Photoacoustic Measurement of Thermal Diffusivity of Polypyrrole Conducting Polymer Composite Films

The thermal diffusivity has been measured for the Polypyrrole-Polyethylene Glycol (PPyPEG) composite films by the open photoacoustic method. The experimental data were analyzed for thermal diffusivity by fitting procedure. The different process parameters such as different concentrations of insulating polymer, monomer and electrolyte and the different voltages used to prepare the PPy-PEG composite films brought changes in thermal diffusivity. The process condition of 0.20 M pyrrole, 0.10 M p-toluene sulfonate and 1×10 M PEG at 1.20 volt (vs SCE) appeared as the optimum condition and the PPy-PEG composite film produced from this optimum condition shows the highest thermal diffusivity (7.88×10 m s).


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°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)×10 −3 M. Theses samples were used to study the effect of PEG concentration on thermal diffusivity of PPy-PEG composite film. From electrical  [8] it shown that the composite film with PEG concentration of 1×10 −3 M gives the highest conductivity value. Therefore the second series of our sample were prepared at fix concentration, of PEG (1×10 −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 (1×10 −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][10][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.
It has been shown that at low modulation frequency the photoacoustic signal for optically opaque samples is given by the expression [12] .
where, A is a constant and b is related to the thermal diffusivity of sample, α and given as By fitting the experimental data to the expression (1), the thermal diffusivity of the sample can be easily calculated. 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 7.88×10 −7 m 2 s −1 . The same thermal diffusivity evaluation procedure has been implied to all of our samples. The results are shown in Fig. 3-6.

RESULTS AND DISCUSSION
The PPy-PEG composite films prepared at various concentrations of PEG ranging from 0.5×10 −3 M-9×10 −3 M in the aqueous solution containing 0.20 M pyrrole and 0.10 M p-toluene sulfonate at a potential of 1.20 volt (vs SCE) has been studied to investigate the effect of PEG concentration on the thermal diffusivity the films. Fig. 3. shows the thermal diffusivity increases from 3.87×10 −7 m 2 s −1 -7.88×10 −7 m 2 s −1 as the PEG concentration in the pyrrole solution increases up to 1×10 −3 M. This indicates that electron migration is an addition process in thermal transport properties, However at higher concentration of PEG it shows a decreasing trend from 7.88×10 −7 m 2 s −1 -4.86×10 −7 m 2 s −1 with further increase of PEG concentration from 1×10 −3 M-9×10 −3 M. This is in a good agreement with the result of other polypyrrole composite films reported by other researcher [12] .
In the second series of samples we investigate the effect of pyrrole concentration on thermal diffusivity of PPy-PEG composite film. The pyrrole concentration has been changed from 0.10 M-0.40 M, with p-toluene sulfonate dopant of 1×10 −3 M of PEG respectively. The film was prepared at voltage of 1.20 Volt (vs SCE). Fig. 4. shows the thermal diffusivity values as a function of pyrrole concentration. A similar trend was also observed where the higher value of thermal diffusivity (7.88×10 −7 m 2 s −1 ) was obtained for 0.20 M of pyrrole concentration. The increase in thermal diffusivity at low   Fig. 5. In this case the highest (7.88×10 −7 m 2 s −1 ) value of thermal diffusivity was obtained for 0.10 M of dopant concentration in PPy-PEG composite film. For concentration higher than 0.10 M, the thermal diffusivity decreases with the increasing of dopant concentration. The higher thermal diffusivity obtained for PPy-PEG film prepared from 0.10 M p-toluene sulfonate may due to higher conjugation length which related to the electron migration process.
Since the applied potential plays an important role in electrochemical polymerization technique, we have prepared the PPy-PEG composite film by changing the potential from 0.40-1.50 volts (vs SCE). For this last series of samples we kept the pyrrole solution, ptoluene sulfonate and PEG at the concentration of 0.20 M, 0.10 M and 1×10 −3 M respectively. Fig. 6. shows the thermal diffusivity of the PPy-PEG composite films obtained at different potential (vs SCE). Obviously, the sample prepared at voltage of 1.20 volts (vs SCE) gives the higher thermal diffusivity (7.88×10 −7 m 2 s −1 ). This can be explained by the fact that the increasing in applied voltage, the conjugation chain length (number of pyrrole monomer units) of PPy increases causing the increase in thermal diffusivity. This decrease in thermal diffusivity at the potential above 1.20 volt (vs SCE) may be due to overoxidation process in the sample.