Researchers have discovered how to make photopolymerization much more efficient

Polymers are materials made up of long chains of repeating molecules, and it is the interaction between these chains that determines most of the physicochemical properties of a polymer. In line with this intuitive understanding of polymers, which dates back to the 1930s, external forces acting on polymers are primarily considered destructive. For example, stretching a polymer can unravel or break some of its constituent chains, weakening the material as a whole.

Over the past decades, scientists have repeatedly demonstrated that external forces can have constructive effects on polymers. Mechanical forces and flow fields, if appropriately exploited, have been shown to confer new functionalities to some polymers by changing their phase, optical properties, and crystallinity. Despite substantial progress in this field, known as mechanochemistry, little is known about how external forces can affect the growth behavior of polymers under external forces.

A research team led by Professor Atsushi Shishido of the Tokyo Institute of Technology in Japan set out to shed light on this topic. In a study published in Macromolecules On July 29, 2024, they investigated how flow fields induced by dynamic UV illumination can influence the photopolymerization process, presenting an interesting and versatile technique to control polymer synthesis.

The photopolymerization reaction in question involved M6BACP as the monomer (the building block of the final polymer) and Irgacure 651, which is a photoinitiator. The latter compound absorbs UV light and breaks down into reactive free radicals, which interact with the monomers and cause them to bond together. But unlike conventional photopolymerization processes, in which the entire solution is uniformly irradiated with UV light, the researchers passed the UV light through a slowly moving slit.

Interestingly, this simple strategy had a profound effect on the resulting polymers, which the researchers demonstrated through various comparative experiments. “Photopolymerization with scanning UV light showed high-molecular-weight polymers, with a 90% reduction in the required exposure dose compared to photopolymerization with static uniform light,” Shishido points out.

The researchers hypothesize that UV light induces molecular fluxes that have two notable effects. First, it causes a slight diffusion of the growing polymers toward the area not yet irradiated, because their concentration there is lower. This allows them to continue growing as radicals become available.

On the other hand, radicals and monomers also diffuse, the former being carried to the unirradiated area and the latter to the irradiated area. Due to this mutual diffusion, the concentration of radicals in monomers tends to decrease in the irradiated areas, which minimizes the probability of termination reactions and limits the growth time of a polymer chain.

Overall, this study not only provides important insights into photopolymerization reactions, but also presents a practical way to improve existing industrial processes and polymer materials.

“The developed method significantly improves polymerization efficiency through a simple procedure of adding motion to the irradiation light, without changing existing compounds or reaction systems. This can reduce the energy cost of photopolymerization, which is used in various industrial applications, and is expected to be applied to manufacturing processes and fundamental technologies for polymer synthesis,” Shishido explains.

It is worth noting that the observed benefits were found not only for M6BACP, but also for various base polymers, such as acrylates. These results could pave the way for more sustainable methods to make even better polymers.