Spectrochimica Acta Part A 60 (2004) 3071–3077 A spectroscopic study on defluorination of poly(tetrafluoroethylene) by alkyllithium/electron-donating solvents Yasuhiro Okuda a,∗, Fumihiro Hayashi a, Hiroshi Sakurai b, Masaru Shiotani b a Electronics & Materials R&D Laboratories, Sumitomo Electric Industries Ltd., 1-1-3 Shimaya, Konohana-Ku, Osaka 554-0024, Japan b Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan Received 26 December 2003; accepted 23 January 2004 Abstract Defluorination of PTFE by alkyllithium/electron-donating solvents such as N,N,N′,N′-tetramethylethylenediamine (TMEDA), hexam- ethylphosphoramide (HMPA) was studied by means of spectroscopy such as ESR, 7Li- and 13C-NMR, XPS, UV-Vis and IR. Based on the experimental results, it was concluded that an electron from radical species, which was generated in the alkyllithium/electron-donating solvent, was transferred onto PTFE molecule so as to eliminate fluorine atoms from the PTFE and to form carbon-centered radicals on the PTFE; concomitantly, the alkyl group of the alkyllithium was transferred onto the PTFE. Combined with the experimental results of the phenyllithium/HMPA system, mechanism of the fluorine atom elimination reactions from PTFE by the radical species is discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Poly(tetrafluoroethylene) (PTFE); Defluorination; Alkyllithium; N,N,N′,N′-tetramethylethylenediamine (TMEDA); Hexamethylphosphoramide (HMPA); Spectroscopy 1. Introduction Fluorinated polymers have excellent chemical and thermal stabilities, low dielectric constant, and/or low surface energy [1,2]. Among the fluorinated polymers, poly(tetrafluoroethylene) (PTFE) is the most widely used in the fields such as electronic, chemical, and medical in- dustries, and a great deal of studies has been performed so far in order to provide further advanced properties to PTFE such as adhesiveness or hydrophilicity. They include physi- cal methods such as �-ray irradiation [3], plasma treatment [4], exposure to ion beams [5], or ultraviolet laser irradi- ation [6]. As chemical methods, defluorination of PTFE with sodium metal dissolved in liquid ammonia [7,8], aro- matic radical anion [9–11], lithium amalgam [12,13] have been studied. In case of the aromatic radical anion such as naphthalene radical anion [9,10] or benzoin dianion [11], the electron transfer from anion radicals to PTFE gives rise to the elimination of fluorine atoms from PTFE to form a dark-brown-colored carbonaceous layer. Sakurai et al. sug- ∗ Corresponding author. Tel.: +81-6-6466-7921; fax: +81-6-6466-1274. E-mail address:
[email protected] (Y. Okuda). gested that the reaction rates of PTFE with anion radicals might be related to the decreasing rate of anion radicals according to the second-order equation [10]. However, the radicals intermediately formed on PTFE during the elim- ination of fluorine atoms from the PTFE could hardly be detected because the reaction of an aromatic radical anion with PTFE might be very fast and the carbonaceous layer might be quickly formed on the treated PTFE. Meanwhile, elimination of the fluorine atom and simultaneous alkyla- tion of PTFE by alkyllithium above 423 K with catalytic N, N, N′, N′-tetramethylethylenediamine (TMEDA) was also reported [14]; the reaction was assumed to be nucleophilic addition of the alkyl anion generated from alkyllithium. However, no clear experimental evidence for an electron transfer from alkyllithium to PTFE and radical generation on the PTFE was provided in the report. Generally, alkyllithiums are associated, e.g. hexameric, or tetrameric, in a hydrocarbon solvent or in the solid state [15]. The coexistence of an electron-donating solvent (ED solvent) such as TMEDA, hexamethylphosphoramide (HMPA), may reduce the degree of association so as to improve the reactivity of the alkyllithium. However, little is known about the radical species, which