Modeling of Poly(3-hexylthiophene) and Its Oligomer's Structure and Thermal Behavior with Different Force Fields: Insights into the Phase Transitions of Semiconducting Polymers

The polymorphism of poly(3-hexylthiophene) (P3HT), one of the reference systems in fundamental studies of polymer semiconductors, is explored by molecular dynamics modeling of selected 3-hexylthiophene (3HT) oligomers, comparing structural and thermal behavior simulation results with rare monodisperse oligomer experimental data. The relative stability of the two crystalline polymorphs and the mechanism of interconversion between them, as the degree of polymerization grows in (3HT)n oligomers (n = 10, 16, 20) to the polymer, can be investigated in infinite periodic oligomer crystals without implicitly imposing infinite molecular weights, as inevitable for polymers. To evaluate the impact of different descriptions of molecular interactions, simulations were performed by using three different force fields specifically adapted to poly(3-alkylthiophenes) (P3ATs). Our results show that MD may adequately describe the key features and relative stability of the different crystal phases and suggest plausible interconversion mechanisms for very rapid solid-solid or melting transitions, albeit with complementary differences among different force fields, which become substantial modeling highly disordered crystal structures or mesophases.