Microstructural Evolution of P(NDI2OD‐T2) Films with Different Molecular Weight during Stretching Deformation
Sichun Wang, Kefeng Zhao, Junhang Li, Xinhong Yu, Qiang Zhang, Yanchun Han
Abstract
Abstract Conjugated polymers exhibit excellent electrical and mechanical properties when their molecular weight ( M w ) is above the critical molecular weight ( M c ). The microstructural changes of polymers under strain are crucial to establish a structure–performance relationship. Herein, the tensile deformation of P(NDI2OD‐T2) is visualized, and cracks are revealed either along the (100) crystal plane of side chain packing or along the main chain direction which depends on the M w is below or above the M c . When M w < M c , the film cracks along the (100) plane under small strains. When M w > M c , the polymer chains first undergo stretch‐induced orientation and then fracture along the main chain direction at large strains. This is attributed to the fact that the low M w film exhibits large crystalline domains and the absence of interdomain connectivity, which are vulnerable to mechanical stress. In contrast, the high M w film displays a nearly amorphous morphology with adequate entanglements, the molecular chains can endure stresses in the stretching direction to release substantial strain energy under greater mechanical deformation. Therefore, the film with M w > M c exhibits the optimal electrical and mechanical performances simultaneously, i.e., the electron mobility is retained under 100% strain and after 100 stretching–releasing cycles.