are generated in alkyllithium/ED solvent solution, and elimination of fluorine 1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2004.01.029 3072 Y. Okuda et al. / Spectrochimica Acta Part A 60 (2004) 3071–3077 atoms from PTFE by the radical species, to the best of our knowledge. In the present study, radical species generated in alkyl- lithium/ED solvent below 273 K, and carbon-centered rad- icals generated on the defluorinated PTFE, were directly observed by electron spin resonance (ESR) spectroscopy when PTFE was treated with the radical species. Identi- fication and electronic structures of the carbon-centered radicals generated on the PTFE, and the mechanism of the fluorine atom elimination were discussed based on the ESR results together with other spectroscopic experimental data such as 7Li- and 13C-nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV-Vis) and infrared (IR) absorption spectroscopy. 2. Experimental 2.1. Materials The following reagents were used as received: PTFE powder of F-104 (average particle size = 560�m, Daikin), solution of n-butyllithium (n-BuLi, 1.6 M solu- tion in hexane, Aldrich), tert-butyllithium (t-BuLi, 1.7 M solution in pentane, Aldrich), phenyllithium (1.8 M so- lution in cyclohexane–ether 70/30, Aldrich), and hex- amethylphosphoramide (HMPA, Aldrich). Diethylamine (DEA, Wako), ethylenediamine (EDA, Wako), and N,N,N′,N′-tetramethylethylenediamine (TMEDA, Wako) were used after distillation (Scheme 1). 2.2. Sample preparations and spectroscopies 2.2.1. Alkyllithium/electron-donating solvent (ED solvent) solution To each of alkyllithium solution was added each ED sol- vent (either amine or HMPA) at a 1:1 molar ratio in a Spec- trosil ESR sample tube at 273 K in an argon atmosphere, and the solution was then shaken for 15 min at the same tem- perature. The solution was subjected to ESR measurement Scheme 1. Electron-donating solvents (ED solvents) used in the present experiments. at 77 K, then at elevated temperatures to room temperature using a JEOL RE-1 spectrometer. 2.2.2. Reaction of alkyllithium/ED solvent solution with PTFE 2.2.2.1. ESR measurement. Five millimoles of the above 1:1 molar ratio mixture solution was added to PTFE powder (50 mg) in a Spectrosil ESR sample tube under an argon atmosphere at 273 K. The mixture was kept for 30 min at the same temperature, and subjected to ESR measurement under an argon atmosphere at 77 K. 2.2.2.2. NMR, XPS, UV-Vis and IR absorption spectra and differential scanning calorimeter (DSC) measurement. After the reaction of the PTFE powder with the alkyl- lithium/HMPA was completed, excess amount of distilled water was added to the mixture to terminate the reaction, then the reacted PTFE powder was removed from the mix- ture by filtering, washed with distilled water at 330 K for 24 h to remove any water-soluble residues, then washed with ethanol for 24 h and finally washed with acetone for 24 h to remove organic residues. The samples prepared in this manner were subjected to 13C-CP-MAS-NMR mea- surement using a Bruker AMX-400 spectrometer, XPS mea- surement with a Perkin-Elmer Physical Electronics 5100 spectrometer, and UV-Vis absorption measurement with a Hitachi U-3400 spectrophotometer. The IR spectrum was obtained by the KBr powder method with a Perkin-Elmer Spectrum 2000 spectrometer. In order to prove the gen- eration of LiF on the reacted PTFE as the fluorine atom elimination product, the reacted PTFE before washing with water was subjected to a 7Li-CP-MAS-NMR measure- ment using a Bruker AMX 400 spectrometer with LiCl as the internal standard. The DSC measurement was carried out for the PTFE samples before and after treatment with alkyllthium/HMPA, using a Shimadzu DSC-50 calorimeter. 