Microwave dielectric sensitive response nanofiber membrane as well as preparation method and application thereof

Abstract

The invention discloses a microwave dielectric sensitive response nanofiber membrane and a preparation method and application thereof. The preparation method comprises the steps of spinning solution preparation, MOF nanoparticle pretreatment and ultrasonic dispersion, electrostatic spinning, nanofiber membrane surface modification, precursor solution preparation, MOF nanoparticle in-situ growth and the like. By accurately controlling parameters of each step, the average fiber diameter of the prepared nanofiber membrane is 150nm, the uniformity deviation of the fiber diameter is within + / -5%, the loading rate of MOF nanoparticles reaches 40%, and the MOF nanoparticles are uniformly distributed. The nanofiber membrane has good mechanical properties, thermal stability, hydrophilicity and microwave dielectric sensitive response performance, and the heating rate in a 2.45 GHz microwave field can reach 5 DEG C / min. Meanwhile, the invention further discloses the nanofiber membrane prepared through the method and application of the nanofiber membrane in microwave food packaging, by reasonably designing the packaging structure, the problem that microwave heating of food is not uniform can be effectively solved, and the food heating efficiency and quality are improved.

Description

Technical Field

[0001] The invention relates to the technical field of nano material preparation, in particular to a microwave dielectric sensitive response nano fiber membrane and a preparation method and application thereof. Background Art

[0002] In the field of modern food processing and packaging, microwave heating has become a widely used heating method due to its high efficiency and convenience. However, the problem of uneven heating of food during microwave heating has always been a major challenge faced by this field. As microwaves propagate inside the food, different parts of the food absorb and scatter microwaves differently, causing some foods to be overheated, resulting in poor taste and loss of nutrients; while some foods are underheated and cannot achieve the expected eating effect, seriously affecting the quality and taste of the food.

[0003] In the prior art, in order to effectively solve the problem of uneven microwave heating, it is urgent to develop a material that can respond sensitively to microwaves and transfer heat evenly. Traditional food packaging materials, such as common paper and plastic packaging materials, cannot meet this special demand due to their own structural and performance limitations. These materials are difficult to interact effectively with microwaves in the microwave field, and cannot achieve efficient conversion and uniform distribution of microwave energy, so it is difficult to improve the heating effect of food.

[0004] As a material with a unique nanoscale structure, nanofiber membranes have shown great application potential in many fields in recent years, and have also provided new ideas for solving microwave heating problems. Nanofiber membranes have high specific surface area, good pore structure and excellent physical and chemical properties. In theory, they can produce specific interactions with microwaves to achieve the absorption, conversion and transmission of microwave energy. However, most of the existing research on nanofiber membranes focuses on their applications in filtration, adsorption, biomedicine and other fields. The exploration of nanofiber membranes and their preparation methods that can achieve microwave dielectric sensitive response, stable performance and simple preparation process is still in its infancy. The existing related research results have many shortcomings in terms of microwave response performance, stability and complexity of preparation process, and cannot meet the needs of actual production and application.

[0005] Therefore, it is necessary to provide a microwave dielectric sensitive nanofiber membrane and a preparation method and application thereof to solve the above technical problems. Summary of the invention

[0006] The purpose of the present invention is to provide a microwave dielectric sensitive response nanofiber membrane and its preparation method and application. By preparing a specific nanofiber membrane, it has good microwave response, mechanical, thermal stability and hydrophilic properties, so as to achieve uniform heating, improve food quality, and meet the packaging requirements in strength, gas exchange, heat transfer and moisture treatment.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] In one aspect, the present invention provides a method for preparing a microwave dielectric sensitive nanofiber membrane, comprising the following steps:

[0009] Step 1: Preparation of spinning solution

[0010] In terms of weight, 18 parts of polyvinylidene fluoride (PVDF) polymer material and 82 parts of N,N-dimethylformamide (DMF) solvent were weighed; the weighed PVDF was slowly added into DMF and stirred at a stirring speed of 200 r / min for 6 hours until the PVDF was completely dissolved to form a uniform spinning solution with a mass fraction of 18%;

[0011] Step 2: Pretreatment and ultrasonic dispersion of MOF nanoparticles

[0012] The pretreated MOF nanoparticles and the spinning solution were prepared as raw materials in a mass ratio of 1:5; the weighed MOF nanoparticles were added to the spinning solution, placed in an ultrasonic disperser, the ultrasonic power was set to 100W, and ultrasonic dispersion was performed for 30 minutes;

[0013] Step 3: Electrospinning

[0014] The spinning voltage of the spinning device is set to 25 kV, the solution flow rate is set to 0.5 mL / h, and the spinning solution prepared in step 2 is loaded into the spinning solution container on the device, the spinning solution container is provided with a nozzle, and the distance between the receiving device on the device and the nozzle is maintained at 15 cm; the electrostatic spinning device is started for spinning, and then a nanofiber membrane is formed on the receiving device, the average fiber diameter of which is 150 nm, and the fiber diameter uniformity deviation is within ±5%;

[0015] Step 4: Nanofiber membrane surface modification

[0016] After the nanofiber membrane is prepared in step 3, the nanofiber membrane is placed in a plasma treatment device to pre-treat its surface for 10 minutes;

