Chapter 4 Results and discussion
4.1 Characteristics of fluorination on PET
4.1.1 Wetting property and morphology of fluorinated PET.
Hydrophobicity was the index to determine the wetting property, and the evaluation of hydrophobicity could be made through water contact angle measurements. The original contact angle of PET utilized in this thesis was about 64.5° while a significant increment to 107.7° after CF4 plasma treatment for 5 minutes at power 60W which indicated the surface became hydrophobic (contact angle>90°) as shown in Fig. 4-1. To understand the dependence of treatment time and hydrophobic characteristic, we extended treatment time was performed from 5 to 15 minutes. Fig. 4-2 shown the contact angle versus different treatment time, the value seemed to be saturated at 106~107°. This result indicated that the PET surface with treatment time within 15 minutes had the same hydrophobicity.
Fig. 4-1 Comparison of contact angle between the (a) untreated PET, and (b) 60W 5 minutes CF4 plasma treatment.
(a) (b)
0 5 10 15 60
70 80 90 100 110 120
contact angle(degree)
treatment time (min)
Fig. 4-2 The relation between plasma treatment time and contact angle.
To learn more about the morphology of CF4 plasma modified PET, the scanning probe microscopy (SPM) had been then used in this work by evaluating the surface area 5×5 µm2. Fig. 4-3 showed the surface morphology of untreated PET with height scale 20 nm and the roughness was only 0.34 nm. The roughness increased to 1.01 nm after CF4 plasma treatment for 15 minutes at power 60W. The variation in surface roughness of treated PET as a function of treatment time was exhibited in Fig. 4-4. Revealed by literature studies, the CF4
plasma modification was described as the sum of two mechanisms: ion etching and fluorination, and the two reactions seemed to be competitive and parallel. The ion etching action had been suggested to be mostly due to ion bombardment which will roughen the surface. [65]
Fig. 4-3(a) The surface roughness of untreated PET, and (b) 15 minutes treatment.
(a) (b)
0 5 10 15 0.2
0.4 0.6 0.8 1.0
surface roughness(nm)
treatment time(min)
Fig. 4-4 The variation in surface roughness as a function of treatment time.
As mention in previous chapter, contact angle would be affected with roughness factor.
However, this phenomenon didn’t appeal in our case because the increment of surface roughness was only 0.67 nm. The slight increase in roughness may attribute to the pure CF4 as the gas source. Several researches concluded that the addition of other gas source will enhance the ion etching process. [66, 67]
4.1.2 Chemical structure and depth of plasma fluorination zone
The chemical structure of plasma fluorinated PET was justified by the resolution of the X-ray Photoelectron Spectroscopy (XPS) spectra. Typically the XPS spectra of PET C 1s had three main peaks shown in Fig. 4-5 and table 4.1.1 C–C/C–H at 284.6eV, C–O at 286.4 eV, O=C at 288.7 eV. After CF4 plasma treatment for 60w 5min, there was a dramatic change exhibited on the XPS spectra which could be attributed to formation of fluorinate carbon functionalities: CF at 289.3 eV, CF2 at 291.1 eV and CF3 at 293.1 eV.[68,69] These results indicated that both the C-C、C-H and C=O group were all likely effected by fluorination. As shown in O (1s) spectra in Fig 4-6 and table 4-2, C=O bond at decreased in the proportion while C-O bond increased relatively. An extra peak at 534.7 eV was assigned as C(O)F. This
peak didn’t appear in the C 1s spectra due to the relative low concentration of C(O)FX group, and the peak position was within the CF2 and CF3 boundary.[70,71] Energy provided by RF power generated the fluorine F* and CFn (n =1, 2, 3) radicals which were easily reacted with PET. The mechanism of plasma etching and surface fluorination was proposed as follows.
Substitution of hydrogen usually initiated with hydrogen abstraction: R-H+F*→R*+HF.
Chain scission in ester group also leaded to the formation of radicals. The fluorine radical generated by CF4 plasma subsequently undergoes fluorination reactions by addition of different fluorinated radical. XPS spectra indicated that 26% of PET component were affected with fluorinated radical for treatment power at 60W for 5 min, and this number evaluated from different treatment time shown within a range 24~26%. This message revealed that the degree of surface fluorination were almost the same during the treatment time 5 to 15 minutes.
The depth of the XPS measurement was about 5 nm in this study, and the component ratio within the detected zone were almost the same for different treatment time. Since the increase in roughness was unobvious during the plasma treatment, the similar hydrophobicity on the surface of PET revealed by contact angle measurement was controlled by the degree of fluorination. Furthermore, we used XRR to quantify the fluorination zone, and the results were arranged in table 4-3. After the CF4 plasma treatment, XPS showed the surface of PET formed a “teflon like” layer. For 5 minutes treatment, a dense layer formed on the top of PET with thickness 22 nm while the density varied from 1.42 to 1.68 g /cm3. The weight increment by fluorination is due to replacement of hydrogen by fluorine in the fluorinated layer. [46,72]
Literature research indicated that the density of polymers was increased greatly under fluorination and usually varied over the range 1.6~2 g /cm3. [46] Corresponding with the extended treatment time, the depth of the fluorinated zone increased from 22 nm to 47 nm as the treatment time varied from 5 to 15 minutes. XRR results revealed that the fluorination wasn’t limited to the surface directly exposed to plasma. [46,73]
Fig. 4-5 The C 1s spectra of (a)untreated PET(b) CF4 plasma treated PET with 60w for 5min.
Table 4-1 The component ratio of untreated and modified PET based on C 1s spectra.
C-C, C-H
C-O C=O C-CFn CF2 CF3
Bonding energies, eV
284.6 286.4 288.7 289.3 291.1 293.1
Untreated PET 46% 27.8% 26.2% － － －
60W 5 min 33.5% 24.1% 16.2% 8.1% 12.5% 5.6%
60W 10 min 33.8% 22.9% 15.8% 5.8% 13.2% 8.5%
60W 15 min 36.3% 22.6% 16.2% 7.5% 11.5% 5.9%
Fig. 4-6 The O 1s spectra of (a) untreated PET (b) CF4 plasma treated PET with 60w for 5min.
(a) (b)
(a) (b)
Table 4-2 The component ratio of untreated and modified PET based on O 1s spectra.
Table 4-3 Density and depth of fluorination zone measured by X-ray Reflectivity (XRR).
Fluorination