Bio-based modifier containing Schiff base as well as preparation method and application of bio-based modifier

Abstract

The invention relates to a bio-based modifier containing Schiff base as well as a preparation method and application thereof, and relates to the field of material science and engineering. The molecular structure of the modifier contains a group derived from a bio-base, so that the modifier is endowed with good biocompatibility and renewability, meanwhile, a Schiff base structure (-C = N-) is introduced into the molecular structure of the modifier, and an imine bond in the Schiff base structure has high reaction activity, so that the modifier can interact with a polylactic acid molecular chain, and the modification effect is improved. Therefore, the performance of the polylactic acid is improved.

Description

Technical Field

[0001] The present invention relates to the field of materials science and engineering, and in particular to a bio-based Schiff base-containing modifier, a preparation method and application thereof. Background Art

[0002] Polylactic acid (PLA) is a biodegradable polyester material made from renewable resources (such as corn, cassava, and other starch raw materials). Due to its excellent biodegradability, microorganisms in the natural environment can completely degrade it into carbon dioxide and water, leaving no pollution to the environment. It is considered an ideal alternative to traditional plastics to address environmental pollution issues. In the packaging field, PLA film can be used for food packaging, shopping bags, etc.; in the biomedical field, it can be used to manufacture sutures and tissue engineering scaffolds; and in the textile field, it can be made into fibers for clothing production. However, conventional PLA materials have the following performance drawbacks: 1. Poor Processing Flow: The relatively rigid PLA molecular chain and high melt viscosity result in poor flowability during processing, making molding difficult and requiring high processing temperatures and pressures. This not only increases processing costs but can also cause thermal degradation of the PLA, impacting product quality. 2. Difficulty Balancing Rigidity and Toughness: Pure PLA materials typically exhibit high rigidity but insufficient toughness, making them susceptible to brittle fracture when subjected to external forces. This limits their use in applications requiring high toughness. 3. Inadequate Barrier Properties: PLA films exhibit poor barrier properties against small molecules such as oxygen and water vapor. This makes them ineffective in extending the shelf life of products requiring high barrier properties (such as high-fat foods and easily oxidizable products). 4. Lack of Antibacterial Properties: PLA inherently lacks antibacterial properties, making it susceptible to microbial attack in some applications (such as food packaging and medical supplies), impacting product safety and longevity. Several technical solutions exist for improving the properties of PLA: 1. Adding plasticizers to improve processing fluidity: By adding small molecule plasticizers, such as citrates and phthalates, interactions between PLA molecular chains can be reduced, improving processing fluidity. For example, the addition of tributyl citrate (TBC) as a plasticizer can improve PLA processing properties to a certain extent. 2. Blending to improve rigidity and toughness: Blending PLA with other polymers, such as polybutylene adipate / terephthalate (PBAT) and polybutylene succinate (PBS), can improve PLA toughness while maintaining a certain level of rigidity. 3. Surface coating to improve barrier properties: Applying barrier coatings, such as silica or polyvinyl alcohol, to the surface of PLA films. Silicon dioxide coatings deposited on PLA films using chemical vapor deposition significantly improve the film's barrier properties. 4. Adding antimicrobial agents to impart antimicrobial properties: Adding antimicrobial agents, such as silver ion antimicrobial agents and quaternary ammonium salt antimicrobial agents, can impart antimicrobial properties to polylactic acid materials. Polylactic acid composite materials with added silver ion antimicrobial agents have good antimicrobial effects.

[0003] However, the above-mentioned technical solutions for improving the performance of polylactic acid also have the following disadvantages: 1. Small molecule plasticizers: However, small molecule plasticizers have problems such as easy migration and volatilization. Over time, the performance of the material will decrease, and excessive addition of plasticizers will also reduce the mechanical properties of the polylactic acid material. 2. Blending modification: The compatibility problem between different polymers in the blending system is often difficult to solve, and phase separation is prone to occur, affecting the stability of the material performance. 3. Surface coating: The coating process is complicated, and the bonding force between the coating and the polylactic acid matrix is ​​limited. The coating is easy to fall off during use, and the cost is high, which limits its large-scale application. 4. Addition of antibacterial agents: It is difficult to find a balance between cost and performance. Summary of the Invention

[0004] In response to the above technical problems, the present invention provides a bio-based Schiff base-containing modifier. The molecular structure of the modifier contains bio-based groups, which gives the modifier good biocompatibility and renewability. At the same time, a Schiff base structure (-C=N-) is introduced into the molecular structure of the modifier. The imine bond in the Schiff base structure has high reactivity, enabling the modifier to interact with the polylactic acid molecular chain, thereby improving the performance of polylactic acid.

[0005] The present invention provides a bio-based Schiff base-containing modifier, the structural formula of which is shown below:

[0006] During research into several existing technologies for improving the properties of polylactic acid (PLA), the inventors discovered the following reasons for the aforementioned shortcomings: 1. Small molecule plasticizers: Phthalate plasticizers, for example, can gradually migrate to the surface of plastic products during use, causing not only stickiness and a negative impact on user experience, but also potentially contaminating any materials they come into contact with. In food packaging applications, this plasticizer migration can potentially migrate into food, posing a potential health hazard. Furthermore, as the plasticizer migrates and evaporates, the processing fluidity of the PLA material gradually reverts to its original, poor state, preventing it from maintaining good processing performance. 2. Blending modification: Taking PLA / PBAT blends as an example, due to the significant differences in molecular structure and polarity between PLA and PBAT, phase separation is prone to occur during the blending process. Phase separation leads to an uneven internal structure in the material. When subjected to external forces, the phase interfaces can easily become stress concentration points, reducing the material's mechanical properties, particularly tensile strength and impact strength. Phase separation can also affect other material properties, such as barrier and optical properties. 3. Surface Coating: The complex and costly surface coating process limits its large-scale application. Chemical vapor deposition (CVD) requires specialized equipment and complex process conditions, placing high demands on operator skills. Furthermore, the bonding between the coating and the PLA substrate relies primarily on physical adsorption or weak chemical bonds. During use, the coating can be easily detached from the substrate due to environmental factors such as temperature and humidity fluctuations and mechanical friction. Once the coating detaches, the barrier properties of the PLA material are significantly reduced, making it unsuitable for practical applications. 4. Antimicrobial Addition: Silver ion antimicrobial agents are expensive, increasing the production cost of PLA materials and hindering their large-scale application. Furthermore, the safety of silver ion release is a concern, as excessive silver ion release may have adverse effects on the environment and human health. While relatively inexpensive, quaternary ammonium antimicrobial agents have a narrow antimicrobial spectrum, inhibiting only some common bacteria and lacking effectiveness against some microorganisms, such as molds and yeasts. In practical applications, especially in areas such as medical care and food packaging that have high requirements for microbial control, it is impossible to provide comprehensive and effective antibacterial protection.

[0007] Based on this, the inventors proposed the above-mentioned bio-based Schiff base-containing modifier, which has a unique chemical structure. The core design of the molecular structure is the organic combination of bio-based groups and Schiff base structures. The bio-based group gives the modifier the advantages of renewability and good biocompatibility, while the Schiff base structure provides the modifier with active sites for interacting with the polylactic acid molecular chain, as well as the key function of improving material performance. Compared with the above-mentioned technologies for improving the performance of polylactic acid, this modifier has the following advantages: 1. Compared with small molecule plasticizers: Existing small molecule plasticizers simply improve processing fluidity by reducing the interaction between polylactic acid molecular chains through physical mixing, and there are problems of migration and volatilization. The multifunctional modifier of the present invention reacts chemically with the polylactic acid molecular chain through the active groups in the chemical structure to form stable chemical bonds or graft copolymers, which not only improves processing fluidity, but also improves the performance of polylactic acid in many aspects, and there is no problem of migration and volatilization. 2. Compared with blending modification: Traditional blending modification mainly involves mechanical mixing of different polymers, which faces compatibility problems. The modifier of this invention is a compound with a specific structure. It interacts with the PLA molecular chains within the PLA matrix to form a relatively stable microstructure, eliminating compatibility issues such as phase separation and more effectively balancing the rigidity and toughness of PLA. 3. Compared with surface coatings: Surface coating processes are complex and the coating is prone to detachment. By melt-blending the modifier with PLA, the modifier and the PLA matrix are thoroughly mixed, extending the lifespan of the modified product. 4. Compared with the addition of antimicrobial agents: While traditional silver ion antimicrobial agents offer significant antibacterial effects, they are expensive, costing 5,000-10,000 yuan / kg, and carry the risk of silver ion dissolution. In vitro cytotoxicity studies have shown that when silver ion concentrations exceed 10 ppm, the survival rate of human fibroblasts drops below 70%. Quaternary ammonium antimicrobial agents have a limited antimicrobial spectrum, with minimum inhibitory concentrations (MICs) as high as 500 ppm against fungi such as Aspergillus niger. The Schiff base structure of the present invention destroys the integrity of the cell membrane by undergoing nucleophilic addition reactions with the sulfhydryl and amino groups on the microbial cell membrane; the natural antibacterial components in the bio-based group (such as the phenolic hydroxyl structure) act synergistically, with an inhibition rate of more than 90% against Escherichia coli and Staphylococcus aureus, and the cost is only 1 / 10 of that of silver ion antibacterial agents. Skin irritation experiments have proven that it has no cytotoxicity.

[0008] The present invention also provides a method for preparing the modifier, comprising the following steps: Synthesis of bio-based intermediates: Under a protective atmosphere, a reaction substrate is mixed with an organic solvent, reacted, a precipitate is collected, washed, filtered, and dried to obtain a bio-based intermediate; the reaction substrate includes a bio-based raw material and 1,4-phenylenediamine; Preparation of a bio-based Schiff base-containing modifier: dissolving a bio-based intermediate and an acid-binding agent in an organic solvent to obtain a bio-based intermediate solution; dissolving trimellitic anhydride chloride in an organic solvent to obtain a trimellitic anhydride chloride solution, adding the trimellitic anhydride chloride solution dropwise to the bio-based intermediate solution to react, precipitating, filtering, washing, and drying to obtain a bio-based Schiff base-containing modifier.

[0009] The preparation method for this modifier utilizes specific reaction steps and conditions, starting from bio-based raw materials and synthesizing them through multiple steps. The introduction of the Schiff base structure and the final polymerization step are particularly crucial for achieving the specific molecular structure.

[0010] In one embodiment, the organic solvent includes at least one of N'N-dimethylformamide, pyridine, tetrahydrofuran, dichloromethane, chloroform, and ethanol; When the organic solvent comprises at least one of N'N-dimethylformamide, tetrahydrofuran, dichloromethane, chloroform, and ethanol, in the step of preparing the bio-based Schiff base-containing modifier, the bio-based intermediate and the acid-binding agent are dissolved in the organic solvent to obtain a bio-based intermediate solution; When the organic solvent includes pyridine, in the step of preparing the bio-based Schiff base-containing modifier, the bio-based intermediate is dissolved in the organic solvent to obtain a bio-based intermediate solution.

[0011] In one embodiment, the bio-based raw material includes vanillin, and the acid-binding agent includes at least one of pyridine and triethylamine.

