Superelastic and Ultralight Aerogel Assembled from Hemp Microfibers

Aerogels with both high elastic strain and fast shape recovery after compression have broad application potentials as thermal regulation, absorbents, and electrical devices. However, creating such aerogels from cellulosic materials requires complicated preparation processes. Herein, a simple strategy for scalable production of hemp microfibers using a top‐down method is reported, which can further be assembled into aerogels with interconnected porous structures via ice‐templating technique. With density as low as 2.1 mg cm−3, these aerogels demonstrate isotropic superelasticity, as exhibited by their fast shape restoration from over 80% compressive strain. Due to the high porosity (99.87%) and structural tortuosity, these aerogels show a low thermal conductivity of 0.0215 ± 0.0002 W m−1 K−1, suggesting their potential in thermal insulation application. Certain hydrophobic modification using silane derivative further endows these aerogels with reduced water affinity. Overall, the proposed strategy to prepare bio‐based microfibers using scalable technology, as well as the assembled aerogels, provides new insights into the design and fabrication of multifunctional bio‐based aerogels for value‐added applications.

convection through these porous materials can be substantially reduced. [3,4] This suggests aerogels' potential application as thermal super-insulating materials, i.e., those with thermal conductivity lower than 0.025 W m −1 K −1 . [5] For instance, aerogels have been commercialized as thermal insulation products including building insulation and thermal insulation fabric. [6][7][8] Conventionally, aerogels have been made from inorganic materials and petrochemical polymers, such as silica, carbon, phenolic resin, and polyimide. [9][10][11][12][13] Recently, studies have identified the feasibility of fabricating sustainable aerogels using bio-based materials, particularly cellulose, [14][15][16][17] the most abundant biopolymer on earth. However, because of low elastic strain and irreversible structural collapse under compression, [18] aerogels made from cellulose are generally not suitable for a flexible thermal insulation application such as fillers for winter jackets. The mechanical properties of aerogels are governed by the intrinsic properties of the building blocks, the interaction between building blocks during assembly, and the geometric morphology of the assembled structure. [19][20][21] A major issue with regard to cellulosic aerogels' poor elasticity is correlated to the geometric feature of the assembled cellular structure. [22] Cellulosic aerogels prepared using ice-templating/freeze drying typically have a honeycomb porous morphology, i.e., cellulose assembles and makes up the walls between neighboring microscale cells. When subject to compression and shearing, plastic deformation easily takes place at the compact cellulosic walls, which accounts for the permanent structural failure and poor elasticity of the aerogel. Rationally designing the microstructure of aerogels helps improve their elasticity. For instance, researchers have developed superelastic aerogels from polymeric and inorganic nanofibers with satisfactory shape recovery from high-strain and cyclic compressions. [23][24][25][26] Their superelasticity is believed to rise from the interconnected fibrous walls, which are different from the compact walls. Particularly, the interconnected fibrous walls, composed of continuous microscale fibers, mitigate the irreversible structural collapse and improve the aerogel's resilience to high-strain compression. Unfortunately, fabricating cellulosic aerogels with similar microscale morphology to these polymeric or inorganic nanofibers aerogel is complex and often relies on bottom-up strategy that involves chemical dissolution and/or assistance

Introduction
With a 3D interconnected porous structure, aerogel represents a type of emerging lightweight monolithic solid, with low density and high porosity. [1,2] Due to the low solid content and high structural tortuosity, the heat conduction and of other synthetic polymers to fabricate continuous fibers by electrospinning as the first step. [27][28][29][30] A recent study proposed a dual ice-templating method to fabricate continuous sub-micron fibers from cellulose nanofibrils for superelastic aerogel application. [22] However, this process is quite energy intensive and time consuming. From a green chemistry point of view, developing a scalable strategy to prepare superelastic cellulosic aerogels from renewable biomass with minimum chemical use is of great interest to sustainable development.
