Arab Journal of Nuclear Science and Applications, 46(5), (106-114) 2013
Structural and Thermal Properties of PTFE Films by Argon and Oxygen
Plasma
A. Atta, H. E. Ali *
Radiation Physics Department, National Center for Radiation Research and Technology
(NCRRT), AEA, Cairo, Egypt
* Radiation chemistry Department, National Center for Radiation Research and Technology
(NCRRT), AEA, Cairo, Egypt
Received 20/11/2012
Accepted 21/5/2013
ABSTRACT
Structural and thermal properties of polytetrafluoroethylene (PTFE) modified by
argon and oxygen plasma. The effects of plasma treatment on physico-chemical
properties of PTFE surface have been determined by different characterization
techniques viz, Scanning electron Microscopy (SEM), Fourier transform infrared
spectroscopy (FTIR), X-ray diffraction technique (XRD), and thermal gravimetric
analysis (TGA). FTIR analysis revealed the defluorination of PTFE modified by
plasma due to the rupture of C–C and C–F bonds. The Plasma beam irradiation
induce chain scission in the polymer, which resulted in the formation of relatively
stable free radical, reduction in thermal stability and decrease in crystallinity of the
polymer.
Keyword: PTFE films, Plasma irradiation, FTIR, XRD, TGA.
INTRODUCTION
Polytetrafluoro ethylene (PTFE) belongs to the family of fluoro-plastics, it is a linear polymer
with no branches and is highly crystalline having melting point of 330 ◦C. It has a very low dielectric
constant. It’s highly electronegativity is attributed to the presence of fluorine in the backbone structure
of the polymer. Fluorinated polymers are widely used as engineering plastics due to their attractive
combination of mechanical properties, chemical inertness, heat resistance and low coefficient of
friction (1)
.
Polytetrafluoroethylene (PTFE) is a polymer used as insulator in cables, connector assemblies
and for printed circuit boards because of its good dielectric properties (2) . Plasma beam bombardment
induces formation and transport of reactive species, which are able to permanently change the
electronic and chemical properties of polymers. The effectiveness of these changes produced in the
polymers depends on the structure of the polymer along with the experimental conditions of ion
implantation like ion energy, fluence and beam current(3). Surface modification of polymer materials
has been of great interest in the past years because of an ever growing application potential in the
fields of materials science, electronics and biomedical physics. Surface modification technology
allows for the change and improvement of the property of a material, consequently making the
processed material more useful in various aspects (4). When PTFE polymer is exposed to RF plasma
irradiation, physical and chemical changes occurred (5). The degree to which the radiation will affect
the polymer depends on many factors, such as the chemical structure, morphology, and plasma
irradiation conditions. Irradiation was used to induce degradation in many fluorinated polymers and
the degraded parts were used for many applications(6-8) . As a consequence of the widespread use of
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Arab Journal of Nuclear Science and Applications, 46(5), (106-114) 2013
degraded fluorinated polymers, the effect of irradiation on structural changes of PTFE needs to be
investigated.
In the present work, an attempt has been made to induce certain modiffcations in PTFE in
order to enhance its applicability. Here, the intention of this study is to investigate the effect of argon
and oxygen exposure times on the PTFE films using SEM, FTIR, XRD and TGA techniques.
EXPRIMENTAL WORK
Irradiation Facilities:
The PTFE films are exposed to argon and oxygen plasma in the reactive ion beam etching
(RIE) system operating at a frequency of 13.56 mHz at National Laboratory of Advanced Technology
and NanoScience (INFM-TASC), Trieste, Italy. The electrodes in this machine are 20cm×12cm and
their spacing is 4 cm. The top electrode is connected to RF power, whereas the bottom electrode which
holds the sample, is connected to ground. The substrate holder is placed on the bottom electrode and
its temperature is kept low during the process using circulating cold water. Oxygen plasma parameters
are the working pressure is 7.3E-3 mbar; Oxygen gas flow rate 25 sccm; and the incident power were
kept constant at 100 W with a corresponding DC self-bias voltage – 275 V and the exposure time was
varied from 0 to 4 minutes. For argon plasma the pressure is 6.8E-3 mbar; gas flow rate 20 sccm;
Power 100W and exposure time from 0 to 4 minutes.