[0017] Step 5: Preparation of precursor solution

[0018] In parts by weight, 5.6 parts of an iron source are weighed, wherein the iron source is ferric nitrate with an iron ion concentration of 0.1 mol / L, and 16.6 parts of an organic terephthalic acid ligand; the weighed iron source and terephthalic acid are added to an appropriate amount of deionized water, stirred and dissolved, and the total volume of the solution is controlled so that the iron ion concentration reaches 0.1 mol / L and the terephthalic acid concentration reaches 0.15 mol / L; then the pH value of the solution is adjusted to 5.5 with dilute hydrochloric acid or sodium hydroxide solution to prepare a precursor solution containing metal ions and organic ligands;

[0019] Step 6: In situ growth of MOF nanoparticles

[0020] The surface-modified nanofiber membrane is immersed in the precursor solution prepared in step five and placed in a reaction container; the temperature of the reaction system is controlled at 80°C by a constant temperature heating device, and the temperature fluctuation range is less than ±0.5°C; the reaction lasts for 12 hours. During the reaction, the growth of MOF nanoparticles is observed by scanning electron microscopy (SEM) every 1-2 hours to ensure that the loading rate of MOF nanoparticles on the nanofiber surface reaches 40% and is evenly distributed.

[0021] Preferably, in step 2, the pretreatment of MOF nanoparticles comprises the following steps:

[0022] S1. Cleaning: Disperse 100 parts of MOF nanoparticles in 600 parts of ethanol by weight, clean them by centrifugation, and repeat the washing for 3 times;

[0023] S2. Drying: The washed MOF nanoparticles are dried in a vacuum drying oven at 40-100° C. for 6-12 h to remove the solvent;

[0024] S3, crushing and grinding: grinding the dried MOF nanoparticles in a ball mill for 0.5-2h;

[0025] S4. Surface modification: The MOF nanoparticles were reacted with 5 parts of silane coupling agent at 50° C. under stirring for 2 h.

[0026] Preferably, in step 4, the thickness of the nanofiber membrane is 20 μm, its tensile strength reaches 10 MPa, and its elongation at break is 30%.

[0027] Preferably, in step 4, the porosity of the nanofiber membrane is 60%.

[0028] Preferably, in step 4, the mass loss of the nanofiber membrane is less than 5% within the temperature range of 20-150°C.

[0029] Preferably, in step 4, the surface of the nanofiber membrane is hydrophilically treated, and its water contact angle is less than 30°.

[0030] Preferably, in Step Four, the nanofiber membrane can generate a heating rate of 5 °C / min in a microwave field of 2.45 GHz.

[0031] Preferably, in Step Six, the uniformity of the distribution of the MOF nanoparticles on the nanofiber membrane is characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), and the elemental distribution deviation is less than 8%.

[0032] On the other hand, the present invention provides a nanofiber membrane, which is prepared according to the preparation method of a nanofiber membrane with microwave dielectric sensitive response provided above. The nanofiber membrane is loaded with MOF nanoparticles, and the MOF nanoparticles are one or more of iron-based, cobalt-based, and nickel-based. The average particle size of the MOF nanoparticles is 50 nm, and they are loaded on the nanofiber membrane through an in-situ growth process.

[0033] On the other hand, the present invention provides a microwave food packaging, which uses the above nanofiber membrane and includes the following preparation steps:

[0034] (1) Packaging structure design: A packaging structure composed of two food-grade packaging layers and a layer of nanofiber membrane with microwave dielectric sensitive response sandwiched in the middle;

[0035] (2) Selection of food-grade packaging layer: The food-grade packaging layer is made of polyethylene (PE) material with a thickness of 30 μm;

[0036] (3) Determination of the arrangement position of the nanofiber membrane: Determine the cold and hot spots inside the food through thermal imaging analysis and simulation calculation;

[0037] (4) Customization of the shape and size of the nanofiber membrane: Customize the shape and size of the nanofiber membrane according to the shape of the food so that the area coverage rate in the area where cold and hot spots are likely to appear in the food reaches 80%.

[0038] Compared with the prior art, the beneficial effects of the present invention are:

[0039] 1. The nanofiber membrane prepared by the present invention can reach a heating rate of 5 °C / min in a microwave field of 2.45 GHz, can quickly respond to microwaves, efficiently convert microwave energy into heat energy, and uniformly heat the food, effectively solving the problem of uneven heating of food during the microwave heating process, and significantly improving the heating efficiency and quality of food.

[0040] 2. The tensile strength of the nanofiber membrane prepared by the present invention reaches 10 MPa, the elongation at break is 30%, and the thickness is 20 μm. Such mechanical properties enable it to withstand a certain external force in practical application scenarios such as packaging, and it is not easy to break, ensuring the reliability and stability of the product.

[0041] 3. The porosity of the nanofiber membrane prepared by the present invention is 60%. During microwave heating, the pore structure is conducive to gas exchange, maintaining the freshness of food, preventing food spoilage due to gas accumulation, and promoting more uniform transfer of heat to the interior of the food.