[0012] In one embodiment, in the step of synthesizing the bio-based intermediate, the mass ratio of the organic solvent to the reaction substrate is (9-12):1, and the molar ratio of the bio-based raw material to the 1,4-phenylenediamine is (1.8-2.2):1; In the step of synthesizing the bio-based intermediate, the reaction conditions include: a stirring speed of 200-250 r / min, a reaction temperature of 60-80° C., and a reaction time of 5-8 h.

[0013] In one embodiment, the molar ratio of the acid binding agent to the 1,4-phenylenediamine is (2.5-4):1, and the molar ratio of the bio-based intermediate to the chlorinated trimellitic anhydride is 1:(1.5-2.5); In the step of preparing the bio-based Schiff base-containing modifier, the reaction conditions include: the reaction temperature is room temperature, and the reaction time is 12-18 hours.

[0014] Precise control of the above reaction conditions is the key to ensuring good performance of the modifier.

[0015] The present invention also provides a modified polylactic acid. The raw materials for preparing the modified polylactic acid include polylactic acid and the bio-based Schiff base-containing modifier.

[0016] In one embodiment, the bio-based Schiff base-containing modifier accounts for 1%-3% of the raw material by mass.

[0017] The present invention also provides a method for preparing the modified polylactic acid, comprising the following steps: blending the bio-based Schiff base-containing modifier with polylactic acid to obtain a mixture, and melt-blending the mixture to obtain the modified polylactic acid.

[0018] The above preparation method applies the modifier to the polylactic acid system through a blending modification method. Optimizing the mixing ratio, mixing method, melt blending conditions, and molding processing conditions allows the modifier to fully function, thereby improving the properties of polylactic acid.

[0019] The present invention also provides a polylactic acid film, which is prepared by using the modified polylactic acid.

[0020] Compared with the prior art, the present invention has the following beneficial effects: The present invention discloses a bio-based Schiff base-containing modifier, a preparation method thereof, and an application thereof. The modifier solves the problems existing in the polylactic acid system in terms of processing flowability, rigidity and toughness balance, barrier properties, and antibacterial properties. Specifically, The modifier's molecular structure contains bio-based groups, endowing it with excellent biocompatibility and renewability. Furthermore, the modifier incorporates a Schiff base structure (-C=N-). The highly reactive imine bond within the Schiff base structure enables the modifier to interact with the polylactic acid (PLA) molecular chain, thereby improving the properties of the PLA. The addition of this modifier significantly improves the processing flowability of the PLA system, reduces processing temperature and pressure, and mitigates the risk of thermal degradation. It also improves the rigidity and toughness of the PLA film, resulting in both high strength and good impact resistance. It also enhances the PLA film's barrier properties to small molecules such as oxygen and water vapor, effectively extending the shelf life of the packaged product. Furthermore, it imparts excellent antibacterial properties to the PLA material, inhibiting the growth and reproduction of common microorganisms and improving product safety and service life. Furthermore, the modifier is made from bio-based raw materials, aligning with the principles of green environmental protection and sustainable development, and provides an effective solution for the high performance and widespread application of PLA materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the infrared spectrum of the bio-based modifier VP@MAH containing a Schiff base structure; Figure 2 This is the 1H NMR spectrum of the bio-based modifier VP@MAH containing a Schiff base structure; Figure 3 The thermogravimetric analysis (TGA) curve of the bio-based Schiff base-containing modifier VP@MAH at 30°C~600°C and a heating rate of 10°C / min; Figure 4 The stress-strain curves of the tensile test of polylactic acid and polylactic acid film with the addition of bio-based Schiff base modifier VP@MAH; Figure 5 The tensile strength results of polylactic acid and polylactic acid films with bio-based Schiff base modifier VP@MAH were obtained by tensile testing. Figure 6 The elongation at break results of tensile tests on polylactic acid (PLA) and polylactic acid films with bio-based Schiff base modifier VP@MAH added. Figure 7 The oxygen transmission rate of the film is obtained by oxygen transmission test of polylactic acid and polylactic acid film added with bio-based Schiff base structure modifier VP@MAH; Figure 8 The water vapor permeability of polylactic acid and polylactic acid films with bio-based Schiff base modifier VP@MAH was obtained by water vapor permeability test. Figure 9 The colony plate after the plate colony test of polylactic acid and polylactic acid film with the addition of bio-based Schiff base structure modifier VP@MAH; Figure 10 The antibacterial ratio results of the film added with the bio-based Schiff base structure modifier VP@MAH. DETAILED DESCRIPTION

[0022] To facilitate understanding of the present invention, the present invention will be described more fully below with reference to the accompanying drawings. Preferred embodiments of the present invention are shown in the accompanying drawings. However, the present invention may be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and comprehensive understanding of the present disclosure.

[0023] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one skilled in the art to which this invention pertains. The terms used in this specification of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0024] source: Unless otherwise specified, the reagents, materials, and equipment used in this example are all commercially available; and the experimental methods, unless otherwise specified, are all conventional experimental methods in the art.

[0025] Example 1 A bio-based Schiff base-containing modifier and a preparation method thereof.

[0026] (1) Chemical structure design of modifier.

[0027] The general chemical structure formula of the bio-based Schiff base structure-containing modifier of the present invention is as follows:

[0028] The modifier contains bio-derived groups within its molecules, which impart excellent biocompatibility and renewability. Furthermore, the introduction of a Schiff base structure (-C=N-) is a key design feature of the invention. The imine bond within the Schiff base structure is highly reactive and can interact with the polylactic acid (PLA) molecular chain, thereby improving the properties of the PLA. (2) Preparation method of modifier.

