MIT chemists, using a new cross-linking molecule, have significantly enhanced the ballistic impact resistance of commonly used polymers, including polystyrene and a type of rubber found in shoe soles. Polystyrene, a rigid polymer, is used in various plastic items like bottles, mugs, and disposable cutlery, and serves as coatings for electronics and in foam form as Styrofoam. Although it carries recycling code No. 6, polystyrene is tough to recycle and seldom reused in the U.S.
The researchers improved the polymer’s impact resistance by introducing weak bonds throughout the material as cross-links, which better dissipate energy upon deformation. When hit by a projectile, these weak bonds break at the impact point, enhancing energy absorption. This method also strengthens styrene-butadiene-styrene rubber, and the team is investigating its effectiveness on other polymers like latex and tire rubber.
Jeremiah Johnson, MIT’s A. Thomas Geurtin Professor of Chemistry, states, “These cross-linkers can substantially increase the amount of energy that the material absorbs under ballistic impact.” The study, authored by Johnson and Keith Nelson, was published in Nature, with contributions from former MIT postdocs Zhen Sang and Suong T. Nguyen and MIT graduate student Kwangwook Ko.
In a 2023 study, Johnson and collaborators from MIT and Duke University made polymers tougher by adding weak cross-linkers throughout a polymer network. These mechanophores break during tearing, helping preserve stronger bonds and dissipate more energy. Johnson explains, “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.”
The recent study focused on mechanophore-enabled strategies to resist rapid deformation like sudden impacts. The researchers incorporated mechanophores as cross-links into common polymers and used Keith Nelson’s laser-induced microprojectile impact testing (LIPIT) to examine the polymers’ response to impacts by firing tiny silica beads at high speeds. The energy absorbed by the material was calculated by measuring changes in the particle’s velocity before and after passing through the film.
Nelson remarks, “We first developed this method to study microparticle impact and penetration into bulk polymer samples.” The experiments demonstrated that mechanophore cross-linked polystyrene absorbed significantly more energy from an impact than regular polystyrene. Johnson notes, “It turned out that the mechanophore leads to substantial increases in energy dissipation compared to both uncross-linked and conventionally cross-linked polystyrene.”
To understand how mechanophores improve impact resistance, the MIT team collaborated with Purdue, Northwestern, and Duke universities. They discovered that a high-speed particle impact raises the temperature at the site, forming a mobile zone where mechanophore bonds break under force, creating pathways that absorb impact energy while leaving surrounding areas stable.
Yoan Simon from Arizona State University, not involved in the research, finds the approach attractive for enhancing ‘off-the-shelf’ plastics with minimal chemistry. The researchers also successfully inserted mechanophores into styrene-butadiene-styrene rubber, observing similar effects. They are now exploring this method with styrene-butadiene rubber used in tires, potentially leading to longer-lasting tires and reduced microplastic generation.
Katharine Covert from the U.S. National Science Foundation highlights the potential of materials with energy-absorbing mechanophores to prevent tire blowouts and protect electronics. The study was funded by the National Science Foundation, U.S. Army Research Office, Schmidt Science Postdoctoral Fellowship, and the U.S. Air Force Office of Scientific Research.
Original Source: news.mit.edu
