Comparison of Low-and Atmospheric-Pressure Radio Frequency

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Comparison of Low- and Atmospheric-Pressure Radio Frequency Plasma Treatments on the Surface Modification of Poly(methyl methacrylate) Plates Shen Tang and Ho Suk Choi* Department of Chemical Engineering, Chungnam National UniVersity, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, Korea ReceiVed: NoVember 27, 2007; In Final Form: January 21, 2008

In this article, the effects of RF (radio frequency) atmospheric pressure plasma (APP), and low pressure plasma (LPP) treatment were compared on the surface of the poly(methyl methacrylate) (PMMA) plate. The APP was proved to be able to improve the surface free energy of the PMMA faster and higher than the LPP under similar treatment condition. Atomic force microscopy revealed extraordinary surface roughness and morphology generated only on the APP-treated PMMA, where the cracks appeared. The surface temperature increased very quickly and exceeded quickly the glass transition temperature (Tg) of PMMA on the APPtreated surface, while it showed a little increase below the Tg on the LPP-treated surface. X-ray photoelectron spectroscopy analysis revealed a novel reaction mechanism is proposed for the formation of the nitrogencontaining groups. It was believed that the generation of new oxygen- and nitrogen-containing groups was dependent on the plasma treatment environment, which provided necessary active species for the formation of the new functional groups.

1. Introduction Poly(methyl methacrylate) (PMMA) is popular polymeric material in industry due to its excellent optical properties, environmental stability, and low cost.1 PMMA needs surface pretreatment2 or soft material coating for improving its surface mechanical and antireflective properties in the field of optical application and microsystem designing.3-5 Thus, many efforts have been made to improve the low surface free energy and morphological and chemical properties of PMMA in previous research.6-10 It has been proved that nonthermal plasmas are efficient in surface processing of various materials without affecting bulk properties,11-14 and lots of research has been performed on PMMA using different plasmas for surface modification. For example, Park et al.6 studied radio frequency (RF) fluorine plasma treatment on the surface of PMMA and proved that plasma was efficient in altering surface functional groups without changing bulk properties of PMMA. Schulz et al.7 reported surface modification of PMMA using direct current (DC) glow discharge and microwave plasma (MW) for improving adhesive coating. They proved that both DC plasma treatment and MW plasma treatment can lead to similar results in improving surface free energy of PMMA and low pressure plasma (LPP) treatment in an Ar/H2O plasma was a convenient way to increase the free surface energy of PMMA. Shenton et al.8 compared the treatment effect of microwave-coupled atmospheric pressure nonequilibrium plasmas (APNEPs) and vacuum plasma on the surface modification of various polymeric materials including PMMA. They proved APNEPs could effectively remove surface contamination and chemically modify polymer surfaces. Although it is an open idea that different plasmas may have different effects on the surface of PMMA, the different effects of RF atmospheric pressure plasma (APP) and LPP on the surface modification of PMMA is nearly not reported. When * To whom correspondence should be addressed. E-mail: [email protected]. Phone: 82-42-821-5689. Fax: 82-42-822-8995.

TABLE 1: Water and DI Contact Angles on PMMA Surface with Respect to Plasma Treatment Time contact angle ((3°) APP treated

LPP treated

plasma treatment time (s)

DW

DI

DW

DI

0 1 3 5 7 10 15 20 30 60

74.9 57.1 48.8 46.4 41.5 32.1 29.3 29.8 36.1 39.7

40.3 39.6 37.8 37.7 37.7 36.3 33.6 35.4 36.6 36.8

74.9

40.3

51.9 51.7 50.4 47.3 40.1 33.8 42.3 42.1

36.7 36.0 35.6 35.4 34.8 33.5 35.7 38.0

plasma technique is applied, the choice of appropriate plasmas and their operation parameters is quite important. Especially different thermal effects on thermosensitive polymers and the glass transition temperature of the polymer substrates10 were seldom discussed, although the APP and LPP both belong to nonthermal plasmas. In this paper, we studied the effects of RF atmospheric and low-pressure plasmas on the surface properties of PMMA, which may contribute to deeply understanding the structural properties in processing of PMMA by plasmas. In previous literature, the plasma oxidation process on PMMA was once reported by Chai. et al.9 However, the nitriding process on PMMA is still not reported. In this paper, through analyzing X-ray photoelectron spectroscopy (XPS) data, a reaction mechanism is proposed for the formation of the nitrogen-containing groups, which brings a clear view in the generation of nitrogen-containing groups by plasma. 2. Experimental 2.1. Preparation of Materials. Transparent PMMA plates (75 mm × 25 mm × 1 mm) were provided by LG Chem. Co. They were first rinsed by 0.1 N HCl and deionized water in the

10.1021/jp711238k CCC: $40.75 © 2008 American Chemical Society Published on Web 03/04/2008

LPP and APP RF Plasma Treatments

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Figure 2. Surface free energy with respect to plasma treatment time.

