A main chain cross-linkable PMR type polyimide resin and its preparation method and application

Abstract

The application discloses a main chain cross-linkable PMR type polyimide resin and a preparation method and application thereof. The PMR type polyimide resin has the structure shown in the following formula: wherein 0 < a <= 1, 0 <= b < 1, a + b = 1, and n is an integer of 1-10. The application simultaneously introduces alkynyl and asymmetric structures into a polyimide resin main chain, so that the resin has good heat resistance and processing performance, and solves the technical problem that the existing thermosetting polyimide resin cannot simultaneously have high heat resistance and good processing performance.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to a PMR type polyimide resin with a crosslinkable main chain, its preparation method, and its application. Background Technology

[0002] Thermosetting polyimide (PI) resins, with their superior overall performance, firmly hold the top position among heat-resistant resin materials, boasting a long-term service temperature range of 280 to 450 °C. This remarkable characteristic, coupled with its excellent dielectric properties, outstanding thermal oxidation stability, good mechanical properties, and relatively ideal molding and processing characteristics, makes it play an increasingly crucial role in the field of advanced polymer composites (APCs), rapidly becoming the most favored and widely adopted core matrix resin in this field.

[0003] As the aerospace industry continues its rapid development towards higher speeds, greater thrust, and longer ranges, the extreme service environments faced by aircraft (such as the high temperatures generated by hypersonic flight and the high thermal loads on engine hot-end components) place unprecedented and almost demanding requirements on the temperature resistance of polymer-based composite materials. Traditional PMR-type polyimide resin systems, such as the widely used PMR-15, are gradually failing to meet the stringent thermal stability requirements of next-generation aerospace equipment. To overcome this temperature resistance bottleneck, much research has focused on introducing molecular groups with higher rigidity and thermal stability (such as rigid units like biphenyl, naphthalene rings, and benzimidazole) into the main chain molecular structure of polyimides. This molecular design strategy can increase the glass transition temperature and thermal decomposition temperature of the resin, thereby improving its heat resistance. However, it is precisely this rigid molecular skeleton structure that endows polyimides with ultra-high heat resistance that severely restricts the mobility and melt flowability of polymer chains, leading to a series of challenging problems during resin processing, such as extremely high melt viscosity, poor solubility, narrow processing window, and high molding pressure. Therefore, how to break through this inherent dilemma of "heat resistance-processability" and successfully develop a new type of PMR polyimide resin system that combines ultra-high heat resistance (meeting the requirements for long-term use above 450 ℃) with good and practical processing performance (such as suitable melt viscosity, wide processing window, and good prepreg preparation properties) has become a key core technology problem that urgently needs to be solved in the field of high-performance composite materials. Summary of the Invention

[0004] The main objective of this invention is to provide a main-chain crosslinkable PMR type polyimide resin, its preparation method, and its application, so as to overcome the shortcomings of the prior art.

[0005] To achieve the aforementioned objectives, the technical solution adopted by this invention includes:

[0006] This invention provides a main-chain crosslinkable PMR type polyimide resin, wherein the PMR type polyimide resin has a structure as shown in formula (I):

[0007]

[0008] Formula (I)

[0009] Where 0 < a ≤ 1, 0 ≤ b < 1, a + b = 1, and n is an integer from 1 to 10;

[0010] A represents a diimide residue, and A is selected from any one or more combinations of the structures shown below:

[0011] , , ;

[0012] In this context, the "—" connecting nitrogen atoms represents the chemical bond connecting A to carbon atoms in the repeating unit;

[0013] R is selected from any one or more combinations of the following structures:

[0014] , , ;

[0015] In this context, the "—" connecting nitrogen atoms represents the chemical bond between R and the carbon atom in the repeating unit.

[0016] This invention also provides a method for preparing the aforementioned main-chain crosslinkable PMR type polyimide resin, comprising:

[0017] In a protective atmosphere, aromatic dianhydride monomer, 4,4'-(acetylene-1,2-diyl)phthalic anhydride, end-capping agent, and solvent are mixed and reacted at 80-95°C to obtain an esterification solution.

