There are two main types of plastics, thermoplastics and thermosets, each with its own pros and cons. The former have great recyclability and malleability, but weak intermolecular interactions; the latter have strong covalent cross-links that remain stable with heat, but usually end up as permanent waste. Covalent adaptable networks (CANs) combine the strong properties of thermosets with the recyclability of thermoplastics by using dynamic cross-links. Based on this concept, Loc Tan Nguyen, Chiel Mertens and Filip Du Prez from Ghent University have found a very simple way to produce recyclable PDMS-based elastomers. 

Dynamic bonds 

Loc Nguyen, a PhD student in Du Prez’s Polymer Chemistry Research Group, explains how the team developed the CANs. ‘Our main goal was to apply β-amino esters [BAEs, ed.] to silicone elastomers which are far more expensive compared to conventional carbon-based elastomers. BAEs are dynamic bonds formed by a reversible reaction between an amine and an acrylate. This means you can create a network of covalent bonds that are not permanent.’ 

The reason for using silicone elastomers is that they are widely used because of their unique properties. ‘Think high flexibility and gas permeability, and stability over a very wide temperature range from -100 to +250 °C’, says Loc Nguyen. ‘Unfortunately, these elastomers are often cured at high temperatures using expensive metal catalysts or toxic reagents. So, simplifying the curing process could lead to very interesting applications.’ 

Five 

For the silicone elastomers, the team used commercially available variants of polydimethylsiloxane, in which they could vary the number of crosslinks by using chain extenders (such as hexylamine) or hardeners (such as tris(2-aminoethyl)amine). Because they combined the elastomer with BAEs, it was very easy to modify and reprocess the corresponding elastomers. Loc Nguyen: ‘We could cut the cross-linked polymer into pieces five times, and each time we were able to re-create the polymer by compression moulding without any loss of properties, showing great recyclability.’ 

For most elastomer applications, you need sufficiently high mechanical strength. ‘The silicone elastomers themselves are not strong enough’, Loc Nguyen explains. ’Hence fillers are typically added to the rubbers to improve their properties. However, there is a danger in using a filler, which is that it reduces the recyclability.’ 

Surprise 

In this case, the researchers used silanol groups as fillers, which increase the electrophilicity of the acrylate carbonyl group through hydrogen bonding interactions. ‘What really surprised us was that the filler didn’t reduce the recyclability,’ says the PhD student. ‘Normally, it’s not possible to combine recyclability and property improvement, but thanks to the functional group on the silica, it works!’ 

It was a difficult task, however. ‘We tried a lot of different fillers and it took us a long time. But eventually we found this one outperformed any others and it proved to be the key for this development. Another advantage of the materials they used is that the whole process is very scalable. We used commercially available building blocks, which makes it easy to apply. Together with the VUB, we’re now looking at making the material healable for soft robotic applications.’ 

Nguyen, L.T., Mertens, C. & Du Prez, F.E. (2024) Macromolecules, DOI: 10.1021/acs.macromol.4c00023