Negative poisson's ratio structured nanocellulose aerogel and method of making, use and mold

Abstract

The application belongs to the technical field of nanocellulose materials, and relates to a negative Poisson's ratio structure nanocellulose aerogel, a preparation method, application and a mold. In view of the technical problems of the existing negative Poisson's ratio structure nanocellulose aerogel, such as poor buffering, shock absorption or impact resistance effect, inability to be produced on a large scale, and high manufacturing cost, the application provides a mold for preparing the negative Poisson's ratio structure nanocellulose aerogel, which comprises a box body and an inner concave hexagonal rod-shaped column. The inner concave angle α of the rod-shaped column is 55-65°, the straight edge half length L is 1-1.5 mm, the side length l is 1.0-1.3 mm, the wall thickness t is 1-2 mm, and the length Z is 20-25 mm. The mold can be used for preparing the negative Poisson's ratio structure nanocellulose aerogel from various raw materials. The obtained negative Poisson's ratio structure nanocellulose aerogel has excellent parameters such as Poisson's ratio, density, elastic modulus and compression hardness coefficient, the preparation method has low cost, and the negative Poisson's ratio structure nanocellulose aerogel can be produced in large quantities and used in the preparation of buffering, shock absorption or impact resistance materials.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of nanocellulose materials, and in particular, relates to a negative Poisson's ratio structure nanocellulose aerogel, a preparation method, applications and a mold. BACKGROUND

[0002] Aerogel refers to a kind of solid material with three-dimensional nanometer open pores obtained by removing the liquid carrier in the gel through a certain drying method, which is assembled by coherent nanoparticles or polymer molecules into a gel network. Among them, nanocellulose aerogel refers to a nanometer porous network structure obtained by drying a solvent using cellulose material with a diameter less than 100 nm and a length of several microns or several hundred nanometers as raw material. The density of such aerogel is extremely low (0.01-0.2 g / cm 3 ), and the porosity is high. Nanocellulose aerogel has the advantages of high compression ratio, ultra-low density, high porosity, convenient manufacturing, low cost, environmental friendliness, recyclability and many other advantages, and is widely used in catalyst carriers, soundproof materials, filter materials, thermal insulation, supercapacitors, adsorbents, template materials and other fields. It has become a new generation of aerogel in addition to inorganic aerogel and organic polymer aerogel. When subjected to external impact, the aerogel can generate a large strain to resist external impact, but the low density makes its structure easy to be damaged and invalid, which limits its application in the field of buffering and energy absorption. The structural improvement of cellulose aerogel can change the compression deformation behavior of cellulose aerogel and effectively improve the compression resistance of the aerogel.

[0003] Negative Poisson's ratio structure is a new type of structure with super mechanics performance, and is the most studied branch in mechanical metamaterial structure. Negative Poisson's ratio material often shows counter-intuitive deformation behavior during deformation, and has many advantages. On the one hand, negative Poisson's ratio structure can produce negative Poisson's ratio effect during compression, so that the material as a whole can respond positively to changes in the external environment in time to resist impact deformation. Specifically, the concave contraction property of negative Poisson's ratio structure makes it densify during compression. This property can make up for the low upper limit of nanocellulose aerogel mechanical properties, poor compression resistance and easy crushing, so that the material quickly concentrates from the periphery to the center during compression, the instantaneous density increases, and a higher stiffness is exhibited to resist external impact. On the other hand, compared with traditional materials, negative Poisson's ratio cellular structure also has unique advantages in strain generation and impact energy absorption. Cellular materials are more likely to achieve large compression deformation during strain, and are ideal structures for efficient energy absorption. The concave polygonal structure is a typical and easy-to-design negative Poisson's ratio structure, which is often used in the design of negative Poisson's ratio structure materials. Some studies have tried to use 3D printing to manufacture negative Poisson's ratio aerogels, such as Chinese invention patent application publication No. CN116162280A, application date December 12, 2022, and name: High negative Poisson's ratio polyimide composite aerogel and shape memory aerogel based thereon and 3D printing method thereof. The method discloses a 3D printable polyamide acid composite ink, which can realize high-precision 3D printing of polyimide aerogel. However, this scheme is limited by the printing height, and the negative Poisson's ratio structure manufactured has a small size, with a height of only 2 mm. In order to pursue a larger negative Poisson's ratio effect, the cellular structure is arranged very densely, which makes it difficult to provide sufficient support if applied to the cushioning field. Although 3D printing has its unique advantages, such as flexible manufacturing conditions, the nozzle advances along the planned printing path, the parameters in the negative Poisson's ratio structure can be quickly adjusted, and the processing precision of 3D printing is generally high, which can manufacture some three-dimensional negative Poisson's ratio structures with high complexity, and is suitable for small-scale and experimental production of aerogel sensors and thin films. However, 3D printing requires that the printing ink can smoothly pass through the printing nozzle, quickly solidify after being sprayed, and not collapse when the number of printing layers is increased, which has high requirements for printing materials. For many aerogel substrates, it is difficult to achieve the above factors, which limits the manufacturing scale of 3D printed aerogels and makes it difficult to adapt to large-scale production. SUMMARY

