polyamide 11/poly(vinylidene fluoride) blends as novel flexible materials for capacitors

Communication

Polyamide 11/Poly(vinylidene fluoride) Blendsas Novel Flexible Materials for Capacitors

Rui Li, Chuanxi Xiong,* Dongliang Kuang, Lijie Dong, Youan Lei,Junlong Yao, Ming Jiang, Liubin Li

A novel capacitor with high dielectric constant (e) has been developed by blending poly-(vinylidene fluoride) (PVDF) with polyamide (PA11). The blends show high dielectric constants(eblend¼40), which give better frequency stability (1 MHz), and excellent mechanical proper-ties. Based on certain volume fractions, themeasured dielectric constants (eblend) were found toexceed those of the corresponding polymers, in contrasted to conventional composites, whereepolymerA< ecomposite< epolymerB. SEM investigations suggest that the enhanced dielectric beha-vior originates from significant interfacial polymer-polymer interactions. DSC and XRDdemonstrate that blending PA11 with PVDF affects the crystalline behavior of each com-ponent. However, the PA11/PVDF blends exhibit aslightly high dielectric loss (tand� 0.17), which isa great disadvantage to a capacitor. Adding acopolymer of styrene and maleic anhydridedecreased the dielectric loss (tand�0.057) andincreased the dielectric constant (eblend¼60).Our findings suggest that the high-e polymericblends created represent a novel type of materialthat is flexible and easy to process, of relativelyhigh dielectric constant, of high breakdownstrength and, moreover, is suited to applicationsin flexible electronics.

R. Li, C. Xiong, L. Dong, Y. Lei, J. Yao, M. Jiang, L. LiSchool ofMaterials Science and Engineering,WuhanUniversity ofTechnology, Wuhan, 430070, ChinaFax: þ86 27 87652879; E-mail: polymerlab@ whut.edu.cnC. Xiong, D. Kuang, L. DongState Key Laboratory of Advanced Technology for MaterialsSynthesis and Processing, Wuhan University of Technology,Wuhan 430070, China

Macromol. Rapid Commun. 2008, 29, 1449–1454

� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Introduction

Poly(vinylidene fluoride) (PVDF) and its polymeric compo-

sites have been extensively studied in piezoelectricity and

dielectric property since their electric properties were

discovered.[1–3] Research into the polar crystal structures

and crystal phase transitions of the polymers has resulted

in an understanding of piezoelectric and dielectric

phenomena.[4–6] A number of studies concerning the

dielectric properties in inorganic-polymer composites,

DOI: 10.1002/marc.200800253 1449

R. Li et al.

1450

such as PZT/PVDF, have been reported. This compound

material is described as having high dielectric proper-

ties.[7–14] Much other work concerning complex materials

comprised of PVDF and other inorganic matters has also

found that the inorganic-polymer composites possess very

distinct dielectric properties that can be applied in

electrics, such as hydroacoustic sensors and medical

applications.[15–18]

Regardless of the mechanical properties of the com-

pounds, themachine-shaping process, necessary due to the

inorganic component, has slowed down progress in

finding applications. As we know, a macromolecule

possesses unique properties due to its architectural

features. Polymer-based composites are now gaining

considerable research attention for applications in resis-

tive, inductive, and capacitive components, because they

have the potential to provide lightweight, flexible and

volume-efficient electrical components required for

embedded passive technologies.[19–23]

The blending of different polymers is an important

and efficient method of obtaining the desired properties

of polymer systems, based on the structure and

property of each component. Scheinbeim et al.[24–25]

reported that the odd-numbered nylons, such as

polyamide (PA11), exhibited fine piezoelectric behavior

after appropriate electroprocessing, especially when the

temperature is raised above the glass transition

temperature.[26–28]

In this paper, we put forward a simple and effective

route for the synthesis of polymeric blends with excellent

dielectric behavior and mechanical properties, especially a

flexible property for a capacitor. At the same time, we add

the compatilizer SMA to the PA11/PVDF blends, and

analyze the electric properties from the structural

mechanism, so as to gain a material with better electric

properties.

Experimental Part

PVDF (Mw ¼37� 105, d¼1.75 g � cm�3), PA11 (Mw ¼4�105,

d¼ 1.03 g � cm�3) and SMA (Mw ¼6� 105, MA¼ 18%) powders

were supplied by the Shanghai Organism Institute, the Guangz-

hou Far East Chemistry Corporation and the Chinese Academy of

Science, respectively.

PA11/PVDF blends of a thickness of 3mmwere produced by co-

melt-pressing at 190 8C and were subsequently quenched in ice

water. Finally the co-melt-pressed-quenched filmswere uniaxially

oriented by drawing to a ratio of 2:1 at room temperature, and

clamping immovably for 30min. The sampleswere circularwith a

diameter of 12 mm.

The dielectric properties of the samples were measured using a

HIOKI3532-50 LCR meter. After being polarized, the samples were

left at room temperature for 24 h, and the piezoelectric properties

were then measured on a ZJ-3A d33 test machine. Fourier-



transform IR (FTIR) spectra of PA11/PVDF blends were measured

by a Nicolet FTIR spectrometer. In order to analyze the

compatibility and flexibility of the blends, the surface topogra-

phies of PA11/PVDF blends with and without SMAwere observed

on a JSM-5610LV scanning electron microscope (SEM), and the

mechanical performances of samples were tested on a RGT-30A

tensile-testing machine.

