MIT explores impact-resistant polymer technology with potential tire applications

Cambridge, Massachusetts – Researchers at the Massachusetts Institute of Technology (MIT) have developed a technique that significantly improves the impact resistance of polymers, including synthetic rubber used in shoe soles, and potentially tires.

In a 3 June article, MIT reported that the research focuses on introducing weak molecular bonds, known as mechanophores, into polymer networks.

These bonds ‘break selectively’ under impact, helping materials absorb and dissipate energy that would otherwise contribute to damage.

The researchers demonstrated the approach in styrene-butadiene-styrene (SBS) rubber, used in shoe soles, asphalt and roofing materials, and are now evaluating its use in other types of polymers such as latex or tire rubbers.

They are also exploring the possibility of impact-resistance plastics using the technology in polystyrene.

"If successful, this technology could yield longer-lasting tires and also cut down on the amount of microplastics generated when tires contact the road," MIT said, noting that tire wear is estimated to account for at least 10% of microplastics in the environment.

Jeremiah Johnson, the A. Thomas Guertin Professor of Chemistry at MIT and one of the senior authors of the study, said the new cross-linking molecules can dramatically improve a material's ability to absorb impact energy.

"These cross-linkers can substantially increase the amount of energy that the material absorbs under ballistic impact," Johnson said. "You can imagine many applications of that, especially if this could be generalised to other polymers."

MIT said the concept builds on earlier work showing that polymers can be strengthened by introducing strategically placed weak links throughout the material.

Rather than weakening the polymer, the bonds help redirect and dissipate energy when the material is subjected to stress.

"As a crack starts to propagate through the material, these mechanophores split in two, which helps to dissipate energy and redirect where the crack goes," Johnson said. "That means you have to put in more energy to tear the material."

For the latest study, researchers incorporated the mechanophores directly into common polymers and then tested their performance using a laser-induced microprojectile impact testing (LIPIT) system developed by co-author Keith Nelson.

The system fires microscopic silica particles at polymer films at around 750 metres per second.

Researchers then measure changes in velocity before and after impact to determine how much energy the material absorbs.

According to MIT, the tests showed that mechanophore-cross-linked polystyrene absorbed substantially more impact energy than either conventional polystyrene or standard cross-linked versions of the material.

"It turned out that the mechanophore leads to substantial increases in energy dissipation compared to both uncross-linked and conventionally cross-linked polystyrene, a behaviour that had not been observed in related previous work," Johnson said.

Further experiments and simulations showed that high-speed impacts create a localised "mobile zone" within the material.

Within this zone, mechanophore bonds break under force, creating what MIT described as "controlled pathways" that absorb impact energy while leaving surrounding areas largely unaffected.

The approach could prove attractive because it can be incorporated into widely used commercial polymers without extensive chemical modification, according to MIT.

"What is particularly attractive about this approach is the ability to bestow these properties upon 'off-the-shelf' commodity plastics, both glassy and elastomeric, with minimal chemistry," said Yoan Simon, associate professor in the School of Molecular Sciences at Arizona State University, who was not involved in the research.

"This study combines an elegant approach while providing an in-depth mechanical analysis of the failure mechanism," Simon added.

Looking ahead, researchers said the technology could lead to more durable products across a range of applications.