Multiscale Assembly of Superinsulating Silica Aerogels Within Silylated Nanocellulosic Scaffolds: Improved Mechanical Properties Promoted by Nanoscale Chemical Compatibilization

Abstract

Silica aerogels are amongst the lightest mesoporous solids known and well recognized for their superinsulating properties, but the weak mechanical properties of the inorganic network structure has often narrowed their field of application. Here, the inherent brittleness of dried inorganic gels is tackled through the elaboration of a strong mesoporous silica aerogel interpenetrated with a silylated nanocellulosic scaffold. To this avail, a functionalized scaffold is synthesized by freeze‐drying an aqueous suspension of nanofibrillated cellulose (NFC)—a bio‐based nanomaterial mechanically isolated from renewable resources—in the presence of methyltrimethoxysilane sol. The silylated NFC scaffold displays a high porosity (>98%), high flexibility, and reduced thermal conductivity (λ) compared with classical cellulosic structures. The polysiloxane layer decorating the nanocellulosic scaffold is exploited to promote the attachment of the mesoporous silica matrix onto the nanofibrillated cellulose scaffold (NFCS), leading to a reinforced silica hybrid aerogel with improved thermomechanical properties. The highly porous (>93%) silica‐NFC hybrids displays meso‐ and macroporosity with pore diameters controllable by the NFCS mass fraction, reduced linear shrinkage, improved compressive properties (55% and 126% increase in Young's modulus and tensile strength, respectively), while maintaining superinsulating properties (λ ≤ 20 mW (m K)–1). This study details a new direction for the synthesis of multiscale hybrid silica aerogel structures with tailored properties through the use of alkyltrialkoxysilane prefunctionalized nanocellulosic scaffolds.

Novel multifunctional polymethylsilsesquioxane–silk fibroin aerogel hybrids for environmental and thermal insulation applications

“…As expected the bulk density is lower for [Si] 3.5 samples (see Table 1) and increases with higher amounts of SF and coupling agent in the composites. However, the densities are comparable with those in previously reported composites such as silica–cellulose nanofiber (CNF),28,29 silica–chitosan32,33 and silica–pectin31 composites, for which only simple tetrafunctional organosilanes have been used as silicon sources. As it is evident from the nitrogen sorption isotherms of selected samples in Fig.…”

“…The power law relationship between the bulk density ( ρ b ) and Young's modulus ( E ) is plotted in Fig. S6(a) and (b)† with an exponent b (1) of 3.84 ( R 2 = 0.85) for [Si] 3.5 and, 4.28 ( R 2 = 0.76) for [Si] 17.5 hybrid aerogels, which are similar to those reported for PU,47 PU–silica,48 cellulose–silica29 and pectin–silica composite aerogels 31. Strongly depending on the synthesis route and network connections, the power law relationships between modulus and density for silica aerogels are reported with an exponent of 3 to 3.7 49–51.…”

Highly Compressible and Hydrophobic Anisotropic Aerogels for Selective Oil/Organic Solvent Absorption

“…920 cm –1 increased significantly, and new bands at the stretching vibrations of Si-C and/or Si-O-Si at ca.770 cm –1 and −CH 3 deformation vibrations at ca. 1272 cm –1 of the polysiloxanes appeared, which indicated the successful formation of polysiloxanes on the substrate surface as expected …”

Thermal superinsulating silica aerogels reinforced with short man-made cellulose fibers

“…In addition, this was achieved with long (several millimeters in length and about 10 microns in width) and much less expensive fiber as compared to nanocellulose. Overall, the moduli of SCD composites obtained in this work are similar or higher compared to those reported for silicabased reinforced SCD aerogels of similar densities [7,28,[31][32][33][34].…”