Results and Discussions

Dielectric Properties

Figure 1a shows the frequency dependence of the dielectric

constant of the PA11/PVDF blends. The dielectric constant

(eblend) of PA11/PVDF blends can reach �20–40, observed

distinctly for the film at room temperature. Because SMA

improves the compatibility between PA11 and PVDF, the

dielectric constants of PA11/PVDF blends containing SMA

increase remarkably (Figure 1b). We demonstrate that,

when the ratio of PA11 to PVDF is 80:20, the eblend (60) of

the blend containing 1% SMA is twelve times higher than

that of PA11 (e¼ 5) and about five times higher than that of

PVDF (e¼ 12).

Figure 1c and d show the frequency dependence of the

dielectric loss tand of the two blends without and with

SMA, respectively; the blend with SMA added is of lower

dielectric loss than the pure PA11/PVDF blend. We

demonstrate that, when the ratio of PA11 to PVDF is

80:20, the tand of the blend containing 1% SMA is 0.057,

which is much lower than the value of 0.17 for pure PA11/

PVDF blends. The PA11/PVDF blends studied here, having

high dielectric constants and low dielectric losses, there-

fore have high potential as dielectric materials to be

employed in the microelectronic package and other

industrial applications.

Piezoelectric Properties

Figure 2 shows the relationship between the piezo-

electric constant, d33, and the PA11 volume fraction.

From the PA11/PVDF blends, we can see that the

blends have higher piezoelectric constants than PA11

(2.0 pC �N�1), but not as high as PVDF (�6 pC �N�1).

Although the two samples do not exhibit superior

piezoelectric properties, a very clear enhancement of the

piezoelectric constant d33 of the PA11/PVDF is observed

by adding SMA at room temperature. When the content

of PA11 is 50%, the d33 value of PA11/PVDF with SMA is

9 pC �N�1, approximately three times higher than that of

PA11/PVDF without SMA, 3.6 pC �N�1. The results show

that, when the content of PA11 is 50%, the degree of

polarization of PA11/PVDF was enhanced by SMA so as

to improve the piezoelectric properties.

DOI: 10.1002/marc.200800253

Polyamide 11/Poly(vinylidene fluoride) Blends as . . .

Figure 1. Frequency dependence of dielectric constant for PA11/PVDF blends: a) without SMA; and, b) with SMA added. Frequencydependence of dielectric loss, tand, for PA11/PVDF blends: c) without SMA; and, d) with SMA added.

Figure 2. The piezoelectric constant, d33, of PA11/PVDF blends.Figure 3. Current-voltage plots for the PA11/PVDF blends.

Current-Voltage Curve

Figure 3 shows the current-voltage behavior of the blends.

We note that the remnant current of is as low as

10�7 mA � cm�2. The remnant current at 0 V of the blend

with SMA is an order of magnitude lower than that of the

blend without SMA. Regardless of depolarizing, charges

could tunnel through the well-connected interface

obtained by SMA into the others to create a lower remnant



current.[12] The effect of macromolecular morphology

alterations in responsive field on their electrical and

charge storage properties can bring new possibilities into

effect for functional composite materials.

Mechanical Properties

The effect of the PA11 volume fraction on the tensile

strength and the tensile elongation of blends are shown in

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R. Li et al.

Figure 4. Variations of the tensile strength and the tensileelongation of PA11/PVDF blends versus volume fraction of PA11.

1452

Figure 4. It can be seen that the tensile strength changed

continually and irregularly with increasing PA11 content.

With increasing PA11 content, the tensile strength of

PA11/PVDF blends decreased slowly, but subsequently

increased sharply, and the lowest tensile strength reached

8.33 MPa when the PA11 content was 60%. It is a typical

curve of incompatible polymeric blends, which explains

why it is necessary for the compatilizer SMA to be added to

the PA11/PVDF blends. Furthermore, the tensile elongation

of the PA11/PVDF blends is from 10 to 80% and the

inorganic-polymer composites are excellent dielectric

materials. It suggests that the PA11/PVDF blends are

surpassed by few other compounds as flexible materials

with high e.
Figure 5. a) DSC results for PA11/PVDF blends at a heating rate of5 8C �min�1. b) XRD spectra of PA11/PVDF blends.
Structure Analysis

Figure 5a shows the differential scanning calorimetry

(DSC) curves of PA11/PVDF with different percentages

(with a heating rate of 5 8C �min�1) for the blends. We

found that the crystallization peaks of the two polymers

existed in the blends and the melting points of crystals

hardly changed, but the melting zones of the crystals in

blends grew broader. The results demonstrate that the

crystals of PA11 and PVDF in the blends are incomplete.We

believe that the NH and C––O in PA11 and polar CF2 groups

in PVDF would interact with each other.