[0004] 1. Technical problems to be solved by the invention

[0005] The application provides a mold for preparing a negative Poisson's ratio structure nanocellulose aerogel, which can be used for preparing the negative Poisson's ratio structure nanocellulose aerogel from various raw materials. The application also provides a negative Poisson's ratio structure nanocellulose aerogel and a preparation method and application thereof. The negative Poisson's ratio structure nanocellulose aerogel has excellent parameters such as Poisson's ratio, density, elastic modulus and compression hardness coefficient, can be produced in large quantities at low cost, and can be used in the preparation of cushioning, shock-absorbing or impact-resistant materials.

[0006] 2. Technical scheme

[0007] To achieve the above-mentioned purpose, the technical scheme provided is:

[0008] The mold for preparing a negative Poisson's ratio structure nanocellulose aerogel comprises a box body and an inner recessed hexagonal rod-shaped column, and an inner recessed hexagonal hole matched with the rod-shaped column is arranged on the inner wall of the bottom of the box body.

[0009] The inner recessed angle a of the rod-shaped column is 55-65°, the half length of the straight side L is 1-1.5 mm, the side length l is 1.0-1.3 mm, the wall thickness t is 1-2 mm, and the length Z is 20-25 mm.

[0010] The rod-shaped column is a plurality of.

[0011] Further, the rod-shaped column is 60-81.

[0012] Preferably, the length or width of the box body is 30-35 mm, and the height is 15-20 mm.

[0013] The preparation method of the negative Poisson's ratio structure nanocellulose aerogel comprises the following steps:

[0014] Preparation of a nanocellulose mixture: a crosslinking agent and a catalyst are added to a nanocellulose suspension, and uniform mixing and stirring are performed to obtain a nanocellulose mixture; the preparation of the suspension: oxidation is performed for nanocellulose carboxylation to obtain a suspension with a mass fraction of 0.7-1.3%, wherein the fiber length-diameter ratio is 120-80:1, and the surface carboxyl content is 1.0-1.35 mmol / L;

[0015] Preparation of a nanocellulose hydrogel: the nanocellulose mixture is poured into the mold to obtain a nanocellulose hydrogel; the mass fraction of the crosslinking agent in the hydrogel is 2.5-12.5% of the mass of the nanocellulose, and the mass fraction of the catalyst is 0.5-2% of the mass of the crosslinking agent;

[0016] Aging and freeze-drying to obtain a nanocellulose aerogel.

[0017] Further, the oxidation in the preparation step of the suspension is mediated by TEMPO.

[0018] Further, the crosslinking agent is one or more of acrylamide, epichlorohydrin, dopamine.

[0019] Further, the catalyst is one or more of potassium persulfate, N,N'-methylenebisacrylamide, cerium ammonium nitrate.

[0020] Further, the process parameters of the aging are 45-55 DEG C for 30-45 min, and cooling to room temperature.

[0021] Further, the process parameters of the freeze-drying are -6 DEG C to -18 DEG C for 24-36 h, and freeze-drying for 36-48 h.

[0022] The negative Poisson's ratio structure nanocellulose aerogel is prepared by using the method.

[0023] The negative Poisson's ratio structure nanocellulose aerogel is applied to the preparation of a cushioning, shock-absorbing or impact-resistant material.

