Humidity-responsive variable stiffness material and method of making same

Abstract

The application discloses a humidity-responsive variable stiffness material and a preparation method thereof, and belongs to the field of intelligent polymer materials. The humidity-responsive PVAm / PEI variable stiffness material is composed of the following components in parts by weight: polyvinylamine 6-9.5 parts, polyethylene imine 0.5-4 parts, and water 90 parts. By blending branched polyethylene imine with a polyvinylamine matrix, the crystallinity and hydrogen bond association degree of the PVAm / PEI variable stiffness material are enhanced, and the material with a large stiffness change is realized. The stiffness change degree of the PVAm / PEI variable stiffness material reaches up to 71000 times when the humidity changes. Meanwhile, the material also has strong intelligent adhesion, shape customization and shape memory performance, and can meet different requirements.

Description

TECHNICAL FIELD

[0001] The present application relates to a humidity-responsive variable-stiffness material and a preparation method thereof, and belongs to the field of intelligent polymer materials. BACKGROUND

[0002] Variable-stiffness materials are intelligent polymer materials that change in stiffness through some stimulus, and have been widely used in soft robots, intelligent adhesives, biomedical minimally invasive devices, and optoelectronic devices. Currently, variable-stiffness materials are mainly prepared through light, electricity, magnetism, temperature, and pressure stimuli, and the range of stiffness change is usually within 10,000 times (Park, Sungjune, et al. "Ultrastretchable elastic shape memory fibers with electrical conductivity." Advanced Science 6.21 (2019): 1901579; Zheng, Xiaoyang, et al. "Minimal-surface-based multiphase metamaterials with highly variable stiffness." Materials & Design 237 (2024): 112548.).

[0003] Compared with light, electricity, magnetism, temperature, and pressure stimuli, humidity comes from air and is an environmentally friendly, non-toxic, low-cost, and inexhaustible stimulus source. However, traditional humidity-responsive materials still have deficiencies in achieving stiffness changes. For example, they greatly expand or dissolve into a liquid state when absorbing humidity; or although they maintain the same volume and solid state after absorbing humidity, the degree of stiffness change is small (10 to 100 times), which cannot meet the actual application. Therefore, it is still challenging to achieve a material with a large degree of stiffness change through humidity while maintaining the same volume and solid state. SUMMARY

[0004] [TECHNICAL PROBLEM]

[0005] In view of the defects and deficiencies of the prior art, the technical problem to be solved by the present application is that the degree of stiffness change is insufficient when the material maintains the same volume and solid state through humidity stimulation.

[0006] [TECHNICAL SCHEME]

[0007] To solve the above technical problems, the present application provides a humidity-responsive variable stiffness material and a preparation method thereof, the humidity-responsive PVAm / PEI variable stiffness material of the present application specifically selects polyvinylamine (PVAm) as a polymer matrix, the polyvinylamine has a primary amine group on each side chain of a repeating unit, has a regular structure, and has intramolecular hydrogen bonds between the primary amine groups, so that the polyvinylamine molecular chain is orderly folded to form a crystalline region, and has good rigidity; the branched polyethyleneimine (PEI) is blended with the polyvinylamine polymer matrix, the polyethyleneimine forms intermolecular hydrogen bonds with the polyvinylamine, so that the crystallinity and hydrogen bond association degree of the PVAm / PEI variable stiffness material are simultaneously increased, and a material with a large stiffness change is realized. Due to the joint action of crystallization and hydrogen bonds, the PVAm / PEI variable stiffness material has strong rigidity (1560.85 MPa) in a dry state; after absorbing moisture, the water molecules destroy the original crystallization and hydrogen bonds, so that the rigidity of the material changes to 0.022 MPa, and the change degree is as high as 71000 times.

[0008] To achieve the above-mentioned purpose, the present application provides a humidity-responsive PVAm / PEI variable stiffness material, the PVAm / PEI variable stiffness material is composed of the following raw materials in parts by weight: polyvinylamine (PVAm) 6-9.5 parts, polyethyleneimine (PEI) 0.5-4 parts, and water 90 parts.

[0009] In an embodiment of the present application, the PVAm / PEI variable stiffness material is composed of the following raw materials in parts by weight: polyvinylamine 8-9.5 parts, polyethyleneimine 0.5-2 parts, and water 90 parts.