Figure 1. Schematic diagram of experimental setup for plasma treatment: (a) APP; (b) LPP.

ultrasonic bath for 2 h, respectively. The cleaned sample was dried through blowing nitrogen on the surface and stalled in vacuum waiting for further treatment. Argon gas, purchased from Praxair, Korea, having ultra purity was used as plasma treatment gas. 2.2. Surface Modification with APP and LPP Treatment. The aforementioned PMMA samples were treated using APP and LPP, as shown in Figure 1, respectively. Plasma power was fixed at 120 W, and argon was used in both plasma systems. During APP treatment, gas flow rate was maintained at 5 L per minute, and gap distance from plasma outlet to sample PMMA was fixed at 5 mm. LPP treatment was conducted at 250 mtorr. The treatment times for both APP and LPP were fixed at 10, 30, 60, 90, and 120 s. Usually, short plasma exposure time is used in surface modification of polymers.15 In this experiment, we chose the treatment time at 10, 30, 60, 90, and 120 s on both APP- and LPP-treated samples for comparing the treatment effect on PMMA. 2.3. Contact Angle Test and Calculation of Surface Free Energy. Contact angle goniometer (Kru¨ss DSA 100, Germany) was used for measuring the surface free energy of PMMA plates. Both deionized water (DW) and diiodomethane (DI) were used

as polar and nonpolar solvents, respectively. The contact angle of each sample was measured at eight different positions selected randomly after exposure for about 5 min in air. The average of those eight values was regarded as the contact angle of sample. The surface free energy obtained from the free energies of polar components (γsp) and nonpolar components (γsd) was obtained through Owen’s method.6,16-17 2.4. Atomic Force Microscopy (AFM) Analysis of PMMA. The surface morphology of PMMA was characterized by AFM (Park Science Instruments Autoprobe CP). The scanning size was fixed at 30 µm, and a three-dimensional image of the surface was obtained. The changes of surface morphology and surface roughness with respect to plasma treatment time were investigated. After plasma treatment, samples were relocated for AFM analysis, so that the effect of exposure time in air for each sample was unavoidable. 2.5. Measurement of Sample Surface Temperature during Plasma Treatment. The change of the PMMA surface temperature during plasma treatment was measured with a thermocouple equipped in temperature controlling device (DX9KcwNR, HunYoung, Korea). 2.6. XPS Analysis of PMMA. XPS (ESCALAB MKII, V. G Scientific), using a monochromatic aluminum X-ray source (Al KR 1486.6 eV) operating at 5 × 10-10 Torr was used to characterize surface functional groups of PMMAs. The analyzed surface area was 1 × 1 mm2, and the photoelectron takeoff angle was 45°. For data analysis and quantification, XPSPEAK 4.1 was used. The binding energies (BEs) were determined by reference to the BE of C1s at 284.6 eV prior to peak fitting. After plasma treatment, samples were moved for XPS characterization so that the effect of exposure time in air for each sample was unavoidable. 3. Results and Discussion 3.1. Changes in Surface Free Energy of PMMA Plate. Table 1 represents the change of contact angles in DW and DI contact angles with respect to the APP and LPP treatment time. For APP-treated samples, the contact angle of DW decreased to a minimum of 29.3° at 15 s, while a minimum contact angle of DW was reached to 33.8° at 20 s for LPP-treated samples. This result indicated that APP treatment could make the PMMA

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Figure 3. Surface morphology of APP-treated PMMA with respect to treatment time: (a) pristine, (b) 10 s, (c) 30 s, (d) 60 s, (e) 90 s, and (f) 120 s.

Figure 4. Surface morphology of LPP-treated PMMA with respect to treatment time: (a) pristine, (b) 10 s, (c) 30 s, (d) 60 s, (e) 90 s, and (f) 120 s.

surface more hydrophilic than LPP treatment under similar treatment conditions. Although the contact angles of DI decreases to minimum at 15 s for the APP-treated sample and 20 s for the LPP-treated sample; however, the minimum values obtained from both plasma treatments show similar values, 33.6 and 33.5°. This observation revealed that the effect of two plasma systems on a nonpolar component was similar, but the enhancement of PMMA hydrophilicity was more effective with APP treatment than with LPP treatment. The contact angles of water and DI are mainly affected by various factors such as surface functionality and surface morphology, etc., and these factors are discussed later.

