H2O Plasma Chemistry for Polyethylene Surface

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Langmuir 2002, 18, 2246-2253

CO2, H2O, and CO2/H2O Plasma Chemistry for Polyethylene Surface Modification Nicolas Me´dard,† Jean-Claude Soutif,‡ and Fabienne Poncin-Epaillard*,† Laboratoire Polyme` res, Colloı¨des et Interfaces (UMR CNRS No. 6120), Universite´ du Maine, avenue O. Messiaen, 72085 Le Mans Cedex 09, France, and Unite´ de Chimie Organique Mole´ culaire et Macromole´ culaire (UMR No. 6011), Universite´ du Maine, avenue O. Messiaen, 72085 Le Mans Cedex 09, France Received September 26, 2001. In Final Form: November 27, 2001 During a cold plasma treatment, a polymer surface is involved in many reactions, such as degradation and functionalization. CO2 plasma treatment leads to carboxylic acid function fixation onto high-density polyethylene surface. Attempts are done to optimize the concentration of these last functions by using CO2/H2O plasmas. By fragmentation and recombination reactions between plasma species, we hope to create new species such as carboxylic acids precursors (CO + OH or CO2 + H) that are able to increase the surface carboxylic density. Spectrochemical quantification and XPS analysis measure the surface functionalization whereas the plasma composition and plasma-polymer interactions are analyzed by mass spectrometry. The surface functionalization results show a decrease of carboxylic acid density whatever the water proportion and the discharge power explained by a mass spectrometry study. Thus, no recombination reaction leading to the formation of COOH plasma species occurred in the plasma phase. During the polyethylene treatment with CO2 plasma, evidence of degradation is also shown by mass spectroscopy (atomic and molecular hydrogen formation in the plasma phase). A correlation done between weight-loss measurement and the atomic oxygen (O3P and/or O1D) proportion in the CO2 plasma showed the significant role of the latter on the surface degradation mechanism.

Introduction Surface treatment and modification of polymeric materials is a domain of growing interest. In this area, pure1 or combined2 chemical and physical processes have been largely used. In the last group, the plasma technique seems to be very powerful because its low temperature could be applied to a large variety of materials and could change the surface properties to a large extent.3 The usual applications are found in the microelectronics industry,4 in improvement of surface adhesion,5 in membrane permselectivity,6,7 and in biocompatibility fields.8 During a cold plasma treatment, the polymer surface is involved in many reactions, divided in two groups.9 The first one is a degradation induced by the absorption of UV-visible radiation emitted by the gaseous excited species. The second one is related to the electron or ion bombardment and leads to a surface functionalization. In a previous paper10 dealing with the synthesis of a new supported metallocene catalyst from a common * Corresponding author. E-mail: fabienne.poncin-epaillard@ univ-lemans.fr. † UMR CNRS No. 6120. ‡ UMR No. 6011. (1) Harper, J. M. E. In Thin Film Process; Vossen, J. L., Kern, W., Eds.; Academic Press: New York, 1978; p 175. (2) Bottin, M. F.; Schreiber, H. P.; Klemberg-Sapieha, J.; Wertheimer, M. R. J. Appl. Polym. Sci., Polym. Symp. 1984, 38, 193-199. (3) d’Agostino, R. Plasma deposition, treatment, and etching of polymers; Academic Press: New York, 1990. (4) Kogoma, M.; Turban, G. Plasma Process. Symp. 1986, 68, 401406. (5) Vallon, S.; Hofrichter, A.; Drevillon, B.; Klemberg-Sapieha, J. E.; Martinu, L.; Poncin-Epaillard, F. Thin Solid Films 1996, 68, 290-291. (6) Poncin-Epaillard, F.; Me´dard, N.; Soutif, J. C. Macromol. Chem. Phys. 2000, 201, 212-19. (7) Marais, S.; Metayer, M.; Labbe, M.; Valleton, J. M.; Alexandre, S.; Saiter, J. M.; Poncin-Epaillard, F. Surf. Coat. 1999, 122, 247-59. (8) Biederman, H.; Osada, Y. Adv. Polym. Sci., Polym. Phys. 1990, 95, 57-65. (9) Poncin-Epaillard, F.; Chevet, B.; Brosse, J. C. Eur. Polym. J. 1990, 26 (3), 333-339.

