Temperature-dependent stress–strain behavior of amorphous and crystalline P3HT†

Abstract

The scalable commercialization of organic electronics wherein π-conjugated polymers serve as the semiconductors hinges on precise control of the material electronic, redox, optical, and mechanical properties, which are each highly influenced by local and long-range morphology. Here, we undertake atomistic molecular dynamics (MD) simulations at three temperatures (150 K, 300 K, and 400 K) to assess the morphological and mechanical response of bulk poly(3-hexylthiophene) (P3HT), a representative homopolymer of interest as an organic semiconductor (OS). As P3HT is a semicrystalline polymer, we characterize mechanical properties for both amorphous and crystalline P3HT models to derive insights into structure–property relationships, including Young's modulus (E) and Poisson's ratio (ν). Mechanical behaviors that arise as a consequence of kinetically induced molecular reorientations/transitions are described, including the determination of entanglement properties over the course of polymer deformation. Specifically, we analyze stress–strain curves to (1) elucidate how, and the extent to which, the rather tangled amorphous domains retain their ductility over temperature ranges that span known phase transitions, and (2) uncover the strength and mechanism of inter-chain mechanical coupling across lamellar stackings as a function of temperature. Generally, this work provides a molecular-level understanding of the thermomechanical behavior of π-conjugated polymers at regions where order or disorder dominates local packing, and prompts a more comprehensive description of the mechanical properties of these systems while recognizing their often inherently semicrystalline nature.