Figure 2 compares the changes of the surface free energy with respect to plasma treatment time for both APP and LPP treatment together with the changes in polar and nonpolar components. Both plasma treatments significantly enhance the polar component of surface free energy but not so remarkably the nonpolar component of surface free energy. A previous research about PMMA reported similar result.18 Other researches also reported that plasma treatment usually more enhanced polar component rather than nonpolar component.6-7,19 After plasma treatment, the treated surface became more hydrophilic through oxidation in air. This resulted in the increase of free energy of polar component. In contrast, the slight decrease of free energy

LPP and APP RF Plasma Treatments

Figure 5. Surface roughness of APP- and LPP-treated PMMA with respect to treatment time.

Figure 6. Surface temperature of APP- and LPP-treated PMMA with respect to treatment time.

of nonpolar component was thought to be due to the change of either surface chemistry or morphology. The surface morphology could be changed after plasma treatment due to the chain-scission and reorientation of molecules on the surface of the PMMA. It was also thought that the increase in the polar components due to oxidation resulted in the decrease of the nonpolar components on the surface. As shown in Figure 2, the change in total surface free energy was mainly consistent with the change in polar surface free energy but not with the change in nonpolar one. This observation further explained the contribution of plasma treatment on the polar component. Figure 2 also shows that APP treatment can more effectively enhance the surface free energy of PMMA than LPP treatment. This is because in situ oxidation occurs simultaneously with APP treatment while oxidation occurs as a postprocess at LPP treatment. 3.2. Changes in Surface Morphology of PMMA Plates. The change of surface morphology after plasma treatment can be affected by various factors such as the type of plasma, the gases used in the plasma, and different experimental substrates.6,8,20-22 Figure 3 shows the change of surface morphology of PMMA plate after APP treatment with respect to time. Shorttime treatment for 10 s did not show any remarkable change of surface morphology. After 30 s of treatment, however, some cracks began to appear and became wider and larger with increasing plasma treatment time to 120 s. The three-dimensional images of PMMA surface clearly show that the surface become rougher and that more and more prominences appeared with

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Figure 7. XPS spectra of (a) pristine, (b) LPP-treated, and (c) APPtreated PMMA (APP treatment time, 15 s at which the surface free energy reached maximum; LPP treatment time, 20 s at which the surface free energy reached maximum).

increasing plasma treatment time. In contrast, the LPP treatment did not show such significant change of surface morphology as shown in parts a-f of Figure 4. No cracks appeared on the surface of PMMA, and even after a long treatment of 120 s, the surface was still not significantly changed. The threedimensional images also showed that the surface became slightly rougher at 30 s. At the plasma treatment time of 60 s, some small prominences appeared on the surface as shown in Figure 4d; however, these small prominences gradually disappeared after further treatment. The changes of surface root-mean-square (rms) roughness after both plasma treatments with respect to time are shown in Figure 5. The surface rms roughness of the APP-treated sample greatly increased with increasing treatment time. In contrast, the surface rms roughness of LPP treated sample only showed a little fluctuation. In general, the change in surface morphology caused by plasma treatment was ascribed to the plasma etching effects.23-24 Plasma etching could modify or design the surface morphology through surface restructuring and cross linking.25 Surface crack formation after plasma treatment, however, was seldom reported. Slight change in surface morphology of the LPP-treated sample shown in Figure 4 was thought to be the result of plasma etching brought by plasma bombardment, which caused slight changes in surface roughness.14,26 The change in surface morphology of the APPtreated sample was thought to be caused by not only the etching effect but also some other factors. Since the cracks appeared at the treatment time of 30 s and they tended to grow after 30 s, it made us associate the change with the thermal effect of plasma. Although APP and LPP are both nonthermal plasmas, the temperature during plasma treatment may be different owing to the difference in discharge density. Measuring temperature changes on both APP- and LPP-treated samples with respect to plasma treatment time (Figure 6) revealed that the surface temperature of the APP-treated sample increased above 100 °C even at 10 s of treatment, which almost reached the glass transition temperature (Tg) of PMMA (105 °C), but the surface temperature of the LPP-treated sample was only around 40 °C even at 120 s of treatment. With further APP treatment, the surface temperature kept increasing and then leveled off at about 280 °C. In contrast, the increase in surface temperature of the LPP-treated sample was quite gentle and slow. In our previous paper,12 we have ever reported that the discharge density of LPP is lower than that of APP due to the dispersion of plasma

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TABLE 2: Percentage of Elemental Composition on the Surface of PMMA Surfacea elemental composition (%) sample

C

N

O

pristine LPP treated APP treated

74.4 73.1 68.4

0.1 0.6 1.4

25.5 26.3 30.2

a APP treatment time, 15 s at which the surface free energy reached maximum; LPP treatment time, 20 s at which the surface free energy reached maximum.

