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ceramics complex material treated by microwave plasma with XPS analysis
Surface and Coatings Technology 131 Ž2000. 294᎐299
Study of the surface-modified Teflonrceramics complex material treated by microwave plasma with XPS analysis R.Q. LiangU , X.B. Su, Q.C. Wu, F. Fang Institute of Plasma Physics, Chinese Academy of Sciences, Box 1126, Hefei 230031, PR China
Abstract A large volume microwave plasma source has been developed in our laboratory. The plasma is generated in a Pyrex glass cylinder with an inner diameter of 30 cm and a height of 50 cm, and the microwave power is coupled into the cylinder by eight equally spaced slot antennae on the inside wall of annular waveguide. The apparatus has been applied to surface modification of the material of PTFErceramic complex dielectrics ŽPTFEC.. The chemical composition of the modified PTFEC was investigated by X-ray photoelectron spectroscopy ŽXPS.. Under plasma treatment, the oxygen content of the surface is found to increase and fluorine content to decrease. XPS analysis proved that interaction between different plasma and PTFEC surface layer exists in etching, cross-linking, oxidizing simultaneously, and especially oxygen plasma etching. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Microwave plasma; Annular waveguide; Slot antennas; Surface modification of materials; XPS
1. Introduction A microwave plasma source that can produce large volume plasma w1,2x has been developed in our laboratory successfully. By this plasma source, the study of surface modification of PTFECrceramics complex material has been carried out. PTFE is a kind of high molecular polymer with white color, non-polarizability, crystal structure and straight chain. Its chemical property is very stable. It has excellent performances on wear resistance, corrosion protection, thermal machine and electronics. Teflonr ceramics complex dielectrics ŽPTFEC. consist of PTFE filled with fine ceramics. Its features include large
dielectric constant and small electromagnetic dissipation and it is easy to mechanically process, making it an ideal material to apply as electronics materials, especially microwave dielectrics w3x. But the surface energy of PTFEC is so low that its hydrophilicity and adhesivity does not satisfy the requirement for industry application. Treated by microwave plasma, the energy and chemical components of the surface of PTFEC can be changed. The surface energy will be raised, resulting in the improvement of hydrophilicity and adhesivity, but its intrinsic advantages will be kept w4᎐7x.
2. Experiment 2.1. Plasma source
U
Corresponding author.
The microwave plasma source is depicted schematically in Fig. 1. The large volume 2.45-GHz discharge
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 7 9 5 - 7
R.Q. Liang et al. r Surface and Coatings Technology 131 (2000) 294᎐299
295
Fig. 1. Schematic diagram of the microwave system.
Fig. 3. The radial profiles of ion density under different gas pressures ŽPi s 500 W, Zs 1.0 cm.. Fig. 2. Schematic diagram of the vacuum chamber.
was excited by the slotted antennae microwave source. The microwave power is coupled into the cylinder to produce plasma through eight slot antennae equidistantly positioned on the inside wall of an annular waveguide Žring cavity.. The width and length of slots are 2.0 mm and 58 mm, respectively, and the distance between two closed slots is the same as the microwave wavelength in the annular waveguide. The ring cavity surrounds the plasma chamber of Pyrex cylinder with
an inner diameter of 30 cm and a height of 50 cm Žsee Fig. 2.. Microwave power coupled to plasma was varied from 200 to 600 W and the pressure of working gas was ranged from 40 to 600 Pa. Langmuir double-probe measurements of the electron density and temperature are performed for argon discharge. The electron temperature generally remained between 0.5 and 3 eV. The maximum ion density can reach 6 = 10 10 cmy3 . Fig. 3 shows the profiles of plasma density under different gas pressures.
Table 1 Concentrations of various elements at PTFEC surface Sample no.
Treating condition
C1s Žat.%.
F1s Žat.%.
O1s Žat.%.
Si2p3 Žat.%.
Ti2p3 Žat.%.
S0 S1 S2 S3 S4 S5 S6
Untreated Ar: 4 min, 500 W Air: 4 min, 500 W AA: 4 min, 500 W O2 : 4 min, 500 W S4 deposited in air for 6 days O2 : 0.5 min, 500 W
34.03 33.09 43.65 34.46 18.5 24.89 35.27
63.95 41.89 40.07 35.82 30.44 47.19 53.08
1.75 19.04 14.89 24.10 37.83 23.10 10.35
0.0 6.1 1.1 5.51 11.4 4.2 1.2
0.28 0.07 0.26 0.12 0.61 0.63 0.21
R.Q. Liang et al. r Surface and Coatings Technology 131 (2000) 294᎐299
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The radial variation of plasma density was found to be less than 25% within a diameter of 16 cm under different gas pressure.