material (high-density polyethylene), a CO2 plasma treatment was selected to functionalize the polymer surface with carboxylic acid functions (COOH). These chemical groups allowed a pure covalent fixation of metallocene catalysts onto the support. In the first part of this study, some attempts were described to optimize the concentration of carboxylic functions to increase the catalytic density. One of them was dealing with the use of gas mixture plasmas. An example of the efficiency of gas mixture plasmas is given by Poncin-Epaillard et al.6 showing that hydrogen addition in nitrogen plasma increased the surface amine function density onto a polypropylene (3.1 sites nm-2 with a N2 plasma against 6.4 sites.nm-2 with N2/H2 (1:2) plasma). This increase was well explained by fragmentation and recombination reactions between N and H atomic species from the plasma phase that led to the formation of plasma amine precursors (N + 2H f NH2). By comparing the nature of the function to be fixed (COOH) and the nature of the species mainly formed into a CO2 plasma (mainly CO2 and CO)11 (Table 1), we concluded that the presence of H or OH species in the plasma might increase the proportion of carboxylic acids precursors (CO + OH or CO2 + H). Therefore, water vapor (H2O) was chosen as the second gas (H2O f OH +H). However, the first results showed that water addition in the CO2 plasma did not increase the proportion of surface carboxylic proportion but, on the contrary, contributed to its large decrease. Therefore, a complete study of the plasma phase composition appears thus necessary to well-characterize fragmentation and recombination reactions occurring in the plasma phase to explain this result. (10) Me´dard, N.; Poncin-Epaillard, F.; Soutif, J. C.; Esteyries, C.; Lado, I. Macromol. Chem. Phys. 2001, 23 (11), 300593016. (11) Poncin-Epaillard, F.; Aouinti, M. Plasma Polym., in press.

10.1021/la011481i CCC: $22.00 © 2002 American Chemical Society Published on Web 02/13/2002

Plasma Chemistry for Surface Modification Table 1. Plasma Chemistry for the Highest Surface Functionalization Pure Gas Plasma N2 plasma: 3.1 NH2.nm-2 attached onto PE surface

CO2 plasma: 3.0 COOH.nm-2 attached onto PE surface

Gas Mixture Plasma N2 + H2 plasma (1/2): 6.4 NH2.nm-2 attached onto PE surface because of the plasma chemistry N + 2H f NH2

CO2 plasma: ? COOH.nm-2 attached onto PE surface with possible plasma chemistry CO2 + H or CO + OH f COOH; w study of CO2 + H2O plasma

The main drawback of the plasma technique is the difficulty to achieve a good understanding of the interactions between plasma species and treated surfaces, necessary to good control and understanding of the surface properties. Thus, this article is an attempt to establish relationships between three sets of parameters: (1) composition and properties of the surface after treatment, especially carboxylic functionalization; (2) the plasma parameters (gas flow and composition, discharge power); (3) composition of the gaseous reactive species with a particular attention to the species leading to surface modification. Surface functionalization is measured by chemical titration and XPS measurement whereas plasma chemistry is described by mass spectrometry. A complete description of the HDPE surface modification (i.e. degradation, cross-linking, and functionalization versus the different plasma parameters) was given in a previous paper12 and can be summarized as follows. Surface functionalization with carboxylic acid groups seems to occur according to a mechanism involving the CO2 plasma species in different excited energy states. The degradation reaction appears to be heterogeneous due to the hardness difference between amorphous and crystalline phases. The plasma treatment also involves a weak increase in surface crystallinity (from 79 to 85% in about 2 min treatment time) and the propagation of chains defects (left conformation within trans conformation chains) leading to the formation of crystalline structures of higher organization. Finally, the presence of surface radicals formed after the activation phase led to a cross-linking gradient from the surface toward the volume. Experimental Section High-Density Polyethylene. High-density polyethylene (HDPE) supplied by Goodfellow was obtained by extrusion and synthesized with a Ziegler-Natta catalyst in the heterogeneous phase. It is an orthorhombic semicrystalline polymer (melting point 115 °C determined by differential scanning calorimetry) with a thickness of 10 µm. The crystalline fraction is between 75 and 77% (determined by FTIR spectroscopy). Before use, the film (16 cm2) is successively washed with ethanol and acetone and dried under vacuum for 1 day. Plasma Equipment. The equipment used here is a microwave plasma apparatus. The plasma excitation is provided by a microwave generator (SAIREM, 433 MHz) with a variable power (from 0 to 250 W), which is coupled to a resonant cavity (surfatron). The incident power (Pi) and the reflected power (Pr) are measured with a powermeter (Hewlett-Packard Nο. 435B). The impedance is adjusted until the reflected power was very low (Pr < 0.02 W). The glow is generated at the top of the reactor. The pumping system is composed of primary (CIT Alcatel Nο. 2012) and oil diffusion (CIT Alcatel Crystal) pumps. A MKS mass flow meter (12) Me´dard, N.; Soutif, J. C.; Poncin-Epaillard, F. Thin Solid Films, submitted for publication.

