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Mechanisms of argon ion-beam surface modification of polystyrene
Surface Science 532–535 (2003) 1040–1044 www.elsevier.com/locate/susc
Mechanisms of argon ion-beam surface modification of polystyrene J. Zekonyte, J. Erichsen, V. Zaporojtchenko, F. Faupel
*
Technical Faculty, Kiel University, Kaiserstrasse 2, Kiel (Gaarden) 24143, Germany
Abstract The surface characteristics of polymers are important factors determining their interfacial properties and their technological performance. Changes in physical and chemical properties of a polymer film may be induced by subjecting the material to a variety of surface modification techniques, one of which is ion-beam modification. In order to understand the underlying mechanisms X-ray photoelectron spectroscopy (XPS) was used to study the alterations of the polystyrene (PS) surface after Ar-ion treatment under well controlled conditions with low ion doses from 1012 to 1016 cm2 . The ion bombardment leads to surface functionalization, loss of aromaticity, and free radical formation. Induced surface cross-linking and the formation of polar groups raised the surface glass transition temperature of PS film. 2003 Elsevier Science B.V. All rights reserved. Keywords: X-ray photoelectron spectroscopy; Ion bombardment; Oxidation
1. Introduction Polymers have become key materials in different technological applications, as they have many unique advantages, including their light weight, corrosion resistance, and low manufacturing cost. As most polymers do not posses required surface properties for industrial applications different kind of surface modification techniques were applied [1, Chapter 1]. One of the most versatile technique is plasma treatment, which could produce unique surface properties not affecting polymer bulk, and which was investigated in different research groups
*
Corresponding author. Tel.: +49-431-880-6225; fax: +49431-880-6229. E-mail address: ff@tf.uni-kiel.de (F. Faupel).
[2–5]. In spite of this advantage the plasma process is extremely complex, and modification is caused by different plasma species such as ions, electrons, excited neutrals, and UV irradiation. This makes it difficult to achieve a good understanding of the interactions between plasma and the surface, to change plasma parameters, and to control the amount of a particular functional group formed on the surface [1, Chapter 6]. In ion-beam treatment, compared to the plasma technique, it is easier to change process parameters such as ion energy and ion beam current, and a more precise ion dose can be calculated. Ion-beam treatment could also induce changes in the chemical structure, reactivity and bonding characteristics of polymer surfaces without affecting the polymer bulk. Modification with reactive gas causes ablation of polymers [6], and functionalization takes place
0039-6028/03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00130-4
J. Zekonyte et al. / Surface Science 532–535 (2003) 1040–1044
within the treatment time [2,5,7,8]. The treatment with inert gas leads to hydrogen abstraction [6], creation of long living free radicals that are responsible for the recombination, unsaturation, or cross-linking [2]. Functional group incorporation is more significant after the exposure of treated polymer to the atmosphere [4]. Using high energy (keV–MeV) ion-beams intensive research was done on the induced structural changes of polymers [9] and on the formation of hydrogenated carbon layers exhibiting interesting electronic properties [10]. Some work was done on low ion energy surface chemical modification such as nitrogen incorporation in PS [7]. The improvement in the wettability and adhesion of various polymers was achieved by ion-assisted reaction treatment [8]. This paper mainly presents results on the PS surface glass transition temperature Tg influenced by the changes in surface structural composition, after Arþ ion treatment with low ion dose from 1012 to 1016 cm2 at a constant ion energy of 1 keV.
