Design and preparation of bio-based dielectric elastomer with polar and plasticized side chains

Article abstract of DOI:10.1039/c5ra06576c

A new dielectric elastomer with large actuated strain driven by low electric field was synthesized from di-n-butyl itaconate and isoprene through free radical redox emulsion polymerization. The effect of the copolymerized proportion of poly(di-n-butyl itaconate-co-isoprene) (PDBII) and the dosage of crosslinking agent on the elastic modulus, dielectric properties, and actuated strain of the elastomer were investigated, and a potential dielectric elastomer candidate containing 70 wt% di-n-butyl itaconate was obtained. The permittivity of the PDBII crosslinked by 3.0 phr of dicumyl peroxide was 5.68 at 10³ Hz, which was higher than that of commercial acrylic and silicone dielectric elastomers. Without any prestrain, an actuated strain of 20% was obtained at an electric field of 30 kV mm^-1. In order to further increase the actuated strain, barium titanate (BaTiO₃), a high-dielectric-constant ceramic powder, was utilized to fill the PDBII to form a BaTiO₃/PDBII composite. The dielectric constant of the composite increased with increasing content of BaTiO₃, and the elastic modulus of the composite was lower than that of the unfilled PDBII, leading to a larger dielectric actuated strain of the composite.

Full text of DOI:10.1039/c5ra06576c

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Design and preparation of bio-based dielectric
elastomer with polar and plasticized side chains†
Cite this: RSC Adv., 2015, 5, 47429
Weiwei Lei,a Runguo Wang,a Dan Yang,d Guanyi Hou,a Xinxin Zhou,a He Qiao,a
a
a
abc
*
Wencai Wang, Ming Tian and Liqun Zhang
A new dielectric elastomer with large actuated strain driven by low electric field was synthesized from di-n-
butyl itaconate and isoprene through free radical redox emulsion polymerization. The effect of the
copolymerized proportion of poly(di-n-butyl itaconate-co-isoprene) (PDBII) and the dosage of
crosslinking agent on the elastic modulus, dielectric properties, and actuated strain of the elastomer
were investigated, and a potential dielectric elastomer candidate containing 70 wt% di-n-butyl itaconate
was obtained. The permittivity of the PDBII crosslinked by 3.0 phr of dicumyl peroxide was 5.68 at
103 Hz, which was higher than that of commercial acrylic and silicone dielectric elastomers. Without any
prestrain, an actuated strain of 20% was obtained at an electric field of 30 kV mmꢀ1. In order to further
increase the actuated strain, barium titanate (BaTiO3), a high-dielectric-constant ceramic powder, was
utilized to fill the PDBII to form a BaTiO3/PDBII composite. The dielectric constant of the composite
increased with increasing content of BaTiO3, and the elastic modulus of the composite was lower than
that of the unfilled PDBII, leading to a larger dielectric actuated strain of the composite.
Received 13th April 2015
Accepted 18th May 2015
DOI: 10.1039/c5ra06576c
www.rsc.org/advances
ꢀ
ꢁ
2
330
U
d
sz ¼ p=Y ¼
(1)
Introduction
Y
where p is the elecrostatic pressure, Y is the elastic modulus, 3 is
the permittivity of the elastomer, 30 is the permittivity of free
space, U is the applied voltage, and d is the thickness of the
lm.3 Elastomers with high dielectric constant and low elastic
modulus are interesting candidates for dielectric using. A large
number of elastomers were tested, including silicone,10 uo-
roelastomer,5 acrylic elastomer11 and some kind of thermo-
plastic elastomers.11 However, none of these materials were
developed for actuator applications because excessively high
operation voltages would be required, causing security risk and
increasing the costs of devices. Great efforts have been made to
develop new materials that can be activated at low voltages.
