Materials Sciences and Applications, 2011, 2, 187-195
doi:10.4236/msa.2011.23023 Published Online March 2011 (
Copyright © 2011 SciRes. MSA
Solvent-Induced Phase-Inversion and Electrical
Actuation of Dielectric Copolymer Films
Yeonju Jang, Toshihiro Hirai
Smart Materials Engineering, Faculty of Textile and Technology, Shinshu University, Nagano, Japan.
Received January 24th, 2011; revised February 8th, 2011; accepted February 11th, 2011.
Block copolymers posses inherently the ability of form a variety of phase-separated microdomain structures. The
lengths of block segments and the selectivity of the solvent are primary factors affecting the resultant morphology. This
paper investigated the effect of casting solvents on the morphologies and electrical actuation of poly(methyl methacry-
late)-poly(n-butyl acrylate)-poly(methyl methacrylate) (PMMA-PnBA-PMMA) triblock copolymer films comprising
PMMA hard segment and PnBA soft segment. Transmission electron microscopy and confocal laser scanning micros-
copy observation revealed that PMMA and PnBA segments were assembled into various micro- and nano-sized phase
structures where either of them formed continuous phase. This implies that continous phase could be inversed by used
casting solvents. Solvent-dependent phase morphologies had a significant effect on the electrical actuation results. In-
crease of the PnBA contents and the continuous phases of PnBA soft segments improved both of electrical actuation
and dielectric constant, indicating that solvent-induced phase separation modulates the electrical actuation of dielectric
films. The significance of the role of solvent selectivity and the major continuous phase of the polymer in defining the
morphology and electrical actuation of the self-assembled block copolymer structure are discussed.
Keywords: Actuator, Acrylic Triblock Copolymers, Electrical Actuation, Dielectric Properties
1. Introduction
Dielectric elastomer actuators (DEAs) behave as actua-
tors by changing their shape in response to electrical
stimulation. Due to their very good electromechanical
capabilities in terms of fast response, large deformation,
reliability and relatively high efficiency, several actua-
tion applications such as loud-speakers, pumps, bio-in-
spired robotic systems and artificial muscles have been
proposed. [1-3]. There are several active materials for
these applications: piezoceramic and magnetostrictive
materials, shape memory alloys, ferroelectric polymers
and electroactive celluloses. The elastomers are com-
posed of block copolymers, which have both hard and
soft segments [4-8]. In particular, triblock copolymers
are widely known as hybrid polymeric materials having a
soft and rubbery matrix with hard, glassy domains. Due
to their potential to form self-assembled structures,
triblock copolymers have been used as active materials.
Films with various morphologies have been prepared by
diverse techniques such as melting, casting from differ-
ent solvents, and solvent casting using different copoly-
mer compositions which is the most widely used tech-
nique [9]. The hard segment can increase the modulus,
and this physical property is related to the ultimate actua-
tion performance [10,11].
To improve the overall electromechanical response of
a dielectric elastomer, suitable compliant electrodes are
usually based on carbon black or graphite powders, either
dispersed in grease and polymer matrices or directly de-
posited as thin layers [12]. When an appropriate voltage
is applied to a DEA, the very soft nature of both the di-
electric elastomer and its electrodes allows for significant
coupling of the mechanical properties of the system with
the electrostatic interactions arising between the induced
polarization charges and the supplied surface free
charges. This actuation mechanism is the so-called ‘Max-
well stress’, as depicted in Scheme 1. A voltage (V) is
applied across the film, which is sandwiched between
two compliant electrodes. The electrostatic force of at-
traction between the oppositely charged electrodes initi-
ates a Maxwell stress that compresses the film in the
transverse (z) direction [13,14]. Like charges along each
film surface repel each other, resulting in further stretch-
Solvent-Induced Phase-Inversion and Electrical Actuation of Dielectric Copolymer Films
Copyright © 2011 SciRes. MSA
Scheme 1. Schematic illustration of a DEA, before (left) and
after (right) electrical actuation. Electrostatic attraction
between the oppositely charged compliant electrodes ap-
plied to opposing surfaces of the D-EAP generates a normal
Maxwell stress upon actuation and compresses the film in
the transverse (z) direction. Under isochoric conditions, the
film extends laterally along the x and y directions. The
thickness of the compliant electrodes is exaggerated here for
ing of the film along the lateral (x and y) directions. The
effective electrostatic (Maxwell) stress is given [15] by
ε0ε(V/t)2, where ε0 is the permittivity of free space, ε
represents the field-invariant dielectric constant of the
DEA film and V/t is the electric field. The stress acting
along the z axis generates a transverse strain, as well as
in-plane stresses along the x and y directions. The extent
and distribution of the accompanying in-plane strains
upon actuation are governed by the elastic modulus (E)
of the DEA and any in-plane anisotropy, which can be
introduced by dissimilar directional pre-straining of the
film [16,17].
