Gamma-phase PVDF / PMMA composite oriented film and preparation method thereof

Abstract

The invention belongs to the technical field of polymer film processing, and particularly relates to a gamma-phase PVDF / PMMA (Polyvinylidene Fluoride / Polymethyl Methacrylate) composite oriented film and a preparation method thereof. The film is prepared from PVDF (Polyvinylidene Fluoride) and PMMA (Polymethyl Methacrylate), wherein the mass percent of the PVDF to the PMMA is 70%-90 And the content of the gamma phase in the film is 85-100%. According to the method, the scientific principles that PMMA inhibits formation of a non-polar alpha phase and melt stretching promotes nucleation and growth of a polar gamma phase are utilized, the PMMA and the polar gamma phase cooperate, the high-content gamma phase is rapidly prepared in a wide melt stretching processing temperature and strain window, and therefore high production flexibility is achieved. The preparation process of the gamma-phase PVDF / PMMA composite oriented film provided by the invention not only has high stability and strong controllability, but also has the characteristics of high efficiency and environmental friendliness, and is suitable for large-scale industrial popularization and application.

Description

Technical Field

[0001] The invention belongs to the technical field of polymer film processing, and in particular relates to a gamma-phase PVDF / PMMA composite oriented film and a preparation method thereof. Background Art

[0002] Among the known electroactive polymers, polyvinylidene fluoride (PVDF) and its copolymers have the best ferroelectricity, piezoelectricity and pyroelectricity, and their film products show broad application prospects in the fields of flexible sensing, intelligent control and energy storage. The electroactivity of PVDF originates from the internal polar crystalline phases, mainly including β phase and γ phase, which have special segment conformations, thus forming a net dipole moment. However, under the usual melt processing and solidification conditions, PVDF mainly produces non-polar α phase, resulting in low or no electroactivity. Therefore, obtaining polar phase PVDF usually requires special processing and manufacturing processes. Compared with the polar β phase, the polar γ phase has higher thermal stability, higher Curie temperature and electrical breakdown strength, thereby improving the performance and life of PVDF-based piezoelectric and ferroelectric thin film devices in high temperature environments. Despite many efforts, it is still a major challenge to simply and efficiently prepare γ-phase PVDF-based films.

[0003] In order to prepare γ-phase PVDF-based films, it is usually necessary to introduce some special organic / inorganic fillers or additives. These fillers or additives can induce the formation of a large amount of polar γ phase in the PVDF matrix by promoting the α→γ solid-state phase transition or by enhancing the nucleation ability of the γ phase. However, the currently disclosed methods still have some shortcomings, including: (1) In order to achieve good dispersion of fillers or additives in the PVDF matrix, solution blending or solvent treatment is often used, involving the use and recovery of solvents, which is not only complex and costly, but also environmentally unfriendly; (2) The growth cycle of the γ phase is generally long, and the key process steps related to its formation process are usually time-consuming, often taking several hours to dozens of hours, which is not conducive to industrial production, whether for solid-state α→γ phase transition or direct nucleation of the melt.

[0004] The inventor has developed and disclosed a method for rapid preparation of a PVDF film with a high γ phase content (CN202410088473.5). This method adopts a melt stretching processing strategy, using the flow field to induce and accelerate the nucleation of the γ phase, thereby achieving rapid preparation of a PVDF film with a high γ phase content within 1 hour. This process does not use any solvent and is highly efficient. However, this technology has the disadvantage of a narrow processing temperature window, which has strict requirements on the processing technology and greatly limits the flexibility of production. The reason for this is mainly because the competitive growth of the non-polar α phase is prone to occur at other temperatures, resulting in the inhibition of the formation of the γ phase, which leads to a very narrow processing temperature window.

