Polyetheretherketone (PEEK) Surface Functionalization by Low

Dec 10, 2003 - Ring Opening of Aromatic Polymers by Remote Atmospheric-Pressure Plasma. E. Gonzalez , M.D. Barankin , P.C. Guschl , R.F. Hicks. IEEE T...
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Langmuir 2004, 20, 157-163

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Polyetheretherketone (PEEK) Surface Functionalization by Low-Energy Ion-Beam Irradiation under a Reactive O2 Environment and Its Effect on the PEEK/Copper Adhesives Sehyun Kim,†,‡ Ki-Jun Lee,‡ and Yongsok Seo*,† Supercomputational Modeling and Simulation Laboratory, Future Technology Research Division, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, Korea 130-650, and School of Chemical Engineering, Seoul National University, Shinlimdong 56-1, Kwanakku, Seoul, Korea 151-742 Received July 30, 2003. In Final Form: October 8, 2003 A low-energy Ar+ ion beam was used to modify the surface of a polyetheretherketone (PEEK) film. The modification reaction proceeded with or without oxygen gas injected during the irradiation. The surface functional groups of the modified PEEK were confirmed with X-ray photoelectron spectroscopy as increasing various oxygen-containing functional groups. The concentration of the functional groups varied rapidly with the irradiation time, reached a maximum value, and then slowly decreased. The surface morphology of PEEK was substantially changed by ion-beam irradiation. Surface smoothening occurred so that the surface roughness reached almost constant value after some irradiation time. The incorporation of functional groups on the PEEK surface and the surface topology change had opposite effects on the adhesion strength between PEEK and copper. Dominance of the former was evident because the lap-shear strength initially increased with the irradiation. The special surface features significantly enhanced the adhesion strength between the evaporated copper layer and the modified PEEK surface. However, the decrease in the surface roughness with a long time irradiation implies a decrease in adhesion strength due to a smaller contact area, and the shear strength due to topology change also slowly decreased after a long time irradiation.

Introduction Polyetheretherketone (PEEK) is a thermoplastic and semicrystalline polymer, which has good mechanical properties, high thermal stability, and outstanding chemical properties, such as high hydrolysis resistance, low flammability, and low toxic gas emission. As a consequence of these advantages, PEEK can be used in a wide variety of fields, such as the aerospace, automotive, electronics, and medicine industries, in various types of not only moldings but also composites and laminates. PEEK also exhibits superior chemical resistance which has allowed it to work effectively as a metal replacement in a harsh environment. Because PEEK is inert to most common solvents and is resistant to a wide range of organic and inorganic liquids, coating metals with PEEK can protect them from corrosion. However, PEEK does not have good adhesion to a metal surface. This limits its application. Many modifications of PEEK by using high-energy species, such as plasmas, ozone, UV light, electrons, and γ-rays, have been applied in order to improve its adhesion to a metal surface.1-4 Pawson et al.1 and Jama et al.2 have used X-ray photoelectron spectroscopy (XPS) and timeof-flight secondary ion mass spectroscopy (ToF-SIMS), respectively, to investigate PEEK surfaces modified by plasma treatment. Inagaki et al.5 studied PEEK surfaces * To whom correspondence should be addressed. E-mail: [email protected]. † Korea Institute of Science and Technology. ‡ Seoul National University. (1) Pawson, D. J.; Ameen, A. P.; Short, R. D.; Denison, P.; Jones, F. R. Surf. Interface Anal. 1992, 18, 13. (2) Jama, C.; Dessaux, O.; Goudmand, P.; Gengembre, L.; Grimblot, J. Surf. Interface Anal. 1992, 18, 751. (3) Baalmann, A.; Vissing, K. D.; Born, E.; Gross, A. J. Adhes. 1994, 46, 57. (4) Mathieson, I.; Bradley, R. H. J. Mater. Chem. 1994, 4, 1157. (5) Inagaki, N.; Tasaka, S.; Horiudhi, T.; Suyama, R. J. Appl. Polym. Sci. 1998, 68, 271.

