Ice templating of gels of cellulose nanofibrils for the design of NFC cellular materials

Cellular materials such as foams or aerogels of nanofibrils of cellulose (NFC) constitute novel low density bio-based materials with high mechanical and insulation properties, and thus have a great potential in several engineering fields. They can be used in the automotive industry as heat/sound insulation systems, cores of structural sandwich composite panels, or for biomedical applications such as filters, scaffolds for tissues. NFC cellular materials can be obtained by ice templating, i.e., by freezing colloidal and concentrated NFC aqueous suspensions (or gels) and sublimation of the formed ice crystals (freeze-drying). The ice templating process has received considerable attention in recent years owing to its simplicity and to the wide variety of porous materials that this technique can provide. However, in the case of NFC gels, growth mechanisms of ice crystals formed during the freezing phase, i.e., when they squeeze and consolidate the remaining NFC gels up to very high NFC concentrations, are complex and still poorly understood. This severely hinders the optimisation of the microstructure of NFC cellular materials. The main objective of this project is to better understand them.

For that purpose, various experimental analyses are carried out to better understand the rheology of concentrated NFC gels and the growth of ice crystals in such gels:

• NFC gels are produced with various NFC types and concentrations, from the concentrated to the hyper-concentrated regimes. Their shear and compression rheology is investigated to assess their elastoviscoplastic behavior. Some rheometry experiments are carried out using 3D in situ imaging of the gel flow at the mesoscale using X-ray microtomography and 3D digital image correlation.

• In parallel, freezing experiments are carried out with the aforementioned gels. The orientation and the morphology of ice-templated crystals are analysed using, respectively, cryo-EBSD (Electron Back Scattering Diffraction) and X-ray microtomography.

• In addition novel freezing experiments are carried out to obtain real-time observations of the ice crystal growth within NFC gels using 3D in situ imaging.