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Doping of ion irradiated poly(aryl ether ether ketone) from water solution of LiCl
PII: S0141-3910(98)00039-1
Polymer Degradation and Stability 62 (1998) 535±540 # 1998 Elsevier Science Limited. All rights reserved Printed in Great Britain 0141-3910/98/$Ðsee front matter
Doping of ion irradiated poly(aryl ether ether ketone) from water solution of LiCl V. SÏvorcÏõÂk,a* V. Rybka,a J. VacõÂk,b V. Hnatowiczb & Y. Kobayashic a
Department of Solid State Engineering, Institute of Chemical Technology, 166 28 Prague, Czech Republic b Nuclear Physics Institute, Academy of Sciences of Czech Republic, 250 68RÏezÏ, Czech Republic c National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan (Received 26 November 1997; accepted 11 January 1998) Poly(aryl ether ether ketone) (PEEK) ®lms were irradiated with 2 MeV O+ ions to ¯uences up to 61014cmÿ2 and then doped from a 5 molar water solution of LiCl. The depth distribution of the incorporated LiCl was investigated using neutron depth pro®ling, and electron paramagnetic resonance was used for the determination of the concentration of free radicals. The structural changes of the modi®ed PEEK were examined by IR spectroscopy. Measurement of the sheet resistivity and its temperature dependence gave information on the electrical properties of the modi®ed PEEK. The results show that the ion irradiation enhances the penetration and incorporation of the LiCl dopant in the radiationdamaged PEEK surface layer. The Li atoms are chemically bound to free radicals created by radiation-induced degradation of PEEK macromolecules. The ion implantation leads to an increase in the PEEK conductance, which is further elevated by the LiCl doping. The conductance enhancement is partly a variablerange hopping mechanism and also electron hopping due to bound Li+ ions. # 1998 Elsevier Science Limited. All rights reserved
to production of excess of unsaturated double bonds9 and ®nal carbonization or graphitization.6,8 The structural and compositional changes induced by ion irradiation lead to changes in macroscopical (mechanical, electrical and optical) properties of polymers, e.g. the ion irradiation enhances electrical conductivity of the modi®ed surface layer,10,11 which may be increased further by doping with a suitable agent.12,13 In this work, the PEEK was irradiated by 2 MeV + O ions and subsequently doped from LiCl water solution. The modi®ed PEEK specimens were investigated by dierent techniques with the aim of ®nding a link between the structural changes due to the ion irradiation and the doping and changes in the electrical conductance.
1 INTRODUCTION Poly(aryl ether ether ketone) (PEEK) is a hightemperature- and radiation-resistant thermoplastic material which is used in an increasing number of technical applications,1 e.g. wire protection in nuclear power stations, production of automotive parts, coating of heat exchangers, medical techniques or tubing for pure media. Ion implantation is widely used in semiconductor processing for introduction of controlled amounts of acceptor or donor impurities into solids. Ion beam processing of polymers has grown rapidly to ful®l needs for diverse technologies covering their mechanical, electronical and optical applications.2 Major processes taking place in ion-irradiated polymers are excitation and ionization of atoms or molecules,3 formation of free radicals,4,5 chain scission,5 and crosslinking3 and outgassing of volatile degradation products.6 Other observed changes are connected with carbonylation7 and dehydrogenation,8 opening of aromatic rings leading
2 EXPERIMENTAL 2.1 Material and irradiation Semicrystalline PEEK ([±O±C6H4±O±C6H4±CO± C6H4±]n) ®lms, 25 m thick, with the density of
*To whom correspondence should be addressed. 535
536
V. SvorcÏõÂk et al.
1.285 g cmÿ3 were kindly supplied by SUMIMOTO Chemical Co Ltd. The temperature of glass transition Tg, the melting temperature Tm and the crystalline enthalpy H were determined by dierential scanning calorimetry on a DSC Du Pont 8900 device as: Tg=148 C, Tm=335 C and H=45.8 J gÿ1. For PEEK crystalline phase H=162 J gÿ1 has been reported,1 so that the PEEK ®lms used in this study contain about 28% of crystalline phase. The PEEK ®lms were irradiated with 2 MeV O+ ions to ¯uences from 31013 to 61014 cmÿ2. The ion beam current density was kept between 3 and 8 nA cmÿ2. 2.2 Measurements The ion-irradiated specimens were divided into two parts, the ®rst serving as a control set, the second being additionally doped with LiCl. For this purpose the ion-irradiated PEEK specimens were exposed to 5 molar water solution of LiCl for 22.5 h. Then the PEEK specimens were submerged in distilled water for 2 h in order to remove excess unbound LiCl dopant. All specimen treatments were performed at room temperature. The IR spectra were measured on an FTIR spectrometer Nicolet 740 and the dierential IR spectra presented in this study were calculated as the dierence between the spectra measured on the modi®ed PEEK and that measured on the pristine PEEK. The electron paramagnetic resonance (EPR) spectra were obtained at room temperature using a Carl±Zeiss 220 device (337 mT magnetic ®eld and 10 mW microwave power). The number of unpaired electrons, characterizing concentration of free radicals, was obtained using a Mn2+/ZnS standard. The sheet resistance Rs was determined by a two-point technique using a Keithley 487 instrument. For the determination of the Rs versus temperature dependence, the specimens were placed into a LN2 cryostat evacuated to about 10ÿ4 Pa. The standard measuring procedure comprises specimen cooling to LN2 temperature and subsequent slow heating. The concentration depth pro®les of the LiCl dopant in the PEEK surface layer, before and after the 2 h leaching in water, were determined using the neutron depth pro®ling method (NDP), making use of the 6Li(nth, 4He) 3H nuclear reaction. The area density (at cmÿ2) of Li atoms was calculated from concentration depth pro®les. The method enables one to determine nondestructively the Li concentration pro®le up to the depths of few micrometers with a depth resolution
