Preparation method of environment-friendly high-performance gas barrier film

Abstract

The invention provides a preparation method of an environment-friendly high-performance gas barrier film, which is characterized in that a solution blending method is adopted to prepare a polyvinyl alcohol / nanocellulose / covalent organic framework nanosheet ternary composite system, and a coating method is adopted to prepare the high-performance gas barrier film under high-temperature evaporation. Multidimensional mixed nanocellulose and covalent organic framework nanosheets are introduced as functional fillers to form a stable intercalation structure with a polyvinyl alcohol matrix, so that the interfacial compatibility of the components is remarkably improved. The multi-component gas barrier film is endowed with high humidity resistance, chemical stability, high gas barrier property and the like by regulating and controlling the heat treatment temperature, the environment-friendly gas barrier film material shows good application prospects in the fields of chemical protective clothing and the like, and a new thought is provided for development of high-performance protective materials.

Description

Technical Field

[0001] The present invention relates to the field of chemical protective materials, and particularly to a preparation method of an environmentally friendly high-performance gas barrier film. Background Art

[0002] In emergency rescue scenarios, rescue personnel such as firefighters and medical staff need to enter chemical accident sites. Chemical protective equipment is their life barrier to resist the invasion of dangerous chemicals and carry out rescue work. Although current chemical protective clothing can meet the protection requirements to a certain extent, there are still problems such as a narrow chemical resistance spectrum, short protection time, and poor wearing flexibility. The fundamental reason lies in the insufficient comprehensive performance of chemical protective fabrics. Therefore, developing multifunctional integrated chemical protective clothing is the key to dealing with complex chemical environments. Multilayer structure design has become the key means for preparing high-performance chemical protective fabrics. As the key to achieving high-performance chemical protective fabrics, the core function of the gas barrier layer is to block the permeation of gases through a low-permeability structure, thereby protecting the contents from the external environment or preventing the leakage of harmful gases. Polyvinyl alcohol has good chemical resistance, oil resistance, and gas barrier properties. It has good film-forming properties, is non-toxic and harmless to the human body, and can be microbially degraded in the soil after special treatment. However, the hydrophilic hydroxyl groups in the macromolecular chain of polyvinyl alcohol are sensitive to moisture and have poor moisture resistance.

[0003] The emergence of mixed-dimensional heterostructures (2D + nD, where n is 0, 1, or 3) provides new ideas for designing high-performance membranes. 2D covalent organic framework nanosheets have excellent thermal stability and chemical stability, and their ultra-thin thickness and large specific surface area can effectively reduce the gas barrier performance of polymer materials. The abundant hydroxyl groups on the surface of 1D nanocellulose materials can be conveniently used for subsequent functionalization and to achieve strong intermolecular interactions, thereby further improving the mechanical strength of the polymer membrane and enhancing its stability and durability in practical applications. By embedding 1D materials into the interlayer of COF nanosheets, the stacking behavior between nanosheets can be reduced, and the position of layered fillers can be effectively fixed by means of the interaction between fillers, thereby extending the gas diffusion path. Summary of the Invention

[0004] The object of the present invention is to provide an environmentally friendly polyvinyl alcohol / nanocellulose / covalent organic framework nanosheet (PVA / CNC / COF) high-performance gas barrier film. By exploring its manufacturing method, physical properties, and potential in the actual chemical environment faced, the important significance of the environmentally friendly new gas barrier film achieving efficient barrier against harsh environments is clarified. The preparation method of the present invention is simple, with remarkable effects, and has broad application prospects in the field of chemical protection.

[0005] The materials used in this invention include: polyvinyl alcohol (PVA), cellulose nanocrystals (CNCs), covalent organic framework (COF) nanosheets, and deionized water (DI). Among them, deionized water is used as the solvent, polyvinyl alcohol serves as the matrix of the gas barrier film, and cellulose nanocrystals and covalent organic framework nanosheets are used as functional fillers.

[0006] A method for preparing an environmentally friendly and high-performance gas barrier film includes the following steps: Step S11: Freeze-dry the cellulose nanocrystals for 3 days, disperse the foamy cellulose nanocrystals in deionized water, disperse them with a homogenizer for 30 seconds, and then ultrasonically treat them with an ultrasonic probe for 15 min to obtain a cellulose nanocrystal solution; Step S12: Pour in the polyvinyl alcohol powder and heat and stir for 2 h under the condition of heating in an oil bath at 95 °C to obtain a polyvinyl alcohol solution; Step S13: After cooling, ultrasonically treat for 1 h in an ultrasonic cleaner to remove bubbles, then scrape and coat on tin foil, and dry at a high temperature for 4 h to obtain a polyvinyl alcohol / cellulose nanocrystal gas barrier film.

[0007] Preferably, the high-temperature drying in step S13 is drying in an oven at 90 °C.

[0008] A method for preparing an environmentally friendly and high-performance gas barrier film includes the following steps: Step S21: Freeze-dry the cellulose nanocrystals for 3 days, disperse the foamy cellulose nanocrystals in deionized water, disperse them with a homogenizer for 30 seconds, and then ultrasonically treat them with an ultrasonic probe for 15 min to obtain a cellulose nanocrystal solution; Step S22: Add the covalent organic framework nanosheets to the dispersion liquid, ultrasonically treat in an ultrasonic cleaner, and take the supernatant after standing overnight to obtain a cellulose nanocrystal / covalent organic framework nanosheet solution; Step S23: Subsequently, pour in the polyvinyl alcohol powder and heat and stir for 2 h under the condition of heating in an oil bath at 95 °C to obtain a polyvinyl alcohol solution; Step S24: After cooling, ultrasonically treat for 1 h in an ultrasonic cleaner to remove bubbles, then scrape and coat on tin foil, and dry at a high temperature for 4 h to obtain a polyvinyl alcohol / cellulose nanocrystal / covalent organic framework nanosheet gas barrier film.