Langmuir, Vol. 18, No. 6, 2002 2247 (type 1259B) controls the gas (carbon dioxide purchased from “Air Liquide” grade N45, purity >99.995%) flow (F in sccm). The pressure (p in mbar) is measured with Penning and Pirani gauges. The reactor is a quartz cylinder of 500 mm length and 76 mm diameter. The reactor is set up on a chamber used for the sample introduction. The substrate can be moved in or out of the plasma volume. We note d (in cm), the distance between the bottom of the excitatory source and the sample. For the H2O plasma, approximately 150 cm3 of cooled distilled water was placed in a 250 cm3 flask equipped with a glass tap and a magnetic stirrer. The water was degassed using a primary pump approximately 3 h to evacuate the residual air. The flask is then fixed at the tube of the gas feeding pipe. To avoid problems of condensation, the feeding pipe is heated at about 35-45 °C using a flexible electric resistance, and the flask is heated to a temperature of 40 °C using an oil bath. The control of the gas flow being difficult, the water vapor flow is fixed at 3 sccm in this study. Mass Spectrometry. The quadripolar mass spectrometer coupled to our plasma reactor is a Balzers PPM 421 model controlled by a computer equipped with Quadstar 421 v.2.0 software. The gas species are extracted from the plasma medium by a system of sampling through microleakage and are admitted in the mass spectrometer where the pressure is approximately 10-7 mbar (against 0.3 mbar in plasma). The experimental parameters are as follows: ionization potential, 70 V; extraction potential, 450 V; focusing potential, 10 V; SEM potential, 2900 V. All the analyses were carried out on a mass range from m/z ) 0 to 542 amu. The results were presented on a mass range from 0 to 80 amu because no signal has been detected beyond 80 amu. It should be noted that this mass spectroscopy study is only based on a comparison between the neutral analysis originated either from the gas (pure or mixture) or plasma phase (pure, mixture, with or without HDPE). Ions are formed by the ionization of neutral fragments or molecules of plasma phase and/or by the fragmentation of plasma species in the ionization chamber of the spectrometer. Surface Carboxylic Group Titration. The treated film was dipped into an ethanol solution of thionin acetate (1 × 10-5 mol.L-1). The reaction solution was stirred at room temperature during 10 h. Then, the film was removed and washed with ethanol and the solution was titrated by measuring its UV absorption (λ ) 604.8 nm,  ) 54 500) and comparing to a blank solution. The difference between the two absorptions was proportional to the carboxylic groups attached onto the film. Spectroscopic titrations were run on an UV-visible spectrometer Varian DMS 100 using a quartz cell (1 cm length). This chemical titration developed either for polyethylene or polypropylene CO2 plasma modification has been compared and correlated with SIMS and XPS analyses.13 X-ray Photoelectron Spectroscopy (XPS). XPS analyses were performed 1 day after the plasma treatment on a Leybold LHS 12 XPS spectrometer (“Institut des Mate´riaux”, CNRS, Nantes, France) with a Mg KR X-ray source (E ) 1.2536 keV, P ) 300 W). The analyzed depth is approximately to a 10 nm. The number of scans is 5 for the total spectrum and 15 for the highresolution spectrum. All spectra are referenced to the C 1s peak for neutral carbon, which is assigned a value of 284.6 eV. The data were collected with a Hewlett-Packard 2436 E, and peak fitting was done using Peak Fit 4.0 software (Jandel Scientific). The full width at half-maximum (fwhm) is maintained constant, and the peak shape was chosen with Gaussian/Lorentzian curves fitting. Gravimetric Measurements. The weight loss generated by the degradation was estimated by weighing the sample before and after the plasma treatment. The effect of the plasma being limited to the extreme surface and the weight loss being independent of the thickness of the film, the unit chosen for this study is the µg.cm-2.