Most studies have focused on the development of high
permittivity polymer composites by loading the elastomer
matrix with insulating or conductive llers. The most
commonly followed approach utilized high permittivity ceramic
llers in the form of powder, such as barium titanate,12 lead–
magnesium niobate,13 and titanium dioxide.14,15 Usually the
dielectric improvement needs a high ceramic ller content,
which would increase the elastic modulus. Another approach
was to prepare percolative composites utilising conductive
llers such as metal nanoparticles,16 polyaniline,17 carbon
nanotubes,18,19 and graphite nanoplates.20 The permittivity can
dramatically increase as the content of conductive ller
approaches the percolation threshold, i.e., less than 20% for a
microcapacitor network in a composite. Such percolative
composites generally have high dielectric losses near the
Dielectric elastomer is a kind of intelligent multifunctional
electroactive polymer. Dielectric elastomers offering perfor-
mance such as large active strain, fast response, high electro-
mechanical coupling efficiency, light weight, freedom from
noise, and low cost1,2 have drawn signicant interest in recent
years, especially aer silicone and acrylic elastomer lms with
strains in excess of 100% have been reported.3,4 Several appli-
cations have already been developed, including articial
muscles,5 sensors,6 micro air vehicles,7 and electrical power
generator.8
A dielectric elastomer actuator consists of a thin lm sand-
wiched between two compliant electrodes.9 When an electric
eld is applied, an electrostatic pressure acts on the lm,
shrinking in the thickness direction and elongating in the
planar direction. The actuated strain in the thickness direction
is given by the following equation:
aState Key Laboratory for Organic–Inorganic Composites, Beijing University of
Chemical Technology, Beijing 100029, China. E-mail: zhanglq@mail.buct.edu.cn;
Fax: +86-10-64456158; Tel: +86-10-64443413
bKey Laboratory of Beijing City for Preparation and Processing of Novel Polymer
Materials, Beijing University of Chemical Technology, Beijing 100029, China
cBeijing Engineering Research Center of Advanced Elastomers, Beijing University of
Chemical Technology, Beijing 100029, China
dBeijing Key Lab of Special Elastomer Composite Materials, Beijing Institute of
Petrochemical Technology, Beijing 102617, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra06576c
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percolation threshold, mainly because large leakage currents transition temperatures polymer and it is a high-efficiency,
are generated between the connecting conductive llers. Efforts renewable biofuel. The isoprene will soen the polymer and
to modify the llers to improve the dielectric performance of provide the crosslinking points. Bio-based isoprene has aroused
elastomer composites include the surface modication of considerable interest and it is already available in the market. A
BaTiO3 (ref. 21–23) and titanium dioxide (TiO2)24 to get a free radical redox emulsion polymerization method34,35 was
homogeneous dispersion and the coating of conductive llers used to synthesize PDBII, as we did our previous work.34 The
with an insulating layer to avoid the direct contact of conductive molar ratio of itaconate to isoprene, crosslinking agent dosage,
llers with one another. Another way to increase the actuated and the content of the high-dielectric ceramic BaTiO3 were
strain and the dielectric strength of dielectric elastomer is the studied to modulate the electromechanical properties.
application of high prestrain,3 which will reduce the thickness
of the dielectric elastomer and lead to the alignment of
macromolecule chains perpendicular to the applied electric
Experimental
Materials
eld. However, a rigid frame or supporting structure was
needed to maintain the structure of the dielectric lm,25 and the
stress relaxation and fatigue of the lm aer prestraining will
take place as time goes by. Previous studies mostly focused on
the ller and ignored the matrix, but the matrix was also critical
for an excellent dielectric elastomer composite to have a low
elastic modulus and high dielectric constant. Despite a great
many of dielectric elastomer composites have been developed,
there was still a severe lack of elastomer with a favorable prole
of mechanical and dielectric properties.
The aim of this study was to design and prepare a novel
dielectric elastomer based on di-n-alkyl itaconate. Di-n-alkyl
itaconic acid ester can be readily polymerized to form a comb-
branched structure, which was an excellent candidate for the
investigation of ordering in amorphous structures. Nanophase
separation of the incompatible main chains and side chains of
poly(di-n-alkyl itaconates) has been extensively studied by
differential scanning calorimetry (DSC),26 dynamic mechanical
analysis (DMA),27,28 dielectric spectroscopy,29 nuclear magnetic
resonance (NMR),30,31 and molecular modeling,31 leading to a
Itaconic acid (purity of 99%) was purchased from Qingdao
Langyatai Group Co., Ltd. Butanol (purity of 97%) was
purchased from Sigma-Aldrich Company. Isoprene (purity of
95%) was purchased from Alfa Aesar Company and distilled to
remove any stabilizer before use. Ferric ethylenediaminetetra-
acetic acid salt (Fe(II) EDTA), sodium dodecyl benzene sulfonate
(SDBS), sodium hydroxymethane sulfonate (SHS), tert-butyl
hydroperoxide (TBH), potassium phosphate tribasic, potassium
chloride, and hydroxylamine were purchased from Sigma-
Aldrich Company and used as received without further puri-
cation. The crosslinking agent dicumyl peroxide (DCP) was
purchased from Beijing Chemical Reagents Co., Ltd., China.