of about 10 nm on the specimen surface. The NDP detection limit for 6Li is about 10ÿ6g gÿ1. 3 RESULTS AND DISCUSSION 3.1 NDP spectroscopy The Li content in the 4 m thick PEEK surface layer for as-doped PEEK specimens and those subsequently leached in water are shown in Table 1 as a function of the ion ¯uence applied. Surprisingly, the amount of incorporated Li atoms in the as-doped specimens is practically independent of the ion ¯uence. The 2 h water leaching leads to a signi®cant decrease in the Li content in the specimens irradiated to ¯uences below 11014 cmÿ2. Li atoms were not detected in the pristine PEEK. However, no such decrease is observed for the PEEK specimens irradiated to higher ¯uences. On the basis of these partial results, it may be concluded that only a part of the initially embeded Li atoms is tightly bound on trapping sites in the PEEK surface layer modi®ed by the ion irradiation and that the fraction of the atoms bound increases rapidly with increasing ion ¯uence. All further results reported in this study were obtained on specimens from which the free, unbound dopant was leached out. The leaching as a very complex process is a subject of another, more thorough investigation which is presently under way and the results of which will be published later. The concentration depth pro®les of the incorporated Li atoms, measured by the NDP technique on the PEEK specimens, irradiated with 2 MeV O+ ions to dierent ¯uences, doped with LiCl and leached in distilled water, are shown in Fig. 1. One can see that most of the dopant molecules are embedded in the PEEK surface layer, which was Table 1. The area density of Li atoms incorporated in the surface layer of the PEEK specimens irradiated with 2 MeV O+ ions to dierent ¯uences and doped in 5 molar water solution of LiCl for 22.5 h at room temperature: A, as-doped; B, after removal of an excess of the LiCl dopant by leaching the specimens in distilled water for 2 h Fluence of O+ ions (cmÿ2) 31013 51013 11014 3.51014 61014
Li concentration (cmÿ2) (A)
(B)
1.31016 1.01016 0.91016 1.91016 1.41016
1.81015 4.91015 0.81016 1.91016 1.41016
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537
into the PEEK surface layer and capture of part of the dopant atoms on traps (e.g. free radicals, excessive free volumes) created by the ion irradiation. For the ion ¯uences below 31014 cmÿ2, the amount of the incorporated Li atoms is an increasing function of the ion ¯uence. For the ion ¯uence of 61014 cmÿ2, the amount of incorporated Li atoms declines by 30%. With increasing ion ¯uence, the portion of the Li atoms trapped near the end of the ion trajectory, where nuclear energy loss mechanism plays a major role, also increases. These results are in general agreement with the data obtained in previous experiments on Li diusion into poly(ethyleneterephtalate)15 and polyethylene.16 In all cases, the Li diusion is aected by an excess of free volume produced by the ion irradiation. Fig. 1. Concentration depth pro®les of Li atoms incorporated in the surface layer of the PEEK specimens irradiated with 2 MeV O+ ions to dierent ¯uences and treated in 5 molar water solution of LiCl. The excess of LiCl dopant was removed by leaching in distilled water.
modi®ed by the ion irradiation. The depth range of this principal component is in rough agreement with the theoretical estimate of the projected range of 2 MeV O+ ions in the PEEK, Rp=2.6 m.14 It should be noted, however, that the width of the principal component increases with increasing ion ¯uence and for the highest ¯uences it exceeds the theoretical ion range signi®cantly. This eect may be due to enhanced deep penetration of the dopant through microvoids or microcracks created by mechanical stress on the interface between the degraded surface layer and the pristine bulk. Besides the principal component of the Li depth pro®le, a much weaker one is also observed, extending to greater depths, which is due to penetration of a part of the LiCl through the modi®ed surface layer and their slow diusion in pristine PEEK bulk. The shape of the principal pro®le component changes strongly with increasing ion ¯uence. The pro®les in the PEEK specimens irradiated to ¯uences below 11014 cmÿ2 exhibit one concentration maximum near the specimen surface. For higher ion ¯uences, another concentration maximum appears, situated near the ion projected range. The relative size of the second maximum increases with increasing ion ¯uence. The observed Li depth pro®le is apparently a result of a complicated penetration/capture process, comprising enhanced penetration of the LiCl water solution
3.2 EPR spectroscopy In order to investigate the mechanism, by which the Li atoms are bound in the PEEK surface layer modi®ed by the ion irradiation, the EPR spectra were measured at the as-irradiated PEEK and the PEEK irradiated and subsequently doped with LiCl. The concentration of free radicals (unpaired electrons) as a function of the ion ¯uence is shown in Fig. 2. One can see that the concentration of free radicals is an increasing function of the ion ¯uence. Doping with LiCl leads to a decline in the free radical concentration by about 50%. The result shows that a signi®cant part of the dopant molecules (or their constituents) combines with free radicals created by the ion irradiation. Complementary
Fig. 2. The dependence of the concentration of free radicals (determined by EPR as the concentration of unpaired electrons) on the ¯uence of O+ ions measured on the as-irradiated specimens and the specimens irradiated and subsequently LiCl doped.