To prepare cellulosic aerogel with microscale fibrous continuity, a top-down strategy may represent a scalable and commercially competitive option. An ideal building block can be a type of cellulosic microfiber with high aspect ratio and abundant defibrillated microfibers. When made into monolithic aerogels, the microfibers would lead to the formation of interconnected fibrous cellular walls. The high aspect ratio and robust mechanical strength make hemp fibers stand out among other types of cellulose feedstocks. [31] Defibrillating hemp fibers into required dimensions through chemical or mechanical treatment may offer a potential candidate for preparing superelastic cellulosic aerogels. Specifically, robust hemp microfibers with high tolerance to shear force can maintain their high aspect ratio during mechanical treatment, while nanofibers can be generated along the microfibers, both being beneficial to generating sufficient entanglement between microfibers when assembling into aerogel. Compared with the previously mentioned methods such as electrospinning and dual ice templating, this strategy involves only the use of mechanical treatment and is expected to be facilely scalable. In addition, the continuous fibrous structure can substantially improve material utilization, holding great promise as an exceptional building block for constructing superelastic aerogels with a connected network.
Herein, a superelastic aerogel was developed from hemp bast fibers using a simple top-down defibrillation treatment, leading to microfibers with high aspect ratio, which will be further assembled by ice-templating and freeze drying. The developed aerogels showed excellent isotropic elasticity at ultralow density (2.1 mg cm −3 ) under 80% compressive strain. The unique cellular wall, consisting of well-bonded and continuous microfibers, endows hemp aerogels with low thermal conductivity (0.0215 ± 0.0002 W m −1 K −1 ) and good superelasticity at an extremely low environmental temperature of −173 °C. Furthermore, it is demonstrated that the aerogel's water stability and hydrophobicity can be easily improved by proper chemical modification. Therefore, such hemp microfiber-based aerogel represents a novel bio-based thermal insulation material that can potentially replace traditional petrochemical-based ones. We believe this simple approach to fabricate superelastic aerogel could be extended to other natural fibers with similar properties.

Fabrication of Superelastic Aerogels
It has been previously shown that, in order to achieve superelasticity, the cell wall of cellulosic aerogel should have an openmesh structure that is formed by continuous interconnected sub-micron or microfibers. [22] In this study, the continuous microfibers are isolated from bulk hemp bast fibers via a topdown approach, and by controlling the fiber concentration, cell walls with an open-mesh like structure can be obtained by ice templating and freeze-drying.
The synthesis pathway of hemp aerogel is displayed in Figure 1a. Hemp bast fibers were chosen as the raw material to construct the elastic fibrous network due to the long fiber morphology. The original hemp bast fiber was first peeled off from   stalk, followed by a delignification and high-speed mechanical blending process to form the microfiber suspension. After adding an in situ crosslinking agent polyamide-epichlorohydrin (PAE), the well-dispersed suspension was frozen and then freeze dried into the aerogel and finally heated to fully crosslink the microfibers with PAE. Particularly, the freezing process was governed by complex and dynamic water-fiber, and fiberfiber interactions, where microfibers in the dispersion accumulated between the growing ice crystals. The densities of the aerogels can be manipulated by changing the concentration of the microfiber suspensions from 1.5 to 21 mg cm −3 . The hempbased aerogel (with density of 2.1 mg cm −3 ) produced were found to be ultralight, superelastic (shape recovery after high strain compression), and flexible, as shown in Figure 1b-d, respectively. Due to the use of ice templating, the materials can be prepared into various shapes depending on the mold used ( Figure 1e). In addition, the aerogel can be easily scalable, as shown by the successful fabrication of a large piece of aerogel with 22 cm in diameter and 2 cm in thickness ( Figure 1f).

