Poly(vinyl chloride) Surface Modification Using Tetrafluoroethylene in

Faculty of Science, Kanazawa University, Kanazawa 920-1161, Japan, and. Department of Chemistry, Sophia University, Chiyoda ku, Tokyo 102, Japan...
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Langmuir 1999, 15, 7055-7062

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Poly(vinyl chloride) Surface Modification Using Tetrafluoroethylene in Atmospheric Pressure Glow Discharge Y. Babukutty,† R. Prat,† K. Endo,‡ M. Kogoma,§ S. Okazaki,§ and M. Kodama*,† Bionic Design Research Group, National Institute for Advanced Interdisciplinary Research, Agency of Industrial Science and Technology, 1-1-4 Higashi, Tsukuba 305-8562, Japan, Faculty of Science, Kanazawa University, Kanazawa 920-1161, Japan, and Department of Chemistry, Sophia University, Chiyoda ku, Tokyo 102, Japan Received January 14, 1999. In Final Form: June 3, 1999 Atmospheric pressure glow plasma was used to modify the inner surface of a commercially available poly(vinyl chloride) (PVC) tube sample to enhance the biocompatible properties by polymerizing tetrafluoroethylene (TFE). The composition and properties of the modified surface were similar to those of PTFE. X-ray photoelectron spectroscopy (XPS) of the TFE-coated surface revealed an F1s peak at approximately 689 eV and a C1s at approximately 292 eV. The TFE-coated surface had a surface fluorine concentration as high as 63% with a conspicuous absence of chlorine. From attenuated total reflectance Fourier transform infrared analysis of the TFE-coated surface, we obtained high absorbance ratios of A1200/A1720 (PTFE absorbance peak at 1200 cm-1 versus that of the substrate peak at 1720 cm-1 due to plasticizer in PVC) indicating a fluorine-rich coated surface. The contact angles of the newly formed surfaces (100-110°) were similar to that of PTFE. Atomic force microscopy showed that the modified surface was less rough than the surface of the substrate PVC. The flow rate of carrier gas (helium) and reactive gas (TFE) influenced the efficiency of surface coating. A helium flow rate of between 500 and 1000 sccm (standard cubic centimeters per minute) and TFE flow rate of between 2 and 3 sccm were optimal for obtaining a uniform and thick surface coating.

Introduction Plasma treatment is very useful for incorporating reactive or inert functional entities onto polymer substrates. It is an important technique for developing surfacespecific materials. Upon plasma treatment, only the surface properties of the substrate change while the bulk properties remain the same.1-5 Since the polymer surfaces are in direct contact with blood in most bioprostheses, they have been modified to enhance their compatibility with blood. Okazaki et al.6-10 found that stable plasma could be formed at atmospheric pressure using an inert gas medium, a low-frequency power source, and two plane electrodes separated by a dielectric material. The advan* To whom correspondence should be addressed. Phone: +81 298 54 2551. Fax: +81 298 54 2560. Email: [email protected]. † National Institute for Advanced Interdisciplinary Research. ‡ Kanazawa University. § Sophia University. (1) Gombotz, W. R.; Hoffman, A. S. Gas discharge techniques for modification of materials. In Critical Reviews in Biocompatibility; Williams, D., Ed.; CRC Press: Boca. Raton, FL, 1987; Vol. 4, pp 1-42. (2) Ratner, B. D.; Chilkoti, A.; Lopez, G. P. Plasma deposition and treatment for biomaterils applications. In Plasma Deposition, Treatment, and Etching of Polymers; d’Agostino, R., Ed.; Academic Press, Inc.: Boston, MA, 1990; pp 463-516. (3) Kiaei, D.; Hoffman, A. S.; Horbett, T. A. J. Biomater. Sci. Polym. Ed. 1992, 4, 35. (4) Lin, J.-C.; Cooper, S. L. Biomaterials 1995, 16, 1017. (5) Lopez, G. P.; Ratner, B. D. Plasmas Polym. 1996, 1, 127. (6) Kanazawa, S.; Kogoma, M.; Moriwaki, T.; Okazaki, S. J. Phys. D: Appl. Phys. 1988, 21, 838. (7) Yokoyama, T.; Kogoma, M.; Kanazawa, S.; Moriwaki, T.; Okazaki, S. J. Phys. D: Appl. Phys. 1990, 23, 374. (8) Okazaki, S.; Kogoma, M. J. Photopolym. Sci. Technol. 1993, 6, 339. (9) Kogoma, M.; Okazaki, S. J. Phys. D: Appl. Phys. 1994, 27, 1985. (10) Kogoma, M.; Koiwa, K.; Okazaki, S. J. Photopolym. Sci. Technol. 1994, 7, 341.