[0042] 4. The nanofiber membrane prepared by the present invention has a mass loss of less than 5% in the temperature range of 20-150°C, indicating that it has good thermal stability. In common temperature change environments such as microwave heating, it can maintain the stability of structure and performance, ensure the safety and reliability of the product, and will not decompose or deteriorate due to temperature fluctuations.

[0043] 5. After the surface of the nanofiber membrane prepared by the present invention is hydrophilic, the water contact angle is less than 30°. Good hydrophilicity enables the nanofiber membrane to better absorb moisture in the food when in contact with the food, preventing moisture from accumulating inside the package and affecting the quality of the food. It also helps the transfer of heat and further improves the uniformity of food heating.

[0044] 6. The average particle size of the MOF nanoparticles loaded by the nanofiber membrane prepared by the present invention through the in-situ growth process is 50nm, the loading rate reaches 40% and the distribution is uniform. The element distribution deviation is less than 8% as characterized by scanning electron microscopy (SEM) and energy spectrum analysis (EDS). The uniformly distributed MOF nanoparticles give full play to the microwave dielectric sensitivity, so that the response of each part of the nanofiber membrane in the microwave field is consistent, thereby ensuring the uniformity of heat transfer and significantly enhancing the microwave dielectric sensitivity of the nanofiber membrane.

[0045] 7. The present invention applies the nanofiber membrane to microwave food packaging. By rationally designing the packaging structure, the nanofiber membrane is composed of two food-grade packaging layers (polyethylene (PE) material with a thickness of 30 μm) and a layer of nanofiber membrane sandwiched in the middle. The location of hot and cold spots inside the food is determined by thermal imaging analysis and simulation calculation, and the shape and size of the nanofiber membrane are customized according to the shape of the food, so that the area coverage rate of the hot and cold spots in the food is 80%, which specifically solves the problem of uneven heating of food during microwave heating, and greatly improves the practicality and functionality of microwave food packaging. DETAILED DESCRIPTION

[0046] The technical solution of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0047] Embodiment 1:

[0048] This embodiment provides a method for preparing a microwave dielectric sensitive nanofiber membrane, comprising the following steps:

[0049] Step 1: Preparation of spinning solution

[0050] In terms of weight, 18 parts of polyvinylidene fluoride (PVDF) polymer material and 82 parts of N,N-dimethylformamide (DMF) solvent were weighed; the weighed PVDF was slowly added into DMF and stirred at a stirring speed of 200 r / min for 6 hours until the PVDF was completely dissolved to form a uniform spinning solution with a mass fraction of 18%;

[0051] Step 2: Pretreatment and ultrasonic dispersion of MOF nanoparticles

[0052] The pretreated MOF nanoparticles and the spinning solution were prepared as raw materials in a mass ratio of 1:5; the weighed MOF nanoparticles were added to the spinning solution, placed in an ultrasonic disperser, the ultrasonic power was set to 100W, and ultrasonic dispersion was performed for 30 minutes;

[0053] Step 3: Electrospinning

[0054] The spinning voltage of the spinning device is set to 25 kV, the solution flow rate is set to 0.5 mL / h, and the spinning solution prepared in step 2 is loaded into the spinning solution container on the device, the spinning solution container is provided with a nozzle, and the distance between the receiving device on the device and the nozzle is maintained at 15 cm; the electrostatic spinning device is started for spinning, and then a nanofiber membrane is formed on the receiving device, the average fiber diameter of which is 150 nm, and the fiber diameter uniformity deviation is within ±5%;

[0055] Step 4: Nanofiber membrane surface modification

[0056] After the nanofiber membrane is prepared in step 3, the nanofiber membrane is placed in a plasma treatment device to pre-treat its surface for 10 minutes;

[0057] Step 5: Preparation of precursor solution

[0058] In parts by weight, 5.6 parts of an iron source are weighed, wherein the iron source is ferric nitrate with an iron ion concentration of 0.1 mol / L, and 16.6 parts of an organic terephthalic acid ligand; the weighed iron source and terephthalic acid are added to an appropriate amount of deionized water, stirred and dissolved, and the total volume of the solution is controlled so that the iron ion concentration reaches 0.1 mol / L and the terephthalic acid concentration reaches 0.15 mol / L; then the pH value of the solution is adjusted to 5.5 with dilute hydrochloric acid or sodium hydroxide solution to prepare a precursor solution containing metal ions and organic ligands;

[0059] Step 6: In situ growth of MOF nanoparticles

[0060] The surface-modified nanofiber membrane is immersed in the precursor solution prepared in step five and placed in a reaction container; the temperature of the reaction system is controlled at 80°C by a constant temperature heating device, and the temperature fluctuation range is less than ±0.5°C; the reaction lasts for 12 hours. During the reaction, the growth of MOF nanoparticles is observed by scanning electron microscopy (SEM) every 1-2 hours to ensure that the loading rate of MOF nanoparticles on the nanofiber surface reaches 40% and is evenly distributed.

