Fabrication of Nano-branched Coaxial Polyaniline / Polyvinylidene Fluoride Fibers via Electrospinning for Strain Sensor

Rong Huang1,2,a, Yunze Long*1,3,b , Chengchun Tang1,4,c, Hongdi Zhang1,d 1College of Physics, Qingdao University, Qingdao 266071, China

2CAS Key Lab for Biological Effects of Nanomaterials and Nanosafety, National Center for NanoScience and Technology, 11 BeiYiTiao, ZhongGuanCun, Beijing 100190, China

3State Key Laboratory Cultivation Base of New Fiber Materials and Modern Textile, Qingdao University, Qingdao 266071, China

4Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

ahuangrong8666@126.com, byunze.long@163.com, ctang.chengchun@163.com, dhongdi_zhang@163.com

Keywords: Conducting polymer; Polyaniline; Nanofiber; Electrospinning; Strain sensor

Abstract. Soft conductive elastomer materials have wide potential applications in material science

and electronic engineering. Through electrospinning and in-situ polymerization, a kind of well-

organized coaxial polyaniline/polyvinylidence fluoride (PANI/PVDF) microfibers with conductivity

about 0.6 S/cm were fabricated, which combined the advantages of conducting polymer and elastic

material. It is found that the resistance of the fibers was changed with the curvature variation. The

results indicate that the PANI/PVDF microfibers could be used as strain sensor with high flexibility,

high sensitivity, and stable repeatability.

Introduction

The development of highly conductive elastomer-like materials is very important to the modern

electronic industry especially in the emerging field of stretchable electronics [1-3]. Materials with

high conductivity and mechanical compliance have widely application prospects on sensory skin for

robotics, artificial muscles, active-matrix displays, stretchable organic transistor, stretchable

integrated circuits, elastic capacitive strain sensor, and stretchable solar cells. Organic-inorganic

composites like carbon nanotubes or metal waves / nets structures embedded in soft elastic organic

materials were considered to be the promising stretchable conductors [1-3]. However, conductive

elastic materials which can maintain high conductivity under significant strain are still a challenge.

Recently, researchers tried to develop all-organic elastic conductors based on conducting polymers

like polyaniline (PANI) and so on. PANI as a kind of conducting polymers has become extensively

studied materials in the last decade. Compared with other conjugate polymers, PANI was excellent in

its cheap price, easily polymerizable and high conductivity [4,5]. But the poor machinability caused

by the strong polarity of this polymer limited its applications in flexible electronic materials

especially the micro-conductors widely demanded in emerging technologies.

Here, through electrospinning and in-situ polymerization, we fabricated a kind of aligned

nano-branched coaxial polyaniline / polyvinylidene fluoride (PANI/PVDF) fibers as a novel strain

sensitive flexible microelectronic material. Electrospinning is a simple manufacturing method for

micro/nanofiber materials fabrication [6]. We used a drum collector to electrospun aligned PVDF

fibers. Then PANI nanobranches were grown on the surface of PVDF fibers. The novel structure was

characterized by scanning electronic microscopy (SEM). Moreover, the conductivity and strain

sensing properties were measured.

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Experimental details

PVDF (Mw 275000) was purchased from Sigma-Aldrich, N,N-dimethylformamide (DMF), acetone,

aniline, ammonium peroxodisulfate (APS), 5-sulfosalicylic acid dihydrate (SSA) were all purchased

from Sinopharm Chemical Reagent, Beijing. All the reagents were used without further purification.

A dc high-voltage power supply (40 kV, Tianjin Dongwen) was used in this electrospinning process.

The electrospinning precursor was prepared by dissolving 2.2 g PVDF in dissolved and in 4.9 g

DMF and 4.9 g acetone. Then it was kept under stirring for 3 hours at 60ºC in order to obtain a

uniform solution. The solution was injected in a syringe and a stainless steel needle (inner diameter is

0.7 mm) was used as positive electrode connected to the power supply. The aligned PVDF fiber was

fabricated by an electrospinning setup with a roller as collector. An eight column roller with the speed

of 600 rpm was applied as negative electrode and used to collect aligned PVDF nanofibers. The

diameter of the column is about 6 cm. The distance from the positive tip to the axis of the roller is

about 13 cm and the applied voltage is 9 kV. The PVDF fibers were transformed to cover the hole (0.8

cm*2 cm) and pasted on the polyethylene plastic film with double-sided adhesive tape. This sample

was prepared for the following in-situ polymerization process.

Firstly, 0.02 mol APS was dissolved adequately in 50 ml deionized water to form the solution A.

Secondly, 0.02 mol aniline was added and stirred into 50 ml deionized water. Thirdly, 0.01 mol SSA

was dissolved thoroughly in it to make up the solution B. And then that poured the solution A into the

solution B and stirred them well to be an equally blended solution. After that, immersed the PVDF

fibers' sample made in the first step in the blended solution and kept steady in the room temperature

for 5 h to generate PANI on the surface of the fibers by chemical oxidative polymerization. Finally,

the samples were taken out and washed by water for three times and dried in drying case for 30 min at

70 oC. The resultant PANI/PVDF fibers were characterized by SEM. And to mesaured the

conductivity and strain sensing property of this fibers, the sample was fixed on a PDMS substrate and

two parallel silver electrodes with an insulating gap of 3 mm were fabricated with elargol on the

fibers. Then a thin PDMS protective film was covered on the sample. The conductivity and strain

sensing property of the fibers were measured by a Keithley 6487 high resistance meter. Fig. 1 shows

the composition of the nano-branched coaxial PANI/PVDF fibers based strain sensors. Among this,

strain intensity were charactered by the curvature of the sample. And the curvature was calculated by

the equation σ =1/R, where σ is the curvature of the sample and R is the radius of the sample. R

represents the distance from the center to the axis of sample.

