Cellulose nanocrystals (CNCs) are one of nature’s most abundant structural building blocks and possess outstanding mechanical properties such as a tensile modulus comparable to Kevlar. It remains challenging to upscale these properties in CNC neat films and nanocomposites due to the difficulty of characterizing interfacial bonding between CNCs that govern stress transfer under deformation. Here we present new analyses based on atomistic simulations of shear and tensile failure of the interfaces between Iβ CNCs in elementary fibrils, providing new insight into factors governing the mechanical behavior of hierarchical nanocellulose materials. We compare the two most relevant crystal surfaces and find that hydrogen bonded surfaces have greater tensile strength compared to the surfaces governed by weaker interactions. On the contrary, shearing simulations show that friction between the atomic interfaces depends not only on surface energy but also the energy landscape along the shear direction. While being a weaker interface, the intersheet plane exhibits greater energy barriers to shear. Analytical models to describe the stick-slip energy landscapes are developed, accounting for size-effects. Our simulations combined with simple theoretical predictions explain experimental observations on the mechanical properties of cellulose nanocomposites and their variation with environmental factors as moisture. Our results pave the way for tailoring hierarchical CNC materials by taking a materials-by-design approach as commonly done for metals, where mechanical properties can be tuned through a deeper understanding of grain boundary physics and nanoscale interfaces.

Stick-slip shear phenomenon of interfaces between cellulose nanocrystals

249th National Meeting (2015)

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