tages of atmospheric pressure glow (APG) discharge include the following: (i) since plasma treatment is performed at atmospheric pressure the complexities of experiments performed at low pressure are avoided; (ii) continuous operation is possible; (iii) it is suitable for materials of any shape or size; (iv) it can be applied to high-vapor pressure and to soft substances such as gum, rubber, etc.; (v) it has a wide developmental scope for industrial applications due to low cost and easy handling. Poly(tetrafluoroethylene) (PTFE) is a widely used biocompatible, nonthrombogenic polymer which has inert surface properties.11,12 We used APG discharge to polymerize tetrafluoroethylene (TFE) monomer and hence developed a PTFE-like structure on the inner surface of a commercial PVC tube sample. We studied how carrier gas (helium) and monomer gas (TFE) flow rates influenced the efficiency at which a totally covered, homogeneous surface was obtained. Our objectives were to suppress plasticizer bleeding from commercial PVC and to enhance its biocompatibility by forming a PTFE-like layer on PVC. Experiments The special electrode used for APG discharge was insulated and consisted of two electrically conductive parallel structures-two copper plates, each ≈3 mm widehelically wound around a cylindrical glass tube (Figure 1). This was embedded in a plastic jacket, and the gap between the electrode and jacket was filled with silicone oil, an insulator. The plastic jacket was sealed at both ends with the copper leads protruding at each end. The high-voltage and ground leads of the power supply were (11) Kiaei, D.; Hoffman, A. S.; Hanson, S. R. J. Biomed. Mater. Res. 1992, 26, 357. (12) Bellon, J. M.; Bujan, J.; Contreras, L. A.; Hernando, A.; Jurado, F. J. Biomed. Mater. Res. 1996, 31, 1.

10.1021/la990039l CCC: $15.00 © 1999 American Chemical Society Published on Web 08/07/1999

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Figure 1. A schematic diagram of the spiral electrode and APG treatment system.

connected to the copper electrode leads. A high-voltage alternating current with a frequency of 20 kHz was applied across the electrode. The sample PVC tube was inserted into the glass tube of the electrode. One end of the sample PVC tube was connected to the gas line which carried the carrier and monomer gases and the other end was opened to the atmosphere; therefore, plasma glow developed at atmospheric pressure. The flow rate of the carrier gas (helium) and monomer gas (TFE) was varied to find optimum treatment conditions. The tested helium flow rates were 200, 400, 500, 600, 800, 1000, 1500, and 2000 sccm (standard cubic centimeters per minute), and the tested TFE flow rates were 2.0, 2.5, 3.0, 4.0, and 5.0 sccm. The plasma treatment time in all experiments was 20 min. The 20 kHz plasma power source was supplied by a SPG 20-500 power generator (Shinko Electric Co. Ltd., Tokyo, Japan) operated at 4.0 kV. The substrate material was a commercially available PVC tube having an outer diameter of 8.4 mm and inner diameter of 6.5 mm. The TFE-modified PVC surface was analyzed by surface spectroscopy, contact angle, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Features of the modified PVC surface were compared to those of the unmodified substrate PVC and standard PTFE data to determine the extent of modification. Surface functional characteristics were evaluated using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. A Janssen Micro FT/IR-200 (JASCO Corp., Tokyo, Japan) with ATR ZnSe crystal was used. Spectra were collected at a resolution of 4 cm-1 over 100 scans. Air-water contact angles were measured with ultrapure water using a contact angle meter (FACE Contact Angle Meter, Kyowa Interface Science Co., Ltd., Tokyo, Japan) in which a syringe was attached to a plunger driven by a micrometer. The PVC sample was fixed to a glass slide with double-sided tape, and the sessile contact angle was measured by placing a water droplet on the flat surface. The angle between the horizontal plane and the tangent to the drop at the point of contact with the substrate was measured. Each value of contact angle was taken as an average of nine measurements. XPS spectra of the surface were obtained using a PerkinElmer (PHI 5400 MC ESCA) instrument. Monochromatic Al KR was used as the X-ray source. The spectrometer was operated at 400 W and 15 kV. The photon energy was 1486.6 eV. Elements were identified and analyzed using survey, high-resolution, and valence band spectra. The pass energy was varied for different types of spectral

