Characterization and ionic conductivity of an amorphouscomblike polymer II: Based on ethylene±maleic anhydride

copolymer backbone with monoethyl ether ofpoly(ethylene glycol) of MW=550 as side chain

Li Qi *, Yunqing Lin, Fosong Wang

Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, P.R. China

Received 22 September 1997; accepted 11 December 1997

Abstract

Using a graft modi®cation method, a comblike polymer host (CBPE550) was synthesized by reacting monomethylether of poly(ethylene glycol) (PEGMA) with ethylene±maleic anhydride copolymer (EMAC) and endcapping the

residual carboxylic acid with methanol. The product was characterized by IR and elementary analysis. Resultshowed that the product was amorphous and semi-ester product is accord with reaction equation. There were twopeaks in the plot of the ionic conductivity against Li salt concentration. The plot of log s against 1/(Tÿ T0) shows

a dual VTF behavior when using the glass transition temperature of PEO of side chain as Tb. The comblikepolymer is a white rubbery solid. It can be well-dissolved in acetone. # 1999 Elsevier Science Ltd. All rightsreserved.

1. Introduction

Since the original proposal that poly(ethylene oxide)

(PEO) doped with alkali metal salts could be used as

an alternative to liquid electrolytes [1]. Many studies

of cell performances have been made, and today the

polymer±LiX solid electrolytes may be regarded as one

of the most promising developments in modern battery

technology [2, 3]. It has been found that the synthesis

of combshaped polymers is an e�ective approach

to minimize crystallization and enhance ionic

conductivity [4]. In the past we synthesized an amor-

phous comblike polymer (CBP) based on methylvinyl

ether/maleic anhydride altering copolymer (MA) back-

bone on oligooxyethylene side chain. The dynamic

mechanical properties and ionic conductivity of the

CBP±Li salt complexes were studied [5, 6]. It has been

found that a-transitions of CBP and its lithium salt

complexes are close to room temperature. For this

reason, the in¯uence of the molecular motion of the

main chain on conductivity at ambient temperature

can be observed. For the sake of increasing conduc-

tivity further and improving mechanical properties we

aim to prepare polymer host for bi-ion conduction

based on ethylene±maleic anhydride copolymer back-

bone with monomethyl ether of poly(ethylene glycol)

side chain. In this paper, we will report the results

obtained in studying IR and elementary analysis of

CBPE550, the between the ionic conductivity and salt

concentration, the temperature dependence of the ionic

conductivities.

European Polymer Journal 35 (1999) 789±793

0014-3057/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0014-3057(98 )00067-6

PERGAMON

* Corresponding author.

2. Experimental

2.1. Materials

Ethylene±maleic anhydride copolymer (EMAC) andmonomethyl ethers of polyethylene glycol with MW of550 (PEGME-550) are the commercial products of

Aldrich. Methylbenzene and cyclohexanone, A.R.grade (Beijing chemical plant). All of them were usedas received without further puri®cation. Para toluene

sulfonic acid (PTSA), C.P. grade (Shanghai 1stChemical Reagent Factory) was dried under vacuumfor 12 h before use. Methyl ethyl ketone (MEK) wasre¯uxed in the presence of C.P. grade P2O5 for 4 h and

then distilled out. Methyl alcohol was at ®rst dehy-drated by using a 4 AÊ molecular sieve, and thenre¯uxed with metallic magnesium for 1 h. Dimethyl

sulfate, C.P. grade (Shanghai Jin Shan ChemicalFactory) was used as received. LiClO4 and LiSCNwere both A.R. grades. LiClO4 was dried in vacuum at

1608C for 24 h before use, and LiSCN was dried invacuum at 608C for 24 h before use.

