Solid State Ionics 109 (1998) 145–150

Study on a comb-like polymer electrolyte based on the backbone ofethylene–maleic anhydride copolymer

*Li Qi , Yunqing Lin, Fosong WangChangchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China

Received 16 September 1997; accepted 11 December 1997

Abstract

A comb-like polymer host(CBPE) as polymer electrolyte was synthesized by reacting poly(ethylene glycol) monomethylether (PEGME) with ethylene–maleic anhydride copolymer(EMAC) and endcapping the residual carboxylic acid withmethanol. The synthetic process was followed by IR and the amorphous product characterized by IR and elemental analysis.There were two peaks in the plot of the ionic conductivity against Li salt concentration. The plot of log s vs. 1 /(T 2 T )0

may exhibit dual VTF behavior when using the glass transition temperature of PEO of side chain as T . The comb-like0

polymer is a white rubbery solid which dissolves in acetone. 1998 Elsevier Science B.V. All rights reserved.

Keywords: Comb-like polymer; Polymer solid electrolyte; Ionic conductivity; PEO; VTF behavior

1. Introduction mediate product was usually insoluble because ofundesirable crosslinking. The present work is an

Since the original proposal that poly(ethylene alternative approach to Rietman’s study, i.e. tooxide) (PEO) doped with alkali metal salts could be prepare a polymer host by reacting poly(ethyleneused as an alternative to liquid electrolytes [1], many glycol) monomethyl ether with poly(vinyl ether alt-other polyether electrolytes have been characterized maleic anhydride) or poly(ethylene alt-maleic an-[2,3]. It has been found that the synthesis of comb- hydride), and endcapping the residual carboxylic acidshaped polymers is an effective approach to mini- group with methanol. If the first step of this reactionmize crystallization and enhance ionic conductivity is carried out in an appropriate solvent, a new type of[4]. Comb-like polymers with oligoethyleneoxide comb-like polymer can be obtained. In the past wependant group attached to various main chain such as have synthesized an amorphous comb-like polymerpolysiloxane [5,6], polymethacrylate [7,8] and poly- (CBP) based on methylvinyl ether /maleic anhydridephosphazene [9] have been investigated. Rietman alternating copolymer backbone with oligoox-and coworkers [4] have synthesized polymer elec- yethylene side chains. The dynamic mechanical

1trolytes with Li -ion conduction through the modi- properties and ionic conductivity of these CBP-Lification of styrene–maleic anhydride or ethylene– salt complexes were studied [10–12]. For the sake ofmaleic–anhydride copolymers. But Rietman’s inter- increasing the conductivity further and improving

mechanical properties, we aim to prepare polymer*Corresponding author. hosts based on ethylene–maleic anhydride copoly-

0167-2738/98/$19.00 1998 Elsevier Science B.V. All rights reserved.PII S0167-2738( 98 )00005-8

146 L. Qi et al. / Solid State Ionics 109 (1998) 145 –150

mer backbone with monomethyl ether of poly- precipitant. Generally, about four reprecipitation(ethylene glycol) side chains. steps were necessary.

2.3. IR and elemental analysis

2. ExperimentalThe IR spectra of the intermediate and end

products were measured on a BIO-RAD FTS-72.1. Materials

spectrometer using KBr plate technique.The elemental analysis was carried out by a Model

Ethylene–maleic anhydride copolymer(EMAC)1106 Elemental Analyzer.

and poly(ethylene glycol) monomethyl ether withMW of 350, 550 and 750 (PEGME-350. PEGME-

2.4. Preparation of the polymer electrolyte550 and PEGME-750) (Aldrich) were used as re-

membranesceived. Para-toluene sulfonic acid, C.P. grade(Shanghai 1st Chemical Reagent Factory) was dried

The polymer was first dissolved in acetone tounder vacuum for 12 h before use. Methyl ethyl

make a definitive concentration of polymer solution.ketone was refluxed in the presence of C.P. grade

Then, according to desired ratio of EO/Li a calcu-P O for 4 h and then distilled out. Methyl alcohol2 5 lated amount of the Li salt solution in acetone waswas at first dehydrated by using 4A molecular sieve,

added to a definitive volume of the polymer solution.and then refluxed with metallic magnesium for 1 h.

After mixing thoroughly, the solution was cast on aDimethyl sulfate, C.P. grade (Shanghai Jin Shan

PTFE dish. Acetone was allowed to evaporate slowlyChemical Factory) was used as received. LiClO and4 and the films obtained were dried thoroughly inLiSCN were both A.R. grades. LiClO was dried in4 vacuum at 708C for about 72 h.vacuum at 1608C for 24 h before use, and LiSCNwas dried in vacuum at 608C for 24 h before use.

