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.