Weaker bonds make for more impact-resistant polymers

In a new MIT study – in partnership with Purdue, Northwestern, and Duke universities – chemists have discovered that inserting weaker bonds into polystyrene actually makes the material more resistant to damage.

The weaker bonds are dispersed throughout the material as cross-links known as mechanophores, and upon a sudden strike, those mechanophores selectively snap at the impact site, absorbing the impact energy more effectively.

In their experiments, the researchers demonstrated how when the cross-linked polystyrene was struck with a high-speed particle, the temperature at the impact site increased high enough to form an adaptable zone that broke, leaving the surrounding area unaffected. As a result, the cross-linked polystyrene absorbed way more energy compared to regular polystyrene.

“These cross-linkers can substantially increase the amount of energy that the material absorbs under ballistic impact. You can imagine many applications of that, especially if this could be generalized to other polymers,” says MIT's Prof. Jeremiah Johnson. “It turned out that the mechanophore leads to substantial increases in energy dissipation compared to both uncross-linked and conventionally cross-linked polystyrene.”

This study builds upon a 2023 study from Johnson and colleagues at MIT and Duke University, which demonstrated that polymers could be toughened by distributing weak cross-links throughout polymer materials that break under slow tearing conditions in a way that perseveres the stronger bonds that bear the load to dissipate more energy.

“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. That means you have to put in more energy to tear the material,” Johnson says.

In this case, the team devised a way to directly merge the mechanophores as cross-links into common polymers. They used an invention created by MIT's Prof. Keith Nelson, to study how the polymers responded to impacts via laser-induced micro-plastic impact testing (LIPIT).

Tiny silica beads about 10 microns in diameter are projected at the polymer at about 750 meters per second (over 1,600 miles per hour) with the amount of energy absorbed measured by calculating the change in particle velocity before and after the beads pass through.

“We first developed this method to study microparticle impact and penetration into bulk polymer samples, where we would monitor particle propagation through about 100 microns of material and analyze after impact how polymer morphology had changed,” Nelson says. “Our new measurements show how much additional information can be extracted from particle velocities before and after penetration through a thin layer. They also show deeply informative deformation patterns both during particle impact and afterward.”

With this cross-linking approach, the researchers can also strengthen styrene-butadiene-styrene rubber which is used in shoe soles, as well as asphalt and roofing materials with the same results. They are additionally studying whether the technology could be applied to other kinds of polymers, like the rubber used in tire production.

“Materials with energy-absorbing mechanophores could one day help keep your vehicle's tires from blowing out on the highway or provide more protective cases for personal electronics,” says Katharine Covert, program director of the US National Science Foundation Centers for Chemical Innovation, which invested in the team’s research. “This work really demonstrates how valuable new insights can be rapidly generated by bringing together researchers with different areas of expertise.”

A paper on the study was recently published in the journal Nature.

Source: MIT