GB2188585A

GB2188585A - Polymeric piezoelectric material

Info

Publication number: GB2188585A
Application number: GB08706535A
Authority: GB
Inventors: Tadahiro Asada; Kenji Hijikata; Takayuki Ishikawa
Original assignee: Polyplastics Co Ltd
Current assignee: Polyplastics Co Ltd
Priority date: 1986-03-26
Filing date: 1987-03-19
Publication date: 1987-10-07
Also published as: GB8706535D0; KR870008933A; JPS62224090A; GB2188585B; SG2690G; KR910006349B1; JPH0575192B2

Abstract

A polymeric piezoelectric material is obtained by heat-electretizing a molded product of a polyester containing an aromatic hydroxycarboxylic acid residue and exhibiting anisotropism in the molten state and/or a polyester containing partly in the same molecular chain a polyester exhibiting anisotropism in the molten state. The material can be used at above 150 DEG C in a variety of applications, eg. pressure sensitive element, movable element, acoustic transducer, medical transducer, infrared and radiation sensor.

Description

SPECIFICATION Polymeric piezoelectric material The present invention relates to a polymeric piezoelectric material obtained by heat-electretizing the molded product of a polyester which contains an aromatic hydroxycarboxylic acid residue and exhibits the anisotropism in the molten state.

It is known to persons skilled in the art that a polymericferroelectric material can be used as a piezoelectric material or pyroelectric material when it is heat-electretized to give an electret element. Known examples of such polymeric piezoelectric material include polyvinylidene fluoride, polytrifluoroethylene, and polyvinylidene cyanide-vinyl acetate copolymer.

These polymeric piezoelectric materials are soft and can be easily made into film of large area. Because of the good moldability characteristic of polymers the inorganic piezoelectric materials do not possess, they are expected to find a wide range of applications.

However, these polymeric piezoelectric materials are limited in temperature at which they are used. This is because the piezoelectric property is derived from the high order structure of the polymer. In otherwords,the direction ofthe dipole moment in the molecular chain is changed and frozen when the polymer is heat-electretized. Therefore, they cannot be used attemperatures higher than about 140 to 1 60'C.

In view ofthe above-mentioned problems, the present inventors carried out extensive studies to develop a new polymeric piezoelectric material which can be used at highertemperatures and can be easily heat-electretized. In their studies, the present inventors paid their attention to the fact that a polymerthat exhibits the anisotropism in the moltent state, or a liquid crystal polymer, is easily mobile for polarization orientation. It was found that the one containing an aromatic hydroxycarboxylic acid residue in the molecular chain has the anisotropism of dipole. This finding led to the present invention.

Accordingly, the present invention relates to a polymeric piezoelectric material which is obtained by heat-electretizing a molded product of a polyester containing an aromatic hyroxycarboxylic acid residue which exhibits the anisotropism in the molten state and/ora polyester containing partly in the same molecularchain a polyester which exhibits the anisotropism in the moltentstate.

The polyester used in this invention is a polymer composition which exhibits the optical anisotropism in the molten state and is capable of thermoplastic melt-processing. It generally falls under the category of thermotropic liquid crystal polymer.

The polymer which forms the anisotropic molten phase has the properties of permitting the polymer molecularchainsto assume regular parallel arrangement in the molten state. The state in which molecules are arranged in such a manner is referred to as the liquid crystal state. The polymer like this is usually produced from a monomerwhich has athin, long, and flat configuration, has a high rigidity along the long axis of the molecule, and has a plurality of chain extension linkages which are coaxial or parallel with one another.

The properties of the anisotropic molten phase can be determined by an ordinary polarization test using crossed nicols. More particularly, the properties can be determined with a Leitz polarizing microscope of 40 magnifications by observing a sample placed on a Leitz hot stage in a nitrogen atmosphere. The polymer is optically anisotropic. Namely, it transmits a light when it is placed between the crossed nicols. When the sample is optically anisotropic, the polarized light can be transmitted through it even in a still state.

