Tunable hierarchical porosity from self-assembled chitin-silica nano-composites

We studied new mesoporous materials with original properties and obtained from self-assembled chitin-silica nano-composites. Our novel synthesis allows the controlled colloidal assembly of alpha-chitin nanorods (bundles of elongated chitin monocrystals) and siloxane oligomers. Calcination of nano-composites results in mesoporous silica materials. Their pore volume fraction phi(POR) (0-0.52) is strongly correlated to the initial chitin content. Using N-2 sorption and TEM data, we identify and characterize primary and secondary textural units related to the imprints of chitin monocrystals (2.5 nm wide) and nanorods (20-30 nm wide) respectively. Primary textural units are preserved over a wide phi(POR) range (linear relationship between pore volume and specific surface area). The coating of monocrystals by siloxane oligomers leads to a siloxane network of fractal nature as deduced from complementary SAXS data. Beyond a critical value phi(POR)' estimated near 0.2, the coating is partial, and the porosity becomes more open and connected. At larger scales, the arrangements of secondary textural units result in complex textures and long-range ordering, showing similarities with textural features found in natural materials. We discuss the competition between entropy-driven transitions typical of anisotropic particles and kinetic arrest due to colloidal gelation and inorganic condensation. Finally, a schematic model for texture formation is given.