Molecular origin of strain-induced chain alignment in PDPP-based semiconducting polymers

Due to the heterogeneous structure and complex solid-state morphology of Donor-acceptor (D-A) type semiconducting polymers, their complete molecular picture under deformation is hard to detect. Here, we probed the molecular orientation of free-standing D-A polymer thin films under tensile alignment, based on a diketopyrrolopyrrole (DPP) polymer. The detailed morphological analysis demonstrates highly aligned polymer crystallites through in-plane rotation towards the strain direction, while the backbone's alignment within the crystalline domain is limited. Surprisingly, the overall chain anisotropy is still low under large deformation. These observations are distinct from traditional semicrystalline polymers like polyethylene due to extinct characteristics of backbone, side-chain, and crystallites in DPP polymers. This study deconvolutes the alignment of multiple components in the polymer microstructure and highlights that crystallite rotation and amorphous chain slippage are the primary chain alignment mechanisms for semiconducting polymers.