ISSN 00181439, High Energy Chemistry, 2014, Vol. 48, No. 4, pp. 282–286. © Pleiades Publishing, Ltd., 2014. Original Russian Text © M.Yu. Yablokov, V.G. Shevchenko, A.B. Gilman, A.A. Kuznetsov, 2014, published in Khimiya Vysokikh Energii, 2014, Vol. 48, No. 4, pp. 326–330.
Dielectric Properties of Polytetrafluoroethylene Films Modified by Direct Current Discharge M. Yu. Yablokov, V. G. Shevchenko, A. B. Gilman, and A. A. Kuznetsov Enikolopov Institute of Synthetic Polymer Materials, Russian Academy of Sciences, ul. Profsoyuznaya 70, Moscow, 117393 Russia email: [email protected]
Received December 25, 2013; in final form, February 15, 2014
Abstract—Dielectric characteristics of polytetrafluoroethylene (PTFE) films modified by direct current dis charge at the anode and cathode have been studied as a function of frequency and temperature. It has been found that the dielectric constant increases slightly as a result of plasma treatment and weakly depends on temperature. It has been shown that the frequency dependences of conductivity at 20°C differ somewhat for the original and treated films, but the closer resemble one another at 80°C, with the difference in the critical conductivity indices becoming more significant. DOI: 10.1134/S0018143914040134
Polytetrafluoroethylene (PTFE) has unique chem ical and physical properties; it does not swell or dis solve in solvents and is resistant to oxidants, acids, and alkalis. The polymer is one of the best dielectrics, its dielectric properties are weakly dependent on the field frequency and remain unchanged over a wide temper ature range. The dielectric permittivity in the fre quency range of 60–1010 Hz is 1.9–2.2 and the dielec tric loss tangent is 0.0002–0.00025 . However, for some applications it is necessary to improve the con tact properties of the polymer surface with retaining its dielectric characteristics, which is undoubtedly a task of great practical importance. There are a large number of papers on the surface modification of PTFE by various methods of high energy chemistry aiming to improve the adhesion characteristics. These are electron beam irradiation ; bombardment with argon and oxygen ions [3, 4]; treatment by lowpressure dielectricbarrier [4, 5], radiofrequency (13.56 MHz) [3, 5], or microwave (2.45 GHz)  discharge; treatment in RF discharge plasma afterglow ; and combined plasma and ozone treatment . We have studied the direct current (dc) discharge treatment of PTFE films and showed that this treatment makes it possible to obtain significantly lower values of contact angles and higher work of adhesion and surface energy as compared with the known PTFE modification methods [9–11]. It was also found that the plasma treatment of PTFE films imparts high adhesion properties: the peel resistance according to the Tpeel test method increased from 30 ± 6 N/m for the initial film to 200 ± 10 or 134 ± 4 N/m after the treatment at the anode or cathode, respectively .
