A bio-based schiff base-containing modifier, and a preparation method and application thereof

Abstract

The present application relates to a kind of bio-based modified agent containing Schiff base and its preparation method and application, it is related to the field of material science and engineering.The molecular structure of the modified agent contains the group of bio-based source, gives the good biocompatibility and renewability of modified agent, simultaneously, the molecular structure of the modified agent is introduced into Schiff base structure (-C=N-), and the imine bond in Schiff base structure has higher reactivity, so that the modified agent can interact with poly-lactic acid molecular chain, to realize the improvement of the performance of poly-lactic acid.

Description

Technical Field

[0001] This invention relates to the field of materials science and engineering, and in particular to a bio-based Schiff base modifier, its preparation method, and its application. Background Technology

[0002] Polylactic acid (PLA) is a biodegradable polyester material made from renewable resources such as corn and cassava starch. Due to its excellent biodegradability, microorganisms in the natural environment can completely degrade it into carbon dioxide and water, causing no environmental pollution. It is considered one of the ideal alternatives to traditional plastics for solving environmental pollution problems. In the packaging field, PLA films can be used for food packaging, shopping bags, etc.; in the biomedical field, it can be used to manufacture sutures, tissue engineering scaffolds, etc.; in the textile field, it can be made into fibers for clothing production. However, conventional PLA materials have the following performance defects:

[0003] 1. Poor processing flow properties: Polylactic acid (PLA) molecules have high rigidity and melt viscosity, resulting in poor flowability during processing. This makes molding difficult, requiring higher processing temperatures and pressures, which not only increases processing costs but may also cause thermal degradation of PLA, affecting product quality. 2. Difficulty in balancing rigidity and toughness: Pure PLA materials typically exhibit high rigidity but insufficient toughness, making them prone to brittle fracture under impact, limiting their use in applications requiring high material toughness. 3. Insufficient barrier properties: PLA films have poor barrier properties against small molecules such as oxygen and water vapor, making them ineffective in extending shelf life when packaging products with high barrier performance requirements (such as high-fat foods and easily oxidized products). 4. Inadequate antibacterial properties: PLA itself does not possess antibacterial properties, making it susceptible to microbial attack in some applications (such as food packaging and medical supplies), affecting product safety and lifespan. Existing technical solutions for improving the properties of polylactic acid (PLA) include: 1. Adding plasticizers to improve processing fluidity: By adding small-molecule plasticizers, such as citrates and phthalates, the interaction between PLA molecular chains can be reduced, improving its processing fluidity. For example, adding tributyl citrate (TBC) as a plasticizer can improve the processing performance of PLA to some extent. 2. Blending modification to improve rigidity and toughness: Blending PLA with other polymers, such as polybutylene adipate / terephthalate (PBAT) and polybutylene succinate (PBS), can improve the toughness of PLA to a certain extent while maintaining a certain degree of rigidity. 3. Surface coating to improve barrier properties: Coating the surface of PLA films with barrier coatings, such as silica coatings or polyvinyl alcohol coatings. Chemical vapor deposition (CVD) is used to prepare silica coatings on the surface of PLA films, significantly improving the barrier properties of the films. 4. Adding antibacterial agents imparts antibacterial properties: Adding antibacterial agents, such as silver ion antibacterial agents and quaternary ammonium salt antibacterial agents, can give polylactic acid (PLA) materials antibacterial properties. PLA composite materials with added silver ion antibacterial agents have better antibacterial effects.

[0004] However, the aforementioned technical solutions for improving the properties of polylactic acid (PLA) also have the following drawbacks: 1. Small molecule plasticizers: However, small molecule plasticizers have problems such as easy migration and volatility, which can lead to a decline in material properties over time. Excessive addition of plasticizers can also reduce the mechanical properties of PLA materials. 2. Blending modification: The compatibility between different polymers in blend systems is often difficult to solve, easily leading to phase separation and affecting the stability of material properties. 3. Surface coating: Coating processes are complex, the bonding force between the coating and the PLA matrix is ​​limited, the coating is prone to peeling off during use, and the cost is high, limiting its large-scale application. 4. Antibacterial agent addition: It is difficult to find a balance between cost and effectiveness. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a bio-based Schiff base modifier. The molecular structure of this modifier contains bio-based groups, which endows it with good biocompatibility and renewability. At the same time, the molecular structure of this modifier introduces a Schiff base structure (-C=N-). 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.

[0006] This invention provides a bio-based Schiff base-containing modifier, the structural formula of which is shown below:

[0007]

