Structural and Dielectric Properties of Polyaniline Doped with Camphor Sulphonic Acid S Saravanan, C Joseph Mathai, M R Anantharaman$, S Venkatachalam*, P V Prabhakaran* Dept. of Physics, Cochin University of Science and Technology, Cochin - 682 022India * PCM, Vikram Sarabhai Space Centre, Trivandrum - 695 022India Email: [email protected]

Abstract Conducting polymers based on polyaniline have been investigated extensively for understanding the underlying physics of the conduction process and also for possible applications. Some of the potential devices based on polyaniline are organic light emitting diodes, low power rechargeable batteries, gas sensors, super capacitors and photo voltaic cells. Bulk polyaniline in its pure form is an insulator and dopants like camphor sulphonic acid, methane sulphonic acid and hydrochloric acid enhance the electrical conductivity of polyaniline. Incorporation of dopants also modifies the structural properties of polyaniline. The modification of structural parameters also plays an important role in improving the conductivity of the polymer. Here we report the synthesis of polyaniline doped with camphor sulphonic acid [Pancsa] and its structural and dielectric characterization. Keywords: Polyaniline, dielectric constant, conducting polymer, FTIR.

Introduction Conventional polymers are excellent electric insulators both at high voltage and high frequencies. Because of their low cost of synthesis and fabrication, flexibility, lightweight, durability, impact resistance with respect to their inorganic counter parts, polymers are preferred to the conventional materials. An essential criterion needed by polymers to attain the significant conductivity change is a conjugated π electron system extending over a large number of monomer units – a characteristic common to polyacetylenes to polyheterocycles such as polypyrrole and polythiophenes and polyanilines [1,2]. Since the focus is on practical applications and due to their appreciable environmental stability most of the studies are now concentrated on heterocyclic compounds like polyaniline. Conjugation is just not enough to make the polymer material conductive. The processes that switch a conjugated polymer from the insulating to the conducting states can be achieved either chemically or electrochemically. This can be carried out with oxidizing agents such as sulphonic acid and protonic acid. The electrical conductivity of conducting polymers and the semiconducting property of doped polymers lead to important practical applications [3,4] which include LEDs, solar cells, transistors, chemical and biological sensors, super capacitors, batteries and electromagnetic interference shielding.

Polyaniline is doped with camphor sulphonic acid and its structural and dielectric properties are evaluated. The variation of permittivity with frequency and temperature is also studied for the doped polyaniline sample with camphor sulphonic acid.

Experimental Sample preparation Polyaniline is prepared by the direct oxidation of aniline using an appropriate chemical oxidant. To a solution containing aniline and aqueous perchloric acid [HClO4] kept at 0-4 °C was added ammonium per sulphate, drop by drop. This mixture was stirred continuously for 2 hours. Further this product was filtered and washed with water and then with methanol. Subsequently polyaniline doped with percholorate was converted to insulating polyaniline emeraldine base (PANIEB) using hydrazine hydrate. The PANIEB is doped using camphor sulphonic acid (CSA). By mixing of camphor sulphonic acid and emeraldine base in a molar ratio of 0.5 CSA to polymer in an agate mortar and pestle. This PANICSA was purified and dried in vacuum oven. The details of this method are cited elsewhere [5,6].

FTIR studies FTIR Spectra of the samples were recorded by using Nicolet 510P FTIR spectrometer. Dielectric measurements Polyaniline doped with camphor sulphonic acid samples were made in the form of pellets with a diameter of 10 mm. The dielectric permittivity studies were carried out on these samples by using a home made four probe dielectric cell and an HP 4285A LCR meter in the frequency range 100KHz to 2MHz in the temperature range 300K to 373K under dynamic vacuum (10-2 Torr). The dielectric measurements were fully automated with the aid of a graphical programming based on a Virtual Instrumentation package called LabVIEW [7]. The dielectric constant of samples was calculated by employing the relation

C=

ε oε r a d

……………………[1]

where a is the surface area of the sample, d the thickness of the sample, εr the dielectric permittivity of the sample, εo the dielectric constant of air and C is the measured capacitance of the sample. It may be noted that the evaluation of permittivity and plotting of graphs have been

made automatic with the help of the virtual instrumentation package.

Results and discussion FTIR Studies Figure 1 shows the FTIR spectrum of polyaniline emeraldine base (dotted line) and polyaniline doped with camphor sulphonic acid (solid Line). In the PANICSA Spectrum the peaks corresponds to 1383, 1541, 1548 cm-1 indicate that the aromatic ring is retained in the polymer also this PANICSA exhibit sharp peaks at 468.76, 578.72, cm-1. These peaks can be attributed to the C-H out of plane deformation [8]. The peak at 1283 cm-1 corresponds to the CH in plane of deformation. Also the peaks at 783.20, 1018.54 cm-1 corresponds to the sulphonic acid group [9]. The existence of peaks at 1674 and 1736 cm-1 reveals that ketone group in the polymer chain is tact. It may also be noted that PANIEB doesn’t exhibit any characteristic peaks corresponding to sulphonic acid and ketone group. Hence it is concluded that the monomer aniline is getting polymerised and the sulphonic acid group is attached. A tentative structure based on FTIR studies is depicted in figure 2.

Fig. 1 FTIR spectrums of PANIEB & PANICSA

(C H 3 ) 2 C O

-

N

+

N

C H 2 SO 3

+ N

N + -

C H 2 SO 3

C H 2S O 3 O

X O

C(C H 3 ) 2

C(CH 3 ) 2

Fig. 2 Structure of PANICSA

160

Dielectric Constant

140

100K hz 500K hz 1M hz 1 .5 M h z 2M hz

120

100

80

60

40

280

300

320

340

T e m p e ra tu r e (K )

360

380

Fig.3 Variation of Dielectric constant with temperature

This is due to the space charge accumulations at the structural interfaces of an inhomogeneous dielectric material. This resulting space charge produces distortions of the microscopic field, which appears as an interfacial polarisation. This effect can be explained by using Maxwell – Wagner theory of polarisation [10].

27 C 40 C 60 C 80 C 100 C

150 135

Dielectric Constant

Dielectric studies The variation of dielectric constant with temperature for five-fixed frequencies is shown in figure 3. When temperature increases, the permittivity also increases. From fig 3 it is seen that in the lower temperature region the change in dielectric constant is small but it rises sharply there after. The variation of dielectric constant with frequency for different temperatures were measured and results are shown in figure 4. The dielectric permitivity varies form 64 to 147. The dielectric constant increases with a decrease in frequency. Also the increase is rapid at higher temperatures.

120 105 90 75 60 45 0 .0 0 0

5 .0 0 0 e + 5

1 .0 0 0 e + 6

1 .5 0 0 e + 6

2 .0 0 0 e + 6

F re q u e n c y

Fig.4 Variation of dielectric constant with frequency

Conclusion From FTIR studies and analysis it can be concluded that the sulphonic acid group is attached to the polyaniline chain without any breaking of the aromatic ring. The retention of the aromatic ring in the polymer enhances the thermal stability of the polymer. The initial permitivity studies on PANICSA indicates that the cross-linked polymers posses good dielectric properties at higher temperatures.

Acknowledgement MRA and SS thank the Department of Space for financial Assistance received in the form of a project under ISRO-RESPOND, Government of India (File. No. 10/03/354 dtd.23-02-1999)

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