NON-REVERSIBLE TUNEABLE ELECTRICAL CONDUCTIVITY OF …m2d/Proceedings_M2D2017/... · spectrometer...

Proceedings of the 7th International Conference on Mechanics and Materials in Design

Albufeira/Portugal 11-15 June 2017. Editors J.F. Silva Gomes and S.A. Meguid.

Publ. INEGI/FEUP (2017)

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PAPER REF: 6617

NON-REVERSIBLE TUNEABLE ELECTRICAL CONDUCTIVITY OF

PANI-BASED CFRP IN THROUGH THICKNESS DIRECTION BY

CONTROLLING DE-DOPING EFFECT OF POLYANILINE

V. Kumar1(*)

, T. Yokozeki1, T. Goto

2, T. Takahashi

2

1Department of Aeronautics and Astronautics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan

2Department of Organic Device Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata, Japan

(*)Email: [email protected]

ABSTRACT

In this work, Carbon Fabric Reinforced Plastics (CFRPs) with Polyaniline (PANI)-based

matrix were prepared and their conductivity through thickness direction was controlled by

controlling the inherent de-doping phenomenon of PANI. PANI was doped with a strong

protonic acid, dodecylbenzenesulfonic acid (DBSA) and mixed with a cross-linking polymer,

divinylbenzene (DVB). This mixture was further used to impregnate the carbon fabric layers

to prepare conductive PANI-based CFRP (CF/PANI) composite. The de-doping phenomenon

of PANI was capitalized to control the electrical conductivity of the CFRP through thickness

direction. The property of PANI to get de-doped at higher temperature and longer annealing

time was established with a series of experiments. Prepared CF/PANI was subjected to

different annealing time to obtain the desired electrical conductivity. Electrical and

mechanical properties of prepared CF/PANI were measured after each annealing time.

Average electrical conductivity value > 1 S/cm in through thickness direction of CF/PANI

was obtained. This conductivity could be reduced by thermal annealing. 3 CF/APNI samples

were subjected to different annealing time to achieve 0.8, 0.5 and 0.3 S/cm electrical

conductivity in through thickness direction. These 3 CF/PANI panels were tested against

simulated thunder lightning strike with an intensity of 40 kA. The effect of electrical

conductivity and mechanical properties of CF/PANI samples against lightning strike are

demonstrated. UV-Vis analysis, FT-IR spectra and micro-images were used to demonstrate

the de-doping phenomenon of PANI. Mechanical properties were measured after each thermal

treatment cycle to determine any deteriorating effect on overall mechanical properties of the

composites and finally the effectiveness of these panels against simulated thunder lightning

test were tested.

Keywords: polyaniline, conductive CFRP, electrical conductivity, thunder lightning

protection.

INTRODUCTION

Composite structures are rapidly gaining ground in our daily life. Automobile, aerospace

industry and energy sectors are just few to name (Tang & Hu 2016). The main reason for

composite’s popularity is mainly due to its superior strength to weight ratio compared to its

counterpart metallic structures. Composites can be designed to impart strengths in the desired

direction without adding any unnecessary weight to the structures. It is well known that the

mechanical properties of the CFRPs can be tailored easily by placing the fibers in specific

directions and other methods(Feng et al. 2017). However, carbon fabric reinforced plastics

Topic-D: Composite and Advanced Materials

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(CFRPs) finds limitations in many applications due to its poor electrical properties. In past,

researchers have tried to make electrical conductive CFRPs by adding conductive nano-fillers

and other additives in to the epoxy resin and other matrices(Yang et al. 2004; Krakovský et al.

2012; Singh et al. 2013). However, process ability, repeatability and low electrical

conductivity at higher loading of fillers values are still big challenges for the researchers

while using such matrices(Shoukai. Wang 2000; Qin et al. 2015).

Controllability of electrical conductivity of the CFRPs is yet to be established. Among

multifunctional roles of CFRPs, other than structural applications, electrical conductive

composites are gaining polularity(Gibson 2010; Cebeci et al. 2009; Wen et al. 2011).

Electrical conductive composites can be used for a vast field of applications namely i.e.

optics, sensors, actuators, EMI shielding, battery/capacitor, anti-static and thunder lightning

protection(Carlson et al. 2010; Luo & Chung 2001; Rea et al. 2005; Lakshmi et al. 2009; D.