[0061] The preparation method of the nanofiber membrane with microwave dielectric sensitive response provided by the present invention can prepare a nanofiber membrane with specific properties through a series of precisely controlled steps. The nanofiber membrane can produce a rapid and stable temperature rise response in a microwave field, and the heating rate can reach 5°C / min, which effectively solves the problem of uneven heating of food during microwave heating. At the same time, the nanofiber membrane has good mechanical properties, with a tensile strength of 10MPa and an elongation at break of 30%, which can meet the strength requirements in practical applications. Its porosity is 60%, which is conducive to gas exchange and heat transfer. Within the temperature range of 20-150°C, the mass loss is less than 5%, and it has good thermal stability. After hydrophilic treatment, the water contact angle is less than 30°, and the surface hydrophilicity is good. In addition, the average particle size of the MOF nanoparticles loaded by the in-situ growth process is 50nm, the loading rate reaches 40% and is evenly distributed, and the element distribution deviation is less than 8%, which significantly enhances the microwave dielectric sensitivity of the nanofiber membrane.

[0062] More specifically, in step 2, the pretreatment of MOF nanoparticles comprises the following steps:

[0063] S1. Cleaning: Disperse 100 parts of MOF nanoparticles in 600 parts of ethanol by weight, clean them by centrifugation, and repeat the washing for 3 times;

[0064] S2. Drying: The washed MOF nanoparticles are dried in a vacuum drying oven at 40-100° C. for 6-12 h to remove the solvent;

[0065] S3, crushing and grinding: grinding the dried MOF nanoparticles in a ball mill for 0.5-2h;

[0066] S4. Surface modification: The MOF nanoparticles were reacted with 5 parts of silane coupling agent at 50° C. under stirring for 2 h.

[0067] In this embodiment, the pretreatment steps of MOF nanoparticles are refined, including washing, drying, crushing and grinding, and surface modification. The washing step removes impurities on the surface of MOF nanoparticles and improves their purity; the drying step completely removes the solvent to prevent interference with subsequent experiments; crushing and grinding make the nanoparticle size more uniform, which is conducive to dispersion in the spinning solution; surface modification enhances the compatibility of MOF nanoparticles with the spinning solution, further improves its dispersibility and stability in the nanofiber membrane, thereby improving the overall performance of the nanofiber membrane.

[0068] More specifically, in step 4, the thickness of the nanofiber membrane is 20 μm, its tensile strength reaches 10 MPa, and its elongation at break is 30%.

[0069] In this embodiment, it is clear that the thickness of the nanofiber membrane is 20 μm, the tensile strength reaches 10 MPa, and the elongation at break is 30%. The appropriate thickness ensures the use effect of the nanofiber membrane in practical applications, and the good mechanical properties enable it to withstand certain external forces in application scenarios such as packaging, and it is not easy to break, thereby improving the reliability of the product.

[0070] More specifically, in step 4, the porosity of the nanofiber membrane is 60%.

[0071] In this embodiment, the porosity of the nanofiber membrane is 60%. The appropriate porosity is conducive to gas exchange and heat transfer. During the microwave heating process, heat can be transferred to the interior of the food more evenly, while ensuring the freshness of the food and preventing food spoilage due to gas accumulation.

[0072] More specifically, in step 4, the mass loss of the nanofiber membrane is less than 5% within the temperature range of 20-150°C.

[0073] In this embodiment, within the temperature range of 20-150°C, the mass loss of the nanofiber membrane is less than 5%, indicating that it has good thermal stability. During microwave heating, the structure and performance can be kept stable within a wide temperature range, and decomposition or performance degradation will not occur due to temperature changes, ensuring the safety and reliability of the product.

[0074] More specifically, in step 4, the surface of the nanofiber membrane is hydrophilically treated, and its water contact angle is less than 30°.

[0075] In this embodiment, the surface of the nanofiber membrane is hydrophilically treated and the water contact angle is less than 30°. Good hydrophilicity is beneficial for the nanofiber membrane to better absorb moisture in food when it comes into contact with food, preventing moisture from accumulating inside the package and affecting food quality. It also helps in heat transfer.

[0076] More specifically, in step 4, the nanofiber membrane can generate a heating rate of 5°C / min in a 2.45 GHz microwave field.

[0077] In this embodiment, the nanofiber membrane can produce a heating rate of 5°C / min in a 2.45GHz microwave field. This characteristic enables the nanofiber membrane to respond quickly during microwave heating, effectively convert microwave energy into thermal energy, and heat food evenly, solving the problem of uneven microwave heating and improving the efficiency and quality of food heating.

[0078] More specifically, in step six, the distribution uniformity of the MOF nanoparticles on the nanofiber membrane is characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), and the element distribution deviation is less than 8%.

[0079] In this embodiment, the element distribution deviation of MOF nanoparticles on the nanofiber membrane is less than 8% through scanning electron microscopy (SEM) and energy spectrum analysis (EDS), indicating that the MOF nanoparticles are evenly distributed on the nanofiber membrane. The evenly distributed MOF nanoparticles can give full play to their microwave dielectric sensitivity, so that the nanofiber membrane responds consistently at all parts in the microwave field, thereby ensuring the uniformity of heat transfer.

