Sensing film, preparation method thereof and prepared sensor

Abstract

The invention belongs to the field of polymer functional materials and polymer processing, and particularly relates to a flexible sensing film with high voltage output performance and a preparation method of the flexible sensing film. The flexible piezoelectric polymer sensing film is prepared by adopting a flash annealing method, the beta crystal content and the piezoelectric output performance of the obtained piezoelectric polymer film are remarkably improved in a short time, and the flexible piezoelectric polymer sensing film has good application potential.

Description

Technical Field

[0001] The present invention belongs to the field of polymer functional materials, and in particular relates to a flexible sensing film with high-voltage electrical output performance, a preparation method thereof, and a sensor prepared using the same. Background Art

[0002] With the rapid development of portable and wearable electronic devices, new demands have been placed on piezoelectric materials in terms of stretchability and flexibility. Polymer piezoelectric materials have become one of the most promising new piezoelectric materials due to their excellent flexibility, lightness, and biocompatibility. In particular, polyvinylidene fluoride (PVDF) and its copolymers are highly favored due to their excellent piezoelectric, pyroelectric, and ferroelectric properties. The development of PVDF-based piezoelectric polymers provides an efficient and multifunctional material platform for applications such as information perception, electro-drive, electro-caloric cooling, and energy harvesting. However, compared with inorganic piezoelectric ceramics, the piezoelectric properties of PVDF-based polymers are still relatively low. Therefore, how to improve piezoelectric performance has become a research focus.

[0003] PVDF and its copolymers have five crystal forms, of which α, β, and γ are the most discussed. Previous studies have shown that the β crystal has the highest dipole moment due to its special all-trans (TTT) conformation, exhibiting excellent piezoelectric and ferroelectric properties. Based on this, the key to improving the piezoelectric properties of PVDF-based polymers is to obtain a higher β crystal content through appropriate processing and post-processing methods. Currently, common processing methods for PVDF-based polymers include phase separation, spin coating, hot pressing, AAO templates, and electrospinning. Electrospinning can apply an electric field to polarize the dipoles while preparing an oriented fiber membrane, and is one of the most effective processing methods for obtaining a high β crystal content.

[0004] The PVDF-based polymer obtained after preliminary processing needs to be post-processed to further increase the β-crystal content. Annealing is a common treatment method that can effectively promote the formation and improvement of crystals. Especially for polyvinylidene fluoride-trifluoroethylene copolymer (PVDF-TrFE), a large number of research works have confirmed that annealing treatment under different temperature and time conditions is conducive to promoting the formation of β-crystals. However, annealing treatments that take several hours or even days, while promoting β-crystals, also greatly increase energy consumption and are not conducive to actual production. Therefore, exploring short-term and efficient annealing treatments has important guiding significance for improving the piezoelectric properties and actual production efficiency of PVDF-TrFE and realizing the large-scale manufacturing of high-performance polymer piezoelectric sensors. Summary of the Invention

[0005] To address the limitations of existing technologies, the present invention applies flash annealing to thin films prepared by solution methods. The authors found that flash annealing significantly promotes the formation of β crystals in thin films prepared by solution methods (electrospinning, spin coating, etc.). Test results confirm that flash annealing effectively enhances intrinsic piezoelectric and ferroelectric properties. Furthermore, thin-film sensors fabricated using this method exhibit superior sensing performance compared to unannealed and long-annealed films.

[0006] The first technical problem to be solved by the present invention is to provide a method for preparing a sensor film. The method comprises the following steps: forming a film from a solution containing a ferroelectric polymer, and then annealing the film at 90-150°C for 0.1-10 minutes to obtain the sensor film.

[0007] Furthermore, the method for preparing the sensing film further includes a step of removing the solvent after the film is formed, and the solvent can be removed by natural evaporation.

[0008] Furthermore, in the above-mentioned method for preparing the sensing film, the film is immersed in liquid nitrogen after the annealing treatment, in order to freeze the internal crystalline structure of the film.

[0009] Specifically, in the above-mentioned method for preparing the sensing film, the ferroelectric polymer is PVDF-TrFE.

[0010] Furthermore, in the above-mentioned method for preparing the sensing film, the ratio of the number of copolymerized units of TrFE in the PVDF-TrFE is 10-50%.

[0011] Specifically, in the above-mentioned method for preparing the sensing film, a mixed solvent of N, N-dimethylformamide and acetone is used to dissolve the polymer to prepare a solution.

[0012] Furthermore, in the above-mentioned method for preparing the sensing film, the volume ratio of the mixed solvent of N, N-dimethylformamide and acetone is 1:4 to 4:1.