D. L. Chung 2001; Hirano et al. 2016). All these applications need different range of

electrical conductivities and even the threshold electrical conductivity to be needed for

thunder lightning protection is unknown. Therefore, controlling electrical conductivity of

CFRPs has become necessity(Lin et al. 2015; Kandare et al. 2015).

Authors demonstrated the manufacturing process of PANI-based conductive thermosetting

matrix and PANI-based CFRPs in their previous works(Kumar et al. 2015; Kumar et al. 2016)

and also demonstrated de-doping phenomenon of PANI-DBSA/DVB matrix subjected to

longer curing time and high temperature(Kumar et al. 2016). Hirano et al. also presented the

effectiveness of the CF/PANI composites against thunder lightning strikes(Hirano et al. 2016)

. In this work, we have used the doping/de-doping property of PANI to control the electrical

conductivity of the CFRPs made by PANI-DBSA/DVB matrix. We controlled the electrical

conductivity of the CFRPs even after complete curing of the composite samples by

controlling the de-doping phenomenon of conductive filler i.e. PANI. We can obtained

different electrical conductivity values of one sample by subjecting it to different annealing

time.

Lightning damage has been a major issue for the aircraft industry working with carbon fiber-

reinforced polymers. Hence, Lightning Strike Protection (LSP) is an inevitable concept. Most

of the research these days are focused on suppression of the lightning damage by using

conducting fillers into the matrix to make conductive CFRP composites. Three CF/PANI

samples with electrical conductivity of 0.8, 0.5 and 0.3 S/cm in thickness directions were

prepared and their effectives against simulated thunder lightning strikes were analyzed.

Hypothesis tested in this work is the ability of PANI-based CFRP to change its electrical

conductivity, without changing its filler content while maintaining its mechanical properties

and their resistance to the lightning strike damage. Quantitative values of de-doping/degree of

doping are presented with the help of UV-vis and FT-IR spectra.

EXPERIMENTS

Materials

Polyaniline (PANI) in the form of emeraldine base was purchased from Regulus Co. Ltd.

Dodecylbenzenesulfonic acid (DBSA) was procured from Kanto Chemical Co. Inc.

Divinylbenzene (DVB) of 80% technical grade was obtained from Sigma-Aldrich Co. For

measuring electrical conductivity silver paste was purchased from Fujikura Kasei Co. Ltd.

Conductive aluminum tape was used as connecting electrode between samples. Plain-woven


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carbon fiber sheets (TR3110M, TR30-3K fibers, 200 g/m2, Mitsubishi Rayon Co., Ltd.) were

used to prepare CF/PANI panels.

Sample preparation

PANI (32.5 wt. %) and DBSA (67.5 wt. %) were mixed in a centrifugal mixer at 2000 rpm for

1 min, 3 times to prepare PANI-DBSA complex. This complex was heated for 4 hours at 60˚C

to achieve uniform semi-doping of PANI. Semi-doping of PANI-DBSA is essential for the

environmental stability of the PANI-DBSA/DVB matrix as explained in our previous

work(Kumar et al. 2016). 50 wt.% of DVB was added with 50 wt. % PANI-DBSA complex

and mixed with centrifugal mixer to achieve uniform dispersion for 3-4 minutes at 1000

RPM. Prepared PANI-DBSA/DVB matrix was further used to impregnate 8 plies of TR30-3K

carbon fabric of size 24 cm × 24 cm each. Hand layup process was used to impregnate layer

by layer and stacked on each other. All 8 plies stacked and cured using hot press machine at

120˚C for 2 hours at 3 MPa pressure. 4 cured CF/PANI composite panel were prepared.

The remaining matrix was used to prepare unreinforced cured matrix (bulk samples of

electrical conductivity measurements & thin films for FT-IR analysis) with same curing

profile as for CF/PANI composite. One CF/PANI composite panel was cut into different

dimensions for various measurements. For each thermal cycle 3 samples for DC electrical

conductivity measurement (25 mm × 25 mm × 2 mm), 3 samples for flexural properties

measurement (80 mm × 15 mm × 2 mm) and 3 samples for inter-laminar strength

measurement (25 mm × 10 mm × 2 mm) were cut from the panel. 5 sets for 5 different

thermal cycles were prepared and subjected to 2, 4, 6, 8 and 10 hours of annealing at 120˚C.

