Harvard researchers develop ‘both stiff and tough’ elastic polymers

Boston, Massachusetts – Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed an elastomer that is both stiff and tough, and can resist tearing under repeated stretching. 

The development involved studying the physical, rather than the chemical bonds that link the polymer chains, explained senior author of the study Zhigang Suo in a paper published in Science magazine 7 Oct.

Polymer chains are made by linking together monomer building blocks. To make a material elastic, the polymer chains are crosslinked by covalent bonds. The more crosslinks, the shorter the polymer chains and the stiffer the material. 

However, instead of crosslinks, the team in this study looked into the physical bonds within the polymer - called entanglements - which up until today were thought to only impact the stiffness rather than the toughness of polymers. 

The researchers found that with enough entanglements, a polymer could become tough without compromising stiffness. 

To create highly entangled polymers, the researchers used a concentrated monomer precursor solution with 10 times less water than other polymer recipes. 

“By crowding all the monomers into this solution with less water and then polymerising it, we forced them to be entangled, like tangled strings of yarn,” said Guogao Zhang, a postdoctoral fellow at SEAS and co-first author the paper. 

“Just like with knitted fabrics, the polymers maintain their connection with one another by being physically intertwined.”

According to the team, with hundreds of these entanglements, just a handful of chemical crosslinks are required to keep the polymer stable. 

“As elastomers, these polymers have high toughness, strength, and fatigue resistance,” said Meixuanzi Shi, a visiting scholar at SEAS and co-author of the paper. 

"When the polymers are submerged in water to become hydrogels, they have low friction, and high wear resistance.”

Features such as high fatigue resistance and high wear resistance increase the durability and lifespan of the polymers, the scientists claim.

The team expects the new understanding of polymer structure to expand opportunities for applications, offering ‘exceptional mechanical properties’ for more sustainable, long-lasting polymer materials.