[0013] Specifically, in the above-mentioned method for preparing the sensing film, the mass fraction of the solution is 10-20%, preferably 12%.

[0014] Specifically, in the above-mentioned method for preparing the sensing film, the film is formed by electrospinning or spin coating.

[0015] Furthermore, in the above-mentioned method for preparing the sensing film, the electrospinning conditions are as follows: spinning solution propulsion rate: 0.5 to 2.5 mL / h (preferably 0.9 mL / h); positive electrode voltage: 8 to 15 kV (preferably 12 kV); negative electrode voltage: -1 to -10 kV (preferably -1 to -3 kV, more preferably -2 kV); spinning distance: 5 to 15 cm (preferably 10 cm); and spinning time: 2 to 5 hours. These conditions are adjusted based on film thickness. After completing the above steps, the collected electrospun film is placed in a ventilated environment to allow for natural evaporation (12 hours) to completely remove the solution.

[0016] Furthermore, in the above-mentioned method for preparing the sensing film, the spin coating conditions are as follows: drop the solution onto a glass plate, activate a vacuum pump to hold the glass plate in place on the spin coater, set the spin coater speed to 500-1000 rpm (preferably 800 rpm), and continue the spin coating process for 1-5 minutes (preferably 2 minutes) to evenly spread the solution on the glass plate. For example, drop 1 mL of the prepared solution onto a 5 × 5 cm glass plate, activate a vacuum pump to hold the glass plate in place on the spin coater, set the spin coater speed to 800 rpm, and continue the spin coating process for 2 minutes to evenly spread the solution on the glass plate. The glass plate is then placed in a ventilated environment for 24 hours to completely remove the solution.

[0017] Furthermore, in the above-mentioned method for preparing the sensing film, the thickness of the film is 60 to 100 μm, preferably 80 μm.

[0018] Furthermore, in the above-mentioned method for preparing the sensing film, the annealing temperature is 130°C.

[0019] Furthermore, in the above-mentioned method for preparing the sensing film, the annealing time is 1 to 5 minutes.

[0020] The second technical problem to be solved by the present invention is to provide a sensing film prepared by the above-mentioned sensing film preparation method.

[0021] The third technical problem to be solved by the present invention is to provide a sensor prepared from the above-mentioned sensing film.

[0022] For example: the PVDF-TrFE sensing film is cut into 1 cm × 1 cm squares, copper foil is used as an electrode to transfer the charge generated by pressure, two copper foils are attached to the two sides of the treated PVDF-TrFE sensing film, and polyimide is used for encapsulation as a shielding layer to protect the device from environmental signal interference. The entire device has a simple structure, is easy to prepare, and has controllable costs, making it suitable for large-scale production.

[0023] Beneficial effects of the present invention:

[0024] (1) The present invention uses flash annealing to treat the polymer film, which significantly improves the β-crystal structure and piezoelectric output performance of the film in a short period of time. The device preparation process is simple, which greatly shortens the annealing time of several hours or even days for such materials in the past, improves production efficiency, reduces production energy consumption, and reduces production costs.

[0025] (2) The piezoelectric sensor obtained in the present invention is derived from a polymer electrospun film, has good biocompatibility, flexibility and lightness, and is easy to carry and use.

[0026] (3) The piezoelectric output of the piezoelectric sensor obtained by the present invention exhibits obvious stable linear characteristics, and the applied mechanical force can be accurately detected based on the calibrated piezoelectric output coefficient.

[0027] (4) The piezoelectric sensor obtained by the present invention can maintain good flexibility and has a stable electrical response to mechanical force. Based on this characteristic, it can be used as a wearable energy harvesting device.

[0028] (5) The piezoelectric sensor obtained by the present invention exhibits good response characteristics to high-frequency micro-vibrations and can be used to detect vibrations and strains in such scenarios as well as for flaw detection of precision equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 FTIR, DSC, and Raman test results of PVDF-TrFE electrospun films obtained in Example 1, Comparative Example 1, and Comparative Example 3

[0030] Figure 2 FTIR and DSC test results of PVDF-TrFE spin-coated films obtained in Example 2, Comparative Example 2 and Comparative Example 4

[0031] Figure 3 FTIR and DSC test results of PVDF electrospun films obtained in Comparative Examples 5, 6 and 7

[0032] Figure 4 Figure 1 shows the piezoelectric properties test results of PVDF-TrFE electrospun films obtained in Example 1, Comparative Example 1 and Comparative Example 3.

