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Superhydrophilic polymer surface modification by low energy reactive ion beam irradiation using a closed electron Hall drift ion source
Surface & Coatings Technology 201 (2007) 8099 – 8104 www.elsevier.com/locate/surfcoat
Superhydrophilic polymer surface modification by low energy reactive ion beam irradiation using a closed electron Hall drift ion source Won-Kook Choi ⁎ Thin Film Materials Research Center, Korea Institute of Science and Technology, Cheongryang P.O. Box 131, Seoul 130-650, South Korea Available online 12 March 2007
Abstract A gridless closed electron Hall drift plasma accelerator with very low energy (b200 eV) and high current density (∼1 mA/cm2) was adopted for polymer surface modification. Ar, N2, O2, and N2O gases were used for the generation of plasma and the ions were irradiated on polyimide and polyvinylidene fluoride with the ion fluence of 5 × 1015/cm2–1 × 1018/cm2. By the irradiation of O+2 at the very low ion fluence of 5 × 1015/cm2 corresponding to 1 s treatment time, the wetting angle of PI and PVDF were greatly reduced from 78° and 61° to less than 2° and N2O+ ion irradiation showed analogous results. The attainment of superhydrophilic polymer surface can be explained by the formation of hydrophilic group induced by low energy reactive ion beam irradiation. From the results, it is revealed that the very low energy ion beam irradiation can be used as an effective and productive ion source compared with conventional gridded ion source for the enhancement of surface energy of polymer. © 2007 Elsevier B.V. All rights reserved. Keywords: Superhydrophilic; Polymer surface; Low energy ion beam; Closed electron Hall drift ion source
1. Introduction Many techniques such as plasma, corona, electron beam [1– 3], gamma ray irradiation [4], and ion irradiation [5,6] have been reported to change hydrophobic polymer surfaces into hydrophilic ones for improving the adhesion to other materials. Recently a low energy ion beam treatment, ion assisted reaction (IAR) [7,8], was introduced as an effective method to greatly increase the adhesion of polymer surface to metal by irradiating with about 1 keV Ar+ ion along with simultaneous flowing of reactive oxygen gas near the substrate. In IAR, it was proposed that Ar+ ion irradiation induces unstable chains by scissoring the bonding of polymer chain and then these chemically active chains would combine with oxygen leading to the formation of hydrophilic groups like C=O etc. On the other hand, IAR generally showed the optimum surface modification at the ion fluence of ∼ 5 × 1016/cm2 which corresponds to quite a long time treatment for a large area polymer and mass production when compared to conventional gridded ion beam source. To increase the productivity, it is necessary to reduce the treatment ⁎ Tel.: +82 2 958 5562; fax: +82 2 958 6720. E-mail address: [email protected]. 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.11.145
time by using the high current density ion beam source. In this study, a closed electron Hall drift plasma accelerator, which is a gridless ion source with high plasma density of 1012/cm2 was adopted [9,10]. This source can generate very low kinetic energy ion beam of 80–300 eV. Polyvinylidene fluoride (PVDF) and polyimide (PI) were chosen as the substrates to be modified because these polymers were known to be as both hydrophobic and widely adopted in the flexible electronics due to its chemical and thermal stability, and electrically insulating property. 2. Experiments A cylindrical closed drift Hall thruster, known as stationary plasma thruster (SPT) with the outer diameter of 70 mm was used as an ion source. Ar, O2, and N2O were used as plasma gas and discharged at Vd = 220–300 eV. Other operational procedures and the principle of the ion source were well described elsewhere in detail [9,10]. Ion beam modification chamber was initially pumped down to 5 × 10− 7 Torr using a turbo molecular pump (Seiko Seiki, STPH2000C) and irradiation was carried out at the working pressure of 0.03–0.07 Pa. Current density J, i.e., ion flux was monitored in-situ using a Faraday cup and
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showed uniformity of 80% within a 10 × 10 cm2 area at a distance of 15 cm from the ion source. J varied from 0.35 to 1.14 mA/cm2 and the ion beam was directed at an incident angle normal to the PVDF and PI (Kapon-E, 25 μm) surfaces without rotation. The surface morphology was examined by an atomic force microscope (Nanoscope, Dimension 3100) and the change of surface energy is calculated by automatically measuring the wetting angle of deionized water and ethylene glycol (EG) using surface wetting anglometer. The induced chemical bonding on the modified polymer surface is identified by Xray photoelectron spectroscopy (hν = 1245.8 eV, PHI-5700). Binding energies of core-levels spectra were calibrated with the reference of C1s core-level peak at 284.6 eV. 3. Results and discussion 3.1. Characterization of SPT A principal diagram of SPT-type ion source is shown in Fig. 1. Main parts of the ion source are as follows. (1) a magnetic system consisted of iron magnetic-line guide and several magnetic coils, which can be operated in series with a main discharge or independently; (2) an annular ceramic channel where main ionization and acceleration processes take place; (3) anode which is combined with a gas distributor in first modifications but in the last ones they are separated [3]; and (4) a cathode which is usually a heated hole cathode with LaB6 pellet for emitter, but may be a simple filament or hole cathode with heating from main discharge or hole cathode with additional discharge which was used here. The fundamentals of plasma generation and other principles are well described elsewhere [9,10]. Fig. 2 shows the dependence ji(Ud) of the ion current density ji on discharge voltage Ud along the beam axis at the distance of 15 cm from the source outlet with the discharge current being
Fig. 2. Variation of ion current density with discharge voltage under constant discharge current for Ar, O2, and N2.
