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Plasma polymerization of styrene with carbon dioxide under glow discharge conditions
Applied Surface Science 253 (2007) 5203–5207 www.elsevier.com/locate/apsusc
Plasma polymerization of styrene with carbon dioxide under glow discharge conditions H.L. Luo, J. Sheng, Y.Z. Wan * School of Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China Received 24 September 2006; received in revised form 28 October 2006; accepted 30 October 2006 Available online 19 December 2006
Abstract Plasma polymerization gains increasing interest for the deposition of films with functional properties suitable for a wide range of modern applications on account of its advantageous features. In this study, carbon dioxide (CO2) was chosen as carrier gas at flow rates of 30 and 60 sccm, respectively and styrene vapor was used as the monomer to prepare polystyrene films on glass substrates. The structure and composition of the plasma polymerized films were characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR) and compared with the film prepared by conventional thermal polymerization. The morphology information of the films was provided by optical microscopy. XPS and FT-IR results reveal that chemical composition of the plasma polymerized films is different from that of the thermal polymerized film and that oxygen content in the plasma polymerized films increases with the flow rate of CO2. Furthermore, the presence of oxygen-containing groups on the surface of plasma polymerized polystyrene films is confirmed. It is also found that the composition and morphology of the plasma polymerized films are controlled by the flow rate of CO2. # 2006 Elsevier B.V. All rights reserved. PACS : 52.77Dq; 82.35.–x Keywords: Plasma polymerization; Polystyrene film; Carbon dioxide
1. Introduction Glow discharge plasma is very useful for surface modification of various polymers and polymerization of organic films. The fact that most common polymers are hydrophobic in nature, which makes them unsuitable for some biomedical applications, makes plasma treatment very attractive as it can improve wettability, adhesion, and biocompatibility of such polymers as a result of the generation of active species in plasma while preserve their desirable bulk properties [1–3]. An important example is surface modification of polystyrene. Though hydrophobic polystyrene is cheap, easily processed, and used in many fields such as packaging, household goods, construction, thermal insulation and so on, hydrophility is needed in some cases like biomedical usage [4,5]. To this end, introduction of functional groups to the surface of polystyrene is the subject of considerable research interest [4–6]. Plasma of
* Corresponding author. Tel.: +86 22 2740 5056; fax: +86 22 2740 5056. E-mail address: [email protected] (Y.Z. Wan). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.10.078
various gases such as argon [5,7,8], oxygen [8], nitrogen [9,10], and carbon dioxide (CO2) [9–11] was applied to polystyrene to change its surface properties. Plasma of CO2 gas is believed effective to make the surface of polystyrene very hydrophilic. For instance, Poncin-Epaillard and co-workers declared that modification of polystyrene with CO2 plasma treatment was more efficient than N2 plasma in terms of degradation, creation of functional groups, roughening of surface [9,10]. Further, they found that the presence of the CO2 active species favored to the formation of carboxylic groups [11]. Different from plasma treatment, plasma polymerization has been widely used for synthesis of films with functional properties suitable for various applications from mechanical, chemical, and physical to electronic, optical, and biomedical fields. Similar to plasma treatment, polymeric films synthesized by plasma polymerization may also have functional groups on their surfaces as similar processes like the generation of active species are involved. It is therefore expected that polystyrene films synthesized by plasma polymerization may have surface functional groups, which makes such polystyrene films useful in biomedical fields without further surface modification. This,
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in turn, shows the advantages of the plasma polymerization over conventional thermal polymerization. The objective of the present study was to prepare polystyrene films by plasma polymerization. In this work, CO2 was selected as the carrier gas and styrene vapor was used as the monomer to prepare plasma polymerized films under glow discharge conditions. The obtained films were then characterized and compared with the film fabricated using conventional thermal polymerization in terms of surface functional groups and chemical composition. 2. Experimental procedures 2.1. Materials CO2 gas of a purity of 99.99% (purchased from Qiaofeng Chemical Co. Ltd., Longkou, Shandong, China) was used as carrying gas for the plasma polymerization process. Styrene in liquid form was used in this work. The styrene monomer specified for synthesis applications (Kewei Chemical Co. Ltd., Tianjin University, Tianjin, China) was used as received without any further purification. Glass plates (20 mm 20 mm 1 mm) were used as substrates. Only one side of the glass plates was deposited with the polystyrene films. 2.2. Plasma polymerization A home-made bell-jar reactor was used for plasma polymerization of styrene and CO2. A schematic diagram of the reactor is presented in Fig. 1. As shown in this figure, the reactor consisted of three parts: vacuum system, gas inlet system, and discharge system. In the discharge chamber, there are two parallel internal electrodes with a diameter of 60 mm. High frequency power operating at 13.56 MHz was used. Each glass plate was cleaned in acetone to remove any possible contaminants, dried and then positioned at the center of the lower plate electrode (ground electrode). Prior to the polymerization process, the reactor was evacuated down to a base pressure of 20–70 Pa. During the reaction, a vacuum gauge was used to measure the pressure inside the plasma chamber. A volume flow controller was used to adjust the CO2 flow rate. The flow rate used in this work was 30 and 60 sccm, respectively. The CO2 was designed to flow into a 250 ml round-bottom flask containing the liquid styrene monomer at 40 8C. The CO2 gas carrying the styrene vapor was then
Fig. 1. A schematic diagram of the reactor for plasma polymerization.
