How Does Nature Evade the “Larger is Weaker” Fate of Ultralong Silk β-Sheet Nanocrystallites

Silk protein builds up one of the strongest fibers superior to most synthetic and natural polymers. However, the strengthening mechanisms of the silk proteins remain largely elusive because of their complex nanocomposite structures. Here, we report an unusual behavior of this kind of material that is distinctively different from those of metals and other polymers. We find that there are multiple interface microcracks nucleating and stacking under the shear loading, dividing the interchain interface into small segments, by which the silk protein can achieve a high strength even with the ultralong chains. This is a new strategy of microstructure design of soft matter that could avoid the "larger is weaker" fate due to the increase of the chain length. This novel mechanism is crucial for building strong polymer materials with long chain molecules and at the same time retaining their complex functional and structural properties.