[0033] Figure 5 Piezoelectric performance test results of PVDF electrospun films obtained in Comparative Examples 5, 6 and 7

[0034] Figure 6 Ferroelectric polarization curves of Example 2 and Comparative Example 4

[0035] Figure 7Energy output density test diagram of PVDF-TrFE electrospun fiber membranes obtained in Example 1, Comparative Example 1 and Comparative Example 3 DETAILED DESCRIPTION

[0036] The specific implementation methods of the present invention are further described below in conjunction with examples, but the present invention is not limited to the scope of the examples.

[0037] Example 1

[0038] The preparation method of the flash annealed PVDF-TrFE electrospun fiber membrane comprises the following steps:

[0039] (1) Preparation of PVDF-TrFE solution: PVDF-TrFE (purchased from Arkema, France, brand FC20) and a mixed solvent (N, N-dimethylformamide: acetone = 3:2, volume ratio) were mixed and magnetically stirred until the solution became transparent to prepare a PVDF-TrFE solution, wherein the mass fraction of PVDF-TrFE was 15%;

[0040] (2) Electrospinning membrane: The PVDF-TrFE solution obtained in step (1) was electrospun using a 23-gauge needle (outer diameter: 0.64 mm, inner diameter: 0.33 mm) for 4 h. The collected fiber membrane was placed in a ventilated environment to dry and remove the solvent to obtain an electrospun membrane. During electrospinning, the positive and negative electrode voltages were +13 kV and -9.5 kV, respectively, and the injection rate was 10 μL / min.

[0041] (3) Annealing treatment: The prepared electrospun membrane was attached to aluminum foil and placed in an oven for flash annealing (annealing for 1 min) to obtain an annealed PVDF-TrFE film. The oven temperature was set to 130 °C.

[0042] (4) Preparation of sensor devices: Cut the film into 1 cm × 1 cm squares, attach copper foil as electrodes to both sides of the film, and use polyimide as a shielding layer to encapsulate the device to make the sensor.

[0043] Example 2

[0044] The method for preparing a flash annealed PVDF-TrFE spin-coated film comprises the following steps:

[0045] The preparation method is the same as that of Example 1, except that the spin coating method is used in step (2), specifically: 1 mL of the PVDF-TrFE solution obtained in step (1) is dropped onto a 5 cm × 5 cm glass plate, the spin coater speed is 800 rpm, and the spin coating is performed for 2 min to evenly spread the solution on the glass plate, and the solution is placed in a ventilated environment to evaporate the solvent to obtain a spin-coated film.

[0046] Comparative Example 1

[0047] The preparation method of the PVDF-TrFE electrospun fiber membrane subjected to long-term annealing treatment comprises the following steps:

[0048] (1) Preparation of PVDF-TrFE solution: PVDF-TrFE was added to a mixed solvent (N, N-dimethylformamide: acetone = 3:2, volume ratio) with a mass fraction of 15%, and magnetic stirring was performed until the solution became transparent.

[0049] (2) Electrospinning membrane: A 23-gauge needle (outer diameter: 0.64 mm, inner diameter 0.33 mm) was used, with the positive and negative voltages of +13 kV and -9.5 kV, respectively, and an injection rate of 10 μL / min for electrospinning for 4 h. The collected fiber membrane was placed in a ventilated environment to dry and remove the solvent.

[0050] (3) Annealing treatment: The prepared electrospun membrane was attached to aluminum foil and placed in an oven for long-term annealing (annealing for 2 h). The oven temperature was set to 130 °C.

[0051] (4) Preparation of sensor devices: Cut the film into 1 cm × 1 cm squares, attach copper foil as electrodes to both sides of the film, and use polyimide as a shielding layer to encapsulate the device.

[0052] Comparative Example 2

[0053] The preparation method is the same as that of Comparative Example 1, the only difference being that the spin coating method is used in (2), specifically: 1 mL of the PVDF-TrFE solution obtained in step (1) is dropped onto a 5 cm × 5 cm glass plate, the spin coater speed is 800 rpm, and the spin coating is performed for 2 min to evenly spread the solution on the glass plate, and the solution is placed in a ventilated environment to evaporate the solvent to obtain a spin-coated film.

[0054] Comparative Example 3

[0055] The preparation method is the same as that of Example 1, except that the annealing treatment in step (3) is omitted.

[0056] Comparative Example 4

[0057] The preparation method is the same as that of Example 2, except that the annealing treatment in step (3) is omitted.

[0058] Comparative Example 5

[0059] The preparation method is the same as that of Comparative Example 3, the only difference being that PVDF is used in step (1).

[0060] Comparative Example 6

[0061] The preparation method is the same as that of Comparative Example 5, the only difference being that the annealing treatment in step (4) is performed by flash annealing, that is, the annealing time is 1 min or less.

