**4. Analyses**

Scanning electron micrographs of both external and internal aerogel surfaces were taken in di fferent magnification after gold sputtering. A Philips ESEM XL30 or a Quanta 250 FEG with energy dispersive analysis X-ray (EDAX) mapping was used, both operated at an acceleration voltage of 5 kV. Evaluation of the surfaces with regard to remaining traces of TBAF was accomplished by detection of nitrogen and fluorine in EDAX mode.

Nitrogen sorption experiments at 77 K were performed using a Micrometrics ASAP 2020 analyzer. All samples were evacuated overnight at room temperature prior to the measurements. Specific surface areas were calculated according to the Brunauer–Emmett–Teller equation using a set of data points of the respective adsorption branches of the isotherms [60]. Average pore diameters were calculated from data points of the desorption branches using the Barrett–Joyner–Halenda (BJH) approach [61]. Nitrogen sorption experiments were complemented by thermoporosimetry using a Mettler-Toledo DSC 823e di fferential scanning calorimeter (DSC) equipped with liquid nitrogen module. DSC calibration for both temperature and enthalpy was performed using open-porous metallic standards (In, Pb, Zn). Aerogels were measured by weighing aliquots (ca. 1–5 mg) into 160 μL aluminum pans and submersing the samples in *o*-xylene. Measurements were carried out in ambient atmosphere using the following temperature program: (i) 0.7 ◦C min−<sup>1</sup> from +25 to −70 ◦C, (ii) 0.7 ◦C min−<sup>1</sup> from −70 to −30 ◦C ≤ Tx ≤ −20 ◦C, (iii) 0.7 ◦C min−<sup>1</sup> from Tx to −70 ◦C and iv) 0.7 ◦C min−<sup>1</sup> from −70 to +25 ◦C. Three thermograms at least were acquired for each sample. Data processing was accomplished using STARe software.

Moisture sensitivity of the cellulosic aerogels, water uptake at di fferent degrees of relative humidity (RH), and possible triggering of shrinkage of the cellulosic aerogels was studied at ambient temperature (20 ± 2 ◦C) in RH-controlled environment for a time period of 84 days after scCO2 drying. Desiccators used for these long-term studies were equipped with either phosphorus pentoxide (0% RH) or a saturated aqueous solution of CaCl2 (30% RH), NH4NO3 (65% RH), and K2SO4 (98% RH) to provide di fferent levels of humidity after equilibration. The actual RH inside the desiccators was monitored by digital hygrometers (Voltcraft data logger DL−120TH) and was found to be largely constant (±5% RH). Throughout the entire experiment, weight (±1 mg) and volume (±0.1 mm3) of the aerogels were recorded periodically.

Uniaxial compression testing in longitudinal direction of the cylindrical aerogels (Ø = 10 mm, l = 20 mm) was performed on a Zwick/Roell Z010 equipment using a feed rate of 2.4 mm min−1. Only samples transferred immediately in argon atmosphere after scCO2 drying were used. Evaluation of the recorded stress strain curves was accomplished using testXpert software. Sti ffness of the aerogels as represented by Young's modulus *E* was determined from the slope of the regression line through the linear elastic region of each stress–strain curve; boundaries were adjusted individually for each curve. A 0.2% offset yield strength was recorded, i.e., the stress at the intersection of stress-strain curve and regression line through the linear elastic region shifted parallel by 0.2% strain as reported earlier [30].

Thermal analysis was conducted using STA 409 CD Skimmer equipment (Netzsch GmbH, Germany) allowing for simultaneous recording of thermogravimetric (TG) and differential scanning calorimetry (DSC) profiles. Aliquots of the samples (approx. 8–10 mg) were weighed into Al2O3 pans and combusted in helium atmosphere (120 mL min−1) using a constant heating rate of 10 ◦C min−<sup>1</sup> over the entire temperature range of 30 to 900 ◦C.