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accumulation. The angle between the X-ray source and analyzer was fixed at 45°. The spot size of the measurement was 3 × 1 mm. Multiscan averaging on a multichannel analyzer was used for the valence-band region since a very low photoelectron emission cross section was observed. A low-energy electron flood gun was used to avoid any charging on the sample surface. The Nanoscope IIIa (Digital Instruments Inc., Santa Barbara, CA) was used for tapping mode AFM. Samples at ∼12 × 12 µm were scanned using a D-scanner. The silicon cantilevers were 125 µm long and had a spring constant between 20 and 50 N/m. AFM images were taken at 256 × 256 pixels in “height” mode, while maintaining a constant force at room temperature. Integral gain, proportional gain, scan rate, scan width, etc. were suitably adjusted. To obtain optimum imaging conditions, the applied force was minimized and stabilized by adjusting height of the cantilever (set point voltage) during sample scanning. Results The efficiency with which stable plasma was produced by APG depended on various treatment parameters, which, in turn, controlled effective monomer polymerization and coating on the surface. The power source frequency, applied voltage, treatment time, and monomer and carrier gas flow rates each affected the stability of the plasma glow. Our objectives were to determine how the monomer and carrier gas flow rates affected coating and to determine the optimum conditions required for stable and uniform substrate coating under the gas and monomer flow rates detailed in the Experiments. The results are described below. Representative surface spectra of untreated PVC, helium-plasma-treated PVC, helium- and TFE-plasmatreated PVC, and PTFE are presented in Figure 2. In the spectrum of the helium- and TFE-plasma-treated sample surface, most of the PVC spectral peaks are replaced by PTFE peaks. The most intense PTFE peaks occur at about 1200 and 1150 cm-1 due to C-C and C-F stretching modes. The spectrum of the modified surface contained these two peaks, although the peak at about 1150 cm-1 was less intense and had fused with the other peak. To approach ATR-FTIR data quantitatively, for each experimental condition we calculated the ratio of the absorbance intensity of two functional groups which represent the substrate and the deposited film surface, respectively; the peak at approximately 1720 cm-1 is typical of the plasticizer in the substrate PVC and the peak at approximately 1200 cm-1 is typical of PTFE. As PTFE-like film is increasingly deposited by TFE polymerization over time, the intensity of the 1720 cm-1 peak intensity decreases while that of the 1200 cm-1 peak increases. The absorbance ratio of these peaks (A1200/A1720) thus serves to suitably monitor the surface modification. Under each experimental condition, we obtained A1200/A1720 values at six representative points on the sample surface and calculated the mean ( standard deviation. Graphs of the (A1200/A1720) values at various monomer and carrier gas flow rates are shown in Figures 3 and 4. At a helium flow rate of 200 sccm, a nonzero absorbance ratio was seen only at the minimum TFE flow rate of 2.0 sccm and was very small. For higher monomer flow rates of 2.5, 3.0, 4.0, and 5.0 sccm, no peak at 1200 cm-1 was seen. At a helium flow rate of 400 sccm, the absorbance ratio was highest at a TFE flow rate of 2.0 sccm, but its value was relatively low and a PTFE peak was not observed at higher TFE flow rates of 4.0 and 5.0 sccm. At

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Figure 4. At each indicated carrier gas (He) flow rate, the variation in absorbance ratio (A1200/A1720) at TFE flow rates of 2.0, 2.5, 3.0, 4.0, and 5.0 sccm is shown.

Figure 2. Surface spectra of untreated PVC, PTFE, and TFEtreated PVC. PVC substrate modification is evident in the spectrum of TFE treated PVC.

Figure 3. At each indicated carrier gas (He) flow rate, the variation in absorbance ratio (A1200/A1720) at TFE flow rates of 2.0, 2.5, 3.0, 4.0, and 5.0 sccm is shown.

a helium flow rate of 500 sccm, surface functional variation was seen at all monomer flow rates. It peaked at the lowest monomer flow rate of 2.0 sccm and decreased gradually as the monomer flow rate was increased. At 600 and 800 sccm helium, a PTFE peak (even a weak signal) was not seen for the maximum (5.0 sccm) monomer flow rate. At a helium flow rate of 600 sccm the maximum absorbance ratio was observed at a TFE flow rate of 2.5 sccm. At a