[0018] Furthermore, the esterification solution is cooled to 40-60°C, and 3-[2-(4-aminophenyl)ethynyl]aniline and solvent are added to continue the reaction, thereby obtaining a PMR-type polyimide resin with a crosslinkable main chain.

[0019] This invention also provides the application of the aforementioned main-chain crosslinkable PMR type polyimide resin as a matrix resin in the preparation of composite materials.

[0020] This invention also provides a cured polyimide resin, which is obtained by curing and crosslinking the aforementioned main-chain crosslinkable PMR type polyimide resin.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] (1) The main chain crosslinkable PMR type polyimide resin provided by the present invention contains alkynyl structure and asymmetric structure. Introducing alkynyl group into the main chain can increase the crosslinking density of the resin, thereby improving the heat resistance of the resin. The asymmetric structure can destroy the stacking of molecular chains and structural rigidity, so that the resin can obtain a lower melt viscosity, so that the polyimide resin has excellent heat resistance and processing performance at the same time. The melt viscosity is adjustable in the range of 1 to 1600 Pa·s. The 5% thermal weight loss temperature of the polyimide resin curing product in air is >520 ℃, and the glass transition temperature of the polyimide resin curing product is >430 ℃. Therefore, the main chain crosslinkable PMR type polyimide resin has both good heat resistance and processing performance, which solves the technical problem of the contradiction between the processing performance and thermal performance of the matrix resin faced by existing thermosetting polyimide composite materials.

[0023] (2) The preparation method provided by the present invention innovatively uses acetylene-containing dianhydride monomers and asymmetric diamine monomers to prepare thermosetting polyimide resins. Introducing acetylene groups into the main chain can increase the crosslinking density of the resin, thereby improving the heat resistance of the resin. The asymmetric structure can destroy the stacking of molecular chains and the structural rigidity, so that the resin can obtain a lower melt viscosity. At the same time, it can adjust the curing density, so that the polyimide resin has both excellent heat resistance and processing performance. Meanwhile, the process uses the relatively mature PMR method to complete the resin preparation. The process is simple and easier to apply in engineering. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a comparison chart of the viscosity-temperature change curves of the resins prepared in Example 1 and Comparative Example 1 of the present invention.

[0026] Figure 2 A comparison of the glass transition temperature curves of the resin cured products prepared in Example 1 and Comparative Example 1 of the present invention;

[0027] Figure 3 This is a comparison of the thermogravimetric curves of the cured resins prepared in Example 1 and Comparative Example 1 of the present invention in air. Detailed Implementation

[0028] In view of the deficiencies of the prior art, the inventors of this case, through long-term research and extensive practice, have proposed the technical solution of this invention. The technical solution of this invention will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0029] Specifically, as one aspect of the technical solution of the present invention, a main-chain crosslinkable PMR type polyimide resin has a structure as shown in formula (I):

[0030]

[0031] Formula (I)

[0032] Where 0 < a ≤ 1, 0 ≤ b < 1, a + b = 1, and n is an integer from 1 to 10;

[0033] A represents a diimide residue, and A is selected from any one or more combinations of the structures shown below:

[0034] , , ;

[0035] In this context, the "—" connecting nitrogen atoms represents the chemical bond connecting A to carbon atoms in the repeating unit;

[0036] R is selected from any one or more combinations of the following structures:

[0037] , , ;

[0038] In this context, the "—" connecting nitrogen atoms represents the chemical bond between R and the carbon atom in the repeating unit.

[0039] Another aspect of the present invention provides a method for preparing the aforementioned main-chain crosslinkable PMR type polyimide resin, comprising:

[0040] In a protective atmosphere, aromatic dianhydride monomer, 4,4'-(acetylene-1,2-diyl)phthalic anhydride, end-capping agent, and solvent are mixed and reacted at 80-95°C to obtain an esterification solution.

[0041] Furthermore, the esterification solution is cooled to 40-60°C and 3-[2-(4-aminophenyl)ethynyl]aniline and solvent are added to continue the reaction, thereby obtaining a PMR type polyimide resin with a crosslinkable main chain.

[0042] In some preferred embodiments, the aromatic dianhydride monomer includes any one or more combinations of 2,3',3,4'-biphenyltetracarboxylic acid dianhydride, 3,4'-oxobisphthalic anhydride, and 2,3',3,4'-benzophenone tetracarboxylic anhydride, and is not limited thereto.

