Journal of Non-Crystalline Solids 351 (2005) 2825–2830
www.elsevier.com/locate/jnoncrysol
Glass transition and ionic conduction in
plasticized and doped ionomers
Shihai Zhang, Shichen Dou, Ralph H. Colby, James Runt
*
Department of Materials Science and Engineering, Materials Research Institute, The Pennsylvania State University, 101 Steidle Building,
University Park, PA 16802, USA
Abstract
The relationship between ionic conduction and matrix glass transition was investigated in selected plasticized and doped
poly((ethylene glycol)n-co-5-sulfoisophthalate lithium) ionomers. Plasticization with a poly(ethylene glycol) oligomer significantly
increases the ionic conductivity, despite a decrease in lithium concentration. Doping with LiClO4 leads to much lower conductivity,
despite the increase in Li+ content. These observations were reconciled by considering the corresponding change in ionomer glass
transition temperature (Tg), and it was found that ionomers with different plasticizer/dopant levels exhibit similar conductivity at
constant T Tg. The fact that Tg plays such a strong role suggests that Li+ ion mobility is controlled by segmental motion of PEO.
2005 Elsevier B.V. All rights reserved.
PACS: 82.45.Wx; 82.35.x
1. Introduction
Due to their flexibility and ease of fabrication, polymeric materials, particularly poly(ethylene oxide),
poly(propylene oxide), and their derivatives, have been
extensively studied as candidate electrolytes for lithium-ion batteries and other electrochemical applications
[1–4]. Although physical mixtures of polymers and salts
(e.g., LiClO4) are easy to prepare and have relatively
high ionic conductivity, they suffer from undesirable
concentration polarization in practical cells, since the
anions also have high mobility and aggregate at the electrode/electrolyte interface over time. This difficulty can
be resolved by chemically attaching the anions to the
polymer backbone to form an ionomer (i.e., polyanion)
structure [5–8]. With ionomers as the electrolytes, only
lithium cations can move and a lithium transference
*
Corresponding author. Tel.: +1 814 863 2749; fax: +1 814 865
2917.
E-mail address: runt@matse.psu.edu (J. Runt).
0022-3093/$ - see front matter 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jnoncrysol.2005.03.075
number of 1 can be achieved. These ionomers are hence
examples of single-ion conductors.
We have recently synthesized a series of well-defined
ionomers by the one-step melt polymerization of
poly(ethylene glycol) (PEG) and dimethyl 5-sulfoisophthalate sodium salt. The sodium can be exchanged to lithium (or other cations) by dialysis [9]
(Scheme 1). By changing the molecular weight of the
starting PEG oligomer, lithium ionomers with different
spacer lengths can be readily obtained. Although the
ionic conductivity (r0) increases as PEG spacer length
increases, preliminary experiments found that r0 is relatively low (6 · 107 S/cm at 25 C) even in the ionomer
with the longest spacer (PEG molecular weight = 900,
i.e., 20 EO repeat units). However, this is not surprising, since the Li+ mobility is relatively low in the ionomer matrix, with Tg = 236 K.
It is well known that miscible small molecules (plasticizers) can efficiently decrease the glass transition temperature (Tg), which can lead to an increase in Li+
mobility and r0 [7,10]. In a continuation of our previous
research, we report in the present paper the effects of
2826
S. Zhang et al. / Journal of Non-Crystalline Solids 351 (2005) 2825–2830
HO (CH2CH2O)m H + CH3O
O
O
C
C OCH
3
SO-3Na+
200-260ºC, Vacuum
Catalyst
O
[(OCH2CH2)m O C
O
than 2% for all samples, suggesting negligible residual
solvent and moisture concentration. Another round
electrode was used to sandwich the ionomer. A Teflon
spacer with thickness 200 lm was used to control the
sample thickness. The electrode-sandwiched assembly
was used in conductivity measurements. Doped samples
were prepared following the same procedure.
2.2. Characterization
C ]n
SO-3Na+
LiCl dialysis
O
[(OCH2CH2)m O C
O
C ]n
SO-3Li+
m = 9, 13, 20
Scheme 1.
plasticization with PEG oligomers on ionomer conductivity. Our results demonstrate that the movement of
Li+ is closely correlated with the matrix glass transition
and segmental relaxation process. This conclusion is
also supported by results from LiClO4-doped ionomers
as well as the application of AngellÕs decoupling scheme
[11].