Fouriertransform IR and Xray photoelectron spectroscopy studies of the composition and structure of the modified PTFE layer showed the formation of oxygencontaining groups –C(O)–F– on the surface [9–11]. Experimentally, it was shown that the thick ness of the modified layer is ~40–50 nm . It is known that the values of dielectric loss tangent (tan δ) and dielectric permittivity (ε') for PTFE are markedly increased as a result of irradiation with γrays or fast electrons [14, 15]. For example, it was shown that the value of tan δ for PTFE films of 45 μm in thickness substantially depends on the dose and the dose rate of γradiation, as well as on the temperature at which measurements are made, and it can increase to 0.03 in comparison with the initial value of 5 × 10–4. It was found that the increase in tan δ for irradiated PTFE is due to the formation of terminal and central peroxide macroradicals as a result of degradation and oxidation processes. Analyzing the literature, we have not found any reports on studying the changes in the dielectric prop erties of PTFE by treatment in lowtemperature plasma. However, studies of this sort were carried out for other polymers, such as Kapton H [16, 17], fluori nated poly(arylene oxide) and a divinylsiloxane–ben zocyclobutane copolymer , and Parylene C . Kapton H films of 50 μm thickness were treated for 50 s by RF (13.56 MHz) discharge in a He (99%) and CHF3 (1%) gas mixture at a discharge power of 100– 400 W . Measurements of the dielectric constant at 1 MHz showed that at 25°C its value gradually decreases depending on the power from 3.15 for the initial film to 2.45 for the film treated at 400 W. There was also an increase in ε' by 0.1–0.2 with an increase in the temperature in the range of 25–200°C. Similar
DIELECTRIC PROPERTIES OF POLYTETRAFLUOROETHYLENE FILMS
The values of water and glycerol contact angles (θ), the work of adhesion (Wa), total surface energy (γ), polar (γp), and dis persion (γd) components for the initial PTFE film and its samples modified by air plasma (50 mA, 60 s, 10–15 Pa) Sample
Wa, mJ/m2 glycerol
At the anode At the cathode
studies with varying the frequency in the range of 100 to 107 Hz and the temperature in the range of 0– 200°С at a frequency of 103 Hz showed that ε' is reduced (within 5%) by increasing frequency and increases at a constant frequency with an increase in the temperature . Trabzon and Awadelkarim  studied the effect of RF (13.56 MHz) or dc discharge treatment on the dielectric properties of films of fluorinated poly(arylene oxide) and a divinylsiloxane–benzocy clobutane copolymer, “1owk” dielectrics used in microelectronics. The films of 1 μm thickness were treated by RF discharge (power, 50–500 W) and by dc discharge (voltage, 180–675 V) in argon or oxygen. The cited authors investigated the optical anisotropy of the films, so they were interested in changes in ε' at an optical frequency (~1015 MHz). It was found that the values of ε' at this frequency increased for both polymers; but the increment was 2.35–2.9 for the former polymer versus 2.55–2.9 for the latter. After the plasma treatment, leakage currents strongly increased for both polymers. Kahouli et al.  studied the change in the dielec tric properties of Parylene C [poly(monochlorop xylylene)] films of 3.7 μm thickness modified by microwave (2.45 MHz) discharge in CF4, O2, or an Ar/H2 mixture (power, 500–900 W; pressure, 525– 675 torr). They found that the treatment by CF4 plasma reduces ε' from 3.55 to 3.15 (25°С, 100 Hz), that by O2 plasma results in a noticeable increase in ε', and the Ar/H2 plasma treatment leads to a decrease in ε', although to a lesser extent than by the fluorinated plasma. In view of the above, the issue of alteration of the dielectric characteristics of PTFE films by plasma treatment is of undoubted importance, especially in the case of their use as dielectric elements of various devices and instruments. This work is devoted to the study of dielectric properties of dc dischargemodified PTFE films. EXPERIMENTAL Samples of PTFE films of 40 μm thickness (GOST (State Standard) 2422280, batch number 300878) were used. HIGH ENERGY CHEMISTRY
The procedure of film modification by dc discharge is detailed in . The samples were placed at the anode and cathode and treated in the continuousflow mode at a plasma gas (air) pressure of 10–15 Pa and a discharge current of 50 mA for 60 s. The change in the surface properties was character ized by values of contact angles (θ) determined by the goniometer technique using an Easy Drop DSA100 (KRUSS, Germany) instrument and the Drop Shape Analysis V.18.104.22.168 software (accuracy ±1°) for two test liquids, water (deionized) and glycerol. The work of adhesion (Wa), the total surface energy (γ), and its polar (γр) and dispersion (γ d) components were calcu lated on the basis of obtained θ values according to . The real and imaginary parts of permittivity and dielectric loss for the films in the frequency range of 10–2–106 Hz and temperatures of 20 to 100°С were measured using a Novocontro1 A1phaA impedance analyzer and a ZSG A1pha Active Sample Cell dielec tric cell with goldplated disk electrodes of 20 mm diameter. To measure the dielectric characteristics of the polymer films, their surface was coated with an electrically conductive metal layer . It was found that upon metallization with aluminum by vacuum thermal sputtering, substantial data scatter is observed in experiments on studying the dielectric properties of a set of samples obtained under identical conditions. . When silver was deposited, the data scatter was significantly lower, so silver was used for vacuum dep osition of electrodes in this study. The layer thickness was 50–70 nm, the value of surface resistance, which was monitored during deposition, did not exceed 5 Ω . RESULTS AND DISCUSSION The modification of the PTFE film surface by low temperature plasma substantially changes its contact properties. The table shows the values of water and glycerol contact angles (θ), the work of adhesion (Wa), total surface energy (γ), polar (γр) and dispersion (γ d) components for the initial PTFE film and the film modified by dc discharge at the cathode and anode (discharge current, 50 mA; treatment time, 60 s; air pressure, 10–15 Pa). A significant decrease in θ (for both liquids), a significant growth of Wa and γ, and an
YABLOKOV et al.
