Molecular Dynamics Investigation of the Mechanical Properties of Polymer–Fullerene Bulk Heterojunctions

Abstract

We investigate the mechanical properties of π-conjugated polymeric materials composed of regioregular poly(3-hexylthiophene) (P3HT) and fullerene C₆₀ using coarse-grained molecular dynamics simulations. Specifically, we perform tensile simulations of P3HT:C₆₀ composites with varied degrees of polymerization and C₆₀ mass fractions to obtain their stress–strain responses. Decomposition of stress tensor into kinetic energy and virial contributions indicates that the tensile moduli of the pure P3HT samples are greatly dependent on non-bonded interactions and on bonded interactions associated with bond-stretching, while the addition of C₆₀ leads to an increase in the tensile modulus originating from enhanced non-bonded interactions associated with C₆₀. Additionally, the tensile strength of the P3HT:C₆₀ samples correlates well with molecular chain entanglements, which are characterized by the average number of kinks per chain obtained from primitive path analysis. We also find that the upper and lower yield points characterizing strain softening become more pronounced with increasing C₆₀ mass fraction. Persistent homology analysis indicates that the emergence of the yield points correlates well with the coalescence of microvoids in the course of tensile deformation, resulting in the generation of larger voids.