The resin molded product of this invention is characterized in thatthe polyester which exhibits the anisotropism in the molten state is one which contains an aromatic hydroxycarbocylic acid residue and aromatic substituted derivative residue thereof. The hydroxy group and carboxyic acid group should preferably be substituted directly on the aromatic ring, and the hydroxyl group and carboxylic acid group may be on the same aromatic ring or different aromatic rings. In either cases, they should be in the same molecule of the aromatic cyclic compound. The aromatic hydroxycarboxyic acid resin should preferably be a compound composed of one or more kinds selected from the hydroxybenzoic acid residue, hydroxynaphthoicacid residue, and their aromatic substituted derivative residue.The aromatic substituted derivative residue should have the substituent group selected from functional group which imparts the anisotropism to the intramolecular dipole moment of the hydroxycarboxylic acid compound and is atthe substitution position to impart such anisotropism. In addition, the substituent group of the aromatic substituted derivative residue is one which imparts the anisotropism to the dipole moment in the direction of the line connecting the carbon atoms on the aromatic ring to which the hydroxyl group and carboxylicacid group are connected, and is at the substitution position to impart such anisotropism. Preferred examples are those represented by the following formulas (I) to (VII).

(wherein the group consisting of X1, X2, and X3 and the group consisting of Y1, Y2, and3 are separated by a line which intersects at right angles a line connecting the carbon atoms on the aromatic ring to which the hydroxyl group and carboxylic acid group are connected, at the center thereof; each of said groups is one or more kinds selected from substituent groups which differfrom one another in dipole moment; the same group does not contain those which differfrom one another in the direction of dipole moment; and the unsubstituted position in each group represents a hydrogen atom.) The substituent group is selected from cyano group, nitro group, aldehyde group, carboxylic acid ester, carboxylic acid group, hydroxyl group, hydrogen, halogen compound, amino group, imino group, azo grnup# alkoxy group, alky group, phenyl group, acyl group, sulfoxy group, and sulfide group. Preferably, it is selected from hydrogen, cyano group, nitro group, acetoxy group, chlorine, bromine, phenyl group, alkyl group, methoxy group, amino group, and alkylsubstituted amino group.

The above-mentioned polyesterwhich exhibits the anisotropism in the molten state may be a homopolymer or a block or graft copolymer. In the latter case, the segments of the polyester are copolymerized in the other polyester which may not be capable of polarization orientation. The other polyester is one or more kinds selected from aromatic polyester, polycarbonate, polyethersulfone, polyacrylate, and polyalkylene terephthalate.

The polymer which exhibits the anisotropism in the molten state may be contained in the other thermoplastic polymer. It may be dispersed in the miscible form ir immiscible form. Those which are uniformly dispersible are preferable, and their examples include aromatic polyester, polycarbopate, polyether sulfone, polyacrylate, and polyalkylene terephthalate.

The polyester which exhibits the anisotropism in the molten state may be produced by a variety of ester-forming processes.

The monomer compounds can be reacted by melt acidolysis in the absence of any heat exchange fluid. In this process, the monomers are heated to form a melt of reactants. As the reaction proceeds, the solid polymer partices begin to suspend in the melt. In the final stage of the condensation reaction, the reaction system may be evacuated to facilitate the removal of volatile by-products such as acetic acid and water.

A slurry polymerization process may also be employed in the preparation of fully aromatic polyesters suitable for use in the present invention. In this process, the solid product is obtained in the form of suspension in a heat exchange medium.

In either of said melt acidolysis and slurry polymerization processes, the organic monomeric reactants from which the fully aromatic polyesters can be derived may be employed in the reaction in a modified form obtained by esterifying the hydroxyl group oftie monomer at ambienttemperature (i.e., in the form oftheir lower acyl esters). The lower acyl groups have preferably about 2 to 4 carbon atoms. Preferably, acetate esters of the organic monomeric reactants are employed in the reaction. Also, the modified form (i.e., phenol ester) formed by esterifying the carboxylic acid group may be used for the reaction.

Typical exampes of the catalysts that can be used in both of the melt acidolysis and slurry processes include dialkyltin oxides (such as dibutyltin oxide), diaryltin oxides, titanium dioxide, antimonytrioxide, alkoxytitanium silicates, titanium alkoxides, alkali metal and alkaline earth metal salts of carboxylic acids (such as zinc acetate), Lewis acids (such as BF3), and hydrogen halides and other gaseous acids (e.g., HC1).