[0029] 1. Raw material preparation: Select bio-based raw materials vanillin, 1,4-phenylenediamine, chlorotrimellitic anhydride, organic solvent (the organic solvent includes at least one of N'N-dimethylformamide, pyridine, tetrahydrofuran, dichloromethane, chloroform, and ethanol), and acid binding agent (pyridine and / or triethylamine. When pyridine is used as the organic solvent, no additional acid binding agent is required, as pyridine acts as an acid binding agent) for the synthesis of a bio-based multifunctional modifier containing a Schiff base structure. 2. Synthesis of Bio-based Intermediates: Under nitrogen protection, vanillin, 1,4-phenylenediamine (the molar ratio of vanillin to 1,4-phenylenediamine is 2:1), and ethanol (the mass of the organic solvent is 9-12 times the total mass of vanillin and 1,4-phenylenediamine) are added to a three-necked flask. The reaction is carried out under magnetic stirring at a stirring rate of 200-250 r / min and at 60-80°C for 5-8 hours. After the reaction is completed, the precipitate is filtered and washed with ethanol and filtered. Finally, the product is dried in a vacuum oven at 80°C for 24 hours. The resulting orange-yellow powder is the bio-based Schiff base ligand (VP, i.e., the bio-based intermediate). The reaction process is shown in Reaction Equation 1:

[0030] 3. Final preparation of the bio-based Schiff base modifier: Dissolve the bio-based Schiff base ligand in an organic solvent and add an acid binder (the molar number of the acid binder is 2.5-4 times that of 1,4-phenylenediamine). Then, under magnetic stirring, dissolve trimellitic anhydride chloride in the organic solvent in an ice-water bath. Slowly add the bio-based Schiff base ligand dropwise (the molar ratio of the bio-based Schiff base ligand VP to trimellitic anhydride chloride is 1:2). After the addition is complete, react at room temperature for 12-18 hours. After the reaction is complete, the precipitate is filtered, washed, and dried to obtain the bio-based Schiff base modifier VP@MAH. The reaction process is shown in Reaction Equation 2: (3) Modification method of polylactic acid system.

[0031] 1. Blending and Modification Process: The prepared bio-based Schiff base modifier and PLA particles are thoroughly mixed in a high-speed mixer at a specific ratio to ensure that the modifier is evenly dispersed on the surface of the PLA particles. The mixing time is generally 3 minutes at a mixing speed of 80 rpm. 2. Melt blending process: Add the uniformly mixed mixture of polylactic acid and modifier into a twin-screw extruder for melt blending. The temperature settings of each section of the twin-screw extruder are shown in the table below.

[0032] Table 1 Temperature settings for each section of the twin-screw extruder

[0033] The screw speed is controlled at 60 rpm, and the material residence time in the extruder is approximately 2 minutes. Through melt blending, the modifier molecules physically entangle and chemically react with the polylactic acid chains at high temperatures, thereby improving the properties of the polylactic acid system. The extrudate is then water-cooled, stretched, and pelletized to produce modified polylactic acid pellets.

[0034] 3. Molding process: The modified polylactic acid particles are made into the required polylactic acid film products by extrusion method. The modified polylactic acid particles are heated and melted in a single-screw extruder and extruded into a film through a die head. The temperature of the single-screw extruder is set to 130°C to 160°C, and the temperature settings of each section of the twin-screw extruder are shown in the following table. The traction speed is 4 m / min and the temperature of the cooling roller is 30°C, thereby obtaining a polylactic acid film with good performance. Table 2 Temperature settings for each section of the twin-screw extruder

[0035] (4) Mechanism of action of modifiers.

[0036] 1. Improved processing fluidity: The bio-based groups in the bio-based Schiff base-containing modifier molecules have low surface energy, which can act as a lubricant in the polylactic acid melt, reducing the friction between the polylactic acid molecular chains, thereby improving the processing fluidity of the polylactic acid system. At the same time, the active groups in the modifier molecules may chemically react with the hydroxyl or carboxyl groups at the ends of the polylactic acid molecular chains to form graft copolymers, increasing the flexibility of the molecular chains and further improving processing performance. 2. Balancing Rigidity and Toughness: The presence of the Schiff base structure enables the modifier molecules to form strong interactions with the PLA molecular chains, such as hydrogen bonding and π-π stacking. These interactions act as physical crosslinks within the PLA matrix, enhancing the material's rigidity. Furthermore, the flexible, long-chain structure of the bio-based groups absorbs energy through deformation when subjected to external impact, improving the material's toughness and ultimately achieving a balance between rigidity and toughness in the PLA material. 3. Enhanced barrier properties: Modifier molecules form a physical barrier within the polylactic acid matrix, hindering the diffusion path of small molecules such as oxygen and water vapor. On the one hand, the long chain structure and irregular arrangement of the bio-based groups increase the diffusion resistance of small molecules within the material; on the other hand, the interaction between the Schiff base structure and the polylactic acid molecular chain makes the material's microstructure more dense, further improving barrier properties. 4. Imparting antibacterial properties: The Schiff base structure has certain biological activity and can interact with proteins or enzymes on the surface of microbial cell membranes, destroying the integrity of the cell membranes and thus inhibiting the growth and reproduction of microorganisms. At the same time, some natural antibacterial components that may be contained in the bio-based group also work synergistically to give the polylactic acid material good antibacterial properties.

[0037] Example 2 Based on the preparation method described in Example 1, a bio-based Schiff base-containing modifier was prepared.

