Synthesis and proton conductivity of anhydrous dendritic electrolytes

DOI: 10.2478/s11532-007-0016-xResearch article

CEJC 5(2) 2007 546–556

Synthesis and proton conductivity of anhydrousdendritic electrolytes

Mehmet Senel, Metin Tulu∗, Ayhan Bozkurt†

Department of Chemistry,Fatih University,

34500 Buyukcekmece, Istanbul-Turkey

Received 17 November 2006; accepted 26 January 2007

Abstract: Water soluble PEG cored dendritic hexa-acid which comprises peripheral carboxylic acidicgroups were prepared via nucleophilic substitution reactions. Novel anhydrous proton conductingelectrolytes were prepared by incorporation of the heterocyclic protogenic solvent imidazole (Im) intoPEG cored dendritic hexa acid, (PEG-HA), at several molar ratios of Im to -COOH units of PEG-HA.The complexation of PEG-HA and Im was illustrated by infrared spectroscopy (FT-IR). The materials arethermally stable up to 150 ◦C as evidenced by thermogravimetry analysis (TGA). Differential scanningcalorimetry (DSC) results verified that the organic electrolytes are homogeneous and amorphous. Theproton conductivities were characterized by means of AC impedance spectroscopy and a maximumconductivity of 1 × 10−3 S/cm was measured at 120 ◦C in the anhydrous state.c© Versita Warsaw and Springer-Verlag Berlin Heidelberg. All rights reserved.

Keywords: Dendritic electrolyte, imidazole, proton conductivity, thermal properties

1 Introduction

Anhydrous proton conducting gel electrolytes are becoming crucial due to their applica-

tion in electrochemical devices such as supercapacitors and electrochromic devices [1, 2].

Recently, several water-free proton conductors were produced either through doping of ba-

sic polymers with strong acid, i.e., H3PO4 [3–5] or acidic polymers with heterocycles such

as imidazole or benzimidazole [6–8]. Both systems have already been shown to possess

high proton conductivity in the dry state. It is very well known that the success of the ion

conducting electrolytes under water-free conditions is expected to depend strongly on the

∗ E-mail: [email protected]† E-mail:[email protected]

M. Senel et al. / Central European Journal of Chemistry 5(2) 2007 546–556 547

nature of the host matrix and the charge carrier. Especially, polymer electrolytes which

comprise ethylene oxide (EO) units with various molecular architectures were considered

to be most promising solutions in terms of electrical properties [9–11]. In that context,

anhydrous PEO/H3PO4complexes were reported to have maximum proton conductivity

of 4×10−5 S/cm at room temperature and 3×10−4 S/cm at 50 ◦C in the absence of mois-

ture [12]. The conductivity isotherms of the PEO/H3PO4 system follow VTF behavior

which indicated a positive contribution of the polymer segmental relaxations to proton

conductivity. Przyluski et al. studied PEO-PMMA-H3PO4 host-guest systems where the

materials exhibited a maximum conductivity of 2.7 × 10−2 S/cm at 50 ◦C [13].

HN

O Ot-Bu

OO t-Bu

O Ot-Bu

NH

OOt-Bu

OOt-Bu

OOt-Bu

O O

O

O

OO

O

O

Cl

Cl

nnTHF, Et3N

N NH

NH

O O

O

O

OO

HN

OO

O

O

OO

O O

O

O

HN

HN

+

NH NH+-

NHNH

+

HNHN +

HNHN +

-

-

-

--

n

HCO2H

a) PEG-HA

b) PEG-HA x Im

NH

NH

+-

HN

O OH

OO H

O OH

NH

OOH

OOH

OOH

O O

O

O

n

NH2

O

Ot-Bu OO

t-Bu

O

O t-Bu

Scheme 1 Synthesis of PEG cored dendric hexa ester and corresponding hexa acid and

PEG-HA1Im (b).

In the present work, A newkome type dendron, di-tert-butyl 4-(2-(tert-butoxycarbonyl)-

ethyl)-4-aminoheptanedioate [14], was coupled with freshly prepared PEG dicarboxylic

acid chloride [15]. Formed PEG coored dendritic hexa ester was hydrolysed to obtain

proton conducting dendritic gel electrolytes consisting of an acidic group, PEG cored

dendritic hexa acid, which is abbreviated as PEG-HA (Scheme 1). Later different ratios

of imidazole were doped into the PEG-HA and their thermal and proton conductivities

were investigated.

