Elastic polymers, known as elastomers, can be stretched and released repeatedly and are used in applications such as gloves and heart valves, where they need to last a long time without tearing. But elastic polymers can be stiff, or they can be tough, but they can't be both. This stiffness-toughness conflict is a challenge for scientists developing polymers that could be used in applications including tissue regeneration, bioadhesives, bioprinting, wearable electronics, and soft robots.
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. If there are only a few crosslinks, the chains are longer and the material is tough but too soft to be useful.
Researchers have resolved that conflict and developed an elastomer that is both stiff and tough. The team looked to physical, rather than chemical bonds to link the polymer chains. These physical bonds, called entanglements, have been known in the field for almost as long as polymer science has existed but they've been thought to only impact stiffness, not toughness.
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 polymerizing it, they forced them to be entangled. Just like with knitted fabrics, the polymers maintain their connection with one another by being physically intertwined. With hundreds of these entanglements, just a handful of chemical crosslinks are required to keep the polymer stable.
As elastomers, the polymers have high toughness, strength, and fatigue resistance. When the polymers are submerged in water to become hydrogels, they have low friction and high wear resistance. That high fatigued resistance and high wear resistance increase the durability and lifespan of the polymers.