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

### **Christian B. Schimper 1, Paul S. Pachschwoell 1, Hubert Hettegger 1, Marie-Alexandra Neouze 2, Jean-Marie Nedelec 3, Martin Wendland 4, Thomas Rosenau 1,5 and Falk Liebner 1,6,\***


Academic Editors: Carlos A. García-González, Pasquale Del Gaudio and Ricardo Starbird Received: 2 March 2020; Accepted: 5 April 2020; Published: 7 April 2020

**Abstract:** Biopolymer aerogels of appropriate open-porous morphology, nanotopology, surface chemistry, and mechanical properties can be promising cell scaffolding materials. Here, we report a facile approach towards the preparation of cellulose phosphate aerogels from two types of cellulosic source materials. Since high degrees of phosphorylation would afford water-soluble products inappropriate for cell scaffolding, products of low DSP (ca. 0.2) were prepared by a heterogeneous approach. Aiming at both i) full preservation of chemical integrity of cellulose during dissolution and ii) utilization of specific phase separation mechanisms upon coagulation of cellulose, TBAF·H2O/DMSO was employed as a non-derivatizing solvent. Sequential dissolution of cellulose phosphates, casting, coagulation, solvent exchange, and scCO2 drying afforded lightweight, nano-porous aerogels. Compared to their non-derivatized counterparts, cellulose phosphate aerogels are less sensitive towards shrinking during solvent exchange. This is presumably due to electrostatic repulsion and translates into faster scCO2 drying. The low DSP values have no negative impact on pore size distribution, specific surface (*S*BET ≤ 310 m<sup>2</sup> g<sup>−</sup>1), porosity (Π 95.5–97 vol.%), or stiffness (*E*ρ ≤ 211 MPa cm<sup>3</sup> g<sup>−</sup>1). Considering the sterilization capabilities of scCO2, existing templating opportunities to afford dual-porous scaffolds and the good hemocompatibility of phosphorylated cellulose, TBAF·H2O/DMSO can be regarded a promising solvent system for the manufacture of cell scaffolding materials.

**Keywords:** cellulose phosphate; cellulose phosphate aerogel; interconnected porosity; supercritical carbon dioxide; tetrabutylammonium fluoride; TBAF/DMSO
