“By crowding all the monomers into this solution with less water and then polymerizing it, we forced them to be entangled, like tangled strings of yarn. Just like with knitted fabrics, the polymers maintain their connection with one another by being physically intertwined.”Guogao Zhang
Scientists from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed an elastic polymer, also known as elastomer, that is both stiff and tough in an approach based on sustainable and long-lasting polymer materials that could reduce plastic consumption around the world.
Polymer science has been responsible for the development of everyday things such as rubber tires, Teflon and Kevlar, and the ubiquitous plastic water bottle. Elastic polymers can be stretched and released repeatedly, and are used in applications such as gloves and heart valves, where they need to be dependable and long-wearing. However, the long-standing problem with elastic polymers is that they can be either stiff or tough, but not both. This conflict is a challenge in producing polymers for innovative technologies such as tissue regeneration, bioadhesives, bioprinting, wearable electronics and soft robots.
This new study, reported in Science [Kim et al. Science (2021) DOI: 10.1126/science.abg6320], showed resolved this conflict to potentially help reduce plastic pollution. To produce a polymer with the required stiffness and toughness, the team investigated physical, rather than chemical, bonds to link the polymer chains. These physical bonds, called entanglements, have long been known about but have been thought to only affect stiffness and not toughness.
However, the team discovered that with sufficient entanglements a polymer could become tough without compromising stiffness. To develop highly entangled polymers, they used a concentrated monomer precursor solution with 10 times less water than other polymer recipes. Polymer chains are produced by linking together monomer building blocks – to ensure a material is elastic, these polymer chains are crosslinked by covalent bonds, with the greater the number of crosslinks the shorter the polymer chains and the stiffer the material.
Here, the team synthesized polymers where there are many more entanglements than crosslinks, which allows transmission of tension in a polymer chain along its length, as well as to other chains. The sparse crosslinks stop the polymer chains from disentangling, so that the polymers have useful toughness, strength and fatigue resistance.
As Guogao Zhang, co-first author of the paper, explained, “By crowding all the monomers into this solution with less water and then polymerizing it, we forced them to be entangled, like tangled strings of yarn. Just like with knitted fabrics, the polymers maintain their connection with one another by being physically intertwined.”
From hundreds of such entanglements, only a small number of chemical crosslinks are needed to keep the polymer stable. When they are immersed in water to become hydrogels, they have low friction and high wear resistance. The team hope that the better understanding of polymer structure will bring new applications and more sustainable, long-lasting polymer materials with excellent mechanical properties.
A highly entangled hydrogel (left) and a regular hydrogel (right).