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Effect of oxygen gas on polycarbonate surface in keV energy Ar1 ion irradiation Jun-Sik Cho, Won-Kook Choi, Hyung-Jin Jung, and Seok-Keun Koh Division of Ceramics, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea

Ki Hyun Yoon Department of Ceramic Engineering, Yonsei University, Seoul 120-701, Korea (Received 3 September 1995; accepted 4 December 1995)

Ar1 ion irradiation on a polycarbonate (PC) surface was carried out in an oxygen environment in order to investigate the effects of surface chemical reaction, surface morphology, and surface energy on wettability of PC. Doses of Ar1 ion were changed from 5 3 1014 to 5 3 1016 at 1 keV ion beam energy by a broad ion beam source. Contact angle of PC was not reduced much by Ar1 ion irradiation without flowing oxygen gas, but decreased significantly as Ar1 ion was irradiated with flowing 4 sccm (mlymin) oxygen gas and showed a minimum of 12± to water and 5± to formamide. A newly formed polar group was observed on the modified PC surface by Ar1 ion irradiation with flowing oxygen gas, and it increased the PC surface energy. On the basis of x-ray photoelectron spectroscopy analysis, the formed polar group was identified as a hydrophilic C°°O bond (carbonyl group). In atomic force microscopy ˚ to (AFM) study, the root mean square of surface roughness was changed from 14 A ˚ by Ar1 ion irradiation without flowing oxygen gas and 26 –30 A ˚ by Ar1 ion 22–27 A irradiation with flowing 4 sccm oxygen gas. It was found that wettability of the modified PC surface was not greatly dependent on the surface morphology, but on an amount of hydrophilic group formed on the surface in the ion beam process.

I. INTRODUCTION

Surface characteristics of polymers determine their interfacial properties and technological applications such as adhesion to metal, wettability, solubility, etc. Previously many methods containing plasma, corona, arc discharge, dc or rf sputter etching, g-ray, ultraviolet light, ion beam, etc.1–8 have been executed to modify the surface properties of polymers. Changes in the chemical and physical properties of polymer at the surface region may be easily derived by chemical and/or structural modifications which can be achieved by variations of chemical functionality, surface texture, and wettability of polymers through the above-mentioned various surface treatment techniques. Many extensive studies have been reported about improving wettability of polymer by using plasma discharge, ion beam modification with a few MeV and 100s keV energy,9–12 and they have tried to explain relations among the changes in different physical/chemical characteristics of the polymer surface in view of surface morphology or interaction between the charged particle and the bulk of polymer. According to Fakes et al.,13 it was reported that the mean contact angle of water to modified PMMA (polymethylmethacrylate) surface by using O2 plasma discharge decreased from 70.7± to 49.5±, and this drop J. Mater. Res., Vol. 12, No. 1, Jan 1997

of contact angle was explained by the oxygenation of the surface layers, which means that the hydrophilic group was induced by chemical reaction. Wrobei et al.14 treated PET (poly ethylene terephthalate) by using plasma initiated in various reactive gases: nitrogen, oxygen, carbon dioxide, and ammonia. They explained that the increase of wettability was mainly due to the change of surface roughness by forming micropores. As mentioned above, so far the change of wettability for polymer has been explained in terms of chemical reaction or surface morphology change, respectively, but those independent explanations could not manifestly clarify the change of wettability. Therefore, the change of wettability should be investigated via associating chemical reaction together with the change of surface roughness. In the previous works,15,16 we modified PMMA with 1 keV energy Ar1 ion beam irradiation in oxygen environment, which is relatively quite lower energy than the conventional surface ion beam modification techniques using a few MeV and 100s keV energy. The contact angle of previously modified PMMA was changed from 69± to 8±, and a much greater increase in wettability was observed than those treated by plasma, corona discharge, etc.13,14 Based on the results of PMMA, PC was also  1997 Materials Research Society

