静電気学会誌，45, 1（2021）28-33

J. Inst. Electrostat. Jpn.

論

文

Local Evaluation of Piezoelectricity of Polyester Polymer
Processed with Microwave Heating
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Jun TAKARADA , Yifu TANG , Takaaki TONE , Kohei TAKATANI , Shingo TANAKA ,
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＊＊
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Yoshihiko NISHIZAWA , Masamichi ANDO , Daisuke MANAI ,
＊＊＊
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＊＊＊
Masayuki KABATA , Tomoya MIZUMORI , Makoto KUSUNOKI ,＊＊＊＊Takanori MATSUYAMA
andYoshiro TAJITSU（Received September 24, 2020; Accepted December 17, 2020）

Dipoles of novel polyester polymer film were oriented by instant heating with high power microwaves in order to obtain thepolymer with piezoelectricity. Response displacement indicating the piezoelectricity was confirmed from results of measuring theapplied voltage dependencies by piezoresponse force microscopy. The reason is that the fundamental wave component of response
displacement is about 10 times larger than the second harmonic component of response displacement. However, it was also
confirmed that the area observing the piezoelectric response was local. The amplitude of the response displacement in an area
shifted a few mm from an origin area was 1/10 times smaller than that in the origin area with large piezoelectric response. In
addition, it was confirmed that there were a few particles whose piezoelectric response was larger than other particles in the area
where large displacement response was obtained. Microwave processing has the capability to partially orient the polyester polymer.

1．Introduction

and the intermolecular interaction including van der Waals

Piezoelectric polymers have attracted attention with theforce. However, PVDF crystals are known to be stabilized byadvent of polyvinylidene fluoride（PVDF）. The piezoelectric

van der Waals forces, because the Coulomb forces are almost

polymers are now widely recognized as the key material ofbalanced. Therefore, the thermal resistance is low, and thewearable device sensors. However, the upper limit

ferroelectricity deteriorates significantly at around 140℃ .

temperature of the polymer is less than 80℃, which hinders

When trifluorinated ethylene（TrFE）, which disturbs thethe practical development. In the past several decades, thestructure of the crystal, is randomly inserted into the molecule

research for piezoelectric polymers to replace PVDF has been

chain, the intermolecular distance of vinylidene fluoride

focused on imide

based（VDF）/ TrFE copolymer with mol％ ration of 55/45 become

polymers. In order to exhibit the ferroelectricity, the dipoles inlarger than that of PVDF. The different crystal phase becomesthe molecules chain in the crystallites wrapped amorphousstable（paraelectric phase）at low temperature, and themust be oriented by the external electric field. It is necessary

ferroelectricity disappears at Curie point of 80℃, because thefor PVDF to break the hydrogen bond pinning the dipole in

Coulomb force of the dipole-dipole interaction becomes small.

the molecule chain and the dipole is switched only by the

The structure of the crystal was verified from the experimental

internal rotation of the molecular chain. In other words, theresult of an endothermic peak in a thermal analysis

external electric field must change the space position of the

Weiss law related to temperature characteristics in dielectric

molecule stabilized by the Coulomb force of the dipole, theconstanthydrogen bond, the molecular interaction with covalent bond,polarization

1）

2-5）

, urea

3, 6）

7-10）

and lactic acid7, 11-14）

15）, Curie-

16-18）, a rapid disappearance of the remnant

17, 19-21）

19, 20）20-22）, and a large change in lattice spacing. Thebalance of each energy such as PVDF is a miracle in the case
Keyword：Microwave Heating, Piezoresponse Force Microscopy,
Polyester Polymer＊
Department of Electrical and Electronic Engineering,Kansai University, 3-3-35, Yamate-cho,Suita-city, Osaka,564-8680, Japan＊＊
Murata Manufacturing Co., Ltd., 2288, Oshinohara, Yasucity, Shiga, 520-2393, Japan＊＊＊
Osaka Organic Chemical Ind., Ltd., 18-8, Katayama-cho,Kashiwara-city, Osaka, 582-0020, Japan1
takarada@kansai-u.ac.jp
DOI：https://doi.org/10.34342/iesj.2021.45.1.28

of a complex structure of polymer. The dipoles associated withthe molecular chain cannot be effectively switched and aligned
in one direction although many researchers have expended to
select molecular structures in imide and urea polymers.
Therefore, there was no emergence of ferroelectric polymers
exceeding the thermal resistance of PVDF. Therefore, in thisresearch, we set the following goals.
・Avoid strong hydrogen bonds.
・Control the influence of Coulomb force.

