13C NMR and conductivity measurements of ethylene oxide-epychloridrine copolymer doped with LiClO4

SOLID STATE ELSEVIER

loNlcs

Solid State Ionics 8.5 (1996) 219-223

13C NMR and conductivity measurements of ethylene oxideepychloridrine copolymer doped with LiClO, A.E. Wolfenson”‘“,

R.M. Torresib, T.J. Bonagambaa,

“Institute de Fisica

de SC?0 Carlos-LISP,

“Institute de Quirnica de Go ‘Institute

Carlos-USP,

de Quimica-UNICAMP,

Caixa Postal 369, Cairn Postal

M.-A. De Paoli”, H. Panepucci”

13560-970

Sr7o Carlos

780, 13560-970

Caixa Postal 61.54, 13083-970

&io Carlos

Campinas

(SP), Brazil (SP), Brazil

(SP), Brazil

Abstract The ethylene oxide-epychloridrine copolymer has interesting properties as an ionic conducting polymer, principally due to its low glass transition temperature (Tr = - 40°C). The effect of dopant concentration (LiClO,) was studied by “C NMR spectra and conductivity measurements. These studies allowed us to identify the different carbon atoms in the copolymer and their interaction with lithium cations and solvent molecules. The analysis of “C spectra and conductivity measurements show that the solvent participates of the solid electrolyte, modifying the charge transport process by the change in the diffusion coefficient. Kepwrds:

Poly(epychloridrine-ethylene)

oxide; Ion conductivity;

1. Introduction Polymeric solid electrolytes, ion-conducting solid solutions of ionic salts, have received considerable attention over the last few years due to their possible technological applications in electrochemical devices (solid state lithium batteries, electrochromic displays, etc.) [I]. Poly(oxyethylene) (PEO) and modified polymers from PEO have been extensively investigated because of their properties as matrix material [2]. However, the low ionic conductivity and poor mechanical properties at temperatures above the crystalline-amorphous phase transition (around 65°C) of PEO, limit their application [3]. In order to improve the mechanical and electrical properties of PEO, it is necessary to modify the polymer structure *Corresponding 0167-2738/96/$15.00 PII

author.

Copyright 01996

SOl67-2738(96)00062-8

“C NMR

to obtain a larger proportion of the amorphous phase [4]. One way to generate this modification would be to use a substituted monomers in order to form another polymer, a copolymer or a terpolymer. Kohjiya et al. [5] and Silva et al. [6] have studied the characteristics of a PEO copolymer, the poly(epychloridrine-co-ethylene oxide) (PEPI-PEO), as polymeric solid electrolyte. PEPI-PEO has a glass transition temperature at ca. - 40°C [7] and is amorphous at room temperature. A good ionic conductivity is expected when ionic salts are dissolved into the copolymer matrix, In this work, nuclear magnetic resonance (NMR) and conductivity measurements were performed with the PEPI-PEO copolymer doped with different concentrations of lithium perchlorate using different non-aqueous solvents in the preparation process. The aim of these studies is to identify the influence of the

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A.E. Wolfenson et al. I Solid State Ionics 8.5 (1996) 219-22-T

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solvent and the effect of the ionic salt concentration on the doped copolymer conductivity. 13C NMR spectra were used to study the chemical process involved.

2. Experimental I

PEPI-PEO copolymer

was provided by DINACO (Brazil) under the brand-name Hydrin. The solid solutions (PEPI-PEO)-LiClO, were prepared by dissolution of different quantities of PEPI-PEO copolymer and the ionic salt in three non-aqueous solvents [tetrahydrofuran (THF), acetonitrile (ACN) and acetone] under a pure nitrogen stream. The typical [O]/[Li] ratio range studied was between 600 and 0.3. The 13C high-resolution solid-state NMR experiments were carried out at 21.343 MHz in a homemade spectrometer using Doty double-resonance prove. The pulse sequences employed were programmed with a Tecmag Libra system. The basic RF pulse sequence consisted of a 7.5 ys followed by 136 ms proton 90” 13C excitation decoupled acquisition. Chemical shift were measured with respect to a TMS reference. Conductivity measurements were carried out at 25°C using a 1286 Solartron electrochemical interface combined with a Frequency Response Analyzer Solar&on 1250. The conductivity values were obtained by the a.c. impedance technique over the frequency range of 0.01-65000 Hz.