constant. Measured ji for Ar in the studied SPT source is of the order of 1 mA/cm2, which is 102 times higher than that measured in conventional Kaufman-type ion source. The results of ion energy analysis are presented in Fig. 3. The average ion energy (Ea) and energy dispersion (σ) for Ar calculated using the plot of Fig. 3 are EaAr = 180 eV (75% of Ud) and σAr = ± 20 eV. The results for N2 and O2 are analogous to those of Ar. 3.2. PI surface Fig. 4 presents the changes in the wetting angle of PI surfaces irradiated by various ions at different ion fluences. As shown in Fig. 4, the wetting angle of PI surface decreased from 78° (non-
Fig. 1. Schematic diagram of the SPT-type plasma source.
W.-K. Choi / Surface & Coatings Technology 201 (2007) 8099–8104
Fig. 3. Energy distribution of the Ar+ ions in the SPT ion beam with the discharge voltage 230 V.
treated PI) to 32° when Ar+ ion beam is irradiated. When PI is irradiated with reactive N2+ ion beam at the low current density 0.5 mA/cm2, the wetting angle quickly dropped to around 25° until the ion fluence of 1 × 1017/cm2, but it decreases less than10° for the fluence higher than 1 × 1017/cm2. Whereas, the wetting angle is remarkably reduced to below 10° at the very low fluence of reactive O2+ and N2O+ ions with the current density of 0.57 mA/cm2. As the fluence of O2+ ion beam increases up to 5 × 1016/cm2, the wetting angle decreases
Fig. 4. Variations of wetting angles at various ion fluences for Ar+, O+2, and N2O+ ion irradiated polyimide surface with180 eV impinging energy.
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gradually to as small as 22°, and further down to 8° at the fluence of 1 × 1017/cm2. At higher than 1 × 1017/cm2, it becomes too small to be precisely measured. In case of N2O+ ion beam irradiation, the wetting angle drops off to 28°, similar to decreasing behavior with O2+ ion irradiation until the ion fluence of 1 × 1016/cm2. But at the ion fluence higher than 1 × 1016/cm2, the wetting angle abruptly decreases to 2°, which indicates that N2O+ ion is an effective ion species to reduce the wetting angle at relatively lower fluence than O2+ ion irradiation. When the current density of O2+ is increased up to 0.8 mA/cm2, the wetting angle dramatically drops down to about 2° even at the very low ion fluence of 5 × 1015/cm2. Wetting angle is known to correlate with surface energy (γ), which is usually expressed as the sum of both a polar force (γp) and a dispersion force (γd). Using two wetting angle parameters of DI water and EG, surface energies of ion irradiated PI can be calculated by the adoption of the Owens–Wendt equation [11] and these results are presented in Fig. 5. As seen in Fig. 5, the surface energy of PI increases from 37 erg/cm to 69 erg/cm by Ar+ ion irradiation. On the other hand, it is greatly raised up to 81.2 erg/cm by the irradiation of O2+ or N2O+ ion beam, which is more than two times larger tan that of bare PI. This reveals that surface energy of the PI can be efficiently increased by reactive ion irradiation compared with an inert Ar+ ion irradiation. This result agrees with the previous reports that keV ion irradiation on polymer surface in reactive gas environment increases the surface energy, in particular, the polar force by the increment of the amounts of hydrophilic groups (i.e., C=O) on modified surface [12,13]. In this study, O2+ ion beam energy of 180 eV is still so high enough to scissor the polymer chain on PI surface and in addition reactive atomic oxygen species result from dissociation on the collision with PI surface might participate in the formation of new chemical
Fig. 5. Variations of surface energy at various ion fluences for Ar+, O+2, and N2O+ ion irradiated polyimide surface with180 eV impinging energy.
