Role of Secondary Thermal Relaxations in Conjugated Polymer Film Toughness

Conjugated polymers have proven to be an important class of materials for flexible and stretchable electronics. To ensure long-term thermal and mechanical stability of associated devices, there is a need to determine the origin of the polymer ductility and toughness. In this work, we investigate a variety of high-performance conjugated polymers and relate their thermomechanical behavior to film toughness. Dynamic mechanical analysis (DMA) is used to probe thermomechanical relaxations of the conjugated polymers. Film ductility is measured as a function of temperature to determine the temperature that corresponds to a significant loss in film toughness. We systematically study polymers with changes to the side-chain structure, backbone structure, and crystallinity. We also compare polymers that have a clear glass transition (T g) to those that do not. It is found that secondary thermal relaxations (sub-T g) play a critical role in film toughness. This sub-T g is found to be a local molecular relaxation that appears to relate to side-chain and backbone mobility. We also find that many of the polymers considered continue to show moderate ductility below their sub-T g, which is attributed to crystallites or aggregates that have active slip systems. These results provide new insights into how conjugated polymer structure and related thermal relaxations influence film toughness that will assist in realizing mechanically robust devices.