Preparation method of low-degradation PVDF (Polyvinylidene Fluoride) extended-chain crystal

Abstract

The invention provides a preparation method of a low-degradation PVDF (Polyvinylidene Fluoride) extended-chain crystal. The problems that the content of the extended-chain crystal is low under low-temperature processing and the material is obviously thermally degraded under high-temperature processing exist in preparation of the PVDF extended-chain crystal by a traditional high-pressure method are solved. The preparation method comprises the following steps: firstly, doping a certain mass fraction of ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate) into PVDF (Polyvinylidene Fluoride), and carrying out high-temperature melt blending; then carrying out hot press molding on the obtained blend to prepare a PVDF / ionic liquid sheet; and finally, applying a high-temperature (205-225 DEG C) and high-pressure (0.4 GPa) effect on the sheet for a certain time (10 minutes), and naturally cooling in a high-pressure state to obtain the PVDF material containing the extended-chain crystal. According to the invention, the ionic liquid is introduced, so that more and plenty of extended-chain crystals can rapidly grow out of PVDF under a relatively mild low-temperature condition, and the thermal degradation of the material is greatly reduced.

Description

Technical Field

[0001] The present invention belongs to the field of high-performance polymer processing, and particularly relates to a method for preparing low-degradation PVDF extended-chain crystals. Background Art

[0002] As an electroactive polymer, polyvinylidene fluoride (PVDF) not only has good processability, chemical corrosion resistance and high mechanical strength, but also exhibits excellent ferroelectric and piezoelectric properties. It is often used to prepare polymer piezoelectric materials and has broad application prospects in the fields of flexible intelligent sensing, energy harvesting, etc.

[0003] The electroactive function of PVDF stems from its polar crystalline phase. Depending on the crystallization conditions, PVDF can form five crystal structures, including non-polar α and ε crystals and polar β, γ and δ crystals. Among them, the β crystal has the highest macroscopic dipole moment due to its all-trans molecular chain conformation, showing strong polarity, which is crucial for realizing the excellent piezoelectric and ferroelectric properties of PVDF materials. The polar β phase obtained by conventional processing methods is mostly folded-chain crystals with relatively low thermal stability. Through high-temperature and high-pressure melt processing, polar β-phase extended-chain crystals can be prepared. Extended-chain crystals are considered to be the most thermodynamically stable crystalline structure, which have low defects and high crystallinity, show stronger piezoelectric activity, and at the same time have a high melting point and a high Curie temperature, which can significantly improve the high-temperature service performance of piezoelectric PVDF materials.

[0004] For PVDF materials, existing high-pressure processing methods for obtaining a large amount of polar extended-chain crystals usually involve relatively harsh process conditions. It is necessary to apply a high-pressure field at a relatively high melt temperature, which will inevitably cause serious thermal degradation of PVDF molecular chains. When a high-pressure field is applied at a lower temperature, although the thermal degradation of the material can be weakened, the content of the obtained extended-chain crystals is less. These seriously restrict the popularization and application of extended-chain crystal PVDF in the field of high-performance piezoelectric materials. Therefore, it is of great significance to develop a relatively mild high-pressure processing method to achieve efficient preparation of PVDF materials with less thermal degradation and a high content of extended-chain crystals. Summary of the Invention

[0005] The purpose of the present invention is to provide a high-pressure processing and preparation method for PVDF materials with low degradation and a high content of extended-chain crystals in view of the deficiencies of the prior art.

[0006] To this end, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing low-degradation PVDF extended-chain crystals, comprising the following steps:

[0008] (1) Incorporate an ionic liquid with a certain mass fraction into PVDF, conduct melt blending in a mixer, and then obtain PVDF / ionic liquid sheets by hot pressing.

[0009] (2) Transfer the sheets obtained in step (1) to a high-pressure device, heat them above the melting point under low-pressure conditions, and hold for a period of time to eliminate the thermal history.

