Molecular Dynamics Simulation of Amorphous Poly(3-hexylthiophene)

Molecular dynamics (MD) simulations are employed to study the effect of chain length and temperature on the density and conformational properties of regioregular poly(3-hexylthiophene), also denoted as RR-P3HT, in its pure amorphous phase. First, several widely used all-atom force fields (FFs) currently available in the literature are evaluated by comparing their predictions for the density, mean-square chain end-to-end distance, mean-square chain radius-of-gyration, and persistence length of RR-P3HT oligomers at temperatures above their melting point with the limited available experimental data in the literature. Then, with one of the most promising from these FFs, we extend the MD simulations to higher-chain-length P3HT systems (containing up to 150 monomers per chain) at various temperatures. The MD results indicate that the density and persistence length of amorphous P3HT increase slightly with chain length approaching limiting asymptotic values equal to 0.788 +/- 0.003 g cm(-3) and 21 +/- 0.4 angstrom, respectively, at temperature T = 700 K and pressure P = 1 atm. This is attributed to excess chain end free volume effects that are significant at low molecular weights. On the contrary, the effective conjugation length, which is found to become larger than the persistence length only above a certain molecular weight, shows a stronger dependence on chain length. Both of these characteristic lengths are found to increase with decreasing temperature due to the increasing relative population of planar (cis and trans) conformational states of the inter-ring torsion angle. The probability distribution of the maximum length of conjugated segments along a P3HT chain coincides with the theoretical distribution of a longest run of heads in a coin-flip experiment. Our MD results suggest that short-chain-length RR-P3HT chains in their bulk amorphous phase are semiflexible but, as their molecular weight increases, they adopt more and more random coil conformations, especially at higher temperatures.