Although it is possible to create materials that are either very strong or very stretchy, it is difficult to make materials with both these properties. Now, Gareth McKinley and colleagues of the Massachusetts Institute of Technology have achieved this by studying spider silk and trying to mimic its properties. Spider silk is both stretchy and strong thanks to the fact that it is filled with many tiny crystals that are oriented towards, and strongly adhere to, the stretchy protein that forms their surrounding polymeric matrix. These crystals reinforce the structure of the material on the nanoscale.

McKinley and co-workers focused on trying to reinforce a commercially available polyurethane elastomer, called Elasthane, which is a rubbery substance, with nanosized clay platelets (laponite). The researchers developed a process to embed these clay chips (which are naturally arranged in stacks – like poker chips) in the rubbery polymer. They did this by first dissolving the clay chips in water and then slowly exchanging the water for a solvent, dimethylacetamide (DMAc), in which polyurethane also dissolves. Finally, they added the polyurethane to the DMAc-clay chip mixture and then removed the solvent.

The resulting nanocomposite was a mixture of stiff clay particles dissolved throughout a stretchy matrix that was now tougher and stronger. Using transmission electron microscopy, the researchers observed that the clay platelets were distributed randomly in the material. This means that the material is reinforced in all directions and distorts very little, even when heated to more than 150 °C.

The molecular composites could be suitable for new lightweight membranes and gas barriers, which might be used in fuel cells, says Evangelos Manias of the University of Pennsylvania. This is because the hard clay structure provides extra mechanical support and prevents the material degrading at high temperatures.

The new materials could find use in military applications too, including tear-resistant films or components for body armour. Textile companies have also already shown an interest in the composites for making fibres similar to stretchy materials like nylon or Lycra. Finally, the new technique for producing such nanocomposites could be applied to making biocompatible polymers for use in biomedical devices.

The researchers reported their work in Nature Materials.