[0024] 3. Beneficial effects

[0025] Compared with the prior art, the technical scheme provided by the present application has the following beneficial effects:

[0026] (1) The mold for preparing the negative Poisson's ratio structure nanocellulose aerogel is suitable for various aerogel raw materials to prepare nanocellulose aerogels with a negative Poisson's ratio structure, such as CNF / epichlorohydrin aerogels or CNF / dopamine aerogels, etc., which can integrally form the aerogels, has low manufacturing cost, high processing efficiency, and does not have as strict restrictions on the substrate forming performance and material manufacturing height as 3D printing, and is particularly suitable for large-scale mass production.

[0027] (2) The preparation method of the negative Poisson's ratio structure nanocellulose aerogel introduces a negative Poisson's ratio structure into the nanocellulose aerogel material through the mold, has high accuracy of the negative Poisson's ratio shape, and when the nanocellulose aerogel is subjected to a large instantaneous impact load, the pre-designed pores can produce a negative Poisson's ratio effect, causing the nanocellulose aerogel to be internally concave and dense, resisting external pressure, and at the same time, the large strain of the negative Poisson's ratio nanocellulose aerogel can absorb impact load energy, further improving the impact resistance of the nanocellulose aerogel material. The preparation method of the present application is not limited to CNF aerogels or CNF / PAM aerogels, but can also be extended to all aerogels that have similar rheological properties and carboxyl nanocellulose and are prepared by freeze-drying.

[0028] (3) The negative Poisson's ratio structure nanocellulose aerogel of the present application has excellent performance in parameters such as density, elastic modulus, compression hardness coefficient, etc. In the impact resistance test, it can withstand the impact load of a 25g small ball (120 times the mass of the nanocellulose aerogel itself) with an energy of 260-270mJ.

[0029] (4) The application of the negative Poisson's ratio structure nanocellulose aerogel of the present application is to apply the negative Poisson's ratio structure nanocellulose aerogel to the preparation of cushioning, shock absorption or impact resistant materials. The ultra-low density nanocellulose aerogel material with negative Poisson's ratio structure has both lightness and excellent impact resistance, and can be applied to the field of impact resistance, such as human protection, spacecraft landing, etc. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 Structure diagram of rod-shaped column in the mold for preparing the negative Poisson's ratio structure nanocellulose aerogel;

[0031] Figure 2 Structure diagram of the mold for preparing the negative Poisson's ratio structure nanocellulose aerogel;

[0032] Figure 3 Flowchart of the preparation method of the negative Poisson's ratio structure nanocellulose aerogel;

[0033] Figure 4 Stress-strain curve of the negative Poisson's ratio structure nanocellulose aerogel;

[0034] Figure 5 Compression ratio-stress diagram of the negative Poisson's ratio structure nanocellulose aerogel;

[0035] Figure 6 Impact resistance test and cushioning effect photo of the negative Poisson's ratio structure nanocellulose aerogel. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0037] Abbreviations and explanations of terms

[0038] NPRS (Negative Poisson's Ratio Structure): negative Poisson's ratio structure.

[0039] TEMPO (2,2,6,6-tetramethylpiperidine-1-oxy): 2,2,6,6-tetramethylpiperidine-1-oxide.

[0040] PAM (Polyacrylamide, Polyacrylic amide): polyacrylamide.

[0041] EPI (Epichlorohydrin): epichlorohydrin.

[0042] DA (Dopamine): dopamine.

[0043] The application relates to a preparation method of a nanocellulose aerogel with a negative Poisson's ratio structure.

[0044] (1) a mold for preparing a nanocellulose aerogel with a negative Poisson's ratio structure, which comprises a box body and an inner concave hexagonal rod-shaped column. The rod-shaped column is used for constructing the negative Poisson's ratio structure, and the rod-shaped column is composed of two straight edges and two inner concave V-shaped structures. The inner concave angle alpha of the inner concave hexagonal hole is 55-65 DEG, the half length L of the straight edge is 1-1.5 mm, the side length l is 1.0-1.3 mm, the wall thickness t is 1-2 mm, and the length Z is 20-25 mm. When the top of the hole in the negative Poisson's ratio structure constructed by the rod-shaped column is pressed, the inner concave V-shaped structure will shrink inward (the inner concave angle alpha decreases), and the side edge will move inward transversely, so that the compression-shrinkage phenomenon, that is, the negative Poisson's ratio effect, is generated.

