One-step solution casting method for preparing polyvinylidene fluoride-based pyroelectric polymer film

Abstract

A one-step solution casting method for preparing a PVDF-based pyroelectric polymer film is provided, which belongs to the technical field of functional material preparation. The method comprises steps of: treating a substrate with a hydrophilic reagent to obtain a hydrophilically-modified substrate, and then casting the organic solution of polyvinylidene fluoride (PVDF) or its copolymer on the hydrophilically-modified substrate. After cured, the as-casted PVDF-based film shows pyroelectricity without undergoing any stretching or poling post-treatment, indicates that the dipoles of the one-step prepared film are aligned. The self-polarization of the prepared film is attributed to a hydrogen bond induced layer-by-layer electrostatic self-assembly growth mechanism. The method is simple, low cost, high efficient, high capability to produce thick and large-area film with smooth morphology and ease to be scalized.

Claims

1. A method for preparing a polyvinylidene fluoride (PVDF)-based pyroelectric polymer film, comprising steps of: treating a substrate with a hydrophilic reagent, introducing a hydrophilic group on a surface of the substrate to obtain a hydrophilically-modified substrate; casting a PVDF-based polymer solution onto the hydrophilically-modified substrate to form a liquid film, wherein dipoles in a first polymer layer closest to the substrate are orderly aligned through hydrogen bonds formed between the F atoms in the PVDF-based polymer and the hydrophilic group modified on the substrate, thus induces subsequent polymer layers to be aligned in a layer-by-layer manner by dipole-dipole interactions, in such a manner that a PVDF-based polymer film with orderly arranged dipoles is obtained by curing the liquid film via controlling a casting temperature and casting time to volatilize solvent in the liquid film.

2. The method for preparing a PVDF-based pyroelectric polymer film, as recited in claim 1, wherein the hydrophilic group modified on the substrate surface comprises: a hydroxyl group (—OH), an amino group (—NH.sub.2) or a carboxyl group (—COOH).

3. The method for preparing a PVDF-based pyroelectric polymer film, as recited in claim 1, wherein the hydrophilic reagent used to treat the substrate comprises a mixed solution of concentrated sulfuric acid and hydrogen peroxide or a mixed solution of aqueous ammonia, hydrogen peroxide and water.

4. The method for preparing a PVDF-based pyroelectric polymer film, as recited in claim 1, wherein the substrate comprises a glass substrate or a silicon wafer.

5. The method for preparing a PVDF-based pyroelectric polymer film, as recited in claim 1, wherein the PVDF-based polymer comprises polyvinylidene fluoride (PVDF), a copolymer of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] and a copolymer of poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)].

6. The method for preparing a PVDF-based pyroelectric polymer film, as recited in claim 1, wherein a concentration of the PVDF-based polymer solution is at a range of 3 wt %-40 wt %.

7. The method for preparing a PVDF-based pyroelectric polymer film, as recited in claim 1, wherein a casting temperature is at a range of 40° C.-60° C., and casting time is at a range of 2-4 hours.

8. A method for preparing a PVDF-based pyroelectric polymer film, comprising steps of: step (1): preparing PVDF-based pyroelectric polymer solution, comprising: according to application demands, selecting a suitable pyroelectric polymer material comprising PVDF and its copolymers: dissolving pyroelectric polymer powder or particles into a suitable solvent; and stirring evenly under heating to obtain a pyroelectric polymer solution; step (2): treating a substrate with a hydrophilic reagent; comprising: putting the substrate into a beaker containing the hydrophilic reagent; then placing the beaker into a water bath with a constant temperature for a fixed period of time; taking the substrate out, and then washing and drying with deionized water; step (3): preparing the PVDF-based pyroelectric polymer film; comprising: casting the pyroelectric polymer solution on the substrate which is treated by the hydrophilic reagent to form a liquid film, and curing the liquid film by controlling a casting temperature and casting time, in such a manner that the solvent in the liquid film is volatilized, thereby curing to obtain a PVDF-based pyroelectric polymer film having orderly aligned dipoles; wherein when the substrate (4) is treated with the hydrophilic reagent, a hydrophilic group is modified on the substrate surface; due to great difference in electronegativity between an oxygen atom and a hydrogen atom, the hydrogen atom of a hydroxyl group (5) on the surface of the substrate (4) is an electron-deficient site and positively charged; wherein the fluorine atom (1) of the PVDF-based pyroelectric polymer film is most electronegative and is negatively charged; the CF.sub.2 group of the polymer which is half of a dipole, is negatively charged; while its counterpart, the CH.sub.2 group is positively charged.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view showing an arrangement of a first layer of polyvinylidene fluoride (PVDF) molecules on a substrate which is modified with hydroxyl groups.