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Ppm ITMS; Fig. 1. ‘H decoupled “C NMR spectrum of pure PEPI-PEO copolymer. Line assigned and chemical structural formulae are inserted in the figure.

45.6 ppm:

secondary ine atom

carbon bonded to the chlor-

The presence of residual THF was not detected. Fig. 2 shows the solid solution conductivity as a function of the [O]/[Li] ratio. For a large [0] /[Li] ratio, the conductivity increases with LiClO, concentration, and reaches a maximum for a ratio of [O]/[Li] of approximately 2. For [0] /[Li] lower than 2, the conductivity decreases, probably due to ionpair formation produced by the high concentration of salt in the copolymer matrix. It is important to remark that at room temperature the PEPI-PEO lo44

3. Results and discussion Fig. 1 shows the ‘H decoupled 13C spectrum of pure PEPI-PEO copolymer dissolved in THF and then dried in an inert atmosphere. Three lines centered at 80.3, 71.7 and 45.6 ppm [TMS] can be observed. As shown in Fig. 1 these were assigned as follows: 80.3 ppm: 7 1.2 ppm:

secondary carbon bonded to an oxygen atom primary carbon bonded to an oxygen atom

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[OMLi] ratio Fig. 2. Conductivity values as a function of the [O]/[Li] PEPI-PEO copolymer prepared with THF.

ratio for

A.E. Wolfenson et al. I Solid State Ionic-s 85 (1996)

copolymer shows higher conductivity values than does the PEO polymer [8]. In order to analyze the effect of the amount of ionic salt on the polymeric chain, 13C NMR spectra were carried out at different LiClO, concentrations. Fig. 3 shows NMR spectra for different [O]/[Li] ratios. For low concentrations of Lif (between 552 and 62 [O]/[Li] ratio), the three lines in the 13C spectra are broadened with the increase in the concentration of Li+. This is usually observed in ion conducting polymers [9] and it is possibly due to dipolar interaction with the spin of Li. Conformational changes in the polymeric chain by the presence of Li+ lead to a reduced mobility of the polymer. For more concentrated solutions, additional lines appear in the “C spectra centered around 108, 32 and 24 ppm TMS. Simultaneously the typical polymer lines are completely broadened. With the purpose of identifying these additional lines, ‘H decoupled 13C NMR spectra were obtained in different steps of the sample preparation, there are

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shown in Fig. 4. Fig. 4a shows the “C NMR spectrum for the pure copolymer dissolved in THF and Fig. 4b shows the spectrum of a 0.05 M solution of LiClO, in THF that is identical to the spectrum of pure THF. On the other hand, Fig. 4c shows the spectrum of a liquid solution of PEPI-PEO copolymer-LiClO, in THF ([O]/[Li] = 100). It can be seen that copolymer and solvent lines can be distinguished in this spectrum and that additional lines are present, probably due to the interaction of the host structure with the ionic salt and solvent molecules. Fig. 4d-e show the spectra after drying for 24 and 72 h, respectively. Both spectra show residual solvent in the copolymer solid phase. The most important fact to point out is that THF always remains in the copolymer when the ionic salt is present. This behavior indicates the physico-chemi7 I

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ITMSI

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pm [TMSI Fig 3. ‘H decoupled “C spectra of PEPI-PEO copolymer with different concentrations of LiCIO,. The samples were prepared with THF.

Fig. 4. ‘H decoupled “C spectra carried out on different steps of the preparation. (a) Pure PEPI-PEO. (b) 0.05 M LiCIO, in THF. (c) PEPI-PEO copolymer with [O]/[Li] = 100 in THF. (d) The same solution as (c) after 24 h of the drying process. (e) The same solution as (c) after 72 h of the drying process.