modified by using a remote oxygen plasma. They observed that degradation products were formed on the PEEK surface by the oxygen plasma treatment and pointed out the following important aspect of the surface modification: PEEK is susceptible to plasma actions, and as a result, the degradation of PEEK and the introduction of oxygen functionality occur simultaneously on the surfaces of PEEK films. Polymer surface modification by ion-beam irradiation was intensively investigated in the 1990s.6-10 However, its application to polymers was limited due to undesirable side reactions such as polymer degradation or crosslinking.7,10 Recently, we developed a novel process, lowenergy ion-beam irradiation under a reactive gas environment for polymer surface modification.11,12 Most other vacuum technology methods use a high-energy ion-beam irradiation,7 but our method employs a low-energy ion beam (less than 5 keV), which is strong enough to activate the polymer surface and additional gas molecules.13 This low-energy ion-beam irradiation reduces both chain degradation and the cross-linking of irradiated polymers. Added gas molecules, such as oxygen, are then chemically adsorbed onto the surface to form some functional groups. These functional groups can react or interact with other (6) Young, R. P.; Slemp. W. S. In Irradiation of Polymeric Materials; Reichmanis, E., Frank, C. W., O’Donnell, J. H., Eds.; ACS Symposium Series No. 527, American Chemical Society: Washington DC, 1993; p 278. (7) Lee, E. H.; Rao, G. R.; Hanser, L. K. Trends Polym. Sci. 1996, 4, 229. (8) Mahlberg, R.; Niemi, H. E.; Denes, F. S.; Rowell, R. M. Langmuir 1999, 15, 2985. (9) DuPont-Gillain, Ch. C.; Adriaensen, Y.; Derclaye, S.; Rouhxet, P. G. Langmuir 2000, 16, 8194. (10) Kim, H.; Urban, M. W. Langmuir 1999, 15, 3499. (11) Kim, H. J.; Lee, K.; Seo, Y.; Kwak, S.; Koh, S. Macromolecules 2001, 34, 2546. (12) Kim, H. J.; Lee, K.; Seo, Y. Macromolecules 2002, 35, 1267. (13) Kim, S.; Lee, K.; Seo, Y. Langmuir 2002, 18, 6185.

10.1021/la035396h CCC: $27.50 © 2004 American Chemical Society Published on Web 12/10/2003

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Figure 1. Schematic diagram of the ion-beam irradiation reactor.

functional groups or with metals evaporated onto the polymer surface. Oxygen-containing polymers have been reported to form complexes with metals, which may significantly enhance the adhesive strength between a polymer and a metal.14-17 In this study, we used a low-energy ion-beam irradiation method to modify the PEEK surface in order to improve the adhesion between PEEK and copper, which has some applications in microelectronics. Ion-beam irradiation of the PEEK surface should introduce functional groups onto the PEEK surface and modify it so that it becomes a hydrophilic surface. Ar+ ion-beam irradiation (with and without a reactive O2 gas) was used. The chemical and the physical properties of modified polymers and the effect of PEEK modification on the adhesion between PEEK and copper were investigated. Experimental Section Materials. Materials employed in this study were commercial PEEK sheets (Sumitomo Inc., Japan) and copper sheets with a thickness of 0.35 mm. The sheets were cleaned with methanol in an ultrasonic water bath and then dried in a vacuum oven at 100 °C for 24 h to remove any residual solvent. Argon and oxygen gases of 99.99% purity were used. Ion-Beam Irradiation. The ion-beam-assisted reaction system was composed of a conventional ion-beam system, a reactive gas feeding system, and the polymer sample (powder, pellet, or film) mixing bowl. Figure 1 shows a schematic diagram of the ion-beam irradiation reactor. The working pressure in the reaction chamber was kept under 10-4 Torr. The Ar+ ion beam was generated from a 5-cm cold-hollow cathode ion source, and its potential energy was maintained at less than 1 keV. The currents of the ions were controlled by the discharge voltage and the ion-beam potential. The discharge current was 0.4 A, and the ion fluence was varied between 1.6 × 1018 and 1.3 × 1020 ions/cm2. The ion fluence was measured by using a Faraday cup placed slightly above the polymer samples. The flow rate of Ar gas, which was ionized to Ar+ by the ion source, was fixed at 2 sccm. The mixing bowl was equipped with a rotor blade for uniform mixing during ion-beam irradiation. Reactive O2 gas was constantly injected from the bottom of the chamber. The flow rate of the O2 gas was 3 sccm and was controlled by using a mass flow controller (MassFlo 9121). Dried PEEK samples were put in the ion-beam chamber, and a vacuum under 10-4 Torr was applied. It was covered with a circular Faraday cup to protect the ion beam before irradiation. Argon (14) Inagaki, N.; Tasaka, S.; Umehara, T. J. Appl. Polym. Sci. 1999, 71, 2191. (15) Inagaki, N.; Tasaka, S.; Narushima, K.; Mochizuki, K. Macromolecules 1999, 32, 8566. (16) Tamada, Y.; Yamada, T.; Tasaka, S.; Inagaki, N. Macromolecules 1996, 29, 4331. (17) Inagaki, N.; Tasaka, S.; Park, Y. W., J. Adhes. Sci. Technol. 1997, 12, 1105.