2. Experimental Monodisperse polystyrene (PS) (Mw ¼ 212 000 g/mol, Mw =Mn < 1:1) used in the experiments was obtained from Aldrich Chemical Company as powder. The polymer films (thickness of approximately 200 nm) were prepared by solving the polymer powder in toluene (27 g/l) and spin coating it on a silicon wafer with its native oxide layer. The influence of film thickness on the glass transition is negligible [11]. PS samples were annealed under UHV conditions for 40 min at a temperature of 130 C and then slowly cooled down to room temperature with constant cooling rate of 1 K/min. X-ray photoelectron spectroscopy (XPS) was chosen to investigate polymer surface decomposition under argon ion bombardment with different ion doses, and to measure the surface glass transition temperature, Tg , using the metal cluster embedding method, developed in our group [12]. XPS measurements were performed using electron spectrometer (Omicron) equipped with a nonmonochromated Mg Ka source. To investigate induced surface chemical changes the C 1s and
1041
O 1s core level spectra were recorded with 20 eV pass energy. The detailed description of the experiment for metal particle embedding temperature measurement was reported in Refs. [12,13]. Briefly, gold was evaporated from a heated molybdenium crucible mounted in the sputter chamber (Omicron Full Lab) onto the PS films at room temperature. The deposition rate and the nominal thickness of the evaporated metal were monitored by a quartz crystal microbalance. After the metal deposition samples were transferred to the analytical chamber and there heated with the rate of 1 K/min up to 423 K. At the same time a series of XPS spectra of Au 4f and C 1s lines with pass energy of 100 eV were taken to check the embedding of the metal nanoparticles. Prior to taking XPS spectra, and evaporating metal onto the polymer, ion bombardment was performed. Using a ISE10 sputter ion gun polymer samples were treated at an ion energy of 1 keV with different ion doses from 1012 to 1016 cm2 . During the treatment pressure in the preparation chamber, filled with argon, was 6 · 106 mbar, while normally pressure in the analytical chamber was 1010 mbar.
3. Results and discussion The surface glass transition temperature Tg was measured using the noble metal cluster embedding method [12,13]. The change in the intensity ratio ðIðAu 4fÞ=IðC 1sÞÞ vs. temperature reflects the kinetics of the embedding process, which requires long range mobility of the polymer chains and hence cannot take place below Tg . Fig. 1 represents experimental results on the embedding of Au nanoclusters (solid squares) vs. ion dose. During the bombardment energetic ions impinge onto the polymer surface and transfer sufficient kinetic energy to break chain bonds, forming low-molecular-weight species on polymer surface which tends to reduce surface Tg . On the other hand, free radicals, created during the collision of Arþ ions with polymer chains, may react with other surface radicals or with other chains increasing polymer molecular weight or form crosslinks raising surface Tg . The present experimental results show an increase in surface Tg with an ion
1042
J. Zekonyte et al. / Surface Science 532–535 (2003) 1040–1044
Fig. 1. Embedding temperature of Au nanoparticles in PS vs. Arþ ion dose (j). Degree of cross-links vs. ion dose (s).
dose. Sample treatment at such low ion dose as 5 · 1012 cm2 was already sufficient to increase the surface Tg above the bulk glass transition temperature Tg;bulk , while untreated sample showed surface Tg to be 7 K lower than the Tg;bulk [12]. PS is known to be a polymer that undergoes cross-linking during surface irradiation rather than chain scission [9] due to the presence of the aromatic ring which increases polymer stability by trapping the excitation energy [14], in such way reducing the polymer degradation. The presence of cross-links tends to reduce the specific volume of the polymer which means that the free volume is reduced and so Tg is raised because molecular motion is made more difficult [15]. Our experimental results suggest that ion bombardment induces cross-links, as the enhancement in surface Tg was detected. The degree of cross-linking, x, (the ratio between the number of cross-links and the number of backbone atoms) as a function of the ion dose was calculated using the empirical relation [16]: x Tg;crl Tg;0 ¼ 1:2Tg;0 ; ð1Þ 1x where Tg;0 and Tg;crl are the surface glass transition temperature of uncross-linked and cross-linked polymer, respectively. The corresponding embedding temperatures were taken as the surface glass transition temperatures of the untreated and ionbeam treated polymer surfaces. The calculated values vs. ion dose are represented as open circles in