3. Results and discussion The concentration of the chemical components on
Fig. 5. Temporal evolution of the contact degree of water on the surface of the samples treated by plasma.
Fig. 4. XPS survey scan of PTFEC.
PTFEC surface both before and after plasma treating were analyzed by X-ray photoelectron spectroscopy ŽXPS. using the Model of England VG ESCALAB MK II. Fig. 4 shows the full spectra of XPS scanning on PTFEC surface treated under different conditions. From the peak area and sensitive factor, the concentration of the components in PTFEC surface may be known by calculating the sensitive factors of C1s Ž284.9 eV., F1s Ž684.9 eV., O1s Ž533.1 eV., Ti2p3 Ž453.8 eV. and Si2p3 Ž103.5 eV. are 0.25, 1.00, 0.66, 1.20 and 0.27, respectively. The calculated results are shown as Table 1. XPS analysis shows there are rather weak O1s and Ti2P3 peaks in untreated sample besides the dominant peaks of C1s and F1s, which indicates there is little TiO 2 existing in the surface. The components of PTFEC include Al 2 O 3 and SiO 2 , but we didn’t find them in the XPS spectrum. The reason is that XPS can only examine the range of maximum depth beneath the material surface around 3᎐5 nm, but no Al 2 O 3 or SiO 2 exists in the surface layer. After the surfaces were treated by Ar, O 2 , air and AA Žacrylic acid. plasmas, the O1s peak became apparently stronger and F1s became weaker. At the same time Si2p3 peak emerged. Among four kinds of plasma discharged in different gases mentioned above, the treatment with O 2 plasma is most effective, which is because oxy-group oxidized PTFEC surface and the effect of etching on PTFEC surface made SiO 2 exposure to surface layer. The samples S4᎐S6 were treated by oxygen plasma with 500 W microwave power, and the plasma treating times of 4.0 min and 0.5 min, respectively. From Fig. 3 and Table 1, it may be seen that as the treating time increased, the concentrations of carbon and fluorine were found to decrease further, whereas oxygen and Si2p3 increased. This verified that oxygen plasma could be used for etching. Samples S4 and S5 are both one-specimen with different periods during which the treated sample is exposed to air. XPS analysis results
R.Q. Liang et al. r Surface and Coatings Technology 131 (2000) 294᎐299
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Table 2 Component of Ža. C1s and Žb. O1s spectra of PTFEC surface Peak no.
Sample no.
(a) C1s spectra C1 at.% Center FWHM
S0
S1
S2
S3
S4
S5
S6
9.8 284.4 2.2
8.4 285.2 1.7
6.84 285.0 1.6
25.5 285.6 1.5
11.2 285.7 1.4
4.6 285.0 1.5
6.4 285.2 1.5
C2
at.% Center FWHM
᎐ ᎐ ᎐
10.5 286.6 1.5
26.0 286.3 1.6
12.1 286.8 1.5
7.2 286.5 1.5
6.5 286.4 1.5
7.9 286.6 1.5
C3
at.% Center FWHM
᎐ ᎐ ᎐
12.3 288.0 1.5
9.1 288.45 1.5
6.8 288.0 1.5
5.24 287.8 1.5
3.7 288.2 1.5
8.2 288.5 1.5
C4
at.% Center FWHM
᎐ ᎐ ᎐
᎐ ᎐ ᎐
11.6 287.6 1.5
9.5 289.0 1.7
᎐ ᎐ ᎐
᎐ ᎐ ᎐
᎐ ᎐ ᎐
C5
at.% Center FWHM
᎐ ᎐ ᎐
11.6 289.6 1.5
8.6 289.5 1.5
6.8 289.8 1.7
7.0 290.0 1.7
5.4 289.6 1.5
8.5 289.5 1.5
C6
at.% Center FWHM
90.2 292.3 1.75
47.7 292.5 1.7
31.5 292.4 1.8
30.3 292.45 1.7
58.5 292.5 1.7
69.6 292.5 1.7
58.8 292.5 1.7
C7
at.% Center FWHM
᎐ ᎐ ᎐
9.6 294.2 1.7
6.1 294.45 1.5
7.1 294.25 1.7
10.9 294.1 1.7
10.2 293.3 1.7
9.1 294.35 1.7
100 532.75 2.75
13.8 532.1 2.00
8.5 531.4 1.8
6.8 531.3 1.5
5.6 531.1 1.7
9.0 531.1 2.00
9.5 531.2 1.5
(b) O1s spectra O1 at.% Center FWHM O2
at.% Center FWHM
᎐ ᎐ ᎐
81.6 533.7 2.00
86.5 533.5 1.6
91.1 533.4 2.3
94.4 533.7 2.0
91.0 533.6 2.2
83.9 533.3 2.6
O3
at.% Center FWHM
᎐ ᎐ ᎐
14.6 535.7 2.00
5.0 535.5 2.45
3.1 535.3 1.5
᎐ ᎐ ᎐
᎐ ᎐ ᎐
6.7 535.6 1.8
