CN116178194B - Polybenzoxazine oligomer, high-thermal-conductivity aromatic ester crosslinked liquid crystal polybenzoxazine and preparation method thereof - Google Patents

Polybenzoxazine oligomer, high-thermal-conductivity aromatic ester crosslinked liquid crystal polybenzoxazine and preparation method thereof Download PDF

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CN116178194B
CN116178194B CN202310156959.3A CN202310156959A CN116178194B CN 116178194 B CN116178194 B CN 116178194B CN 202310156959 A CN202310156959 A CN 202310156959A CN 116178194 B CN116178194 B CN 116178194B
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polybenzoxazine
liquid crystal
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鲁在君
拉蕊
孟真
张其坤
王敏
廖广明
高天正
王义峰
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Shandong University
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Abstract

The invention provides a polybenzoxazine oligomer, high-thermal-conductivity aromatic ester crosslinked liquid crystal polybenzoxazine and a preparation method thereof. The invention takes p-hydroxybenzoic acid, amine compounds and paraformaldehyde as raw materials, synthesizes benzoxazine monomers through Mannich reaction, and then obtains the polybenzoxazine oligomer through ring-opening polymerization and hydroxyl acetylation reaction. The polybenzoxazine oligomer is self-crosslinked to produce the crosslinked product polybenzoxazine containing aromatic ester bond with liquid crystal structure and high heat conductivity for the first time. The invention has simple synthesis process, mild reaction condition, high yield and low equipment requirement, and is suitable for large-scale production. The polybenzoxazine obtained by the invention has a liquid crystal structure, high heat conduction and heat resistance, and is a material with important functions, which can be used in the fields of integrated circuits, light-emitting diodes and the like.

