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Characterization by optical emission spectroscopy of an oxygen plasma used for improving PET wettability
Vacuum 84 (2010) 902–906
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Characterization by optical emission spectroscopy of an oxygen plasma used for improving PET wettability Espedito Vassallo*, Anna Cremona, Francesco Ghezzi, Daria Ricci Istituto di Fisica del Plasma – Consiglio Nazionale delle Ricerche (CNR), via R. Cozzi 53, 20125 Milano, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 September 2009 Received in revised form 15 December 2009 Accepted 17 December 2009
Polymers have excellent bulk physical and chemical properties but usually poor surface properties. For wettability improvement plasma technology is one of the most promising techniques. Several studies about surface modifications of polyethylene terephthalate (PET) exposed to an oxygen plasma have been already carried out. In this work an analysis of the plasma phase by optical emission spectroscopy (OES) has been employed in order to establish a correlation with the surface effects induced by plasma exposition on PET chemical composition and wettability, investigated by X-ray photoelectron spectroscopy (XPS) and water contact angle measurements, respectively. The treatment has been carried out for a time of 60 s at a constant pressure (15 Pa) and at different process powers ranging from 20 to 200 W. As expected, the best performance has been obtained at a power of 200 W due to the larger presence of oxygen radicals (OI) with the assistance of ionic species (OII, Oþ 2 ) which create dangling bonds on the substrate surface. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Optical emission spectroscopy (OES) Oxygen plasma PET Water contact angle X-ray photoelectron spectroscopy (XPS)
1. Introduction Polymer surfaces are generally characterized by poor adhesive properties. In order to improve adhesion between a polymer and a coating, the surface of the polymer must be treated. Chemical treatments are often employed but solvent processes produce large volumes of waste. Some solvents can be recycled, but the majority must be disposed by incineration or landfill. Plasma technology is environmentally friendly and it is an innovative technology which can be alternative to conventional methods. During the process, the polymer exposed to the plasma undergoes interactions with ions and molecules. The treatment produces a synergetic result of cleaning (removal of surface contaminants) and activation (formation of new surface chemical groups) of the polymer. Different plasma parameters and gases (Ar, O2, H2, N2, NH3, N2O3, etc..) can be varied to improve the performance of the material. It is customary to define plasmas in different groups depending on: the operative region of the current-voltage characteristic of the discharge [1], the dominant excitation mechanism, the operating pressure and the electrodes geometry. Pressure is one of the most important process parameters. The recent development of cold atmospheric pressure plasma (Dielectric Barrier Discharges) [2,3] has allowed to develop a plasma process which does not require
* Corresponding author. E-mail address: [email protected] (E. Vassallo). 0042-207X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2009.12.008
a vacuum system. Although the treatment at atmospheric pressure has a lower effective cost, more contamination problems occur and non uniform treatments are generated. In facts, at atmospheric pressure, dust and micron-scale asperities on the surface of the insulator covering the electrodes promote the formation of filaments or micro-discharges which generate a non-uniform overall effect on the exposed polymer and sometimes can even damage it. Low pressure plasma is a more efficient system. In this work an oxygen low pressure plasma, obtained with a parallel electrodes radiofrequency (RF) capacitive-coupled reactor, has been employed to enhance the wettability of PET foil and a correlation between the surface modifications of the material and the plasma phase is investigated by optical emission spectroscopy (OES). 2. Experimental details 2.1. Process reactor The apparatus consists of a parallel-electrodes, capacitivecoupled plasma enhanced chemical vapour deposition (PECVD) system [4], made up of a cylindrical stainless steel vacuum chamber of 25 cm inner diameter with an asymmetric electrode configuration. The powered electrode is connected to a 13.56 MHz power supply, associated to an automatic impedance matching unit, while the other electrode is grounded and works as sample holder. The treatment was performed for 60 s at a total plasma process pressure of about 15 Pa, kept constant by balancing the incoming
E. Vassallo et al. / Vacuum 84 (2010) 902–906
