Large Enhancement of the Ionic Conductivity in an
Electron-Beam-Irradiated [Poly(ethylene glycol)]
x
LiClO
4
Solid Polymer Electrolyte
TH. JOYKUMAR SINGH,
1
GANESHSANJEEV,
2
K. SIDDAPPA,
2
S. V. BHAT
1
1
Department of Physics, Indian Institute of Science, Bangalore 560 012, India
2
Microtron Centre, Mangalore University, Mangalagangotri 574 199, India
Received 23 July 2003; revised 14 October 2003; accepted 19 November 2003
ABSTRACT: The effect of electron-beam (4 – 8 MeV) irradiation on the ionic conductivity
of a solid polymer electrolyte, poly(ethylene glycol) complexed with LiClO
4
, was studied.
A large enhancement of the conductivity of nearly two orders of magnitude was
observed for the highest dose of irradiation (15 kGy) used. The samples were charac-
terized with differential scanning calorimetry, matrix-assisted laser desorption/ioniza-
tion, and electron spin resonance spectroscopy. Although no free radicals were present
in the irradiated samples, a decrease in the glass-transition temperature and an
increase in the amorphous fraction were observed. Even though pure poly(ethylene
glycol) underwent considerable fragmentation, unexpectedly, no significant fragmen-
tation was observed in the polymer–salt complexes. The enhancement of the conduc-
tivity was attributed to an increase in the amorphous fraction of the systems and also
to an increase in the flexibility of the polymer chains due to the irradiation.
© 2004 Wiley
Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1299 –1311, 2004
Keywords: solid polymer electrolyte; electron beam irradiation; MALDI; glass tran-
sition; ionic conductivity; (PEG)
x
LiClO
4
INTRODUCTION
During the last two decades, solid polymer elec-
trolytes (SPEs) have attracted considerable scien-
tific and technological attention because of their
interesting physics and wide application poten-
tials in high-energy density batteries, electrochro-
mic displays, sensors, and fuel cells.
1– 4
Most of
these studies have concerned high-molecular-
weight polymers [e.g., poly(ethylene oxide) (PEO);
molecular weight ⬃ 10
6
] complexed with alkali
metal salts (lithium salts being the most stud-
ied
5,6
), and not much attention has been paid to
the somewhat low-molecular-weight polymers
(molecular weight ⬃ 2000). It is known that poly-
mers below a critical molecular weight (⬃3200)
have different viscosity and diffusion behaviors. A
study on the effect of the molecular weight of a
polymer on cation mobility by Shi and Vincent
7
has shown that even though the molecular weight
has no significant effect on cation mobility above
a critical limit of approximately 3200, below that
an additional cation transport mechanism could
be operating. In this low-molecular-weight region,
called the Rouse region, there is a possibility of
polymer chain diffusion in addition to segmental
motion. Bearing these facts in mind, we have
prepared a new SPE based on poly(ethylene gly-
col) (PEG; molecular weight ⫽ 2000) complexed
Correspondence to: S. V. Bhat (E-mail: svbhat@physics.
iisc.ernet.in)
Journal of Polymer Science: Part B: Polymer Physics, Vol. 42, 1299 –1311 (2004)
© 2004 Wiley Periodicals, Inc.
1299
with lithium perchlorate salt. LiClO
4
has been
chosen because it fulfils the electrochemical sta-
bility criteria
8
and has low lattice energy (723 kJ
mol
⫺1
), which is favorable for the formation of
polymer–salt complexes.
It is generally accepted that polymer–salt com-
plexes consist of three coexisting phases: a crys-
talline polymer, a crystalline polymer–salt com-
plex, and an amorphous polymer–salt complex.
9
Most of the easily solvating host polymers, such
as PEO and PEG, used for complexing with low-
lattice-energy salts to form SPEs are partially
crystalline in nature. It has been shown that the
amorphous regions mainly contribute to the ob-
served ionic conductivity.