[0010] (3) Cool or heat the melt after eliminating the thermal history in step (2) to a specified temperature, then rapidly apply a constant pressure, conduct heat treatment for a period of time under constant temperature and pressure, and then continue to cool naturally to room temperature under constant pressure to prepare the resulting low-degradation PVDF material containing a large number of extended-chain crystals.

[0011] Further, the type of ionic liquid in step (1) is 1-ethyl-3-methylimidazolium tetrafluoroborate.

[0012] Further, the mass fraction of the ionic liquid in step (1) is 0.1 - 0.2%, preferably, the mass fraction of the ionic liquid is 0.1%.

[0013] Further, the melt blending temperature in step (1) is 210 - 230 °C, the blending time is 10 min, and the rotation speed of the mixer is 30 rpm.

[0014] Further, the low-pressure condition in step (2) is 0.005 - 0.02 GPa.

[0015] Further, the temperature for eliminating the thermal history in step (2) is 220 °C, and the holding time for eliminating the thermal history is 5 - 10 min.

[0016] Further, the specified temperature in step (3) is 205 - 225 °C.

[0017] Further, the loading rate of the constant pressure in step (3) is 0.1 - 0.2 GPa / s.

[0018] Further, the constant pressure in step (3) is 0.4 GPa.

[0019] Further, the heat treatment time under constant temperature and pressure in step (3) is 10 min.

[0020] Preferably, the specified temperature is 215 °C, the constant pressure is 0.4 GPa, and the loading rate of the constant pressure is 0.2 GPa / s.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] To prepare PVDF materials with a straight-chain crystal content exceeding 60% using prior art, a high-pressure field needs to be applied at a PVDF melt temperature higher than 235°C, which can cause severe thermal degradation of the material. In the present invention, by simply adding a very small amount of ionic liquid (mass fraction 0.1 - 0.2%), a large number of straight-chain crystals can be rapidly prepared at a relatively low temperature (215°C). The process conditions are relatively mild, effectively reducing or avoiding thermal degradation of the material. At the same time, the ionic liquid is inexpensive and the blending process is simple. Using the present invention, it is possible to efficiently prepare PVDF materials with less thermal degradation and a high straight-chain crystal content, thus promoting the popularization and application of highly heat-resistant electroactive PVDF functional devices. Description of the Drawings

[0023] Figure 1 It is a process flow chart for the preparation of low-degradation PVDF straight-chain crystals in the present invention.

[0024] Figure 2 It is a differential scanning calorimeter test curve of the low-degradation PVDF straight-chain crystals in Examples 1 - 2 and the high-pressure PVDF material products in Comparative Examples 1 - 7 of the present invention, where a is the low-degradation PVDF straight-chain crystals in Examples 1 - 2, b is the high-pressure PVDF material products in Comparative Examples 1 - 4, and c is the high-pressure PVDF material products in Comparative Examples 5 - 7.

[0025] Figure 3 It is a photo of the high-pressure products in Examples 1 - 2, Comparative Examples 1 - 4 and the initial PVDF of the present invention. Detailed Description of the Invention

[0026] The present invention will be described in more detail with specific examples as follows:

[0027] The following are the specific instruments and raw materials used in the following examples, but it should be noted that the present invention is not limited to being implemented through the following devices:

[0028] PVDF pellets (weight average molecular weight 1.5 - 2.0×10 5 g / mol); ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate, Beijing Innochem Science & Technology Co., Ltd.); internal mixer (HAAKE PolyLab Rheomix 600OS, Germany); vacuum molding press (Y-002, Zhengzhou Craftsman Machinery Equipment Co., Ltd.); differential scanning calorimeter (Q2000, TA); high-pressure equipment (HYY-2000).

[0029] Example 1

[0030] A method for preparing low-degradation PVDF straight-chain crystals is carried out according to the following steps:

[0031] 1) The ionic liquid and PVDF were melt-blended using a Banbury mixer. The mass fraction of the ionic liquid was 0.1%, the blending temperature was 220 °C, the blending time was 10 min, and the rotation speed of the Banbury mixer was 30 rpm.

[0032] 2) The PVDF / ionic liquid blend obtained in step 1) was pressed into a 1-mm-thick sheet using a vacuum molding press. The pressing temperature was 220 °C and the pressure was 5 MPa.