[0045] The inner concave hexagonal hole on the inner wall of the bottom of the box body is used in cooperation with the rod-shaped column and is used for shaping the negative Poisson's ratio structure. The length or width of the box body is 30-35 mm, and the height is 15-20 mm. The rod-shaped column is inserted into the inner concave hexagonal hole to be combined into the mold for preparing the nanocellulose aerogel with the negative Poisson's ratio structure. The rod-shaped column is a plurality of rod-shaped columns.

[0046] (2) preparing a nanocellulose suspension: TEMPO-mediated oxidation is adopted to perform carboxylation on nanocellulose to obtain a suspension with a mass fraction of 0.7-1.8%. The length-diameter ratio of the nanocellulose is 120-80:1, and the surface carboxyl content is 1.0-1.35 mmol / L.

[0047] (3) a crosslinking agent: one or more of acrylamide, epichlorohydrin and dopamine.

[0048] (4) a catalyst: one or more of potassium persulfate, N, N'-methylene bisacrylamide and cerium ammonium nitrate.

[0049] (5) Preparation of the nanocellulose mixture, casting: adding crosslinking agent and catalyst into the nanocellulose suspension, uniformly mixing and stirring to obtain the nanocellulose mixture, and uniformly casting into the mold to obtain the nanocellulose hydrogel, wherein the mass fraction of the crosslinking agent in the nanocellulose hydrogel is 2.5-12.5% (solid mass), and the mass fraction of the catalyst in the crosslinking agent is 0.5-2% (solid mass).

[0050] (6) Aging: placing the nanocellulose hydrogel at 45-55℃ for 30-45 min, and then cooling to room temperature, so that the nanocellulose forms a crosslinked network with the polyacrylamide, generates strength, and provides structural support.

[0051] (7) Freeze-drying: keeping the nanocellulose hydrogel obtained in (6) at -6 to -18℃ for 24-36 h, and freeze-drying for 36-48 h, so that the nanocellulose hydrogel is converted into a nanocellulose aerogel.

[0052] (8) Taking out the nanocellulose aerogel from the mold to obtain a nanocellulose aerogel with a negative Poisson's ratio structure.

[0053] Example 1

[0054] The rod-shaped mold for preparing the nanocellulose aerogel with a negative Poisson's ratio structure in this example has an inner recessed angle a of the inner recessed hexagonal hole of 60°, a straight edge half length L of 1.25 mm, a side length l of 1.0 mm, a wall thickness t of 1.5 mm, and a length Z of 20 mm. The length and width of the box body are 30*30 mm, and the height is 15 mm. The rod-shaped column is 81, and in actual preparation, the number of rod-shaped columns can be adjusted according to the size of the box body, and in other embodiments, it can also be 60-81, and both can achieve the same technical effect.

[0055] comprises the following steps:

[0056] Preparation of the nanocellulose suspension: TEMPO-mediated oxidation is used for carboxylation of nanocellulose to obtain a suspension with a mass fraction of 0.7-1.3%, wherein the fiber aspect ratio is 120-80:1, and the surface carboxyl content is 1.0-1.35 mmol / L.

[0057] The crosslinking agent is acrylamide.

[0058] The catalyst is potassium persulfate, and in parallel tests, N,N'-methylenebisacrylamide or cerium ammonium nitrate can also achieve similar technical effects.

[0059] Preparation of nanocellulose mixture, casting molding: crosslinking agent and catalyst were added into nanocellulose suspension, uniformly mixed and stirred to obtain nanocellulose mixture, and uniformly cast into a mold to obtain nanocellulose hydrogel, wherein the mass fraction of the crosslinking agent in the nanocellulose hydrogel is 2.5% (solid mass), and the mass fraction of the catalyst is 1% (solid mass) of the mass of the crosslinking agent. In parallel tests, the mass fraction of the crosslinking agent is 2.5-12.5% of the nanocellulose, and the mass fraction of the catalyst is 0.5-2% of the mass of the crosslinking agent, which can achieve similar technical effects.

