High Energy Chemistry, Vol. 39, No. 5, 2005, pp. 342–345. Translated from Khimiya Vysokikh Energii, Vol. 39, No. 5, 2005, pp. 392–395. Original Russian Text Copyright © 2005 by Kuvaldina, Rybkin, Titov, Shutov.
PLASMA CHEMISTRY
Kinetics of Structural and Chemical Changes in Poly(ethylene terephthalate) Films Treated in Oxygen and Nitrogen Plasmas E. V. Kuvaldina, V. V. Rybkin,1 V. A. Titov, and D. A. Shutov Ivanovo State University of Chemical Technology, pr. F. Engel’sa 7, Ivanovo, 153000 Russia Received May 27, 2004
Abstract—The results of measurements of the kinetics of formation of various functional groups in the surface layer of poly(ethylene terephthalate) under the action of a dc-discharge plasma in oxygen and nitrogen are reported. Based on these data, the mechanisms of polymer modification and degradation are hypothesized. 1
A number of publications have been devoted to various aspects of the action of a low-temperature plasma on the surface of poly(ethylene terephthalate) (PET) [1–8]. In all of these publications, it was noted that plasma treatment in gases different in chemical activity (air [1], oxygen [2], argon [2, 3], and Ar–O2 [4] or He– O2 [5] mixtures) improved the hydrophilicity of the material. Using attenuated total internal reflection (ATR) IR spectroscopy, Drachev et al. [1] did not detect considerable chemical changes in the surface layers of PET samples after plasma treatment. The enhancement of the wettability of the material, which was observed in this case, was related to the formation of experimentally demonstrated charge states on the PET surface [1]. A correlation between surface charge density and contact angle was also found. At the same time, Inagaki and coauthors [2, 3] found that polar carboxyl and hydroxyl groups appeared on the surface of a plasmatreated polymer. They found that an argon plasma provided higher concentrations of hydroxyl groups, as compared with carboxyl groups, than an oxygen plasma. Data obtained by electron spectroscopy for chemical analysis (ESCA) supported an increase in the degree of surface oxidation (the ratio between the atomic concentrations of oxygen and carbon [O]/[C]) as a result of plasma treatment [2]. The measurements were performed after treatment for 60 s when the contact angles on the polymer surface practically ceased to change. The kinetics of changes in the chemical composition and properties of surface is of paramount importance for comparing the efficiency of plasma-chemical modification of polymers under various conditions. Published data on a nonmonotonic change in contact angles upon the plasma treatment of polymers are available [9, 10]. This change can be a consequence of the nonlinear kinetics of reactions that lead to changes in the chemi1 E-mail:
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cal composition and structure of a modified surface [10]. This is consistent with data on the kinetics of formation of gaseous products at the initial steps of the oxygen-plasma treatment of the PET surface [11]. However, there is almost no published data on the kinetics of structural and chemical changes in the modified PET layer. The aim of this work was to study the kinetics of changes in the surface composition of PET under exposure to a reduced-pressure plasma in oxygen and nitrogen. EXPERIMENTAL A plasma was generated by initiating a dc discharge in a cylindrical reactor of made from S-52 glass with a radius of R = 1.5 cm. Polymer samples were treated at a gas pressure of 50 Pa and a discharge current of 80 mA. The linear velocity of a gas flow was 30 cm/s on an NTP basis. A PET film (GOST 24 234-80) specimen with an area of 18.8 cm2 and 30 µm in thickness was mounted as a ring on the reactor wall in the positive column region. The wall temperature was maintained with the use of an external heat exchanger. The temperature of the sample surface facing the plasma was measured with a glass-embedded copper–constantan thermocouple arranged along the reactor wall. The surface temperature of the polymer sample was equal to 357 ± 1 K. The composition of the gas phase was determined using an MX 7304 monopole mass spectrometer. The experimental setup and the procedure of mass-spectrometric measurements were described in detail elsewhere [12]. The composition of the polymer surface layer was studied by Fourier-transform IR ATR spectroscopy with the use of a Nicolet Avatar 360 spectrometer. A 12-bounce zinc selenide crystal was used; a signal accumulation mode (32 scans) was employed; the resolution was 2 cm–1. Because the aromatic and aliphatic moieties of the elementary unit of PET could be degraded
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in a plasma at different rates, the measured absorbance was normalized using two procedures: by relating it to the absorbance due to the stretching vibrations of C=C bonds in the benzene ring (ν = 1504 cm–1) and the antisymmetric stretching vibrations of C–H bonds in the −ëç2– groups of the aliphatic moiety of the repeat unit (ν = 2968 cm–1). The absorbance measured from the spectra of plasma-treated polymers was averaged over data obtained in ten or more samples. The time between the extraction of a sample from the reactor and the spectral measurement was no longer than 10 min. Each of experimental data points in the plots of absorbance as a function of time was obtained with the use of PET samples that were not exposed to a plasma. Minimum and maximum treatment times were 1 s and 10 min, respectively. The assignment of bands in the spectra was performed based on published data [13, 14].