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information is obtained from IR spectra measured on the PEEK specimens irradiated with the 2 MeV O+ions to dierent ¯uences and doped with LiCl. 3.3 FTIR spectroscopy The relevant part of dierential IR spectra is shown in Fig. 3. It is seen that the intensity of the absorption maximum at 417 cmÿ1, attributed to the C±Li group,17 increases with increasing ion ¯uence. Another absorption maximum at 518 cmÿ1, also attributed to the C±Li bond,17 is situated in a complex spectral region and its intensity was extracted with large error. Nevertheless, the observation of this absorption maximum and its evolution with increasing ion ¯uence also con®rm the presence of C±Li groups in the LiCl doped PEEK specimens. It may be concluded, therefore, that at least a part of the Li atoms is chemically bound on the defects (free radicals) created by the ion irradiation on the PEEK chains. No absorption maxima (e.g. at 610, 631 and 692 cmÿ1) typical for C±Cl bond18 were observed. Because of conservation of electrical neutrality, the Cl atoms should also be present in the modi®ed PEEK surface layer, but they are probably bound in more complex structures. It should also be noted that, in both NDP and RBS measurements no Cl atoms were detected, but this negative result may be explained by the very low sensitivity of both techniques in this case.
Fig. 3. Dierential IR spectra in the region from 400± 445 cmÿ1 measured on the PEEK specimens irradiated with O+ ions to dierent ¯uences (indicated in ®gure) and doped with LiCl.
3.4 Electrical resistance Another property, which is strongly aected by the ion irradiation and/or doping, is the polymer electrical resistance. The dependence of the sheet resistance Rs on the ion ¯uence is shown in Fig. 4 for the as-irradiated PEEK and the PEEK irradiated and then doped with LiCl. The irradiation to ¯uences below 11014 cmÿ2 results in a slow Rs decrease which is followed by a very rapid one at higher ¯uences. The resistance Rs of the PEEK specimen irradiated to the ¯uence of 61014 cmÿ2 is four orders of magnitude lower than that of the pristine, unirradiated PEEK. The rapid Rs decline at higher ion ¯uences may be due to the production and increased concentration of conductive structures (conjugated double bonds, graphitized regions etc.).19 Similar dependence of the Rs on the ion ¯uence was observed earlier on polyimide irradiated with N+ ions.20 The strong decrease in the polymer resistance caused by the irradiation with heavy ions is a general phenomenon, depending only weakly on the polymer initial structure. Introduction of LiCl dopant into the irradiated PEEK leads to another resistance decrease, which is more pronounced for the PEEK specimens irradiated to higher ion ¯uences. The nature of this additional resistance decrease is still unclear. The dependence of the Rs on the temperature for the PEEK specimen irradiated to the ¯uence of 61014 cmÿ2 and the specimen irradiated to the same ¯uence and then LiCl doped is shown in Fig. 5. Because of the limited sensitivity of the present apparatus, only the sheet resistivities above 110 K could be measured. One can see that the
Fig. 4. The dependence of the sheet resistance Rs on the O+ ion ¯uence for the as-irradiated PEEK specimens and those irradiated and subsequently LiCl doped.
Doping of ion irradiated poly(aryl ether ether ketone)
539
Fig. 6. The same data as in Fig. 5 but plotted in ln(Rs) versus Tÿ1/4 representation (T being the specimen temperature). Fig. 5. Temperature dependence of the sheet resistivity Rs for the PEEK specimen irradiated to the ¯uence of 61014 cmÿ2 and that irradiated to the same ¯uence and LiCl doped.
