Due to their optoelectronic properties, conjugated polymers play a crucial role in flexible electronics, organic light-emitting technology, and organic photovoltaics. These specialty polymers, however, share the same degradation and recycling problems as commodity plastics. Scientists from Huazhong University of Science and Technology in China have recently created a conjugated polymer that completely degrades within a week when exposed to air and sunlight.
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The ever-increasing use of polymers has supported the rapid economic growth in numerous economic sectors over the last decades. Because of their reduced weight and easily tunable properties, polymer materials are extensively used in a diverse range of applications, such as the automotive industry, packaging, and consumer goods manufacturing.
The use of plastics in the modern economy is projected to double from its current level to a staggering 800 million tons per year by 2040. With landfills and incineration no longer regarded as viable options for plastics' end-of-life treatment in a circular economy, polymer recycling, material recovery and upcycling become of paramount importance to minimize the environmental impact of industrial plastics.
Self-Degradable Polymers Reduce Plastic Waste
The main problem for the environment is the durability and stability of modern polymer materials. In contrast to various biomaterials, such as cellulose and chitin, polymers cannot easily be broken down by microorganisms and cause significant environmental pollution. Large plastic fragments can be ingested by animals, while micro-and nano plastics, present in all our surroundings, harm human and ecosystem health.
While mechanical and chemical recycling, such as solvolysis (treating the polymer with solvents and reagents that cause depolymerization) and pyrolysis, are being utilized on an industrial scale, the possibilities of biochemical and photochemical polymer degradation and recycling remain relatively unexplored.
Developing strategies for the degradation and upcycling of postconsumer polymers in an environmentally friendly manner, without leaving behind fragments or harmful products, has become a focal point of numerous industrial and academic research groups in the last decade.
Unveiling the Potential of Conjugated Polymers
Scientists from Huazhong University of Science and Technology in Wuhan, China, led by Professors Liang Luo and Ben Zhong Tang, recently created a novel self-degradable conjugated polymer that decomposes rapidly when exposed to sunlight and air without any microplastic fragments left in the environment.
Conjugate polymers exhibit optical and electrical properties similar to those of inorganic semiconductors – good thermal stability, high electrical conductivity, and reversible color change when subjected to electrical current (electrochromic behavior).
A conjugated carbon chain consists of alternating single and double bonds, where the highly delocalized π-bonds (formed by the overlap of p-orbitals on adjacent carbon atoms) are responsible for the material's electrical and optical properties. The delocalized electrons are shared by all carbon atoms along the polymer backbone chain and become the charge carriers that enable conductivity. Typical conductive polymers include polyacetylene, polyaniline, polypyrrole, polythiophene, and others.
Combined with the attractive properties associated with conventional polymers, such as easy processability, low cost, and lightweight, the unique optical and electrical properties of the conjugated polymers lend them to a rapidly expanding range of applications, including:
- Electrochromic devices
- Organic light-emitting diodes (OLEDs)
- Photovoltaic cells
- Wearable electronics
Polymers Turn into Succinic Acid
Initially, Prof. Luo and his collaborators were developing a pH-sensitive color-changing conjugated polymer for chemical sensing applications. One of the compounds they created was a soluble polydiacetylene derivative with short carboxyl side chains called PDDA.
Polydiacetylene is one of the earliest discovered conjugated polymers with a conjugated backbone composed of double and triple carbon-carbon bonds. The material created by the Chinese scientist is a flexible plastic with a deep red color owing to its highly π-conjugated backbone chain. The researchers established that PDDA's unique optoelectronic features remained unchanged when the material was kept in the dark or under an inert atmosphere (nitrogen), thus validating the material's potential use in organic electronics applications.
Strikingly, when a piece of PDDA thin film was immersed into water, the polymer film disintegrated into small fragments rapidly when exposed to sunlight, and finally disappeared within one week. Similar degradation occurred when PDDA was immersed in a weak acidic solution and irradiated by artificial white light. The plastic film rapidly broke apart, and the material's natural red color faded, indicating depolymerization of the conjugated backbone chain.
To track the changes in PDDA's molecular structure in real-time, the researchers performed a comprehensive set of spectroscopic and nuclear magnetic resonance (NMR) measurements at different stages of the degradation process. The results indicated a rapid decrease in the molecular weight of the degrading plastic (polymer chains broken into smaller fragments), while the majority of the final degradation products (more than 60%) was succinic acid, with no trace of environmentally harmful microplastics left in the solution. Succinic acid is a naturally occurring compound that could be commercially upcycled in the pharmaceutical and food industries.
Nearly-Ideal Plastic Self-Degradation by Sunlight and Oxygen
When analyzed the experimental data further, Prof. Luo's team concluded that the cause for the PDDA degradation is a photo-oxidative process, where absorbed sunlight breaks the polymer’s double- and triple-bonds along the backbone chain, unlike other self-degrading polymers that typically rely on hydrolysis of ester or amide bonds.
Prof. Luo emphasized that only 20% of the carbon atoms were lost during the polymer degradation, promising an excellent economy of the upcycling process.
Importantly, the conjugated π-electron backbone chains seem to facilitate the photochemical production of reactive oxygen species without the need for photo-sensitizing dyes or other harmful additives to the polymer.
The researchers hope that they could establish similar photo-oxidative degradation processes in other widely used conjugated polymers. They aim to develop a universal strategy for degrading postconsumer conjugated polymers at ambient conditions in a natural environment.
References and Further Reading
S. Tian, et al. (2021) Complete Degradation of a Conjugated Polymer into Green Upcycling Products by Sunlight in Air. J. Am. Chem. Soc. 143, 27. https://doi.org/10.1021/jacs.1c04611
R. Jefferson (2021) New Eco-Friendly Plastic Degrades on Sunlight and Oxygen in Just One Week Developed by Experts. [Online] www.sciencetimes.com Available at: https://www.sciencetimes.com/articles/32343/20210719/new-eco-friendly-plastic-degrades-sunlight-oxygen-one-week-developed.htm (Accessed on 8 August 2021).
A. McDermott (2021) Degradable plastic polymer breaks down in sunlight and air. [Online] www.blog.pnas.org Available at: https://blog.pnas.org/2021/07/degradable-plastic-polymer-breaks-down-in-sunlight-and-air (Accessed on 8 August 2021).
Wei, R., et al. (2020) Possibilities and limitations of biotechnological plastic degradation and recycling. Nat. Catal. 3, 867–871. Available at: https://doi.org/10.1038/s41929-020-00521-w
Lebreton, L., Andrady, A. (2019) Future scenarios of global plastic waste generation and disposal. Palgrave Commun. 5, 6. Available at: https://doi.org/10.1057/s41599-018-0212-7