[0009] Preferably, the ultrasonic treatment time of the covalent organic framework nanosheet solution in step S22 is 1 - 5 h.

[0010] Preferably, the concentration of the covalent organic framework nanosheet solution in step S22 is 0.5 - 2.0 wt %.

[0011] Preferably, the variable temperature range of the high-temperature drying in step S24 is 120 - 180 °C.

[0012] Preferably, the concentration of the nanocellulose solution is 0.2 to 1.2 wt%.

[0013] Preferably, the concentration of polyvinyl alcohol is 12 wt%.

[0014] Beneficial effects:

[0015] 1. The preparation method of the present invention is simple and environmentally friendly, and is green and pollution-free to the environment; 2. In the high-performance gas barrier film prepared by the present invention, the covalent organic framework nanosheets and nanocellulose in the system form a binary hybrid system, so that the high-performance gas barrier film has a high crystallinity (crystallinity is 74.9%). Compared with the polyvinyl alcohol gas barrier film, the gas performance of the polyvinyl alcohol / nanocellulose / covalent organic framework nanosheet high-performance gas barrier film is improved by 62% (the gas barrier performance is 0.49 × 10 -11 g·cm / (cm 2 ·s·Pa)).

[0016] 3. The introduction of the covalent organic framework nanosheets in the present invention plays multiple roles. It not only enhances the gas barrier performance of the polyvinyl alcohol-based gas barrier film, but also endows the barrier film with excellent additional properties such as strong tensile strength, high temperature / low temperature resistance and high humidity resistance in cooperation with nanocellulose, greatly broadening its application range.

[0017] 4. The gas barrier film prepared by the present invention has broad-spectrum chemical protection performance, can have excellent chemical stability and broad-spectrum properties within a wide range of chemical reagents, and has potential applications in chemical protection. Description of the drawings

[0018] Figure 1 It is the electron microscope image of the covalent organic framework nanosheets under different ultrasonic times.

[0019] Figure 2 It is the transmission electron microscope image of the binary hybrid solution of nanocellulose and covalent organic framework nanosheets and the performance characterization image of the high-performance gas barrier film.

[0020] Figure 3 It is the surface and cross-section microscopic schematic diagrams and performance characterization images of different gas barrier films.

[0021] Figure 4 They are respectively the stress-strain diagrams and Young's modulus diagrams of the polyvinyl alcohol gas barrier film, the polyvinyl alcohol / nanocellulose gas barrier film, and the polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier film after heat treatment at 160°C.

[0022] Figure 5 They are respectively the stress-strain curve diagrams of different gas barrier films after being treated at a high temperature of 70°C and a low temperature of -60°C for 8 hours.

[0023] Figure 6 Analysis diagrams of the relationship with water vapor barrier performance under different reaction conditions respectively.

[0024] Figure 7 Macromorphology comparison diagrams, electron microscopy diagrams after immersion, and mechanical strength comparison diagrams before and after 4 h of soaking in different chemical reagents for polyvinyl alcohol gas barrier film, polyvinyl alcohol / nanocellulose gas barrier film, and polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier film respectively. Detailed implementation manners

[0025] The present invention will be further described below in conjunction with specific embodiments.

[0026] A preparation method of an environmentally friendly high-performance gas barrier film includes the following steps: Step S11: Freeze-dry nanocellulose for 3 days, disperse the foamy nanocellulose in deionized water, disperse it with a homogenizer for 30 seconds, and then ultrasonically treat it with an ultrasonic probe for 15 min to obtain a nanocellulose solution; the concentration of the nanocellulose solution is 0.2 - 1.2 wt%.

[0027] Step S12: Pour in polyvinyl alcohol powder, and heat and stir for 2 h under the condition of heating in an oil bath at 95°C to obtain a polyvinyl alcohol solution; the concentration of polyvinyl alcohol is 12 wt%.

[0028] After cooling, ultrasonically treat it in an ultrasonic cleaner for 1 h to remove air bubbles, then scrape it onto tin foil and dry it in an oven at 90°C for 4 h to obtain a polyvinyl alcohol / nanocellulose gas barrier film.

[0029] A preparation method of an environmentally friendly high-performance gas barrier film includes the following steps: Step S21: Freeze-dry nanocellulose for 3 days, disperse the foamy nanocellulose in deionized water, disperse it with a homogenizer for 30 seconds, and then ultrasonically treat it with an ultrasonic probe for 15 min to obtain a nanocellulose solution; the concentration of the nanocellulose solution is 0.2 - 1.2 wt%.

[0030] Step S22: Add covalent organic framework nanosheets to the dispersion liquid, ultrasonically treat it in an ultrasonic cleaner, and take the supernatant after standing overnight to obtain a nanocellulose / covalent organic framework nanosheet solution; the ultrasonic treatment time of the covalent organic framework nanosheet solution is 1 - 5 h, and the concentration of the covalent organic framework nanosheet solution is 0.5 - 2.0 wt% Step S23: Then pour in polyvinyl alcohol powder, and heat and stir for 2 h under the condition of heating in an oil bath at 95°C to obtain a polyvinyl alcohol solution; the concentration of polyvinyl alcohol is 12 wt%.

[0031] Step S24: After cooling, ultrasonically clean for 1 h in an ultrasonic cleaner to remove air bubbles, then scrape and coat on tin foil, and dry at a high temperature of 120 - 180 °C for 4 h to obtain a polyvinyl alcohol / nanocellulose / covalent organic framework nanosheet gas barrier film.