introduced into the stainless steel plasma reaction chamber for polymerization. During polymerization, the distance between the two parallel electrodes was 35 mm. The power of the glow discharge was kept at 85 W and the discharge time was set at 15 min. 2.3. Characterization of the films The chemical structure of the films was analyzed by means of Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The FT-IR analysis was conducted with a BIO-RAD FTS 6000 spectroscope (USA). The instrument for XPS analysis was a PHI-1600 model machine (Physical Electronics, USA) using a monochromatic Mg Ka X-ray and a passing energy of 23.5 eV. Calibration of the spectra of the plasma polymerized films was performed by using the C 1s peak (binding energy 285.0 eV) as an internal reference. The morphology of the films was observed by a BX51TF Olympus optical microscope. 3. Results and discussion 3.1. FT-IR analysis Typical FT-IR spectra obtained in this work are given in Fig. 2. The assignments of typical peaks are listed in Table 1 according to the literature [12]. For the sake of comparison, a spectrum (curve (a)) of a thermal polymerized polystyrene film is included in this figure. Curves (b) and (c) are spectra of the plasma polymerized films formed at CO2 flow rates of 30 and 60 sccm, respectively. As shown in Fig. 2, the spectra of the plasma polymerized films are very similar to the spectrum of the thermal polymerized polystyrene film. However, differences are observed among the three spectra. First, four intense aromatic finger bands located at 1735, 1800, 1870, and 1942 cm 1 are observed in spectrum (a). In spectra (b) and (c), the intensity of these four peaks weakens. The reduction of the strength of the four characteristic peaks indicates that some
Fig. 2. FT-IR spectra of various films: (a) thermal polymerized; (b) plasma polymerized at 30 sccm; (c) plasma polymerized at 60 sccm.
H.L. Luo et al. / Applied Surface Science 253 (2007) 5203–5207 Table 1 Assignments of typical peaks presented in Fig. 2 Wavenumber (cm 1)
Type of vibration
3296 3082, 3059, 3026 2963, 2868 2925, 2850 1709 1674–1942
Hydroxyl O–H stretching Aromatic C–H stretching Methyl (CH3) C–H stretching Methylene C–H stretching Carbonyl C O stretching C–H out-of-plane deformation bending of aromatic ring Methyl C–H bending; in-plane phenyl ring-bending mode Out-of-plane phenyl ring-bending mode
1449 700, 759
Table 2 Relative atomic proportions (%) of plasma and thermal polymerized films Preparation conditions
C
O
O/C
Thermal polymerization Plasma polymerization (30 sccm) Plasma polymerization (60 sccm)
96.1 87.8 85.5
3.9 12.2 14.5
0.04 0.14 0.17
phenyl rings in molecules are destroyed by the plasma polymerization process. Second, the intensity of the two bands positioned at 2925 and 2850 cm 1 corresponding to methylene C–H stretching is substantially reduced such that the 2850 cm 1 band disappears in the spectra of the plasma polymerized films. Third, two new peaks positioned at 2963 and 2868 cm 1, which correspond to unsymmetrical and symmetrical stretching vibrations of methyl (–CH3) [13], are noted in the spectra of the plasma polymerized films. The presence of –CH3 is an indicator of extensive branching of plasma polystyrene. The formation of –CH3 in the plasma polymerized films is most likely due to destruction of benzene rings as a result of bombardment of plasma [14]. Furthermore, introduction of CO2 is believed to accelerate the destruction of benzene rings [15]. Last, two other new peaks at 1709 and 3296 cm 1, which are ascribed to C O and O–H groups, respectively, are observed in the spectra of the plasma polymerized films. Furthermore, the intensity of the two new peaks increases with the increase of the flow rate of CO2. This finding indicates that CO2 has participated in the plasma polymerization of styrene. 3.2. XPS analysis XPS, which is one of the most widely employed techniques for the characterization of polymer films, was used to analyze the