[0043] In some preferred embodiments, the capping agent includes any one or more combinations of 4-phenylethynyl phthalic anhydride, ethynyl phthalic anhydride, and norbornenedicarboxylic anhydride, but is not limited thereto.

[0044] In some preferred embodiments, the solvent includes any one or more combinations of ethanol, n-butanol, isopropanol, methanol, and isobutanol, but is not limited thereto.

[0045] In some preferred embodiments, the solvents used in the two reactions can be the same or different; preferably, the solvents used in the two reactions are the same.

[0046] In some preferred embodiments, the protective atmosphere includes an inert atmosphere, which may include a nitrogen atmosphere and / or an argon atmosphere, but is not limited thereto.

[0047] In some preferred embodiments, the molar ratio of the aromatic dianhydride monomer, 4,4'-(acetylene-1,2-diyl)phthalic anhydride, and the capping agent is 0.1~0.9:0.1~0.9:2.

[0048] In some preferred embodiments, the molar ratio of the aromatic diamine monomer to 3-[2-(4-aminophenyl)ethynyl]aniline is 1~9:1~9.

[0049] In some preferred embodiments, the preparation method specifically includes:

[0050] S1. Under a protective atmosphere, aromatic dianhydride monomer, 4,4'-(acetylene-1,2-diyl)diphthalic anhydride, end-capping agent and solvent are mixed and reacted at 80~95℃ for 2~10h to obtain esterification solution;

[0051] S2. Cool the esterification solution in step S1 to 40~60℃, add 3-[2-(4-aminophenyl)ethynyl]aniline and solvent, and continue stirring for 3~9h to obtain PMR type polyimide resin solution.

[0052] Furthermore, the solid content of the esterification solution is 40~90wt%.

[0053] Furthermore, the solid content of the PMR type polyimide resin solution is 30~80wt%.

[0054] Another aspect of the present invention provides the application of the aforementioned main-chain crosslinkable PMR type polyimide resin as a matrix resin in the preparation of composite materials.

[0055] Another aspect of the present invention provides a polyimide resin cured product, which is obtained by curing and crosslinking the aforementioned main-chain crosslinkable PMR type polyimide resin.

[0056] In some preferred embodiments, the curing and crosslinking is carried out at 360~390°C for 1~4 hours.

[0057] Furthermore, the curing and crosslinking conditions include: curing at 370°C for 2 hours and curing at 380°C for 2 hours.

[0058] The technical solution of the present invention will be further described in detail below with reference to several preferred embodiments and accompanying drawings. This embodiment is implemented on the premise of the technical solution of the invention, and provides detailed implementation methods and specific operation processes. However, the protection scope of the present invention is not limited to the following embodiments.

[0059] Unless otherwise specified, the experimental materials used in the examples below can be purchased from conventional biochemical reagent companies.

[0060] Example 1

[0061] Under nitrogen protection, 1.9094 g (6.0 mmol) of 4,4'-(acetylene-1,2-diyl)phthalic anhydride, 1.4894 g (6.0 mmol) of 4-phenylethynylphthalic anhydride, and 8 g of ethanol were added to a 100 ml reaction flask and placed in an oil bath. The mixture was stirred at 85 °C for 4 h. After the solution became clear, it was cooled to 50 °C, and then 1.8743 g (9.0 mmol) of 3-[2-(4-aminophenyl)ethynyl]aniline and 5 g of ethanol were added. The mixture was stirred for 5 h to obtain the final resin solution.

[0062] The thermosetting polyimide resin prepared in Example 1 has a minimum melt viscosity of 51 Pa·s, and the viscosity-temperature curve of the resin is shown below. Figure 1 As shown, the glass transition temperature of the cured products obtained after curing at 370℃ / 2h and 380℃ / 2h is 503℃, and the 5% thermogravimetric temperature in air is 560℃. The glass transition temperature curves of the cured products are shown below. Figure 2 As shown, the thermogravimetric curve of the solidified material in air is as follows: Figure 3 As shown.