[0032] The covalent organic framework nanosheets of the present invention are purchased from Suzhou Baolidi Material Technology Co., Ltd., and the model is NKCOF - 41.

[0033] Comparative Example 1: Pour 4.8 g of PVA powder into 40 g of deionized water, and heat and stir for 2 h under the condition of heating in a 95 °C oil bath to obtain a polyvinyl alcohol solution. After cooling, ultrasonically clean for 1 h in an ultrasonic cleaner to remove air bubbles, then scrape and coat on tin foil, and dry in an oven at 90 °C for 4 h to obtain a polyvinyl alcohol gas barrier film.

[0034] Example 1:

[0035] First, subject the CNCs to freeze - drying treatment for 3 days. Disperse 0.2 wt% of the CNCs in 40 g of deionized water, disperse for 30 s using a homogenizer, and then ultrasonically treat with an ultrasonic probe for 15 min. Subsequently, pour in 4.8 g of PVA powder, and heat and stir for 2 h under the condition of heating in a 95 °C oil bath to obtain a polyvinyl alcohol solution. After cooling, ultrasonically clean for 1 h in an ultrasonic cleaner to remove air bubbles, then scrape and coat on tin foil, and dry in an oven at 90 °C for 4 h to obtain a polyvinyl alcohol / nanocellulose gas barrier film.

[0036] Example 2: First, subject the CNCs to freeze - drying treatment for 3 days. Disperse 0.5 wt% of the CNCs in 40 g of deionized water, disperse for 30 s using a homogenizer, and then ultrasonically treat with an ultrasonic probe for 15 min. Subsequently, pour in 4.8 g of PVA powder, and heat and stir for 2 h under the condition of heating in a 95 °C oil bath to obtain a polyvinyl alcohol solution. After cooling, ultrasonically clean for 1 h in an ultrasonic cleaner to remove air bubbles, then scrape and coat on tin foil, and dry in an oven at 90 °C for 4 h to obtain a polyvinyl alcohol / nanocellulose gas barrier film.

[0037] Example 3: First, subject the CNCs to freeze - drying treatment for 3 days. Disperse 0.7 wt% of the CNCs in 40 g of deionized water, disperse for 30 s using a homogenizer, and then ultrasonically treat with an ultrasonic probe for 15 min. Subsequently, pour in 4.8 g of PVA powder, and heat and stir for 2 h under the condition of heating in a 95 °C oil bath to obtain a polyvinyl alcohol solution. After cooling, ultrasonically clean for 1 h in an ultrasonic cleaner to remove air bubbles, then scrape and coat on tin foil, and dry in an oven at 90 °C for 4 h to obtain a polyvinyl alcohol / nanocellulose gas barrier film.

[0038] Example 4: First, the CNCs were freeze-dried for 3 days. 1.0 wt% of CNCs was dispersed in 40 g of deionized water and dispersed with a homogenizer for 30 seconds, followed by sonication with a sonicator probe for 15 min. Subsequently, 4.8 g of PVA powder was poured in, and the mixture was heated and stirred for 2 h under the condition of 95 °C oil bath heating to obtain a polyvinyl alcohol solution. After cooling, it was sonicated in an ultrasonic cleaner for 1 h to remove air bubbles, then scrape-coated on tin foil, and dried in an oven at 90 °C for 4 h to obtain a polyvinyl alcohol / nanocellulose gas barrier film.

[0039] Example 5: First, the CNCs were freeze-dried for 3 days. 1.2 wt% of CNCs was dispersed in 40 g of deionized water and dispersed with a homogenizer for 30 seconds, followed by sonication with a sonicator probe for 15 min. Subsequently, 4.8 g of PVA powder was poured in, and the mixture was heated and stirred for 2 h under the condition of 95 °C oil bath heating to obtain a polyvinyl alcohol solution. After cooling, it was sonicated in an ultrasonic cleaner for 1 h to remove air bubbles, then scrape-coated on tin foil, and dried in an oven at 90 °C for 4 h to obtain a polyvinyl alcohol / nanocellulose gas barrier film.

[0040] Example 6: First, the CNCs were freeze-dried for 3 days. 0.7 wt% of CNCs was dispersed in 40 g of deionized water and dispersed with a homogenizer for 30 seconds, followed by sonication with a sonicator probe for 15 min. 0.5 wt% of COF particles was added to the dispersion, sonicated in an ultrasonic cleaner for 3 h, and the supernatant was taken after standing overnight to obtain a CNC / COF dispersion solution. Subsequently, 4.8 g of PVA powder was poured in, and the mixture was heated and stirred for 2 h under the condition of 95 °C oil bath heating to obtain a polyvinyl alcohol solution. After cooling, it was sonicated in an ultrasonic cleaner for 1 h to remove air bubbles, then scrape-coated on tin foil, and dried in an oven at 90 °C for 4 h to obtain a polyvinyl alcohol / nanocellulose / covalent organic framework nanosheet gas barrier film.

[0041] Example 7: First, the CNCs were freeze-dried for 3 days. 0.7 wt% CNCs were dispersed in 40 g of deionized water and dispersed with a homogenizer for 30 seconds, followed by sonication with a sonication probe for 15 min. 1.0 wt% of COF particles were added to the dispersion and sonicated in an ultrasonic cleaner for 3 h. After standing overnight, the supernatant was taken to obtain the CNC / COF dispersion solution. Subsequently, 4.8 g of PVA powder was poured in, and the mixture was heated and stirred at 95 °C in an oil bath for 2 h to obtain a polyvinyl alcohol solution. After cooling, it was sonicated in an ultrasonic cleaner for 1 h to remove air bubbles, then spin-coated on tin foil and dried in an oven at 90 °C for 4 h to obtain a polyvinyl alcohol / nanocellulose / covalent organic framework nanosheet gas barrier film.