5205
surface component of the plasma polymerized films. The results are summarized in Tables 2 and 3 and Fig. 3. Table 2 shows the elemental composition of the thermal and plasma polymerized polystyrene films. As given in Table 2, the thermal polymerized film shows the existence of 96.1% carbon and 3.9% oxygen. The existence of oxygen in the thermal polymerized film is probably due to the quench effect during the preparation process [16,17]. Note that the oxygen content of the plasma polymerized films is higher than that of the thermal polymerized one, being 12.2 and 14.5%, respectively, at flow rates of 30 and 60 sccm. The higher oxygen content indicates that introduction of CO2, as carrier gas, results in more oxygen incorporation into the films. The residual oxygen in the chamber (due to relative low vacuum) may also contribute a little to the increased oxygen content. Fig. 3 shows the fitted C 1s spectra of various films. The functional groups and their contents of these films obtained from Fig. 3 are given in Table 3. Obviously, C–C and/or C–H (at 285.0 eV) and p–p* (at 291.8 eV) bands are present on all films. The characteristic p–p* shake-up satellite is due to resonance of the aromatic rings of polystyrene [18,9], indicating polystyrene films are synthesized by plasma polymerization under our experimental conditions. It is interesting to notice that three additional peaks which corresponds to C–O–C and/or C–OH (286.5 eV), C O, H– C O, and/or aromatic-OH (288.0 eV), and COOH and/or COOR (289.2 eV) are observed on the plasma polymerized films. Therefore, it is inferred that CO2 has been incorporated into the plasma polymerized films. This finding agrees well with FT-IR results. 3.3. Morphology of plasma polymerized films Fig. 4 shows the morphology of the plasma polymerized films obtained by optical microscopy. As shown in this figure, the plasma polymerized films formed on the glass substrates exhibit a wave-like pattern. A similar wave-like pattern has been reported by Bowden et al. [19]. The wave-like pattern of the plasma polymerized films is believed to be ascribed to the buckling phenomenon of the films as a result of compressive forces [19]. A comparison between Fig. 4a and b reveals that the wavelength (width of two neighboring waves) is related to the flow rate of CO2. A higher flow rate corresponds to a larger wavelength. The plasma polymerized film at 60 sccm possesses a larger wavelength than that at 30 sccm. As described in literature [20], the wavelength is dependent on the properties of the films. Consequently, the observed difference in wavelength indicates that the film properties can be adjusted by changing the flow rate of CO2.
Table 3 Functional groups and their contents (%) of plasma and thermal polymerized films Position (eV)
Assignment
Thermal polymerized film
Plasma polymerized film (30 sccm)
Plasma polymerized film (60 sccm)
285.0 286.5 288.0 289.2 291.8
C–C, C–H C–O–C, C–OH C O, H–C O, aromatic-OH COOH, COOR p–p* shake-up satellite
97.31 – – – 2.69
84.76 10.30 2.35 0.58 2.01
81.07 12.40 3.38 1.09 2.05
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Fig. 4. Images of plasma polymerized polystyrene films at CO2 flow rate of (a) 30 sccm and (b) 60 sccm.
Though how the wave-like pattern is formed and even how the films are polymerized are not fully understood, the observed patterned thin films are believed useful as models for analysis of mechanical properties in materials science field [21,22]. We believe that more work is worthy to gain insight into the formation mechanisms, properties and applications of the polystyrene films.
4. Conclusions
Fig. 3. C 1s spectra of various films: (a) thermal polymerized; (b) plasma polymerized at 30 sccm; (c) plasma polymerized at 60 sccm (1, C–C and/or C– H; 2, C–O–C and/or C–OH; 3, C O, H–C O, and/or aromatic-OH; 4, COOH and/or COOR; 5, p–p* shake-up satellite).