The catalyst is generally used in an amount of about 0.001 to 1 wt%, particularly about 0.01 to 0.2 wt%, based on the monomer.

The aromatic polymers suitable for use in the present invention aresubstantiallyinsolube in ordinary solvents and, therefore, they are unsuitable for use in a solution processing. However, these polymers can be processed easily by the ordinary melt processing. Particularly preferred aromatic polymers are soluble in pentafluorophenol to some extent.

The aromatic polyesterwhich is preferably used in the present invention have a weight-average molecular weight of about 1,000 to 200,000, preferably about 2,000to 50,000, particularly about 3,000to 25,000.

The molecularweight may be determined by gel permeation chromatography or other standard methods which need no polymer solution, such as a method in which terminal groups are determined by infrared spectroscopy using a compression-molded film sample. The molecularweight may also be determined by the light-scattering method using a solution of pentafluorophenol.

The above-mentioned aromatic polyester has an inherent viscosity (I.V.) of at least about 0.5 dl/g,for example, about 0.5 to 10.0 dl/g as measured in a0.1 wt%solution in pentafluorophenol at60#.

The polyester which exhibits the anisotropism in the molten state may contain anotherferroelectric substance. Such ferroelectric compound enhances the properties of the polyester used as a ferroelectric material. The ferroelectric com pound may be inorganic com pounds, organic com pou nds, or polymeric compounds.

Examples of the inorganic compounds include quartz, lead zirconate titanate, potassium hydrogen phosphate, barium titanate, lead titanate, lead niobate, lithium niobate, lithium tantalate, strontium barium niobate, Pb(B1 B2)03, and PbTi03~PbZr03~Pb(B1~B2)03 (where B1 represents Mg, Co, Ni, Mn, or Zn; and B2 represents Nb,Ta,Sb,orW).

Examples ofthe organic compounds include low molecular liquid crystal compounds, Rochelle salt, and triglycin sulfate. Expanations on low-molecular liquid crystal compounds will be found in "Ekisho no Saishin Gijutsu" (Latest Technology of Liquid Crystals) by Matsumoto and Tsunoda (Kogyo Chosakai) and "Handbook of Liquid Crystals by K. Kelkerand R. Hatz (Weinheim,1980).

Examples ofthe polymeric compound incude polymers and copolymers ofvinylidenefluoride, trifluoroethylene, vinylidene cyanide, and chloroacrylonitrile.

The polyester obtained in this invention is usually heat-electretized when it is formed into sheet orfilm.

After the heat-electretizing, it may be used in the form of powder having polarization orientation. This powder may also be dispersed into a thermosetting or thermoplastic resin.

Examples ofthethermosetting resin include phenolic resin, epoxy resin, melamine resin, urea resin, unsaturated polyester resin, and alkyd resin.

Examples of the thermoplastic resin include polyethyene, polypropyene, polybutylene, polybutadiene, polyisoprene, polyvinyl acetate. polyvinyl chloride, polyvinylidene chloride, polystyrene, acrylic resin, ABS resin, AS resin, BS resin, polyurethane, silicone resin, fluoroplastic, cyan resin, polyacetal, polycarbonate, polyethyene terephthalate, polybutylene terephthalate, aromatic polyester, polyamide, polyacrylonitrile, polyvinyl alcohol, polyvinyl ether, polyetherimide, polyamideimide, polyetherimide, polyetherketone, polyethersulfone, polysulfone, polyphenylene sulfide, and polyphenylene oxide.

Preferred ones are aromatic polyester, polycarbonate, polyethersulfone, polyacrylate, polyalkylene terephthalate, and polymers and copolymers derived from vinylidene fluoride, trifluoroethylene, vinylidene cyanide, and chloroacrylonitrile.

In another usage, the composite material containing a heat-electretized powder may be further heat-electretized.

The heat-electretizing can be carried out by the method known to persons skilled in the art. Both sides of the sheetorfilm ofthe polyester ofthis invention are treated for electric condution, and a voltage is applied to the sheet orfilm with heating. The voltage application may be performed continuously or intermittenty (in pulse). The intermittent application in pulse is preferred for the ease of polarization orientation. The object is achieved with minimum heating sufficient to impart kinetic energy that causes the polarization inversion. For complete polarization inversion, the heating temperature should be higher than the melting poipt of the polyester.At such a temperature the polarization orientation takes place very quickly because ofthe properties of liquid crystal the polyester originally possesses. Where the polarized anisotropic strain is required, it is possible to control the temperature and the intensity of electricfield.