[0038] (1) Under nitrogen, 0.02 mol of vanillin, 0.01 mol of 1,4-phenylenediamine, and 50 ml of ethanol were added to a 150 mL three-necked flask (in this example, the molar ratio of vanillin to 1,4-phenylenediamine was 2:1). The reaction was carried out under magnetic stirring at a stirring rate of 200 r / min and a temperature of 60°C for 5 h. After the reaction, the precipitate was filtered, washed with ethanol, filtered, and dried in a vacuum oven at 80°C for 24 h. The resulting orange powder was the bio-based Schiff base ligand VP with a yield of 96.3%. (2) Dissolve 0.01 mol VP and 0.035 mol triethylamine (acid-binding agent) in 30 g N'N-dimethylformamide (in this example, the molar ratio of the acid-binding agent to 1,4-phenylenediamine is 3.5:1). Then, under magnetic stirring, dissolve 0.02 mol of trimellitic anhydride chloride in 30 g of N'N-dimethylformamide in an ice-water bath at a stirring rate of 200 r / min. Slowly add the bio-based Schiff base ligand solution dropwise (in this example, the molar ratio of the bio-based Schiff base ligand VP to trimellitic anhydride chloride is 1:2). The addition is completed within 60 min. After the addition is complete, the mixture is reacted at room temperature for 12 h. After the reaction is complete, the precipitate is placed in ice water, filtered, washed, and dried to obtain a bio-based Schiff base multifunctional modifier. The resulting yellow powder product is VP@MAH with a yield of 78.1%. Take a small amount of product for FT-IR test, the results are as follows Figure 1 , the NH stretching vibration peak (3446 cm - ¹) and the C=O stretching vibration peak of the aldehyde group (-CHO) of vanillin (1700 cm - ¹) completely disappeared after the reaction, indicating that the condensation reaction between the amino group and the aldehyde group was complete. - The characteristic absorption peak attributed to the Schiff base C=N bond appears at ¹, confirming the successful formation of the imine bond. During the anhydride chloride substitution reaction stage, the hydroxyl (-OH) broad peak (3423 cm - ¹) The intensity decreases significantly and the peak at 1739 cm - ¹ and 1192 cm - Stretching vibration peaks of the ester group C=O and COC were detected at ¹, indicating an esterification reaction between the hydroxyl group and trimellitic anhydride chloride. These results confirm that the Schiff base (C=N) and ester bond (COC) in the VP@MAH molecule were introduced according to the designed pathway, highly consistent with the target structure.

[0039] At the same time, the obtained VP@MAH was subjected to 1H NMR test, and the solvent was deuterated chloroform. The results are as follows Figure 2 As shown in the 1H NMR spectrum of VP@MAH, the singlet (9) at 3.91 ppm is attributed to two chemically equivalent methoxy groups (-OCH3), indicating the presence of symmetrically distributed methoxy groups in the molecule. The characteristic peak of the proton (7) in the imine group appears at 8.31 ppm, indicating the presence of an aromatic ring. Protons (4, 5, 6) appear at 7.29-7.59 ppm, while 8.14-8.79 ppm represent the chemical shift of protons (1, 2, 3). The doublet (8) at 7.67 ppm further supports symmetrical substitution. These results confirm the successful synthesis of VP@MAH.

[0040] The processing thermal stability of the obtained VP@MAH was characterized by TGA. Figure 3 , the curve shows the 5% mass loss temperature of VP@MAH ( T 5% ) is 199.2℃, the maximum degradation rate temperature ( T max ) is 251.7℃, and the residual weight at 600℃ is 5.61%. The surface MBC has good processing thermal stability and carbon forming properties.

[0041] Example 3 Based on the preparation method described in Example 1, a bio-based Schiff base-containing modifier was prepared.

[0042] (1) Under nitrogen, 0.02 mol of vanillin, 0.01 mol of 1,4-phenylenediamine, and 50 ml of ethanol were added to a 150 mL three-necked flask (in this example, the molar ratio of vanillin to 1,4-phenylenediamine was 2:1). The reaction was carried out under magnetic stirring at a stirring rate of 250 r / min and at 60°C for 6 h. After the reaction, the mixture was filtered, and the resulting precipitate was washed with ethanol and filtered. Finally, the resulting product was dried in a vacuum oven at 80°C for 24 h. The resulting orange-yellow powder was the bio-based Schiff base ligand VP, with a yield of 94.2%. (2) Dissolve 0.01 mol VP and 0.025 mol triethylamine (acid-binding agent) in 35 g of dichloromethane (in this example, the molar ratio of acid-binding agent to 1,4-phenylenediamine is 2.5:1). Then, under magnetic stirring at 220 r / min, dissolve 0.02 mol of trimellitic anhydride chloride in 35 g of dichloromethane in an ice-water bath. Add the bio-based Schiff base ligand solution dropwise slowly over 40 min (in this example, the molar ratio of bio-based Schiff base ligand VP to trimellitic anhydride chloride is 1:2). After the addition is complete, react at room temperature for 12 h. After completion of the reaction, place the mixture in ice water to precipitate, filter, wash, and dry to obtain a bio-based Schiff base multifunctional modifier. The resulting yellow powder product, VP@MAH, is obtained with a yield of 73.9%. The structure of the resulting VP@MAH is consistent with that in Example 1.

[0043] Example 4 Based on the preparation method described in Example 1, a bio-based Schiff base-containing modifier was prepared.

[0044] (1) Under nitrogen, 0.02 mol of vanillin, 0.01 mol of 1,4-phenylenediamine, and 50 ml of ethanol were added to a 150-mL three-necked flask (in this example, the molar ratio of vanillin to 1,4-phenylenediamine was 2:1). The reaction was carried out under magnetic stirring at a stirring rate of 240 r / min and at 80°C for 8 h. After the reaction was completed, the precipitate was filtered, washed with ethanol, filtered, and dried in a vacuum oven at 80°C for 24 h. The resulting orange-yellow powder was the bio-based Schiff base ligand VP with a yield of 86.8%.

