Thermochromic polyester film with memory function and method for manufacturing the same

Abstract

This invention relates to a thermochromic polyester film with shape memory function. The film is characterized by being composed of a shape memory polyester layer containing chemically bonded thermochromic microcapsules. The film is capable of undergoing both shape and color changes under temperature stimulation. The thermochromic microcapsules of this invention are firmly connected to the shape memory polyester network through chemical bonding, fundamentally solving the problems of poor compatibility, easy detachment, and rapid functional decay in physical blending, thus achieving long-term stability of dual functions. Because the microcapsules form an integral whole with the matrix, they can move synchronously with the deformation and recovery of the matrix during shape memory deformation, without affecting their color-changing function. This allows the material to simultaneously and synergistically produce shape and color changes under external temperature stimulation, resulting in a higher level of intelligence.

Description

Technical Field

[0001] This invention belongs to the field of smart materials technology, specifically relating to a polyester film that can simultaneously change color and shape in response to temperature stimulation, namely a thermochromic polyester film with memory function, and its preparation method. Background Technology

[0002] Smart materials are materials that can sense changes in the external environment (such as temperature, light, electricity, magnetism, pH value, etc.) and respond accordingly. Among them, shape memory polymers (SMPs) and thermochromic materials are two important types of smart materials. Shape memory polymers can deform and remain fixed above a specific temperature (transition temperature), returning to their original shape upon reheating. Thermochromic materials, on the other hand, can change color with temperature variations. Combining these two technologies to develop materials that possess both shape memory and thermochromic functions has broad application prospects in fields such as smart packaging, anti-counterfeiting labels, medical devices, and sensor displays.

[0003] Currently, research on shape memory and thermochromic composite materials is still in its early stages. A common preparation method involves simply physically blending thermochromic pigments or microcapsules into a shape memory polymer matrix, followed by melt processing. For example, some literature reports the preparation of composite films by blending thermochromic microcapsules with shape memory polyurethane. However, this simple physical blending method has the following problems: First, the thermochromic microcapsules have poor compatibility with the polymer matrix, easily agglomerating in the matrix, leading to uneven color change and impairing the material's mechanical and shape memory properties; second, the microcapsules are only physically embedded in the matrix, resulting in weak interfacial bonding. During repeated shape memory cycles (deformation-recovery), the microcapsules easily detach from the matrix and rupture, causing the thermochromic function to rapidly decay or even fail; third, simple blending cannot effectively control the dispersion state and interfacial interactions of the microcapsules, making it difficult to achieve synergistic and stable dual functions.

[0004] Therefore, how to achieve uniform and stable dispersion of thermochromic functional units in shape memory polymer matrices and establish a strong bond to realize long-term stability and synergy of dual functions is a current technical challenge in this field. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a thermochromic polyester film with shape memory function, wherein the thermochromic unit and the shape memory polyester matrix are chemically bonded together to achieve stable coexistence and synergistic response of dual functions, and a method for its preparation is provided.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] A thermochromic polyester film with memory function is characterized in that the film is composed of a shape memory polyester layer, wherein the shape memory polyester layer contains chemically bonded thermochromic microcapsules, and the film can undergo dual changes in shape and color under temperature stimulation.

[0008] The core innovation of this invention lies in "chemical bonding." The thermochromic microcapsules are no longer simply embedded in the matrix; instead, they are covalently linked to the molecular chains of the polyester matrix through active functional groups on their surface. This makes the microcapsules part of the matrix network, greatly improving compatibility, enhancing interfacial bonding, and enabling the microcapsule's response to synergistically with the matrix's deformation.

[0009] Preferably, the shape memory polyester is a polylactic acid-polyethylene glycol block copolymer (PLA-PEG-PLA) or a polycaprolactone-polyethylene terephthalate block copolymer (PCL-PET). By adjusting the ratio of soft segments (such as PEG, PCL) and hard segments (such as PLA, PET), its shape memory transition temperature Tm can be precisely controlled between 35-60°C, making it closer to human body temperature or daily ambient temperature, thus facilitating its application.

[0010] Preferably, the wall material of the thermochromic microcapsule is a polymer that can react with a shape memory polyester matrix, and the core material is a ternary composite thermochromic composition containing an electron donor, an electron acceptor, and a solvent. This microcapsule exhibits typical reversible thermochromic properties, and the color-changing temperature can be adjusted by regulating the core material composition.

[0011] Preferably, the wall material is an acrylate copolymer containing epoxy groups or carboxyl groups. These active groups can react with the terminal carboxyl or hydroxyl groups of the polyester during polymerization to form chemical bonds. In the core material, the electron donor is crystal violet lactone (CVL), the electron acceptor is bisphenol A, and the solvent is a higher fatty alcohol (such as tetradecyl alcohol or hexadecyl alcohol). The melting point of the solvent determines the color change temperature.