BaTiO3 nanoparticles (average particle size 30 nm) were
purchased from Sigma-Aldrich Company. All the other agents
were commercial products without further purication.
Methods
Synthesis of di-n-butyl itaconate (DBI). Preparation of di-n-
deeper understanding of the relative roles of intramolecular butyl itaconate monomer. Into a 1 L, three-neck glass ask
constraints and intermolecular interactions in the solid-state equipped with a condenser, a thermometer, a mechanical
organization and properties of these polymers. The dielectric agitator, and a Dean–Stark decanter, 130.1 g of itaconic acid,
elastomers based on di-n-alkyl itaconate are readily polarized, 160 ml of butanol, and 39.0 g of cyclohexane were added. Aer
and the alkyl groups provide the nanoheterogeneity and the the mixture was heated to 115–130 ꢁC, 1.38 g of sulfuric acid was
plasticizing effect.29,31,32 The structure of PDBII is shown in added as catalyst. The reaction was considered complete when
Scheme 1, this structure is similar to that of acrylic elastomer. the quantity of byproduct (i.e., water) reached approximately
Itaconic acid, which has been categorized as one of the “top 36.0 ml. The product was washed by deionized water in a
12” building block molecules in advanced bioreneries,33 is a separating funnel and distilled in vacuum to obtain di-n-butyl
monomer obtained at low cost from renewable resources by itaconate.
fermentation. Butanol and isoprene were used in systhize PDBII
Synthesis of PDBII by redox-initiated free radical emulsion
together with itaconic acid. Butanol was used because it has the polymerization. The polymerization reactor was a 1 L, four-neck
appropriate side chain length to polymerize a low glass glass ask equipped with a reux condenser, a sampling device,
a nitrogen inlet, and a two-bladed anchor-type impeller.
According to the formulation in Table 1, deionized water, SDBS,
potassium phosphate tribasic solution, potassium chloride
solution, Fe(II) EDTA solution, and SHS solution were added
into the ask under high-speed stirring (400 rpm). Subse-
quently, pre-blended di-n-butyl itaconate and isoprene were
added into the ask. A stable and homogeneous latex was
obtained aer 45 min of stirring. Later, TBH solution was
injected into the ask and the stirring speed reduced to
200 rpm. The polymerization was allowed to proceed at 20 ꢁC for
18 h to form the target PDBII latex, and hydroxylamine solution
groups (red circles) and flexible alkyl groups (blue lines) from itaconate. was added to terminate the reaction. The PDBII latex was
Scheme 1 Synthesis of dielectric elastomer with easily polarized ester
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coagulated by using an excess of calcium chloride solution. The
wet product was washed with ethyl alcohol and water for ve
times and dried at 60 ꢁC in vacuum until a constant weight was
obtained.
Differential scanning calorimetry (DSC). The thermal prop-
erties of pure PDBII were determined by using a Perkin-Elmer
Pyris 1 series differential scanning calorimeter. The tempera-
ture was calibrated with indium and zinc, and the heat ow was
calibrated with indium. A small section of a lm was cut and
weighed to 2–5 mg by using a Perkin-Elmer microbalance and
sealed in crimped aluminum pan.ꢀT1he sample was heated from
25 to 150 ꢁC at a rate of 10 ꢁC min and cooled from 150 ꢁC to
Preparation of cured PDBII and BaTiO3/PDBII composites.
The uncured PDBII was mixed with the crosslink agent DCP and
the ller on a 6-in two-roll mill. The lled PDBII was cured at a
pressure of 15 MPa for its optimum cure time at 165 ꢁC as
determined by a disk oscillating rheometer (P3555B2, Beijing
Huanfeng Chemical Machinery Experimental Factory, China).
