2D, mechanically interlocked, polymer resembles chainmail

The first, 2D, mechanically interlocked material is reported to have been made at Northwestern University, USA.

The material is said to contain 100 trillion mechanical bonds per square centimetre, the highest density of mechanical bonds ever achieved, according to the group.

The chainmail-like polymer could offer a future material for armour, says the research team. It has potential for high-performance uses that demand lightweight, flexible, yet tough materials.

Professor William Dichtel at Northwestern says the polymer structure is similar to chainmail because it cannot easily rip as the mechanical bonds have some freedom to move. Pulling it dissipates the applied force in multiple directions, so to rip it would require the bonds to break in multiple places.

To coax polymers to form such mechanical bonds, the team started with X-shaped monomers – the building blocks of polymers – and arranged them into a specific, highly ordered crystalline structure.

Once the molecules are organised in this way, they introduce a second molecule that links each X-shaped molecule into the 2D-interlocked polymer.

Dichtel explains, 'The second monomer is a dimethyldichlorosilane – this reacts with each pair of hydrogen-bonded OH groups to make a covalent, 2D-interlocked structure.'

The resulting crystals have layers of 2D-interlocked polymer sheets. Within each sheet, the ends of the X-shaped monomers are bonded to each other, with more monomers threaded through the gaps. 

Despite its rigidity, the polymer is surprisingly flexible, reports the team. They have also found that dissolving the polymer in solution causes the layers of interlocking monomers to peel off. The crystal dissolves but each 2D layer holds together.

First author and PhD candidate Madison Bardot invented the concept. '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.'

They became aware of the interlocked crystal structure of X-shaped monomers because of research from China a few years ago.

Dichtel adds, 'We hypothesised that the reaction with a dichlorosilane could be a new and efficient way to form this unique mechanically interlocked architecture.

'But this sort of polymerisation, in which a second monomer infiltrates the molecular crystal and reacts selectively to polymerise the crystal and form all the mechanical bonds, was a new idea.'

The team at Northwestern suggests that the material can be produced in large quantities.

Previous polymers with mechanical bonds have often been prepared in very small quantities using methods that are unlikely to scale. Over a year, they improved the synthesis, preparing 0.5kg worth of material in 100-150g batches. They believe larger amounts are possible.

Dichtel says, 'It should not be too onerous to scale up production of this material. The monomer synthesis is efficient and provides relatively pure materials without the use of chromatography for purification.'

He notes that crystallisation of organic molecules is also straightforward and routinely completed at scale, while the polymerisation uses inexpensive silanes.

Inspired by its inherent strength, collaborators at Duke University, USA, added it to Ultem, a material in the same family as Kevlar. Just 2.5% of the 2D polymer added to these fibres is reported to increase their tensile modulus by 45% and ultimate stress by 22%.

Dichtel believes the polymer could be a specialty material for lightweight body armour and ballistic fabrics.