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Fig. 6. Evolution of (A) C1s, (B) O1s, and (C) N1s core-level spectra taken over the bare and ion beam impinged PI surfaces by 1 × 1016/cm2(Ar+), 5 × 1015/cm2(O+2) and 5 × 1016/cm2 (N2O+). In each core-level spectra, a–d's mean bare, Ar+, O+2 (high), and N2O+ ion irradiated cases, respectively.
W.-K. Choi / Surface & Coatings Technology 201 (2007) 8099–8104
bonding with unstable broken chains. This assumption can be sufficiently supported by the increment of polar force in the O2+ irradiated PI surface compared to that of Ar+ irradiated. Ar+. In case of N2O+ ion beam, two aspects of both chemical reaction and high mass transfer should be considered. Moreover, in a similar case of O2+ ion beam impingement N2O+ ion would be broken into several species like NO, O, N2 and N at the collision on the PI surface. These reactive species actively take part in forming the chemical bonding such as aromatic carbon bonded with O and N, carbonyl carbon, and ether carbon etc and some of which improve the polar force of the PI surface. This dual effects of N2O+ ion beam irradiation makes the wetting angle abruptly decreased to 2° at even low N2O+ ion fluence of 5 × 1016/cm2. At this fluence, Ar+ and O2+ ion irradiation merely decrease the wetting angles to as much as 44° and 25°,
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respectively. From the wetting angle measurement, low energy ion beam irradiation on PI shows much better performance for increasing the surface energy compared with previously reported surface treatment methods of plasma [1] and reactive ion etching [14]. Fig. 6 shows the evolution of C1s, N1s, and O1s core-level spectra taken over the bare and ion beam impinged PI surfaces by 1 × 1016/cm2 Ar+, 5 × 1015/cm2 O2+ and 5 × 1016/cm2 N2O+ ions. C1s is comprised of four components, hereafter C1, C2, C3, and C4 features constrained to their values for clean PI. The component peak C1, aromatic carbon, at 284.5 eV is attributed to the carbon atoms of two benzene rings of the ODA. The component peak C2, aromatic carbon bonded N, at 285.3 eV is related to carbon atoms of the benzene ring in the PMDA and C–N bonding. Peak C3 at 286.1 eV corresponds to a C–O single
Fig. 7. XPS core-level spectra of bare and ion beam irradiated PVDF.