oxygen flux with the system pumping speed. Commercially (99.998% purity) available oxygen was supplied into the discharge vessel through a mass flow controller. The total gas pressure was measured by a capacitive vacuum gauge. Experiments were performed using samples of PET foil (DuPont) of 400 mm thickness and 1 1 cm2 size. Plasma treatment was performed as a function of RF power in the range of 20–200 W. In order to evaluate a degradation condition of polymer, a plasma treatment at high RF power (400 W) was performed. 2.2. Optical emission spectroscopy The plasma phase has been investigated through optical emission spectroscopy. The experimental apparatus consists of a scanning monochromator (Horiba Jobin Yvon iHR550) of the Czerny–Turner type, with a focal length of 0.55 m, built around a holographic diffraction grating with 1800 grooves/mm, coupled with a CCD (Synapse Horiba Jobin Yvon) camera, thermoelectrically cooled to 70 C. The light from the plasma was collected from the bulk volume of the discharge through a quartz window with a plane-convex convergent lens of 1 in. diameter and focused through an optical fiber (length 3 m, core 600 mm, numerical aperture 0.22) onto the entrance slit of the monochromator, keeping the slit aperture fixed at 50 mm. Colored glass filters were used to remove higher diffraction orders from the light. The spectra were acquired in the wavelength range of 300–1000 nm with an integration time of 5 s. The spectral resolution of this system is 0.06 nm. 2.3. X-ray photoelectron spectroscopy The surface chemical characterization was carried out by means of X-ray photoelectron spectroscopy (XPS). The core level spectra have been acquired using a non-monochromatized Al anode X-ray source (hn ¼ 1486.6 eV) VSW model TA10 and a hemispherical analyzer VSW model CLASS 100, equipped with a single channel detector, operating in constant pass energy mode (22 eV) with 0.9 eV of overall resolution. The overall experimental resolution (accounting for lifetime, phonon, source and instrumental broadening) was previously determined using a polycrystalline Ag sample. The surface of the Ag sample was cleaned by Ar sputtering at 4 keV until only the Ag core lines were detectable. Then the analyzer resolution was determined for every pass energy. The core lines fitting was performed through Gaussian-Lorentzian product profile with the Lorentzian function, accounting for the lifetime broadening and a Gaussian function, accounting for the finite instrumental resolution, phonon and photon source broadening. The fitting routine includes also a Shirley background, reproducing the secondary electron background. The survey scans were acquired with an overall experimental resolution of about 2 eV and with a take-off angle of 90 with respect to the surface plane. All the spectra were referenced to the same energy scale determined by the calibration of the Ag 3d5/2 line at 368.3 eV [5]. The pressure inside the analysis chamber was kept at w8 108 Pa. The samples were analysed after 10 min of Ar sputtering (primary beam energy 3 keV; beam current z 0.5 mA) in order to remove the surface contaminated layers due to the air exposure. After the XPS analysis the sputtered depth was measured and estimated as w50 nm. All the samples were affected by charging effects. This effect was compensated by manual shift of the binding energy (BE) scale assigning to the alkyl component of the C 1s photoelectron peak BE ¼ 285 eV [6]. For each element the relative atomic concentration of the species was estimated from the area below the prominent spectral lines and after normalization to the atomic sensitivity factors [7] regardless of the specific chemical state. We remark that
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in the present work the main issues deal with the relative trends and not with the absolute values of the concentrations. This justifies the use of sensitivity factors related to a reference spectrometer rather than ours. 2.4. Water contact angle measurements Contact angle measurements were carried out by an FKV (Bergamo, Italy) dataphysics OCA20 goniometer. The surface wettability was evaluated from the value of the contact angle measured with a drop of distilled water on the surface of the samples. The modifications of the surface wettability were followed for 30 days. 3. Results and discussion 3.1. Plasma phase characterization The emission spectra have been acquired in the wavelength range of 300–1000 nm at different process powers, for brevity we report here only the results of two power levels, i.e. 20 and 200 W. Fig. 1 shows the 777 nm OI triplet which increases of a 40 factor when power increases from 20 to 200 W. In the same range of RF power, the 533 nm OI triplet (Fig. 2) increases of a 75 factor. The higher increase of the 533 nm triplet in comparison with the 777 nm triplet, as a funcion of RF power, is a consequence of the higher energies involved in collisional events occurring in the plasma at high power. In facts, while the energy of the upper levels corresponding to the 533 nm transitions is 13.06 eV, the energy of the upper levels corresponding to the 777 nm transitions is only 10.74 eV. Fig. 2 shows also a large band in the 519–531 nm range, which can be interpreted as the overlapping of vibronic transitions (wavelength of the first heads at 529.57, 527.47, 525.9, 525.1, 524.1, 523.4 nm) belonging to the Oþ 2 first 4 negative system (b 4S g a Pu). This band, which is weakly visible at 20 W, rises of a very large amounts (240 factor) as power increasing. As well-known, in this type of plasma the electrons transfer the energy, absorbed from the external electric field, through collisions with the discharge molecole causing their ionization and dissociation. As RF power increases, the electron energy distribution function (EEDF) changes shape. In particular, the electrons in the high-energy tail of distribution increase enhancing the ionization probability. The larger increase of Oþ 2 species compared to atomic oxygen species agree with the increase of the particles energy at higher power. The small band with band head at 519.82 nm is due to the (0,2) vibronic transition belonging to the CO
Fig. 1. Emission spectra in the 775–780 nm range at 20 and 200 W.