[0005] In summary, the current technology for efficiently preparing γ-phase PVDF-based films based on pure melt processing is still immature and rarely reported, which limits the large-scale application of γ-phase PVDF films as functional film devices, and more effective processing strategies need to be explored. Summary of the invention

[0006] The purpose of the present invention is to provide a γ-phase PVDF / PMMA composite oriented film and a preparation method thereof, which can realize the rapid preparation of a γ-phase film within a wider melt stretching processing temperature and strain window, thereby greatly improving the production flexibility and processing controllability, and is more conducive to the large-scale promotion and application of PVDF-based films.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0008] A γ-phase PVDF / PMMA composite oriented film is composed of PVDF and PMMA, the mass percentages of which are 70%-90%:10%-30%; the γ-phase content in the film is 85%-100%.

[0009] The PVDF is a high molecular weight linear homopolymer with a weight average molecular weight of 500000-600000. The use of high molecular weight PVDF can obtain high melt strength, which is conducive to the implementation of high temperature melt stretching operation.

[0010] The weight average molecular weight of the PMMA is 100,000-500,000, preferably 200,000, which ensures that the melt strength of the film can be improved while taking into account the ease of processing to a greater extent.

[0011] Preferably, the mass percentage of PVDF and PMMA in the film is 70%:30%. The present invention preferably has a higher PMMA ratio, which corresponds to a lower lower limit of the permissible melt stretching temperature, that is, a wider processing temperature window.

[0012] The present invention further provides a method for preparing the γ-phase PVDF / PMMA composite oriented film, which uses a mixture of PVDF and PMMA as a raw material, first performs melt blending and extrusion to obtain a prefabricated film, then heats the prefabricated film until it is fully melted and the thermal history is eliminated, and then cools the prefabricated film to perform melt stretching to control the orientation degree of the molecular chain; stops stretching and maintains a constant temperature to complete the isothermal crystallization of the PVDF component, and then cools and solidifies to obtain a PVDF / PMMA composite oriented film with a high γ-phase content.

[0013] In the mixture of PVDF and PMMA, the mass percentage of the two is 70%-90%: 10%-30%.

[0014] The melt blending extrusion can be carried out conventionally using a twin-screw extruder, such as but not limited to the following: the feeding section temperature is 170-190°C, the compression section temperature is 180-200°C, the homogenization section temperature is 200-220°C, the die head temperature is 180-190°C, and the screw speed is 30-60r / min. Of course, the present invention can also use polymer processing equipment such as a torque rheometer and a vacuum molding machine for melt blending and preparation of PVDF / PMMA prefabricated films.

[0015] The heating rate of the prefabricated film is 10-20°C / min, the melting temperature for eliminating the thermal history is 210-230°C, and the heat preservation time is 10min.

[0016] Further, the temperature is lowered to 150-168°C for melt stretching.

[0017] Furthermore, melt stretching is performed to achieve a molecular chain orientation of 30-35%. The molecular chain orientation is controlled by the applied tensile strain, and too high or too low orientation is not conducive to the formation of the γ phase. The molecular chain orientation is characterized by wide-angle X-ray scattering, and the molecular chain orientation is calculated by the orientation order parameter.

[0018] The strain rate applied during melt stretching is preferably 10-20s -1 , strain is 1.0-2.5.

[0019] The melt stretching equipment used in the present invention can be a conventional clamp-pull equipment, or a roller pull equipment commonly used in continuous industrial production. When a continuous roller pull equipment is used, it can be closely connected with a melt extrusion equipment to achieve continuous batch production.

[0020] Further, the stretching is stopped and the temperature is maintained for 15 minutes to complete the isothermal crystallization of the PVDF component. The purpose of this step is to achieve sufficient γ phase nucleation and crystal growth, followed by cooling and solidification. It should be noted that PVDF / PMMA remains stretched during the cooling and solidification process.

[0021] Specifically, the method for preparing the film of the present invention comprises:

[0022] (1) melt-blending and extruding PVDF and PMMA to prepare a prefabricated film;

[0023] (2) The prefabricated film is heated until it is fully melted and the thermal history is eliminated, and then the temperature is cooled to 150-168°C and a strain rate of 20s is applied. -1 , the stretching effect with strain of 1.0-2.5 achieves a molecular chain orientation of 30-35%;

[0024] (3) After the stretching stops, the temperature is maintained at a constant level for 15 minutes to complete the isothermal crystallization of the PVDF component, and then cooled and solidified to obtain a PVDF / PMMA composite oriented film with a high γ phase content.

