(694i) Predicting Mechanical Properties of Organic Semiconductors from Molecular Dynamics Simulations

The mechanical properties of organic semiconductors are of critical importance for roll-to-roll production and thermomechanical reliability of organic electronic devices. Here, we describe 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. We show that the resolution of the coarse-grained model has a strong effect on the predicted properties. In particular, we find that a three-site model treating each 3-hexylthiophene unit by three coarse-grained beads predicts values of density and tensile modulus that are much closer to experimental measurements than a one-site model. The three-site model also correctly predicts the strain-induced alignment of chains as well as the vitrification of P3HT by C₆₀ and the corresponding increase in the tensile modulus. We also observe a decrease in the radius of gyration and the density of entanglements of the P3HT chains with the addition C₆₀ which may contribute to the experimentally noted brittleness of the composite material. Although extension of the model to poly(3-alkylthiophenes) (P3ATs) containing side chains longer than hexyl groups correctly predicts the experimental trend of decreasing modulus with increasing length of the side chain, obtaining absolute agreement could not be accomplished by a straightforward extension of the three-site model, indicating limited transferability of such models. Nevertheless, the accurate values obtained for P3HT and P3HT:C60 blends suggest that coarse graining is a valuable approach for predicting the thermomechanical properties of organic semiconductors of similar or more complex architectures.