[0080] A microwave food package using the nanofiber membrane provided in this embodiment comprises the following preparation steps:

[0081] (1) Packaging structure design: The packaging structure consists of two food-grade packaging layers and a layer of microwave dielectric sensitive nanofiber membrane sandwiched in between;

[0082] (2) Food-grade packaging layer selection: The food-grade packaging layer is made of polyethylene (PE) material with a thickness of 30 μm;

[0083] (3) Determine the location of the nanofiber membrane: Determine the hot and cold spots inside the food through thermal imaging analysis and simulation calculation;

[0084] (4) Customization of nanofiber membrane shape and size: The shape and size of the nanofiber membrane can be customized according to the appearance of the food, so that its area coverage rate in the hot and cold spots of the food can reach 80%.

[0085] Performance testing and evaluation of the nanofiber membrane in this example:

[0086] 1. Microwave dielectric properties test: The microwave dielectric properties of the prepared nanofiber membrane loaded with MOF nanoparticles were tested using vector network analyzer and other equipment; the test results showed that the real part of its complex dielectric constant reached 6-8 in the frequency range of 2-4GHz, and the imaginary part reached 3-5 in the frequency range of 4-6GHz, indicating that the nanofiber membrane has excellent microwave dielectric sensitivity, can achieve precise directional control in the microwave field, and can respond quickly and produce corresponding physical changes in the microwave environment.

[0087] 2. Mechanical properties testing: The nanofiber membrane was subjected to a tensile test using a material testing machine, and the tensile strength was found to be 10MPa and the elongation at break was 30%. It has good mechanical properties and can meet the mechanical requirements of stretching, bending, etc. in practical applications. It is not easy to break or be damaged during use.

[0088] 3. Porosity detection: The porosity of the nanofiber membrane was detected by mercury intrusion method and other methods, and the results showed that the porosity was 60%. This pore structure is conducive to gas exchange and material transmission. Under the action of microwaves, the pores have a certain scattering and absorption effect on microwaves, which enhances the microwave dielectric response effect and enables the nanofiber membrane to interact with microwaves more effectively.

[0089] 4. Thermal stability test: The thermal stability of the nanofiber membrane was tested using a thermogravimetric analyzer. The mass loss was less than 5% in the temperature range of 20-150°C, indicating that the nanofiber membrane has good thermal stability and can maintain the stability of its structure and performance under common high-temperature environments such as microwave heating, ensuring that its function will not be affected by temperature changes in actual applications.

[0090] 5. Hydrophilicity test: The water contact angle on the surface of the nanofiber membrane is tested by a contact angle meter. After hydrophilic treatment, the water contact angle is less than 30°. The hydrophilic surface is conducive to contact with water-containing substances such as food. During microwave heating, it can improve the heat transfer efficiency, promote the uniformity of food heating, and avoid local overheating or insufficient heating.

[0091] 6. Distribution uniformity detection: Scanning electron microscopy (SEM) and energy dispersive spectrum analysis (EDS) were used to characterize the distribution uniformity of MOF nanoparticles on the nanofiber membrane, and the element distribution deviation was less than 8%. This uniform distribution ensures the consistency of the overall microwave dielectric properties of the nanofiber membrane, so that each part of the membrane can exhibit similar response characteristics in the microwave field.

[0092] 7. Durability test: The nanofiber membrane was subjected to 50 microwave cycles, each cycle included heating in the microwave field for a certain period of time and then cooling to room temperature; after treatment, its microwave dielectric properties were tested, and the results showed that the retention rate of its microwave dielectric properties was greater than 85%; good durability and stability enable it to be used in microwave-related fields for a long time, such as microwave food packaging, microwave catalysis, etc., reducing the problem of frequent material replacement due to performance degradation.

[0093] Embodiment 2:

[0094] The difference between Example 2 and Example 1 is only that:

[0095] Step 2, pretreatment and ultrasonic dispersion of MOF nanoparticles: 100g of MOF nanoparticles that had been washed (dispersed in 600g of ethanol, centrifuged and washed three times), dried in a vacuum oven at 100°C for 6h, ground in a ball mill for 0.5h, and stirred with 5g of silane coupling agent at 50°C for 2h were mixed with 500g of the above spinning solution at a mass ratio of 1:5. The mixed solution was placed in an ultrasonic disperser and dispersed at 100W ultrasonic power for 30min.

[0096] Performance test of Example 2: The prepared nanofiber membrane has a thickness of 20 μm, a tensile strength of 9.8 MPa, an elongation at break of 29.8%, a porosity of 59.8%, a mass loss of 4.6% in the temperature range of 20-150°C, a water contact angle of 29° after hydrophilic treatment on the surface, a heating rate of 4.9°C / min in a 2.45 GHz microwave field, and an element distribution deviation of MOF nanoparticles on the nanofiber membrane of 7.8%.

[0097] Comparative Example 1:

[0098] Step 1, preparation of spinning solution: Weigh 18g of polyvinylidene fluoride (PVDF) polymer material and 82g of N,N-dimethylformamide (DMF) solvent. Slowly add PVDF into DMF and stir at a stirring speed of 150r / min for 8h to form a uniform spinning solution with a mass fraction of 18%.

[0099] Step 2: Pretreatment and ultrasonic dispersion of MOF nanoparticles: 100 g of MOF nanoparticles that have not been washed, dried, crushed, ground, or surface-modified are mixed with 500 g of the above spinning solution at a mass ratio of 1:5. The mixed solution is placed in an ultrasonic disperser and dispersed at 80 W ultrasonic power for 40 min.

