Multiscale microstructure of physical cellulose hydrogels by SAXS, WAXD, AFM and X-ray μCT

Understanding the microstructure of cellulose hydrogels is critical for tailoring their material design. This study aims at gaining further understanding of the links between the initial solution state (semi-dilute, concentrate, anisotropic) and the resulting self-organization of the hydrogel microstructure. SAXS, WAXD, liquid-state AFM and μCT helped covering a broad range of scales ranging from ~1 nm to 1 mm. The presence of crystalline cellulose hydrates a few nanometre thick was evidenced by WAXD and their size was mostly influenced by the initial degree of polymerization, as confirmed by SAXS. Moreover, the microstructure of the physical cellulose hydrogels displayed a fractal organization at both nanoscale and microscale. The SAXS characterisations highlighted structures of characteristic lengths ca. 13–30 nm and provided a holistic picture of the self-assembly mechanism in which concentration and DP are mere expressions of the underlying physics. Liquid-state AFM also evidenced the existence of internal textures with fractal dimensions between 2.3 and 2.9, with a tendency to form globular aggregates (as opposed to more planar ones) at low concentrations. Finally, μCT gave further information as to the role of shrinkage while demonstrating the existence of another microstructural scale in the 100–400 μm range.