c, Yiping Qiu a,c,* a
Key Laboratory of Textile Science and Technology, Ministry of Education, China Key Laboratory of Science & Technology of Eco-Textiles, Ministry of Education, China c Department of Textile Materials Science and Product Design, College of Textiles, Donghua University, Shanghai 201620, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 April 2009 Received in revised form 31 July 2009 Accepted 3 September 2009 Available online 11 September 2009
Polyamide 6 (PA 6) films are treated with helium(He)/CF4 plasma at atmospheric pressure. The samples are treated at different treatment times. The surface modification of the PA 6 films is evaluated by water contact angle, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The etching rate is used to study the etching effect of He/CF4 plasma on the PA 6 films. The T-peel strengths of the control and plasma treated films are measured to show the surface adhesion properties of the films. As the treatment time increases, the etching rate decreases steadily, the contact angle decreases initially and then increases, while the T-peel strength increases first and then decreases. AFM analyses show that the surface roughness increases after the plasma treatment. XPS analyses reveal substantial incorporation of fluorine and/or oxygen atoms to the polymer chains on the film surfaces. ß 2009 Elsevier B.V. All rights reserved.
PACS: 52.40.Hf 81.05.Lg 81.65.Mq Keywords: Polyamide Atmospheric pressure plasma Surface XPS AFM
1. Introduction Plasma treatments have been widely used in recent years as an effective means to modify polymer surfaces without affecting the bulk properties [1–3]. The modification effects of plasma treatments may include improving surface hydrophilicity and roughness for enhanced wetting, dyeing, printing and adhesion properties [4–6]. Under the bombardment of active species generated by a homogeneous plasma, polymer surfaces can be modified by removing surface contaminations [7,8], introducing new chemical functional groups [9–14], and depositing a thin coating [15–17]. However, most of the plasma systems operate under a low pressure and require an expensive and complicated vacuum device [18]. To overcome this drawback, atmospheric pressure plasma devices have been developed, one of which is an atmospheric pressure plasma jet (APPJ) invented to produce a homogeneous plasma at a low temperature [19,20].
* Corresponding author. Tel.: +86 21 67792744; fax: +86 21 67792627. E-mail address: [email protected] (Y. Qiu). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.09.010
Carbon tetrafluoride (CF4) is often chosen as a reactive gas due to its mild etching and highly fluorinating characteristics [21]. Several fundamental studies on characterization of CF4 plasma treated surfaces have been published [22–26]. The reactive species of CF4 plasmas are reported to be mainly F atoms and relatively low concentrations of CFn (n = 1, 2, 3) radicals [20,21]. With a low CFn/F ratio, CF4 does not polymerize in plasma, but it can initiate fluorination through a direct grafting of F atoms onto polymer surfaces [24,25]. Previous studies have shown that the CF4 plasma modification is described as a synergistic effect of two mechanisms: degradation and fluorination [27]. Dreux et al. [28] have treated polyamide 12 films using low pressure CF4 and CF4 + H2 plasmas to reduce their water vapor permeability and reported that CF4 plasma treatment reduces water permeability of PA 12 films more significantly than CF4 + H2 plasma does while the later is more effective in plasma etching of PA 12 films because H2 reacts with atomic fluorine, facilitating degradation. Saloum et al. [29] have analyzed the composition of plasmas when treating PA 6 films with low pressure N2–Ar plasma and found that N and N+ increase significantly as a result of disassociation of the amide groups on the polymer surface when the plasma interacted with PA 6 surface. Yip et al. [30] have
Z. Gao et al. / Applied Surface Science 256 (2009) 1496–1501
reported that when PA 6 fibers are treated with low pressure O2 plasma, sub-micron ripples perpendicular to the fiber axis are formed on the fiber surfaces as a result of plasma etching due to existence of a stress field. Pappas et al. [31] have treated PA 6 fibers with N2/He and C2H2/He atmospheric pressure plasmas and found that the plasmas induce transformation of hydrocarbon and carbonyl groups into carboxylic groups while enhancing fiber surface roughness. This paper is intended to investigate the etching and surface modification behavior of He/CF4 atmospheric pressure plasma treatment to PA 6 film surfaces in terms of the etching rate, surface chemical composition, wettability, and topography. In particular, the effect of plasma treatment duration on the outcome of the plasma treatment was investigated. The surface wettabilities of the PA 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 etching rate was also studied after plasma treatment and the T-peel strength was carried out to show the surface adhesion properties of the films. 