[0045] (2) 0.01 mol of VP was dissolved in 20 g of dichloromethane and 40 g of pyridine (pyridine served as an organic solvent and also acted as an acid binder). Then, 0.02 mol of trimellitic anhydride chloride was dissolved in 35 g of dichloromethane in an ice-water bath under magnetic stirring at 250 r / min. The bio-based Schiff base ligand solution (in this example, the molar ratio of the bio-based Schiff base ligand VP to trimellitic anhydride chloride was 1:2) was slowly added dropwise over 60 min. After the addition was complete, the mixture was allowed to react at room temperature for 18 h. After the reaction was complete, the precipitate was placed in ice water, filtered, washed, and dried to obtain the bio-based Schiff base multifunctional modifier. The resulting yellow powder product, VP@MAH, was obtained with a yield of 76.7%. The structure of the resulting VP@MAH was consistent with that in Example 1.

[0046] Example 5 Based on the preparation method described in Example 1, a bio-based Schiff base-containing modifier was prepared.

[0047] (1) Under nitrogen, 0.02 mol of vanillin, 0.01 mol of 1,4-phenylenediamine, and 50 ml of ethanol were added to a 150 mL three-necked flask (in this example, the molar ratio of vanillin to 1,4-phenylenediamine was 2:1). The reaction was carried out under magnetic stirring at a stirring rate of 260 r / min and at 60°C for 8 h. After the reaction, the precipitate was filtered, washed with ethanol, filtered, and dried in a vacuum oven at 80°C for 24 h. The resulting orange-yellow powder was the bio-based Schiff base ligand VP with a yield of 85.1%. (2) Dissolve 0.01 mol VP and 0.03 mol triethylamine (acid-binding agent) in 40 g tetrahydrofuran (in this example, the molar ratio of acid-binding agent to 1,4-phenylenediamine is 3:1). Then, under magnetic stirring at 250 rpm, dissolve 0.02 mol of trimellitic anhydride chloride in 40 g tetrahydrofuran in an ice-water bath. Add the bio-based Schiff base ligand solution dropwise slowly over 30 min (in this example, the molar ratio of bio-based Schiff base ligand VP to trimellitic anhydride chloride is 1:2). After the addition is complete, react at room temperature for 16 h. After completion of the reaction, place the mixture in ice water to precipitate, filter, wash, and dry to obtain a bio-based Schiff base multifunctional modifier. The resulting yellow powder product, VP@MAH, is obtained with a yield of 76.5%. The structure of the resulting VP@MAH is consistent with that in Example 1.

[0048] Example 6 Based on the preparation method described in Example 1, a bio-based Schiff base-containing modifier was prepared.

[0049] (1) Under nitrogen, 0.02 mol of vanillin, 0.01 mol of 1,4-phenylenediamine, and 50 ml of ethanol were added to a 150 mL three-necked flask (in this example, the molar ratio of vanillin to 1,4-phenylenediamine was 2:1). The reaction was carried out under magnetic stirring at a stirring rate of 250 r / min and at 70°C for 7 h. After the reaction, the precipitate was filtered, washed with ethanol, filtered, and dried in a vacuum oven at 80°C for 24 h. The resulting orange powder was the bio-based Schiff base ligand VP with a yield of 88.7%. (2) Dissolve 0.01 mol VP and 0.035 mol pyridine (acid-binding agent) in 40 g of chloroform (in this example, the molar ratio of acid-binding agent to 1,4-phenylenediamine is 3.5:1). Then, under magnetic stirring at 250 r / min, dissolve 0.02 mol of trimellitic anhydride chloride in 40 g of chloroform in an ice-water bath. Add the bio-based Schiff base ligand solution dropwise slowly over 60 min (in this example, the molar ratio of bio-based Schiff base ligand VP to trimellitic anhydride chloride is 1:2). After the addition is complete, react at room temperature for 14 h. After completion of the reaction, place the mixture in ice water to precipitate, filter, wash, and dry to obtain a bio-based Schiff base multifunctional modifier. The resulting yellow powder product, VP@MAH, is obtained with a yield of 72.1%. The structure of the resulting VP@MAH is consistent with that in Example 1.

[0050] Example 7 Based on the preparation method described in Example 1, a bio-based Schiff base-containing modifier was prepared.

[0051] (1) Under nitrogen, 0.02 mol of vanillin, 0.01 mol of 1,4-phenylenediamine, and 50 ml of ethanol were added to a 150 mL three-necked flask (in this example, the molar ratio of vanillin to 1,4-phenylenediamine was 2:1). The reaction was carried out under magnetic stirring at a stirring rate of 220 r / min and at 70°C for 8 h. After the reaction, the precipitate was filtered, washed with ethanol, filtered, and dried in a vacuum oven at 80°C for 24 h. The resulting orange powder was the bio-based Schiff base ligand VP with a yield of 92.6%. (2) Dissolve 0.01 mol of VP in 50 g of pyridine (pyridine serves as an organic solvent and also acts as an acid binder). Then, under magnetic stirring, dissolve 0.02 mol of trimellitic anhydride chloride in 50 g of pyridine in an ice-water bath at 230 r / min. Slowly add the bio-based Schiff base ligand solution dropwise (in this example, the molar ratio of the bio-based Schiff base ligand VP to trimellitic anhydride chloride is 1:2) over 30 min. After the addition is complete, react at room temperature for 14 h. After completion of the reaction, place the mixture in ice water to precipitate, filter, wash, and dry to obtain the bio-based Schiff base multifunctional modifier. The resulting yellow powder product, VP@MAH, is obtained with a yield of 76.4%. The structure of the resulting VP@MAH is consistent with that in Example 1.