Hemp Fibers Morphologies and Suspension Properties
The morphology of hemp bast fibers before and after treatment were characterized by polarized optical microscopy (Figure 2a-c). After delignification, the hemp fiber bundle was liberated from the hemp bast fiber at a yield of 78.7%, which can be further fragmented into short microfibers by high-speed mechanical blending. The chemical compositions of hemp bast and delignified hemp fibers are given in Table S1 (Supporting  Information). NaClO 2 treatment progressively removed lignin (50%), while kept the hemicellulose content almost unchanged. Due to the removal of non-cellulosic components, the cellulose content increased after delignification. Figure 2d demonstrated that the lateral dimensions of the fiber greatly changed after blending. Apparently, most of the microfibers possessed a peak diameter of ≈6.7 µm, much smaller than that of the delignified hemp fibers (≈38.1 µm). The average length of microfiber was 761 µm ( Figure S1, Supporting Information). Thus, the aspect ratio of hemp microfiber was ≈110, proving that robust hemp fibers can maintain their high aspect ratio during mechanical treatment. XRD was used to examine the crystallinity of the samples by different treatments. A major diffraction peak for 2θ ranging between 22 and 23° is attributed to the (200) crystallographic planes of cellulose Iβ. The other two peaks were found at 15.2° and 16.6°, corresponding to (110) and (110) crystallographic planes of cellulose Iβ, respectively. [32] Hemp bast fibers exhibited a crystallinity index of 68%. In contrast, delignified hemp fibers gave a higher crystallinity index of 74%, which can be attributed to the removal of non-cellulosic components. Further mechanical blending appeared to barely change the crystallinity of the hemp microfiber, as indicated by a similar XRD profile and a similar crystallinity index of 73%. The zeta potential of the hemp microfiber suspension was found to be −30.3 ± 3.8 mV, which is close to the microfluidizer derived nanocellulose from alkaline treated hemp, and the negative charges are deemed to be from the carboxylate groups in the remaining hemicellulose in the cell walls. [33] This can be  further confirmed by the FTIR (Figure S2, Supporting Information). The peak at 1735 cm −1 , which is commonly related to the stretching vibration of carbonyl groups in hemicellulose, [34,35] is still prominent after NaClO 2 treatment. Such strong electrostatic repulsion of the negatively charged functional groups of hemp microfibers contributed to the colloidal stability of the suspension, as indicated by no gravity sedimentation of the suspension after one month (Figure 2f).
To enhance the bonding between hemp microfibers and improve the mechanical stability of the assembled aerogel, PAE was introduced into the system as an in situ crosslinking agent at a low dosage of 1 wt.% with respect to hemp microfibers. Previous studies have revealed that PAE can trigger both homo-crosslinking and hetero-crosslinking. [36,37] PAE can act as a wet strength enhancement by forming covalent ester bonds between carboxyl groups of hemp microfibers and azetidinium groups of PAE. Also, the azetidinium groups and secondary amines of PAE can develop into water-insoluble networks during heating. These self-crosslinking networks play a vital role in supporting the structure by inhibiting fiber-bond detachment when being re-wetted in water and so improving the wet strength and structural integrity. FTIR analysis ( Figure S3a, Supporting Information) confirmed the introduction of PAE. The main difference between two is the presence of a new peak at 1550 cm −1 (shadowed in yellow), which is attributed to the −NH groups of the PAE cross-linker. [38] When immersed in water, the PAE-strengthened hemp aerogel maintained its structural integrity, overcoming water-induced swelling and structural collapse ( Figure S3b, Supporting Information). In addition, adding PAE roughly doubled the aerogel's compressive stress as compared to that of PAE-free aerogel under 80% strain ( Figure S3c, Supporting Information).

Characterization of Aerogel's Superelasticity
Preliminary tests found that the hemp aerogel's elasticity is directly associated with its apparent density. Compared to the aerogel with higher density (21 mg cm −3 , Figure 3a) that cannot recover to its original shape after compression, the aerogel with a low density of 2.1 mg cm −3 (Figure 3b) is able to recover to its original shape. Insight into what happened to the aerogel's microscale structure during compression is given by SEM images. Before compression, both aerogels showed a honeycomb structure with a major cellular pore size of ≈200 µm (Figure 3e,h). However, only the ultralow density one (2.1 mg cm −3 ) preserved the honeycomb structure after compression, while the majority of the cellular structures were damaged for the denser aerogel (Figure 3f,i). Such densitydependent superelasticity can be explained by the difference in their cell wall structures. Unlike the compact cell wall with all fibers stacked together in the denser aerogel (Figure 3j), the cell walls of the lighter aerogel, which consisted of an inter-connected hemp microfiber network, exhibited porous, highly bonded, and fiber-entangled structures ( Figure 3g). As noted previously, such unique fibrous cell walls gain stronger structural resistance and undergo elastic deformation under compression, resulting in superelasticity of the corresponding aerogels. However, it should be noted that the 0.1 wt.% aerogel (with density of 1.5 mg cm −3 ) showed poorer elasticity ( Figure S4, Supporting Information) as compared to the 0.2% aerogel, which could be due to limited microfiber entanglement at such low microfiber concentration. More importantly, since the hemp aerogel was prepared under isotropic freezedrying conditions, it exhibited superelasticity in all directions, i.e., good shape recovery after compression release regardless of the direction of the applied force (Figure 3c; Movies S1 and S2, Supporting Information). Such isotropic superelasticity makes the hemp aerogel distinctive from other aerogels that only show superelasticity in a particular direction. [39,40] Moreover, the hemp aerogel's isotropic superelasticity was found to be well preserved even in an extremely cold environment, for example, −173 °C (Figure 3d; Movies S3 and S4, Supporting Information). In addition, the addition of PAE makes no contribution to the superelastic property. The result showed aerogel without PAE still preserved superelastic properties ( Figure S5, Supporting Information), confirming that the superelasticity is originated from the interconnected fibrous structure within the aerogel.