[0063] Example 2

[0064] Under nitrogen protection, 0.8827 g (3.0 mmol) of 2,3',3,4'-biphenyltetracarboxylic acid dianhydride, 0.9547 g (3.0 mmol) of 4,4'-(ethynyl-1,2-diyl)phthalic anhydride, 1.4894 g (6.0 mmol) of 4-phenylethynylphthalic anhydride, and 7 g of n-butanol were added to a 100 ml reaction flask and placed in an oil bath. The mixture was stirred at 90 °C for 4 h. After the solution became clear, it was cooled to 60 °C, and then 1.6660 g (8.0 mmol) of 3-[2-(4-aminophenyl)ethynyl]aniline and 6 g of ethanol were added. The mixture was stirred for 5 h to obtain the final resin solution.

[0065] The thermosetting polyimide resin prepared in Example 2 had a minimum melt viscosity of 45 Pa·s, and the glass transition temperature of the cured product obtained after curing at 370℃ / 2h and 380℃ / 2h was 470℃, with a 5% thermal weight loss temperature in air of 540℃.

[0066] Example 3

[0067] Under nitrogen protection, 0.2942 g (1.0 mmol) of 2,3',3,4'-biphenyltetracarboxylic acid dianhydride, 1.5912 g (5.0 mmol) of 4,4'-(acetylen-1,2-diyl)phthalic anhydride, 1.4894 g (6.0 mmol) of 4-phenylethynylphthalic anhydride, and 13 g of ethanol were added to a 100 ml reaction flask and placed in an oil bath. The mixture was stirred at 90 °C for 8 h. After the solution became clear, it was cooled to 40 °C, and then 1.6660 g (8.0 mmol) of 3-[2-(4-aminophenyl)ethynyl]aniline and 6 g of ethanol were added. The mixture was stirred for 3 h to obtain the final resin solution.

[0068] The thermosetting polyimide resin prepared in Example 3 had a minimum melt viscosity of 71 Pa·s, and the glass transition temperature of the cured product obtained after curing at 370℃ / 2h and 380℃ / 2h was 485℃, with a 5% thermal weight loss temperature in air of 545℃.

[0069] Example 4

[0070] Under nitrogen protection, 1.5511 g (5.0 mmol) of 3,4'-oxophthalic anhydride, 0.3182 g (1.0 mmol) of 4,4'-(ethynyl-1,2-diyl)phthalic anhydride, 1.0328 g (6.0 mmol) of ethynylphthalic anhydride, and 9 g of isopropanol were added to a 100 ml reaction flask and placed in an oil bath. The mixture was stirred at 90 °C for 2 h. After the solution became clear, it was cooled to 55 °C, and then 1.6660 g (8.0 mmol) of 3-[2-(4-aminophenyl)ethynyl]aniline and 4 g of isopropanol were added. The mixture was stirred for 3 h to obtain the final resin mixture solution.

[0071] The thermosetting polyimide resin prepared in Example 4 had a minimum melt viscosity of 500 Pa·s, and the glass transition temperature of the cured product obtained after curing at 370℃ / 2h and 380℃ / 2h was 455℃, and the 5% thermal weight loss temperature in air was 521℃.

[0072] Example 5

[0073] Under nitrogen protection, 1.2889 g (4.0 mmol) of 2,3',3,4'-benzophenone tetracarboxylic dianhydride, 0.6365 g (2.0 mmol) of 4,4'-(ethynyl-1,2-diyl)phthalic anhydride, 1.4894 g (6.0 mmol) of 4-phenylethynylphthalic anhydride and 14 g of ethanol were added to a 100 ml reaction flask and placed in an oil bath. The mixture was stirred at 87 °C for 6 h. After the solution became clear, it was cooled to 60 °C, and then 1.6660 g (8.0 mmol) of 3-[2-(4-aminophenyl)ethynyl]aniline and 6 g of ethanol were added. The mixture was stirred for 6 h to obtain the final resin solution.

[0074] The thermosetting polyimide resin prepared in Example 5 had a minimum melt viscosity of 13 Pa·s, and the glass transition temperature of the cured product obtained after curing at 370℃ / 2h and 380℃ / 2h was 464℃, with a 5% thermal weight loss temperature in air of 556℃.

