New research uses light-induced molecular flow to produce heavier polymer chains with less energy.
A team of chemists and materials scientists from the Tokyo Institute of Technology has demonstrated how flow fields induced by dynamic UV lighting can influence the process of photopolymerization, resulting in a novel and versatile technique to control polymer synthesis.
External forces acting on polymers have often been regarded as primarily destructive. For example, stretching a polymer can disentangle or break apart some of its constituent chains, weakening the overall material.
Over the past few decades, however, research has shown that external forces can have constructive effects on polymers. Mechanical forces and flow fields, if leveraged appropriately, can impart new functionalities in certain polymers by altering their phase, optical properties and crystallinity. Despite substantial progress in this field—known as mechanochemistry—not much is known about how external forces can affect the growth behavior of polymers under external forces.
That’s why the research team, led by professor Atsushi Shishido, began investigating the photopolymerization reaction between M6BACP (a monomer) and the photoinitiator Irgacure 651. Unlike typical photopolymerization processes, in which the entire solution is irradiated uniformly with UV light, the researchers shone UV light through a slowly moving slit instead.
According to the researchers, this simple change had a profound effect on the resulting polymers, which they demonstrated through various comparative experiments. “Photopolymerization with scanning UV light exhibited high molecular weight polymers, with a reduction of 90% of the required exposure dose compared to photopolymerization with static uniform light,” said Shishido in a press release.
The researchers theorize that UV light induces molecular flows with two notable effects. First, it causes growing polymers to diffuse slightly towards the yet-unirradiated area, as their concentration there is lower. This allows them to keep growing as radicals become available. Second, the radicals and monomers also diffuse, with the former being supplied to the unirradiated area and the latter to the irradiated area. As a result of this mutual diffusion, the radical-to-monomer concentration tends to become lower in irradiated areas, which minimizes the probability of termination reactions from taking place, putting a cap on how long a polymer chain can grow.
Overall, this study not only provides important insights into photopolymerization reactions, but also showcases a convenient way of improving existing industrial processes and polymeric materials.
“The developed method significantly improves polymerization efficiency with a simple procedure of adding movement to the irradiation light, without changing existing compounds or reaction systems,” Shishido explained. “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 foundational technologies for polymer synthesis,”
It’s also worth noting that the observed advantages were found not only for M6BACP, but also for various commodity polymers, such as acrylates.
The research is published in the journal Macromolecules.