Films Preparation and Characterization Techniques:
PTFE films with thickness 100 µm, supplied by Technopack Co., Egypt, were used in the
present study. They were cleaned using acetone, rinsed with de-ionized water and allowed to dry at
room temperature in order to remove any contamination on the polymeric surface. FTIR spectra of the
pristine and the irradiated samples were investigated using (FTIR-Beckman-4250) spectrophotometer
in the range 400 cm-1 to 4000 cm-1.SEM (Model JEOL, JSM-5400, Japan) was used to investigate the
surface morphology of the pristine and irradiated PTFE surfaces. XRD, scanning was carried out by a
fully computerized X-ray diffractometer, (Shimadzu type XD-DI). The thermal stability of the PTFE
films was investigated by thermogravimetric analysis (TGA) (Shimadzu, Japan). The TGA
measurements were carried out in N2 atmosphere from room temperature to 1000 C using a heating
rate of 10 0C/min.
RESULTS AND DISCUSSION
Morphological Characteristics of the Irradiated Films
In order to investigate the change in the surface morphology for the pristine and irradiated
PTFE by argon and oxygen plasma, SEM was performed as shown in Figs. 1 and 2 respectively. The
pristine sample showed a smooth surface, as shown in Figs 1 and 2 for argon and oxygen plasma
respectively. The SEM micrographs of the irradiated samples reveal that there is a change in the
morphology as the RF plasma time increased. It can be seen after 1 min of exposure time by argon and
oxygen plasma numerous microfibers and small voids were formed on the surface of the irradiated
PTFE. After 2 min, of plasma treatment time more voids on the surface of the irradiated sample. It was
observed that more chain chains were broken by incident Argon and oxygen ions after exposed to 3
min of plasma. Microfibers found over the surfaces of the polymer after 4 min. of irradiated as a result
of main chains breakage. The SEM pictures clearly show that the morphology of the PTFE surface
changed significantly during plasma treatment. This remarkable change of surface structure during
argon and oxygen plasma treatment was assigned to the surface structural change.
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Arab Journal of Nuclear Science and Applications, 46(5), (106-114) 2013
Infrared Absorption Spectra Measurements (IR)
Figs. 3 and 4 show the IR spectra of PTFE polymer before and after they had been exposed to
various minutes of argon and oxygen plasma respectively. As shown in Fig. 3, the IR spectrum of
un_irradiated PTFE shows two main distinctive absorption bands around 500-700 and 1200 cm-1
beside a weak absorption band at about 2300-2500 cm-1 . The absorption band at 1158-1200 cm-1 is
due to C-C stretching while the other at 500-700 cm-1 is due to C-F stretching(9) . In this regard, the
latter absorption band is similar to that of alkyl halides which usually give a weak band in the
fingerprint region of the IR spectra from 500 to 1430 cm-1. The argon and oxygen plasma irradiation
of PTFE leads mainly to partial degradation through the formation of acid fluoride groups COF.
Fig 1: SEM of the pristine and irradiated
PTFE films using Argon plasma
Fig 2: SEM of the pristine and irradiated
PTFE films using oxygen plasma
This process is particularly expected in the near-surface regions. The COF groups are
hydrolysed to carboxylic acids (COOH) in the presence of atmospheric humidity after irradiation. The
intensity of the different characteristic bands of the IR spectra of PTFE before and after exposure to
argon and oxygen irradiation was measured as shown in the Table 1. It can be seen that the intensity of
the C=O band at 2365 cm-1 increases with increasing tplasma exposure time, due to increased
degradation of the PTFE(9). As the exposure time is increased the de-fluorination effect is aggravated.
Therefore, it may be concluded that the plasma irradiation leads to C-C bond splitting and, at the
same time, to liberation of CF2 groups. The later together with other degradation products containing
C and F were detected earlier during the PTFE irradiation (10) .
