Natural rubber, one of the world's most widely manufactured biomaterials, has been used by humans for thousands of years. Its resistance to crack growth, however, hasn't improved much for decades.
Until now. Materials researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have devised a way to produce natural rubber that retains its key properties of stretchiness and durability while greatly improving its ability to resist cracking, even after repeated cycles of use.
The work is published in Nature Sustainability and is led by Zhigang Suo, the Allen E. and Marilyn M. Puckett Professor of Mechanics and Materials at SEAS.
“Improving crack resistance will extend the material’s service lifetime and therefore improve its sustainability,” said first author and former SEAS postdoctoral researcher Guodong Nian.
Natural rubber is a durable polymer material that’s in too many products to count: gloves, tires, shoes, medical devices, conveyor belts.
Derived from natural rubber latex, a milk-like substance from the Hevea tree, rubber is harvested, coagulated, dried, mixed with additives, shaped, and heated to trigger vulcanization. This process creates short polymer chains within the material that are densely crosslinked, or chemically bonded.
The researchers modified this longstanding, high-intensity process to induce a gentler transformation that retains long polymer chains in their natural state, rather than cutting them into shorter chains. Resembling tangled spaghetti, their so-called rubber “tanglemer” endows the new product with heightened durability by outnumbering crosslinks with entanglements.
“We used a low-intensity processing method, based on latex processing methods, that preserved the long polymer chains,” Nian said.
When a crack forms in the new material, the long spaghetti strands spread out the stress by sliding past each other, allowing more rubber to crystallize as it stretches, and overall making the material stronger and more resistant to cracking.
Their results blew away their expectations, said paper co-first author Zheqi Chen, a former SEAS postdoctoral researcher. The rubber became four times better at resisting slow crack growth during repeated stretching. It became 10 times tougher overall.
“We imagined that the properties would be enhanced maybe twice or three times, but actually they were enhanced by one order of magnitude,” Chen said.
The work highlights the benefit of preserving the natural state of long polymer chains. But issues remain: the new material processing involves a large amount of water evaporation, yielding a smaller volume of material than would be desirable for products like tires. It is currently more suited to thin rubber products like gloves or condoms. Other possibilities the new process opens up are flexible electronics or soft robotics parts.
The paper was co-authored by Xianyang Bao (co-first), Matthew Wei Ming Tan, and Yakov Kutsovsky. The research had federal support from the National Science Foundation under the Materials Research Science and Engineering Centers (DMR-2011754) and the Air Force Office of Scientific Research under award No. GA9550-20-1-0397.
More information: https://www.youtube.com/watch?v=UvGIVUFnsjg