Results and Discussion To improve the carboxylic acid functions fixation on the HDPE surface, the effect of water (H2O) in carbon dioxide (13) Me´dard, N.; Aouinti, M.; Poncin-Epaillard, F.; Bertrand, P. Surf. Interface Anal. 2001, 31, 1042-1047.

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Figure 2. C 1s high-resolution peak of HDPE treated in CO2/ H2O plasma (P ) 50 W; t ) 1 min; d ) 8 cm; F(CO2) ) F(H2O) ) 3 sccm; p ) 2 × 10-1 mbar). Table 2. XPS Analysis of Virgin or Treated HDPE (P ) 50 W; t ) 1 min; d ) 8 cm)

Figure 1. Dependence of the carboxylic functionalization of HDPE treated in CO2/H2O plasma versus gas composition and discharge power.

(CO2) plasma was first studied. The plasma conditions were the following: Pi, variable; Pr < 0.02 W; t ) 1 min; d ) 8 cm; F(H2O) ) 3 sccm; F(CO2) variable. The carboxylic acid fixation versus the CO2/H2O proportion was calculated from an acid-base spectroscopic quantification:6,14

plasma none CO2, F(CO2) ) 6 sccm CO2/H2O, F(CO2) ) F(H2O) ) 3 sccm a

Figure 1 clearly indicated that the addition of H2O in the CO2 plasma did not improve the rate of surface carboxylic species. On the contrary, the reduction of the attached acid groups density for small H2O proportions highlights its inhibiting character. This effect is more marked when the discharge power increases. The treatment of the HDPE surface by CO2/H2O (1:1) plasma results in surface atomic oxygen incorporation (37%) as observed by XPS analyses. The examination of the C 1s highresolution peak showed the existence of four components corresponding to different natures of carbon bonds15 (Figure 2). The main contribution fixed at 284.6 eV is related to the hydrocarbon structure of the polymer and represents the C-C and C-H bonds. The second component located at 286.1 eV indicates the presence of species containing C-O bonds such as alcohol, ether, and (hydro)peroxide. The component at 287.6 eV reveals the presence of CdO bonds assigned to aldehyde and ketone. Last, the peak at 288.4 eV corresponds to carboxylic functions.16 The (14) Ivanov, V. B.; Behnisch, J.; Mehdorn, F. Surf. Interface Anal. 1996, 24, 257-262. (15) Beamson, G.; Briggs, D. High-Resolution XPS of Organics Polymers; Wiley & Sons: Chichester, U.K., 1992. (16) Inagaki, N.; Tasaka, S.; Hibi, K. Polym. Prepr. 1990, 31 (2), 380-381.

bonding energy (eV)

assgnt

284.6 284.6 285.8 286.8 288.4 284.6 285.8 286.8 288.4

C-C, C-H C-C, C-H C-O CdO COO C-C, C-H C-O CdO COOH

R (Gaussian/ σ (eV)a Lorentzian) 1.31 1.65 1.65 1.65 1.65 1.60 1.60 1.60 1.60

0.3 0.542 0.542 0.542 0.542 0.536 0.536 0.536 0.536

rel intensity (%) C, 94; O, 6 78.2 10.0 7.5 4.3 88.2 8.9 1.7 1.2

σ: full width at half-maximum.

presence of ester functions (COOR: 288.4 eV) was considered negligible due to the absence of alkyl groups (R) in the plasma phase as well as in the degradation byproducts as observed by mass spectrometric and optical emission spectroscopy.12 In addition, no ester peak was characterized from SIMS measurement on a treated sample.13 To evaluate the efficiency of the CO2/H2O plasma treatment for the surface carboxylic acid functionalization, an XPS analysis was also carried out on a sample treated by pure CO2 plasma under same conditions. The results (Table 2) showed same behavior as previously obtained with the spectrochemical titration, i.e., a lower functionalization in the CO2/H2O plasma mixture. This latter event seemed mainly to induce the formation of alcohol, ether, and (hydro)peroxide functions rather than carboxylic acid one’s. This behavior was similar to pure H2O plasma treatment, which mainly generates surface alcohol species.17 Consequently, the water seemed acting in a role of a weaker oxidizing agent. Thus, contrary to what we supposed, addition of water in CO2 plasmas did not improve the carboxylic density on the HDPE surface. The active species formed by recombination in CO2/H2O plasma mixtures did not lead to the formation of plasma carboxylic precursors. In situ analyses of the plasma composition by mass spectrometry appeared necessary to characterize plasma fragmentation and recombination reactions to explain this latter result. Mass Spectrometry. Mass spectroscopy analyses were performed only on the neutral species present in the plasma phase because of the small ion proportions (1 ion for approximately 105-106 neutrals) and their low probability to reach the mass analyzer without any applied (17) Vargo, T.; Hook, D. J.; Gardella, J. A. J. Polym. Sci. 1991, 29, 535-545.