helium flow rate of 800 sccm, the absorbance ratio gradually increased as the monomer flow rate was increased, it reached a peak at a TFE flow rate of 3.0 sccm, decreased sharply, and then showed no evidence of modification at 5.0 sccm monomer flow rate. At a helium flow rate of 1000 sccm, surface modification was seen under all monomer flow rates. The maximum absorbance ratio was observed at a TFE flow rate of 3.0 sccm and then it decreased gradually, but this ratio was lower than the maximum absorbance ratio under the 800 sccm helium flow rate. At the helium flow rate of 1500 sccm, there was a difference in the absorbance ratio of the surface at each monomer flow rate, but the absorbance ratios were relatively small and uniform. At a helium flow rate of 2000 sccm, a PTFE peak was not seen at a TFE flow rate of 2 sccm, but all other monomer flow rate conditions yielded relatively small, uniform values of the absorbance ratio. Most of the contact angles for treated samples (Table 1) ranged between 90 and 115°. The contact angle of substrate PVC is ∼95°, and on APG treatment with carrier gas (helium) alone, the contact angle is decreased to ∼75°. For a given helium flow rate, there was little variation in the contact angle of different TFE flow rates. However, for given monomer flow rates, a pattern was seen in the variation of contact angle. For all monomer flow rates, the contact angle peaked at a helium flow rate of either 1500 or 2000 sccm. Under all monomer flow rates at helium flow rates of 200, 400, 500, 600, 800, and 1000 sccm, many of the contact angles were close to 100°, taking into account the standard deviation; for all monomer flow rates at helium flow rates of 1500 and 2000 sccm, the contact angles were close to 110°. The contact angle of PTFE is approximately 115°. Therefore, our contact angles are in the range of that of a PTFE-like structure. The XPS spectra of a PVC sample treated with 600 sccm helium and 2.5 sccm TFE, substrate PVC, and a sample treated with 600 sccm of helium alone, were obtained and the results are summarized in Table 2. An entirely new surface elemental spectral pattern, which was different from those of the substrate PVC and helium plasma-treated PVC, was observed. The original PVC substrate had a surface concentration of carbon of 79%, and on the helium and TFE-coated surface it was reduced to 35%. The PVC substrate had a chlorine concentration of 8.5%, which was completely covered by the new surface coating. The new surface had a fluorine concentration of 63%. The original oxygen concentration on the PVC substrate due to the contribution of plasticizer was about 13%, but that on the modified surface was only 2%. The helium-plasma-treated surface had slightly higher concentrations of carbon and oxygen than the surface of the

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Table 1. Contact Angles for Different Monomer and Carrier Gas Flow Rates He flow rate (sccm)

2

2.5

monomer flow rate (sccm) 3

4

5

200 400 500 600 800 1000 1500 2000

96.7 ( 1.5 94.8 ( 1.7 98.2 ( 1.1 101.9 ( 2.6 93.4 ( 2.5 88.9 ( 2.8 110.7 ( 2.2 113.6 ( 2.1

75.0 ( 4.0 101.1 ( 1.9 101.7 ( 1.8 99.2 ( 1.2 99.8 ( 3.2 105.4 ( 3.0 110.8 ( 2.5 110.3 ( 2.4

90.4 ( 2.1 100.9 ( 1.2 100.6 ( 1.5 100.4 ( 2.4 102.7 ( 2.3 101.2 ( 1.6 111.9 ( 2.3 109.9 ( 1.7

96.5 ( 2.4 99.8 ( 2.3 102.7 ( 2.1 100.8 ( 1.3 103.4 ( 2.4 101.4 ( 1.6 108.9 ( 2.5 108.7 ( 2.6

95.1 ( 2.7 97.7 ( 3.1 97.8 ( 3.1 98.4 ( 2.1 102.2 ( 2.9 101.6 ( 0.6 108.1 ( 4.5 111.1 ( 2.6

Table 2. Surface Atomic Concentrations of Untreated and TFE-Treated PVC Samples by XPS Analysis surface atomic concentration (%) PVC