Kim et al. gas was injected into ion gun, and by adjusting the discharge voltage and the ion-beam potential, current density for the appropriate treatment was set. After the ion gun reached a stable condition and the current density had a steady value, ion-beam irradiation was started. Reactive oxygen gas was injected from the bottom of the mixing bowl. After a predetermined period, gas injection was terminated. Water Contact Angle. Contact angles of the virgin PEEK and Ar+ ion beam irradiated PEEK samples with reactive O2 gas environment were measured using a contact angle goniometer (ERMA, model G-1) at room temperature. The water droplets made of 0.025 mL of distilled water were dropped at seven different sites on each sample, and the measured values of the contact angle were averaged. Atomic Force Microscopy (AFM). The AFM images in this article were obtained from a multimode scanning probe microscope (Digital Instruments, Inc.). Tapping mode was used to obtain height imaging data with 125 µm long cantilevers. The cantilever has a very small tip radius of 5-10 nm. The lateral scan frequency was about 1.0 Hz. The samples were moved in the x-y plane, and a voltage was applied, which moved the piezo driver over the z axis, to keep the probing force constant, resulting in a three-dimensional height image of the samples surface. X-ray Photoelectron Spectroscopy (XPS). Chemical components on the surface ion beam irradiated PEEK were analyzed by XPS. XPS spectrum was recorded by Surface Science 2803-S spectrometer (hv ) 1.5 keV). The basic pressure of 2 × 10-10 Torr was maintained during analysis. Energy resolution of 0.48 eV was kept. The XPS spectra were referenced to the main component of the C 1s peak of PEEK at a binding energy of 284.6 eV. The irradiation generally resulted in a small shift of all the peaks (up to ca. 0.6 eV) toward higher binding energies, implying an increased conductivity of the modified surfaces. The overlapping peaks were resolved by the peak synthesis method based on Gaussian peaks. Adhesive Strength Measurement. A copper layer of about 2000 Å in thickness was thermally evaporated onto the virgin PEEK and the ion-beam irradiated PEEK by a vacuum thermal evaporation technique. The deposition was carried out under a pressure of 10-6 Torr and at a deposition rate of about 2 Å/s. Then, the deposited surface was adhered to a copper sheet (0.35 mm thickness) using an epoxy adhesive. The assembly was cured at 100 °C for 6 h and then was subjected to lap-shear adhesion test (ASTM D 1002) using an Instron Universal Testing Machine (model 4204) at a room temperature. Crosshead speed of 10 mm/ min was used. All the reported results are averages of at least 10 measurements.

Results and Discussion Various methods have been employed to modify the properties of polymer surface. In our previous works,11-13 we showed that low-energy ion-beam irradiation was a very efficient method for modifying the properties of polymer surfaces through the incorporation of functional groups. A polymer surface irradiated with an ion beam depositing a high-energy density can be characterized as a highly reactive system in which the addition of a reactive gas will induce many chemical reactions and introduce functional groups. This is a heterogeneous, solvent-free and environmentally favorable process. Since the modification proceeds at a relatively shallow depth below the surface, the physical properties of the ion-beam-treated polymer do not change remarkably. X-ray Photoelectron Spectroscopy and Surface Functionalization. Ion-beam irradiation is able to modify the surface of a PEEK film by making it hydrophilic. Ion-beam irradiation involves the following two main processes: oxidation, which forms hydrophilic groups on the PEEK film, and degradation, which forms low molecular weight products on the surface of the PEEK film. The first process contributes to the hydrophilic modification through the formation of polar groups, and the second process disturbs the hydrophilic surface modification