Fig. 1. Supposing that untreated PS has no crosslinks, the ion dose of 5 · 1013 cm2 was enough to increase the degree of cross-linking by about 18%. This caused an increase in the surface Tg by 27 K. We note that Eq. (1) attributes the increase in Tg completely to cross-linking and reflects contributions due to an increase in chain stiffness resulting from new functionalities (see below). The crosslinking density in Fig. 1 is hence an upper limit. The embedding of Au nanoparticles into the PS sample treated at higher ion dose of 1014 cm2 was not observed (not shown in the graph), at least in the temperature range (up to 423 K) where the experiment was performed. The embedding could occur at higher temperatures, or the density of cross-links is so high that the range of the transition region is broadened and the surface glass transition may not occur at all [15]. Moreover, the mesh size could be as high as to exclude penetration of the probe clusters (size typically 1 nm) into the polymer. Together with the molecular architecture the chemical structure of polymer also affects Tg . The important factor is chain flexibility that is governed by the nature of the chemical groups which constitute the main chain [15]. The presence of side groups, for example polar ones, increases Tg through the restriction of bond rotation. Does ion bombardment incorporate polar functionalities on the PS surface, in such way together with crosslinks raising surface glass transition? The induced chemical changes during the argon-ion treatment were investigated using XPS. A peak fit (performed with Origin Software) of the untreated C 1s core level is shown in Fig. 2a. Two fitted peaks represent carbon of aliphatic and aromatic character [17] with the ratio that is expected from the polymer molecular structure. Table 1 reports percentage values of induced functionalities in the C 1s core level during Arþ ion treatment. With increasing ion dose the intensity of the shake-up satellite was reduced. At an ion dose of 1015 cm2 the p–p* shake-up transition was eliminated completely suggesting ring opening, formation of saturated carbon bonding environment on the PS surface and hence the loss of aromaticity within the XPS attenuation depth of about 3 nm. The primary carbon peak became
J. Zekonyte et al. / Surface Science 532–535 (2003) 1040–1044
Fig. 2. C 1s core level peak fits PS: (a) untreated and (b) treated at 1016 cm2 .
broader, the full width half maximum (FWHM) increased from 1.27 to 1.85 eV with ion dose. The broadening suggests possible chain scission, creation of new bonds or even the formation of some functional groups.
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Treatment of polymers with an inert gas plasma leads to the formation of free radicals but does not add new chemical functionalities from the gas phase [2]. Functionalization, for example incorporation of oxygen, occurs after the treated sample was exposed to atmosphere [4] or other environment containing oxygen. Nevertheless, several new features overlapping with the main C 1s peak were observed in the C 1s spectra. This could be due to the formation of carbon–oxygen groups, as the increase in some oxygen content, i.e. increase in intensity of O 1s line >20%, on PS film was observed. It is common phenomenon that oxygen is incorporated on polymer surfaces after and during non-oxygen-plasma treatments [1, Chapter 6]. Taking into account that the experiments were performed in vacuum, PS surface oxidation during Arþ ion bombardment could occur as a consequence of reaction between free radicals, formed