New, recyclable plastic lets you weld pieces together with a hairdryer
Organic polymers can be used to create materials with very distinct properties (both good and bad). For example, thermoset materials resist heat and solvents, making them extremely durable and allowing them to be used in the oven. The downside is that, once they’re made, that’s it—no recycling. Thermoplastics are stable below a set temperature, but they can be melted, allowing them to be remade into new materials. Unfortunately, they don’t hold up very well to solvents.
Now, researchers are saying they’ve created a third option, one that acts like a thermoplastic at high temperatures but can hold up to most solvents. The material’s secret? An embedded catalyst that allows chemical bonds to constantly rearrange. The material’s desired properties can be tuned based on the polymer it’s made from and how much catalyst remains.
A bit like epoxy
The chemistry involved is similar to epoxy, which is a type of thermosetting polymer that solidifies irreversibly (the precise starting material is bisphenol linked to glycerol at each end). In the new material, molecules have four reactive sites that allow them to be crosslinked into a polymer by chemicals that carry two or more carboxylic acid groups. However, these reactions don’t go to completion, leaving lots of both the starting material and the crosslinker available for further reactions.
That means, as the authors note, that there’s a “chemical equilibrium between bond breaking and reforming” in the resulting polymer. Lower the rate of this reaction enough, and the system acts as a permanent polymer. Raise the reaction rate, and the polymer will start to rearrange itself as its components swap partners. “The key is to design the chemistry so that at high temperature, exchange reactions enable stress relaxation and malleability and upon cooling, the exchanges become so slow that the topology of the network is essentially fixed and the system behaves like a soft solid,” they write.