Fig. 1 The composition of the nano-branched coaxial polyaniline / polyvinylidene fluoride fibers

based strain sensors. (A) Diagrammatic sketch of the sample and the electronic test equipments. (a

and b) in the picture (B) are the photograph of the sensor before and after strain respectively. (C)

Schematic diagram for the strain sensor.

80 Materials Science, Machinery and Energy Engineering

Results and discussion

Fig. 2 SEM images and the schematic diagram of the materials. (a) SEM image of electrospun PVDF

fibers; (b) SEM image of coaxial PANI/PVDF fibers after in situ chemical polymerization; (c) SEM

images of a single composite fiber and the schematic diagram for its structure.

The microstructure of the PVDF fibers and surface features of coaxial fibers were characterized in

Fig. 2 by scanning electronic microscopy. As shown in this figure, the electrospun PVDF fibers (Fig.

2A) were used to provided surfaces for PANI polymerization and provided supporting for the coaxial

fibers to improve the mechanical properties as well. We supported that in the in situ chemical

polymerization process, PANI were covered on the surface of the PVDF fibers because of interfacial

compatibility of PANI to PVDF. Some bumps rised up for the intrinsic non-uniformity of PANI

membrane that is half amorphous and half crystallized. And then PANI nanobranches were grown on

it to combine the novel struture of coaxial fibers (Fig. 2B-C). This special structure increased the

content of PANI in this composite fibers to improve the conductivity of the material. In addition, the

structure also made this material to have a big specific surface area, so that the material may shows

excellent performances in some contact-dependent experiments like gas sensitivity test and so on.

The current-voltage (I-V) curve of the sample was measured by scanning the voltage from 0 V to 3

V and showed a linear relationship. And the average conductivity of the fibers calculated to be about

0.6 S/cm without curved. The strain sensing property of the coaxial PANI/PVDF fibers web sample

was tested. The result has been given by the Fig. 3. Fig. 3A gives us a strain sensitivity graph

character by the relationship of relative resistance change rate and curvature. Where R0 is the

resistance of the sample without deformation, the relative resistance change rate ((ΔR/R0)*100) is the

differential of the resistance before and after curved. And the Fig. 3B shows a stable and sensitive

response performance of this material in the repeated curving and straightening experiment.

According to the results, this strain sensor would have high flexibility, high sensitivity, stable

repeatability and other significant advantages.

Fig. 3 The strain sensitive property of the PANI/PVDF fibers web sample: (A) The strain sensitivity

graph for this sample. (B) The sensitivity response graph of the repeated curving (σ = 0.0594 mm-1

)

and straightening experiment.

We consider that the strain sensitive property of these fibers is due to the break of the PANI parcel

layers in the curving process. The principle can be easily understood by Fig. 4A. Because the fibers

Advanced Materials Research Vol. 853 81

were on the top of the sample’s axis, the fibers would be stretched when the sample curved. The

elongation (ΔL) was calculated by the equation ΔL=θR-L0, where R is the radius (the distance from

the center to fibers’ flat), θ is curve angle and L0 is the original length of the fibers between the

electrodes. Because of the poor tensile properties, the PANI parcel layers of the coaxial fibers can not

stretch with the PVDF fibers, and they will be broken in this process as shown in Fig. 4B. The

Elongation can also be defined as the sum of the fracture lengths. These fracture would hindered the

contact of conductors then reduced the conductivity of the material. And the conductivity will reduce

with the degree of bending is increased.

Fig. 4 Schematic diagram of the microfibers based strain sensor: (A) Side view of the curving process.

The fibers would be stretched in this process since they were on the top of the sample’s axis. (B) The

PANI parcel layers broken process when fibers were stretched.

Conclusion

In summary, novel nano-branched coaxial PANI/PVDF fibers were fabricated by electrospinning and

in situ chemical oxidative polymerization methods. These conducting materials were proved to have

a larger surface area and superior strain sensitive property. Due to high flexibility, high sensitivity,

stable repeatability and other significant advantages of this material, we believe it would have

enormous applications prospects in electronic engineering especially in the field of microelectronic

technology for precision measurements, highly sensitive robot sensors such as biologically inspired

skin, human-benign devices, personal health monitoring, disaster warning, touch panel and so on.

Acknowledgments

This work was supported by Natural Science Foundation of Shandong Province for Distinguished

Young Scholars (JQ201103), Taishan Scholars Program of Shandong Province (ts20120528),

National Natural Science Foundation of China (11074138, 11004114 and 51373082) and National

Key Basic Research Development Program of China (973 preliminary study plan, 2012CB722705).

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82 Materials Science, Machinery and Energy Engineering

Materials Science, Machinery and Energy Engineering 10.4028/www.scientific.net/AMR.853 Fabrication of Nano-branched Coaxial Polyaniline / Polyvinylidene Fluoride Fibers via Electrospinning

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