[0062] Comparative Example 7

[0063] The preparation method is the same as that of Comparative Example 5, the only difference being that the annealing treatment in step (4) is long annealing, i.e., the annealing time is 2 h.

[0064] The test results of FTIR, DSC and Raman of PVDF-TrFE electrospun membranes obtained in Example 1, Comparative Example 1 and Comparative Example 3 of the present invention are as follows: Figure 1 As shown, Figure a is an FTIR spectrum, Figure b is a DSC curve, and Figure c is a Raman spectrum. Among them, flash annealing corresponds to Example 1, long annealing corresponds to Example 1, and no annealing corresponds to Example 3. The infrared spectrum in Figure a and the DSC curve in Figure b were quantitatively analyzed, wherein the infrared spectrum was normalized and the area ratio of the characteristic peak of β crystal was calculated, and the enthalpy value of the Curie transition peak in the DSC curve was integrated and calculated to analyze the change in the relative content of β crystal. The statistical results are shown as follows: Figure 1 As shown in Figure d, the results show that the β-crystal content of the PVDF-TrFE electrospun film changes significantly after a short annealing treatment, and the β-crystal content does not continue to increase after a long annealing treatment, but rather decreases. -1 The peak intensity of the characteristic peak belonging to β crystal is significantly improved after flash annealing. As the annealing time is further extended, the peak intensity decays. This is consistent with the analysis results of FTIR and DSC, verifying the accuracy of the conclusion.

[0065] The test results of FTIR and DSC of the PVDF-TrFE spin-coated films obtained in Example 2, Comparative Example 2 and Comparative Example 4 are as follows: Figure 2 As shown, Figure a is the FTIR spectrum, and Figure b is the DSC curve. Among them, flash annealing corresponds to Example 2, long annealing corresponds to Example 2, and no annealing corresponds to Example 4. Figure 2 The spin-coated films exhibited similar properties to the electrospun films. Flash annealing significantly increased both the Curie transition temperature and the Curie transition enthalpy. The characteristic peaks of the β-crystals in the infrared spectrum were enhanced. However, further increases in the β-crystals and TTT conformation did not occur with extended annealing time.

[0066] The test results of FTIR and DSC of the PVDF electrospun films obtained in Comparative Examples 5, 6 and 7 of the present invention are as follows: Figure 3 As shown, Figure a is the FTIR spectrum, and Figure b is the DSC curve. Among them, no annealing corresponds to Example 5, flash annealing corresponds to Example 6, and long annealing corresponds to Example 7. Figure 3It can be seen that the characteristic peaks in the FTIR spectrum remain almost unchanged before and after annealing, and the thermal enthalpy parameters obtained by DSC testing also do not change significantly. Therefore, for PVDF homopolymer, flash annealing does not significantly change its internal aggregate structure, and the β-crystal content does not significantly increase. Therefore, PVDF homopolymer does not have the characteristics explored in the examples.

[0067] In addition, the present invention conducted a piezoelectric performance test on the films obtained in Example 1, Comparative Example 1 and Comparative Example 3, and the results are as follows: Figure 4 As shown, Figure a is the piezoelectric output performance test results, and Figure b is the piezoelectric coefficient |d 33 |The value of. Figure 4 It can be seen that: for PVDF-TrFE electrospun film, after using flash annealing process, its piezoelectric performance is significantly improved from 28.1mV / N to 96.9mV / N, which is much higher than the output performance (18.2mV / N) of commercial piezoelectric film (purchased from PolyK Technology, USA). If the annealing time is further extended, the piezoelectric output performance decays to 72.1mV / N. 33 The instrument measures the piezoelectric coefficient |d of PVDF-TrFE film 33 |, reaching the same conclusion. The absolute value of the piezoelectric coefficient of the unannealed film was 33 pC / N. After flash annealing, this coefficient increased to 68 pC / N. Further extension of the annealing time resulted in a decrease in the piezoelectric performance, with the absolute value of the piezoelectric coefficient reaching 45 pC / N. The results of the piezoelectric performance tests are consistent with the previous structural test results.

[0068] The piezoelectric properties of the films obtained in Comparative Examples 5, 6 and 7 of the present invention were tested, and the results are as follows: Figure 5 As shown, Figure a is the piezoelectric output performance test results, and Figure b is the piezoelectric coefficient |d 33 |The value of. Figure 5 It can be seen that: for PVDF electrospun film, after the flash annealing process, its piezoelectric performance is not significantly improved, only from 22.0 mV / N to 26.3 mV / N, and after long-term annealing treatment, it shows a higher output performance of 40.5 mV / N. 33 The instrument measures the piezoelectric coefficient of PVDF film |d 33 The absolute value of the piezoelectric coefficient of the unannealed PVDF sample was measured to be 11.3 pC / N. After flash annealing, this coefficient increased to 16.7 pC / N. Further extension of the annealing time significantly improved the piezoelectric performance, reaching 41.0 pC / N. The consistent results of the two piezoelectric performance tests indicate that flash annealing does not significantly improve the piezoelectric performance of PVDF.