[0008] During their research on several existing technologies for improving the properties of polylactic acid (PLA), the inventors discovered the following reasons for the aforementioned drawbacks: 1. Small molecule plasticizers: Taking phthalate plasticizers as an example, they gradually migrate to the surface of plastic products during use, not only making the material surface sticky and affecting the user experience, but also potentially contaminating substances they come into contact with. In food packaging applications, the migration of plasticizers may enter the food, posing a potential health hazard. Moreover, as plasticizers continue to migrate and volatilize, the processing fluidity of PLA materials gradually returns to its original poor state, making it impossible to maintain good processing performance. 2. Blending modification: Taking the PLA / PBAT blend system as an example, due to the significant differences in molecular structure and polarity between PLA and PBAT, phase separation is prone to occur during blending. Phase separation leads to inhomogeneous internal structure of the material, and when the material is subjected to external forces, the phase interface easily becomes a stress concentration point, thereby reducing the mechanical properties of the material, especially tensile strength and impact strength. In addition, phase separation also affects other properties of the material, such as barrier properties and optical properties. 3. Surface Coating: Surface coating processes are complex and costly, limiting their large-scale application. Chemical vapor deposition (CVD) for preparing silica coatings requires specialized equipment and complex process conditions, demanding high levels of operator skill. Furthermore, the adhesion between the coating and the polylactic acid (PLA) matrix relies primarily on physical adsorption or weak chemical bonds. During material use, external environmental factors (such as temperature and humidity changes, and mechanical friction) can easily cause the coating to detach from the substrate surface. Once the coating detaches, the barrier properties of the PLA material will significantly decrease, failing to meet practical application requirements. 4. Antibacterial Agent Addition: Silver ion antibacterial agents are expensive, increasing the production cost of PLA materials and constraining their large-scale application due to economic factors. Simultaneously, the safety of silver ion release is a major concern; excessive silver ion release may have adverse effects on the environment and human health. While quaternary ammonium salt antibacterial agents are relatively inexpensive, their antibacterial spectrum is narrow, only inhibiting some common bacteria and showing poor inhibitory effects against molds, yeasts, and other microorganisms. In practical applications, especially in fields with high requirements for microbial control such as medical and health care and food packaging, it cannot provide comprehensive and effective antimicrobial protection.

[0009] Based on this, the inventors propose the aforementioned bio-based Schiff base modifier, which possesses a unique chemical structure. The core design of its molecular structure is the organic combination of bio-based groups and Schiff base structures. The bio-based groups endow the modifier with advantages such as renewability and good biocompatibility, while the Schiff base structure provides active sites for interaction with polylactic acid (PLA) molecular chains, as well as a key function for improving material properties. Compared with the aforementioned technologies for improving PLA properties, this modifier has the following advantages: 1. Compared with small molecule plasticizers: Existing small molecule plasticizers simply improve processing fluidity by physically mixing to reduce the intermolecular forces of PLA molecular chains, and suffer from migration and volatilization problems. The multifunctional modifier of this invention, however, reacts chemically with PLA molecular chains through active groups in its chemical structure to form stable chemical bonds or graft copolymers, not only improving processing fluidity but also enhancing PLA properties in multiple ways, without migrating or volatilization issues. 2. Compared with blending modification: Traditional blending modification mainly involves mechanically mixing different polymers, facing compatibility challenges. The modifier of this invention is a compound with a specific structure. Within a polylactic acid (PLA) matrix, it can form a relatively stable microstructure through interaction with PLA molecular chains, eliminating compatibility issues such as phase separation and more effectively balancing the rigidity and toughness of PLA. 3. Compared to surface coatings: Surface coating processes are complex and coatings are prone to peeling. This invention, by melt-blending the modifier with PLA, ensures thorough mixing between the modifier and the PLA matrix, extending the lifespan of the modified product. 4. Compared to antibacterial agents: While traditional silver ion antibacterial agents have significant antibacterial effects, they are expensive (5000-10000 RMB / kg) and pose a risk of silver ion leaching. In vitro cytotoxicity experiments show that when the silver ion concentration exceeds 10 ppm, the survival rate of human fibroblasts drops below 70%. Quaternary ammonium salt antibacterial agents have limited antibacterial spectrums, with minimum inhibitory concentrations (MICs) against fungi such as Aspergillus niger as high as 500 ppm. The Schiff base structure of this invention disrupts cell membrane integrity through nucleophilic addition reactions with thiol and amino groups on microbial cell membranes; the natural antibacterial components (such as phenolic hydroxyl structures) in the bio-based group work synergistically to achieve an inhibition rate of over 90% against Escherichia coli and Staphylococcus aureus, and the cost is only 1 / 10 of that of silver ion antibacterial agents. Skin irritation tests have proven that it has no cytotoxicity.

[0010] The present invention also provides a method for preparing the modifier, comprising the following steps:

[0011] Synthetic bio-based intermediate: Under a protective atmosphere, the reaction substrate is mixed with an organic solvent and reacted. The precipitate is collected, washed, filtered, and dried to obtain the bio-based intermediate. The reaction substrate includes bio-based raw materials and 1,4-phenylenediamine.

[0012] Preparation of bio-based Schiff base-containing modifier: A bio-based intermediate and an acid-binding agent are dissolved in an organic solvent to obtain a bio-based intermediate solution; trimellitic anhydride chloride is dissolved in an organic solvent to obtain a trimellitic anhydride chloride solution; the trimellitic anhydride chloride solution is added dropwise to the bio-based intermediate solution to react, a precipitate is formed, filtered, washed, and dried to obtain the bio-based Schiff base-containing modifier.

[0013] The above-mentioned modifier was prepared using specific reaction steps and conditions, starting from bio-based raw materials and synthesized through multiple reaction steps. In particular, the introduction of the Schiff base structure and the final polymerization reaction step are key to obtaining the specific molecular structure.