Description

Polybenzoxazine oligomer, high-thermal-conductivity aromatic ester crosslinked liquid crystal polybenzoxazine and preparation method thereof
Technical Field
The invention relates to a polybenzoxazine oligomer, high-thermal-conductivity aromatic ester crosslinked liquid crystal polybenzoxazine and a preparation method thereof, and belongs to the technical field of high polymer materials.
Background
Electronic devices in integrated circuits are currently moving towards higher power and lighter, thinner, smaller size. The reduction in size and improvement in performance of electronic devices can result in the generation of large amounts of heat, which can lead to a number of potential problems such as failure, reduced durability, and even explosion of equipment. In recent years, heat dissipation has become a critical issue limiting integrated circuit development and Light Emitting Diode (LED) development. Therefore, a polymer material having excellent Thermal Conductivity (TC) and high heat resistance is demanded to effectively dissipate heat.
However, most polymeric materials have an ultra-low thermal conductivity and are considered insulators. Traditionally, there are two approaches to improving the thermal conductivity of polymers: first, highly thermally conductive inorganic particles such as Boron Nitride (BN), aluminum oxide (Al 2O3) and Carbon Nanotubes (CNTs) are added to a polymer matrix. Although a very high thermal conductivity is obtained, a very large amount (> 30 vol%) of filler is required, which greatly reduces the mechanical and processing properties of the polymer composite. Another most efficient approach is to incorporate crystalline or liquid crystal structures in the polymer
Recently, a new type of thermosetting resin called benzoxazine resin has emerged internationally. It has high heat resistance, high solvent resistance, low dielectric constant and low cost. It also shows excellent dimensional stability and ring-opening polymerization is performed in curing without releasing any by-products. It is actively developed into high potential electronic materials, adhesives, precision mechanical parts, carbon fiber reinforced composites, and aerospace materials. However, polybenzoxazines are similar to other resins and have very low thermal conductivity.
In order to improve the intrinsic thermal conductivity of benzoxazine resins, many chemists have explored liquid crystalline benzoxazines. For example, ishida et al first synthesized Liquid Crystal (LC) benzoxazines containing cyanobiphenyl intermediates and reported their phase behavior. Kawauchi et al synthesized LC benzoxazine monomers. Ito et al report increasing the temperature range of LC by introducing more rigid biphenyl groups. The above reports, although successful, often lose liquid crystalline behavior after polymerization of LC benzoxazine monomers.
Therefore, there is a need to develop a polybenzoxazine having a liquid crystal structure, high heat conductivity and heat resistance.
Disclosure of Invention
Aiming at the defects existing in the prior art, the invention provides a polybenzoxazine oligomer, high-thermal-conductivity aromatic ester crosslinked liquid crystal polybenzoxazine and a preparation method thereof. The invention takes p-hydroxybenzoic acid, amine compounds and paraformaldehyde as raw materials, synthesizes benzoxazine monomers through Mannich reaction, and then obtains the polybenzoxazine oligomer through ring-opening polymerization and hydroxyl acetylation reaction. The polybenzoxazine oligomer is self-crosslinked to produce the crosslinked product polybenzoxazine containing aromatic ester bond with liquid crystal structure and high heat conductivity for the first time. The invention has simple synthesis process, mild reaction condition, high yield and low equipment requirement, and is suitable for large-scale production. The polybenzoxazine obtained by the invention has a liquid crystal structure, high heat conduction and heat resistance, and is a material with important functions, which can be used in the fields of integrated circuits, light-emitting diodes and the like.
The invention is realized by the following technical scheme:
a polybenzoxazine oligomer having the structure of formula I:
wherein R 1 is selected from one of the following ：-CH3,-(CH2)11CH3,-CH2CH2OH,-CH2-CH＝CH2,
R 2 is selected from one of the following: -CH 2CH3,-CH3;
n is 3-6.
The preparation method of the polybenzoxazine oligomer comprises the following steps:
(1) Dissolving p-hydroxybenzoic acid, amine compounds and paraformaldehyde in a low-polarity solvent, and carrying out reflux reaction to obtain benzoxazine monomers;
Wherein R 1 and R 1 in the compound of formula I have the same meaning;
(2) Dissolving benzoxazine monomer and initiator in solvent, and performing ring-opening polymerization reaction to obtain oligomeric benzoxazine; then adding anhydride, acid catalyst and solvent, fully mixing and dispersing uniformly, and obtaining the polybenzoxazine oligomer (I) through acylation reaction and drying.
According to a preferred embodiment of the present invention, in the step (1), the amine compound has a structural formula of R 1-NH2; wherein R 1 and R 1 in the compound of formula I have the same meaning.
According to the invention, in the step (1), the molar ratio of the parahydroxybenzoic acid, the amine compound and the paraformaldehyde is preferably 1:2:1 to 1:2:6.
According to the present invention, preferably, in the step (1), the low polarity solvent is one or a combination of two or more of toluene, xylene, ethanol or dioxane; the volume ratio of the molar quantity of the parahydroxybenzoic acid to the low-polarity solvent is 1:1-1:20 mol/L.
According to the present invention, in the step (1), the reflux reaction temperature is 80 to 110℃and the reflux reaction time is 10 to 24 hours.
According to the present invention, in the step (1), the post-treatment method of the reaction liquid obtained by the reflux reaction comprises the steps of: deionized water is added into the reaction liquid, and then the benzoxazine monomer is obtained through filtration, washing and drying.
Preferably, in step (2) according to the present invention, the solvent is tetrahydrofuran or toluene; the volume ratio of the mass of the benzoxazine monomer to the solvent is 0.1-1 g/mL.
According to a preferred embodiment of the invention, in step (2), the initiator is Benzoyl Peroxide (BPO), 2' -Azobisisobutyronitrile (AIBN) or tert-butylphenol (TBP). The addition amount of the initiator is 0.1-15% of the mass of the benzoxazine monomer.
According to the invention, in the step (2), the ring-opening polymerization reaction temperature is 130-200 ℃ and the reaction time is 2-5 h. The polymerization temperature depends on the kind of amine compound used, and the use of an amine compound having a long hydrocarbon chain can lower the polymerization temperature.