2. Experimental details 2.1. Material PA 6 films (1.134 g/cm3) used in this study have molecular weight of 17,200–41,000, provided by Dongsu Group, China. The samples were cut into 2 cm 5 cm pieces, soaked in acetone for 10 min to remove the contamination and finishes, and then dried in a desiccator for 24 h at room temperature. 2.2. Plasma treatment The plasma treatments were carried out using an atmospheric pressure plasma jet (APPJ) apparatus (Atomflo-R, Surfx Company, USA) with He/CF4 gas. 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 CF4 was added as the reactive gas. The APPJ employs a capacitively coupled electrode design and produces a stable discharge at atmospheric pressure and 13.56 MHz radio frequency. More detailed information about the plasma machine was given in [19,20]. After the plasma treatments, the specimens were immediately placed into clean plastic bags to minimize potential contamination. 2.3. Contact angle measurement
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2.5. AFM analysis Atomic force microscopy (Multimode Nanoscope IIIa, Digital Instrument, USA) was used to examine the morphology of the PA 6 film surfaces before and after the plasma treatments. All of the AFM images were acquired in air using the tapping mode to prevent significant deterioration of the film surface. 2.6. Analysis of surface chemical composition The chemical composition changes of the PA 6 film surfaces were examined using X-ray photoelectron spectroscopy (XPS) model ESCALAB 250 (Thermo Electron VG Scientific, USA). The Xray source was Mg Ka (1253.6 eV), operating at 400 W. The pressure within the XPS chamber was 10 7 10 8 Pa. Photo emitted electrons were collected at a take-off angle of 458 and the deconvolution analysis of C1s peaks was carried out using an XPS Peak analysis software. 2.7. T-peel strength In order to study the effect of He/CF4 plasma treatment on the adhesion properties of the PA 6 film surfaces, a standard Tpeel test was carried out using a Universal Testing Machine at a rate of 100 mm/min. For this study an adhesive tape of 2 cm width was stuck over a length of 4.0 cm on the specimen. To minimize potential air gaps or wrinkles, each specimen was pressed with 1.0 kg dead weight for 10 min. To measure the Tpeel strength, one end of the specimen was clamped in one jaw and the adhesive tape end with a piece of paper adhered to it in the other jaw. T-peel strengths were recorded as the force of peeling per centimeter of the specimen width. For each sample, five measurements were performed and the mean value was taken as the T-peel strength. 3. Results and discussion 3.1. Contact angle measurement The surface properties of the PA 6 films were analyzed by water contact angle measurement. To evaluate the effect of He/CF4 plasma treatment duration, PA 6 films were treated for different lengths of time. Fig. 1 shows that the variation of contact angles for the treated PA 6 film surfaces as a function of the treatment time. The contact angle initially decreased. This may be explained as a result of cleaning of the film surface by plasma etching or oxidation before the reactive cites had enough time to react with fluorine as
The contact angle was measured to determine the wettability of the PA 6 film surface using sessile drop method. A 2 ml drop of distilled water was put on the surface with a microliter syringe and observed through an optical microscope. The value of contact angle was an average of at least five readings at different locations on the surface of each sample. 2.4. Etching rate of the plasma treated PA 6 film surfaces To calculate the plasma etching rate, the PA 6 films before and after the plasma treatments were weighed by an electronbalance (FA1004, Shanghai Precision & Scientific Instrument CO. LTD, China). The percent of mass loss was used to calculate the etching rate as the percent mass loss = (W0 W1)/W0 100%, where W0 and W1 are the masses of untreated and plasma treated samples, respectively. The thickness reductions of the films were then derived from the mass losses using the density of the PA 6 film.
Fig. 1. Influence of plasma treatment time on contact angle of PA 6 film.
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Z. Gao et al. / Applied Surface Science 256 (2009) 1496–1501 Table 1 Surface roughness of PA 6 films before and after plasma treatment. Sample
Control He/CF4, 30 s He/CF4, 60 s He/CF4, 90 s
RMS (nm) Average
Standard deviation
12.1 20.0 29.1 36.3
1.5 2.1 2.9 3.3
3.2. Etching rate of the plasma treated PA 6 film surfaces
Fig. 2. Influence of plasma treatment time on etching rate of PA 6 film.
suggested by Dreux et al. [28] who have also found an increase of hydrophilicity after low pressure CF4 plasma treatment of PA 12 for a very short time. When the treatment time further increased, the contact angle increased with the treatment time, indicating introduction of hydrophobic fluorinated groups on to the PA 6 film surfaces.
