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Nanofibrillated cellulose aerogels are low-density bio-based materials that present a great potential in several fields. The properties of aerogels are a consequence of their microstructure. The understanding and control of the structure is therefore a priority for the preparation of aerogels with specific properties. This study aims at investigating how freeze-drying conditions affect the microstructure of nanofibrillated cellulose aerogels and how their microstructure affects their thermal insulating properties. TEMPO-oxidized nanofibrillated cellulose aerogels were prepared by freeze-drying using two different moulds in order to vary the cooling rate and the temperature gradient. The microstructure of the nanofibrillated cellulose aerogels obtained was investigated using both scanning electron microscopy and nitrogen adsorption–desorption. Controlling solvent solidification has a drastic effect on aerogel microstructure. Different temperature gradients result in different distributions of pore size, each with its specific shape and connectivity. The thermal insulation properties of aerogels were evaluated using the hot strip technique. The resulting original structures revealed very different thermal insulation properties. Aerogels with a lamellar microstructure oriented in the direction of the temperature gradient showed porous channels. As a consequence, they had the poorest performance in terms of thermal insulating properties, with a minimal thermal conductivity of 0.038 W/(m·K). Aerogels with a cellular microstructure had smaller pores and reached a minimal thermal conductivity of 0.024 W/(m·K). Graphical Abstract

Nanocelluloses occur under various crystalline forms that are currently being selectively used for a wide variety of high performance materials. In the present study, two cellulose nanofibers (CF-I) were mercerized by alkaline treatment (CF-II) without degradation, the same molar mass of 560,000 g/mol was measured. Both samples were acid hydrolyzed, leading to cellulose nanocrystals in native (CNC-I) and mercerized (CNC-II) forms. This study focuses on the detailed characterization of these two nanoparticle morphologies (light and neutron scattering, TEM, AFM), surface chemistry (zetametry and surface charge), crystallinity (XRD, 13 C NMR), and average molar mass coupled to chromatographic techniques (SEC–MALLS-RI, A4F-MALLS-RI), revealing variations in the packing of the crystalline domains. The crystal size of CNC-II is reduced by half compared to CNC-I, with molar masses of individual chains of 41,000 g/mol and 22,000 g/mol for CNC-I and CNC-II, respectively, whereas the same surface charge density is measured. This study gives an example of complementary characterization techniques as well as results to help decipher the mechanism involved in mercerization.