Low-energy yellow light powering the polymerization process. Image: Anh Thy Bui/NC State.
Low-energy yellow light powering the polymerization process. Image: Anh Thy Bui/NC State.

A team of researchers from North Carolina (NC) State University has demonstrated a way to use low-energy, visible light to produce polymer gel objects from pure monomer solutions. This study, reported in a paper in Chem, not only offers a potential solution to current challenges in producing these materials, it also sheds further light on the ways in which low-energy photons can combine to produce high-energy excited states.

Polymer products – primarily plastics – are used for everything from water bottles to medical applications, with billions of pounds of these materials being produced annually. Select polymers can be produced via a process called free radical polymerization, in which a monomer solution is exposed to ultraviolet (UV) light. The high energy of UV light powers the polymerization reaction, forming the polymer. The advantages of this process over other forms of polymerization include fewer chemical waste by-products and less environmental impact.

However, this method is not without its drawbacks. The high-energy UV light used to generate these polymers can also degrade plastics and is unsuitable for producing certain materials.

Felix Castellano, professor in the Department of Chemistry at NC State, had previously shown that it was possible to combine the excited states of lower-energy molecules to achieve more potent excited states. In this study, Castellano and his team applied a process called homomolecular triplet-triplet annihilation to polymer production, by using lower-energy yellow or green light to create polymer gels.

The team dissolved zinc(II) meso-tetraphenylporphyrin (ZnTPP) into two different pure monomers – trimethylolpropane triacrylate (TMPTA) and methyl acrylate (MA) – then exposed the solution to yellow light. Energy from the light creates the homomolecular triplets in ZnTPP, and when those triplets combine, they create an extremely short-lived excited state, known as S2, that has enough energy to power the polymerization process.

"While triplets are really long lived in chemical terms – they live for milliseconds – the S2 excited state only lives for picoseconds, which is nine orders of magnitude less," Castellano says. "One of the important facets of this work is demonstrating that if you have a pure liquid you can utilize this potent, short-lived excited state to facilitate important transformations. The neat liquid ensures that electrons are transferred efficiently."

The team conducted spectroscopic analysis of the solution, establishing the existence of the S2 excited state in the presence of yellow and green light. "We used ZnTPP because it allows you to see light emission from two different excited states and we could differentiate between lower-energy S1 and higher-energy S2 states," Castellano says. "We know that polymer formation is a direct result of the S2 excited state, but we can also show that's what happening spectroscopically."

This story is adapted from material from North Carolina State University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.