The low-density hemp aerogel's (density of 2.1 mg cm −3 ) mechanical properties were further characterized using a unidirectional compression test. The stress-strain curves (Figure 4a) obtained during compression exhibited three characteristic regions, which is typical for a cellular monolith: a linear elastic range at ε < 10%, a subsequent plateau stage at medium strain range (10% < ε < 65%), and a densification stage with sharp increase in stress (ε > 65%). 24 The hemp aerogel showed good elastic properties under all different strains, with an increased hysteresis loop under higher strain. The highly elastic aerogel also possessed durable cycling performance. Though certain plastic deformation was observed in the cyclic compression test as shown in the hysteresis curves (Figure 4b), no significant reduction in strength was found for aerogels after the 2nd compression all the way up to 80th cycle. As shown in Figure 4c, hemp aerogels still preserved over 90% of the initial stress, showing good structural integrity. A relatively high energy loss coefficient of 58.9% was found in the first compressive cycle, probably due to the collapse of the less stable or less inter-connected fibers within cell walls. After the first cycle, the energy loss coefficient did not decrease much further until 80th cycle, indicating that the entangled structure can effectively withstand  repeated compression. At 100th cycle, the ultimate stress and energy loss coefficient decreased slightly, which could be ascribed to some structural collapse from repeated compression. Overall, the low-density hemp aerogel exhibits excellent elastic performance under high strain and during cyclic compressive tests. In addition to the vertical direction, the mechanical properties of the aerogel at the transverse direction were also characterized. The ultimate stress (0.125 kPa) is much lower as compared to the vertical direction ( Figure S6, Supporting Information), which may suggest some structural anisotropy exists. After 100 cycles of compression tests, the aerogel showed 22% unrecovered strain, and much higher energy loss coefficient as compared to the vertical direction. Nevertheless, the aerogel still maintained good elasticity after 100 cycles of compressive tests.
The compressive mechanical performance of aerogels with different apparent densities was investigated as well. Compressive stress-strain curves with varying hemp microfibers concentrations are shown in Figure 4d. All aerogels can be compressed over 80% without cracking, showing superb flexibility. The increased density will lead to more interconnection between adjacent fibers, which results in dramatically increased mechanical performance. The specific modulus, Young's modulus, and yield stress increased with increasing hemp concentrations (Figure 4e,f), which can be up to 32.9, 690, and 26 kPa for 2% aerogels, respectively. Therefore, it proves that by simply controlling the initial concentration of hemp microfibers, we can manipulate the aerogels from rigid to superelastic textures.

Hydrophobic Modification of Hemp Fiber Aerogels
Waterproof properties are an important consideration for allweather applications of hemp aerogel, particularly to prevent a reduction in its mechanical and thermal insulation performance in high-humidity environments. The hydrophilic nature of hemp microfiber makes it necessary to apply a hydrophobic treatment for enhanced moisture resistance. In this study, the hydrophobic modification was achieved by chemical vapor deposition of methyltrimethoxysilane (MTMS) onto hemp aerogel, which resulted in a uniform hydrophobic coating throughout the aerogel. Successful silane coating was confirmed by FTIR (Figure 5a), showing characteristic peaks at 1262, 776, and 798 cm −1 (shadowed in yellow), which corresponded to the formation of SiCH 3 and SiOSi bonds. [22] The presence of Si element can be clearly observed from the EDX mapping image, showing homogeneous coverage on the modified aerogel, and the weight concentration of Si is determined at 5.33% ( Figure S7, Supporting Information).