resistivity decreases rapidly with increasing temperature, so that both specimens exhibit semiconducting properties. Lower resistance is observed for the specimen additionally doped with LiCl throughout the whole temperature region, the dierence between as-irradiated and additionally doped specimens being an increasing function of the temperature. The enhanced conductance of the ion-irradiated polymers is well known to be contributed by the variable-range hopping (VRH) model,21 according to which the temperature dependence of the resistance Rs should be described by Mott's relation Rs T exp To =T1=4
1
where T is the specimen temperature. In order to check the validity of the VRH model in the present case, the data in Fig. 5 are plotted as the ln R(T) versus Tÿ1/4 dependence in Fig. 6. One can see that the dependence is linear, as is supposed by the VRH model, only for lower temperatures. It may be concluded, therefore, that the VRH mechanism is of major importance only in the low temperature region and that other, unspeci®ed processes aect electrical charge transport at higher temperatures. There is no principal dierence in the electrical charge transport between the as-irradiated and the irradiated and doped specimens. The present results (see Figs 4±6) clearly show that the Li atoms chemically bound in the PEEK surface layer modi®ed by the ion irradiation raises electrical conductance. It is known that
the reaction of Li+ salt with modi®ed macromolecular chain leads to binding of Li cation on the polymer.22 It may be supposed, therefore, that the additional increase in the PEEK conductance observed after doping with LiCl is caused mainly by the electron hopping on Li cation. The total conductance, however, is mainly due to the VRH mechanism and a minor contribution of the electron hopping by Li+ bound ions cannot aect signi®cantly the observed dependence of Rs on the specimen temperature (Figs 5 and 6). 4 CONCLUSION The PEEK specimens were irradiated with 2 MeV O+ ions to dierent ¯uences and the irradiated specimens were treated for 22.5 h in 5 molar water solution of LiCl. Then the excess of LiCl dopant was removed by leaching of specimens in distilled water. Enhanced penetration and incorporation of LiCl in the PEEK surface layer modi®ed by the ion irradiation was observed. The amount of incorporated and ®xed dopant molecules increases with increasing ¯uence of the O+ ions, reaches a maximum for the ¯uence of 3.51014 cmÿ2 and then it declines slightly. EPR measurements show that the introduction of LiCl dopant reduces the concentration of free radicals created by the ion irradiation by about 50%. The IR spectroscopy proves that a signi®cant part of the incorporated Li is bound to the PEEK macromolecular chain, probably on the sites where the free radicals, created by the ion irradiation, were originaly situated. The ion irradiation reduces the PEEK sheet resistance by several orders of magnitude. The measurement of
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the temperature dependence of the sheet resistance Rs shows that VRH plays a major role in the conductance enhancement. Another, minor resistance reduction is observed after LiCl doping, which is due to electron hopping by Li+ ions. ACKNOWLEDGEMENT This work was supported by the Grant Agency of the Czech Republic under Project No. 202-96-0077. REFERENCES 1. Hartwig, A., Hunnekuhl, I., Vitr, G., Diecho, S., Vohnwinkel, F. and Hennenmann, O. D., J. Appl. Polym. Sci., 1997, 64, 1091. 2. Lee, E. H., Rao, G. R., Lewis, M. B. and Masur, L. K., Nucl. Instrum. Meth., 1993, B74, 326. 3. Venkatesan, T., Calgano, L., Elman, B. S. and Foti, G., Ion Beam Modi®cation of Insulators. Elsevier, Amsterdam, 1987. 4. SÏvorcÏõÂk, V., EndrsÏ t, R., Rybka, V. and Hnatowicz, V., Mater. Lett., 1996, 28, 441. 5. Ferain, E. and Legras, R., Nucl. Instrum. Meth., 1993, B83, 163. 6. Lee, E. H., Lewis, M. B. and Masur, L. K., J.Mater. Res., 1991, 6, 610.
7. SÏvorcÏõÂk, V., EndrsÏ t, R., Rybka, V., Arenholz, E. and Hnatowicz, V., Eur. Polym. J., 1995, 31, 189. 8. Moliton, A., Lucas, B., Moreau, C., Friend, R. H. and Francois, B., Phil. Mag., 1994, B69, 1155. 9. SÏvorcÏõÂk, V., Rybka, V., Stibor, I. and Hnatowicz, V., J.Electrochem. Soc., 1995, 142, 590. 10. Calgano, L., Compagnini, G. and Foti, G., Nucl. Instrum. Meth., 1992, B65, 413. 11. Beddell, C. J., Se®eld, C. J., Bridwell, L. B. and Brown, I. M., J.Appl. Phys., 1990, 67, 1736. 12. Jankovskij, O., SÏvorcÏõÂk, V., Rybka, V. and Hnatowicz, V., J. Appl. Polym. Sci., 1996, 60, 1455. 13. Davenas, J. and Xu, X. L., Nucl. Instrum. Meth., 1992, B71, 23. 14. Kobayashi, Y., Kojima, T., Suzuki, T., Asari, E. and Katajima, M., Phys. Rev., 1995, B52, 823. 15. Hnatowicz, V., VacõÂk, J., CÏervenaÂ, J., SÏvorcÏõÂk, V., Rybka, V., Fink, D. and Klett, R., Phys. Stat. Sol. (a), 1997, 159, 327. 16. SÏvorcÏõÂk, V., Rybka, V., Jankovskij, O. and Hnatowicz, V., J. Appl. Polym. Sci., 1996, 61, 1097. 17. West, R. and Glaze, E., J. Am. Chem. Soc., 1961, 83, 3580. 18. Owen, E. D., Degradation and Stabilization of PVC. Elsevier, New York, 1984. 19. SÏvorcÏõÂk, V., ProsÏ kovaÂ, K., Rybka, V., Hnatowicz, V., Vacik, J. and Kobayashi, Y., 1998 Mater. Lett., in press. 20. SÏvorcÏõÂk, V., Arenholz, E., Rybka, V. and Hnatowicz, V., Nucl. Instrum. Meth., 1997, B122, 663. 21. Mott, N., Metal Insulator Transitions. Taylor & Francis, London, 1990. 22. SÏvorcÏõÂk, V., Rybka, V., VacõÂk, J., Hnatovicz, V., OÈchsner, R. and Ryssel, H., Nucl. Instrum. Meth., submitted.