Recipe 1 contained 2.5 phr (parts per hundred of rubber) of DCP
and 100 phr of PDBII-70, PDBII-50, or PDBII-30; recipe 2 con-
tained 100 phr of PDBII-70 and different contents of DCP (0.5,
1.0, 1.5, 2.0, 3.0, 4.0, 5.0, and 6.0 phr); recipe 3 contained
100 phr of PDBII-70, 5 phr of DCP, and different contents of
BaTiO3 (0, 10, 30, 50, 70, and 90 phr).
ꢀ100 ꢁC at a rate of ꢀ20 C minꢀ1 under a nitrogen purge at a
ꢁ
ow rate of 20 ml minꢀ1. The glass transition temperature was
determined from the heating curve from ꢀ100 to 100 ꢁC at a rate
of 10 C minꢀ1. A baseline was recorded with matching empty
ꢁ
pan. Certied indium and sapphire were used for temperature
and heat ow calibration.
Tensile properties. Tensile tests were performed on
dumbbell-shaped samples (original length 25 mm) according to
ASTM D412 by using a CMT 4104 electrical tensile apparatus
(Shenzhen SANS Test Machine Co., Ltd., China) at a crosshead
speed of 50 mm minꢀ1 and 25 ꢁC. The elastic modulus of a
sample was determined from the slope of the stress–train curve
at 5% strain.
Dielectric analysis. The dielectric properties of the samples
were measured with an impedance analyzer (E4980A, Agilent,
USA) over the frequency range of 10 to 106 Hz under a voltage of
1 V at room temperature.
Electric actuation tests. The samples for actuated strain tests
were 0.4 mm thick. The compliant electrodes were fabricated by
spraying with an airbrush a graphite suspension composed of
graphite, silicone oil, and a crosslinking agent on the two
surfaces of a dielectric elastomer lm xed on a circular frame.
The strain was dened as the number of pixels (A) of the elec-
trodes' area divided by the number of pixels before being
applied voltage (A0). The voltage was supplied by a high-voltage
direct current generator (DTZH-60, Wuhan Dotek Electric Co.,
Ltd., China). A video camera was used at constant focal length to
capture the actuator plane before and aer the voltage was
applied, and then the captured video pictures were processed
with photoshop soware to determine the change in the
number of pixels on the electrodes. The planar strain can be
calculated by
Measurements
FTIR spectroscopy. The FTIR spectra of raw PDBII rubber
were recorded on a Bruker Tensor 27 spectrometer (Bruker,
Germany) to determine the structure of PDBII. The samples
were prepared by cutting raw PDBII rubber into pieces. The
spectra were obtained between 4000 and 400 cmꢀ1 at a resolu-
tion of 4 cmꢀ1 and over 32 scans.
1H-NMR spectroscopy. The 1H-NMR spectra of elastomers
were recorded with a Bruker AV400 NMR spectrometer (Bruker,
Germany) to determine the chemical structural of PDBII and
polyisoprene at a frequency of 400 MHz and with CDCl3 as
solvent.
Gel permeation chromatography (GPC). The molecular
weight and polydispersity of PDBII were determined by gel
permeation chromatography analysis on a Perkin-Elmer 200 gel
permeation chromatography instrument with a PL mixed-B10m
column and a reective index detector. Tetrahydrofuran at a
ow rate of 1.0 ml minꢀ1 and 25 ꢁC was used as the eluent, and
the calibration was carried out with polystyrene standards.
Table 1 Formulation for redox-initiated emulsion polymerization
Sp ¼ (A ꢀ A0)/A0 ꢂ 100%
(2)
Ingredients (concentration)
Amount (g)
where A is the actuated planar area and A0 is the original area.
As it is difficult to accurately measure the change in thickness,
we instead measured the change in planar area Sp to evaluate
the thickness actuated strain Sz.
Di-n-butyl itaconate
Isoprene
Deionized water
SDBS solution (10%)
Variablea
Variablea
30% solid content
5% weight of
monomers
2
Based on the law of volume constancy,
Potassium phosphate
tribasic solution (10%)
Potassium chloride (10%)
SHS solution (10%)
Fe(II)EDTA solution (1%)
TBH solution (10%)
(1 + Sp)(1 + Sz) ¼ 1
(3)
5
2
4
0.5
0.4
Rearrangement of eqn (3) gives an expression for the planar
strain:
Hydroxylamine solution (50%)
1
Sp ¼
ꢀ 1
(4)
a
Di-n-butyl itaconate and isoprene were copolymerized in mass ratios
of 70/30 (PDBII-70), 50/50 (PDBII-50), and 30/70 (PDBII-30).