after the interaction of ions with polymer chains, and oxygen-containing residual gases in the sputtering chamber. The fitted functionalities, relative to the aliphatic hydrocarbon peak corrected to 285 eV [17], after the treatment with ion dose of 1016 cm2 are represent in Fig. 2b. PS treatment up to an ion dose of 1013 cm2 shows reduction in surface aromaticity and broadening in the main carbon peak, which could be due to the formation of the new bonds. Modification at so low ion doses does not incorporate oxygen, or the oxygen content is so low (Table 1) that formation of high-molecular-weight species or crosslinks could be the main factor affecting surface glass transition. The treatment with ion dose of 1013 cm2 starts to incorporate carbon–oxygen functionalities which increases with the modification dose. Together with increasing density of
Table 1 Percentage values of carbon–oxygen functionalities in the C 1s induced by Arþ ion bombardment Ion dose [cm2 ]
C (aliphatic)
C (aromatic)
Untreated 1012 1013 1014 1015 1016
25.32 37.30 40.85 42.97 46.57 48.67
70.77 59.66 55.52 50.61 38.34 29.84
Alcohol, ether
1.07 4.63 9.76 12.46
Epoxide
Aldehyde, ketone
COOR, COOH
CO2 3
p–p* 3.91 3.04 2.56 1.79
2.48 4.22
1.57 2.51
1.01 1.30
1.0
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J. Zekonyte et al. / Surface Science 532–535 (2003) 1040–1044
cross-links which reduces free volume thus making molecular motion more difficult, interactions of oxygen containing polar groups restrict bond rotation raising surface Tg above 423 K. 4. Conclusion Ion-beam treatment by means of 1 keV Ar-ions was performed under well controlled conditions to investigate the changes in polymer the surface properties. Argon ion treatment induced degradation and rearrangement of polymer surface. During ion bombardment of PS film surface oxidation, destruction of aromatic ring, formation of new bonds, and surface cross-linking take place. Treatment with increasing ion dose up to 5 · 1013 cm2 increased the degree of cross-linking raising the surface Tg above the bulk glass transition temperature as the molecular motion became more difficult. Besides the mesh size could be high enough excluding the penetration of the probe clusters into the polymer. Additionally to cross-links, carbon–oxygen functionalities induced at higher ion doses restricted bond rotation thus also raising the surface Tg to higher values. References [1] C.-M. Chan, Polymer Surface Modification and Characterization, Hanser Publishers, Munich Vienna, NY, 1994.
[2] E.M. Liston, L. Martinu, M.R. Wertheimer, in: M. Strobel, C. Lyons, K.L. Mittal (Eds.), Plasma Surface Modification of Polymers, VSP, 1994, pp. 3–39. [3] L.J. Gerensen, in: M. Strobel, C.S. Lyons, K.L. Mittal (Eds.), Plasma Surface Modification of Polymers: Relevance to Adhesion, VSP, 1994, pp. 43–64. [4] R.M. France, R.D. Short, Langmuir 14 (1998) 4827. [5] R.W. Paynter, Surf. Interface Anal. 29 (2000) 56–64. [6] F. Clouet, F. Shi, J. Appl. Polym. Sci. 46 (1992) 1955– 1966. [7] L. K€ ohler, S. Scaglione, R. Giorgi, J. Riga, P. Rudolf, R. Caudano, Surf. Interface Anal. 29 (2000) 647– 652. [8] S.K. Koh, J.S. Cho, K.H. Kim, S. Han, Y.W. Beag, J. Adhesion Sci. Technol. 16 (2) (2002) 129–142. [9] L. Calgano, G. Compagnini, G. Foti, Nucl. Instrum. Meth. Phys. Res. B 65 (1992) 413–422. [10] J. Davenas, P. Thevenard, G. Boiteux, M. Fallavier, X.L. Lu, Nucl. Instrum. Meth. Phys. Res. B 46 (1990) 317– 323. [11] J.A. Forrest, R.A.L. Jones, in: A. Karim, S. Kumar (Eds.), Polymer Surfaces, Interfaces and Thin Films, World Scientific, 2000. [12] V. Zaporojtchenko, T. Struskus, J. Erichsen, F. Faupel, Macromolecules 34 (5) (2001) 1125. [13] R. Weber, K.-M. Zimmermann, M. Tolan, J. Stettner, W. Press, O.H. Seeck, J. Erichsen, V. Zaporojtchenko, T. Strunskus, F. Faupel, Phys. Rev. E 64 (2001) 061508. [14] A. Chapiro, Nucl. Instrum. Meth. Phys. Res. B 32 (1988) 111. [15] R.J. Young, P.A. Lovell, Introduction to Polymers, Chapman & Hall, 1991, p. 292. [16] D.W. van Krevelen, Properties of Polymers, third ed., Elsevier Science, 1990, pp. 147–148. [17] D. Briggs, Surface Analysis of Polymers by XPS and Static SIMS, Cambridge University Press, 1998.
