1-millimeter-thick rubber that becomes as hard as an acrylic plate after absorbing carbon dioxide. (courtesy of Yohei Miwa, Professor at Gifu University)
One of chemistry's enduring appeals is its capacity to create entirely new materials with distinct functions. A research team at Gifu University and others has developed a rubber that turns into plastic when it absorbs carbon dioxide (CO2), a greenhouse gas.
The innovative approach was featured in the British scientific journal Nature Communications. It offers a new avenue not only for capturing and storing CO2 as part of efforts to address global warming, but also for putting the gas to productive use.
A CO2-Absorbing Rubber
The newly developed material is known as a "CO2-curable elastomer." An elastomer is a polymer that can be easily stretched and compressed while retaining its elasticity — in other words, rubber.
This rubber naturally absorbs CO2 from its surroundings. When formed into a one-millimeter-thick sheet and placed in a bag filled with CO2, it absorbed about 220 milligrams of CO2 per gram of its own weight within a few days.
After absorbing CO2, the material becomes more than 1,000 times harder, effectively transforming into plastic. Its rigidity is comparable to that of an acrylic sheet.
As rubber, its surface offers enough friction to function as a slip-proof material. Once converted into plastic, however, it becomes smooth.
A Reversible Transformation
During the experiment, when ultraviolet light was shone on the plastic after it had absorbed CO2, its blue fluorescence intensified.
When heated to 70–80 degrees Celsius (158–176°F) in its plastic state, the material began releasing CO2. At 100 degrees, CO2 escaped almost instantly, and the plastic reverted to rubber.
But even without heating, the gas gradually dissipated over time, and the plastic eventually returned to its original form.
The team is now developing a version that can retain absorbed CO2 and maintain its plastic state, and has already drawn up a production plan.
Chemistry Behind the Breakthrough
Chemically, the rubber was created by bonding polyethyleneimine (PEI) with polydimethylsiloxane (PDMS). On its own, PEI reacts with CO2 but absorbs only about 1 milligram per gram.
When PDMS was chemically bonded to PEI, the absorption capacity rose sharply to about 220 milligrams per gram. Because PDMS readily allows CO2 to pass through, combining it with PEI enables the gas to penetrate beyond the surface and reach internal PEI through PDMS "passageways." This significantly boosts overall absorption.
Still, challenges remain. Chemically bonding PEI and PDMS requires the use of chloroform, a toxic solvent. PEI is highly water-soluble, while PDMS does not dissolve in water, much like oil and water.
Linking them at the molecular level, therefore, demands a solvent capable of dissolving both. For now, chloroform is the only known option.
Once used as an anesthetic, chloroform is now generally avoided in manufacturing. The research team is currently searching for safer alternatives.
From Curiosity to Discovery
According to Yohei Miwa, a professor at Gifu University, the latest research began when a young scientist observed a phenomenon of CO2 absorption.
The team then set out to identify conditions that would enhance absorption, ultimately arriving at the combination of PEI and PDMS.
Nature Communications, the journal that accepted their paper, is said to have recognized the increased CO2 uptake achieved through this novel pairing.
In recent years, the push for "carbon neutrality" — reducing net greenhouse gas emissions to zero — has spurred a range of proposals for capturing and storing CO2. Parallel efforts have also explored using the gas as a raw material for polymers or converting it into fuels such as methanol and ethanol.
The latest achievement did not begin as application-driven research, but grew out of fundamental inquiry sparked by scientific curiosity. As the work advances, it will be worth watching how such basic science may evolve into practical use.
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Author: Shinji Ono, The Sankei Shimbun
(Read this article in Japanese)