(2) FIG. 2 is a schematic view showing the arrangement of the first layer of PVDF molecules on the substrate which is not treated with hydrophilic reagent.

(3) FIG. 3 is a diagram showing pyroelectric response signal of a PVDF film provided in Embodiment 1 of the present invention.

(4) References in the Figs: 1—fluorine atom in PVDF; 2—carbon atom in PVDF; 3—hydrogen atom in PVDF; 4—substrate; 5—hydroxyl group on the substrate; 6—hydrogen bond.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(5) The technical solutions of the present invention will be clearly and completely described in the following with reference to the accompanying drawings and preferred embodiments, in such a manner that those skilled in the art are capable of understanding the principles and characteristics of the present invention.

(6) A one-step solution casting method for preparing a PVDF-based pyroelectric polymer film is characterized in comprising following steps of:

(7) step (1): preparing PVDF-based pyroelectric polymer solution, comprising:

(8) according to application demands, selecting a suitable pyroelectric polymer material, wherein in the present invention, PVDF and its copolymers thereof are mainly selected; dissolving the pyroelectric polymer powder or particles into a suitable solvent; and stirring evenly under heating to obtain a pyroelectric polymer solution;

(9) step (2): treating the substrate with a hydrophilic reagent; comprising:

(10) putting a clean substrate into a beaker containing a hydrophilic reagent; then placing the beaker into a water bath with a constant temperature for a fixed period of time; taking the substrate out, and then washing and drying with deionized water;

(11) step (3): preparing the PVDF-based pyroelectric polymer film; comprising:

(12) casting the pyroelectric polymer solution on the substrate which is treated by the hydrophilic reagent to form a liquid film, and curing the liquid film by controlling a casting temperature and casting time, in such a manner that the solvent in the liquid film is volatilized, thereby curing to obtain a PVDF-based pyroelectric polymer film having orderly aligned dipoles.

(13) When the substrate 4 is treated with a hydrophilic reagent, a hydrophilic group such as hydroxyl group (—OH) 5 is modified on the substrate surface. Due to the great difference in electronegativity between an oxygen atom and a hydrogen atom, the hydrogen atom of a hydroxyl group 5 on the surface of the substrate 4 is an electron-deficient site and positively charged. On the contrary, the fluorine atom 1 of the PVDF-based polymer in the liquid film has strongest electronegativity and is negatively charged. That is to say, the CF.sub.2 group of the polymer which is half of a dipole, is negatively charged; while its counterpart, the CH.sub.2 group is positively charged. Therefore, when a PVDF-based polymer is casted on a hydroxyl group 5 modified substrate 4, the fluorine atom 1 in the polymer as well as the CF.sub.2 group is attracted by the hydroxyl group 5 on the substrate 4, moves closing to it and forms a hydrogen bond 6 with it; while the CH.sub.2 group is repelled by the hydroxyl group 5 and moves in the direction away from the substrate surface. The opposite moving directions of the CF.sub.2 and CH.sub.2 groups drive the carbon chain in the PVDF-based polymer to rotate, facilitates the formation of a TTT conformation, namely, the β phase. When the hydrogen bonds 6 are eventually formed, the first layer of the polymer molecules is immobilized on the substrate surface. In such a way the fluorine atoms 1 together with the CF.sub.2 groups are orderly arranged at the substrate 4 side, whilst the hydrogen atoms 3 together with the CH.sub.2 groups are orderly arranged at the film side, as illustrated in FIG. 1. This means that the dipoles of the first molecular layer of the PVDF-based polymer deposited on the substrate 4 are aligned. Thus, the new grown solid surface is also positively charged, although not by hydroxyl groups but by CH.sub.2 dipoles. It attracts the CF.sub.2 dipoles of the polymer in the solution via a dipole-dipole interaction, induces the film further grows in a layer-by-layer mode at the effect of an electrostatic self-assembly mechanism. The layer-by-layer growth process is finished at the liquid film stage, then the film is fully cured, the PVDF-based polymer with aligned dipoles is realized.