2.1 20°C 100°C 2.0
Fig. 1. Dielectric permittivity at 20 and 100°C (frequency f = 100 Hz) for the initial (I) and processed at the anode (A) and cathode (C) PTFE film.
increase by many times in the polar component γр are seen, indicating that the originally hydrophobic sur face becomes hydrophilic. Figure 1 shows the change in the dielectric con stant (ε') of the PTFE film processed at the anode and cathode by dc discharge, depending on the tempera ture (20 and 100°С) at a frequency of 100 Hz. It is seen that ε' slightly increases as a result of plasma treatment and is weakly dependent on temperature. Previously, we have found that a chemically modi fied layer with a thickness of dm ≈ 40–50 nm is formed on the PTFE film surface during treatment by low temperature plasma . The XPS technique showed that the oxygen content in the layer is ~11% for the film treated at the anode . It was found that elec tret states appear in the film . Apparently, an increase in the dielectric constant of the plasmamod ified film is due to the appearance of dipoles formed by oxygencontaining groups with a dipole moment of p = 4D . The permittivity of the modified layer (εm) was determined in accordance with the Lan gevin–Debye formula . Assuming that the dipoles are uniformly distributed in the bulk of the modified layer and are oriented per pendicular to the surface, the orientation polarizabil ity for an oxygen group (α) can be defined by α = p2/kT, where p is the dipole moment, k is Boltzmann’s constant, and Т is the absolute temperature . In calculating the volume concentration of the dipoles in the modified layer the density of the layer material, was considered equal to that of the PTFE amorphous phase, 2.01 g/cm3, which is less than the PTFE density of 2.2 g/cm3 usually quoted in the literature . The permittivity of the lowtemperature plasma modified layer (εm) under such conditions was εm = 12.