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Fig. 2. Emission spectra in the 515–535 nm range at 20 and 200 W.
Angstrom system B 1S A 1P. This feature, which slightly growths when power increases from 20 to 200 W, is ascribable to the interaction of oxygen radicals with the substrate surface. To evaluate the degradation of the polymer, we have followed the temporal evolution of the CO band and we have observed that it keeps approximately constant also after a time of 10 min, while with a process power of 400 W the signal intensity increases significantly during the time and, after 10 min, the specimen is completely degraded. So we can suppose that the treatment carried out for a time of 60 s with a process power of 200 W does not damage the polymer. This result is in accordance with the XPS analysis, which has demonstrated, as we will discuss in the following paragraph, that the treatment has not caused substantial modification of the chemical composition of the material. Fig. 3 shows the rotational structure of the Fraunhofer A-band centered near 761.9 nm, corresponding to the (0–0) magnetic 3 þ dipole transition of the O2 b 1S g X Sg atmospheric band system. The intense line at 763.51 nm is due to the presence of argon desorbed off the walls of the vessel. Fig. 4 shows the presence of OII lines (energy of first ionization of atomic oxygen 13.62 eV) in the spectrum related to the plasma generated at 200 W which are totally absent in the 20 W spectrum. This figure shows also the Hg line due to the water vapour present in the vessel. While the intensity of the OI, OII and Oþ 2 emitting species grows considerably
Fig. 3. Emission spectra in the 758–770 nm range at 20 and 200 W.
Fig. 4. Emission spectra in the 433–439 nm range at 20 and 200 W.
when power increases from 20 to 200 W, the intensity of the O2 Fraunhofer A-band increases lightly (2 factor). Because the plasma pressure does not change during the experiment, we ascribe this effect to a strong quenching of the O2 excited level, probably due to collisions with ArI atoms, as the considerable growth (w10 factor) of the 763.51 nm line suggests. 3.2. Solid phase characterization Quantitative analysis was performed to obtain the relative concentrations of C1s, O1s components in the surface layers of the as-received and plasma treated PET films. Fig. 5 shows the C1s core level of the untreated PET. It is composed of three main components at a binding energy, respectively, of 285 eV which corresponds to C–C and C–H bonds (carbon atoms in phenyl ring), 286.5 eV which corresponds to C–O bonds (methylene carbon atoms singly bonded to oxygen atoms), and 288.9 eV which corresponds to O]C–O bonds (ester carbon atoms). The atomic ratio (O/C), calculated from the areas under the O1s and C1s core level lines, is 0.43. Fig. 6 shows the spectra of the treated PET as a function of RF plasma power. Plasma treatment produced surface modifications with the incorporation of hydrophilic functional groups as carbonyl [C]O] and carboxylic [–COOH] groups (both at ca. 287.5 eV) [8]. However, two different mechanisms take place at low and high
Fig. 5. XPS spectrum of the as received PET film.
E. Vassallo et al. / Vacuum 84 (2010) 902–906
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Fig. 6. High resolution C1s spectra of plasma treated PET workpieces as a function of RF power.
Fig. 8. XPS spectra of the untreated PET and of PET subjected to a weak ionic bombardment.
power. At low power level (20 W) the plasma is mainly rich of radical species, as showed by the OES, which react with methylene (C–O) and ester groups (O–C]O) present in the polymeric chains producing a slight etching of them and new hydrophilic species as carbonyl groups. At high power level (200 W), due to the synergistic effect of ions and radicals, as showed by the OES, a more efficient incorporation of hydrophilic species into the PET surface was observed. This effect took place at expenses of the C–C and C–H bounds on the phenyl ring (C1s spectrum at ca. 285 eV), as revealed in Fig. 6. These rationales explain the increasing of the atomic ratio (O/C) and the decreasing of the contact angle at any plasma power (Fig. 7 and Fig. 9). In order to compare plasma treatment to the surface modifications induced by a sputtering process, an XPS spectrum of untreated PET [9] subjected for 30 seconds to ionic bombardment with an Ar beam of weak intensity (3 1012 ions cm2 s1) has been observed. Fig. 8 shows the changes of the carbon relative concentrations with respect to untreated PET. It is clear that ion beam induces a strong degradation of the polymer chain and so a substantial modification of its chemical composition. The atomic ratio (O/C) decreases to 0.27. The broadening of the main peak can be due to carbonization processes. So plasma treatment, when compared to ionic bombardment, appears a softer process because it does not involve a drastic modification of the chemical composition of the polymer. Plasma treatment has been employed to enhance the adhesive properties of PET. In order to analyse the treatment effectiveness, the surface wettability was evaluated before and after the plasma process. Since polar functional groups produced during the process tend to degrade slowly as a consequence of the rearrangement of
surface polar groups due to diffusion phenomena from the surface into the bulk and chemical reactions with the atmosphere, wettability versus the ageing time was evaluated (Fig. 9). The lowest value for the water contact angle was obtained at high power (q < 10 ). This indicates that a great number of polar functional groups was created on the sample surface. In any case, even with the lowest power a low contact angle (q < 40 ) was obtained. The contact angle of untreated PET was estimated 71. The surface ageing was observed for all the samples. Results indicate that the ageing of oxidized functional groups was very high in the first day after the treatment and then, after about 2 days, the surface was stabilized. The contact angle remained constant around 56 even after 30 days.
