SYNTHESIS TECHNIQUE OF CONDUCTING POLYMERS AND THIN FILMS USED AS ELECTRODE MATERIAL FOR SUPERCAPACITORS

The synthesis of the materials is required for the study of polymer composites. The development of systematic studies for the synthesis of polymer composites and polymer nanocomposites is a current challenge. A detailed description of the synthesis of conjugated polymers by various methods and the characterization techniques employed is given in this chapter. Characterization can take the form of actual materials testing or analysis. Although the techniques to be used depends upon the type of material and information one needs to know, usually one is interested in first knowing the structural properties and composition and then the chemical state, optical properties, DC conductivity, and other properties. A wide range of techniques are available in each of these areas and the systematic use of these tools leads to a complete understanding of the system. The information obtained from these techniques can be processed to yield images or spectra which reveal the structural, chemical, and physical details of the materials. To study the optoelectronic properties of synthesized semiconductor materials synthesized, a range of techniques such as UV-visible absorption spectroscopy and photoluminescence are used. The structural properties are studied using Fourier transform infrared spectroscopy, X-ray diffraction measurements.

CHEMICAL OXIDATION METHOD

Conducting polymers can be synthesized either chemically or electro- chemically.36 Chemical synthesis involves either condensation polymerization where the growth of polymer chains proceeds by a condensation reaction or addition polymerization where the growth is dependent on radical, anion, or cation formation at the end of the polymer chain. The conducting polymers produced chemically are in their undoped insulating states and the doping process can switch them into their conductive states. Chemical synthesis is preferred when it is necessary to synthesize or modify conducting polymers at large scale, or to obtain conducting polymers with defined structures.37 In this work, the polymer has been synthesized by in situ oxidative polymerization of the monomer with hydrochloric acid (HC1), and by using ammonium persulfate ((NH4) S2Os) as an oxidant. The monomer is doubly distilled prior to use. The oxidant to monomer ratio is (1:1). The monomer (1M) was dissolved in 10 mL of HC1 (1M) taken in 200 mL round bottom flask and stirred well. Further, finely ground dopant material powder taken in different weight% with respect to monomer concentration was added to the previous mixture under vigorous stilling in order to keep dopant material suspended in solution. The reaction mixture was then cooled up to 5°C, and the precooled solution of (NH4) S,Os (1M) was added dropwise over a period of 30 min. The reaction was allowed to proceed for 6-8 h. The mixture was further cooled down to 4°C for 24-36 h. It was then filtered and washed until the filtrate was colorless. The dark-colored polymer powder so obtained was dried thoroughly in an oven at 100°C until a constant weight was attained, then grinded and sieved. Undoped polymer was synthesized in the same fashion with the absence of the dopant material. The chemical reaction for the synthesis of PANI is shown in Figure 10.1.

Schematic scheme proposed for the formation of PANI

FIGURE 10.1 Schematic scheme proposed for the formation of PANI.

The same method was employed to synthesize poly(o-toluidine) and poly(m-toluidine) powders. The monomers o-toluidine and m-toluidine are the derivatives of aniline in which methyl gr oup is attached at ortho (o-) and met a (m-) positions of aniline ring. The commercially available monomers were procured (Merck Chemicals) and the polymers were prepared for the present study. The monomers and polymers of o- and m- substituted anilines are shown in Figure 10.2.

The ortho- and meta- substituted anilines and their polymers

FIGURE 10.2 The ortho- and meta- substituted anilines and their polymers.

The polymer powders so obtained were found to be very less soluble in common organic solvents like THF (tetrahydrofuran), CHClr etc.

Flowchart of chemical synthesis of {poly(o-toluidine)/poly(m-toluidine)/PANI}.

RADIO FREQUENCY (RF) PLASMA POLYMERIZATION FOR THE PREPARATION OF POLYMER THIN FILMS

Plasma polymerized thin films from different monomer precursors were prepared by employing the RF plasma polymerization technique.38-39 Plasma polymerized thin films of monomers on ultrasonically cleaned glass and silicon wafer substrates were obtained by polymerizing of monomers (99.9% purity) under RF plasma discharge in a home built set up. The setup consists of a custom-manufactured glass deposition chamber coupled to a vacuum system, RF amplifier, and a monomer feed-through set up. For maximum deposition, a novel setup has been designed. The position from where evacuation takes place and the position of monomer feed-through are crucial in this setup; a schematic of which shown in Figure 10.3. Feed-through setup consists of one on/off valve and one needle valve. The on/off valve is to create a quick vacuum in monomer pot and needle valve regulates the flow of monomer vapors into the chamber. This helps us to achieve the maximum deposition rate on substrates.40

The RF-plasma polymerization setup

FIGURE 10.3 The RF-plasma polymerization setup.

The monomer vapors enter slowly into the chamber through the needle valve. During a typical experiment, the glass and silicon substrates are placed on the lower electrode. The system was evacuated to lower than 10'3 torr and argon gas was introduced into the chamber for plasma pretreatment for 20 s. The RF power applied was 15 W for about 1 h. Fragmentation of monomer takes place because of the argon plasma created between the electrodes. The deposition rate is estimated to 3.33 nm/min under constant deposition conditions.

 
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