C1s

O1s

Cl2p

blank He treated He + TFE treated

78.6 82.3 35.3

12.9 15.1 2.0

8.5 2.6

F1s

62.6

original substrate, which was due to surface oxidation and the reduction in surface concentration of chlorine. To better understand the change in surface functional groups, we compared the C1s scan spectra of the TFEcoated surface with those of the substrate PVC and heliumplasma-treated surface (Figure 5). On the C1s scan spectra of substrate PVC, carbon-containing functional groups which consisted of vinyl groups and functional groups from the plasticizer, were seen mainly at approximately 285, 287, and 289 eV. In spectra obtained for the TFE-coated surface, an important carbon peak was seen at approximately 292 eV corresponding to the -CF2 group. The other small peaks at approximately 294, 290, and 287.5 eV were attributed to various fragments of fluorine-bound carbon groups formed due to plasma polymerization. Since the structure and pattern of the carbon peaks are identical to those of fluorinated carbon functional groups and are not similar to those of chlorinated carbon functional groups, the new surface can be considered to be functionally similar to a fluorinated surface, as further evidenced by valence band spectra (Figure 6). A valence band peak for Cl3p was found in the spectra of the substrate PVC, similar to that which is present in the spectra of nonplasticized PVC.13 On the treated surface, the observed peak corresponded to that of the F2s structure of PTFE. The XPS spectra of the substrate showed a peak between 5 and 10 eV, which corresponds to Cl3p, and the XPS spectra of the coated surface showed a peak between 35 and 40 eV, which corresponds to the F2s structure. The XPS study thus clearly indicated that the TFE-coated surface was enriched with fluorine. Valence band spectra of the TFE-coated surface, those of a PTFE surface, and simulated spectra of a trimer model are shown in Figure 7 and discussed in a later section. A TFE-treated sample having a thick surface coating after treatment under 600 sccm helium and 2.5 sccm TFE flow rates was analyzed by AFM (Figure 8). We evaluated the surface morphology and found that the untreated surface had a highly uneven topography while the TFEtreated surface had a uniform topography. Surface smoothness was analyzed by evaluating the average deviation of the surface from the mean profile line. The TFE-modified surface, having a mean deviation of 0.7 nm and a maximum deviation of 19 nm, was less rough than the untreated surface, which had a mean deviation of 6.5 nm and maximum deviation of 58 nm; this difference is (13) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers; John Wiley & Sons: New York, 1992.

Figure 5. XPS (C1s) spectra of untreated PVC, helium-plasmatreated PVC, and TFE-coated PVC surface. The shift in peak position is due to the presence of fluorine.

Figure 6. XPS (valence band) spectra of untreated PVC and TFE-coated PVC surface. The peak at approximately 35 eV corresponds to fluorine.

significant when considering nanometer-level smoothness. Surface topography found by “section analysis” also yielded interesting results (Figure 9). The surface profile of the TFE-treated PVC was nearly uniform, indicating an even surface, and the maximum vertical distance was only 2.7 nm. The surface profile of an untreated surface showed an uneven distribution with a maximum vertical distance of nearly 25 nm.

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Figure 7. XPS (valence band) spectra of PVC with a TFEcoated surface and PTFE are compared with the simulated spectrum of a trimer model molecule.

Discussion APG plasma was applied to develop thin films of organic and metallic origin on various substrates as reported by Okazaki et al.6-10,14 Yokoyama et al.14 proposed that the stability of APG plasma is maintained by the following important processes. The insulated electrode enables a buildup of charge and the resulting opposite voltage halts its discharge, generating a pulsed discharge. A suitable frequency causes the discharge to show a microdischarge on the electrode, enabling several pulse currents every cycle that cutoff discharge before transition to an arc discharge. The use of helium, which has a low breakdown voltage and long transition time, is also important. The high-energy metastable state of helium atoms may help to extend microdischarge points because high-energy helium ionizes mixed molecules. The combination of all of these processes prevents the discharge from attaining an arc formation, stabilizing plasma glow at atmospheric pressure.7,14 The APG apparatus and technique shown in Figure 1 enabled selective treatment of the inner surface of a PVC tube while leaving the outer surface and bulk tube properties unchanged. To overcome the limitations of quantitative evaluation of data from ATR-FTIR, we used the ratio of the absorbance of two specific, separate peaks in the same spectrum, as described earlier and elsewhere.15,16 This ratio, A1200/A1720, is suitable for monitoring the nature of surface modification. A high ratio indicates enrichment of PTFE-like surface functional groups. Although an exact correlation does not exist, the absorbance ratio gives a rough indication of the thickness of the coated film. The standard deviation gives information on surface uniformity. Helium flow rates that were very low or very high did not yield a thick, PTFE-like coating. The energy required to activate the substrate and to dissociate TFE monomer molecules may be derived from He plasma glow. In APG, the monomer molecule is excited by highly excited, long(14) Yokoyama, T.; Kogoma, M.; Moriwaki, T.; Okazaki, S. J. Phys. D: Appl. Phys. 1990, 23, 1125. (15) Babukutty, Y.; Kodama, M.; Nomiyama, H.; Kogoma, M.; Okazaki, S. Influence of flow rates of carrier and reactive gases in the surface modification of biomedical materials using APG treatment. In Advances in Polymeric Biomaterials Science; Akaike, T., Okano, T., Akashi, M., Terano, M., Yui, N., Eds.; CMC: Tokyo, 1997; pp 279-283. (16) Lee, J. H.; Khang, G.; Lee, J. W.; Lee, H. B. J. Biomed. Mater. Res. 1998, 40, 180.

