Interplay of the Electrical and Mechanical Properties of Conjugated Polymers
Doktorsavhandling, 2022

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.

mechanical properties


conjugated polymers


organic semiconductors


conducting fibers

electrical properties


organic electronics

Lecture room Vasa A-salen
Opponent: Sahika Inal, Associate Professor, Organic Bioelectronic Laboratory, King Abdullah University of Science and Technology


Sepideh Zokaei

Chalmers, Kemi och kemiteknik, Tillämpad kemi, Christian Müller Group

Tuning of the elastic modulus of a soft polythiophene through molecular doping

Materials Horizons,; Vol. 9(2022)p. 433-443

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Electrically Conducting Elastomeric Fibers with High Stretchability and Stability

Small,; Vol. 18(2022)

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Toughening of a Soft Polar Polythiophene through Copolymerization with Hard Urethane Segments

Advanced Science,; Vol. 8(2021)

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Hultmark S., Craighero M., Zokaei S., Kim D., Järsvall E., Farooqi F., Marina S., Kroon R., Martin J., Zozoulenko I., Müller C., Decoupling of the electrical and mechanical properties of a polythiophene through doping with bistriflimidic acid

Polymer based materials such as plastics are vastly used in our daily life.  Plastics usually are electrical insulators and they can display a wide range of mechanical properties. For instance, soft Sculpey clay for making handicraft items and stiff kitchen appliances are made out of plastics. However, 50 years ago a class of polymers, i.e. conjugated polymers, was discovered that has the ability to conduct electricity, if mixed with additives, so called dopants. To name a few potential applications of conjugated polymers, they can be employed in solar cells that convert light to electricity, as well as wearable electronics such as e-textiles for health care that allow to monitor and measure various health metrics.

However, the mechanical properties of these polymers need to be optimized for different applications without unduly compromising their electrical properties. This thesis focuses on modulating the mechanical properties of a soft conjugated polymer using different strategies.

First, the mechanical and electrical properties of a soft conjugated polymer is studied after introduction of a hard non-conducting domains that form a network within the conjugated polymer. Furthermore, the addition of an insulating polymer to improve the stiffness and melt-processability or an insulating elastomer to prepare stretchable conducting fibers for e-textiles is explored. Additionally, the introduction of cellulose nanomaterials from wood is shown to increase the stiffness of the material while maintaining the electrical conductivity. Finally, it is shown how the incorporation of dopants that are needed to increase the electrical conductivity of conjugated polymers, can also modify their mechanical properties. Therefore, through the addition of a single additive, both the desired electrical and mechanical properties may be achieved.

This thesis explores the structural, electrical and mechanical modifications of a soft conjugated polymer using the aforementioned methods. The results of this study will open up new possibilities for modulating the property portfolio of conjugated polymers suitable for, e.g., wearable electronics.









Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5114



Lecture room Vasa A-salen


Opponent: Sahika Inal, Associate Professor, Organic Bioelectronic Laboratory, King Abdullah University of Science and Technology

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