Synthesis and characterization of a new network polymer electrolyte containing polyether in the main chains and side chains

European Polymer Journal 44 (2008) 2376–2384

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Synthesis and characterization of a new network polymer electrolyte containing polyether in the main chains and side chains Yu-Hao Liang a, Cheng-Chien Wang b, Chuh-Yung Chen a,* a b

Department of Chemical Engineering, National Cheng-Kung University, Tainan 70148, Taiwan Department of Chemical and Material Engineering, Southern Taiwan University, Tainan 710, Taiwan

a r t i c l e

i n f o

Article history: Received 2 August 2007 Received in revised form 12 May 2008 Accepted 14 May 2008 Available online 21 May 2008

Keywords: Network polymer Polymer electrolyte Macroinitiator Comb-like

a b s t r a c t A new network polymer electrolyte matrix with polyether in the side chains and main chains was synthesized by the azo-macroinitiator method and urethane reaction. The macroinitiator, polymer and network polymer were confirmed by Fourier-transform infrared (FT-IR) spectroscopy and 1H NMR. FT-IR was also used to study the environment of lithium ions doped in these network polymer electrolytes. Three important groups are considered: N–H, carbonyl, and ether groups. The thermal properties of the polymer electrolytes were measured by differential scanning calorimetry and thermogravimetric analysis. The Tg value of this polymer is less than that of a general comb-like polymer. Added lithium ions interact with the oxygen atoms on ether groups, causing the Tg of the polymer electrolyte to increase. Moreover, the interaction between lithium ions and ether groups decreases the decomposition temperature of the polymer. The conductivity measured by AC impedance reached a maximum of 104 S cm1. A plot of conductivity vs. temperature fit the Vogel– Tamman–Fulcher equation, indicating that ionic mobility in this network polymer electrolyte is coupled to segmental chain movements. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Since Wright and Armand [1,2] found that poly(ethylene oxide) (PEO) dissolved salts and had ionic conductivity of 108–107 S cm1 at ambient temperature, solid polymer electrolytes have been extensively studied. Ionic conductors for use in electrochemical devices [1] have to satisfy several requirements, including high ionic conductivity, electrochemical stability, and good mechanical properties. Among the polymer electrolyte materials available, PEO and its derivatives are the most widely studied [3,4] as a host for ions because PEO contains ether coordination sites and has a flexible macromolecular structure, which assists in dissociation of salts incorporated in the polymer and promoted facile ionic transport. Unfortunately, the ionic conductivity of PEO–salt complexes is * Corresponding author. Tel.: +886 6 2757575x62643; fax: +886 6 2344496. E-mail address: [email protected] (C.-Y. Chen). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.05.006

poor at room temperature. There are two reasons for this low ionic conductivity. First, ionic transport depends on the motion of the main chain, which rapidly diminishes with decreasing temperature [5,6]. Second, the semi-crystalline nature of PEO interferes with ionic transport. The ionic conductivity of a polymer electrolyte is facilitated in elastomeric amorphous phases owing to motion of the polymer chain [7]. Several approaches have been used to solve these problems. Comb-like polymers with PEO side chains have recently attracted most interest [7–13]. These polymers posses the desired combination of structural strength and low Tg, such that greater segmental motion of the PEO side chains leads to increased mobility of dissolved ions. The highest conductivity at ambient temperature for comb-like polymer electrolytes is currently approximately 105 S cm1. In our previous studies [12–14] to identify a good solvent-free solid polymer electrolyte, we developed comblike polymer electrolytes in which the polar subunits are attached along the polymer chain. Polar subunits, such as

