Method Makes Generic Polymers Luminescent

A Hokkaido University research team has developed a method to give luminescent properties to generic polymers, potentially transforming existing commodity polymers into more valuable functional materials. The technique is based on mechanical force, bypassing sophisticated organic synthetic methods for the preparation of luminescent polymers — which are used in applications such as organic lasers (lasing), solar cells, sensors, and bioimaging.

The preparation of luminescent polymers commonly requires multiple chemical synthesis steps, which are time and labor intensive, said Hajime Ito, an author of a study describing the method. Ito is vice director of the Institute for Chemical Reaction Design and Discovery (WPI-ICReDD) at Hokkaido University.

Ball-milling generic polymers with pre-fluorescent radical reactants yielded luminescent polymers. Courtesy of Koji Kubota et al.

“It is well known that mechanically stimulating polymers, for example by grinding or crushing them, generates reactive species called free radicals,” said Koji Kubota, an associate professor from Hokkaido University and a paper co-author. Kubota referenced the group’s previous research into mechanical-force-induced luminescence and reactions as an impetus for the current investigation into a simpler method of preparation for functional luminescent materials.

The team placed the polymer and pre-fluorescent radical reactants in a ball-milling jar that contained stainless steel balls. Members then shook the jar, causing the balls to grind the solid compounds and initiate a reaction. In the process, the covalent bonds in the polymer chains were cleaved and the pre-fluorescent molecules were inserted into the polymer, gaining significantly higher emission intensity.

Polystyrene and polyethylene were among the generic polymers to which the researchers applied the method. “With further development, the method could potentially be adapted to introduce other functions to generic polymers,” said Mingoo Jin, an assistant professor at Hokkaido University.

Ball-milling the mixture of polystyrene and pre-fluorescent radical reactants yielded luminescent polymers. The mixture is shown before (left) and after (right) the reaction, under UV light. Courtesy of Koji Kubota et al.

Ito said the group additionally hopes to use its method to develop novel sensing and recording materials that change color in response to mechanical stimuli. Such research would chart a course for “smart” materials that could be used for things such as bioimaging reagents and pressure-sensitive sensors.

The research was published in Angewantde Chemie (www.doi.org/10.1002/anie.202105381).