Hydrophobic and thermal-insulating aerogels based on rigid cellulose nanocrystal and elastic rubber

Introducing terminal alkyne groups at the reducing end of cellulose nanocrystals by aldimine condensation for further click reaction

In recent years, click reactions with cellulose nanocrystals (CNC) participation have gradually become a research hotspot. Carboxylamine condensation is the most used method to introduce terminal alkyne groups at the reducing end of CNC as reaction sites for click reactions. However, hydroxyl groups on CNC surface would be slightly oxidized during the carboxyamine condensation process, inducing the potential positions of introduced alkynes would be not only at the reducing end but also on CNC surface. Here, aldimine condensation was proposed to introduce terminal alkyne groups just at the reducing end of CNC, and a systematic comparison analysis was conducted with carboxylamine condensation. Firstly, the selectivity and extent of alkynylation were characterized by XPS and EA. Secondly, the end aldehyde content in these CNC samples was measured by the BCA method, which quantitatively explained the grafting efficiency of aldimine condensation and further verified its feasibility. Thirdly, the clickability of the modified CNC samples was confirmed through XPS analysis of the products after a pre-designed click reaction. In sum, aldimine condensation was proven to be a simple and effective strategy for introducing terminal alkyne groups at the reducing end of CNC, which could be used as reaction sites for further click reactions.

Cellulose nanocrystals in the development of biodegradable materials: A review on CNC resources, modification, and their hybridization

The escalating demand for sustainable materials has propelled cellulose into the spotlight as a promising alternative to petroleum-based products. As the most abundant organic polymer on Earth, cellulose is ubiquitous, found in plants, bacteria, and even a unique marine animal-the tunicate. Cellulose polymers naturally give rise to microscale semi-crystalline fibers and nanoscale crystalline regions known as cellulose nanocrystals (CNCs). Exhibiting rod-like structures with widths spanning 3 to 50 nm and lengths ranging from 50 nm to several microns, CNC characteristics vary based on the cellulose source. The degree of crystallinity, crucial for CNC properties, fluctuates between 49 and 95 % depending on the source and synthesis method. CNCs, with their exceptional properties such as high aspect ratio, relatively low density (≈1.6 g cm-3), high axial elastic modulus (≈150 GPa), significant tensile strength, and birefringence, emerge as ideal candidates for biodegradable fillers in nanocomposites and functional materials. The percolation threshold, a mathematical concept defining long-range connectivity between filler and polymer, governs the effectiveness of reinforcement in nanocomposites. This threshold is intricately influenced by the aspect ratio and molecular interaction strength, impacting CNC performance in polymeric and pure nanocomposite materials. This comprehensive review explores diverse aspects of CNCs, encompassing their derivation from various sources, methods of modification (both physical and chemical), and hybridization with heterogeneous fillers. Special attention is devoted to the hybridization of CNCs derived from tunicates (TCNC) with those from wood (WCNC), leveraging the distinct advantages of each. The overarching objective is to demonstrate how this hybridization strategy mitigates the limitations of WCNC in composite materials, offering improved interaction and enhanced percolation. This, in turn, is anticipated to elevate the reinforcing effects and pave the way for the development of nanocomposites with tunable viscoelastic, physicochemical, and mechanical properties.

Ambient Pressure Drying to Construct Cellulose Acetate/Benzoxazine Hybrid Aerogels with Flame Retardancy, Excellent Thermal Stability, and Superior Mechanical Strength Resistance to Cryogenic Temperature

Cellulose aerogels are highly attractive candidates in various applications, such as thermal insulation, adsorption separation, biomedical field, and as carriers, due to their intrinsic merits of low density, high porosity, biodegradability, and renewability. However, the expensive cost of the supercritical drying process and poor mechanical properties limit their practical applications. Herein, a new method was presented to fabricate cellulose acetate/benzoxazine hybrid aerogels (CBAs) with low cost, low drying shrinkage, excellent mechanical properties under cryogenic condition (-196 °C), outstanding thermal insulation, flame retardancy, and good thermal stability by ambient pressure drying. In more detail, the weighted drying shrinkage rate of CBAs-T2 can be controlled to 6.8% (the average value along the radial and axial directions), mainly due to the enhanced skeleton, by introducing polybenzoxazine networking chains. The resultant CBAs-T2 exhibit outstanding mechanical properties at room temperature because of the presence of the polybenzoxazine hybrid in the cellulose networking system. CBAs-T2 still have good mechanical properties even after subjecting them to liquid nitrogen treatment. In addition, the optimal value of thermal conductivity (0.033 W m^-1 K^-1) is gained easily because of the uniform cross-linking networking structure and small pore size. A superior flame retardance of CBAs-T2 is endowed to achieve self-extinguishment after ignition, which is attributed to the presence of the aromatic ring in the backbone structure. Moreover, the good thermal stability of CBAs-T2 is attributed to the fact that polybenzoxazine components could resist the decomposition of cellulose acetate and inhibit heat release during the combustion process. Our study would provide a novel method for obtaining biomass aerogels including the cellulose-based materials system with low drying shrinkage and superior mechanical properties despite bearing a cryogenic environment by the low-cost ambient pressure drying approach.

Advanced cellulose nanocrystals (CNC) and cellulose nanofibrils (CNF) aerogels: Bottom-up assembly perspective for production of adsorbents

The most common and abundant polymer in nature is the linear polysaccharide cellulose, but processing it requires a new approach since cellulose degrades before melting and does not dissolve in ordinary organic solvents. Cellulose aerogels are exceptionally porous (>90 %), have a high specific surface area, and have low bulk density (0.0085 mg/cm³), making them suitable for a variety of sophisticated applications including but not limited to adsorbents. The production of materials with different qualities from the nanocellulose based aerogels is possible thanks to the ease with which other chemicals may be included into the structure of nanocellulose based aerogels; despite processing challenges, cellulose can nevertheless be formed into useful, value-added products using a variety of traditional and cutting-edge techniques. To improve the adsorption of these aerogels, rheology, 3-D printing, surface modification, employment of metal organic frameworks, freezing temperature, and freeze casting techniques were all investigated and included. In addition to exploring venues for creation of aerogels, their integration with CNC liquid crystal formation were also explored and examined to pursue "smart adsorbent aerogels". The objective of this endeavour is to provide a concise and in-depth evaluation of recent findings about the conception and understanding of nanocellulose aerogel employing a variety of technologies and examination of intricacies involved in enhancing adsorption properties of these aerogels.

Enhanced Toughness and Sound Absorption Performance of Bio-Aerogel via Incorporation of Elastomer

In this study, Arabic gum/ carboxylic butadiene-acrylonitrite latex aerogels (AG/XNBRL) hybrid aerogel was successfully prepared by a green method, i.e., the combination of latex compounding and vacuum freeze-drying process. After that, the obtained composites were subjected to a high temperature treatment to crosslink the rubber phase. It was found that the AG in the AG/XNBRL hybrid aerogel could act as a framework to improve the dimensional stability of the aerogel, while the XNBRL phase could significantly improve the mechanical flexibility of the ensuing composite. Compared to the AG aerogel which is highly brittle in nature, the AG/XNBRL hybrid aerogel not only exhibits significantly enhanced toughness, but also shows improved thermal stability and sound absorption performances; for instance, the half weight loss (50%) temperature and average sound adsorption coefficient for aerogel containing 30 wt% XNBRL is 344 °C and 0.585, respectively, which are superior to those of neat AG aerogel. Overall, this work provides novel inspiration to prepare the mechanical robust bio-based aerogel for the sound absorption application.