Y.-H. Liang et al. / European Polymer Journal 44 (2008) 2376–2384

acrylonitrile, maleic anhydride and carbonate, can be introduced in a comb-like polymer as another strategy to increase the dissociation of lithium salt and thereby increase the ionic conductivity. However, the conductivity improved by the polar subunit is less. Because the polar groups in the polymer electrolytes decrease the monition of polymer chains. Azo-based macroinitiators (azo-macroinitiators) are widely used for the preparation of block copolymers. First an azo-containing acid chloride, e.g., ‘‘4,40 -azobis(4-cyanopentanoyl chloride) (ACPC), has to be prepared, in this case by the reaction of 4,40 -azobis(4-cyanopentanoic acid) (ACPA) with PCl5 or thionyl chloride according to the procedure of Furukawa et al. [15] or Ueda and Nagai [16]. Then ACPC is reacted with amines or alcohols to prepare the macroinitiator. In thermal addition polymerization, a macroinitiator is used as the telechelic polymer with an active azo-radical chain that connects other vinyl monomers for preparing a block copolymer. In the present study, a new polymer electrolyte with polyether in the side chains and main chains was synthesized using a macroinitiator. It was expected that fast molecular motion of the polymer side chains would contribute to fast ionic transport. Moreover, the polyether in the main chain improves the dissociation of lithium salts and acts as a plasticizer to increase segmental motion. Additionally, the crosslink point in the network polymer can provide mechanical strength for employing it. To prepare the macroinitiator, ACPA was reacted with polyethylene glycol by esterification to obtain an initiator containing polyether groups. This was used to initiate polymerization of the comb-like monomer poly(ethylene glycol-methyl methacrylate) (PEGMEM). The polymer was further reacted with 4,40 -methylene diphenyl diisocyanate (MDI) and pentaerythritol to product a network polymer host. We report here the characterization and ionic conductivity for these network polymer electrolytes.

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SOCl2 was then removed by distillation, and PEG (0.04 mol) and dichloromethane (160 ml) were added to reaction flasks with reflux condenser. The reaction was controlled at 4 ± 0.1 °C for 24 h. The yellow liquid obtained was filtered to remove solid ACPA, which does not react with PEG. Finally, the product was purified by precipitating the reaction mixture in ethylene ether, and the solid product was dried under vacuum for 6 h. The product was named ACPA–PEG (yield 72.3%) and the Mn measured by gel permeation chromatography (GPC) was 7340. Moreover, the ACPA/PEG 4000 ratio (1:1.65) in the macroinitiator ACPA–PEG was estimated from the 1H NMR. 2.3. Polymer preparation PEGMEM (0.04 mol) and THF (50 ml) were mixed and added to a reactor that was previously purged with nitrogen gas to yield an oxygen-free atmosphere. ACPA–PEG (5.87 g) was used as the initiator. The reaction was performed in reaction flasks with reflux condenser, and the reaction temperature was controlled at 60 ± 0.1 °C. After 24 h of polymerization, the flask was cooled to ambient temperature. The polymer was purified by precipitating the reaction mixture into hexane, which was controlled at 78 °C. The precipitate was dried in a vacuum oven at 70 °C for 48 h. The product was named PEGMEM–PEG (yield 91.2%) and Mn measured by GPC was 33,210. Additionally, the PEGMEM/PEG 4000 ratio in the polymer (PEGMEM–PEG) estimated from the 1H NMR spectrum was 26.12:1. PEGMEM–PEG (10.00 g) and DMAC (30 ml) were mixed and added to a reactor previously purged with nitrogen gas. A DMAC (5 ml) solution of MDI (0.6 mmol) containing stannous octoate as the catalyst was added drop-wise into the reactor at 70 °C and reacted for 2 h. Then a DMAC (5 ml) solution of pentaerythritol (0.15 mmol) was added to the reactor and left for 12 h at 90 °C. The products were purified by Soxhlet extraction using ethanol as the solvent for 7 days. Finally, the products were dried in a vacuum oven at 70 °C for 6 days (yield 65.6%).

2. Experimental 2.4. Polymer electrolyte preparation 2.1. Materials Polyethylene glycol (PEG 4000; Fluka) was dehumidified in a vacuum system. Tetrahydrofuran (THF; J.T. Baker), and N,N-dimethyl acetamide (DMAC; Mallinckrodt) were distilled twice and stored over molecular sieves (4 Å). Ethylene ether (J.T. Baker), dichloromethane (J.T. Baker), hexane (J.T. Baker) ACPA (Fluka), thionyl chloride (SOCl2; Fluka), PEGMEM (Mn = 475; Aldrich), MDI (Aldrich) and pentaerythritol (Fluka) were used without purification. Lithium perchlorate (LiClO4; Fluka) was dried in a vacuum oven prior to use. The reaction processes are presented in Scheme 1. 2.2. Macroinitiator preparation ACPA (0.02 mol) and THF (10 ml) were mixed in a reactor equipped with a nitrogen purge. SOCl2 (20 ml) was then slowly added to the reactor while the temperature was maintained at 4 ± 0.1 °C and stirred for 24 h. The excess