3. Results and discussion 3.1. Solution of alkyllithium in ED solvent All the mixture solutions of the n-BuLi or t-BuLi, each containing DEA, EDA, TMEDA or HMPA, gave a singlet ESR signal at g = 2.004 with a line width of 1.4–2.2 mT at 77 K as shown in Fig. 1a and Fig. 2a. Upon warming, the ESR singlet disappeared at 150 K for the solution with DEA as the amine, whereas for the other solutions with EDA, TMEDA or HMPA, the ESR singlets persisted even at the higher temperature of 273 K. The latter amines are known to chelate lithium atoms. Thus, the chelation might contribute to the higher stability of the ESR active species at the higher temperature. Here, we conclude that the alkyllithium/ED solvent mix- ture generates ESR-detectable radical species at 77 K. The Y. Okuda et al. / Spectrochimica Acta Part A 60 (2004) 3071–3077 3073 Fig. 1. ESR spectra of the t-BuLi/HMPA mixture solution with a 1:1 molar ratio recorded at 77 K (a) before, and (b) after reaction with the PTFE powder at 77 K. The sample was then recorded at elevated temperatures of (c) 250 K, and (d) 300 K. (e) The theoretical spectrum calculated using the 1H-hf splittings given in the text. Note that the ESR singlet similar to (a) was also observed for the n-BuLi/X, and t-BuLi/X (X: TMEDA, EDA, and DEA) systems at 77 K. reactivity of the radical species with PTFE is the subject of Section 3.2. 3.2. Reaction of alkyllithium/ED solvent mixture with PTFE 3.2.1. ESR study Among all the possible combinations of alkyllithium and ED solvent, only when the solutions of alkyllithium/TMEDA and alkyllithium/HMPA were added to PTFE powder, the color of PTFE varied from the original white to dark-brown after about 30 min under an argon atmosphere at 273 K. The broad ESR singlet of the t-BuLi/HMPA mixture solution dis- appeared and concomitantly a triplet with an isotropic hy- perfine (hf) splitting of 2.3 mT appeared as seen in Fig. 1b at 77 K. Upon warming the sample to 300 K, each of the triplet lines was further split into 10 hf lines with a smaller split- ting of 0.16 mT (Fig. 1d). It may be possible to attribute the triplet (2.3 mT) to two equivalent �-fluorine atoms of PTFE, and the 10 hf lines (0.16 mT) to the nine �-hydrogen atoms Fig. 2. ESR spectra of the n-BuLi/HMPA mixture solution with a 1:1 molar ratio recorded at 77 K (a) before, and (b) after reaction with the PTFE powder at 77 K, and then at elevated temperature of (c) 240 K. (d) The theoretical spectrum was calculated using the 1H-hf splittings given in the text. of the tert-butyl (t-Bu) group (C(CH3)3). The result suggests that the t-Bu group could be transferred from t-BuLi onto the PTFE and the carbon-centered radical species was formed on the PTFE as illustrated in Fig. 1e. When n-BuLi/HMPA was used instead of t-BuLi/HMPA, a similar ESR triplet with a hf splitting of 2.36 mT appeared (Fig. 2). However, no further hf splittings could be resolved even upon warm- ing the solution to 240 K in this system. We believe that, in the same way with the case of t-BuLi/HMPA, the n-Bu group might be transferred from n-BuLi onto the PTFE, but the 1H-hf splittings due to two �-hydrogens of n-Bu group are too small to be resolved. Thus, the present ESR results suggest that the reac- tion of the PTFE with the radical species generated from the alkyllithium/HMPA mixture solution generates the carbon-centered radicals on the PTFE and alkyl group is transferred from alkyllithium onto the treated PTFE. In addition to the above results, with the further progress of the defluorination reaction, the original dark-brown color of the reacted PTFE was getting further dark and finally became black color after 24 h, and a new singlet ESR peak 3074 Y. Okuda et al. / Spectrochimica Acta Part A 60 (2004) 3071–3077 Fig. 3. 7Li-CP-MAS-NMR spectrum of the n-BuLi/HMPA/PTFE system measured before washing the sample with water. LiCl was used as the internal standard of the 7Li-NMR chemical shift (δ (ppm)). with g = 2.0067 appeared. The newly generated singlet may originate from carbonaceous layers accumulated on the defluorinated PTFE. 3.2.2. 