Conjugated polymers are in high demand for a wide range of applications from wearable electronics to bioelectronics. While the optimization of their electrical properties is increasingly well understood, the design of materials with balanced mechanical properties has received less attention. One way to adjust the stiffness of a conjugated polymer is through the addition of a filler, such as cellulose nanofibrils. Many conjugated polymers with a state-of-the-art electrical performance are processable from organic solvents, while cellulose nanofibrils are typically processed from water. This solvent mismatch complicates compounding from solution.
Here, the preparation of composites through in-situ polymerization is presented. Direct arylation polymerization is used to prepare a thienothiophene-based copolymer in the polar solvent N-butyl-2-pyrrolidone, which also contains dispersed cellulose nanofibrils. After removal of the solvent, which acted as both the reaction and dispersion medium, a fine composite is obtained. Considerable reinforcement is observed, as evidenced by an increase in elastic modulus from about 0.2 GPa for the neat polymer to more than 1.1 GPa for a dry composite containing 15 vol% cellulose nanofibrils. The stiffness of composites could be reversibly reduced by at least 50% through water uptake, enabling the design of organic semiconducting materials with switchable mechanical properties.