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 Å, 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.

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We introduce a powerful Monte Carlo (MC) algorithm for the atomistic simulation of bulk models of oligo-and poly-thiophenes by redesigning MC moves originally developed for considerably simpler polymer structures and architectures, such as linear and branched polyethylene, to account for the ring structure of the thiophene monomer. Elementary MC moves implemented include bias reptation of an end thiophene ring, flip of an internal thiophene ring, rotation of an end thiophene ring, concerted rotation of three thiophene rings, rigid translation of an entire molecule, rotation of an entire molecule and volume fluctuation. In the implementation of all moves we assume that thiophene ring atoms remain rigid and strictly co-planar; on the other hand, inter-ring torsion and bond bending angles remain fully flexible subject to suitable potential energy functions. Test simulations with the new algorithm of an important thiophene oligomer, α-sexithiophene (α-6T), at a high enough temperature (above its isotropic-to-nematic phase transition) using a new united atom 2 model specifically developed for the purpose of this work provide predictions for the volumetric, conformational and structural properties that are remarkably close to those obtained from detailed atomistic Molecular Dynamics (MD) simulations using an all-atom model. The new algorithm is particularly promising for exploring the rich (and largely unexplored) phase behavior and nanoscale ordering of very long (also more complex) thiophene-based polymers which cannot be addressed by conventional MD methods due to the extremely long relaxation times characterizing chain dynamics in these systems.