Dynamic crosslinks: The future of plastic recycling

In a perfect world, different types of plastics would be kept separate from the time of their disposal to their polymer recovery, but the recycling story that many people are taught is optimistic: It’s nearly impossible to keep all plastic types separate, reducing the amount of recycled material and thus increasing the need for techniques to mix incompatible polymers.

One way to achieve this is through the addition of crosslinks, made of anything from two monomers to long chains of these building blocks. Permanent, covalent crosslinks can help, but Hanrahan et al. demonstrated the improved efficacy of dynamic crosslinks in compatibilizing polymer blends through simulations.

“Crosslinking softens the interface between polymer types. Then the dynamic on-and-off of the links helps the two species rearrange,” said author Francis Starr. “That allows the species to ‘breathe’ and find a more mixed state.”

To determine how dynamic crosslinks alter the mixing properties of polymers, the researchers used molecular dynamics simulations of a two-species polymer blend that was immiscible below a certain temperature. When the dynamic crosslinks were added, they found that the temperature at which the two polymers separated decreased dramatically, which should make reprocessing more viable.

“We also showed that when you add in these crosslinks, you not only make the system mix better, but you actually reduce the surface tension between the two phases when they are phase-separated,” said author Max Hanrahan.

By reducing the surface tension, it becomes easier to mix the system with simple mechanical agitation, like stirring. Additionally, the researchers found that as long as there was a small preference for crosslinks to form between two different polymers rather than between the same type of polymer, that simple difference was enough to improve the miscibility.

The team is in conversation with experimental chemists who plan to continue to study the properties of dynamic crosslinks in the lab.

Source: “Phase behavior, self-assembly, and interfacial tension of a dynamically-linked polymer blend,” by Max K. Hanrahan, Evelyn Grandfield, Sanat K. Kumar, Jack F. Douglas, and Francis W. Starr, Journal of Chemical Physics (2026). The article can be accessed at https://doi.org/10.1063/5.0325449 .

This paper is part of the Festschrift in honor of Kurt Kremer: Computational Polymer Science and Soft Matter Physics Collection, learn more here .