A university in the United States has developed a special super-strong new material resembling chainmail. This material boasts the highest micro-mechanical key density to date, demonstrating outstanding flexibility and strength. Scientists anticipate using it in the next generation of bulletproof vests or other objects requiring lightweight, flexible, and resilient materials.
For years, researchers have been attempting to develop mechanically interlocked polymer molecules. Late Nobel laureate and then Northwestern University professor Fraser Stoddart had tried in the 1980s to enhance the performance of polymers by interlocking them in a manner resembling the chainmail worn by ancient European knights.
However, Stoddart’s idea was not realized during his lifetime. It was not until a team led by Northwestern University doctoral student Madison Bardot and Professor William Dichtel successfully overcame this challenge that a new, efficient, and scalable two-dimensional mechanical interlocking material (MIM) was created, resembling a molecular ‘chainmail’.
The research findings were published in the journal “Science” on January 17th, with over 8,500 downloads and coverage from over thirty news media outlets, ranking in the top 5% of all research outcomes on Altmetric.
The breakthrough for the team came unexpectedly when Bardot discovered a monomer with an X-shaped appearance called triphenylene metal-organic frame (TPE-PhOH) while experimenting with a new crystal growth method. It was found that this monomer would bind to the end of another monomer, forming a specific layered crystal structure with additional monomers passing through the gaps.
Furthermore, it was discovered that dissolving the polymer in a solvent would cause the interlocking monomer layers to peel from each other.
To enhance this crystal, the team injected it with di(chlorodimethylsilane) molecules, forming strong siloxane bonds, creating a 2D MIM with robust mechanical interlocking akin to chainmail or woven nets.
Using electron microscopy and instruments, the team observed the material’s microstructure. Results showed that within one square centimeter, the polymer contained over a million molecular-level interlocked structures. When gently stretched, it behaves like silk, but under intense impact, it transforms into a hard substance, providing properties highly sought after in bulletproof vests.
Scientists found that by adding 2.5 weight percentage (wt%) of 2D MIM material to polyetherimide fiber (Ultem), the material’s elasticity during stretching increased by 45%, and the ultimate stress (deformation amount) increased by 22%. However, adding over 5 wt% of 2D MIM to Ultem was found to decrease the overall performance.
This performance enhancement is excellent news for soft body armor used to protect against handguns. By adding a small amount of 2D MIM material, the weight of the body armor can be significantly reduced while increasing its overall bulletproof performance. Currently, a qualified soft body armor for protecting against handguns typically requires 15 to 30 layers of Kevlar fiber, with a total thickness between 6 and 8 millimeters and a weight ranging from 2.5 to 4 kilograms.
Polyetherimide fiber (Ultem) and Kevlar fiber both possess high-temperature resistance, abrasion resistance, and insulation properties, capable of withstanding extreme temperatures, acidic conditions, and corrosive chemicals. Therefore, they are commonly used in high-temperature gas and liquid filters, firefighting gear, pilot uniforms, bulletproof vests, automotive parts, and aerospace equipment.
Northwestern University researchers found that this 2D MIM material can be mass-produced, breaking the previous difficulty of mass-producing mechanically interlocked polymers. Currently producing half a kilogram, they plan to manufacture more of these materials for use in lightweight bulletproof vests, sports protective equipment, medical devices, and construction materials.
The communication author of this research, Professor William Dichtel from Northwestern University, stated to the university’s news outlet, “We have created a brand-new polymer structure similar to the chainmail from the Middle Ages. Each mechanical bond has a certain degree of flexibility, making it more resistant to tearing. If you want to tear it apart, you would have to break it from multiple points as it can distribute force in various directions.”
This research was primarily funded by the US Defense Advanced Research Projects Agency and Northwestern University’s IIN (Ryan Fellow Program).