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lived He atoms17 and, hence, if the plasma glow does not contain enough excited He atoms to meet the energy requirement, the surface coating efficiency deteriorates. At a very low helium flow rate, the number of energyrich He atoms may be insufficient to generate the energy needed to completely decompose a sufficient number of TFE molecules and to activate the substrate surface to the level required for interaction with monomer radicals. Hence, there is an insufficient number of dissociated monomer molecules to properly interact with the surface. The same effect is seen if the monomer flow rate exceeds a certain limit. This assumption explains the poor performance at higher monomer flow rates. A 5 sccm monomer flow rate yielded no result up to a helium flow rate of 800 sccm except for the odd behavior at the helium flow rate of 500 sccm. Similarly, at a monomer flow rate of 4 sccm, comparatively lower values or no value of the absorbance ratio was seen for He flow rates up to 800 sccm. At very high helium flow rates, the faster movement does not allow ions and radicals to remain in the reaction system, although there are a sufficient number of activated helium species. The residence time of ions and radicals in the reaction system influences the monomer dissociation and subsequent propagation of polymerization and deposition on the substrate.18 The higher He gas flow rate may result in slight cooling in the reaction system, leading to quenching on plasma and decreasing the efficiency of development of a polymerized surface coating. Clark19 also reported that at higher flow rates it may be difficult to sustain the plasma and the deposition rate may be negligible. Under optimal conditions, plasma glow provides enough activated He atoms with sufficient energy to dissociate the monomer and activate the substrate surface. Optimal conditions enable excited He atoms to reside for a considerable length of time as activated radicals to initiate and propagate polymerization. The optimal conditions for different monomer and carrier gases depend on the ionization energy of the carrier gas and the monomer bond dissociation energy. Plasma treatment generally increases surface wettability and adhesion by increasing surface energy.20 In contrast, fluorinated surfaces developed by plasma treatment have a high contact angle;21 this is supported by our results which showed that the contact angles of the TFEtreated surface were higher than those of the substrate PVC. The contact angle of the PVC substrate treated with plasma without monomer was approximately 75°, whereas all of the samples that had been treated with TFE under all conditions yielded angles of 100° or greater except for one value. Given that APG treatment of PVC samples without monomer lowers the contact angle to 75°, we could successfully develop a PTFE-like coating on the PVC substrate. In all cases where a maximum absorbance ratio was seen on FTIR, the contact angles fell in a narrow range between 98 and 103°, which is perhaps due to increased surface reactivity of the newly formed, plasmapolymerized surface structure. In the samples treated with (17) Masuda, S. J. Surf. Sci. Soc. Jpn. 1993, 14, 132. (18) Yasuda, H. Plasma Polymerization; Academic Press: Orlando, FL, 1985. (19) Clark, D. T. Pure Appl. Chem. 1982, 54, 415. (20) Klemberg-Sapieha, J. E.; Martinu, L.; Sapieha, S.; Wertheimer, M. R. Control and modification of surfaces and interfaces by corona and low-pressure plasma. In The Interfacial Interactions in Polymeric Composites; Akovali, G., Ed.; Kluwer Academic Publishers: The Netherlands, 1993; pp 201-222. (21) Liston, E. M. J. Adhes. 1989, 30, 199.

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Figure 8. Tapping mode AFM image of PVC surface morphology: (a) TFE-treated PVC; (b) untreated PVC.

He flow rates of 1500 and 2000 sccm, even though the absorbance ratios were small, the contact angles were large, which was possibly due to the relatively thin PTFE surface film on the substrate surface. Since ATR-FTIR penetrates samples to the micrometer level, the substrate may also be sampled along with the thin surface coating during measurement. The value of a contact angle is highly surface sensitive, and ATR-FTIR with such penetration may provide some insight into the bulk

properties of the substrate. A higher helium flow rate may help the surface to attain equilibrium by decreasing the number of reactive radicals and surface reactivity. Surfaces with higher contact angles should also be PTFE-like structure, but with thin surface films stabilized by the higher helium flow. Unlike the ATR-FTIR data, considerable variation in contact angle was not seen when the helium flow rate was kept constant and the monomer flow rate was varied.

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Figure 9. Tapping mode AFM profile of PVC surface section analysis: (A) TFE-treated PVC, (a) uniform distribution around the mean line of profile demonstrates the evenness of the surface; (b) surface overview; (B) untreated PVC, (a) nonuniform distribution around the mean line of profile showing the unevenness of the surface, (b) surface overview.