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bond and peak C4, carbonyl carbon, at 288.5 eV pertains to a C=O double bond. In the oxygen O1s peak, it consists of two components of the carbonyl oxygen O1 (C=O, 531.9 eV) and the ether oxygen O2 (C–O–C, 533.2 eV). N1s is fitted into the imide group (400.1 eV) and iso-imide group (C–N–C, 398.8 eV). The energy difference 1.3 eV of fitted value coincides with the previous report on XPS spectra of PI irradiated by 0.1 keVoxygen ion beam [15]. Especially the ratio of carbonyl and ether oxygen by the fitting of O1s core-level spectra is estimated and the oxygen contents in all the samples are increased from 14% for bare PI to higher than 29% after O2+ (high) ion irradiation, and that is about 28% for N2O+ ion irradiated PI surface at ion fluence 5 × 1016/cm2. From the results, the ratio of carbonyl O (C=O) increases up to18.3% and 17.08% for O2+ and N2O+ ion impinged PI surfaces respectively from 9.9% of the bare PI surface. Direct irradiation of low energy reactive ion beam on PI was proved effectively to increase the atomic contents of hydrophilic C=O groups. 4. PVDF surface The change of the wetting angle and surface energy of PVDF are investigated by various ion beam irradiation at the ion fluence of 5 × 1015/cm2 corresponding to just 1 s treatment time. In case of Ar+ ion irradiation, the wetting angle for deionized water is just slightly reduced from 61° to 40° and this means that inert ion irradiation is not effective in reducing the wetting angle. On the other hand, when the PVDF surfaces are irradiated with N2O+ or O2+ the wetting angles greatly decreased to below 4° or 2° respectively, which represents the lower value than ever been reported in the modification of PVDF surface. In latter cases, the wetting angles to EG drop are hardly measurable because the EG seems to be entirely spread on the PVDF surfaces modified by N2O+ and O2+ ion beams. Calculated surface energy γpris for the pristine PVDF is 44 mN/m, but it increases up to 68 mN/m for Ar+ ion irradiation and rises up to 81 mN/m corresponding to almost double to that of γpris for both N2O+ andO2+ ion impingement. Park et al. [16] reported that when chemically activated Ar, H2, and O2 plasma with rf power of 25–100 W was remotely introduced on PVDF surface, the wetting angle of PVDF was reduced from 90° to only 55°, 53° and 71° respectively. Similarly Duca et al. [17] also modified the PVDF surface with Ar plasma excited by rf at 5 cm apart and decreased the wetting angle from 71° to 36° and increased the surface energy from 33 mN/m to 60 mN/m. Moreover, Han et al. [18] reported that Ar+ ion irradiation with an energy of 1 keV decreased the wetting angle from initial 75° to 50°, but that with the blowing of reactive O2 gas around the sample surface only reduced the wetting angle to 30°, by IAR method. To date there is still no method to effectively reduce the wetting angle of PVDF lower than 30°. Fig. 7 represents the C1s, F1s, O1s, and N1s core-level spectra of bare and ion irradiated PVDF surfaces. In C1s spectra, with Ar+ on irradiation the full-width at half maximum becomes smaller than those of any others. This result from the defluoridation of F atoms by ion beam impingement into the vacuum which makes the degree of carbonization of PVDF
enhanced. In case of O2+ irradiation, peak intensity around the binding energy of 287.6–288 eV like C=O is increased and this results from the increment of the bonding between C and O which was typically well observed in low energy ion beam irradiation. N2O+ ion irradiation broadly increase the peak intensity around 286–286.5 eV which is related to C–O–N and C–O bonding. From the O1s spectra, the peak intensity of carbonyl group is largely increased with O2+ ion irradiation, but instead N–O bonding around 530 eV is increased with the irradiation of N2O+ ion which is confirmed again by observing the strong peak at 398.5 eV in N1s spectra. 5. Conclusion A closed electron Hall drift plasma accelerator having very low energy ion beam less than 200 eV and high current density was adopted for enhancing the wettability of PVDF and PI surfaces. With the irradiation of O2+ and N2O+ ion within the low ion fluence of 5 × 1015/cm2 corresponding to very short treatment time of 1 s, the wetting angle of PVDF and PI was greatly reduced from 61° and 78° to 2° respectively. Such a successful achievement of superhydrophilicity is closely related to the formation of hydrophilic groups by the irradiation of high flux reactive ion beam with even low kinetic energy. Acknowledgement The author, Won-Kook Choi, would like to appreciate the financial support from KIST, Flextronics Policy Program. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
R.P. Livi, Nucl. Instrum. Methods B 10/11 (1985) 545. S. Jacobson, B. Johnson, B. Sunquist, Thin Solid Films 107 (1983) 89. T.A. Tombrello, Nucl. Instrum. Methods 218 (1983) 679. I.V. Mitchell, G. Nyberg, R.G. Elliman, Appl. Phys. Lett. 45 (1984) 137. C.R. Wie, C.R. Shi, M.H. Mendenshall, R.P. Livi, T. Vreeland, T.A. Tombrello, Nucl. Instrum. Methods B 9 (1985) 20. B. Leclercq, M. Sotton, A. Baszkin, L. Ter-Minassian-Saraga, Polymer 18 (1977) 675. W.K. Choi, S.K. Koh, H.J. Jung, J. Vac. Sci. Technol. A 14 (1996) 2366. S.K. Koh, S.K. Song, W.K. Choi, H.J. Jung, S.N. Han, J. Mater. Res. 10 (1995) 2390. J.Y. Park, Y.S. Jung, Yu.A. Ermakov, W.K. Choi, Nucl. Instrum. Methods B 239 (2005) 440. A.I. Morozov, A.I. Bougrova, V.T. Niskine, A.V. Dessijatskov, D. Valentian, (US5581155). D.K. Owens, R.C. Wendt, J. Appl. Polym. Sci. 13 (1969) 1741. W.K. Choi, S.K. Koh, H.J. Jung, J. Vac. Sci. Technol. A 14 (1996) 2366. S.K. Koh, S.C. Park, S.R. Kim, W.K. Choi, H.J. Jung, K.D. Pae, J. Appl. Polym. Sci. 64 (1997) 1913. S. Iwamori, T. Miyashita, S. Fukuda, S. Nozaki, K. Sudoh, N. Fukuda, Vaccum 51 (1998) 615. R. Flitsch, D-Y. Shin, J. Vac. Sci. Technol. A 8 (1990) 2376. Y.W. Park, N. Inagaki, Polymer 44 (2003) 1569. M.D. Duca, C.L. Plosceau, T. Pop, Polym. Degrad. Stab. 61 (1998) 65. S. Han, W.K. Choi, K.H. Yoon, S.K. Koh, J. Appl. Polymer Sci. 72 (1997) 41.