[0014] In one embodiment, the organic solvent includes at least one of N'N-dimethylformamide, pyridine, tetrahydrofuran, dichloromethane, chloroform, and ethanol;

[0015] When the organic solvent includes at least one of N'N-dimethylformamide, tetrahydrofuran, dichloromethane, chloroform, and ethanol, in the step of preparing the bio-based Schiff base modifier, the bio-based intermediate and the acid-binding agent are dissolved in the organic solvent to obtain a bio-based intermediate solution.

[0016] When the organic solvent includes pyridine, in the step of preparing the bio-based Schiff base modifier, the bio-based intermediate is dissolved in the organic solvent to obtain a bio-based intermediate solution.

[0017] In one embodiment, the bio-based raw material includes vanillin, and the acid-binding agent includes at least one of pyridine and triethylamine.

[0018] 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.

[0019] In the step of synthesizing bio-based intermediates, the reaction conditions include: a stirring speed of 200-250 r / min, a reaction temperature of 60-80℃, and a reaction time of 5-8 h.

[0020] 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 trimellitic anhydride chloride is 1:(1.5-2.5).

[0021] In the step of preparing the bio-based Schiff base modifier, the reaction conditions include: a reaction temperature of room temperature and a reaction time of 12-18 hours.

[0022] Precise control of the above reaction conditions is key to ensuring that the modifier has good performance.

[0023] The present invention also provides a modified polylactic acid, wherein the raw materials for preparing the modified polylactic acid include polylactic acid and the aforementioned bio-based Schiff base-containing modifier.

[0024] In one embodiment, the bio-based Schiff base modifier accounts for 1%-3% of the mass percentage of the raw material used in the preparation.

[0025] The present invention also provides a method for preparing the modified polylactic acid, comprising the following steps: blending the bio-based Schiff base modifier with polylactic acid to obtain a mixture, and then melt-blending the mixture to obtain the modified polylactic acid.

[0026] The above preparation method applies the modifier to the polylactic acid (PLA) system through blending modification. Optimizing the combination of process parameters such as mixing ratio, mixing method, melt blending conditions, and molding processing conditions allows the modifier to fully exert its effect, which is beneficial for improving the properties of PLA.

[0027] The present invention also provides a polylactic acid film prepared using the modified polylactic acid described above.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] This invention discloses a bio-based Schiff base-containing modifier, its preparation method, and its application. This modifier addresses problems in the processing flow properties, rigidity and toughness balance, barrier properties, and antibacterial properties of polylactic acid (PLA) systems. Specifically,

[0030] The modifier contains bio-based groups in its molecular structure, endowing it with good biocompatibility and renewability. Simultaneously, the introduction of a Schiff base structure (-C=N-) into the molecular structure, with its highly reactive imine bonds, allows the modifier to interact with the polylactic acid (PLA) molecular chain, thereby improving PLA's properties. Adding the modifier of this invention significantly improves the processing flow properties of PLA systems, reduces processing temperature and pressure, and decreases the risk of PLA thermal degradation; improves the rigidity and toughness of PLA film materials, giving them high strength and good impact resistance; enhances the barrier properties of PLA films against small molecules such as oxygen and water vapor, effectively extending the shelf life of packaged products; and imparts good antibacterial properties to PLA materials, inhibiting the growth and reproduction of common microorganisms, thus improving product safety and service life. Furthermore, the modifier of this invention is prepared using bio-based raw materials, conforming to the concepts of green environmental protection and sustainable development, and providing an effective solution for the high-performance and widespread application of PLA materials. Attached Figure Description

[0031] Figure 1 Infrared spectrum of VP@MAH, a bio-based modifier containing a Schiff base structure;

[0032] Figure 2 The 1H NMR spectrum of VP@MAH, a bio-based modifier containing a Schiff base structure;

[0033] Figure 3 Thermogravimetric analysis (TGA) curves of the bio-based Schiff base modifier VP@MAH at a heating rate of 10℃ / min from 30℃ to 600℃.

[0034] Figure 4 Stress-strain curves of polylactic acid (PLA) films with added bio-based Schiff base modifier VP@MAH were obtained from tensile tests.

[0035] Figure 5 Tensile strength results of polylactic acid (PLA) films with added bio-based Schiff base-containing modifier VP@MAH.

[0036] Figure 6 The results of tensile tests on polylactic acid (PLA) films and PLA films modified with bio-based Schiff base-containing modifier VP@MAH show the elongation at break.

[0037] Figure 7 The oxygen permeability of polylactic acid (PLA) films and PLA films modified with bio-based Schiff base-containing modifier VP@MAH is obtained by oxygen permeability testing.

[0038] Figure 8 The water vapor transmission rate of polylactic acid (PLA) films and PLA films modified with bio-based Schiff base-containing modifier VP@MAH was obtained by water vapor transmission test.

[0039] Figure 9 Colony plates after plate colony testing of polylactic acid (PLA) films with added bio-based Schiff base-containing modifier VP@MAH;

[0040] Figure 10 The graph shows the antibacterial ratio of films modified with the bio-based Schiff base-containing modifier VP@MAH. Detailed Implementation

[0041] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0042] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0043] source:

[0044] Unless otherwise specified, all reagents, materials, and equipment used in this embodiment are commercially available; unless otherwise specified, all test methods are conventional test methods in this field.

[0045] Example 1

[0046] A bio-based Schiff base modifier and its preparation method.

[0047] (a) Chemical structure design of modifiers.