(14) If the substrate is not treated with a hydrophilic reagent, the arrangement of the first molecular layer of polyvinylidene fluoride on the substrate is schematically shown in FIG. 2. It can be seen that the dipoles in the first polymer layer on the substrate 4 are disorderly arranged, which causes the arrangement of the dipoles in the subsequent polymer layers to be disordered as well.

Embodiment 1

(15) Dissolving polyvinylidene fluoride (PVDF) powder in a solvent of N,N-dimethylformamide (DMF) to prepare a solution having a PVDF mass percentage of 15%, magnetically stirring for 2 hours under heating at 60° C. in a water bath until the PVDF solute is completely dissolved;

(16) preparing a Piranha solution as a hydrophilic reagent by mixing 98% concentrated sulfuric acid and 30% hydrogen peroxide with a volume ratio of 7:3 in a breaker; immersing a clean glass substrate in the Piranha solution; then sending the breaker into a water bath and keeping a constant temperature of 50° C. for 1 hour, then taking out the glass substrate and washing with deionized water and drying to obtain a glass substrate modified with hydroxyl groups;

(17) casting the PVDF solution prepared above on the glass substrate modified with hydroxyl groups to form a liquid film; adjusting a temperature of a controllable heating plate to 40° C. to evaporate the DMF solvent in the liquid film, solidifying for 4 hours to obtain a PVDF film with a size of 20×20 cm.sup.2 and a thickness of 52 μm.

(18) In the Embodiment 1, in order to test a pyroelectric response of the as-casted PVDF film, each side of the PVDF film is deposited on an aluminum electrode with a size of 15×15 mm.sup.2 by vacuum evaporation, and then the two aluminum electrodes are connected to a signal amplifier through conductive leads, and the signal amplifier is connected to an oscilloscope by a coaxial cable. A specific process of the test is as follows.

(19) The present invention utilizes an infrared laser emitter to emit an infrared laser with a spot diameter of ϕ3 mm, and an output power and an output frequency of the infrared laser are modulated by a signal generator to 80 mW and 1 Hz respectively. The infrared laser is vertically irradiated on an upper electrode of the PVDF film prepared in the Embodiment 1 and the corresponding signals collected by the oscilloscope is shown in FIG. 3. It can be seen that the frequency of the recorded signal is also 1 Hz, indicating that the signal is a pyroelectric response to the pulsed laser irradiation loaded on the PVDF film. It demonstrates that the PVDF film not only has high content of polar phase but also the polar phase is dipolar oriented, otherwise it can't show pyroelectricity. It should be note that the PVDF film prepared in Embodiment 1 does not undergo any electric poling process, implies that the as-casted film is self-polarized. In Embodiment 1, the pyroelectric response is 3.1 V.

Contrast Embodiment

(20) For comparison, a clean glass substrate which is not treated with a hydrophilic reagent is employed as a substrate. A DMF solution with a mass percentage of PVDF of 15% is casted on the surface of the glass substrate using identical casting parameters as in Embodiment 1 to prepare a PVDF film as a contrast embodiment. And then the contrast film is tested by the identical laser irradiation method to measure an open circuit voltage between the two electrodes with each on one side of the film. However, the oscilloscope does not record any signal, which indicates that the dipoles in the film on an unmodified substrate are disorderly arranged, so that pyroelectric signals are not capable of being generated.