Mechanisms of argon ion-beam surface modification of polystyrene J. Zekonyte, J. Erichsen, V. Zaporojtchenko, F. Faupel
*
Technical Faculty, Kiel University, Kaiserstrasse 2, Kiel (Gaarden) 24143, Germany
Abstract The surface characteristics of polymers are important factors determining their interfacial properties and their technological performance. Changes in physical and chemical properties of a polymer film may be induced by subjecting the material to a variety of surface modification techniques, one of which is ion-beam modification. In order to understand the underlying mechanisms X-ray photoelectron spectroscopy (XPS) was used to study the alterations of the polystyrene (PS) surface after Ar-ion treatment under well controlled conditions with low ion doses from 1012 to 1016 cm2 . The ion bombardment leads to surface functionalization, loss of aromaticity, and free radical formation. Induced surface cross-linking and the formation of polar groups raised the surface glass transition temperature of PS film. 2003 Elsevier Science B.V. All rights reserved. Keywords: X-ray photoelectron spectroscopy; Ion bombardment; Oxidation
1. Introduction Polymers have become key materials in different technological applications, as they have many unique advantages, including their light weight, corrosion resistance, and low manufacturing cost. As most polymers do not posses required surface properties for industrial applications different kind of surface modification techniques were applied [1, Chapter 1]. One of the most versatile technique is plasma treatment, which could produce unique surface properties not affecting polymer bulk, and which was investigated in different research groups
*
Corresponding author. Tel.: +49-431-880-6225; fax: +49431-880-6229. E-mail address: ff@tf.uni-kiel.de (F. Faupel).
[2–5]. In spite of this advantage the plasma process is extremely complex, and modification is caused by different plasma species such as ions, electrons, excited neutrals, and UV irradiation. This makes it difficult to achieve a good understanding of the interactions between plasma and the surface, to change plasma parameters, and to control the amount of a particular functional group formed on the surface [1, Chapter 6]. In ion-beam treatment, compared to the plasma technique, it is easier to change process parameters such as ion energy and ion beam current, and a more precise ion dose can be calculated. Ion-beam treatment could also induce changes in the chemical structure, reactivity and bonding characteristics of polymer surfaces without affecting the polymer bulk. Modification with reactive gas causes ablation of polymers [6], and functionalization takes place
0039-6028/03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00130-4
J. Zekonyte et al. / Surface Science 532–535 (2003) 1040–1044
within the treatment time [2,5,7,8]. The treatment with inert gas leads to hydrogen abstraction [6], creation of long living free radicals that are responsible for the recombination, unsaturation, or cross-linking [2]. Functional group incorporation is more significant after the exposure of treated polymer to the atmosphere [4]. Using high energy (keV–MeV) ion-beams intensive research was done on the induced structural changes of polymers [9] and on the formation of hydrogenated carbon layers exhibiting interesting electronic properties [10]. Some work was done on low ion energy surface chemical modification such as nitrogen incorporation in PS [7]. The improvement in the wettability and adhesion of various polymers was achieved by ion-assisted reaction treatment [8]. This paper mainly presents results on the PS surface glass transition temperature Tg influenced by the changes in surface structural composition, after Arþ ion treatment with low ion dose from 1012 to 1016 cm2 at a constant ion energy of 1 keV.