[0025] There is no strict limit to the thickness of the prefabricated membrane, and the thickness has no effect on the γ phase content of the final product.

[0026] In the industrial processing of polymers, such as injection molding, extrusion, spinning, etc., flow fields are usually involved. The flow field can not only regulate the crystal phase structure, but also promote nucleation and accelerate crystallization, thus directly affecting the structure and properties of the final material.

[0027] The present invention introduces a second amorphous polymer component, PMMA, which is thermodynamically compatible with PVDF and can be well dispersed in the PVDF matrix by melt blending without the assistance of solvents; the presence of PMMA can effectively inhibit the nucleation and growth of the non-polar α phase, thereby giving the γ phase a competitive advantage in formation. Melt stretching significantly enhances the nucleation and growth rate of the γ phase by inducing molecular chain orientation. Therefore, the present invention comprehensively utilizes PMMA to inhibit the formation of the non-polar α phase, and combines the flow field, i.e., melt stretching, to promote the nucleation and growth of the γ phase. The two work together to achieve rapid preparation of a high content of γ phase within a wider melt stretching processing temperature and strain window, with high process stability, thus having higher production flexibility. In addition, in this implementation process, no solvent treatment is required, the steps are simple and controllable, and it is easy to expand to continuous industrial production.

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

[0029] (1) PMMA and PVDF in the present invention are thermodynamically compatible and can be well dispersed directly by melt blending without the need for solvent to assist dispersion. The process is simple, the operation is convenient, and the environment is friendly.

[0030] (2) PMMA in the present invention has strong electrical insulation properties, which can improve the electrical breakdown resistance of the γ-phase film under strong electric fields and reduce leakage current, thereby enhancing the reliability of the film in dielectric energy storage and other electrical applications;

[0031] (3) The addition of PMMA in the present invention can effectively inhibit the formation of non-polar α phase, thereby giving the γ phase nucleation and growth advantages, while melt stretching further enhances the nucleation and growth rate of the γ phase through molecular chain orientation. The two work together to achieve the preparation of the γ phase within a wider melt stretching processing temperature and strain window, with high process stability, strong controllability, and high production flexibility.

[0032] (4) The γ-phase composite film prepared in the present invention not only has a high polar γ-phase content, but also has a short preparation process. In particular, the key process steps involving the growth of the γ-phase only take 15 minutes, which is significantly lower than the preparation time of other existing γ-phase PVDF-based composite films. It is highly efficient and has good prospects for industrial application. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 The present invention is a process flow chart for preparing a γ-phase PVDF / PMMA composite oriented film;

[0034] Figure 2 This is a Fourier transform infrared spectrum of the PVDF / PMMA composite oriented film prepared by melt stretching in Example 1-9;

[0035] Figure 3 This is a Fourier infrared spectrum of the PVDF / PMMA composite oriented film prepared by melt stretching in Comparative Example 1-8;

[0036] Figure 4 This is a scanning electron microscope image of the PVDF / PMMA composite oriented film prepared by melt stretching in Example 14. DETAILED DESCRIPTION

[0037] The technical solution of the present invention is described below with specific embodiments, but the protection scope of the present invention is not limited thereto:

[0038] Examples 1-9

[0039] A method for preparing a γ-phase PVDF / PMMA composite oriented film comprises the following specific steps:

[0040] 1) 700 g of PVDF with a weight average molecular weight of 550,000 and 300 g of PMMA with a weight average molecular weight of 200,000 were weighed, melt-blended and extruded in a twin-screw extruder (model Haake PolyLab) to prepare a PVDF / PMMA pre-film, the pre-film thickness was 250 μm, and the mass fraction of PMMA was 30%.

[0041] The feeding section temperature of the twin-screw extruder is 180°C, the compression section temperature is 190°C, the homogenization section temperature is 210°C, the die head temperature is 190°C, and the screw speed is 30r / min.

[0042] 2) The prefabricated film obtained in step 1) is mounted and fixed on a melt stretching device (developed by Zhengzhou University, with the same structure as CN212964416U), and then heated to 220°C at a heating rate of 20°C / min to achieve complete melting of the prefabricated film, and kept warm for 10 minutes to eliminate the thermal history.

