The development of well-characterized cellulose nanofibrils (CNFs) has allowed for the development of the bottom-up engineering of a wide range different materials with tailored properties. A common denominator in these materials is the need for a controlled self-assembly of the CNFs in the wet state and a subsequent preservation of the so created structure in the dry form. In this respect the preparation of foams and aerogels has attracted a lot of attention since the arrested states of the fibrils in the foams or in the liquid state can basically be preserved with these techniques with a subsequent freeze drying, controlled liquid exchange or different approaches using chemical crosslinking. In our work we have been using basically all these different techniques and by combining high quality CNF dispersions with nano-CaCO3 and alginate, a freezing in a standard freezer at -18°C and thawing using acidic acetone it was possible to prepare aerogels/foams with as low density as 2 kg/m3 (1). In another approach, specially prepared cationic fibrils were combined with nano-MOFs (nZIF-8), the use of directional freezing using liquid nitrogen followed by lyophilization we were able to prepare aerogels with excellent mechanical properties at a composition of 10 % CNFs and 90 % MOFs. The so prepared aerogels had a specific surface areas of 1470 m2/g and could be used for gas separation, water cleaning and for fire retardancy (2). After carbonization at 800-100 °C these aerogels also could be used as symmetrical electrodes in two electrode capacitors and showed to have a capacitance of 270 F/g at 50 mV/s and a very good stability over 20 000 cycles (2). In a final example we were also able to show that dialdehyde modified fibrils could be used to form wet-stable foams/aerogels by freezing at -18°C, thawing and liquid exchange to acetone and drying. By using this technique, it was possible to form low density materials with shape memory and also to form low density materials with excellent dry and wet mechanical properties and basically any shape (3)
1.Rostami, J. et al., Adv. Materials 2022, 2204800
2. Rostami, J. et al., Materials Today 2021,48, 47-58
3. Erlandsson, J. et al., Biomacromolecules 2019, 20, 728−737
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