2. Experimental section
2.1. Sample preparation
Lithium ionomers with PEG spacer molecular
weights of 400, 600, and 900 [9] were used in this study
and are termed PE400Li, PE600Li, and PE900Li,
respectively. The molar mass of these polymers was
determined to be 4400, 6600, and 4500, respectively,
by 1H NMR. The PEG oligomer with MW = 400 was
used as the plasticizer. PEG400/PE400Li and PEG400/
PE600Li mixtures containing 10 and 20 wt% PEG400
were investigated in the present study, as were LiClO4/
PE900Li mixtures. For the latter materials, the composition (5/100 or 10/100) is defined as the molar ratio of
added LiClO4 to ethylene oxide (EO) units. Samples
were prepared by mixing 5% methanol solutions of the
parent ionomer and PEG400 and stirring for 2 h, resulting in a clear homogeneous solution. After evaporating
most of the solvent in a hood, the viscous solution was
poured onto a round gold-sputtered stainless steel electrode and transferred to a vacuum oven. The solvent
was further removed under vacuum at 70 C for 2 days,
followed by 100 C for 2 h. Thermogravimetric analysis
(TGA) found that the weight loss below 200 C is less
A Novocontrol GmbH Concept 40 broadband dielectric spectrometer was used to measure the ionic conductivity. AC voltage of 0.5 V was used for measurement of
all samples. Frequency sweeps were performed isothermally from 10 MHz to 0.01 Hz in the temperature range
from 393 K to the corresponding Tg. Temperature stability during data acquisition was better than ±0.1 C.
Immediately before the conductivity measurement,
Tg was measured with a TA Q100 DSC instrument using
a heating rate of 10 C/min. Tg was defined as the midpoint of the heat capacity jump.
3. Results
3.1. Glass transition
A single DSC Tg was observed for all plasticized and
doped ionomers and these are summarized in Table 1.
Since the ions can serve as physical crosslinking points,
Tg decreases with increasing PEG spacer length for the
neat ionomers: Tg = 285 K for PE400Li, decreases to
258 K for PE600Li, and further to 237 K for PE900Li
(high molecular weight PEO has Tg = 221 K).
As expected, Tg also decreases with increasing PEG
plasticizer content as a consequence of dilution of the ionic stickers. The addition of 20% PEG400 decreases the
Tg of PE400Li and PE600Li by 34 K and 16 K, respectively. On the other hand, doping increases the Tg of
PE900Li by 29 K in the mixture having LiClO4/
EO = 10/100. The change in Tg directly affects the ionic
conduction, as discussed in the next section.
3.2. Ionic conduction
Fig. 1 displays spectra of the real part of conductivity
(r 0 ) and the imaginary portion of the dielectric modulus
(M00 ) of plasticized and unplasticized PE400Li at 298 K.
The ionic conductivity can be read directly from the
plateau value of the r 0 vs. frequency spectrum and it is
evident that r0 increases with increasing plasticizer
concentration, despite the simultaneous decrease in
Li+ concentration. Adding 20% PEG400 improves the
conductivity of PE400Li by 900· at 298 K. Similar
r0 enhancement is also observed for the plasticized
PE600Li ionomer (i.e., a 20-fold increase for the 20/
S. Zhang et al. / Journal of Non-Crystalline Solids 351 (2005) 2825–2830
2827
Table 1
Characterization of thermal and electrical properties of plasticized/doped ionomers
A (S/cm)
2
B (K)
T0 (K)
Tg (K)
r0 (S/cm) at 298 K
PE400-Li
10/100 PEG400
20/100 PEG400
1.1 · 10
1.3 · 102
2.4 · 102
1161
1144
1077
234
219
209
285
262
251
1.6 · 1010
6.9 · 109
1.4 · 107
PE600-Li
10/100 PEG400
20/100 PEG400
8.1 · 103
8.8 · 103
1.16 · 102
1045
983
946
215
210
204
258
249
242
2.7 · 108
1.2 · 107
4.4 · 107
PE900-Li
5/100 LiClO4
10/100 LiClO4
1.1 · 102
0.8
5.9
995
1390
1794
196
205
213
237
250
266
6.3 · 107
2.8 · 107
4.2 · 109
(a) PEG400/PE400Li
σ0 (S/cm)
10-5
10-7
10-9
10-11
0/100
10/100
10-13
240
20/100
280
320
360
400
Temperature (K)
(b) PEG400/PE600Li
Fig. 1. Conductivity r 0 (filled symbols) and dielectric modulus M00
(unfilled symbols) spectra of plasticized and unplasticized PE400Li at
298 K. Plasticizer content is provided in the figure legend.
in which B is the activation parameter for conduction, A
represents the intrinsic conductivity at very high temperature, and T0 is the temperature at which r0 ! 0.