show that the longer the period is, the higher the concentration of carbon and fluorine on the surface becomes, but the concentration of oxygen decreases. The change of the concentration may explain why the
hydrophilicity and adhesivity of PTFEC decrease when the sample is exposed to air after being treated. The temporal evolution of the contact angle of water on the treated surface is shown in Fig. 5. The contact angle
Table 3 Possible carbon functional group corresponding binding energy Binding energy ŽeV.
285.0
286.6" 0.2
287.8" 0.2
289.0" 0.2
289.8" 0.2
292.2" 0.2
294.1" 0.2
Possible groups
C᎐CŽH.
C᎐O C᎐OH
CsO O᎐C᎐O
O᎐CsO COOH
᎐CF᎐
᎐CF2᎐ ᎐CF᎐O᎐
᎐CF3 ᎐CF2᎐O᎐
298
R.Q. Liang et al. r Surface and Coatings Technology 131 (2000) 294᎐299
Fig. 6. The spectrums obtained of sample 2 are deconvoluted into multiple sub-peaks of C1s and O1s contained in different functional group using Gaussian᎐Lorentzian fit.
changing from small to large indicates that the surface hydrophilicity is getting worse gradually along the time of sample deposited in air. In order to survey the variation of concentration on PTFEC in more detail, the fine scanning of spectra to the C1s and O1s peaks in Fig. 3 has been done. The spectra ŽFig. 6. obtained are deconvoluted into multiple sub-peaks of C1s and O1s contained in different functional groups using Gaussian᎐Lorentzian fit. From the area and position of each sub-peak, the concentration of each chemical component with C1s or O1s were calculated Žsee Tables 2 and 3.. Table 3 shows the possible carbon group corresponding to the bind energy of C1s in Table 2a. The binding energy of O1s in Table 2b correspond to the possible functional groups as follows w8x: 532.00᎐532.40 eV vs. CsO, 533.60᎐533.70 eV vs. C᎐O, 534.80᎐535.00 eV vs. ᎐CF᎐O᎐. For S0 Žuntreated sample., ᎐CF2᎐ occupies 90.2% in total of carbon group, the rest is C᎐CŽH.. The data in Table 2 indicate that after plasma treatment, the concentration of ᎐CF2 decreased, and C᎐C ŽH., ᎐CF᎐ and oxy-group increased at same time. The results indicate that plasma treating to PTFEC has the functions of defluorine, and oxy-group was guided into the molecular chain on PTFEC surface. The lowest concentration of ᎐CF2 ᎐ in samples S1᎐S4 was the one treated by AA plasma. The possible reason may be the reaction of graft taking place in PTFEC surface by AA plasma. Samples S4 and S5 show that with an increase in
exposure time to air after sample being treated, ᎐CF2 ᎐ increased, and at the same time the oxy-group decreased. This means that: Ž1. after the sample was treated there are still much active radicals remaining on PTFEC surface, these radicals may continuously cause chemical reactions; and Ž2. due to free rotation of high molecular bonds, the oxy-group, which is hydrophilic, rotated into the inside of the polymer. Samples S4 and S6 show that with the exposure time increasing after samples being treated, ᎐CF2 ᎐ decreased, ᎐CF᎐, oxy-group increased, and the hydrophilic performance was improved. The mechanism of surface modification by plasma is a process of interaction between the polymer surface and plasma in which there are an amount of positive and negative ions, metastable molecules, excited atoms, and wide band electromagnetic waves. The energy of some particles in a plasma may exceed the binding energy of chemical bonds in PTFEC. In the plasma-treating process, many particles bombard the PTFEC surface, some large molecular bonds are broken and become radicals on the surface. These radicals will further react with small excited molecules. So plasma modification in polymer surfaces may be considered a process of energy exchange caused by the energetic plasma particles impinging upon the treated surface to produce an amount of large molecular radicals.