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Figure 2. XPS C 1s spectra: (a) virgin PEEK; (b) only Ar+ ion beam irradiation for 120 s; (c) Ar+ irradiation with reactive O2 gas for 120 s; (d) XPS O 1s spectra of virgin PEEK.

through the formation of degradation products. These two processes compete with each other. If degradation is dominant, the outermost layer of the PEEK film will be stripped off by the degradation, and the PEEK film’s surface will be covered with degradation products. Therefore, we believe that a key factor for achieving a highly hydrophilic surface is to depress the degradation of the PEEK film. Another preliminary point to be discussed concerns the homogeneity within the XPS sampling depth of the modified samples. The penetration depth of an ion strongly relies on the class of the ion, the initial energy of the ion, and the material of the substrate to be treated. The estimated sampling depth for C 1s photoelectrons (for Al KR photons at 1486.6 eV) is about 60 Å and is slightly less for N 1s and O 1s photoelectrons.18 We used the TRIM96 code18 to calculate the penetration depth of low-energy 1 keV Ar+ ions into PEEK. The maximum penetration depth calculated was 80 Å. Furthermore, there was no significant variation of the energy deposition along the track for a 1 keV Ar+ ion; thus, the collisional term dominated. These two facts convinced us that the modification of the irradiated layer was essentially homoge(18) Ziegler, J.F.; Biersack, J. P.; Littmak, U. The Stopping and Range of Ions in Solids; Pergamon Press: New York, 1985.

neous within the XPS sampling depth and that the thickness of the modified layer on the PEEK film was quite thin. The kinds of oxygen functionalities formed by ion-beam irradiation are handily investigated by using an X-ray photoelectron spectroscope. Figure 2 shows the characteristic modification trend of C 1s and O 1s peaks for the virgin PEEK surface and for the ion-beam-irradiated PEEK surface. The peak synthesis and assignment of components were performed using data in the literature.5,10,19-23 Figure 2a shows the spectra of virgin PEEK. It can be deconvoluted into three main components: 285.0 eV (CH groups), 286.5 eV (CsO groups), and 287.9 eV (CdO groups). The relative concentrations of the CH, the CsO, and the CdO components for the virgin PEEK film are 73.7, 21.1, and 5.2%, respectively. Within the experimental errors, the results of the deconvolution procedure (19) Greenwood, O. D.; Hopkins, J.; Badyal, J. P. Macromolecules 1997, 30, 1091. (20) Silanes and Other Coupling Agents; Mittal, K. L., Ed.; VSP: Utrecht, 1992. (21) Marletta, G.; Iacona, F.; Toth, A. Macromolecules 1991, 25, 3190. (22) Ektessabz, A. M.; Hakamota, S. Thin Solid Films 2000, 377, 621. (23) Kim, H. J. Ph.D. Dissertation, Seoul National University, 2001.

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Table 1. The Relative Concentrations of C 1s and O 1s Components of Ion Beam Irradiated PEEK Filma oxygen (%)

CH

CsO CdO COO OdC OsC

30 120 300 30 120 300

73.7 74.7 71.3 72.3 71.3 67.2 66.2 66.3

21.1 20.7 20.3 19.6 19.8 21.4 22.1 22.1

(theoretical) virgin Ar+ Ar+ + O2

a

carbon (%)

irradiation time, s

5.2 5.1 5.6 5.1 5.1 7.4 7.4 7.7

2.8 3.0 3.8 4.0 4.3 3.9

33.0 31.5 21.4 21.0 20.0 14.6 15.0 23.7

67.0 68.5 78.6 79.0 80.0 85.4 85.0 76.3

Relative peak area in %. Table 2. The Atomic Ratio of Ion Beam Irradiated PEEK Film