The polyester and composite product thereof obtained in this invention may be incorporated with a variety of additives by the method known to persons skilled in the art. The additives include plasticizer, antioxidant, UV light absorber, antistatic agent, flame retardant, dye and pigment, surface treatmenttherefor, and reinforcing fiber and inorganicfiller.

The present invention provides a polymeric piezoelectric material which is obtained by heat-electretizing a molded product of a polyester which contains an aromatic hydroxycarboxylic acid residue and exhibits the anisotropism in the molten state and/ora polyester containing partly in the same molecuarchain a polyester which exhibits the anisotropism in the molten state. The polymeric piezoelectric material has several features. It has extremely high heat resistance. The molecules are extremely mobile in the molten state because the molecular chain is rigid. It is quickly responsive to the heat-electretizing. Therefore, the polymeric piezoelectric material of this invention finds a large variety of applications. It can be used in a hot environment above 1 500C in which the ordinaryferroelectric polymeric materials cannot be used.It can be used as a heat-resistant piezoelectric material such as a pressure-sensitive element, movable element, acoustic transducer, and a medical transducer. It can also be used as a pyroelectric material as an infrared sensor and radiation sensor. Detailed explanations on these application will be found in "Fine Electronics and Highfunctional Materials", edited by Higaki (CMC Co., Ltd., June 1983), P. 168.

The invention is now described in more detail with reference to the following examples, which should not be construed as limiting the scope of the invention.

Example I 1261 parts by weight of 4-acetoxybenzoic acid and 691 parts by weight of 6-acetoxy-2-naphthoic acid were placed in a reactor provided with a stirrer, a nitrogen inlet tube, and a distillation tube. The mixture was heated to 2500C under a nitrogen stream and stirred vigorously atthattemperature for 3 h and then at2B00C for 2 h while acetic acid was distilled off from the reactor. The temperature was elevated at320 C and the feeding of nitrogen was stopped. The pressure in the reactor was reduced gradually to 0.1 mmHg after20 min. The mixture was stirred atthattemperature underthat pressure for 1 h.

The resulting polymer had an intrinsic viscosity of 5.4 as determined in pentafluorophenol at a concentration of 0.1 wt% at 60 C.

The resulting polymer has the following constitutional units.

The resulting polymer was made into a 20film thick film by using aT-die extruder. The extruder and T-die were set at 3000C and 2900C respectively. The film was stretched at a draw ratio of 1/10. Both sides ofthefilm were coated with silver by va pour deposition. The film was heated at 2600C and subjected to the application of Doc voltage (100 kV/cm) for 60 minutes. Afterthat, the film was cooled rapidly. The resulting test piece had a piezoelectric constant d31 of 8.5 x 10.8 CGSesu and a pyroelectric constant of 5.7 x 10.10 C/cm K. Ad31 measured in the same manner at 160'was 10 x 10#8CGSesu.

Example2 138.2 parts by weight of polyethyeneterephthaiate (having an intrinsic viscosity of 0.36) was added to 162 parts by weight of polyester (having an intrinsic viscosity of 0.77) preliminarily polymerized at 260"for3 hours in the same manner as in Example 1.The reaction was continued in the same reactorat280 Cfor4 hours with vigorous agitation. The reacation temperature was raised to 320 C and the feeding of nitrogen was stopped. The reactor was evacuated to 0.1 mmHg after 15 minutes. Stirring was continued for 1 hourat this temperature and pressure.

The resulting polymer had an intrinsic viscosity of 2.9 as determined in pentafluorophenol at a concentration of 0.1 wt%at60 C.

The resulting polymer was found to be composed of 40 mol% of polyethylene terephthalate and 60 mol% of hydroxybenozic acid and hydroxynaphthoic acid.

Atest piece was prepared in the same manner as in Example 1, and itwas heated at2000C and subjected to the application of DC voltage (100kV/cm) for 60 minutes. The resulting test piece had a piezoelectric constant d31 of 9.1 x 10-8 CGSesu and a pyroelectric constant of 6.2 x 10.10 C/cm K. A d31 measured in the same manner at 160 C was 12 x 10~3CGSesu.