[0048] The general chemical formula of the bio-based Schiff base-containing modifier of the present invention is shown below:

[0049]

[0050] The modifier molecule contains bio-based groups, which endow the modifier with good biocompatibility and renewability. Meanwhile, the introduction of the Schiff base structure (-C=N-) is one of the key design features of this invention. The imine bonds in the Schiff base structure have high reactivity and can interact with the polylactic acid molecular chain, thereby improving the performance of polylactic acid.

[0051] (ii) Preparation method of modifier.

[0052] 1. Raw material preparation: Select bio-based raw materials vanillin, 1,4-phenylenediamine, trimellitic anhydride chloride, organic solvents (organic solvents include at least one of N'N-dimethylformamide, pyridine, tetrahydrofuran, dichloromethane, chloroform, and ethanol), and pyridine and / or triethylamine (when pyridine is used as the organic solvent, no additional pyridine is needed, as pyridine acts as the pyridine chelating agent) for the synthesis of bio-based Schiff base-containing multifunctional modifiers.

[0053] 2. Synthesis of bio-based intermediates: Under nitrogen protection, vanillin and 1,4-phenylenediamine (molar ratio of vanillin to 1,4-phenylenediamine 2:1) and ethanol (the mass of the organic solvent is 9-12 times the total mass of vanillin and 1,4-phenylenediamine) were added to a three-necked flask. The reaction was carried out under magnetic stirring at a rate of 200-250 r / min at 60-80℃ for 5-8 h. After the reaction was completed, the mixture was filtered, and the precipitate was washed with ethanol and filtered again. Finally, the product was dried in a vacuum oven at 80℃ for 24 h. 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 1:

[0054]

[0055] 3. Final preparation of the bio-based Schiff base modifier: The bio-based Schiff base ligand was dissolved in an organic solvent, and an acid-binding agent (the molar amount of the acid-binding agent was 2.5 to 4 times that of 1,4-phenylenediamine) was added. Then, trimellitic anhydride chloride was dissolved in the organic solvent in an ice-water bath under magnetic stirring. The bio-based Schiff base ligand (the molar ratio of bio-based Schiff base ligand VP to trimellitic anhydride chloride was 1:2) was slowly added dropwise. After the addition was complete, the reaction was carried out at room temperature for 12 to 18 hours. After the reaction was completed, the precipitate was filtered, washed, and dried to obtain the bio-based Schiff base modifier VP@MAH. The reaction process is shown in reaction formula 2:

[0056]

[0057] (III) Modification methods for polylactic acid systems.

[0058] 1. Blending Modification Process: The prepared bio-based modifier containing Schiff base structure is thoroughly mixed with polylactic acid particles in a high-speed mixer at a certain ratio, so that the modifier is uniformly dispersed on the surface of the polylactic acid particles. The mixing time is generally 3 minutes, and the mixing speed is 80 rpm.

[0059] 2. Melt Blending Process: The uniformly mixed polylactic acid and modifier are added to a twin-screw extruder for melt blending. The temperature settings for each section of the twin-screw extruder are shown in the table below.

[0060] Table 1 Temperature settings for each section of the twin-screw extruder

[0061]

[0062] 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 can undergo physical entanglement and chemical reaction with the polylactic acid (PLA) molecular chains at high temperatures, thereby improving the performance of the PLA system. After water cooling, stretching, and pelletizing, the extrudate yields modified PLA granules.

[0063] 3. Molding and processing: Modified polylactic acid granules are extruded to form the desired polylactic acid film products. The modified polylactic acid granules are heated and melted in a single-screw extruder and extruded into a film through a die. The temperature of the single-screw extruder is set to 130℃ to 160℃. The temperature settings of each section of the twin-screw extruder are shown in the table below. The traction speed is 4 m / min and the cooling roller temperature is 30℃, thereby obtaining a polylactic acid film with good performance.

[0064] Table 2 Temperature settings for each section of the twin-screw extruder

[0065]

[0066] (iv) Mechanism of action of modifiers.

[0067] 1. Improved processing flow properties: The bio-based groups in bio-based Schiff base modifier molecules have low surface energy, which can act as a lubricant in polylactic acid (PLA) melts, reducing the friction between PLA molecular chains and thus improving the processing flow properties of the PLA system. Simultaneously, the active groups in the modifier molecules may react chemically with the hydroxyl or carboxyl groups at the ends of the PLA molecular chains to form graft copolymers, increasing the flexibility of the molecular chains and further improving processing performance.

[0068] 2. Balancing Rigidity and Toughness: The presence of the Schiff base structure enables the modifier molecules to form strong interactions with the polylactic acid (PLA) molecular chains, such as hydrogen bonds and π-π stacking. These interactions act as physical cross-linking points in the PLA matrix, enhancing the material's rigidity. Simultaneously, the flexible long-chain structure of the bio-based groups allows the material to absorb energy through deformation when subjected to external impact, improving its toughness and thus achieving a balance between rigidity and toughness in PLA materials.

[0069] 3. Enhanced Barrier Properties: The modifier molecules form a physical barrier within the polylactic acid matrix, hindering the diffusion paths of small molecules such as oxygen and water vapor. On 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 chains makes the material's microstructure more compact, further improving its barrier properties.

