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Inelastic neutron scattering study of proton dynamics in polyanilines
N u Tfi."IK==irlIC ImTCRILS ELSEVIER
Synthetic Metals 81 (1996) 211--214
Inelastic neutron scattering study of proton dynamics in polyanilines R. Baddour-Hadjean a,., F. Fillaux ~, Ph. Colomban ~, A. Gruger ~, A. Regis a, S.F. Parker b,
L.T. Yu ° Laboratoire de Spectrochimie Infrarouge et Raman, Centre National de la Recherche Scientifique, 2-8 rue Henri Dunant, 94320 Thiais, France b Isis Facility, RutherfordAppleton Laboratory, Chilton, Didcot 0 X l l OQX, UK ° Laboratoire d'Electrochimie, Catalyse et Synth~se Organique, Centre National de la Recherche Scientifique, 2-8 rue Henri Dunant, 94320 Thiais, France
Abstract
The inelastic neutron scattering (INS) spectra between 16 and 4000 cm -I of chemically prepared non-doped (PANI 2A) and 50% protonated (PANI 2S) polyaniline at 20 K are reported. Band frequencies are compared to those estimated from the IR reftectivity. Different types of protons are distinguished in the spectra: (i) For the 'water-containing' 2S sample, INS features are consistent with the presence of a crystalline arrangement of the H20 network within the PANI lattice. (ii) The spectra of the 2S and 2A samples are dominated by bands assigned to vibrations of the benzoic and quinoidal protons. (iii) A continuum of intensity, underneath the bands of the aromatic protons, is tentatively attributed to the recoil of 'quasi-free' protons. These protons could be of great relevance to the explanation of the correlation between the protonation degree and the electronic and protonic conductivity. Keywords: Polyaniline;Spectroscopy;Protondynamics; Inelasticneutron scattering
1. Introduction Polyaniline (PANI) has attracted much attention as one of the most promising conducting polymers [ 1 ]. The most spectacular property of this compound lies in the 'insulator-toconductor' transition which occurs as a function of the protonation level [2-4]. The base form of PANI may be described with the general formula: [ ( - B - N H - B - N - H - ) r - - - ( - B - N = Q = N - ) 1- r ] n reduced unit
( 1)
oxidized unit
where B and Q refer to the C6H4 rings in the benzoic and quinoidal forms, respectively. The average oxidation state is given by the parameter 1 - y. Parameter y can be varied from y = 1 (leucoemeraldine base) to y = 0 (pernigraniline base). For y = 0.5, the material is referred to as 'emeraldine base' (PAN] 2A). PANI is unique among the known conducting polymers in that neutral insulating forms such as PANI 2A can be converted into conductive PAN[ 2S by treatment in non-oxidizing acidic solutions. A drastic increase of electronic conductivity is observed after protonation (typically from 10 - s up to 1-100 S / c m ) . Furthermore, conductivity increases when water molecules are absorbed by the polymer. * Correspondingauthor. E-mail:[email protected] 0379-6779/96/$15.00 © 1996 ElsevierScienceS.A. All rights reserved PHS0379-6779 (96) 03750-2
In spite of these essential characteristics, the role of the protonic species on the transport properties of PANI remains largely unknown. Several theoretical studies, based upon the existence ofpolaronic and/or bipolaronic species [5,6], have been proposed to describe the conductivity mechanism in PANI. However, these molecular models lack experimental evidence. Previous IR and Raman studies remained essentially limited to the spectroscopic fingerprint of different forms of PANI [7-10]. Some of us have recently investigated the IR and Raman spectra of PANI 2S and its ---CrD 4- and - N D deuterated derivatives [ 11-14]. Some unusual features have been observed in the IR spectra of KBr pellets: sharp transmission windows. These features were first questioned as Evans or Fano profiles and then re-assigned to multiple reflection effects which were further analysed using a KramersKrrnig treatment [ 14]. Therefore, this technique only provides information limited to the sample surfaces and, in addition, is difficult to analyse. Raman spectroscopy, on the other hand, is hampered by resonance due to the manifold of electronic absorptions from U'V to near-IR. Inelastic neutron scattering (INS) is the only technique which may pro+tide clear information on the proton dynamics in PANI. Because the scattering cross section of hydrogen atoms is about ten times greater than that for C and N atoms, the spectra should be dominated by signals due to proton
212
R. Baddour-Hadjean et al. / Synthetic Metals 81 (1996) 211-214
motions, and totally free of the side effects (reflection, resonance) encountered with optical techniques. This proton selectivity can be further exploited because deuterium atoms have a very much smaller cross section than protons: selective deuteration of the protons bound to the conjugated rings allow for a detailed view of the dynamics of those protons, supposed to be bound to N atoms, and those added by protonation. Therefore, INS provides simpler spectra which can be analysed with greater confidence. However, previous INS studies of PANI are very few and limited by energy-transfer range and resolution [ 15,16]. We have thus undertaken an INS study of PANI and its ring-deuterated analogues. In the present paper, we present the INS spectra of hydrogenated emeraldine base (2A) and the 50% protonated salt (2S), with two hydration degrees. Kramers-Krtnig spectra (g' = f(v) ) calculated from reflectivity measurements are also examined. A full band assignment scheme is given. INS band splitting is correlated to ring electronic modifications between the 2S and 2A forms. Finally, a broad continuum is observed for both samples, which is tentatively assigned to the recoil of 'nearly free' protons.
