Synthesis of textured polysaccharide–silica nanocomposites: a comparison between cellulose and chitin nanorod precursors†
The properties and formation mechanisms of silica-based nanocomposites textured by polysaccharide nanorods have been studied from a series of new materials obtained using cellulose nanocrystals (CNCs), and their comparison with those obtained using equivalent chitin nanorods. Stable ethanolic co-suspensions of CNCs and siloxane oligomers have been processed by spray-drying to form hybrid cellulose–silica nanocomposites. The materials’ surface and internal texture, before and after calcination, are highly informative about the distribution and templating role of the polysaccharide nanorods (cellulose or chitin). Both electron microscopy and nitrogen sorption analyses highlight the role of the particle dimensions in defining the porosity of calcined silica replicas. In addition, the structures of the polysaccharide crystals inside the nanocomposites were investigated by XRD and 13C solid-state NMR. From these results, it is concluded that both polysaccharide crystalline precursors form the same sort of silica-based nanocomposites, with no visible effect of their different surface chemistry towards templating. There are strong interactions between the polysaccharide surface and the siloxane oligomers that can be explained by the high density of hydroxyl groups on the sugar cycles. During the sol–gel processing, these interactions favour the formation of a silica shell around the nanocrystals accompanied by a slight contraction of the cellulose and chitin crystal structures, not observed until now. When the spray-dried nanocomposites are subjected to ammonia vapours, the siloxane condensation increases and, concomitantly, the role of the interactions at the siloxane–polysaccharide interface is decreased. This study leads therefore to new insights and opportunities for the design of nanocomposites or porous materials, the properties of which can be tuned by the choice of polysaccharide nanorods with specific dimensions and functionalities.