2. Experimental Monodisperse polystyrene (PS) (Mw ¼ 212 000 g/mol, Mw =Mn < 1:1) used in the experiments was obtained from Aldrich Chemical Company as powder. The polymer films (thickness of approximately 200 nm) were prepared by solving the polymer powder in toluene (27 g/l) and spin coating it on a silicon wafer with its native oxide layer. The influence of film thickness on the glass transition is negligible [11]. PS samples were annealed under UHV conditions for 40 min at a temperature of 130 C and then slowly cooled down to room temperature with constant cooling rate of 1 K/min. X-ray photoelectron spectroscopy (XPS) was chosen to investigate polymer surface decomposition under argon ion bombardment with different ion doses, and to measure the surface glass transition temperature, Tg , using the metal cluster embedding method, developed in our group [12]. XPS measurements were performed using electron spectrometer (Omicron) equipped with a nonmonochromated Mg Ka source. To investigate induced surface chemical changes the C 1s and
1041
O 1s core level spectra were recorded with 20 eV pass energy. The detailed description of the experiment for metal particle embedding temperature measurement was reported in Refs. [12,13]. Briefly, gold was evaporated from a heated molybdenium crucible mounted in the sputter chamber (Omicron Full Lab) onto the PS films at room temperature. The deposition rate and the nominal thickness of the evaporated metal were monitored by a quartz crystal microbalance. After the metal deposition samples were transferred to the analytical chamber and there heated with the rate of 1 K/min up to 423 K. At the same time a series of XPS spectra of Au 4f and C 1s lines with pass energy of 100 eV were taken to check the embedding of the metal nanoparticles. Prior to taking XPS spectra, and evaporating metal onto the polymer, ion bombardment was performed. Using a ISE10 sputter ion gun polymer samples were treated at an ion energy of 1 keV with different ion doses from 1012 to 1016 cm2 . During the treatment pressure in the preparation chamber, filled with argon, was 6 · 106 mbar, while normally pressure in the analytical chamber was 1010 mbar.
3. Results and discussion The surface glass transition temperature Tg was measured using the noble metal cluster embedding method [12,13]. The change in the intensity ratio ðIðAu 4fÞ=IðC 1sÞÞ vs. temperature reflects the kinetics of the embedding process, which requires long range mobility of the polymer chains and hence cannot take place below Tg . Fig. 1 represents experimental results on the embedding of Au nanoclusters (solid squares) vs. ion dose. During the bombardment energetic ions impinge onto the polymer surface and transfer sufficient kinetic energy to break chain bonds, forming low-molecular-weight species on polymer surface which tends to reduce surface Tg . On the other hand, free radicals, created during the collision of Arþ ions with polymer chains, may react with other surface radicals or with other chains increasing polymer molecular weight or form crosslinks raising surface Tg . The present experimental results show an increase in surface Tg with an ion
1042
J. Zekonyte et al. / Surface Science 532–535 (2003) 1040–1044
Fig. 1. Embedding temperature of Au nanoparticles in PS vs. Arþ ion dose (j). Degree of cross-links vs. ion dose (s).
dose. Sample treatment at such low ion dose as 5 · 1012 cm2 was already sufficient to increase the surface Tg above the bulk glass transition temperature Tg;bulk , while untreated sample showed surface Tg to be 7 K lower than the Tg;bulk [12]. PS is known to be a polymer that undergoes cross-linking during surface irradiation rather than chain scission [9] due to the presence of the aromatic ring which increases polymer stability by trapping the excitation energy [14], in such way reducing the polymer degradation. The presence of cross-links tends to reduce the specific volume of the polymer which means that the free volume is reduced and so Tg is raised because molecular motion is made more difficult [15]. Our experimental results suggest that ion bombardment induces cross-links, as the enhancement in surface Tg was detected. The degree of cross-linking, x, (the ratio between the number of cross-links and the number of backbone atoms) as a function of the ion dose was calculated using the empirical relation [16]: x Tg;crl Tg;0 ¼ 1:2Tg;0 ; ð1Þ 1x where Tg;0 and Tg;crl are the surface glass transition temperature of uncross-linked and cross-linked polymer, respectively. The corresponding embedding temperatures were taken as the surface glass transition temperatures of the untreated and ionbeam treated polymer surfaces. The calculated values vs. ion dose are represented as open circles in