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Table 3. Normalized Intensities of Mass Spectroscopy Peaks for Different Gases and Plasmas Species (without PE Film) CO2 gas

O+

CO22+

CO+

O2+

CO2 plasma

O+ 0.1 H2+ 4 H2+ 182 H2+ 0.03 H2+ 10

CO22+ 0.05 O+ 4 O+ 82 O+ 0.001 O+ 2

CO+ 25 OH+ 12 OH+ 82 OH+ 10 OH+ 5

O2+ 2 O2+ 4 O2+ 82 H2O+ 100 H2O+ 100

H2O gas H2O plasma CO2/H2O gas CO2/H2O plasma

CO2+ 100 CO2+ 100 H2O+ 100 H2O+ 100 CO+ 0.3 CO+ 1 × 104

HCO+ 7 × 10-4 HCO+ 2

H2CO+ 2 × 10-5 H2CO+ 1

CO2+ 100 CO2+ 500

a CO gas: F(CO ) ) 10 sccm, p ) 3.5 × 10-1 mbar. CO plasma: P ) 100 W, F(CO ) ) 10 sccm, p ) 3.5 × 10-1 mbar. H O gas: F(H O) 2 2 2 2 2 2 ) 3 sccm, p ) 2 × 10-1 mbar. H2O plasma: P ) 100 W, F(H2O) ) 3 sccm, p ) 2 × 10-1 mbar. CO2/H2O gas: F(CO2) ) F(H2O) ) 3 sccm, -1 -1 p ) 2 × 10 mbar. CO2/H2O plasma: P ) 100 W, F(CO2) ) F(H2O) ) 3 sccm, p ) 2 × 10 mbar.

potential on the extraction hole. The study of the two gases and plasmas taken separately (CO2 and H2O) will be first carried out. Then, their mixture will be described. All analyses were run versus the two main plasma parameters: the plasma discharge power and the gas flow. Characterization of the CO2 Plasma. The mass spectrum of the CO2 gas shows the presence of an intense and unique signal at m/z ) 44 that was assigned to the carbon dioxide molecule (CO2+)18,19 (Table 3). It should be noted that first spectrum carried out without any preliminary purification had shown water vapor traces (H2O+, 18 amu; OH+, 17 amu; O+, 16 amu; H2+, 2 amu) and also some peaks related to the presence of nitrogen (N2+, 28 amu; N+, 14 amu). Specific plasma parameters (100 W, 10 sccm) were chosen to locate the mass spectrometer extraction hole in the plasma phase (minimal distance between surfatron and extraction head equal to 25 cm). Compared to the gas one, the CO2 plasma spectrum (Table 3) revealed the presence of new gaseous species (O+, 16 amu; CO22+, 22 amu; CO+, 28 amu; O2+, 32 amu). Apart from the diionization of the carbon dioxide molecule (CO22+) caused by the electronic bombardment in the spectrometer ionization chamber, the presence of these new oxygenated species resulted from dissociation and recombination reactions according to the following equations:20 e-

CO2 y\ z CO + O E dl

e-

z O2 Oads + O y\ E dl

e-

Oads + O y\ z O2 E dl

(1) (2) (3)

e-

zO+O O2 y\ E dl

Here Ed is the dissociation energy of the molecule (Ed1 ) 5.5 eV and Ed2 ) 5.2 eV). According to Clark et al.,21 molecular oxygen could be formed from recombination reactions of atomic oxygen produced by either the dissociation of carbon dioxide or adsorbed on the reactor wall (Oads). When the CO2 gas flow increased, the plasma tended toward homogenization (Figure 3). From a 10 sccm gas flow, it appears fairly stable. The dissociation and (18) Kato, S.; Yamamoto, Y.; Okuyama, M. Joint Power Gen. Conf. 1997, 1, 497-499. (19) Kato, S.; Lee, S. J. Macroscale Thermophys. Eng. 1997, 1, 245251. (20) Wu, D.; Outlaw, R. A.; Ash, R. L. J. Vac. Sci. Technol. 1996, A14 (2), 408-414. (21) Clark, D. T.; Wilson, R. J. Polym. Sci. 1983, 21, 837-853.