Polystyrene thin films have been successfully prepared by plasma polymerization technique by using carbon dioxide, as carrier gas and styrene vapor, as monomer. The current experimental results show that oxygen-containing groups (C– O, C O, and COO) are present in the plasma polymerized polystyrene films, suggesting that chemical reactions have happened between CO2 and styrene. XPS analysis further confirms that the content of these oxygen-containing groups increases with CO2 flow rate. Optical microscope observations demonstrate that the plasma polymerized styrene films show a wave-like pattern and that the wavelength increases with CO2 flow rate. Further investigation on the polymerization mechanisms, the electrical and mechanical properties of the plasma polymerized films, and their applications is underway.
H.L. Luo et al. / Applied Surface Science 253 (2007) 5203–5207
Acknowledgment This study was financially supported by the National Natural Science Foundation (Grant No. 50390090). References [1] I. Ganacarz, G. Pozniak, M. Bryjak, Eur. Polym. J. 35 (1999) 1419. [2] U. Schulz, P. Munzert, N. Kaiser, Surf. Coat. Technol. 142–144 (2001) 507. [3] M.P. Ferraz, J.C. Knowles, I. Olsen, F.J. Monteiro, J.D. Santos, Biomaterials 21 (2000) 813. [4] T.G. Van Kooten, H.T. Spijker, H.J. Busscher, Biomaterials 25 (2004) 1735. [5] S.C. Baker, N. Atkin, P.A. Gunning, N. Granville, K. Wilson, D. Wilson, J. Southgate, Biomaterials 27 (2006) 3136. [6] S.A. Mitchell, M.R. Davidson, R.H. Bradley, J. Colloid Interf. Sci. 281 (2005) 122. [7] R.K. Wells, J.P.S. Badyal, I.W. Drummond, K.S. Robinson, F.J. Street, J. Adhesion Sci. Tech. 7 (1993) 1129. [8] S. Guruvenket, G.M. Rao, M. Komath, A.M. Raichur, Appl. Surf. Sci. 236 (2004) 278.
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[9] M.J. Wang, Y.I. Chang, F. Poncin-Epaillard, Surf. Interf. Anal. 37 (2005) 348. [10] N. Medard, J.C. Soutif, F. Poncin-Epaillard, Surf. Coat. Technol. 160 (2002) 197. [11] M.J. Wang, Y.I. Chang, F. Poncin-Epaillard, Surf. Interf. Anal. 37 (2005) 325. [12] D.Y. Shen, Applications of Infrared Analysis Methods in Polymers, Scientific Publisher, Beijing, 1983, p. 69. [13] K. Jesch, J.E. Bloor, P.L. Kronick, J. Polym. Sci. Part A 4 (1966) 1487. [14] M. Tibbitt, M. Shen, A.T. Bell, J. Vac. Sci. Technol. 5 (1978) 338. [15] J.T. Kim, K.B. Lim, D.C. Lee, Surf. Coat. Technol. 182 (2004) 1. [16] A. Kettle, A. Becj, L. Toole, F. Jones, R. Short, Compos. Sci. Technol. 57 (1996) 1023. [17] N. Shahidzadeh, F. Arefi-Khonsari, M.M. Chehimi, J. Amouroux, Surf. Sci. 352–354 (1996) 888. [18] A. Nakao, Y. Suzuki, M. Iwaki, J. Colloid Interf. Sci. 197 (1998) 257. [19] N. Bowden, W.T.S. Huck, K.E. Paul, G.M. Whitesides, Appl. Phys. Lett. 75 (1999) 2557. [20] J. Groenewold, Physica A 298 (2001) 32. [21] C.M. Stafford, C. Harrison, K.L. Beers, A. Karim, E.J. Amis, M.R. Vanlandingham, H.C. Kim, W. Volksen, R.D. Miller, E.E. Simonyi, Nat. Mater. 3 (2004) 546. [22] X. Wang, G. Grundmeier, Plasma Proces. Polym. 3 (2006) 39.