Other 3 CF/PANI panels were used for lightning strike simulation test. Other CF/PANI

composite samples were trimmed to 150 x 150 mm2 size for simulated lightning test.

Characterization techniques

The electrical conductivity (DC measurement) of the samples and flexural modulus of the

samples were measured according to the procedure mentioned elsewhere (Cheng et al. 2016).

Matrix used to impregnate CF was analyzed using FT-IR spectra using IRAffinity-1S

spectrometer from Shimadzu Co. Thin film of the matrix were prepared and subjected to

aforementioned annealing time. UV-vis Spectra on UV-2600 UV-vis spectrometer from

Shimadzu Co. Measurements were taken after 0 min, 10 min, 1 hour, 2 hours, 4 hours, 6

hours and 8 hours of thermal treatment. Spectra were recorded and plotted in the same graph

to show the change in the electronic structure of PANI-DBSA after doping and de-doping

phenomenon takes place.

De-doping phenomenon was also studied with the micrographs obtain from optical

microscope (Olympus BX-50) fitted with a heating plate system. Images were taken w.r.t

increasing temperature at regular interval. Samples were prepared as per the procedure

mentioned elsewhere (Goto et al. 2011). Differential scanning calorimeter (DSC-60 Plus)

from Shimadzu Co. was used to obtain the heat flux during curing profile. DSC analysis was

performed from 0 to 250°C under nitrogen atmosphere. DSC is used to show the complete

curing of the matrix at certain curing time. Uncured and cured (2, 4, 6 and 8 hours curing)

PANI-DBSA/DVB matrix (unreinforced) were analyzed using DSC.

An impulse generator (developed by Otowa Electric Co., Ltd., owned by National Composite

Center Japan at Nagoya University) was used to apply a simulated lightning current to the

specimens. In these experiments, a peak current of -40 kA was applied to all the samples.


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RESULTS & DISCUSSION

Electrical Conductivity

Figure 1 clearly shows a decrement in electrical conductivity of the CF/PANI composite w.r.t

thermal treatment. This degradation of the electrical conductivity of the CF/PANI can be

assigned to the de-doping phenomenon occurring in the PANI present in the matrix. CF/PANI

after 2 hours of curing has shown average electrical conductivity value of 1.35 S/cm in the

thickness direction. This is the highest electrical conductivity value ever reported for

thermosetting matrix based CFRPs in through thickness direction. Thermally doped PANI-

DBSA is known for its high electrical conductivity (Poussin et al. 2003)and therefore doped

PANI-DBSA complex present between carbon fibers act as the passage for electric current.

This imparts electrical conductivity in the through thickness direction of the CF/PANI

composite panel. The same composite sample when subjected to further thermal treatment has

shown a uniform degradation of electrical conductivity, which confirm the controllability of

the samples even after it gets completely cured. The de-doping behavior of PANI is explained

in the next few sections. The electrical conductivity value of prepared CF/PANI is found to be

very high as compared to the electrical conductivity value of epoxy based CFRPs (CF/epoxy).

CF/epoxy generally has the conductivity values in the range of 0.01 S/cm - 0.05 S/cm in

thickness direction due to the insulating nature of epoxy resins.

Fig. 1 - Degradation of electrical conductivity of PANI-based CFRP with different curing time.

Mechanical properties

To know the effect of annealing on the mechanical properties of CF/PANI, 3-point bending

test was performed after each thermal cycle. Flexural behavior and ILSS test was conducted

for the CF/PANI composites. Flexural modulus, flexural strength and ILSS strength are

plotted in Figure 2, Figure 3 and Figure 4, respectively. Not much change in the flexural

modulus of the composite samples was observed with annealing time. However, flexural

strength and ILSS values increases up to 4 hours of curing and becomes stagnant on further

annealing. These results confirms that the matrix gets completely cured between after 4 hours

of curing time. It can also be seen that there is no harmful effect of thermal treatment on

mechanical properties. DSC analysis has been employed to support this result and shown in

next section. Mechanical properties of CF/PANI are lower than the mechanical properties of

CF/epoxy at similar volume fraction (Yokozeki et al. 2015). This reduction can be assigned to

the very high content of PANI-DBSA complex in the matrix. PANI-DBSA portion accounts

