Preparation And Structure Characterization Of Highly Conductive Poly(3,4-ethylenedioxythiophene)/ Lignosulfonic Acid Complexes

As by-product of pulping and papermaking process, lignosulfonate(LS) is an amphiphilic polymer with good adsorption and dispersion performance, so it is widely used in industrial fields. LS is a polyanion and can form polyelectrolyte complexes with polycation. Poly(3,4-ethylenedioxythiophene)(PEDOT) attracts wide attention because of its high conductivity, great stability and high transparency in thin film. Utilizing LS to disperse and dope PEDOT can not only solve its processing problem causing by insolubility and infusibility,but alsoexpand the application areas of lignin. However, the conductivity of the PEDOT/LS dispersion prepared at present is very low, limiting its application. In this work, Pickering emulsion polymerization and ethylene glycol treatment method were used to improve the conductivity of PEDOT/LS dispersion, then expanding its application as well. The main conclusions were as follows.PEDOT/LS latex nanoparticles synthesized by traditional emulsion polymerization were doped again as stabilizer to prepare new PEDOT/LS complexes(PEDOT/LS-PEP) by Pickering emulsion polymerization. Compared to pristine PEDOT/LS complexes, the conductivity of PEDOT/LS-PEP complexes increased by two orders of magnitude. In order to explore the improvement mechanism, the structures of pristine PEDOT/LS and PEDOT/LS-PEP complexes were characterized. The results showed that the total content of PEDOT in PEDOT/LS-PEP nucleus increased, making the number of carriers also increase. Meanwhile, the PEDOT chains in its nucleus intertwined to form a larger network structure, which in favour of the transmission of carriers between PEDOT chains. Therefore, the electrical conductivity of the complexes improved. The optimal Pickering emulsion polymerization conditions were as follows: the mass ratio of PEDOT/LS nanoparticles to EDOT was 0.5:1, the molar ratio of oxidant to EDOT was 1.3:1, the solid content of reaction solution was 2.50%. Under this conditions, the conductivity of the prepared conductive complexes was up to 3.846 S/cm, increased by nearly 5,000 times compared to pristine PEDOT/LS complexes.When using ethylene glycol(EG) to improve the conductivity of PEDOT/LS complexes, three different methods of treatment:(1) directly adding EG to the PEDOT/LS dispersion;(2) directly immersing PEDOT/LS film in the EG solution;(3) a combination of these two methods were employed and the results were compared. The optimal additive amount of EG was 6 wt%. And the conductivity of PEDOT/LS complexes increased by 26.63, 33.56, 34.90 times respectively after treatment by above methods. In order to investigate the enhancement mechanism, the structure and morphology of PEDOT/LS film before and after treatment by EG were characterized. The results showed that the content of insulating LS on the film reduced, presenting obvious phase separation and conducting PEDOT-rich granular networks after treatment by EG. Meanwhile, the resonance structure of PEDOT chains transformed from a benzoid structure to a quinoid structure. The above two factors together resulted in the improvement of conductivity by enhanced the transmission of carriers between PEDOT chains.