4. Conclusion
XPS analysis indicates the following: Ž1. After plasma treatment the concentration of fluorine on the PTFEC surface decreased, and that of oxygen increased. There are different polarized oxygen functional groups appeared on PTFEC surface. With the treating time and microwave power increasing, ᎐CF2 ᎐ functional group decreased with oxygen functional group increased simultaneously. Ž2. The mechanism of microwave plasma modification to PTFEC surface can be considered as the process of energy exchange caused by the various fractures in plasma impinging on the PTFEC surface. With the process of energy exchange the reaction of large molecular radical occurs, oxygen-functional groups are produced in the surface, and small molecules are removed out from the surface. This leads to PTFEC surface polarization to be enhanced, so the hydrophilicity and adhesivity are improving. Ž3. The interaction between PTFEC surface and the plasma generated by different gas discharge results in the effects of etching, defluorine, crosslink, and oxidizing. In particular, the plasma of oxide gas gives a maximum activation of PTFEC surface.
R.Q. Liang et al. r Surface and Coatings Technology 131 (2000) 294᎐299
Acknowledgements We thank the National Nature Science Foundation of China for financial support for this study Žgrant no. 19875055.. References w1x F. Werner, D. Korzec, J. Engemann, J. Vac. Sci. Technol. A 14 Ž1996. 3065. w2x M. Moisan et al., J. Microwave Power Electromagnetic Energy 30 Ž1995. 58.
299
w3x H.Q. Zhou, M. Liu, N.R. Yang, Functional Mater. Žin Chinese. 28 Ž1997. 78. w4x J.W. Brennan, J.W. Feast, Polymer 32 Ž1991. 1527. w5x T.A. Groux, S.L. Cooper, J. Appl. Polym. Sci. 43 Ž1991. 145. w6x H.R. Hansen, J. Polym. Sci. A 3 Ž1984. 2205. w7x R.F. Zhang, Y.L. Yu, X.S. Liu et al., J. High Molec. Žin Chinese. 3 Ž1991. 293. w8x J.Q. Wang, W.H. Wu, D.M. Feng, Introduction to Electronic Energy SpectroscopyŽXPSrXAESrUPS. Žin Chinese., N.D. Industry Press, Beijing, 1992.
Study of the surface-modified Teflonrceramics complex material treated by microwave plasma with XPS analysis R.Q. LiangU , X.B. Su, Q.C. Wu, F. Fang Institute of Plasma Physics, Chinese Academy of Sciences, Box 1126, Hefei 230031, PR China
Abstract A large volume microwave plasma source has been developed in our laboratory. The plasma is generated in a Pyrex glass cylinder with an inner diameter of 30 cm and a height of 50 cm, and the microwave power is coupled into the cylinder by eight equally spaced slot antennae on the inside wall of annular waveguide. The apparatus has been applied to surface modification of the material of PTFErceramic complex dielectrics ŽPTFEC.. The chemical composition of the modified PTFEC was investigated by X-ray photoelectron spectroscopy ŽXPS.. Under plasma treatment, the oxygen content of the surface is found to increase and fluorine content to decrease. XPS analysis proved that interaction between different plasma and PTFEC surface layer exists in etching, cross-linking, oxidizing simultaneously, and especially oxygen plasma etching. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Microwave plasma; Annular waveguide; Slot antennas; Surface modification of materials; XPS
1. Introduction A microwave plasma source that can produce large volume plasma w1,2x has been developed in our laboratory successfully. By this plasma source, the study of surface modification of PTFECrceramics complex material has been carried out. PTFE is a kind of high molecular polymer with white color, non-polarizability, crystal structure and straight chain. Its chemical property is very stable. It has excellent performances on wear resistance, corrosion protection, thermal machine and electronics. Teflonr ceramics complex dielectrics ŽPTFEC. consist of PTFE filled with fine ceramics. Its features include large
dielectric constant and small electromagnetic dissipation and it is easy to mechanically process, making it an ideal material to apply as electronics materials, especially microwave dielectrics w3x. But the surface energy of PTFEC is so low that its hydrophilicity and adhesivity does not satisfy the requirement for industry application. Treated by microwave plasma, the energy and chemical components of the surface of PTFEC can be changed. The surface energy will be raised, resulting in the improvement of hydrophilicity and adhesivity, but its intrinsic advantages will be kept w4᎐7x.