Local Evaluation of Piezoelectricity of Polyester Polymer Processed with Microwave Heating（Jun TAKARADA et.al）

29

・Secure a certain ordered region where ferroelectricity isexpressed.
・Stabilize the molecule by hydrogen bonding after dipole
orientation to increase thermal resistance.
We will develop a novel polyester polymer that combine
dipolar orientation and stabilization by inserting fluorine

Fig.1 Chemical structure of（a）PVDF and（b）Polyester polymer.

groups into side chain molecules with high degree of freedom
instead of main chain molecules with low freedom as shownin Fig. 1. The hydrogen bond between the dipoles of thepolyester polymer is smaller than that of PVDF because theend of the side chain of the polyester polymer has a carbonylgroup in addition to the methyl and the fluorine group.
However, it is not easy to orient the dipole in the direction of
the electric field due to the hydrogen bonding although the
dipole is in the side chain. Therefore, we tried to control the
Coulomb force by applying energy of 10 kJ/mol order to thesample by high power microwave irradiation to break theFig.2 D-E curve of polyester polymer shown in Fig.1（b）.

hydrogen bond. Furthermore, we try to orient the fluorine

However, the dispersion frequency of its orientationgroup in the region by applying a DC voltage. The origin ofpolarization is considered to be sufficiently lower than thethe orientation of this polyester polymer is the rotation of thefrequency of microwaves. Therefore, dipoles cannot follow

side chain while the origin of orientation of PVDF is thethe microwave AC electric field, and the orientation does not

rotation of the main chain. Therefore, the degree of freedom ofchange with the microwave AC electric field. The temperature

dipole rotation of this polymer is higher than that of PVDF. In

rises locally and instantaneously due to the dielectric loss ofaddition, this polymer has more amorphous components than

atomic polarization and electronic polarization although

PVDF. Based on the above, this polymer was selected in this

dipoles cannot be followed. The dipoles are oriented by astudy. We aim at orientating the dipole in the region withstatic electric field while giving a rapid temperature change bycertain ordered amorphous particles around crystals by thesuperimposition. Few example of microwave heating have

local and instant heating and cooling. We succeeded inbeen applied to heating polymers although microwave heating

securing a certain ordered region to express thehas been used to sinter ceramics . Local heating by microwaves

piezoelectricity because the piezoelectric response voltage wascan instantly break hydrogen bonds between molecules in thisconfirmed in the local region from the measurement result ofpolymer. While the total amount of thermal energy given to

Piezoresponse Force Microscopy（PFM）. In addition, thisthe sample by microwaves is the same as that by normal heat

polyester polymer has a thermally stable structure since thetreatment, the heat absorption and radiation rate bymelting point of this polyester polymer is 200℃ or more.

microwaves are faster than that by normal heat treatment. In

23）addition, microwave heating can prevent the crystallization of

2．Orientation processing of polyester polymer film
by microwave heatingHigh voltage application is required to orient the film.

the sample compared to normal heat treatment that conducts
heat from outside the sample because microwaves heat theinside of the sample uniformly.