Figure 9. N1s XPS spectra of (a) pristine, (b) LPP-treated, (c) APPtreated PMMA. (APP and LPP treatment conditions are the same as described in Figure 7.)

Figure 10. Deconvolved N1s XPS spectra of (a) LPP-treated and (b) APP-treated PMMA. Peak 1 -CdNH, peak 2 -C-NH2, peak 3 -CO-NH2, peak 4 R-NdO. (APP and LPP treatment conditions are the same as described in Figure 7.)

Figure 8. Deconvolved C1s XPS spectra of (a) pristine, (b) LPPtreated, (c) APP-treated PMMA. Peak 1 C-C, peak 2 C-O, peak 3 CdO, peak 4 O-CdO. (APP and LPP treatment conditions are the same as described in Figure 7.)

particles in vacuum chamber. This characteristic can weaken the burn-up effects during LPP treatment and make the surface temperature much lower than that during APP treatment. The difference in thermal effects during two plasma treatments enables us to relate the surface cracks in Figure 3 with surface temperature. When the surface temperature reached the Tg of PMMA, surface PMMA chains became fluidic but the bulk phase was still at solid state. After plasma treatment, rapid

LPP and APP RF Plasma Treatments

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Figure 11. A possible reaction scheme for generating nitrogen-containing groups. (a) The activation of PMMA with plasma treatment. (b) The formation of stable nitrogen compounds with plasma treatment.

cooling in air caused surface cracks due to residual stress difference between surface and bulk. The longer the plasma treatment time was, the higher the surface temperature increased and the thicker the fluidic layer became. Therefore, the cracks on the APP-treated surface became wider and larger with increasing treatment time, which resulted in significant increase in surface roughness. In contrast, the surface temperature of the LPP-treated sample was always below the Tg of PMMA. Thus, no cracks appeared on the surface. 3.3. Changes in Surface Functionalities of PMMA Plate. Figure 7 shows the result of XPS analysis. While carbon and oxygen mainly appeared on the surface of pristine PMMA, a small amount of nitrogen also appeared on the surface after both plasma treatments. Table 2 shows percentage contents of surface elements before and after both plasma treatments. As shown in Figure 7 and Table 2, oxygen content increases with subsequent decrease of carbon content after both plasma treatments. As observed from the change of surface free energy, APP treatment more effectively increased the amount of oxygen-containing groups on the surface of PMMA plate than LPP treatment. Since the pristine PMMA already contains many O-CdO groups, the increase of oxygen-containing groups does not seem to be prominent. Unlike this, the increase of nitrogen-containing groups seems to be more prominent, even if just a small percentage, after both plasma treatments. During APP treatment, the surface of PMMA can easily contact N2 in air. In discharge area, the N2 can be activated and immobilized on the surface of the PMMA. During LPP treatment, nitrogen-containing groups are thought to be generated by the activation of the residual N2 adhered on the surface of plasma chamber. The C1s is the main element as a backbone that constitutes the PMMA, and the change in the constitution of C1s (Figure 8) further reveals the two plasma effects on the surface

TABLE 3: Relative Area of Different Components in the C1s Peaka

sample

peak 1 C-C 248.6 eV

peak 2 C-O and C-N 286.3 eV

peak 3 CdO and CdN 288.1 eV

peak 4 O-CdO and CONH2 288.6 eV

pristine LPP treated APP treated

63.7% 61.2% 56.6%

21.7% 25.8% 26.4%

1.3% 2.8% 3.8%

13.3% 10.2% 13.2%

a APP and LPP treatment conditions are the same as described in Table 2.