XPS is often used to study exceptionally thin surfaces and is useful for identifying and analyzing the chemical structure of the surface.19,22,23 This is particularly suited to fluoropolymer study because photoelectrons emitted by C1s display large chemical shifts due to the high electronegativity of fluorine.24 Survey spectra of the substrate PVC- and TFE-coated surface showed substantial compositional differences (Table 2). The Cl2p peak on the substrate was partially removed by helium plasma treatment and was completely replaced with the F1s peak upon TFE treatment. A second feature is the reduced oxygen concentration on the surface after TFE treatment. Due to the presence of oxygencontaining functional groups in the plasticizer, the substrate had a surface oxygen concentration of 13% (Table (22) Gengenbach, T. R.; Griesser, H. J. J. Polym. Sci.: Part A: Polym. Chem. 1998, 36, 985. (23) Ratner, B. D.; McElroy, B. J. Electron spectroscopy for chemical analysis: Applications in the biomedical sciences. In Spectrscopy in the Biomedical Sciences; Gendreau, R. M., Ed.; CRC Press: Boca Raton, FL, 1986; p 107.

2), while the surface oxygen concentration on the TFEcoated surface was 2% even after the post-plasmatreatment surface oxidation. Differences in the chemical structure of the surface of the substrate and the TFE-modified surface can be better understood by analyzing C1s spectra (Figure 5). On the C1s spectra of substrate PVC, the main contribution of the C1s peak occurs at 285.0 eV, which is associated with C-C and C-H bonded carbon. The other peaks at 286.5 and 289.2 eV represent the chlorine-bound carbon in PVC and the oxygen-bound carbon in the plasticizer, respectively. We assigned the peak at 289.2 eV to the carboxyl group20 and that at 286.5 eV to the C-Cl group.13 The C1s spectra of the helium-plasma-treated surface showed nearly the same pattern as that of the substrate, except for an increased contribution from oxygen-bound carbon (24) d’Agostino, R.; Cramarossa, F.; Fracassi, F. Plasma polymerization of Fluorocarbons. In Plasma Deposition, Treatment, and Etching of Polymers; d’Agostino, R., Ed.; Academic Press Inc.: Boston, MA, 1990; pp 95-162.

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due to surface oxidation that occurs during the plasma treatment process. Plasma-polymerized fluorinated surfaces are more complex than those of conventional polymers due to the fusion of several components with different binding energies.24 In TFE polymerization, fragments such as -CF3, -CF2, -CF, and -C-CF contribute to the C1s spectra. The C1s spectra of the TFEcoated surface differed from the C1s spectra of the substrate and did not contain a peak centered at 285.0 eV, which indicates complete covering of the substrate. The important peak observed at approximately 292.0 eV is due to -CF2 (Figure 5), which corresponds to the -CF2 peak at 292.5 eV in the PTFE spectra.13 The small shift in the peak position is reasonable, given that the fragments were formed during plasma treatment. The other small peak centered at approximately 287.5 eV in the spectra of the TFE-treated PVC, was assigned to C-C bonding of fluorine-bonded carbon atoms (C-CF).24 The second small peak at approximately 294 eV represents -CF3 formed due to fragmentation during plasma polymerization. The peak at about 290 eV can be assigned to the C-F group. Similar results of TFE polymerization by radio frequency glow discharge (RFGD) have been reported.3,25,26 Previous studies have reported the -CF3 group to be a major entity contributing to the structure of the C1s spectra, along with -CF2. In the present study, the -CF2 peak was most prominent in the C1s spectral composition of the TFEtreated sample, confirming that the surface-coating structure is similar to that of PTFE, which has a -(CF2-CF2)n- structure.13 In valence band spectra (Figure 6), the strong peak at 30-40 eV of the TFE-treated PVC surface is typical of F2s contribution and corresponds to the vertical ionization potentials (VIP) of the sσ(F2s-C2s) and pσ (F2s-C2p) bonding orbitals. The peak at 20 eV, though very broad and weak, is due to the VIP of sσ (C2s-C2s) bonding orbitals resulting from the main chain -C-C- group.27 Endo et al.28 presented and compared a theoretical (simulated) trimer molecule spectral model and theoretical model of PTFE valence band spectra. To simulate the valence band XPS of polymers theoretically, a superposition of peaks centered on the vertical ionization potential (VIP) was constructed.27-29 The authors observed that the valence band spectra of PTFE closely resembled that of the simulated trimer molecule spectral model, suggesting a possible trimer structure for PTFE. In the present study, the simulated trimer spectral pattern was structurally similar to the valence band XPS of the coated surface (Figure 7), indicating that the surface coating has a PTFElike trimer structure. AFM analysis was conducted to clarify the surface morphology and topography. The surface of polymers such as polyethylene and poly(tetrafluoroethylene) has been studied to determine their chain order in molecular resolution30,31 and their surface orientation.32 Tapping (25) Castner, D. G. Surface characterization of oriented afterglow fluorocarbon films. In Plasma Processing Polymers; d’Agostino, R., Favia, P., Fracassi, F. Eds.; Kluwer Academic Publishers: London 1997; pp 221-230. (26) Golub, M. A.; Wydeven, T.; Finney, L. S. Plasmas Polym. 1996, 1, 173-194. (27) Endo, K.; Kaneda, Y.; Inoue, C.; Aida, M.; Chong, D. P. J. Phys. Chem. Solids 1995, 56, 1131. (28) Endo, K.; Inoue, C.; Kaneda, Y.; Aida, M.; Kobayashi, N.; Chong, D. P. Bull. Chem. Soc. Jpn.1995, 68, 528. (29) Aida, M.; Kaneda, Y.; Kobayashi, N.; Endo, K.; Chong, D. P. Bull. Chem. Soc. Jpn. 1994, 67, 2972. (30) Magonov, S. N.; Qvarnstrom, K.; Eilings, V.; Cantow, H.-J. Polym. Bull. 1991, 25, 689. (31) Jandt, K. D.; Buhk, M.; Miles, M. J.; Petermann, J. Polymer 1994, 35, 2458.

