Synthesis and Coating of Electroactive Polymers: Wet vs. Dry Processing

Electroactive polymers have been of the subject of great interest in recent decades due to their ability to show metal-like conductivity while retaining polymer-like flexibility at the same time. They have found broad applications in microelectronics, medicine and energy sectors, particularly where biocompatibility is involved. This is due to their soft and flexible nature, making them suited to human interface compared to their inorganic counterparts. Since these polymers are often used in the form of a thin coating layer on an underlying substrate, widespread research has been done on their synthesis and processing. In this regard, two different techniques have been developed simultaneously with and without the use of solvent for their processing. Each of these techniques have strengths and challenges related to the type of polymer, polymer film formation, film properties, substrate choice and interfacial phenomena. This thesis aims to first provide an overview of electroactive polymers and their processing techniques namely wet and dry routes. The merits and demerits of each route is discussed in detail based on the state-of-the-art literature. Their scientific progress and technological relevance has been critically compared. This thesis investigated each method to study different aspects of these polymers. In particular, an electroactive unit (triphenylamine) was chosen as the building block for design and synthesis of different polymer architectures and how it affects their electrochemical energy storage performance. These polymers were investigated using wet processing, since their dry processing would be extremely challenging. In the second part of this thesis, the dry processing technique of oxidative chemical vapor deposition (oCVD) is comprehensively discussed from the early works which were less than two decades ago up to its current state-of-the-art. A unique contribution of this thesis is the synthesis of polypyrrole using oCVD with an in-depth molecular characterization to gain insight into its processing condition and thin film properties. The application for conductive coatings of polypyrrole was studied in the context of electrochemical energy storage and piezoresistive sensing devices using porous or delicate substrates

Oxidative chemical vapor deposition of polypyrrole onto carbon fabric for flexible supercapacitive electrode material

Polypyrrole has been a promising conjugated polymer for application in electrochemical energy storage devices. One primary feature is its pseudocapacitive behavior, which makes it suitable for hybridization with traditional carbon-based electrical double layer capacitive materials. The processing condition for such a hybridization is a critical aspect for the electrode performance in long term. Oxidative chemical vapor deposition was used to deposit polypyrrole onto 3D carbon fiber fabric. This allowed uniform and conformal deposition of polypyrrole on individual fibers as well as a control over its thickness depending on the reaction time. The obtained composite was characterized for electrochemical energy storage application using cyclic voltammetry and galvanostatic charge discharge measurements. Additionally, the stability of the polypyrrole-carbon fiber electrode was tested using microscopy and energy dispersive spectroscopy in order to obtain insights into physical and chemical degradation of polypyrrole during electrochemical aging. Results showed thickness-dependence of electrode stability, tuning of which in the correct voltage window is necessary for optimal long-term performance

All-dry, one-step synthesis, doping and film formation of conductive polypyrrole

Oxidative chemical vapor deposition (oCVD) is an extremely effective method for solvent-free deposition of highly conductive polypyrrole films, where polymer synthesis, doping, and film formation are combined in a single step. Here we show that by carefully tuning the reaction parameters, namely the deposition temperature, the reactor pressure and the oxidant to monomer flow rate ratio, homogeneous polypyrrole films with a record conductivity of 180 S cm-1 for a solvent-free method were produced. Fourier transform infrared spectroscopy, UV-vis spectrophotometry, X-ray photoelectron spectroscopy, scanning electron microscopy, and four-probe surface resistivity measurements were performed to gain insights into the relationship between different reaction conditions and the structure of oCVD-deposited polypyrrole, the development of defects, the film morphology and its physical properties

Electrically Conductive and Highly Stretchable Piezoresistive Polymer Nanocomposites via Oxidative Chemical Vapor Deposition

Electrically conductive polymer nanocomposites have been the subject of intense research due to their promising potential as piezoresistive biomedical sensors, leveraging their flexibility and biocompatibility. Although intrinsically conductive polymers such as polypyrrole (PPy) and polyaniline have emerged as lucrative candidates, they are extremely limited in their processability by conventional solution-based approaches. In this work, ultrathin nanostructured coatings of doped PPy are realized on polyurethane films of different architectures via oxidative chemical vapor deposition to develop stretchable and flexible resistance-based strain sensors. Holding the substrates perpendicular to the reactant flows facilitates diffusive transport and ensures excellent conformality of the interfacial integrated PPy coatings throughout the 3D porous electrospun fiber mats in a single step. This allows the mechanically robust (stretchability &gt; 400%, with fatigue resistance up to 1000 cycles) nanocomposites to elicit a reversible change of electrical resistance when subjected to consecutive cycles of stretching and releasing. The repeatable performance of the strain sensor is linear due to dimensional changes of the conductive network in the low-strain regime (ϵ ≤ 50%), while the evolution of nano-cracks leads to an exponential increase, which is observed in the high-strain regime, recording a gauge factor as high as 46 at 202% elongational strain. The stretchable conductive polymer nanocomposites also show biocompatibility toward human dermal fibroblasts, thus providing a promising path for use as piezoresistive strain sensors and finding applications in biomedical applications such as wearable, skin-mountable flexible electronics.</p

Oxidative chemical vapor deposition for synthesis and processing of conjugated polymers: a critical review

Oxidative chemical vapor deposition (oCVD) has developed progressively in the last two decades as a solvent-free (or dry) methodology for synthesis and thin film deposition of conjugated polymers. This method has offered new opportunities beyond traditional solution processing methods in the research of these materials. It is crucial to have a clear understanding of the differences between the solvent-free vs. solvent-based methodologies for synthesis and thin film deposition of conjugated polymers. Herein, the strengths and limitations of each procedure are compared in order to provide guidelines for future research and development. This review systematically approaches this comparison by first characterizing the thin films in terms of their chemical and physical properties. Then, the interfacial properties of a conjugated polymer thin film with the underlying substrate are critically compared when two different processing methods are exploited. Finally, the effect of the substrate on the coating properties and performance is reviewed

Polytriphenylamine composites for energy storage electrodes: Effect of pendant vs. backbone polymer architecture of the electroactive group

Polymers are an increasingly used class of materials in semiconductors, photovoltaics and energy storage. Polymers bearing triphenylamine (TPA) or its derivatives in their structures have shown promise for application in electrochemical energy storage devices. The aim of this work is to systematically synthesize polymers bearing TPA units either as pendant groups or directly along the backbone of the polymer and evaluate their performance as electrochemical energy storage electrode materials. The first was obtained via radical polymerization of an acrylate monomer bearing TPA as a side group, resulting in a non-conjugated polymer with individual redox active sites (rP). The latter was obtained by oxidative polymerization of a substituted TPA, resulting in a conjugated polymer with TPA units along its backbone (cP). These polymers were then developed into electrodes by separately blending them with multi-wall carbon nanotubes (rC and cC). The electrodes were characterized and their charge storage stability and mechanical properties were investigated for up to 1000 cycles by cyclic voltammetry, galvanostatic charge–discharge measurements and nanoindentation. The results show that cC offers a higher initial charge capacity than rC as well as improved carbon nanotube dispersion due to its conjugated structure. Although the improved dispersion results in a higher elastic modulus for cC (compared to rC), the stiffer nature of cP made it more vulnerable to degrade upon repetitive volumetric change, while with rP, the decoupled acrylate monomer remained more protected when its redox active units of TPA underwent charge–discharge cycling