modification. The deconvolved C1s peak is composed of peak 1 C-C (284.6 eV), peak 2 C-O and C-N (286.3 eV), peak 3 CdO and CdN (288.1 eV), and peak 4 O-CdO and OdC-N (288.6 eV). The percentages of different bonding contributions, which are obtained from the ratios of the peak area of Figure 8, are represented in Table 3. It can be seen that C-C groups decreased after both APP and LPP treatments but more prominently after APP treatment. The amount of oxycarbides such as C-O and CdO increased more on APP-treated PMMA than LPP-treated PMMA. Table 3 also represented that the O-CdO group decreased after both plasma treatments, which made us suppose that during plasma treatment, the O-CdO group was decomposed by the plasma ions and electrons, and some new peroxides or nitrides were generated thereafter. The generation of oxygen-containing groups was studied by Chai et al.,9 and they put forward a possible reaction mechanism for oxygen plasma treatment of PMMA. They suggested that the plasma treatment could open a C-H bond in the polymer chain, and in the oxygen environment, the plasma-generated radicals could induce a series of reactions, generating various oxygencontaining groups, e.g., C-O, CdO, and O-CdO groups.

4718 J. Phys. Chem. C, Vol. 112, No. 12, 2008 However, the reactions on branched chains were not considered. Since the delocalization bonds exist in the O-CdO group, carbon atoms are susceptible to nucleophilic attack and oxygen atoms are susceptible to electrophilic attack. Under strong plasma discharge, O-CdO groups could be attacked following with the formation of carbon and oxygen radicals, which were able to react with the active species such as activated oxygen and nitrogen atoms. The activation of the O-CdO is discussed later. Table 2 also shows that the content of nitrogen increases from 0.1 to 0.6 and 1.4% on the LPP- and APP-treated samples, respectively. Figure 9 shows the XPS spectra of N1s, which further reveals the change in the nitrogen groups after both plasma treatments. It was reckoned that the newly generated nitrogen-containing groups were the product of surface radicals and active nitrogen species in plasma environment. Since plasma treatment could generate various radicals on the surface, after reaction with active nitrogen in the plasma environment, various nitrogen-containing compounds could be formed. When APP was applied, the source of active nitrogen was from air, and in the case of LPP, the active nitrogen was thought to be from residual nitrogen in vacuum plasma chamber. Since the amount of the nitrogen in air was much more than that in the plasma chamber of LPP, the possibility of generating new nitrogencontaining groups was much higher when APP was applied. Therefore, the content of nitrogen groups on the APP treated sample was higher than that on the LPP-treated one. Parts a and b of Figure 10show the deconvolved N1s peaks after APP and LPP treatments, respectively, which are composed of peak 1 -CdNH (398.9 eV), peak 2 -C-NH2 (399.9 eV), peak 3 -CO-NH2 (401.1 eV), and peak 4 -R-NdO (402 eV).27-28 On the untreated sample, since the nitrogen content is only 0.1%, it was thought that the N1s could be neglected and the deconvolution of the N1s was omitted. It can be seen that the absolute area of each peak in Figure 10b was larger than that in Figure 10a, which further explained that APP generated more nitrogen-containing groups than LPP. A possible plasma reaction mechanism for the formation of various nitrogen-containing groups is shown in parts a and b of Figure 11. It is well-known that the recombination of electrons and ions is much faster than that of radicals, and the lifetime of radicals is longer than that of ions and electrons during plasma treatment. Therefore, it is believed that the active nitrogen atoms could react with plasmagenerated radicals during plasma treatment. As shown in Figure 11a, the radicals are first generated on the backbone of C-C bonds with the abstraction of hydrogen in CH2 or on the O-Cd O bonds with breaking the delocalized bonds. Then after combination with active nitrogen atoms, active nitrogencontaining groups are generated. These newly generated nitrogencontaining groups are quite unstable since the nitrogen atom was unsaturated with electrons, and they could further react with proton or oxygen atoms to form more stable structure shown in Figure 11b. During plasma treatment, the reactions occurred on the surface are thought to be very complex, and Figure 11 suggests the possible mechanism for the formation of the nitrogen compounds. It was thought that the mechanisms of generating nitrogen compounds using APP and LPP were similar, and the amount of active nitrogen species in contact with radicals was a crucial factor in the formation of nitrogen compounds. 4. Conclusions The effects of both APP and LPP treatment on the PMMA were comparatively studied. Both plasma systems could improve