[0043] 3) The PVDF / PMMA melt obtained in step 2) was cooled to melt stretching temperatures of 150°C, 153°C, 156°C, 159°C, 162°C, 165°C, and 168°C, respectively, and then stretched for 20 seconds. -1 The stretching rate is stretched to 1.0, 1.5, 2.0, 2.5 strains to induce partial molecular chain orientation in the melt. Other equipment such as clamp pulling and roller pulling can also be used to achieve the same effect.

[0044] 4) The deformed melt obtained in step 3) is maintained at the stretching temperature for 15 minutes in a stretched state to complete isothermal crystallization, and then cooled and solidified to obtain different PVDF / PMMA composite oriented films.

[0045] The process flow chart of the preparation of the γ-phase PVDF / PMMA composite oriented film of the present invention is shown in Figure 1 .

[0046] The composite oriented films prepared in the above examples were characterized by Fourier infrared spectroscopy (model Nicolet6700), wide-angle X-ray diffractometer (model D8 Discovery) and scanning electron microscope (model CzechTESCAN MIRALMS). The results showed that the internal crystal form of the film was mainly γ phase, indicating that the above method can quickly prepare PVDF / PMMA composite oriented films with high γ phase content.

[0047] The characterization methods of Fourier infrared spectroscopy, wide-angle X-ray diffractometer and scanning electron microscope are all well known in the art.

[0048] Table 1 lists information such as the content of γ phase in the composite oriented films obtained in Examples 1-9 using different melt stretching temperatures, melt stretching strains, and molecular chain orientation degrees.

[0049] Table 1

[0050]

[0051] Examples 10-18

[0052] A method for preparing a γ-phase PVDF / PMMA composite oriented film comprises the following specific steps:

[0053] 1) PVDF with a weight average molecular weight of 550,000 and PMMA with a weight average molecular weight of 200,000 were weighed, with a total mass of 1000 g, wherein the mass fractions of PMMA were 10% and 20%, respectively. The mixture was melt-blended and extruded in a twin-screw extruder (model Haake PolyLab) to prepare a PVDF / PMMA pre-film having a thickness of 250 μm.

[0054] The feeding section temperature of the twin-screw extruder is 180°C, the compression section temperature is 190°C, the homogenization section temperature is 210°C, the die head temperature is 190°C, and the screw speed is 30r / min.

[0055] 2) The prefabricated film obtained in step 1) is mounted and fixed on a melt stretching device, and heated to 220° C. at a heating rate of 20° C. / min to achieve complete melting of the prefabricated film, and then kept warm for 10 minutes to eliminate the thermal history.

[0056] 3) The PVDF / PMMA melt obtained in step 2) was cooled to melt stretching temperatures of 156°C, 159°C, 162°C, 165°C, and 168°C, respectively, and then stretched for 20 seconds. -1 The tensile rate was stretched to 1.0, 1.5, and 2.0 strains to induce partial molecular chain orientation in the melt.

[0057] 4) The deformed melt obtained in step 3) is maintained at the stretching temperature for 15 minutes in a stretched state to complete isothermal crystallization, and then cooled and solidified to obtain different PVDF / PMMA composite oriented films.

[0058] The composite oriented films prepared in the above examples were characterized by Fourier infrared spectroscopy, wide-angle X-ray diffractometer and scanning electron microscope, and the results showed that the internal crystal form of the film was mainly γ phase, indicating that the above method can quickly prepare PVDF / PMMA composite oriented films with high γ phase content.

[0059] Table 2 lists the information such as the content of γ phase in the composite oriented films obtained in Examples 10-18 using different PMMA mass fractions, melt stretching temperatures, melt stretching strains, and molecular chain orientation degrees.

