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The development of biobased materials with lower environmental impact has seen an increased interest these last years. In this area, nanocelluloses have shown a particular interest in research and industries. Cellulose nanocrystals (CNC) and cellulose nanofibrils (CNF) are both known to stabilize oil-water interfaces, forming the so-called Pickering emulsions which are surfactant-free, highly stable emulsions armored by a layer of solid particles. This work describes the emulsion's characteristics and properties according to particle size, shape and surface chemistry in order to produce controlled micro- and nanoemulsions stabilized by nanocelluloses. For this purpose, four nanocelluloses which vary in source, length, width, and surface charge density were used. Isolated droplets were produced by CNCs and interconnected droplets by CNFs that led to distinct drop size (micro- and nanosized), organization of nanoparticles at the surface of the droplets, stability, and mechanical properties through rheological measurements. This work gives a precise description of the resulting emulsions and shows the ability to produce nanosized droplets for CNC and TEMPO oxidized CNF but not for the less fibrillated CNF using HP-homogenizer. Individual noncreaming droplets with average diameters as low as 350 nm were achieved for cotton CNCs and TEMPO oxidized CNFs.

Thermal superinsulating properties of biobased materials are investigated via the structuration of aerogels through a biphasic system. Highly stable Pickering emulsions are produced using TEMPO-oxidized cellulose nanofibrils (NFC) adsorbed at an oil/water interface. NFCs form an entangled system of clusters of droplets that lead to excellent mechanical properties. The emulsions produced are strong gels that are further used as template to form aerogels. The freeze-dried emulsions result in porous bioaerogels with extremely low densities (0.012-0.030 g/cm). We describe a hierarchical morphology with three levels of porosity: an alveolar organization of larger macropores due to ice crystals, spherical smaller macropores induced by the emulsion template, and mesoporous domains localized at the pore walls level. The low-density bioaerogels have compression moduli as high as 1.5 MPa and can be deformed up to 60% strain before the structure collapse. NFC aerogels have thermal superinsulating properties; the lowest thermal conductivity obtained is 0.018 W/(m·K). In the context of the development of sustainable materials, we demonstrate that NFC-stabilized Pickering emulsions are excellent templates to produce fully biobased, mechanically strong thermal superinsulating materials.

International audienceNanofibrillated 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)