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

Abstract

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.

Alignment of Cellulose Nanofibers: Harnessing Nanoscale Properties to Macroscale Benefits

“…There are three approaches for ice-templating (Figure a–c): bulk freezer freezing (FF), unidirectional gradient freezing (UGF), and bidirectional gradient freezing (BGF). ,− Chen et al compared these three methods of ice templating and determined that temperature-gradient-directed methods produce repeatable aligned structures. , One of the easiest and most common methods utilized is UGF (Figure b) . A temperature gradient is applied in one direction, and ice crystals grow slowly parallel to the freezing direction, forming channels between crystals.…”

“…(d–f) SEM images of three different directions (XZ, YZ, and XY) of TOCNF foams from TOCNF-FF, TOCNF-UGF, and TOCNF-BGF techniques. Adapted with permission ref . Copyright 2019 American Chemical Society.…”

Biobased foams for thermal insulation: material selection, processing, modelling, and performance

“…For these cases, the most common method employed is freeze drying. [13][14][15][61][62][63] Mixing neat cellulose with water and other additives (e.g. surfactant, nanoparticles) and then freezing causes ice crystals to nucleate and grow surrounded by the cellulose matrix.…”

“…placing inside a freezer). However, there are downsides to this method of freezing as it can lead to cell collapse 61 and is generally not scalable or easily reproducible. It is also possible in homogeneous freezing conditions to form an open-cell, more disordered structure as opposed to a closed-cell structure as would typically be observed in EPS.…”

Superelastic, Hygroscopic, and Ionic Conducting Cellulose Nanofibril Monoliths by 3D Printing

“…With a green and sustainable requirement for material synthesis, cellulose nanofibril (CNF), as an abundant, renewable, and biodegradable material, has been extensively explored as a good candidate for 3D printing. ,, However, these 3D-printed CNF monoliths are generally rigid without satisfactory elastic and compressive shape recovery behavior under dry conditions. Although elastic CNF aerogels and foams have been successfully fabricated by silane vapor hydrophobic modification, − chemical cross-linking, and directional freeze-casting, these strategies are ineffective for 3D-printed monoliths due to their complex hierarchical structure. So far, elastic 3D-printed cellulose monoliths have not been reported and warrant further investigation to overcome this limitation.…”