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Infrared and Raman characterization of polyethylene oxide complexes of sulfamide
SOLID STATE
Solid State lonics 61 (1993) 219-225 North-Holland
IONICS Infrared and Raman characterization of polyethylene oxide complexes of sulfamide V. De Zea Bermudez, G. Lucazeau, L. Abello, C. Poinsignon, J.Y. Sanchez and M. Armand Laboratoire d'lonique et d'Electrochimie du Solide de Grenoble, ENSEEG/INPG, Domaine Universitaire, BP 75, 38402 St. Martin d'Hkres, France
The Raman and infrared spectra of poly(ethylene oxide): PEO, and PEO-based complexes of sulfamide of general formula P(EO)nNH2SO2NH2 (where n=O/S stands as the ratio of monomer units per sulfamide molecule) with compositions equal to n = 2, 2.5, 4, 5, 20 and 30 have been recorded at 300 K in the 4000-100 cm-~ range. The evolution of sulfamide-polymer interactions with salt concentration is studied mainly by analysing the spectral features of the NH and -C-C-O stretching region. For compositions n = 4 and 5 splittings of the NH Raman bands indicate the existence of definite interactions between sulfamide and PEO. For the more diluted samples, the behaviour of the salt traduces the absence of specific interactions and the complete orientational disorder accounts for the weak character of the hydrogen bonds, a situation also encountered in sulfamide solutions of tetraethyleneglycoldimethylether(tetraglyme) and in molten sulfamide. In situ micro Raman technique allows to evidence the coexistence of phases for n = 2. The influence of the substrate and of the thickness of the films on its structure is presented. The Raman spectra of compositions n = 4 and 5 have shown that doping with a deprotonating agent results in increasing the disorder of the materials, for n = 2 instead of a mixture of phases, one obtains a homogeneous amorphous phase.
1. Introduction A i m i n g at their use in all-solid state electrochemical devices, basic to neutral proton conducting polymers are systems more adapted than the corresponding acidic electrolytes [ 1 - 5 ] , especially in the presence of insertion materials such as oxides and hydroxides, whose stability d o m a i n ranges from p H = 4 to p H = 12. Owing to the fact that the sulfonamide group ( SO2NH2) exhibits PKa's o f about l0 (corresponding to the equilibrium RSOENH2c>H + + [ RSOENH ] - ), we have recently proposed [6] that sulfamide-type salts constitute ideal cases, since they can play the role o f alcaline bases which do not form strong complexes with PEO, the favoured host-polymer in studies o f conductivity in polymer electrolytes. We have thus introduced a series o f so-called proton-vacancy conducting polymers based on a three c o m p o n e n t s system: a solvating polymer ( P O E ) , a proton source (sulfamide, NH2SO2NH2) and a deprotonating agent or proton-vacancy inducer (guanid i n i u m cation) [6]. The phase diagram established for the
P (EO)nNH2SO2NH2 binary system on the basis o f A T D techniques [6] (where n = O / S stands as the ratio o f m o n o m e r units per sulfamide molecule) indicates the presence o f at least three eutectics (found for compositions somewhere around n = 2, 4 and 30) and three intermediate crystalline compounds (found for compositions somewhere around n=2.5, 5 and 2O). At room temperature and in the absence o f dopant, the eutectic compositions exhibit good conductivities which may be explained on the basis o f sulfamide self-protonation, most probably followed by a Grotthus-type conduction mechanism associated with the segmental motion o f the polymeric chain, which plays the role of charge carrier. 2H2NSO2NH2 [ H2 NSO2 NH3 ] + + [ H2 NSO2 N H ] - .
(1)
Hence, under such conditions, conduction is ensured by the presence of both protons and protonvacancies.
In the presence o f the deprotonating agent, conductivity is greatly enhanced, being now solely due to
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14 De Zea Bermudez et al. / Polyethylene oxtde comph'xes qfsul/'arnide
the motion o f proton-vacancies. For a doping level (represented by the ratio N / H of guanidine molecules a d d e d per extracted proton ) o f 5% remarkable room temperature conductivities higher than I 0-- 5 ~-- 1 c m - ~ result. Nevertheless, the samples are in a metastable a m o r p h o u s state and slow crystallization kinetics make them evolve with time, being responsible for a drastic decrease of conductivity after storage (6 m o n t h s ) , for it is accepted that conduction in semi-crystalline polymers takes place exclusively in the elastomeric phase [8]. This unavoidable room t e m p e r a t u r e crystallinity typical o f many PEO-based systems leads to the heterogeneous character of these materials. In the first part o f this paper the influence of salt concentration on the PEO-sulfamide interactions will be discussed, the effect o f the thickness o f the sampies, as well as o f shaping on the degree o f crystallinity will be analysed in the second part. Finally, the third part is devoted to the study of the consequences of doping on the structure o f the complexes. The discussion that follows is mostly based on two works presented previously dealing with spectroscopical studies of crystalline and molten sulfamide [7,9] and o f a tetraglyme-based solution o f the same salt [ 10]. It was concluded that while in crystalline sulfamide, the protons of each amine group were found to be non equivalent, the spectrum of molten sulfamide and that of a solution of this salt in tetraglyme traduces the absence o f specific interactions, the weak character of the hydrogen bonds in both liquid and melt.