[0052] Example 8 (1) The bio-based Schiff base-containing modifier prepared in the above embodiment was used to modify polylactic acid (a product of Anhui Fengyuan Biomaterials Co., Ltd., brand PLA FY802). By changing the mass percentage ratio of the raw materials (as shown in Table 3), 1 to 3 application examples were obtained. The specific preparation process was as follows: after the polylactic acid pellets were dried in a blast oven for 6 to 12 hours, polylactic acid, bio-based Schiff base-containing modifier VP@MAH and antioxidant (Suzhou Kaibaoli New Materials Co., Ltd., tetrakis(2,4-di-tert-butyl)-4,4'-biphenyl diphosphite (PEPQ)) were weighed according to the ratio, mixed evenly, and added to a twin-screw extruder (model LTE26 / ). 40, LabTech, Germany) were first blended and extruded to obtain pellets, and then a single-screw extruder (model LTE20-30, LabTech, Germany) was used to extrude the film; the temperatures of the screw barrel of the twin-screw extruder were as follows: first zone 130°C, second zone 140°C, third zone 150°C, fourth zone 160°C, fifth zone 160°C, sixth zone 160°C, seventh zone 160°C, eighth zone 160°C, ninth zone 160°C, the temperature of the die head was 160°C, and the main engine speed was 280 r / min. The film extrusion conditions are as follows: the obtained pellets are dried in a blast oven for 6 to 12 hours, the temperatures of each zone of the twin-screw extruder barrel are: first zone 130°C, second zone 150°C, third zone 155°C, fourth zone 160°C, fifth zone 160°C, sixth zone 160°C, seventh zone 160°C, eighth zone 160°C, and ninth zone 160°C, the main engine speed is 80 r / min, and the pressure roller speed is 6 r / min.

[0053] The bio-based Schiff base-containing modifier of the present invention contains anhydride groups at the molecular ends, resulting in a low surface energy. When added to a polylactic acid system, this multifunctional modifier tends to distribute at the interface of the polylactic acid matrix, combining with the matrix to form a cross-linked network. Furthermore, its low molecular weight makes it easier to penetrate between polymer chains, promoting molecular chain disentanglement and more effectively improving the processing rheology of the polylactic acid system. Simultaneously, because the modifier is uniformly dispersed in the polymer matrix, it ensures the effective transmission of external stress, thereby improving the rigidity and toughness of the resulting material. Furthermore, the bio-based Schiff base-containing modifier can enhance the equilibrium torque of the modified polylactic acid.

[0054] (2) Performance verification.

[0055] Comparative Example 1 used pure polylactic acid without the addition of the bio-based Schiff base-containing multifunctional modifier VP@MAH. Comparative Example 2 used polylactic acid modified with Joncryl® ADR 4468 (Shanghai Liangsu Environmental Protection Technology Co., Ltd.), a commonly used polylactic acid chain extender. The weight percentages of the raw materials are shown in Table 3.

[0056] 1. Detection of equilibrium torque: After simple mixing of the raw materials according to the formula in Table 3, use the RTOI-55 / 20 torque rheometer of Guangzhou Putong Experimental Analytical Instrument Co., Ltd. to test at a temperature of 160°C, a speed of 50 r / min, and a test time of 10 min.

[0057] 2. The tensile properties of the film material were tested according to GB / T 1040.3-2006 standard using a BTIFRO10TH.A50 tensile testing machine from Zwick Roe11, Germany. The sample size was 120 mm x 10 mm x 4 mm and the tensile rate was 50 mm / min.

[0058] 3. The water vapor permeability of the film material was tested according to GB / T 30412-2013 using a US MOCON 221MD at 38°C and 90% relative humidity.

[0059] 4. The oxygen permeability of film materials was tested according to GB / T1038-2000 using a Jinan Labthink BTY-B2P air permeability tester at 23°C.

[0060] 5. The antibacterial properties of the film material were tested according to QB / T 2591-2003. The film was cut into 2×2 cm 2 size, immersed in 2 mL of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) suspension (10 5 After the bacterial solution was fixed for 4 h, the film sample was washed three times with phosphate buffered saline (PBS) and then ultrasonicated in 10 mL PBS solution. 100 μL of the ultrasonicated liquid was spread on an agar plate and incubated at 37°C for 12 h before observing the growth of the colonies.

[0061] The tensile strength, elongation at break, and fracture energy of the film materials obtained by applying the equilibrium torque of Examples 1 to 3 and Comparative Examples 1 to 2 are shown in Table 4. The water vapor and oxygen permeability and plate colony count results of the film materials obtained by applying Examples 1 to 3 and Comparative Examples 1 to 2 are shown in Table 5. The stress-strain curves of the tensile test of polylactic acid and polylactic acid film with the addition of bio-based Schiff base structure modifier VP@MAH are shown in Figure 4 As shown in the figure, the tensile strength results of the polylactic acid film and the polylactic acid film with the addition of bio-based Schiff base structure modifier VP@MAH are as follows: Figure 5 As shown in the figure, the elongation at break of the polylactic acid film and the polylactic acid film with the addition of bio-based Schiff base structure modifier VP@MAH were subjected to tensile testing. Figure 6As shown in the figure, the oxygen permeability of the polylactic acid film and the polylactic acid film with the addition of the bio-based Schiff base structure modifier VP@MAH was tested. Figure 7 As shown in the figure, the water vapor permeability of the polylactic acid film and the polylactic acid film with the addition of the bio-based Schiff base structure modifier VP@MAH was tested. Figure 8 As shown in the figure, the colony plate of PLA film after the plate colony test with PLA film containing bio-based Schiff base structure modifier VP@MAH is as follows: Figure 9 As shown in Figure 2, the antibacterial ratio of the film with the addition of the bio-based Schiff base structure modifier VP@MAH is improved compared with the polylactic acid film. Figure 10 shown.

