Molecular Dynamics Simulations of the Orientation and Reorientational Dynamics of Water and Polypyrrole Rings as a Function of the Oxidation State of the Polymer

Abstract

Summary: Polypyrrole is one of the most widely‐studied conducting polymers due to its steady electrochemical response and good chemical stability in different solvents, including organic and inorganic ones. In this work, we provide for the first time valuable information in atomic detail concerning the steady and dynamic properties of pyrrole rings as a function of the oxidation state of the polymer. The study was carried out by Classical molecular dynamics simulation, where the system was modelled by 256 polypyrrole chains of 10 pyrrole rings each. Water was explicitly introduced in our simulations. Besides the uncharged or reduced state, two steady oxidation states of the polymer have been simulated by introducing a net charge (+1) on 85 and 256 of the polypyrrole chains. To balance the charges emerging in these oxidised states, 85 and 256 chloride ions (Cl−1) respectively, were introduced into the system. From an analysis of the simulated trajectories, the orientation and relaxation times of water and pyrrole rings were evaluated for the different oxidation states of the polymer across the polypyrrole/water interface. The calculated densities for different oxidation states describe the swelling or shrinking process during electrochemical oxidation or reduction respectively. The rotational relaxation times calculated for the polypyrrole rings decrease with increasing oxidation of the polymer, which is in a good agreement with experimental electrochemical data. Almost no variation in pyrrole ring orientation was measured for the different oxidation states of the polymer, even compared with polypyrrole bulk. As regards the water structure in the vicinity of the polypyrrole/water interface, both the orientation and orientation relaxation time were strongly affected by the presence of charges in the polymer. Thus, the water dipole was strongly orientated in the vicinity of the water/polypyrrole interface and its orientational relaxation time increased by one order of magnitude compared with bulk water, even when only one‐third of the total polymer chains were oxidised. The results attained in this work were validated with experimental results, when they were available.Polypyrrole ring orientation and water orientation at the polypyrrole/water interface. (a) 256 rPPy and (b)171 rPPy + 85 oPPy.imagePolypyrrole ring orientation and water orientation at the polypyrrole/water interface. (a) 256 rPPy and (b)171 rPPy + 85 oPPy.

Physicochemical study of the acetonitrile insertion into polypyrrole films

“…Information related distances, charge distribution, and angles used to model oPPY can be consulted elsewhere. 10 Once the polypyrrole film was modeled, the two gaps above and below the amorphous polypyrrole film were filled in with an equilibrated acetonitrile box containing 1728 acetonitrile molecules. The acetonitrile was modeled using the three point charges model proposed by Bernardini and Stassen 27 based on the previous Edwards et al 28 model.…”

“…7 In this study, we focused our interest on the polypyrrole/ acetonitrile system based on previous simulations of polypyrrole in aqueous solution [8][9][10] and molecular dynamics ͑MD͒ simulation of acetonitrile in bulk. [11][12][13] Thus a great number of laboratories around the world focused their interest on the study of the electrochemical response of this conducting polymer using acetonitrile as solvent [14][15][16][17][18] due to its reasonable polar and aprotic nature, permitting to avoid the use of water that could eventually interfere in the electrochemical response of these conducting polymers.…”

Molecular dynamics simulation of polypyrrole film in an acetonitrile solution

“…The long-range interactions were treated by a Lennard-Jones potential using a cut-off of 1 nm, and the electrostatic interactions were evaluated using the Fast Particle-Mesh Ewald method [22,23]. In this regard, the LJ parameters and force field used in this work may be obtained elsewhere [7]. All the simulations were carried out using an integration leap frog algorithm with a constant integration time step of 2 fs.…”

“…In this regard, based on previous simulations of polypyrrole films at different oxidation states [5][6][7], we propose the study of an acetonitrile/polypyrrole interface with lithium perchlorate in solution at two different oxidation states of the polymer. From simulation, valuable information concerned the translational diffusion coefficient of ions in solution and the interior of the polymer, coordination number of acetonitrile around ions in their solvation shell, acetonitrile residence time in the solvation shell, and the shape and thickness of the polypyrrole/solution interface are available from simulation.…”

Coarse‐grained modelling of polypyrrole morphologies

“…Molecular dynamics (MD) simulations of solvated reduced and oxidized polypyrrole were performed by Cascales et al [12,13]. In these simulations, the standard GROMOS torsion potential connecting repeat units was used for the neutral as well as the doped polymer.…”