Polarization reversal could not be observed from DE loop of

Figure 3 shows a microwave heating and applying DC

the film as shown in Fig. 2 although an electric field exceeding

voltage system that has been applied to the polylactic acid film

the coercive electric field of PVDF was applied to the sample.

in previous research . The microwave used for heating is 2.4
24）

Therefore, we tried to heat the polymer rapidly with highGHz. A magnetron was used to generate high power

power microwave and simultaneously applied high DC voltage

microwaves. The magnetron can oscillate high intensity andto orient the molecules in the polymer. The origin of thestable microwaves. Microwaves are output from the magnetronorientation of this polymer is the rotation of the side chain.

in a rectangular waveguide. Microwaves transmitted in the

静電気学会誌 第45巻 第 1 号（2021）

30

Fig.4 FEM simulation（a）model and（b）result of resonant
electric field amplitude at 2.4 GHz inputting the microwave
24）
power of 300 W .

150 to 300 K during 20 ms. Actually, some of the sample isFig.3 Manufacturing apparatus applying microwave and DC
24）
voltage to polyester polymer .

melted when the microwave is incident to the sample. In

rectangular waveguide are converted to ones transmitted in amonomer can be 20 - 40 kJ/mol. The energy can cut hydrogencoaxial line with an input impedance of 50 Ω because

bond. The heat instantaneously generated by the microwave is

microwaves transmitted in coaxial line are easier to form aabsorbed by the conductive roller, and the sample isbias-T, couplers, a resonant and a load termination part in the

instantaneously cooled. This rapid temperature change

processing system than microwaves transmitted in theprevents complete crystallization of the polymer and creates arectangular waveguide. For the high power transmission,

space in which crystal defects and molecules move. At theradius of the center conductor of the coaxial line is 10 mm.

same time, we aim to orient the molecules by applying a DC

The microwaves are input to the sample through a power

voltage. The normal orientation process is performed while themonitoring coupler and a bias voltage application bias-T.

sample is warmed by thermal conduction from the heater. On

addition, the energy given by microwaves per mol of the

Figure 4（a）shows the film and the conductor formed by

the other hand, the proposed orientation process is performed

the finite element method（FEM）simulation model thatwhile the microwave directly heats the sample. The crystal

exactly represents resonant and input part in the microwave

phase of the sample changes when the same thermal energy asheating system. The film is nipped and transported by athe proposed orientation process is applied during the normal

conductor roller. The conductor roller is a λ/2 resonator, aorientation process. In normal process, the crystallization is

microwave whose amplitude amplified ten through hundredpromoted and the molecule is fixed, and the dipole cannot betimes by the resonator is applied to the film. Figure 4（b）

aligned with only high DC voltage. In proposed process, highshows the FEM calculation result of the resonant electric field

DC voltage can align the dipole with the help of the microwave.

strength at 2.4 GHz when the microwave power of 300 W is
input. The 7 MV/m electric field strength was applied to the

3．Evaluation of Piezoelectricity of Polyester polymer

film . Therefore, a polymer with small loss tangent can be

According to the requirements mentioned in Chapter 1, we

heated rapidly. The measured dielectric loss tangent of thesynthesized a polymer whose main component is a molecule

sample was 0.003. The sample absorbs the power about 2 W

having a carbonyl group and a fluorine group at the end of theand replaces the power with thermal energy when the sample

side chain as shown in Fig. 1（b）
. The main component wasreceives the power of 600W. The power consumption actuallycopolymerized with a molecule having a carbonyl group and ameasured was also 1 to 2W. When the conductive roller moves

methyl group at the end of the side chain as a softener. The

the sample at 5 mm/s in the width direction with a width of 0.1

molar ratio of each molecule having a fluorine group m and

mm at which the sample contacts the roller, the sample

molecule having a methyl group n is 4:1. A film was fabricatedcontacts the roller during 20 ms and obtains the energy of 40

by solvent casting. The film size and thickness were 5.0×5.0

mJ by microwave. The film thickness of the sample and themm and 20 μm, respectively. The sample film was set in thelength of contact between the sample and the roller are 20 μm

production apparatus shown in Fig. 3, and a microwave of 600

and 30 mm, respectively. Assuming that the sample has a

W was input to the film. At the same time, a DC voltage of -2

specific heat and density of a common polymer, thekV was applied to the sample. The topographic andtemperature of the microwave incident area can increase frompiezoelectric image of the 2 × 2 μm region were observed by