Figure 3. Dependence of CO2 plasma composition versus gas flow (P ) 100 W; d ) 25.5 cm).

recombination reactions appeared to be dependent on the gas flow. Various evolutions of the plasma species can be explained in the following way. At a given power, a given quantity of electrons, produced by the carbon dioxide molecule ionization (CO2 f CO2+ + e-), initiated the dissociation reaction of carbon dioxide (eq 1). At low gas flow, electron density was higher than that for carbon dioxide. Thus, with a 5 sccm gas flow, carbon monoxide concentration was higher than that for carbon dioxide. The gas flow increase led to higher CO2 proportion in the plasma phase. From 10 sccm gas flow, the CO, O2, and O species densities remain constant meaning that the dissociation reaction (eq 1) was complete due to the limitation of the provided electrons densities. Later on, the increase of the gas flow involved only an increase of CO2 proportion in the plasma. Another observation was carried out concerning monoand dioxygenated species (O and O2). Whatever the flow, the molecular oxygen proportion was always higher than its monatomic counterpart ([O2]/[O] ∼ 100). Thus, the atomic oxygen recombination reactions (eqs 2 and 3) were almost quantitative indicating its high reactivity, as already noted by other authors.21,22 When the discharge power and consequently the quantity of induced electrons increases (Figure 4), the (22) Podhajny, R. M. J. Plast. Film 1987, 4, 177-188.

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Figure 4. Dependence of CO2 plasma composition versus the discharge power (F(CO2) ) 10 sccm; p ) 3.5 × 10-1 mbar; d ) 25.5 cm).

Figure 6. Temporal dependence of CO2 plasma composition with or without HDPE (P ) 100 W; F(CO2) ) 20 sccm; p ) 7.5 × 10-1 mbar).

Figure 5. Dependence of CO2 dissociation rate in the plasma versus parameter W/FM (R(CO2) ) 100I(CO2)/(I(CO2) + I(CO) + I(O2) + I(O)).

presented in Figure 6. At the time of the plasma activation, a fast dissociation of the carbon dioxide molecule characterized by the presence of previously described oxygenated species was observed (Figure 6a). With the polymer film (Figure 6b), the spectrum clearly shows the appearance of two new species: atomic (H+) and molecular (H2+) hydrogen. The surface degradation characterized initially, via bond scissions, by the elimination of hydrogen atoms12 explained the presence of such species. If some other fragments are formed in the analyzed mass range (0-542 amu), they were in too low concentration to be detected. This surface degradation previously observed was often associated with oxygenated plasmas (O2 or CO2) that produce atomic oxygen:24

partial pressures of oxygenated species, except for carbon dioxide, increased due to more important CO2 molecule ionization and dissociation. These dissociation processes observed versus the gas flow and the discharge power can also be described from a universal parameter W/FM,23 where W is the discharge power, F the gas flow, and M its molar mass. This parameter represents the energy provided/gas mass unit. Figure 5 clearly indicated that the carbon dioxide dissociation varied in a linear way with the discharge power and was inversely proportional to the gas flow. Characterization of CO2 Plasma with Polyethylene. Mass spectrometry can also provide information on species ejected from the surface during the plasma treatment. In this study, high power (P ) 100 W) and gas flow (F(CO2) ) 20 sccm) were chosen because these conditions favor the degradation and the plasma stability, respectively. The results of the temporal variations of the plasma species in the presence or not of HDPE are

Here O(3P) and O(1D) represent the initial and metastable states of atomic oxygen, respectively, O2*(A3Σu+) and O2*(B3Σu+) are the two excited states of the molecule, and E is the recombination reaction energy. Figure 7 presents

(23) Yasuda, H. Plasma Polymerization; Academic Press Inc.: Orlando, FL, 1985; p 168.