Polymer Degradation and Stability 62 (1998) 535±540 # 1998 Elsevier Science Limited. All rights reserved Printed in Great Britain 0141-3910/98/$Ðsee front matter
Doping of ion irradiated poly(aryl ether ether ketone) from water solution of LiCl V. SÏvorcÏõÂk,a* V. Rybka,a J. VacõÂk,b V. Hnatowiczb & Y. Kobayashic a
Department of Solid State Engineering, Institute of Chemical Technology, 166 28 Prague, Czech Republic b Nuclear Physics Institute, Academy of Sciences of Czech Republic, 250 68RÏezÏ, Czech Republic c National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan (Received 26 November 1997; accepted 11 January 1998) Poly(aryl ether ether ketone) (PEEK) ®lms were irradiated with 2 MeV O+ ions to ¯uences up to 61014cmÿ2 and then doped from a 5 molar water solution of LiCl. The depth distribution of the incorporated LiCl was investigated using neutron depth pro®ling, and electron paramagnetic resonance was used for the determination of the concentration of free radicals. The structural changes of the modi®ed PEEK were examined by IR spectroscopy. Measurement of the sheet resistivity and its temperature dependence gave information on the electrical properties of the modi®ed PEEK. The results show that the ion irradiation enhances the penetration and incorporation of the LiCl dopant in the radiationdamaged PEEK surface layer. The Li atoms are chemically bound to free radicals created by radiation-induced degradation of PEEK macromolecules. The ion implantation leads to an increase in the PEEK conductance, which is further elevated by the LiCl doping. The conductance enhancement is partly a variablerange hopping mechanism and also electron hopping due to bound Li+ ions. # 1998 Elsevier Science Limited. All rights reserved
to production of excess of unsaturated double bonds9 and ®nal carbonization or graphitization.6,8 The structural and compositional changes induced by ion irradiation lead to changes in macroscopical (mechanical, electrical and optical) properties of polymers, e.g. the ion irradiation enhances electrical conductivity of the modi®ed surface layer,10,11 which may be increased further by doping with a suitable agent.12,13 In this work, the PEEK was irradiated by 2 MeV + O ions and subsequently doped from LiCl water solution. The modi®ed PEEK specimens were investigated by dierent techniques with the aim of ®nding a link between the structural changes due to the ion irradiation and the doping and changes in the electrical conductance.
1 INTRODUCTION Poly(aryl ether ether ketone) (PEEK) is a hightemperature- and radiation-resistant thermoplastic material which is used in an increasing number of technical applications,1 e.g. wire protection in nuclear power stations, production of automotive parts, coating of heat exchangers, medical techniques or tubing for pure media. Ion implantation is widely used in semiconductor processing for introduction of controlled amounts of acceptor or donor impurities into solids. Ion beam processing of polymers has grown rapidly to ful®l needs for diverse technologies covering their mechanical, electronical and optical applications.2 Major processes taking place in ion-irradiated polymers are excitation and ionization of atoms or molecules,3 formation of free radicals,4,5 chain scission,5 and crosslinking3 and outgassing of volatile degradation products.6 Other observed changes are connected with carbonylation7 and dehydrogenation,8 opening of aromatic rings leading
2 EXPERIMENTAL 2.1 Material and irradiation Semicrystalline PEEK ([±O±C6H4±O±C6H4±CO± C6H4±]n) ®lms, 25 m thick, with the density of
*To whom correspondence should be addressed. 535
536
V. SvorcÏõÂk et al.
1.285 g cmÿ3 were kindly supplied by SUMIMOTO Chemical Co Ltd. The temperature of glass transition Tg, the melting temperature Tm and the crystalline enthalpy H were determined by dierential scanning calorimetry on a DSC Du Pont 8900 device as: Tg=148 C, Tm=335 C and H=45.8 J gÿ1. For PEEK crystalline phase H=162 J gÿ1 has been reported,1 so that the PEEK ®lms used in this study contain about 28% of crystalline phase. The PEEK ®lms were irradiated with 2 MeV O+ ions to ¯uences from 31013 to 61014 cmÿ2. The ion beam current density was kept between 3 and 8 nA cmÿ2. 2.2 Measurements The ion-irradiated specimens were divided into two parts, the ®rst serving as a control set, the second being additionally doped with LiCl. For this purpose the ion-irradiated PEEK specimens were exposed to 5 molar water solution of LiCl for 22.5 h. Then the PEEK specimens were submerged in distilled water for 2 h in order to remove excess unbound LiCl dopant. All specimen treatments were performed at room temperature. The IR spectra were measured on an FTIR spectrometer Nicolet 740 and the dierential IR spectra presented in this study were calculated as the dierence between the spectra measured on the modi®ed PEEK and that measured on the pristine PEEK. The electron paramagnetic resonance (EPR) spectra were obtained at room temperature using a Carl±Zeiss 220 device (337 mT magnetic ®eld and 10 mW microwave power). The number of unpaired electrons, characterizing concentration of free radicals, was obtained using a Mn2+/ZnS standard. The sheet resistance Rs was determined by a two-point technique using a Keithley 487 instrument. For the determination of the Rs versus temperature dependence, the specimens were placed into a LN2 cryostat evacuated to about 10ÿ4 Pa. The standard measuring procedure comprises specimen cooling to LN2 temperature and subsequent slow heating. The concentration depth pro®les of the LiCl dopant in the PEEK surface layer, before and after the 2 h leaching in water, were determined using the neutron depth pro®ling method (NDP), making use of the 6Li(nth, 4He) 3H nuclear reaction. The area density (at cmÿ2) of Li atoms was calculated from concentration depth pro®les. The method enables one to determine nondestructively the Li concentration pro®le up to the depths of few micrometers with a depth resolution