Tang and Choi the surface free energy of PMMA. The change in surface free energy was closely related to the free energy change of the polar component on the surface during both plasma treatments. The free energy of the polar component could be increased more using APP since more active oxygen species were available in air. Surface morphology and roughness of PMMA characterized by AFM revealed that, during APP treatment, the thermal effect was an important factor for thermosensitive materials, especially under a long plasma treatment. However, the thermal effect in LPP treatment was negligible when compared to APP treatment. Through XPS analysis, it was confirmed that new oxygen- and nitrogen-containing groups were generated on the surface of PMMA using both plasma treatments. The reaction mechanisms in generating new oxygen- and nitrogen-containing groups through APP and LPP treatments are thought to be similar, but the difference in the increment of oxygen and nitrogen groups in two plasma systems is considered to be ascribed to the plasma treatment environment, which provides active species for generating new functional groups. Acknowledgment. This study was financially supported by research fund of Chungnam National University in 2007. References and Notes (1) Yoshimura, R.; Hikita, M.; Tomaru, S.; Imamura, S. J. LightwaVe Technol. 1998, 16, 1030. (2) Kaless, A.; Schulz, U.; Munzert, P.; Kaiser, N. Surf. Coat. Technol. 2005, 200, 58. (3) Blanc, D.; Last, A.; Franc, J.; Pavan, S.; Loubet, J. L. Thin Solid Films 2006, 515, 942. (4) Rout, B.; Kamal, M.; Dymnikov, A. D.; Zachry, D. P.; Glass, G. A. Nucl. Instr. Meth. Phys. Res. B 2007, 260, 366. (5) Gottmann, J.; Kreutz, E. W. Surf. Coat. Technol. 1999, 116-119, 1189-1194. (6) Park, S. J.; Cho, K. S.; Choi, C. G. J. Colloid Interface Sci. 2003, 258, 424. (7) Schulz, U.; Munzert, P.; Kaiser, N. Surf. Coat. Technol. 2001, 142144, 507. (8) Shenton, M. J.; Stevens, G. C.; Wright, N. P.; Duan, X. J. Polym. Sci.: Polym. Chem. 2002, 40, 95. (9) Chai, J.; Lu, F.; Li D. B.; Kwok, Y. Langmuir 2004, 20, 10919. (10) Lim, H.; Cho, Y.; Han, S.; Cho, J.;Kim, K. J. J. Vac. Sci. Technol. A 2001, 19(4), 1490. (11) Tang, S.; Kwon, O. J.; Lu, N.; Choi, H. S. Surf. Coat. Technol. 2005, 195, 298. (12) Tang, S.; Lu, N.; Wang, J. K.; Ryu, S. K.; Choi, H. S. J. Phys. Chem. C 2007, 111, 1820. (13) Siow, K. S.; Britcher, L.; Kumar, S.; Hans J. Plasma Process. Polym. 2006, 3, 392. (14) Kwon, O. J.; Tang, S.; Myung, S. W.; Lu, N.; Choi, H. S. Surf. Coat. Technol. 2005, 192, 1. (15) Zhao, Y.; Tang, S.; Myung, S. W.; Lu, N.; Choi, H. S. Polym. Test. 2006, 25, 327. (16) Zenkiewicz, M.; Adhes, J. Sci. Technol. 2001, 15, 1769. (17) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741. (18) Kaminska, A.; Kaczmarek, H.; Kowalonek, J. Eur. Polym. J. 2002, 38, 1915. (19) Kim, J. S.; Kim, Y. K.; Lee, K. H. J. Colloid Interface Sci. 2004, 271, 187. (20) Lu, C. A.; Chang, L. Mater. Chem. Phys. 2005, 92, 48. (21) Singh, L. S. S.; Tiwary, K. P.; Purohit, R. K.; Zaidi, Z. H.; Husain, M. Curr. Appl. Phys. 2005, 5, 351. (22) Li, L. H.; Tian, J. Z.; Cai, X.; Chen, Q. L.; Xu, M.; Wu, Y. Q.; Fu, R. K. Y.; Chu, P. K. Surf. Coat. Technol. 2005, 196, 241. (23) Krump, H.; Hdec, I.; Luyt, A. S. Int. J. Adhes. Adhes. 2005, 25, 269. (24) Liu, J. T.; Nemchuk, N. I.; Ast, D. G.; Couillard, J. G. J. NonCryst. Solids 2004, 342, 110. (25) Bonaccurso, E.; Graf, K. Langmuir 2004, 20, 11183. (26) Cui, N. Y.; Brown, N. M. D. Appl. Surf. Sci. 2002, 189, 31. (27) Tatoulian, M.; Bouloussa, O.; Moriere, F.; Arefi-Khonsari, F.; Amouroux, J.; Rondelez, F. Langmuir 2004, 20, 10481. (28) Zhang, W.; Ji, J.; Zhang, Y.; Yan, Q.; Chu, P. K. Plasma Process. Polym. 2007, 4, 158.