Polyisoprene was also synthesized for comparison.
1 þ Sz
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According to eqn (1),
double bonds for subsequent reaction. Absorptions at 4.17,
4.10, 1.63, 1.61, 1.40 and 1.38 ppm are due to the d, e, f, g, h, i
protons of the methylenes groups of the capped product,
respectively. The absorptions at 0.95 and 0.93 ppm belong to the
–CH3 protons of butyl units. In conclusion, di-n-butyl itaconate
was successfully produced.
Sp ¼ 3r30U/(dY ꢀ 3r30U)
(5)
Every experimental data point of mechanical properties,
dielectric properties, and electromechanical strain in this work
was the average of the results obtained from at least ve
samples under the same conditions.
Molecular weight and structural characteristics of PDBII
The PDBIIs with different di-n-butyl itaconate content were
synthesized, and Fig. 1(e) shows photographs of the dry PDBIIs.
As the isoprene content increased, the polymer became less
Results and discussion
Synthesis and structure of di-n-butyl itaconate
Itaconic acid was capped with butanol, a renewable monomer, consolidated and so. The GPC results listed in Fig. S2† showed
to restrain the chain the chain transfer effect of carboxyl groups. three traces with broad single peaks suggesting only one
In Fig. S1,† the resonances at 5.69 and 6.32 ppm are both component exists. The number-average molecular weight of
assigned to the C]CH2 protons of itaconate, indicative of PDBII-70, PDBII-50, PDBII-30 were 19.3 ꢂ 104, 18.1 ꢂ 104, and
Fig. 1 Samples of PDBII with different itaconate to isoprene feed ratios: (a) FTIR spectra, (b) 1H-NMR spectra, (c) DSC thermograms, (d) chemical
structure, and (e) sample pictures (DBI: di-n-butyl itaconate, i.e., 30% DBI means 30 wt % DBI in the feed).
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Fig. 2 Frequency dependence of (a) dielectric constant and (b) loss tangent of crosslinked PDBII. (70% DBI stands for 70% DBI in the feed).
carbonyl moieties (C]O) and the peak at 1170 cmꢀ1 repre-
senting the asymmetric vibration of C–O–C indicate that DBI
has been successfully copolymerized. Because most of the CH3
and CH2 absorption peaks are overlapping, the detailed struc-
ture of PDBII, in particular the copolymerization ratio, should
1
be conrmed by H-NMR.
In Fig. 1(b), the polyisoprene proton signals at 5.12 ppm
(]CH–, peak a), 4.88 ppm (]CH2, peak b), and 4.67 ppm
(]CH2, peak c) represent the 1,4-units, 1,2-units, and 3,4-units,
respectively. The peak areas indicate that the polyisoprene
prepared by redox emulsion polymerization consists of 93% 1,4-
units, 3.3% 1,2-units, and 3.7% 3,4-units. The area under peak e
(1.59 ppm, –CH3) is larger than that under peak d (1.68 ppm,
Fig. 3 Actuated strain of crosslinked PDBII at different DBI contents.
–CH3) because there are more trans-1,4-units than cis-1,4-units.
The sharp peak g (0.92 ppm, –CH3) corresponds to the –CH3 in
the DBI unit. From the areas under peaks g and a, we can
deduce that PDBII-70, PDBII-50, and PDBII-30 consist of 72.0%,
12.3 ꢂ 104 respectively, ensuring a sufficiently high elasticity for
PDBII to be used as elastomers aer being crosslinked.
57.7%, and 39.8% DBI, respectively. The chemical structures of
As shown in Fig. 1(a), although the most of the absorbance
PDBIIs are summarized in Fig. 1(d).
peaks of polyisoprene are overlapping, the peak at 1380 cmꢀ1
representing the trans-1,4- and the cis-1,4-units of isoprene in
the main chain can still be distinguished, and the intensity
decreases as the di-n-butyl itaconate (DBI) content increases
Thermal properties such as crystallization and glass transi-
tion are pivotal for dielectric elastomers. As shown in Fig. 1(c),
the PDBII and polyisoprene synthesized by redox emulsion
polymerization are amorphous in the testing range of
from 30% to 70%. The peak at 1730 cmꢀ1 representing the
Fig. 4 Frequency dependence of (a) dielectric constant and (b) loss tangent of PDBII (with 70% DBI) crosslinked by different amounts of DCP.