[0075] Example 6

[0076] Under nitrogen protection, 0.8700 g (2.7 mmol) of 2,3',3,4'-benzophenone tetracarboxylic dianhydride, 0.0955 g (0.3 mmol) of 4,4'-(ethynyl-1,2-diyl)phthalic anhydride, 1.4894 g (6.0 mmol) of 4-phenylethynylphthalic anhydride and 15 g of ethanol were added to a 100 ml reaction flask and placed in an oil bath. The mixture was stirred at 65 °C for 10 h. After the solution became clear, it was cooled to 40 °C, and then 1.2496 g (6.0 mmol) of 3-[2-(4-aminophenyl)ethynyl]aniline and 5 g of ethanol were added. The mixture was stirred for 9 h to obtain the final resin mixture.

[0077] The thermosetting polyimide resin prepared in Example 6 had a minimum melt viscosity of 2 Pa·s, a glass transition temperature of 451 °C for the cured product obtained after curing at 370 °C / 2h and 380 °C / 2h, and a 5% thermal weight loss temperature in air of 527 °C.

[0078] Example 7

[0079] Under nitrogen protection, 0.9667 g (3.0 mmol) of 2,3',3,4'-benzophenone tetracarboxylic dianhydride, 8.5925 g (27.0 mmol) of 4,4'-(ethynyl-1,2-diyl)phthalic anhydride, 0.9850 g (6.0 mmol) of norbornene dicarboxylic anhydride, and 65 g of isobutanol were added to a 250 ml reaction flask and placed in an oil bath. The mixture was stirred at 80 °C for 9 h. After the solution became clear, it was cooled to 42 °C, and then 6.8726 g (33.0 mmol) of 3-[2-(4-aminophenyl)ethynyl]aniline and 9 g of isobutanol were added. The mixture was stirred for 8 h to obtain the final resin solution.

[0080] The thermosetting polyimide resin prepared in Example 7 had a minimum melt viscosity of 1663 Pa·s, and the glass transition temperature of the cured product obtained after curing at 370℃ / 2h and 380℃ / 2h was 430℃, and the 5% thermal weight loss temperature in air was 520℃.

[0081] Example 8

[0082] Under nitrogen protection, 0.0883 g (0.3 mmol) of 2,3',3,4'-biphenyltetracarboxylic acid dianhydride, 0.8592 g (2.7 mmol) of 4,4'-(acetylen-1,2-diyl)phthalic anhydride, 1.4894 g (6.0 mmol) of 4-phenylethynylphthalic anhydride and 11 g of ethanol were added to a 100 ml reaction flask and placed in an oil bath. The mixture was stirred at 90 °C for 9 h. After the solution became clear, it was cooled to 30 °C, and then 1.2496 g (6.0 mmol) of 3-[2-(4-aminophenyl)ethynyl]aniline and 9 g of ethanol were added. The mixture was stirred for 6 h to obtain the final resin solution.

[0083] The thermosetting polyimide resin prepared in Example 8 had a minimum melt viscosity of 9 Pa·s, a glass transition temperature of 491 °C after curing at 370 °C / 2h and 380 °C / 2h, and a 5% thermal weight loss temperature in air of 543 °C.

[0084] Comparative Example 1

[0085] Under nitrogen protection, 1.7653 g (6.0 mmol) of 2,3',3,4'-biphenyltetracarboxylic acid dianhydride, 1.4894 g (6.0 mmol) of 4-phenylethynylphthalic anhydride, and 8 g of ethanol were added to a 100 ml reaction flask and placed in an oil bath. The mixture was stirred at 85 °C for 4 h. After the solution became clear, it was cooled to 50 °C, and then 1.9106 g (9.0 mmol) of 2,2'-dimethyl-4,4'-diaminobiphenyl and 5 g of ethanol were added. The mixture was stirred for 5 h to obtain the final resin solution.

[0086] The thermosetting polyimide resin prepared in Comparative Example 1 had a minimum melt viscosity of 72 Pa·s, and the viscosity-temperature curve of the resin is shown below. Figure 1 As shown, the glass transition temperature of the cured products obtained after curing at 370℃ / 2h and 380℃ / 2h is 424℃, and the 5% thermogravimetric temperature in air is 513℃. The glass transition temperature curves of the cured products are shown below. Figure 2 As shown, the thermogravimetric curve of the solidified material in air is as follows: Figure 3 As shown.

