The Rise of Nanocellulose Aerogels
As the most abundant, biocompatible, and biodegradable biopolymer on Earth, cellulose represents a promising raw material to replace polymers derived from fossil resources. This versatility provides considerable potential for the development of innovative and high value-added materials, especially aerogels.
Pioneered by Kistler in 1931, aerogels are porous materials obtained by replacing the liquid component of a gel with air while maintaining their three-dimensional network structure. These materials exhibit unique characteristics such as high porosity, large specific surface area, low density, low thermal conductivity, and low dielectric constant. Combining these properties with those of cellulose offers considerable potential for developing innovative, lightweight, and environmentally-friendly materials covering a wide range of applications.
Nanocellulose-based aerogels are an emerging subclass of aerogels composed of cellulosic materials with a dimension in the nanometer range. These aerogels combine the unique properties of cellulose with the specific characteristics of nanoscale materials. They can be produced using various nanoscale fibrillar crystalline domains extracted from different types of cellulose, commonly classified into three primary groups: cellulose nanofibers (CNFs), cellulose nanocrystals (CNCs), and bacterial nanocellulose.
The structure and the morphology of nanocellulose play a significant influence on the method of hydrogel gelation, which can, in turn, have a significant impact on the aerogel’s properties. Furthermore, the selection of an appropriate drying method to minimize the collapse of the network structure is of crucial importance. This determining step influences the morphology of the material and confers distinctive characteristics to the nanocellulose-based aerogels, while preserving their structural integrity.
The Impact of Composition and Drying Method
This study delves into the impact of the composition (lignocellulose nanofibers–LCNFs and CNFs) and the drying methods (supercritical drying and freeze-drying) on the morphology and the properties of nanocellulose-based aerogels. The investigation evaluates the concentrations of nanofibers and the influence of lignin, a constituent of LCNFs recognized for enhancing the rigidity of plant cell walls, on the aerogel’s properties.
Hydrogel Preparation and Aerogel Synthesis
Lignocellulose and cellulose nanofibers, designated as LCNFs and CNFs, respectively, were extracted from Eucalyptus cellulose pulp through a homogenization process. Hydrogels were synthesized via physical crosslinking involving the freeze-thawing of water suspensions containing different concentrations of nanofibers (1%, 1.5%, and 2% by weight).
Aerogels were produced from nanocellulose hydrogels by solvent exchange using two different organic solvents: acetone and ethanol. Supercritical carbon dioxide (Sc-CO₂) drying was carried out by placing the nanocellulose alcogels in an autoclave, while freeze-drying involved freezing the hydrogels at -20°C prior to lyophilization.
Aerogel Characterization
The bulk density, skeletal density, porosity, specific surface area, and pore size distribution of the nanocellulose aerogels were thoroughly analyzed. Scanning electron microscopy (SEM) was used to examine the microstructure of the aerogels, and uniaxial compression tests were performed to evaluate their mechanical properties.
Shrinkage and Density
The evolution of the volume shrinkage of aerogels obtained by different drying methods reveals that supercritical drying generates significant volumetric shrinkage, which decreases with a higher content of nanofibers. This is attributed to the enhanced rigidity conferred by the reinforced nanofiber network.
Interestingly, the shrinkage rate is notably reduced in aerogels subjected to solvent exchange with acetone, despite its lower interfacial surface tension compared to ethanol. This can be attributed to the development of a more rigid hydrogel during the solvent exchange process, as acetone promotes stronger interactions with cellulose nanofibers.
In contrast, freeze-drying generates the lowest volume shrinkage, as the solvent passes directly from the solid to the gaseous state, minimizing disruption of the material structure.
The bulk density of the samples increases proportionally with nanocellulose concentration, as the mass concentration of nanofibers rises. However, the average skeletal density remains consistent across all compositions and drying conditions, aligning with literature values.
Porosity and Specific Surface Area
The porosity of the different aerogels is highly dependent on the nanofiber concentration and the drying method. For aerogels containing similar nanofiber concentrations but synthesized using different drying methods, the porosity slightly increases with decreasing shrinkage rate.
Comparing batches of aerogels synthesized with the same drying method, an inverse correlation is observed: as the nanofiber mass concentration increases, the shrinkage rate decreases, but the porosity rate also decreases. This can be explained by the thickening of pore walls with increasing nanofiber concentration, leading to a reduction in the available pore space.
Regarding the specific surface area, aerogels prepared via supercritical drying exhibit a decrease with increasing nanofiber concentration, linked to higher rates of interactions and thicker pore walls. Solvent exchange with acetone results in a significant increase in the specific surface area, particularly for aerogels with low nanofiber concentration, due to the reduced shrinkage.
The specific surface area of freeze-dried aerogels was relatively low, around ten times lower than those obtained from supercritically dried aerogels. This is likely attributed to the destruction of the pore structure by the sublimation of water crystals during the freeze-drying process.
Microstructure
SEM analysis reveals that both drying methods result in honeycomb-shaped matrices with irregular pores. However, a slight difference can be observed in terms of pore sizes. Aerogels that have undergone supercritical drying exhibit a range of pore sizes from meso- to macropore, while freeze-dried aerogels show larger “open channels”.
The presence of lignin in the nanofibers also influences the microstructure, yielding smoother and thicker pore walls. This is attributed to the reduced rigidity of the hydrogel network formed with lignocellulose nanofibers, which creates distinct interaction zones and modifies pore formation and arrangement.
Mechanical Properties
Uniaxial compression tests demonstrate that nanocellulose-based aerogels possess notable flexibility and high compressive modulus. The incorporation of lignin into cellulose nanofibers significantly enhances the mechanical properties of the resulting aerogels.
LCNF-based aerogels exhibit a higher compressive modulus than their CNF-based counterparts. This improvement in mechanical strength stems from the unique characteristics conferred by the presence of lignin, which adds rigidity and complexity to the structural network.
Insights and Implications
This study provides a comprehensive understanding of the intricate factors shaping nanocellulose aerogel properties, paving the way for the development of innovative and environmentally-friendly materials.
The choice between freeze-drying and supercritical drying significantly impacts aerogel properties, with the former preserving structure and minimizing shrinkage, while the latter yields a higher specific surface area. The incorporation of lignin into cellulose nanofibers has a notable influence, enhancing mechanical properties but reducing porosity due to the formation of a less rigid hydrogel network.
These findings highlight the importance of considering both the composition and the drying method when designing nanocellulose-based aerogels for targeted applications. The ability to tailor the properties of these materials through careful selection of raw materials and processing techniques opens up a wide range of possibilities, from lightweight insulation to advanced filtration and beyond.
As the demand for sustainable, high-performance materials continues to grow, the insights gained from this study on cellulose nanofiber aerogels will be invaluable in guiding the development of the next generation of innovative, eco-friendly materials.