[0010] In an embodiment of the present application, the PVAm / PEI variable stiffness material is composed of the following raw materials in parts by weight: polyvinylamine 8-9.5 parts, polyethyleneimine 0.5-2 parts, and water 90 parts.

[0011] In an embodiment of the present application, the PVAm / PEI variable stiffness material is composed of the following raw materials in parts by weight: polyvinylamine 8-9.5 parts, polyethyleneimine 0.5-2 parts, and water 90 parts.

[0012] In an embodiment of the present application, the molecular weight Mw of the polyvinylamine is 1 million-3 million, preferably 3 million; and the molecular weight Mw of the polyethyleneimine is 10,000-50,000, preferably 25,000.

[0013] Another object of the present application is to provide a preparation method of the humidity-responsive PVAm / PEI variable stiffness material, and the preparation method specifically includes the following steps:

[0014] According to the weight ratio, polyvinylamine and polyethyleneimine are configured with water to obtain a composite solution, and then the composite solution is poured into a mold, dried, demolded, and a humidity-responsive PVAm / PEI variable stiffness material is obtained.

[0015] In an embodiment of the present application, the mass percentage of the polyethyleneimine is 5-40%, and the mass percentage of the polyethyleneimine is calculated with respect to the total of the polyvinylamine and the polyethyleneimine; preferably 15-20%.

[0016] In an embodiment of the present application, the drying condition is 40-60°C for 8-24h; preferably 60°C for 12h.

[0017] A third object of the present application is to provide an application of the above-mentioned PVAm / PEI variable stiffness material in intelligent adhesion, shape locking and shape memory.

[0018] A fourth object of the present application is to provide a PVAm / PEI composite material with shape memory, which is composed of the following raw materials in weight parts: polyvinylamine 6-9.5 parts, polyethyleneimine 0.5-4 parts, and water 90 parts.

[0019] In an embodiment of the present application, the PVAm / PEI composite material is composed of the following raw materials in weight parts: polyvinylamine 8-9.5 parts, polyethyleneimine 0.5-2 parts, and water 90 parts.

[0020] In an embodiment of the present application, the PVAm / PEI composite material is composed of the following raw materials in weight parts: polyvinylamine 8-8.5 parts, polyethyleneimine 1.5-2 parts, and water 90 parts.

[0021] In an embodiment of the present application, the PVAm / PEI composite material is composed of the following raw materials in weight parts: polyvinylamine 8.5 parts, polyethyleneimine 1.5 parts, and water 90 parts.

[0022] In an embodiment of the present application, the molecular weight Mw of the polyvinylamine is 1 million-3 million, preferably 3 million; and the molecular weight Mw of the polyethyleneimine is 10,000-50,000, preferably 25,000.

[0023] Advantages:

[0024] (1) The present application prepares PVAm / PEI variable stiffness material by one-step method, which is low in cost, fast in speed, energy-saving and environment-friendly; and the obtained PVAm / PEI variable stiffness material has strong humidity response performance, the Young's modulus of the material is 1560.85 MPa when it is dry, the Young's modulus after reaching humidity equilibrium is 0.022 MPa, and the stiffness change degree is as high as 71000 times.

[0025] (2) The PVAm / PEI variable stiffness material of the present application also has strong adhesion, can be shaped into various complex shapes, and can be shaped into the required shape by heating to meet different needs; the soft variable stiffness material can completely copy details and lock them by removing humidity, and even can be shaped from simple shapes such as square and triangle to complex pentagonal shape. Since the stiffness change is as high as 10 4 orders of magnitude, the shape-locked composite material can bear a certain external load; it shows strong load-bearing capacity, shape memory, and due to its two-dimensional scalability, it can be re-locked into complex 3D shapes.

