- Email: [email protected]
A new assessment of the crystalline structure of undoped and doped aniline oligomers and polymers
Synthetic
ELSEVIER
Metals
69 (1995)
167-169
A new assessment of the crystalline structure of undoped and doped aniline oligomers and polymers F. aSchool of Chemistry, “Department
of Physics and Mathematics, Qeparlment
Lux' , E.J. Samuelsenb and E.T. KangC
University of Bristol, Cantock’s University of Trondheim,
of Chemical Engineering,
Close, Bristol BS8 lTS, United Kingdom
Norwegian
Institute of Technology,
National University of Singapore,
N-7034 Trondheim,
Norway
Kent Ridge, Singapore 0511
Abstract We describe here the crystalline structure of aniline oligomers and polymers, derived from the aniline dimer, paminodiphenylamine, and aniline. It is shown that while the crystalline structure of the undoped samples is generally in reasonable accord with the fmdings of Pouget et al. for their EBII-structure, especially the crystalline arrangement of the doped materials does not fit the picture, given by these authors for the ES&salt. The latter fact might be rationalized by the simple exchange of the two main peaks of the ESIJ arrangement, thus giving the P2r22r arrangement, proposed by Pouget et al. but not found in their experimental studies.
1. Introduction Polyaniline ( PANI ) is one of the few electronically conductive polymers, having successtirlly survived the basically “scientitic exploration step”. However, although some industrial companies have put a lot effort into the development of more or less sophisticated applications of this material, some of the fimdamental problems of it have not been resolved yet. The latter fact has been highlightened only recently by one of us ( 1 ). One of the still not frilly explored aspects of PANI concerns the atomic arrangement in semicrystalline PANI grades. Despite the fact that a lot of characterization work about this subject has been published in some detail by an american/Gench group [ i.e. Pouget et al. ( 2 , 3 )], the structure of PANI crystals still appears not thoroughly enough investigated, because other groups have found quite different X-my difGaction patterns for their semi-crystalline PANI grades ( e.g. 4 ). We report here on the crystalline arrangement of PANI oligomers and polymers, which have been successGrlly prepared from mixtures of the aniline dimer, paminodiphenylamine ( ADPA ), and aniline via fractionation of the respective polymerization products. It is shown that while most part of the crystalline arrangements of the undoped powders might be reasonably explained by somewhat of an EBII arrangement ( Pouget et al. ), especially the crystalline structure of the H2SO4doped salts differ considerably Gom that one, proposed for the ESII salt by these authors. A reasonable explanation for this discrepancy might be found by exchanging the two main crystalline peaks of the X-my ditTYaction patterns of Pouget et al., thus giving a pattern more close to the P2122r one, assumed by Pouget et al. in their considerations but not found experimentally in their studies.
2. Experimental Aniline oligomers and polymers were synthesized adopting the principles of the usual polymerization route to PANI. The
characterization of the resulting powders, using W, FTIR-, X-ray diffraction and X.P.S. spectroscopy, respectively, were performed using state-of-the-art equipment.
3. Results and discussion 3.1. UV-Vis-N&Spectra of undoped PANI fractionsThe UV-Vis-NIR-spectra have shown that at least two major absorption peaks ( located at ca. 320 mn and 620 mn, respectively ) exist in all spectra. The exact positions are listed in Tab. 1. As has been discussed before by different authors ( 6, 7, 8 ), the band at about 320 mn can be attributed to the x-x*-transition between benzoid rings and the band at ca. 620 mu is indicative of a charge-transfer exciton-like transition from the highest occupied levels on beruoid rings to the lowest occupied energy levels on quinoid rings, respectively. Both bands shift to higher wavelengths when the chain length of the molecules is increased. However, the absorption at ca. 620 mn is also intluenced by the oxidation state of the molecules. Thus, it can be inferred Gom the UV-spectra that an increase in chainlength takes place when going Gom the diethylether/benzene ( soluble/insoluble ) to the DMF ( soluble/insoluble ) Gactions. Furthermore, differences in the oxidation states of the individual molecules are to be expected between different fractions. Note that the shoulders in most of the spectra below 320 nm are probably due to the existence of low molecular weight residues in the samples and the shoulder in the methanol ( soluble ) fraction at ca. 430 mn probably results from the oxidative degradation of the molecules. 3.2. FTLR spectra of undoped and doped PANI fractions” The FTIR spectra have shown that the different fractions contain the main JR-features of PANI.. Most important are the bands at 1587, 1510,830,740 and 690 cm-’ ( nndoped and doped samples ) and the band at 1140 cm-’ ( doped samples ). The band at 1587 cm” is *see