Example3 900 parts by weight of 4-acetoxybenzoic acid, 431 parts by weightor 4-acetoxy-3-chlorobenzoic acid, and 690 parts by weight of 6-acetoxy-2-naphthoic acid were placed in a reactor provided with a stirrer, a nitrogen inlet tu be, and a distillation tube. The mixture was heated to 2500C under nitrogen stream and stirred vigorously at that temperature for 3 h and then at 2800C for 2 h while acetic acid was distilled off from the reactor. The temperature was elevated to 3200C and the feeding of nitrogen was stopped. The pressure in the reactor was reduced gradually to 0.1 mmHg after 20 min. The mixture was stirred at that temperature under that pressure for 1 h.

The resulting polymer had an intrinsic viscosity of 5.0 as determined in pentafluorophenol at a concentration of 0.1 wt% at600C.

Atest piece was prepared in the same manner as in Example 1, and itwas heated at 220 C and subjected to the application of DC voltage (100 kV/cm) for 60 minutes. The resulting test piece had a piezoelectricconstant d31 of 8.8 x 10.8 CGSesu and a pyroelectric constant of 5.9 x 10.10 C/cm#K. A d31 measured in the same manner at160 Cwas12x 108CGSesu.

Comparative Example 1 166 parts by weight of terephthalic acid, 166 parts by weight of isophthalic acid, and 250 parts by weight of diacetoxymethylhydroquinone were placed in a reactor provided with a stirrer, a nitrogen inlet tube, and a distillation tube. The mixture was heated to 260C under a nitrogen stream and stirred vigorously atthat temperature for 2.5 h and then at 2800C for 3 h while acetic acid was distilled offfrom the reactor. The temperature was elevated at 320 C and the feeding of nitrogen was stopped. The pressure in the reactor was reduced gradually to 0.1 mmHg after 15 min. The mixturewas stirred at that temperature under that pressure for 1 h.

The resulting polymer had an intrinsic viscosity of 0.87 as determined in a 1:1 mixed solvent of tetrachloroethane and phenol at a concentration of 0.5 wt%.

The resulting polymer was heat-electretized in the same manner as in Example 3. The piezoelectric constant d31 was 9.2 x 108CGSesu Example 4 The polymer obtained in Example 1 was slowly and uniformly incorporated with ceramics powder of lead zirconate-titanate (PbZrO3-PbTiO3) while heating at 280 C with hot rolls. The amount of the ceramics powder was 30vol% in the resulting composite material. The resulting composite was made into a 50- > mthickfilm by using a hot press. The resulting film was made into a test piece in the same manner as in Example 1.The test pieces at 2200C was subjected to the application of DC electric field (200 kV/cm) for 60 minutes.The piezoelectric constant d31 was 2.6 > c CGSesu.

Example 5 The polymer obtained in Example 2 was made into a 50- > mthickfilm in the same manner as in Example 4.

The hot rolls were heated to 220 C. The test pieces at 220 C was subjected to the application of DC electric field (200 kV/cm) for 60 minutes. The piezoelectric constant d31 was 2.7 x 1 ### CGSesu.

Example 6 The polymer obtained in Example 1 was heated to 2800C using a hot press. While being held between copper plates, the polymer at 2800C was subjected to the application of DC electric field (100 kV/cm) for 60 minutes, followed by rapid cooling. The resulting film underwent cryogenic grinding at-600C. There was obtained needle powder. The powder was uniformly incorporated into polyacetal ("Duracon M-90", a product of Polyplastics Co., Ltd.) using hot rolls at 1 80#C. The amount of the powder was 40 vol% based on the resulting composite. The resulting composite was made into a 50- m thick film using a hot press. The film at 1 000C was subjected to the application of DC electric field (200kV/cm) for 60 minutes in the same manner as in Example 1. The piezoelectric constant d31 was 2.2 x 10.8 CGSesu. Incidentally, the piezoelectric constant of the polyacetal measured in the same manner was 7.7 x 10.10 CGSesu.