The results of fitting Eq. (1) to the data in Fig. 2 are
σ0 (S/cm)
10-7
10-9
10-11
10-13
240
0/100
10/100
20/100
280
320
360
400
Temperature (K)
10-3
(c) LiClO4/PE900Li
10
σ0 (S/cm)
100 mixture (see Table 1)). However, although Li+ concentration is obviously increased in the LiClO4 doped
PE900Li ionomer, r0 decreases significantly, i.e., it is
150 times smaller for 10/100 LiClO4/EO compared with
the original PE900Li at 298 K.
Due to the increase in Li+ mobility, r0 increases rapidly with temperature for all mixtures. Fig. 2 presents the
temperature dependence of r0. The conductivity of the
plasticized ionomers is invariably higher than their
unplasticized counterparts at all temperatures. As observed for many electrolytes, this dependence is
non-Arrhenius and r0 changes more rapidly as the temperature approaches Tg, similar to the temperature
dependence of the viscosity and segmental relaxation
time of polymers. This is usually described by the wellknown Vogel–Fulcher–Tammann (VFT) equation [12]:
B
r0 ðT Þ ¼ A exp ð1Þ
T T0
10
-5
-5
10-7
10-9
LiClO4/EO
10-11
10-13
240
0/100
5/100
10/100
280
320
360
400
Temperature (K)
Fig. 2. Temperature dependence of the ionic conductivity of
PEG400 plasticized: (a) PE400Li, (b) PE600Li, and (c) LiClO4 doped
PE900Li.
S. Zhang et al. / Journal of Non-Crystalline Solids 351 (2005) 2825–2830
(a) PEG400/PE400Li
10-5
10-7
σ0 (S/cm)
presented in Table 1. It should be noted that PE900Li is
crystallizable; it crystallizes near room temperature during cooling. Therefore r0 for PE900Li below 298 K
changes with time and the low temperature data were
excluded from the VFT fitting.
The Vogel parameters A, B, and T0 exhibit different
trends with plasticizer or LiClO4 content. First, the
intrinsic conductivity A continuously increases with
increasing PEG400 and LiClO4 concentration. While
this is logical in doped ionomers, it is rather surprising
in plasticized systems considering the decreased Li+ content (1.6 mol/L for PE400Li vs. 1.3 for 20/100 PEG400/
PE400Li). At high temperatures, since Li+ essentially
has the same mobility in the different ionomers, higher
Li+ content would have been expected to lead to higher
r0. Apparently, addition of PEG400 increases the concentration of free Li+ ions. Secondly, B is reduced at
higher PEG400 contents, but raised with increasing
LiClO4. This is reasonable since B characterizes the
barrier for Li+ movement, which is lower in materials
with lower Tg. Finally, T0 exhibits the same trend with
PEG400 and LiClO4 content as observed for Tg with
Tg T0 = 35–55 K.
10-9
10-11
10-13
0/100
10/100
20/100
0
30
60
90
120
T-Tg (K)
(b) PEG400/PE600Li
10-5
10-7
σ0 (S/cm)
2828
10-9
10-11
10-13
0/100
10/100
20/100
0
30
60
90
120
T-Tg (K)
4. Discussion
10-3
(c) LiClO4/PE900Li
10-5
σ0 (S/cm)
The plasticized and doped ionomers have significantly different r0, as well as different Tg (and T0), and
the dependence of r0 on PEG400 and LiClO4 content
follows Tg. Stimulated by the previously reported correlation between ion mobility and matrix glass transition
[10], we explore the dependence of r0 on the normalized
temperature, T Tg, in Fig. 3. It is immediately evident
that the different plasticized ionomers have approximately the same r0 as that of their parent ionomer at
the same T Tg. This is also the case for the doped
ionomers.
