From the Nanostructure and the mechanics of NanoFibrils of cellulose to the Gel-like Rheology of Concentrated NFCs Suspensions

Because of their large aspect ratio, great mechanical properties, and abundance, nanofibrils of cellulose (NFCs) are promising biosourced nanofibers for structural or functional materials. However, all the processes using NFC suspensions for the production of NFC-reinforced materials such as film casting, injection, extrusion, or compression molding, encounter several rheological problems due to the fact that the flow acts upon NFCs, inducing deformations and organisation phenomena which are likely to have an impact on the mechanical properties of the final material.

Thus, it is important to better understand and be able to model the deformation of the gels and their flow-induced nanostructures during the forming processes.

Using rheometry combined with ultrasonic velocimetry, a semi analytical multiscale rheological model was recently developed with a discrete representation of NFC systems. However, the model does not account properly for the transient elastic effects induced by the NFC deformations which is an important drawback.

Thus, in this project, an original multiscale and numerical methodology is proposed to overcome this deficiency:

• At the nanoscale, Molecular Dynamics (MD) simulations will be used to build realistic molecular models of NFCs constituted with highly crystalline regions and kinks. MD simulations will also provide estimations of the NFC (non-)linear stiffness tensor, as well as an appropriate curvilinear representation of the NFCs that will be idealized as anisotropic beams with possibly enriched kinematics.

• At the microscale, the NFC beam model will be integrated into an in-house Discrete Elements Method code (DEM) developed for the mechanics of entangled fibrous materials. Using a realistic generation procedure, representative volumes of NFC gels, modeled as concentrated networks of elastic nanofibers that interact by repulsive electrostatic and hydrodynamic forces, will be subjected to various mechanical loadings. For the first time, this will lead to the estimation of both the (non)linear stiffness tensors and the yield stress surfaces of NFC gels.

The simulated mechanical responses and the spatial organizations of the gels will be compared with rheology and X-ray and light scattering experiments.