Plasma polymerization of styrene with carbon dioxide under glow discharge conditions H.L. Luo, J. Sheng, Y.Z. Wan * School of Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China Received 24 September 2006; received in revised form 28 October 2006; accepted 30 October 2006 Available online 19 December 2006
Abstract Plasma polymerization gains increasing interest for the deposition of films with functional properties suitable for a wide range of modern applications on account of its advantageous features. In this study, carbon dioxide (CO2) was chosen as carrier gas at flow rates of 30 and 60 sccm, respectively and styrene vapor was used as the monomer to prepare polystyrene films on glass substrates. The structure and composition of the plasma polymerized films were characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR) and compared with the film prepared by conventional thermal polymerization. The morphology information of the films was provided by optical microscopy. XPS and FT-IR results reveal that chemical composition of the plasma polymerized films is different from that of the thermal polymerized film and that oxygen content in the plasma polymerized films increases with the flow rate of CO2. Furthermore, the presence of oxygen-containing groups on the surface of plasma polymerized polystyrene films is confirmed. It is also found that the composition and morphology of the plasma polymerized films are controlled by the flow rate of CO2. # 2006 Elsevier B.V. All rights reserved. PACS : 52.77Dq; 82.35.–x Keywords: Plasma polymerization; Polystyrene film; Carbon dioxide
1. Introduction Glow discharge plasma is very useful for surface modification of various polymers and polymerization of organic films. The fact that most common polymers are hydrophobic in nature, which makes them unsuitable for some biomedical applications, makes plasma treatment very attractive as it can improve wettability, adhesion, and biocompatibility of such polymers as a result of the generation of active species in plasma while preserve their desirable bulk properties [1–3]. An important example is surface modification of polystyrene. Though hydrophobic polystyrene is cheap, easily processed, and used in many fields such as packaging, household goods, construction, thermal insulation and so on, hydrophility is needed in some cases like biomedical usage [4,5]. To this end, introduction of functional groups to the surface of polystyrene is the subject of considerable research interest [4–6]. Plasma of
* Corresponding author. Tel.: +86 22 2740 5056; fax: +86 22 2740 5056. E-mail address: [email protected] (Y.Z. Wan). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.10.078
various gases such as argon [5,7,8], oxygen [8], nitrogen [9,10], and carbon dioxide (CO2) [9–11] was applied to polystyrene to change its surface properties. Plasma of CO2 gas is believed effective to make the surface of polystyrene very hydrophilic. For instance, Poncin-Epaillard and co-workers declared that modification of polystyrene with CO2 plasma treatment was more efficient than N2 plasma in terms of degradation, creation of functional groups, roughening of surface [9,10]. Further, they found that the presence of the CO2 active species favored to the formation of carboxylic groups [11]. Different from plasma treatment, plasma polymerization has been widely used for synthesis of films with functional properties suitable for various applications from mechanical, chemical, and physical to electronic, optical, and biomedical fields. Similar to plasma treatment, polymeric films synthesized by plasma polymerization may also have functional groups on their surfaces as similar processes like the generation of active species are involved. It is therefore expected that polystyrene films synthesized by plasma polymerization may have surface functional groups, which makes such polystyrene films useful in biomedical fields without further surface modification. This,
5204
H.L. Luo et al. / Applied Surface Science 253 (2007) 5203–5207
in turn, shows the advantages of the plasma polymerization over conventional thermal polymerization. The objective of the present study was to prepare polystyrene films by plasma polymerization. In this work, CO2 was selected as the carrier gas and styrene vapor was used as the monomer to prepare plasma polymerized films under glow discharge conditions. The obtained films were then characterized and compared with the film fabricated using conventional thermal polymerization in terms of surface functional groups and chemical composition. 2. Experimental procedures 2.1. Materials CO2 gas of a purity of 99.99% (purchased from Qiaofeng Chemical Co. Ltd., Longkou, Shandong, China) was used as carrying gas for the plasma polymerization process. Styrene in liquid form was used in this work. The styrene monomer specified for synthesis applications (Kewei Chemical Co. Ltd., Tianjin University, Tianjin, China) was used as received without any further purification. Glass plates (20 mm 20 mm 1 mm) were used as substrates. Only one side of the glass plates was deposited with the polystyrene films. 2.2. Plasma polymerization A home-made bell-jar reactor was used for plasma polymerization of styrene and CO2. A schematic diagram of the reactor is presented in Fig. 1. As shown in this figure, the reactor consisted of three parts: vacuum system, gas inlet system, and discharge system. In the discharge chamber, there are two parallel internal electrodes with a diameter of 60 mm. High frequency power operating at 13.56 MHz was used. Each glass plate was cleaned in acetone to remove any possible contaminants, dried and then positioned at the center of the lower plate electrode (ground electrode). Prior to the polymerization process, the reactor was evacuated down to a base pressure of 20–70 Pa. During the reaction, a vacuum gauge was used to measure the pressure inside the plasma chamber. A volume flow controller was used to adjust the CO2 flow rate. The flow rate used in this work was 30 and 60 sccm, respectively. The CO2 was designed to flow into a 250 ml round-bottom flask containing the liquid styrene monomer at 40 8C. The CO2 gas carrying the styrene vapor was then
Fig. 1. A schematic diagram of the reactor for plasma polymerization.