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J-S. Cho et al.: Effect of oxygen gas on polycarbonate surface in keV energy

modified by using the 1 keV Ar1 ion irradiation in oxygen environment, and significant enhancement of wettability was also observed.17,18 It is well known that wettability is closely related to surface energy, which is expressed by the sum of dispersion force and polar force. Usually, the larger the surface energy is, the better wettability is. One purpose of this work is to examine thoroughly the reciprocal effects of chemical reaction and surface roughness on change of wettability, and investigate which force terms will dominantly contribute to increase surface energy of the modified PC surface. In this article, we investigate contact angle variations of two different polar liquids (water and formamide) to PC surface, and surface energies for untreated and modified PC surfaces by various doses of Ar1 ion at 1 keV energy in oxygen environment are calculated, respectively. Chemical bonds for PC surface modified by Ar1 ion irradiation are identified by x-ray photoelectron spectroscopy (XPS) using high resolution C1s and O1s spectra. Change of surface morphology was examined by atomic force microscope (AFM). II. EXPERIMENTAL A. Materials

A commercial polycarbonate (PC) was cut into 10 3 10 3 3 mm3 . These blocks were rinsed by a soap and cleaned by Iso Propyl Alcohol (Baker Analyzed HPLC Reagent) and were washed by distilled water. In order to remove residual stress, the PC samples were annealed at 100 ±C for 24 h. The chemical formula of PC used in the experiment is as follows:

time. This experimental setup was described in detail previously.15–18 Oxygen gas (99.99%) was introduced near the PC surface during energetic Ar1 ion irradiation, in order to compare the chemical change of PC surface by Ar1 irradiation with that of PC surface by Ar1 irradiation with flowing oxygen gas. C. Measurement of contact angle

Change of contact angles in the sessile drop method was measured by Contact Anglemeter (ERMA, Goniometer type), and the values of contact angle were taken as average values of four drops at different places on untreated or modified PC samples. Measured wetting value was advancing contact angle, and test liquids were triple distilled water and formamide (Junsei Chemical Co., Ltd.). From the data of contact angles of water and formamide, dispersion and polar force were calculated, and consequently changes of surface energy for the modified PC samples were quantitatively investigated. D. XPS and AFM analyses

X-ray photoelectron spectroscopy (XPS) was performed to examine the chemical bond environment for the modified PC surface, using a Surface Science Instrument 2803-S spectrometer which had a base pressure of 2 3 10210 Torr. XPS data were obtained with nonochromatic Al Ka x-ray (hy 1486.6 eV), x-ray monochromator, and a concentric hemispherical analyzer, and the performance could reach up to an energy resolution of 0.48 eV. Atomic force microscope (Park Scientific Instrument) was employed to measure surface morphology for modified PC samples. The root mean square, Rrms , of surface roughness was averaged from 10 times scanning over 1 mm2 at different places. III. RESULTS AND DISCUSSION

≤

B. Ion source

A 5-cm diameter cold-hollow cathode ion source was used to ionize Ar gas. Ar1 ions were irradiated on the PC surface at room temperature without or with flowing oxygen gas whose flow rate was fixed at 4 sccm. Ion beam potential of Ar1 ions was applied at 1 keV. Current of ion beam was controlled by discharge current and measured by a Faraday cup biased 230 V in order to prevent background electron from entering it. Doses of ions were changed from 5 3 1014 to 5 3 1016 ionsycm2 at fixed ion beam current with irradiating 278

Figure 1 shows the change of contact angles of water and formamide as a function of irradiating ion dose which were varied from 5 3 1014 to 5 3 1016 ionsycm2 without and with flowing 4 sccm oxygen gas. After Ar1 ion irradiation only, contact angles of water were changed from 78± to 48± [ssd line] and those of formamide from 63± to 32± [( ) line]. Contact angles do not largely decrease with changing ion dose, and these values are in good agreement with those obtained by other methods, which are 100s keV to a few MeV ion irradiation, plasma, corona discharge, dc and rf sputtering, etc. In Fig. 1, the open rectangular [shd line] and close rectangular [sjd line] show an effect of oxygen gas on contact angle as Ar1 ions irradiate PC surface. Contact angles of water are significantly reduced to 12± [shd line] and those of formamide to 5± [sjd line] when oxygen gas was flowed at the rate of 4 sccm, which explained that the introduction of oxygen gas

J. Mater. Res., Vol. 12, No. 1, Jan 1997


and for resolving the surface energy into contribution from dispersion and polar force. Dispersion force contained the Keesom forces arising from molecules with permanent dipoles, Debye forces caused by a molecule polarization, and London dispersion forces arising from instantaneous dipoles produced by the motion of electrons within molecule. London forces are ubiquitous and account for a major part if not all of the strength of such polymers as polyethylene. Polar force is induced by dipole-dipole interaction and is mainly hydrogen bonds in water. Their conclusion was that dispersion force and polar force of solids could be calculated by using two polar liquids. By using Owens’ method and measuring the contact angle sud of two different pure liquids on solid, the values of gsd and gsp could be evaluated. Therefore, the total surface energy of solid can be obtained by the sum of gsd and gsp : gs gsd 1 gsp .