The network polymer was prepared as circular films of 1.74 cm in diameter. LiClO4 and THF were mixed to form LiClO4 solutions of various concentrations. Dry polymer films were then immersed into the LiClO4 solutions. Finally, the hybrid films were dried in a vacuum oven at 80 °C for 48 h. 2.5. Characterization of the macroinitiator, polymer and polymer electrolytes High-resolution NMR measurements were performed on a Bruker AMX-400 spectrometer with 1H and 13C resonance frequencies at 400.13 and 100.61 MHz, respectively. The macroinitiator (ACPA–PEG) and polymer (PEGMEM– PEG) were dissolved in deuterium oxide (D2O) and chloroform (CDCl3), respectively. The 1H and 13C chemical shifts were referenced to tetramethylsilane (TMS) at 0.0 ppm. GPC was performed on a Waters 410 differential refractometer calibrated with linear polystyrene (PS) standards.

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Macroinitiator preparation: O H H CH3 2 2 N HO C C C C CN O H H CH3 2 2 N Cl C C C C CN

N

N

O CH3 H2 H2 C C C C OH CN

O H H CH3 2 2 N Cl C C C C CN

excess SOCl2 THF

4º C

CH3 O H2 H2 H2 H2 C C C C Cl + HO C C O H y CN

dichlormethane 4º C CH3 O H2 H2 N C C C C O

O H H CH3 2 2 C C C C N CN

O CH3 H2 H2 C C C C Cl CN

N

H2 H2 C C O y

CN

m

Polymer preparation: HO

CH 3 C

CH 2

C

O

CH 3 H2 C C

H2 H2 C C O y

ACPA-PEG

H2 H2 O C C OH y

n

C

O

O

CH 2CH 2O

-78º C

THF, 60º C

CH 3

Hexane

8

O

CH 2CH 2O

precipitate

(termination by combination)

CH 3 8

CH 3 H2 C C

H2 H2 O C C OH y

n

C

O

O

CH 2CH 2O

CH 3 8

(termination by disproportionation) OH precipitate

+ OCN

H2 C

DMAC, 70º C

HO C OH OH

SnOct

90º C, 12h

NCO 2h

H2 H2 C C O

:

:

MDI

n CH3 H2 HC C

:

IDM C MDI MDI

C

O

O

CH2CH2O CH3 8

Scheme 1. Reaction process.

THF (HPLC-grade) was used as the eluent at a flow rate of 0.7 ml min1. Thermal analysis of the samples was carried out in a DuPont 2910 differential scanning calorimeter

(DSC) from 120 to 120 °C at a heating rate of 10 °C min1. Fourier-transform infrared (FT-IR) spectra were recorded at room temperature using a Bio-Rad FT-IR system coupled

Y.-H. Liang et al. / European Polymer Journal 44 (2008) 2376–2384

to a computer at a resolution of 2 cm1 for 64 scans per sample. Spectra were collected in the range between 600 and 4000 cm1. Thermogravimetric analysis (TGA) experiments were performed using a DuPont TGA Q50 instrument at a scan rate of 10 °C min1 up to 800 °C under a nitrogen atmosphere. The thermal decomposition temperature (Td) in this study is defined as the temperature at 5% weight loss of polymer. 2.6. Conductivity measurements The ionic conductivity of the network polymer electrolyte was determined using an electrochemical cell consisting of the electrolytic film sandwiched between two stainless steel electrodes. The cell was placed inside a thermostat under an Ar atmosphere. Impedance analysis was recorded from 30 to 90 °C using Autolab PGSTAT 30 equipment (Eco Chemie B.V., The Netherlands) with frequency response analysis (FRA) software using an oscillation potential of 10 mV from 100 kHz to 10 Hz in a thermostatic cell. 3. Results and discussion 3.1. Characteristics of the macroinitiator and polymer The macroinitiator (ACPA–PEG) was prepared by reacting ACPC with PEG 4000. Its structure was confirmed by FT-IR and 1H NMR. In the FT-IR spectra of ACPA–PEG (Fig. 1b), the peak that appeared at 1740 cm1 corresponds to –C@O stretching of the ester group. Meanwhile, the – C@O stretching of the acid group (1717 cm1) in ACPA (Fig. 1a) disappeared in ACPA–PEG spectrum (Fig. 1b), indicating the formation of an ester linkage between PEG 4000 and ACPA. In the 1H NMR spectrum (Fig. 2a), the remarkable downfield shift of the peak from 3.7 to 4.2 ppm, which is attributed to methylene protons next to the hydroxyl group, further confirms the esterification reaction between ACPA and PEG 4000 and the synthesis of ACPA–PEG. Polymer PEGMEM–PEG was synthesized by radical polymerization of PEGMEM. Fig. 1c shows the FT-IR spectrum of PEGMEM–PEG. The spectrum displays peaks at 1731 and 1110 cm1 corresponding to the carbonyl ester and –

