Rheological, thermal and electrical properties
of Poly(ethylene oxide) / MMT
nanocomposites
Martin Reading*, Alun S. Vaughan
High Voltage Laboratory
Electronics and Computer Science Department,
University of Southampton,
Southampton,
SO17 1BJ
*mdr06r@ecs.soton.ac.uk
Abstract- Polyethylene oxide (PEO) has been used for
several years as an electrolyte solvent in lithium polymer
cells. One beneficial quality is its high viscosity, that
remains high up to temperatures just short of its solidstate Tm, allowing the production of thin electrolyte layers.
PEO batteries are sometimes preferred over lithium ion
due to their greater power/weight ratio. PEO is a polar
polymer and is water-soluble, which is beneficial for
composite dispersion. Its solubility suggests that, with
addition of a nanofiller during the solution phase, it could
provide easily implemented high levels of dispersion. With
the availability of many different average molecular
weights it constitutes an ideal model system for studies.
This paper details an investigation into the physical,
thermal and electrical properties of PEO and PEO-MMT
nanocomposites. Three different molecular weight PEO
systems were used as the polymer matrix- 100,000 g mol-1,
400,000 g mol-1 and 1,000,000 g mol-1. Properties
investigated include- rheological behaviour of solutions;
crystallisation and melting; the AC ramp breakdown
strength and dielectric response. Dispersion was achieved
during initial mixing and the solution stage in the
rheometer. After previous investigations, this dispersion
method has been seen to be acceptable.
I.
INTRODUCTION
Plastics could be considered to be one of the most important
discoveries over the last few hundred years, with society today
using variations in almost every part of life. For example, in
2006 the United States generated 14 million tons of plastici.
Polymers and plastics have been hugely successful in a
multitude of areas for many reasons. As packaging, for
example, they are light, strong, easily moulded, cost effective,
re-usable and flexible in their application. Despite the
excellent physical benefits of using polymers, this should not
overshadow the dielectric benefits. Metals have strong
electronic conduction, whereas polymers have a significantly
lower response, underlying their use as excellent insulators.
Although the bulk conductivity of most polymers is low, more
subtle effects can be observed, such as space charge, and
experimental techniques have been developed to evaluate
electrical breakdown strength, dielectric loss, permittivity etcii.
However, the dielectrics of polymers are not yet fully
understood, such as the cause of electrical breakdown, which
has drawn many theories from scientists and engineers.
In this investigation, aluminium pillared montmorillonite clay
(MMT) was dispersed in a distilled water/PEO solution by use
of a cup-bob rheometer, and viscosity data was collected.
Samples were melted and crystallised using power
compensated differential scanning calorimetry (DSC).
Emphasis was placed on any variation seen of the Tc and Tm
between the virgin PEO samples and PEO-MMT composites.
Samples were also pressed into thin films and tested
electrically. For electrical studies, a comparison arbitrary PE
sample was also used. Breakdown data was analysed using a
Weibull distribution. For comparison of dielectric properties
Cole-Cole plots were used.
II.
EXPERIMENTAL
In this investigation, rheometry served two purposes; to
characterise the shear rate - shear stress behaviour of the
solution and to aid in dispersion of the filler. The
rheometer used was a RHEOLAB MC1 Paar Physica.
The PEO was supplied by Sigma-Aldrich, as was the
MMT (aluminium pillared clay), both in granular form.
Samples were mixed in glass flasks and a weight-weight
ratio was employed. Solutions contained 20 ml of
distilled water and the required amount of PEO/MMT.
Initial hand mixing provided basic dispersion. For
higher molecular weight solutions a longer mixing time
was required before the solution was visually dispersed.
To counter any water lost through evaporation, the
flask’s total weight was recorded and checked, with
addition of distilled water where necessary. In addition
to providing rheological data, the rheometer helped to
break up any agglomeration within the solution and
provide good dispersion of the final system. The
rheological behaviour and experiment details are
discussed elsewhere iii.