Fig. 2 shows the influence of plasma treatment time on the etching rate. The etching rate exhibited a steady decrease as the treatment time increased, which could be due to the competition of etching and redeposition processes during plasma treatments. As plasma etching proceeded the fragmented pieces etched away from the polymer surface could be re-deposited on the film [32], which could not only reduce the thickness reduction as a result of plasma etching but also partially shield the polymer surface, reducing the etching efficiency. This could be confirmed by increased fluorine concentration on the film surface as the plasma treatment time increased shown in the following sections.
Fig. 3. AFM images of PA 6 film surfaces: (a) control; (b) plasma treated for 30 s; (c) plasma treated for 60 s; (d) plasma treated for 90 s (the magnitude of the vertical axis is 100 nm/division).
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Table 2 XPS elemental analysis of the control and plasma treated PA 6 films. Sample
Control He/CF4, 30 s He/CF4, 60 s He/CF4, 90 s
Chemical composition (%)
Atomic ratio (%)
C1s
O1s
N1s
F1s
O/C
N/C
F/C
77.4 70.9 66.9 61.1
14.5 17.8 18.8 19. 5
8.1 9.0 11.2 9.4
– 2.3 3.2 10.0
18. 8 25.1 28.0 31.8
10.4 12.7 16.8 15.4
– 3.3 4.7 16.4
3.3. Surface morphology and roughness
Fig. 4. X-ray photoelectron survey spectra for the control and treated PA 6 films.
The morphology and the root mean square roughness (RMS) of the PA 6 films under various plasma treatment durations were investigated by AFM. The topographical images of the PA 6 film surfaces are shown in Fig. 3. The surface roughness increased with plasma treatment time. The increase in the roughness was believed to be caused by etching effect of the radical fluorine and other activated species provided by He/CF4 plasma. In this experiment, He participated in the Penning reaction which promoted dissociation and ionization of the CF4. The root mean square roughness (RMS) of the control and plasma treated PA 6 film is presented in Table 1. It can be seen that the values of RMS gradually increased as
Fig. 5. Deconvoluted XPS C1s peaks for PA 6 film: (a) control; (b) plasma treated for 30 s; (c) plasma treated for 60 s; (d) plasma treated for 90 s.
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Table 3 Results of deconvolution analysis of C1s peaks of Control and plasma treated PA 6 films. Sample
Control He/CF4, 30 s He/CF4, 60 s He/CF4, 90 s
Relative area corresponding to different chemical bonds (%) C–C (284.6 eV)
C–N (285.4 eV)
C–O/C–CF2 (286.5 eV)
CONH/CF (287.9eV)
O–C5 5O (288.5 eV)
CF3 (293.0 eV)
58.4 48.2 40.0 37.3
31.8 30.1 27.8 24.7
– 9.4 10.4 12.9
9.8 (no CF) 12.4 17.8 19.0
– – – 4.5.