548 M. Senel et al. / Central European Journal of Chemistry 5(2) 2007 546–556

2 Experimental

2.1 Materials

All chemicals and solvents are reagent grade and used without further purification, unless

otherwise indicated. Polyethylene glycols (600 g/mol), Nitromethane, 1,2-Dimethoxy

ethane and tert-butylacrylate were from Fluka. Raney Nickel catalyst (Al-Ni alloy 50%

w/w), triton-B, triethylamine, thionylchloride were obtained from Merck, methanol (imi-

bn) was prepared according to literature [14]. Reactions were monitored by thin layer

chromatography (TLC) on silica gel 60 F254 (E. Merck, Darmstadt) and spots were

detected either by UV-absorption or by charring with I2 vapor. Dialysis was performed

by Spectrum membrane filtration having MWCO 1KD, 2KD and 3KD in water.

2.2 Synthesis of PEG-HA

To a stirred solution of di-tert-butyl 4-(2-(tert-butoxycarbonyl) ethyl)-4-aminoheptane-

dioate (1.74 g, 4.2 mmol) and Et3N (0.43 g, 4.2 mmol) in dry THF was added freshly

prepared PEG diacetylchloride (1.31 g, 2 mmol). The mixture was stirred first for 30 min

at 0 ◦C and then 6 h at 25 ◦C. The solution was first filtered to remove the Et3N salt,

and then the excess Et3N and solvents were evaporated to obtain the crude product. The

product was dialysed in Methanol by using a membrane with a Molecular Weight Cut Off

(MWCO): of 1 KD to afford (75%) of pure PEG hexaester as viscous oil (Scheme 1). Then

1 g of this dendritic ester was hydrolysed in 5 mL of formic acid at room temperature

for 12 hours. After evaporation excess formic acid and formed tert-butanol, subsequent

PEG-HA was obtained quantitatively without further purification. 1H NMR (CDCl3)

δ: 1.28, (s, OCH2CO), 1.85 (t, CH2CH2COOH), 2.19 (s, OCH2CH2O in POE), 3.69

(t, CH2COOH), 4.09 (s, NH); IR, (KBr), cm− : 3300-3200 (NH and OH), 1777 acidic

(C=O), 1632 amide (C=O) and 1145 (C-O); Elemental Analysis, (%): Calculated: C:

59.4, H: 9.13, N: 1.98; Found: C: 57.22, H: 8.12, N: 1.92

2.3 PEG-HA-Im electrolytes

A stoichiometric amount of PEG-HA, and imidazole (Im) were dissolved in tetrahydro-

furan (THF) with different molar ratios x which were determined by x = [ Im]/ [-COOH

unit of PEG-HA]. The [Im] and [-COOH unit of PEG-HA] are molar concentrations of

Im and carboxylic group in PEG-HA, respectively. The solutions with x=0.5 and x=1

were prepared and cast on the polished, Polytetrafluoroethylene (PTFE) plates and dried

under vacuum at 60 ◦C. Transparent, homogeneous and hygroscopic gels were obtained

and were stored under nitrogen atmosphere.


2.4 Measurements

NMR spectra were recorded on an Inova 500 MHz Varian system spectrometer using

solutions of CDCl3. The IR spectra were recorded with a Mattson Genesis II spectropho-

tometer by casting the gels onto silicon wafers. Thermal properties of the gel electrolytes

were studied by Thermogravimetry analyses (TGA) using a Mettler-Toledo TG-50 and

Differential Scanning Calorimetry (DSC) using a Mettler-Toledo DSC 30. All the thermal

measurements were carried out at a temperature scanning rate of 10 ◦C/min and under

a nitrogen atmosphere. Proton conductivity of PEG-DA-Im mixed gels was measured

using a SI 1260-Schlumberger impedance analyzer. The measurements were carried out

in the Max-Planck Institute for Polymer Research Mainz-Germany. The gels were placed

between platinum electrodes and their conductivities were measured within the frequency

range from 1 Hz to 1 MHz at various temperatures.

3 Results and discussion

Water soluble PEG cored dendritic hexa acid which comprises peripheral carboxylic acidic

groups were prepared via a nucleophilic substitution reaction. First Newkome type three

branched dondron, (di-tert-butyl 4-amino-4-(3-tert-butoxy-3-oxopropyl)heptanedioate),

was synthesized [14]. This dendron was successfully coupled with freshly chlorinated

PEG dicarboxylic acid [15]. The subsequent hexaester was hydrolyzed from the esteric

groups to make it water soluble. The 1H NMR spectra of the PEG-HA is shown in

Figure 1. Possible carboxylic protons could not be observed due to a dimerization effect

due to H bonding betwen the carboxylic acids. However amidic protons at 4.09 ppm,

dendritic protons at 1.85 and 3.69 ppm respectively, and repeating units protons of PEG

at 2.19 ppm were correlated with previous studies [14–16].