Transmittance

(a) -C=O of acid

(b) -C=O

-OH

(c)

-C-O-C-OH

-C=O -C-O-C-

(d)

-N-H -C=O -C-O-C-

4000

3500

3000

2500

2000

1500

1000

Wavenumber (cm-1) Fig. 1. FT-IR spectra of composite copolymers: (a) initiator (ACPA), (b) macroinitiator (ACPA–PEG), (c) polymer (PEGMEM–PEG) and (d) network polymer.

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CH2–O–CH2– units, respectively, in the polymer. In the 1 H NMR spectrum (Fig. 2b), the resonance peaks at 0.8– 1.2, 1.6–2.1 and 3.3 ppm are assigned to the –CH3, –CH2 and –CH3 (side chain) protons of PEGMEM, whereas the peaks at 3.6–3.9 ppm are assigned to –CH2–O–CH2– protons. The peak at 4.2 ppm is attributed to methylene protons next to the hydroxyl group on PEGMEM. Both the FT-IR and NMR spectra reveal that a PEGMEM–PEG polymer with PEG in the main chain and side chain was successfully synthesized. The network polymer was synthesized by reaction of PEGMEM–PEG, MDI and pentaerythritol. Fig. 1d shows the FT-IR spectrum of the network polymer. An absorption peak appeared at 3310 cm1, corresponding to N–H stretching. Meanwhile, the –OH stretching (3470 cm1) in PEGMEM–PEG disappeared (Fig. 1c), indicating the formation of a urethane linkage in the network polymer. These results confirm that the network polymer was successfully synthesized. 3.2. IR analysis of network polymer electrolytes FT-IR was further used to study the environment of lithium ions doped in the network polymer electrolytes. Three important regions of the spectrum are considered: N–H stretching vibrations at 3100–3700 cm1, –C@O stretching vibrations at 1650–1800 cm1, and ether stretching vibrations at 1000–1200 cm1 s. 3.2.1. N–H stretching region Fig. 3 shows IR spectra of the N–H stretching region for external doping with LiClO4 at different concentrations. Table 1 shows the positions of the peaks for three vibration modes. According to the literature [17–19], peak 1 at 3481 cm1 is assigned to free N–H stretching vibration. Peak 2 at 3314 cm1 is assigned to N–H groups hydrogen-bonded to the carbonyl oxygen. Peak 3 at 3250 cm1 is assigned to N–H groups hydrogen-bonded to the ether oxygen. Peak 4 at 3188 cm1 is assigned to the sp2 C–H stretching vibration of MDI. Table 1 and Fig. 3 indicate significant changes in the N–H stretching vibration with increasing LiClO4. The band of hydrogen bonding between N–H and carbonyls (peak 2) was shifted from 3314 to 3340 cm1 with increasing LiClO4 concentration. Because the band position is related to the strength of the Hbonded N–H band, the shift to higher frequency with increasing LiClO4 concentration indicates an increase in the strength of the N–H bond. This is likely due to the localization of electron-rich oxygens through coordination of the Li+ cation with the hydrogen-bonded species (Scheme 2b). Thus the strength of hydrogen bonding between N–H and the carbonyls is weakened, resulting in a shift to higher frequency for the N–H band affected by carbonyl groups. Note that Table 1 reveals that the band position for hydrogen bonding of N–H to ether oxygens (peak 3) was shifted from 3250 to 3273 cm1 with increasing LiClO4 concentration, implying that an increased amount of LiClO4 induces greater strength of the N–H bond. This is likely due to the coordination of non-bonded electrons on the ether oxygens with the Li+ cation, leading to weakening of the hydrogen bond strength between N–H and the ether oxygens (Scheme 2c). In addition, for the amount

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Fig. 2. The 1H NMR spectrum of (a) macroinitiator (ACPA–PEG) and (b) polymer (PEGMEM–PEG).