Other methods have been proposed recently to en-
hance the actuation performance. The most widely ado-
pted methods are based on the production of composites,
by loading an elastomer matrix with insulating or con-
ductive fillers (either organic or inorganic). However, the
fillers make the film stiffer, thus low stretchability is
obtained [18,19]. To bypass such limitations, blending a
dielectric elastomer with a highly polarizable phase,
rather than loading it with filler, has been proposed more
recently [20]. However, the contents of a highly polariz-
able phase are difficult to control for blend uniformity.
For DEA films that have an irregular structure, suffi-
ciently accurate values of the dielectric constant, which is
one of the important factors determining actuation per-
formance, are difficult to obtain [21]. To improve the
actuation performance of a dielectric elastomer it is thus
necessary to focus on those properties, namely the elastic
modulus and dielectric constant, that directly control
both stress and strain. In addition, investigation is needed
of the phase morphology of DEA films, which influences
deformation, elastic modulus and response to strain.
When the hard segment make continuous phase, the film
getting harder while the soft segment forms continuous
phase, the flexible film is obtained.
In the present study a simple approach is proposed to
make the films with controllable morphologies. New
dielectric elastomer films with improved actuation per-
formance were obtained by controlling the casting sol-
vents without any additives. The systems that were stud-
ied were two poly(methyl methacrylate)-poly(n-butyl a-
crylate)-poly(methyl methacrylate)(PMMA-PnBA-PMMA)
block copolymers with different composition. The for-
mulated materials were then characterized in terms of
their dielectric constant, tensile properties and electrical
actuation. To clarify the relation between phase mor-
phologies and physical properties, the phase structures of
films were observed by transmission electron microscopy
(TEM) and confocal laser scanning microscopy to find
the morphologies both of micro- and nanosize.
2. Experimental
2.1. Materials
Poly(methyl methacrylate)-b-poly(n-butyl acrylate)-b-poly
(methyl methacrylate) (PMMA-PnBA-PMMA) triblock
copolymers with different block lengths were employed in
conjunction with various solvents. Two PMMA-PnBA-
PMMA copolymers, supplied by Kuraray Co. Ltd., Japan,
were synthesized by living anionic polymerization [22,
23] with 23% PMMA (molecular weight 72 000 g·mol1)
and 31% PMMA (molecular weight 132 000 g·mol1):
the corresponding dielectric elastomer films are denoted
TAPM23 and TCPM31, respectively. Toluene, chloro-
form, tetrahydrofuran and acetone (Wako Pure Chemical
Industries, Ltd., Japan) were used as casting solvents to
obtain phase inversion with different continuous phase
either of PMMA or PnBA because of their different af-
finities. The three dimensional solubility parameters of
the homopolymers (PMMA and PnBA) and the solvents
are listed in Table 1. Graphite powder (99.9995% metal
basis, Alfa Aesar) and silicone grease (Shin Etsu Chemi-
Table 1. Solubility parameters of various solvents and
polymers dt: total solubility parameter, dd: dispersive pa-
rameter, dp: polar parameter, dh: hydrogen bonding pa-
Name Molecular
formula dt d
d dp dh
(n-butylacrylate) C7H12O2 10.18 16.386.635.77
Toluene C7H8 18.2
Chloroform CHCl3 18.84 17.693.075.73
THF C4H8O 19.47 16.815.737.98
Acetone CH3COCH3 20.1 15.510.47.0
(methylmethacrylate) (CH2C(CH3)CO2CH3)n 22.7 18.610.57.5
Solvent-Induced Phase-Inversion and Electrical Actuation of Dielectric Copolymer Films
Copyright © 2011 SciRes. MSA
cal Co., Ltd.) were used as the compliant electrode mate-
rials in the actuation experiment.
