Japanese Scientists Develop New Plastic That Dissolves In Saltwater

Plastics are known for their durability and strength, making them ideal for everyday use. However, their long-lasting nature becomes a significant environmental issue when they end up in nature. Traditional plastics, made with strong covalent bonds, are difficult to break down and can remain in the environment for decades or even centuries, causing harm to wildlife and humans through microplastics.

In an effort to tackle this problem, Researchers led by Takuzo Aida at the RIKEN Center for Emergent Matter Science (CEMS) in Japan, have developed a new type of plastic that maintains its strength during use but dissolves quickly in saltwater. Once dissolved, the plastic breaks down into harmless compounds, reducing the environmental impact.

In their new study, Aida and his team focused on solving this problem with supramolecular plastics-polymers with structures held together by reversible interactions. The new plastics were made by combining two ionic monomers that form cross-linked salt bridges, which provide strength and flexibility. In the initial tests, one of the monomers was a common food additive called sodium hexametaphosphate and the other was any of several guanidinium ion-based monomers. Both monomers can be metabolized by bacteria, ensuring biodegradability once the plastic is dissolved into its components.

"While the reversable nature of the bonds in supramolecular plastics have been thought to make them weak and unstable," says Aida, "our new materials are just the opposite." In the new material, the salt bridges structure is irreversible unless exposed to electrolytes like those found in seawater. The key discovery was how to create these selectively irreversible cross links.

As with oil with water, after mixing the two monomers together in water, the researchers observed two separated liquids. One was thick and viscous and contained the important structural cross linked salt bridges, while the other was watery and contained salt ions. For example, when sodium hexametaphosphate and alkyl diguanidinium sulfate were used, sodium sulphate salt was expelled into the watery layer. The final plastic, alkyl SP2, was made by drying what remained in the thick viscous liquid layer.

The "desalting" turned out to be the critical step; without it, the resulting dried material was a brittle crystal, unfit for use. Resalting the plastic by placing it in salt water caused the interactions to reverse and the plastic's structure destabilized in a matter of hours. Thus, having created a strong and durable plastic that can still be dissolved under certain conditions, the researchers next tested the plastic's quality. 

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