10
Therefore, one of the
most promising ways of achieving better ionic
conductivity is increasing the amorphous fraction
in the system. Different methods have been used
to achieve this goal. Plasticization with a low-
molecular-weight polymer
4,11
and the dispersion
of insulating nanoparticle fillers into SPEs are
two of the ways of increasing the amorphous frac-
tion.
12,13
The irradiation of SPE systems with
high-energy beams can also be used to achieve an
increased amorphous fraction and, therefore, en-
hance the ionic conductivity. In principle, radia-
tion damage can result in both chain scission (or
chain fragmentation) and crosslinking. Extensive
chain scission will lead to a reduction in molar
mass and higher flexibility of the chains, whereas
crosslinking will usually increase the molar mass,
leading to a less flexible product.
14
Different types
of irradiation [
␥
-ray irradiation, photoirradiation,
electron-beam irradiation (e-beam irradiation),
and ion-beam irradiation] can be used to irradiate
polymeric systems. A few experimental studies on
the effect of irradiation, mostly
␥
-ray irradiation
and photoirradiation, on SPEs have been re-
ported to date, and they have focused mainly on
crosslinking. McCallum et al.
15
␥
-irradiated com
-
plexes formed by high-molecular-weight (10
6
)
PEO and LiClO
4
. The product thus obtained
showed ambient ionic conductivity somewhat
lower than that of unirradiated samples. This was
understood to be a result of decreased chain flex-
ibility caused by crosslinking, as evidenced by an
increase in the glass-transition temperature (T
g
).
However, Song et al.
16
in their study of a PEO–
LiClO
4
system, over a wide range of
␥
-irradiation
doses, found an increase in the ionic conductivity
up to 6.8 ⫻ 10
⫺4
S/cm associated with crosslink
-
ing. It appears that crosslinking may lead to ei-
ther an increase in the amorphous fraction or a
reduction in the flexibility of the polymer seg-
ments. Therefore, one may observe either a reduc-
tion in the ionic conductivity
15
or an enhancement
when the crosslinking is optimal, as achieved by
Song et al.
16
Zhang et al.
17
recently reported the
effect of radiation dose on the destruction of the
crystallinity of PEO. They found, using wide-an-
gle X-ray diffraction and calorific measurements,
that about 3.5 ⫻ 10
6
Gy of
␥
-radiation was re
-
quired to completely destroy the crystallinity and
that this resulted in highly crosslinked, fragile,
and glassy products. As is well known, such
glassy specimens are mechanically unsuitable as
useful polymer hosts. Also, because of the pres-
ence of excessive crosslinks, the ambient ionic
conductivity was still rather low. The
␥
-radioly-
sis-induced chain degradation of PEO and the
production of carbonyl groups at the end of the
cleaved polymer chain were reported by Okamato
and Cho.
18
They observed an increase in the con
-
ductivity by a factor of approximately 3 at room
temperature upon
␥
-irradiation and reported that
the effect of e-beam irradiation was similar. The
photoirradiation control of some specific SPEs
was also carried out by Kobayashi et al.
19
There
are also a number of patents
20,21
related to the
use of irradiation for enhancing crosslinking in
polymer electrolytes and, therefore, increasing
their mechanical stability. In this study, we used
e-beam irradiation to cause chain scission and
thus an increase in the flexibility of the polymer
systems and enhanced ionic conductivity. We ob-
served a large enhancement (nearly two orders of
magnitude) in the ionic conductivity upon irradi-
ation. To the best of our knowledge, there is no
detailed report on the effect of e-beam irradiation
leading to such a large enhancement of the ionic
conductivity of an SPE.
We earlier investigated a (PEG)
x
LiClO
4
system
(molecular weight of PEG ⫽ 2000; x is the ratio of
ether oxygens to Li
⫹
) and found that the room-
temperature ionic conductivity had a maximum
value of 7.27 ⫻ 10
⫺7
S/cm for x ⫽ 46.
22
In this
article, we report detailed studies on the effect of
e-beam irradiation on the morphology, thermal
properties, and the ionic conductivity of the SPE.
We use three different doses: 5, 10, and 15 kGy.
We have found that the maximum ionic conduc-
tivity increases as the irradiation dose increases.