[0070] 4. Imparting antibacterial properties: The Schiff base structure possesses certain biological activity, enabling it to interact with proteins or enzymes on the surface of microbial cell membranes, disrupting cell membrane integrity and thus inhibiting microbial growth and reproduction. Simultaneously, some natural antibacterial components that may be present in the bio-based groups also work synergistically, endowing polylactic acid materials with excellent antibacterial properties.

[0071] Example 2

[0072] Based on the preparation method described in Example 1, a bio-based Schiff base modifier was prepared.

[0073] (i) Under nitrogen protection, 0.02 mol vanillin, 0.01 mol 1,4-phenylenediamine, and 50 mL 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 at 60 °C for 5 h. After the reaction was completed, the mixture was filtered, and the precipitate was washed with ethanol and filtered again. Finally, the 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 96.3%.

[0074] (II) 0.01 mol VP and 0.035 mol triethylamine (acid-binding agent) were dissolved in 30 g N'N-dimethylformamide (in this embodiment, the molar ratio of acid-binding agent to 1,4-phenylenediamine was 3.5:1). Then, under magnetic stirring, 0.02 mol trimellitic anhydride chloride was dissolved in 30 g N'N-dimethylformamide in an ice-water bath at a stirring rate of 200 r / min. A bio-based Schiff base ligand solution was slowly added dropwise (in this embodiment, the molar ratio of bio-based Schiff base ligand VP to trimellitic anhydride chloride was 1:2). The addition was completed within 60 min. After the addition was complete, the reaction was carried out at room temperature for 12 h. After the reaction was complete, the precipitate was placed in ice water to precipitate, filtered, washed, and dried to obtain a bio-based Schiff base multifunctional modifier. A yellow powder product, VP@MAH, was obtained with a yield of 78.1%.

[0075] A small amount of the product was subjected to FT-IR testing, and the results are as follows: Figure 1 The NH stretching vibration peak of the primary amino group (-NH2) in the raw material 1,4-phenylenediamine (3446 cm⁻¹) - ¹) and the C=O stretching vibration peak of the aldehyde group (-CHO) of vanillin (1700 cm⁻¹). - ¹) The complete disappearance after the reaction indicates that the condensation reaction between the amino and aldehyde groups is complete. Simultaneously, at 1626 cm⁻¹... - The presence of a characteristic absorption peak at position ¹, attributed to the Schiff base C=N bond, confirms the successful formation of the imine bond. During the anhydride chloride substitution reaction stage, the vanillin derivative exhibits a broad hydroxyl (-OH) peak (3423 cm⁻¹). - ¹) The strength is significantly reduced, and at 1739 cm - ¹ and 1192 cm - Stretching vibration peaks of the ester group C=O and COC were detected at ¹, indicating that the hydroxyl group undergoes an esterification reaction with trimellitic anhydride chloride. The above results verify that the Schiff base (C=N) and ester bond (COC) in the VP@MAH molecule were introduced along the designed path, which is highly consistent with its target structure.

[0076] Simultaneously, the obtained VP@MAH was subjected to 1H NMR analysis using deuterated chloroform as the solvent, and 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 presence of the aromatic ring is indicated by the characteristic peak of the proton (7) in the imine group at 8.31 ppm. Protons (4, 5, 6) appear at 7.29–7.59 ppm, while 8.14–8.79 ppm represents 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.

[0077] The processing thermal stability of the obtained VP@MAH was characterized using TGA, and the results are as follows: Figure 3 The curve shows the 5% mass loss temperature of VP@MAH ( T 5% The temperature at which the maximum degradation rate is 199.2℃ is also mentioned. T max The temperature is 251.7℃, and the residual weight at 600℃ is 5.61%. The surface MBC has good processing thermal stability and char-forming properties.

[0078] Example 3

[0079] Based on the preparation method described in Example 1, a bio-based Schiff base modifier was prepared.

[0080] (i) Under nitrogen protection, 0.02 mol vanillin, 0.01 mol 1,4-phenylenediamine, and 50 mL 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 at 60 °C for 6 h. After the reaction was completed, the mixture was filtered, and the precipitate was washed with ethanol and filtered again. Finally, the 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%.

[0081] (II) 0.01 mol VP and 0.025 mol triethylamine (acid-binding agent) were dissolved in 35 g of dichloromethane (in this embodiment, the molar ratio of acid-binding agent to 1,4-phenylenediamine was 2.5:1). Then, under magnetic stirring, 0.02 mol trimellitic anhydride chloride was dissolved in 35 g of dichloromethane in an ice-water bath at a stirring rate of 220 r / min. A bio-based Schiff base ligand solution was slowly added dropwise (in this embodiment, the molar ratio of bio-based Schiff base ligand VP to trimellitic anhydride chloride was 1:2). The addition was completed within 40 min. After the addition was completed, the reaction was carried out at room temperature for 12 h. After the reaction was completed, the precipitate was placed in ice water to precipitate, filtered, washed, and dried to obtain a bio-based Schiff base multifunctional modifier. The resulting yellow powder product was VP@MAH, with a yield of 73.9%. The structure of the obtained VP@MAH was consistent with that in Example 1.

[0082] Example 4

[0083] Based on the preparation method described in Example 1, a bio-based Schiff base modifier was prepared.