[0026] (3) Since the stiffness of the PVAm / PEI variable stiffness material can change quickly and continuously, the hygroscopic material can easily support about 1.7 kg of weight by adhering to the plastic bottle cap and drying quickly. The PVAm / PEI composite material can quickly change its stiffness, deform in a flexible and controllable way, and quickly recover its shape memory and locking. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 Figure 1 is a schematic diagram of the PVAm / PEI polymer network prepared for Example 3 of the present application; (a) a schematic diagram of the PVAm / PEI polymer network; (b) a schematic diagram of the humidity-triggered PVAm / PEI variable stiffness material switching between rigid and soft states;

[0028] Figure 2 Figure 2 is a characterization diagram of the crystallinity and hydrogen bonding degree of the humidity-responsive PVAm / PEI variable stiffness material prepared for the examples and comparative examples of the present application; (a) XRD results of different PVAm / PEI variable stiffness materials in dry state and (b) their corresponding crystallinity data diagram; (c) infrared spectrum curve of the PVAm / PEI variable stiffness material in the range of 2900-3700 cm -1 ; and (d) their corresponding hydrogen bonding degree data diagram; (e) infrared spectrum curve of the PVAm / PEI variable stiffness material at 1560-1700 cm -1 ; and (f) their corresponding hydrogen bonding degree data diagram;

[0029] Figure 3Figures of moisture absorption rate of PVAm / PEI variable stiffness materials with different proportions of the present application under different relative humidity (RH); (a) 25℃, 25% RH; (b) 25℃, 55% relative humidity; (c) 25℃, 75% relative humidity;

[0030] Figure 4 Figures of stress-strain curves of PVAm / PEI variable stiffness materials with different proportions of the present application under 75% RH and different moisture absorption time; (a) 0 min, (b) 10 min, (c) 20 min, (d) 30 min, (e) 60 min, (f) 120 min;

[0031] Figure 5 Figures of mechanism characterization of the humidity-responsive PVAm / PEI variable stiffness material prepared in Example 3 of the present application; (a) XRD of 15% PVAm / PEI variable stiffness material; (b) figure of crystallinity data under different moisture absorption time; (c) figure of change in transparency of variable stiffness material before and after moisture absorption; (d) in-situ infrared one-dimensional spectrum of 15% PVAm / PEI variable stiffness material at 20-80℃; (d) two-dimensional synchronous infrared correlation spectrum of 15% PVAm / PEI variable stiffness material measured at 1750-1500 cm –1 Figures of mechanism characterization of the humidity-responsive PVAm / PEI variable stiffness material prepared in Example 3 of the present application; (a) XRD of 15% PVAm / PEI variable stiffness material; (b) figure of crystallinity data under different moisture absorption time; (c) figure of change in transparency of variable stiffness material before and after moisture absorption; (d) in-situ infrared one-dimensional spectrum of 15% PVAm / PEI variable stiffness material at 20-80℃; (d) two-dimensional synchronous infrared correlation spectrum of 15% PVAm / PEI variable stiffness material measured at 1750-1500 cm

[0032] Figure 6 Figures of application effect of the humidity-responsive PVAm / PEI variable stiffness material prepared in Example 3 of the present application in smart adhesion, shape locking and shape memory; (a) schematic diagram of variable stiffness material reconfigured into a load-bearing 3D shape, after heating and moisture evaporation, the 3D shape is locked, and after absorbing moisture, it returns to the original state; (b) figure of adhesion performance of variable stiffness material; (c) figure of angle change of 15% PVAm / PEI variable stiffness material after absorbing moisture for different time under 25℃, 75% relative humidity. DETAILED DESCRIPTION

[0033] The preferred embodiments of the present application are described below, and it should be understood that the embodiments are for better explaining the present application and are not used to limit the present application.

[0034] Test methods involved in the present application

[0035] The crystallinity of the PVAm / PEI variable stiffness material is characterized by X-ray diffraction (XRD);

[0036] The hydrogen bonding degree of the PVAm / PEI variable stiffness material is characterized by Fourier transform infrared spectroscopy (FTIR);

[0037] The mechanical properties of the PVAm / PEI variable stiffness material were tested by a universal tensile testing machine.

[0038] After 2 hours of standing at 25°C, 75% RH, the samples were tested in tension. The stress-strain curve can be divided into two regions - the elastic region, where the curve is linear, and the plastic region. In the elastic region, the stress-strain curve of most materials is a straight line. This means that the strain is directly proportional to the applied stress. Hooke's law gives the relationship between stress and strain in this linear elastic region. The ratio between stress and strain is called Young's modulus, also called the elastic modulus. Young's modulus (E = stress / strain) was used as a measure of the stiffness of the variable stiffness material.