2. Experiment 2.1. Plasma source
U
Corresponding author.
The microwave plasma source is depicted schematically in Fig. 1. The large volume 2.45-GHz discharge
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 7 9 5 - 7
R.Q. Liang et al. r Surface and Coatings Technology 131 (2000) 294᎐299
295
Fig. 1. Schematic diagram of the microwave system.
Fig. 3. The radial profiles of ion density under different gas pressures ŽPi s 500 W, Zs 1.0 cm.. Fig. 2. Schematic diagram of the vacuum chamber.
was excited by the slotted antennae microwave source. The microwave power is coupled into the cylinder to produce plasma through eight slot antennae equidistantly positioned on the inside wall of an annular waveguide Žring cavity.. The width and length of slots are 2.0 mm and 58 mm, respectively, and the distance between two closed slots is the same as the microwave wavelength in the annular waveguide. The ring cavity surrounds the plasma chamber of Pyrex cylinder with
an inner diameter of 30 cm and a height of 50 cm Žsee Fig. 2.. Microwave power coupled to plasma was varied from 200 to 600 W and the pressure of working gas was ranged from 40 to 600 Pa. Langmuir double-probe measurements of the electron density and temperature are performed for argon discharge. The electron temperature generally remained between 0.5 and 3 eV. The maximum ion density can reach 6 = 10 10 cmy3 . Fig. 3 shows the profiles of plasma density under different gas pressures.
Table 1 Concentrations of various elements at PTFEC surface Sample no.
Treating condition
C1s Žat.%.
F1s Žat.%.
O1s Žat.%.
Si2p3 Žat.%.
Ti2p3 Žat.%.
S0 S1 S2 S3 S4 S5 S6
Untreated Ar: 4 min, 500 W Air: 4 min, 500 W AA: 4 min, 500 W O2 : 4 min, 500 W S4 deposited in air for 6 days O2 : 0.5 min, 500 W
34.03 33.09 43.65 34.46 18.5 24.89 35.27
63.95 41.89 40.07 35.82 30.44 47.19 53.08
1.75 19.04 14.89 24.10 37.83 23.10 10.35
0.0 6.1 1.1 5.51 11.4 4.2 1.2
0.28 0.07 0.26 0.12 0.61 0.63 0.21
R.Q. Liang et al. r Surface and Coatings Technology 131 (2000) 294᎐299
296
The radial variation of plasma density was found to be less than 25% within a diameter of 16 cm under different gas pressure.
3. Results and discussion The concentration of the chemical components on
Fig. 5. Temporal evolution of the contact degree of water on the surface of the samples treated by plasma.
Fig. 4. XPS survey scan of PTFEC.
