Structurally Integrated Properties of Random, Uni-Directional, and Bi-Directional Freeze-Dried Cellulose/Chitosan Aerogels

Abstract

Aerogels as a thermally insulating material have attracted extensive consideration in recent years due to prevailing energy consumption in building, industrial sector, and aerospace applications. Cellulose contains all the favorable properties to be used as a thermally resistant substance but due to structural instability, it needs to be impregnated with other resilient substances. We report herein the preparation of cellulose nanofiber (CNF)/chitosan (CS) isotropic and anisotropic composite aerogels (CCSA) by employing three different: random, uni-directional, and bi-directional freezing techniques, ensuing freeze-drying process to investigate the structural modification effect on the thermal and mechanical properties. Aerogels behaved distinctively along different axis due to holding entirely eccentric porous micro-orientation along lateral (perpendicular to ice growth) and axial (parallel to ice growth) directions. Randomly frozen aerogels (r-CCSA) contained uneven porous networks because of random ice crystal formation within the microstructure and resulted in isotropic characteristics with thermal conductivity (λ) of 0.038Wm-1K-1 in both axial and radial direction. whereas unidirectional frozen aerogels (u-CCSA) exhibited anisotropic microstructure and performance with lamellas in axial and honeycombed in radial direction resulting in λ value of 0.040 Wm-1K-1 and 0.034 Wm-1K-1 respectively. Similarly, the controlled temperature gradient during the preparation of bidirectional aerogels (b-CCSA) presented an anisotropic sheet-like microstructure and resulted in ultra-low thermal conductivity along the axial and radial geometry with λ of 0.33 Wm-1K-1 and 0.027 Wm-1K-1 respectively because of the Knudsen effect. CCSA reported in this work resulted in ultra-low density (5 to 16 mgcm-3), high porosity (~99.6%), and robustness mechanically with an endurance of 60% strain. Such a composite bio-mass aerogel preparation method will offer a clear insight into the adoption of the right methodology according to target practical applications with consideration of environmental safety.