There is considerable scientific and commercial interest in developing conjugated polymer materials like the alkoxy-functionalized thiophene poly(3,4-ethylenedioxythiophene) (PEDOT) and its derivatives due to their high chemical stability and charge transport properties. They also conduct charge both electronically and ionically making them useful as interfacing materials in biomedical devices. As a result, these polymers have found a variety of applications like organic bioelectronics, light emitting diodes and electrochromic displays.<br/>Electrochemical polymerization is commonly used to deposit thin-films of these conjugated polymers due to its ability to coat patterned surfaces. However, the nature of the electrochemical polymerization reaction and the factors that determine the development of different morphologies are not yet well established making it difficult to optimize device performance. We have used in-situ, low dose Transmission Electron Microscopy to monitor the early-stage nucleation and growth processes of the mobile oligomeric clusters occurring at the electrode-solution interface. We use commercially available electrochemical holders from Protochips, Inc. and Hummingbird Scientific to image these electrochemical reactions in-situ allowing us to look at a thin section of liquid (monomer solution) sandwiched between two microfabricated chips, where the top chip has micropatterned electrodes and the spacer chip is essentially used to confine the liquid.<br/>In previous studies, it has been determined that introducing a macromolecular counter-ion poly(acrylic acid) (PAA) during the electrodeposition facilitates the formation of highly anisotropic nano-fibrils of PEDOT thereby reducing the overall impedances due to increase in effective surface area. The mechanism of nanofibril formation is not yet well understood. In-situ electron microscopy has helped us observe, quantify and understand the fundamental nucleation and growth processes of individual nanofibrils. We observed variations in film morphology and density that reveal details of the transition from locally mobile liquid-like to sessile solid-like behavior as the deposition proceeds. We have also examined a more hydrophilic carboxylic-acid substituted variant of EDOT (EDOT-acid) and a more ordered conducting polymer poly(5,6-dimethoxyindole-2-carboxylic acid) (PDMICA) both of which demonstrate drastically different nucleation and growth processes as compared to the homopolymer PEDOT.

Quantitative observations of PEDOT nanofibril nucleation and growth using liquid <i>in-situ</i> transmission electron microscopy 

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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Quantitative observations of PEDOT nanofibril nucleation and growth using liquid in-situ transmission electron microscopy