The improved hydrophobicity of the MTMS treated aerogel was confirmed by water contact angle analysis. Figure 5b showed the change in water contact angle (CA) within 30 min. After modification, the surface was repellent toward water, with a static CA of 142°, and a contact angle hysteresis θ s of 5 ± 3.4° (Figure 5c). It has been demonstrated that such water repellency is applicable to various types of water-based liquids, which all maintained spherical geometries on the aerogel's surface and could not penetrate into the aerogel (Figure 5d). Moreover, such hydrophobicity existed not only at the outer surface of the aerogel but also at the inner surface of a tornapart aerogel ( Figure S8, Supporting Information), confirming a homogenous hydrophobic treatment of the chemical vapor deposition method. Due to the deposition of non-polar methyl groups during MTMS treatment, aerogels can selectively absorb non-polar liquids. The aerogel can absorb and remove soybean oil (dyed by Nile red) from water (Movie S5, Supporting Information), suggesting that this aerogel can be used for oil-water separation purposes as well. Due to the high volumetric porosity of aerogels up to 99.87% ( Figure S9, Supporting Information), the silane-modified superelastic aerogel exhibited excellent absorption capacities, ranging from 200 to 400 g g −1 (shown in Figure 5e), toward a wide range of oils and organic solvents. For aerogel with larger pore volume, the theoretical absorption capacity mainly depends on the pore volume and can be calculated by Equation 1, by assuming all air-occupied pores can be filled by the liquid: [43] Theoretical absorption capacity porosity For the case of silane-modified hemp aerogel, it was found that ≈67% of the pore volume was responsible for absorbing non-polar liquids. The rest of the unfilled pores are considered un-accessible to liquids or trapped by air. While for more polar liquids, like acetone and dimethyl sulfoxide (DMSO), only ≈51% of the calculated pore volume was occupied, which was less than that of non-polar liquids. This can be attributed to the improved hydrophobicity of the MTMS modified aerogels.
The compression-recovery resilience of the hemp microfiber aerogel gives it the ability to cyclically absorb and desorb liquid; results are summarized in Figure 5f. For each cycle, the aerogel was allowed to absorb a maximum amount of hexane, which was then squeezed by hand to release the absorbed solvent. It can be seen that the absorption capacity exhibited a slight decrease after the initial several cycles and then plateaued at ≈169 g g −1 , which was ≈70% of the initial absorption capacity. This can be attributed to the good stability of the well-bonded fibrous structure. The results showed that the aerogel maintained good recyclability for cyclic usages in oil removal.

Thermal Insulating Properties of Hemp Aerogel
Due to the high porosity, the ultralow density hemp aerogel represents a promising candidate for thermal insulation applications. As shown in Figure 6a, the hemp aerogel (0.2 wt.%) had an ultralow thermal conductivity of 0.0215 ± 0.0002 W m −1 K −1 , which is even lower than that of air (0.025 W m −1 K −1 ). This kind of low thermal conductivity was comparable to other reported cellulose-based thermal insulation aerogels (  Information). We attributed this result to the high porosity of the aerogel (99.87%, Figure S9, Supporting Information). With further decreased hemp microfibers concentration to 0.1 wt.%, the assembled 0.1 wt.% hemp aeogel showed thermal conductivity of 0.0259 ± 0.002 W m −1 K −1 . This may be due to the loose structure and large pores of the aerogel, leading to large contribution of the air convection. Thus, hemp aerogel with concentration of 0.2 wt.% was chosen as the thermal insulation material. The thermal insulation performance of the ultralight aerogel was demonstrated by placing the aerogel on either a hot (80 °C) or cold (−20 °C) plate. After a slight change within the first 3 min, the upper temperature of the aerogel stabilized. Notably, the aerogel was able to maintain a consistent 51 °C and 23 °C temperature difference for the hot and cold scenarios, respectively (Figure 6b,c), suggesting good thermal insulation performance. An Infrared (IR) camera was also used to reveal the thermal insulation properties of the aerogel. Figure 6d showed the IR image to visualize the process of heat transfer through aerogel. After placing the aerogel on an 80 °C hot plate, heat transfer through the bottom surface can be observed at the first 0.5 min. However, the heat penetration depth did not further increase within 120 min, suggesting effective thermal insulation performance.