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Arab Journal of Nuclear Science and Applications, 46(5), (106-114) 2013
Table (1): Intensity of the characteristic bands of the IR spectra of PTFE before
and after irradiation by argon and oxygen plasma
Treatemet time
(minutes)
Pristine
1min argon
2min argon
2min argon
4 min argon
Treatemet time
(minutes)
C=O (2365 cm-1)
0.440
0.442
0.443
0.460
0.510
Pristine
1 min oxygen
2 min oxygen
3 min oxygen
4 min oxyge
C=O (2365 cm-1)
0.440
0.445
0.450
0.470
0.475
Fig 3. FTIR of the pristine and irradiated PTFE films using Argon plasma
Figure 4: FTIR of the pristine and irradiated PTFE films using negyxo plasma
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Arab Journal of Nuclear Science and Applications, 46(5), (106-114) 2013
X-ray diffraction (XRD) and Structural investigations:
The intensity of diffraction peak for irradiated PTFE films decreases with increasing exposure
time as shown in figures 5&6 for argon and oxygen plasma respectively. It indicates the partially decrease
in crystalline of PTFE by plasma beam irradiation. The decrease in intensity of the main peak indicated the
destruction of the polymer matrix due to the irradiation, thus, reducing its crystallinity. The loss of
crystallinity after plasma irradiation could be due to main chain scission processes of PTFE, which could
lead to the disruption in the packing.On the other hand, the shape of the peaks i.e. the value of the full
width at half maximum (FWHM) of the peaks, reflects the degree of crystallinity. The larger the crystals
of a given material are, the sharper the peaks on the XRD spectra (11,12) . Scherrer equation relates the
FWHM (b), in radians, of a XRD peak to the crystallite size (L) as the following (13) .
L
K
b cos 
(1)
Where λ is the wavelength of the x-ray beam (in our case λ = 0.15425 nm of CuKα1), θ is the
corresponding Bragg angle and K is Scherrer constant. The value of K, in general, depends on the
crystallite shape, and normally it is taken in the order of unity for PTFE films (14) . The crystallite sizes,
corresponding to the most intense peak of the pristine and irradiated PTFE films, were calculated as
shown in table 2. It is clearly shown that as the treatment time increases, the FWHM increases,
consequently, the crystallite size decreases from 28.40 nm for the pristine sample to 20.56 nm after 4
min of argon plasma and decrease to 22.00 nm after 4 minutes of oxygen plasma as shown in table 2
Table (2): Position of the intense peak (2θ), FWHM and the crystallite size (L) for the pristine
and irradiated PTFE films with argon and oxygen plasma.
Treatemet time
FWHM
d (A0)
L (nm)
2
(minutes)
(Radian)
Pristine
2 min argon
3 min argon
4min argon
Pristine
2 min oxygen
3 min oxygen
4 min oxygen
17.94
18.22
18.13
18.52
17.94
18.58
17.99
18.71
0.0055
0.0073
0.0075
0.0076
0.0055
0.0068
0.0069
0.0071
110
4.94
4.86
4.88
4.78
4.94
4.77
4.92
4.73
28.40
21.40
20.82
20.56
28.39
22.98
22.63
00.00
Arab Journal of Nuclear Science and Applications, 46(5), (106-114) 2013
5. XRD of pristine and irradiated PTFE Fig 6. XRD of pristine and irradiated
films using Argon plasma.
PTFE films using oxygen plasma.
Thermo Gravimetric Analysis:
Thermogravimeteric analysis may be considered the most practical widely used method to
illustrate the thermal stability of polymers over a wide range of temperature. Moreover, it can be used
to determine the kinetic parameters such as activation energy and order of reaction. These parameters
can be used to give a better understanding of the thermal stability of polymers. Figures 7., 8 shows the
TGA curves for PTFE films irradiaded by argon and oxygen plasma respectively. It is clear that
PTFE exhibited good thermal stability, since no obvious weight loss was found on the differential
thermogravimetric (TGA) curve below 500 °C. The improvement of the thermal stability of the PTFE
by increasing plasma exposure up to 2 min. can be based on the fact that these materials have
inherently good thermal stability and also due to the strong interaction/chemical bonding that exposed
the PTFE. After 2 min of radiation there is decrease in thermal stability in PTFE was due to chain
scissioning by argon and oxygen plasma irradiation in the polymer, which reduced its strength.
TGA thermograms, various kinetic parameters of degradation reaction have been determined
by adopting most commonly used method of Horowitz –Metzger(15) .The activation energies of PTFE
polymer estimated using the expression.