of about 10 nm on the specimen surface. The NDP detection limit for 6Li is about 10ÿ6g gÿ1. 3 RESULTS AND DISCUSSION 3.1 NDP spectroscopy The Li content in the 4 m thick PEEK surface layer for as-doped PEEK specimens and those subsequently leached in water are shown in Table 1 as a function of the ion ¯uence applied. Surprisingly, the amount of incorporated Li atoms in the as-doped specimens is practically independent of the ion ¯uence. The 2 h water leaching leads to a signi®cant decrease in the Li content in the specimens irradiated to ¯uences below 11014 cmÿ2. Li atoms were not detected in the pristine PEEK. However, no such decrease is observed for the PEEK specimens irradiated to higher ¯uences. On the basis of these partial results, it may be concluded that only a part of the initially embeded Li atoms is tightly bound on trapping sites in the PEEK surface layer modi®ed by the ion irradiation and that the fraction of the atoms bound increases rapidly with increasing ion ¯uence. All further results reported in this study were obtained on specimens from which the free, unbound dopant was leached out. The leaching as a very complex process is a subject of another, more thorough investigation which is presently under way and the results of which will be published later. The concentration depth pro®les of the incorporated Li atoms, measured by the NDP technique on the PEEK specimens, irradiated with 2 MeV O+ ions to dierent ¯uences, doped with LiCl and leached in distilled water, are shown in Fig. 1. One can see that most of the dopant molecules are embedded in the PEEK surface layer, which was Table 1. The area density of Li atoms incorporated in the surface layer of the PEEK specimens irradiated with 2 MeV O+ ions to dierent ¯uences and doped in 5 molar water solution of LiCl for 22.5 h at room temperature: A, as-doped; B, after removal of an excess of the LiCl dopant by leaching the specimens in distilled water for 2 h Fluence of O+ ions (cmÿ2) 31013 51013 11014 3.51014 61014
Li concentration (cmÿ2) (A)
(B)
1.31016 1.01016 0.91016 1.91016 1.41016
1.81015 4.91015 0.81016 1.91016 1.41016
Doping of ion irradiated poly(aryl ether ether ketone)
537
into the PEEK surface layer and capture of part of the dopant atoms on traps (e.g. free radicals, excessive free volumes) created by the ion irradiation. For the ion ¯uences below 31014 cmÿ2, the amount of the incorporated Li atoms is an increasing function of the ion ¯uence. For the ion ¯uence of 61014 cmÿ2, the amount of incorporated Li atoms declines by 30%. With increasing ion ¯uence, the portion of the Li atoms trapped near the end of the ion trajectory, where nuclear energy loss mechanism plays a major role, also increases. These results are in general agreement with the data obtained in previous experiments on Li diusion into poly(ethyleneterephtalate)15 and polyethylene.16 In all cases, the Li diusion is aected by an excess of free volume produced by the ion irradiation. Fig. 1. Concentration depth pro®les of Li atoms incorporated in the surface layer of the PEEK specimens irradiated with 2 MeV O+ ions to dierent ¯uences and treated in 5 molar water solution of LiCl. The excess of LiCl dopant was removed by leaching in distilled water.
modi®ed by the ion irradiation. The depth range of this principal component is in rough agreement with the theoretical estimate of the projected range of 2 MeV O+ ions in the PEEK, Rp=2.6 m.14 It should be noted, however, that the width of the principal component increases with increasing ion ¯uence and for the highest ¯uences it exceeds the theoretical ion range signi®cantly. This eect may be due to enhanced deep penetration of the dopant through microvoids or microcracks created by mechanical stress on the interface between the degraded surface layer and the pristine bulk. Besides the principal component of the Li depth pro®le, a much weaker one is also observed, extending to greater depths, which is due to penetration of a part of the LiCl through the modi®ed surface layer and their slow diusion in pristine PEEK bulk. The shape of the principal pro®le component changes strongly with increasing ion ¯uence. The pro®les in the PEEK specimens irradiated to ¯uences below 11014 cmÿ2 exhibit one concentration maximum near the specimen surface. For higher ion ¯uences, another concentration maximum appears, situated near the ion projected range. The relative size of the second maximum increases with increasing ion ¯uence. The observed Li depth pro®le is apparently a result of a complicated penetration/capture process, comprising enhanced penetration of the LiCl water solution
3.2 EPR spectroscopy In order to investigate the mechanism, by which the Li atoms are bound in the PEEK surface layer modi®ed by the ion irradiation, the EPR spectra were measured at the as-irradiated PEEK and the PEEK irradiated and subsequently doped with LiCl. The concentration of free radicals (unpaired electrons) as a function of the ion ¯uence is shown in Fig. 2. One can see that the concentration of free radicals is an increasing function of the ion ¯uence. Doping with LiCl leads to a decline in the free radical concentration by about 50%. The result shows that a signi®cant part of the dopant molecules (or their constituents) combines with free radicals created by the ion irradiation. Complementary
Fig. 2. The dependence of the concentration of free radicals (determined by EPR as the concentration of unpaired electrons) on the ¯uence of O+ ions measured on the as-irradiated specimens and the specimens irradiated and subsequently LiCl doped.