An analysis of the FTIR-ATR spectra of untreated PET films demonstrated that the polymer contained both crystalline and amorphous phases. In addition to the intense absorption bands of the intrinsic groups of the polymer, weak bands corresponding to other functional groups were also present. These are absorption bands with maximums at 3550 and 3620 cm–1, which are indicative of the presence of terminal hydroxyl groups involved in intramolecular hydrogen bonding and free OH groups, respectively. In the wave number region ν = 3150–3380 cm–1, a broad band appeared due to the presence of alcoholic hydroxyl groups that form intermolecular hydrogen bonds. The intrinsic absorption band due to the stretching vibrations of the ester C=O bond (ν = 1727 cm–1) is broadened and exhibits a shoulder at ν = 1660–1680 cm–1. The intrinsic absorption bands due to the stretching vibrations of the C–O bonds in crystalline and amorphous polymer phases (bands at wave numbers of ν = 1243, 1120, and 1097 cm–1) behave in the same behavior. Thus, in addition to the ester group, other oxygen-containing groups, which were presumably due to the history of the material, were present in the polymer. Figure 1a demonstrates the dependence of the ratio between absorbance due to aliphatic and aromatic moieties of the PET repeat unit (A2968/A1504) on the plasma treatment time. Qualitatively, the shapes of curves obtained by polymer treatment in the nitrogen and oxygen environments were the same. These data indicate that the degradation of the PET repeat unit primarily began from its aliphatic moiety. It is likely that molecular hydrogen is the primary gaseous product of this process. Previously [11], it was found that molecular hydrogen was released into the gas phase at a maximum rate at short times of plasma treatment. However, the ratio between the rates of degradation of aliphatic and aromatic moieties of the PET repeat unit changed in the course of the subsequent treatment and the absorbance Vol. 39
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Fig. 1. Dependence of the ratio between the absorbances (a) at 2968 and 1504 cm–1 or (b) at 3241 and 1504 cm–1 on the time of treatment of PET in an (1) oxygen or (2) nitrogen plasma. Straight line 3 corresponds to the (a) A2968/A1504 or (b) A3241/A1504 ratio in the parent polymer.
ratio (A2968/A1504) at a treatment time of ~10 min became approximately equal to that for the parent polymer. Even at a short time of plasma treatment, a broad band appeared in the spectrum of PET, indicating the formation of bound OH groups (ν = 3241 cm–1). The absorbance at this band, which is proportional to the concentration of hydroxyl groups, increased with the increasing duration of polymer treatment in both gas plasmas (Fig. 1b). The band shape remained unchanged during the treatment of the polymer in an oxygen plasma, whereas the band broadened toward greater wave numbers in a nitrogen plasma. This may be due to the appearance of nitrogen-containing functional groups in the modified PET layer. Indeed, absorption bands corresponding to the stretching vibrations of bonds in amine and imine groups lie in the wave number region ν = 3200–3500 cm–1 [13]. In the spectral region of absorption carbonyl groups (1620–1800 cm–1), the main band is an intrinsic absorption band with a maximum at ν = 1727 cm–1, which corresponds to the vibrations of the C=O bond in the ester group of PET. The relative absorbance at this band (A1727/A2968 and A1727/A1504) increased with the time of polymer exposure to a plasma. The rate of change in the absorbance depends on the normalization procedure.