Superhydrophilic polymer surface modification by low energy reactive ion beam irradiation using a closed electron Hall drift ion source Won-Kook Choi ⁎ Thin Film Materials Research Center, Korea Institute of Science and Technology, Cheongryang P.O. Box 131, Seoul 130-650, South Korea Available online 12 March 2007
Abstract A gridless closed electron Hall drift plasma accelerator with very low energy (b200 eV) and high current density (∼1 mA/cm2) was adopted for polymer surface modification. Ar, N2, O2, and N2O gases were used for the generation of plasma and the ions were irradiated on polyimide and polyvinylidene fluoride with the ion fluence of 5 × 1015/cm2–1 × 1018/cm2. By the irradiation of O+2 at the very low ion fluence of 5 × 1015/cm2 corresponding to 1 s treatment time, the wetting angle of PI and PVDF were greatly reduced from 78° and 61° to less than 2° and N2O+ ion irradiation showed analogous results. The attainment of superhydrophilic polymer surface can be explained by the formation of hydrophilic group induced by low energy reactive ion beam irradiation. From the results, it is revealed that the very low energy ion beam irradiation can be used as an effective and productive ion source compared with conventional gridded ion source for the enhancement of surface energy of polymer. © 2007 Elsevier B.V. All rights reserved. Keywords: Superhydrophilic; Polymer surface; Low energy ion beam; Closed electron Hall drift ion source
1. Introduction Many techniques such as plasma, corona, electron beam [1– 3], gamma ray irradiation [4], and ion irradiation [5,6] have been reported to change hydrophobic polymer surfaces into hydrophilic ones for improving the adhesion to other materials. Recently a low energy ion beam treatment, ion assisted reaction (IAR) [7,8], was introduced as an effective method to greatly increase the adhesion of polymer surface to metal by irradiating with about 1 keV Ar+ ion along with simultaneous flowing of reactive oxygen gas near the substrate. In IAR, it was proposed that Ar+ ion irradiation induces unstable chains by scissoring the bonding of polymer chain and then these chemically active chains would combine with oxygen leading to the formation of hydrophilic groups like C=O etc. On the other hand, IAR generally showed the optimum surface modification at the ion fluence of ∼ 5 × 1016/cm2 which corresponds to quite a long time treatment for a large area polymer and mass production when compared to conventional gridded ion beam source. To increase the productivity, it is necessary to reduce the treatment ⁎ Tel.: +82 2 958 5562; fax: +82 2 958 6720. E-mail address: [email protected]. 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.11.145
time by using the high current density ion beam source. In this study, a closed electron Hall drift plasma accelerator, which is a gridless ion source with high plasma density of 1012/cm2 was adopted [9,10]. This source can generate very low kinetic energy ion beam of 80–300 eV. Polyvinylidene fluoride (PVDF) and polyimide (PI) were chosen as the substrates to be modified because these polymers were known to be as both hydrophobic and widely adopted in the flexible electronics due to its chemical and thermal stability, and electrically insulating property. 2. Experiments A cylindrical closed drift Hall thruster, known as stationary plasma thruster (SPT) with the outer diameter of 70 mm was used as an ion source. Ar, O2, and N2O were used as plasma gas and discharged at Vd = 220–300 eV. Other operational procedures and the principle of the ion source were well described elsewhere in detail [9,10]. Ion beam modification chamber was initially pumped down to 5 × 10− 7 Torr using a turbo molecular pump (Seiko Seiki, STPH2000C) and irradiation was carried out at the working pressure of 0.03–0.07 Pa. Current density J, i.e., ion flux was monitored in-situ using a Faraday cup and