0379.6779/95/$09.50
0 1995 Elsevier
SSDI 0379-6779(94)02406-O
Science
S.A. All rights
note on the end of the paper
reserved
F. LLU et al. I Synthetic Metals 69 (1995) 167-169
168
indicative of stretching vibrations in quinoid rings ( -N=Q=N- ), whereas the 1510 cm-’ vibration represent the same mode in benzoid rings ( -NH-B-NH- ). On this basis it can be concluded that all the different undoped samples have an oxidation degree lower than 0.5, i.e. the individual chains contain more bcnzoid rings than quinoid ones. On the other hand, the vibrations between 650 and 850 cm-’are indicative of C-H vibrations on 1,2,4 substituted rings, with the 830 cm” vibration the only one, indicative of neat 1,4-substituted rings. As has been discussed by Kang et al. ( 6 ), the appearence of the bands at 740 and 690 cm-’ is primarily indicative for the existence of an unproportional high fraction of monosubstituted rings, i.e. lower molecular weight species. On this basis the first evidence is established for the fact that the chain length is increased when going t?om the diethylether ( soluble/insoluble ) to the DMF ( soluble/insoluble ) powders. Note, however, that the 740 and 690 cm-’ bands might be also representive for some sort of branching on the individual molecules. Finally, the existence of the strong band at 1140 cm -I ( appearing in ah the doped samples ) is indicative for doped PANI samples, as this vibration is correlated to some sort of -Q=NH+=Q-and -B-W.-B- vibrations. 3.3. X.P.S. spectra of undoped and doped samples* X.P.S. spectroscopy was used to establish further evidence for differences in the oxidation state of the molecules of different fractions and to estimate the amount of branching/crosshnking, involved. The results allow the estimation of the proportion of imine nitrogens ( =N-; binding energy = 398.2 eV ), amine nitrogens ( -NH-; binding energy = 399.4 eV ) and positively charged nitrogens ( “-N-“; binding energy > 400 eV ), respectively. Note that the latter species is generally viewed as an indicator for the degree of doping of the individual molecules. Of course, this point of view has been adopeted also in this paper. However, it should be beared in mind that the latter species might also enclose charged nitrogens, resulting fkom undesired side reactions, like oxygen uptake, crosslinking and branching. Fig. 4, showing the N(ls) spectra of the undoped samples, indicate that no matter of the sample all fractions contain some amount of doped material [ usually < 10 %; in case of the methanol ( soluble/insoluble ) samples ca. 20 %, which probably results corn an insutTicent undoping procedure, as it was used in the course of the PANI preparation 1, On the other hand, adding the fraction of “doped” material residues to the fraction of imine nitrogens ( because the imine nitrogens are the preferred species, transfered to positive nitrogens upon doping ) gives a quite straightforward indication for the increase in the oxidation state when going from the smaller molecules [ diethylether/benzene ( soluble/insoluble ) ] to the bigger ones [ DMF ( soluble/insoluble ) 1. The exact tigures are listed in Tab. 2. So, on this basis it can bc stated that all the different PANT tractions have an oxidation state between the leucoemeraldine and emeraldine oxidation state [ according to Green&Woodhead ( 5 ) 1. This point of view is further supported by the fact that the HrSOr doped samples ( Fig. 5 ) show only a medium (S)/(N)-ratio of about 0.36 ( 0.31 - 0.43 ) i.e. clearly below that one of doped polyemeraldine [ (S)/(N) = 0.5 1. 3.4. X-ray diffraction patterns of undoped and doped PANI fractions* The X-ray difiaction patterns of undoped and HzSO4doped
PANI powders show that most part of the undoped samples have got a u: see note on the end of the paper
diffraction patterns close to the one, proposed by Pouget et al. for the EBII arrangement ( i.e. main peaks at ca. 20 = 19,23 and 29 ’ ). only the very small molecules, i.e. the diethylether ( soluble ) and methanol ( soluble ) powders fail to tit this picture. Gn the other hand, upon treating the different powders with 2M HrS04 a common diffraction pattern evolves, independent of the differences in the difiaction patterns of the undoped materials. Thus, it can be stated that there must exist also some sort of correlation between the rather different diffraction patterns of the lower molecular weight species and the bigger molecules. Comparing the diffraction pattern of the doped samples with the ESII arrangement of Pouget et al., it becomes apparent that there is no correlation at all, except for one exchanges the two main peaks, centered at ca. 20 = 19 and 25 ‘, respectively. Another explanation for the differences in the difhaction patterns of both, this work and the ones of Pouget et al., might bc offered by differences in the underlying space groups. As can be deduced from Pouget’s et al.‘s main paper, these authors dealt originally with two different space groups for the ESII salts. One of those groups ( P21221) might be able to account for the X-ray diffraction patterns of the PANI powders of this work. However, although a P21221 arrangement of the doped PANT molecules is more reasonable than the one found by Pouget et al ( Pc2a ), these authors did not observe it at all. soluble