FIG. 1. Change of contact angles of water and formamide for PC as a function of Ar1 ion dose at 1 keV without flowing oxygen gas and with flowing 4 sccm oxygen gas.

makes contact angle considerably decrease, and that some chemical reaction would happen between oxygen and PC surface. Changes of contact angle with changing flow rate of oxygen gas and dose of Ar1 or O21 ion have been investigated in our early works where contact angles of water were changed from 78± to 10±–12± with the dose of Ar1 and O21 ions in various oxygen contents.17,18 In order to explain the reaction mechanism, we have suggested a two-step model. The first step is a creation of unstable chains by Ar1 ions impact on PC surface by keV Ar1 ions irradiation sufficient to scissor bonding of a polymer chain whose bonding energy is about a few eV. The second step is a formation of hydrophilic bond by the interaction between scissored unstable chains and oxygen. Many workers have developed a number of methods for measuring the surface energy of solid, which is closely related to wettability. Fowkes19 demonstrated that surface free energy consisted of different intermolecular forces at the surface and that the work of adhesion was related to the surface energy of solid sgsd d and liquid sgld d by a geometric mean approximation. Further work proved that polar interaction of solid and liquid existed, assuming that a geometric mean approximation was also appropriate for polar force. Owens and Wendt20 developed a method for measuring the surface energy of solid

(1)

From Eq. (1) and the data of contact angle in Fig. 1, we calculate the value of surface energy for modified PC surfaces without flowing oxygen gas and with flowing 4 sccm oxygen gas. Dispersion force sgld d and polar p force sgl d of surface energy for test liquids are 21.8, 51.0 ergycm2 to water and 39.5, 18.7 ergycm2 to formamide, respectively. Figure 2 and Figure 3 represent the change of surface energy as a function of ion dose for the modified PC surfaces without flowing oxygen gas and with flowing 4 sccm oxygen gas. These show that dispersion force is changed from 15 to 20 ergycm2 in both cases and is not affected much by Ar1 ion irradiation, but polar force is significantly changed and the values of polar force increase from 14 to 29 ergsycm2 in Ar1 ions irradiation and to 55 ergsycm2 in Ar1 ions irradiation with flowing 4 sccm oxygen. It is apparent that the introduction of oxygen gas effectively increases polar force term in surface energy by dipole-dipole interaction of hydrogen bond between the newly formed polar group and two liquids. From this result, the large decrease of contact angle can be explained directly by means of dominant increase of polar force in the surface energy of the modified PC surfaces. Compared with other methods such as corona, arc, plasma discharge, etc.21,22 the values of surface energy by Ar1 ion irradiation only are similar to those by other methods, but the higher values of surface energy than other methods are obtained by Ar1 ion irradiation with oxygen gas. The significant increase of surface energy mainly due to polar force should contribute to good wettability with reducing contact angles, as shown in Fig. 1. In order to identify chemical reaction on PC surface by Ar1 ion irradiation in an oxygen environment, XPS analysis was performed. Figures 4 and 5 show C1s and O1s core level spectra of the untreated PC sample and


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the modified PC sample which was irradiated at an ion dose of 1 3 1016ycm2 with oxygen flow rate of 4 sccm. Figure 4(a) shows the C1s peak of an untreated PC sample in which the C–C peak is located at 284.6 eV, C–O at 286.01 eV, and C°°O at 287.61 eV, and the ratio of peak area is 84%, 13%, and 3%, respectively. In the case of the modified sample [Fig. 4( b)], binding energies of carbon peaks are shifted to higher energy position, and the ratio of each peak area is changed compared with that of the untreated sample. In particular, the area ratio of the C° °° °O is significantly increased from 3% to 17%. It can be explained that unstable chains generated by Ar1 ion irradiation react with oxygen and thus more C°°O group is formed. A similar result is shown in O1s spectra. Figure 5(a) represents O1s spectrum of °° untreated sample, and the peak position of C° °O and C–O is 532 eV and 533.66 eV, respectively. For the modified sample [Fig. 5( b)], an area ratio and intensity of C°°O peak largely increase, which indicates the formation of more C°°O group. Through XPS analysis, it is well supported that the increase of polar force on PC surface is due to the formation of a new polar group, especially C°°O bond (carbonyl group) by a chemical reaction of unstable chains and oxygen. FIG. 2. Changes of surface energy, dispersion, and polar force for PC surface as a function of Ar1 ion dose at 1 keV without oxygen gas.