2.2. Synthesis of comblike polymer

The synthetic method of comblike polymer and

preparation of the polymer electrolyte membrane fol-lowed Ref. [7], and were improved. The esteri®cationof EMAC with PEGME was achieved at ®rst by dis-

solving EMAC and PEGME-350 (molar ratio=1:2) inthe freshly distilled MEK and cyclohexanone (volumeratio=2:1), then injecting a de®nite volume of MEK

solution of TPSA to the reaction mixture under a ¯owof nitrogen. The mixture was stirred continuously for24 h at 808C. After the reaction came to the end, themixture was transferred to a rotary evaporator in

order to remove the MEK and cyclohexanone, then anexcess amount of freshly distilled methyl alcohol (ca.200-folds) and 0.2 ml of dimethyl sulphate were added

to the residue so as to endcap the unreacted carboxylgroup thoroughly. The reaction proceeded with con-tinuous stirring for another 24 h at 608C. The reaction

mixture was then retransferred to the rotary evapor-ator for the removal of the excess methyl alcohol.Thereafter, the product was puri®ed by reprecipitation

method by using cyclohexanone as solvent, andmethylbenzene as precipitant. Generally, about fourtimes of reprecipitation were necessary.

2.3. IR

The IR spectra of the intermediate and ®nalproducts were measured on a BIO-RAD FTS-7spectrometer using KBr plate technique.

2.4. Elemental analysis

The elemental analysis of CBPE550 was carried outby Model. 1106 Elemental Analyzer.

2.5. Preparation of the polymer electrolyte membranes

The polymer was ®rst dissolved in acetone to makea de®nitive concentration of polymer solution. Then,according to desired ratio of EO/Li a calculated

amount of the Li salt solution in acetone was added toa de®nitive volume of the polymer solution describedabove. After mixing thoroughly, the solution was caston a PTFE dish. Acetone was allowed to evaporate

slowly until the solution did not ¯ow. The samplesobtained were dried thoroughly in vacuum at 708C forabout 72 h.

2.6. Conductivity measurement

The polymer electrolyte ®lm sandwiched betweentwo stainless steel electrodes of 1 cm2 area was put in aglass vessel equipped with a temperature controlled

heater. The precision of the temperature reading waswithin20.58C. Impedance measurements were taken onHewlett Packard 4275A multi-frequency LCR meter.

The frequency range tested was from 104 Hz to 107 Hz.

3. Results and discussion

3.1. Study of IR spectra

The esteri®cation of EMAC with PEGME was

achieved at ®rst. After the ®rst step esteri®cation reac-tion the IR spectrum of the intermediate productshowed the characteristic absorption of anhydride at

1846 cmÿ1. This was an evidence that a part of anhy-dride remained unreacted. The IR spectrum of the pur-i®ed ®nal product CBPE550 is shown in Fig. 1. In this

Fig. 1. The IR spectrum of the puri®ed ®nal product

CBPE550.

L. Qi et al. / European Polymer Journal 35 (1999) 789±793790

Table 2

Elemental analysis of CBPE550(C32H60O16)

Calculated Found

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

54.84 8.63 54.98 8.54

®gure, the absorption peak at 1846 cmÿ1 is absent, in-dicating that the anhydride has been converted com-

pletely. In addition, there are strong absorptions at1728.8 and 1197.7 cmÿ1, showing that the product isan ester of poly(ethylene glycol). The assignment of

the main absorption bands appeared in the IR spec-trum of CBPE550 is listed in Table 1. It should benoted that sometimes a discernible absorption shoulder

at 1710 cmÿ1 appeared in the IR spectrum ofCBPE550 due to incomplete conversion of the car-boxyl group. If an in®nitesimal amount of dimethyl

sulfate is added as a catalyst in the second step ofesteri®cation, no such a shoulder can be detected, Itmeans that the resulting CBPE550 contained no free-COOH group. In addition, the absorption peaks of

PEO crystallization bands at 1463, 1364, 1283, 1244,1149, 1062, 965 cmÿ1 are absent in the CBPE system.The result shown that the side chain of CBPE550 is

non-crystallization.