For DSC the previously mixed and dried samples were
cut into samples weighing around 5-6 mg and placed
into a Perkin Elmer DSC can. The can was heated to
melt the sample to the base of the can, a lid was added
and the can sealed. The DSC used was a power
compensated Perkin-Elmer DSC 7 with a nitrogen feed
using Perkin-Elmer Pyris software. The DSC was
calibrated using melt traces from an Indium sample, a
method commonly used for calibrationiv. For calculation
of Tc values an isothermal scan was taken from 80-20 0C
and from 20-80 0C for Tm. To avoid detection of thermal
history effects on the samples, all isothermal scans were
performed after a 5 minute ‘hold’ command at a high
temperature.
leaving a ring of uncoated sample at the edge to avoid
pure conduction across the electrodes. The data obtained
from dielectric spectroscopy was used to generate five
graphs for each sample. The natural log of impedance
was plotted against frequency to ensure collected data
was acceptable, and should show a straight line
decreasing with frequency. The natural log of real
relative permittivity and natural log of imaginary
relative permittivity were plotted as a function of
frequency. In both cases, a background scan was
subtracted from the raw data to remove any
contributions to the measured impedance resulting from
the cables or the measurement cell itself. Also graphs
were generated for tan delta against frequency and ColeCole plots with real relative permittivity against
imaginary relative permittivity.
For AC breakdown strength tests, thin air-free discs
were required, therefore all samples were de-gassed in a
vacuum oven held at 80 0C. This process worked well
for the 1,000,000 g mol-1 and 400,000 g mol-1 PEO
systems, the 100,000 g mol-1 samples appeared to
undergo a thermal aging process before the air was
successfully removed. The behaviour of this aged PEO
will be reported elsewhere. A Grazeby Spec-ac press
was then used to press samples to a thickness of around
100 m. For most polymers this process simply
involves melting the sample within a press, pressing the
sample to the desired thickness, followed by quenching
the sample in cold water. However the solubility of PEO
meant that this quenching method could not be used.
Instead, quenching was achieved using a separate
chilled press that the sample was placed into straight
from the hot press. After initial tests this proved quite
effective.
To analyse the data, three Weibull equations were
considered, allowing for one, two or three
parameters.v In general, the one parameter Weibull
fails to accurately fit the data and the three parameter
is considered to “over-parameterise”. This is the
belief of many researchers, such as Chunsheng et.
al.vi. To evaluate uncertainty, two types of confidence
bounds were considered – Fisher matrix and Maximum
Likelihood. Fisher matrix confidence bounds are
sometimes considered to be too optimistic, an opinion
expressed by Greenvii, therefore Likelihood bounds will
be used.viii
For dielectric spectroscopy tests thicker discs were
produced, using the same method as above, of around
400 m. These samples were gold coated to provide a
better contact for the electrodes using a gold sputter
coater. Initial tests provided an estimate of the required
time/current to use to generate a coating with reasonably
low resistance. Both sides of the samples were coated,
RESULTS
Initial results strongly suggested shear re-dispersion
within the solutions, followed by repeatable data. As
expected, with increasing PEO content came more shear
thinning, starting from near-Newtonian characteristics at
1% to a large shear thinning curve at 10%. Generated
viscosity values for the samples were plotted on ln-ln
graphs to ascertain any effect the MMT may have on the
molecular mobility within the system. The addition of
MMT to any of the varying molecular weight PEO
solutions showed no noticeable effect. Further
discussion of these results can be found here ix.
1000
1000
C105
100
100
10
Viscosity / Pa.s
10
Viscosity / Pa.s
8-12 discs were produced for each sample, with 5
breakdown sites designated for each, providing a
healthy population for analysis. In the test cell silicone
oil was used to cover the sample and lower electrode.
The two electrodes were 6.3 mm steel ball bearings and
were changed after every other sample. To check the
linearity of the AC ramp rate a thick plastic plate was
inserted in between the ball bearings and the voltage
was measured as a function of time. Once calibrated for
a linear AC ramp rate all sites on samples were broken
down and data collected.
III.
C55
1
C25
0.1
0.001
0.001
0.01
0.1
1
10
.
Shear rate / s-1
C210
0.1
C15
0.01
C510
1
100
C110
0.01
1000 10000
0.001
0.01
0.1
1
10
100
1000
.