[0060] Table 2

[0061]

[0062]

[0063] Examples 19-23

[0064] A method for preparing a γ-phase PVDF / PMMA composite oriented film comprises the following specific steps:

[0065] 1) Weigh 700 g of PVDF with a weight average molecular weight of 550,000 and 300 g of PMMA with weight average molecular weights of 100,000, 200,000 and 500,000, respectively, melt blend and extrude them in a twin-screw extruder to prepare a PVDF / PMMA pre-film, the pre-film thickness is 250 μm, and the mass fraction of PMMA is 30%. Others are the same as in Example 1.

[0066] Step 2) is the same as in Example 1.

[0067] 3) The PVDF / PMMA melt obtained in step 2) was cooled to a melt stretching temperature of 165°C and then stretched for 20 seconds. -1 The tensile rate was stretched to 1.5, 2.0, and 2.5 strains to induce partial molecular chain orientation in the melt.

[0068] 4) The deformed melt obtained in step 3) is maintained at the stretching temperature for 15 minutes in a stretched state to complete isothermal crystallization, and then cooled and solidified to obtain different PVDF / PMMA composite oriented films.

[0069] The composite oriented film prepared in the above steps was characterized by Fourier infrared spectrometer, wide-angle X-ray diffractometer and scanning electron microscope, and the results showed that the internal crystal form of the film was mainly γ phase, indicating that the above method can quickly prepare PVDF / PMMA composite oriented film with high γ phase content.

[0070] Table 3 lists information such as the content of γ phase in the composite oriented films obtained in Examples 19-23 using different PMMA molecular weights, melt tensile strains, and molecular chain orientation degrees.

[0071] Table 3

[0072]

[0073] Comparative Examples 1-8

[0074] The steps and characterization methods are the same as those in the embodiment, except that the tensile strain is different, resulting in different molecular chain orientations. The selected comparative example is a film whose molecular chain orientation deviates from the preferred range.

[0075] Table 4 shows the information of PMMA mass fraction, PMMA molecular weight, melt stretching temperature, melt stretching strain, molecular chain orientation degree and γ phase content in the composite oriented film in Comparative Examples 1-8.

[0076] Table 4

[0077]

[0078] Comparative Examples 9-13

[0079] The steps and characterization methods are the same as those of Example 1-23, except that the PMMA content is 0%, that is, the raw material is pure PVDF. The selected comparative example is a film with the highest γ phase content obtained by pure PVDF at various melt stretching temperatures.

[0080] Table 5 shows the information of PMMA mass fraction, PMMA molecular weight, melt stretching temperature, melt stretching strain, molecular chain orientation degree and γ phase content in the composite oriented film in Comparative Examples 9-13.

[0081] Table 5

[0082]

[0083] According to Examples 1-9, when the mass fraction of PMMA is 30%, within a wide melt stretching temperature window of 150-168°C, by applying a strain of 1.0-2.5 and controlling the molecular chain orientation degree at 30-35%, a γ phase content of 85%-100% can be obtained, thereby preparing a PVDF / PMMA composite oriented film with a high γ phase content.

[0084] According to Examples 10-18, when the mass fraction of PMMA is within 10-20%, by applying a strain of 1.0-2.0 and controlling the molecular chain orientation degree at 30-35%, a γ phase content of 85%-100% can be obtained, thereby preparing a PVDF / PMMA composite oriented film with a high γ phase content. The higher the proportion of PMMA, the lower the lower limit of the allowable melt stretching temperature, that is, the wider the processing temperature and strain window.

[0085] According to Examples 19-23, under the same stretching temperature, stretching strain rate and PMMA mass fraction conditions, when the PMMA molecular weight is in the range of 100000-500000, by applying 1.5-2.5 strain and controlling the molecular chain orientation degree at 30-35%, a γ phase content of 85%-100% can be obtained, thereby preparing a PVDF / PMMA composite oriented film with a high γ phase content.

[0086] Comparing Examples 1-9 with Comparative Examples 9-13, when PMMA is not added, to obtain a high γ phase content of 85%-100% in pure PVDF, a melt stretching temperature of 165°C and a tensile strain of 2.0 are required, and the processing window is extremely narrow. After the introduction of PMMA, the temperature window for obtaining a γ phase content of 85%-100% can be expanded to 150-168°C, and the tensile strain window can be expanded to 1.0-2.5. Therefore, the addition of PMMA greatly expands the temperature and tensile strain window range of the melt stretching processing of PVDF-based films with high γ phase content, and realizes the rapid preparation of oriented films with high γ phase content within a wider melt stretching temperature and strain window range, which is conducive to large-scale industrial promotion and application.