PTFEC surface both before and after plasma treating were analyzed by X-ray photoelectron spectroscopy ŽXPS. using the Model of England VG ESCALAB MK II. Fig. 4 shows the full spectra of XPS scanning on PTFEC surface treated under different conditions. From the peak area and sensitive factor, the concentration of the components in PTFEC surface may be known by calculating the sensitive factors of C1s Ž284.9 eV., F1s Ž684.9 eV., O1s Ž533.1 eV., Ti2p3 Ž453.8 eV. and Si2p3 Ž103.5 eV. are 0.25, 1.00, 0.66, 1.20 and 0.27, respectively. The calculated results are shown as Table 1. XPS analysis shows there are rather weak O1s and Ti2P3 peaks in untreated sample besides the dominant peaks of C1s and F1s, which indicates there is little TiO 2 existing in the surface. The components of PTFEC include Al 2 O 3 and SiO 2 , but we didn’t find them in the XPS spectrum. The reason is that XPS can only examine the range of maximum depth beneath the material surface around 3᎐5 nm, but no Al 2 O 3 or SiO 2 exists in the surface layer. After the surfaces were treated by Ar, O 2 , air and AA Žacrylic acid. plasmas, the O1s peak became apparently stronger and F1s became weaker. At the same time Si2p3 peak emerged. Among four kinds of plasma discharged in different gases mentioned above, the treatment with O 2 plasma is most effective, which is because oxy-group oxidized PTFEC surface and the effect of etching on PTFEC surface made SiO 2 exposure to surface layer. The samples S4᎐S6 were treated by oxygen plasma with 500 W microwave power, and the plasma treating times of 4.0 min and 0.5 min, respectively. From Fig. 3 and Table 1, it may be seen that as the treating time increased, the concentrations of carbon and fluorine were found to decrease further, whereas oxygen and Si2p3 increased. This verified that oxygen plasma could be used for etching. Samples S4 and S5 are both one-specimen with different periods during which the treated sample is exposed to air. XPS analysis results
R.Q. Liang et al. r Surface and Coatings Technology 131 (2000) 294᎐299
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Table 2 Component of Ža. C1s and Žb. O1s spectra of PTFEC surface Peak no.
Sample no.
(a) C1s spectra C1 at.% Center FWHM
S0
S1
S2
S3
S4
S5
S6
9.8 284.4 2.2
8.4 285.2 1.7
6.84 285.0 1.6
25.5 285.6 1.5
11.2 285.7 1.4
4.6 285.0 1.5
6.4 285.2 1.5
C2
at.% Center FWHM
᎐ ᎐ ᎐
10.5 286.6 1.5
26.0 286.3 1.6
12.1 286.8 1.5
7.2 286.5 1.5
6.5 286.4 1.5
7.9 286.6 1.5
C3
at.% Center FWHM
᎐ ᎐ ᎐
12.3 288.0 1.5
9.1 288.45 1.5
6.8 288.0 1.5
5.24 287.8 1.5
3.7 288.2 1.5
8.2 288.5 1.5
C4
at.% Center FWHM
᎐ ᎐ ᎐
᎐ ᎐ ᎐
11.6 287.6 1.5
9.5 289.0 1.7
᎐ ᎐ ᎐
᎐ ᎐ ᎐
᎐ ᎐ ᎐
C5
at.% Center FWHM
᎐ ᎐ ᎐
11.6 289.6 1.5
8.6 289.5 1.5
6.8 289.8 1.7
7.0 290.0 1.7
5.4 289.6 1.5
8.5 289.5 1.5
C6
at.% Center FWHM
90.2 292.3 1.75
47.7 292.5 1.7
31.5 292.4 1.8
30.3 292.45 1.7
58.5 292.5 1.7
69.6 292.5 1.7
58.8 292.5 1.7
C7
at.% Center FWHM
᎐ ᎐ ᎐
9.6 294.2 1.7
6.1 294.45 1.5
7.1 294.25 1.7
10.9 294.1 1.7
10.2 293.3 1.7
9.1 294.35 1.7
100 532.75 2.75
13.8 532.1 2.00
8.5 531.4 1.8
6.8 531.3 1.5
5.6 531.1 1.7
9.0 531.1 2.00
9.5 531.2 1.5
(b) O1s spectra O1 at.% Center FWHM O2
at.% Center FWHM
᎐ ᎐ ᎐
81.6 533.7 2.00
86.5 533.5 1.6
91.1 533.4 2.3
94.4 533.7 2.0
91.0 533.6 2.2
83.9 533.3 2.6
O3
at.% Center FWHM
᎐ ᎐ ᎐
14.6 535.7 2.00
5.0 535.5 2.45
3.1 535.3 1.5
᎐ ᎐ ᎐
᎐ ᎐ ᎐
6.7 535.6 1.8
show that the longer the period is, the higher the concentration of carbon and fluorine on the surface becomes, but the concentration of oxygen decreases. The change of the concentration may explain why the
hydrophilicity and adhesivity of PTFEC decrease when the sample is exposed to air after being treated. The temporal evolution of the contact angle of water on the treated surface is shown in Fig. 5. The contact angle
Table 3 Possible carbon functional group corresponding binding energy Binding energy ŽeV.
285.0
286.6" 0.2
287.8" 0.2
289.0" 0.2
289.8" 0.2
292.2" 0.2
294.1" 0.2
Possible groups
C᎐CŽH.