– – – 1.6
the treatment time increased. Longer treatment time also led to a larger surface roughness, indicating that plasma etching of the surface of the PA 6 films could be selective at microscopic level. This increased roughness for longer plasma treatment time was also reported by Yip et al. [30] who treated PA 6 fibers with low
pressure O2 plasma. The etching could be more effective in certain areas on the film surface, where the molecules are less organized such as amorphous phase dominant areas or where the molecules are stressed as reported in literature [30]. 3.4. XPS analysis The surfaces of the control and plasma treated PA 6 films for different treatment times were examined by XPS. In term of binding energy, the peaks at 285 eV, 400 eV, 532 eV and 685 eV correspond to the C1s, N1s, O1s and F1s, respectively [33–35]. Fig. 4 shows the X-ray photoelectron survey spectra for the control and the treated samples with He/CF4 plasma for different treatment times. Compared to the control, the O1s intensities increased and the F1s peaks were introduced onto the PA 6 film surfaces after He/CF4 plasma treatment. The surface elemental composition and the atomic ratios of O/C and F/C for the samples are presented in Table 2. For the 30 s and 60 s groups, the F/C ratios were 3.3% and 4.7%, whereas for the 90 s group, the F/C ratio reached 16.4%, more than tripled from that of the 60 s sample. However, the O/C ratios jumped from 18.8% for the control to 25.1% and 28.0% for the 30 s and 60 s samples but only further increased slightly to 31.8% for the 90 s sample. This clearly implied that plasma etching and oxidation to the PA 6 film surface could be more dominant reactions at the early stage of the plasma treatment while fluorination reaction was probably more dominant for longer treatment durations consistent with the literature [28]. Incorporation of F into the polymer molecules could be achieved through a substitution mechanism between H and F atoms during CF4 plasma treatment of a polymer [36]. In order to reveal the change of surface functional groups, deconvolution analysis of the C1s peaks was performed as shown in Fig. 5. The PA 6 film surfaces after He/CF4 plasma treatments had the following carbon-containing components with binding energies of 284.6 eV (C–C), 285.4 eV (C–N), 286.5 eV (C–O/C–CF2), 287.9 eV (CONH/CF), 288.5 eV (O–C5 5O) and 293.0 (CF3) according to the literature [18,22,31,37,38]. Table 3 gives the relative areas corresponding to different carbon-containing chemical bonds. The peak areas of C–O/C–CF2 and CONH/CF all increased whereas that of C–C/C–N decreased after the plasma treatment. Obviously this was resulted from fluorination and disassociation of the amide groups originally in PA 6 [29]. The deconvolution analysis of F1s is shown in Fig. 6 and the relative areas corresponding to different fluorine chemical bonds are presented in Table 4 [39,40]. For the 30 s and 90 s samples, about 2/3 fluorine is in C–CF2 and 1/3 in CF whereas for the 90 s sample CF3 also appeared while the amount of C–CF2 decreased by half, indicating further fluorination of C–CF2 Table 4 Results of deconvolution analysis of F1s peaks of plasma treated PA 6 films. Sample
Fig. 6. Deconvoluted XPS F1s peaks for PA 6 film: (a) plasma treated for 30 s; (b) plasma treated for 60 s; (c) plasma treated for 90 s.
He/CF4, 30 s He/CF4, 60 s He/CF4, 90 s
Relative area corresponding to different chemical bonds (%) C–CF2 (684.23 eV)
CF (685.84 eV)
CF3 (688.33 eV)
69.3 63.6 30.7
30.7 36.4 49.8
– – 19.6
Z. Gao et al. / Applied Surface Science 256 (2009) 1496–1501
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and the plasma etching rate steadily decreases as the plasma treatment time prolongs. Acknowledgements 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), and the Program of Introducing Talents of Discipline to Universities (No. B07024). References
Fig. 7. Dependence of T-peel strength on plasma treatment time.
into CF3. Meanwhile, O–C5 5O groups were also introduced on to PA 6 film surface for the 90 s sample (Table 3) which was a further oxidation product of other oxygen containing groups. From these results, it can be concluded that the longer treatment time, the more oxygen- and fluorine-containing groups incorporated on to the PA 6 film surfaces due to a less dominant role of etching and a more dominant role of fluorination and oxidation [28]. 3.5. T-peel strength PA 6 films were subjected to T-peel strength measurement as a function of treatment time to understand the effect of the He/CF4 plasma treatment on bonding strength. The values of peel strength are plotted against treatment time in Fig. 7. The T-peel strength initially increased but soon decreased as the plasma treatment time further increased. This is consistent with the water contact angle and XPS analysis results which showed that surface wettability and polar group increased for a short plasma treatment time while for a longer treatment time, surface wettability decreased and hydrophobic fluorinated groups greatly increased on the film surface. In addition, the increase of surface roughness as shown in AFM analysis could also increase the adhesion strength for a hydrophilic surface and hinder it for a hydrophobic surface. 4. Conclusions He/CF4 plasma was used to treat PA 6 films at atmospheric pressure. For a short treatment time, a decrease in contact angle and an increase in T-peel strength are observed corresponding to a relatively large increase in surface oxygen content and relatively small increase in surface fluorine content. However, as the treatment time further increases, the contact angle increases and T-peel strength decreases accompanied by a large increase of fluorine content and a relatively small increase in surface oxygen content. In addition, the surface roughness continuously increases
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Surface modification of a polyamide 6 film by He/CF4 plasma using atmospheric pressure plasma jet Zhiqiang Gao a,c, Jie Sun b, Shujing Peng a,c, Lan Yao a,c, Yiping Qiu a,c,* a
Key Laboratory of Textile Science and Technology, Ministry of Education, China Key Laboratory of Science & Technology of Eco-Textiles, Ministry of Education, China c Department of Textile Materials Science and Product Design, College of Textiles, Donghua University, Shanghai 201620, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 April 2009 Received in revised form 31 July 2009 Accepted 3 September 2009 Available online 11 September 2009
Polyamide 6 (PA 6) films are treated with helium(He)/CF4 plasma at atmospheric pressure. The samples are treated at different treatment times. The surface modification of the PA 6 films is evaluated by water contact angle, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The etching rate is used to study the etching effect of He/CF4 plasma on the PA 6 films. The T-peel strengths of the control and plasma treated films are measured to show the surface adhesion properties of the films. As the treatment time increases, the etching rate decreases steadily, the contact angle decreases initially and then increases, while the T-peel strength increases first and then decreases. AFM analyses show that the surface roughness increases after the plasma treatment. XPS analyses reveal substantial incorporation of fluorine and/or oxygen atoms to the polymer chains on the film surfaces. ß 2009 Elsevier B.V. All rights reserved.