1.35

1.151.03

0.9 0.825

0

0.5

1

1.5

2

2 4 6 8 10

Ele

ctri

cal

Co

nd

uct

ivit

y (

S/c

m)

Curing Time (in Hours)

Electrical Conductivty (S/cm)


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only for the electrical conductivity of the system and DVB content is directly related to the

mechanical properties of the matrix. Therefore, we can also change the overall mechanical

properties of the CF/PANI composite by altering the DVB content in the matrix during

manufacturing process itself as matrix properties directly affects the mechanical properties of

CFRPs. We showed in our previous work that higher would be the DVB content, higher

would be the mechanical properties with the penalty of electrical conductivity in PANI-

DBSA/DVB matrix system.

Fig. 2 - Flexural modulus of PANI-based CFRP with different curing time.

Fig. 3 - Flexural strength of PANI-based CFRP with different curing time.

Fig. 4 - ILSS value of PANI-based CFRP with different curing time.

48.1 48.81 48.5 48.1

0

10

20

30

40

50

60

2 4 6 8

Fle

xu

ral

Mo

du

lus

(MP

a)

Curing Time (Hours)

Flexural Modulus (MPa)

238.66

289.6333.3

346.66

0

100

200

300

400

2 4 6 8

Fle

xura

l S

tre

ng

th (

MP

a)

Curing TIme (In hours)

Flexural Strength (MPa)

15.96

19.3121.43

22.7 22.9

0

5

10

15

20

25

2 4 6 8 10

ILS

S (

MP

a)

Curing Time (in Hours)

ILSS (MPa)


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MATERIAL CHARACTERIZATION

DSC Analysis

Curing behavior of the PANI-DBSA/DVB matrix was studied using DSC analysis.

Unreinforced PANI-DBSA/DVB cured matrix subjected to different curing time was also

analyzed to determine the complete curing profile and results are plotted in Figure 5. 2

distinguish exothermic peaks at ̴ 85°C and ̴ 176°C can be seen in case of uncured matrix. The

first peak corresponds to the doping of the PANI(Pan et al. 2005)and second peak

corresponds to the curing of DVB. Peak at ̴ 243°C could be due to the dopant decomposition

(Belaabed et al. 2010; Dumitrescu et al. 2009). DSC plots of 4 hours of curing does not show

any peak at ̴ 85°C confirming the complete doping within 2 hours. However, a joint peak

present at > 170°C indicates uncured DVB present in the system and DBSA decomposition.

This peak diminish at 6 hours or higher hours of curing. This results support our mechanical

properties results of CF/PANI that curing completes after 2 hours of extra annealing.

Fig. 5 - DSC of unreinforced uncured/cured PANI-DBSA/DVB matrix after thermal treatment.

FT-IR spectroscopy

Electronic structure and degradation of electrical of the CF/PANI w.r.t annealing time is

analyzed with FT-IR analysis of the uncured and cured PANI-DBSA/DVB unreinforced

matrix. Peaks at 1500 cm-1

and 1590 cm-1 can be assigned to benzenoid and quinoid ring

respectively. These are the characteristic peaks of emeraldine base form of PANI and shift of

these peaks to the lower wavenumber signifies the transition to emeraldine salt due to doping

(Afzal et al. 2010). Thin film of PANI-DBSA/DVB matrix were prepared by subjecting the

matrix for different annealing time under pressure, using hot press machine. FT-IR spectra of

annealed sample is shown in Figure 6 and the inset plot shows the FT-IR spectra of uncured

matrix. In inset plot, the characteristic peaks present at 1454 cm-1

and 1556 cm-1

can be

assigned to the C=C stretching vibration of benzenoid and quinoid ring respectively. The shift

of the peaks to the lower wavenumber indicates that PANI was already in semi-doped state

when mixed with DVB. Peaks at 2924 cm-1

and 2852 cm-1

can be attributed to the presence of

DBSA within the matrix, which can be assigned to the C-H stretching of CH2 and CH3 of

DBSA molecule. The absorption band at lower wavenumber indicates the degree of

protonation of the PANI chains, which represent the conducting form of PANI. The intensity

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

30 80 130 180 230

He

at

Flo

w (

mW

)

Temperature (°C)

0 Minute 2 Hours 4 Hours 6 Hours 8 Hours


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of this band reaches maximum just after 10 minutes of heating which indicates the highest

electrical conductivity. The reduction in the intensity of this band w.r.t annealing time clearly

shows the de-protonation of PANI. This band was absent in case of uncured PANI-

DBSA/DVB matrix due to very low conductivity. One another very effective way to know the

degree of doping is to study the ratio of the intensity of quinoid and benzenoid rings (Saini &

Arora 2013). The reduction in the intensity ratio of peaks I1556/ I1454 is presented in table 1.

