Fine-fibrous cellulose II aerogels of high specific surface from pulp solutions in TBAF·H₂O/DMSO

Abstract

Lightweight cellulose II aerogels featuring densities of about 40–70 mg cm−3 were prepared from 1 to 3% solutions of different pulps in hot (60°C) tetra-n-butylammonium fluoride (TBAF)·H2O/dimethyl sulfoxide (DMSO) by (i) the coagulation of cellulose with EtOH to afford self-standing, transparent and homogeneous gels, (ii) gel ripening and washing, (iii) solvent exchange and (iv) supercritical carbon dioxide (scCO2) drying. Size exclusion chromatography (SEC) analyses confirmed that the chemical integrity of cellulose is largely preserved at short dissolution times. Dissolution of more than 2% of cellulose at a sufficiently low viscosity for solution, casting was achieved after the water content of TBAF was reduced to a value equaling that of the monohydrate. Intriguingly, the obtained aerogels featured higher specific surfaces (≤470 m2 g−1) than comparable materials prepared from other cellulose solvents. This is due to the particular morphology of TBAF aerogels, which is supposedly formed by spinodal decomposition of the cellulose/solvent mixture upon exposure to the cellulose antisolvent. As a result, largely homogeneous three-dimensional (3D) networks of agglomerated cellulose spheres were formed, which simultaneously acted as supporting scaffolds for interconnected micron-size voids. As cellulose spheres are composed of very small interwoven nanofibers, TBAF-derived aerogels contain a high portion of micropores and small amounts of mesopores, too.

Aerogels from Cellulose Phosphates of Low Degree of Substitution: A TBAF·H2O/DMSO Based Approach

“…Immersion of the CL and hwPHK hydrogel samples in water of incrementally increasing ethanol content (50%, 75%, 96%, and 100%) caused the gels to shrink by up to 18 vol.% ( Figure 1, Table 1). This is in agreement with previous work [36] and has been observed for other cellulose II gels too. Examples include gels formed by water-induced coagulation of cellulose from solvent systems like NMMO·H 2 samples in water of incrementally increasing ethanol content (50%, 75%, 96%, and 100%) caused the gels to shrink by up to 18 vol.% ( Figure 1, Table 1).…”

“…Examples include gels formed by water-induced coagulation of cellulose from solvent systems like NMMO·H 2 samples in water of incrementally increasing ethanol content (50%, 75%, 96%, and 100%) caused the gels to shrink by up to 18 vol.% ( Figure 1, Table 1). This is in agreement with previous work [36] Interestingly, both types of hydrogels prepared from the respective phosphorylated cellulosic materials suffered from less shrinkage during the solvent exchange sequence than their P-free counterparts ( Figure 1). This is presumably due to the presence of negatively charged phosphate groups on the surface of cellulose II nanoparticles that form and aggregate spontaneously during coagulation to afford three-dimensional networks [7].…”

“…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 agreement 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.…”

“…31 P NMR as exemplarily conducted for the hwPHK sample confirmed the expected selective substitution of the primary alcohol groups in C6 position of the anhydroglucose units (data not shown). Previously, we have shown that a wide range of cellulosic source materials can be dissolved up to 3 wt.% cellulose content within comparatively short dissolution time [36]. Optically clear solutions of sufficiently low viscosity, and hence, good workability, were obtained.…”

“…Processing of the cellulosic source materials into lightweight open-porous aerogels gives rise to a series of differences in material properties. Compared to the cellulosic fibers of the source materials, the diameters of the aerogel network forming fibrils is much smaller [36] and the degree of crystallinity lower [32]. This translated into much higher specific surface areas and moisture sensitivity.…”