[0042] Example 8: First, the CNCs were freeze-dried for 3 days. 0.7 wt% CNCs were dispersed in 40 g of deionized water and dispersed with a homogenizer for 30 seconds, followed by sonication with a sonication probe for 15 min. 1.5 wt% of COF particles were added to the dispersion and sonicated in an ultrasonic cleaner for 3 h. After standing overnight, the supernatant was taken to obtain the CNC / COF dispersion solution. Subsequently, 4.8 g of PVA powder was poured in, and the mixture was heated and stirred at 95 °C in an oil bath for 2 h to obtain a polyvinyl alcohol solution. After cooling, it was sonicated in an ultrasonic cleaner for 1 h to remove air bubbles, then spin-coated on tin foil and dried in an oven at 90 °C for 4 h to obtain a polyvinyl alcohol / nanocellulose / covalent organic framework nanosheet gas barrier film.

[0043] Example 9: First, the CNCs were freeze-dried for 3 days. 0.7 wt% CNCs were dispersed in 40 g of deionized water and dispersed with a homogenizer for 30 seconds, followed by sonication with a sonication probe for 15 min. 2.0 wt% of COF particles were added to the dispersion and sonicated in an ultrasonic cleaner for 3 h. After standing overnight, the supernatant was taken to obtain the CNC / COF dispersion solution. Subsequently, 4.8 g of PVA powder was poured in, and the mixture was heated and stirred at 95 °C in an oil bath for 2 h to obtain a polyvinyl alcohol solution. After cooling, it was sonicated in an ultrasonic cleaner for 1 h to remove air bubbles, then spin-coated on tin foil and dried in an oven at 90 °C for 4 h to obtain a polyvinyl alcohol / nanocellulose / covalent organic framework nanosheet gas barrier film.

[0044] Example 10: First, the CNCs were freeze-dried for 3 days. 0.7 wt% of CNCs was dispersed in 40 g of deionized water and dispersed with a homogenizer for 30 seconds, followed by sonication with a sonicator probe for 15 min. 1.5 wt% of COF particles were added to the dispersion and sonicated in an ultrasonic cleaner for 3 h. After standing overnight, the supernatant was taken to obtain a CNC / COF dispersion solution. Subsequently, 4.8 g of PVA powder was poured in, and the mixture was heated and stirred at 95 °C in an oil bath for 2 h to obtain a polyvinyl alcohol solution. After cooling, it was sonicated in an ultrasonic cleaner for 1 h to remove air bubbles, then cast on tin foil and dried in an oven at 120 °C for 4 h to obtain a high-performance gas barrier film of polyvinyl alcohol / nanocellulose / covalent organic framework nanosheets.

[0045] Example 11: First, the CNCs were freeze-dried for 3 days. 0.7 wt% of CNCs was dispersed in 40 g of deionized water and dispersed with a homogenizer for 30 seconds, followed by sonication with a sonicator probe for 15 min. 1.5 wt% of COF particles were added to the dispersion and sonicated in an ultrasonic cleaner for 3 h. After standing overnight, the supernatant was taken to obtain a CNC / COF dispersion solution. Subsequently, 4.8 g of PVA powder was poured in, and the mixture was heated and stirred at 95 °C in an oil bath for 2 h to obtain a polyvinyl alcohol solution. After cooling, it was sonicated in an ultrasonic cleaner for 1 h to remove air bubbles, then cast on tin foil and dried in an oven at 140 °C for 4 h to obtain a high-performance gas barrier film of polyvinyl alcohol / nanocellulose / covalent organic framework nanosheets.

[0046] Example 12: First, the CNCs were freeze-dried for 3 days. 0.7 wt% of CNCs was dispersed in 40 g of deionized water and dispersed with a homogenizer for 30 seconds, followed by sonication with a sonicator probe for 15 min. 1.5 wt% of COF particles were added to the dispersion and sonicated in an ultrasonic cleaner for 3 h. After standing overnight, the supernatant was taken to obtain a CNC / COF dispersion solution. Subsequently, 4.8 g of PVA powder was poured in, and the mixture was heated and stirred at 95 °C in an oil bath for 2 h to obtain a polyvinyl alcohol solution. After cooling, it was sonicated in an ultrasonic cleaner for 1 h to remove air bubbles, then cast on tin foil and dried in an oven at 160 °C for 4 h to obtain a high-performance gas barrier film of polyvinyl alcohol / nanocellulose / covalent organic framework nanosheets.

[0047] Example 13: First, the CNCs were freeze-dried for 3 days. 0.7 wt% of CNCs was dispersed in 40 g of deionized water and dispersed for 30 seconds using a homogenizer, followed by ultrasonic treatment with a probe sonicator for 15 min. 1.5 wt% of COF particles was added to the dispersion and sonicated in an ultrasonic cleaner for 3 h. After standing overnight, the supernatant was taken to obtain a CNC / COF dispersion solution. Subsequently, 4.8 g of PVA powder was poured in, and the mixture was heated and stirred in an oil bath at 95 °C for 2 h to obtain a polyvinyl alcohol solution. After cooling, it was sonicated in an ultrasonic cleaner for 1 h to remove air bubbles, then cast on tin foil and dried in an oven at 180 °C for 4 h to obtain a high-performance gas barrier film of polyvinyl alcohol / nanocellulose / covalent organic framework nanosheets.