(theoretical) virgin Ar+ Ar+ + O2

irradiation time, s

atomic ratio O/C

30 120 300 30 120 300

0.13 0.134 0.155 0.224 0.175 0.260 0.287 0.272

show a close agreement with the theoretical values (74.1, 20.7, and 5.1% for the CH, the CsO, and the CdO groups, respectively). The ion beam irradiated PEEK film shows a similar C 1s spectrum but has new peaks. These peaks can be deconvoluted into four components: 285.0 eV (CH group), 286.5 eV (CsO group), 287.9 eV (CdO group), and 289.0 eV (COsO group) (parts b and c of Figure 2). The ion beam irradiated PEEK film possesses higher concentrations of the CsO group than the original PEEK film. The O 1s spectrum for the original PEEK is deconvoluted into two components: 531.9 eV (CdO group) and 533.1 eV (CsO group) (Figure 2d). The relative concentrations of the CdO and the CsO components are 31.5 and 68.5%, in agreement with the theoretical values calculated from the repeating unit of the PEEK film. Comparison of the C 1s spectra shows that the ion beam irradiated PEEK film possesses more of the CsO group than the original PEEK film and possesses slightly less of the CdO group. The functional group ratios of PEEK before and after irradiation were calculated from the integrated intensities of the peaks of each functional group and are presented in Table 1. The amount of CdO decreased with the irradiation time while that of CsO increased. This indicates that the ion beam irradiation predominantly forms CsO groups rather than the CdO group on the PEEK film surface. The effect of irradiation time is not easily discernible until after a long time. The atomic ratios (relative ratios of atoms on the surface within the XPS region) of the PEEK surface after ionbeam irradiation were calculated from the integral intensities of the peaks of each element and are summarized in Table 2. It is evident from the Ar+ ion beam irradiated sample that the elemental ratio of O to C increased with irradiation time. After a long time, it decreased slightly because of an increase in the amount of C due to carbonization. The amount of oxygen noticeably increased when oxygen gas was added, but it decreased slightly again after a long irradiation time because of an increase in the amount of C due to surface carbonization.19 After irradiation under an oxygen gas, the amount of oxygen was increased to more than twice that in the as-received PEEK film. This result indicates that the oxygen atoms originally contained

Figure 3. The contact angle of ion beam irradiated PEEK with the irradiation time: only Ar+ ion beam irradiation (b); Ar+ irradiation with reactive O2 gas (0).

in the PEEK film were sputtered less than the carbon atoms in the benzene ring. (This can be checked by the relative concentrations of CdO and COO in Table 1 for the case of only Ar+ ion beam irradiation.) This sputtering of benzene rings and the incorporation of oxygen gas caused the increase in the relative ratio of O/C. The late-time increase in C-H is due to the formation of a C-C-C structure at 285 eV. 9,19 The hydrocarbon component was substantially depleted by the ion-beam irradiation whereas the carbonylic component and the ether-like bond showed apparent stability, which was attributed to the high radiation resistance of linkages stabilized by two contiguous benzene rings.21,22 The slight change in the full width at half-maximum of the C 1s peak (from 1.2 to 1.5 eV) shows that C-C-C bonds were created owing to breakage of the benzene structure and rearrangement of the scattered carbon atoms.23 Hence, the major structural change in PEEK is believed to occur through the breakage of the benzene structure forming the carbon-oxygen bond and through the carbonized surface. The most remarkable difference between the samples before and after irradiation was the emergence of the COO- group. This formation of the COO- group on the PEEK film’s surface contributes to the formation of a hydrophilic surface. None of the component peaks shows a significant broadening in the full width at half-maximum after irradiation. For the case of a polyimide film, the C 1s peak corresponding to the CdO shows significant broadening, which is attributed to random collisions of the incident ions with the CdO neighboring structures.13,24 Random collisions of ions creates different structures adjacent to the CdO bond. However, the XPS spectra of the PEEK film do not show this kind of structural change. Water Contact Angle. Changes in the functionalgroup concentrations on the PEEK film’s surface lead to increases in the surface free energy. The effect of irradiation on the surface polarity is shown in Figure 3. It shows the change of the contact angle between water droplets and the virgin PEEK surface and the contact angle between water droplets and the ion beam irradiated PEEK (24) Carter, G.; Vishnyakov, V. Phys. Rev. B 1996, 54, 17647.

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Figure 4. The surface roughness of ion beam irradiated PEEK with the irradiation time: only Ar+ ion beam irradiation (b); Ar+ irradiation with reactive O2 gas (0).