The x-ray diffraction (XRD) patterns of PA11/PVDF

blends are shown in Figure 5b. We note that the diffraction

peak of PA11 just appears as the content of PA11 added is

near to 60%. It illuminates that the crystallization

transmutation of PA11 from a to d0 is controlled by the

PVDF content; that is, for the melt-quenched PA11/PVDF

blends, the PA11 content has the dominant effect (> 50%)



on the blend properties, which can be attributed to the fact

that the d0 crystal structure of PA11 is good at polarization,

and so to produce electricity.

Figure 6a is a surface photograph of the PA11/PVDF

blend. Figure 6b and c show the cross-sectional topography

of PA11/PVDF blends after 24 h at room temperature. In

Figure 6b, there is a remarkable layer-layer interface, like

the pages of a book, a typical surface photograph of which

is shown by the Figure 6a. Especially, it exhibited a typical

two phase separation, which indicates PA11 has poor

compatibility with PVDF in the blends, and that the

dielectric properties result from the PA11/PVDF, rather

than the individual components.

Figure 6c exhibits a good compatibility, in comparison

with Figure 6a and b, in the composite. A well-connected

interface can obviously be observed in virtue of the

compatible properties of SMA. This can be ascribed to the

DOI: 10.1002/marc.200800253

Polyamide 11/Poly(vinylidene fluoride) Blends as . . .

Figure 6. SEM micrographs of: a) the surface of PA11/PVDF; b) thePA11/PVDF blend; c) the PA11/PVDF blend with SMA.

Figure 7. FTIR spectrum of PA11and PA11-g-SMA.

interfacial tension between PA11 and PVDF being sharp

decreased by the PA11-g-SMA. In addition, the addition of

SMA made the dispersion more even and of smaller

dimension. That is to say that the presence of SMA in

blends can improve interface adhesive properties formed

as the interface intermolecular react.

To explore effect of the compatilizer, we investigated the

functionalization of the SMA using far-infrared FTIR

spectroscopy. Figure 7 shows the IR spectra of PA11 and



PA11-g-SMA (the thickness of the sample was 1.5 mm).

With the addition of SMA, we found that the characteristic

peak of acid anhydride of SMA became weaker in the

bands at 1 190 and 1279 cm�1, and the peak at 1 414 cm�1

indicates the formation of the cyclic amide group.[29] This

phenomena indicated that a new composite (PA11-g-SMA)

was produced by the reaction, acting as the real

compatilizer to promote the reaction between SMA and

PA11. Therefore, adding a proper amount of SMA is an

important measure to improve the electrical properties.

The graft reaction of PA11 and SMA can be exhibited by

ref.[29] As an effective compatilizer, PA11-g-SMA can make

PA11 and SMA compatible. At the same time, PVDF is also

compatible with SMA. The reason is that two carbonyls

and one oxygen atom with pentavalent rings are

coexistent in the SMA molecular chain, which can react

with the a-phase hydrogen atom in PVDF. The hydrogen

atom of SMA may also simultaneously react with the

electronegative fluoride atom in PVDF. Those intermole-

cular functions improve the compatibility between the

PA11 and the PVDF.

Conclusion

Through the measurements of dielectric constant, e,dielectric loss, tand, and piezoelectric constant, d33, after

polarizing at room temperature, it was confirmed that

PA11/PVDF blends with SMA added have higher dielectric

constant, e, and lower dielectric loss, tand, than those

without SMA, and also exhibited good frequency stability.

Compared with the samples without SMA, a very large

enhancement in dielectric and piezoelectric response for

the PA11/PVDF came into being. Itmay be attributed to the

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R. Li et al.

1454

compatibility between PA11and PVDF being increased by

the SMA, which makes the electron move among crystal-

lization grains so as to improve the polarization and

electric properties in sequence. The SMA plays an

important role in these blends.

However, the result is different from those in the

literature and deviates, to some extent, from general

knowledge. In theory, the dielectric constant of PVDF/PA11

blends should lie between those of PVDF and PA11,

whereas the dielectric performance testing in this work

was unexpectedly far better than the two individual units;

a feasible explanation is the influence of the polarization

between two phases on the piezoelectric and dielectric

properties. Inquiring into the interface intermolecular

microstructures, typical phase separation appears on the

interface intermolecular configuration. The purpose of the

compatilizer (i.e., SMA), applied to this system to reactwith

both polymers (PA11 and PVDF), is to improve the

compatibility between PA11 and PVDF. The PA11/PVDF

blends have excellent flexibility. The dielectric and piezo-

electric properties of the blends, with or without SMA,

were determined and may be suitable for application in

capacitors, sensors and so on.

Acknowledgements: This work was supported by the NationalNatural Science Foundation of China (grant no. 50572081) and theSchool of Material Science and Engineering of Wuhan University ofTechnology.

Received: April 28, 2008; Revised: June 19, 2008; Accepted: June20, 2008; DOI: 10.1002/marc.200800253

Keywords: dielectric properties; electrical properties; flexible;intermolecular interactions; PA11/PVDF blends; SMA



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DOI: 10.1002/marc.200800253

polyamide 11/poly(vinylidene fluoride) blends as novel flexible materials for capacitors

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