insoluble
Diethyletber
315; 584 run
315; 607nm
Methanol
314; 599 nm
319; 613 run
THF
321; 613 nm
320; 615 nm
DMF
320; 615 nm
323; 622 nm
Tab. 1: Absorption maxima of different PANJINMP solutions
Note This paper does not include any figures, because of the unreasonable restriction on the length of poster presentations, imposed by the publisher. So, interested readers should ask the authors more comprehensive versions of it, including figures. soluble
insoluble
Diethylether
0.24
0.30
Methanol
0.30
0.33
THF
0.40
0.42
DMF
0.45
0.46
Tab.2: hnine nitrogen parts of different PANI tractions
References 1.F. Lux, Polymer, in press 2. J.P. Pouget, M.E. Jozefowicz, A. J. Epstein, X. Tang and A.G.
F. Lux et al. / Synthetic Metals 69 (1995) 167-169
MacDiarmid, Macromolecules, 24 ( 1991 ) 779 3. J.P. Pouget, M. Laridjani, M.E. Jozefowicz, A. J. Epstein E.M. Scherr, and A.G. MacDiarmid, Synth. Met., 5 1 ( 1992 ) 95 4. F. Lux, Ph.D. Thesis, TU Berlin, Gernany, 1993 5. A.G. Green and A.E. Woodhead, J. Chem. Sot. Trans., 10 ( 1912) 1117
169
6. K.G. Neoh, E.T. Kang andK.L. Tan, Polymer, 34 ( 1993 ) 3921 7. Y. Cao, S. Li, Z. Xue and D. Guo, Synth. Met., 16 ( 1986 ) 305 8. W.S. Huang and A.G. MacDiarmid, Polymer, 34 ( 1993 ) 1833
ELSEVIER
Metals
69 (1995)
167-169
A new assessment of the crystalline structure of undoped and doped aniline oligomers and polymers F. aSchool of Chemistry, “Department
of Physics and Mathematics, Qeparlment
Lux' , E.J. Samuelsenb and E.T. KangC
University of Bristol, Cantock’s University of Trondheim,
of Chemical Engineering,
Close, Bristol BS8 lTS, United Kingdom
Norwegian
Institute of Technology,
National University of Singapore,
N-7034 Trondheim,
Norway
Kent Ridge, Singapore 0511
Abstract We describe here the crystalline structure of aniline oligomers and polymers, derived from the aniline dimer, paminodiphenylamine, and aniline. It is shown that while the crystalline structure of the undoped samples is generally in reasonable accord with the fmdings of Pouget et al. for their EBII-structure, especially the crystalline arrangement of the doped materials does not fit the picture, given by these authors for the ES&salt. The latter fact might be rationalized by the simple exchange of the two main peaks of the ESIJ arrangement, thus giving the P2r22r arrangement, proposed by Pouget et al. but not found in their experimental studies.
1. Introduction Polyaniline ( PANI ) is one of the few electronically conductive polymers, having successtirlly survived the basically “scientitic exploration step”. However, although some industrial companies have put a lot effort into the development of more or less sophisticated applications of this material, some of the fimdamental problems of it have not been resolved yet. The latter fact has been highlightened only recently by one of us ( 1 ). One of the still not frilly explored aspects of PANI concerns the atomic arrangement in semicrystalline PANI grades. Despite the fact that a lot of characterization work about this subject has been published in some detail by an american/Gench group [ i.e. Pouget et al. ( 2 , 3 )], the structure of PANI crystals still appears not thoroughly enough investigated, because other groups have found quite different X-my difGaction patterns for their semi-crystalline PANI grades ( e.g. 4 ). We report here on the crystalline arrangement of PANI oligomers and polymers, which have been successGrlly prepared from mixtures of the aniline dimer, paminodiphenylamine ( ADPA ), and aniline via fractionation of the respective polymerization products. It is shown that while most part of the crystalline arrangements of the undoped powders might be reasonably explained by somewhat of an EBII arrangement ( Pouget et al. ), especially the crystalline structure of the H2SO4doped salts differ considerably Gom that one, proposed for the ESII salt by these authors. A reasonable explanation for this discrepancy might be found by exchanging the two main crystalline peaks of the X-my ditTYaction patterns of Pouget et al., thus giving a pattern more close to the P2122r one, assumed by Pouget et al. in their considerations but not found experimentally in their studies.