LiClO4 doping of 60.5 mmol g1 polymer, the free N–H stretching vibration is shifted to a higher frequency, presumably due to weakening of the hydrogen bond strength [20]. However, when the concentration LiClO4 exceeds 0.5 mmol g1 polymer, the free N–H peak returns to a lower frequency because the Li+ ions further coordinate with

the lone pair of electrons on the nitrogen atom [21], as shown in Scheme 2a. 3.2.2. C@O stretching region Fig. 4 shows FT-IR spectra of the C@O stretching region for the polymer electrolytes. The band centered at

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H N C

R

O R

O

Absorbance

(c)

ClO4

Li+

C (b)

O

Li+

Peak 3

Peak 2

Peak 1

Peak 4

R N C 3700

ClO4-

H

(a) 3600

3500

3400

3300

3200

3100

Li+

Fig. 3. N–H stretching for the network polymer electrolytes doped with LiClO4: (a) 0.0, (b) 0.5 and (c) 1.5 mmol g1 polymer.

H2 C O

Table 1 Deconvolution results of N–H stretching of the network polymer electrolytes

ClO4H2 C

Peak position (cm1) 1

2

3

3481 3483 3485 3474 3472 3472

3314 3317 3320 3325 3335 3343

3250 3253 3254 3263 3267 3273

1731 cm1 is associated with stretching of the carbonyl groups in PEGMEM. The band at 1727 cm1 is assigned to the stretching of free urethane carbonyl groups, while the peak at 1707 cm1 corresponds to stretching of the hydrogen-bonded urethane carbonyl groups. Table 2 presents deconvolution data for the carbonyl stretching region of the polymer electrolytes by Gaussian–Lorentzian sum. The intensity of the free urethane carbonyl band (peak 2) decreases with increasing LiClO4 concentration because the Li+ ions coordinate with the urethane carbonyl groups [22,23]. According to the literature [23,24], the absorption band for carbonyl groups would be shifted to lower frequency due to the ionization of functional groups. Hence, the decrease in intensity for peak 2 (1727 cm1) should partially correspond to an increase for peak 3 (1707 cm1), which is located at lower frequency. It is evident that the intensity of the hydrogen-bonded urethane carbonyl band (peak 3) obviously increases. However, peak 1 does not change obviously with the LiClO4 concentration because the interactions between the lithium ion and the ester groups of PEGMEM are relatively weak or negligible [12,13]. 3.2.3. Ether stretching region Fig. 5 shows FT-IR spectra of the ether stretching region for the polymer electrolytes. The absorption band of the ether oxygen at 1108 cm1 shifts to lower frequency as the LiClO4 concentration increases. This change is expected

H R N C O

O R

Scheme 2. Schematics for the possible coordination of lithium salt with network polymer.

Peak 2 Peak 1 (c)

Absorbance

[LiClO4] (mmol g1 polymer)

O R

O

Wavenumber (cm-1)

0 0.25 0.5 1.0 1.5 2.0

-

Peak 3

(b)

(a) 1820

1800

1780

1760

1740

1720

1700

1680

1660

1640

Wavenumber (cm-1) Fig. 4. Carbonyl stretching for the network polymer electrolytes doped with LiClO4: (a) 0.0, (b) 0.5 and (c) 1.5 mmol g1 polymer.

owing to the well-known coordination of Li+ ions to the ether oxygen, which has also been observed elsewhere [21,23]. The FT-IR results indicate that the Li+ ion can interact with N–H groups, urethane carbonyl groups, and ether groups in the network polymer electrolyte. Therefore, the

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Table 2 Deconvolution results of carbonyl stretching of the network polymer electrolytes [LiClO4] (mmol g1 polymer)