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showed uniformity of 80% within a 10 × 10 cm2 area at a distance of 15 cm from the ion source. J varied from 0.35 to 1.14 mA/cm2 and the ion beam was directed at an incident angle normal to the PVDF and PI (Kapon-E, 25 μm) surfaces without rotation. The surface morphology was examined by an atomic force microscope (Nanoscope, Dimension 3100) and the change of surface energy is calculated by automatically measuring the wetting angle of deionized water and ethylene glycol (EG) using surface wetting anglometer. The induced chemical bonding on the modified polymer surface is identified by Xray photoelectron spectroscopy (hν = 1245.8 eV, PHI-5700). Binding energies of core-levels spectra were calibrated with the reference of C1s core-level peak at 284.6 eV. 3. Results and discussion 3.1. Characterization of SPT A principal diagram of SPT-type ion source is shown in Fig. 1. Main parts of the ion source are as follows. (1) a magnetic system consisted of iron magnetic-line guide and several magnetic coils, which can be operated in series with a main discharge or independently; (2) an annular ceramic channel where main ionization and acceleration processes take place; (3) anode which is combined with a gas distributor in first modifications but in the last ones they are separated [3]; and (4) a cathode which is usually a heated hole cathode with LaB6 pellet for emitter, but may be a simple filament or hole cathode with heating from main discharge or hole cathode with additional discharge which was used here. The fundamentals of plasma generation and other principles are well described elsewhere [9,10]. Fig. 2 shows the dependence ji(Ud) of the ion current density ji on discharge voltage Ud along the beam axis at the distance of 15 cm from the source outlet with the discharge current being
Fig. 2. Variation of ion current density with discharge voltage under constant discharge current for Ar, O2, and N2.
constant. Measured ji for Ar in the studied SPT source is of the order of 1 mA/cm2, which is 102 times higher than that measured in conventional Kaufman-type ion source. The results of ion energy analysis are presented in Fig. 3. The average ion energy (Ea) and energy dispersion (σ) for Ar calculated using the plot of Fig. 3 are EaAr = 180 eV (75% of Ud) and σAr = ± 20 eV. The results for N2 and O2 are analogous to those of Ar. 3.2. PI surface Fig. 4 presents the changes in the wetting angle of PI surfaces irradiated by various ions at different ion fluences. As shown in Fig. 4, the wetting angle of PI surface decreased from 78° (non-
Fig. 1. Schematic diagram of the SPT-type plasma source.
W.-K. Choi / Surface & Coatings Technology 201 (2007) 8099–8104
Fig. 3. Energy distribution of the Ar+ ions in the SPT ion beam with the discharge voltage 230 V.
treated PI) to 32° when Ar+ ion beam is irradiated. When PI is irradiated with reactive N2+ ion beam at the low current density 0.5 mA/cm2, the wetting angle quickly dropped to around 25° until the ion fluence of 1 × 1017/cm2, but it decreases less than10° for the fluence higher than 1 × 1017/cm2. Whereas, the wetting angle is remarkably reduced to below 10° at the very low fluence of reactive O2+ and N2O+ ions with the current density of 0.57 mA/cm2. As the fluence of O2+ ion beam increases up to 5 × 1016/cm2, the wetting angle decreases
Fig. 4. Variations of wetting angles at various ion fluences for Ar+, O+2, and N2O+ ion irradiated polyimide surface with180 eV impinging energy.
