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Polymer science has made rubber tires, PTFE and Kevlar fibers, plastic water bottles, and nylon jackets many other ubiquitous features in daily life. Elastic polymers, called elastomers, can be stretched and released repeatedly and are used in applications such as gloves and heart valves that need to be used for a long time without tearing. But there is a problem that has plagued polymer scientists for a long time: Elastic polymers can be very hard or very hard, but not both. 

This stiffness-toughness conflict is a challenge for scientists developing polymers that can be used in applications such as tissue regeneration, bioadhesives, bioprinting, wearable electronics, and soft robotics. 

In a paper published today in the journal Science, researchers at Harvard University’s John A. Paulson School of Engineering and Applied Sciences (SEAS) resolved this long-standing conflict and developed a Hard and tough elastomer. 

Why are some polymers tough while others are brittle? How do we make the polymer tear-resistant under repeated stretching?

"In addition to developing polymers for emerging applications, scientists also face an urgent challenge: plastic pollution," said Allen E. and Marilyn M. Puckett Professor of Mechanics and Materials Suo Zhigang, the senior author of the study. "The development of biodegradable polymers once again brings us back to the basic question-why are some polymers hard and others brittle? How do we make polymers tear-resistant under repeated stretching?" 

Polymer chains are made by connecting monomer building blocks together. In order to make the material elastic, the polymer chains are cross-linked by covalent bonds. The more crosslinks, the shorter the polymer chain and the harder the material. 

"As your polymer chains become shorter, the energy you can store in the material becomes less and less, and the material becomes fragile," said Junsoo Kim, a graduate student at SEAS and co-first author of the paper. "If you only have a few crosslinks, the chain will be longer and the material is tough, but it is too soft to be useful."

In order to develop a polymer that is both hard and tough, the researchers focused on using physical bonds rather than chemical bonds to connect polymer chains. These physical bonds, called entanglements, have existed for almost the same time as polymer science and have been known in the field, but they are believed to only affect stiffness, not toughness. 

But the SEAS research team found that if there are enough entanglements, the polymer can become tough without affecting its stiffness. To make highly entangled polymers, the researchers used a concentrated monomer precursor solution with a water content 10 times less than other polymer formulations. 

Zhang Guogao, a postdoctoral researcher at SEAS and the co-first author of the paper, said: “By squeezing all the monomers into the solution with less water and then polymerizing them, we force them to become entangled, like tangled yarns. The same." "Like knitted fabrics, polymers keep in touch with each other through physical interweaving."

Each polymer chain has a large number of entanglements along its length (left), with crosslinks at each end. The stretched polymer (middle) shows the transfer of tension to the other chains. (Source: Suo Lab/Harvard SEAS)

Highly tangled hydrogel (left) and ordinary hydrogel (right). (Source: Suo Lab/Harvard SEAS)

There are hundreds of such entanglements, and only a small amount of chemical crosslinking is needed to keep the polymer stable. 

"As elastomers, these polymers have high toughness, strength and fatigue resistance," said Shi Meixuanzi, a SEAS visiting scholar and co-author of the paper. "When polymers become hydrogels when immersed in water, they have low friction and high abrasion resistance."

High fatigue resistance and high abrasion resistance improve the durability and service life of the polymer. 

We hope that this new understanding of polymer structure will expand the opportunities for application and pave the way for more sustainable and durable polymer materials with these outstanding mechanical properties.

"Our research shows that by using entanglement instead of crosslinking, we can reduce the consumption of certain plastics by improving the durability of the material," Zhang said.

"We hope that this new understanding of polymer structure will expand the opportunities for applications and pave the way for more sustainable and durable polymer materials with these excellent mechanical properties," Jin said.

The Harvard Office of Technology Development has protected the intellectual property rights associated with the project and is exploring commercialization opportunities.

This research was partially supported by Harvard MRSEC with grant number DMR-2011754.

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