Controlling the Backbone Flexibility of Conjugated Polymer to Achieve Superior Backbone Tensile Alignment

Recent reports have shown increased interest in the capacity of conjugation-break spacers (CBS) to soften relatively rod-like conjugated polymers (CP) while maintaining charge mobility. Here, we provide a holistic approach to understand the thermomechanical influence of CBS length on an n-type NDI-based conjugated polymer, denoted by PNDI-Cx. CBS lengths are varied from C0 (fully conjugated) to C7 with the CBS engineered into each repeat unit for systematic evaluation. Solution small angle neutron scattering and melt shear rheology were employed to provide the first quantitative evidence of CBS influence over CP chain rigidity and entanglement molecular weight (M_e), demonstrating an increase in chai flexibility, as well as a M_e independent of CBS length. Thermomechanical property, including: modulus, glass transition temperature, and melting temperature were all shown to decrease with increasing CBS length, while a high ductility was observed for PNDI-C4 which we attribute to a large number of entanglements. Furthermore, a high degree of backbone alignment was observed upon strain through GIXD, polarized UV-vis, and AFM. We will also discuss coalignment in a blend system of PNDI-C0 (tie-chain) in a ductile PNDI-C4 (matrix) to yield enhanced charge mobilities.