2.5. Conductivity measurement

2.2. Synthesis of comb-like polymer host The polymer electrolyte film, sandwiched between2two stainless steel electrodes of 1 cm area, was put

The esterification of EMAC with PEGME without in a glass vessel equipped with a temperaturegel formation was achieved at first by dissolving controlled heater. The precision of the temperatureEMAC and PEGME-350 (mole ratio 5 1:2) in fresh- reading was within 60.58C. Impedance measure-ly distilled methyl ethyl ketone and cyclohexanone ments were taken on Hewlett Packard 4275A multi-(volume ratio 5 2:1), then injecting a definite volume frequency LCR meter. The frequency range tested

4 7of paratoluene sulfonic acid in methyl ethyl ketone was from 10 Hz to 10 Hz. The conductivity datainto the reaction mixture under a flow of nitrogen. were analyzed by an AC-immittance analysis systemThe mixture was stirred continuously for 24 h at of EG&G PARC.808C. After the reaction was complete, the mixturewas transferred to a rotary evaporator in order toremove the solvents. An excess amount of freshly 3. Results and discussiondistilled methyl alcohol (ca. 200-fold) and 0.2 ml ofdimethyl sulphate were added to the residue so as to 3.1. Synthesis of the comb-like polymer hostthoroughly endcap the unreacted carboxyl group.The reaction proceeded with continuous stirring for The reaction was as given in Scheme 1.another 24 h at 608C. The reaction mixture was then Under the catalytic action of externally addedtransferred again to the rotary evaporator for the acid, the esterification of the ethylene–maleic an-removal of the excess methyl alcohol. Thereafter, the hydride copolymer with low molecular weight al-product was purified by reprecipitation using cohol can usually only form a semi-ester, i.e., onlycyclohexanone as solvent, and methylbenzene as one of the two carboxylic acids can be converted into

L. Qi et al. / Solid State Ionics 109 (1998) 145 –150 147

Scheme 1.

ester [13]. In our case, using PEGME as an esterifi-cation agent, a similar result was obtained, as hasalso been reported by Rietman [4]. However, incontrast with the low MW monofunctional alcohol,the use of PEGME often results in an insolubleproduct if no suitable solvent is available. Theformation of an insoluble product brings about manydifficulties in the later operations, e.g., purification,film formation and characterization. We believed thatthe gel formation may be caused by the presence of atrace impurity of unmethylated PEG, which is verydifficult to remove completely from the bulk of

Fig. 1. The IR spectrum of CBPE-350 for the intermediate productPEGME. We tried to overcome this difficulty byafter the first step of reaction.allowing the esterification to occur in the presence of

an appropriate solvent, and have succeeded in ob-taining completely soluble products. Several organic shown. This is an evidence that an appreciablesolvents such as DMF, cyclohexanone and methyl amount of the anhydride has been esterified, al-ethyl ketone have been selected for the solution though a part remains unreacted. The IR spectrum ofesterification. Among them, it was found that methyl the purified final product is shown in Fig. 2. Here

21ethyl ketone with cyclohexanone seemed to be the both the absorption peaks at 1846 cm are absent,most favorable. It was found that after the comple- indicating that the anhydride has been convertedtion of the first step of the reaction, the intermediateproduct was very difficult to isolate since it readilybecame an insoluble gel even if it was dried in anoven at the temperature as low as 508C. Thus thesecond step of the reaction was started by adding anexcess amount of methyl alcohol right after theremoval of the solvents.

3.2. Study of IR spectra and elemental analysis

The synthesis process was followed by IR (Fig.1). For the intermediate product, the characteristic

21absorption of anhydride at 1846 cm has beenappreciably lowered, while a strong characteristic Fig. 2. The IR spectra of the purified final product. (a) CBPE-350,

21absorption of the ester group at 1728.8 cm is (b) CBPE-550, (c) CBPE-750.

148 L. Qi et al. / Solid State Ionics 109 (1998) 145 –150

completely. In addition, there are strong absorption 3.3. The unusual dependence of ionic conductivity21at 1728.8 and 1197.7 cm , showing that the product on Li salt content

is an ester of poly(ethylene glycol). The assignmentof the main absorption bands appearing in the IR The ionic conductivities of LiClO and LiSCN4

spectrum is listed in Table 1. It should be noted that complexes have been investigated over the Li salt21a discernible absorption shoulder at 1710 cm concentration range from EO/Li 5 80 to EO/Li 5 6.

sometimes appeared in the IR spectrum due to Generally, the plot of the ionic conductivity of solidincomplete conversion of the carboxyl group. If a polymer electrolytes vs. salt content exhibited asmall amount of dimethyl sulfate is added as a single maximum [14]. However, the relation betweencatalyst in the second step of esterification, no such a the ionic conductivities and salt content for theseshoulder can be detected, shows that the polymer materials differs considerably from that of usualcontained no free COOH group. In addition, the solid polymer electrolytes. As shown in Figs. 3 and 4absorption peaks of PEO crystallization bands at

211463, 1364, 1283, 1244, 1149, 1062, 965 cm areabsent.