We have characterized the samples, showing the
largest ionic conductivity (i.e., the one irradiated
with 15 kGy) by matrix-assisted laser desorption/
ionization (MALDI), electron spin resonance
(ESR), and differential scanning calorimetry
1300 SINGH ET AL.
(DSC) studies. We also offer a possible explana-
tion for the enhanced ionic conductivity.
EXPERIMENTAL
Sample Preparation
PEG (molecular weight ⫽ 2000; Fluka) and Li-
ClO
4
(Fluka) were used without further purifica
-
tion. LiClO
4
was dried in an oven overnight at
100 –110 °C to remove the moisture before use.
The (PEG)
x
LiClO
4
(x ⫽ 20 –500) samples were
prepared with the solution-casting method with
methanol (analytical-reagent-grade) as the com-
mon solvent. The solutions were magnetically
stirred for 7– 8 h at room temperature, and this
was followed by another hour at approximately 50
°C in a nitrogen atmosphere. The viscous solu-
tions were then poured into Teflon rings (diame-
ter ⬃ 7 mm, thickness ⬃ 500
m to 1 mm) and
kept in a glove box under a nitrogen atmosphere
over night, and the solvent was allowed to evap-
orate slowly. The solidified samples were trans-
ferred to a vacuum desiccator for vacuum drying
under continuous pumping. These methods were
used to dry the samples thoroughly, as they were
hygroscopic. For the same reason, they were also
stored inside a vacuum desiccator before the var-
ious experiments were performed.
E-Beam Irradiation of the Samples
Thin film samples with various salt concentra-
tions were irradiated in a 4–8-MeV pulsed elec-
tron accelerator (Microtron Centre, Mangalore
University, Mangalagangotri, India). The elec-
tron beam was emitted from a LaB
6
single crystal.
The samples were sealed in ultrathin and trans-
parent polythene sheets and were irradiated di-
rectly in air at room temperature. Typically, the
pulse duration was 2.3
s with a repetition fre-
quency of 50 Hz. A dose rate of 1 kGy/min was
used. The samples were irradiated in one stretch.
Still, there was no noticeable heating of the sam-
ples, mostly because of the very small average
beam wattage. The samples were irradiated for a
few minutes to achieve three different doses: 5,
10, and 15 kGy. The doses were monitored with a
current integrator.
X-Ray Diffraction (XRD)
XRD patterns for unirradiated and irradiated
PEG 2000 were recorded (Scintag XDS 2000,
United States) at a scanning rate of 10°/min in
the 2
range of 10 – 80° to study the changes in the
crystalline fraction. This slightly high scanning
rate was used so that the samples did not absorb
moisture. The samples were prepared on glass
slides so that their insertion into the XRD ma-
chine would be easier. The areas of the films on
the glass slides were kept the same for both un-
irradiated and irradiated PEG 2000 so that a
comparison of intensities would be possible.
Mass Spectrometry
Matrix-assisted laser desorption/ionization time-
of-flight mass spectrometry (MALDI-TOF MS) is
a soft ionization technique in which the energy
from a laser is dissipated in volatilizing the ma-
trix rather than in degrading the polymer. The
matrix material also leads to the isolation of the
polymer molecules from one another.
23
We used a
MALDI-TOF MS instrument (Kompact SEQ,
Kratos Analytical Instruments, Manchester,
United Kingdom) to analyze the molecular weight
distribution of unirradiated and irradiated PEG
2000 and (PEG)
x
LiClO
4
SPE samples. This mass
spectrometer was equipped with a pulsed nitro-
gen laser (
⫽ 337 nm, pulse width ⬃ 4 ns). The
instrument was operated in the positive polarity
mode with a linear flight path. The samples were
dissolved in methanol before being loading onto
the mass spectrometer. The solvent prevented the
aggregation of the polymer. Under vacuum condi-
tions, the solvent was removed, and cocrystallized
polymer molecules were left behind, homoge-
neously dispersed within the matrix molecules.
2,5-Dihydroxybenzoic acid (Gentisic Acid, Sigma
Chemicals) was used as the matrix.