It is possible to assess the contribution of the dielectric permittivity of the modified layer to the dielectric constant of the whole film (εf) (with a thick ness df) assuming that the film consists of two layers: the modified one with a thickness dm and a dielectric constant εm, and the intact layer with a thickness dt and permittivity εt. To estimate the dielectric constant of the whole polymer film, we used an expression derived from the formula for the total capacity of two plane capacitors connected in series: εf = εt /[1 + (εt/εm – 1)dm/df], believing that the thickness of the modified layer is substantially less than that of the film, dt ≈ df. From this expression it follows that at a small value of the ratio of the modified layer thickness to the film thick ness dm/df, the change in the dielectric properties of the modified layer has no influence on the dielectric properties of the film as a whole, assuming that the change in the dielectric properties of the film is only due to the formation of polar groups in the modified layer during plasma treatment. In our case, dm/df = 10–3 and the relative increase in the dielectric permittivity is ≤0.1%. Similar calculations for polymer coatings of ~1 μm thickness treated by oxygen RF discharge plasma give a value of ~4%, which is close to that obtained in . Note, however, that changes in the dielectric constant of polymer films by plasma treat ment can be caused not only by the formation of oxy gencontaining groups in the surface layer, but also by the generation of charge states localized in the poly mer on structural defects of different kinds . Thus, the dc glow discharge treatment of PTFE films, resulting in a substantial increase in the surface wettability and improvement in the contact properties, causes a slight increase in their dielectric constant. Figure 2 shows the dependence of electrical con ductivity (σ) on the frequency at 20°C (a) and 80°С (b) for the initial film and the film treated at the anode and cathode. It is seen that these dependences differ at 20°С, but they are almost identical at 80°С, possibly because of a higher mobility of charge states induced in the plasmamodified films [23, 28]. The dependence of the conductivity (σ) on the fre quency for hopping transport in disordered systems is described by the equation: σ = Af s, where A is a coeffi cient characterizing the zerofrequency conductivity, f is the frequency, and s is the exponent (critical index) characterizing the distribution of defects providing the hopping conduction mechanism in the material. Figure 3 shows the change in the critical index s for the initial PTFE film and the film treated at the anode and cathode. It is seen that its magnitude is weakly dependent on the temperature for the initial film and the film treated at the anode, i.e., the number of defects and average distance between them in these films are probably almost unchanged, as well as the number of carriers and/or their mobility. For the film modified at the cathode, the magnitude of the critical HIGH ENERGY CHEMISTRY
DIELECTRIC PROPERTIES OF POLYTETRAFLUOROETHYLENE FILMS (а)
σ, Ω–1 cm–1
σ, Ω–1 cm–1
101 102 103 Frequency, Hz
Fig. 2. Dependence of electrical conductivity (σ) on the frequency at (a) 20 and (b) 80°C for (䊏) the initial PTFE film and the film treated (䊉) at the anode and (䉱) cathode.
index decreases significantly upon heating. Appar ently, this can be due to freezingout of the mobility of the injected charge carriers by heating and degradation of the electret state formed in the film by direct current glow discharge treatment . This behavior is proba bly due to the fact that the changes occur in a thin sur face layer, the influence of which on the dielectric properties of the entire film is small. Thus, the results demonstrate that the modification of the polymer films by dc discharge slightly affects their dielectric characteristics, wheres the surface con tact properties undergo substantial changes. s 1.0 100°C 20°C 0.9