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guarantee a high elasticity. Thus, crosslinked PDBII with a DBI
content of 70% will be selected in the following research.
The actuated strain of a dielectric elastomer under a given
voltage is an important property. The actuated plane strain of
PDBII as a function of applied electric eld is shown in Fig. 3. It
can be seen that the actuated strain increases as the electric
eld increases. PDBII with 70% DBI displays a higher actuated
strain than PDBII with 50% or 30% because the ester groups
have strong polarity, which enhances the dielectric constant,
and PDBII with 70% DBI has fewer double bonds and hence a
lower crosslink density than PDBII with a lower DBI content.
Effect of crosslink density
The tensile strength, elastic modulus, electric breakdown
strength, actuated strain, and response speed aer stimulation
are closely related to the crosslink density. Meanwhile, a high
actuated strain at a low electric eld is actively pursued. PDBII
with a DBI content of 70%, crosslinked with different amounts
of DCP, were further investigated. The Fig. S4† shows that the
PDBII is effectively crosslinked more than 1.5 phr of DCP in the
PDBII matrix, and the optimum cure time were summarized in
Table S3.† And the torque at optimum cure time increases as
the DCP dosage increases.
Fig.
5 Stress–strain curves of PDBIIs (70% DBI) crosslinked by
different contents of DCP and (inset) plots of elastic modulus versus
DCP dosage and ratio of dielectric constant to elastic modulus versus
DCP dosage.
temperature, and the glass transition temperatures decreases
with increasing isoprene content.
Effect of molar ratio of di-n-butyl to isoprene
Fig. 4(a) shows the frequency dependence of dielectric
The curing characters of PDBIIs with different DBI content in constant of PDBII at ambient temperature. With increasing DCP
the feed were listed in Fig. S3 and Table S2.† The frequency content, the dielectric constant rst increases, probably
dependence of dielectric constant and loss tangent of cross- because the free volume of the material decreases with
linked PDBII in the frequency range of 10 to 106 Hz is shown in increasing degree of crosslinking.36 However, the dielectric
Fig. 2. The dielectric constant increases with increasing DBI constant decreases with further increases of DCP dosage
content over the whole frequency range. As there are many because a high crosslink density limits the polarization of the
polarizable ester groups on the side chains, DCP-crosslinked polymer chains. Meanwhile, the dielectric constant decreases
PDBII with 70% DBI exhibits a high dielectric constant of signicantly with increasing frequency at frequencies above 104
5.6 at 103 Hz, which is higher than those of two commonly used Hz because the polarization of the polymer chains do not have
dielectric elastomers: silicone elastomer (Nusil CF19-2186, 2.8)3 enough relaxation time to catch up with the frequency change of
and acrylic elastomer (3 M VHB, about 4.8)3. Generally, the an applied electric eld.37 Fig. 4(b) shows that the crosslink
more abundant the ester groups on the polymer side chain, the density has no signicant effect on the loss tangent.
higher the dielectric constant of the polymer. The alkyl groups
Fig. 5 shows that the elongation at break decreases from
on the side chain always provide some plasticizing effect, but 623% to 51% and the elastic modulus increases almost linearly
the amount of DBI on the polymer chain have to be optimized to from 0.092 MPa to 0.55 MPa with increasing DCP dosage. The
Fig. 6 (a) Actuated strain of PDBII (70%DBI) crosslinked by different contents of DCP and (b) comparison between experimental and theoretical
values of actuated strain.
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Fig. 7 (a) Curing curves and (b) stress–strain curves of BaTiO3-filled PDBIIs.
modulus of crosslinked PDBII is of the same order of magni- content of DCP, as shown in Fig. 6(a), probably because of the
tude as two commercial dielectric elastomers: VHB 4910 decrease in sample defects with increasing DCP content.