7Li-MAS-NMR study—evidence for elimination of fluorine atom In order to confirm the generation of LiF that must be the reaction product of Li+ with fluoride ion (F−) elim- inated from the PTFE [13], the PTFE after reaction with t-BuLi/HMPA was subjected to a 7Li-MAS-NMR measure- ment before washing the sample with distilled water. A 7Li resonance absorption peak appeared at −0.30 ppm (see Fig. 3), which can be attributed to the Li atom of LiF [16]. Thus, we confirmed that the fluoride ion (F−) eliminated from the PTFE reacted with Li+ from the alkyllithium to yield the LiF salt during the reaction of PTFE with the alkyl- lithium/HMPA solution. 3.2.3. 13C-CP-MAS-NMR study—evidence for the alkyl group transfer Fig. 4 shows the 13C-CP-MAS-NMR spectra measured for the PTFE powder, before and after treatment with the t-BuLi/HMPA solution. The PTFE powder before the treat- ment gave a broad singlet at 142 ppm (Fig. 4a). The 13C chemical shift is attributed to the CF2 carbon of PTFE [17]. After the treatment with t-BuLi/HMPA, two sharp peaks newly appeared at 27.2 and 37.5 ppm in addition to the broad singlet (Fig. 4b). The signals are attributable to the C(CH3)3 carbons from t-BuLi [17]. When n-BuLi was used instead of t-BuLi, we could observe four different 13C-NMR signals at 13.7, 22.3, 28.5, and 36.6 ppm (Fig. 4c). The chemical shifts are attributed to –CH2CH2CH2CH3 carbons from n-BuLi [17]. These 13C-CP-MAS-NMR results are consistent with the results derived from above ESR measurement, and we could confirm the transfer of alkyl groups from the alkyl- lithium onto the defluorinated carbon atoms of the PTFE during treatment with the alkyllithium/HMPA solutions. Fig. 4. 13C-CP-MAS-NMR spectra of (a) PTFE, and (b) PTFE after treatment with the t-BuLi/HMPA mixture solution, and (c) PTFE after treatment with the n-BuLi/HMPA mixture solution. 3.2.4. UV-Vis and IR spectroscopy—evidence for the conjugated C=C bond formation The UV-Vis absorption spectra were also obtained for the PTFE powder, before and after it was treated with alkyl- lithium/HMPA mixture solution. After the treatment a broad Y. Okuda et al. / Spectrochimica Acta Part A 60 (2004) 3071–3077 3075 Fig. 5. FT-IR spectra of (a) non-treated PTFE, (b) n-BuLi/HMPA-treated PTFE, and (c) t-BuLi/HMPA-treated PTFE. Absorption peaks at 1718–1722 cm−1 as stretching vibration of monofluorinated C=C [11], at 2936, 2960, and 2876 cm−1 as stretching vibration of C−H, at 1243, 1231, and 1152 cm−1 as stretching vibration of C−F, and at 510–640 cm−1 as deformation vibration of C−F were identified. absorption band was newly developed in the 200–400 nm range. This absorption is characteristic of �–�∗ transition of C=C bonds [18]and suggests the conjugated C=C bond formation as a result of fluorine atom elimination from the PTFE. The corresponding IR spectra are shown in Fig. 5. When PTFE was reacted with n-BuLi/HMPA or t-BuLi/HMPA, new absorption peak appeared around 1720 cm−1. Consis- tent with the UV-Vis spectra, the IR absorption peak is at- tributable to the C=C stretching vibration of the monoflu- orinated C=C bond [11]. Thus, both the UV-Vis and IR absorption spectra suggested the conjugated C=C bond for- mation which was caused by the fluorine atom elimination reaction from PTFE. 3.2.5. XPS study—evidence for the C–H bond formation Fig. 6 shows the C 1s XPS spectra measured for the PTFE powder before and after treatment with the t-BuLi/HMPA Fig. 6. XPS spectra of (a) PTFE, and (b) PTFE after treatment with t-BuLi/HMPA mixture solution. mixture solution. In addition to a strong absorption peak at 292 eV, which is attributed to the CF2 carbon of the origi- nal PTFE [19], a peak at 284.5 eV appeared after the treat- ment. The latter peak is attributed to carbons of C(CH3)3 (t-Bu) formed in the treated PTFE sample. These XPS re- sults are consistent with the present ESR, NMR, UV-Vis, and IR spectroscopic ones and we