Fig. 7. O/C atomic ratio as a function of RF power.
Fig. 9. Water contact angle as a function of the ageing time at different plasma powers.
3.3. Correlation between plasma phase and solid phase analysis The improvement of PET wettability at 200 W is ascribable to the combined effect of ionic species (OII and Oþ 2 ) and neutral species (OI radicals), as observed by optical emission spectroscopy. Ions generated at high power in the bulk plasma are accelerated across the plasma sheath surrounding the PET workpiece and then interact with the surface breaking the polymer chains and creating
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dangling bonds. The bond scission of the polymer promotes chemical reactions with the radicals created in plasma phase and so the generation of oxygen functionalities which improve the adhesive properties of the polymer is very efficient. Because the electrode configuration of the reactor is highly asymmetric (the powered electrode has a smaller surface area), the sheath potential drop between the plasma and the workpiece located on the grounded electrode is small and so the ionic bombardment at the surface is not too energetic and does not damage the material but is anyway sufficient to activate it. 4. Conclusion Polymer surfaces are generally characterized by a low surface energy and hence poor adhesive and wettability properties. Polyethylene terephthalate (PET) samples have been treated by means of an oxygen plasma to improve their wettability. As indicated by the plasma phase analysis with OES, at low power the heavy species are essentially neutrals (OI radicals), while at high power ionic species (OII and Oþ 2 ) together with a larger quantity of oxygen radicals contribute to the process purpose, giving a better result. The interaction between the substrate and the ions produces a removing of surface organic contaminants and a surface activation with formation of dangling bonds which react with the oxygen radicals present in plasma phase and cause a surface functionalization, as resulted from XPS analysis, which has shown a growth of the concentration of oxygenated functional groups with power increasing. (Fig. 7). Contact angle measurements confirm the enhancement of the surface wettability as a function of RF power. Moreover the interaction between the ions and the substrate has been valued to be less drastic than an ionic bombardment treatment with an Ar beam of weak intensity (3 1012 ions cm2 s1) because it does not modify in a substantial way the chemical
composition of the PET polymer chains but it limit itself to induce a surface oxygenation process which enhances the PET wettability. This is confirmed by the OES analysis of the plasma phase. In fact the optical diagnostic has shown that the small CO band at 519.82 nm due to the chemical etching made by oxygen radicals does not change significantly during the time, and this indicates that plasma treatment has not caused a degradation of the polymer. Since for industrial application it is fundamental to know the duration of the surface wettability, the ageing of treated PET was studied for 30 days. Results indicate that the ageing was very high in the first day after the treatment and then, after about 2 days, the surface was stabilized. In any case, the contact angle remained constant around 56 even after 30 days. When required by the application, an improvement of the ageing of the plasma treated PET is possible with a modification of the process parameters or with a further cross-linking [10] process (branching of the surface molecules). References [1] Roth JR. Industrial plasma engineering, vol. 1. Bristol/Philadelphia: Institute of Physics Publishing; 1995. [2] Conrads H, Schmidt M. Plasma Sources Sci Technol 2000;9:441. [3] Yang S, Gupta MC. Surf Coat Technol 2004;187:172. [4] Vassallo E, Laguardia L, Catellani M, Cremona A, Dellera F, Ghezzi F. Plasma Process Polym 2007;4(S1):801. [5] Romand R, Roubin M, Deloume JP. J Electron Spectrosc Relat Phenom 1978:229. [6] Beamson G, Briggs D. The XPS of polymer database. Manchester: SurfaceSpectra Ltd.; 2000. [7] Moulder JF, Stckle WF, Sobol PE, Bomben KD. Handbook of X-ray photoelectron spectroscopy. Minnesota: Physical Electronics, Inc.; 1995. [8] Strobel M, Lyons CS, Mittal KL, editors. Plasma surface modification of polymers: relevance to adhesion;, ISBN 90-6764-164-2; 1994. p. 43–64. [9] Grant JL, Dunn DS, McClure DJ. J Vac Sci Technol 1988;A6:2213. [10] Arefi-Khonsari F, Tatoulian M, Kurdi J, Ben-Rejeb S, Amouroux J. J Photopoly Sci Technol 1998;2:277.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Characterization by optical emission spectroscopy of an oxygen plasma used for improving PET wettability Espedito Vassallo*, Anna Cremona, Francesco Ghezzi, Daria Ricci Istituto di Fisica del Plasma – Consiglio Nazionale delle Ricerche (CNR), via R. Cozzi 53, 20125 Milano, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 September 2009 Received in revised form 15 December 2009 Accepted 17 December 2009