Example 7 The electretized needle powder obtained in Example 6 was incorporated with polyvinylidenefluoride resin (KF-1 100 made by Kureha Chemical Co., Ltd.) using hot rolls in the same manner as in Exampe 6. The amount ofthe powder was 50 vol% based on the resulting composite. The test piece at 1 000C was subjected to the application of DC electric field (200 kV/cm) for 60 minutes. The piezoelectric constantd31 was 1.5 x 110-7 CGSesu. Incidentally, the piezoelectric constant ofthe polyvinylidene fluoride alone measured in the same mannerwas 1.0 x 107CGSesu.

Example 8 The polymer obtained in Example 1 underwent cryogenic grinding at -600C. There was obtained needle powder. The powder was uniformly incorporated into polybutyene terephthalate ("Duranex 2002", a product of Polyplastics Co., Ltd.) using hot rolls at 230"C. The amount of the powder was 50 vol% based on the resulting composite. The resulting composite was made into a 50-m thicktest piece using a hot press. The test piece at 2000C was subjected to the application of DC electric field (200 kV/cm) for 60 minutes in the same manner as in Example 1. The piezoelectric constant d31 was 4.0 x 10-8 CGSesu. Incidentally, the piezoelectric constant ofthe polybutylene terephthalate measured in the same manner was 8.0 x 1 0#10 CGSesu.

Example 9 The polymer obtained in Example 2 was made into needle powder in the same manner as in Example 8, and the powder was made into a test piece which was subsequently electretized. The piezoelectric constant d31 was 3.8 x 108 CGSesu.

Claims

1. A polymer piezoelectric material which is obtained by heat-electretizing a molded product of a polyester containing aromatic hyroxycarboxylic acid residue which exhibits the anisotropism in the molten state and/or a polyester containing partly in the same molecuar chain a polyesterwhich exhibits the anisotropism in the molten state.

2. A polymeric piezoelectric material as setforth in Claim 1, wherein the aromatic hydroxycarboxylic acid residue is one or more kinds of compounds selected from hydroxybenzoic acid residue, hydroxynaphthoic acid residue, and aromatic substituted derivative residues thereof.

3. A polymeric piezoelectric material as setforth in Claim 1,wherein the polyester containing an aromatic hydroxycarboxylicacid resin and also containing partly in the same molecular chain a polyester which exhibits the anisotropism in the molten state is a copolymercomposed of a polyesterwhich exhibits the anisotropism in the molten state and one or more kinds selected from other aromatic polyesters, polycarbonate, polyethersulfone, polyacrylate, and potyalkyleneterephthalate.

4. A polymeric piezoelectric material as set forth in Claim 1, wherein the polyester containing an aromatic hydroxycarboxyic acid residue which exhibits the anisotropism in the molten state and/or the polyester containing partly in the same molecular chain a polyester which exhibits the anisotropism in the molten state is contained in other thermoplastic polymer.

5. A polymeric piezoelectric material as setforth in Claim 2, wherein the substituent group ofthe aromatic substituted derivative residue is one which imparts the anisotropism to the intramolecular dipole moment of the hydroxycarboxylic acid compound and is at the substitution position to impart such anisotropism.

6. A polymeric piezoelectric material as setforth in Claim 2, wherein the substituent group of the aromatic substituted derivative residue is one which imparts the anisotropism to the dipole moment in the direction of the line connecting the carbon atoms on the aromatic ring to which the hydroxyl group and carboxylic acid group are connected, and is at the substitution position to impart such anisotropism.

7. A polymeric piezoelectric material as set forth in Claim 2, wherein the aromatic hydroxycarboxylic acid residue is composed of one or more kinds selected from the following formulas (I) to (Vll).

(wherein the group consisting of X1, X2, and X3 and the group consisting of Y1, Y2, and Y3 are separated bya line which intersects at right angles a line connecting the carbon atoms on the aromatic ring to which the hydroxyl group and carboxylic acid group are connected, at the center thereof; each of said groups is one or more kinds selected from substituent groups which differfrom one another in dipole moment; the same group does not contain those which differfrom one another in the direction of dipole moment; and the unsubstituted position in each group represents a hydrogen atom.)

8.A polymeric piezoelectric material as set forth in any of Claims 5to 7, wherein the substituent group is one or more kinds selected from cyano group, nitro group, aldehyde group, carboxylic acid ester, carboxylic acid group, hydroxyl group, hydrogen, halogen compound, amino group, imino group, azo group, alkoxy group, alkyl group, phenyl group, acyl group, sulfoxy group, and sulfide group.