[0048] Performance analysis of the high-performance gas barrier film of polyvinyl alcohol / nanocellulose / covalent organic framework nanosheets: (1) The morphology of covalent organic framework nanosheets under different ultrasonic times was observed using a field emission scanning electron microscope. As Figure 1 shown, a is the original covalent organic framework, b is the covalent organic framework nanosheets after 1 h of sonication, c is the covalent organic framework nanosheets after 2 h of sonication, d is the covalent organic framework nanosheets after 3 h of sonication, e is the covalent organic framework nanosheets after 4 h of sonication, and f is the covalent organic framework nanosheets after 5 h of sonication. It can be found that as the ultrasonic time increases, the covalent organic framework particles gradually separate into sheet-like structures. When the ultrasonic time reaches 3 h ( Figure 1 d in the figure), the covalent organic framework nanosheets clearly disperse into a "melon seed-like" morphological structure. Thus, it can be seen that when the ultrasonic time is too short ( Figure 1 b-c in the figure) or too long ( Figure 1 e-f in the figure), there is an easy stacking phenomenon among the covalent organic framework nanosheets, which is attributed to the strong van der Waals forces between the covalent organic framework nanosheets. Too short a dispersion time is difficult to separate the nanosheets, and too long a dispersion time may exacerbate this attraction, causing the nanosheets to re-aggregate.

[0049] (2) Figure 2 Figure 18 shows the transmission electron microscopy images of the binary mixed solution of nanocellulose and covalent organic framework nanosheets and the performance characterization images of the high-performance gas barrier film. a is the transmission electron microscopy image of the binary mixed solution of covalent organic framework nanosheets and nanocellulose, b is the Fourier transform infrared spectroscopy image of the high-performance gas barrier film of polyvinyl alcohol / nanocellulose / covalent organic framework nanosheets, and c is the X-ray diffraction pattern of the high-performance gas barrier film of polyvinyl alcohol / nanocellulose / covalent organic framework nanosheets. The distribution state of the binary mixed nanofiller of nanocellulose / covalent organic framework nanosheets was observed using transmission electron microscopy. As Figure 2As shown in a, its structure is manifested as a dense one-dimensional nanocellulose covering a planar covalent organic framework nanosheet. The Fourier transform infrared spectrometer and X-ray diffraction were used to analyze the microstructures of polyvinyl alcohol, nanocellulose, covalent organic framework nanosheets, and the high-performance gas barrier film of polyvinyl alcohol / nanocellulose / covalent organic framework nanosheets in Example 12. As Figure 2 shown in b, the vibration peak of polyvinyl alcohol at is attributed to the stretching vibration of -OH, while the band at about corresponds to the C-O stretching vibration in the crystalline and amorphous regions of polyvinyl alcohol. The crystalline peak of the polyvinyl alcohol / nanocellulose / covalent organic framework nanosheet high-performance gas barrier film in Example 12 at is sharper, which is attributed to the strong interaction between nanocellulose and the hydroxyl groups of polyvinyl alcohol, as well as the dehydration condensation of hydroxyl groups to form ether bonds (C-O-C) during the heat treatment process. In addition, the vibration peak of C=C appears in the band of the polyvinyl alcohol / nanocellulose / covalent organic framework nanosheet high-performance gas barrier film in Example 12 at , indicating that covalent organic framework nanosheets have been successfully introduced into the high-performance gas barrier film. As Figure 2 shown in c, semi-crystalline polyvinyl alcohol forms partial crystalline regions, but with the addition of nanocellulose and covalent organic framework nanosheets, the intensity of the main crystalline peak of polyvinyl alcohol weakens. At the same time, the diffraction vibration peaks originally belonging to covalent organic framework nanosheets in the range of 3-10° disappear, which is attributed to the interlaced stacking of the layered structure of covalent organic frameworks after ultrasonic treatment, and the effective dispersion of nanocellulose and covalent organic framework nanosheets in the polyvinyl alcohol matrix.

[0050] (3) Figure 3 are the surface, cross-section micrographs, and performance characterization diagrams of different gas barrier films. The (a) surface and (e) cross-section of the polyvinyl alcohol gas barrier film, the (b) surface and (f) cross-section of the polyvinyl alcohol / nanocellulose gas barrier film, the (c) surface and (g) cross-section of the polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier film, the (d) surface and (h) cross-section of the polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier film after heat treatment at 160 °C; the (i) X-ray diffraction spectra, (j) crystallinity diagrams, (k) thermal stability diagrams, and (l) water contact angle diagrams of different gas barrier films.