[0039] The raw materials involved in the examples and comparative examples of the present application are as follows:

[0040] Polyvinylamine, molecular weight Mw 3 million.

[0041] Polyethyleneimine, molecular weight Mw 25,000.

[0042] Example 1

[0043] A humidity-responsive PVAm / PEI variable stiffness material was prepared as follows:

[0044] Take 9.5 parts of polyvinylamine and 0.5 parts of polyethyleneimine, then configure them into a solution with 90 parts of water, each part according to 1 g, pour the solution on a polystyrene culture dish, and dry it at 60°C for 12 hours, to obtain the humidity-responsive PVAm / PEI variable stiffness material.

[0045] Example 2

[0046] A humidity-responsive PVAm / PEI variable stiffness material was prepared as follows:

[0047] Take 9 parts of polyvinylamine and 1 part of polyethyleneimine, then configure them into a solution with 90 parts of water, each part according to 1 g, pour the solution on a polystyrene culture dish, and dry it at 60°C for 12 hours, to obtain the humidity-responsive PVAm / PEI variable stiffness material.

[0048] Example 3

[0049] A humidity-responsive PVAm / PEI variable stiffness material was prepared as follows:

[0050] Take 8.5 parts of polyvinylamine and 1.5 parts of polyethyleneimine, then configure them into a solution with 90 parts of water, each part according to 1 g, pour the solution on a polystyrene culture dish, and dry it at 60°C for 12 hours, to obtain the humidity-responsive PVAm / PEI variable stiffness material.

[0051] Example 4

[0052] A humidity-responsive PVAm / PEI variable stiffness material is prepared as follows:

[0053] Take 8 parts of polyvinylamine and 2 parts of polyethyleneimine, then configure them into a solution with 90 parts of water, each part according to 1 g, pour the solution on a polystyrene culture dish, and dry it at 60°C for 12 hours, to obtain a humidity-responsive PVAm / PEI variable stiffness material.

[0054] Example 5

[0055] A humidity-responsive PVAm / PEI variable stiffness material is prepared as follows:

[0056] Take 6 parts of polyvinylamine and 4 parts of polyethyleneimine, then configure them into a solution with 90 parts of water, each part according to 1 g, pour the solution on a polystyrene culture dish, and dry it at 60°C for 12 hours, to obtain a humidity-responsive PVAm / PEI variable stiffness material.

[0057] Comparative Example 1

[0058] The difference from Example 3 is only that the polyethyleneimine is omitted, and other conditions and parameters are the same as in Example 3.

[0059] Comparative Example 2

[0060] The difference from Example 3 is only that the polyvinylamine is omitted, and other conditions and parameters are the same as in Example 3.

[0061] Comparative Example 3

[0062] The difference from Example 3 is only that the polyvinylamine is 4 parts and the polyethyleneimine is 6 parts, and other parameters and conditions are the same as in Example 3.

[0063] Comparative Example 4

[0064] The difference from Example 3 is that the polyethyleneimine is replaced by chitosan, and other conditions and parameters are the same as in Example 3.

[0065] Comparative Example 5

[0066] The difference from Example 3 is only that the polyethyleneimine is replaced by polyvinyl alcohol (Mw = 23000), and other conditions and parameters are the same as in Example 3.

[0067] Result performance analysis

[0068] Figure 1Polymer network and physical picture of the humidity-responsive variable-stiffness material prepared in Example 3; (a) Schematic diagram of the PVAm / PEI polymer network with crystallization and hydrogen bonding; (b) Humidity-triggered PVAm / PEI variable-stiffness material switching between rigid and soft states.

[0069] Schematic diagram of the polymer network of the PVAm / PEI variable-stiffness material is shown in Figure 1 In the dry state, the PVAm / PEI variable-stiffness material exhibits rigidity due to the crystalline regions within the PVAm molecules (intramolecular hydrogen bonds between in situ amine groups on PVAm) and the intermolecular hydrogen bonds between the amine groups of PVAm and PEI that are tightly crosslinked. Upon water absorption, the crystalline regions of PVAm and the intermolecular hydrogen bonds are destroyed by water molecules, causing the PVAm / PEI variable-stiffness material to change from rigid to soft and flexible. When the humidity is removed, the intramolecular hydrogen bonds reform, driving the PVAm molecular chains to fold to form crystalline regions; at the same time, the intermolecular hydrogen bonds between PVAm and PEI are also rebuilt. Therefore, the PVAm / PEI variable-stiffness material recovers from soft to its original rigid state Figure 1 a). These optical photographs show the change in stiffness of the PVAm / PEI variable-stiffness material in dry and wet states, as shown in Figure 1 b. Upon water absorption, the PVAm / PEI variable-stiffness material becomes soft and flexible; it can easily change shape and quickly recover to the initial state after twisting and folding. When the PVAm / PEI variable-stiffness material dries, it will break at the twisted angle and cannot return to the initial state. It is worth noting that these "rigid" and "soft" states can quickly change to the expected rigid state according to individual needs and basic stimulus-response conditions.