24）

2

2

Local Evaluation of Piezoelectricity of Polyester Polymer Processed with Microwave Heating（Jun TAKARADA et.al）31

atomic force microscopy（AFM）and PFM. In previous study,
the author has observed the local polarization reversal
phenomenon of P（VDF/ TrFE）film by PFM ．Thus, the25）orientation of the particles in the polyester polymer film wasobserved by PFM．The scan in contact with the surface was
performed with a conductive cantilever with a spring constantand a resonant frequencies of 0.1 N/m and 37 kHz, respectively.

Fig.5 Topographic images（a）before and（b）after the microwave
and DC processing.

A voltage with a frequency of 10 kHz was applied to the sample.
The applied voltage amplitude Figure 5 shows topographic
images before and after the microwave and DC processing. The
surface roughness before and after the processing are 2.26 and225 nm, respectively. The particles are region surrounded by thewhite line in Fig. 5. The particle sizes are about 20 through 30
nm and 100 through 200 nm, respectively. The particles growabout 10 times by the processing.
Figure 6 shows the response displacement images obtainedin the area of Fig. 5（b）
（Area 1）. Figures 6（a）and 6（b）
show fundamental wave amplitude images of displacement
when voltage amplitudes 2 and 5 V were applied. Figures （6 c）
and 6（d）show the second harmonic amplitude images ofdisplacement. The response displacement amplitude increases

Fig.6 Microscopic images in region with large amplitude of
response voltage:（a）and（b）displacement response
images of fundamental wave amplitude applying voltage
amplitude of 2 and 5 V,（c）and（d）displacement response
images of second harmonic amplitude applying voltage
amplitude of 2 and 5 V.

as the applied voltage amplitude increases. In addition, thewidth of the maximum and the minimum value of thedisplacement amplitude also increase. This result suggests theoccurrence of piezoelectricity in the local area of this film.
However, there are regions that cannot measure piezoelectric
response when the observed position changes.
Figure 7 shows the topographic and displacement response
images of an area a few mm away from Area 1（Area 2）. The
fundamental wave amplitude of the displacement response andthe width of the maximum and minimum values shown in Fig.
（
7 b）and （7 c）are smaller than those shown in Fig. （6 a）and6（b）. The second harmonic amplitude of the displacement
shown in Fig. （7 d）and （7 e）can hardly measure the response
due to noise. Next, the sample was turned upside down, andmicrowave and DC voltage of -2 kV are applied to the sample.
Figure 8 shows the topographic and displacement response
images on the top surface of two samples. Figures 8（a）
（c）
show the topographic, displacement amplitude and phase

Fig.7 Microscopic images in region with small amplitude of
response voltage:（a）Topographic image,（b）and（c）
displacement response images of fundamental wave
amplitude applying voltage amplitude of 2 and 5 V,（d）and（e）displacement response images of second harmonic
amplitude applying voltage amplitude of 2 and 5 V.

images before turning the direction in which the voltage was

Figures 9（a）and（b）show topographic data and theapplied, and Figs. 8（d）
（f）
show those after turning thefundamental wave amplitude changing the applied voltage at

direction in which the voltage was applied（Area 3）. The

the location described by the white solid line in Area 1. The

displacement response is reversed since the average value of the

response voltage become locally large for the particles on thephase calculated from the displacement phase images shown insurface located in the region of 0.70 to 0.90 μm. This result

Figs. 8（c）and 8（f）is 20.9 and -167.8 degrees, respectively.

suggests that the microwave processing locally orients the

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静電気学会誌 第45巻 第 1 号（2021）

Fig.8 Microscopic images on the top surface of two samples:（a）（c）one sample images before turning direction in which
DC voltage of -2 kV was applied,（d）（f）
another sample
images after turning direction in which DC voltage of -2 kV
was applied,（a）and（d）topographic images,（b）and（e）
displacement response amplitude images, and（c）and（f）
displacement phase images.