PACS: 52.40.Hf 81.05.Lg 81.65.Mq Keywords: Polyamide Atmospheric pressure plasma Surface XPS AFM
1. Introduction Plasma treatments have been widely used in recent years as an effective means to modify polymer surfaces without affecting the bulk properties [1–3]. The modification effects of plasma treatments may include improving surface hydrophilicity and roughness for enhanced wetting, dyeing, printing and adhesion properties [4–6]. Under the bombardment of active species generated by a homogeneous plasma, polymer surfaces can be modified by removing surface contaminations [7,8], introducing new chemical functional groups [9–14], and depositing a thin coating [15–17]. However, most of the plasma systems operate under a low pressure and require an expensive and complicated vacuum device [18]. To overcome this drawback, atmospheric pressure plasma devices have been developed, one of which is an atmospheric pressure plasma jet (APPJ) invented to produce a homogeneous plasma at a low temperature [19,20].
* Corresponding author. Tel.: +86 21 67792744; fax: +86 21 67792627. E-mail address: [email protected] (Y. Qiu). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.09.010
Carbon tetrafluoride (CF4) is often chosen as a reactive gas due to its mild etching and highly fluorinating characteristics [21]. Several fundamental studies on characterization of CF4 plasma treated surfaces have been published [22–26]. The reactive species of CF4 plasmas are reported to be mainly F atoms and relatively low concentrations of CFn (n = 1, 2, 3) radicals [20,21]. With a low CFn/F ratio, CF4 does not polymerize in plasma, but it can initiate fluorination through a direct grafting of F atoms onto polymer surfaces [24,25]. Previous studies have shown that the CF4 plasma modification is described as a synergistic effect of two mechanisms: degradation and fluorination [27]. Dreux et al. [28] have treated polyamide 12 films using low pressure CF4 and CF4 + H2 plasmas to reduce their water vapor permeability and reported that CF4 plasma treatment reduces water permeability of PA 12 films more significantly than CF4 + H2 plasma does while the later is more effective in plasma etching of PA 12 films because H2 reacts with atomic fluorine, facilitating degradation. Saloum et al. [29] have analyzed the composition of plasmas when treating PA 6 films with low pressure N2–Ar plasma and found that N and N+ increase significantly as a result of disassociation of the amide groups on the polymer surface when the plasma interacted with PA 6 surface. Yip et al. [30] have
Z. Gao et al. / Applied Surface Science 256 (2009) 1496–1501
reported that when PA 6 fibers are treated with low pressure O2 plasma, sub-micron ripples perpendicular to the fiber axis are formed on the fiber surfaces as a result of plasma etching due to existence of a stress field. Pappas et al. [31] have treated PA 6 fibers with N2/He and C2H2/He atmospheric pressure plasmas and found that the plasmas induce transformation of hydrocarbon and carbonyl groups into carboxylic groups while enhancing fiber surface roughness. This paper is intended to investigate the etching and surface modification behavior of He/CF4 atmospheric pressure plasma treatment to PA 6 film surfaces in terms of the etching rate, surface chemical composition, wettability, and topography. In particular, the effect of plasma treatment duration on the outcome of the plasma treatment was investigated. The surface wettabilities of the PA 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 etching rate was also studied after plasma treatment and the T-peel strength was carried out to show the surface adhesion properties of the films. 2. Experimental details 2.1. Material PA 6 films (1.134 g/cm3) used in this study have molecular weight of 17,200–41,000, provided by Dongsu Group, China. The samples were cut into 2 cm 5 cm pieces, soaked in acetone for 10 min to remove the contamination and finishes, and then dried in a desiccator for 24 h at room temperature. 