2. Experimental
The INS spectra were obtained with the TFXA spectrometer (ISIS, Rutherford Appleton Laboratory, Chilton, UK). This spectrometer has excellent resolution (Ao)/~o< 2%). About 5 g of each sample were wrapped in aluminium foil and loaded in a cryostat at about 20 K. The spectra were normalized to the weight of sample in the beam. The 2S sample was chemically prepared from solution in sulfuric acid, according to Ref. [ 17]. The 2A material was obtained by treatment of the 2S salt with aqueous ammonia. Two samples of 2S salts, differing by their hydration level, were studied: one dried under static vacuum until a 'dry-looking' powder (about 60% water content in weight) was obtained, the other dried under dynamic vacuum until constant weight. These samples are referred to as 2Shydand 2S, respectively. The dielectric functions d' of PANI pellets were recorded on a Fourier transform ILS 113 Blqiker spectrometer. These spectra were treated by the standard Kramers-Krtnig method.
3. Results and discussion
3.1. Hydrated species in 2S salts The INS spectrum of the 'water containing' 2Shy~sample is shown in Fig. 1. Assignments are given in Table 1. This spectrum is dominated by bands due to a rather well-crystallized form of water, similar to ice Ih [ 18,19]. In the translational region (below 400 era- ~), the acoustic modes give a very sharp peak at about 56 cm- ~with a very weak shoulder
4
go
o= 2
0
1
0
I
1000
i
I
2000
i
I
3000
4000
Energy Transfer (cm"1) Fig. 1. INS spectrumof hydratedPANI2S,~asalt (dried under staticvacuum) at about20 K. The dashedlinedefinesthe continuumof intensity. at about 102 cm -1. The weaker peaks at 141,230 and 307 cm-1 have been attributed to O'..O stretching modes [ 19]. All these bands have counterparts in the IR and Raman spectra of ice Ih. At higher frequency, the librational modes of water molecules are observed between 500 and 1000 cm-i. The broad band at about 1600 cm-1 is attributed to the internal bending mode of water molecules. Ill-defined broad bands above 2000 cm- 1 are due to overtones and combinations. The stretching modes are barely visible at about 3300 cm- 1. Such features indicate that, at 20 K, there is a crystalline arrangement of the hydrated species within the static dried 2S lattice. This is in line with the higher correlation length deduced from X-ray Bragg peaks recorded at room temperature for static-dried powder [ 12]. In contrast to pure ice Ih, the INS spectrum of 2Shyareveals several bands superimposed on the ice-like spectrum, at about 416, 521,830, 890 and 960 cm-I, and beneath an important continuum of intensity (dashed line). These features correspond to the vibrational fingerprint of PANI protons, which is discussed in the next section.