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gradually to as small as 22°, and further down to 8° at the fluence of 1 × 1017/cm2. At higher than 1 × 1017/cm2, it becomes too small to be precisely measured. In case of N2O+ ion beam irradiation, the wetting angle drops off to 28°, similar to decreasing behavior with O2+ ion irradiation until the ion fluence of 1 × 1016/cm2. But at the ion fluence higher than 1 × 1016/cm2, the wetting angle abruptly decreases to 2°, which indicates that N2O+ ion is an effective ion species to reduce the wetting angle at relatively lower fluence than O2+ ion irradiation. When the current density of O2+ is increased up to 0.8 mA/cm2, the wetting angle dramatically drops down to about 2° even at the very low ion fluence of 5 × 1015/cm2. Wetting angle is known to correlate with surface energy (γ), which is usually expressed as the sum of both a polar force (γp) and a dispersion force (γd). Using two wetting angle parameters of DI water and EG, surface energies of ion irradiated PI can be calculated by the adoption of the Owens–Wendt equation [11] and these results are presented in Fig. 5. As seen in Fig. 5, the surface energy of PI increases from 37 erg/cm to 69 erg/cm by Ar+ ion irradiation. On the other hand, it is greatly raised up to 81.2 erg/cm by the irradiation of O2+ or N2O+ ion beam, which is more than two times larger tan that of bare PI. This reveals that surface energy of the PI can be efficiently increased by reactive ion irradiation compared with an inert Ar+ ion irradiation. This result agrees with the previous reports that keV ion irradiation on polymer surface in reactive gas environment increases the surface energy, in particular, the polar force by the increment of the amounts of hydrophilic groups (i.e., C=O) on modified surface [12,13]. In this study, O2+ ion beam energy of 180 eV is still so high enough to scissor the polymer chain on PI surface and in addition reactive atomic oxygen species result from dissociation on the collision with PI surface might participate in the formation of new chemical
Fig. 5. Variations of surface energy at various ion fluences for Ar+, O+2, and N2O+ ion irradiated polyimide surface with180 eV impinging energy.
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Fig. 6. Evolution of (A) C1s, (B) O1s, and (C) N1s core-level spectra taken over the bare and ion beam impinged PI surfaces by 1 × 1016/cm2(Ar+), 5 × 1015/cm2(O+2) and 5 × 1016/cm2 (N2O+). In each core-level spectra, a–d's mean bare, Ar+, O+2 (high), and N2O+ ion irradiated cases, respectively.
W.-K. Choi / Surface & Coatings Technology 201 (2007) 8099–8104
bonding with unstable broken chains. This assumption can be sufficiently supported by the increment of polar force in the O2+ irradiated PI surface compared to that of Ar+ irradiated. Ar+. In case of N2O+ ion beam, two aspects of both chemical reaction and high mass transfer should be considered. Moreover, in a similar case of O2+ ion beam impingement N2O+ ion would be broken into several species like NO, O, N2 and N at the collision on the PI surface. These reactive species actively take part in forming the chemical bonding such as aromatic carbon bonded with O and N, carbonyl carbon, and ether carbon etc and some of which improve the polar force of the PI surface. This dual effects of N2O+ ion beam irradiation makes the wetting angle abruptly decreased to 2° at even low N2O+ ion fluence of 5 × 1016/cm2. At this fluence, Ar+ and O2+ ion irradiation merely decrease the wetting angles to as much as 44° and 25°,
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respectively. From the wetting angle measurement, low energy ion beam irradiation on PI shows much better performance for increasing the surface energy compared with previously reported surface treatment methods of plasma [1] and reactive ion etching [14]. Fig. 6 shows the evolution of C1s, N1s, and O1s core-level spectra taken over the bare and ion beam impinged PI surfaces by 1 × 1016/cm2 Ar+, 5 × 1015/cm2 O2+ and 5 × 1016/cm2 N2O+ ions. C1s is comprised of four components, hereafter C1, C2, C3, and C4 features constrained to their values for clean PI. The component peak C1, aromatic carbon, at 284.5 eV is attributed to the carbon atoms of two benzene rings of the ODA. The component peak C2, aromatic carbon bonded N, at 285.3 eV is related to carbon atoms of the benzene ring in the PMDA and C–N bonding. Peak C3 at 286.1 eV corresponds to a C–O single
Fig. 7. XPS core-level spectra of bare and ion beam irradiated PVDF.
