Hierarchical porous structures and properties of cellulose aerogels

Abstract

Cellulose-based biocomposites from nature exhibit remarkable mechanical properties which inspire to prepare synthetic biocomposites. The detailed studies of the porous cellulose materials having hierarchical structures are essential to understand how the cellulose nanofiber network and interconnected macro- and mesoporous structures exclusively influence the mechanical properties. We designed the hierarchical open porous structures of cellulose aerogels by a novel method in which an intricate trap of oil droplets in the cellulose-dissolved molten salt hydrate acts as a structural template assisting the formation of interconnected macroporous structures. After washing, the wet alcogels of cellulose were employed in super critical drying technique to prepare aerogels. We have employed the surfactants with different hydrophilic-lipophilic balance (HLB) values in order to vary the hierarchical structures. The diversity in physical properties of the cellulose aerogels were characterized by X-ray μ-computed tomography, density analyzer, scanning electron microscopy, nitrogen adsorption–desorption analysis, X-ray powder diffraction, FTIR spectroscopy, mechanical testing and gas permeability. In the resulting products, the variations in physical properties were the size of the macropores produced by the emulsion template, the change in surface features of the cell walls and the presence of nanopores in the cell walls. The mechanical properties of hierarchically porous cellulose materials were diverged from lightweight soft materials to hard, stiff and dense material if the drying technique was changed to freeze drying or ambient drying. This new method opens design routes for cellulose-based materials suitable for applications in new type of filters, since the emulsion template method allows tailoring the gas permeability and the nanopores of the cell walls act as absorbers.