2.2. Plasma treatment The plasma treatments were carried out using an atmospheric pressure plasma jet (APPJ) apparatus (Atomflo-R, Surfx Company, USA) with He/CF4 gas. 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 CF4 was added as the reactive gas. The APPJ employs a capacitively coupled electrode design and produces a stable discharge at atmospheric pressure and 13.56 MHz radio frequency. More detailed information about the plasma machine was given in [19,20]. After the plasma treatments, the specimens were immediately placed into clean plastic bags to minimize potential contamination. 2.3. Contact angle measurement
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2.5. AFM analysis Atomic force microscopy (Multimode Nanoscope IIIa, Digital Instrument, USA) was used to examine the morphology of the PA 6 film surfaces before and after the plasma treatments. All of the AFM images were acquired in air using the tapping mode to prevent significant deterioration of the film surface. 2.6. Analysis of surface chemical composition The chemical composition changes of the PA 6 film surfaces were examined using X-ray photoelectron spectroscopy (XPS) model ESCALAB 250 (Thermo Electron VG Scientific, USA). The Xray source was Mg Ka (1253.6 eV), operating at 400 W. The pressure within the XPS chamber was 10 7 10 8 Pa. Photo emitted electrons were collected at a take-off angle of 458 and the deconvolution analysis of C1s peaks was carried out using an XPS Peak analysis software. 2.7. T-peel strength In order to study the effect of He/CF4 plasma treatment on the adhesion properties of the PA 6 film surfaces, a standard Tpeel test was carried out using a Universal Testing Machine at a rate of 100 mm/min. For this study an adhesive tape of 2 cm width was stuck over a length of 4.0 cm on the specimen. To minimize potential air gaps or wrinkles, each specimen was pressed with 1.0 kg dead weight for 10 min. To measure the Tpeel strength, one end of the specimen was clamped in one jaw and the adhesive tape end with a piece of paper adhered to it in the other jaw. T-peel strengths were recorded as the force of peeling per centimeter of the specimen width. For each sample, five measurements were performed and the mean value was taken as the T-peel strength. 3. Results and discussion 3.1. Contact angle measurement The surface properties of the PA 6 films were analyzed by water contact angle measurement. To evaluate the effect of He/CF4 plasma treatment duration, PA 6 films were treated for different lengths of time. Fig. 1 shows that the variation of contact angles for the treated PA 6 film surfaces as a function of the treatment time. The contact angle initially decreased. This may be explained as a result of cleaning of the film surface by plasma etching or oxidation before the reactive cites had enough time to react with fluorine as
The contact angle was measured to determine the wettability of the PA 6 film surface using sessile drop method. A 2 ml drop of distilled water was put on the surface with a microliter syringe and observed through an optical microscope. The value of contact angle was an average of at least five readings at different locations on the surface of each sample. 2.4. Etching rate of the plasma treated PA 6 film surfaces To calculate the plasma etching rate, the PA 6 films before and after the plasma treatments were weighed by an electronbalance (FA1004, Shanghai Precision & Scientific Instrument CO. LTD, China). The percent of mass loss was used to calculate the etching rate as the percent mass loss = (W0 W1)/W0 100%, where W0 and W1 are the masses of untreated and plasma treated samples, respectively. The thickness reductions of the films were then derived from the mass losses using the density of the PA 6 film.
Fig. 1. Influence of plasma treatment time on contact angle of PA 6 film.