[0051] Figure 3 It can be found from that the scanning electron microscopy characterized that the surface of the polyvinyl alcohol gas barrier film in Comparative Example 1 showed an uneven morphology under heating evaporation ( Figure 3 in a), but the cross-section had a unique smooth and flat morphology ( Figure 3In e), the surface of the polyethylene / nanocellulose gas barrier film in Example 3 has granular protrusions with the addition of nanocellulose ( Figure 3 In b), the cross-section also shows a large number of nanocellulose protrusions ( Figure 3 In f), this is attributed to the hydrogen bonding between nanocellulose and polyvinyl alcohol under heat treatment, causing nanocellulose to agglomerate in the polyvinyl alcohol matrix. The introduction of covalent organic framework nanosheets makes the surface of the polyethylene / nanocellulose / covalent organic framework nanosheet gas barrier film in Example 8 appear smoother and flatter ( Figure 3 In c), similar "melon seed-shaped" covalent organic framework nanosheets can also be found in the cross-section ( Figure 3 In g). After heat treatment at 160 °C, the surface of the polyethylene / nanocellulose / covalent organic framework nanosheet high-performance gas barrier film in Example 12 shows an uneven shape ( Figure 3 In d), with a surface morphology similar to that of the polyvinyl alcohol gas barrier film in Comparative Example 1, but its cross-section shows an interlayer structure in which covalent organic framework nanosheets and nanocellulose are tightly combined ( Figure 3 In h), this phenomenon indicates that the hydrogen bond network formed by polyvinyl alcohol molecular chains and rod-shaped nanocellulose effectively fixes the position of covalent organic framework nanosheets. At the same time, good chemical cross-linking and physical cross-linking result in good interfacial compatibility between the filler and the matrix. As Figure 3 shown in i, obvious diffraction peaks are observed at 19.5° and 41.5° at 2θ, which are attributed to the crystallization peaks of semi-crystalline polyvinyl alcohol. After adding nanocellulose, the main crystallization peak of polyvinyl alcohol weakens, and the crystallinity increases from 80.2% to 70.4%, indicating that the addition of nanocellulose interferes with the crystallization of polyvinyl alcohol. After adding covalent organic framework nanosheets, the crystallinity of the polyethylene / nanocellulose / covalent organic framework nanosheet high-performance gas barrier film in Example 12 increases to 74.9%. After heat treatment, the main crystal peak of the high-performance gas barrier film at 19.5° is enhanced, and new crystal peaks appear at 16.0° and 40.5°, which are attributed to the recrystallization of covalent organic framework and two-dimensional nanocellulose fillers in the polyvinyl alcohol matrix after heat treatment, forming more crystal structures. Its crystallinity also increases to 79.5% ( Figure 3 In j), the thermogravimetric curve is used to analyze the thermal stability of the gas barrier film. From Figure 3As can be seen from k, the thermal decomposition temperature of the polyvinyl alcohol gas barrier film in Comparative Example 1 is 250 °C. When the thermal decomposition temperature is reached, 4.67% of the gas barrier film decomposes. After the decomposition stabilizes, 3.9% remains; the thermal decomposition temperature of the polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier film in Example 12 is 241 °C. When the thermal decomposition temperature is reached, 3.71% of the high-performance gas barrier film decomposes, and 4.05% remains after the decomposition stabilizes. By measuring the water contact angle diagrams of different gas barrier films, it is found that the highest water contact angle of the polyvinyl alcohol gas barrier film in Comparative Example 1 is 57.3°. The surface of nanocellulose is rich in hydroxyl groups (-OH) and has strong hydrophilicity. Adding it to polyvinyl alcohol will increase the overall hydrophilicity of the gas barrier film, resulting in a decrease in the water contact angle to 36.8°. The addition of covalent organic framework nanosheets with high crystallinity increases the water contact angle of the polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier film in Example 12 to 41.5°. Heat treatment further regularizes the crystal sequence in the composite film, making the overall crystallinity similar to that of the polyvinyl alcohol film, and its water contact angle increases to 56.1°. The overall result is consistent with the change in crystallinity.

[0052] (4) Mechanical strength: The polyvinyl alcohol gas barrier film in Comparative Example 1, the polyvinyl alcohol / nanocellulose gas barrier film in Example 3, and the polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier film in Example 12 were fixed on a universal tensile testing machine, and the tensile rate was controlled at 100 mm / min. Figure 4 Respectively, are the (a) stress-strain diagram and (b) Young's modulus diagram of the polyvinyl alcohol gas barrier film, the polyvinyl alcohol / nanocellulose gas barrier film, and the polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier film after heat treatment at 160 °C. As Figure 4 shown, the tensile strength of the polyvinyl alcohol gas barrier film in Comparative Example 1 is 109.57 MPa, and the Young's modulus is 22.4 GPa. Due to the addition of nanocellulose resulting in a decrease in crystallinity, the maximum tensile strength of the polyvinyl alcohol / nanocellulose gas barrier film in Example 3 is 97.52 MPa, and the Young's modulus is 20.55 GPa. Compared with the polyvinyl alcohol gas barrier film in Comparative Example 1, it decreases slightly, which is attributed to the addition of nanocellulose possibly interfering with the arrangement of polyvinyl alcohol molecular chains and reducing the crystallinity of the polyvinyl alcohol matrix. However, there is a strong interfacial adhesion force between nanocellulose, covalent organic framework nanosheets, and polyvinyl alcohol. The interfacial interaction optimizes the microstructure of the high-performance gas barrier film, reduces defects, and improves the overall performance. Therefore, the tensile strength of the polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier film in Example 12 is increased to 137.21 MPa, and the Young's modulus is 29.17 GPa.