2.2. Preparation of PMMA-PnBA-PMMA
Triblock Copolymer Films
The general procedures for preparing dielectric thin films
were identical in each case, but the triblock copolymer
and solvent were varied. The PMMA-PnBA-PMMA trib-
lock copolymer films were cast from 6.25% (w/v) each
of solvents for 1 day; then the solution was transferred to
a PTFE laboratory dish at room temperature. After 7
days of solvent evaporation, transparent circular films
with radius 8 cm and nominal thickness 100 μm were
formed (Figure 1).
2.3. Dielectric Characterization
Electromechanical impedance measurement was used to
determine the dielectric constant of the fabricated films.
The apparatus used for the measurements was a Solarton
Co. model 1250 frequency response analyzer coupled
with a model 1286 potentiostat via an IEEE interface
(National Instruments, Austin, TX) to a 586 PC (Solar-
tron software Zplot and Zview). Impedance measurement
was performed in the frequency range 0 to 25 000 Hz
with signal amplitude 100 mV. A Pt plate electrode is
used as counter electrode, and Ag/AgCl as reference
electrode. Circular disk-shaped specimens 10mm in dia-
meter and 1mm thick were used.
2.4. Morphology Observation
The nanoscale phase-separated morphologies of films
were observed with a transmission electron microscope
(Hitachi High-Technologies Corporation. model H-7100
FA). The micrographs were obtained using 100 kV ac-
celeration voltage. The sample films were prepared in
ultra-thin sections by means of a cryo-ultra microtome,
and the PMMA phase was selectively stained with phos-
Figure 1. Fabricated dielectric thin film. The transparent
circular film has a radius of 8 cm and nominal thickness of
100 μm was formed.
photungstic acid (PTA). The microscale phase-separated
surface was examined using a confocal laser scanning
microscope (LEXT OLS3100, Olympus Co., Japan).
Confocal images were obtained using a x100 objective
with a 408 nm incident laser beam.
2.5. Mechanical Characterization
Measurements were performed using a Tensilon me-
chanical testing machine (RTC tensile tester from A&D
Co. Ltd). Sample films with standard size [ISO 37-2 (JIS
K 6251-6)] were drawn at 25 mm·min at 20˚C. The
variation of tensile stress with draw ratio was recorded.
Elastic modulus, elongation at break, and stress at break
were determined as averages of five independent ex-
periments performed under the same conditions.
2.6. Electrical Actuation
Electrical actuation was conducted with a circular actua-
tor (radius R = 50 mm). To make an actuator, fabricated
films were 100% biaxially pre-strained with a custom-
made tensile jig designed to reduce the thickness, and
therefore the voltage level required for actuation. Gener-
ally, dielectric elastomer films are routinely prestrained
uniaxially to improve their actuation performance [17,
24-27]. Pre-straining tends to reduce the dielectric con-
stant while simultaneously enhancing the dielectric break-
down strength [25-27]. Mechanically compliant elec-
trodes (a mixture of graphite powder and silicone grease)
were deposited on both sides of the film, leaving the
border area free of conduction paths to avoid possible
arcing (electrical short). The two electrodes were con-
nected to a high voltage supply by means of thin alumi-
num tape. The effects of the application of an electric
field were investigated by increasing the voltage in steps
of 500 voltages. A video recorder was used to record the
circular actuation behavior of the films, and the nominal
radial strain of the coated area was measured and ana-
lyzed using the ImageJ image analysis program.
