Intrinsically stretchable naphthalenediimide–bithiophene conjugated statistical terpolymers using branched conjugation break spacers for field–effect transistors

Abstract

The development of conjugated polymers through the statistical terpolymerization of conjugation break spacers (CBSs) has received great attention because of their synergistic potential in enhancing fracture strain and tensile strength. In this study, a series of bulky and branched CBSs with ester functional groups and dimethyl (P1), methyl/propyl (P2), ethyl/butyl (P3), and dibutyl (P4) alkyl groups were incorporated into naphthalenediimide–bithiophene-based conjugated polymers through statistical terpolymerization. Morphological studies, including atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXD) studies, were applied to pristine and stretched polymer films to decipher the structure–stretchability relationship; further, their mobility–stretchability relationship was investigated by studying their field-effect transistor (FET) characteristics. We found that polymers with bulky and branched CBSs exhibit reduced backbone rigidity, allowing polymer chains to entangle more easily than those of stiffer polymers with similar molecular weights. Therefore, the polymer film exhibits a more uniform load distribution throughout the channel layer, and the branched CBSs serve as lubricants that deter crack propagation during deformation. Consequently, P4 with symmetrically extended alkyl groups shows good ductility and mobility–stretchability properties, with orthogonal μ_e,‖/μ_e,⊥ retention of approximately 20–30% with 60% strain and 30–40% after 400 stretching–releasing cycles with 60% strain. Our results indicated that statistical terpolymerization with branched CBSs is a versatile and effective methodology for controlling orientational and stacking patterns to achieve good mobility–stretchability properties in conjugated polymers.