This behavior is surprising, since plasticization dilutes
the Li+ concentration. It is conceivable that PEG400
facilitates Liþ –SO
3 dissociation and increases the effective mobile Li+ concentration. Since PEO has a dielectric constant of 5 in the amorphous state, this
scenario requires some specific solvation interaction between PEO and Li+, which has been observed by IR,
Raman, and NMR studies [2].
Fig. 3 qualitatively supports the close correlation between ionic conductivity and the segmental process of
the matrix. To quantify this correlation, Angell has defined a decoupling index, Rs = ss/sr, where ss is the segmental relaxation time of the matrix (quantifying chain
mobility) and sr is the relaxation time of the electrical
stress associated with ion mobility [11]. In electrolytes
for which Rs is of order unity or smaller, ion mobility
is coupled with matrix relaxation. If polymer segmental
10-7
10-9
LiClO4/EO
10-11
10-13
0/100
5/100
10/100
0
30
60
90
120
T-Tg (K)
Fig. 3. Tg-normalized temperature dependence of the ionic conductivity in plasticized: (a) PE400Li, (b) PE600Li, and (c) LiClO4 doped
PE900Li. Tg data are provided in Table 1.
motion was a Debye process with a single ss, the meaning of the decoupling index would be clear. However,
polymers are well known to exhibit a broad distribution
of segmental relaxation times, with some exceeding
1000ss. Thus to be fully decoupled, Rs must exceed 1000.
To determine Rs, both sr and ss are required. The former is defined as:
e1 e0
sr ¼
;
ð2Þ
r0
where e1 and e0 are the relaxed (low frequency) dielectric constant of the matrix and of free space, respec-
S. Zhang et al. / Journal of Non-Crystalline Solids 351 (2005) 2825–2830
2829
tively. Since the contribution from the ionic conduction
process to the dielectric spectra is much larger than that
from the dipolar relaxation, e*(x) can be approximated
by e1 ir0/e0x. Thus, the dielectric modulus M* can be
written as:
"
#
2
1
1
ðsr xÞ
isr x
0
00
M ¼ M þ iM ¼ ¼
þ
;
e
e1 1 þ ðsr xÞ2 1 þ ðsr xÞ2
ð3Þ
where x is the angular frequency. From this relation,
M 0 = M00 at x = 1/sr, where M00 also has a local
maximum. Therefore, sr can be obtained in three approaches: the known e1e0/r0, the crossover of M 0 and
M00 , or directly from the peak location in the M00
spectrum.
On the other hand, ss is usually difficult to define in
ionomers since strong ionic conduction can mask the
segmental relaxation process in dielectric loss (e00 ) spectra. Fortunately, ionic (dc) conduction effects can in
principle be minimized considering their simple relationship with the measurement frequency:
e00 ðtotalÞ ¼ r0 =e0 x þ e00 ðdipole orientationÞ.
ð4Þ
However, direct subtraction of conduction losses
from e00 is unreliable in this case since it involves subtracting large numbers to obtain a much smaller one.
Recently, this separation has been accomplished by taking the derivative of the real part of the dielectric permittivity [13]:
e00der ¼ p oe0 ðxÞ
e00 .
2 o ln x
Fig. 4. Dielectric spectra of 20/100 PEG400/PE600Li. Top: M00 (filled
squares) and r 0 (open squares) at 0 C. Bottom: derivative spectra e00der
at selected temperatures (in C). The unfilled circles represent the
original e00 spectrum at 0 C, in which the segmental process is
completely masked by ionic conduction. At 0 C, electrode polarization occurs at around 0.03 Hz; the dotted line represents the frequency
of the segmental process as defined by the derivative spectrum; and the
dashed line denotes the electrical relaxation process as defined by the
M00 spectrum.
ð5Þ
e00der is the dc conduction-free loss and allows the segmental relaxation process to be readily defined in our situation. As an example, Fig. 4 presents e00der and e00 for 20/
100 PEG400/PE600Li. In the original loss spectrum at
0 C, the segmental process is not visible, while a peak
at 1/2pss 120 Hz emerges in the corresponding e00der
spectrum.