2. Experimental
and polymer-sulfamide complexes on silicon plates. Evaporation of the solvent was carried out in the open air. 2.2. Infrared spectroscopy Mid-infrared spectra were acquired at room temperature using a Nicolet 710 F T I R system coupled to a microcomputer. The spectra were collected over the range 4000-400 c m - 1 by averaging 200 scans at a m a x i m u m resolution of 4 c m J. Samples were analysed as thin films in transmission mode. 2.3. R a m a n spectroscopy Data were collected at room temperature on a XY.Dilor (Lille, France) multichannel spectrometer over the 4 0 0 0 - 5 0 c m 1. The 488 nm line o f an argon laser light source was used. Samples were illuminated with 10-20 m W through the objective o f a microscope, which also serves to collect the R a m a n light, and were analysed as both thick and thin films. The materials under study were found to exhibit rather high levels of fluorescence (most probably connected not only to the presence of antioxidants in PEO, but also to ageing of the latter), leading to a very, poor signal/noise ratio. This drawback was partly overcome by systematically submitting the samples to the influence o f the laser beam for about a m i n i m u m o f 15 min prior to recording o f the spectra. Such procedure allowed to considerably improve the quality of the spectral signature, without increasing significantly the temperature of the sample: moreover, the point irradiated by the intense light source was not damaged as a result o f either the pretreatment or the experiment itself.
2. I. Synthesis o f electrolytes High molecular weight ( 9 × 105) PEO was supplied by Aldrich. Sulfamide, NH2SO2NH2, was commercially available at Janssen. Both products were used without further purification. Synthesis o f electrolytes, ac conductivity differential calorimetry' measurements are performed according to the procedure given previously [6]. U n d o p e d and doped thick films (40 to 140 ~tm thick) and thin films (approximately 5-10 ~m thick) were prepared by casting acetonitrile solutions of uncomplexed polymer
3. Results and discussion 3.1. Phases identlJTcation The evolution of the NH stretching bands with sulfamide concentration for thick samples of the P ( E O ) ~N H2SOzNH2 system at room temperature is presented in fig. 1; compositions vary from 2, 2.5.4, 5 to 20. Undoubtedly, the NH stretching modes undergo
V. De Zea Bermudez et al. /Polyethylene oxide complexes of sulfamide
3336 il~ NH2SO2NH2 f! : 1 3324
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O/S=2
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3500 3300 cm q Fig. 1. R o o m temperature NH stretching region R a m a n spectra of sulfamide and of P ( E O ) , N H 2 S O 2 N H 2 complexes of compositions n = 2 , 2.5.4, 5, 20.
many changes all through the whole series. However, three main types of behaviour may be easily identified: (a) existence of one asymmetric broad band presenting a shoulder at higher frequency; (b) presence of four narrow components; (c) appearance of two equally intense well separated bands characteristic of pure crystalline sulfamide. The first situation is characteristic of composition n = 20. Actually, the corresponding spectrum closely resembles that of the tetraglyme solution of sulfamide, meaning that in the P(EO)2oNH2SO2NH2 complex the amine groups of the molecule must interact weakly, as though they were isolated. Consequently,
221
the only existing NH couplings must be definitely the intramolecular ones. The feature of the spectra of the PEO-based complexes with compositions n = 4 and 5 (fig. 3a and c) is totally different, traducing situation (b). In these compounds, the vNH bands present a fine structure. The multiplicity of the bands and the small half-width observed can be interpreted in terms of dynamical or static disorder (for the time scale of optical spectroscopy, any fluctuation taking place in time longer than 10-12s will appear as static disorder). We are thus led to conclude that the spectroscopical signature of this apparently organised building is in good agreement with thermodynamical considerations connected to the phase diagram which associates composition n = 5 with a defined stoichiometric crystalline complex, as mentioned above. Though some typical sulfamide bands are present, compounds with compositions n = 20 and 30 exhibit spectra which are essentially identical to that of the uncomplexed PEO and sulfamide in solution (fig. 3 in [10]). On the contrary, the Raman spectra of P (EO)4NH2SO2NH2 and P (EO) sNH2SO2NH2 are intermediate between those of the liquid and solid states, i.e., between the situation observed in a tetraglyme solution ofsulfamide (or in molten sulfamide) and in crystalline sulfamide, respectively. Consequently, the spectra do not result from the simple addition of the spectra of the constituents. Moreover, the majority of the PEO bands is markedly widened for these compositions. Such result clearly indicates that the introduction of more salt in the polymer must possibly induce not only stronger host polymer-sulfamide interactions, but also some desorganisation for the PEO. The Raman spectra of complexes corresponding to compositions n = 2 and 2.5 (fig. 4) result roughly from the superposition of the spectra of uncomplexed PEO and crystalline sulfamide. However when considering the whole spectrum obtained by micro Raman spectroscopy (see section 3.2 and fig. 4) one observes that some grains present a spectrum identical to that of compositions 4 and 5, as if for the eutectic compound n = 2 the phase in presence were n = 4 in small proportion, pure sulfamide and pure PEO. Therefore, the former compound does not seem to corroborate ATD results. In fact, if for n = 2 there
222
V. De Zea Bermudez el al. / Polyethylene oxide complexes c~[sul/itmide
were indeed an eutectic, it would originate from the addition of the spectrum o f pure sulfamide and the spectrum of complex P(EO)2.sNHzSO2NH2, which is not the case.