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With normalization to absorbance at 2968 cm–1, the rate was higher at short treatment times because of the predominant degradation of the aliphatic moiety of the repeat unit. The band at 1727 cm–1 broadened with treatment time. An absorption band with a maximum at 1700 cm–1 appeared, which can be attributed to the stretching of the C=O bond in saturated carboxylic acids (1725–1700 cm–1) or aromatic acids (1700– 1680 cm–1) [13, 14]. In this region, possible contributions of arylketones (1700–1680 cm–1) and arylaldehydes (1715–1695 cm–1) cannot also be excluded. Thus, the changes in the IR-ATR spectra suggest that not only the total concentration of oxygen-containing groups in the modified PET layer increased but also various new carbonyl groups were formed. The new functional groups are difficult to identify because of the superposition of absorption bands. Figure 2 demonstrates the kinetics of changes in the total relative concentration of these groups, as found by integrating absorbance in the wave number range ν = 1620– 1800 cm–1. The total concentration of carbonyl groups attainable by treatment of the polymer in a nitrogen plasma was much lower than that in an oxygen plasma. Moreover, the degradation of oxygen-containing groups prevailed over the formation of these groups at short treatment times in a nitrogen plasma. Similar changes were observed in the spectral region of absorption bands that mainly correspond to the vibrations of the C–O bond in various functional groups (ν = 1027–1322 cm–1). The relative absorbance at the intrinsic absorption bands of the ester group (ν = 1243, 1120, and 1097 cm–1) increased with treatment
time, and the bands broadened. Overlapping bands with maximums at ν = 1209 and 1187 cm–1 and a band at 1066 cm–1 appeared. According to Bellamy [13], O–H bending and C–O stretching vibrations characteristic of phenols are observed at ν = 1200 cm–1. Note that the absorbance at 1209 and 1187 cm–1 changed symbatically to the absorbance at 3241 cm–1 in the course of polymer treatment. This suggests the formation of phenolic hydroxyl groups. Yoshiki et al. [6] arrived at the same conclusion based on the ESCA analysis of the PET surface after atmospheric-pressure plasma treatment in a mixture of helium with oxygen. The absorbance at ν = 1066 and 1700 cm–1 changed synchronously in the course of treatment. It is believed that both of the bands correspond to the formation of the same functional group –C(=O)–O– (carboxyl or ester group). Our data are insufficient for the unambiguous identification of these groups. However, with consideration for published ESCA data on PET [2, 3, 6], it is most likely that these are carboxyl groups. Kabajev et al. [7], based on an analysis of the IR-ATR spectra of modified samples, pointed to the possibility of formation of carboxyl groups during the oxygen-plasma treatment of PET. Figure 2 demonstrates the kinetics of changes in the total relative concentration of functional groups, including C–O bonds, as found by integrating a spectrum over the wave number range ν = 1027–1322 cm–1. The concentration of these groups increased to approach a stationary value in 8–10 min of treatment. Note that the qualitative composition of functional groups in the PET surface layer was the same after modification in oxygen and nitrogen plasmas. The formation of new oxygen-containing groups after the treatment of PET in inert-gas plasmas was reported in [2, 15]. Both the reactions of macroradicals with oxygen and water vapor after the removal of samples from the