A dynamic boronic ester bond toughened polyvinyl alcohol film and a preparation method thereof

Abstract

The application discloses a kind of dynamic borate ester bond toughened polyvinyl alcohol film and preparation method thereof, and the polyvinyl alcohol film is mainly composed of polyvinyl alcohol main chain and phenyldiboronic acid crosslinking structure.The toughened polyvinyl alcohol film prepared by the method of solution reaction-annealing heat treatment has the following main characteristics: when preparing, dehydration condensation reaction occurs between phenyldiboronic acid and the hydroxyl group of polyvinyl alcohol, forming dynamic reversible crosslinking structure, weakening the hydrogen bond interaction between polyvinyl alcohol molecular chains and inhibiting the crystallization of polyvinyl alcohol;During annealing heat treatment, dynamic borate ester bond can undergo dynamic bond exchange at high temperature, promoting the agglomeration of crosslinking agent phenyldiboronic acid and forming a stacking structure of benzene ring π-π conjugation, achieving the toughening of polyvinyl alcohol film.The polyvinyl alcohol film based on dynamic borate ester bond crosslinking provided by the application has good mechanical properties and self-repairing performance, and the preparation method is simple.

Description

Technical Field

[0001] This invention belongs to the field of polyvinyl alcohol film technology, specifically relating to a dynamically borate bond-toughened polyvinyl alcohol film and its preparation method. Background Technology

[0002] Polyvinyl alcohol (PVA) is a biodegradable polymer that can decompose under natural conditions to obtain non-toxic products, avoiding the problem of white pollution. Furthermore, it can be produced on a large scale via non-petroleum routes, possessing significant economic and ecological value. PVA has a simple and regular structure, with numerous hydroxyl groups forming various forms of hydrogen bonds between the molecular chains and increasing its crystallinity. While this structure provides sufficient mechanical strength, it limits the material's toughness, making it strong but brittle. Its elongation at break is extremely low, hindering flexible applications such as bending and torsion, thus limiting its application scenarios. Therefore, toughening modification of PVA is necessary while maintaining a certain level of strength.

[0003] Traditional methods for toughening polyvinyl alcohol (PVA) primarily involve physical and chemical toughening modifications. Physical toughening involves introducing modifiers to prepare composite materials or optimizing the network structure design to adjust the crystallinity and intermolecular interactions of PVA, thereby achieving toughening. The former involves introducing materials with different structures and properties from PVA into the matrix resin, altering the hydrogen bonds between PVA molecular chains and introducing new interactions or forming new molecular aggregation states to achieve toughening. The latter involves adjusting and optimizing the preparation process through strategies such as freeze casting, mechanical tempering, salting out, and solvent replacement to achieve network systems with directional molecular arrangements, multi-scale networks, and layered structures, thus achieving PVA toughening. Chemical toughening, on the other hand, involves introducing certain substances that react chemically with PVA, altering its molecular chain structure and connection methods to improve PVA's performance. Chemically modified PVA forms a uniform and dense polymer network, and the introduced covalent bonds change intermolecular interactions, significantly improving the mechanical properties of PVA materials. Aldehydes and carboxylic acids / anhydrides are two of the most commonly used substances for chemical toughening modification of PVA. However, both physical toughening and chemical toughening have their advantages and disadvantages. How to combine the advantages of traditional physical and chemical toughening modifications to achieve toughening of polyvinyl alcohol remains a challenge.

[0004] (Benzene)boronic acid possesses a specific affinity for cis-dihydroxyl groups, selectively binding with diols to form dynamic borate ester bonds. Under certain conditions, these dynamic borate ester bonds can undergo reversible breakage and recombination through hydrolysis / re-esterification or transesterification between borate ester bonds, altering the molecular structure of the material. Utilizing the dynamic reversibility of borate ester bonds, their introduction into polyvinyl alcohol (PVA) can not only endow the material with self-healing and remodeling properties but also promises to achieve high toughness in PVA films. Summary of the Invention

[0005] To overcome the shortcomings of traditional physical and chemical modification methods for PVA, this invention aims to provide a dynamically borate bond-toughened polyvinyl alcohol film and its preparation method. The method of this invention is simple to operate, the reaction is easily controlled, and it innovatively proposes a new strategy for toughening polyvinyl alcohol films using dynamic borate bonds.