Blocking of Li+ and other charges at the sample/electrode interface can lead to frequency-dependent electrode polarization, which cannot be removed by use of
Eq. (5). However, as shown in the top panel in Fig. 4, this
process is only significant at low frequencies, and the
conductivity r 0 exhibits a big drop at sEP due to the significant reduction in Li+ concentration in the bulk of the
sample. For the plasticized 20/100 PEG400/PE600Li
sample, the electrode polarization process begins at
0.03 Hz at 0 C. At the same condition, the segmental
relaxation defined by the resolved spectrum occurs at
120 Hz, more than 104 times faster than the electrode
polarization process. The conductivity relaxation process is much faster and it takes place at 5000 Hz. Hence
the relaxation at 120 Hz is the segmental relaxation of
PEO, with ss = 1/(2p120 Hz) = 1.3 ms.
We take sr to be the reciprocal of the frequency at
which there is a local maximum in the M00 spectra. With
both ss and sr available, the decoupling index Rs at different temperatures can be calculated. Fig. 5 provides
the results for plasticized PE600Li ionomers, and similar
results are obtained for the other systems under investigation here.
For all materials studied, 1 < Rs < 100 at all temperatures. This suggests that fast motions in the segmental
distribution are controlling the conduction. The reduction of Rs as temperature increases makes sense because
the distribution of segmental relaxation times narrows
as temperature is raised. Fig. 3 clearly shows a strong
correlation between conductivity and Tg. Therefore,
we conclude that, even though Rs > 1 at all temperatures, conduction is controlled by segmental motion.
The fastest PEO regions allow motion of Li+ and such
fast motion dominates the measured conductivity.
Finally, the ionic conductivities of the materials under investigation are too low for practical purposes,
even for the plasticized ionomers. Based on the trends
observed in the present study, r0 can undoubtedly be enhanced by adding more plasticizer. In a similar study,
Xu et al. obtained r0 > 0.001 S/cm for a PEG-based
ionomer plasticized with 90% ethylene carbonate (EC)
and/or propylene carbonate (PC) [7]. Investigation of
plasticization with higher dielectric constant solvents
and with higher plasticizer concentrations is currently
in progress in our lab.
2830
S. Zhang et al. / Journal of Non-Crystalline Solids 351 (2005) 2825–2830
102
60
(a) PE600Li
100
τs/τσ
τs,τσ (s)
40
10-2
10-4
20
10
-6
10-8
250
280
310
0
340
Temperature (K)
10
2
(b) 10/100
PEG400/PE600Li
10
60
-2
40
τs/τσ
τs,τσ (s)
100
80
T Tg: ionomers with different plasticizer/dopant levels
have essentially the same r0 at the constant normalized
temperature. This evidence supports a strong correlation
between ionic conduction and the matrix glass transition. At constant temperature, Li+ has higher mobility
in ionomers with lower Tg.
The phenomenological decoupling index, correlating
ion conduction with segmental relaxation, was calculated with ss and sr defined by the derivative spectra
and the peak in M00 , respectively. Rs decreases with
increasing temperature, but it is always greater than 1,
even at Tg + 90 K. This suggests that the Li+ jump rate
falls on the higher frequency end of the ionomer segmental relaxation distribution. Conduction is dominated by the fastest moving ions, and these ions reside
in the most mobile regions of the PEO matrix.
Acknowledgments
10-4
10-8
240
This work was supported by Penn State Materials
Research Institute and the Penn State MRSEC under
NSF grant DMR 0213623, as well as by the NSF Polymers Program (DMR-0211056).
20
10-6
270
300
330
0
Temperature (K)
102
(c) 20/100
PEG400/PE600Li
References
60
10-2
40
10-4
20
10-6
10-8
240
τs/τσ
τs,τσ (s)
100
80
270
300
330
0
Temperature (K)
Fig. 5. Conductivity relaxation time sr (diamonds), segmental relaxation time ss (triangles) and their ratio ss/sr (squares) for plasticized
PE600Li as a function of temperature.
5. Conclusions
PEG-based lithium ionomers were plasticized with
PEG400 oligomer or doped with LiClO4. Plasticization
decreases the ionomer Tg and consequently enhances
ion conduction. Although doping increases the Li+ content, it also increases Tg and a reduction in r0 was observed. This demonstrates that Tg plays a critical role
in ionic conductivity by influencing Li+ mobility. This
was further confirmed by comparing r0 at constant
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