Microstructural Evolution of P(NDI2OD-T2) Films with Different Molecular Weight During Stretching Deformation.

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, we visualized the tensile deformation of P(NDI2OD-T2) and revealed cracks 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 exhibited the optimal electrical and mechanical performances simultaneously, i.e., the electron mobility was retained under 100% strain and after 100 stretching-releasing cycles. This article is protected by copyright. All rights reserved.