Cellulose nanocrystals : From gels to solid materials

Overview & Synopsis

Cellulose nanocrystals (CNC) are solid rod-like particles with a large negative surface charge. When suspended in water in the presence of salt, CNCs aggregate and form gels. Such gels can be used as soft precursors in the design of biosourced solid materials, to which they transmit part of their remarkable mechanical properties.The goal of the present thesis is, first, to probe the mechanical properties of CNC gels using rheology. The study of the gelation dynamics of such gels and the establishment of time-connectivity and time-composition superposition principles have allowed us to show (i)~the existence of a universal scenario for the formation and aging of CNC gels, and (ii)~the structural equivalence between the salt concentration of a gel and its age. Then, we have studied the shear-induced failure and fluidization of CNC gels. In particular, we have shown that the failure can be abrupt (``fragile'') or gradual (``ductile'') depending on the gel age and on the applied shear rate.Finally, we have performed a preliminary study of the transition from a soft material to a solid material, through two different solidification processes: drying and freezing. We have shown that the two processes induce the formation of specific microscopic structures inside the solid material, partially due to CNCs alignment, whose characteristics can be controlled by the boundary conditions or by the solidification speed.