In Chicago, a plastic has been developed that can be hard or soft depending on the temperature at which it is synthesised. A useful tool for a trip to Mars?
For more than three thousand years, metalworkers have played with temperature to adjust the hardness of iron alloys. But in the world of plastics, people have tended to look for a polymer with the best physico-chemical properties for each application. That’s fine as long as you can order the ideal plastic from the factory, but it gets tricky when you want to make your own spare parts with a 3D printer. Especially if you’re in the middle of the ocean or in a spaceship with a limited collection of plastic filaments.
In Science, Nicholas Boynton, Stuart Rowan and colleagues at the University of Chicago now present a prototype of what they call a ’pluripotent’ polymer. Hardness and elasticity can be adjusted by the temperature at which the components react with each other, between 60 and 120°C. At room temperature, it retains these properties for at least a month. However, if you heat it up to 140°C, the structure disintegrates and the components can be reused several times.
It all revolves around a classic thia-Michael addition, in which two ingredients are linked by a C-S bond. One is a benzalcyanoacetate with two reactive sites and the other is a tetrathiol with four arms; together they form a network. By adding a dithiol, you can increase the chain length between the links. Such a Michael addition has a distinct dynamic, especially if no catalyst is added. As the temperature rises, the network starts to form slowly, but as the temperature rises further, the equilibrium shifts and the bonds break more and more easily. You can set the limits of this temperature range by cleverly choosing the molecular architecture.
If, after a 24-hour simmer, the result is cooled quickly below the starting temperature (‛tempering’, as metallurgists call it), the structure freezes, so to speak. Exactly why this happens is still not entirely clear; the authors thought it might be a combination of kinetics and steric hindrance, but AFM images point more in the direction of dynamic reaction-induced phase separation between strongly and less strongly cured regions. Either way, it seems to work. The material hardened at 60°C is hard and brittle at room temperature, while the exact same composition at 110°C produces something that can be used as an adhesive. It even manages to create different zones with different properties in one test strip by heating one side hotter than the other.
Polymer pioneer Piet Lemstra, professor emeritus at TU Eindhoven and still active as a consultant and commentator on LinkedIn, is not overly impressed. ‘I think the story is a bit overblown, but you have to do something to get attention’, he says. He says the idea of using such materials in resource-poor areas, such as on a spaceship, is a bit far-fetched. And he points out that the idea of ’dynamic’ polymers is not an original one. ’My TUE colleague Bert Meijer has done a lot of work on replacing some of the covalent bonds in polymer chains with hydrogen bonds. Depending on the temperature, these were released, so that during processing or moulding you could work with a low-viscosity system. When it cooled down, it was once again a polymer with the desired properties. In this way, you could adjust the chain length and thus the physical properties.’
A more recent example is French researcher Ludvik Leibler’s so-called vitrimers; thermosetting resins in which the cross-links behave dynamically, making the material deformable and thus recyclable at elevated temperatures. Ghent professor Filip Du Prez is currently trying to refine this concept to make reusable wind turbine blades, for example. But as far as Lemstra knows, such systems have not yet become a breakthrough commercial success anywhere. ’It’s very complicated.’