[0060] Aging: the nanocellulose hydrogel was placed at 50°C for 30 min. In parallel tests, the process parameters for aging can also be 45-55°C for 30-45 min, which can achieve the effect of aging.

[0061] Freeze-drying: the nanocellulose hydrogel after aging was kept at -18°C for 24 h, and freeze-dried for 36 h, so that the nanocellulose hydrogel was converted into nanocellulose aerogel. In parallel tests, -6 to -18°C for 24 to 36 h, freeze-drying for 36 to 48 h, can achieve similar freeze-drying effects.

[0062] The nanocellulose aerogel with a negative Poisson's ratio structure was obtained by taking the nanocellulose aerogel out of the mold.

[0063] The nanocellulose aerogel with a negative Poisson's ratio structure prepared in this example has the performance shown in Table 1.

[0064] Example 2

[0065] The mold and preparation method for preparing the nanocellulose aerogel with a negative Poisson's ratio structure in this example are basically the same as in Example 1. The difference between the steps and Example 1 is that the inner recess angle α of the inner recess hexagonal pore is 55°, the straight edge half length L is 1.0 mm, the side length l is 1.0 mm, the wall thickness t is 1.0 mm, and the length Z is 20 mm. The length and width of the box body are 30*30 mm, and the height is 15 mm. The number of rod-shaped columns is 60.

[0066] The crosslinking agent is acrylamide.

[0067] The catalyst is N,N'-methylenebisacrylamide.

[0068] Preparation of nanocellulose mixture, casting molding: the mass fraction of the crosslinking agent in the nanocellulose hydrogel is 5.0% (solid mass), and the mass fraction of the catalyst is 0.5% (solid mass) of the mass of the crosslinking agent.

[0069] Aging: 45°C for 33 min.

[0070] Freeze-drying: keep at -12℃ for 26h, and freeze-drying for 38h, to transform the nanocellulose hydrogel into nanocellulose aerogel.

[0071] The nanocellulose aerogel with negative Poisson's ratio structure prepared in this example has the performance shown in Table 1.

[0072] Example 3

[0073] The mold and preparation method for preparing the nanocellulose aerogel with negative Poisson's ratio structure in this example are basically the same as in Example 1, and the difference from Example 1 is that the inner recess angle a of the inner recess hexagonal pore is 58°, the half length L of the straight side is 1.2mm, the side length l is 1.1mm, the wall thickness t is 1.2mm, and the length Z is 21mm. The length and width of the box body are 32*32mm, and the height is 16mm. The number of rod-shaped columns is 65.

[0074] The crosslinking agent is acrylamide.

[0075] The catalyst is cerium ammonium nitrate.

[0076] Preparation and casting molding of nanocellulose mixture: the mass fraction of the crosslinking agent in the hydrogel is 7.5% (solid mass) of the nanocellulose, and the mass fraction of the catalyst is 1.25% (solid mass) of the mass of the crosslinking agent.

[0077] Aging: keep at 50℃ for 38min.

[0078] Freeze-drying: keep at -12℃ for 26h, and freeze-drying for 38h.

[0079] The nanocellulose aerogel with negative Poisson's ratio structure prepared in this example has the performance shown in Table 1.

[0080] Example 4

[0081] The mold and preparation method for preparing the nanocellulose aerogel with negative Poisson's ratio structure in this example are basically the same as in Example 1, and the difference from Example 1 is that the inner recess angle a of the inner recess hexagonal pore is 60°, the half length L of the straight side is 1.3mm, the side length l is 1.2mm, the wall thickness t is 1.5mm, and the length Z is 22mm. The length and width of the box body are 33*33mm, and the height is 18mm. The number of rod-shaped columns is 70.

[0082] The crosslinking agent is acrylamide.

[0083] The catalyst is potassium persulfate.

[0084] Preparation and casting molding of nanocellulose mixture: the mass fraction of the crosslinking agent in the hydrogel is 10% (solid mass) of the nanocellulose, and the mass fraction of the catalyst is 1.5% (solid mass) of the mass of the crosslinking agent.

[0085] Aging: 52℃ for 40min.

[0086] Freeze-drying: -10℃ for 30h, and freeze-drying for 40h.

