Influence of Acceptor Side-Chain Length and Conjugation-Break Spacer Content on the Mechanical and Electronic Properties of Semi-Random Polymers

Conjugation-break spacers (CBS) have been shown to enhance the mechanical properties of conjugated polymers. In particular, incorporation of CBS units into semi-random polymers has revealed high ductility and low elastic moduli, attributed to the combined influence of the CBS units and the semi-random architecture. To further elucidate the structure-property relationships in these polymers, two new families of semi-random polymers are reported here. In the first, poly(3-hexylthiophene)-based semi-random polymers incorporating diketopyrrolopyrrole (DPP) units were synthesized in which CBS units with 4-10 carbons were incorporated from 10 to 40% with an equivalent content of 2-decyltetradecyl-DPP (dtdDPP) to overcome solubility limitations previously observed with 2-ethylhexyl-DPP (ehDPP). These polymers had much higher solubility and could attain higher molecular weights, formed films with high integrity, and displayed extraordinary mechanical properties, with elastic moduli as low as 5.45 MPa and fracture strains as high as 398%. In the second family, the content of ehDPP was held constant at 10%, while the CBS content was varied from 10 to 50% (with an eight-carbon spacer) to deconvolute the influence of CBS and DPP content on mechanical properties. Polymer solubility, molecular weight, and processability were not shown to improve dramatically relative to the previous generation of ehDPP polymers with matched DPP and CBS content, but the mechanical properties of this series were quite notable, with elastic moduli as low as 4.08 MPa, an increase in toughness, and fracture strains as high as 432%. The extraordinary mechanical properties exhibited by these polymers can serve as a guide in the judicious selection of monomers and backbone architectures in the future synthesis of semiconducting polymers for flexible electronic applications.