Shear rate / s-1
Fig. 2. Comparing samples with 0%, 5% and 10% MMT loading
showed little effect on the viscosity
Having made no effect physically within the solution, it
was expected that Tc and Tm curves should also show no
effect, due to the proposal that the MMT has not
affected the mobility within the system. As seen in
figures 2 and 3, the Tc and Tm respectively showed no
noticeable variation with 5 or 10% MMT. However it
was observed that the 400,000 g mol-1 had the highest Tc
and Tm values, followed closely by 1,000,000 g mol-1
and finally 100,000 g mol-1. Future research using
Avrami analysis may hopefully confirm this unchanged
mobility.
10000
30
99.0
99.0
95.0
95.0
70.0
70.0
50
Weibull Breakdown Probability (%)
20
15
Virgin PEO
10
48
5
0
Approx. Tc
30
40
50
60
70
46
44
Weibull Breakdown Probability (%)
25
50.0
20.0
10.0
5.0
1.0
0.5
50.0
20.0
10.0
5.0
1.0
Arbitrary PE sample
0.5
42
1,000,000 g mol-1 PEO
-1
1,000,000 g mol PEO
-1
400,000 g mol PEO
0.1
40
0.0
0.2
0.4
0.6
0.8
1.0
0.1
30
1.2
50
60
70
80
30
Breakdown Strength (kV/mm)
Molecular weight /106 g mol-1
40
50 60 70 80
100 120140
Breakdown Strength (kV/mm)
Fig. 5. A small but distinct difference was observed with varying
molecular weight (Left). PEO is seen to have a breakdown strength
around half that of a comparison PE sample (Right)
30
25
50
40
20
15
MMT-PEO Composites
10
48
99.0
99.0
95.0
95.0
70.0
70.0
5
40
50
60
70
44
42
40
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Weibull Breakdown Probability (%)
Approx. Tc
30
Weibull Breakdown Probability (%)
0
46
50.0
20.0
10.0
5.0
1.0
0.5
Molecular Weight / 106 g mol-1
+
50.0
20.0
10.0
5.0
1.0
0.5
1,000,000 g mol-1 PEO
With 5% MMT
With 10% MMT
+
0.1
Fig. 3. Little change was seen between the Tc values for virgin PEO
and PEO-MMT samples
400,000 g mol-1 PEO
With 5% MMT
With 10% MMT
0.1
30
40
50
60
70
80
30
Breakdown Strength (kV/mm)
40
50
60
70
80
Breakdown Strength (kV/mm)
Fig. 6. Little change was observed with addition of 5% or even 10%
MMT to either 1,000,000 g mol-1(Left) or 400,000 g mol-1 (Right)
70
Approx. Tc or Tm
65
Despite observing no MMT effect in the breakdown
data, it was hopeful that the MMT would perhaps affect
the disc electrically as a whole, more than at specific
points. For virgin PEO samples the permittivity and tan
delta were almost identical, and independent of
molecular weight, Fig 7.
60
Virgin PEO Tm
55
Virgin PEO Tc
50
30
45
Real Relative Permittivity
20
40
0.0
0.2
0.4
0.6
0.8
1.0
1.2
10
Molecular Weight /106 g mol-1
Fig. 4. Similar distributions of Tm and Tc values were observed for
molecular weight, with 400,000 g mol-1 having the highest.
5
3
1
2
1e-2
1e-1
1e+0
1e+1
1e+2
1e+3
1e+4
1e+5
1e+6
1e+7
Frequency (Hz)
Tan delta
With no apparent effect on thermal and physical
properties, addition of MMT may still affect the
electrical properties of the composite. A quick test for
reliability of the data was to check that the beta value
generated by the Reliasoft Weibull program for each
data set was higher than 10, which it was for all
samples. Comparison of varying PEO molecular
weights showed that the higher molecular weights
exhibited a decrease in breakdown strength.
Comparison of the MMT-PEO composites to their
virgin counter-parts, again showed little to no change.