The reduction in this ratio can be interpreted as the deprotonation of the PANI w.r.t annealing

time. These results confirms the de-doping phenomenon of the PANI.

Fig. 6 - FT-IR spectra of PANI-DBSA/DVB cured composite film after thermal treatment (inset is the FT-IR

spectra of uncured matrix).

UV-Vis analysis

UV-Vis analysis was performed on PANI-DBSA/DVB matrix w.r.t annealing time and

change in the spectra were recorded. These plots are shown in Figure 7. The inset figure

shows the UV-VIS spectra of PANI-EB dissolved at 0.1 wt. % in NMP solution. The peaks

present at 327 nm, due to the π→ π* transition of benzenoid amine ring and at 630 nm, due to

the πB→πQ excitation absorption in the quinoid imine ring are the characteristic peaks of the

emeraldine base form of PANI(de Souza & Soares 2006).

Fig. 7 - UV-Vis spectra of PANI-DBSA/DVB cured matrix after thermal treatment (inset is the UV-VIS spectra

of PANI-EB).

0,10

0,25

0,40

0,55

0,70

0,85

500150025003500

Ab

sorb

an

ce (

a.u

)

Wavenumber (cm-1)

FT-IR 10 min 1 hr 2 hrs 4 hrs 6 hrs 8 hrs

0,8

1,5

2,2

2,9

3,6

4,3

5,0

325 525 725 925

Ab

sorb

an

ce (

a.u

)

Wavenumber (nm)

UV-vis 10min 8h 6h 4h 2h 1h

-0.2

0.3

0.8

1.3

1.8

2.3

300 500 700 900 1100 1300

Abso

rban

ce (a

.u)

wavenumber (nm)

PANI-EB PANI-EB


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The peaks at 320 nm is replaced by a shoulder like peak at 440 nm after thermal doping. This

new peak indicates the attachment of the DBSA with PANI. Similarly, a new peak appears at

830 nm. This peak can be assigned to the formation of polaron band due to protonation of

PANI. The ratio of the intensity of the peaks I440/I830 can also be used as the indicator of

degree of doping of PANI (Bilal et al. 2012). The values obtained for this ratio at different

annealing time is shown in the table 1. It can be seen from the plots that the degree of doping

decreases w.r.t thermal treatment. Results obtained from UV-Vis analysis reestablished the

findings of the FT-IR spectroscopy.

Table 1 - Quantitative data of degree of doping using FT-IR and UV-Vis analysis.

Optical thermal microscopy

Optical thermal microscopic images have been used to demonstrate the de-doping

phenomenon w.r.t curing temperature. Images were taken during in-situ doping and curing of

PANI-DBSA/DVB matrix. Three images taken at (a) 30°C, (b) 120°C and (c) 180°C are

shown in Figure 8. Image taken at 30°C shows the un-doped form of PANI-DBSA (dark blue

color) dispersed into DVB (yellow color) matrix. After 10 minutes of heating, PANI changed

its form from emeraldine base to emeraldine salt which is represented by the color change of

PANI from dark blue to bright green. This confirms the doping process of PANI. On further

increase in the curing temperature, DBSA start getting detaching from the PANI backbone

causing de-doping of the system. This can be confirmed by the change of the color of the

complex from bright green to dark green.

Fig. 8 - Thermal in-situ optical micrograph of PANI-DBSA/DVB w.r.t curing temperature.

