Photo credit: www.sciencedaily.com

Innovative Advances in Supramolecular Polymers: A Study by Japanese Researchers

The behavior of polymers is influenced by the interactions between the folding of their chains and their tendency to aggregate. Groundbreaking research from a team in Japan has introduced folded supramolecular polymers capable of spontaneous interchain aggregation. By employing atomic force microscopy, the researchers were able to observe that the process of interchain aggregation is facilitated by the unfolding of the polymers’ main chains. Notably, they found that photoisomerization of an azobenzene unit triggers the unfolding, thereby accelerating the aggregation process.

In the realm of polymers, the balance between chain folding and aggregation shapes their mechanical, thermal, and electrical characteristics. A comprehensive understanding of this dynamic interaction could pave the way for the invention of polymer materials with specific, tailored properties.

This is also relevant for supramolecular polymers (SPs), which are non-covalent in nature and offer potential for new responsive materials. Typically, SPs maintain a one-dimensional linear shape that encourages interchain aggregation, with limited examples of SPs capable of forming diverse higher-order structures through chain folding. The advancement of SPs that can achieve both folding and aggregation would significantly enhance the development of novel materials engineered for specific functionalities.

A recent publication in the Journal of the American Chemical Society on July 25, 2024, presented a novel folded SP that can spontaneously aggregate into crystalline forms. Led by Professor Shiki Yagai of Chiba University—along with doctoral student Kenta Tamaki—the research explored the intricate relationship between chain unfolding and aggregation.

“We began with a monomer that polymerizes into a spiral form. By modifying the chemical structure of the monomer’s polymerization-driving unit, we discovered an unexpected phenomenon: the spiral would spontaneously unfold, causing different chains to bind together. Incorporating a photo-switchable element allowed this unfolding to occur at controlled light-triggered moments,” explains Prof. Yagai, sharing the inspiration for their study.

In their design, the research team utilized twistable biphenyl units combined with photoresponsive azobenzene units. These components self-assembled into the desired SPs, which began in a folded state and gradually reordered over a period of half a day, leading to crystalline aggregation. The inclusion of azobenzene units not only prompted unfolding due to photoinduction but also expedited the aggregation process by easing the stabilization of the folded structure.

The researchers noted that upon allowing the folded SP solution to remain at 20°C for several days, the polymers underwent a spontaneous transition, resulting in precipitation. When examined with AFM, they identified an interesting intermediate state characterized by the fusion of curved chains transitioning towards uniform straight fibrils. This observation drew parallels to biological systems, where misfolding in proteins leads to the creation of amyloid fibrils.

The study further elucidated the mechanisms underlying this structural transformation. It involved intrachain ordering prompted by conformational changes in the biphenyl units and interchain ordering stemming from the alignment of exterior aliphatic tails on the main chains. This crystallization behavior mirrors that observed in traditional covalent polymers. The team validated this mechanism through the photoisomerization of azobenzene; exposure to UV light induced a rapid unfolding of the main chain, significantly facilitating the ensuing interchain aggregation.

This research contributes new insights into the phenomena of folding and aggregation within polymer science. The mesoscale SPs, resulting from the collective self-assembly of numerous molecules, can serve as effective model systems for investigating molecular-level interactions between individual chains. This could lead to groundbreaking advancements in the materials science sector.

“Traditionally, such phenomena have been probed through spectroscopic or macroscopic techniques, which reflect the average behavior of the entire system. Creating observable mesoscale models is anticipated to greatly enhance material science research. We hope these findings will spur the development of meso-scale molecular assemblies with significant higher-order structures,” concluded Professor Yagai.

Source
www.sciencedaily.com