Supported by DARPA, the nanoscale material could also find applications that require lightweight, flexible and tough materials. The study is detailed in Science. Madison Bardot, a PhD candidate in Dichtel’s laboratory and IIN Ryan Fellow, is the study’s first author.
The study marks several firsts in its field: it is the first 2D mechanically interlocked polymer, and it contains 100 trillion mechanical bonds per 1cm2, which is the highest density of mechanical bonds ever achieved. The researchers produced this material using a new, highly efficient and scalable polymerisation process.
“We made a completely new polymer structure,” said Northwestern’s William Dichtel, the study’s corresponding author. “It’s similar to chainmail in that it cannot easily rip because each of the mechanical bonds has a bit of freedom to slide around. If you pull it, it can dissipate the applied force in multiple directions. And if you want to rip it apart, you would have to break it in many, many different places. We are continuing to explore its properties and will probably be studying it for years.”
Researchers have previously attempted to develop mechanically interlocked molecules with polymers but found it challenging to coax polymers to form mechanical bonds.
To overcome this challenge, Dichtel’s team started with X-shaped monomers - the building blocks of polymers - and arranged them into a specific, highly ordered crystalline structure. Then, they reacted these crystals with another molecule to create bonds between the molecules within the crystal.
“I give a lot of credit to Madison because she came up with this concept for forming the mechanically interlocked polymer,” Dichtel said in a statement. “It was a high-risk, high-reward idea where we had to question our assumptions about what types of reactions are possible in molecular crystals.”
The resulting crystals comprise layers of 2D interlocked polymer sheets. Within the polymer sheets, the ends of the X-shaped monomers are bonded to the ends of other X-shaped monomers. Then, more monomers are threaded through the gaps in between. Despite its rigid structure, the polymer is surprisingly flexible. Dichtel’s team also found that dissolving the polymer in solution caused the layers of interlocked monomers to peel off each other.
“After the polymer is formed, there’s not a whole lot holding the structure together,” said Dichtel. “So, when we put it in solvent, the crystal dissolves, but each 2D layer holds together. We can manipulate those individual sheets.”
To examine the structure at the nanoscale, collaborators at Cornell University, led by Professor David Muller, used electron microscopy techniques. The images revealed the polymer’s high degree of crystallinity, confirmed its interlocked structure and indicated its high flexibility.
Dichtel’s team also found the new material can be produced in large quantities. Previous polymers containing mechanical bonds typically have been prepared in very small quantities using methods that are unlikely to be scalable. Dichtel’s team made half a kilogram of their new material and assume larger amounts are possible.
Inspired by the material’s inherent strength, Dichtel’s collaborators at Duke University, led by Professor Matthew Becker, added it to Ultem, a material that can withstand extreme temperatures plus acidic and caustic chemicals. The researchers developed a composite material of 97.5 per cent Ultem fibre and 2.5 per cent of the 2D polymer. That small percentage is said to have ‘dramatically increased’ Ultem’s overall strength and toughness.
Dichtel envisions his group’s new polymer might have a future as a specialty material for lightweight body armour and ballistic fabrics.
The study, “Mechanically interlocked two-dimensional polymers,” was supported also by Northwestern’s IIN (Ryan Fellows Program).
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