Novel Supramolecular Plastic with Controlled Degradation in Marine Environments

Researchers at the RIKEN Center for Emergent Matter Science have developed a new type of plastic that is durable yet environmentally friendly. Unlike traditional plastics, this biodegradable material breaks down in seawater, preventing ocean pollution. The research was published in the journal Science.

The new plastic is expected to help reduce harmful microplastic pollution, which accumulates in soil and oceans and eventually makes its way into the food chain.

In an effort to replace conventional plastics, which are environmentally harmful and non-sustainable, scientists have been working on creating safe and sustainable alternatives. While some recyclable and biodegradable plastics exist, significant challenges remain.

PLA and other biodegradable polymers are water-insoluble, often ending up in the ocean where they cannot break down. As a result, microplastics—plastic particles smaller than 5 mm—damage aquatic life and enter the food chain, including human bodies.

Aida and his colleagues' recent work focused on using supramolecular plastic polymers, where structures are held together by reversible interactions, to address this problem. Two ionic monomers were combined to create cross-linked salt bridges, which provide the new polymers with strength and flexibility.

In initial tests, one of the monomers was sodium hexametaphosphate, a common food ingredient, while the other was based on guanidinium ions. Both monomers can be metabolized by bacteria, ensuring the material’s biodegradability once the plastic breaks down into its constituent parts.

While the reversible nature of the bonds in supramolecular plastics have been thought to make them weak and unstable, our new materials are just the opposite.

Takuzo Aida, Center for Emergent Matter Science, RIKEN

Unless exposed to electrolytes like those found in seawater, the new material's salt bridge structure remains irreversible. The key discovery was the development of a method to create these selectively irreversible cross-links.

When the two monomers were combined in water, the researchers observed two distinct layers, similar to oil and water. One layer was liquid and contained salt ions, while the other was thick and viscous and contained the structural cross-linked salt bridges.

For example, when sodium hexametaphosphate and alkyl diguanidinium sulfate were used, sodium sulfate salt was released into the watery layer. The remaining thick, viscous liquid layer was then dried to create the final plastic, alkyl SP₂.

Without the critical "desalting" step, the dried substance became a brittle crystal and was unusable. After being re-salted by submerging it in saltwater, the plastic's structure became unstable, and the material began to break down within a few hours. The researchers then assessed the plastic's quality after producing a strong, durable material that could still degrade under specific conditions.

Like existing thermoplastics, the new plastics can be reshaped at temperatures above 120 °C and are non-toxic and non-flammable, meaning they emit no CO₂.

The researchers produced plastics with a range of tensile strengths and hardnesses by experimenting with various guanidinium sulfates, all of which performed similarly to or better than traditional plastics.

This suggests that the new plastic can be tailored for specific applications, allowing for the creation of flexible plastics with low tensile strength, rubber-like materials, hard, scratch-resistant plastics, and strong, weight-bearing polymers.

Additionally, the team used polysaccharides that form cross-linked salt bridges with guanidinium monomers to create ocean-degradable plastics. These materials could be used in medical or health-related applications, as well as for 3D printing.

Finally, the researchers examined the new plastic's biodegradability and recyclability. After dissolving the plastic in saltwater, they successfully recovered 91 % of the hexametaphosphate and 82% of the guanidinium as powders, indicating that recycling is both simple and effective. Over ten days, sheets of the new plastic broke down in soil, releasing phosphorus and nitrogen, similar to the effects of fertilizer.

With this new material, we have created a new family of plastics that are strong, stable, recyclable, can serve multiple functions, and, importantly, do not generate microplastics.

Takuzo Aida, Center for Emergent Matter Science, RIKEN

Journal Reference:

Cheng, Y., et al. (2024) Mechanically strong yet metabolizable supramolecular plastics by desalting upon phase separation. Science. doi.org/10.1126/science.ado1782.