ESR
ESR (ER 200D-SRC, Bruker) spectra for the glass
rod used to mount the unirradiated and 15-kGy-
irradiated PEG 2000 and (PEG)
46
LiClO
4
samples
were recorded at a scanning rate of 60 Gauss/s
from 0 to 6000 Gauss in forward and reverse
magnetic field scans at room temperature. The
modulation amplitude was 4 Gauss, and the mi-
crowave power was 150 mW.
DSC
DSC experiments were carried out to compare the
thermal properties of the unirradiated and irra-
diated systems. An MDSC 2920 (TA Instruments)
[PEG]
x
LiCLO4 SOLID POLYMER ELECTROLYTE 1301
machine in the standard mode was used. Samples
(10 –12 mg) were heated to 90 °C at the rate of 10
°C/min, cooled to ⫺90 °C, and then heated again.
An empty aluminum pan was used as a reference.
Dry nitrogen gas was used to purge the DSC
sample cell at a rate of 25 mL/min.
Ionic Conductivity Measurements
The ionic conductivity of the unirradiated and
irradiated samples was measured with the com-
plex impedance method. A vector lock-in amplifier
(PAR 5210) in the frequency range of 2 Hz to 120
kHz with a signal of 500 mV was used for this
purpose. The sample was loaded into the sample
cell with a spring-fit stainless steel blocking elec-
trode and was kept in a vacuum desiccator for the
room-temperature ionic conductivity measure-
ments. The samples were soft and were deposited
into Teflon rings to maintain the intactness of the
shape during the conductivity measurements.
The measurements were also carried from 250 to
315 K. For the temperature variation studies, the
sample cell was kept in a glass dewar, and cold
nitrogen gas produced from boiling liquid nitro-
gen flowed continuously. The temperature was
changed at intervals of 5 K during heating with a
Bruker VT-1000 temperature controller. After the
desired temperature was set, the cell was kept at
that temperature for 20–25 min to stabilize at the
set value. All the experimental spectra were an-
alyzed with Boukamp’s Equivalent Circuit soft-
ware.
24
RESULTS AND DISCUSSION
XRD Patterns
XRD patterns for unirradiated and irradiated
PEG 2000 are shown in Figure 1. The two prom-
inent peaks at 2
⫽ 19.18° and 2
⫽ 23.36°
25
are
present in both patterns, indicating the crystal-
line nature of PEG 2000. The intensity of these
two peaks for irradiated PEG 2000 is reduced and
slightly broadened. Most of the weaker peaks also
disappear after irradiation. This result signifies a
decrease in the crystalline fraction of the polymer
after irradiation.
Mass Spectroscopy
The molecular weight distributions of unirradi-
ated and irradiated PEG 2000 and (PEG)
x
LiClO
4
(x ⫽ 20, 46, or 100) were determined with
MALDI-TOF MS. Table 1 gives the MALDI re-
sults for unirradiated and irradiated PEG 2000
and (PEG)
46
LiClO
4
.
Similar results (not shown)
were also obtained for samples with x ⫽ 20 or x
⫽ 100. The average molecular weight of unirra-
diated PEG 2000 quoted by Fluka was approxi-
mately 2000. This was confirmed with MALDI-
TOF MS. A mass/charge (m/z) distribution from
1600 to 2600 can be observed for commercially
obtained PEG 2000 with the molecular weight at
the maximum peak value, M
p
, at 2062.1 [Fig.
2(a)]. MALDI for 15-kGy-irradiated PEG 2000
shows a different distribution of m/z from 300 to
2600 with a new M
p
value at 1010 [Fig. 2(b)]. In
addition, the m/z distribution for irradiated PEG
Figure 1. XRD patterns for unirradiated and 15-kGy-irradiated PEG 2000.
1302
SINGH ET AL.
2000 is broader than that of the unirradiated
PEG 2000. Thus, MALDI-TOF MS shows the
fragmentation of PEG 2000 after the irradia-
tion and a decrease in its molecular weight.