3.2. Vibrational band assignment of aromatic rings The INS spectra of PANI 2S and 2A are presented in Figs. 2 and 3, respectively. Band assignments in terms of group vibrations, including INS and IR frequencies, are given in Table 1, using the Wilson notation for aromatic rings [20]. The INS spectra of 2S and 2A are comparable (see Figs. 2 and 3): below 400 cm -], several bands are observed for the 2S sample at about 206, 284 and 350 cm-1, which correspond to the vibrations of the polymer backbone. These modes are barely visible in the INS spectrum of the 2A sample, which is presumably related to the amorphous character of this compound, as evidenced by X-ray diffraction [ 12]. Above 400 cm-l, the spectra reveal intense bands, which correspond to various aromatic proton modes. There is a quite good accordance between the INS and IR frequencies observed for both PANI 2A and 2S (see Table 1). In INS, there are no symmetry-based selection rules and only modes involving proton displacements have significant intensities.
R. Baddour-Hadjean et al. / Synthetic Metals 81 (1996) 211-214
213
Table 1 IR reflectivity (at 300 K) and INS (at about 20 K) band frequencies (in cm -1) and assignments a for hydrogenated PANI 2S and 2A and their --C6D~derivatives, (D)2S and (D)2A, respectively 2Shyd
2S
INS
IR
INS
56vs 102sh 141b 206vw 230m 284vw 307m 350vw 416b 521m 600-1000vs
415b 518s 600b
350w 420s 522s 606m
710sh
714vw
718m
830sh 890sh
806s-824s 883m
830s 890sh
(D)2S
2A
IR
IR
102 141vw 206w
157s
(D)2A INS
Assignments
IR
102b 138w
284w 358w 415s 507s
422s 506s-533s 600w-644w
627m 773vw 822m
630 715vw
717w
832s
819s-835s 866sh, 891sh
770vw 820m
863s 960m
~ 1000vw
972s
1179vs
1177s
1259m 1315s 1339sh
1288s
1500m 1587s-1611s
1500m 1580m 3100m
859m ~ 1000vw
1137w ~ 1260vs
1113s-1166s 1219m-1237m
958s 1014sh 1135s-1182s 1238vw
I127w
1316s
1317m
1287vs
t380w 1508vs 1591s
1394w 1480w
1332sh 1414vs-1471s 1568vs
1355s 1415s 1559-1575s
3100m
T(H20) + "tC~t--I5 T(H20) ext. mode T(H20) ext. mode T(H20) ext. mode 7CC, 16a 3,CC,16b H20 + 8CC,6a 3CD, 11 3~C,4 TCD,17a 3CH, 11 + ~5CC,1 ~CD,9a )CH,17a 3CH,18a 8CH,9a vB-N vB'-N 3,14 vSQ-N vQ-N vCC,19a vCC,Sa vCH
a vs: very strong; s: strong; m: medium; w: weak; vw: very weak; b: broad; sh: shoulder; T: translation; ~-: torsion; % & out-of-plane, in-plane deformations, respectively; v: stretching.
x¢4 _/
0
I
0
r
T
1
2000 3000 Energy Transfer (crn 1)
I000
0
4000
Fig. 2. INS spectrum of PANI 2S salt (dried under dynamic vacuum up to constant weight) at about 20 K. The dashed line defines the continuum of intensity.
For PAN[ 2S, intense INS bands are observed for the bending CH modes (11, 17a, 9a at about 830, 970 and 1180 cm -t, respectively) and for the stretching vCH mode at about 3100 cm- 1. However, significant intensities are also observed for the bending CC modes (16a, 16b, 6a, 4) and stretching CC and CN modes between 1200 and 1600 cm- 1, because these
I
0
I
r
P
1000 2000 3000 Energy Transfer (cm "1)
4000
Fig. 3. INS spectrum of PANI 2A at about 20 K. The dashed line defines the continuum of intensity.