(24) Dzioba, S.; Este, G.; Naguib, H. M. J. Electrochem. Soc. 1982, 129 (11), 2537-2541.

e-

CO2 y\ z CO + O(3P or 1D) E dl

e-

e-

O2 + e- 98 O2*(A3Σu+) 98 O(3P) + O(3P) + ee-

e-

O2 98 O2*(B3Σu+) 98 O(3P) + O(1D) + e-

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Figure 7. Correlation between the surface weight loss and the partial pressure of atomic oxygen (m/z ) 16) in CO2 plasma (F(CO2) ) 10 sccm; p ) 3.5 × 10-1 mbar).

the dependence of HDPE degradation (estimated by gravimetric measurement) versus the O+ peak intensity (measured by mass spectrometry). The fairly good linear correlation (r2 ) 0.991) between these two parameters confirmed the significant role of atomic oxygen (O(3P) and/ or O(1D)) on the degradation mechanism as already shown with other studies such as titration in the gas phase25 and ESR.26 Characterization of the H2O Plasma with or without Polyethylene. The H2O gas mass analysis (Table 3) showed, apart of the water parent peak (H2O+, 18 amu), three signals (HO+, 17 amu; O+, 16 amu; H2+, 2 amu) relative to the water decomposition in the ionization chamber of the mass spectrometer (Ed(H2O) ) 5.1 eV against 5.5 eV for the CO2 molecule).27 Compared to the gas analysis, the mass H2O plasma spectrum exhibited only one additional peak (O2+, 32 amu). However, the important evolution of the partial pressures of the species initially present indicated that, during the plasma phase study, the water fragmentation occurring in the ionization chamber was negligible. Thus, the water dissociation in the plasma phase could be described in the following way: e-

HO2 y\z OH + H e-

OH y\z O + H e-

Oads + O y\z O2 e-

Oads + Oad y\z O2 e-

O2 y\z O + O Here Oads represents an oxygen atom adsorbed on the wall of the quartz cylinder. The absence of atomic hydrogen indicated its significant reactivity in the following recombination reaction:

2H f H2 (25) Battey, J. F. IEEE Trans. Electron Devices 1977, 24 (2), 140145. (26) Koretsky, M. D.; Reimer, J. A. J. Appl. Phys. 1992, 72 (11), 5081-5088. (27) Lide, D. R. Handbook of Chemistry and Physics, 80th ed.; Weast, R. C., Ed.; CRC Press: Washington, DC, 1999; pp 9-51.

Figure 8. Dependence of H2O plasma composition versus the discharge power (F(H2O) ) 3 sccm; p ) 2 × 10-1 mbar).

Figure 9. Dependence of H2O dissociation rate in the plasma versus W/FM parameter (R(H2O) ) 100I(H2O)/(I(H2O) + I(HO) + I(O2) + I(O) + I(H2)).

Increasing the discharge power enlarges the density of the gaseous reactive species (Figure 8), but the dissociation rate of these species seems to evolve slightly mostly due to the easy initial fragmentation of the water molecule. Contrary to what was observed with CO2, the H2O fragmentation versus the W/FM parameter (Figure 9) takes place rapidly with the increase of the discharge power whatever the gas flow is. The different values of dissociation and recombination energies probably explain this observation. Increasing the discharge power increases the density of the gaseous reactive species. The H2O plasma contains molecular hydrogen in important concentration (IH2 ) 105-106 cps). During the study of H2O plasma/HDPE surface interaction, no trace of atomic hydrogen that characterizes the existence of surface degradation was detected. The weak degrading behavior of the H2O plasma associated with the important reactivity of atomic hydrogen leading to its recombination into molecular hydrogen can explain this observation. So a fast reaction of hydrogen recombination could be taking place:

Hplasma + Hdeg f H2

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Here Hplasma and Hdeg represent the hydrogen atoms resulting from the plasma phase and from the degradation of the polymer, respectively. Characterization of the CO2/H2O Plasma. In CO2/H2O gas phase, mass studies revealed the appearance of unexpected peaks (Table 3) compare to those in the mass spectra of CO2 and H2O alone. The peak at 28 amu corresponds to CO+ species probably formed from a fragmentation reaction of the carbon dioxide induced by that of a water molecule during electronic bombardment in the ionization chamber. The presence of species at 29 and 30 amu was more difficult to explain. Ihara et al.28 showed also in CO2/H2O plasma the formation of methanol (32 amu) and, in a smaller proportion, of formaldehyde (30 amu). The presence of hydrogen peroxide (34 amu) and of oxalic acid (90 amu) was characterized by chromatographic analysis of the gas products. In our case, only the formaldehyde (29 and 30 amu) was identified. The formation of such a product is described hereafter:

Figure 10. Dependence of the CO2 dissociation versus its concentration (P ) 100 W; F(H2O) ) 3 sccm; F(CO2) variable).

