For four years, mechanochemistry has been the focus of PhD candidate Annelore Aerts’s attention. This branch of chemistry is concerned with the study of behavior of materials subject to mechanical stress. In short, she developed an improved way of revealing material fatigue in plastics. Although that is a highly simplified explanation, laughs Aerts. As the backdrop to her online interview, she is using a white, freshly plastered wall – a likely candidate for the use of polymers, as it happens – because as well as having a PhD conferral ceremony to prepare for, the Flemish scientist is in the middle of moving house. She hasn’t long had the keys to her new home, situated 15 minutes’ drive from the Dutch border.
“Plastics are made up of polymers, chains of the same, repeating molecules. Polymers are cheap, easy to produce and easy to use, and so they are used in an increasing range of applications. We are familiar with many plastic utensils, but remember, plastics are also in things like building materials and aircraft parts. In cases like these, before you switch to a polymer, you must know whether the new material meets the necessary requirements. With a polymer frame a very lightweight aircraft wing can be produced – and fuel can be saved – but the construction must provide guaranteed safety for a certain number of years. The consequences may otherwise be catastrophic.”
To study in detail when and where a polymer fails, Aerts produced various model polymers in which she incorporated smart, fluorescent building blocks. These building blocks, properly called mechanophores, break when subjected to a certain force. They change color, emit light – or in Aerts’ research – become fluorescent. Working from calculations, you can design the mechanophore in such a way that its connection is slightly less robust than that of the polymer in which it sits, explains Aerts. “You set things up so that the mechanophore is the weakest link. You want to be absolutely sure that the mechanophore will break before the polymer fails.”
Strip of light
Aerts first tested her method on elastic polymers, before making the switch to what are known as glassy polymers. “Polycarbonate and polystyrene, for example, which disposable beakers are made from. You know about these, right? And you know how these beakers break right where you squeeze them? This localized breakage, the way the material behaves like this, makes it particularly difficult to use a mechanophore to study stresses in these types of polymer.” Eventually, she developed two fluorescent mechanophores. One is what is known as a supramolecular mechanophore, whose molecules have no bonds attaching them to one other, which accelerates their breakage when you exert force on them. This building block is therefore ideal for revealing minimal stress in a material. The second mechanophore, by contrast, does contain covalent bonds between its molecules, but some tweaking by Aerts made it well suited to testing glassy polymers.
In preparation for testing, Aerts worked in the lab to transform her powdery polymer-mechanophore mix into little pieces of thin, transparent film. Using a diamond nib and a predetermined force, she then etched a miniscule scratch on the plastic film. And after many experiments and much finetuning, Aerts was rewarded. Under the fluorescence microscope she saw ‘the light’: a colored line exactly where the scratch was applied with the most force. Right there her mechanophore had broken. “Mechanochemistry has really taken off over the past ten years, but it is applied primarily to elastic polymers. Now a highly sensitive technique has been used to demonstrate that material stress can also be revealed in glassy polymers. And we have been able to show that the noticeable difference in fracture behavior is caused at molecular level. These polymers often turn whitish before they break; just try bending a clear plastic object yourself. For the first time, we now have effective evidence that this ‘crazing’ is accompanied by the breaking of molecular bonds.”
The fluorescent building blocks developed by Aerts and her colleagues are providing many new insights into materials. But industrial applications, such as checking a crash helmet for damage after an accident, still sound a long way off. “We also have to consider how materials are produced industrially, often at high temperatures. So, first we have to develop mechanophores that are thermally stable, as well as reversible. Otherwise in any entirely fluorescent crash helmet, the potential fracture lines won’t be visible.”
Before too long, however, Aerts hopes that her method can also be applied to composites. These are polymers to which an additional component has been added in order to produce even better, stronger or lighter plastics. Initial steps in this direction have already been taken, but regrettably Aerts has no more time to complete this work herself; her research time is up. Next week, she will defend her thesis, the last member of the ‘PhD squad’ to do so. Four years ago, a group of doctoral candidates started their doctorates at around the same time within the Supramolecular Polymer Chemistry group. Since then they have shared plenty of ups and downs. Now, with the easing of corona restrictions, she becomes the first PhD squad member to have a closing ceremony everyone can attend. And so she – between the plastering and the painting - and her PhD buddies will be able, after all, to say their goodbyes with a fabulous party.