modes produce large motions of the neighbouring protons due to a riding action. The INS spectrum of the 2A material also exhibits intense features for the aromatic CH vibrations. Although the band frequencies are quite similar, several INS band splittings are observed for the 2A sample (see Fig. 3). The 7CC,16b and g>CH,9a modes split into two components at about 506-533
214
R. Baddour-Hadjean et aL / Synthetic Metals 81 (1996) 211-214
and 1135-1182 c m - l, respectively, the 3CH, 11 mode into four components a+ound about 830 c m - 1. This band splitting is not observed for the 2S sample (see Fig. 2). This arises presumably from the disordered character of PANI 2S, which can have different origins: (i) from the electronic point of view, PANI 2S is usually described as a heterogeneous material made of conducting islands dispersed in a low conducting medium (intermediate between 2S and 2A forms); (ii) from the crystallographic point of view, PANI 2S is described as a material made of crystalline regions (about 100 × 50 × 20 A3) dispersed in an amorphous medium [12]. However, correlations between the two descriptions are not straightforward. The fact that the 2A material seems spectroscopically more ordered suggests a dynamical disorder between quinoidal and semi-quinoidal species in the 2S material. The relative INS intensity of the bending CC modes between 400 and 750 c m - 1 is comparable for both samples, whereas that of the stretching CN and CC modes has significantly decreased for PANI 2A (the band at 1316 cm -~ in the 2A INS spectrum corresponds to a protonic mode, 6CH,3). This probably comes from the different nature of the riding protons: aromatic protons are involved in the bending CC motions, whereas protons added by protonation are those involved in the CN and CC stretching modes. As a result, the splitting of the ~,CN mode is well evidenced in the INS spectrum of PANI 2S: there are two INS components, at 1288 and t355 cm -1, which correspond to the benzoic and semi-quinoidal counterparts, v B ' - N and ~,SQ-N respectively. For PANI 2A, comparison with the IR spectrum allows us to assign the two CN stretching frequencies: v B - N and v Q - N at about 1238 and 1394 c m - l , respectively. It comes out that the 2A frequency splitting for the pnng-N mode, A v ~ 150 c m - ~, is lowered to 70 c m - ~ for PANI 2S, which reflects the greater similarity of benzoic and semi-quinoidal rings in the latter material. The INS spectra of 2S and 2A PANIs reveal an important continuum of intensity below the bands due to bound oscillators (see Figs. 2 and 3), which suggests that mechanically mobile protons recoil. It is worthwhile stressing that proton recoil cannot be observed with optical techniques. The INS spectra of the perdeuterated -C6D 4- derivatives clearly demonstrate the existence of this continuum [ 21]. Therefore, the existence of free protons analogous to those previously observed in 7MnO2 [221, coals [23], more or less dehydrated tungstophosphoric acids [24] and hydrated H-/3A1203 [25] can be speculated. This supposes a large delocalization of the companion electrons. This could be a straightforward explanation for the remarkable correlation between the protonation degree and the electronic conductivity. Furthermore,
these mobile protons could play an important role in the proton conductivity as straightforward charge carriers. These preliminary conclusions deserve further studies. Analysis of the INS spectra of partially deuterated PANIs should provide a better description of the mobile proton dynamics.