3. 2. InJluence q f the substrate and~or the fihn thickness on polymer structure: comparison oJ'IR and Raman data A completely different situation is found when analysing the room temperature infrared spectra of pure PEO and of the P(EO)nNH2SO2NH2 system obtained for compositions n = 2 , 4, 5, 20, 30 represented in fig. 3 in [6]. The intensity of the NH stretching bands in this series is roughly directly proportional to sulfamide concentration. On the other hand, as salt concentration is enhanced from n = 30 up to 2, neither variations in the frequency, nor splittings o f the two NH stretching bands are observed. The NH stretching bands in the complex with composition n = 30 indicate that each N H 2 group of the sulfamide molecule is symmetrical. It corresponds to the situation of an ideal solution where the salt is completely dissolved in the solvent. For compositions n = 4 and 5, contrarily to Raman data, there is no indication of specific interactions between sulfamide and its surrounding. Moreover, Raman [7,11 ] and infrared spectra [6] show a clear evolution o f the polymer with increasing amounts of sulfamide. The bands associated with PEO are narrower for compositions n = 30, 20 and are on the contrary rather wide for composition tz = 2, 4 and 5. F u r t h e r m o r e the IR shift observed for stretching u ( C - C - O - ) mode from 1148cm -~ in pure PEO to 1129cm ~ for n = 2 gives evidence for interaction of sulfamide with the oxygen of the ether function contributing on the disorder o f this hostpolymer thus an enhancement of its a m o r p h o u s character [ 1 l ]. The above IR observations must be interpreted as the result of the absence of any specific anisotropic interactions between sulfamide in PEO, just like in a solution. These facts are thus in contradiction with both R a m a n and A T D data. One of the reasons for this inconsistency may lie in the different morphological state o f the samples analysed by R a m a n and infrared techniques. In par-
ticular, the thickness has most probably a great influence. The room temperature Raman spectra of P(EO)4NH2SO2NH2 under the form of thick film ( u p p e r part) and thin film (lower part) depicted in fig. 2 for the high and mid-frequency regions confirm our claim. The corresponding deconvoluted NH stretching bands represented in fig. 3 show how the set of well-defined uNH stretching components of the thick film ( ,~ 100 lam) becomes a broad vNH band in the thin film ( ~ 10 ~tm), suggesting that any specific interactions existing between salt and polymerhost (alike in the infrared spectra already discussed) are thus lost and that the latter mixture may be considered as an a m o r p h o u s solution of sulfamide in PEO. Obviously a reduction of the signal intensity is observed in the spectrum of the thin film, since the a m o u n t of substance analysed is smaller. These results suggest that the structure of the compounds (and consequently the proton conduction) is not the same tbr thick and thin film samples. Therefore it seems fundamental to perform Raman measurements on the samples to be studied by electrochemical methods. X-ray diffraction data obtained on thin films [ 1 l ] indicate a preferred orientation of the sulfamide along the 001 axis for pure sulfamide and along 010 for P(EO)2NH2SO2NH2 and P(EO)25NH2SO2NH> This effect of texture gives account for the variable relative intensity of the high and low frequency NH stretching components observed both in IR and R a m a n spectra of pure sulfamide and P ( E O ) complexes.
3.3. Lffect o f deprotonation The effect of the doping agent on the samples is discussed next. Fig. 3b and d (relative to compositions O / S = 4 and 5) allows to conclude that the presence of dopant leads to the loss o f the fine structure of the uNH bands and to the situation of a solution of sulfamide in PEO. Apparently the a d d i t i o n of guanidine carbonate induces a disorder similar to that observed for thin films. Since it is accepted that polymer disorder is associated with good conductivity, it is not surprising to find that the latter increases as the doping agent is introduced. For measurements performed two months after
V. De Zea Bermudez et al. I Polyethylene oxide complexes of sulfamide
223
r-,
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Fig. 2. Room temperature Raman spectra of P(EO)4NH2SO2NH2 under the form of (left) high-frequency region and (right) midfrequency region: (a) thick film ( 100/am), (b) thin film ( l0/am).
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Fig. 3. Room temperature Raman spectra of P(EO),NHzSO2NH2 n = 4 (left) and n = 5 (righl); (a), (c) undoped: (a): 5-3129cm -~, 4-3240 cm -l, 3-3276 cm -~, 2-3314 cm -~, 1-3371 cm -1. (c): 5-3129 c m - ~, 4-3240 cm ~, 3-3275 cm -I, 2-3315 cm -1, 1-3370 cm -l. (b), (d) doped. ( N / H = 5 % ) : ( b ) : 3 - 3 1 1 8 c m - I , 2 - 3 2 5 8 c m - l , l - 3 3 5 5 c m - L ( d ) : 3 - 3 1 2 4 c m ~,2-3260cm - ~ , l - 3 3 5 6 c m - L
224
I4 De Zea Bermudez et al. / Polyethylene oxide complexes qf sul/amMe
synthesis, in the absence o f doping agent the best room temperature conductivity ( 2 × 1 0 - s f l -~ cm I) is found for composition n = 4 (fig. 5a), whereas for the doped samples, the best conductor is n = 2, exhibiting values o f 6;4 1 0 - 5 f l -~ cm -~ at room temperature for a doping level o f 5% (fig. 5b). The great similarity between the spectrum o f such an amorphous phase and those of sulfamide in a tetraglyme solution or in the molten state [10] suggests the equivalence o f the protons and the absence of specific interactions, allowing both Grotthus and vacancy mechanisms. A more detailed correlation between conducting and R a m a n data versus temperature will be presented in [ 1 1 ]. We have therefore investigated the latter material more carefully by performing m i c r o - R a m a n analysis on a complex with an almost identical composition ( n = 2 . 5 ) . Fig. 4 shows the room temperature Raman spectra obtained for different grains (ca. 1 tam 2) of a sample of undoped P (EO)2.sNH2SO2NH2 complex. While the upper spectrum traduces a mixture of non-interacting c~'stalline sulfamide and PEO, the lower spectrum corresponds to grains o f nearly pure crystalline sulfamide (orientation effects of the light relative to the crystal axes are observed). Finally, the middle spectrum exhibits a behaviour characteristic
o f the intermediate crystalline c o m p o u n d s of composition n = 4 or 5. This example proves that the c o m p o u n d is highly heterogeneous, and that microR a m a n spectroscopy is the best adapted technique for such investigations.