[0012] Preferably, the thermochromic microcapsules have an average particle size of 1-10 μm and a mass fraction of 5-20% in the film. Within this range, the film can achieve a significant color change with minimal impact on the mechanical properties of the substrate.

[0013] Preferably, the initial shape of the film is planar, and it can be deformed into a second shape (such as bending or spiraling) by external force when the temperature is above the transition temperature. After cooling, the shape can be fixed. When it is heated again to above the transition temperature, the film can return to the initial planar shape, and the color undergoes a reversible change (such as changing from colored to colorless, or changing from one color to another).

[0014] The present invention also provides a method for preparing the above-mentioned thermochromic polyester film with memory function, comprising the following steps: (1) Preparing reactive thermochromic microcapsules: using interfacial polymerization or in-situ polymerization, a polymer containing reactive functional groups (such as epoxy-containing acrylate resin) is used as the wall material to coat a thermochromic core material composed of crystal violet lactone, bisphenol A and tetradecyl alcohol, to obtain thermochromic microcapsules with epoxy groups on the surface. The particle size of the microcapsules is controlled by controlling the emulsification stirring speed. (2) Preparing thermochromic polyester chips: using monomers (lactide, PEG) or prepolymers for preparing shape memory polyester (such as PLA-PEG-PLA) as raw materials, the reactive thermochromic microcapsules and catalyst from step (1) are added, and ring-opening polymerization is carried out in a reactor. During the polymerization process, the epoxy groups on the surface of the microcapsules react with the carboxyl or hydroxyl groups at the ends of the PLA chains, so that the microcapsules are chemically bonded to the polyester molecular chains. After the reaction is completed, the thermochromic polyester chips are obtained by extrusion, pelletizing and drying. (3) Preparation of thermochromic polyester film: After drying the thermochromic polyester chips obtained in step (2), they are melt-extruded and cast into sheets. Then, they are stretched unidirectionally or bidirectionally at a temperature below the transition temperature (e.g., room temperature) to orient the molecular chains, thereby giving the material macroscopic shape memory ability. The stretched film is heat-set at a temperature above the transition temperature to eliminate internal stress and fix its macroscopic shape (e.g., planar).

[0015] The key to this method is that reactive microcapsules are directly involved in the polymerization reaction, making them part of the polymer network, thus ensuring their chemical bonding and uniform distribution in the matrix.

[0016] Preferably, in step (1), the particle size and particle size distribution of the microcapsules can be precisely controlled by adjusting the ratio of the aqueous phase and the oil phase, the type and amount of emulsifier, the stirring speed, etc.

[0017] Preferably, in step (2), the polymerization reaction temperature needs to be determined according to the selected monomer and catalyst. For example, the ring-opening polymerization temperature of lactide is 130-180℃.

[0018] Preferably, in step (3), the biaxial stretching temperature must be lower than the transformation temperature of the shape memory polyester to ensure that the orientation of the molecular chains can be effectively "frozen" in a non-equilibrium state during the stretching process. The stretching ratio is 2.0-3.5 times. The heat setting temperature needs to be higher than the transformation temperature so that the molecular chains obtain sufficient energy to perform micro Brownian motion, thereby fixing the macroscopic shape after stretching.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] High integration and stability of dual functions: Thermochromic microcapsules are firmly connected to the shape memory polyester network through chemical bonding, which fundamentally solves the problems of poor compatibility, easy shedding and rapid functional decay in physical blending, and achieves long-term stability of dual functions.

[0021] Intelligent Synergistic Response: Because the microcapsules form an integral whole with the matrix, they can move synchronously with the deformation and recovery of the matrix during shape memory deformation, without affecting their color-changing function. This allows the material to simultaneously and synergistically produce changes in shape and color under external temperature stimuli, resulting in a higher level of intelligence.

[0022] Excellent overall performance: The chemical bonding method ensures uniform dispersion of microcapsules, minimizing the negative impact on the mechanical properties of the matrix, while the film still maintains good flexibility and strength.

[0023] Designable response temperature: By changing the type and content of the soft segments of the shape memory polyester, as well as the type of solvent in the thermochromic microcapsule core material, the shape memory transition temperature and thermochromic temperature of the material can be designed independently or synergistically, achieving on-demand customization.

[0024] The preparation process is feasible: The preparation method provided by this invention is based on existing polymerization and thin film processing technologies. By introducing the concept of reactive microcapsules, chemical modification is achieved. The process route is clear and has industrialization prospects.