[0062] Table 3 Mass percentage ratio of raw materials in Application Examples 1-3 and Comparative Examples 1-2

[0063] Table 4 Equilibrium torque of polylactic acid system and mechanical properties of film materials obtained in application examples and comparative examples

[0064] Table 5 Water vapor and oxygen transmission rates and plate colony count results of the films obtained from the application examples and comparative examples

[0065] As shown in Table 4, compared with the comparative example, the bio-based Schiff base-containing multifunctional modifier VP@MAH of the present invention not only improves the processing flow properties of the polylactic acid system, but also has a more significant effect on enhancing the tensile strength, elongation at break, and fracture energy of the polylactic acid film. Compared with comparative example 1, the equilibrium torque of application example 3 decreased from 2.6 N·m to 1.3 N·m, a decrease of 50%; the tensile strength increased from 7.3 MPa to 10.3 MPa, a 41.1% increase; the elongation at break increased from 528% to 723%, a 36.9% increase; and the fracture energy increased from 26.5 to 39.9, a 50.5% increase. Compared with comparative example 2, which used the commercially available polylactic acid modifier Joncryl® ADR 4468, application example 3 not only achieved a similar decrease in equilibrium torque, but also increased the tensile strength, elongation at break, and fracture energy of the resulting film by 17.7%, 19.9%, and 13.0%, respectively.

[0066] As shown in Table 5, Application Example 3, which employed the bio-based, Schiff-base-containing multifunctional modifier VP@MAH according to the present invention, reduced the water vapor transmission rate by 76.3% and the oxygen transmission rate by 13.3% compared to Comparative Example 1, which did not incorporate the VP@MAH. Furthermore, Application Example 3, which incorporated the bio-based, Schiff-base-containing multifunctional modifier VP@MAH according to the present invention, exhibited growth inhibition against Escherichia coli and Staphylococcus aureus by 94% and 92%, respectively, compared to Comparative Example 1. Compared to Comparative Example 2, which employed the commercially available polylactic acid modifier Joncryl® ADR 4468, the film material obtained in Application Example 3 exhibited growth inhibition against Escherichia coli and Staphylococcus aureus by 93% and 92%, respectively.

[0067] In summary, the bio-based, Schiff-base-containing multifunctional modifier VP@MAH exhibits liquid crystal orientation viscosity reduction properties and small molecule lubricity, significantly improving the processing fluidity of polylactic acid systems and reducing their processing difficulty. It also enhances the tensile strength, elongation at break, and toughness of composite materials, strengthens the water vapor and oxygen barrier properties of polylactic acid films, and imparts antibacterial properties. This multifunctional modifier offers excellent comprehensive performance and is highly applicable in glass fiber-reinforced polymer blending and modification.

[0068] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0069] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A bio-based Schiff base-containing modifier, characterized in that: The structural formula of the modifier is shown below: 。 2. A method for preparing the modifier according to claim 1, characterized in that: The following steps are involved: Synthesis of bio-based intermediates: Under a protective atmosphere, a reaction substrate is mixed with an organic solvent, reacted, a precipitate is collected, washed, filtered, and dried to obtain a bio-based intermediate; the reaction substrate includes a bio-based raw material and 1,4-phenylenediamine; Preparation of a bio-based Schiff base-containing modifier: dissolving a bio-based intermediate and / or an acid-binding agent in an organic solvent to obtain a bio-based intermediate solution; dissolving trimellitic anhydride chloride in an organic solvent to obtain a trimellitic anhydride chloride solution; adding the trimellitic anhydride chloride solution dropwise to the bio-based intermediate solution to react, precipitate, filter, wash, and dry to obtain a bio-based Schiff base-containing modifier.

3. The preparation method according to claim 2, characterized in that The organic solvent includes at least one of N'N-dimethylformamide, pyridine, tetrahydrofuran, dichloromethane, chloroform, and ethanol; When the organic solvent comprises at least one of N'N-dimethylformamide, tetrahydrofuran, dichloromethane, chloroform, and ethanol, in the step of preparing the bio-based Schiff base-containing modifier, the bio-based intermediate and the acid-binding agent are dissolved in the organic solvent to obtain a bio-based intermediate solution; When the organic solvent includes pyridine, in the step of preparing the bio-based Schiff base-containing modifier, the bio-based intermediate is dissolved in the organic solvent to obtain a bio-based intermediate solution.

4. The preparation method according to claim 3, characterized in that The bio-based raw material includes vanillin, and the acid-binding agent includes at least one of pyridine and triethylamine.

5. The preparation method according to any one of claims 2 to 4, characterized in that In the step of synthesizing the bio-based intermediate, the mass ratio of the organic solvent to the reaction substrate is (9-12):1, and the molar ratio of the bio-based raw material to the 1,4-phenylenediamine is (2.0-3.1):1; In the step of synthesizing the bio-based intermediate, the reaction conditions include: a stirring speed of 200-250 r / min, a reaction temperature of 60-80° C., and a reaction time of 5-8 h.

6. The preparation method according to any one of claims 2 to 4, characterized in that The molar ratio of the acid binding agent to the 1,4-phenylenediamine is (2.5-4):1, and the molar ratio of the bio-based intermediate to the chlorinated trimellitic anhydride is 1:(2.0-2.8); In the step of preparing the bio-based Schiff base-containing modifier, the reaction conditions include: the reaction temperature is room temperature, and the reaction time is 12-18 hours.

7. A modified polylactic acid, characterized in that: The raw materials for preparing the modified polylactic acid include polylactic acid and the bio-based Schiff base-containing modifier according to claim 1.

8. The modified polylactic acid according to claim 7, characterized in that The bio-based Schiff base-containing modifier accounts for 1% to 3% of the raw material by mass.

9. A method for preparing the modified polylactic acid according to any one of claims 7 to 8, characterized in that: The method comprises the following steps: blending the bio-based Schiff base-containing modifier according to claim 1 with polylactic acid to obtain a mixture, and melt-blending the mixture to obtain modified polylactic acid.

10. A polylactic acid film, characterized in that: The modified polylactic acid is prepared by using the modified polylactic acid described in any one of claims 7-8.

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