[0084] (i) Under nitrogen protection, 0.02 mol vanillin, 0.01 mol 1,4-phenylenediamine and 50 mL 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 at 80 °C for 8 h. After the reaction was completed, the mixture was filtered, and the precipitate was washed with ethanol and filtered again. Finally, the 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 86.8%.

[0085] (II) 0.01 mol VP was dissolved in 20 g dichloromethane and 40 g pyridine (pyridine acts as both an organic solvent and an acid-binding agent). Then, under magnetic stirring, 0.02 mol trimellitic anhydride chloride was dissolved in 35 g dichloromethane in an ice-water bath at a stirring rate of 250 r / min. A bio-based Schiff base ligand solution was slowly added dropwise (in this example, the molar ratio of bio-based Schiff base ligand VP to trimellitic anhydride chloride was 1:2). The addition was completed within 60 min. After the addition was completed, the reaction was carried out at room temperature for 18 h. After the reaction was completed, the precipitate was placed in ice water to precipitate, filtered, washed, and dried to obtain the bio-based Schiff base multifunctional modifier. The resulting yellow powder product was VP@MAH, with a yield of 76.7%. The structure of the obtained VP@MAH was consistent with that in Example 1.

[0086] Example 5

[0087] Based on the preparation method described in Example 1, a bio-based Schiff base modifier was prepared.

[0088] (i) Under nitrogen protection, 0.02 mol vanillin, 0.01 mol 1,4-phenylenediamine, and 50 mL 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 at 60 °C for 8 h. After the reaction was completed, the mixture was filtered, and the precipitate was washed with ethanol and filtered again. Finally, the 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 85.1%.

[0089] (II) 0.01 mol VP and 0.03 mol triethylamine (acid-binding agent) were dissolved in 40 g tetrahydrofuran (in this example, the molar ratio of acid-binding agent to 1,4-phenylenediamine was 3:1). Then, under magnetic stirring, 0.02 mol trimellitic anhydride chloride was dissolved in 40 g tetrahydrofuran in an ice-water bath at a stirring rate of 250 r / min. A bio-based Schiff base ligand solution was slowly added dropwise (in this example, the molar ratio of bio-based Schiff base ligand VP to trimellitic anhydride chloride was 1:2). The addition was completed within 30 min. After the addition was completed, the reaction was carried out at room temperature for 16 h. After the reaction was completed, the precipitate was placed in ice water to precipitate, filtered, washed, and dried to obtain a bio-based Schiff base multifunctional modifier. The resulting yellow powder product was VP@MAH, with a yield of 76.5%. The structure of the obtained VP@MAH was consistent with that in Example 1.

[0090] Example 6

[0091] Based on the preparation method described in Example 1, a bio-based Schiff base modifier was prepared.

[0092] (i) Under nitrogen protection, 0.02 mol vanillin, 0.01 mol 1,4-phenylenediamine, and 50 mL 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 at 70 °C for 7 h. After the reaction was completed, the mixture was filtered, and the precipitate was washed with ethanol and filtered again. Finally, the 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 88.7%.

[0093] (II) 0.01 mol VP and 0.035 mol pyridine (acid-binding agent) were dissolved in 40 g chloroform (in this example, the molar ratio of acid-binding agent to 1,4-phenylenediamine was 3.5:1). Then, 0.02 mol trimellitic anhydride chloride was dissolved in 40 g chloroform in an ice-water bath under magnetic stirring at a stirring rate of 250 r / min. A bio-based Schiff base ligand solution was slowly added dropwise (in this example, the molar ratio of bio-based Schiff base ligand VP to trimellitic anhydride chloride was 1:2). The addition was completed within 60 min. After the addition was completed, the reaction was carried out at room temperature for 14 h. After the reaction was completed, the precipitate was placed in ice water to precipitate, filtered, washed, and dried to obtain a bio-based Schiff base multifunctional modifier. The resulting yellow powder product was VP@MAH, with a yield of 72.1%. The structure of the obtained VP@MAH was consistent with that in Example 1.

[0094] Example 7

[0095] Based on the preparation method described in Example 1, a bio-based Schiff base modifier was prepared.

[0096] (i) Under nitrogen protection, 0.02 mol vanillin, 0.01 mol 1,4-phenylenediamine, and 50 mL 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 at 70 °C for 8 h. After the reaction was completed, the mixture was filtered, and the precipitate was washed with ethanol and filtered again. Finally, the 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 92.6%.

[0097] (II) 0.01 mol VP was dissolved in 50 g pyridine (pyridine acts as both an organic solvent and an acid-binding agent). Then, under magnetic stirring, 0.02 mol trimellitic anhydride chloride was dissolved in 50 g pyridine in an ice-water bath at a stirring rate of 230 r / min. A bio-based Schiff base ligand solution was slowly added dropwise (in this example, the molar ratio of bio-based Schiff base ligand VP to trimellitic anhydride chloride was 1:2). The addition was completed within 30 min. After the addition was completed, the reaction was carried out at room temperature for 14 h. After the reaction was completed, the precipitate was placed in ice water to precipitate, filtered, washed, and dried to obtain a bio-based Schiff base multifunctional modifier. The resulting yellow powder product was VP@MAH, with a yield of 76.4%. The structure of the obtained VP@MAH was consistent with that in Example 1.