Since Ihara28 characterizes formaldehyde only in the plasma, its presence in the gaseous medium was surprising. However, its formation can be explained by dissociation and recombination reactions of the water and carbon dioxide molecules in the mass spectrometer ionization chamber. Compared to the gaseous phase, two additional peaks, CO22+ (22 amu) and O2+ (32 amu), were observed in the CO2/H2O plasma phase. Significant increase of the formaldehyde peaks (29 and 30 amu) suggests that, in this case, this last was mainly formed in the plasma. We also noted the absence of COOH+ peaks (45 amu) indicating that, contrary to our supposition (Table 1), no recombination reaction occurred between CO2 and H2O plasma species. This absence of carboxylic functions precursor species explained why no increase of surface carboxylic groups density occurred. Only CO2 species led to the formation of surface carboxylic groups. However, this observation did not explain why the functionalization decreased. Complementary studies were needed to follow the evolution of CO2 plasma species (responsible of the carboxylic functionalization) versus the plasma composition. The evolution of the CO2 species density versus the W/FM parameter (Figure 11) let it appear that the discharge power and also the water gas flow widely influence CO2 dissociation. This carbon dioxide dissociation by water is associated with a mechanism of excitation transfer already described by Ricard.29 In reactive gases mixtures, excitation transfers between active species occurred and are added to electronic collisions in the discharge. They become predominant in postdischarge, where there is no more electric field to give energy to electrons. Among all observable excitation transfers, the ionizing transfers are important with mixtures of rare gas (Ar, He) and molecular gas (N2, O2). In our case, water seems to play a role similar to that of rare gas by (28) Ihara, T.; Ouro, T.; Ochiai, T.; Kiboku, M.; Irayama, Y. Bull. Chem. Soc. Jpn 1996, 69, 241-243. (29) Ricard, A. Reactive Plasmas; SFV: Paris, 1995; p 55.

Figure 11. Dependence of the CO2 dissociation rate in CO2/ H2O plasma versus W/FM parameter (R(CO2) ) 100I(CO2)/ (I(CO2) + I(CO) + I(O2) + I(O)).

transferring its energy for CO2 dissociation probably with the following such mechanisms29 Penning ionization:

M* + CO2 f M + CO+ + O + eor

M* + CO2 f M + CO + O+ + eHere M and M* are respectively water stable and metastable species (H2O*, OH*, ...) with an energy close to 10-20 eV. Charge transfer:

M+ + CO2 f M + CO+ + O or

Plasma Chemistry for Surface Modification

Langmuir, Vol. 18, No. 6, 2002 2253

M+ + CO2 f M + CO + O+

Conclusion In this study, an attempt to increase to surface carboxylic functions was performed by using a CO2/H2O plasma with the goal of inducing the formation of a new plasma species (COOH) responsible of the surface carboxylic increase. The results of spectrochemical quantification and of XPS analysis show however the large decrease of carboxylic acid whatever the water proportion and the discharge power. The mass spectrometry study exhibited that no recombination reactions between CO2 and H, CO, and OH radicals leading to the formation of the COOH plasma species occurs in such a plasma. But a mechanism of excitation transfer was observed leading to the dissociation of the carbon dioxide molecule to CO and O species in the presence of water vapor. CO plasma species react with water fragments to give formaldehyde byproduct. Recombination processes displaying at the CO2 plasma/ polyethylene interface show the appearance of atomic and molecular hydrogen. By correlation with gravimetric measurements, the atomic oxygen of excited level O3P and/or O1D created by the discharge CO2 plasma seems largely responsible of the degradation process.

This type of transfer can also be nondissociative. Most of time, its rate constant is much more significant than that of Penning reactions. Finally, the nonionizing transfer:

M+ + CO2 f M + CO* + O or

M+ + CO2 f M + CO + O* This reaction occurs when the metastable energy is much lower than the ionization energy of gas. The dissociation of the carbon dioxide molecule into carbon monoxide and oxygen species by water plasma explained thus the decrease of the concentration of surface carboxylic acids characterized previously. Indeed, this CO2 plasma species is directly associated with surface functionalization with carboxylic acid.

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