Ion Diffusion and Morphology of PEDOT:PSS. Insight from Molecular Dynamics Simulations

Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is one of the most important mixed electron-ion conducting polymers, where the efficiency of the ion transport is crucial for many of its applications. Despite of the impressive experimental progress in determination of the ionic mobilities in PEDOT:PSS, the fundamentals of ion transport in this material remain poorly understood and the theoretical insight of the ion diffusion on the microscopical level is completely missing.<br/><br/>In the present paper, Martini 3 coarse grained molecular dynamics (MD) model for PEDOT:PSS is developed and applied to calculate the ion diffusion coefficients and ion distribution in the film.¹ A prerequisite to study the ion diffusion in PEDOT:PSS is to build its adequate morphological model properly describing its two-phase morphology as well as water intake and ion exchange. Using the coarse-grained calculations we developed this model by mimicking experimental process of film formation by drying the emulsion of PEDOT:PSS particles.² We demonstrate that PEDOT:PSS film is an essentially three component system, consisting of positively charged PEDOT chains, PSS chains with mostly deprotonated sulfonate groups, and protonated PSS chains.³ PEDOT-rich regions are predominantly composed of PEDOT and deprotonated PSS chains, whereas PSS-rich regions are composed of protonated PSS chains. Our calculations unravel how PEDOT-rich and PSS-rich regions are formed from the solution phase during drying process. We show that when the dry polymer film is immersed in water its swells by nearly 60%, and we demonstrate that the origin of swelling is related to deprotonation of the sulfonate groups in the PSS-rich regions. It is mostly PSS-rich regions that swell while the PEDOT-rich regions remain rather unchanged.<br/><br/>Using the obtained morphology we calculated the ion diffusion coefficients for Na⁺ ions and we found that they are practically the same in the PEDOT-rich and PSS-rich regions and do not show sensitivity to the oxidation level. We compare the calculated diffusion coefficients with available experimental results. Based on this comparison, and based on the MD morphology simulation of PEDOT:PSS revealing the formation of pores inside the film, we revised a commonly accepted granular morphological model of PEDOT:PSS. Namely, we argue that PEDOT: PSS films, in addition to PEDOT-rich and PSS-rich regions, must contain a network of pores, where the ion diffusion takes place.<br/><br/>We believe that our results demonstrate the power of the MD simulations for organic mixed electron-ion conductors providing the essential insight into polymer morphology and ion diffusion that is difficult to obtain by other means.<br/><br/>(1) Sedghamiz, T.; Mehandzhiyski, A. Y.; Modarresi, M.; Linares, M.; Zozoulenko, I. What Can We Learn about PEDOT:PSS Morphology from Molecular Dynamics Simulations of Ionic Diffusion? <i>Chem. Mater.</i> <b>2023</b>, <i>35</i> (14), 5512–5523. https://doi.org/10.1021/acs.chemmater.3c00873.<br/>(2) Jain, K.; Mehandzhiyski, A. Y.; Zozoulenko, I.; Wågberg, L. PEDOT:PSS Nano-Particles in Aqueous Media: A Comparative Experimental and Molecular Dynamics Study of Particle Size, Morphology and z-Potential. <i>J. Colloid Interface Sci.</i> <b>2021</b>, <i>584</i>, 57–66. https://doi.org/10.1016/j.jcis.2020.09.070.<br/>(3) Modarresi, M.; Mehandzhiyski, A.; Fahlman, M.; Tybrandt, K.; Zozoulenko, I. Microscopic Understanding of the Granular Structure and the Swelling of PEDOT:PSS. <i>Macromolecules</i> <b>2020</b>, <i>53</i> (15), 6267–6278. https://doi.org/10.1021/acs.macromol.0c00877.