[0053] (5) High temperature / low temperature performance:Figure 5 Stress-strain curves of different gas barrier films after being treated at a high temperature of 70 °C and a low temperature of -60 °C for 8 h, where (a) is the polyvinyl alcohol gas barrier film, (b) is the polyvinyl alcohol / nanocellulose gas barrier film, (c) is the high-performance polyvinyl alcohol / nanocellulose / covalent organic framework gas barrier film after being heat-treated at 160 °C, and (d) is the change rate of the strength of different gas barrier films after being treated at high / low temperatures. It shows the changes in the mechanical strength of the polyvinyl alcohol gas barrier film in Comparative Example 1, the polyvinyl alcohol / nanocellulose gas barrier film in Example 3, and the high-performance polyvinyl alcohol / nanocellulose / covalent organic framework gas barrier film in Example 12 after being placed in a high temperature environment of 70 °C and a low temperature environment of -60 °C for 8 h. After the low-temperature treatment, the mechanical strength of the composite films all decreased. The strength reduction rates of the polyvinyl alcohol gas barrier film in Comparative Example 1, the polyvinyl alcohol / nanocellulose gas barrier film in Example 3, and the high-performance polyvinyl alcohol / nanocellulose / covalent organic framework gas barrier film in Example 12 were 90.63%, 94.89%, and 97.52% respectively ( Figure 5 in a-d), while the strength of the films increased after the high-temperature treatment. The strength growth rates of the polyvinyl alcohol gas barrier film in Comparative Example 1, the polyvinyl alcohol / nanocellulose gas barrier film in Example 3, and the high-performance polyvinyl alcohol / nanocellulose / covalent organic framework gas barrier film in Example 12 were 22.83%, 17.08%, and 2.61% respectively ( Figure 5 in d). It shows that the addition of functional nanocellulose and covalent organic framework nanosheet fillers combined with the heat treatment process enhances the interfacial bonding force. The combination of multi-dimensional fillers effectively disperses external stress, reduces stress concentration, and the good interfacial compatibility realizes stress transfer, reduces interfacial defects, and effectively achieves the excellent high-temperature / low-temperature performance of the high-performance gas barrier film.

[0054] (6) Gas barrier performance: Figure 6 It is a comprehensive analysis chart of the relationship between different contents of functional nanoscale fillers and gas barrier performance. Among them, (a) is the analysis chart of the relationship between different nanocellulose ratios in the gas barrier film and water vapor barrier performance, (b) is the analysis chart of the relationship between different covalent organic framework nanosheet ratios in the gas barrier film and water vapor barrier performance when the nanocellulose content is 0.7%, and (c) is the analysis chart of the relationship between the high-performance gas barrier film and water vapor barrier performance under different heating times. The gas barrier performance of the polyvinyl alcohol gas barrier film in Comparative Example 1 is 1.29 × 10 -11 g·cm / (cm 2·s·Pa). Among them, multi-component nanofillers are introduced into the polymer matrix to form an "intercalated structure" as a physical barrier, which can effectively extend the diffusion path of gas molecules in the composite membrane and improve the gas barrier performance. When the content of nanocellulose in the gas barrier membrane reaches 0.7%, the gas barrier performance of the polyvinyl alcohol / nanocellulose gas barrier membrane in Example 3 reaches 0.98 × 10 -11 g·cm / (cm 2 ·s·Pa). When it exceeds 0.7%, due to the excessive content of nanocellulose, the nanocellulose in the membrane agglomerates, forming "interface voids", which provides a convenient path for gas diffusion. When the content of covalent organic framework nanosheets in the gas barrier membrane reaches 1.5%, the gas barrier performance reaches the best, and the value of the polyvinyl alcohol / nanocellulose / covalent organic framework nanosheet gas barrier membrane in Example 8 is 0.75 × 10 -11 g·cm / (cm 2 ·s·Pa). When the treatment temperature is 160 °C, the gas barrier performance of the polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier membrane in Example 12 reaches the lowest at 0.49 × 10 -11 g·cm / (cm 2 ·s·Pa).