Fig. 1. Supposing that untreated PS has no crosslinks, the ion dose of 5 · 1013 cm2 was enough to increase the degree of cross-linking by about 18%. This caused an increase in the surface Tg by 27 K. We note that Eq. (1) attributes the increase in Tg completely to cross-linking and reflects contributions due to an increase in chain stiffness resulting from new functionalities (see below). The crosslinking density in Fig. 1 is hence an upper limit. The embedding of Au nanoparticles into the PS sample treated at higher ion dose of 1014 cm2 was not observed (not shown in the graph), at least in the temperature range (up to 423 K) where the experiment was performed. The embedding could occur at higher temperatures, or the density of cross-links is so high that the range of the transition region is broadened and the surface glass transition may not occur at all [15]. Moreover, the mesh size could be as high as to exclude penetration of the probe clusters (size typically 1 nm) into the polymer. Together with the molecular architecture the chemical structure of polymer also affects Tg . The important factor is chain flexibility that is governed by the nature of the chemical groups which constitute the main chain [15]. The presence of side groups, for example polar ones, increases Tg through the restriction of bond rotation. Does ion bombardment incorporate polar functionalities on the PS surface, in such way together with crosslinks raising surface glass transition? The induced chemical changes during the argon-ion treatment were investigated using XPS. A peak fit (performed with Origin Software) of the untreated C 1s core level is shown in Fig. 2a. Two fitted peaks represent carbon of aliphatic and aromatic character [17] with the ratio that is expected from the polymer molecular structure. Table 1 reports percentage values of induced functionalities in the C 1s core level during Arþ ion treatment. With increasing ion dose the intensity of the shake-up satellite was reduced. At an ion dose of 1015 cm2 the p–p* shake-up transition was eliminated completely suggesting ring opening, formation of saturated carbon bonding environment on the PS surface and hence the loss of aromaticity within the XPS attenuation depth of about 3 nm. The primary carbon peak became
J. Zekonyte et al. / Surface Science 532–535 (2003) 1040–1044
Fig. 2. C 1s core level peak fits PS: (a) untreated and (b) treated at 1016 cm2 .
broader, the full width half maximum (FWHM) increased from 1.27 to 1.85 eV with ion dose. The broadening suggests possible chain scission, creation of new bonds or even the formation of some functional groups.
1043
Treatment of polymers with an inert gas plasma leads to the formation of free radicals but does not add new chemical functionalities from the gas phase [2]. Functionalization, for example incorporation of oxygen, occurs after the treated sample was exposed to atmosphere [4] or other environment containing oxygen. Nevertheless, several new features overlapping with the main C 1s peak were observed in the C 1s spectra. This could be due to the formation of carbon–oxygen groups, as the increase in some oxygen content, i.e. increase in intensity of O 1s line >20%, on PS film was observed. It is common phenomenon that oxygen is incorporated on polymer surfaces after and during non-oxygen-plasma treatments [1, Chapter 6]. Taking into account that the experiments were performed in vacuum, PS surface oxidation during Arþ ion bombardment could occur as a consequence of reaction between free