Wageningen Team Shatters Materials Theory With New Plastic

Drop a drinking glass and you will have to grab a dustpan and brush to sweep up the shards. A plastic cup, by contrast, bounces lightly off the floor and can go straight back into the cupboard. Convenient, but plastics have their drawbacks too: they are difficult to form into precise shapes, it typically requires a mould. Glass, on the other hand, can be blown and shaped at high temperatures. Wageningen researchers have now combined the best of both worlds in a novel material. For now, the team only has a few grams, but the effect is clear. The amber-coloured plastic can be kneaded and blown like glass once heated, while remaining impact-resistant like plastic. The researchers call this new class of material a compleximer.

A mystery in physics

The researchers themselves were surprised when they first saw the results. "It cast a completely new light on something scientists have been trying to understand for decades," says Jasper van der Gucht, Professor of Physical Chemistry and Soft Matter. For years, a rule of thumb has applied to so-called glassy materials - a category that includes both plastics and glass: the more slowly a material melts, and the easier it is to process, the more brittle it becomes. "But now we have something that completely defies that rule," says Van der Gucht. A material that melts slowly yet can withstand impact.

For Van der Gucht, the real excitement lies in this fundamental discovery, which challenges long-standing assumptions in materials science. Still, he also sees future possibilities if he allows his imagination to roam. Because compleximers are impact-resistant and easy to process, they can also be repaired quickly and easily. Think of roofing panels, garden furniture or even a car body made from compleximers. "Got a serious crack? Just heat it up with a hairdryer, press it together, and the gap is sealed again."

The difference from conventional plastics lies in how their molecular building blocks are connected. On a molecular scale, plastics consist of long chains. Normally, these chains are 'glued' together by chemical cross-links. In the new material, the chains are held together by physical attractive forces instead. One half of the chains carries a positive charge, the other a negative charge. These opposite charges attract each other, much like magnets. "That's how the chains stay together, without being chemically fixed," explains Sophie van Lange, first author of the publication.

Space between the chains

The researchers compared their material with substances described in the literature that also contain charged components, such as so-called ionic liquids, which conduct electricity and are used in applications including solar panels and batteries. They found that other charged materials also seemed to behave differently, although scientists had not previously considered this. "That was strange, but at the same time exciting," says Van der Gucht. Materials with charges, it seems, can display an entirely new kind of behaviour.

Exactly why this happens is not yet clear. The researchers suspect it has to do with the distance between the molecular chains that make up the plastic. In traditional plastics, chemical crosslinks pull the long chains tightly together. The attraction between positively and negatively charged chains works over a greater distance, leaving more space between them. On a molecular scale, this gives the material a very different structure, which may explain its unusual behaviour. "But," Van Lange stresses, "for now this is only a hypothesis."

A biobased version

The research team is keen to take the work further. In follow-up studies, two researchers will delve deeper into the underlying physics to better understand these mysterious materials. They will also investigate how the properties of compleximers can be tuned for specific applications. Sustainability is another priority. For the time being, the compleximers are made from fossil-based raw materials, but Van der Gucht wants to change that. "We have several ideas for a biobased version that we want to develop over the coming years," he says.

For now, (biobased) compleximers will not yet appear on the market. Even so, applied plastics researchers at WUR are already eager to explore their potential. "I'm very enthusiastic about the work of our colleagues," says Wouter Post, senior researcher in Sustainable Plastic Technology. "Their research contributes to a fundamentally better understanding of materials, and plastics in particular. That is essential for the transition to more sustainable use." According to Post, the study also highlights a blind spot in current plastics research. "Most applied research focuses on improving recycling, whereas this work opens the door to plastics that are easy to repair or even break down biologically very quickly."

For now, Van der Gucht and Van Lange are not too focused on practical applications. For them, the greatest value lies elsewhere. "Showing that charged materials can behave fundamentally differently from what we expected is what excites me most at this stage," says Van der Gucht. "We started with a material that, according to current theory, should not exist, and we end with new questions about how materials behave. That is where the real work begins."

Read the scientific publication:

Ionic glass formers show an inverted relation between fragility and non-exponential alpha-relaxation (Nature Communications)