[0100] Step 3, electrospinning: Set the spinning voltage of the spinning equipment to 20 kV, the solution flow rate to 0.8 mL / h, and put the prepared spinning solution into the spinning solution container on the equipment. The spinning solution container is provided with a nozzle, and the distance between the receiving device on the equipment and the nozzle is maintained at 12 cm. Start the electrospinning equipment for spinning, and form a nanofiber membrane on the receiving device.

[0101] Step 4: Surface modification of nanofiber membrane: Place the prepared nanofiber membrane into a plasma treatment device for pretreatment for 8 minutes.

[0102] Step 5. Preparation of precursor solution: weigh 5.6 g of ferric nitrate (iron ion concentration 0.1 mol / L), 16.6 g of terephthalic acid organic ligand, add appropriate amount of deionized water and stir to dissolve, control the total volume of the solution so that the iron ion concentration reaches 0.1 mol / L, the terephthalic acid concentration reaches 0.15 mol / L, adjust the pH value of the solution to 5.0 with dilute hydrochloric acid, and prepare a precursor solution.

[0103] Step 6: In-situ growth of MOF nanoparticles: Immerse the surface-modified nanofiber membrane in the precursor solution and place it in a reaction vessel. The temperature of the reaction system is controlled at 75°C by a constant temperature heating device, and the temperature fluctuation range is less than ±1°C. The reaction lasts for 10 hours. The growth of MOF nanoparticles is observed every 2 hours by scanning electron microscopy (SEM).

[0104] Performance test of Comparative Example 1: The prepared nanofiber membrane has a thickness of 22 μm, a tensile strength of 8 MPa, an elongation at break of 25%, a porosity of 55%, a mass loss of 8% in the temperature range of 20-150°C, an unhydrophilic surface treatment, a water contact angle of 60°, a heating rate of 3°C / min in a 2.45 GHz microwave field, and an element distribution deviation of MOF nanoparticles on the nanofiber membrane of 15%.

[0105] Comparative Example 2:

[0106] Step 1, preparation of spinning solution: weigh 20g polyvinylidene fluoride (PVDF) polymer material and 80g N,N-dimethylformamide (DMF) solvent. Slowly add PVDF into DMF and stir at a stirring speed of 250r / min for 5h to form a uniform spinning solution with a mass fraction of 20%.

[0107] Step 2: Pretreatment and ultrasonic dispersion of MOF nanoparticles: 100 g of MOF nanoparticles that had been washed (dispersed in 600 g of ethanol, centrifuged and washed three times), dried in a vacuum oven at 60°C for 9 h, and ground in a ball mill for 1 h but not surface-modified were mixed with 500 g of the above spinning solution at a mass ratio of 1:5. The mixed solution was placed in an ultrasonic disperser and dispersed at an ultrasonic power of 120 W for 25 min.

[0108] Step 3, electrospinning: Set the spinning voltage of the spinning equipment to 28 kV, the solution flow rate to 0.3 mL / h, and put the prepared spinning solution into the spinning solution container on the equipment. The spinning solution container is provided with a nozzle, and the distance between the receiving device on the equipment and the nozzle is maintained at 18 cm. Start the electrospinning equipment for spinning, and form a nanofiber membrane on the receiving device.

[0109] Step 4: Surface modification of nanofiber membrane: Place the prepared nanofiber membrane in a plasma treatment device for pretreatment for 12 minutes.

[0110] Step 5. Preparation of precursor solution: weigh 6 g of ferric nitrate (iron ion concentration 0.1 mol / L), 18 g of terephthalic acid organic ligand, add appropriate amount of deionized water and stir to dissolve, control the total volume of the solution so that the iron ion concentration reaches 0.1 mol / L, and the terephthalic acid concentration reaches 0.18 mol / L, adjust the solution pH to 6.0 with sodium hydroxide solution, and prepare a precursor solution.

[0111] Step 6: In-situ growth of MOF nanoparticles: Immerse the surface-modified nanofiber membrane in the precursor solution and place it in a reaction vessel. The temperature of the reaction system is controlled at 85°C by a constant temperature heating device, and the temperature fluctuation range is less than ±0.8°C. The reaction lasts for 14 hours. The growth of MOF nanoparticles is observed every hour by scanning electron microscopy (SEM).

[0112] Performance test: The prepared nanofiber membrane has a thickness of 18μm, a tensile strength of 9MPa, an elongation at break of 28%, a porosity of 58%, a mass loss of 6% in the temperature range of 20-150°C, a water contact angle of 35° after hydrophilic treatment on the surface, a heating rate of 4°C / min in a 2.45GHz microwave field, and an element distribution deviation of MOF nanoparticles on the nanofiber membrane of 12%.