4. C o n c l u s i o n
The R a m a n and infrared spectra o f PEO complexes of sulfamide have been recorded at room temperature for compositions equal to 2, 2.5, 4, 5, 20 and 30. The structure of the complexes varies with the shaping o f the sample (thick or thin film). For low salt concentrations ( n = 2 0 and 30), homogeneous solid solutions o f sulfamide in PEO are evidenced. Sulfamide seems to be simply dissolved and apparently not involved in any particular interaction with PEO, as suggested by the NH stretching region R a m a n spectra of both complexes which are roughly identical to those previously obtained for molten sulfamide and for a T E G D M E solution o f sulfamide. On the other hand, PEO exhibits a tendency for rcorganisation. For compositions n = 4 and 5 the formation o f a well organised c o m p o u n d is observed whenever the sample is in the bulk state, c o r -
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Fig. 4. Room temperature micro-Raman spectra of P (EO)2.sNH2SO2NH2: (left) mid-frequency region and (right) high frequency region. Different phases have been identified with a spatial resolution of ca. 1 ~m. (a) Point 1, (b) point 2 and (c) point 3.
K De Zea Bermudez et al. / Polyethylene o_vide comph'xes ofsulfamide
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High c o n c e n t r a t i o n s in s u l f a m i d e ( n = 2 and 2.5) lead on the c o n t r a r y to a m i x t u r e o f phases, as conf i r m e d by D S C , on aged samples [ 1 1 ]. T h e high spatial resolution ( 1 ~tm) o f R a m a n m i c r o p r o b e allows to isolate not only d o m a i n s o f pure s u l f a m i d e and pure P E O but also o f an i n t e r m e d i a t e crystalline c o m p o u n d , m o s t likely with a c o m p o s i t i o n equal to 4or5.
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We wish to t h a n k the C o m m i s s i o n o f the European C o m m u n i t i e s for h a v i n g s u p p o r t e d this work through a B R I T E / E U R A M P h D G r a n t . Dr. J.C1. Lassbgues is w a r m f u l l y a c k n o w l e d g e d for discussions a n d c o m m e n t s .
3.4 3.5
1000/T(K -]) Fig. 5. Arrhenius conductivity plot for undoped and doped for: (a) P(EO)4NH2SO2NH2 complexes; (b) P(EO)2NH2SO2NH2 complexes. r o b o r a t i n g p r e v i o u s t h e r m o d y n a m i c a l results [6]. Yet, the s a m e c o m p o s i t i o n s are also f o u n d to be a m o r p h o u s w h e n d o p e d with g u a n i d i n e c a r b o n a t e and w h e n a n a l y s e d u n d e r the f o r m o f thin films dep o s i t e d on silicon plates.
References [ 1 ] F. Defendini, PhD. Thesis (University of Grenoble, France, 1987). [2] J.C. Lassegues, B. Desbat, O. Trinquet, F. Gruege and C. Poinsignon, Solid State lonics 35 (1989) 17. [ 3 ] O. Trinquet, PhD. Thesis ( University of Bordeaux, France, 1990). [4] R. Tanaka, T. lwase, T. Hori and S. Saito, Proc. Intern. Congr. Polym. Electrolytes, St. Andrews, Scotland, 1987. [ 5 ] Y. Charbouillol, D. Ravaine, M. Armand and C. Poinsignon, J. Non Cryst. Solids 103 (1988) 325. [6 ] V. De Zea Bermudez, M. Armand, C. Poinsignon, L. Abello and J.Y. Sanchez, Electrochim. Acta 27 (1992) 1603. [ 7 ] V. De Zea Bermudez, PhD. Thesis ( University of Grenoble, France, 1992 ). [8] C. Berthier, W. Gorecki, M. Minier, M.B. Armand, J.M. Chabagno and P. Rigaud, Solid State lonics l 1 ( 1983 ) 91. [9] V. De Zea Bermudez, G. Lucazeau, L. Abello and C. Poinsignon, Mol. Strum., to be published. [10]V. De Zea Bermudez, G. Lucazeau, L. Abello and C. Poinsignon, Mol. Struct., to be published. [ 11 ] V. De Zea Bermudez, C. Poinsignon, G. Lucazeau and L. Abello, Mol. Struct., to be published.
