Solid Material: Hyperelastic Cellulose Aerogel

Aerogels, known as the lightest solid material, have many applications in the fields of thermal insulation, particulate matter capture and accurate sensors. Combined with the advantages of high porosity and environmental protection, cellulose aerogel is an ideal substitute for traditional non renewable gel. The cellulose based aerogels with high aspect ratio can achieve large deformation. Therefore, cellulose aerogel is a substitute for silica and phenolic resin aerogels. Due to the existence of a large number of hydroxyl groups in the dehydrated glucose ring of cellulose, abundant dynamic hydrogen bonds are formed, which leads to the decrease of structural stability and viscosity. Due to the difficulty of separation of nanofibers after close contact, fatal structural collapse of cellulose aerogels usually occurs after large deformation. The low elasticity and structural instability limit the practical application of cellulose aerogels in heat insulation and air filtration.

At present, researchers have developed some methods to construct elastic cellulose aerogels. For example, there is a top-down method to remove lignin and hemicellulose from natural wood by chemical treatment. Silane modified or petroleum based polymer compositions can also improve the elasticity by shielding hydrogen bonds. However, the chemicals introduced have potential impacts on the environment and human health and weaken the advantages of cellulose as an all natural and safe biomass product, especially when used in health-related areas such as masks and air purification. In the aspect of structure, it is an effective way to improve the mechanical properties of porous materials to design multi-layer hierarchical structure on multiple scales. In addition, since no petrochemical products are involved, this design principle can maintain the biodegradability of cellulose. For example, the recently reported double ice template strategy can be used to produce elastic cellulose aerogels. However, the strategy requires a total of six steps, which involve two rounds of freezing and freeze-drying, as well as intermediate redispersion and final hydrophobic modification. Therefore, it is still a challenge to develop a more sustainable and simple method to produce elastic cellulose aerogels completely free of petrochemical products.

In view of this, the team of academician Yu Shuhong from University of science and technology of China (USTC) has prepared the super elastic anisotropic cellulose hierarchical aerogels (AChA) through the non Petrochemical strategy based on ice template. Biopolymer polyhydroxyalkanoates (PHA) particles were introduced into the cellulose network to avoid excessive cell wall densification. After thermal etching, PHA becomes large pores, which not only reduces the rigidity and viscosity of the wall, but also acts as a defect site to guide micro deformation and disperse internal stress during macro deformation. In addition, heat induced dehydration of cellulose also leads to hydrogen bonding. All of these measures help to reduce the rigidity of the wall and the adhesion between nanofibers, thus making aerogels hyperelastic. Due to the large aspect ratio of the components, the structural resilience is enhanced by the bending or elastic buckling of the cell wall. The anisotropic and multi-layered porous structure can reduce the local strain and make AChA have better deformation ability under large strain. The aerogels have excellent stability, including temperature independent elasticity, fatigue resistance (about 5% plastic deformation after 100000 cycles), and high angular recovery rate (1475.4 ° s-1), which is superior to most cellulose based aerogels. This benign strategy can be used to produce multi-layered hierarchical porous materials with good mechanical properties, thermal insulation, particle filtration and other properties while retaining the biosafety. The study was published in the latest issue of Angewandte Chemistry International Edition as a paper entitled "a petrochemical free route to superelastic hierarchical cellular aerogel".

Preparation and structure of AChA

In order to construct a multilayer hierarchical structure, the authors developed a collaborative strategy combining unidirectional freeze casting and thermal etching (Fig. 1a). Firstly, a uniform and stable suspension containing cellulose nanofibers (CNF) and PHA particles with large aspect ratio was prepared by ball milling, and then unidirectionally frozen on a cold copper platform. CNF and PHA were compacted by ice crystals, and then formed aerogel prepolymer (p-acha) after freeze-drying. Thanks to the large aspect ratio, bacterial CNF can be easily assembled into flexible, tangled networks with improved mechanical integrity. However, the strong hydrogen bond between CNF usually leads to high density of cell wall, which hinders the uniform distribution of stress during deformation. As shown by scanning electron microscopy (SEM) images, poorly water-soluble PHA particles act as a spacer that can be encapsulated by CNF to prevent excessive cell wall densification (Fig. 4C, d). The porous structure can soften the wall and improve the flexibility of the structure.

Fig. 1. Preparation, characterization and properties of AChA.