[0006] To achieve the above-mentioned technical objectives and effects, the present invention is implemented through the following technical solution:

[0007] A method for preparing a dynamically borate bond-toughened polyvinyl alcohol film includes the following steps:

[0008] Polyvinyl alcohol and crosslinking agent are dissolved in organic solvents respectively, mixed evenly at room temperature, dried, annealed, and cooled to obtain the polyvinyl alcohol film toughened by dynamic borate ester bonds;

[0009] The crosslinking agent is phenyl diboronic acid, and its structure is shown in structural formulas I-III:

[0010]

[0011] Wherein, R1 = at least one of -H, -F, -OH, -CH3, -CH2OCH3; R2 = at least one of -H, -F, -OH, -CH3, -CH2OCH3; and R3 = at least one of -H and -CH3.

[0012] Preferably, the degree of alcoholysis of the polyvinyl alcohol is 88-99%, and the degree of polymerization is 1700-2000.

[0013] Preferably, the organic solvent is at least one selected from dimethyl sulfoxide, methanol, chloroform, glycerol, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0014] Preferably, the ratio of polyvinyl alcohol to organic solvent is 1-10 wt%, the dissolution temperature is 60-90°C, and the dissolution time is 1-3 h.

[0015] Preferably, the ratio of crosslinking agent to organic solvent is 0.1-1 wt%, the dissolution temperature is 20-40°C, and the dissolution time is 0.5-2 h.

[0016] Preferably, the mass ratio of the crosslinking agent to polyvinyl alcohol is 1-7 wt%; the mixing time of the crosslinking agent and polyvinyl alcohol is 2 h, and the mixing temperature is 15-35 °C.

[0017] Preferably, the drying step is performed at a temperature of 60–90°C for 10–24 hours.

[0018] Preferably, the annealing step is performed at a temperature of 90–120°C for 20 min–2 h.

[0019] The polyvinyl alcohol film toughened by dynamic borate ester bonds was prepared by the above method.

[0020] By adopting the above technical solution, the present invention has the following advantages and beneficial effects:

[0021] 1. This invention utilizes the dehydration condensation reaction of phenyl diboronic acid with the hydroxyl groups of PVA chains to form a dynamic and reversible cross-linking network, which weakens the hydrogen bonds between molecular chains and reduces the crystallization ability of PVA. During subsequent heat treatment, the dynamic borate ester bonds undergo associative bond exchange induced by heat to form a benzene ring π-π conjugated aggregate structure. This structure can act as a sacrificial bond to dissipate energy during stretching, thereby improving the toughness of the cross-linked polyvinyl alcohol film.

[0022] 2. The polyvinyl alcohol film prepared by this invention adopts dynamic covalent cross-linking, which can realize the recycling and remodeling of the film; at the same time, the biodegradability of the polyvinyl alcohol film will not cause environmental pollution.

[0023] 3. This invention introduces dynamic borate ester bonds to toughen conventional polymer materials, taking into account the advantages of traditional physical or chemical modification methods, and providing a new strategy for toughening polymer materials.

[0024] 4. The polyvinyl alcohol film based on dynamic borate ester crosslinking provided by the present invention has good mechanical properties and self-healing properties, and the preparation method is simple. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of a conjugated structure formed by stacked benzene rings.

[0026] Figure 2 Here is a picture of the ultraviolet fluorescence sample, in which... Figure 2 a is the UV fluorescence image of the sample that has not undergone annealing treatment; Figure 2 b is the UV fluorescence image of the sample annealed at 120℃ for 1 hour.

