Molecular Engineering for Mechanically Resilient and Stretchable Electronic Polymers and Composites

Establishing the design criteria for elasticity and ductility in conjugated polymers and composites by analysis of the structural determinants of the mechanical properties.

Air Force Research Laboratory, Arlington, Virginia

The ability to predict the mechanical properties of organic semiconductors is of critical importance for roll-to-roll production and thermomechanical reliability of organic electronic devices. This research describes the use of coarse-grained molecular dynamics simulations to predict the density, tensile modulus, Poisson ratio, and glass transition temperature for poly(3-hexylthiophene) (P3HT) and its blend with C₆₀. In particular, it is shown that the resolution of the coarse-grained model has a strong effect on the predicted properties.

It was found that a one-site model, in which each 3-hexylthiophene unit is represented by one coarse-grained bead, predicts significantly inaccurate values of density and tensile modulus. In contrast, a three-site model, with one coarse-grained bead for the thiophene ring and two for the hexyl chain, predicts values that are very close to experimental measurements (density = 0.955 g cm⁻³, tensile modulus = 1.23 GPa, Poisson ratio = 0.35, and glass transition temperature = 290 K). The model also correctly predicts the strain-induced alignment of chain, as well as the vitrification of P3HT by C60 and the corresponding increase in the tensile modulus (tensile modulus = 1.92 GPa, glass transition temperature = 310 K).