[0070] Figure 2 Characterization of the crystallinity and hydrogen bonding degree of the humidity-responsive variable-stiffness material prepared in the present application. (a) XRD results of different PVAm / PEI variable-stiffness materials in the dry state and (b) their corresponding crystallinity; (c) Infrared spectrum curves of the PVAm / PEI variable-stiffness materials in the range of 2900-3700 cm -1 and (d) their corresponding hydrogen bonding degrees; (e) Infrared spectrum curves of the PVAm / PEI variable-stiffness materials at 1560-1700 cm -1 and (f) their corresponding hydrogen bonding degrees.

[0071] Five PVAm / PEI materials with 5%, 10%, 15%, 20%, and 60% PEI were selected to study the effect of PEI on the crystallization and hydrogen bonding of PVAm / PEI variable stiffness materials, corresponding to PEI mass ratios of 5wt%, 10wt%, 15wt%, 20wt%, and 60wt% (the percentage of PEI in the total mass of PVAm and PEI). X-ray diffraction (XRD) analysis of the PVAm / PEI variable stiffness materials in the dry state was performed. Figure 2 a) It shows a broad amorphous peak near 25° and a narrower crystalline peak at 33°. The crystallinity of the PVAm / PEI variable stiffness material was calculated ( Figure 2 (b) The crystallinity of the pure PVAm film is approximately 9.41%, gradually increasing and then decreasing with the addition of PEI. The crystallinity of the PVAm / PEI variable stiffness material reaches its maximum value of approximately 16.55% when the PEI concentration is 15%. This crystallinity decreases with further increases in PEI concentration, indicating that excessive PEI addition may disrupt the crystalline regions of PVAm. The addition of PEI increases intermolecular hydrogen bonding in the system; strong molecular forces contribute to the ordered arrangement of chain segments, which is beneficial for the formation of crystalline regions. However, excessive PEI addition may form even stronger intermolecular hydrogen bonds, leading to reduced PVAm chain mobility and thus limiting PVAm crystallization. Figure 2 The Fourier transform infrared spectrum in c shows 3223 and 3120 cm⁻¹. -1 The NH stretching vibration at this point is attributed to the major amine group on PVAm. Both v(NH) values ​​are below the normal v(NH) range (3300 to 3500 cm⁻¹). -1 This is because PVAm contains intramolecular hydrogen bonds. When PEI is added at 15 wt%, v(NH) becomes a broad peak, ranging from 3200 to 3500 cm⁻¹. -1 This is because excessive PEI leads to stronger intermolecular hydrogen bonds, disrupting the crystallization of PVAm (intramolecular hydrogen bonds), which is consistent with the XRD results.

[0072] The hydrogen bonding degree (HBA) of PVAm / PEI in PVAm / PEI variable stiffness materials is determined by calculating the ratio of split and fitted peak areas. Figure 2 d). The highest degree of hydrogen bonding (HBA, intramolecular hydrogen bonding) is found in pure PVAm (59.11%) due to the abundance of major amino groups in the compound. However, as the PEI content gradually increases to 10%, HBA decreases to 547%, at which point intermolecular crosslinks begin to form between PVAm and PEI; with a PEI content reaching 15%, HBA decreases further at 3200–3500 cm⁻¹. -1a broader PVAm and PEI intermolecular hydrogen bonding peak appears, intermolecular hydrogen bonding is gradually formed, at this time HBA (intermolecular hydrogen bonding) is about 32.12%, and finally gradually increases to 39% (60% PVAm / PEI). Therefore, when 15% of PEI is added to the system, intermolecular hydrogen bonding is considered to be almost saturated. In addition, the peak located at 1660 cm -1 wavenumber is attributed to N-H bending vibration Figure 2 e), and the gradual increase and red shift of σ(N-H) further prove the formation of intermolecular hydrogen bonding as PEI is added. When the PEI content increases from 10% to 15%, HBA rapidly increases from 13.3% to 18.4%, and then remains essentially unchanged Figure 2 f). Therefore, based on XRD and FTIR, it is concluded that the addition of 15wt% of PEI has the highest number of crystallization and hydrogen bonding.