10
0.1
0.01
400,000 g mol-1 Tan delta
1,000,000 g mol-1 Tan delta
0.001
1e-2
1e-1
1e+0
1e+1
1e+2
1e+3
1e+4
1e+5
1e+6
Frequency (HZ)
Fig. 7. Little change in Tan delta or relative real permittivity (inset)
was observed with varying molecular weigh
400
1e+5
100
100
1e+4
50
20
10
5
10
3
1e-2
1e-1
1e+0
1e+1
1e+2
1e+3
1e+4
1e+5
1e+6
1e+7
Frequency (Hz)
Tan delta
1
0.1
Relative Imaginary Permittivity
Real Relative Permittivity
200
1e+3
1e+2
1e+1
400,000 g mol-1 with 10% MMT
1,000,000 g mol-1 with 10% MMT
1,000,000 g mol-1 with 5% MMT
1e+0
1e-1
0.01
0.001
1e-2
400,000 g mol-1 PEO
400,000 g mol-1 with 10% MMT
1e-2
5
1e-1
1e+0
1e+1
1e+2
1e+3
1e+4
1e+5
1e+6
400
Real Relative Permittivity
200
100
100
50
20
100
200
500
Figures 8-9 outline the increased tan delta and real
permittivity with addition of MMT. The Cole-Cole plot
figure 10 shows the resultant two PEO-MMT samples
containing 10% MMT behaved almost identically
dielectrically. It was also observed that the amount of
MMT added was irrelevant, figure 11 showing very
close correlation between 5% and 10% weightings.
10
10
5
3
1e-2
Tan delta
50
Fig. 11. Cole-Cole plots for samples with MMT. Real and imaginary
permittivity appears independent to amount of MMT added
1e+7
Fig. 8. Addition of MMT to 400,000 g mol-1 causes a large increase to
real permittivity at low frequencies (inset) as well as a flatter
tan delta curve compared to virgin PEO
1e-1
1e+0
1e+1
1e+2
1e+3
1e+4
1e+5
1e+6
IV.
1e+7
Conclusions
Frequency (Hz)
1
0.1
0.01
1,000,000 g mol-1 PEO
1,000,000 g mol-1 with 10% MMT
1e-1
1e+0
1e+1
1e+2
1e+3
1e+4
1e+5
1e+6
1e+7
Frequency (HZ)
Fig. 9. Addition of MMT to 1,000,000 g mol-1 again causes an
increase to real permittivity at low frequencies (inset) as well as a
flatter tan delta curve
1000
1e+5
1,000,000 g mol-1 PEO
400,000 g mol-1 PEO
100
10
1
0.1
With the observation that the viscosity of the PEOMMT composite was the same as virgin PEO, this
suggests that the molecular mobility was not affected.
This belief was confirmed by thermal melting and
crystallization data by DSC. The addition of MMT was
deemed to cause no change in the electrical breakdown
strength, however an increase in molecular weight
exhibited a decrease in breakdown strength. The
dielectric data however, showed a large increase in tan
delta and real permittivity with addition of MMT. This
shows an increase in conduction when viewed as a
Cole-Cole plot. Further investigations into a new ColeCole method will determine whether another relaxation
process is present.x
-1
1,000,000 g mol
with 10% MMT
-1
400,000 g mol
with 10% MMT
1e+4
Relative Imaginary Permittivity
Relative Imaginary Permittivity
20
Relative Real Permittivity
Frequency (HZ)
0.001
1e-2
10
1e+3
REFERENCES
i
Municipal Solid Waste (MSW), U.S.
Environmental Protection Agency
1e+2
1e+1
www.epa.gov/msw/plastic.htm
ii
“Electrical properties of polymers”
iii
ME!
1e+0
Tony Blythe, David Bloor
1e-1
“An evaluation of the transition temperature
range of super-elastic orthodontic NiTi springs
using differential scanning calorimetry”
iv
0.01
1e-2
2
5
10
20 30
Relative Real Permittivity
2
5 10 20
50 100 250 600
Relative Real Permittivity
Fig. 10. Cole-Cole plots for virgin PEO samples (Left) displaying a
single relaxation with conduction, and PEO-MMT samples (Right)
displaying possibly two relaxations and conduction.
Otto Barwart, Judith M Rollinger,
Artur Burger 1999
European Journal of Orthodontics, Volume
21, pp. 497-502
v
“Life Data Analysis Reference”
Reliasoft 2005
“Influence of Threshold Stress on the
Estimation of the Weibull Statistics”
vi
Chunsheng Lu, Robert Danzer, Franz
Dieter Fischer 2002
Journal of the American Ceramic Society,
Vol 85, issue 6, pp 1640-1642
vii
CHRIS
viii
“Life Data Analysis Reference”
Reliasoft 2005
ME
x
“A new graphical representation for dielectric data”
Yan-Zhen Wei, S Sridhar
J. Chem. Phys. 99 (4) 1993
ix