[0087] In addition, the inventors of this case also conducted experiments with other raw materials, process operations, and process conditions described in this specification, referring to the aforementioned embodiments, and obtained relatively ideal results in all cases.

[0088] It should be understood that the technical solutions of the present invention are not limited to the specific embodiments described above. Any technical modifications made to the technical solutions of the present invention without departing from the spirit and scope of the claims are within the scope of protection of the present invention.

Claims

1. A PMR type polyimide resin with a main chain capable of crosslinking, characterized in that, The PMR type polyimide resin has the structure shown in formula (I): ； Formula (I); Where 0 < a ≤ 1, 0 ≤ b < 1, a + b = 1, and n is an integer from 1 to 10; A represents a diimide residue, and A is selected from any one or more combinations of the structures shown below: 、 、 ； In this context, the "—" connecting nitrogen atoms all represent chemical bonds connecting A to carbon atoms in the repeating unit; R is selected from any one or more combinations of the following structures: 、 、 ； In this context, the "—" connecting nitrogen atoms represents the chemical bond between R and the carbon atom in the repeating unit.

2. The method for preparing the main-chain crosslinkable PMR type polyimide resin as described in claim 1, characterized in that, include: In a protective atmosphere, aromatic dianhydride monomer, 4,4'-(acetylene-1,2-diyl)phthalic anhydride, end-capping agent, and solvent are mixed and reacted at 80-95°C to obtain an esterification solution. Furthermore, the esterification solution is cooled to 40-60°C and 3-[2-(4-aminophenyl)ethynyl]aniline and solvent are added to continue the reaction, thereby obtaining a PMR type polyimide resin with a crosslinkable main chain.

3. The preparation method according to claim 2, characterized in that, Specifically, it includes: S1. Under a protective atmosphere, aromatic dianhydride monomer, 4,4'-(acetylene-1,2-diyl)diphthalic anhydride, end-capping agent and solvent are mixed and reacted at 80~95℃ for 2~10h to obtain esterification solution; S2. Cool the esterification solution in step S1 to 40~60℃, add 3-[2-(4-aminophenyl)ethynyl]aniline and solvent, and continue stirring for 3~9h to obtain PMR type polyimide resin solution.

4. The preparation method according to claim 3, characterized in that: The aromatic dianhydride monomers include any one or a combination of 2,3',3,4'-biphenyltetracarboxylic acid dianhydride, 3,4'-oxobisphthalic anhydride, and 2,3',3,4'-benzophenone tetracarboxylic acid dianhydride. And / or, the capping agent comprises any one or more combinations of 4-phenylethynyl phthalic anhydride, ethynyl phthalic anhydride, and norbornenedicarboxylic anhydride.

5. The preparation method according to claim 3, characterized in that: The solvent includes any one or more combinations of ethanol, n-butanol, isopropanol, methanol, and isobutanol; And / or, the protective atmosphere includes an inert atmosphere, including a nitrogen atmosphere and / or an argon atmosphere.

6. The preparation method according to claim 3, characterized in that: The molar ratio of the aromatic dianhydride monomer, 4,4'-(acetylene-1,2-diyl)phthalic anhydride, and the capping agent is 0.1~0.9:0.1~0.9:2; And / or, the molar ratio of the aromatic dianhydride monomer to 3-[2-(4-aminophenyl)ethynyl]aniline is 1~9:1~9.

7. The preparation method according to claim 3, characterized in that: The solid content of the esterification solution is 40~90wt%; And / or, the solid content of the PMR type polyimide resin solution is 30~80wt%.

8. The application of the main-chain crosslinkable PMR type polyimide resin according to claim 1 in the preparation of composite materials as a matrix resin.

9. A cured polyimide resin, characterized in that, The cured polyimide resin is obtained by curing and crosslinking the main chain crosslinkable PMR type polyimide resin as described in claim 1.

10. The polyimide resin cured product according to claim 9, characterized in that: The curing and crosslinking temperature is 360~390℃, and the time is 1~4h.

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