[0055] (7) Chemical stability: Figure 7 Respectively, they are the macroscopic morphology comparison diagrams and SEM images after immersion of polyvinyl alcohol gas barrier membrane, polyvinyl alcohol / nanocellulose gas barrier membrane and polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier membrane before and after 4 h of immersion in different chemical reagents, where (a) acetone, (b) acetonitrile, (c) dichloromethane, (d) methanol, (e) tetrahydrofuran, (f) toluene, (g) 30% sodium hydroxide, (h) sodium hypochlorite, (i) immersion in deionized water, and the stress-strain curves of the gas barrier membrane after immersion in different chemical reagents for 4 h, where (j) polyvinyl alcohol gas barrier membrane, (k) polyvinyl alcohol / nanocellulose gas barrier membrane, (l) polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier membrane after heat treatment at 160 °C. It shows that the polyvinyl alcohol gas barrier membrane in Comparative Example 1, the polyvinyl alcohol / nanocellulose gas barrier membrane in Example 8 and the polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier membrane in Example 12 are respectively immersed in acetone, acetonitrile, dichloromethane, methanol, tetrahydrofuran, toluene, 30% sodium hydroxide and sodium hypochlorite for 4 h, and the shape change, microscopic change and performance change before and after immersion in chemical reagents are compared to evaluate their broad-spectrum and chemical stability. Figure 7The physical pictures of a-i before and after immersion show that after being treated in chemical reagents for 4 h, the polyvinyl alcohol gas barrier film in Comparative Example 1, the polyvinyl alcohol / nanocellulose gas barrier film in Example 8, and the polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier film in Example 12 maintain stable morphologies and show no obvious changes. Further observation of the microscopic damage of the gas barrier films by scanning electron microscopy reveals that after immersion in organic solvents, the surface roughness of the polyvinyl alcohol gas barrier film in Comparative Example 1 and the polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier film in Example 12 increases. The situation of the polyvinyl alcohol / nanocellulose gas barrier film in Example 3 is more obvious. However, no obvious holes and cracks appear in the overall situation of the polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier film in Example 12, indicating that the high-performance gas barrier film has excellent chemical stability towards organic solvents. Measuring the changes in the mechanical strength of the three gas barrier films after immersion in organic solvents (100% means the performance remains unchanged after testing), the strength of the polyvinyl alcohol gas barrier film in Comparative Example 1 decreases after being immersed in methanol and toluene, and the reagent with the lowest strength decrease rate is toluene (97.2%); the strength increases after being immersed in acetone, acetonitrile, dichloromethane, and tetrahydrofuran, and the reagent with the highest strength increase rate is acetone (131.50%). The strength of the polyvinyl alcohol / nanocellulose gas barrier film in Example 3 decreases after being immersed in acetonitrile, dichloromethane, and toluene, and the reagent with the lowest strength decrease rate is acetonitrile (71.71%); the strength increases after being immersed in acetone, methanol, and tetrahydrofuran, and the reagent with the highest strength increase rate is tetrahydrofuran (123.00%). The strength of the polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier film in Example 12 decreases after being immersed in dichloromethane, tetrahydrofuran, and toluene, and the reagent with the lowest strength decrease rate is toluene (81.04%). The strength increases after being immersed in acetone, acetonitrile, and methanol, and the reagent with the highest strength increase rate is methanol (113.84%). The surfaces of the polyvinyl alcohol gas barrier film in Comparative Example 1 and the polyvinyl alcohol / nanocellulose gas barrier film in Example 3 immersed in sodium hydroxide show obvious swelling morphologies, and the surface of the polyvinyl alcohol / nanocellulose gas barrier film in Example 3 immersed in sodium hypochlorite also has obvious wrinkles and cracks, indicating that the acid-base solutions are corrosive to the polyvinyl alcohol gas barrier film in Comparative Example 1 and the polyvinyl alcohol / nanocellulose gas barrier film in Example 3. The corrosion situation of the polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier film in Example 12 is much lower than that of the other two gas barrier films, which is attributed to the excellent chemical stability of the covalent organic framework nanosheets, enhancing the stability of the high-performance gas barrier film in acid-base solutions.After soaking the polyvinyl alcohol gas barrier film in Comparative Example 1 and the polyvinyl alcohol / nanocellulose gas barrier film in Example 3 in deionized water, the films showed obvious curling. Further observation of their microscopic morphology revealed obvious wrinkles and protrusions on the film surface, indicating that the polyvinyl alcohol gas barrier film in Comparative Example 1 and the polyvinyl alcohol / nanocellulose gas barrier film in Example 3 swelled in water. However, the polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier film in Example 12 did not change, and an increase in surface roughness was observed in the microscopic morphology, with little change in the overall morphology. However, the strength of the polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier film in Example 12 decreased severely after treatment with sodium hydroxide (25.80%), sodium hypochlorite (35.38%), and deionized water (45.83%). In summary, the high-performance gas barrier film prepared with nanocellulose and covalent organic framework nanosheets as fillers has excellent chemical stability in organic reagents. Compared with the polyvinyl alcohol gas barrier film in Comparative Example 1, the water resistance of the polyvinyl alcohol / nanocellulose / covalent organic framework high-performance gas barrier film in Example 12 was significantly improved, further expanding the application scope.

Claims

1. A method for preparing an environmentally friendly high-performance gas barrier film, characterized in that: The following steps are involved: Step S11, freeze-drying the nanocellulose for 3 days, dispersing the foamy nanocellulose in deionized water, using a homogenizer to disperse for 30 seconds, and then sonicating with an ultrasonic probe for 15 minutes to obtain a nanocellulose solution; Step S12, pouring polyvinyl alcohol powder, and heating and stirring in a 95° C. oil bath for 2 h to obtain a polyvinyl alcohol solution; Step S13: After cooling, ultrasonic cleaning is performed in an ultrasonic cleaning machine for 1 h to remove bubbles, followed by coating on tin foil, and high-temperature drying for 4 h to obtain a polyvinyl alcohol / nanocellulose gas barrier film.

2. The preparation method according to claim 1, characterized in that: The high temperature drying in step S13 is drying in an oven at 90°C.

3. The preparation method according to claim 1, characterized in that: The concentration of the nanocellulose solution is 0.2-1.2 wt %.

4. The preparation method according to claim 1, characterized in that: The concentration of polyvinyl alcohol was 12 wt %.

5. A method for preparing an environmentally friendly high-performance gas barrier film, characterized in that: The following steps are involved: Step S21, freeze-drying the nanocellulose for 3 days, dispersing the foamy nanocellulose in deionized water, using a homogenizer to disperse for 30 seconds, and then sonicating with an ultrasonic probe for 15 minutes to obtain a nanocellulose solution; Step S22, adding the covalent organic framework nanosheets to the dispersion, ultrasonically treating it in an ultrasonic cleaning machine, letting it stand overnight, and taking the supernatant to obtain a nanocellulose / covalent organic framework nanosheet solution; Step S23, then pour in polyvinyl alcohol powder, and heat and stir in an oil bath at 95° C. for 2 h to obtain a polyvinyl alcohol solution; Step S24: After cooling, ultrasonic cleaning is performed in an ultrasonic cleaning machine for 1 h to remove bubbles, followed by coating on tin foil, and high-temperature drying for 4 h to obtain a polyvinyl alcohol / nanocellulose / covalent organic framework nanosheet gas barrier film.

6. The preparation method according to claim 5, characterized in that: The concentration of the nanocellulose solution is 0.2-1.2 wt %.

7. The preparation method according to claim 5, characterized in that: The concentration of polyvinyl alcohol was 12 wt %.

8. The preparation method according to claim 5, characterized in that: The ultrasonic time of the covalent organic framework nanosheet solution in step S22 is 1 to 5 h.

9. The preparation method according to claim 5, characterized in that: The concentration of the covalent organic framework nanosheet solution in step S22 is 0.5-2.0 wt %.

10. The preparation method according to claim 5, characterized in that: The temperature range of high temperature drying in step S24 is 120-180°C.

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