Stretchable Conjugated Polymers: A Case Study in Topic Selection for New Research Groups.

The field of π-conjugated (semiconducting) polymers has been underwritten largely because of the promise of flexible (and increasingly, stretchable) devices for energy and health care. Our research group has spent much of the past six years studying the mechanical properties of conjugated polymers. Mechanically robust materials can extend the life spans of devices such as solar cells and organic light-emitting diode (OLED) panels and enable high throughput processing techniques such as roll-to-roll printing. Additionally, wearable and implantable devices, including electronic skin, implantable pressure sensors, and haptic actuators, benefit by having moduli and extensibilities close to those of biological tissue. At the time of our laboratory's inception, however, the optoelectronic properties of conjugated polymers were understood in much greater depth than their mechanical properties. We therefore set out, as our laboratory's first research topic, to understand the molecular and microstructural determinants of the mechanical properties of conjugated polymers. This is an Account not only of our scientific findings but also of the pragmatic aspects, including personnel, funding, and time constraints, behind our studies as a nascent research group. We hope that this Account will provide information to newly independent scientists about the process of starting a new research laboratory. We identify three main stages of our scientific growth. (1) We began by conducting proof-of-concept experiments to identify basic correlations between chemical structure and mechanical properties and to determine whether high optoelectronic performance and mechanical robustness were mutually exclusive. (2) We then added new metrological techniques to enable more rapid and robust measurements, such as obtaining full stress-strain curves for conjugated polymer thin films, characterizing modes of thin film failure, and simplified identification of the glass transition temperature. (3) Finally, we incorporated new capabilities, such as organic synthesis and molecular dynamics simulations, into the toolkit of our group. These stages corresponded with increased funding, personnel commitment, and flexibility to take on long-term projects. Our research efforts identified polythiophene-based semiconducting polymers capable of both achieving high power conversion efficiencies and accommodating high degrees of strain. Additionally, we identified several chemical and microstructural determinants of the mechanical properties of conjugated polymer films, such as the chemical composition and structure of side chains and a high degree of dependence on amorphous packing structure. While the field has not yet produced stretchable materials that retain state-of-the-art electronic properties with high elastic range and repeated deformation, we hope that our work and the work of others in the field has provided a foundation for future advances.