Anisotropic cellulosic aerogels constitute an emerging subclass of bio-based materials that holds promise for novel applications owing to their unique properties. Amongst them cellulose II aerogels are of particular interest as their preparation is less expensive and allows for better tunability compared to their cellulose I counterparts. Cellulose II aerogels are frequently prepared by coagulation of cellulose from dilute solutions and subsequent scCO2 drying. Depending on the cellulose solvent and antisolvent employed, the nanomorphology of the obtained aerogels can vary to a large extent which renders them attractive materials for a wide range of applications including tissue engineering, high performance thermal insulation or advanced separation [Pircher et al, Cellulose, 2016].
Recently, we presented a novel simple and energy efficient route towards the preparation of anisotropic cellulose II gels and aerogels, respectively, employing the principles of diffusion-driven self-assembly. Cellulose was dissolved in the ionic liquid TMGH[OAc] and cast in semipermeable cylindrically shaped molds which were submersed in the antisolvent ethanol. From the obtained cylindrically shaped cellulose II gels cross sectional cuts along and perpendicular to the cylinder axis were performed before drying. The resulting samples were investigated with respect to porosity (BET), morphology (SAXS, SEM) and mechanical properties (compression test) [Plappert et al, Biomacromolecules, 2018].
Lately we followed the change of the nano cellulose network of such oriented samples by in-situ Synchrotron SAXS experiments during compression up to 80 % compressive strain. The change in the network, i.e. elastic and plastic deformation, as seen in the stress-strain curves can be related to the change of the network properties on the nanoscale. The compression in general results in samples more rigid to further mechanical load but with similar inner surface, smaller pore size and higher anisotropy. Surface roughness, network dimensionality and main orientation of the fibres highly depend on the degree of compression. This indicates a simple post processing where after basic orientation final materials morphology as well as mechanical and optical properties are tailored directly by a controlled compression step. This would allow to match exactly applicational needs, like e.g. templating or tissue engineering [Rennhofer and Plappert et al, Soft Matter, 2019].