[0087] By comparing Example 10 with Comparative Examples 1-5, and Example 14 with Comparative Examples 6-8, it can be seen that under the same PMMA molecular weight, PMMA mass fraction, and stretching temperature conditions, excessive or excessive strain will cause the molecular chain orientation degree to be higher or lower than the preferred range of 30-35%, and PVDF / PMMA composite oriented films with a high γ phase content (85%-100%) cannot be prepared.

[0088] Depend on Figure 2It can be seen from the Fourier transform infrared spectrum of the PVDF / PMMA composite oriented film prepared in Example 1-9 that the γ phase in the film is located at 832 and 1233 cm -1 The absorption peaks at 763 and 1208 cm -1 The absorption peak at is weak, indicating that the content of γ phase is high.

[0089] from Figure 3 The Fourier transform infrared spectra of the PVDF / PMMA composite oriented film prepared in Comparative Example 1-8 show that the α phase in the prepared film is located at 763 and 1208 cm -1 The absorption peak at is stronger, indicating that the content of γ phase is lower.

[0090] Figure 4 The crystal morphology of the γ-phase PVDF / PMMA film prepared in Example 14 was observed using a scanning electron microscope, and a typical curled crystal morphology of the γ-phase was observed.

[0091] The characterization methods of the Fourier infrared spectrometer, wide-angle X-ray diffractometer and scanning electron microscope are well known and will not be described in detail.

Claims

1. A γ-phase PVDF / PMMA composite oriented film, characterized in that: The film is composed of PVDF and PMMA, and the mass percentage of the two is 70%-90%:10%-30%; the content of γ phase in the film is 85%-100%.

2. The γ-phase PVDF / PMMA composite oriented film according to claim 1, characterized in that: The PVDF is a high molecular weight linear homopolymer with a weight average molecular weight of 500,000-600,000.

3. The γ-phase PVDF / PMMA composite oriented film according to claim 1, characterized in that: The weight average molecular weight of the PMMA is 100,000-500,000.

4. The γ-phase PVDF / PMMA composite oriented film according to any one of claims 1 to 3, characterized in that: The mass percentages of PVDF and PMMA in the film are 70%:30%.

5. The method for preparing the γ-phase PVDF / PMMA composite oriented film according to any one of claims 1 to 4, characterized in that: Using a mixture of PVDF and PMMA as raw materials, melt blending and extrusion are first performed to obtain a prefabricated film, and then the prefabricated film is heated to be fully melted and the thermal history is eliminated, and the temperature is cooled to perform melt stretching to control the orientation degree of the molecular chain; the stretching is stopped and the temperature is maintained at a constant temperature to complete the isothermal crystallization of the PVDF component, and then cooled and solidified to obtain a PVDF / PMMA composite oriented film with a high γ phase content.

6. The method for preparing the γ-phase PVDF / PMMA composite oriented film according to claim 5, characterized in that: The temperature is lowered to 150-168°C for melt stretching.

7. The method for preparing the γ-phase PVDF / PMMA composite oriented film according to claim 6, characterized in that: Melt stretching is performed to achieve a molecular chain orientation degree of 30-35%.

8. The method for preparing the γ-phase PVDF / PMMA composite oriented film according to claim 5, characterized in that: The strain rate applied during melt stretching is 10-20 s -1 , strain is 1.0-2.

5.

9. The method for preparing the γ-phase PVDF / PMMA composite oriented film according to claim 5, characterized in that: Stop stretching and keep the temperature constant for 15 min to complete the isothermal crystallization of the PVDF component.

10. The method for preparing the γ-phase PVDF / PMMA composite oriented film according to claim 5, characterized in that: The heating rate of the prefabricated film is 10-20°C / min, the melting temperature for eliminating the thermal history is 210-230°C, and the holding time is 10 min.

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