3.2. Elemental analysis

The elemental analysis for the puri®ed ®nal products(CBPE550) has been carried out. The data were listedin Table 2. If semi-esters are yielded, the formula of

CBPE-550 can be expressed by C32H60O16. For speci-men CBPE-550, C, 54.84%; H, 8.63%. These data arefairly close to the calculated values listed in the

Table 2.Thus, under the reaction conditions adopted the

structure of the polymer hosts obtained is near the

following one:

Table 1

Assignment of main absorptions in IR spectrum of CBPE550

Absorption peak (cmÿ1) Assignment Remarks

2943 -CH3 ASCH3

2875.5 -CH3 SCH3

2875 -CH2- ASCH2

1728.8 -CO- ÿ1452 -CO±OCH2 AS-CH2-

1437 -CO±OCH3 AS-CH3

1349.7 -CO±OCH3 AS-CH3

1197 -C±CO±O- AS-C±O-

1095.6 -C±O±C- AS

3.3. Salt concentration dependence of ionic conductivity

The ionic conductivity of the CBPE550±Li salt com-plexes have been investigated over the Li salt concen-tration range from EO/Li+ =6 to EO/Li+ =80 at

the di�erent temperature. Generally the plot of theionic conductivity of solid polymer electrolytes vs saltcontent exhibited a maximum [8]. However, the re-lation between the ionic conductivity of the CBPE550±

Li salt complexes and the salt content exists two peaksas shown in Fig. 2: for the CBPE550±LiClO4 complex,one of two peaks is located in the EO/Li+ =8,

another one in the EO/Li+ =30 (molar ratio) (seeFig. 2(a)). For the CBPE±LiSCN complex, the same isthe case (see Fig. 2(b)). This phenomenon have not

been observed before. The result is similar to that ofrelation between ionic conductivity of the CBP±Li saltcomplexes and the salt content [9] (i.e. Tg a�ects theionic conductivity). It is found that the minimum in Ta

is just corresponding to the maximum in the ionic con-ductivity. After the minimum, the value of Ta rises toa maximum with increasing the concentration, while

the ionic conductivity s falls down to another mini-mum in the same salt concentration region. We believethat this is due to the same reason as CBP±Li salt

complexes. Because the macromolecule backbone ofEMAC is more ¯exible than that of MA. The higherionic conductivity of CBPE±Li salt are fairly reason-

able. It is concluded that the ¯exibility of the macro-molecule main chain in the host polymer±Li saltcomplexes has considerable in¯uence on the migrationof ions in the polymer matrix.

L. Qi et al. / European Polymer Journal 35 (1999) 789±793 791

3.4. Temperature dependence of ionic conductivity

The ionic conductivity of the CBPE550±LiClO4 and

CBPE550±LiSCN complexes increases with increasing

temperature (see Fig. 3). It can not be expressed by the

Arrhenius relationship because the plot of log a vs 1/T

is not a straight line (see Fig. 3). As is well known, for

an amorphous solid polymer electrolyte, the tempera-

ture dependence of the ionic conductivity generally fol-

lows VTF equation (s= AT ÿ1/2 exp{ÿEb/(Tÿ T0)}).

We took glass transition (ÿ508C) of side chain PEO as

T0. The results are shown in Fig. 4. It can be seen

Fig. 2. (a) Salt concentration dependence of the ionic conductivity for CBPE550-LiClO4. (b) Salt concentration dependence of the

ionic conductivity for CBPE550-LiSCN.

Fig. 3. (a) The Arrhenius plot for CBPE550±LiClO4. (b) The Arrhenius plot for CBPE550±LiSCN.

L. Qi et al. / European Polymer Journal 35 (1999) 789±793792

from Fig. 4 that every ®gure obtained is a very goodin¯ected line which is formed by intersection of twostraight lines. These results indicate that the tempera-

ture dependence of ionic conductivity of theCBPE550±LiClO4 and CBPE550±LiSCN systems notonly obeys VTF relationship but also exhibits a special

dual VTF behavior. And it was also found that theintersections for most of in¯ected lines are near 318 K(408C). It is quite obvious that the dual VTF behaviorof the CBPE550±Li system is due to glass transition of

main chain in this temperature range. Therefore, ionicconductivity of CBPE550±Li system was a�ected bythe glass transition of the main chain. Acetone was

adopted as the solvent of make the ®lm of CBPE550and CBPE550±Li complexes. In addition, the solventenable this complexes to process easily. The complexes

have the highest conductivity 2.58�10ÿ5 S/cm at roomtemperature and 1.45�10ÿ3 S/cm at 1008C.

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Fig. 4. (a) VTF plot for the CBPE550±LiClO4 complexes. (b) VTF plot for the CBPE550±LiSCN complexes.

L. Qi et al. / European Polymer Journal 35 (1999) 789±793 793