2. Experimental Aniline oligomers and polymers were synthesized adopting the principles of the usual polymerization route to PANI. The
characterization of the resulting powders, using W, FTIR-, X-ray diffraction and X.P.S. spectroscopy, respectively, were performed using state-of-the-art equipment.
3. Results and discussion 3.1. UV-Vis-N&Spectra of undoped PANI fractionsThe UV-Vis-NIR-spectra have shown that at least two major absorption peaks ( located at ca. 320 mn and 620 mn, respectively ) exist in all spectra. The exact positions are listed in Tab. 1. As has been discussed before by different authors ( 6, 7, 8 ), the band at about 320 mn can be attributed to the x-x*-transition between benzoid rings and the band at ca. 620 mu is indicative of a charge-transfer exciton-like transition from the highest occupied levels on beruoid rings to the lowest occupied energy levels on quinoid rings, respectively. Both bands shift to higher wavelengths when the chain length of the molecules is increased. However, the absorption at ca. 620 mn is also intluenced by the oxidation state of the molecules. Thus, it can be inferred Gom the UV-spectra that an increase in chainlength takes place when going Gom the diethylether/benzene ( soluble/insoluble ) to the DMF ( soluble/insoluble ) Gactions. Furthermore, differences in the oxidation states of the individual molecules are to be expected between different fractions. Note that the shoulders in most of the spectra below 320 nm are probably due to the existence of low molecular weight residues in the samples and the shoulder in the methanol ( soluble ) fraction at ca. 430 mn probably results from the oxidative degradation of the molecules. 3.2. FTLR spectra of undoped and doped PANI fractions” The FTIR spectra have shown that the different fractions contain the main JR-features of PANI.. Most important are the bands at 1587, 1510,830,740 and 690 cm-’ ( nndoped and doped samples ) and the band at 1140 cm-’ ( doped samples ). The band at 1587 cm” is *see
0379.6779/95/$09.50
0 1995 Elsevier
SSDI 0379-6779(94)02406-O
Science
S.A. All rights
note on the end of the paper
reserved
F. LLU et al. I Synthetic Metals 69 (1995) 167-169
168
indicative of stretching vibrations in quinoid rings ( -N=Q=N- ), whereas the 1510 cm-’ vibration represent the same mode in benzoid rings ( -NH-B-NH- ). On this basis it can be concluded that all the different undoped samples have an oxidation degree lower than 0.5, i.e. the individual chains contain more bcnzoid rings than quinoid ones. On the other hand, the vibrations between 650 and 850 cm-’are indicative of C-H vibrations on 1,2,4 substituted rings, with the 830 cm” vibration the only one, indicative of neat 1,4-substituted rings. As has been discussed by Kang et al. ( 6 ), the appearence of the bands at 740 and 690 cm-’ is primarily indicative for the existence of an unproportional high fraction of monosubstituted rings, i.e. lower molecular weight species. On this basis the first evidence is established for the fact that the chain length is increased when going t?om the diethylether ( soluble/insoluble ) to the DMF ( soluble/insoluble ) powders. Note, however, that the 740 and 690 cm-’ bands might be also representive for some sort of branching on the individual molecules. Finally, the existence of the strong band at 1140 cm -I ( appearing in ah the doped samples ) is indicative for doped PANI samples, as this vibration is correlated to some sort of -Q=NH+=Q-and -B-W.-B- vibrations. 3.3. X.P.S. spectra of undoped and doped samples* X.P.S. spectroscopy was used to establish further evidence for differences in the oxidation state of the molecules of different fractions and to estimate the amount of branching/crosshnking, involved. The results allow the estimation of the proportion of imine nitrogens ( =N-; binding energy = 398.2 eV ), amine nitrogens ( -NH-; binding energy = 399.4 eV ) and positively charged nitrogens ( “-N-“; binding energy > 400 eV ), respectively. Note that the latter species is generally viewed as an indicator for the degree of doping of the individual molecules. Of course, this point of view has been adopeted also in this paper. However, it should be beared in mind that the latter species might also enclose charged