NH

OOH

O

OH

OOH

PEG-HAOO

O

10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5ppm

1.281.

85

2.19

3.69

7.28

n

CD

Cl 3

Fig. 1 1H NMR spectra of PEG-HA


The solutions of PEG-HA and Im were cast onto the poly(tetrafluoroethylene), PTFE,

plates and the solvent was evaporated to produce PEG-HAxIm dendritic electrolytes. The

materials were in gel form irrespective to composition and very hygroscopic.

4000 3000 2000 1000

PEG-HA

CO2HCO2

-

x=1.0

x=0.5

% T

rans

mitt

ance

(a.u

.)

Wavenumbers (cm-1)

Fig. 2 Infrared (IR) spectra of the gel electrolytes, PEG-HAxIm.

Figure 2 shows the Infrared (IR) spectra of pure PEG-HA and PEG-HA-Im mixed

materials after blending. As the analytical bands, pure dendritic electrolyte gives a broad

peak arround 3300-3200 cm−1 due to amide NH and free acidic OH and a strong peak

near 1717 cm−1 due to C=O stretching of the terminal carboxylic acid and 1632 cm−1

due to amide C=O. Subsequent to blending, a new broad peak appears at 1653 cm−1

and the intensity of the carbonyl stretching at 1720 cm−1 decreased for x=0.5 and com-

pletely disappeared when x=1. This new peak arises from the asymmetric stretching of

deprotonated carboxylic acid units. Also, a medium peak became clear at 1300 cm−1 due

to in plane deformation of C–O–H in both blends. These results show that the proton

transfer reactions between PEG-HA and Im form complexes and the final structure can

be illustrated as in Scheme 1b.

Thermal properties of the gel electrolytes were investigated for both thermogravimetry

and differential scanning calorimetry. Prior to measurements all the samples were dried

under vacuum and the experiments were performed under nitrogen atmosphere. PEG-HA

x Im (x=0.5 and x = 1) complexes exhibited a thermal stability up to 150 ◦C (Fig. 3).

The weight loss above this temperature can be attributed to decomposition of the gel

electrolytes.

Figure 4 shows the DSC traces of PEG-HAxIm electrolytes. The Tg of the PEG-

HA0.5Im is -4 ◦C and that of PEG-HA1Im is -22 ◦C. Shifting of the Tg can be attributed


0 100 200 300 400 500 600

0

20

40

60

80

100

% W

eigh

t

Temperature (oC)

x=0.5 x=1.0

Fig. 3 TG curves of PEG-HAx Im recorded under N2 atmosphere at a heating rate of 10◦C/min.

-150 -100 -50 0 50 100 150

(Hea

t Exc

hang

e) E

xo >

Temperature ( oC)

0.5 1.0

Fig. 4 DSC curves of PEG-HAxIm recorded under N2 atmosphere at a heating rate of

10 ◦C /min.

to the softening effect of Im as described in our previous work [7]. It is clearly seen that

a single phase model is a reasonable assumption for anhydrous PEG-HAxIm complexes.


10-2 10-1 100 101 102 103 104 105 106 107

1E-6

1E-5

1E-4

1E-3120 oC

80 oC

60 oC

40 oC

20 oC

ac (S

cm

-1 )

Frequency (Hz)

x=1

Fig. 5 AC conductivity of PEG-HA1 Im at several temperatures.

Figure 5 displays the frequency and temperature dependence of the alternating current

AC conductivity (σac) of PEG-HA1Im. The curves comprise broad frequency independent

conductivity plateaus which shift toward higher frequencies with increasing temperature.

The direct current, DC, conductivities (σdc) of the samples were estimated from the AC

conductivity plateaus according to previous literature [8, 10]. The temperature depen-

dences of the proton conductivity of the PEG-HAxIm are illustrated in Figure 6. All

the dentritic electrolytes showed a positive temperature-conductivity dependency within

the temperature range of the measurements. The curved conductivity isotherms were

explained with the Vogel–Tamman–Fulcher (VTF) equation Eq. 1.

log σ = log σo − B

k(T − To)(1)

where σo is the conductivity at infinite temperature, k Boltzman constant, B and To

are empirical parameters. The VTF fits are inserted in Figure 6 (dot lines) and the fit

parameters are listed in Table 1.

Table 1 VTF parameters.

