Lignin-kappa-carrageenan cryogels and aerogels: Structure and swelling analysis

Due to the abundance of lignin, its valorisation for the synthesis of various materials may be important from an environmental perspective. This work proposes the synthesis of porous carbon materials using sodium lignosulphonate and a polysaccharide with thermal transitions (kappa-carrageenan), assessing how the precursor's viscoelasticity and the drying process (supercritical CO2 or freeze-drying) can tune the properties of the obtained material. Rheological and infrared results indicate the formation of a network based on electrostatic interactions, leading to strong gels (G′ over 100,000 Pa). Furthermore, the use of supercritical CO2 promotes the obtention of materials with a nanofibrous mesoporous structure, as opposed to the chaotic macroporous structure obtained through freeze drying. The nanofibrous network reduces the pore size compared to the macroporous structure by three orders of magnitude (10 nm vs 30 μm). The nanofibrous network also improves the surface area of the material (values between 60 and 90 m2·g−1) compared to the null area of the macroporous solid. Furthermore, thermogravimetric results indicated that lignin can be removed from the network before drying with supercritical CO2 due to the solvent exchange step. Finally, materials dried with supercritical CO2 have the highest swelling ratio (ranging from 400 % to 1000 % depending on the lignin ratio), following pseudo-second-order kinetics. This phenomenon is numerically studied using computational fluid dynamics (volume of fluid method), which determines the material's permeability and observes how the fluid replaces the air inside the pores of the dried material with supercritical CO2 in <1 s.