A.E. Wolfenson

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cal interaction between LiClO, and THF, since for the pure copolymer all solvent is removed by the drying process (Fig. 4a). The above interaction between THF and the ionic salt could be related to the lithium solvation process. Lithium ion is a strong Lewis acid [lO,ll], that is to say, Li+ aligns some solvent molecules in its near neighborhood. In this case, the formed complex cation can be written as Li( THF): , where m is the number of THF molecules bonded to Li+ by electrostatic interaction [ 121. In order to study the influence of the solvent nature, the same experiment was performed using acetone and acetonitrile as solvents. Fig. 5a shows the spectrum of the pure copolymer dissolved in acetone and then dried. Fig. 5b shows the spectrum of a solution of acetone and LiClO,. Fig. 5c-e show

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Table 1 Conductivity values of dried PEPI-PEO copolymer [O]/[Li] = 100 prepared with different solvents Solvent

Conductivity

THF Acetonitrile Acetone

5. lo-’ 7.10-l 3. 10-h

with

[S]

the solid solution of the PEPI-PEO with LiClO, after drying for 24, 48 and 72 h, respectively. The same bordering effects occur but no solvent is observed after drying. The NMR spectra cIearly showed that in the case of PEPI-PEO copolymer, the solid solution formed in the presence of THF contains the copolymer matrix, the dopant and an important amount of solvent. This can modify the mobility of the ionic species due to the formation of a solvation layer principally around the cationic species, thus reducing the diffusion coefficient. Table 1 shows the conductivity values of three samples of a solid solution of PEPI-PEO-LiClO, obtained using different solvents with an [O]/[Li] ratio equal to 100. It is observed that the highest value of conductivity is obtained in the sample prepared with acetone and that the lowest value corresponds to the sample prepared with THF.

4. Conclusions

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Fig. 5. ‘H decoupled “C spectra carried out on different steps of the preparation. (a) Pure PEPI-PEO. (b) 0.05 M LiCIO, in acetone. (c) PEPI-PEO copolymer with [O]/[Li] = 100 in acetone after 24 h of the drying process. (d) The same solution as (c) after 48 h of the drying process. (e) The same solution as (c) after 72 h of the drying process.

Conductivity measurements show that the PEPIPEO copolymer doped with LiClO, is a good ionic conductor at room temperature. Variation of the [O]/[Li] ratio shows that there is an optimal concentration of ionic salt. The 13C NMR spectra show that it is important to take into account the nature of the solvent because, even using a careful drying process, the solvent can remain in the solid electrolyte. The analysis of those spectra combined with conductivity measurements show that the solvent participates of the solid electrolyte, modifying the charge transport process by changing the diffusion coefficient. Comparing both experiments, with THF and acetone, it is evident that the interaction between the ionic salt and the host polymeric matrix can be

A.E. Wolfenson

et al. I Solid State tonics

perturbed by the solvent when it remains in the solid solution. This phenomenon could lead to wrong conclusions about the interpretations of broadening and shift of 13C lines. Results show that, depending on the chemical nature of the solvent, the interaction of lithium ions will be strong, leading to the formation of a different structure than that obtained if one considers only the presence of lithium ions inside the copolymer matrix.

We thank E.L.G. Vidoto for his technical support. This research was supported by FAPESP, FINEP and CNPq (Brazilian Agencies).

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[I] Electrochemical Science and Technology of Polymers, ed. R.G. Linford (Elsevier Applied Science, London, 1987).

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(VCH, New York, 1991). C. Berthier, W. Gorecki, M. Minier, M.B. Armand, J.M. Chahagno and P. Rigaud, Solid State Ionics I I (1983) 91. M. Armand, Solid State Ionics 69 (1994) 309. S. Kohjiya, T. Horiuchi and S. Yamashita, Electrochim. Acta 37 (1992) 1721. G.G. Silva, C.M.N. Polo da Fonseca, E.A. Rezende Duek, R.C.D. Peres and M.A. De Paoli, 4” Simposio Latino-Americane de Polimeros, Gramado, Brazil. DINACO Importacao Comercio S/A, BFGoodrich Hydrin Elastomers Technical Report. M. Watanabe, S. Magano, K. Sanui and N. Ogata, Solid State Ionics 18-19 (1986) 338. J.F. O’Gara, G. Nazri and K.M. MacArthur, Solid State Ionics 47 (1991) 87. Y. Zhang, Inorg. Chem. 21 (1982) 3886. Y. Zhang, Inorg. Chem. 21 (1982) 3889. Y. Marcus, Ion Solvation (Wiley-Interscience. London, 1985).

References

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121F.M. Gray, Solid Polymer Electrolytes

PI

Acknowledgments

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