[0033] 3) The PVDF / ionic liquid sheet obtained in step 2) was placed in an aluminum mold and fixed to a high-pressure device. Subsequently, it was heated to 220 °C and melted under a low pressure condition of 0.01 GPa, and held for 5 min to eliminate the thermal history.

[0034] 4) The melt after eliminating the thermal history in step 3) was cooled to the specified constant pressure temperature of 215 °C, and then a constant pressure of 0.4 GPa was applied at a loading rate of 0.2 GPa / s. After that, it was heat-treated at 215 °C and 0.4 GPa under constant temperature and constant pressure for 10 min, and finally naturally cooled to room temperature under a constant pressure of 0.4 GPa, thus obtaining the target product.

[0035] The preparation process of the present invention is shown in Figure 1 .

[0036] Example 2

[0037] The method for preparing the low-degradation PVDF extended-chain crystals in this example is different from that in Example 1 only in that the mass fraction of the ionic liquid is 0.2%.

[0038] The target products obtained in the present invention were tested by a differential scanning calorimeter to characterize the content information of the extended-chain crystals in the materials.

[0039] Table 1 shows the content of the extended-chain crystals and the material degradation information of the products prepared in Examples 1-2.

[0040] Table 1

[0041]

[0042] Comparative Examples 1-4

[0043] A method for preparing a PVDF material at high temperature and high pressure is carried out according to the following steps:

[0044] The process steps and characterization methods of Comparative Examples 1-4 are the same as those in Example 1, except that the raw material is pure PVDF, that is, the mass fraction of the ionic liquid is 0%, and four different specified constant pressure temperatures, namely 175, 215, 235, and 255 °C, are selected.

[0045] Table 2 shows the information on the content of extended-chain crystals and the degradation of the materials in Comparative Examples 1-4.

[0046] Table 2

[0047]

[0048] Comparative Examples 5-7

[0049] A method for preparing PVDF materials at high temperature and high pressure is carried out according to the following steps:

[0050] The process steps and characterization methods of Comparative Examples 5-7 are the same as those of Example 1, except that the mass fractions of the selected ionic liquids are 0.5%, 1% and 2% respectively.

[0051] Table 3 shows the information on the content of extended-chain crystals and the degradation of the materials in Comparative Examples 5-7.

[0052] Table 3

[0053]

[0054] The relative content of extended-chain crystals in the high-pressure products is obtained by testing with a differential scanning calorimeter. The characteristic melting peak of the extended-chain crystals is located near 196 °C, and the relative content of the extended-chain crystals is the ratio of its melting enthalpy to the total melting enthalpy of the crystals in the material.

[0055] The degradation of the high-pressure products is directly reflected by the color change of the products ( Figure 3 ): When the color of the high-pressure product is close to the color before high-pressure processing, it indicates that the product is rarely degraded; when the color of the high-pressure product is brownish-yellow, it indicates that the product is severely degraded; when the color of the high-pressure product is brownish-brown, it indicates that the product is violently degraded.

[0056] According to Examples 1 and 2, under relatively mild low-temperature processing conditions of 215 °C, by adding a small amount of ionic liquids (0.1% and 0.2%), the characteristic melting peaks of the extended-chain crystals in the high-pressure products near 196 °C are both strong ( Figure 2 a), that is, a large amount (72% and 63%) of extended-chain crystals are generated, and at the same time, the color of the material is close to that of the initial PVDF, indicating that very little degradation occurs.

[0057] According to Comparative Examples 1-4, with the increase of the constant-pressure temperature, the intensity of the characteristic melting peak of the extended-chain crystals in the pure PVDF high-pressure products near 196 °C gradually increases ( Figure 2 b). This indicates that high temperature is beneficial to the formation of extended-chain crystals. However, when the content of extended-chain crystals exceeds 60%, a constant-pressure temperature higher than 235 °C is required, which will cause the color of the material to turn brownish-brown, indicating that the material has undergone violent degradation.