[0087] The negative Poisson's ratio structure nanocellulose aerogel prepared in this example has the performance shown in Table 1.

[0088] Example 5

[0089] The mold and preparation method for preparing the negative Poisson's ratio structure nanocellulose aerogel in this example are basically the same as in Example 1, and the difference from Example 1 is that the inner recess angle a of the inner recess hexagonal pore is 65°, the half length of the straight edge L is 1.5mm, the side length l is 1.3mm, the wall thickness t is 2.0mm, and the length Z is 25mm. The length and width of the box body are 35*35mm, and the height is 20mm. The number of rod-shaped columns is 81.

[0090] The crosslinking agent is acrylamide.

[0091] The catalyst is N,N'-methylenebisacrylamide.

[0092] Preparation and casting molding of nanocellulose mixture: the mass fraction of the crosslinking agent in the hydrogel is 12.5% (solid mass) of the nanocellulose, and the mass fraction of the catalyst is 2.0% (solid mass) of the mass of the crosslinking agent.

[0093] Aging: 55℃ for 45min.

[0094] Freeze-drying: -6℃ for 36h, and freeze-drying for 48h.

[0095] The negative Poisson's ratio structure nanocellulose aerogel prepared in this example has the performance shown in Table 1.

[0096] Example 6

[0097] The mold and preparation method for preparing the negative Poisson's ratio structure nanocellulose aerogel in this example are basically the same as in Example 1, and the difference is that the crosslinking agent is epichlorohydrin, and the mass fraction of the crosslinking agent in the hydrogel is 5% (solid mass) of the nanocellulose.

[0098] The negative Poisson's ratio structure nanocellulose aerogel prepared in this example has the performance shown in Table 1.

[0099] Example 7

[0100] The mold and preparation method for preparing the negative Poisson's ratio structure nanocellulose aerogel of the embodiment are basically the same as those of embodiment 1, except that dopamine is used as the crosslinking agent, and the mass fraction of the crosslinking agent in the hydrogel is 5% (solid mass) of the nanocellulose.

[0101] The negative Poisson's ratio structure nanocellulose aerogel prepared in the embodiment has the performance shown in Table 1.

[0102] Comparative Example 1

[0103] The comparative example is basically the same as embodiment 2, except that no crosslinking agent is added.

[0104] The nanocellulose aerogel prepared in the comparative example has the performance shown in Table 1.

[0105] Comparative Example 2

[0106] The comparative example is basically the same as embodiment 2, except that no crosslinking agent is added, and no mold is used, and the negative Poisson's ratio structure is not provided.

[0107] The nanocellulose aerogel prepared in the comparative example has the performance shown in Table 1.

[0108] Table 1 Performance parameters of materials prepared in examples and comparative examples

[0109]

[0110] As can be seen from the examples, the nanocellulose aerogel prepared in the application has a Poisson's ratio of -0.121 to -0.203, showing a negative Poisson's ratio effect. The density of the negative Poisson's ratio nanocellulose aerogel is only 13.0 to 27.6 mg / cm 3 , the elastic modulus is 1.20 to 6.20 MPa, and the compression hardness coefficient is 1.25 to 6.30; in the impact resistance test, it can withstand an impact load of 25g small balls (120 times the mass of the nanocellulose aerogel itself) with an energy of 260 to 270 mJ, and the glass on one side of the negative Poisson's ratio nanocellulose aerogel is intact, as shown in Figure 6 Under laboratory small-scale freeze-drying conditions, 216 cm 3 of negative Poisson's ratio nanocellulose aerogel can be produced in a single run (36h).

[0111] It can be seen from the comparative examples that flexible polymers such as PAM can improve the impact resistance of nanocellulose aerogels. In pure nanocellulose aerogels, due to the flaky structure of nanocellulose after drying, high brittleness and poor elasticity, the strain cannot be efficiently transmitted after the pure nanocellulose aerogel is impacted, and the stress is often concentrated in one place, causing local damage to the aerogel, and the advantages of the negative Poisson's ratio structure cannot be fully utilized. After adding flexible polymers, the flexible polymers are cross-linked between the nanocellulose layers, making strain transmission more efficient and uniform, increasing the elastic modulus, and also increasing the internal support of the aerogel, thereby improving the impact resistance.

