Studies on synthesis, characterization, physico-chemical and electrical properties of some conducting polymers

Abstract

Polyaniline samples have been prepared by chemical oxidation method in three different acid media under different conditions of temperature, pH and bath composition. The solubility test shows that polyaniline is insoluble in both polar and non-polar solvents such as CHJOH, C2HsOH, H20, CCI4, CS2 but soluble in partially polar solvents Iike DMF and DMSO. From this it can be said it is soluble in aprotic solvent when dielectric constants are greater than those of CCl4 (0.0) & CS2(2.2) but less than those of CHJOH (175.47), C2HsOH (159.05) and H20(78.6). The thermogravimetric analysis shows three steps decomposition of the polymer corresponding to loss of absorbed water molecules, loss of acid counter ion and degradation of the polymer. The thermal stability depends on the polymerization condition and the counter ion associated with the polymer salt. The stability of synthesized dopped polyaniline (PANI) is found in the order: cr >S04-2 > CI04-. X-ray diffraction pattern of polymers shows crystalline as well as amorphous region in the crystal lattice. The crystallinity of the polymer is found in the order: H-PANI > S-PANI > P-PANI. This may be due to the smaller size of counter ions (Cn that leads to higher crystallinity as they can enter easily into the polymer lattice. The study of electrical conductivity reveals that the polymers synthesized using monomer : oxidant ratio 1:1.15 and acetonitrile-acid concentration ratio 1: 3 have higher conductivity than the polymer synthesized under other condition. The UV-visible spectra of the polymer shows two absorption bands. Absorption at 355-410 nm (3.49-3.02 ev) corresponds to the n; - n;* II transition and a broad band absorption at around 560-615 nm (2.21-2.02 ev) correspond to n~1t* transition forming exciton band. The polyaniline synthesized in bath composition of monomer and oxidant ratio I: 1.15 has the lowest band gap and exciton energy (3.18 ev) and (2.10 ev) respectively which, indicates ease transition of 1t - 1t* and n~1t*. This implies that polyaniline synthesized in this ratio would be better conductive. Decrease of polymerization temperature refelTed to as bathochromic shift i.e. lowering the band gap energy leading to easy n~1t* conjugation. The increase of acidity in polymerization bath increases C-N-C bond angle and decrease the steric repulsion between the ortho hydrogen and the adjacent rings. This enables the ring to attain a more planer structure and increase the conjugation, which lowers the band gap. The infrared spectra of polyaniline under investigation testi fy that the sequence of functional groups attachment in the polymer are mostly identical with very few exception in position of particular assignment among the polymers due to difference in condition ofpolymerization. The study of scanning electron micrographs reveal that for the synthesis of better crystalline polyaniline at least in the case of HCI medium larger amount of oxidant (> I: 1) and higher percentage of HCI (> I: I) are to be used. The dissolution of polyaniline in DMF and (DMF-H20) solvents increase the values of viscosity co-efficient. This indicates the positive interaction of poly aniline with DMF and DMF-H20 mixture. From the study of B-coefficient of John-Dole equation it is observed that there is no systematic changes in the values of B-coefficient. So, John-Dole equation is not applicable to the solution of poly aniline salt. 111 ,. The average molecular weight of synthesized polyaniline' was determined using calculated values of intrinsic viscosity and was found to be 4580. The increased enthalpy of activation (l1Hn#) values with the increase of concentration of polymer corresponds to the intensification of the structure by the addition of poly aniline. The decrease of free energy change at higher percentage of DMF (60-80%) indicates the insufficient number of polyaniline molecules interact with large number of DMF molecules. The increase of free energy change with the increase of polyaniline contents indicates polaron or bipolaron structure of polyaniline molecules which activates interaction between the two solvents. The initial increase of entropy (Tl1Sn#) values with the increase of percentage of DMF indicates that energetic effect is more significant than geometric effect particularly at lower percentage of DMF. But the decrease of Tl1S at higher percentage of DMF (above 50%) indicates the geometric effect is more significant. The addition of polyaniline shows no significant change of entropy of activation. The negative molar excess volume (yE) over the entire range of composition at all measured temperature indicates the positive interaction between the water and DMF. The values of conductivity is significantly small so the solution of polyaniline is feebly electrically conductive. In the adsorption study of organic dyes such as Kedenthrene Blue (KB), Best Acid Red (BAR), Modem Direct Brown (MDB) and inorganic pigment such as nickel sulfate hexahydrate on experimental polymeric IV matrices indicate that the polymeric matrices have the property of adsorption of the experimental dyes: KB, BAR and MDB under the experimental condition of pH 6.9. Adsorption of dyes, Best Acid Red, by P2-PANI, S2-PAN1 and H2- PANI is not identical to that of Kedenthrene Blue (KB) although increase of dye absorption with increasing experimental time is a common phenomena. The following sequence is observed from the adsorption pattem of Modem Direct Brown (MDB) by HrPANI, PrPANI and SrPAN1 : HrPANI > S2-PANI > PrPANI. This is completely reverse in the case of Kedenthrene Blue (KE). The inorganic dye NiS04.6H20 was not found to be adsorbed on polymeric matrix used in this study during the total exposure time.