[0058] According to Comparative Examples 5-7, as the mass fraction of the ionic liquid (higher than 0.5%) increases, the characteristic melting peak of the extended-chain crystals in the high-pressure products gradually weakens near 196 °C. Figure 2 c). This indicates that increasing the mass fraction of the ionic liquid is not conducive to the formation of extended-chain crystals under high temperature and high pressure.

[0059] Comparing Examples 1-2 with Comparative Examples 2 and 5-7, under the constant temperature and constant pressure conditions of 215 °C and 0.4 GPa, compared with pure PVDF, adding ionic liquids with mass fractions of 0.1% and 0.2% can significantly promote the formation of extended-chain crystals, increasing the content of extended-chain crystals from 21% to 63% and 72%, respectively, while effectively avoiding the thermal degradation of the material. However, excessive ionic liquid is not conducive to the formation of extended-chain crystals. When the mass fractions of the added ionic liquid are 0.5%, 1.0%, and 2.0%, the contents of extended-chain crystals are reduced to 39%, 24%, and 3%, respectively.

[0060] Generally speaking, incorporating a small amount of ionic liquid into the PVDF products prepared under relatively mild low temperature and high pressure conditions can not only maintain a high content of extended-chain crystals but also avoid significant thermal degradation of the material.

[0061] In summary, the present invention provides a method for preparing low-degradation PVDF extended-chain crystals. Compared with the traditional method, under the same pressure, the thermal degradation of the prepared PVDF high-pressure products is significantly reduced, while the content of extended-chain crystals is significantly increased.

[0062] As described above, the above are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the common technical knowledge and general methods in the art, and based on the technical solution and inventive concept of the present invention, making equivalent substitutions or changes should be covered within the protection scope of the present invention.

Claims

1. A method for preparing low-degradation PVDF straight chain crystals, characterized in that: The following steps are involved: (1) A certain mass fraction of ionic liquid is added to PVDF, melt-blended in an internal mixer, and then hot-pressed to obtain a PVDF / ionic liquid sheet; (2) moving the sheet obtained in step (1) to a high-pressure device, heating it to above the melting point under low pressure conditions, and keeping it warm for a period of time to eliminate the thermal history; (3) Cooling or heating the melt after eliminating the thermal history in step (2) to a specified temperature, then quickly loading a constant pressure, and heat treating it at a constant temperature and pressure for a period of time, and then continuing to cool naturally to room temperature at a constant pressure, thereby preparing a low-degradation PVDF material containing a large number of extended chain crystals.

2. The method for preparing low-degradation PVDF extended chain crystals according to claim 1, characterized in that: The type of ionic liquid in step (1) is 1-ethyl-3-methylimidazolium tetrafluoroborate.

3. The method for preparing low-degradation PVDF extended chain crystals according to claim 1, characterized in that: The mass fraction of the ionic liquid in step (1) is 0.1-0.2%.

4. The method for preparing low-degradation PVDF extended chain crystals according to claim 1, characterized in that: The melt blending temperature in step (1) is 210-230 o C, the blending time was 10 min, and the mixer speed was 30 rpm.

5. The method for preparing low-degradation PVDF extended chain crystals according to claim 1, characterized in that: The low pressure condition in step (2) is 0.005-0.02 GPa.

6. The method for preparing low-degradation PVDF extended chain crystals according to claim 1, characterized in that: The temperature for eliminating the thermal history in step (2) is 220 o C, the holding time to eliminate thermal history is 5-10min.

7. The method for preparing low-degradation PVDF extended chain crystals according to claim 1, characterized in that: The temperature specified in step (3) is 205-225 o C.

8. The method for preparing low-degradation PVDF extended chain crystals according to claim 1, characterized in that: The constant pressure loading rate in step (3) is 0.1-0.2 GPa / s.

9. The method for preparing low-degradation PVDF extended chain crystals according to claim 1, characterized in that: The constant pressure in step (3) is 0.4 GPa.

10. The method for preparing low-degradation PVDF extended chain crystals according to claim 1, characterized in that: The heat treatment time at constant temperature and pressure in step (3) is 10 minutes.

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