9. A polymeric piezoelectric material as set forth in any of Claims 5 to 7, wherein the substituent group is one or more kinds selected from hydrogen, cyano group, nitro group, acetoxy group, chlorine, bromine, phenyl group, alkyl group, methoxy group, amino group, and alkyl-substituted amino group.

10. A polymeric piezoelectric material as set forth in Claim 1, wherein the polyester which exhibits the anisotropism in the molten state is one which has a molecular weight of 2,000 to 50,000.

11. A polymeric piezoelectric material as set forth in Claim 1, wherein the molded product (to be heat-electretized) of the polyesterwhich contains an aromatic hydroxycarboxylic acid residue and exhibits the anisotropism in the molten state and/or a polyester containing party in the same molecular chain a polyesterwhich exhibits the anisotropism in the moltentstate, isonewhich contains aferroelectric compound.

12. A polymeric piezoelectric material as setforth in Claim 11,wherein theferroelectric compound is an organic compound.

13. A polymeric piezoelectric material as setforth in Claim 11, wherein theferroelectric compounds is an inorganic compound.

14. A polymeric piezoelectric material as set forth in Claim 13,wherein the inorganic compound is one or more compounds selected from quartz, lead zirconate titanate, potassium hydrogen phosphate, barium titanate, lead titanate, lead niobate, lithium niobate, lithium tantalate, strontium barium niobatq, Pb(B1.B2)03, and PbTiO3-PbZrO3-Pb(B-B2) (where B1 represents Mg. Co, Ni, Mn, or Zn; and B2 represents Nb; Ta, Sb, orW).

15. A polymeric piezoelectric material as setforth in Claim 12, wherein the organic compound is Rochelle saltortriglycin sulfate.

16. A polymeric piezoelectric material as setforth in Claim 12, wherein the organic compound is a polymericferroelectric material.

17. A polymeric piezoelectric material as setforth in Claim 12,wherein the organic compound is a low-molecuarferroelectric liquid crystal compound.

18. A polymeric piezoelectric material as setforth in Claim 16, wherein the polymericferroelectric material is one or more kinds selected from vinylidene fluoride, triffuoroethylene, vinylidene cyanide, and chloroacrylonitrile.

19. A polymeric piezoelectric material as set forth in Claim 1, which is in the form of sheet orfilm.

20. A polymeric piezoelectric material as set forth in Claim 1, which is in the form of powder orfiber.

21. A polymeric piezoelectric material as setforth in Claim 19, wherein both sides of the filmy polymeric piezoelectric material are coated with electroconductivefilm.

22. A polymeric piezoelectric material as set forth in Claim 1, wherein the molded product is heat-electretized by the application of direct current or both direct current and alternate current.

23. A polymeric piezoelectric material as set forth in Claim 1, wherein the heat-electretized polymeric piezoelectric material is in the form of fiber.

24. A polymeric piezoelectric material as set forth in Claim 1, wherein the heat-electretized polymeric piezoelectric material is in the form of powder and/orshortfiber.

25. A polymeric piezoelectric material as set forth in Claim.24, wherein the heat-electretized polymeric piezoelectric material in the form of powder and/or shortfiber is dispersed in other resin.

26. A polymeric piezoelectric material as set forth in Claim 24, wherein the heat-electretized polymeric piezoelectric material is dispersed in a thermosetting resin.

27. A polymeric piezoelectric material as setforth in Claim 24, wherein the heat-electretized polymeric piezoelectric material is dispersed in thermoplastic resin.

28. A polymeric piezoelectric material as set forth in Claim 27, wherein the thermoplastic resin is one which has a lower melting point than the resin to be heat-electretized.

29. A polymeric piezoelectric material as set forth in Claim 27, wherein the thermoplastic resin is one or more kinds selected from polyacetal, polybutylene terephthalate, polyethylene terephthalate, vinylidene fluoride, trifluoroethylene, vinylidene cyanide, and chloroacrylonitrile.

30. A polymeric piezoelectric material as set forth in Claim 25, wherein the resin composition in which the heat-electretized resin is dispersed is further heat-electretized.