[0027] Figure 3 The images show the fluorescence absorption spectra of pure PVA films after heat treatment at different temperatures for 1 hour.

[0028] Figure 4 The fluorescence absorption spectra of the thin film after heat treatment at different temperatures are shown when the crosslinking agent content is 1%.

[0029] Figure 5 The diagram shows the self-healing performance of the film when the crosslinking agent content is 3%. Detailed Implementation

[0030] To enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be further described in detail below with reference to the preferred embodiments. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0031] This invention provides a dynamically borate bond-reinforced polyvinyl alcohol film and its preparation method. Preferably, the polyvinyl alcohol has a degree of polymerization of 1700 and a degree of alcoholysis of 88%.

[0032] Example 1

[0033] (1) Weigh 1.8g of polyvinyl alcohol solid and dissolve it in 58.2g of dimethyl sulfoxide. Stir at 80℃ for 2h to obtain a polyvinyl alcohol solution.

[0034] (2) Weigh 0.1g of terephthalic acid powder and dissolve it in 9.9g of dimethyl sulfoxide. Stir at room temperature for 2h to obtain terephthalic acid solution.

[0035] (3) Add the solution from step (2) to the solution from step (1) at a mass ratio of 1% for terephthalic acid / polyvinyl alcohol, and stir for 2 hours at room temperature; pour the well-mixed solution into a glass dish and dry it in an oven at 80°C for 13 hours to remove the solvent and obtain a polyvinyl alcohol film.

[0036] (4) The film obtained in step (3) is placed in a vacuum oven for heat treatment. The heating temperature is 120°C and the heating time is 20 min. After cooling to room temperature, the toughened polyvinyl alcohol film can be obtained.

[0037] Example 2

[0038] Steps (1) and (2) are the same as in Example 1;

[0039] (3) Add the solution from step (2) to the solution from step (1) at a mass ratio of 3% for terephthalic acid / polyvinyl alcohol, and stir for 2 hours at room temperature; pour the well-mixed solution into a glass dish and dry it in an oven at 80°C for 13 hours to remove the solvent and obtain a polyvinyl alcohol film.

[0040] (4) The film obtained in step (3) is placed in a vacuum oven for heat treatment. The heating temperature is 120°C and the heating time is 1 hour. After cooling to room temperature, the toughened polyvinyl alcohol film can be obtained.

[0041] Example 3

[0042] Steps (1) and (2) are the same as in Example 1;

[0043] (3) Add the solution from step (2) to the solution from step (1) at a mass ratio of 7% for terephthalic acid / polyvinyl alcohol, and stir for 2 hours at room temperature; pour the well-mixed solution into a glass dish and dry it in an oven at 80°C for 13 hours to remove the solvent and obtain a polyvinyl alcohol film.

[0044] (4) The film obtained in step (3) is placed in a vacuum oven for heat treatment. The heating temperature is 100°C and the heating time is 1 hour. After cooling to room temperature, the toughened polyvinyl alcohol film can be obtained.

[0045] Example 4

[0046] (1) Weigh 1.8g of polyvinyl alcohol solid and dissolve it in 58.2g of dimethyl sulfoxide. Stir at 80℃ for 2h to obtain a polyvinyl alcohol solution.

[0047] (2) Pour the solution into a glass dish and dry it in an oven at 80°C for 13 hours to remove the solvent and obtain an uncrosslinked pure polyvinyl alcohol film.

[0048] (3) The film from step (2) is placed in a vacuum oven for heat treatment at a temperature of 120°C for 1 hour. After cooling to room temperature, the heat-treated pure polyvinyl alcohol film can be obtained.

[0049] Comparative Example 1

[0050] (1) Weigh 1.8g of polyvinyl alcohol solid and dissolve it in 58.2g of dimethyl sulfoxide. Stir at 80℃ for 2h to obtain a polyvinyl alcohol solution.