Due to good thermal insulation and superelastic properties, the hemp aerogel developed in this study has considerable potential to be used as a filler in winter jackets. The ultralow density aerogels can fit in fabric, and the flexibility endows them to bend into different angles ( Figure 6e). As shown in the IR photo, the covered area mitigated heat loss from the body and showed a temperature similar to the background environment, compared to the uncovered forearm showing a typical skin temperature value. The temperature change within fabric was also measured ( Figure S10, Supporting Information). The fabric shows an excellent warmth-retention effect and the temperature of the skin covered by the fabric increased significantly in few seconds. Also, the fabric can effectively inhibit temperature exchange between skin to room temperature, allowing a constant 11 °C temperature difference between the two.

Conclusion
In summary, we demonstrated a simple and effective way to make long microfibers from hemp bast fibers through a topdown method. Such long microfibers can be further assembled into aerogels with interconnected fibrous architectures. The resulting aerogel offers high porosity (99.87%) and low density (2.1 mg cm −3 ) yet mechanical durability and superelasticity. Consequently, the hemp aerogel showed shape recovery under high strain and achieved 92.5% strength retention after 60 loadingunloading cycles. Such superelasticity can still be preserved under extremely cold environment. In addition, the aerogel showed excellent thermal insulation properties, with measured thermal conductivity as low as 0.0215 ± 0.0002 W m −1 K −1 . With their ultralow density, temperature invariant superelasticity, low thermal conductivity, and hydrophobicity, this kind of aerogel will open broad technological applications in thermal insulation, absorbents, and flexible electrical devices. The preparation process of the hemp aerogel proposed in this study can be adopted to valorize other types of natural fibers to fabricate bio-based aerogels with lightweight and superelasticity for insulation purposes.
Delignification of Hemp Fibers: The hemp bast fibers were first immersed in hot water (100 °C) and then left to cool down overnight. The water soaked in hemp bast fibers (15 g) were further treated with a 1.7 wt.% NaClO 2 solution (500 mL) and buffered with acetate acid solution (0.06 mmol L −1 ) at pH 4.6 at 80 °C for 5 h. The delignified hemp bast fibers were then washed with copious DI water and dried at ambient condition.
Fabrication and Surface Modification of Hemp Aerogel: Hemp microfiber suspensions were obtained by mechanical blending (Vitamix 5200, 15 min) of the delignified hemp fibers. After blending, 1 wt.% PAE (with respect to hemp) was added into the suspension. Then the hemp microfiber suspension was diluted to various concentrations of 0.1%, 0.2%, 0.5%, 0.8%, 1.0%, 1.5%, and 2%. The suspended hemp microfibers were frozen at −20 °C and then freeze-dried to get the aerogel. The aerogels were then heated in a vacuum oven at 120 °C for 3 h to complete the cross-linking process. The obtained aerogels were hydrophobized by chemical vapor deposition of MTMS in a sealed container (containing 1 mL MTMS and 0.5 mL water) at 80 °C for 6 h.
Characterizations: The chemical structures of samples were observed through FTIR (Bruker Optics). The zeta potential was measured without ionic strength adjustment by a Zetasizer Nano S90 (Malvern Instrument). Crystallinity information of samples was determined by XRD analysis (Bruker, Billerica). The crystallinity index (CrI) was calculated based on Segal empirical equation. [33] Morphologies of hemp fibers and assembled aerogels were visualized by Polarized optical microscopy (POM; POLYVAR) and Scanning Electron Microscope (SEM, Helios NanoLab 650 FIB-SEM). Thermal conductivity and diffusivity of the aerogels were determined by a thermal conductivity analyzer (TPS 2500 S) at ambient condition. Infrared images were taken using a handheld IR Thermal Imagining camera (RoHS HT-19). The water contact angle was quantified using Theta Flex (Biolin Scientific).
Chemical compositions of hemp bast and delignified fibers was analyzed using TAPPI standard T-22 om-88 method. The acid-insoluble lignin (AIL) content was weighted out using fritted glass crucible and the acid-soluble lignin (ASL) was measured by the absorbance at 205 nm. The result composition content was determined by HPLC (ICS-3000).

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.