radicals, formed after the interaction of ions with polymer chains, and oxygen-containing residual gases in the sputtering chamber. The fitted functionalities, relative to the aliphatic hydrocarbon peak corrected to 285 eV [17], after the treatment with ion dose of 1016 cm2 are represent in Fig. 2b. PS treatment up to an ion dose of 1013 cm2 shows reduction in surface aromaticity and broadening in the main carbon peak, which could be due to the formation of the new bonds. Modification at so low ion doses does not incorporate oxygen, or the oxygen content is so low (Table 1) that formation of high-molecular-weight species or crosslinks could be the main factor affecting surface glass transition. The treatment with ion dose of 1013 cm2 starts to incorporate carbon–oxygen functionalities which increases with the modification dose. Together with increasing density of
Table 1 Percentage values of carbon–oxygen functionalities in the C 1s induced by Arþ ion bombardment Ion dose [cm2 ]
C (aliphatic)
C (aromatic)
Untreated 1012 1013 1014 1015 1016
25.32 37.30 40.85 42.97 46.57 48.67
70.77 59.66 55.52 50.61 38.34 29.84
Alcohol, ether
1.07 4.63 9.76 12.46
Epoxide
Aldehyde, ketone
COOR, COOH
CO2 3
p–p* 3.91 3.04 2.56 1.79
2.48 4.22
1.57 2.51
1.01 1.30
1.0
1044
J. Zekonyte et al. / Surface Science 532–535 (2003) 1040–1044
cross-links which reduces free volume thus making molecular motion more difficult, interactions of oxygen containing polar groups restrict bond rotation raising surface Tg above 423 K. 4. Conclusion Ion-beam treatment by means of 1 keV Ar-ions was performed under well controlled conditions to investigate the changes in polymer the surface properties. Argon ion treatment induced degradation and rearrangement of polymer surface. During ion bombardment of PS film surface oxidation, destruction of aromatic ring, formation of new bonds, and surface cross-linking take place. Treatment with increasing ion dose up to 5 · 1013 cm2 increased the degree of cross-linking raising the surface Tg above the bulk glass transition temperature as the molecular motion became more difficult. Besides the mesh size could be high enough excluding the penetration of the probe clusters into the polymer. Additionally to cross-links, carbon–oxygen functionalities induced at higher ion doses restricted bond rotation thus also raising the surface Tg to higher values. References [1] C.-M. Chan, Polymer Surface Modification and Characterization, Hanser Publishers, Munich Vienna, NY, 1994.
[2] E.M. Liston, L. Martinu, M.R. Wertheimer, in: M. Strobel, C. Lyons, K.L. Mittal (Eds.), Plasma Surface Modification of Polymers, VSP, 1994, pp. 3–39. [3] L.J. Gerensen, in: M. Strobel, C.S. Lyons, K.L. Mittal (Eds.), Plasma Surface Modification of Polymers: Relevance to Adhesion, VSP, 1994, pp. 43–64. [4] R.M. France, R.D. Short, Langmuir 14 (1998) 4827. [5] R.W. Paynter, Surf. Interface Anal. 29 (2000) 56–64. [6] F. Clouet, F. Shi, J. Appl. Polym. Sci. 46 (1992) 1955– 1966. [7] L. K€ ohler, S. Scaglione, R. Giorgi, J. Riga, P. Rudolf, R. Caudano, Surf. Interface Anal. 29 (2000) 647– 652. [8] S.K. Koh, J.S. Cho, K.H. Kim, S. Han, Y.W. Beag, J. Adhesion Sci. Technol. 16 (2) (2002) 129–142. [9] L. Calgano, G. Compagnini, G. Foti, Nucl. Instrum. Meth. Phys. Res. B 65 (1992) 413–422. [10] J. Davenas, P. Thevenard, G. Boiteux, M. Fallavier, X.L. Lu, Nucl. Instrum. Meth. Phys. Res. B 46 (1990) 317– 323. [11] J.A. Forrest, R.A.L. Jones, in: A. Karim, S. Kumar (Eds.), Polymer Surfaces, Interfaces and Thin Films, World Scientific, 2000. [12] V. Zaporojtchenko, T. Struskus, J. Erichsen, F. Faupel, Macromolecules 34 (5) (2001) 1125. [13] R. Weber, K.-M. Zimmermann, M. Tolan, J. Stettner, W. Press, O.H. Seeck, J. Erichsen, V. Zaporojtchenko, T. Strunskus, F. Faupel, Phys. Rev. E 64 (2001) 061508. [14] A. Chapiro, Nucl. Instrum. Meth. Phys. Res. B 32 (1988) 111. [15] R.J. Young, P.A. Lovell, Introduction to Polymers, Chapman & Hall, 1991, p. 292. [16] D.W. van Krevelen, Properties of Polymers, third ed., Elsevier Science, 1990, pp. 147–148. [17] D. Briggs, Surface Analysis of Polymers by XPS and Static SIMS, Cambridge University Press, 1998.