[0051] (2) Weigh 0.1g of terephthalic acid powder and dissolve it in 9.9g of dimethyl sulfoxide. Stir at room temperature for 2h to obtain terephthalic acid solution.

[0052] (3) Add the solution from step (2) to the solution from step (1) at a mass ratio of 1% for terephthalic acid / polyvinyl alcohol, and stir for 2 hours at room temperature; pour the well-mixed solution into a glass dish and dry it in an oven at 80°C for 13 hours to remove the solvent and obtain a polyvinyl alcohol film.

[0053] Comparative Example 2

[0054] Steps (1) and (2) are the same as in Comparative Example 1;

[0055] (3) Add the solution from step (2) to the solution from step (1) at a mass ratio of 3% for terephthalic acid / polyvinyl alcohol, and stir for 2 hours at room temperature; pour the well-mixed solution into a glass dish and dry it in an oven at 80°C for 13 hours to remove the solvent and obtain a polyvinyl alcohol film.

[0056] Comparative Example 3

[0057] Steps (1) and (2) are the same as in Comparative Example 1;

[0058] (3) Add the solution from step (2) to the solution from step (1) at a mass ratio of 7% for terephthalic acid / polyvinyl alcohol, and stir for 2 hours at room temperature; pour the well-mixed solution into a glass dish and dry it in an oven at 80°C for 13 hours to remove the solvent and obtain a polyvinyl alcohol film.

[0059] Comparative Example 4

[0060] (1) Weigh 1.8g of polyvinyl alcohol solid and dissolve it in 58.2g of dimethyl sulfoxide. Stir at 80℃ for 2h to obtain a polyvinyl alcohol solution.

[0061] (2) Pour the solution into a glass dish and dry it in an oven at 80°C for 13 hours to remove the solvent and obtain an uncrosslinked pure polyvinyl alcohol film.

[0062] The polyvinyl alcohol films prepared in the examples and comparative examples were made into standard specimens, and their tensile strength and elongation at break were tested using a universal testing machine. The test data results are shown in Table 1.

[0063] Table 1 shows the mechanical property data of the examples and comparative examples. The tensile yield strength σ is in MPa, the elongation at break ε is in %, and the toughness τ is in MJ / m. 3 .

[0064] Table 1 Mechanical properties of the embodiments and comparative examples

[0065]

[0066] Figure 2 a shows the UV fluorescence images of the samples that have not undergone annealing treatment. From left to right, these are samples with crosslinking agent addition amounts of 0%, 1%, 3%, 5%, and 7%. Figure 2 b shows the UV fluorescence images of the samples annealed at 120℃ for 1 hour, with samples from left to right representing crosslinking agent additions of 0%, 1%, 3%, 5%, and 7%. As can be seen from the image, compared to the initial film, the film exhibits more pronounced fluorescence after annealing at 120℃ for 1 hour. The introduction of the crosslinking agent disrupts the structural regularity and crystallinity of pure PVA; however, the strong fluorescence emitted by the crosslinked film preliminarily confirms the formation of a benzene ring conjugated structure.

[0067] pass Figure 3 and Figure 4 The comparison shows that the pure PVA film only has a small peak at about 325 nm, which corresponds to the n-π of C=O. *The transition, combined with the hydrogen bonding within the structure, contributes to the fluorescence phenomenon even in pure PVA films. For the heat-treated crosslinked films, a distinct benzene ring conjugated absorption peak appears in the 250-300 nm range, and the intensity of the absorption peak increases significantly with increasing heat treatment temperature. This is because dynamic bond exchange and rearrangement occur during the annealing heat treatment process, gradually forming a stacked benzene ring conjugated structure, which enhances this effect. Therefore, the fluorescence absorption of the film in this wavelength range is significantly increased.

