Ice-templated freeze-dried cryogels from tunicate cellulose nanocrystals with high specific surface area and anisotropic morphological and mechanical properties

High aspect ratio cellulose nanocrystals (CNCs) extracted from tunicate were used to create so-called cryogels from an ice-templating directional freeze-drying process. The structure of the resulting solid foam was investigated at the micro- and nanoscales by scanning electron microscopy and nitrogen adsorption measurements were used to extract the specific surface area. The mechanical properties were probed by compression tests. To highlight the specificities of tunicate CNC-based cryogels, results were compared with the one obtained from two other types of nanocellulose, namely cellulose nanofibrils (CNFs) from wood and CNCs from cotton, which exhibit different dimensions, aspect ratio, flexibility and crystallinity. While CNF- and cotton CNC-based cryogels exhibited a classical morphology characterized by a sheet-like structure, a particular honeycomb organization with individual particles was obtained in the case of tunicate CNC cryogels. The latter cryogels presented a very high specific surface area of about 122 m² g⁻¹, which is unexpected for foams prepared from a water-based process and much higher than what was obtained for CNF and cotton CNC cryogels (25 and 4 m² g⁻¹, respectively). High mechanical resistance and stiffness were also obtained with such tunicate CNC cryogels. These results are explained by the high crystallinity, aspect ratio and rigidity of the tunicate CNCs combined with the particular honeycomb architecture of the cryogel.