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information is obtained from IR spectra measured on the PEEK specimens irradiated with the 2 MeV O+ions to dierent ¯uences and doped with LiCl. 3.3 FTIR spectroscopy The relevant part of dierential IR spectra is shown in Fig. 3. It is seen that the intensity of the absorption maximum at 417 cmÿ1, attributed to the C±Li group,17 increases with increasing ion ¯uence. Another absorption maximum at 518 cmÿ1, also attributed to the C±Li bond,17 is situated in a complex spectral region and its intensity was extracted with large error. Nevertheless, the observation of this absorption maximum and its evolution with increasing ion ¯uence also con®rm the presence of C±Li groups in the LiCl doped PEEK specimens. It may be concluded, therefore, that at least a part of the Li atoms is chemically bound on the defects (free radicals) created by the ion irradiation on the PEEK chains. No absorption maxima (e.g. at 610, 631 and 692 cmÿ1) typical for C±Cl bond18 were observed. Because of conservation of electrical neutrality, the Cl atoms should also be present in the modi®ed PEEK surface layer, but they are probably bound in more complex structures. It should also be noted that, in both NDP and RBS measurements no Cl atoms were detected, but this negative result may be explained by the very low sensitivity of both techniques in this case.
Fig. 3. Dierential IR spectra in the region from 400± 445 cmÿ1 measured on the PEEK specimens irradiated with O+ ions to dierent ¯uences (indicated in ®gure) and doped with LiCl.
3.4 Electrical resistance Another property, which is strongly aected by the ion irradiation and/or doping, is the polymer electrical resistance. The dependence of the sheet resistance Rs on the ion ¯uence is shown in Fig. 4 for the as-irradiated PEEK and the PEEK irradiated and then doped with LiCl. The irradiation to ¯uences below 11014 cmÿ2 results in a slow Rs decrease which is followed by a very rapid one at higher ¯uences. The resistance Rs of the PEEK specimen irradiated to the ¯uence of 61014 cmÿ2 is four orders of magnitude lower than that of the pristine, unirradiated PEEK. The rapid Rs decline at higher ion ¯uences may be due to the production and increased concentration of conductive structures (conjugated double bonds, graphitized regions etc.).19 Similar dependence of the Rs on the ion ¯uence was observed earlier on polyimide irradiated with N+ ions.20 The strong decrease in the polymer resistance caused by the irradiation with heavy ions is a general phenomenon, depending only weakly on the polymer initial structure. Introduction of LiCl dopant into the irradiated PEEK leads to another resistance decrease, which is more pronounced for the PEEK specimens irradiated to higher ion ¯uences. The nature of this additional resistance decrease is still unclear. The dependence of the Rs on the temperature for the PEEK specimen irradiated to the ¯uence of 61014 cmÿ2 and the specimen irradiated to the same ¯uence and then LiCl doped is shown in Fig. 5. Because of the limited sensitivity of the present apparatus, only the sheet resistivities above 110 K could be measured. One can see that the
Fig. 4. The dependence of the sheet resistance Rs on the O+ ion ¯uence for the as-irradiated PEEK specimens and those irradiated and subsequently LiCl doped.
Doping of ion irradiated poly(aryl ether ether ketone)
539
Fig. 6. The same data as in Fig. 5 but plotted in ln(Rs) versus Tÿ1/4 representation (T being the specimen temperature). Fig. 5. Temperature dependence of the sheet resistivity Rs for the PEEK specimen irradiated to the ¯uence of 61014 cmÿ2 and that irradiated to the same ¯uence and LiCl doped.