𝑊0 − 𝑊
𝐿𝑛 {Ln (𝑊 − 𝑊𝑓 ) } =
𝑡
𝑓
Ea ×
(2)
R×T2s
where W0 , Wf are the initial and final weights, Wt is the remaining weight at temperature T,
Ea is the activation energy, R is gas constant ( R = 8.314 J k-1 mol-1 ) , and θ = T-Ts with Ts as the
reference temperature corresponding to Wt - Wf / W0 – Wf = 1/e. In the light of the equation (2), the
activation energy Ea can be calculated from the slope of the linear fitted line between Ln ( ln [ W 0 – Wf
/ Wt - Wf ] ) and θ as illustrated, for pristine and irradiated PTFE samples . The values of activation
energies so determined for pristine as well as irradiated samples have been enlisted in Table 3. There
is increase in the values of activation energy with the increasing irradiation time, such increase may
be attributed to the initialization of chain scissioning, possible evaporation of volatile side groups
resulting in significant reduction of packing density, reorganization of molecular arrangements etc. in
the polymeric sample which signifies the decrease in thermal stability of the polymer (16,17) . The
increase in the activatin energy with exposure time also may be due to the formation of COF groups (9)
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Arab Journal of Nuclear Science and Applications, 46(5), (106-114) 2013
. These groups are hydrolysed with atmospheric humidity to free and associated -COOH groups (at
ambient temperature). These groups have average dissociation energy higher than both -C-C- and C-F
groups. The intensity of the absorption bands of these groups increased with increasing irradiation
dose as seen from FTIR spectra in Figs. 3 and 4 and Table 3.
Fig 7. TGA of pristine and irradiated PTFE films using Argon plasma
Fig 8. TGA of pristine and irradiated PTFE films using oxygen plasma
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Arab Journal of Nuclear Science and Applications, 46(5), (106-114) 2013
Table (3) The activation energy Ea and Thermal decomposition for PTFE before and after
irradiation by argon and oxygen plasma.
Treatemet time
(minutes)
Pristine
Weight loss
Ea
w0
Wf
Wt
1.238
1.455
0.0243
0.646
3.425
1.883
0.0546
0.849
12.892
1.730
0.0763
0.795
13.293
2.344
0.0994
1.075
1.238
1.455
0.0243
0.646
4.982
2.016
0.0546
0.907
4.402
2.872
0.0107
1.255
3.165
3.328
0.0239
1.461
2 min argon
3 min argon
4min argon
Pristine
2 min oxygen
3min oxygen
4 min oxygen
Tmax
600
40.24
700
625.00
800
94.73
625.0
36.87
636.00
93.71
636.0
40.17
624.50
95.75
624.5
45.31
624.00
99.90
624.0
40.24
625.00
94.73
625.0
41.25
636.30
92.33
636.3
60.21
622.00
96.40
622.0
60.93
621.00
97.48
621.0
CONCULSION
Thus, in general it can be concluded that different properties of PTFE, viz. free radical
formation, emergence of new absorption bands, thermal decomposition behavior and crystallinity are
modified mainly ocuured as result of chain scission in the polymer arising from the irradiation by
argon and oxygen radio frequency plasma. The plasma induced degradation of PTFE extends its
applicability as lubricants and in production of perform intermediates useful in textile industries. The
XRD patterns of the irradiated PTFE revealed a decrease in its intensity which implied a decrease in
crystallinity of the polymer due to plasma irradiation. The TGA thermograms indicated a decrease in
thermal stability of PTFE after the irradiation. These studies support the fact that the chain-scission is
the dominant phenomenon caused by the irradiation in this polymer. The chain-scission led to the
formation of low molecular products, which decrease the strength of the polymer, thereby decreasing
its ability to withstand high temperatures
ACKNOWLDEGMENT
The authors wish to thanks Dr. Gianluca Crenci, Dr. Simone DalZilio (National Laboratory of
Advanced Technology and NanoScience (INFM-TASC), SS. 14 km 163,5, Basovizza, 34012 Trieste,
Italy for her kind help with plasma technique ''reactive ion etching (RIE) .
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