Solid State lonics 61 (1993) 219-225 North-Holland
IONICS Infrared and Raman characterization of polyethylene oxide complexes of sulfamide V. De Zea Bermudez, G. Lucazeau, L. Abello, C. Poinsignon, J.Y. Sanchez and M. Armand Laboratoire d'lonique et d'Electrochimie du Solide de Grenoble, ENSEEG/INPG, Domaine Universitaire, BP 75, 38402 St. Martin d'Hkres, France
The Raman and infrared spectra of poly(ethylene oxide): PEO, and PEO-based complexes of sulfamide of general formula P(EO)nNH2SO2NH2 (where n=O/S stands as the ratio of monomer units per sulfamide molecule) with compositions equal to n = 2, 2.5, 4, 5, 20 and 30 have been recorded at 300 K in the 4000-100 cm-~ range. The evolution of sulfamide-polymer interactions with salt concentration is studied mainly by analysing the spectral features of the NH and -C-C-O stretching region. For compositions n = 4 and 5 splittings of the NH Raman bands indicate the existence of definite interactions between sulfamide and PEO. For the more diluted samples, the behaviour of the salt traduces the absence of specific interactions and the complete orientational disorder accounts for the weak character of the hydrogen bonds, a situation also encountered in sulfamide solutions of tetraethyleneglycoldimethylether(tetraglyme) and in molten sulfamide. In situ micro Raman technique allows to evidence the coexistence of phases for n = 2. The influence of the substrate and of the thickness of the films on its structure is presented. The Raman spectra of compositions n = 4 and 5 have shown that doping with a deprotonating agent results in increasing the disorder of the materials, for n = 2 instead of a mixture of phases, one obtains a homogeneous amorphous phase.
1. Introduction A i m i n g at their use in all-solid state electrochemical devices, basic to neutral proton conducting polymers are systems more adapted than the corresponding acidic electrolytes [ 1 - 5 ] , especially in the presence of insertion materials such as oxides and hydroxides, whose stability d o m a i n ranges from p H = 4 to p H = 12. Owing to the fact that the sulfonamide group ( SO2NH2) exhibits PKa's o f about l0 (corresponding to the equilibrium RSOENH2c>H + + [ RSOENH ] - ), we have recently proposed [6] that sulfamide-type salts constitute ideal cases, since they can play the role o f alcaline bases which do not form strong complexes with PEO, the favoured host-polymer in studies o f conductivity in polymer electrolytes. We have thus introduced a series o f so-called proton-vacancy conducting polymers based on a three c o m p o n e n t s system: a solvating polymer ( P O E ) , a proton source (sulfamide, NH2SO2NH2) and a deprotonating agent or proton-vacancy inducer (guanid i n i u m cation) [6]. The phase diagram established for the
P (EO)nNH2SO2NH2 binary system on the basis o f A T D techniques [6] (where n = O / S stands as the ratio o f m o n o m e r units per sulfamide molecule) indicates the presence o f at least three eutectics (found for compositions somewhere around n = 2, 4 and 30) and three intermediate crystalline compounds (found for compositions somewhere around n=2.5, 5 and 2O). At room temperature and in the absence o f dopant, the eutectic compositions exhibit good conductivities which may be explained on the basis o f sulfamide self-protonation, most probably followed by a Grotthus-type conduction mechanism associated with the segmental motion o f the polymeric chain, which plays the role of charge carrier. 2H2NSO2NH2 [ H2 NSO2 NH3 ] + + [ H2 NSO2 N H ] - .
(1)
Hence, under such conditions, conduction is ensured by the presence of both protons and protonvacancies.
In the presence o f the deprotonating agent, conductivity is greatly enhanced, being now solely due to
0167-2738/93/$ 06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
220
14 De Zea Bermudez et al. / Polyethylene oxtde comph'xes qfsul/'arnide
the motion o f proton-vacancies. For a doping level (represented by the ratio N / H of guanidine molecules a d d e d per extracted proton ) o f 5% remarkable room temperature conductivities higher than I 0-- 5 ~-- 1 c m - ~ result. Nevertheless, the samples are in a metastable a m o r p h o u s state and slow crystallization kinetics make them evolve with time, being responsible for a drastic decrease of conductivity after storage (6 m o n t h s ) , for it is accepted that conduction in semi-crystalline polymers takes place exclusively in the elastomeric phase [8]. This unavoidable room t e m p e r a t u r e crystallinity typical o f many PEO-based systems leads to the heterogeneous character of these materials. In the first part o f this paper the influence of salt concentration on the PEO-sulfamide interactions will be discussed, the effect o f the thickness o f the sampies, as well as o f shaping on the degree o f crystallinity will be analysed in the second part. Finally, the third part is devoted to the study of the consequences of doping on the structure o f the complexes. The discussion that follows is mostly based on two works presented previously dealing with spectroscopical studies of crystalline and molten sulfamide [7,9] and o f a tetraglyme-based solution o f the same salt [ 10]. It was concluded that while in crystalline sulfamide, the protons of each amine group were found to be non equivalent, the spectrum of molten sulfamide and that of a solution of this salt in tetraglyme traduces the absence o f specific interactions, the weak character of the hydrogen bonds in both liquid and melt.