Embodiment 2

(21) Dissolving a copolymer of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] powder with VDF to TrFE mole ratio of 80:20 in a solvent of N,N-dimethy lacetamide (DMAc) to prepare a solution having a P(VDF-TrFE) mass percentage of 15%, magnetically stirring for 2 hours under heating at 60° C. in a water bath until the P(VDF-TrFE) solute is completely dissolved;

(22) putting a clean glass substrate into a beaker containing a Piranha solution (mixture of 98% concentrated sulfuric acid and 30% hydrogen peroxide with a volume ratio of 3:1); then sending the breaker into a water bath and keeping a constant temperature of 50° C. for 3 hours, then taking out the glass substrate and washing with deionized water and drying to obtain a glass substrate modified with hydroxyl groups;

(23) casting the P(VDF-TrFE) solution prepared above on the glass substrate modified with the hydroxyl groups to form a liquid film; adjusting a temperature of a controllable heating plate to 50° C. to evaporate the DMAc solvent in the liquid film, solidifying for 4 hours to obtain a P(VDF-TrFE) film with a size of 20×20 cm.sup.2 and a thickness of 56 μm.

(24) Adopting the identical test method as in Embodiment 1, infrared laser is irradiated onto the P(VDF-TrFE) film prepared in Embodiment 2. The oscilloscope records a pulsed pyroelectric response signal of 1 Hz with an amplitude of 3.3V.

Embodiment 3

(25) Dissolving polyvinylidene fluoride (PVDF) powder in a solvent of N,N-dimethylformamide (DMF) to prepare a solution having a PVDF mass percentage of 10%, magnetically stirring for 2 hours under heating at 60° C. in a water bath until the PVDF solute is completely dissolved;

(26) putting a clean (100) monocrystalline silicon wafer into a beaker containing a mixed solution of 25% aqueous ammonia, 30% hydrogen peroxide solution and deionized water with a volume ratio of 1:2:7; then sending the breaker into a water bath and keeping a constant temperature of 90° C. for 1 hour, then taking out the silicon wafer and washing with deionized water and drying to obtain a monocrystalline silicon wafer which is modified with hydroxyl groups;

(27) casting the PVDF solution prepared above on the monocrystalline silicon wafer which is modified with the hydroxyl groups to form a liquid film; adjusting a temperature of a controllable heating plate to 40° C. to evaporate the DMF solvent in the liquid film, solidifying for 4 hours to obtain a PVDF film with a diameter of 8 inches and a thickness of 38 μm. Adopting the identical test method as in Embodiment 1, infrared laser is irradiated onto the PVDF film prepared in Embodiment 3. The oscilloscope records a pulsed pyroelectric response signal of 1 Hz with an amplitude of 1.7 V.

Embodiment 4

(28) Dissolving a copolymer of Poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] powder with VDF to HFP mole ratio of 90:10 in a solvent of N,N-dimethy lacetamide (DMAc) to prepare a solution having a P(VDF-HFP) mass percentage of 15%, magnetically stirring for 2 hours under heating at 60° C. in a water bath until the P(VDF-HFP) solute is completely dissolved;

(29) putting a clean (100) monocrystalline silicon wafer into a beaker containing a mixed solution of 25% aqueous ammonia, 30% hydrogen peroxide solution and deionized water with a volume ratio of 1:2:7; then sending the breaker into a water bath and keeping a constant temperature of 90° C. for 1 hour, then taking out the silicon wafer and washing with deionized water and drying to obtain a monocrystalline silicon wafer which is modified with hydroxyl groups;

(30) casting the P(VDF-HFP) solution prepared above on the monocrystalline silicon wafer which is modified with hydroxyl groups to form a liquid film; adjusting a temperature of a controllable heating plate to 40° C. to evaporate the DMAc solvent in the liquid film, solidifying for 4 hours to obtain a P(VDF-HFP) film with a diameter of 8 inches and a thickness of 55 μm.

(31) Adopting the identical test method as in Embodiment 1, infrared laser is irradiated onto the P(VDF-HFP) film prepared in Embodiment 4. The oscilloscope records a pulsed pyroelectric response signal of 1 Hz with an amplitude of 2.7 V.

(32) One skilled in the art will understand that the embodiment of the present invention as shown in the drawings and described above is exemplary only and not intended to be limiting.

(33) It will thus be seen that the objects of the present invention have been fully and effectively accomplished. Its embodiments have been shown and described for the purposes of illustrating the functional and structural principles of the present invention and is subject to change without departure from such principles. Therefore, this invention includes all modifications encompassed within the spirit and scope of the following claims.