3. Results and Discussion
3.1. Dielectric Characterization
To elucidate the general response of TAPM23 and
TCPM31 films under loading during actuation, various
electric field strengths were used. The relative dielectric
constant was measured for all of the formulations. Se-
lected results for various systems are shown in Figures 2
and 3. The dielectric constant of PMMA film, a dielectric
polymer, was measured as a control and compared with
those of the copolymer films. Unlike the other fabricated
dielectric films, PMMA film was measured with no
pre-strain, because attempts to strain PMMA film to the
same extent caused the film to break due to its fragility.
Solvent-Induced Phase-Inversion and Electrical Actuation of Dielectric Copolymer Films
Copyright © 2011 SciRes. MSA
Figure 2. Relative dielectric constant as a function of fre-
quency for TAPM23 films with a various casting solvents.
The solid line identifies dielectric constant of PMMA film.
Figure 3. Relative dielectric constant as a function of fre-
quency for TCPM31 films with a various casting solvents.
The solid line identifies dielectric constant of PMMA film.
The dielectric constants of the TAPM23 films were
higher than the values for the TCPM31 films overall; the
observed difference depended on molecular weight and
composition of the triblock copolymers (the molecular
weight of TCPM31 was twice that of TAPM23). As a
general rule, solubility decreases as the molar mass of the
solute increases [28]. As electrical forces due to pola-
rizability and polar moment determine the cohesive en-
ergy, a correlation between dielectric constant and solu-
bility parameter may be expected [29]. Interestingly, both
TAPM23 film and TCPM31 film had very low values of
the dielectric constant when acetone was used as casting
solvent. It appears that when acetone was used, the
polymer film was formed with PMMA as the continuous
phase due to the close affinity of acetone and PMMA
(Table 1).
Another important observation can be made in relation
to the spectra of Figure 2 and Figure 3. The films made
from TAPM23 showed higher dielectric constants overall
except for those made in acetone. This behavior is related
to the solubility parameters shown in Table 1. The solu-
bility parameter of acetone is much closer to that of
PMMA than to the value for PnBA. Considering that
acetone has a weak dispersive contribution, the other
components (the polar component and the hydrogen
bonding component) have a strong effect in the polymer
solutions. We recognize also that to achieve good disper-
sion, the polar component of the solubility parameter is
important, while the other components have been re-
ported by Jing et al. [30]. This corresponds to the dielec-
tric constant results for the fabricated films. The details
of the relationship of the solubility parameter to the di-
electric constant of the polymer are considered in the
morphology section. The dielectric constant results for
the TCPM31 films showed a similar trend to those of
TAPM23 films (Figure 2), which suggests that the solu-
bility parameter has a non-negligible effect. However,
the TCPM31 film cast from chloroform showed anoma-
lous behavior, which can be related to the electrical ac-
tuation results shown in Figure 8. The TCPM31 film
cast from chloroform had the smallest dielectric constant
(Figure 3) and a corresponding small nominal radial
strain (%) in its electrical actuation result (Figure 8).
Zhang and coworkers have proposed an elastic energy
density to measure both the stress and strain generation
capability of an actuator material [31]. The energy den-
sity is UE = 1/2Kε0E2 where E is the applied field, ε0 is
the vacuum dielectric constant (= 8.85 × 1012 F/m), and
K is the dielectric constant of the polymer. The dielectric
constants of all fabricated dielectric films correspond to
the electrical actuation results of those films.
3.2. Mechanical Properties of
Figures 4 and 5 show the stress-strain responses of
TAPM23 films and TCPM31 films, respectively. All of
the curves show an up-swing at high elongations. How-
ever, a marked reduction of the rupture point was ob-
served for the TAPM23 film that was cast from acetone.
Such behavior can be ascribed to the affinity of the ace-
tone casting solvent for the PMMA blocks in the triblock
copolymers. The other TAPM23 films retained the
up-swing at high elongations, indicative of close affinity
Solvent-Induced Phase-Inversion and Electrical Actuation of Dielectric Copolymer Films
Copyright © 2011 SciRes. MSA
Figure 4. Stress-strain tensile mechanical properties for
TAPM23 films with a various casting solvents.
