Why weaker links could lead to more durable polymers

3 July 2023 | Muriel Cozier

Researchers call for increased investment on tyre wear research as concerns over impact of microplastics grows.

Chemists from Massachusetts Institute of Technology (MIT), and Duke University, US, have found that introducing a number of weaker bonds into a polymer increases its resistance to tearing up to tenfold. The researchers say that this discovery could lead to rubber tyres that last longer and produce fewer microplastics.

Publishing their work in Science, the researchers used polyacrylate elastomers, to which cyclobutane cross-linkers were added. These are much easier to break than normal carbon-carbon bonds. It was found that joining the acrylate building blocks with the cyclobutane crosslinkers in random locations the material became more resistant to tearing.

The researchers concluded that the random distribution of the weaker bonds creates junctions between otherwise strong bonds throughout the material. When the material is stretched to breaking point, cracks that develop propagate through the weaker bonds within the material, avoiding the stronger bonds. This leaves more of the stronger bonds intact, maintaining the material’s integrity.

It was found that with the weak crosslinks making up only 2% of the overall composition of the material, the polyacrylates incorporating them were nine to ten times harder to tear than polyacrylates with stronger cross linking bonds. It was also noted that the change in composition did not impact any of the materials other properties such as resistance to breaking when heated.

Stephen Craig, Professor of Chemistry at Duke University and a senior author of the research paper said: ‘Polymer engineers know how to make materials tougher, but it invariably involves changing some property of the material you don’t want to change. Here the toughness enhancement comes without any other significant change in physical properties – at least that we can measure – and it is brought about through the replacement of only a small fraction of the overall material.’

The researchers are now investigating whether this approach could be used to improve the toughness of a range of materials, including rubber.

The impact of microplastics is a major source of concern, and earlier this year researchers from Imperial College London’s Transition to Zero Pollution Initiative released a briefing warning that while electric vehicles removed the issue of emissions from fuel, particulates from tyre wear was set to remain a problem.

The briefing document: Tyre wear particles are toxic for us and the environment calls for as much investment to go into tyre wear research as goes into reducing fuel emissions. Lead author of the document, Dr Zhengchu Tan, from Imperial College Department of Mechanical Engineering said: ‘Tyre wear particles pollute the environment, the air we breathe, the water run-off from roads, and has compounding effects on waterways and agriculture. Even if all our vehicles eventually become powered by electricity instead of fossil fuels, we will still have harmful pollution from vehicles because of tyre wear.’

Imperial researchers are now calling for a more ‘concerted research effort to establish standardised testing protocols that would allow us to critically determine the toxicity levels of tyre wear particles and leachate chemicals in the environment’.

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