Osaka, Japan - Biodegradable plastics should be tough enough for use, yet able to break down at an appropriate rate afterward. Researchers at the University of Osaka showed that changing the size of rings threaded onto a biodegradable polymer can tune both its toughness and enzymatic degradation, offering a way to design material lifetimes. This study was accepted for publication in ACS Sustainable Chemistry & Engineering.
Polycaprolactone (PCL) is a biodegradable polyester with potential applications in sustainable materials. However, conventional methods used to improve its mechanical performance, such as blending and copolymerization, can alter the molecular structure and chain packing that govern degradation. It has therefore remained difficult to combine practical toughness with controllable end-of-life behavior. To address this challenge, the researchers investigated whether movable molecular crosslinks could reinforce PCL while allowing its degradation rate to be tuned through molecular design.
The team focused on cyclic poly (phenylene sulfide), c[n]PS, an industrial by-product of poly (phenylene sulfide) (PPS) production. They isolated three ring sizes—c [5] PS, c [7] PS, and c [9] PS - and incorporated them into PCL through solvent-free ring-opening polymerization. As PCL chains formed through the rings, pseudorotaxane-based "movable crosslinks" were created. These rings can slide along polymer chains, helping redistribute stress and dissipate energy.
At 0.5 wt%, c [7] PS nearly doubled PCL toughness while preserving its Young's modulus and thermoplastic reprocessability. Ring size also strongly affected enzymatic degradation: c [5] PS modestly accelerated degradation, c [7] PS slowed it, and c [9] PS degraded fastest, with no residual film mass after 48 hours. Control samples without movable crosslinks did not show the same ring-size dependence. The results are consistent with a mechanism in which ring size changes polymer-chain mobility and the accessibility of amorphous regions to enzymatic attack.
This approach could help create biodegradable plastics that remain durable during use but degrade at a rate suited to their intended end-of-life pathway. It also upcycles a PPS manufacturing by-product into a supramolecular material, supporting circular use of polymer resources.
Senior author Professor Yoshinori Takashima noted that making materials durable and making them break down quickly may seem like contradictory goals. He hopes to control both so polymer lifetimes can be tailored to different applications.
