Photoacoustic measurement of thermal diffusivity of polypyrrole conducting polymer composite films

American Journal of Applied Sciences, Feb, 2009 by M.Y. Lim, M.Y.W. Mahmood, A. Kassim, H.N.M.E. Mahmud

INTRODUCTION

Recently, Photoacoustic spectroscopy (PAS) found important applications in research and analysis of inorganic, organic and biological solids and semisolid. It is a non-destructive technique that is applicable to almost all types of samples. It offers a minimal sample preparation and has the ability to look at opaque and scattering samples. In addition it has the capability to perform depth profiling experiments. PAS can be used for both qualitative and quantitative analysis. Photoacoustic is the production of acoustic waves by the absorption of light energy. The photoacoustic effect is based on the sensitive detection acoustic waves launched by the absorption of modulated laser radiation(1-2).

The PAS constitutes a simple, very reliable, experimental tool (3) and has been widely used in measuring thermal diffusivity. The sensitivity tool of photoacoustic technique make this technique is suitable for studying the effect of synthesizing conditions in chemistry very often occurs in polymer synthesis (4-5).

In our present study, the polypyrrole-polyethylene glycol (PPy-PEG) composite films were prepared by electrochemical method (6-7).The main purpose of this study is to investigate the thermal diffusivity as a function of process parameters such as different concentrations of insulating polymer, monomer and electrolyte and the different applied voltages effects used to prepare PPy-PEG composite film.

MATERIAL AND METHODS

The aqueous solution in the one-compartment cell containing pyrrole monomer, p-toluene sulfonate electrolyte and the insulating polymer PEG, was electrochemically polymerized at a constant voltage (vs SCE) at room temperature for 2 h to form PPy-PEG composite film. The composite film thus produced on the ITO glass surface as an insoluble film was rinsed thoroughly with distilled water and then peeled off from the electrode. It was then dried in the oven at 60[degrees]C for 24 h.

Four series of PPy-PEG film samples were prepared by electrochemical polymerization technique. In the first series we fix the concentration of polypyrrole (0.20 M), p-toluene sulfonate (0.10 M) and voltage (vs SCE) (1.2 volts) while the PEG was varied from (0.5-9)x[10.sup.-3] M. Theses samples were used to study the effect of PEG concentration on thermal diffusivity of PPy-PEG composite film. From electrical measurement (8) it shown that the composite film with PEG concentration of 1x[10.sup.-3] M gives the highest conductivity value. Therefore the second series of our sample were prepared at fix concentration, of PEG (1x[10.sup.-3]) M and let the concentration of polypyrrole change from (0.10-0.40) M. Again the electrical measurement shown that the sample with 0.20 M concentration of polypyrrole gives the highest value of conductivity (8). Furthermore we prepared the third series of PPy-PEG composite film by keeping the concentration of PEG and polypyrrole at (1x[10.sup.-3]) M and 0.20 M respectively and change the concentration of p-toluene sulfonate from 0.05 M-0.30 M. However in electrochemical polymerization technique the voltage (vs SCE) is one of the important parameter to produce a good uniformity film and to control the thickness of the sample. With this in mind we have prepared the forth series of PPy-PEG composite at different value of voltage (vs SCE).

Photoacoustic technique was used to measure thermal diffusivity of the prepared conducting composite films. Photoacoustic is the production of acoustic waves by the absorption of light. This effect is described as the periodic heating generated in the sample by a nonradiative deexcitation (9-11) process due to the absorption of a modulated light source. In this study a heat transmission configuration known as open photoacoustic cell (OPC) was used. The sample was illuminated by the chopped laser beam which heats the surface and subsequently heats the gas in the cell. The periodic heating generates pressure variation in the surrounding gas medium. The experimental set up is shown in Fig. 1.

[FIGURE 1 OMITTED]

It has been shown that at low modulation frequency the photoacoustic signal for optically opaque samples is given by the expression (12)

S = [A/f]exp ( - b[square root of [f]]) (1)

where, A is a constant and b is related to the thermal diffusivity of sample, a and given as

b = [1.sub.s][square root of [[pi]/a]] (2)

By fitting the experimental data to the expression (1), the thermal diffusivity of the sample can be easily calculated.

RESULTS AND DISCUSSION

Figure 2 shows a typical photoacoustic signal as a function of modulated frequencies measured for frequency range of 5 Hz-100 Hz. of PEG-Ppy composite sample prepared at 1.20 Volts (vs SCE). The solid line is the calculated values obtained by using Eq. (1). In this case the fitting results thermal diffusivity of PPy-PEG sample as 7.88x[10.sup.-7] [m.sup.2][s.sup.-1]. The same thermal diffusivity evaluation procedure has been implied to all of our samples. The results are shown in Fig. 3-6.

[FIGURE 2 OMITTED]