[0113] Parameter comparison between the above comparative examples and embodiments:

[0114]

[0115]

[0116] Test performance comparison:

[0117]

[0118] By comparing and analyzing the experimental data of Example 1, Example 2, Comparative Example 1, and Comparative Example 2, the following conclusions can be drawn:

[0119] 1. The preparation method has a significant impact on the performance of the nanofiber membrane

[0120] - Spinning solution preparation parameters: In Examples 1 and 2, the spinning solution formed under the conditions of a stirring speed of 200 r / min, a stirring time of 6 h, and a specific ratio of PVDF to DMF is helpful for preparing a nanofiber membrane with good performance. In Comparative Example 1, the stirring speed and time were changed, and in Comparative Example 2, the ratio of PVDF to DMF was changed, resulting in the nanofiber membrane having inferior performance in terms of tensile strength, elongation at break, porosity, microwave heating rate, etc. compared to the examples.

[0121] - MOF nanoparticle pretreatment parameters: In Examples 1 and 2, the MOF nanoparticles were cleaned, dried, pulverized and ground, and surface modified, making the nanofiber membrane perform well in terms of mechanical properties, microwave dielectric sensitivity properties, etc. In Comparative Example 1, no pretreatment was carried out, and in Comparative Example 2, no surface modification was carried out, resulting in a large deviation in the elemental distribution of the MOF nanoparticles in the nanofiber membrane and a slow microwave heating rate, with overall poor performance.

[0122] - Ultrasonic dispersion parameters: In the examples, ultrasonic dispersion at a power of 100 W for 30 min is beneficial for the uniform dispersion of MOF nanoparticles in the spinning solution, thereby improving the performance of the nanofiber membrane. In Comparative Example 1, the power was 80 W and the time was 40 min, and in Comparative Example 2, the power was 120 W and the time was 25 min, neither of which achieved the best dispersion effect, affecting the performance of the nanofiber membrane.

[0123] - Electrospinning parameters: In Examples 1 and 2, with a spinning voltage of 25 kV, a solution flow rate of 0.5 mL / h, and a distance between the nozzle and the receiving device of 15 cm, the prepared nanofiber membrane has good uniformity of the average fiber diameter and excellent comprehensive performance. After changing these parameters in Comparative Examples 1 and 2, the performance of the nanofiber membrane decreased to varying degrees.

[0124] - Nanofiber membrane surface modification time: In the examples, a surface modification time of 10 min makes the nanofiber membrane perform well in the subsequent in-situ growth of MOF nanoparticles and overall performance. In Comparative Example 1, it was 8 min, and in Comparative Example 2, it was 12 min, and the performance was inferior to that of the examples.

[0125] - Precursor solution preparation parameters: In Examples 1 and 2, the amounts of iron source ferric nitrate and terephthalic acid and the condition of a pH value of 5.5 are beneficial for the in-situ growth of MOF nanoparticles and the improvement of the performance of the nanofiber membrane. After changing these parameters in Comparative Examples 1 and 2, the performance of the nanofiber membrane was affected.

[0126] -MOF nanoparticle in-situ growth parameters: In Examples 1 and 2, the reaction temperature of 80°C, the reaction time of 12h and SEM observation every 1-2h made the MOF nanoparticle loading rate reach 40% and the distribution was uniform, which enhanced the microwave dielectric sensitivity of the nanofiber membrane. After these parameters were changed in Comparative Examples 1 and 2, the MOF nanoparticle distribution deviation was large and the performance decreased.

[0127] 2. The preparation method of the present invention has advantages: According to the preparation method of the present invention (Examples 1 and 2), the prepared nanofiber membrane has an appropriate thickness, a tensile strength of about 10MPa, an elongation at break of about 30%, a porosity of about 60%, a mass loss of less than 5% in the temperature range of 20-150°C, good surface hydrophilicity (water contact angle less than 30°), a heating rate of about 5°C / min in a 2.45GHz microwave field, and an element distribution deviation of MOF nanoparticles on the nanofiber membrane of less than 8%. These performance indicators show that the nanofiber membrane prepared by the present invention has excellent mechanical properties, thermal stability, hydrophilicity and microwave dielectric sensitive response performance, and can effectively solve the problem of uneven microwave heating of food and improve the efficiency and quality of food heating.

[0128] 3. Parameter fine-tuning has a certain impact on the performance but does not change the overall advantage: Examples 1 and 2 have fine-tuning differences in some parameters, such as the drying temperature and grinding time in the pretreatment of MOF nanoparticles, but the overall performance is similar, indicating that within the parameter range of the present invention, the nanofiber membrane can still maintain good performance by appropriately fine-tuning some parameters, and has a certain degree of process flexibility.

[0129] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the present invention can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, the embodiments should be considered exemplary and non-restrictive in all respects, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims be included in the present invention.