References [ 1] E.M. Geni~s, A. Boyle, M. Lapkowski and C. Tsintavis, Synth. Met., 36 (1990) 139. [2] J.P. Travers, J. Chroboczek,F. Devreux, F. Genoud, M. Nechtschein, A.A. Syed,E.M. Geni~s and C. Tsintavis, MoL Cryst., Liq. Cryst., 121 (1985) 195. [3] A.G. MacDiarmid, J.C. Chiang, M. Halpern, W.S. Huang, S.L. Mu, N.L.D. Somasiri, W. Wu and S.I. Yaniger, Mol. Cryst., Liq. Cryst., 121 (1985) 173. [4] AJ. Epstein, J.M. Grinder, F. Zuo, R.W. Bigelon, H.S. Woo, D.B. Tanner, A.F. Richter, W.S. Huang and A.G. MacDiarmid,Synth, Met., 18 (1987) 303. [5] M.N. Bussac and L. Zuppiroli, Phys. Rev. B, 49 (1994) 5876. [6] M.N. Bussacand L. Zuppiroli, Synth. Met., 69-71 (1995) 693. [7] M. Ohira, T. Sakai, M. Takeushi, Y. Kabayashiand M. Tsuyi, Synth. Met., 18 (1987) 347. [8] I. Harada, Y. Furukawaand F. Veda, Synth. Met., 29 (1989) E303. [9] S. Quillard, G. Louarn, J.P. Buisson, S. Lefrant, J. Masters and A.G. MacDiarmid, Synth. Met., 49-50 (1992) 525. [ 10] T. Kukuda,H. Takezoe, K. Ishikawaand A. Kukuda,Synth. Met., 6971 (1995) 247. [ 11] Ph. Colomban,A. Gruger,A. Novak and A. Regis,Z Mol. Struct., 317 (1994) 261. 112] Ph. Colomban, S. Folch, A. Gruger, A. Regis and D. Michel, C.R. Acad. Sci. Paris, t322, Sea lib (1996) 63. [ 13] A. Gruger,A. Novak,A. Regis and Ph. Colomban,J. Mol. Struct., 328 (1994) 153. [ 14] Ph. Colomban, A. Gruger and A. Regis, C.R. Acad. Sci. Paris, Ser. lib, 321 (1995) 247. [ 15] J.L. Sauvagot, D. Djurado, A.J. Dianoux,J.E. Fisher, E.M. Scherrand A.G. MacDiarmid,Phys. Rev. t3, 47 (1993) 4959. [ 16] K. Prassides, C.J. Bell, A.J. Dianoux, C.G. Wu and M.G. Kanatzidis, Physica B, 180--181 (1992) 668. [ 17] C. Fite, Y. Cao and A.J. Heeger,SolidState Commun., 70 (1989) 245. [ t8] J.C. Li, D.K. Ross, L. Howe, P.G. Hall and J. Tomldnson,Physica B, 156-157 (1989) 376. [ 19] J.C. Li, J.D. Londono,D.K. Ross,J.L, Finney,J. Tomkinsonand W.F. Sherman, J. Chem. Phys., 94 (1991) 6770. [20] G. Varsanyi, Assignments for Vibrational Spectra of Benzene Derivatives, Vol. 1, Adam Hilger, Bristol, 1987. [21] R. Baddour-Hadjeanet al., in preparation. [22] F. Fillaux, H. Ouboumour,J. Tomkinson and L.T. Yu, Chem. Phys., 149 (1991) 459. [23] F. Fillaux, R. Papoular, A. Laurie and J. Tomkinson, Carbon, 32 (1994) 1325. [24] U.B. Mioc, Ph. Colomban, M. Davidovic and J. Tomkinson, J. MoL Struct., 326 (1994) 99. [25] Ph. Colomban,F. Fillaux, J. Tomkinson and G.J. Kearley,SolidState lonics, 77 (1995) 45.
Synthetic Metals 81 (1996) 211--214
Inelastic neutron scattering study of proton dynamics in polyanilines R. Baddour-Hadjean a,., F. Fillaux ~, Ph. Colomban ~, A. Gruger ~, A. Regis a, S.F. Parker b,
L.T. Yu ° Laboratoire de Spectrochimie Infrarouge et Raman, Centre National de la Recherche Scientifique, 2-8 rue Henri Dunant, 94320 Thiais, France b Isis Facility, RutherfordAppleton Laboratory, Chilton, Didcot 0 X l l OQX, UK ° Laboratoire d'Electrochimie, Catalyse et Synth~se Organique, Centre National de la Recherche Scientifique, 2-8 rue Henri Dunant, 94320 Thiais, France
Abstract
The inelastic neutron scattering (INS) spectra between 16 and 4000 cm -I of chemically prepared non-doped (PANI 2A) and 50% protonated (PANI 2S) polyaniline at 20 K are reported. Band frequencies are compared to those estimated from the IR reftectivity. Different types of protons are distinguished in the spectra: (i) For the 'water-containing' 2S sample, INS features are consistent with the presence of a crystalline arrangement of the H20 network within the PANI lattice. (ii) The spectra of the 2S and 2A samples are dominated by bands assigned to vibrations of the benzoic and quinoidal protons. (iii) A continuum of intensity, underneath the bands of the aromatic protons, is tentatively attributed to the recoil of 'quasi-free' protons. These protons could be of great relevance to the explanation of the correlation between the protonation degree and the electronic and protonic conductivity. Keywords: Polyaniline;Spectroscopy;Protondynamics; Inelasticneutron scattering
1. Introduction Polyaniline (PANI) has attracted much attention as one of the most promising conducting polymers [ 1 ]. The most spectacular property of this compound lies in the 'insulator-toconductor' transition which occurs as a function of the protonation level [2-4]. The base form of PANI may be described with the general formula: [ ( - B - N H - B - N - H - ) r - - - ( - B - N = Q = N - ) 1- r ] n reduced unit
( 1)
oxidized unit
