(336d) Revealing Phase Formation and Structure Evolution of Conjugated Polymers Using Cryogenic Electron Microscopy

¹Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania, 16802, USA

Organic mixed ionic-electronic conducting (OMIEC) polymers have attracted a tremendous amount of attention due to their easy tunability and potential for various applications, such as neuromorphic computing and biosensing. Among different OMIECs, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is the most widely used polymeric system due to its commercial availability and ease of processibility as an aqueous dispersion. PEDOT is a p-type conjugated polymer with poor solubility in both polar and nonpolar solvents, hence it is usually processed along with PSS which also serves as a dopant to induce positively charged electronic carriers in PEDOT. It has been shown that both the electronic and mechanical properties of PEDOT:PSS can be modulated through post-processing or the application of different additives. Despite its wide range of applications the underlying mechanisms behind the improvement in the mechanical and electrical properties of PEDOT:PSS in the presence of additives remains poorly understood. In this work, we use cryogenic electron microscopy (cryo-EM) to investigate micro- and nano-structure of PEDOT:PSS in different hydrated and swollen states.

Here, we show that common methods used to improve electrical conductivity and mechanical properties of PEDOT:PSS, such as ionic additives and high boiling point solvents, lead to the formation of long-range fibers in both solution and solid states. Cryo-EM allowed us to capture the self-assembly of PEDOT:PSS in a solution phase (Figure 1a and Figure 1c). The darker regions in the cryo-EM micrograph show the PEDOT:PSS micelles which self-assemble to form elongated fibers. Furthermore, cryo-EM for the first time reveals that these fibers grow during the film formation and, similar to the solution state, consist of core-shell PEDOT:PSS micelles that self-assemble to form an interconnected network of amorphous polymer embedding semi-crystalline PEDOT:PSS phases (Figure 1b and Figure 1d). Our results further suggest that in a hydrated state PSS rich regions of the PEDOT:PSS swell while the PEDOT-rich phases maintain an interconnected network which highlights the importance of a charge percolation pathway as well as the amorphous phase connectivity to achieve high mobility and stretchable OMIECs. Our findings pave the way to a deeper understanding of OMIECs and their structure-function relationships.

Caption: a) schematic of the vitrification process used for preparation of solution phase cryo-EM samples. b) Schematic of solid-state cryo-EM. c) Cryo-EM micrographs of PEDOT:PSS in a solvated state. d) Cryo-EM micrograph of thin film PEDOT:PSS in a solid state.