[0025] In summary, this invention provides a novel, functionally synergistic, and stable thermochromic polyester film with memory function, offering a new product form and preparation approach for the field of smart materials, and possessing significant theoretical and application value. Detailed Implementation

[0026] To further illustrate the present invention, the following detailed description of the thermochromic polyester film with memory function and its preparation method is provided in conjunction with specific embodiments. Unless otherwise specified, all raw materials used in the embodiments are commercially available chemically pure or industrial-grade products. Example 1

[0027] (1) Preparation of reactive thermochromic microcapsules:

[0028] Oil phase: Mix 10g crystal violet lactone (CVL), 20g bisphenol A, and 60g cetyl alcohol (melting point approximately 50℃), and heat to 70℃ to dissolve, serving as the core material. Add 30g of epoxy-containing methacrylate copolymer (serving as the wall material prepolymer) and 2g initiator AIBN, and stir until homogeneous to form the oil phase.

[0029] Aqueous phase: Dissolve 5g of polyvinyl alcohol (PVA, emulsifier) ​​in 200g of deionized water.

[0030] The oil phase was slowly added to the aqueous phase, and emulsified at 60°C with high-speed stirring (1000 rpm) for 30 minutes to form a stable O / W emulsion. The temperature was then raised to 75°C and the reaction was carried out for 4 hours to allow the wall material prepolymer to polymerize and solidify. After the reaction, the prepolymer was centrifuged, washed, and dried to obtain thermochromic microcapsules with epoxy groups on the surface, with an average particle size of approximately 5 μm.

[0031] (2) Preparation of thermochromic polyester chips (PLA-PEG-PLA matrix): In a dry polymerization flask, add 100g L-lactide, 20g polyethylene glycol (PEG, Mn=4000), 0.1g stannous octoate catalyst, and 10g of the microcapsules prepared in step (1). Replace the air in the flask three times with high-purity nitrogen. Stir the reaction in an oil bath at 150℃ for 24 hours. After the reaction is complete, pour the melt into a polytetrafluoroethylene mold to cool, then crush and pelletize to obtain thermochromic polyester chips. The mass fraction of microcapsules in the chips is approximately 7.7%.

[0032] (3) Preparation of thermochromic polyester film: Thermochromic polyester chips were vacuum dried at 50°C for 24 hours. An appropriate amount of dried chips were placed in a flat vulcanizing machine and melt-pressed at 160°C. Then, the chips were quickly cooled in ice water to obtain an amorphous sheet with a thickness of about 1 mm. The sheet was cut into appropriate sizes, preheated in an oven at 40°C (50°C below the melting point of cetyl alcohol), and then stretched unidirectionally by 2.5 times using a manual stretching machine. The stretched film was then cooled in air to fix the stretched shape. The stretched film was placed in an oven at 60°C (above the transition temperature) for heat setting for 15 minutes to obtain a uniaxially stretched thermochromic polyester film with memory function. Example 2

[0033] This embodiment is basically the same as Embodiment 1, except that in the core material of the thermochromic microcapsule, hexadecyl alcohol is replaced with tetradecyl alcohol (melting point about 38°C), which lowers the color change temperature. In step (2), the polymerization reaction time is 20 hours. In step (3), the stretching temperature is 35°C and the heat setting temperature is 55°C. Example 3

[0034] This embodiment is basically the same as Embodiment 1, except that the shape memory polyester matrix is ​​replaced with a PCL-PET block copolymer. First, a carboxyl-terminated PCL prepolymer is synthesized, and then it undergoes an ester exchange reaction with a PET prepolymer to obtain the PCL-PET block copolymer. The reactive microcapsules from step (1) are added during the ester exchange reaction. The stretching temperature for film preparation is set to 45°C based on the melting point of the PCL segments, and the heat setting temperature is set to 65°C.

[0035] Comparative Example 1

[0036] A comparative film was prepared using a physical blending method. Ordinary thermochromic microcapsules (with non-reactive functional groups PMMA wall material) prepared in Example 1 and without surface grafting of active groups were melt-blended with pure PLA-PEG-PLA chips (without microcapsules) synthesized in Example 1 at 160°C in a Hacker rheometer. The chips were then pressed into sheets and stretched and heat-set according to the steps in Example 1 to obtain the comparative film.

[0037] Performance Testing and Evaluation

[0038] The following performance tests were performed on the films prepared in Examples 1-3 and Comparative Example 1:

[0039] Thermochromic properties: Place the film on a heating stage and heat it from room temperature to 70°C. Observe the color change and the color change temperature. Record the color difference before and after the color change.

[0040] Shape memory performance: The film was cut into strips, heated in an oven 20°C above the transition temperature, bent into a U-shape, and quickly immersed in ice water to fix the shape. Then it was heated again to above the transition temperature, and its shape recovery time and recovery rate were recorded.

[0041] Microcapsule dispersibility: The dispersion of microcapsules was observed by examining the brittle fracture surface of the film using a scanning electron microscope (SEM).