[0098] Example 8

[0099] (I) The bio-based Schiff base modifier prepared using the above examples was used for the modification of polylactic acid (product of Anhui Fengyuan Biomaterials Co., Ltd., brand name PLA FY802). By changing the mass percentage ratio of the raw materials (as shown in Table 3), 1-3 application examples were obtained. The specific preparation process was as follows: after drying the polylactic acid granules in a forced-air drying oven for 6-12 hours, polylactic acid, the bio-based Schiff base modifier VP@MAH, and the antioxidant (Suzhou Kaibaoli New Material Co., Ltd., tetrakis(2,4-di-tert-butyl)-4,4'-biphenyl diphosphite (PEPQ)) were weighed according to the formula, mixed evenly, and then added to a twin-screw extruder (model LTE26 / ). 40, LabTech GmbH (Germany) First, the material is blended and extruded to obtain granules, and then a single-screw extruder (model LTE20-30, LabTech GmbH, Germany) is used to extrude a film; the temperature of each zone of the twin-screw extruder barrel is as follows: Zone 1 130℃, Zone 2 140℃, Zone 3 150℃, Zone 4 160℃, Zone 5 160℃, Zone 6 160℃, Zone 7 160℃, Zone 8 160℃, Zone 9 160℃, Die head 160℃, and main engine speed 280 r / min. The film extrusion conditions are as follows: the obtained granules are dried in a forced-air drying oven for 6-12 hours, and the temperatures of each zone of the twin-screw extruder barrel are as follows: zone 1 130℃, zone 2 150℃, zone 3 155℃, zone 4 160℃, zone 5 160℃, zone 6 160℃, zone 7 160℃, zone 8 160℃, and zone 9 160℃. The main extruder speed is 80 r / min, and the pressure roller speed is 6 r / min.

[0100] The bio-based Schiff base modifier of this invention contains anhydride groups at the molecular ends, exhibiting low surface energy. When added to a polylactic acid (PLA) system, this multifunctional modifier tends to distribute at the PLA matrix interface, forming a cross-linked network with the matrix. Furthermore, its low molecular weight allows for easier penetration between polymer chains, promoting chain deentanglement and more effectively improving the processing rheology of the PLA system. Simultaneously, the uniform dispersion of the modifier within the polymer matrix ensures effective stress transfer, thereby improving the rigidity and toughness of the resulting material. In addition, the bio-based Schiff base modifier can enhance the equilibrium torque of the modified PLA.

[0101] (ii) Performance verification.

[0102] Pure polylactic acid (PLA) without the addition of the bio-based Schiff base multifunctional modifier VP@MAH was used as Comparative Example 1. PLA was modified with the commercially available PLA chain extender Joncryl® ADR 4468 (Shanghai Liangsu Environmental Protection Technology Co., Ltd.) as Comparative Example 2. The mass percentage composition of the raw materials is shown in Table 3.

[0103] 1. Testing the balance torque: After simply mixing the raw materials according to the formula in Table 3, the torque was tested using an RTOI-55 / 20 torque rheometer from Guangzhou Putong Experimental Analytical Instrument Co., Ltd. The temperature was 160℃, the rotation speed was 50r / min, and the test time was 10min.

[0104] 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 120mm x 10mm x 4mm, and the tensile rate was 50mm / min.

[0105] 3. The water vapor permeability of the thin film material was tested according to GB / T 30412-2013 using a MOCON 221MD (USA) at 38℃ and 90% relative humidity.

[0106] 4. The oxygen permeability of the membrane material was tested at 23℃ using the Jinan Labthink BTY-B2P air permeability tester according to GB / T1038-2000.

[0107] 5. The antibacterial properties of the film material were tested according to QB / T 2591-2003, with the film cut into 2×2 cm pieces. 2 The samples, of different sizes, were immersed in 2 mL of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) suspensions (10... 5 After the bacterial suspension was fixed for 4 h, the above film sample was washed three times with phosphate buffered saline (PBS) and then sonicated in 10 mL of PBS solution. 100 μL of the sonicated liquid was then spread onto an agar plate and incubated at 37 ℃ for 12 h before observing the colony growth.

[0108] The tensile strength, elongation at break, and fracture energy of the film materials obtained using the equilibrium torque of Examples 1-3 and Comparative Examples 1-2 are shown in Table 4. The water vapor and oxygen permeability and plate colony count results of the film materials obtained using Examples 1-3 and Comparative Examples 1-2 are shown in Table 5. The stress-strain curves of polylactic acid films with polylactic acid modified by adding a bio-based Schiff base-containing modifier VP@MAH are shown in Table 5. Figure 4 As shown, the tensile strength results of polylactic acid (PLA) films modified with bio-based Schiff base-containing modifier VP@MAH are obtained from tensile tests. Figure 5 As shown, the elongation at break results of polylactic acid (PLA) films modified with bio-based Schiff base-containing modifier VP@MAH are obtained through tensile testing. Figure 6As shown, the oxygen permeability of polylactic acid (PLA) films modified with bio-based Schiff base-containing modifier VP@MAH was tested, and the resulting oxygen permeability was as follows: Figure 7 As shown, the water vapor transmission rate of polylactic acid (PLA) films modified with bio-based Schiff base-containing modifier VP@MAH was obtained through water vapor transmission testing. Figure 8 As shown, the colony plates of polylactic acid (PLA) films modified with bio-based Schiff base-containing modifier VP@MAH were tested. Figure 9 As shown, the antibacterial ratio of the film with the addition of the bio-based Schiff base modifier VP@MAH is improved compared to that of the polylactic acid film. Figure 10 As shown.

