Dual-Enhanced Hydrophobic and Mechanical Properties of Long-Range 3D Anisotropic Binary-Composite Nanocellulose Foams via Bidirectional Gradient Freezing

Inspired by the structured architecture of natural materials, research has focused on the assembly of long-range three-dimensional (3D) anisotropic aligned structure through the synergy of silylated binary-composite and bidirectional gradient freezing using renewable and biocompatible cellulose nanofibrils. Low-cost methyltrimethoxysilane (MTMS) was introduced to reinforce the cross-linking strength between nanofibrils, simultaneously improving the surface hydrophobicity of cellulose foams. A copper coldfinger with a thermal insulative polydimethylsiloxane (PDMS) wedge was used to build bidirectional anisotropic aligned porous structures using bitemperature gradients to control the nucleation and propagation of ice crystals. This two-step method successfully assembled the cellulose nanofibrils into ultralight and ultraporous foams. The effects of freezing techniques, including freezer freezing, unidirectional gradient freezing, and bidirectional gradient freezing on the internal morphology and surface structure of modified foams have been thoroughly investigated by micro-CT and SEM characterizations. The developed 3D anisotropic honeycomb-like foams exhibited excellent compressive elasticity and enhanced ultraporous properties. Moreover, the synergistic effect of chemical techniques and freezing methods has realized a dual enhancement of the surface hydrophobicity and mechanical properties of cellulose foams. Our methodology could provide an effective way of achieving precise control of the final architecture of high-aspect-ratio fibril materials. Moreover, it offers a flexible process for preparing various functional composites: in particular, advanced materials such as for energy storage, thermal insulation, and composites requiring a higher level of structure control.