[0073] Figure 3 Moisture absorption rate of PVAm / PEI variable stiffness material prepared by the present application under different relative humidity (RH). Moisture absorption rate of PVAm / PEI variable stiffness material at 25℃ and (a) 25% RH, (b) 55% relative humidity, (c) 75% relative humidity.

[0074] In order to characterize the humidity absorption rate, the composite film in dry state is put into a programmable constant temperature and humidity test box, the temperature is set to 25℃, and the humidity is set to 25% RH, 55% RH and 75% RH respectively, and the humidity absorption test is carried out. The composite film in humidity absorption state shows a "fast first and slow later" trend Figure 3 ), indicating that rapid humidity absorption can be achieved in a relatively short time.

[0075] Figure 4 Stress-strain curves of PVAm / PEI variable stiffness material prepared by the present application under 75% RH and different moisture absorption times. (a) 0 minutes, (b) 10 minutes, (c) 20 minutes, (d) 30 minutes, (e) 60 minutes, (f) 120 minutes.

[0076] According to the stress-strain curves of various variable stiffness material components Figure 4), the change of Young's modulus and the range of stiffness change before and after moisture absorption were calculated. The results show that each component of the PVAm / PEI composite film exhibits "rigidity and brittleness" characteristics in the dry state; the maximum stiffness can reach 1560.85 MPa, and the corresponding strain is about 2.9%; after 10 minutes of moisture absorption, water molecules penetrate into the composite film from the surface, destroy the hydrogen bonds between the amine groups, and gradually destroy the crystalline region. The stress of different components of the PVAm / PEI variable stiffness material decreases by 10 times. However, the presence of water molecules and the formation of hydrogen bonds between the main amine groups enhance the toughness of the film, making its maximum strain increase by about 3800% relative to the dry state, about 1310 times the original, exhibiting "soft and tough" characteristics. As the moisture absorption time increases, the stress decreases more slowly. After 120 minutes of moisture absorption, the crystalline region is basically destroyed, and the moisture basically replaces the hydrogen bonds between the original amine groups. The stiffness of the 15% PVAm / PEI variable stiffness material before and after moisture absorption is 1560.85 MPa and 0.022 MPa, respectively, and the stiffness change is about 7.1×10 4 ; see Table 1 for specific results.

[0077] Figure 5 Mechanism characterization of the humidity-responsive variable stiffness material obtained in Example 3 of the present application. (a) XRD of 15% PVAm / PEI variable stiffness material and (b) crystallinity at different moisture absorption times. (c) Change in transparency of the variable stiffness material before and after moisture absorption. (d) In-situ infrared one-dimensional spectrum of 15% PVAm / PEI variable stiffness material at 20-80°C. (d) Two-dimensional synchronous infrared correlation spectrum of 15% PVAm / PEI variable stiffness material measured at 1750-1500 cm –1 -1. (e) Two-dimensional correlation spectrum of synchronous spectrum and (f) asynchronous spectrum. (g) Molecular simulation and binding energy calculation of PVAm chain, PEI chain and water molecule.