Figure 5. Stress-strain tensile mechanical properties for
TCPM31 films with a various casting solvents.
between PnBA blocks and their casting solvents (Table
Markedly different mechanical properties were found
for the TCPM31 film cast from acetone compared to
those of the other TCPM31 films of Figure 5. The stress-
strain curve of the TCPM31 film cast from acetone re-
flects hard and tough polymer characteristics, which are
intimately related to the lamellar phase morphology of
that film (Figure 6(h)) [32]. The mechanical response of
the TCPM31 films for various solvents can be ascribed to
their polymer-solvent affinities and correlated with their
phase morphologies. The tensile stress-strain curves of
TCPM31 films cast from toluene, chloroform and tetra-
hydrofuran, which have close affinity to the PnBA phase,
show soft property behavior. In the case of TCPM31
Figure 6. TEM photographs of ultra-thin cross-sections of
triblock copolymer films. The bright regions represent the
PnBA phase and the dark regions correspond to the PMMA
phase. Top line (a-d) show the TAPM23 films cast from
THF, chloroform, toluene and acetone, respectively. Bottom
line (e-h) represent to the TCPM31 films cast from chlo-
roform, toluene, THF and acetone, respectively.
Solvent-Induced Phase-Inversion and Electrical Actuation of Dielectric Copolymer Films
Copyright © 2011 SciRes. MSA
films cast from toluene and tetrahydrofuran, the stress-
strain curves showed a similar trend to cylindrical mor-
phology, though lamellar phase morphology is included
in their structure. It appears that the cylindrical mor-
phology is generated in preference to lamellar morphol-
ogy when they coexist. Moreover, the degree of align-
ment of the lamellar structure has an effect on their me-
chanical properties. The TCPM31 film cast from tetra-
hydrofuran, which had a more ordered lamellar structure,
was stiffer than the TCPM31 film cast from toluene.
3.3. Morphologies of PMMA-PnBA-PMMA
Because of the incompatibility between PnBA and PMMA,
the triblock copolymers were expected to show phase
separation. To visualize this effect, TEM imaging was
carried out on thin films prepared in ultrathin sections by
means of acryo-ultra microtome, and the PMMA phase
was selectively stained with phosphotungstic acid (PTA).
The various phase structures were obtained for each of
films and are shown in Figures 6(a-h). From the view-
point of comparing TAPM23 and TCPM31 films, in-
creasing the molecular weight of the triblock copolymers
led to more oriented phase morphology. The morphology
of TAPM23 copolymer films with PMMA content 23%,
cast from various solvents, showed PnBA cylinders dis-
persed in the PMMA phase.
On the other hand, the TCPM31 copolymer with 69%
of PnBA showed a more oriented cylindrical morphology,
excluding the TCPM31 film cast from acetone. This dif-
ference of film microstructure arose from the affinity
between polymer segments and solvent, because the mi-
crostructure is strongly influence by interfacial interac-
tions at the polymer-solvent interface [33,34]. The cast-
ing solvents used in this study, excluding acetone, had
higher affinity to PnBA segments than to PMMA seg-
ments (Table 1). It can be inferred that the preferential
interaction of the solvent to one of the blocks in block
copolymers has a strong effect on self-assembly in films
of the block copolymers. The micro-phase separated
structure is due to the incompatibility between the dif-
ferent connected block chains. The compatibility of sol-
vent and block copolymer is a function of the interaction
parameter (XAB) between the component polymers A and
B. The interaction parameter; XAB is related to the solu-
bility parameters by the relation [35,36].
where Vr is the molar volume of the smaller repeat unit,
R the universal gas constant, T absolute temperature, and
δA and δB are the solubility parameters of polymers A and
B, respectively. The expression for δ includes contribu-
tions from dispersive forces (δd), dipole/dipole (δp) and
hydrogen bonding (δh) interactions. The total solubility
parameter δt is related to these individual components by
The morphology of TCPM31 copolymer film cast
from acetone has a close affinity with PMMA than PnBA
(Table 1). For this reason, the phase structure of this film
showed the lamellar structure with a continuous phase of
PMMA. This ordered structure was observed on micro
scale by confocal laser scanning microscopy as shown in
Table 2. All of the points had nearly equal separation of
on average of 0.6 μm. A similar pattern for the lamellar
structure means that the structure was regularly ordered.
