*2.4. Aerogel Morphology*

Selected aerogel samples were split open by gently pulling apart the two halves of the cylindrical specimen using a small fork with narrow teeth. Scanning electron microscopy (SEM) of the interior of these aerogels revealed the presence of an isotropic network largely composed of interconnected spherical particles (Figure 3). Its appearance resembles that of other cellulose gels formed by spinodal decomposition, such as from cellulose solutions in ionic liquids (e.g., [EMIm][OAc]) or molten NMMO·H2O) [43,44]. The isotropic networks are formed from (partially elongated) single-digit micron-sized clusters of cellulosic spheres being a few hundred nanometers in diameter. The voids in between the clusters are interconnected and of similar spatial shape and dimension (diameter 2–4 μm, partially elongated). A closer look revealed the coexistence of a further substructure for all of the studied aerogels. In particular, the micrograph of Figure 3E suggests that the spherical submicron particles are composed of finer fibrils, which is in agreemen<sup>t</sup> with previous studies [32,36]. Phosphorylation at the low degrees of substitution envisaged in this study had, however, virtually no impact on microscale morphology (*cf.* Figure 3A vs. Figures 3C and 3E vs. Figure 3G).

SEM micrographs of the cutting edges close to the exterior surface revealed that a comparably dense skin had formed in case of the aerogels from the non-derivatized cellulosic materials. It has an average thickness of 10–50 μm and is deficient in micron-size pores (Figures 3B and 3F). Skin formation is evidently less pronounced in the cellulose phosphate aerogels. In particular, the hwPHK-P sample features a largely skin-free open-porous flaky structure (Figure 3H). Skin formation is a well-known phenomenon occurring during cellulose processing from solution state, such as in the course of wet spinning of *Lyocell* dopes. Different coagulation kinetics across the diameter of the extruded dope strands result in significant morphological variation, typically comprising of a compact fiber core, a porous middle zone, and a semipermeable fiber skin [45].

Although relying on different coagulation mechanisms, also membranes prepared from cellulose solutions in mixtures of sodium hydroxide and urea exhibited morphological differences between surface and core of the respective materials. Their extent differed with the type of antisolvent used for cellulose coagulation and was most pronounced for ethanol [46]. Formation of a layered structure comprising of denser outer and looser inner zones has been shown also for cellophane films (sheet-extruded viscose rayon; [47]). Anisotropic cellulose II aerogels (Hermans orientation factor 0.46) obtained by self-assembly of cellulose in super-cooled ionic liquid under the impact of decelerated antisolvent infusion [28] feature skin formation too.

**Figure 3.** SEM micrographs of the interior (**A**, **C**, **E**, **G**) and near-surface breaking edge (**B**, **D**, **F**, **H**) of aerogels from non-derivatized cellulose (CL: **A**, **B**; hwPHK: **E**, **F**) and their phosphorylated counterparts (CL-P: **C**, **D**; hwPHK-P: **G**, **H**) at different magnification.