Fig. 3. Critical index s at 20 and 100°С for the initial (I) and processed (A) at the anode and (C) at the cathode PTFE film. HIGH ENERGY CHEMISTRY
REFERENCES 1. Entsiklopediya polimerov (Polymer Encyclopedia), Moscow: Sovetskaya Entsiklopediya, 1977, vol. 3. 2. Lunkwitz, K., Lappan, U., and Lehman, D., Radiat. Phys. Chem., 2000, vol. 57, nos. 3–6, p. 373. 3. Kim, S.R., J. Appl. Polym. Sci., 2000, vol. 77, no. 9, p. 1913. 4. Liu, C.Z., Wu, J.Q., Ren, L.Q., Tong, J., Li, J.Q., Cui, N., Brown, N.M.D., and Meenan, B.J., Mater. Chem. Phys., 2004, vol. 85, nos 23, p. 340. 5. Pappas, D., Bujanda, A.A., Orlicki, J.A., and Jensen, R.E., Surf. Coat. Technol., 2008, vol. 203, nos. 5–7, p. 830. 6. Xu, H., Hu, Zh., Wu, Sh., and Chen, Y., Mater. Chem. Phys., 2003, vol. 80, no. 1, p. 278. 7. Park, Y.W., Tasaka, S., and Inagaki, N., J. Appl. Polym. Sci., 2002, vol. 83, no. 6, p. 1258. 8. Tu, Ch.Y., Liu, Y.L., Lee, K.R., and Lai, J.Y., Polymer, 2005, vol. 46, no. 18, p. 6976. 9. Piskarev, M.S., Gilman, A.B., Shmakova, N.A., and Kuznetsov, A.A., High Energy Chem., 2008, vol. 42, no. 2, p. 137. 10. Gilman, A., Piskarev, M., Shmakova, N., Yablokov, M., and Kuznetsov, A., Mater. Sci. Forum, 2010, vol. 636/637, p. 1019. 11. Piskarev, M.S., Batuashvili, M.R., Yablokov, M.Yu., Kechek’yan, A.S., Gil’man, A.B., and Kuzne tsov, A.A., Khim. Khim. Tekhnol., 2012, vol. 55, no. 4, p. 35. 12. Yablokov, M.Yu., Sokolov, I.V., Malinovskaya, O.S., Gil’man, A.B., and Kuznetsov, A.A., High Energy Chem., 2013, vol. 47, no. 1, p. 32. 13. Yablokov, M.Yu., Kechek’yan, A.S., Bazhenov, S.L., Gilman, A.B., Piskarev, M.S., and Kuznetsov, A.A., High Energy Chem., 2009, vol. 43, no. 6, p. 512.
YABLOKOV et al.
14. Matveev, V.K., Noifekh, A.I., Klinshpont, E.R., and Milinchuk, V.K., Khim. Vys. Energ., 1992, vol. 26, no. 2, p. 130. 15. Matveev, V.K., Smirnova, N.A., and Milinchuk, V.K., Vysokomol. Soedin., 1993, vol. 35, no. 6, p. 297. 16. Park, S.J., Sohn, H.J., Hong, S.K., and Shin, G.S., J. Colloid Interface Sci., 2009, vol. 332, no. 1, p. 246. 17. Park, SJ., Lee, EJ., and Kim, BJ., J. Colloid Interface Sci., 2008, vol. 319, no. 1, p. 365. 18. Trabzon, L. and Awadelkarim, O.O., Microelectronic Eng., 2003, vol. 65, no. 4, p. 463. 19. Kahouli, A., Sylvestre, A., Laithier, J.F., Pairis, S., Garden, J.L., Andre, E., Jomni, F., and Yangui, B., J. Phys. D: Appl. Phys., 2012, vol. 45, no. 21, p. 215306. 20. Richkov, D., Yablokov, M., and Richkov, A., Appl. Phys. A: Mater. Sci. Process., 2012, vol. A107, no. 3, p. 589. 21. Wu, S., Polymer Interfaces and Adhesion, New York: Marcel Dekker, 1982. 22. Lushcheikin, G.A., Metody issledovaniya elek tricheskikh svoistv polimerov (Investigation Techniques
24. 25. 26. 27. 28.
for Electrical Properties of Polymers), Moscow: Khimiya, 1988. Rychkov, A.A. and Boitsov, V.G., Elektretnyi effekt v strukturakh polimermetall (Electret Effect in Metal– Polymer Structures), St. Petersburg: RGPU im. A.I. Gertsena, 2000. Blythe, A.R. and Bloor, D., Electrical Properties of Polymers, Cambridge: Cambridge Univ. Press, 2005, 2nd ed. Borsenberger, P.M., Gruenbaum, W.T., O’Regan, M.B., and Rossi, L.J., J. Polym. Sci., Part B: Polym. Phys., 1995, vol. 33, no. 15. p. 2143. Kittel, C., Introduction to Solid State Physics, New York: Wiley, 1974, 4th ed. Panshin, Yu.A., Malkevich, S.G., and Dunaevskaya, Ts.S., Ftoroplasty (Fluoroplastics), Leningrad: Khimiya, 1978. Rychkov, A.A., Yablokov, M.Yu., Kuznetsov, A.E., Gil man, A.B., and Kuznetsov, A.A., High Energy Chem., 2010, vol. 44, no. 4, p. 347. Translated by V. Makhaev
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