(ref. 38) (about 0.1 MPa) and DC 3481 (ref. 7) (Dow Corning
Fig. 6(b) compares the theoretical and experimental actuated
SILASTIC 3481, about 0.35 MPa). With about the same elastic strains of PDBII dielectric elastomer. The theoretical planar
modulus, the PDBII dielectric elastomers can load about the actuated strain Sp was calculated by eqn (5). The theoretical
same stress as the two commercial dielectric elastomers. values are higher than the experimental values under the same
However, further increases in elastic modulus will lead to a low applied electric eld. During the process of actuation, a small
driving factor for dielectric elastomer, which is dened as the part of the electric energy is converted into thermal energy by
ratio of dielectric constant to elastic modulus.
mechanical loss and dielectric loss. The discrepancy between
The actuated strains of PDBIIs with different contents of DCP the experimental and theoretical values increases with
are shown in Fig. 6. With decreasing DCP content, the actuated increasing electric eld because the mechanical loss increases
strain at a given electric eld dramatically increases, mainly as the actuated strain increasing and thus more electric energy
because a decrease in DCP content reduces the elastic modulus is transformed into thermal energy, in agreement with previous
and thus increases the ratio of dielectric constant to elastic studies.39,40
modulus. The highest actuated strain at 20 kV mmꢀ1, 30 kV
mmꢀ1, and 42 kV mmꢀ1 are 14.7%, 20.0%, and 25.5%, respec-
tively, without any prestrain. This actuated strain is relatively
BaTiO3/PDBII composites
high compared with that of reported polymer dielectric mate- Fig. 7 show the curing characteristics and stress–strain curves of
rials under the condition of no prestrain (see ESI†). However,
BaTiO3/PDBII composites. Usually, a polymer composite lled
the electric breakdown strength increases with increasing
with inorganic nanoparticles has a higher elastic modulus and a
Fig. 8 (a) Scanning electron microscope and (b) transmission electron microscope micrographs of PDBII elastomers filled with different
contents of BaTiO3.
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Fig. 9 Schematic diagram of dispersion of different weight contents of BaTiO3 in PDBII elastomer: (a) 10%, (b) 50%, and (c) 90%.
lower elongation at break than the polymer matrix, and the
elastic modulus increases and the elongation at break decreases
with increasing ller content. The inorganic nanoparticles act
as physical crosslinks in the polymer matrix to cause a rein-
forcing effect and hinder the mobility of polymer chains.
However, as shown in Fig. 7(b), the PDBIIs lled with 10 to 70
phr of BaTiO3 have lower elastic modulus and higher elongation
at break than the pure PDBII. Elastic modulus and curing
character were summarized in Table S4.† Similar results were
rst reported by Carpi14 for silicone dielectric elastomer lled
with titanium dioxide. Furthermore, as the BaTiO3 dosage
increases to 90 phr, the elongation at break increases sharply to
higher than 400% and the elastic modulus comes close to that
at a BaTiO3 dosage of 10 phr.
This unusual phenomena are mainly attributed to the
decrease in crosslink density. The crosslink density can be
illustrated by the torque increment (DS) between the maximum
torque (Smax) and minimum torque (Smin) in the curing curve.
The curing curves of PDBII elastomers with different dosage of
BaTiO3 are shown in Fig. 7(a), and the data of Smax, Smin, DS,
optimum cure time and elastic modulus are summarized in
Table S4.† As reported previously, the elastic modulus of
BaTiO3/PDBII is controlled by two factors: the soening effect as
a result of reduced crosslinking and the hardening effect of the
reinforcement of the ller network.41 Two mechanisms for the
decrease in crosslink density in TiO2-lled dielectric elastomers
were proposed: polymer chain cleavage at the TiO2 particles42
and the combination of the radicals produced by the TiO2
Fig. 11 Comparison of theoretical (red dots) and experimental (black
squares) actuated strains of BaTiO3/PDBII composites at 15 kV mmꢀ1
.
particles with the radicals produced by DCP.41 Although the
pure PDBII presents the highest elastic modulus (see Fig. 7(b))
as a result of high crosslink density, the torque of the lled
PDBIIs (see Fig. 7(a)) and the elastic modulus (see Fig. 7(b))
increase as the BaTiO3 dosage increases from 10 phr to 70 phr
because of the ller hardening effect. The lower elastic modulus
and higher elongation at break at 90 phr of BaTiO3 than at lower
dosages were probably due to secondary structures such as
nanoparticle aggregates, which further hindered the
crosslinking.