2. Experimental
and polymer-sulfamide complexes on silicon plates. Evaporation of the solvent was carried out in the open air. 2.2. Infrared spectroscopy Mid-infrared spectra were acquired at room temperature using a Nicolet 710 F T I R system coupled to a microcomputer. The spectra were collected over the range 4000-400 c m - 1 by averaging 200 scans at a m a x i m u m resolution of 4 c m J. Samples were analysed as thin films in transmission mode. 2.3. R a m a n spectroscopy Data were collected at room temperature on a XY.Dilor (Lille, France) multichannel spectrometer over the 4 0 0 0 - 5 0 c m 1. The 488 nm line o f an argon laser light source was used. Samples were illuminated with 10-20 m W through the objective o f a microscope, which also serves to collect the R a m a n light, and were analysed as both thick and thin films. The materials under study were found to exhibit rather high levels of fluorescence (most probably connected not only to the presence of antioxidants in PEO, but also to ageing of the latter), leading to a very, poor signal/noise ratio. This drawback was partly overcome by systematically submitting the samples to the influence o f the laser beam for about a m i n i m u m o f 15 min prior to recording o f the spectra. Such procedure allowed to considerably improve the quality of the spectral signature, without increasing significantly the temperature of the sample: moreover, the point irradiated by the intense light source was not damaged as a result o f either the pretreatment or the experiment itself.
2. I. Synthesis o f electrolytes High molecular weight ( 9 × 105) PEO was supplied by Aldrich. Sulfamide, NH2SO2NH2, was commercially available at Janssen. Both products were used without further purification. Synthesis o f electrolytes, ac conductivity differential calorimetry' measurements are performed according to the procedure given previously [6]. U n d o p e d and doped thick films (40 to 140 ~tm thick) and thin films (approximately 5-10 ~m thick) were prepared by casting acetonitrile solutions of uncomplexed polymer
3. Results and discussion 3.1. Phases identlJTcation The evolution of the NH stretching bands with sulfamide concentration for thick samples of the P ( E O ) ~N H2SOzNH2 system at room temperature is presented in fig. 1; compositions vary from 2, 2.5.4, 5 to 20. Undoubtedly, the NH stretching modes undergo
V. De Zea Bermudez et al. /Polyethylene oxide complexes of sulfamide
3336 il~ NH2SO2NH2 f! : 1 3324
t!
~'! f' I
O/S=2
J/,'ll 0/S=4
•i U ? ' ,
o,==,
.d
IS:20
~< ¢ ~ ! ~ / s = 3 0
3500 3300 cm q Fig. 1. R o o m temperature NH stretching region R a m a n spectra of sulfamide and of P ( E O ) , N H 2 S O 2 N H 2 complexes of compositions n = 2 , 2.5.4, 5, 20.
many changes all through the whole series. However, three main types of behaviour may be easily identified: (a) existence of one asymmetric broad band presenting a shoulder at higher frequency; (b) presence of four narrow components; (c) appearance of two equally intense well separated bands characteristic of pure crystalline sulfamide. The first situation is characteristic of composition n = 20. Actually, the corresponding spectrum closely resembles that of the tetraglyme solution of sulfamide, meaning that in the P(EO)2oNH2SO2NH2 complex the amine groups of the molecule must interact weakly, as though they were isolated. Consequently,
221
the only existing NH couplings must be definitely the intramolecular ones. The feature of the spectra of the PEO-based complexes with compositions n = 4 and 5 (fig. 3a and c) is totally different, traducing situation (b). In these compounds, the vNH bands present a fine structure. The multiplicity of the bands and the small half-width observed can be interpreted in terms of dynamical or static disorder (for the time scale of optical spectroscopy, any fluctuation taking place in time longer than 10-12s will appear as static disorder). We are thus led to conclude that the spectroscopical signature of this apparently organised building is in good agreement with thermodynamical considerations connected to the phase diagram which associates composition n = 5 with a defined stoichiometric crystalline complex, as mentioned above. Though some typical sulfamide bands are present, compounds with compositions n = 20 and 30 exhibit spectra which are essentially identical to that of the uncomplexed PEO and sulfamide in solution (fig. 3 in [10]). On the contrary, the Raman spectra of P (EO)4NH2SO2NH2 and P (EO) sNH2SO2NH2 are intermediate between those of the liquid and solid states, i.e., between the situation observed in a tetraglyme solution ofsulfamide (or in molten sulfamide) and in crystalline sulfamide, respectively. Consequently, the spectra do not result from the simple addition of the spectra of the constituents. Moreover, the majority of the PEO bands is markedly widened for these compositions. Such result clearly indicates that the introduction of more salt in the polymer must possibly induce not only stronger host polymer-sulfamide interactions, but also some desorganisation for the PEO. The Raman spectra of complexes corresponding to compositions n = 2 and 2.5 (fig. 4) result roughly from the superposition of the spectra of uncomplexed PEO and crystalline sulfamide. However when considering the whole spectrum obtained by micro Raman spectroscopy (see section 3.2 and fig. 4) one observes that some grains present a spectrum identical to that of compositions 4 and 5, as if for the eutectic compound n = 2 the phase in presence were n = 4 in small proportion, pure sulfamide and pure PEO. Therefore, the former compound does not seem to corroborate ATD results. In fact, if for n = 2 there
222
V. De Zea Bermudez el al. / Polyethylene oxide complexes c~[sul/itmide
were indeed an eutectic, it would originate from the addition of the spectrum o f pure sulfamide and the spectrum of complex P(EO)2.sNHzSO2NH2, which is not the case.