[0078] According to Figure 5 a, the 15% PVAm / PEI variable stiffness material in the dry state also shows a significantly narrower crystalline peak at 2θ = 22° and 33°, with a crystallinity of about 17% ( Figure 5 b). After 5 minutes of moisture absorption, the crystalline peak at 2θ = 33° disappears, and the crystalline region gradually deteriorates, with a crystallinity of about 14.3% at this time. As the moisture absorption time increases, the crystalline peak gradually widens and the crystallinity gradually decreases; after 60 minutes of moisture absorption, the crystalline structure is almost destroyed, and the crystallinity decreases to about 6.5%. Due to the moisture absorption process, the crystalline region is destroyed, resulting in a decrease in the stiffness of the crystallinity. The transmittance of the variable stiffness material in the dry state is only 30%, showing a white opaque state; however, after 180 minutes of moisture absorption, the variable stiffness material becomes transparent, and the transmittance increases to 70%, indicating a change in the crystallinity Figure 5 c). Figure 5d shows one-dimensional infrared spectra with an initial temperature of 20 °C and an increase to 80 °C. At a temperature of 20 °C, doublets at 3230 cm -1 and 3073 cm -1 are observed, which are attributed to the N-H anti-symmetrical and symmetrical stretching vibrations of the primary amine groups in PEI and PVAm, respectively, while a singlet at 1693 cm -1 is attributed to the N-H bending vibration of the primary amine groups. The formation of hydrogen bonds reduces the bonding constant between hydrogen and nitrogen atoms, resulting in a decrease in the vibration frequency. This is the reason why the curve shows a clear red shift and broadening of the absorption peak as the temperature increases. To further understand the network dynamics of the 15% PEI rigidifying material during heating, 2D correlation calculations were performed using spectral wavenumbers between 20 °C and 80 °C. Synchronous and asynchronous 2D correlation spectra of the thin film at 1750-1500 cm -1 are shown in Figure 5 e and 5f, respectively. In Figure 5 e, two positive autocorrelation peaks can be seen at [1688, 1688 cm -1 ] and [1556, 1556 cm -1 ], respectively. These represent the formation of hydrogen-bonded N-H moieties and the free N-H bending vibration absorption peak of the primary amine groups. In the off-diagonal positions of Figure 5 f, two positive cross peaks can be observed at [1680, 1540 cm -1 ] and [1680, 1596 cm -1 ], respectively. According to Noda's principle, it can be seen that the change in the free N-H bond first occurs at 1680 cm -1 , followed by the change in the N-H bond at 1556 cm -1 , which has already formed a hydrogen-bonded connection, indicating that the free N-H bond first begins to relax and moves to the adjacent hydrogen-bonded chain to form more intermolecular hydrogen bonds. Therefore, the number of intermolecular hydrogen bonds gradually increases, and the HBA also rises, which is consistent with the previous infrared results. In addition, molecular dynamics simulations were performed, which demonstrated that the hydrogen bonds between amine groups in the composite thin film were replaced by hydrogen bonds between water molecules after moisture absorption Figure 5g), the molecular dynamics behavior of the composite film mixed with PVAm chains, PEI chains and water molecules before and after moisture absorption was simulated using COMPASS II force field. The binding energy between 30 PVAm chains and 10 PEI chains in the composite film was 1448 kcal / mol, while the binding energy between 30 PVAm chains and 1077 water molecules was 4645 kcal / mol, and the binding energy between 30 PEI chains and 540 water molecules was 1788 kcal / mol. The binding energy between PVAm, PEI chains and water molecules was greater than the binding energy between PVAm and PEI chains. This is sufficient to show that the hydrogen bonds between amine groups in the variable stiffness material are replaced by hydrogen bonds after the water molecules are absorbed.

[0079] Figure 6 Application of the humidity-responsive variable stiffness material obtained in Example 3 of the present application in smart adhesion, shape locking and shape memory. (a) Change diagram of the variable stiffness material reconfigured into a load-bearing 3D shape; after heating and water evaporation, the 3D shape is locked; after absorbing moisture, the film can recover to the original state; (b) Adhesion performance diagram of the variable stiffness material; (c) Real object diagram of the angle change of the 15% PVAm / PEI variable stiffness material after absorbing moisture for different times at 25°C and 75% relative humidity.

