Molecular Dynamics Simulation of Amorphous Poly(3-hexylthiophene)

Abstract

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.

Chain Stiffness of Donor–Acceptor Conjugated Polymers in Solution

“…Here, we will compare the persistence lengths extracted from the SANS scattering curves to the theoretical predictions calculated following an approach by Milner et al Dihedral potentials were computed via density functional theory (DFT) calculations using Spartan 18 with the ωB97X-D long-range exchange correlation functional and a 6-31G*­(d,p) diffuse, polarized, split-valence double-ζ basis set. Although limiting, , for computational efficiency, the torsional potentials U (ϕ) were calculated by using optimized scans of the central dihedral angle(s) ϕ of a repeat unit (rather than longer oligomers), and the side chains were replaced with methyl groups.…”

Molecular Dynamics Study of the Thermodynamics of Integer Charge Transfer vs Charge-Transfer Complex Formation in Doped Conjugated Polymers

“…Our simulation cell consists of 6 stacks of 12 P3HT chains (for a total of 72 chains) that each have 24 thiophene monomer units (1728 monomers in total), along with various numbers of F 4 TCNQ molecules in different doping geometries, which with periodic boundary conditions in all three directions provides a reasonable approximation to the bulk material. We note that, although there have been many computational studies examining the doping of conjugated polymers, most either looked only at single oligomers and dopant molecules in the gas phase − or examined the structure and mechanical properties of the pure P3HT polymer without including dopant molecules. − As far as we know, this represents the first work using simulation methods to study the bulk structure with dopant molecules and the thermodynamics of the doping process for semiconducting polymers.…”

Temperature-Dependent Conformation Behavior of Isolated Poly(3-hexylthiopene) Chains

“…The fact that this force field was designed for long chains makes it particularly suitable for our investigations. As discussed in recent in-depth force field comparisons by the authors of [25][26][27], many other force fields, in contrast, are designed rather for short oligomers. Here, we therefore adjusted the force field of Bhatta et al to form long, but finite chains of P3HT by adding uncharged hydrogen atoms at the end.…”