In-Situ Imaging of Electrochemical Polymerization of Functionalized Thiophenes Using Transmission Electron Microscopy
Authors: Subramanian Vivek , Liu Jinglin , Wei Bin , Martin David Charles 
Published: February 2020

There is considerable continuing interest in developing conjugated polymer materials like the functionalized thiophene poly(3,4-ethylenedioxythiophene) (PEDOT) to interface ionically conducting living tissue with electronically conducting metallic or semiconducting biomedical devices. These versatile conjugated polymers can also be used in organic solar cells, anti-static coatings and flexible organic electronic devices.

Electrochemical polymerization can be used to synthesize thin-films of conjugated polymers. Typically, a 3-electrode system comprising of a working, counter and reference electrodes is used for this process. It is initiated by the electrochemical oxidization of the monomers, followed by the formation of higher molecular weight products that precipitate from solution, forming a thin-film of the conjugated polymer on the working electrode (anode). The reaction can be controlled either potentiostatically (constant voltage) or galvanostatically (constant current).This technique has several advantages over vapor-phase and chemical polymerization such as the facile control of polymer morphology (rough and bumpy under normal conditions) and the ability to coat patterned surfaces. However, the factors that determine the development of these structures during electrodeposition are not yet well established. This makes it difficult to design new systems and optimize device performance. Considering that the morphology of electrochemically-polymerized thin-films can be fine-tuned by controlling the early stage nucleation and growth of the oligomeric clusters, we have used in-situ, low dose Transmission Electron Microscopy to image and obtain a detailed understanding of the fundamental processes occurring at the electrode-solution interface, especially the evolution of the mobile oligomers that precede solid polymer film formation.

The observation and quantification of these electrochemical reactions are made possible using a commercial holder designed by Protochips, Inc. The liquid (monomer solution) is sandwiched between a top chip and a spacer chip. The spacer chip is used to control the amount of liquid to be imaged (typically 500 nm – 1000 nm). This system can either be used in a static mode (no flow) or flowing mode depending on the requirements. The top chip is equipped with the electrical connections required to perform electrochemical reactions and watch the process in-situ.

During the experiments, we observed individual nuclei forming, merging and growing in size and thickness with increasing charge density. These droplets correspond to more mobile, oligomer-rich clusters which further react with the monomers in the solution to form the solid polymer (PEDOT) deposit. This mechanism of precipitation and solidification provides a reasonable explanation for the usual bumpy morphology of the electrochemically polymerized films. We observed that the deposition occurs in 2 regimes. The first regime corresponds to an increase in nucleation density (the nucleation regime) and the second (the growth regime) where the clusters start to merge and increase in size causing a decrease in the nucleation density. During the deposition, certain droplets were found to gradually decrease in size and mass thickness, indicating some degree of reversibility of the process which was not previously appreciated. In addition to this, we have found substantial variations when the chemistry of the monomer is changed, including a dramatic increase in the nucleation density when a more hydrophilic, carboxylic acid substituted EDOT was used as a comonomer. The total electron doses we used for these experiments were around 10-20 mC/cm2. These were well below the critical dose for electron damage of the PEDOT, which is about 0.1 C/cm2.

Further, using 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 the effective surface area. The basic understanding is that PAA prevents the lateral growth of PEDOT which results in the nano-fibrillar morphology. Using operando electron microscopy, we have observed the nucleation and growth of these fibrils in an oriented, dendritic manner at the solution-electrode interface, allowing us to better understand the nano-fibril formation mechanisms.

These experiments have presented us with unprecedented insights about the processes occurring at the nanoscale during the early stage nucleation and growth of electrodeposition of PEDOT and PEDOT copolymers.

Figure 1