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Z. Gao et al. / Applied Surface Science 256 (2009) 1496–1501 Table 1 Surface roughness of PA 6 films before and after plasma treatment. Sample
Control He/CF4, 30 s He/CF4, 60 s He/CF4, 90 s
RMS (nm) Average
Standard deviation
12.1 20.0 29.1 36.3
1.5 2.1 2.9 3.3
3.2. Etching rate of the plasma treated PA 6 film surfaces
Fig. 2. Influence of plasma treatment time on etching rate of PA 6 film.
suggested by Dreux et al. [28] who have also found an increase of hydrophilicity after low pressure CF4 plasma treatment of PA 12 for a very short time. When the treatment time further increased, the contact angle increased with the treatment time, indicating introduction of hydrophobic fluorinated groups on to the PA 6 film surfaces.
Fig. 2 shows the influence of plasma treatment time on the etching rate. The etching rate exhibited a steady decrease as the treatment time increased, which could be due to the competition of etching and redeposition processes during plasma treatments. As plasma etching proceeded the fragmented pieces etched away from the polymer surface could be re-deposited on the film [32], which could not only reduce the thickness reduction as a result of plasma etching but also partially shield the polymer surface, reducing the etching efficiency. This could be confirmed by increased fluorine concentration on the film surface as the plasma treatment time increased shown in the following sections.
Fig. 3. AFM images of PA 6 film surfaces: (a) control; (b) plasma treated for 30 s; (c) plasma treated for 60 s; (d) plasma treated for 90 s (the magnitude of the vertical axis is 100 nm/division).
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Table 2 XPS elemental analysis of the control and plasma treated PA 6 films. Sample
Control He/CF4, 30 s He/CF4, 60 s He/CF4, 90 s
Chemical composition (%)
Atomic ratio (%)
C1s
O1s
N1s
F1s
O/C
N/C
F/C
77.4 70.9 66.9 61.1
14.5 17.8 18.8 19. 5
8.1 9.0 11.2 9.4
– 2.3 3.2 10.0
18. 8 25.1 28.0 31.8
10.4 12.7 16.8 15.4
– 3.3 4.7 16.4
3.3. Surface morphology and roughness
Fig. 4. X-ray photoelectron survey spectra for the control and treated PA 6 films.
The morphology and the root mean square roughness (RMS) of the PA 6 films under various plasma treatment durations were investigated by AFM. The topographical images of the PA 6 film surfaces are shown in Fig. 3. The surface roughness increased with plasma treatment time. The increase in the roughness was believed to be caused by etching effect of the radical fluorine and other activated species provided by He/CF4 plasma. In this experiment, He participated in the Penning reaction which promoted dissociation and ionization of the CF4. The root mean square roughness (RMS) of the control and plasma treated PA 6 film is presented in Table 1. It can be seen that the values of RMS gradually increased as
Fig. 5. Deconvoluted XPS C1s peaks for PA 6 film: (a) control; (b) plasma treated for 30 s; (c) plasma treated for 60 s; (d) plasma treated for 90 s.
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Table 3 Results of deconvolution analysis of C1s peaks of Control and plasma treated PA 6 films. Sample
Control He/CF4, 30 s He/CF4, 60 s He/CF4, 90 s
Relative area corresponding to different chemical bonds (%) C–C (284.6 eV)
C–N (285.4 eV)
C–O/C–CF2 (286.5 eV)
CONH/CF (287.9eV)
O–C5 5O (288.5 eV)
CF3 (293.0 eV)
58.4 48.2 40.0 37.3
31.8 30.1 27.8 24.7
– 9.4 10.4 12.9
9.8 (no CF) 12.4 17.8 19.0
– – – 4.5.