Fig.10 Average value of response voltage changing applied
voltage in Area 1-3:（a）Fundamental wave in Area 1 and
2,（b）second harmonic component in Area 1 and 2, and（c）Fundamental wave in Area 3.

surface has a positive correlation with the applied voltage. On
the other hand, the top surface response after microwave
treatment with -2 kV applied to the bottom surface has anegative correlation with the applied voltage. Therefore, it
assumes that these responses are piezoelectric responses due
Fig.9 Line plot at the location described by the solid line in Figure
5（b）:（a）Topographic Height,（b）fundamental wave
amplitude changing the applied voltage.

to the orientation of the polyester polymer processed by
microwave heating. However, there are also the region withalmost no voltage dependence in one film, as shown in Fig. 7.

particles on the surface. In addition, the base response voltage

The plots described by circles in Figs. 10（a）and 10（b）

increases with the increase of the applied voltage. This result

indicate the average value of the response voltage of thesuggests that the microwave processing orients the particles

particles on the line shown in Fig. 7（a）when the applied

inside the film.

voltage is changed. The slopes of fundamental and second

The plots described by triangles in Figs. 10（a）and（b）

harmonic components in the region with large response areindicate the average value of the response voltage in Fig. （9 b）

13.2 and 9.9 times larger than those in the region with small

when the applied voltage is changed. The average value was

response, respectively. From the result, microwave processing

calculated from the voltage value between 0.70 and 0.90 μm

partially orients this polyester polymer. Areas 1 and 3 selectedon the particle with particularly large response voltage

regions that passed just in the center of the roller where thementioned above. The fundamental and second harmonic

microwave processing was performed. Area 2 selected acomponents of the average voltage are fitted by the first andregion that passed a few mm outside from the center. The

second order functions to the applied voltage as shown in Figs.

reason for selection is that the intensity of the microwave

10（a）and 10（b）, respectively. The fundamental average

electric field generated in the center of the roller is thevoltage is larger than the second one. Figure 10（c）shows thestrongest from the finite element analysis result. It is possible

applied voltage dependency of the average value of the cosinethat the piezoelectricity has increased due to the effect ofcomponent of the response voltage in Area 3. The averagethermal energy from microwaves, however detailed

value was calculated from the response voltage value on solid

verification has not yet been achieved. The evaluation ofwhite line described in Fig. 8（d）
. The top surface response

dependence of processed location is a future issue.

after microwave processing with -2 kV applied to the top

We aim to uniformly orient this polyester film by microwave

Local Evaluation of Piezoelectricity of Polyester Polymer Processed with Microwave Heating（Jun TAKARADA et.al）heating, and to create a macro piezoelectric polymer film in
the future although it is limited to partially orienting thispolyester-based polymer in this report. It is considered that thepiezoelectricity was obtained only in some small regions andsome particles since the crystallization of the sample has
progressed as a whole. We planned to rotate the dipole side
chain of the amorphous region. However, there were no
defects or spaces in which the side chains could rotate because
many ordered crystals were formed. The piezoelectricity will
be obtained by changing the processing speed and microwave
power since the temperature change rate was still slow.

4．Conclusions
Dipoles of polyester polymer film were oriented by instantheating with high power microwaves in order to obtain thepolymer with piezoelectricity. The displacement response
indicating the piezoelectricity was confirmed as a result of
measuring the piezoelectricity with Atomic Force Microscope.
The reason is that the fundamental wave component ofresponse displacement is about 10 times larger than the second
harmonic component of response displacement. However, it
was also confirmed that the area observing the piezoelectric
response was local. The amplitude of the response displacement
in an area shifted a few mm from an origin area was 1/10 times
smaller than that in the origin area with large piezoelectric
response. In addition, it was confirmed that there were a few
particles whose piezoelectric response was larger than otherparticles in the area where large displacement response wasobtained. Microwave processing has the capability to partiallyorient the polyester polymer.
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