[0112] It can be seen from the examples and comparative examples that Example 2 is the best embodiment, with the highest specific modulus, balancing the low density and high impact strength of nanocellulose aerogels, and the Poisson's ratio is not lost much. On the basis of Example 2, continue to add polyacrylamide, the elastic modulus of nanocellulose aerogel increases significantly, and the density still increases, and exceeds 20 mg / cm 3 If too much is added, the low density advantage of nanocellulose aerogel will be gradually lost. Considering the use scenario, the impact resistance of aerogel needs to be evaluated from the following aspects: ① has obvious negative Poisson's ratio effect, Poisson's ratio should be less than 0, and produces internal densification to resist external pressure when compressed (on the premise of not affecting other mechanical properties, the smaller the Poisson's ratio, the better, but blindly pursuing lower Poisson's ratio will reduce the compression resistance, and the aerogel cannot be applied in impact resistance, and it will not be worth the loss); ② has sufficient compression resistance, i.e. the elastic modulus needs to be high to achieve the buffering effect; ③ has the advantage of low density of aerogel, and cannot add too much flexible polymer to cause the aerogel to lose its own low density advantage. Therefore, the specific modulus is introduced for comprehensive evaluation, i.e. the higher the specific modulus, the higher the elastic modulus of the aerogel per unit density, and the better the impact resistance that can be achieved with the same mass.

Claims

1. A method for preparing negative Poisson's ratio structured nanocellulose aerogel, characterized in that: Includes the following steps: Preparation of nanocellulose mixture: A crosslinking agent and a catalyst are added to a nanocellulose suspension and mixed and stirred uniformly to obtain a nanocellulose mixture; Preparation of the suspension: Nanocellulose is carboxylated by oxidation to obtain a suspension with a mass fraction of 0.7%~1.8%, wherein the fiber aspect ratio is 120~80:1 and the surface carboxyl content is 1.0 mmol / L~1.35 mmol / L; Preparation of nanocellulose hydrogel: The nanocellulose mixture is poured into a mold for preparing negative Poisson's ratio structure nanocellulose aerogel to obtain nanocellulose hydrogel; the mass fraction of the crosslinking agent in the hydrogel is 2.5%~12.5% ​​of the mass of nanocellulose, and the mass fraction of the catalyst is 0.5%~2% of the mass of the crosslinking agent. The crosslinking agent is one or more of acrylamide, epichlorohydrin, and dopamine; The catalyst is one or more of potassium persulfate, N,N'-methylenebisacrylamide, and cerium ammonium nitrate. Aging and freeze-drying yielded nanocellulose aerogels; The mold for preparing negative Poisson's ratio structured nanocellulose aerogel includes a box and a concave hexagonal rod-shaped column. The inner wall of the bottom of the box is provided with a concave hexagonal hole that cooperates with the rod-shaped column. The concave angle α of the rod-shaped column is 55°~65°, the half length of the straight side L is 1 mm~1.5 mm, the side length l is 1.0 mm~1.3 mm, the wall thickness t is 1 mm~2 mm, and the length Z is 20 mm~25 mm. The number of rod-shaped columns is 60 to 81.

2. The method for preparing negative Poisson's ratio structured nanocellulose aerogel according to claim 1, characterized in that: In the preparation step of the suspension, oxidation is mediated by TEMPO.

3. The method for preparing negative Poisson's ratio structured nanocellulose aerogel according to any one of claims 1-2, characterized in that: The aging process parameters are: 45 ℃~55 ℃ for 30 min~45 min, then cooled to room temperature.

4. The method for preparing negative Poisson's ratio structured nanocellulose aerogel according to claim 3, characterized in that: The freeze-drying process parameters are: -6 ℃ to -18 ℃ for 24 h to 36 h, freeze-drying for 36 h to 48 h.

5. A negative Poisson's ratio structured nanocellulose aerogel, characterized in that: It is prepared using the method described in any one of claims 1-4.

6. Application of negative Poisson's ratio structured nanocellulose aerogel, characterized by: The negative Poisson's ratio structured nanocellulose aerogel described in claim 5 can be applied to the preparation of cushioning, shock absorption, or impact-resistant materials.

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