FIG. 3. Changes of surface energy, dispersion, and polar force for PC surface as a function of Ar1 ion dose with flowing 4 sccm oxygen gas. 280

FIG. 4. Evolution of C1s spectra of PC surface: (a) before Ar1 ion irradiation and ( b) after Ar1 ion irradiation with flowing 4 sccm oxygen.



(a)

FIG. 5. Evolution of O1s spectra of PC surface: (a) before Ar1 ion irradiation and ( b) after Ar1 ion irradiation with flowing 4 sccm oxygen.

In order to carefully examine the dependence of contact angle on surface roughness, AFM analysis was carried out. From Fig. 6, the root mean square of surface ˚ roughness sRrms d for untreated PC sample (a) is 14 A and that for PC surface (b) irradiated without oxygen ˚ In gas at a dose of 5 3 1016 ionsycm2 is 22–27 A. the case of Ar1 ion irradiation with flowing 4 sccm oxygen gas at dose of 5 3 1016 ionsycm2, Rrms for the ˚ and hardly modified PC surface (c) is just 26–30 A changes compared with that of Ar1 ion irradiation only. If the changes of surface roughness directly affect those of contact angles, the decrease of contact angle for modified PC surface without flowing oxygen gas may be caused by an increase of Rrms . However, the Rrms of modified PC surface with flowing 4 sccm oxygen gas is not significantly changed compared with that modified without flowing oxygen gas. This means that an increase of surface roughness by 1 keV Ar1 ion irradiation can only reduce contact angle to some extent from 78± to 48±, but cannot completely explain the reduction of contact angle as small as 10± –12± for the modified PC surface with oxygen flow showing nearly the same surface roughness. Therefore, it is evident that the change of

( b)

(c) FIG. 6. AFM images of surface morphology for PC surface: (a) without Ar1 ion irradiation, ( b) Ar1 ion irradiation without oxygen gas at dose of 5 3 1016ycm2 , and (c) Ar1 ion irradiation with flowing 4 sccm oxygen gas at dose of 5 3 1016ycm2 .


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contact angle is more dependent on polar force in surface energy rather than surface roughness in this modification process. IV. CONCLUSIONS

Ar1 ion only irradiation without flowing oxygen gas reduces the contact angles of PC surface from 78± to 48± for water and to 32± for formamide, and in the case of Ar1 ion irradiation with flowing 4 sccm oxygen gas, those decrease 12± for water and 5± for formamide. In the former process, the root mean square of surface ˚ to 22–27 A, ˚ and it also roughness is varied from 14 A ˚ appears about 26 –30 A in the latter case. Such a small difference of surface roughness in both cases could not well explain the large reduction of contact angle in the latter case by only a change of surface morphology. Through XPS analysis, it is found that much more polar groups (C°°O bond) on the PC surface are formed by Ar1 ion irradiation with oxygen gas than Ar1 ion only irradiation, and the formation of polar group is believed to result from the chemical reaction between PC surface and oxygen gas. Dispersion force is measured 15 –20 ergycm2 in both cases, but polar force increases from 14 to 29 ergsycm2 in Ar1 ion only irradiation and to 55 ergsycm2 in Ar1 ion irradiation with oxygen gas. Consequently, the wettability of the PC surface modified by Ar1 ion irradiation with oxygen gas is improved by the increase of surface energy in which polar force increases due to dipole-dipole interaction between the polar group and two liquids. REFERENCES 1. R. P. Livi, Nucl. Instrum. Methods B10/11, 545 (1985). 2. S. Jacobson, B. Johnson, and B. Sundqvist, Thin Solid Films 107, 89 (1983).

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