where B and Q refer to the C6H4 rings in the benzoic and quinoidal forms, respectively. The average oxidation state is given by the parameter 1 - y. Parameter y can be varied from y = 1 (leucoemeraldine base) to y = 0 (pernigraniline base). For y = 0.5, the material is referred to as 'emeraldine base' (PAN] 2A). PANI is unique among the known conducting polymers in that neutral insulating forms such as PANI 2A can be converted into conductive PAN[ 2S by treatment in non-oxidizing acidic solutions. A drastic increase of electronic conductivity is observed after protonation (typically from 10 - s up to 1-100 S / c m ) . Furthermore, conductivity increases when water molecules are absorbed by the polymer. * Correspondingauthor. E-mail:[email protected] 0379-6779/96/$15.00 © 1996 ElsevierScienceS.A. All rights reserved PHS0379-6779 (96) 03750-2
In spite of these essential characteristics, the role of the protonic species on the transport properties of PANI remains largely unknown. Several theoretical studies, based upon the existence ofpolaronic and/or bipolaronic species [5,6], have been proposed to describe the conductivity mechanism in PANI. However, these molecular models lack experimental evidence. Previous IR and Raman studies remained essentially limited to the spectroscopic fingerprint of different forms of PANI [7-10]. Some of us have recently investigated the IR and Raman spectra of PANI 2S and its ---CrD 4- and - N D deuterated derivatives [ 11-14]. Some unusual features have been observed in the IR spectra of KBr pellets: sharp transmission windows. These features were first questioned as Evans or Fano profiles and then re-assigned to multiple reflection effects which were further analysed using a KramersKrrnig treatment [ 14]. Therefore, this technique only provides information limited to the sample surfaces and, in addition, is difficult to analyse. Raman spectroscopy, on the other hand, is hampered by resonance due to the manifold of electronic absorptions from U'V to near-IR. Inelastic neutron scattering (INS) is the only technique which may pro+tide clear information on the proton dynamics in PANI. Because the scattering cross section of hydrogen atoms is about ten times greater than that for C and N atoms, the spectra should be dominated by signals due to proton
212
R. Baddour-Hadjean et al. / Synthetic Metals 81 (1996) 211-214
motions, and totally free of the side effects (reflection, resonance) encountered with optical techniques. This proton selectivity can be further exploited because deuterium atoms have a very much smaller cross section than protons: selective deuteration of the protons bound to the conjugated rings allow for a detailed view of the dynamics of those protons, supposed to be bound to N atoms, and those added by protonation. Therefore, INS provides simpler spectra which can be analysed with greater confidence. However, previous INS studies of PANI are very few and limited by energy-transfer range and resolution [ 15,16]. We have thus undertaken an INS study of PANI and its ring-deuterated analogues. In the present paper, we present the INS spectra of hydrogenated emeraldine base (2A) and the 50% protonated salt (2S), with two hydration degrees. Kramers-Krtnig spectra (g' = f(v) ) calculated from reflectivity measurements are also examined. A full band assignment scheme is given. INS band splitting is correlated to ring electronic modifications between the 2S and 2A forms. Finally, a broad continuum is observed for both samples, which is tentatively assigned to the recoil of 'nearly free' protons.
2. Experimental
The INS spectra were obtained with the TFXA spectrometer (ISIS, Rutherford Appleton Laboratory, Chilton, UK). This spectrometer has excellent resolution (Ao)/~o< 2%). About 5 g of each sample were wrapped in aluminium foil and loaded in a cryostat at about 20 K. The spectra were normalized to the weight of sample in the beam. The 2S sample was chemically prepared from solution in sulfuric acid, according to Ref. [ 17]. The 2A material was obtained by treatment of the 2S salt with aqueous ammonia. Two samples of 2S salts, differing by their hydration level, were studied: one dried under static vacuum until a 'dry-looking' powder (about 60% water content in weight) was obtained, the other dried under dynamic vacuum until constant weight. These samples are referred to as 2Shydand 2S, respectively. The dielectric functions d' of PANI pellets were recorded on a Fourier transform ILS 113 Blqiker spectrometer. These spectra were treated by the standard Kramers-Krtnig method.