Polymers have excellent bulk physical and chemical properties but usually poor surface properties. For wettability improvement plasma technology is one of the most promising techniques. Several studies about surface modifications of polyethylene terephthalate (PET) exposed to an oxygen plasma have been already carried out. In this work an analysis of the plasma phase by optical emission spectroscopy (OES) has been employed in order to establish a correlation with the surface effects induced by plasma exposition on PET chemical composition and wettability, investigated by X-ray photoelectron spectroscopy (XPS) and water contact angle measurements, respectively. The treatment has been carried out for a time of 60 s at a constant pressure (15 Pa) and at different process powers ranging from 20 to 200 W. As expected, the best performance has been obtained at a power of 200 W due to the larger presence of oxygen radicals (OI) with the assistance of ionic species (OII, Oþ 2 ) which create dangling bonds on the substrate surface. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Optical emission spectroscopy (OES) Oxygen plasma PET Water contact angle X-ray photoelectron spectroscopy (XPS)
1. Introduction Polymer surfaces are generally characterized by poor adhesive properties. In order to improve adhesion between a polymer and a coating, the surface of the polymer must be treated. Chemical treatments are often employed but solvent processes produce large volumes of waste. Some solvents can be recycled, but the majority must be disposed by incineration or landfill. Plasma technology is environmentally friendly and it is an innovative technology which can be alternative to conventional methods. During the process, the polymer exposed to the plasma undergoes interactions with ions and molecules. The treatment produces a synergetic result of cleaning (removal of surface contaminants) and activation (formation of new surface chemical groups) of the polymer. Different plasma parameters and gases (Ar, O2, H2, N2, NH3, N2O3, etc..) can be varied to improve the performance of the material. It is customary to define plasmas in different groups depending on: the operative region of the current-voltage characteristic of the discharge [1], the dominant excitation mechanism, the operating pressure and the electrodes geometry. Pressure is one of the most important process parameters. The recent development of cold atmospheric pressure plasma (Dielectric Barrier Discharges) [2,3] has allowed to develop a plasma process which does not require
* Corresponding author. E-mail address: [email protected] (E. Vassallo). 0042-207X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2009.12.008
a vacuum system. Although the treatment at atmospheric pressure has a lower effective cost, more contamination problems occur and non uniform treatments are generated. In facts, at atmospheric pressure, dust and micron-scale asperities on the surface of the insulator covering the electrodes promote the formation of filaments or micro-discharges which generate a non-uniform overall effect on the exposed polymer and sometimes can even damage it. Low pressure plasma is a more efficient system. In this work an oxygen low pressure plasma, obtained with a parallel electrodes radiofrequency (RF) capacitive-coupled reactor, has been employed to enhance the wettability of PET foil and a correlation between the surface modifications of the material and the plasma phase is investigated by optical emission spectroscopy (OES). 2. Experimental details 2.1. Process reactor The apparatus consists of a parallel-electrodes, capacitivecoupled plasma enhanced chemical vapour deposition (PECVD) system [4], made up of a cylindrical stainless steel vacuum chamber of 25 cm inner diameter with an asymmetric electrode configuration. The powered electrode is connected to a 13.56 MHz power supply, associated to an automatic impedance matching unit, while the other electrode is grounded and works as sample holder. The treatment was performed for 60 s at a total plasma process pressure of about 15 Pa, kept constant by balancing the incoming
E. Vassallo et al. / Vacuum 84 (2010) 902–906
oxygen flux with the system pumping speed. Commercially (99.998% purity) available oxygen was supplied into the discharge vessel through a mass flow controller. The total gas pressure was measured by a capacitive vacuum gauge. Experiments were performed using samples of PET foil (DuPont) of 400 mm thickness and 1 1 cm2 size. Plasma treatment was performed as a function of RF power in the range of 20–200 W. In order to evaluate a degradation condition of polymer, a plasma treatment at high RF power (400 W) was performed. 