[0042] Durability test: The film is subjected to 10 shape memory cycles (deformation-fixation-recovery) and its thermochromic properties are tested again to observe whether there is any decay.

[0043] The test results are summarized in Table 1:

[0044]

[0045] The results in Table 4 show that:

[0046] Color-changing performance: The color difference (ΔE) of the embodiments of the present invention is above 24, the color change is significant, and the color-changing temperature range is clear. In contrast, the color difference of Comparative Example 1 is smaller, and the color change is uneven.

[0047] Shape memory performance: The shape recovery time and recovery rate of the embodiments of the present invention are short, indicating that the shape memory function of the matrix is ​​well maintained. In contrast, the recovery rate and recovery time of Comparative Example 1 are worse, indicating that the aggregation of microcapsules and poor interfaces affect the molecular chain movement of the matrix.

[0048] Microcapsule dispersibility: SEM observation showed that the microcapsules in the embodiments of the present invention were uniformly dispersed in the matrix, tightly bound to the matrix, and without obvious interfacial voids. In contrast, the microcapsules in Comparative Example 1 showed obvious aggregation and surrounding voids, indicating poor interfacial bonding.

[0049] Durability: After 10 cycles, the color-changing performance of the embodiment of the present invention only slightly decreased, while the color-changing performance of Comparative Example 1 decreased significantly (ΔE decreased from 18 to 8), proving that chemical bonding greatly improves the durability of the function.

[0050] In summary, the thermochromic polyester film with memory function prepared in the embodiments of the present invention has microcapsules that are uniformly dispersed and firmly bonded in the matrix, achieving excellent dual-function response and good cycling stability, which is superior to the comparative example of physical blending, and fully demonstrates the technical advantages of the present invention.

[0051] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A thermochromic polyester film having a memory function, characterized by comprising: The film is composed of a shape memory polyester layer containing chemically bonded thermochromic microcapsules, and the film can undergo both shape and color changes under temperature stimulation.

2. The thermochromic polyester film having a memory function according to claim 1, characterized by, The shape memory polyester is a polylactic acid-polyethylene glycol block copolymer or a polycaprolactone-polyethylene terephthalate block copolymer, and its shape memory transition temperature Tm is between 35-60℃.

3. The thermochromic polyester film having a memory function according to claim 1, characterized by, The wall material of the thermochromic microcapsule is a polymer that can react with a shape memory polyester matrix, and the core material is a ternary composite thermochromic composition containing an electron donor, an electron acceptor, and a solvent.

4. The thermochromic polyester film having a memory function according to claim 3, characterized by, The wall material is an acrylate copolymer containing epoxy groups or carboxyl groups; in the core material, the electron donor is crystal violet lactone, the electron acceptor is bisphenol A, and the solvent is a higher fatty alcohol.

5. The thermochromic polyester film having a memory function according to claim 1, wherein, The thermochromic microcapsules have an average particle size of 1-10 μm and a mass fraction of 5-20% in the film.

6. The thermochromic polyester film having a memory function according to claim 1, wherein, The film is initially planar in shape. When the temperature is above the transition temperature, it can be deformed into a second shape by external force. After cooling, the shape can be fixed. When it is heated again to above the transition temperature, the film can return to its initial planar shape, and the color undergoes a reversible change.

7. A method of producing the thermochromic polyester film having a memory function according to any one of claims 1 to 6, characterized by, Includes the following steps: (1) Preparation of reactive thermochromic microcapsules: Using interfacial polymerization or in-situ polymerization, a polymer containing reactive functional groups is used as the wall material to coat the thermochromic core material, thereby obtaining thermochromic microcapsules with active functional groups on the surface. (2) Preparation of thermochromic polyester chips: Mix the monomer or prepolymer of shape memory polyester with the reactive thermochromic microcapsules of step (1) and carry out a polymerization reaction under the action of a catalyst, so that the microcapsules are chemically bonded to the polyester matrix through surface functional groups to obtain thermochromic polyester chips. (3) Preparation of thermochromic polyester film: After drying the thermochromic polyester chips obtained in step (2), the thermochromic polyester film with memory function is obtained by melt extrusion, casting, biaxial stretching and heat setting.

8. The method of claim 7, wherein, In step (1), during the preparation of the reactive thermochromic microcapsules, the particle size of the microcapsules is adjusted by controlling the stirring speed.

9. The preparation method according to claim 7, characterized in that, In step (2), the polymerization reaction is a condensation polymerization reaction or a ring-opening polymerization reaction, and the reaction temperature is 150-250℃.

10. The preparation method according to claim 7, characterized in that, In step (3), the biaxial stretching temperature is lower than the transition temperature of the shape memory polyester, the stretching ratio is 2.0-3.5 times, the heat setting temperature is higher than the transition temperature, and the time is 10-60 seconds, in order to fix the macroscopic shape of the film.

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