Interplay of the Electrical and Mechanical Properties of Conjugated Polymers

Abstract: Knowledge about organic semiconductors has drastically developed in the past decades. They have a myriad of applications in areas such as energy harvesting and storage, bioelectronics and wearable electronics. For most of these applications, mechanical flexibility is desirable. Conjugated polymers, a class of organic semiconductors, tend to be brittle and rigid. The latter is a consequence of their planar‑aromatic backbones that endow them with a high glass transition temperature and a tendency to strongly aggregate. Polythiophenes with oligoethylene glycol side chains, on the contrary, tend to be soft materials with a low glass transition temperature and low degree of crystallinity, which limit their use as a bulk free‑standing material. At the same time, they can feature high ionic and electrical conductivity. This thesis explores different strategies to modulate the mechanical properties of polythiophenes with polyethylene glycol side chains without unduly affecting their electrical properties. This thesis will compare the mechanical and electrical properties of a soft polythiophene and a copolymer of the same material with hard urethane blocks, which enable the formation of a reversible network. Then, blending of a doped soft conjugated polymer with melt‑processable insulating polymers such as polycaprolactone is explored with the goal to prepare thermally stable blends for melt-processing. Conducting stretchable fibers of a doped conjugated polymer and a polyurethane elastomer are demonstrated that feature a high degree of electrical and mechanical stability. Further, the properties of composites with cellulose nanomaterials are described. The nanocomposites feature a high elastic modulus, and the presence of cellulose nanofibrils does not affect the electrical conductivity. Finally, the impact of molecular doping, which is an essential step for rendering the conjugated polymers conductive, on the nanostructure and thermomechanical properties of polythiophenes with oligoethylene glycol side chains is explored. In particular, doping is found to strongly increase the elastic modulus of the polymer. Evidently, a wide range of methods such as copolymerization, blending, the use of a reinforcing agent as well as molecular doping itself can be used for the which may facilitate the design of mechanically robust electrical conductors.