introduced into the stainless steel plasma reaction chamber for polymerization. During polymerization, the distance between the two parallel electrodes was 35 mm. The power of the glow discharge was kept at 85 W and the discharge time was set at 15 min. 2.3. Characterization of the films The chemical structure of the films was analyzed by means of Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The FT-IR analysis was conducted with a BIO-RAD FTS 6000 spectroscope (USA). The instrument for XPS analysis was a PHI-1600 model machine (Physical Electronics, USA) using a monochromatic Mg Ka X-ray and a passing energy of 23.5 eV. Calibration of the spectra of the plasma polymerized films was performed by using the C 1s peak (binding energy 285.0 eV) as an internal reference. The morphology of the films was observed by a BX51TF Olympus optical microscope. 3. Results and discussion 3.1. FT-IR analysis Typical FT-IR spectra obtained in this work are given in Fig. 2. The assignments of typical peaks are listed in Table 1 according to the literature [12]. For the sake of comparison, a spectrum (curve (a)) of a thermal polymerized polystyrene film is included in this figure. Curves (b) and (c) are spectra of the plasma polymerized films formed at CO2 flow rates of 30 and 60 sccm, respectively. As shown in Fig. 2, the spectra of the plasma polymerized films are very similar to the spectrum of the thermal polymerized polystyrene film. However, differences are observed among the three spectra. First, four intense aromatic finger bands located at 1735, 1800, 1870, and 1942 cm 1 are observed in spectrum (a). In spectra (b) and (c), the intensity of these four peaks weakens. The reduction of the strength of the four characteristic peaks indicates that some
Fig. 2. FT-IR spectra of various films: (a) thermal polymerized; (b) plasma polymerized at 30 sccm; (c) plasma polymerized at 60 sccm.
H.L. Luo et al. / Applied Surface Science 253 (2007) 5203–5207 Table 1 Assignments of typical peaks presented in Fig. 2 Wavenumber (cm 1)
Type of vibration
3296 3082, 3059, 3026 2963, 2868 2925, 2850 1709 1674–1942
Hydroxyl O–H stretching Aromatic C–H stretching Methyl (CH3) C–H stretching Methylene C–H stretching Carbonyl C O stretching C–H out-of-plane deformation bending of aromatic ring Methyl C–H bending; in-plane phenyl ring-bending mode Out-of-plane phenyl ring-bending mode
1449 700, 759
Table 2 Relative atomic proportions (%) of plasma and thermal polymerized films Preparation conditions
C
O
O/C
Thermal polymerization Plasma polymerization (30 sccm) Plasma polymerization (60 sccm)
96.1 87.8 85.5
3.9 12.2 14.5
0.04 0.14 0.17
phenyl rings in molecules are destroyed by the plasma polymerization process. Second, the intensity of the two bands positioned at 2925 and 2850 cm 1 corresponding to methylene C–H stretching is substantially reduced such that the 2850 cm 1 band disappears in the spectra of the plasma polymerized films. Third, two new peaks positioned at 2963 and 2868 cm 1, which correspond to unsymmetrical and symmetrical stretching vibrations of methyl (–CH3) [13], are noted in the spectra of the plasma polymerized films. The presence of –CH3 is an indicator of extensive branching of plasma polystyrene. The formation of –CH3 in the plasma polymerized films is most likely due to destruction of benzene rings as a result of bombardment of plasma [14]. Furthermore, introduction of CO2 is believed to accelerate the destruction of benzene rings [15]. Last, two other new peaks at 1709 and 3296 cm 1, which are ascribed to C O and O–H groups, respectively, are observed in the spectra of the plasma polymerized films. Furthermore, the intensity of the two new peaks increases with the increase of the flow rate of CO2. This finding indicates that CO2 has participated in the plasma polymerization of styrene. 3.2. XPS analysis XPS, which is one of the most widely employed techniques for the characterization of polymer films, was used to analyze the
5205