2.2. Optical emission spectroscopy The plasma phase has been investigated through optical emission spectroscopy. The experimental apparatus consists of a scanning monochromator (Horiba Jobin Yvon iHR550) of the Czerny–Turner type, with a focal length of 0.55 m, built around a holographic diffraction grating with 1800 grooves/mm, coupled with a CCD (Synapse Horiba Jobin Yvon) camera, thermoelectrically cooled to 70 C. The light from the plasma was collected from the bulk volume of the discharge through a quartz window with a plane-convex convergent lens of 1 in. diameter and focused through an optical fiber (length 3 m, core 600 mm, numerical aperture 0.22) onto the entrance slit of the monochromator, keeping the slit aperture fixed at 50 mm. Colored glass filters were used to remove higher diffraction orders from the light. The spectra were acquired in the wavelength range of 300–1000 nm with an integration time of 5 s. The spectral resolution of this system is 0.06 nm. 2.3. X-ray photoelectron spectroscopy The surface chemical characterization was carried out by means of X-ray photoelectron spectroscopy (XPS). The core level spectra have been acquired using a non-monochromatized Al anode X-ray source (hn ¼ 1486.6 eV) VSW model TA10 and a hemispherical analyzer VSW model CLASS 100, equipped with a single channel detector, operating in constant pass energy mode (22 eV) with 0.9 eV of overall resolution. The overall experimental resolution (accounting for lifetime, phonon, source and instrumental broadening) was previously determined using a polycrystalline Ag sample. The surface of the Ag sample was cleaned by Ar sputtering at 4 keV until only the Ag core lines were detectable. Then the analyzer resolution was determined for every pass energy. The core lines fitting was performed through Gaussian-Lorentzian product profile with the Lorentzian function, accounting for the lifetime broadening and a Gaussian function, accounting for the finite instrumental resolution, phonon and photon source broadening. The fitting routine includes also a Shirley background, reproducing the secondary electron background. The survey scans were acquired with an overall experimental resolution of about 2 eV and with a take-off angle of 90 with respect to the surface plane. All the spectra were referenced to the same energy scale determined by the calibration of the Ag 3d5/2 line at 368.3 eV [5]. The pressure inside the analysis chamber was kept at w8 108 Pa. The samples were analysed after 10 min of Ar sputtering (primary beam energy 3 keV; beam current z 0.5 mA) in order to remove the surface contaminated layers due to the air exposure. After the XPS analysis the sputtered depth was measured and estimated as w50 nm. All the samples were affected by charging effects. This effect was compensated by manual shift of the binding energy (BE) scale assigning to the alkyl component of the C 1s photoelectron peak BE ¼ 285 eV [6]. For each element the relative atomic concentration of the species was estimated from the area below the prominent spectral lines and after normalization to the atomic sensitivity factors [7] regardless of the specific chemical state. We remark that
903
in the present work the main issues deal with the relative trends and not with the absolute values of the concentrations. This justifies the use of sensitivity factors related to a reference spectrometer rather than ours. 2.4. Water contact angle measurements Contact angle measurements were carried out by an FKV (Bergamo, Italy) dataphysics OCA20 goniometer. The surface wettability was evaluated from the value of the contact angle measured with a drop of distilled water on the surface of the samples. The modifications of the surface wettability were followed for 30 days. 3. Results and discussion 3.1. Plasma phase characterization The emission spectra have been acquired in the wavelength range of 300–1000 nm at different process powers, for brevity we report here only the results of two power levels, i.e. 20 and 200 W. Fig. 1 shows the 777 nm OI triplet which increases of a 40 factor when power increases from 20 to 200 W. In the same range of RF power, the 533 nm OI triplet (Fig. 2) increases of a 75 factor. The higher increase of the 533 nm triplet in comparison with the 777 nm triplet, as a funcion of RF power, is a consequence of the higher energies involved in collisional events occurring in the plasma at high power. In facts, while the energy of the upper levels corresponding to the 533 nm transitions is 13.06 eV, the energy of the upper levels corresponding to the 777 nm transitions is only 10.74 eV. Fig. 2 shows also a large band in the 519–531 nm range, which can be interpreted as the overlapping of vibronic transitions (wavelength of the first heads at 529.57, 527.47, 525.9, 525.1, 524.1, 523.4 nm) belonging to the Oþ 2 first 4 negative system (b 4S g a Pu). This band, which is weakly visible at 20 W, rises of a very large amounts (240 factor) as power increasing. As well-known, in this type of plasma the electrons transfer the energy, absorbed from the external electric field, through collisions with the discharge molecole causing their ionization and dissociation. As RF power increases, the electron energy distribution function (EEDF) changes shape. In particular, the electrons in the high-energy tail of distribution increase enhancing the ionization probability. The larger increase of Oþ 2 species compared to atomic oxygen species agree with the increase of the particles energy at higher power. The small band with band head at 519.82 nm is due to the (0,2) vibronic transition belonging to the CO
Fig. 1. Emission spectra in the 775–780 nm range at 20 and 200 W.
