Computational study of Polymerization, Crystallization and Mechanical Properties of Conducting Polymers

Sammanfattning: Nowadays, electronic devices that include conducting polymers, ranging from batteries and OLED panel for TVs and smartphones to bioelectronics devices such as sensors and ion-pumps for drug-delivery are widely used in our life. The use of conducting polymer in many electronic applications was driven by demand for light weight, flexibility or biocompatibility with the performance on pair with conventional inorganic counterparts. As a result, during last two decades conducting polymers have been a subject of significant interest in both academia and industry. Though many aspects of conducting polymers’ nature have been disclosed, it is still challenging to design a conducting polymer that meets required electrical and mechanical properties. It is because these properties are simultaneously influenced by many various factors such as charge carrier concentration, molecular weight, chemical structure. Thus, understanding the polymerization, crystallization and morphology of conducting polymers is a crucial key to realize flexible, stretchable or wearable electric applications based on conducting polymers. Computational methods represent an important tool in studies of conducting polymer since they not only provide information about morphology of polymer films on molecular level, but also can describe physical properties such as thermodynamic potential and pair-wise interaction between chains that experimental studies can rely on. This thesis is focused on two classes of conducting polymers: Thiophene-based polymers (PEDOT and p(g42T-T)) and NDI-based polymers (pNDI-TVT-TET). The former is one of the most versatile p-type materials, while the latter is known to have ambipolar charge transport owing to its donor-acceptor structure. First, we corroborated the mechanism of in-situ chemical polymerization of PEDOT with Fe(TOS)3 as oxidant by reaction energy calculation for the conventional oxidation polymerization mechanism. We found that doping of PEDOT chain became energetically unfavorable beyond of 33% doping level and we explained it in terms of polaron localization. To explore the impact of polymerization temperature on PEDOT length, we developed a polymerization model for in-situ chemical polymerization of PEDOT:TOS. The results demonstrate that the average PEDOT length is 6, 7, and 11 monomer units at 298, 323, and 373K respectively, and we concluded that the diffusivity of reactants was a dominant factor determining the PEDOT length. We also investigated the effect of molecular doping on the morphology of p(g42T-T) films and their mechanical properties. Doping of p(g42T-T) by TFSI from 0% to 10% gradually increases the - stacking between polymers. It is also found that when doped by F4TCNQ, the elastic modulus and electrical conductivity of films increases until the doping level of about 18%. We attribute these results to the increasing of -stacking between inter-polymer backbones upon increasing the doping levels from 0% to 18%. Finally, the impact of the ratio of TVT/TET in pNDI-TVTx-TET1-x on the morphology and mechanical properties was studied. From MD simulations, we find that the π-π stacking between polymers as the TVT content increases till 50% and afterwards slightly decreases. In addition, a thin-film transistor with the TVT content of 60 or 80% shows a better conductivity than the one with 100% content when it is bent. Our findings on polymerization of conducting polymers, evolution of crystalline and mechanical properties provide theoretical insight that can help a practical improvement in the field of flexible organic electric devices.