Peak position (cm1)

0 0.25 0.5 1.0 1.5 2.0

Peak area(%)

1

2

3

1

2

3

1731 1731 1730 1730 1730 1730

1727 1724 1721 1718 1717 1717

1707 1706 1705 1705 1703 1701

60.2 60.7 60.8 60.6 60.5 59.8

34.6 28.8 26.6 22.6 18.9 14.6

5.21 10.5 12.6 16.8 20.6 25.7

Exo

Absorbance

-1

Heat Flow (wg )

(f)

(f) (e) (d)

(e) (d) (c) (b) (a)

(c) (b) -100

(a) 1200

1150

1100

1050

-50

Wavenumber (cm-1) Fig. 5. Ether stretching carbonyl stretching for the network polymer electrolytes doped with LiClO4: (a) 0.0, (b) 0.25, (c) 0.5, (d) 1.0, (e) 1.5 and (f) 2.0 mmol g1 polymer.

0

50

100

Temperature (°C)

1000

Fig. 6. DSC thermograms for the network polymer electrolytes doped with LiClO4: (a) 0.0, (b) 0.25, (c) 0.5, (d) 1.0, (e) 1.5 and (f) 2.0 mmol g1 polymer.

Table 3 DSC results of the network polymer electrolytes

urethane groups in the polymer electrolyte improve the dissociation of LiClO4. 3.3. DSC of the polymer electrolytes The thermal behavior of the polymer electrolytes was studied by DSC, as shown in Fig. 6. All samples showed a glass transition temperature related to the polyether chain. At low LiClO4 concentration, samples exhibited a melting transition at approximately 48 °C during the heating scan. This melting transition is attributed to crystallization of the polyether phase. At higher LiClO4 concentration, coordination of Li+ ions by the polyether chain depressed this crystallization. The dissociation of alkali metal salts by polyethers has been reported to occur via coordination of the alkali metal ions with the ether oxygen of the polyethers, and many studies [8,25] have investigated the effect of such coordination on Tg values for polyether chains. The results of the DSC analysis are summarized in Table 3. The Tg values increase with LiClO4 concentration. This indicates that solvation of Li+ by the ether oxygen of polyether impedes local motion of the polymer segment through the formation of transient crosslinks, thus causing an increase in Tg. Moreover, the value of DTg/DC decreases with LiClO4 concentration. This is attributable to the effect generated by the formation of charge-neutral contact ion

LiClO4 (mmol g1 polymer)

Tg (°C)

DTg/DC

DH (J g1)

0 0.25 0.5 1 1.5 2

68 59 52 44 38 34

– 36 28 16 12 8

2.70 2.22 0.94 – – –

pairs with increasing LiClO4 concentration [12,13,26]. The neutral ion pairs lose their ability to provide ionic crosslinks; hence, any further increase in Tg is insignificant. 3.4. Thermal stability TGA was used to measure the thermal stability of the polymer electrolytes for further application in technological devices. Fig. 7 plots typical TGA curves for the polymer electrolytes. The pure polymer decomposes at 290 °C in one step. However, the degradation temperature (Td) clearly falls to 230–240 °C and decomposition occurs in two steps when LiClO4 is introduced into the polymer. This phenomenon may be attributed to the weakness of the C–O bond caused by a reduction in electronic density due to coordination between the Li+ ions and the ether oxygen of the polyether [27]. Fortunately, the decomposition

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-5.0 100

-5.5 -6.0 -6.5

LiClO4

80

wt (%)

-1

ln(σ) (S cm )

-7.0

60

40

-7.5 -8.0 -8.5 -9.0 -9.5

-10.0 -10.5

20

-11.0 -11.5 0 0

100

200

300

400

500

600

700

2.7

800

2.8

2.9

3.0

3.1

3.2

3.3

1000/T(1/K)

Temperature (°C) Fig. 7. Typical TGA curves of the network polymer electrolytes with various LiClO4 concentrations.