[0109] Table 3. Mass percentage ratio of raw materials in Application Examples 1-3 and Comparative Examples 1-2

[0110]

[0111] Table 4. Equilibrium torque and mechanical properties of the obtained thin film materials in the application examples and comparative examples of polylactic acid systems.

[0112]

[0113] Table 5. Results of water vapor and oxygen permeability and plate colony count of the films obtained in the application examples and comparative examples.

[0114]

[0115] 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 improving the tensile strength, elongation at break, and fracture energy of the polylactic acid film material. 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, an increase of 41.1%; the elongation at break increased from 528% to 723%, an increase of 36.9%; and the fracture energy increased from 26.5 to 39.9, an increase of 50.5%. Compared with Comparative Example 2, which used the commercially available polylactic acid modifier Joncryl® ADR 4468, Application Example 3 not only showed a similar decrease in the equilibrium torque of the blend system, but also achieved increases in the tensile strength, elongation at break, and fracture energy of the resulting film material by 17.7%, 19.9%, and 13.0%, respectively.

[0116] As shown in Table 5, in Application Example 3, which used the bio-based Schiff base-containing multifunctional modifier VP@MAH of the present invention, the water vapor permeability decreased by 76.3% and the oxygen permeability decreased by 13.3% compared to Comparative Example 1 without VP@MAH. Furthermore, Application Example 3, with the addition of the bio-based Schiff base-containing multifunctional modifier VP@MAH of the present invention, showed a 94% and 92% increase in growth inhibition of *Escherichia coli* and *Staphylococcus aureus*, respectively, compared to Comparative Example 1. Compared to Comparative Example 2, which used the commercially available polylactic acid modifier Joncryl® ADR 4468, the film material obtained in Application Example 3 showed a 93% and 92% increase in growth inhibition rates against *Escherichia coli* and *Staphylococcus aureus*, respectively.

[0117] In summary, the bio-based Schiff base-containing multifunctional modifier VP@MAH of this invention, with its liquid crystal orientation viscosity-reducing properties and small molecule lubricating properties, effectively improves the processing flowability of polylactic acid (PLA) systems, reduces the processing difficulty of PLA systems, and improves the tensile strength, elongation at break, and toughness of composite materials. It also enhances the water vapor and oxygen barrier properties of PLA film materials and imparts antibacterial properties. Therefore, it is a multifunctional modifier with excellent comprehensive performance and has significant application value in the blending modification of glass fiber reinforced polymers.

[0118] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above 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.

[0119] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A modified polylactic acid, characterized in that, The raw materials for preparing the modified polylactic acid include polylactic acid and a bio-based Schiff base modifier, wherein the bio-based Schiff base modifier accounts for 1%-3% of the raw materials by mass. The structural formula of the modifier is shown below: 。 2. A method for preparing modified polylactic acid as described in claim 1, characterized in that, The bio-based Schiff base modifier is blended with polylactic acid to obtain a mixture, and the mixture is melt-blended to obtain modified polylactic acid. The preparation method of the bio-based Schiff base-containing modifier includes the following steps: Synthetic bio-based intermediate: Under a protective atmosphere, the reaction substrate is mixed with an organic solvent and reacted. The precipitate is collected, washed, filtered, and dried to obtain the bio-based intermediate. The reaction substrate is selected from bio-based raw materials and 1,4-phenylenediamine, and the bio-based raw material is vanillin. Preparation of bio-based Schiff base-containing modifier: Dissolve the bio-based intermediate and the acid-binding agent in an organic solvent, or dissolve the bio-based intermediate in an organic solvent to obtain a bio-based intermediate solution; dissolve trimellitic anhydride chloride in an organic solvent to obtain a trimellitic anhydride chloride solution; add the trimellitic anhydride chloride solution dropwise to the bio-based intermediate solution to react, precipitate out, filter, wash, and dry to obtain the bio-based Schiff base-containing modifier.

3. The preparation method according to claim 2, characterized in that, The organic solvent is selected from at least one of N'N-dimethylformamide, pyridine, tetrahydrofuran, dichloromethane, chloroform, and ethanol; When the organic solvent is selected from at least one of N'N-dimethylformamide, tetrahydrofuran, dichloromethane, chloroform, and ethanol, in the step of preparing the bio-based Schiff base 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 is selected from pyridine, in the step of preparing the bio-based Schiff base 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 acid-binding agent is selected from at least one of pyridine and triethylamine.

5. The preparation method according to any one of claims 2-4, 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 bio-based intermediates, the reaction conditions include: a stirring speed of 200-250 r / min, a reaction temperature of 60-80℃, and a reaction time of 5-8 h.

6. The preparation method according to any one of claims 2-4, 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 trimellitic anhydride chloride is 1:(2.0-2.8). In the step of preparing the bio-based Schiff base modifier, the reaction conditions include: a reaction temperature of room temperature and a reaction time of 12-18 hours.

7. A polylactic acid film, characterized in that, It was prepared using the modified polylactic acid described in claim 1.

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