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bond and peak C4, carbonyl carbon, at 288.5 eV pertains to a C=O double bond. In the oxygen O1s peak, it consists of two components of the carbonyl oxygen O1 (C=O, 531.9 eV) and the ether oxygen O2 (C–O–C, 533.2 eV). N1s is fitted into the imide group (400.1 eV) and iso-imide group (C–N–C, 398.8 eV). The energy difference 1.3 eV of fitted value coincides with the previous report on XPS spectra of PI irradiated by 0.1 keVoxygen ion beam [15]. Especially the ratio of carbonyl and ether oxygen by the fitting of O1s core-level spectra is estimated and the oxygen contents in all the samples are increased from 14% for bare PI to higher than 29% after O2+ (high) ion irradiation, and that is about 28% for N2O+ ion irradiated PI surface at ion fluence 5 × 1016/cm2. From the results, the ratio of carbonyl O (C=O) increases up to18.3% and 17.08% for O2+ and N2O+ ion impinged PI surfaces respectively from 9.9% of the bare PI surface. Direct irradiation of low energy reactive ion beam on PI was proved effectively to increase the atomic contents of hydrophilic C=O groups. 4. PVDF surface The change of the wetting angle and surface energy of PVDF are investigated by various ion beam irradiation at the ion fluence of 5 × 1015/cm2 corresponding to just 1 s treatment time. In case of Ar+ ion irradiation, the wetting angle for deionized water is just slightly reduced from 61° to 40° and this means that inert ion irradiation is not effective in reducing the wetting angle. On the other hand, when the PVDF surfaces are irradiated with N2O+ or O2+ the wetting angles greatly decreased to below 4° or 2° respectively, which represents the lower value than ever been reported in the modification of PVDF surface. In latter cases, the wetting angles to EG drop are hardly measurable because the EG seems to be entirely spread on the PVDF surfaces modified by N2O+ and O2+ ion beams. Calculated surface energy γpris for the pristine PVDF is 44 mN/m, but it increases up to 68 mN/m for Ar+ ion irradiation and rises up to 81 mN/m corresponding to almost double to that of γpris for both N2O+ andO2+ ion impingement. Park et al. [16] reported that when chemically activated Ar, H2, and O2 plasma with rf power of 25–100 W was remotely introduced on PVDF surface, the wetting angle of PVDF was reduced from 90° to only 55°, 53° and 71° respectively. Similarly Duca et al. [17] also modified the PVDF surface with Ar plasma excited by rf at 5 cm apart and decreased the wetting angle from 71° to 36° and increased the surface energy from 33 mN/m to 60 mN/m. Moreover, Han et al. [18] reported that Ar+ ion irradiation with an energy of 1 keV decreased the wetting angle from initial 75° to 50°, but that with the blowing of reactive O2 gas around the sample surface only reduced the wetting angle to 30°, by IAR method. To date there is still no method to effectively reduce the wetting angle of PVDF lower than 30°. Fig. 7 represents the C1s, F1s, O1s, and N1s core-level spectra of bare and ion irradiated PVDF surfaces. In C1s spectra, with Ar+ on irradiation the full-width at half maximum becomes smaller than those of any others. This result from the defluoridation of F atoms by ion beam impingement into the vacuum which makes the degree of carbonization of PVDF
enhanced. In case of O2+ irradiation, peak intensity around the binding energy of 287.6–288 eV like C=O is increased and this results from the increment of the bonding between C and O which was typically well observed in low energy ion beam irradiation. N2O+ ion irradiation broadly increase the peak intensity around 286–286.5 eV which is related to C–O–N and C–O bonding. From the O1s spectra, the peak intensity of carbonyl group is largely increased with O2+ ion irradiation, but instead N–O bonding around 530 eV is increased with the irradiation of N2O+ ion which is confirmed again by observing the strong peak at 398.5 eV in N1s spectra. 5. Conclusion A closed electron Hall drift plasma accelerator having very low energy ion beam less than 200 eV and high current density was adopted for enhancing the wettability of PVDF and PI surfaces. With the irradiation of O2+ and N2O+ ion within the low ion fluence of 5 × 1015/cm2 corresponding to very short treatment time of 1 s, the wetting angle of PVDF and PI was greatly reduced from 61° and 78° to 2° respectively. Such a successful achievement of superhydrophilicity is closely related to the formation of hydrophilic groups by the irradiation of high flux reactive ion beam with even low kinetic energy. Acknowledgement The author, Won-Kook Choi, would like to appreciate the financial support from KIST, Flextronics Policy Program. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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