[0080] The PVAm / PEI variable stiffness material can be used as a shape memory and locking material. The humidity-responsive PVAm / PEI variable stiffness material prepared can be shaped into various complex shapes in its soft state, and can be shaped into the desired shape by heating; this process is reversible. If the shape needs to be changed, the material can be re-shaped by softening through moisture absorption. The material is universal and can be re-shaped into different shapes by absorbing moisture again to meet different needs. The process of absorbing and releasing moisture to change the shape of the same material is shown in Figure 6 a. The soft variable stiffness material can completely copy the details and lock them by removing moisture, and even from simple shapes such as squares and triangles to complex five-point star shapes. Due to the high stiffness change of up to 10 4 orders of magnitude, the shape-locked variable stiffness material can bear certain external loads. Therefore, the variable stiffness material can have certain load-bearing capacity, shape memory, and due to its two-dimensional scalability, it can be re-locked into complex 3D shapes. In addition, as Figure 6b. As the stiffness of the variable stiffness material can change quickly and continuously, the hygroscopic material can easily support a weight of about 1.7 kg by adhering to the plastic bottle cap and drying quickly. The PVAm / PEI variable stiffness material can quickly change its stiffness, deform in a flexible and controllable manner, and quickly recover its shape memory and locking. The composite shape memory and locking film was placed in a programmable constant temperature and humidity test chamber, set at a temperature of 25°C and a humidity of 75% to absorb moisture to check the shape recovery performance of the variable stiffness material. The change of the angle of the variable stiffness material with time was recorded Figure 6 c). In the initial state, the variable stiffness material maintains the shape-locked upright state, and when the moisture absorption is about 100 seconds, the variable stiffness material gradually absorbs moisture and softens. Within 150-160 seconds, the softened variable stiffness material angle changes rapidly from 20° to 75°, at which time the variable stiffness material can be considered to have almost recovered to the state before shape memory. In other words, the variable stiffness material will basically recover to the original state after absorbing moisture for 220 seconds. Studies have shown that the prepared PVAm / PEI variable stiffness material has timely shape memory and locking performance.

[0081] Table 1. Stiffness performance of PVAm / PEI variable stiffness material

[0082]

[0083] The above provided examples are not intended to limit the scope of the present application, and the described steps are not intended to limit the order of execution. Those skilled in the art can make obvious improvements to the present application in combination with existing common knowledge, which also falls within the protection scope defined by the claims of the present application.

Claims

1. A humidity-responsive PVAm / PEI variable stiffness material, characterized in that, The PVAm / PEI variable stiffness material is composed of the following raw materials in parts by weight: 6-9.5 parts of polyethyleneimine, 0.5-4 parts of polyethyleneimine, and 90 parts of water.

2. The PVAm / PEI variable stiffness material according to claim 1, characterized in that, The PVAm / PEI variable stiffness material is composed of the following raw materials in parts by weight: 8-9.5 parts of polyethyleneimine, 0.5-2 parts of polyethyleneimine, and 90 parts of water.

3. The PVAm / PEI variable stiffness material according to claim 1, characterized in that, The PVAm / PEI variable stiffness material is composed of the following raw materials in parts by weight: 8-8.5 parts of polyethyleneimine, 1.5-2 parts of polyethyleneimine, and 90 parts of water.

4. The PVAm / PEI variable stiffness material according to claim 1, characterized in that, The PVAm / PEI variable stiffness material is composed of the following raw materials in parts by weight: 8.5 parts polyethyleneamine, 1.5 parts polyethyleneimine, and 90 parts water.

5. The PVAm / PEI variable stiffness material according to claim 1, characterized in that, The molecular weight of the polyethyleneamine is 1 million to 3 million; the molecular weight of the polyethyleneimine is 10,000 to 50,000.

6. A method for preparing a humidity-responsive PVAm / PEI variable stiffness material according to any one of claims 1 to 5, characterized in that, The preparation method includes the following: Polyvinylamine and polyethyleneimine are mixed with water according to the weight ratio to obtain a composite solution. The composite solution is then poured into a mold, dried, and demolded to obtain a humidity-responsive PVAm / PEI variable stiffness material.

7. The preparation method according to claim 6, characterized in that, The mass percentage of polyethyleneimine is 5-40%, and the mass percentage of polyethyleneimine is calculated relative to the sum of polyethyleneimine and polyethyleneimine.

8. The preparation method according to claim 6, characterized in that, The drying conditions are 40–60°C for 8–24 hours.

9. An application of the PVAm / PEI variable stiffness material as described in any one of claims 1 to 5 in smart adhesion, shape locking and shape memory.

10. A PVAm / PEI composite material with shape memory, characterized in that, The shape memory PVAm / PEI composite material is composed of the following raw materials in parts by weight: 6-9.5 parts of polyethyleneimine, 0.5-4 parts of polyethyleneimine, and 90 parts of water.

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