Counterion Dependent Side‐Chain Relaxation Stiffens a Chemically Doped Thienothiophene Copolymer

ABSTRACT The electrical conductivity and elastic modulus of doped conjugated polymers tend to increase in tandem, which complicates the design of soft conductors. This work investigates how different dopant counterions influence the electrical and mechanical properties of a thienothiophene copolymer with triethylene glycol side chains. Sequential doping and proton‐coupled electron‐transfer were used to prepare samples with a comparable oxidation level neutralized with different counterions. Highly oxidized films featured a comparable electrical conductivity of about 100 S cm −1 irrespective of the counterion size. Dynamic mechanical analysis revealed that the choice of counterion strongly impacts the sub‐glass transition temperature, which varied from −44°C to −3°C. As a result, the elastic modulus at room temperature ranged from 0.05 GPa to 0.7 GPa for materials with a comparable oxidation level. Evidently, it is possible to decouple the electrical and mechanical properties of doped polymers, which are governed by charge transport along the backbone and side‐chain relaxation, respectively. This insight opens up new opportunities for the design of soft conductors and more sustainable bioelectronic and wearable devices whose various soft and rigid components could be created with the same polymer.