Fig. 10 Frequency dependence of (a) dielectric constant and (b) loss tangent of BaTiO3-filled PDBIIs.
47436 | RSC Adv., 2015, 5, 47429–47438
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The Scanning electron microscope and transmission elec- electromechanical sensitivity (b ¼ 3r/Y). And the experimental
tron microscope micrographs of BaTiO3/PDBII composites are data are in good agreement with theoretical predictions.
shown in Fig. 8(a) and (b). The nanoparticles in the diameter However, the actuated strain increases sharply as the BaTiO3
range of 100–200 nm are uniformly dispersed in the PDBII content increases from 70 wt% to 90 wt%, mainly because the
matrix. However, the nanoparticle tended to aggregate even at decrease in elastic modulus discussed above. The discrepancy
only 10 phr. If the aggregates are sufficiently large and/or between experimental data and theoretical prediction increases
numerous, the particles will come into direct contact with one sharply because ller networks increase the conversion of
another to form agglomerates (circled in red).43 An obvious ller electric energy to thermal energy through mechanical loss and
network appears in Fig. 8(b6) for the composite with 90 wt% dielectric loss.
BaTiO3, in which the crosslink reaction was most hindered.
A simplied model of BaTiO3/PDBII composites is shown in
Fig. 9. The high surface energy of BaTiO3 nanoparticles results
Conclusion
in strong physical adsorption and/or chemisorption, which The novel poly(di-n-butyl itaconate-co-isoprene) (PDBII) dielec-
make it easy for the nanoparticles to form aggregates. As a tric elastomer have been synthesized from itaconic acid,
result, single particles are rare even at low contents of BaTiO3, butanol, and isoprene by free radical redox emulsion polymer-
as shown in Fig. 8(a2), (b2), and 9(a). As the BaTiO3 content ization. Aer being crosslinked by 3.0 phr of dicumyl peroxide
increases, secondary ller structures or ller network which (DCP), PDBII with a di-n-butyl itaconate content of 70%, a
may also be described as agglomerates are formed. However, number-average molecular weight of 310 000, and a glass
ller networks on a macro scale are not formed until the BaTiO3 transition temperature of ꢀ38 ꢁC showed a permittivity of 5.68
exceeds the percolation threshold of 90 phr. Some polymer at 103 Hz. The effect of DCP content on the mechanical,
chains are trapped in the ller networks without being cross- dielectric, and actuation properties was investigated. An actu-
linked. At moderate and high strains, uncrosslinked rubber is ated strain of 20% at an electric eld of 30 kV mmꢀ1 and an
released from ller network as the agglomerates are destroyed, actuated strain of 25% at an electric eld of 43 kV mmꢀ1 were
and acts as the polymer matrix. As a result, the elastic modulus obtained with PDBIIs crosslinked with 3.0 and 5.0 phr of DCP,
of PDBII lled with 90 phr of BaTiO3 decreases and the elon- respectively, without any prestrain. PDBII (mass ratio of di-n-
gation at break increases sharply, as shown in Fig. 7(b).
butyl itaconate to isoprene of 70/30) lled with 10.0 phr of
Fig. 10(a) shows the frequency dependence of the dielectric barium titanate and crosslinked by 5.0 phr of dicumyl peroxide
constant of BaTiO3/PDBII elastomer composites with different had an actuated strain of 8.0% at 15 kV mmꢀ1, 321% higher
contents of BaTiO3. The dielectric constant increases with than the actuated strain of pure PDBII. This research may help
increasing content of BaTiO3 particles at all frequencies establish a new and valuable route for dielectric elastomers.
because of the enhancement of electron polarization and
interface polarization at the interface between the PDBII matrix
and BaTiO3 particles.44 However, the electron polarization and
Acknowledgements
interface polarization will increase not only the dielectric This work was supported by the National Natural Science
constant, but also the dielectric loss of the composites. Foundation of China (Grant nos 50933001, 51173007 and
Fig. 10(b) shows the frequency dependence of the dielectric loss 51221002).
of the PDBII elastomer composites at room temperature. The
dielectric loss of pure PDBII is smaller than those of all BaTiO3/
PDBII composites. As explained above, the semiconductor
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