surface with the irradiation time, respectively. When the Ar+ ion beam irradiation was conducted with or without an O2 environment, the contact angle decreased with the irradiation time owing to the effect of hydrophilicity. The PEEK film surfaces treated with Ar+ ion beam irradiation under a reactive O2 environment showed lower contact angles than those treated without O2. An irradiation time longer than 60 s does not give any additional hydrophilicity; the contact angle remains almost constant.25 The monotonic decrease in the contact angle can be ascribed to two separate causes: formation of hydrophilic functional groups and surface roughness change. Although it has been experimentally proven that ion-beam bombardment roughens a polymer surface, the roughness of PEEK film decreased with irradiation time, in contrast to the PTFE case.26,27 The roughness of a PTFE film increases with irradiation; the water contact angle also increases. More details are given in the next section. Surface Roughness. Figure 4 shows the surface roughness of the PEEK samples. We investigated the influence of the ion-beam irradiation time and the environmental condition (reactive gas atmosphere) on the PEEK surfaces. The surface morphology of PEEK was substantially changed by ion-beam irradiation. An atomic force microscope image of a virgin PEEK film surface shows a relatively rough surface (Figure 5). Surface modification occurs first through ion bombardment that initiates etching and radical production. Etching seems to occur at the hillocks first, smoothing the rough surface. The underlying mechanisms of the microdynamics of surface roughness and pattern formation induced by ion sputtering have been heavily studied for inorganic materials that form semiconductor quantum dots.28 The ion-induced surface instability can be described by a specific term in the erosion equation that is proportional (25) Here the contact angle measurement was done 30 min after the specimen was taken out from the reactor. It is well-known that the contact angle measurement is influenced by aging condition. The oxygen plasma treated PEEK surface showed hydrophobic recovery (increasing contact angle) after a long time (a few days) (Brenman, W. J.; Feast, W. J.; Munro, H. S.; Walker, S. A. Polymer 1991, 32, 1527). This is not peculiar to PEEK, but it happens to most common polymers.9 (26) Kim, S. Ph.D. Dissertation, Seoul National University, 2003. (27) Kim, S. Macromol. Res. 1999, 7, 250. (28) Bradley, R. M.; Harper, J. M. E. J. Vac. Sci. Technol. 1988, A6, 2390.

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to the negative surface tension which tends to maximize the surface, in contrast to surface tension which tends to minimize it.29 It is the competition between the roughening-instability and the diffusive-smoothing mechanisms that governs the buildup of a regular pattern with a characteristic wavelength.30 General theories including negative surface tension and additional smoothening mechanisms such as viscous flow on amorphous surfaces have been developed.24,31 Under a normally incident ion beam, the surface morphology forms a regular matrix of hillocks and depressions with a characteristic spatial frequency. For polymer molecules, this kind of regularity is unexpected because of their large molecular sizes, high viscosities, and low mobilities. However, some small molecules emitted during the ion bombardment can move on the surface.23 We have already observed that some carbonization bonds are produced on the PEEK surface. Diffusion of these small molecules can smoothen the surface. After the surface smoothening process, further ion bombardment etches the surface to produce much smaller hillocks and depressions (Figure 6). The height difference becomes much smaller, and the roughness reaches an almost constant value. Surface smoothing seems to occur more rapidly when oxygen gas is added due to more vigorous reactions on the surface. According to Greenwood et al.’s observation,19 globular features were evident for PEEK surface after oxygen plasma treatment. They found a correlation between the mean diameter of globules and the extent of plasma oxidative attack from the XPS measurement. With increasing level of oxygenation, the mean diameter of globules decreased. From this interdependency, they noticed that the degree of oxygenation determines the final surface morphology. This is in agreement with our result. Increasing the level of oxidation accompanies reduced size of globules and decrease of contact angle. Adhesion Test on PTFE/Cu. Figure 7 shows the adhesive strength for the adhesive joint between the PEEK film and the copper metal as a function of the irradiation time. The adhesion strength increased with the irradiation time. Some facts are worthy of note: (1) Neat PEEK has a low shear strength (∼10 N/cm2) though the surface roughness is larger than it is for ion beam irradiated samples. On the other hand, the lap shear strength increases significantly for all ion beam irradiated PEEK/ copper joints, and the values are quite high (five and six times for Ar+ ion-beam irradiation and Ar+ + O2 irradiation, respectively). (2) The shear strength is significantly affected by the surface functional groups and by the surface topology. The adhesion strengths of the O2-gas-added system were higher than the values for the Ar+ ion-beamonly system, indicating that incorporation of functional groups results in a significant increase in adhesion strength. (3) The shear strength for the Ar+ + O2 system shows a slow decrease with irradiation time. The decrease in the surface area due to the topology change and the incorporation of functional groups formed on the PEEK surface are two important parameters in determining the adhesion strength between PEEK and copper. The resulting lap-shear strength is the outcome of two opposing effects, increased adhesion due to chemical functional groups and diminished adhesion due to a smaller inter(29) Chason, E.; Mayer, T. M.; Kellerman, B. K.; Mcilroy, D. T.; Howard, A. J. Phys. Rev. Lett. 1994, 72, 3040. (30) Cucrmo, R.; Barabasi, A. C. Phys. Rev. Lett. 1995, 74, 4746. (31) Carter, G.; Navinsek, B.; Whitton, L. In Sputtering by Particle Bombardment II; Topics in Applied Physics, Springer-Verlag: New York, 1991; Vol. 64.

