Thermoelectric Properties Of Conductive Polyaniline

Nowadays, most thermoelectric materials being investigated and with application value are inorganic semiconductor materials. Polymers have merits such as light weight, easy synthesis, designable structure, etc., which are unavailable in traditional inorganic materials and could meet the requirements of many special applications. If these polymers can be used as thermoelectric materials with application prospect, it will become a great breakthrough in material research field. Among the numerous polymers, conductive polymers have always been a hotspot for scientific research due to their changeable conductivity from insular to semiconductor or even to conductor. However, until now the investigation of conductive polymers are mainly focused on their optical properties, electrical properties or in the aspects of anticorrosion and biology science, few studies have been reported on their thermoelectric properties.In this paper, conductive polyaniline (PANI) was chosen as the research object and a series of doped polyaniline and poly(aniline-co-ortho-substituted aniline) were synthesized through chemical oxidative polymerization. XRD, IR, UV-Vis characterization were conducted on these products, and the thermoelectric properties of the products were measured too. We have systematically investigated the effects of different doping methods, various acid medium, acid concentration and synthesis temperature on the electrical conductivity, Seebeck coefficient, thermal conductivity and ZT value of doped polyaniline samples and also studied the influence of ortho-substituted groups on the thermoelectric properties of poly(aniline-co-ortho-substituted aniline) samples. The results are as follows:1. In the IR spectrum analysis and UV-Vis absorbtion spectrum analysis results, the peaks of doped PANI samples have exhibit different degrees of red shift compared to the undoped PANI sample, which further comforms the protonation of acids.The electrical conductivity and power factor of the sample synthesized and doped in one step is much higher than the one which prepared in the multi-step method of synthesis-dedoping-redoping. So, we choose the simple one-step method to prepare proton acid doping PANI.2. The studies of acid medium shows that the electrical conductivity and ZT of the PANI samples doped with inorganic acids (HCl,H2SO4,HClO4) and sulfonic acids (p-toluenesulfonic acid and camphor-sulfonic acid) is higher than the ones doped by carboxylic acids (tartaric acid and citric acid). In the range of 303-373K, PTSA doped PANI has the largest ZT value due to its relatively higher electrical conductivity, but in the higher temperature range, its Seebeck coefficient increases much slowly than other samples, which makes the HCl doped PANI has the largest ZT value in 373-393K. When above 393K, the sample doped with HClO4 possesses the best thermoelectric properties, ZT could reach 3.18×10-4 at 423K.3. According to the optimization results of reaction conditions, the optimal concentration and temperature is 1.0M and 15℃when under HCl reaction medium, and the highest ZT is 2.67×10-4 at 423K. While for the HClO4 doped PANI, the sample prepared when HClO4 concentration is 0.5M and under 0-5℃shows the best thermoelectric properties, with the largest ZT is 4.31×10-4 at 423K. However, in the case of PTSA doped PANI samples, the optimal acid concentration is 1.0M, and the studies of the polymerization temperature show that in the range of 303-383K the sample prepared at 15℃has the highest ZT. When above 383K, the product of 0-5℃possesses the best thermoelectric performance and ZTcan reach 2.65 X 10-4 at 423K.4. Poly(aniline-co-o-toluidine) (PAOT) and Poly(aniline-co-o-nitro-aniline) (PAON) samples were synthesized through one-step method. For the PAOT samples, steric effect is dominant after the introduction of o-methyl, which leads the copolymer has lower electrical conductivity and thermal conductivity but higher Seebeck coefficient than pure PANI. When o-toluidine mole concentration is 15%, the copolymer sample has a higher ZT value than pure PANI in the range of 303K-360K. While for the PAON copolymer, all the samples'electrical conductivity and ZT value are much lower than the PANI sample's, which is probably caused by the combined steric effect and electron withdrawing effect of nitro group.