[0069] Ferroelectric polarization and piezoelectric force microscopy (PFM) test results are shown in Figure 6 . Figure a is the ferroelectric polarization curve of comparative example 4, Figure b is the ferroelectric polarization curve of embodiment 2, and Figure c is a comparison of the ferroelectric polarization curves of comparative example 4 and embodiment 2 at the same polarization voltage (1400 kV / cm). The PVDF-TrFE spin-coated film shows an obvious hysteresis loop, verifying that it has good intrinsic ferroelectric properties. The ferroelectric polarization curves of unannealed and flash-annealed PVDF-TrFE under the same polarization electric field conditions (1400 kV / cm) are compared. The flash-annealed sample has a significantly higher degree of polarization, which means that it has a stronger electric polarization response, can obtain a better polarization effect at a lower electric field, and exhibits stronger ferroelectric properties. The ferroelectric test results are consistent with the structural characterization results, both showing the great effect of flash annealing.

[0070] The present invention tests the energy output density of the films obtained in Example 1, Comparative Example 1 and Comparative Example 3 in parallel with resistors of different resistance values. The results are as follows: Figure 7 As shown in Figures a, b, and c, respectively, are the energy output density curves of the PVDF-TrFE electrospun fiber membranes obtained in Comparative Example 3 (no annealing), Example 1 (flash annealing), and Comparative Example 1 (long annealing). When the load resistance is 680 kΩ, the PVDF-TrFE electrospun membrane obtained in Example 1 (flash annealing) reaches a maximum output power density of 7.53 mW / m 2 , which is higher than the films of Comparative Example 3 (no annealing) and Comparative Example 1 (long annealing).

Claims

1. A method for preparing a sensing film, characterized in that: The following steps are involved: A solution containing a ferroelectric polymer is made into a thin film, and then annealed at 90-150° C. for 0.1-10 min to obtain the sensing film.

2. The method for preparing the sensor film according to claim 1, wherein: After the annealing treatment, the film is immersed in liquid nitrogen or other low-temperature treatment steps.

3. The method for preparing the sensor film according to claim 1 or 2, wherein: The ferroelectric polymer is PVDF-TrFE; further, in the PVDF-TrFE, the proportion of the number of copolymerized units of -TrFE is 10 to 50%.

4. The method for preparing the sensor film according to any one of claims 1 to 3, characterized in that: A solution is prepared by dissolving the polymer in a mixed solvent of N, N-dimethylformamide and acetone; further, the volume ratio of the mixed solvent of N, N-dimethylformamide and acetone is 1:4 to 4:1; further, the mass fraction of the solution is 10 to 20%; preferably 12%.

5. The method for preparing the sensor film according to any one of claims 1 to 4, characterized in that: The film is made by a solution processing method such as electrospinning or spin coating; Furthermore, the electrospinning conditions are as follows: spinning solution propulsion speed: 0.5 ~ 2.5 mL / h, preferably 0.9 mL / h; positive electrode voltage: 8 ~ 15 kV, preferably 12 kV; negative electrode voltage: -1 ~ -10 kV, preferably -1 ~ -3 kV, more preferably -2 kV; spinning distance: 5 ~ 15 cm, preferably 10 cm; spinning time: 2 ~ 5 h; Furthermore, the spin coating conditions are as follows: drop the solution onto a glass plate, turn on the vacuum pump to adsorb the glass plate onto the spin coater, set the spin coater speed to 500-1000 rpm, preferably 800 rpm, and continue the spin coating process for 1-5 min, preferably 2 min, to evenly spread the solution on the glass plate.

6. The method for preparing the sensor film according to any one of claims 1 to 5, characterized in that: The thickness of the film is 60 to 100 μm, preferably 80 μm.

7. The method for preparing the sensor film according to any one of claims 1 to 6, characterized in that: The annealing temperature is 130°C.

8. The method for preparing the sensor film according to any one of claims 1 to 7, characterized in that: The annealing time is 1 to 5 minutes.

9. A sensing film prepared by the method for preparing a sensing film according to any one of claims 1 to 8.

10. A sensor prepared from the sensing film according to claim 9.

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