Claims

1. A method for preparing a microwave dielectric sensitive nanofiber membrane, characterized in that: The following steps are involved: Step 1: Preparation of spinning solution In terms of weight, 18 parts of polyvinylidene fluoride (PVDF) polymer material and 82 parts of N,N-dimethylformamide (DMF) solvent were weighed; the weighed PVDF was slowly added into DMF and stirred at a stirring speed of 200 r / min for 6 hours until the PVDF was completely dissolved to form a uniform spinning solution with a mass fraction of 18%; Step 2: Pretreatment and ultrasonic dispersion of MOF nanoparticles The pretreated MOF nanoparticles and the spinning solution were prepared as raw materials in a mass ratio of 1:5; the weighed MOF nanoparticles were added to the spinning solution, placed in an ultrasonic disperser, the ultrasonic power was set to 100W, and ultrasonic dispersion was performed for 30 minutes; Step 3: Electrospinning The spinning voltage of the spinning device is set to 25 kV, the solution flow rate is set to 0.5 mL / h, and the spinning solution prepared in step 2 is loaded into the spinning solution container on the device, the spinning solution container is provided with a nozzle, and the distance between the receiving device on the device and the nozzle is maintained at 15 cm; the electrostatic spinning device is started for spinning, and then a nanofiber membrane is formed on the receiving device, the average fiber diameter of which is 150 nm, and the fiber diameter uniformity deviation is within ±5%; Step 4: Nanofiber membrane surface modification After the nanofiber membrane is prepared in step 3, the nanofiber membrane is placed in a plasma treatment device to pre-treat its surface for 10 minutes; Step 5: Preparation of precursor solution In parts by weight, 5.6 parts of an iron source are weighed, wherein the iron source is ferric nitrate with an iron ion concentration of 0.1 mol / L, and 16.6 parts of an organic terephthalic acid ligand; the weighed iron source and terephthalic acid are added to an appropriate amount of deionized water, stirred and dissolved, and the total volume of the solution is controlled so that the iron ion concentration reaches 0.1 mol / L and the terephthalic acid concentration reaches 0.15 mol / L; then the pH value of the solution is adjusted to 5.5 with dilute hydrochloric acid or sodium hydroxide solution to prepare a precursor solution containing metal ions and organic ligands; Step 6: In situ growth of MOF nanoparticles The surface-modified nanofiber membrane is immersed in the precursor solution prepared in step five and placed in a reaction container; the temperature of the reaction system is controlled at 80°C by a constant temperature heating device, and the temperature fluctuation range is less than ±0.5°C; the reaction lasts for 12 hours. During the reaction, the growth of MOF nanoparticles is observed by scanning electron microscopy (SEM) every 1-2 hours to ensure that the loading rate of MOF nanoparticles on the nanofiber surface reaches 40% and is evenly distributed.

2. The method for preparing a microwave dielectric sensitive nanofiber membrane according to claim 1, characterized in that: In step 2, the pretreatment of MOF nanoparticles includes the following steps: S1. Cleaning: Disperse 100 parts of MOF nanoparticles in 600 parts of ethanol by weight, clean them by centrifugation, and repeat the washing for 3 times; S2. Drying: The washed MOF nanoparticles are dried in a vacuum drying oven at 40-100° C. for 6-12 h to remove the solvent; S3, crushing and grinding: grinding the dried MOF nanoparticles in a ball mill for 0.5-2h; S4. Surface modification: The MOF nanoparticles were reacted with 5 parts of silane coupling agent at 50° C. under stirring for 2 h.

3. The method for preparing a microwave dielectric sensitive nanofiber membrane according to claim 2, characterized in that: In step 4, the thickness of the nanofiber membrane is 20 μm, the tensile strength thereof reaches 10 MPa, and the elongation at break is 30%.

4. The method for preparing a microwave dielectric sensitive nanofiber membrane according to claim 2, characterized in that: In step 4, the porosity of the nanofiber membrane is 60%.

5. The method for preparing a microwave dielectric sensitive nanofiber membrane according to claim 2, characterized in that: In step 4, the mass loss of the nanofiber membrane is less than 5% within the temperature range of 20-150°C.

6. The method for preparing a microwave dielectric sensitive nanofiber membrane according to claim 2, characterized in that: In step 4, the surface of the nanofiber membrane is hydrophilically treated, and its water contact angle is less than 30°.

7. The microwave dielectric sensitive nanofiber membrane and the preparation method thereof according to claim 2, characterized in that: In step four, the nanofiber membrane can generate a heating rate of 5°C / min in a 2.45 GHz microwave field.

8. The method for preparing a microwave dielectric sensitive nanofiber membrane according to claim 2, characterized in that: In step six, the distribution uniformity of the MOF nanoparticles on the nanofiber membrane is characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), and the element distribution deviation is less than 8%.

9. A nanofiber membrane, prepared according to the method for preparing a microwave dielectric sensitive nanofiber membrane according to any one of claims 1 to 8, characterized in that: The nanofiber membrane is loaded with MOF nanoparticles, which are one or more of iron-based, cobalt-based and nickel-based. The average particle size of the MOF nanoparticles is 50 nm, and the MOF nanoparticles are loaded on the nanofiber membrane through an in-situ growth process.

10. A microwave food package using the nanofiber film according to claim 9, characterized in that: The preparation steps include: (1) Packaging structure design: The packaging structure consists of two food-grade packaging layers and a layer of microwave dielectric sensitive nanofiber membrane sandwiched in between; (2) Food-grade packaging layer selection: The food-grade packaging layer is made of polyethylene (PE) material with a thickness of 30 μm; (3) Determine the location of the nanofiber membrane: Determine the hot and cold spots inside the food through thermal imaging analysis and simulation calculation; (4) Customization of nanofiber membrane shape and size: The shape and size of the nanofiber membrane can be customized according to the appearance of the food, so that its area coverage rate in the hot and cold spots of the food can reach 80%.

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