Curing Time FT-IR

(I1556/I1454)

UV-Vis

(I435-440/I830-850)

Electrical Conductivity

of Bulk Sample (S/cm)

1. 10 min 0.92 1.33 NA

2. 1 hour 0.91 1.41 1.04

3. 2 hour 0.89 1.44 0.45

4. 4 hour 0.88 1.46 0.19

5. 6 hour 0.86 1.48 0.11

6. 8 hour 0.83 1.48 0.05


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Previous sections have been dedicated to understand the de-doping behavior of the PANI-

DBSA in presence of DVB due to the prolonged curing time and increased curing

temperature. DVB is a cross linking monomer which follows radical as well as cationic

polymerization. However, semi-doped PANI possess some scavenging effect which can

hinder the radical self-polymerization of DVB. Therefore DVB takes away DBSA proton

from PANI to reinitiate its polymerization. This cause the de-doping of PANI. High

temperature helps the deprotonation of PANI, causing the increased rate of de-doping of

PANI at higher temperature or prolonged heating. This deprotonation of PANI attribute to the

overall decrement of the electrical conductivity of the PANI-DBSA/DVB matrix and

subsequently the electrical conductivity of CF/PANI. However, at particular temperature the

rate of de-doping is found to be almost constant and therefore, the de-doping behavior of

PANI can be controlled by controlled thermal treatment.

Simulated lighting current test

Impulse generator and the sample holder are shown in Figure 9. Top view of the samples

before and after the lighting test are also shown in Figure 10. CF/PANI with different

electrical conductivity in through thickness direction were tested at -40kA current, which

represent the simulated thunder lighting on an aircraft. Circular damage area can be seen

clearly from the images in all the cases. This damage occurred due to the resin evaporation

mainly. Small amount of fiber damage is also visible. No damages after first layer was

observed, which suggest that the damage could not penetrate the remaining layers of

CF/PANI panel. Conductive nature of the CF/PANI helped the simulated lightning current to

flow out of the CFRP samples. These are very interesting result for future of the development

of new technologies for lighting strike protection because, no catastrophic failure was

observed as was the case in CF/epoxy (without lightning strike protection) (Hirano et al.

2016). Based on these observations, we can see that the sample with electrical conductivity of

0.3 S/cm could exhibit strong resistance against lightning strike as well, if provided with

enough mechanical strength. It was also found that the electrical conductivity didn’t changed

much even after lighting strike (high residual electrical conductivity). This property make

CF/PANI capable to withstand multiple lighting strikes without losing its inherent electrical

conductivity through thickness direction. Although electrical conductivity is the main factor

against lighting strike protection, but we have to improve the mechanical properties of the

CF/PANI composite especially its interlaminar properties to make it completely safe against

lighting strikes. Other factors like minimizing the evaporation of the resin will be evaluated in

future.

Fig. 9 - Setup for simulated lightning test: a) Impulse current generator b) Support jig and discharge electrode


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Fig. 10 - Specimens before damage (a) and after damage (b): Sample with 1) 0.8 S/cm, 2) 0.5 S/cm and 3) 0.3

S/cm through thickness electrical conductivity respectively.


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CONCLUSION

DBSA-doped PANI dispersed in DVB has been used as thermosetting matrix to impregnate

carbon fabrics to prepare conductive CF/PANI composite. This composite was subjected to

different annealing time and its electrical and mechanical properties were measured after each

thermal cycle. Stagnation of the mechanical properties (due to complete curing of matrix) and

continuous degradation of electrical properties of the CF/PANI composite (due to the de-

doping effect of PANI) was observed. The de-doping phenomenon of PANI, in presence of

DVB matrix was explained using different analytical techniques (FT-IR, UV-VIS and optical

thermal microscope). A direct relation between degree of de-doping and annealing time and

temperature has been demonstrated systematically. Quantitative data has been presented to

understand the results. The hypothesis to control the electrical conductivity of the CF/PANI

composite without any detrimental effect on its mechanical properties up to certain annealing

time has been proved successfully. This work is important to provide different electrical

conductivity values of one sample without changing in its manufacturing process or structure.

CF/APNI samples were subjected to different annealing time to achieve 0.8, 0.5 and 0.3 S/cm

electrical conductivity in through thickness direction. These 3 CF/PANI panels were tested

against simulated thunder lightning strike with an intensity of -40 kA. The effect of electrical

conductivity and mechanical properties of CF/PANI samples against lightning strike are

demonstrated.

ACKNOWLEDGEMENT

The authors acknowledge JSPS for the financial support of this project (Grant-in-Aid for

Scientific Research, 16H02424). This project was also supported by JSPS and DST under the

Japan -India Science Cooperative Program.

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