Transforming Styrofoam into high-value polymers for electronics

This transformative chemical reaction is detailed in a recent publication in JACS Au, showcasing how upgraded plastic waste can be effectively incorporated into functional electronic devices, including silicon-based hybrid solar cells and organic electrochemical transistors.

The research team, led by Laure Kayser, an assistant professor in UD’s Department of Materials Science and Engineering with a joint appointment in the Department of Chemistry and Biochemistry, frequently works with PEDOT due to its dual electronic and ionic conductivity. Their quest to synthesise this material from plastic waste took a significant step forward after connecting with Argonne chemist David Kaphan during a UD-hosted event.

The teams hypothesised that PEDOT could be produced by sulfonating polystyrene, a synthetic plastic commonly found in disposable containers and packing materials. Sulfonation, a process where a hydrogen atom is replaced by sulfonic acid, is typically used to create various products such as dyes, drugs, and ion exchange resins. This reaction can be ‘hard’, requiring caustic reagents for high efficiency, or ‘soft’, using milder materials but with lower efficiency.

Kayser’s team sought a middle ground: "A reagent that is efficient enough to get really high degrees of functionalisation but that doesn't mess up your polymer chain.”

The researchers initially turned to a method from a previous study that used 1,3-Disulfonic acid imidazolium chloride ([Dsim]Cl) for sulfonating small molecules. However, applying this method to polymers posed additional challenges due to the complexity of separating unwanted byproducts and the sensitivity of the polymer chain to errors.

Through extensive trial and error, the team optimised the reaction conditions to achieve high degrees of sulfonation with minimal defects, using a mild sulfonating agent. This process allowed them to efficiently convert waste polystyrene, specifically Styrofoam, into PEDOT.

Once synthesised, the waste-derived PEDOT was tested against commercially available PEDOT. Chun-Yuan Lo, a chemistry doctoral candidate and the study’s first author, remarked: “In this paper, we looked at two devices – an organic electronic transistor and a solar cell. The performance of both types of conductive polymers was comparable and shows that our method is a very eco-friendly approach for converting polystyrene waste into high-value electronic materials.”

Detailed analyses were conducted using X-ray photoelectron spectroscopy (XPS) at UD's surface analysis facility, film thickness analysis at the UD Nanofabrication Facility, and solar cell evaluation at the Institute of Energy Conversion. Argonne's advanced spectroscopy equipment, such as carbon NMR, was utilised for detailed polymer characterisation. Additional support came from materials science and engineering professor Robert Opila and David C. Martin, the Karl W. and Renate Böer Chaired Professor of Materials Science and Engineering.

A notable finding in the study was the ability to use stoichiometric ratios during the sulfonation reaction, which typically requires an excess of harsh reagents. Kelsey Koutsoukos, a materials science doctoral candidate and second author, explained: “Typically, for sulfonation of polystyrene, you have to use an excess of really harsh reagents. Here, being able to use a stoichiometric ratio means that we can minimise the amount of waste being generated.”

This efficiency opens avenues for further research into ‘fine-tuning’ the degree of sulfonation to impact the electrical properties of PEDOT and other applications like fuel cells or water filtration devices.

“For the electronic devices community, the key takeaway is that you can make electronic materials from trash, and they perform just as well as what you would purchase commercially,” Kayser said. “For the more traditional polymer scientists, the fact that you can very efficiently and precisely control the degree of sulfonation is going to be of interest to a lot of different communities and applications.”

The research underscores significant potential for contributing to global sustainability efforts by providing a new method to convert waste products into valuable materials. Lo concluded: “Many scientists and researchers are working hard on upcycling and recycling efforts, either by chemical or mechanical means, and our study provides another example of how we can address this challenge.”