could further confirm the defluorination from PTFE and the alkyl group transfer from alkyllithium onto the PTFE. 3.2.6. Crystallinity and molecular weight change of PTFE The crystallinity of the PTFE was evaluated using dif- ferential scanning calorimeter (DSC) at heating rate of 10 K min−1 under nitrogen atmosphere. The endothermic peak arising from melting was observed at 601 K for the PTFE before and after treatment with alkyllithium/HMPA solution. And the heat of fusion for the PTFE crystal was 18.4 and 18.1 J g−1 for the PTFE before and after the treat- ment, respectively. It is well known that the crystallinity of the PTFE is dependent on the molecular weight as shown in Suwa’s equation [20]. The results suggest that both the crystallinity and the molecular weight of the PTFE did not change significantly by the defluorination reaction. As com- pared with the �-ray irradiation that causes degradation or molecular weight decrease of the PTFE, the defluorination with alkyllithium/HMPA shows a marked contrast. 3.2.7. ESR of phenyllithium and HMPA mixture In order to obtain further information about the reac- tion mechanism of the alkyllithium/HMPA with PTFE, ESR 3076 Y. Okuda et al. / Spectrochimica Acta Part A 60 (2004) 3071–3077 Fig. 7. (a) Experimental ESR spectrum of the PhLi/HMPA mixture at room temperature, and (b) assuming that the biphenyl anion radical is responsible for the experimental spectrum, the theoretical spectrum was calculated using the 1H-hf constants given in the text. Scheme 2. Generation of biphenyl anion radical from the phenyllithium and HMPA mixture. spectra for the phenyllithium (PhLi) and HMPA mixture so- lution were measured at room temperature. The ESR spec- trum consists of the following 1H-hf splittings: aH(2) = 0.265, aH(3) = 0.043, and aH(4) = 0.529 mT as shown in Fig. 7. The observed 1H-hf splittings are very close to those of biphenyl anion radical [21]. Considering the compounds involved in the reaction, the spectrum could be attributable to biphenyl anion radical. A possible reaction mechanism to form the biphenyl anion radical can be as follows. It has been suggested that the carbon–lithium bond of the alkyllithium could be cleaved when the alkyllithium is mixed with HMPA or TMEDA [22–24]. Referring these papers, we propose, when phenyllithium is mixed with HMPA, the carbon–lithium bond of phenyllithium can be cleaved prob- ably through chelation of the lithium by HMPA, and radical species denoted as [n-(HMPA)-Li·] in Scheme 2 is formed, although exact structure of the radical has not been clarified yet. The phenyl radical ( •) can react with each other to form biphenyl, i.e. reaction (2). Thus, formed biphenyl then reacts with the electron, which may be released from the radical denoted as [n-(HMPA)-Li·], reaction (3), to generate the biphenyl radical anion, reaction (4). Consistent with the present reaction, biphenyl is known as a good electron ac- ceptor because of its high electron affinity (i.e. 1.096 eV as phenyl [25]). Scheme 3. Generation of radical species from the alkyllithium/HMPA mixture, and defluorination of PTFE. Y. Okuda et al. / Spectrochimica Acta Part A 60 (2004) 3071–3077 3077 Fig. 8. A two-dimensional pictorial representation of the presumed struc- ture of defluorinated PTFE after treatment with radical species generated from alkyllithium (RLi)/HMPA. Here, R stands for the alkyl group of RLi. Referring the biphenyl formation from phenyllithium/ HMPA system as shown in Scheme 2, the possible reaction mechanism of the fluorine atom elimination from PTFE in alkyllithium/HMPA/PTFE mixture can be as follows. When butyllithium (n-BuLi or t-BuLi) is mixed with HMPA, the carbon–lithium bond scission as reaction (1) in Scheme 3 may be expected similar to the case in the phenyllithium/HMPA system. However, we could not ob- tain any spectroscopic evidence for the formation of the butyl radical anion [(C4H9)•], or the coupled product of the octyl radical anion [(C8H18)•−]; the latter corresponds to the biphenyl radical anion in the case of PhLi/HMPA. This means that an electron released from the radical species, [n-(HMPA)-Li•], is neither transferred to [(C4H9)•] the coupled product [(C8H18)]; the reaction (4) does not take place. This can be rationalized by considering the poor electron affinity of butyl group (0.75 eV as hydrogen atom, and 0.08 eV as CH3 [25]) compared to the biphenyl. When the radical species, [n-(HMPA)-Li•], reacted with PTFE, a carbon–fluorine bond of PTFE was easily reduced to eliminate the fluorine atom from PTFE, so as to form the defluorinated PTFE and a fluoride ion (F−), reaction (5). The proposed reaction is based on the higher electron affin- ity of the fluorine atom of 3.40 eV [25] compared to that of hydrogen atom of butyl group. (Scheme 3) 4. Conclusion We conclude the present spectroscopic results as fol- lows. We could succeed the direct ESR observation of the radical species formed in the alkylltihium/HMPA solution. When PTFE was treated with the radical species in the solution, carbon-centered radicals on defluorinated PTFE were formed. Meanwhile, the mixture of phenyllithium and HMPA resulted in generation of biphenyl radical anions. These results suggest that the carbon–lithium bond of the alkyllithium is cleaved when the alkylltihum is solved in the ED solvent, and a single electron is transferred from the lithium to the solvent, and an ESR-detectable radical species is generated. Because PTFE is a stronger electron acceptor than the solvent, a single electron is then transferred to the PTFE so as to form the carbon radicals on PTFE. The fluorine atom elimination from the PTFE was separately detected by the 7Li-NMR. Thus, we could confirm that the 19F elimination was followed by the generation of the carbon-centered radicals and the conjugated C=C bond as detected by the present ESR, UV-Vis, and IR spectroscopies. The alkyl group introduction onto the defluorinated PTFE from alkyllihtium was also detected as revealed by the ESR, 13C-NMR, and C 1s XPS spectra. The DSC measurement suggested that the crystallinity and the molecular weight of the PTFE did not change by the defluorination reaction. Based on the present experimental results, we propose a two-dimensional pictorial representation for the molecular structure for the PTFE after treatment with radical species generated in alkyllithium/HMPA as illustrated in Fig. 8. References [1] T. Satokawa (Ed.), Fussojushi (Fluororesin) Handbook, Nikkankogyo Shimbunsha, Tokyo, 1992, p. 147. [2] L.A. Wall (Ed.), Fluoropolymers, Wiley-Interscience, New York, 1972, p. 475. [3] El.-S.A. Hegazy, N.H. Tahler, H. Kamal, J. Appl. Polym. Sci. 38 (1989) 1229. [4] H. Shonhorn, R.H. Hansen, J. Appl. Polym. Sci. 11 (1967) 1461. [5] C.-A. Chang, J.E.E. Baglin, A.G. Schrott, K.C. Lin, Appl. Phys. Lett. 51 (1987) 103. [6] H. Yanaura, H. Kurokawa, T. Fujimoto, F. 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A spectroscopic study on defluorination of poly(tetrafluoroethylene) by alkyllithium/electron-donating solvents Introduction Experimental Materials Sample preparations and spectroscopies Alkyllithium/electron-donating solvent (ED solvent) solution Reaction of alkyllithium/ED solvent solution with PTFE ESR measurement NMR, XPS, UV-Vis and IR absorption spectra and differential scanning calorimeter (DSC) measurement Results and discussion Solution of alkyllithium in ED solvent Reaction of alkyllithium/ED solvent mixture with PTFE ESR study 7Li-MAS-NMR study-evidence for elimination of fluorine atom 13C-CP-MAS-NMR study-evidence for the alkyl group transfer UV-Vis and IR spectroscopy-evidence for the conjugated CC bond formation XPS study-evidence for the CH bond formation Crystallinity and molecular weight change of PTFE ESR of phenyllithium and HMPA mixture Conclusion References