surface component of the plasma polymerized films. The results are summarized in Tables 2 and 3 and Fig. 3. Table 2 shows the elemental composition of the thermal and plasma polymerized polystyrene films. As given in Table 2, the thermal polymerized film shows the existence of 96.1% carbon and 3.9% oxygen. The existence of oxygen in the thermal polymerized film is probably due to the quench effect during the preparation process [16,17]. Note that the oxygen content of the plasma polymerized films is higher than that of the thermal polymerized one, being 12.2 and 14.5%, respectively, at flow rates of 30 and 60 sccm. The higher oxygen content indicates that introduction of CO2, as carrier gas, results in more oxygen incorporation into the films. The residual oxygen in the chamber (due to relative low vacuum) may also contribute a little to the increased oxygen content. Fig. 3 shows the fitted C 1s spectra of various films. The functional groups and their contents of these films obtained from Fig. 3 are given in Table 3. Obviously, C–C and/or C–H (at 285.0 eV) and p–p* (at 291.8 eV) bands are present on all films. The characteristic p–p* shake-up satellite is due to resonance of the aromatic rings of polystyrene [18,9], indicating polystyrene films are synthesized by plasma polymerization under our experimental conditions. It is interesting to notice that three additional peaks which corresponds to C–O–C and/or C–OH (286.5 eV), C O, H– C O, and/or aromatic-OH (288.0 eV), and COOH and/or COOR (289.2 eV) are observed on the plasma polymerized films. Therefore, it is inferred that CO2 has been incorporated into the plasma polymerized films. This finding agrees well with FT-IR results. 3.3. Morphology of plasma polymerized films Fig. 4 shows the morphology of the plasma polymerized films obtained by optical microscopy. As shown in this figure, the plasma polymerized films formed on the glass substrates exhibit a wave-like pattern. A similar wave-like pattern has been reported by Bowden et al. [19]. The wave-like pattern of the plasma polymerized films is believed to be ascribed to the buckling phenomenon of the films as a result of compressive forces [19]. A comparison between Fig. 4a and b reveals that the wavelength (width of two neighboring waves) is related to the flow rate of CO2. A higher flow rate corresponds to a larger wavelength. The plasma polymerized film at 60 sccm possesses a larger wavelength than that at 30 sccm. As described in literature [20], the wavelength is dependent on the properties of the films. Consequently, the observed difference in wavelength indicates that the film properties can be adjusted by changing the flow rate of CO2.
Table 3 Functional groups and their contents (%) of plasma and thermal polymerized films Position (eV)
Assignment
Thermal polymerized film
Plasma polymerized film (30 sccm)
Plasma polymerized film (60 sccm)
285.0 286.5 288.0 289.2 291.8
C–C, C–H C–O–C, C–OH C O, H–C O, aromatic-OH COOH, COOR p–p* shake-up satellite
97.31 – – – 2.69
84.76 10.30 2.35 0.58 2.01
81.07 12.40 3.38 1.09 2.05
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Fig. 4. Images of plasma polymerized polystyrene films at CO2 flow rate of (a) 30 sccm and (b) 60 sccm.
Though how the wave-like pattern is formed and even how the films are polymerized are not fully understood, the observed patterned thin films are believed useful as models for analysis of mechanical properties in materials science field [21,22]. We believe that more work is worthy to gain insight into the formation mechanisms, properties and applications of the polystyrene films.
4. Conclusions
Fig. 3. C 1s spectra of various films: (a) thermal polymerized; (b) plasma polymerized at 30 sccm; (c) plasma polymerized at 60 sccm (1, C–C and/or C– H; 2, C–O–C and/or C–OH; 3, C O, H–C O, and/or aromatic-OH; 4, COOH and/or COOR; 5, p–p* shake-up satellite).
Polystyrene thin films have been successfully prepared by plasma polymerization technique by using carbon dioxide, as carrier gas and styrene vapor, as monomer. The current experimental results show that oxygen-containing groups (C– O, C O, and COO) are present in the plasma polymerized polystyrene films, suggesting that chemical reactions have happened between CO2 and styrene. XPS analysis further confirms that the content of these oxygen-containing groups increases with CO2 flow rate. Optical microscope observations demonstrate that the plasma polymerized styrene films show a wave-like pattern and that the wavelength increases with CO2 flow rate. Further investigation on the polymerization mechanisms, the electrical and mechanical properties of the plasma polymerized films, and their applications is underway.
H.L. Luo et al. / Applied Surface Science 253 (2007) 5203–5207
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