Precise polymer 'knots' uncover hidden slack for designing ultra-tough and responsive smart materials

Precise polymer 'knots' uncover hidden slack for designing ultra-tough and responsive smart materials Sadie Harley Scientific Editor Robert Egan Associate Editor From household plastic packaging to the flexible frameworks that support wearable electronics, polymer materials form the invisible backbone of modern life. At a microscopic level, polymers consist of long, ribbon-like molecular chains that are entangled into a disorganized mass resembling a bowl of cooked noodles. For decades, these unpredictable molecular twists and knots have made it difficult for scientists to control, map, or customize the behavior of the final material. A research team led by Professor Yufeng Wang and Professor Ho Yu Au-Yeung from the Department of Chemistry at The University of Hong Kong (HKU) has achieved a breakthrough to address this challenge. By using discrete molecular rings as precise structural models of polymer knots, the team untangled the complex relationship between molecular architecture and material properties, allowing them to correlate characteristics such as stiffness, strength, and elasticity with the specific structures and topologies of the molecular rings. Their findings were published in the Journal of the American Chemical Society. Tuning materials with a 'metal switch' At the heart of this research is the discovery of the "hidden length" of the rings, a form of molecular slack within the material's structure that releases when pulled under force. Much like a seatbelt catching to absorb an impact or a spring snapping back into place, different molecular architectures respond to mechanical stress in very different ways, thereby altering how the final material behaves. By replacing the unstructured tangles in conventional polymers with molecular rings of precise structures, the researchers were able to observe how different architectures store and release energy. Simple macrocyclic rings, for instance, are highly flexible and harbor significant hidden length; when the material is subjected to stress, this internal slack unfurls to absorb the impact, resulting in exceptional toughness and durability. In contrast, mechanically interlocked rings, known as catenanes, adopt a much more constrained and compact configuration. The team found that because these interlocked rings have less "slack" to unfurl, they behave like rapid-response springs. This creates a material with high elasticity, allowing it to snap back efficiently to its original shape after being stretched. The team took the research a step further by demonstrating that these materials can be tuned on demand. By introducing copper ions to the molecular rings, the internal slack can be effectively locked in place to increase rigidity. This ability to manipulate structural rigidity enables a material's properties—such as stiffness and elasticity—to be dynamically altered in a controllable, responsive manner. Paving the way for soft robotics and tissue engineering This discovery provides a blueprint for creating a new generation of "smart" materials with highly specialized functions. By identifying these distinct mechanical pathways, the HKU team has provided a new framework for guiding the design of new materials with specific properties. Professor Ho Yu Au-Yeung from the HKU Department of Chemistry said the research helps scientists gain a deeper understanding of how entanglements at the molecular level influence material properties, opening up new possibilities for designing materials with specialized functions for different applications. "By choosing the right molecular 'knots' and controlling their 'hidden length,' we are now able to design materials with specific functions tailored to different needs." Professor Yufeng Wang from the HKU Department of Chemistry added that the findings could have important implications for fields such as soft robotics, tissue engineering and wearable electronics. "For example, soft robots require materials that are both flexible and strong; tissue engineering materials need to mimic the complex and dynamic movements of human muscles; while wearable electronic devices require both high durability and elasticity. This research provides scientists with a new framework for designing smart materials with specialized functions for different applications." Publication details Tianjing Luo et al, Role of Molecular Topology Elucidated in Unified Gels, Journal of the American Chemical Society (2026). DOI: 10.1021/jacs.6c01062 Journal information: Journal of the American Chemical Society Provided by The University of Hong Kong