Sample To (K) Log σo B (eV)

PEG-HA 0.5 Im 219 -1,6 0.04PEG-HA1.0 Im 223 -1.2 0.06

The proton conductivity of anhydrous PEG-HAxIm electrolytes improved through in-


2.4 2.6 2.8 3.0 3.2 3.4

-7.0

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

Log

dc (S

/cm

)

1000 / T (K-1)

x=1 x=0.5

Fig. 6 Temperature dependences of the proton conductivity of the dendritic electrolytes

PEG-HAx Im.

corporation of Im. The conductivity difference between PEG-HA0.5Im and PEG-HA1Im

is more pronounced at lower temperatures and approaches each other at higher temper-

atures. In this system conductivity increase was found to depend on the glass transition

temperature rather than imidazole volume fraction [5, 17]. This is proved by plotting σdc

versus T-Tg and presented in Figure 7.

It is reasonable that the proton transport may occur by two routes, that is, the

proton transport between imidazole units through protonic defects (free nitrogen) and

ethylene oxide units of the oligomer groups that the proton interacts with through hydro-

gen bonding. FT-IR of PEG-HAxIm confirmed the partial protonation of imidazole from

the “free” nitrogen side. The proton exchange reactions between neighboring protonated

and unprotonated imidazoles is described by structure diffusion (Grotthuss mechanism)

where imidazole acts as proton donor and acceptor in the conduction process [18]. The

protonic defects may cause local disorder by forming a hydrogen bonded network, i.e.,

Him-(HimH+)-imH), which enhance long range proton transport. Similar mechanisms

were reported for other studies, i.e., Polymers have been doped with amphoteric hetero-

cyclic structures such as imidazole [19, 20], benzimidazole [21] and pyrazole [22].

Maximum conductivity of the blends based on poly(acrylic acid)/imidazole is 1 ×10−3 S/cm at 120 ◦C [7] and alginic acid/imidazole is 2 × 10−3 S/cm at 160 ◦C [6]. The

current system PEG-HA1Im showed a maximum proton conductivity of 10−3 S/cm at

120 ◦C.

The use of imidazole in dendritic acid PEG-HA increased the concentration of defects

and enhanced the proton conductivity. The physical properties (conductivity and elas-


20 40 60 80 100 120 140-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

Log

dc (S

/cm

)

T-Tg (K)

x=1 x=0.5

Fig. 7 Conductivities of the dendritic PEG-HaxIm electrolytes as a function of the re-

duced temperature.

ticity) of this system may allow for practical applications to electrochromic devices [23]

and supercapacitors [24].

4 Conclusion

New proton conducting dendritic electrolytes based on PEG cored dendritic acid, PEG-

HA and Im were synthesized by means of complexation. Thermogravimetry analyses

indicated that these materials are thermally stable up to 150 ◦C. Differential scanning

calorimetry results proved that the organic electrolytes are homogeneous and amorphous.

From IR spectra and conductivity isotherms, it can be concluded that proton conduc-

tivity occurs through structure diffusion (Grotthuss mechanism). Anhydrous dendritic

electrolytes present relatively high proton conductivity of 1× 10−3 S/cm at 120 ◦C. Our

future work will focus on the dendrimers comprising EO cores and heterocyclic peripheral

units.

Acknowledgment

This work was supported by TUBITAK – 104M220 , DPT (State Planning Center) and

Fatih University Research Support Officce( P50020603). The authors thank C. Sieber

and P. Rader in the Max-Planck Institute for Polymer Research, Mainz - Germany for

technical assistance and Dr. Bayram Cakir from Ruhr-Universitat Bochum-Germany for

the NMR measurements.


References

[1] Y.G. Wang and X.G. Zhang: “All solid-state supercapacitor with phosphotungstic

acid as the proton-conducting electrolyte”, Solid State Ionics, Vol. 166, (2004), pp.

61–67.

[2] A. Bozkurt: “Application of Proton Conducting Polymer Electrolytes to Elec-

trochromic Devices”, Turk. J. Chem., Vol. 26, (2002), pp. 663–668.

[3] P. Jannasch: “Recent developments in high-temperature proton conducting polymer

electrolyte membranes”, Curr. Opin. Colloid In., Vol. 8, (2003), pp. 96–102.

[4] A. Bozkurt and W.H. Meyer: “Proton conducting blends of poly(4-vinylimidazole)

with phosphoric acid”, Solid State Ionics, Vol. 138, (2001), pp. 259–265.

[5] M.F.H. Schuster and W.H. Meyer: “Anhydrous proton conducting polymers”, Annu.

Rev. Mater. Res., Vol. 33, (2003), pp. 233–261.

[6] M. Yamada and I. Honma: “Alginic acid–imidazole composite material as anhydrous

proton conducting membrane”, Polymer, Vol. 45, (2004), pp. 8349–8354.