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mode AFM (TMAFM) is very useful for studying polymer surfaces, because the tip does not damage the surface. The cantilever oscillates vertically near its resonance frequency, so the tip is in contact with the sample only briefly during each cycle of oscillation. Short, intermittent tip and sample contact reduces lateral force during scanning, prevents damage to the sample, and enables imaging of weakly bound surface layers and structural details.33 The variation in surface morphology upon introduction of new surface functional groups has been studied by AFM.34 The results of AFM revealed greater uniformity in the fine structure of the TFE-treated surface compared to that of the untreated substrate. The area analyzed was only 1 × 1 µm, meaning the result must be evaluated at the micrometer level. However, information on the specific molecular arrangement can be obtained, as reported by Hansma et al.,32 if the analysis is confined to a small area. The tapping mode AFM image of the TFE-treated surface (Figure 8a) revealed a very fine surface; the unfiltered image even showed the minute, fine structures of TFE coating. The untreated surface we examined was highly uneven. A filtered and flattened image of the untreated surface is shown in Figure 8b for comparison. Gengenbach et al.35 reported the formation of such fine, additional structures on plasma-treated surfaces. The mean surface roughness of our treated surface was ≈0.70 nm. Given that the typical intermolecular spacing of the PTFE crystal lattice is 0.49 nm,36 the structure provides near-molecular resolution. The results of “section analysis” (Figure 9) also revealed differences in the surface topography of TFEtreated and untreated surfaces. The points on the TFEcoated surface (Figure 9A) were distributed very close to the mean profile line, whereas the points on the substrate PVC showed greater divergence from the mean profile line, indicating surface unevenness. Conclusions APG discharge treatment of TFE produced a thick and uniform surface coating of a PTFE-like structure on a PVC substrate. Surface polymerization depended on the plasma conditions and especially on the flow rate of the monomer and carrier gases. Helium flow rates of 500 to 1000 sccm and monomer flow rates of 2 to 3 sccm were found to be suitable to obtain a uniform coating. XPS revealed a surface structure consisting mainly of -CF2 groups, confirming the PTFE-like structure. The valence band XPS of the modified surface and simulated spectra of a trimer molecule were very similar. This similarity suggests that the surface coating may have a trimer structure. AFM analysis showed nanometer-level smoothness and uniform topography of the modified surface. From the present study, we found this technique to be very useful and promising for the development of new surface-specific materials. Acknowledgment. Y.B. is grateful to the New Energy and Industrial Technology Development Organization (NEDO) for financial support and Dr. I. Kojima of National Institute of Materials and Chemical Research, for the use of an XPS instrument. LA990039L (32) Hansma, H.; Motamedi, F.; Smith, P.; Hansma, P.; Wittman, J. C. Polymer 1992, 33, 647. (33) Sheiko, S. S.; Moller, M.; Cantow, H.-J.; Magonov, S. N. Polym. Bull. 1993, 31, 693. (34) Bar, G.; Thomann, Y.; Whangbo, M.-H. Langmuir 1998, 14, 1219. (35) Gengenbach, T. R.; Xie, X.; Chatelier, R. C.; Griesser, H. J. J. Adhes. Sci. Tehnol. 1994, 8, 305. (36) Bunn, C. W.; Howells, E. R. Nature 1954, 18, 549.