resistivity decreases rapidly with increasing temperature, so that both specimens exhibit semiconducting properties. Lower resistance is observed for the specimen additionally doped with LiCl throughout the whole temperature region, the dierence between as-irradiated and additionally doped specimens being an increasing function of the temperature. The enhanced conductance of the ion-irradiated polymers is well known to be contributed by the variable-range hopping (VRH) model,21 according to which the temperature dependence of the resistance Rs should be described by Mott's relation Rs T exp To =T1=4
1
where T is the specimen temperature. In order to check the validity of the VRH model in the present case, the data in Fig. 5 are plotted as the ln R(T) versus Tÿ1/4 dependence in Fig. 6. One can see that the dependence is linear, as is supposed by the VRH model, only for lower temperatures. It may be concluded, therefore, that the VRH mechanism is of major importance only in the low temperature region and that other, unspeci®ed processes aect electrical charge transport at higher temperatures. There is no principal dierence in the electrical charge transport between the as-irradiated and the irradiated and doped specimens. The present results (see Figs 4±6) clearly show that the Li atoms chemically bound in the PEEK surface layer modi®ed by the ion irradiation raises electrical conductance. It is known that
the reaction of Li+ salt with modi®ed macromolecular chain leads to binding of Li cation on the polymer.22 It may be supposed, therefore, that the additional increase in the PEEK conductance observed after doping with LiCl is caused mainly by the electron hopping on Li cation. The total conductance, however, is mainly due to the VRH mechanism and a minor contribution of the electron hopping by Li+ bound ions cannot aect signi®cantly the observed dependence of Rs on the specimen temperature (Figs 5 and 6). 4 CONCLUSION The PEEK specimens were irradiated with 2 MeV O+ ions to dierent ¯uences and the irradiated specimens were treated for 22.5 h in 5 molar water solution of LiCl. Then the excess of LiCl dopant was removed by leaching of specimens in distilled water. Enhanced penetration and incorporation of LiCl in the PEEK surface layer modi®ed by the ion irradiation was observed. The amount of incorporated and ®xed dopant molecules increases with increasing ¯uence of the O+ ions, reaches a maximum for the ¯uence of 3.51014 cmÿ2 and then it declines slightly. EPR measurements show that the introduction of LiCl dopant reduces the concentration of free radicals created by the ion irradiation by about 50%. The IR spectroscopy proves that a signi®cant part of the incorporated Li is bound to the PEEK macromolecular chain, probably on the sites where the free radicals, created by the ion irradiation, were originaly situated. The ion irradiation reduces the PEEK sheet resistance by several orders of magnitude. The measurement of
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the temperature dependence of the sheet resistance Rs shows that VRH plays a major role in the conductance enhancement. Another, minor resistance reduction is observed after LiCl doping, which is due to electron hopping by Li+ ions. ACKNOWLEDGEMENT This work was supported by the Grant Agency of the Czech Republic under Project No. 202-96-0077. REFERENCES 1. Hartwig, A., Hunnekuhl, I., Vitr, G., Diecho, S., Vohnwinkel, F. and Hennenmann, O. D., J. Appl. Polym. Sci., 1997, 64, 1091. 2. Lee, E. H., Rao, G. R., Lewis, M. B. and Masur, L. K., Nucl. Instrum. Meth., 1993, B74, 326. 3. Venkatesan, T., Calgano, L., Elman, B. S. and Foti, G., Ion Beam Modi®cation of Insulators. Elsevier, Amsterdam, 1987. 4. SÏvorcÏõÂk, V., EndrsÏ t, R., Rybka, V. and Hnatowicz, V., Mater. Lett., 1996, 28, 441. 5. Ferain, E. and Legras, R., Nucl. Instrum. Meth., 1993, B83, 163. 6. Lee, E. H., Lewis, M. B. and Masur, L. K., J.Mater. Res., 1991, 6, 610.
7. SÏvorcÏõÂk, V., EndrsÏ t, R., Rybka, V., Arenholz, E. and Hnatowicz, V., Eur. Polym. J., 1995, 31, 189. 8. Moliton, A., Lucas, B., Moreau, C., Friend, R. H. and Francois, B., Phil. Mag., 1994, B69, 1155. 9. SÏvorcÏõÂk, V., Rybka, V., Stibor, I. and Hnatowicz, V., J.Electrochem. Soc., 1995, 142, 590. 10. Calgano, L., Compagnini, G. and Foti, G., Nucl. Instrum. Meth., 1992, B65, 413. 11. Beddell, C. J., Se®eld, C. J., Bridwell, L. B. and Brown, I. M., J.Appl. Phys., 1990, 67, 1736. 12. Jankovskij, O., SÏvorcÏõÂk, V., Rybka, V. and Hnatowicz, V., J. Appl. Polym. Sci., 1996, 60, 1455. 13. Davenas, J. and Xu, X. L., Nucl. Instrum. Meth., 1992, B71, 23. 14. Kobayashi, Y., Kojima, T., Suzuki, T., Asari, E. and Katajima, M., Phys. Rev., 1995, B52, 823. 15. Hnatowicz, V., VacõÂk, J., CÏervenaÂ, J., SÏvorcÏõÂk, V., Rybka, V., Fink, D. and Klett, R., Phys. Stat. Sol. (a), 1997, 159, 327. 16. SÏvorcÏõÂk, V., Rybka, V., Jankovskij, O. and Hnatowicz, V., J. Appl. Polym. Sci., 1996, 61, 1097. 17. West, R. and Glaze, E., J. Am. Chem. Soc., 1961, 83, 3580. 18. Owen, E. D., Degradation and Stabilization of PVC. Elsevier, New York, 1984. 19. SÏvorcÏõÂk, V., ProsÏ kovaÂ, K., Rybka, V., Hnatowicz, V., Vacik, J. and Kobayashi, Y., 1998 Mater. Lett., in press. 20. SÏvorcÏõÂk, V., Arenholz, E., Rybka, V. and Hnatowicz, V., Nucl. Instrum. Meth., 1997, B122, 663. 21. Mott, N., Metal Insulator Transitions. Taylor & Francis, London, 1990. 22. SÏvorcÏõÂk, V., Rybka, V., VacõÂk, J., Hnatovicz, V., OÈchsner, R. and Ryssel, H., Nucl. Instrum. Meth., submitted.