An international joint research team led by Group Director Takuzo Aida (Distinguished Professor at the University of Tokyo), Student Trainee (at the time of the study) Yiren Cheng, Research Part-time Worker II Eiji Hirano, and Research Scientist (at the time of the study) Hubiao Huang of the Emergent Soft Matter Function Research Group at the RIKEN Center for Emergent Matter Science have developed colorless, transparent, ultrahigh density, glassy supramolecular plastics that are very tough, yet break down to raw materials metabolizable by living organisms through simple immersion in seawater. The work was published in Science.
Aida said, "Our plastics have almost no effects on the ecosystem because they break down into sodium hexametaphosphate, which is used as a food additive and for other purposes, and guanidinium sulfate, which is biochemically metabolizable, by simple immersion in salt water. This approach is expected to find application in various fields. I think we were able to demonstrate a significant subset of how plastics should be."
Supramolecular polymers can be easily converted back to the starting monomers because their bonds are reversible. However, supramolecular polymers have long been believed to be usable only as soft materials, such as rubber, and unsuitable as alternative materials to meet the demand for plastics.
Aida's research was based on a new concept of suppressing the reversibility of bonding by utilizing the two raw material ionic monomers that have a feature of undergoing phase separation as soon as they are cross-linked. The international joint research team mixed two biochemically metabolizable ionic monomers in water at room temperature. The mixture of the two raw materials undergoes phase separation into upper and lower phases while the raw materials adhere to each other through electrostatic interactions strengthened by hydrogen bonding (salt bridging) to form a cross-linked structure.
The upper phase (aqueous phase) incorporates the inorganic counterions of the monomers in dense water (desalting), while a cross-linked structure attached by electrostatic interactions (salt bridging) is formed in the lower phase, which transforms into the condensed phase. This phase separation stabilizes the cross-linked structure and prevents the cross-linked structure from being dissociated from the raw materials unless salt is added externally. They found that a colorless, transparent, ultrahigh density, glassy supramolecular plastic was obtained almost quantitatively after separating and drying the condensed phase.
"Salt bridges are very strong bonds because these cross-links are mediated by both ionic and hydrogen bonds, but they break down easily when salt comes in contact with them. The yield is 95%, which means that most of the raw materials used turn into plastics." (Aida)
Various physical properties of supramolecular plastics can also be modulated by designing the structure of the guanidinium sulfate monomer. In this study, a supramolecular plastic with an upper-temperature limit of 315℃, a supramolecular plastic with a hardness indicated by Young's modulus of 18 gigapascals, and a supramolecular plastic with a tensile strength of 36 megapascals were produced using guanidinium sulfate monomers differing in structure. All of the supramolecular plastics developed in this study were solid yet easily moldable into even complex shapes by heating and confirmed to have physical properties compared favorably with those of existing plastics.
Meanwhile, when placed in salt water, they are rapidly dissociated from the raw material monomers and are biochemically metabolizable by bacteria and other organisms. Thus, these plastics are not harmful to the environment. Sodium hexametaphosphate, one of the starting monomers, is widely used as food additives and for agricultural applications and costs only 20,000 yen per ton. Guanidinium sulfate, the other raw material monomer, can be partially synthesized from naturally occurring amines.
"Phosphorus and nitrogen contained in these monomers are important as fertilizers, and phosphorus is in short supply in the oceans, except adjacent waters. Thus, I don't see a problem if they flow into the ocean." (Aida)
Furthermore, these plastics can be synthesized in a single step and broken down into raw materials by simply placing them in salt water. Thus, they are very easy to not only produce but also recycle. Moreover, they can be used in the same way as ordinary plastics if the surface is coated with a hydrophobic coating.
The international joint research team also developed a polysaccharide-based supramolecular plastic using sodium chondroitin sulfate, a naturally occurring polysaccharide, in place of sodium hexametaphosphate, expanding the potential of this supramolecular polymerization approach. The polysaccharide-based supramolecular plastic has a better tensile strength of 9 megapascals and is applicable for 3D printing by adjusting the density.
Journal Information
Publication: Science
Title: Mechanically strong yet metabolizable supramolecular plastics by desalting upon phase separation
DOI: 10.1126/science.ado1782
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.