reactor and the presence of minor impurities of O2 and H2O in a plasma gas were considered as conceivable reasons for the above phenomenon. Under conditions of our experiments, the latter reason seems to be more likely. As found by mass-spectrometric measurements, when a discharge in nitrogen was excited in the absence of a sample from the reactor, the base gas contained ~0.2% water vapor and oxygen. Desorption from the reactor walls and electrodes was primarily responsible for the appearance of these impurities. The partial pressures of oxygen and water vapor decreased during polymer treatment; thus indicating the consumption of these molecules. The main gaseous products of PET degradation in a nitrogen plasma were H2 and CO. Carbon dioxide molecules were not detected in the gas phase probably for the following two reasons: because of a low rate of their formation and as a consequence of rapid dissociation under electron impact. The dissociation of gaseous products of heterogeneous reactions resulted in the appearance of atomic oxygen and its subsequent participation in redox processes. HIGH ENERGY CHEMISTRY
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Note that the FTIR-ATR spectra measured after storage of treated samples in air for various times (from 10 min to 30 days) were practically identical; that is, the chemical structure of the PET layer modified by oxygen- or nitrogen-plasma treatment is sufficiently stable. ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research (project no. 04-02-17525). REFERENCES 1. Drachev, A.I., Gil’man, A.B., Pak, V.M., and Kuznetsov, A.A., Khim. Vys. Energ., 2002, vol. 36, no. 2, p. 143 [High Energy Chem., 2002, vol. 36, no. 2, p. 116]. 2. Inagaki, N., Tasaka, S., Narushima, K., and Kobayashi, H., J. Appl. Polym. Sci., 2002, vol. 85, no. 14, p. 2845. 3. Inagaki, N., Narushima, K., Tsutsui, Y., and Ohyama, Y., J. Adhesion Sci. Technol., 2002, vol. 16, no. 8, p. 1041. 4. Carlotti, S. and Mas, A., J. Appl. Polym. Sci., 1998, vol. 69, no. 12, p. 3221. 5. Borcia, G., Arefi-Khonsari, F., Amourouroux, J., and Popa, G., Proceedings of 14th International Symposium on Plasma Chemistry, Prague, 1999, vol. 4, p. 1815. 6. Yoshiki, H., Oki, A., Ogawa, H., and Horike, Y., J. Vac. Sci. Technol., A, 2002, vol. 20, no. 1, p. 24.
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7. Kabajev, M., Prosycevas, I., Kazakeviciute, G., and Valiene, V., Mater. Sci., 2004, vol. 10, no. 2, p. 173. 8. Kumagai, H., Hiroki, D., Fujii, N., and Kobayashi, T., J. Vac. Sci. Technol., A, 2004, vol. 22, no. 1, p. 1. 9. Gil’man, A.B., Kuznetsov, A.A., Vengerskaya, L.E., Lopukhova, G.V., Tuzov, L.S., and Potapov, V.K., Khim. Vys. Energ., 1995, vol. 29, no. 4, p. 294 [High Energy Chem., 1995, vol. 29, no. 4, p. 270]. 10. Kutepov, A.M., Zakharov, A.G., and Maksimov, A.I., Vakuumno-plazmennoe i plazmenno-rastvornoe modifitsirovanie polimernykh materialov (Vacuum-Plasma and Plasma–Solution Modification of Polymers), Moscow: Nauka, 2004. 11. Rybkin, V.V., Kuvaldina, E.V., Smirnov, S.A., Ivanov, A.N., and Titov, V.A., Khim. Vys. Energ., 2001, vol. 35, no. 1, p. 42 [High Energy Chem., 2001, vol. 35, no. 1, p. 39]. 12. Kuvaldina, E.V., Maksimov, A.I., Rybkin, V.V., and Lyubimov, V.K., Khim. Vys. Energ., 1990, vol. 24, no. 5, p. 422. 13. Bellamy, L.T., The Infra-Red Spectra of Complex Molecules, London: Methuen, 1954. 14. Tarutina, L.I. and Pozdnyakova, F.O., Spektral’nyi analiz polimerov (Spectral Analysis of Polymers), Leningrad: Khimiya, 1986. 15. France, R.M. and Short, R.D., J. Chem. Soc., Faraday Trans., 1997, vol. 93, no. 17, p. 3173.