[7] A. Bozkurt, W.H. Meyer and G. Wegner: “PAA/imidazol-based proton conducting

polymer electrolytes”, J. Power Sources, Vol. 123, (2003), pp. 126–131.

[8] A. Bozkurt: “Anhydrous Proton Conductive Polystyrene Sulfonic Acid Membranes”,

Turk J. Chem., Vol. 29, (2005), pp. 117–123.

[9] P. Donoso, W. Gorecki, C. Berthier, F. Defendini, C. Poinsignon and M.B. Armand:

“NMR, conductivity and neutron scattering investigation of ionic dynamics in the an-

hydrous polymer protonic conductor PEO(H3PO4)x)”, Solid State Ionics, Vol. 28/30,

(1988), pp. 969–974.

[10] W.H. Meyer: “Polymer Electrolytes for Lithium-Ion Batteries”, Adv. Mater., Vol.

10, (1998), pp. 439–448.

[11] M.F.H. Schuster, W.H. Meyer, G. Wegner, H.G. Herz, M. Ise, M. Schuster, K.D.

Kreuer and J. Maier: “Proton mobility in oligomer-bound proton solvents: imidazole

immobilization via flexible spacers”, Solid State Ionics, Vol. 145, (2001), pp. 85–92.

[12] G.R. Newkome, R.K. Behera, C.N. Moorefield and G.R. Baker: “Cascade polymers:

Synthesis and characterisation of four-directional spherical dendritic macromolecules

based on adamantane”, J. Org. Chem., Vol. 57, (1992), pp. 358–362.

[13] J.J. Przyluski, W. Wieczorec and S. Glowinkowski: “Novel Proton Polymer Ionic

Conductors”, Electrochim. Acta, Vol. 37, (1992), pp. 1733–1745.

[14] G.R. Newkome, C.N. Moorefield, G.R. Baker, R.K. Behara, G.H. Escamilia and

M.J. Saunders: “Supramolecular self-assemblies of two-directional cascade molecules

: automorphogenesis”, Angew. Chem., Vol 31, (1992), pp. 917-919.

[15] H. Namazi and M. Adeli: “Novel linear–globular thermoreversible hydrogel ABA

type copolymers from dendritic citric acid as the A blocks and poly(ethyleneglycol)

as the B block”, Eur. Polym. J., Vol. 39, (2003), pp. 1491–1500.


[16] M. Tulu and K.E. Geckeler: “Synthesis and properties of hydrophilic polymers.

Part 7. Preparation, characterization and metal complexation of carboxy-functional

polyesters based on poly(ethylene glycol)”, Polymer Int., Vol. 48, (1999), pp. 909–

914.

[17] A. Bozkurt and T. Pakula: “Dielectric and dynamic mechanical relaxations in

polymer-heterocycle hybrid materials”, Chem. Phys. Lett., Vol. 422, (2006) pp. 496–

499.

[18] W. Munch, K.D. Kreuer, W. Silvestri, J. Maier and G. Seifert: “The diffusion mech-

anism of an excess proton in imidazole molecule chains: first results of an ab initio

molecular dynamics study” Solid State Ionics, Vol. 145, (2001), pp. 437–443.

[19] C. Yang, P. Costamagna, S. Srinivasan, J. Benziger and A.B. Bocarsly: “Approaches

and technical challenges to high temperature operation of proton exchange membrane

fuel cells”, J. Power Sources, Vol. 103, (2001), pp. 1–9.

[20] F. Sevil and A. Bozkurt: “Proton conducting polymer electrolytes on the basis of

poly(vinylphosphonic acid) and imidazole”, J. Phys. Chem. Solids, Vol. 65, (2004),

pp. 1659–1662.

[21] A. Bozkurt and T. Pakula: “Dielectric and dynamic mechanical relaxations in

polymer–heterocycle hybrid materials”, Chem. Phys. Lett., Vol. 422, (2006), pp.

496–499.

[22] M. Yamada and I. Honma: “Proton conducting acid–base mixed materials under

water-free condition”, Electrochim. Acta, Vol. 48, ( 2003), pp. 2411–2415.

[23] K.D. Kreuer, A. Fuchs, M. Ise, M. Spaeth and J. Maier: “Imidazole and pyrazole-

based proton conducting polymers and liquids”, Electrochim. Acta, Vol. 43, (1998),

pp. 1281–1288.

[24] M. Morita, J .L. Qiao, N. Yoshimoto and M. Ishikawa: “Application of proton con-

ducting polymeric electrolytes to electrochemical capacitors”, Electrochim. Acta, Vol.

50, (2004), pp. 837–841.

Synthesis and proton conductivity of anhydrous dendritic electrolytes

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