3. 2. InJluence q f the substrate and~or the fihn thickness on polymer structure: comparison oJ'IR and Raman data A completely different situation is found when analysing the room temperature infrared spectra of pure PEO and of the P(EO)nNH2SO2NH2 system obtained for compositions n = 2 , 4, 5, 20, 30 represented in fig. 3 in [6]. The intensity of the NH stretching bands in this series is roughly directly proportional to sulfamide concentration. On the other hand, as salt concentration is enhanced from n = 30 up to 2, neither variations in the frequency, nor splittings o f the two NH stretching bands are observed. The NH stretching bands in the complex with composition n = 30 indicate that each N H 2 group of the sulfamide molecule is symmetrical. It corresponds to the situation of an ideal solution where the salt is completely dissolved in the solvent. For compositions n = 4 and 5, contrarily to Raman data, there is no indication of specific interactions between sulfamide and its surrounding. Moreover, Raman [7,11 ] and infrared spectra [6] show a clear evolution o f the polymer with increasing amounts of sulfamide. The bands associated with PEO are narrower for compositions n = 30, 20 and are on the contrary rather wide for composition tz = 2, 4 and 5. F u r t h e r m o r e the IR shift observed for stretching u ( C - C - O - ) mode from 1148cm -~ in pure PEO to 1129cm ~ for n = 2 gives evidence for interaction of sulfamide with the oxygen of the ether function contributing on the disorder o f this hostpolymer thus an enhancement of its a m o r p h o u s character [ 1 l ]. The above IR observations must be interpreted as the result of the absence of any specific anisotropic interactions between sulfamide in PEO, just like in a solution. These facts are thus in contradiction with both R a m a n and A T D data. One of the reasons for this inconsistency may lie in the different morphological state o f the samples analysed by R a m a n and infrared techniques. In par-
ticular, the thickness has most probably a great influence. The room temperature Raman spectra of P(EO)4NH2SO2NH2 under the form of thick film ( u p p e r part) and thin film (lower part) depicted in fig. 2 for the high and mid-frequency regions confirm our claim. The corresponding deconvoluted NH stretching bands represented in fig. 3 show how the set of well-defined uNH stretching components of the thick film ( ,~ 100 lam) becomes a broad vNH band in the thin film ( ~ 10 ~tm), suggesting that any specific interactions existing between salt and polymerhost (alike in the infrared spectra already discussed) are thus lost and that the latter mixture may be considered as an a m o r p h o u s solution of sulfamide in PEO. Obviously a reduction of the signal intensity is observed in the spectrum of the thin film, since the a m o u n t of substance analysed is smaller. These results suggest that the structure of the compounds (and consequently the proton conduction) is not the same tbr thick and thin film samples. Therefore it seems fundamental to perform Raman measurements on the samples to be studied by electrochemical methods. X-ray diffraction data obtained on thin films [ 1 l ] indicate a preferred orientation of the sulfamide along the 001 axis for pure sulfamide and along 010 for P(EO)2NH2SO2NH2 and P(EO)25NH2SO2NH> This effect of texture gives account for the variable relative intensity of the high and low frequency NH stretching components observed both in IR and R a m a n spectra of pure sulfamide and P ( E O ) complexes.
3.3. Lffect o f deprotonation The effect of the doping agent on the samples is discussed next. Fig. 3b and d (relative to compositions O / S = 4 and 5) allows to conclude that the presence of dopant leads to the loss o f the fine structure of the uNH bands and to the situation of a solution of sulfamide in PEO. Apparently the a d d i t i o n of guanidine carbonate induces a disorder similar to that observed for thin films. Since it is accepted that polymer disorder is associated with good conductivity, it is not surprising to find that the latter increases as the doping agent is introduced. For measurements performed two months after
V. De Zea Bermudez et al. I Polyethylene oxide complexes of sulfamide
223
r-,
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Fig. 2. Room temperature Raman spectra of P(EO)4NH2SO2NH2 under the form of (left) high-frequency region and (right) midfrequency region: (a) thick film ( 100/am), (b) thin film ( l0/am).