Anomalous phase morphology was observed for
TCPM31 film cast from chloroform. To the best of our
knowledge, non-equilibrium structure have trapped dur-
ing the progress of the coexitence phases with cylindrical
and lamellar morphologies like the phase morphology of
TCPM31, made from tetrahydrofuran because tetrahy-
drofuran and chloroform are easy to quickly vaporize.
This anomalous phase morphology is reflected in the
anomalous performance of the film in the electrical ac-
tuation experiment. Comparison of the TEM images for
TCPM31 films cast from tetrahydrofuran and toluene
shows that the film cast from tetrahydrofuran had a more
ordered lamellar structure with cylindrical morphology
than the TCPM31 film cast from toluene, which is con-
sistent with the respective electrical actuation results. We
conclude that the phase morphologies are significantly
associated with the electrical actuation results.
3.4. Electrical Actuation of
To explore the relationship between phase morphology
and actuation behavior of the films, the responses of cir-
cular actuators, made from TAPM23 and TCPM31 co-
polymers with various casting solvents, to electric fields
Table 2. Confocal laser scanning microscope image and the
distances of phases of TCPM31 film cast from acetone.
points Distance (mm)
1 0.563
2 0.629
3 0.562
4 0.563
5 0.629
6 0.712
Solvent-Induced Phase-Inversion and Electrical Actuation of Dielectric Copolymer Films
Copyright © 2011 SciRes. MSA
were observed and measured. Pre-strained films were
fixed into the circular frame, and the circular center of
the films was coated with compliant electrodes to form
an electrical contact on both sides of the films. When a
voltage was applied across the film, the electrostatic
force of attraction between the oppositely charged elec-
trodes initiated a Maxwell stress that compressed the film.
The results of nominal radial strain (%) of TAPM23 and
TCPM31 films are presented in Figures 7 and 8, re-
specttively. The TAPM23 films had greater deflections
than those of TCPM31 films. The films that formed a
PnBA continuous phase with cylindrical structure showed
large (almost 14%) nominal radial strain and reproduce-
ble electrical actuation behavior was observed. Moreover,
the films started to actuate at a lower critical electric field
strength. However, the TAPM23 film cast from acetone
reached dielectric breakdown at lower electric fields.
This behavior can be ascribed to the low dielectric con-
stant of that film, compared with other films, and the
highly ordered phase morphology. The structure formed
with PMMA continuous phase can be a non-negligible
Interesting performance was shown by the TCPM31
films (Figure 8). In the cases of films with lamellar mi-
crostructure, the deformation of the films exhibited a
stepwise response with increasing electric field strength.
However, the TCPM31 film cast from toluene had a less
ordered lamellar structure, hence the deformation showed
an almost quadratic relationship with the electric field.
The presence of the ordered lamellar PMMA domains
significantly obstructed the actuation [36]. Overall, the
TCPM31 film was in a stable state in high electric fields,
whereas the nominal radial strain (%) was small.
4. Conclusions
Relevant properties of two triblock copolymers with dif-
Figure 7. Electrical actuation results for TAPM23 films
with various casting solvents.
Figure 8. Electrical actuation results for TCPM31 films
with various casting solvents.
ferent molecular weights, prepared by casting from vari-
ous solvents, were studied and compared by examining
the dielectric constant and electrical actuation. For the
PMMA-PnBA-PMMA triblock copolymer films, the
electrical actuation results were consistent with the di-
electric constants and phase morphology and the tensile
stress-strain data. Smaller content of hard segments and
the soft segment as a continuous phase made it possible
to obtain remarkable performance in their actuation be-
haviors by increasing the dielectric constant.
Further study is in progress, with the objective of bet-
ter clarifying the influence of various soft segments and
soft segment contents on the electromechanical perfor-
5. Acknowledgements
This work is supported by a Grant-in-Aid for Global
COE program by the Ministry of Education, Culture,
Sports, Science, and Technology. We thank the Kuraray
Co. Ltd., for providing materials ans we thank Syuji
Kobukata for his support for the TEM analysis.
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