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Fig. 2. Emission spectra in the 515–535 nm range at 20 and 200 W.
Angstrom system B 1S A 1P. This feature, which slightly growths when power increases from 20 to 200 W, is ascribable to the interaction of oxygen radicals with the substrate surface. To evaluate the degradation of the polymer, we have followed the temporal evolution of the CO band and we have observed that it keeps approximately constant also after a time of 10 min, while with a process power of 400 W the signal intensity increases significantly during the time and, after 10 min, the specimen is completely degraded. So we can suppose that the treatment carried out for a time of 60 s with a process power of 200 W does not damage the polymer. This result is in accordance with the XPS analysis, which has demonstrated, as we will discuss in the following paragraph, that the treatment has not caused substantial modification of the chemical composition of the material. Fig. 3 shows the rotational structure of the Fraunhofer A-band centered near 761.9 nm, corresponding to the (0–0) magnetic 3 þ dipole transition of the O2 b 1S g X Sg atmospheric band system. The intense line at 763.51 nm is due to the presence of argon desorbed off the walls of the vessel. Fig. 4 shows the presence of OII lines (energy of first ionization of atomic oxygen 13.62 eV) in the spectrum related to the plasma generated at 200 W which are totally absent in the 20 W spectrum. This figure shows also the Hg line due to the water vapour present in the vessel. While the intensity of the OI, OII and Oþ 2 emitting species grows considerably
Fig. 3. Emission spectra in the 758–770 nm range at 20 and 200 W.
Fig. 4. Emission spectra in the 433–439 nm range at 20 and 200 W.
when power increases from 20 to 200 W, the intensity of the O2 Fraunhofer A-band increases lightly (2 factor). Because the plasma pressure does not change during the experiment, we ascribe this effect to a strong quenching of the O2 excited level, probably due to collisions with ArI atoms, as the considerable growth (w10 factor) of the 763.51 nm line suggests. 3.2. Solid phase characterization Quantitative analysis was performed to obtain the relative concentrations of C1s, O1s components in the surface layers of the as-received and plasma treated PET films. Fig. 5 shows the C1s core level of the untreated PET. It is composed of three main components at a binding energy, respectively, of 285 eV which corresponds to C–C and C–H bonds (carbon atoms in phenyl ring), 286.5 eV which corresponds to C–O bonds (methylene carbon atoms singly bonded to oxygen atoms), and 288.9 eV which corresponds to O]C–O bonds (ester carbon atoms). The atomic ratio (O/C), calculated from the areas under the O1s and C1s core level lines, is 0.43. Fig. 6 shows the spectra of the treated PET as a function of RF plasma power. Plasma treatment produced surface modifications with the incorporation of hydrophilic functional groups as carbonyl [C]O] and carboxylic [–COOH] groups (both at ca. 287.5 eV) [8]. However, two different mechanisms take place at low and high
Fig. 5. XPS spectrum of the as received PET film.
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Fig. 6. High resolution C1s spectra of plasma treated PET workpieces as a function of RF power.
Fig. 8. XPS spectra of the untreated PET and of PET subjected to a weak ionic bombardment.
power. At low power level (20 W) the plasma is mainly rich of radical species, as showed by the OES, which react with methylene (C–O) and ester groups (O–C]O) present in the polymeric chains producing a slight etching of them and new hydrophilic species as carbonyl groups. At high power level (200 W), due to the synergistic effect of ions and radicals, as showed by the OES, a more efficient incorporation of hydrophilic species into the PET surface was observed. This effect took place at expenses of the C–C and C–H bounds on the phenyl ring (C1s spectrum at ca. 285 eV), as revealed in Fig. 6. These rationales explain the increasing of the atomic ratio (O/C) and the decreasing of the contact angle at any plasma power (Fig. 7 and Fig. 9). In order to compare plasma treatment to the surface modifications induced by a sputtering process, an XPS spectrum of untreated PET [9] subjected for 30 seconds to ionic bombardment with an Ar beam of weak intensity (3 1012 ions cm2 s1) has been observed. Fig. 8 shows the changes of the carbon relative concentrations with respect to untreated PET. It is clear that ion beam induces a strong degradation of the polymer chain and so a substantial modification of its chemical composition. The atomic ratio (O/C) decreases to 0.27. The broadening of the main peak can be due to carbonization processes. So plasma treatment, when compared to ionic bombardment, appears a softer process because it does not involve a drastic modification of the chemical composition of the polymer. Plasma treatment has been employed to enhance the adhesive properties of PET. In order to analyse the treatment effectiveness, the surface wettability was evaluated before and after the plasma process. Since polar functional groups produced during the process tend to degrade slowly as a consequence of the rearrangement of
surface polar groups due to diffusion phenomena from the surface into the bulk and chemical reactions with the atmosphere, wettability versus the ageing time was evaluated (Fig. 9). The lowest value for the water contact angle was obtained at high power (q < 10 ). This indicates that a great number of polar functional groups was created on the sample surface. In any case, even with the lowest power a low contact angle (q < 40 ) was obtained. The contact angle of untreated PET was estimated 71. The surface ageing was observed for all the samples. Results indicate that the ageing of oxidized functional groups was very high in the first day after the treatment and then, after about 2 days, the surface was stabilized. The contact angle remained constant around 56 even after 30 days.