nitrogens, resulting fkom undesired side reactions, like oxygen uptake, crosslinking and branching. Fig. 4, showing the N(ls) spectra of the undoped samples, indicate that no matter of the sample all fractions contain some amount of doped material [ usually < 10 %; in case of the methanol ( soluble/insoluble ) samples ca. 20 %, which probably results corn an insutTicent undoping procedure, as it was used in the course of the PANI preparation 1, On the other hand, adding the fraction of “doped” material residues to the fraction of imine nitrogens ( because the imine nitrogens are the preferred species, transfered to positive nitrogens upon doping ) gives a quite straightforward indication for the increase in the oxidation state when going from the smaller molecules [ diethylether/benzene ( soluble/insoluble ) ] to the bigger ones [ DMF ( soluble/insoluble ) 1. The exact tigures are listed in Tab. 2. So, on this basis it can bc stated that all the different PANT tractions have an oxidation state between the leucoemeraldine and emeraldine oxidation state [ according to Green&Woodhead ( 5 ) 1. This point of view is further supported by the fact that the HrSOr doped samples ( Fig. 5 ) show only a medium (S)/(N)-ratio of about 0.36 ( 0.31 - 0.43 ) i.e. clearly below that one of doped polyemeraldine [ (S)/(N) = 0.5 1. 3.4. X-ray diffraction patterns of undoped and doped PANI fractions* The X-ray difiaction patterns of undoped and HzSO4doped
PANI powders show that most part of the undoped samples have got a u: see note on the end of the paper
diffraction patterns close to the one, proposed by Pouget et al. for the EBII arrangement ( i.e. main peaks at ca. 20 = 19,23 and 29 ’ ). only the very small molecules, i.e. the diethylether ( soluble ) and methanol ( soluble ) powders fail to tit this picture. Gn the other hand, upon treating the different powders with 2M HrS04 a common diffraction pattern evolves, independent of the differences in the difiaction patterns of the undoped materials. Thus, it can be stated that there must exist also some sort of correlation between the rather different diffraction patterns of the lower molecular weight species and the bigger molecules. Comparing the diffraction pattern of the doped samples with the ESII arrangement of Pouget et al., it becomes apparent that there is no correlation at all, except for one exchanges the two main peaks, centered at ca. 20 = 19 and 25 ‘, respectively. Another explanation for the differences in the difhaction patterns of both, this work and the ones of Pouget et al., might bc offered by differences in the underlying space groups. As can be deduced from Pouget’s et al.‘s main paper, these authors dealt originally with two different space groups for the ESII salts. One of those groups ( P21221) might be able to account for the X-ray diffraction patterns of the PANI powders of this work. However, although a P21221 arrangement of the doped PANT molecules is more reasonable than the one found by Pouget et al ( Pc2a ), these authors did not observe it at all. soluble
insoluble
Diethyletber
315; 584 run
315; 607nm
Methanol
314; 599 nm
319; 613 run
THF
321; 613 nm
320; 615 nm
DMF
320; 615 nm
323; 622 nm
Tab. 1: Absorption maxima of different PANJINMP solutions
Note This paper does not include any figures, because of the unreasonable restriction on the length of poster presentations, imposed by the publisher. So, interested readers should ask the authors more comprehensive versions of it, including figures. soluble
insoluble
Diethylether
0.24
0.30
Methanol
0.30
0.33
THF
0.40
0.42
DMF
0.45
0.46
Tab.2: hnine nitrogen parts of different PANI tractions
References 1.F. Lux, Polymer, in press 2. J.P. Pouget, M.E. Jozefowicz, A. J. Epstein, X. Tang and A.G.
F. Lux et al. / Synthetic Metals 69 (1995) 167-169
MacDiarmid, Macromolecules, 24 ( 1991 ) 779 3. J.P. Pouget, M. Laridjani, M.E. Jozefowicz, A. J. Epstein E.M. Scherr, and A.G. MacDiarmid, Synth. Met., 5 1 ( 1992 ) 95 4. F. Lux, Ph.D. Thesis, TU Berlin, Gernany, 1993 5. A.G. Green and A.E. Woodhead, J. Chem. Sot. Trans., 10 ( 1912) 1117
169
6. K.G. Neoh, E.T. Kang andK.L. Tan, Polymer, 34 ( 1993 ) 3921 7. Y. Cao, S. Li, Z. Xue and D. Guo, Synth. Met., 16 ( 1986 ) 305 8. W.S. Huang and A.G. MacDiarmid, Polymer, 34 ( 1993 ) 1833