Synthesis and characterization of novel sustainable dual crosslinked sodium carboxymethyl cellulose aerogels

Aerogels are remarkable nanoporous materials with unique properties such as low density, high porosity, high specific surface area, and interconnected pore networks. In addition, their ability to be synthesized from various precursors such as inorganics, organics, or hybrid, and the tunability of their properties make them very attractive for many applications such as adsorption, thermal insulation, catalysts, tissue engineering, and drug delivery. The physical and chemical properties and pore structure of aerogels are crucial in determining their application areas. Moreover, it is possible to tailor the aerogel properties to meet the specific requirements of each application. Synthesis of novel carboxymethyl cellulose (CMC) aerogels for thermal insulation applications is an essential area of research in finding alternative sustainable insulation materials instead of petroleum-based ones. Previous studies have been mainly focused on polysaccharide aerogels, especially cellulose aerogels. Although extensive research has been carried out on cellulose ether hydrogels or films, cellulose ethers and the effects of their functional groups on gelation mechanism, aerogels' morphology, and thermal properties have not been widely investigated. Moreover, when these cellulose ethers are used to synthesize aerogels, their functional groups can play a crucial role in controlling the morphology of the resulting aerogel. Modifying the synthesis parameters such as solvent choice, concentration, pH, and temperature makes it possible to tune the interactions between cellulose ethers and the solvent, leading to the formation of aerogels with tailored morphologies. Furthermore, adding crosslinking agents or other additives can also influence cellulose ether-based aerogels' gelation kinetics and final morphology. This tunability makes cellulose ether-based aerogels promising materials for various applications, including insulation, drug delivery, and tissue engineering. The objective of this study is to synthesize CMC aerogels and also, at the same time, preserve their 3D porous structure and shape. To obtain CMC hydrogels with a 3D porous structure, the freeze-thaw induced gelation method was used and effecting factors were investigated. Since CMC aerogels obtained via supercritical drying have not been studied extensively in the literature, solvent exchange conditions for CMC hydrogels were determined. We aimed to elucidate the pore size distribution, specific surface area, and porosity of these novel materials, thereby paving the way for future exploration of their potential applications in diverse fields. Our work expands the available synthesis methods for CMC aerogels and provides crucial insights into their fundamental pore characteristics, contributing to the ongoing development of sustainable and environmentally friendly aerogels. In this study, sustainable maleic anhydride crosslinked CMC aerogels were synthesized by freeze-thaw induced gelation method. The effect of precursor concentration, crosslinker concentration, and gelation parameters on the aerogel properties, such as density, volumetric shrinkage, surface area, pore diameter, and thermal properties, were investigated. The prepared aerogels exhibited low density between 0.051 g/cm3 and 0.204 g/cm3, high porosity (> 93%), and high specific surface areas up to 214 m2/g. The crosslinking mechanisms in CMC aerogels were examined by Fourier Transform Infrared Spectroscopy (FTIR) and Nuclear Magnetic Resonance (NMR) analysis. The lowest thermal conductivity was measured as 0.038 W/mK. A theoretical equation was also developed to calculate the thermal conductivity of aerogels. The results indicated that it is possible to tune the aerogel properties by adjusting biopolymer or crosslinker concentration and changing the freeze-thaw parameters. PVA was incorporated into CMC aerogels to promote intermolecular crosslinking and, therefore, reduce the pore diameter. CMC/PVA hybrid aerogels from 4 and 6 wt% aqueous solutions were prepared, and the porous structures of hybrid aerogels were investigated. CMC/PVA hybrid aerogels had low bulk density and high porosity (> 95%). FTIR and NMR analyses were conducted to confirm the esterification reaction. The thermal conductivity of CMC/PVA hybrid aerogel was measured as 0.055 W/mK. Finally, sulfuric acid catalyzed aerogels were synthesized, and it was seen that it is possible to reduce the gelation time with an acid catalyst. The effect of acid concentration on the aerogel properties, such as density, volumetric shrinkage, surface area, pore diameter, and thermal properties, was investigated. The crosslinking reaction was confirmed by FTIR analysis. Contrary to expectations, sulfuric acid catalyzed aerogels had low porosity, low surface area, and high pore size. However, these results shed light on the potential to tailor aerogel properties using acid catalysts precisely. These findings provide valuable insights for optimizing aerogel synthesis tailored to specific applications.