MALDI results for unirradiated and irradiated
(PEG)
46
LiClO
4
are shown in Figure 2(c,d). The
polymer–salt complexes do not undergo any sig-
nificant fragmentation of the polymer chains, un-
like pure PEG, and this signifies the radiation
resistance of PEG complexed with the salt.
We also calculated the number-average molec-
ular weight (M
n
), weight-average molecular
weight (M
w
), and polydispersity index (PDI; i.e.,
M
w
/M
n
) of the various samples (Table 1).
In comparison with unirradiated PEG and
polymer–salt complexes, the irradiated samples
show larger PDI values. This means that the ir-
radiated samples are more disperse and inhomo-
geneous. Furthermore, this increase in PDI after
irradiation is much greater for pure PEG than for
the complexes. In this context, it is worth men-
tioning that the measurements of the self-diffu-
sion coefficient of polymer chains show that the
wider the molecular weight distribution is, the
faster the polymer chains diffuse.
26
This first MALDI study of irradiation effects in
an SPE has led to a couple of unique results. First
is the absence of any sign of crosslinking. Earlier
studies have shown that irradiation with
␥
-rays
15,16
or an electron beam
18
can lead to chain
scission, crosslinking, or both, depending on the
irradiation conditions. Although this needs to be
confirmed by further studies, there is some evi-
dence that if the irradiation is carried out in
vacuo, crosslinking results, whereas if it is done
in the presence of oxygen or air, scission is more
likely.
27
In our experiments, the samples were
irradiated with an electron beam in the presence
of air, and this explains the observed chain frag-
mentation for the pure PEG samples.
The second and more difficult to understand
result is the absence of any chain fragmentation
in the PEG–LiClO
4
complexes, whereas the pure
PEG sample underwent chain scission. We can
only speculate about the possible cause of this
difference at present. Although short-living PEG
radicals, which induce chain scission, are gener-
ated in both PEG and PEG–salt complexes upon
e-beam irradiation, it is likely that in the pres-
ence of the salt, the dissipation of energy is more
effective, and this could result in a much shorter
lifetime and thus limited the chain reactions that
may contribute to fragmentation. We thank one of
the referees for pointing out this possibility. It is
also likely that because of the enhanced ionic
conductivity of the PEG–salt complexes, the re-
combination process may be more effective and,
therefore, reduce the radical lifetimes.
Thermal Properties
The DSC curves for unirradiated and irradiated
PEG 2000 and the SPE (PEG)
46
LiClO
4
are shown
in Figure 3. The curves for the other compositions
(not shown) are similar to that of the x ⫽ 46 com-
position. PEG 2000 and all the SPE samples exhibit
a relatively sharp endothermic peak, which could be
attributed to the melting of a PEG-rich crystalline
phase. The melting temperature (T
m
) decreases as
the salt concentration increases for the (PEG)
x
Li
-
ClO
4
systems.
28
Also, a minimal decrease in T
m
for
the irradiated systems can be observed, and so it
can be concluded that the thermal stability of the
polymeric systems does not degrade after irradia-
tion. A weaker second peak can be observed for pure
PEG 2000 in the second heating cycle. This could be
due to the segregation of two types of crystalline
regions after the first heating and cooling cycle.
Further studies have to be carried out to verify this
conjecture.
Table 1. MALDI Results for PEG 2000 and (PEG)
46
LiClO
4
Sample Dose (kGy) M
p
M
n
M
w
PDI
PEG 2000 Unirradiated 2062.1 2056.7 2071.6 1.007
PEG 2000 5 1889.2 1996.9 2024.6 1.014
PEG 2000 10 2023.7 1967.8 1998.5 1.016
PEG 2000 15 1009.5 1131.0 1337.4 1.182
(PEG)
46
LiClO
4
Unirradiated 1919.5 1926.6 1950.2 1.012
(PEG)
46
LiClO
4
5 1830.3 1896.8 1921.4 1.013
(PEG)
46
LiClO
4
10 2007.3 1974.2 2003.4 1.015
(PEG)
46
LiClO
4
15 1919.2 1962.6 1995.1 1.017
[PEG]
x
LiCLO4 SOLID POLYMER ELECTROLYTE 1303
