The Mechanical Performance of a Biomimetic Nanointerface Made of Multilayered Polyelectrolytes

Integrating organic and inorganic components into hybrid materials is a promising pathway towards unique and useful combinations of stiffness, strength, and toughness. Nature provides superb examples of such materials: bone, teeth, and mollusk shells are made of stiff inorganic mineral inclusions bonded by more compliant proteins and polysaccharides that play a critical role by providing large deformations and energy absorption. Nanometer-thick polyelectrolyte multilayers (PEMs) provide an elegant approach to mimicking natural organic materials, but experimental data on their mechanical properties is scarce. In this work we have for the first time measured the shear performance and fracture toughness of PEM nanointerfaces that join two silicon substrates. Most of the properties of PEMs fall between those of a typical office tape and an engineering epoxy. However, the energy for failure in shear in PEMs exceeds the other two adhesives by far, because of breakage and reformation of electrostatic bonds. Interestingly, the properties of PEMs can be tuned by, for example, depositing a preliminary (3-aminopropyl)triethoxysilane layer or by adjusting the degree of hydration. They can also partially heal, and we demonstrate how only a fraction of the initial toughness is lost upon reassembly after fracture. This set of mechanical properties will greatly facilitate the design and optimization of future hybrid materials inspired from nature.