3. Results and discussion
3.1. Hydrated species in 2S salts The INS spectrum of the 'water containing' 2Shy~sample is shown in Fig. 1. Assignments are given in Table 1. This spectrum is dominated by bands due to a rather well-crystallized form of water, similar to ice Ih [ 18,19]. In the translational region (below 400 era- ~), the acoustic modes give a very sharp peak at about 56 cm- ~with a very weak shoulder
4
go
o= 2
0
1
0
I
1000
i
I
2000
i
I
3000
4000
Energy Transfer (cm"1) Fig. 1. INS spectrumof hydratedPANI2S,~asalt (dried under staticvacuum) at about20 K. The dashedlinedefinesthe continuumof intensity. at about 102 cm -1. The weaker peaks at 141,230 and 307 cm-1 have been attributed to O'..O stretching modes [ 19]. All these bands have counterparts in the IR and Raman spectra of ice Ih. At higher frequency, the librational modes of water molecules are observed between 500 and 1000 cm-i. The broad band at about 1600 cm-1 is attributed to the internal bending mode of water molecules. Ill-defined broad bands above 2000 cm- 1 are due to overtones and combinations. The stretching modes are barely visible at about 3300 cm- 1. Such features indicate that, at 20 K, there is a crystalline arrangement of the hydrated species within the static dried 2S lattice. This is in line with the higher correlation length deduced from X-ray Bragg peaks recorded at room temperature for static-dried powder [ 12]. In contrast to pure ice Ih, the INS spectrum of 2Shyareveals several bands superimposed on the ice-like spectrum, at about 416, 521,830, 890 and 960 cm-I, and beneath an important continuum of intensity (dashed line). These features correspond to the vibrational fingerprint of PANI protons, which is discussed in the next section.
3.2. Vibrational band assignment of aromatic rings The INS spectra of PANI 2S and 2A are presented in Figs. 2 and 3, respectively. Band assignments in terms of group vibrations, including INS and IR frequencies, are given in Table 1, using the Wilson notation for aromatic rings [20]. The INS spectra of 2S and 2A are comparable (see Figs. 2 and 3): below 400 cm -], several bands are observed for the 2S sample at about 206, 284 and 350 cm-1, which correspond to the vibrations of the polymer backbone. These modes are barely visible in the INS spectrum of the 2A sample, which is presumably related to the amorphous character of this compound, as evidenced by X-ray diffraction [ 12]. Above 400 cm-l, the spectra reveal intense bands, which correspond to various aromatic proton modes. There is a quite good accordance between the INS and IR frequencies observed for both PANI 2A and 2S (see Table 1). In INS, there are no symmetry-based selection rules and only modes involving proton displacements have significant intensities.
R. Baddour-Hadjean et al. / Synthetic Metals 81 (1996) 211-214
213
Table 1 IR reflectivity (at 300 K) and INS (at about 20 K) band frequencies (in cm -1) and assignments a for hydrogenated PANI 2S and 2A and their --C6D~derivatives, (D)2S and (D)2A, respectively 2Shyd
2S
INS
IR
INS
56vs 102sh 141b 206vw 230m 284vw 307m 350vw 416b 521m 600-1000vs
415b 518s 600b
350w 420s 522s 606m
710sh
714vw
718m
830sh 890sh
806s-824s 883m
830s 890sh
(D)2S
2A
IR
IR
102 141vw 206w
157s
(D)2A INS
Assignments
IR
102b 138w
284w 358w 415s 507s
422s 506s-533s 600w-644w
627m 773vw 822m
630 715vw
717w
832s
819s-835s 866sh, 891sh
770vw 820m
863s 960m
~ 1000vw
972s
1179vs
1177s
1259m 1315s 1339sh
1288s
1500m 1587s-1611s
1500m 1580m 3100m
859m ~ 1000vw
1137w ~ 1260vs
1113s-1166s 1219m-1237m
958s 1014sh 1135s-1182s 1238vw
I127w
1316s
1317m
1287vs
t380w 1508vs 1591s
1394w 1480w
1332sh 1414vs-1471s 1568vs
1355s 1415s 1559-1575s
3100m
T(H20) + "tC~t--I5 T(H20) ext. mode T(H20) ext. mode T(H20) ext. mode 7CC, 16a 3,CC,16b H20 + 8CC,6a 3CD, 11 3~C,4 TCD,17a 3CH, 11 + ~5CC,1 ~CD,9a )CH,17a 3CH,18a 8CH,9a vB-N vB'-N 3,14 vSQ-N vQ-N vCC,19a vCC,Sa vCH
a vs: very strong; s: strong; m: medium; w: weak; vw: very weak; b: broad; sh: shoulder; T: translation; ~-: torsion; % & out-of-plane, in-plane deformations, respectively; v: stretching.
x¢4 _/
0
I
0
r
T
1
2000 3000 Energy Transfer (crn 1)
I000
0
4000
Fig. 2. INS spectrum of PANI 2S salt (dried under dynamic vacuum up to constant weight) at about 20 K. The dashed line defines the continuum of intensity.