[0068] Figure 5 The self-healing performance curve of the film shows that even with a small amount of water, the cut film can still form new borate ester crosslinking structures at the interface. As the healing time increases, the yield strength of the film gradually increases, reaching 90 MPa after 24 hours, with a self-healing efficiency of approximately 84%, demonstrating excellent self-healing properties. The self-healing efficiency is defined as the ratio of the yield strength of the repaired film to the yield strength of the film before repair.

[0069] As shown in Table 1, pure polyvinyl alcohol (PVA) films exhibit typical characteristics of being hard and brittle, with high mechanical strength but extremely high brittleness and very low elongation at break. During heat treatment, the mobility of the internal molecular chains of PVA increases, and the system further crystallizes, thus significantly improving the mechanical strength of the pure PVA film, but also resulting in poor toughness. Introducing terephthalic acid into PVA consumes a large number of hydroxyl groups in the main chain and simultaneously forms a dynamic cross-linked network structure based on borate ester bonds. The reduction of hydrogen bonds and the formation of the cross-linked network reduce the crystallinity of the system, which is detrimental to improving mechanical strength; however, increasing the degree of cross-linking can improve mechanical strength to some extent. Therefore, with the increase of the cross-linking agent content, the strength of the PVA film shows a trend of first increasing and then decreasing under the influence of crystallinity and cross-linking degree. Furthermore, comparing Example 2 and Comparative Example 2, it is evident that the strength and elongation at break of the heat-treated film are significantly increased, with a significant toughening effect. This is because during the subsequent heat treatment process, the dynamic borate ester bonds undergo associative bond exchange, forming a π-π stacked conjugated structure (e.g., ...). Figure 1 As shown in the diagram, this structure can act as a sacrificial bond to dissipate energy during stretching, thereby improving the toughness of cross-linked polyvinyl alcohol (PVA) films. This invention provides a dynamically borate bond-toughened PVA film and its preparation method, which significantly improves the mechanical properties of PVA films. The preparation process is simple and easy to control, and by introducing dynamic borate bonds, it achieves toughening of conventional polymer materials, providing a new strategy for toughening traditional polymer materials.

[0070] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to specific implementations. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects to describe these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for preparing a dynamically borate bond-toughened polyvinyl alcohol film, characterized in that, Includes the following steps: Polyvinyl alcohol and crosslinking agent are dissolved in organic solvents respectively, mixed evenly at room temperature, dried, annealed, and cooled to obtain the polyvinyl alcohol film toughened by dynamic borate bond; The crosslinking agent is phenyl diboronic acid, and its structure is shown in structural formulas I-III: ； Wherein, R1 = at least one of -H, -F, -OH, -CH3, -CH2OCH3; R2 = at least one of -H, -F, -OH, -CH3, -CH2OCH3; R3 = at least one of -H, -CH3; The organic solvent is at least one selected from dimethyl sulfoxide, methanol, chloroform, glycerol, N,N-dimethylformamide, and N,N-dimethylacetamide. The mass ratio of the crosslinking agent to polyvinyl alcohol is 1-7 wt%. The annealing temperature is 90–120°C, and the time is 20 min–2 h.

2. The preparation method according to claim 1, characterized in that, The degree of alcoholysis of the polyvinyl alcohol is 88-99%, and the degree of polymerization is 1700-2000.

3. The preparation method according to claim 1, characterized in that, The ratio of polyvinyl alcohol to organic solvent is 1–10 wt%, the dissolution temperature is 60–90℃, and the dissolution time is 1–3 h.

4. The preparation method according to claim 1, characterized in that, The ratio of crosslinking agent to organic solvent is 0.1–1 wt%, the dissolution temperature is 20–40 °C, and the dissolution time is 0.5–2 h.

5. The preparation method according to claim 1, characterized in that, The crosslinking agent and polyvinyl alcohol were mixed for 2 hours at a temperature of 15-35°C.

6. The preparation method according to claim 1, characterized in that, The drying step is performed at a temperature of 60–90°C for 10–24 hours.

7. A polyvinyl alcohol film with dynamic borate bond toughening prepared according to any one of claims 1 to 6.

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