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Figure 5. The surface roughness of virgin PEEK with only Ar+ ion beam irradiation and Ar+ ion beam irradiation with reactive O2 gas.

Figure 6. Surface roughness of virgin PEEK (a), only Ar+ irradiation (b), and Ar+ irradiation with reactive O2 gas (c).

acting area32 The XPS analysis showed that the CdO peak intensities decreased whereas the CsO peak intensity increased (Table 1). Cu is known to react with CsO, but much less with CdO.33 Thus, the total amount of interacting Cu initially increases with irradiation time, and the shear strength due to chemical interactions also increases. However, the decrease in the surface roughness with ion-beam irradiation implies a decrease in adhesion strength due to a smaller contact area, and the shear strength due to topology change also slowly decreases. We can conclude, in contrast to the PTFE/copper system, that the former dominates the latter in the PEEK/copper system. In the PTFE/copper system, the surface roughness increased with the ion-beam irradiation due to severe defluorination (weak C-F bonding).26 The contacting surface area between PTFE and copper then increased (32) The negative effect by the increase of short chains and burial of functional groups is presumably negligible since the ion-beam irradiation does modify only the shallow surface molecules and the copper deposition was carried out right after the ion-beam irradiation. (33) Kim, D. H.; Jo, W. H. Macromolecules 2000, 33, 3050. Kim, D. H.; Kim, K. H.; Jo, W. H.; Kim, J. Macromol. Chem. Phys. 2000, 201, 2699.

Figure 7. The adhesive strength of ion beam irradiated PEEK/ Cu with the irradiation time: only Ar+ ion beam irradiation (b); Ar+ irradiation with reactive O2 gas (0).

Surface Functionalized PEEK/Copper Adhesion

with the ion beam irradiation time. The evaporated copper filled the roughened surface. Thus, the adhesion strength between PTFE and copper is enhanced while that between PEEK and copper is weakened due to a small contact area. In the PTFE/copper system, the adhesion strength due to topological change is more significant, and the surface topology change and the functional groups generated on the surface have a synergistic effect.26 On the other hand, they are in competition for a PEEK/copper system and a chemical functional group effect dominates that of interacting area change. Conclusions In this study, we used a low-energy ion-beam irradiation method to modify the surface of PEEK to improve the adhesion between PEEK and copper. Surface modification was done by using Ar+ ion beam irradiation with and without a reactive O2 gas. The contact angle decreased with the irradiation time owing to the increased hydrophilicity. Incorporation of oxygen onto the surface layer occurred through depletion of hydrocarbons in benzene rings. The XPS results showed that hydrophilic groups were formed on the PEEK surface during the modification process. Incorporation of oxygen in the surface layers and emergence of the COO- group with the depletion of

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hydrocarbons made the surface quite hydrophilic. The surface became smoother with increasing ion beam irradiation time. The incorporation of functional groups on the PEEK surface and the surface topology change have opposite effects on the adhesion strength between PEEK and copper. Dominance of the former was evident because the lap-shear strength increased with the irradiation time (hence, the induction of functional groups on the surface) while the contact surface area decreased. The lap-shear strengths of the irradiated PEEK/copper system were significantly improved (five and six times for Ar+ ion beam irradiation only and Ar+ ion beam plus O2 gas irradiation, respectively). In conclusion, we have demonstrated that PEEK surface modification by using a low-energy Ar+ ion beam with and without a reactive gas (O2) caused special surface features which significantly enhanced the adhesion strength between the evaporated copper layer and the modified PEEK surface. Acknowledgment. This work was supported by MOST (2N26010). Special thanks go to Dr. Hyoung-jun Kim for enlightening discussion and experimental help. LA035396H