Fig. 7. O/C atomic ratio as a function of RF power.
Fig. 9. Water contact angle as a function of the ageing time at different plasma powers.
3.3. Correlation between plasma phase and solid phase analysis The improvement of PET wettability at 200 W is ascribable to the combined effect of ionic species (OII and Oþ 2 ) and neutral species (OI radicals), as observed by optical emission spectroscopy. Ions generated at high power in the bulk plasma are accelerated across the plasma sheath surrounding the PET workpiece and then interact with the surface breaking the polymer chains and creating
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dangling bonds. The bond scission of the polymer promotes chemical reactions with the radicals created in plasma phase and so the generation of oxygen functionalities which improve the adhesive properties of the polymer is very efficient. Because the electrode configuration of the reactor is highly asymmetric (the powered electrode has a smaller surface area), the sheath potential drop between the plasma and the workpiece located on the grounded electrode is small and so the ionic bombardment at the surface is not too energetic and does not damage the material but is anyway sufficient to activate it. 4. Conclusion Polymer surfaces are generally characterized by a low surface energy and hence poor adhesive and wettability properties. Polyethylene terephthalate (PET) samples have been treated by means of an oxygen plasma to improve their wettability. As indicated by the plasma phase analysis with OES, at low power the heavy species are essentially neutrals (OI radicals), while at high power ionic species (OII and Oþ 2 ) together with a larger quantity of oxygen radicals contribute to the process purpose, giving a better result. The interaction between the substrate and the ions produces a removing of surface organic contaminants and a surface activation with formation of dangling bonds which react with the oxygen radicals present in plasma phase and cause a surface functionalization, as resulted from XPS analysis, which has shown a growth of the concentration of oxygenated functional groups with power increasing. (Fig. 7). Contact angle measurements confirm the enhancement of the surface wettability as a function of RF power. Moreover the interaction between the ions and the substrate has been valued to be less drastic than an ionic bombardment treatment with an Ar beam of weak intensity (3 1012 ions cm2 s1) because it does not modify in a substantial way the chemical
composition of the PET polymer chains but it limit itself to induce a surface oxygenation process which enhances the PET wettability. This is confirmed by the OES analysis of the plasma phase. In fact the optical diagnostic has shown that the small CO band at 519.82 nm due to the chemical etching made by oxygen radicals does not change significantly during the time, and this indicates that plasma treatment has not caused a degradation of the polymer. Since for industrial application it is fundamental to know the duration of the surface wettability, the ageing of treated PET was studied for 30 days. Results indicate that the ageing was very high in the first day after the treatment and then, after about 2 days, the surface was stabilized. In any case, the contact angle remained constant around 56 even after 30 days. When required by the application, an improvement of the ageing of the plasma treated PET is possible with a modification of the process parameters or with a further cross-linking [10] process (branching of the surface molecules). References [1] Roth JR. Industrial plasma engineering, vol. 1. Bristol/Philadelphia: Institute of Physics Publishing; 1995. [2] Conrads H, Schmidt M. Plasma Sources Sci Technol 2000;9:441. [3] Yang S, Gupta MC. Surf Coat Technol 2004;187:172. [4] Vassallo E, Laguardia L, Catellani M, Cremona A, Dellera F, Ghezzi F. Plasma Process Polym 2007;4(S1):801. [5] Romand R, Roubin M, Deloume JP. J Electron Spectrosc Relat Phenom 1978:229. [6] Beamson G, Briggs D. The XPS of polymer database. Manchester: SurfaceSpectra Ltd.; 2000. [7] Moulder JF, Stckle WF, Sobol PE, Bomben KD. Handbook of X-ray photoelectron spectroscopy. Minnesota: Physical Electronics, Inc.; 1995. [8] Strobel M, Lyons CS, Mittal KL, editors. Plasma surface modification of polymers: relevance to adhesion;, ISBN 90-6764-164-2; 1994. p. 43–64. [9] Grant JL, Dunn DS, McClure DJ. J Vac Sci Technol 1988;A6:2213. [10] Arefi-Khonsari F, Tatoulian M, Kurdi J, Ben-Rejeb S, Amouroux J. J Photopoly Sci Technol 1998;2:277.