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Fig. 3. Room temperature Raman spectra of P(EO),NHzSO2NH2 n = 4 (left) and n = 5 (righl); (a), (c) undoped: (a): 5-3129cm -~, 4-3240 cm -l, 3-3276 cm -~, 2-3314 cm -~, 1-3371 cm -1. (c): 5-3129 c m - ~, 4-3240 cm ~, 3-3275 cm -I, 2-3315 cm -1, 1-3370 cm -l. (b), (d) doped. ( N / H = 5 % ) : ( b ) : 3 - 3 1 1 8 c m - I , 2 - 3 2 5 8 c m - l , l - 3 3 5 5 c m - L ( d ) : 3 - 3 1 2 4 c m ~,2-3260cm - ~ , l - 3 3 5 6 c m - L
224
I4 De Zea Bermudez et al. / Polyethylene oxide complexes qf sul/amMe
synthesis, in the absence o f doping agent the best room temperature conductivity ( 2 × 1 0 - s f l -~ cm I) is found for composition n = 4 (fig. 5a), whereas for the doped samples, the best conductor is n = 2, exhibiting values o f 6;4 1 0 - 5 f l -~ cm -~ at room temperature for a doping level o f 5% (fig. 5b). The great similarity between the spectrum o f such an amorphous phase and those of sulfamide in a tetraglyme solution or in the molten state [10] suggests the equivalence o f the protons and the absence of specific interactions, allowing both Grotthus and vacancy mechanisms. A more detailed correlation between conducting and R a m a n data versus temperature will be presented in [ 1 1 ]. We have therefore investigated the latter material more carefully by performing m i c r o - R a m a n analysis on a complex with an almost identical composition ( n = 2 . 5 ) . Fig. 4 shows the room temperature Raman spectra obtained for different grains (ca. 1 tam 2) of a sample of undoped P (EO)2.sNH2SO2NH2 complex. While the upper spectrum traduces a mixture of non-interacting c~'stalline sulfamide and PEO, the lower spectrum corresponds to grains o f nearly pure crystalline sulfamide (orientation effects of the light relative to the crystal axes are observed). Finally, the middle spectrum exhibits a behaviour characteristic
o f the intermediate crystalline c o m p o u n d s of composition n = 4 or 5. This example proves that the c o m p o u n d is highly heterogeneous, and that microR a m a n spectroscopy is the best adapted technique for such investigations.
4. C o n c l u s i o n
The R a m a n and infrared spectra o f PEO complexes of sulfamide have been recorded at room temperature for compositions equal to 2, 2.5, 4, 5, 20 and 30. The structure of the complexes varies with the shaping o f the sample (thick or thin film). For low salt concentrations ( n = 2 0 and 30), homogeneous solid solutions o f sulfamide in PEO are evidenced. Sulfamide seems to be simply dissolved and apparently not involved in any particular interaction with PEO, as suggested by the NH stretching region R a m a n spectra of both complexes which are roughly identical to those previously obtained for molten sulfamide and for a T E G D M E solution o f sulfamide. On the other hand, PEO exhibits a tendency for rcorganisation. For compositions n = 4 and 5 the formation o f a well organised c o m p o u n d is observed whenever the sample is in the bulk state, c o r -
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,
3000
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K De Zea Bermudez et al. / Polyethylene o_vide comph'xes ofsulfamide
T(°C) 60
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225
High c o n c e n t r a t i o n s in s u l f a m i d e ( n = 2 and 2.5) lead on the c o n t r a r y to a m i x t u r e o f phases, as conf i r m e d by D S C , on aged samples [ 1 1 ]. T h e high spatial resolution ( 1 ~tm) o f R a m a n m i c r o p r o b e allows to isolate not only d o m a i n s o f pure s u l f a m i d e and pure P E O but also o f an i n t e r m e d i a t e crystalline c o m p o u n d , m o s t likely with a c o m p o s i t i o n equal to 4or5.
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We wish to t h a n k the C o m m i s s i o n o f the European C o m m u n i t i e s for h a v i n g s u p p o r t e d this work through a B R I T E / E U R A M P h D G r a n t . Dr. J.C1. Lassbgues is w a r m f u l l y a c k n o w l e d g e d for discussions a n d c o m m e n t s .
3.4 3.5
1000/T(K -]) Fig. 5. Arrhenius conductivity plot for undoped and doped for: (a) P(EO)4NH2SO2NH2 complexes; (b) P(EO)2NH2SO2NH2 complexes. r o b o r a t i n g p r e v i o u s t h e r m o d y n a m i c a l results [6]. Yet, the s a m e c o m p o s i t i o n s are also f o u n d to be a m o r p h o u s w h e n d o p e d with g u a n i d i n e c a r b o n a t e and w h e n a n a l y s e d u n d e r the f o r m o f thin films dep o s i t e d on silicon plates.
References [ 1 ] F. Defendini, PhD. Thesis (University of Grenoble, France, 1987). [2] J.C. Lassegues, B. Desbat, O. Trinquet, F. Gruege and C. Poinsignon, Solid State lonics 35 (1989) 17. [ 3 ] O. Trinquet, PhD. Thesis ( University of Bordeaux, France, 1990). [4] R. Tanaka, T. lwase, T. Hori and S. Saito, Proc. Intern. Congr. Polym. Electrolytes, St. Andrews, Scotland, 1987. [ 5 ] Y. Charbouillol, D. Ravaine, M. Armand and C. Poinsignon, J. Non Cryst. Solids 103 (1988) 325. [6 ] V. De Zea Bermudez, M. Armand, C. Poinsignon, L. Abello and J.Y. Sanchez, Electrochim. Acta 27 (1992) 1603. [ 7 ] V. De Zea Bermudez, PhD. Thesis ( University of Grenoble, France, 1992 ). [8] C. Berthier, W. Gorecki, M. Minier, M.B. Armand, J.M. Chabagno and P. Rigaud, Solid State lonics l 1 ( 1983 ) 91. [9] V. De Zea Bermudez, G. Lucazeau, L. Abello and C. Poinsignon, Mol. Strum., to be published. [10]V. De Zea Bermudez, G. Lucazeau, L. Abello and C. Poinsignon, Mol. Struct., to be published. [ 11 ] V. De Zea Bermudez, C. Poinsignon, G. Lucazeau and L. Abello, Mol. Struct., to be published.