C᎐O C᎐OH
CsO O᎐C᎐O
O᎐CsO COOH
᎐CF᎐
᎐CF2᎐ ᎐CF᎐O᎐
᎐CF3 ᎐CF2᎐O᎐
298
R.Q. Liang et al. r Surface and Coatings Technology 131 (2000) 294᎐299
Fig. 6. The spectrums obtained of sample 2 are deconvoluted into multiple sub-peaks of C1s and O1s contained in different functional group using Gaussian᎐Lorentzian fit.
changing from small to large indicates that the surface hydrophilicity is getting worse gradually along the time of sample deposited in air. In order to survey the variation of concentration on PTFEC in more detail, the fine scanning of spectra to the C1s and O1s peaks in Fig. 3 has been done. The spectra ŽFig. 6. obtained are deconvoluted into multiple sub-peaks of C1s and O1s contained in different functional groups using Gaussian᎐Lorentzian fit. From the area and position of each sub-peak, the concentration of each chemical component with C1s or O1s were calculated Žsee Tables 2 and 3.. Table 3 shows the possible carbon group corresponding to the bind energy of C1s in Table 2a. The binding energy of O1s in Table 2b correspond to the possible functional groups as follows w8x: 532.00᎐532.40 eV vs. CsO, 533.60᎐533.70 eV vs. C᎐O, 534.80᎐535.00 eV vs. ᎐CF᎐O᎐. For S0 Žuntreated sample., ᎐CF2᎐ occupies 90.2% in total of carbon group, the rest is C᎐CŽH.. The data in Table 2 indicate that after plasma treatment, the concentration of ᎐CF2 decreased, and C᎐C ŽH., ᎐CF᎐ and oxy-group increased at same time. The results indicate that plasma treating to PTFEC has the functions of defluorine, and oxy-group was guided into the molecular chain on PTFEC surface. The lowest concentration of ᎐CF2 ᎐ in samples S1᎐S4 was the one treated by AA plasma. The possible reason may be the reaction of graft taking place in PTFEC surface by AA plasma. Samples S4 and S5 show that with an increase in
exposure time to air after sample being treated, ᎐CF2 ᎐ increased, and at the same time the oxy-group decreased. This means that: Ž1. after the sample was treated there are still much active radicals remaining on PTFEC surface, these radicals may continuously cause chemical reactions; and Ž2. due to free rotation of high molecular bonds, the oxy-group, which is hydrophilic, rotated into the inside of the polymer. Samples S4 and S6 show that with the exposure time increasing after samples being treated, ᎐CF2 ᎐ decreased, ᎐CF᎐, oxy-group increased, and the hydrophilic performance was improved. The mechanism of surface modification by plasma is a process of interaction between the polymer surface and plasma in which there are an amount of positive and negative ions, metastable molecules, excited atoms, and wide band electromagnetic waves. The energy of some particles in a plasma may exceed the binding energy of chemical bonds in PTFEC. In the plasma-treating process, many particles bombard the PTFEC surface, some large molecular bonds are broken and become radicals on the surface. These radicals will further react with small excited molecules. So plasma modification in polymer surfaces may be considered a process of energy exchange caused by the energetic plasma particles impinging upon the treated surface to produce an amount of large molecular radicals.
4. Conclusion
XPS analysis indicates the following: Ž1. After plasma treatment the concentration of fluorine on the PTFEC surface decreased, and that of oxygen increased. There are different polarized oxygen functional groups appeared on PTFEC surface. With the treating time and microwave power increasing, ᎐CF2 ᎐ functional group decreased with oxygen functional group increased simultaneously. Ž2. The mechanism of microwave plasma modification to PTFEC surface can be considered as the process of energy exchange caused by the various fractures in plasma impinging on the PTFEC surface. With the process of energy exchange the reaction of large molecular radical occurs, oxygen-functional groups are produced in the surface, and small molecules are removed out from the surface. This leads to PTFEC surface polarization to be enhanced, so the hydrophilicity and adhesivity are improving. Ž3. The interaction between PTFEC surface and the plasma generated by different gas discharge results in the effects of etching, defluorine, crosslink, and oxidizing. In particular, the plasma of oxide gas gives a maximum activation of PTFEC surface.
R.Q. Liang et al. r Surface and Coatings Technology 131 (2000) 294᎐299
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