– – – 1.6
the treatment time increased. Longer treatment time also led to a larger surface roughness, indicating that plasma etching of the surface of the PA 6 films could be selective at microscopic level. This increased roughness for longer plasma treatment time was also reported by Yip et al. [30] who treated PA 6 fibers with low
pressure O2 plasma. The etching could be more effective in certain areas on the film surface, where the molecules are less organized such as amorphous phase dominant areas or where the molecules are stressed as reported in literature [30]. 3.4. XPS analysis The surfaces of the control and plasma treated PA 6 films for different treatment times were examined by XPS. In term of binding energy, the peaks at 285 eV, 400 eV, 532 eV and 685 eV correspond to the C1s, N1s, O1s and F1s, respectively [33–35]. Fig. 4 shows the X-ray photoelectron survey spectra for the control and the treated samples with He/CF4 plasma for different treatment times. Compared to the control, the O1s intensities increased and the F1s peaks were introduced onto the PA 6 film surfaces after He/CF4 plasma treatment. The surface elemental composition and the atomic ratios of O/C and F/C for the samples are presented in Table 2. For the 30 s and 60 s groups, the F/C ratios were 3.3% and 4.7%, whereas for the 90 s group, the F/C ratio reached 16.4%, more than tripled from that of the 60 s sample. However, the O/C ratios jumped from 18.8% for the control to 25.1% and 28.0% for the 30 s and 60 s samples but only further increased slightly to 31.8% for the 90 s sample. This clearly implied that plasma etching and oxidation to the PA 6 film surface could be more dominant reactions at the early stage of the plasma treatment while fluorination reaction was probably more dominant for longer treatment durations consistent with the literature [28]. Incorporation of F into the polymer molecules could be achieved through a substitution mechanism between H and F atoms during CF4 plasma treatment of a polymer [36]. In order to reveal the change of surface functional groups, deconvolution analysis of the C1s peaks was performed as shown in Fig. 5. The PA 6 film surfaces after He/CF4 plasma treatments had the following carbon-containing components with binding energies of 284.6 eV (C–C), 285.4 eV (C–N), 286.5 eV (C–O/C–CF2), 287.9 eV (CONH/CF), 288.5 eV (O–C5 5O) and 293.0 (CF3) according to the literature [18,22,31,37,38]. Table 3 gives the relative areas corresponding to different carbon-containing chemical bonds. The peak areas of C–O/C–CF2 and CONH/CF all increased whereas that of C–C/C–N decreased after the plasma treatment. Obviously this was resulted from fluorination and disassociation of the amide groups originally in PA 6 [29]. The deconvolution analysis of F1s is shown in Fig. 6 and the relative areas corresponding to different fluorine chemical bonds are presented in Table 4 [39,40]. For the 30 s and 90 s samples, about 2/3 fluorine is in C–CF2 and 1/3 in CF whereas for the 90 s sample CF3 also appeared while the amount of C–CF2 decreased by half, indicating further fluorination of C–CF2 Table 4 Results of deconvolution analysis of F1s peaks of plasma treated PA 6 films. Sample
Fig. 6. Deconvoluted XPS F1s peaks for PA 6 film: (a) plasma treated for 30 s; (b) plasma treated for 60 s; (c) plasma treated for 90 s.
He/CF4, 30 s He/CF4, 60 s He/CF4, 90 s
Relative area corresponding to different chemical bonds (%) C–CF2 (684.23 eV)
CF (685.84 eV)
CF3 (688.33 eV)
69.3 63.6 30.7
30.7 36.4 49.8
– – 19.6
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and the plasma etching rate steadily decreases as the plasma treatment time prolongs. Acknowledgements 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), and the Program of Introducing Talents of Discipline to Universities (No. B07024). References
Fig. 7. Dependence of T-peel strength on plasma treatment time.
into CF3. Meanwhile, O–C5 5O groups were also introduced on to PA 6 film surface for the 90 s sample (Table 3) which was a further oxidation product of other oxygen containing groups. From these results, it can be concluded that the longer treatment time, the more oxygen- and fluorine-containing groups incorporated on to the PA 6 film surfaces due to a less dominant role of etching and a more dominant role of fluorination and oxidation [28]. 3.5. T-peel strength PA 6 films were subjected to T-peel strength measurement as a function of treatment time to understand the effect of the He/CF4 plasma treatment on bonding strength. The values of peel strength are plotted against treatment time in Fig. 7. The T-peel strength initially increased but soon decreased as the plasma treatment time further increased. This is consistent with the water contact angle and XPS analysis results which showed that surface wettability and polar group increased for a short plasma treatment time while for a longer treatment time, surface wettability decreased and hydrophobic fluorinated groups greatly increased on the film surface. In addition, the increase of surface roughness as shown in AFM analysis could also increase the adhesion strength for a hydrophilic surface and hinder it for a hydrophobic surface. 4. Conclusions He/CF4 plasma was used to treat PA 6 films at atmospheric pressure. For a short treatment time, a decrease in contact angle and an increase in T-peel strength are observed corresponding to a relatively large increase in surface oxygen content and relatively small increase in surface fluorine content. However, as the treatment time further increases, the contact angle increases and T-peel strength decreases accompanied by a large increase of fluorine content and a relatively small increase in surface oxygen content. In addition, the surface roughness continuously increases
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