For PAN[ 2S, intense INS bands are observed for the bending CH modes (11, 17a, 9a at about 830, 970 and 1180 cm -t, respectively) and for the stretching vCH mode at about 3100 cm- 1. However, significant intensities are also observed for the bending CC modes (16a, 16b, 6a, 4) and stretching CC and CN modes between 1200 and 1600 cm- 1, because these
I
0
I
r
P
1000 2000 3000 Energy Transfer (cm "1)
4000
Fig. 3. INS spectrum of PANI 2A at about 20 K. The dashed line defines the continuum of intensity.
modes produce large motions of the neighbouring protons due to a riding action. The INS spectrum of the 2A material also exhibits intense features for the aromatic CH vibrations. Although the band frequencies are quite similar, several INS band splittings are observed for the 2A sample (see Fig. 3). The 7CC,16b and g>CH,9a modes split into two components at about 506-533
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R. Baddour-Hadjean et aL / Synthetic Metals 81 (1996) 211-214
and 1135-1182 c m - l, respectively, the 3CH, 11 mode into four components a+ound about 830 c m - 1. This band splitting is not observed for the 2S sample (see Fig. 2). This arises presumably from the disordered character of PANI 2S, which can have different origins: (i) from the electronic point of view, PANI 2S is usually described as a heterogeneous material made of conducting islands dispersed in a low conducting medium (intermediate between 2S and 2A forms); (ii) from the crystallographic point of view, PANI 2S is described as a material made of crystalline regions (about 100 × 50 × 20 A3) dispersed in an amorphous medium [12]. However, correlations between the two descriptions are not straightforward. The fact that the 2A material seems spectroscopically more ordered suggests a dynamical disorder between quinoidal and semi-quinoidal species in the 2S material. The relative INS intensity of the bending CC modes between 400 and 750 c m - 1 is comparable for both samples, whereas that of the stretching CN and CC modes has significantly decreased for PANI 2A (the band at 1316 cm -~ in the 2A INS spectrum corresponds to a protonic mode, 6CH,3). This probably comes from the different nature of the riding protons: aromatic protons are involved in the bending CC motions, whereas protons added by protonation are those involved in the CN and CC stretching modes. As a result, the splitting of the ~,CN mode is well evidenced in the INS spectrum of PANI 2S: there are two INS components, at 1288 and t355 cm -1, which correspond to the benzoic and semi-quinoidal counterparts, v B ' - N and ~,SQ-N respectively. For PANI 2A, comparison with the IR spectrum allows us to assign the two CN stretching frequencies: v B - N and v Q - N at about 1238 and 1394 c m - l , respectively. It comes out that the 2A frequency splitting for the pnng-N mode, A v ~ 150 c m - ~, is lowered to 70 c m - ~ for PANI 2S, which reflects the greater similarity of benzoic and semi-quinoidal rings in the latter material. The INS spectra of 2S and 2A PANIs reveal an important continuum of intensity below the bands due to bound oscillators (see Figs. 2 and 3), which suggests that mechanically mobile protons recoil. It is worthwhile stressing that proton recoil cannot be observed with optical techniques. The INS spectra of the perdeuterated -C6D 4- derivatives clearly demonstrate the existence of this continuum [ 21]. Therefore, the existence of free protons analogous to those previously observed in 7MnO2 [221, coals [23], more or less dehydrated tungstophosphoric acids [24] and hydrated H-/3A1203 [25] can be speculated. This supposes a large delocalization of the companion electrons. This could be a straightforward explanation for the remarkable correlation between the protonation degree and the electronic conductivity. Furthermore,
these mobile protons could play an important role in the proton conductivity as straightforward charge carriers. These preliminary conclusions deserve further studies. Analysis of the INS spectra of partially deuterated PANIs should provide a better description of the mobile proton dynamics.
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