-2.5 -3.0

The conductivity of the solid polymer electrolyte was evaluated by AC impedance and studied as a function of the LiClO4 concentration. Conductivity data as a function of temperature are displayed in Fig. 8a. In Arrhenius plots, all the data show positively curved profiles, indicating Vogel–Tamman–Fulcher (VTF) behavior. A VTF plot of the ionic conductivity is shown in Fig. 8b, and the VTF equation,

r ¼ AT 1=2 exp½B=kðT  T 0 Þ; where A is a constant proportional to the number of carrier ions, B denotes the pseudo-activation energy associated with the motion of the polymer, k is the Boltzmann constant, and T0 is taken as the idealized temperature corresponding to zero configurational entropy [28]. According to the literature on polyether-based polymer electrolytes, in which T0 was found to be approximately Tg – 45 [29], T0 is fixed at Tg – 45 for the electrolytes resulting in the good fit to the data. This result indicates that ionic mobility in this polymer electrolyte is coupled to segmental chain movements. Fig. 9 shows the ionic conductivity at 30 °C for the polymer electrolyte as a function of LiClO4 concentration. The ionic conductivity increases, passes a maximum, and then decreases. As reported by Nishimoto et al. [10], ionic conductivity is proportional to the product of the number of charge carries and their mobility. Therefore, at low LiClO4 concentration, the salt is completely dissociated and the number of mobile ions increases with LiClO4 concentration. However, at higher LiClO4 concentration, the dissociated Li+ and ClO4 ions can form neutral contact ion pairs, which decreases the number of mobile ions, thus decreasing the conductivity. Moreover, based on previous discussions, Tg increases with the LiClO4 concentration. The increase in Tg not only reduces the segmental motion of the polymer, but also directly reduces the ionic mobility.

1/2

-4.0 -4.5 -5.0 -5.5 -6.0 -6.5 -7.0 -7.5 -8.0 -8.5 -9.0 6

7

8

9

10

11

12

13

14

15

16

1000/(T-T0) (1/K) Fig. 8. The (a) Arrhenius plot and (b) VTF plot for network polymer electrolyte doped with LiClO4 (j) 0.25, (s) 0.5, (4) 1.0, (5) 1.5 and (q) 2.0 mmol g1 polymer.

0.00012 0.00010 0.00008 -1

3.5. Conductivity

σ(S cm )

temperature is higher than the practical operating temperature of the polymer electrolytes.

-1

1/2

ln(σ Τ ) (S cm K )

-3.5

0.00006 0.00004 0.00002 0.00000 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

-1

LiClO4 (mmole g polymer) Fig. 9. The ionic conductivity for (j) network polymer electrolytes (s) conventional comb-like polymer electrolyte with various LiClO4 concentrations.

Therefore, the maximum conductivity, as shown in Fig. 9, is determined by these effects. The highest conductivity at 30 °C is 1.13  104 S cm1 for the network polymer

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electrolyte doped with 1.5 mmol LiClO4 g1, which is higher than the value of 105 S cm1 for a conventional comblike polymer electrolyte. (The conventional comb-like polymer was synthesized by free radical polymerization of the comb-like monomer PEGMEM). Moreover, the maximum conductivity of the network polymer is observed at 1.5 mmol LiClO4 g1, while the maximum for the conventional comb-like polymer is obtained at 0.5 mmol LiClO4 g1. This result establishes the possibility of improving the dissociation of LiClO4 by introducing PEG and urethane groups in the comb-like polymer host. 4. Conclusion In the present study, an azo-macroinitiator containing PEG was successfully synthesized by esterification reaction between ACPA and PEG 4000. This macroinitiator was used for polymerization of PEGMEM to synthesize a polymer (PEGMEM–PEG) containing a polyether structure in the main chain and side chain. Moreover, a new network polymer matrix was successfully synthesized by condensation reaction between PEGMEM–PEG, pentaerythritol and MDI. FT-IR results indicate that Li+ ions interact with N– H, carbonyl, and ether groups in the network polymer electrolytes. The interaction between Li+ ions and ether groups increases Tg and decreases Td of the polymer. In this study the highest conductivity at 30 °C was 1.13  104 S cm1 for a polymer electrolyte doped with 1.5 mmol LiClO4 g1 polymer. Acknowledgements The authors would like to thank the National Science Council of the Republic of China (NSC 95-221-E-006-188) and the Ministry of Economic Affairs of the Republic of China (TDPA: 95-EC-17-A-05-S1-0014) for financially supporting this research.

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

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