Mechanical properties of AChA

Three types of aerogels with the same density were selected to further prove the effect of porous structure on mechanical properties. As control samples, DCA and ACA showed worse elasticity than ahca (Fig. 2a-f). Due to the overall plastic buckling of the wall, the DCA suffered severe structural collapse, as shown by significant permanent deformation and reduced compressive stress (Fig. 2a, d-f). From the structural point of view, ACA with oriented microchannels shows improved elastic properties, especially in stress reduction and plastic deformation. (FIGS. 2B, D and E). For AChA, the elasticity of the material has been improved and is superior to the disordered and anisotropic cellular structure (Fig. 2C). In the same cyclic compression test, the strength of AChA is reduced by about 5% (Fig. 2D, e), and the hysteresis loop between the loading unloading curves is also smaller than that of DCA and ACA, which indicates that the energy storage capacity of multi-layer hierarchical structure is improved (Fig. 2c, f). As a result, the maximum stress, plastic deformation and energy loss coefficient of highly multilayered graded aerogels have been significantly reduced (Fig. 2d-f).

Figure 2. Characterization of mechanical properties.

Then, the reasons for the excellent compressive elastic behavior of AChA are studied at the micro scale. As shown in SEM images, at 50% compression, the cell structure of AChA was severely deformed, and the cell wall curled into a more compact microstructure through large in-plane deformation (Fig. 3a). Once the compression is released, the aerogels recover completely without rupture, collapse or adhesion, showing stable and firm microstructure. The authors also studied the mechanical behavior of a single wall using a nanomanipulator, which moves from the bottom to the top, trying to tilt the wall (Fig. 3b). Generally, the dense cellulose wall is more rigid structure, and it is easy to cause local stress concentration and structural damage when overloaded. Porous walls are softer, more flexible and tend to deform locally (Fig. 3C). This can avoid structural fracture or collapse caused by stress concentration, thus forming a more stable structure. Due to the stable structure and flexibility of the wall, AChA can bend along the direction of the directional channel. The anisotropic structure can adapt to compression and tensile deformation in an accordion like manner (Fig. 3D). In situ SEM images showed that the compression and bending of cell wall occurred in the internal contraction side. In terms of stretching, the interconnections of the dendritic walls contribute to recovery (Fig. 3e).

Fig. 3. Macro and micro structural deformation of AChA.

Air filtration performance of AChA

Structural hyperelasticity, parallel channels and electrostatic nanofibers make AChA a a biodegradable air filtration material, which can withstand high velocity without structural collapse. The PM capture performance of AChA with thickness of 5mm and 10mm was studied and compared with commercial activated carbon mask. At a flow rate of 1 lmin-1, 10 mm thick AChA showed comparable removal efficiencies of PM2.5 (95.3 ± 2.4%) and masks (97.1 ± 0.3%), which met the high efficiency standard of 95% removal rate (FIG. 4A). Even after 30 rounds of testing, AChA can maintain high PM removal efficiency (pm0.3 and PM2.5 removal rates > 90%) and low pressure drop (~ 70 PA), which indicates that it has good reusability and structural stability (Fig. 4C). In addition, real smoke retention experiments demonstrate the effective filtration capacity of AChA (Fig. 4D). When passing through the channel, PM can be absorbed on the cell wall due to electrostatic interaction of oxygen rich groups (Fig. 4e). SEM images showed that the AChA filter turned slightly yellow and dispersed PM particles deposited on the surface of the porous cells (Fig. 4F). In the sustainable future, biocompatible cellulose filters will be a safer alternative to petroleum based filter materials. This kind of aerogel, which is environmentally friendly and harmless to human body, has broad application prospects in health related fields.

Figure 4. Air filtration performance of AChA.

Summary

In this work, a petrochemical free strategy was developed to fabricate anisotropic hierarchical cellulose aerogels by combining freeze casting technology with thermal etching of biopolymer particles. Directional channels in the material consolidate the entire architecture. The porous wall of dehydrated cellulose produced by thermal etching not only reduces the rigidity and viscosity, but also guides the micro deformation, reduces local large strain and prevents structural collapse. Cellulose aerogels exhibit temperature independent hyperelasticity from room temperature to low temperature (- 196 ℃), excellent fatigue resistance (only about 5% permanent set after 100000 compression cycles at 50% strain), large strain flexibility (including folding and torsion) and high angle recovery rate (1475.4 ° s-1). This aerogel has great thermal insulation potential in harsh environment and can be used as air filter material for masks and equipment. The materials used in this route are all sustainable biomass, so it is expected to solve the environmental pollution problems caused by energy intensive technologies and petrochemical materials. The strategy will be a powerful and environmentally friendly tool for manufacturing multi-layered hierarchical porous materials with good mechanical properties, thermal insulation, particle filtration and other properties.