The elemental analysis for the purified final prod-ucts has been carried out (Table 2). If semi-esters arefound, the formula can be expressed by C H O ,22 40 11

C H O and C H O , respectively for the32 60 16 42 80 21

PEGME-350, -550 and -750 based materials, corre-sponding to the structures given in Scheme 2.

Table 1Assignment of main absorptions in IR spectrum of CBPE-350

21Absorption peak (cm ) Assignment Remarks

2943 –CH AsCH3 3

2875.5 –CH sCH3 3

2875 –CH – AsCH2 2Fig. 3. Dependence of the ionic conductivity on the1728.8 –CO– –LiClO concentration for polymer hosts based on PEGME-350.41452 –CO–OCH As–CH –2 2

1437 –CO–OCH As–CH3 3

1349.7 –CO–OCH As–CH3 3

1197 –C–CO–O– As–C–O–1095.6 –C–O–C– As

Table 2Elemental analysis

Calculated Found

C (%) H (%) C (%) H (%)

CBPE350 (C H O ) 54.98 8.39 54.86 8.1222 40 11

CBPE550 (C H O ) 54.84 8.63 54.98 8.5432 60 16

CBPE750 (C H O ) 54.77 8.75 55.40 8.7542 80 21

Fig. 4. Dependence of the ionic conductivity on the LiSCNScheme 2. concentration for polymer hosts based on PEGME-350.

L. Qi et al. / Solid State Ionics 109 (1998) 145 –150 149

there are two peaks in the ionic conductivity /con- by the Arrhenius relationship but as with othercentration plots: one is located at a concentration of amorphous solid polymer electrolytes, the tempera-EO/Li 5 8, another at a concentration of EO/Li 5 ture dependence of the ionic conductivity generally30, as noted by us previously [13]. follows the ‘Vogel–Tammann–Fulcher’ (VTF) equa-

tion, namely,3.4. Temperature dependence of the ionic

21 / 2s 5 AT exp 2 E /(T 2 T )h jconductivity b 0

in which A and E are fitted constants and T isb 0The ionic conductivity increases with increasingrelated to the equilibrium state glass transition

temperature (Figs. 5 and 6). It can not be expressedtemperature. Therefore, we took glass transition(2508C) of side chain PEO as to [15,16]. The resultsof the fitting are shown in Fig. 5 and Fig. 6,respectively for LiClO and LiSCN electrolytes. If4

we refer to the Cowie and Martin’s results [17], Fig.5 and Fig. 6 will be linear.

4. Conclusion

The complexes have the highest conductivity25 212.58310 S cm at room temperature and 1.453

23 2110 S cm at 1008C.

Acknowledgements

This work was carried out with financial supportFig. 5. VTF plots for the LiClO -containing polymer electrolytes4 from the National Natural Science Foundation ofbased on PEGME-350.

China.

References

[1] M. Armand, J.M. Chabagno, M.J. Dudot, in: P. Vashishta,J.N. Mundy, G.K. Shenoy (Eds.), Fast Ion Transport inSolids, North-Holland, Amsterdam, 1979, p. 131.

[2] A. Hooper, J.M. North, Solid State Ionics 9–10 (1983) 1161.[3] A. Takeoka, H. Ohno, E. Tsuchida, Polym. Adv. Tech. 4

(1993) 53.[4] E.A. Rietman, M.L. Kaplan, J. Polym. Sci: Part C: Polym.

Lett. 28 (1990) 187.[5] P.G. Hall, G.R. Davies, I.M. Ward et al., Polym. Comm. 27

(1986) 98.[6] D. Fish, I.M. Khan, E. Wu, J. Smid, Br. Polym. J. 20 (1988)

281.[7] D.W. Xia, D. Soltz, J. Smid, Solid State Ionics 14 (1984)

221.[8] E. Tsuchida, N. Kobayashi, H. Ohno, Macromolecules 21

(1988) 96.Fig. 6. VTF plots for the LiSCN-containing polymer electrolytes [9] P.M. Blonsky, D.F. Shriver, Solid State Ionics 18–19 (1986)based on PEGME-350. 258.

150 L. Qi et al. / Solid State Ionics 109 (1998) 145 –150

[10] L. Qi, Y. Lin, F. Wang, Acta Polymerica Scinica 4 (1995) [14] N. Kobayashi, H. Ohno, E. Tsuchida, Nippon Kagaku Kaishi459. 441 (1986) 1986.

[11] L. Qi, Y. Lin, F. Wang, Chinese Chemical Lett. 7 (1996) 653. [15] D.F. Shriver, M.A. Ratner, Chem. Rev. 88 (1988) 109.[12] Y. Lin, L. Qi, D. Chen, F. Wang, Solid State Ionics 90 (1996) [16] B. Qian, G. Xu, F. Yu, Polymeric Transition and Relaxation,

307. Science Press, Beijing, 1986, p. 505.[13] M. Ratzsch, Prog. Polym. Sci. 13 (1988) 277. [17] J.M.G. Cowie, C.S. Martin, Polymer Comm. 26 (1985) 298.