According to the present invention, preferably, in the step (2), the post-treatment method of the crude product obtained by the ring-opening polymerization reaction comprises the steps of: the crude product is cooled to room temperature, then is dissolved in normal hexane, is added dropwise into methanol for precipitation, and is filtered and dried to obtain the oligobenzoxazine.
Preferably, in step (2) according to the present invention, the anhydride is acetic anhydride, propionic anhydride, acetyl chloride or propionyl chloride; the molar ratio of the anhydride to the oligobenzoxazine is 1:1-2:1. The anhydride is preferably acetic anhydride or propionic anhydride; if acetyl chloride or propionyl chloride is used, hydrogen chloride is formed as a by-product.
Preferably, in step (2) according to the present invention, the acidic catalyst is an aqueous solution of an acid, which is sulfuric acid, sulfonic acid or hydrochloric acid; the concentration of the aqueous solution of the acid is 0.005-0.05 mol/L; the molar ratio of the acid catalyst to the oligobenzoxazine is 1:500-1:820.
Preferably, in step (2) according to the present invention, the solvent is dimethylacetamide; the ratio of the molar amount of the oligobenzoxazine to the volume of the solvent is 0.2-1 mol/L.
According to the present invention, in the step (2), the acylation reaction temperature is 100 to 150 ℃, the acylation reaction time is 2 to 6 hours, and the acylation reaction is performed in an inert atmosphere under stirring. Preferably, the inert atmosphere is a nitrogen or argon atmosphere.
According to the invention, in the step (2), the drying is vacuum drying, the drying temperature is 60-130 ℃, and the drying time is 20-30 hours.
A high thermal conductivity aromatic ester crosslinked liquid crystal polybenzoxazine has a structure shown in the following formula II:
Wherein R 1 and R 1 in the compound of formula I have the same meaning.
The preparation method of the high-thermal-conductivity aromatic ester crosslinked liquid crystal polybenzoxazine comprises the following steps: and (3) dissolving the polybenzoxazine oligomer (I) and a catalyst in an organic solvent, and heating, crosslinking and curing to obtain the high-thermal-conductivity aromatic ester crosslinked liquid crystal polybenzoxazine.
According to the invention, the catalyst is preferably calcium acetate, sodium acetate, manganese acetate or zinc acetate; the mass ratio of the catalyst to the polybenzoxazine oligomer (I) is 0.01:1-0.05:1.
According to the invention, the organic solvent is preferably dimethylacetamide, N-dimethylformamide or N-methylpyrrolidone; the ratio of the molar amount of the polybenzoxazine oligomer (I) to the volume of the organic solvent is 0.5 to 1mol/L.
According to the invention, the method further comprises the steps of reflux stirring and drying before the heating crosslinking curing. The purpose of reflux stirring is to allow better mixing of the raw materials, to heat the solution without loss of the raw materials, and to increase the yield. If the solution is heated without reflux stirring, there may be a loss of material. The drying is to remove impurities such as by-products.
Preferably, the reflux stirring temperature is 110-180 ℃, the reflux stirring time is 2-6 h, and the reflux stirring is carried out in inert atmosphere under stirring conditions; further preferably, the inert atmosphere is a nitrogen or argon atmosphere.
Preferably, the drying is vacuum drying, the drying temperature is 60-100 ℃, and the drying time is 20-30 h.
According to the present invention, preferably, the heat crosslinking curing is performed in air; the heating crosslinking curing method comprises the following steps: heating to 160-273 ℃ at a heating rate of 8-12 ℃/min, and then preserving heat for 0-2 h; or the heat crosslinking curing method comprises the following steps: directly preserving the temperature at 160-273 ℃ for 6 min-2 h.
The synthetic route of the polybenzoxazine oligomer and the high-thermal-conductivity aromatic ester crosslinked liquid crystal polybenzoxazine is as follows.
Wherein R 1 is selected from one of the following ：-CH3,-(CH2)11CH3,-CH2CH2OH,-CH2-CH＝CH2,
R 2 is selected from one of the following: -CH 2CH3,-CH3;
n is 3-6.
The invention has the technical characteristics and beneficial effects that:
1. the invention adopts commercial raw materials and has low cost; the synthesis route is simple, the working procedure is not inflammable and explosive, the production is safe, the reaction condition is mild, the equipment requirement is low, the yield is high, and the method is favorable for industrial production.
2. The invention takes p-hydroxybenzoic acid, amine compounds and paraformaldehyde as raw materials, synthesizes benzoxazine monomers through Mannich reaction, and then obtains the polybenzoxazine oligomer through ring-opening polymerization and hydroxyl acetylation reaction. The polybenzoxazine oligomer is self-crosslinked to obtain the crosslinked product polybenzoxazine. The liquid crystal structure is formed by introducing the rigid aromatic ester bond, wherein the rigid aromatic ester bond forms an ordered structure within the temperature range of 160-273 ℃. The synthesized polybenzoxazine can maintain a liquid crystal structure during polymerization, and a common liquid crystal benzoxazine monomer usually loses the liquid crystal structure during polymerization.
3. The ordered arrangement of the polybenzoxazine liquid crystal structure improves the thermal conductivity of the material. In the present invention, the thermal conductivity of the aromatic ester-crosslinked liquid crystalline polybenzoxazine is 21% and 16% higher than that of the oligobenzoxazines and polybenzoxazine oligomers, respectively.
4. The invention synthesizes aromatic ester bond crosslinked polybenzoxazine products which are formed as a result of self-crosslinking. The results show that heat resistance is improved while the crosslink density is improved. The glass transition temperature tg=269 ℃ of the product synthesized according to the invention is much higher than that of other resins.
5. The variety of the amine compound influences the ring-opening polymerization reaction temperature, and has certain influence on the liquid crystal structure, the thermal performance and the like of the finally obtained polybenzoxazine. The acid anhydride type used in the invention affects the acetylation reaction and has a certain influence on the crosslinking effect of the esterification reaction and the formation of a liquid crystal structure. In the self-crosslinking process of the polybenzoxazine oligomer, the crosslinking curing temperature and time have important influence on the formation of a liquid crystal structure; meanwhile, all groups are combined to prepare the aromatic ester crosslinked liquid crystal polybenzoxazine with a liquid crystal structure and high heat conduction and heat resistance.
Drawings
FIG. 1 is a SEC chromatogram of BZ-COOH, OBZ-COOH prepared in example 1.
FIG. 2 shows the 1H-NMR spectra of (A) BZ-COOH, (B) OBZ-COOH, and (C) OBZ-AC prepared in example 1.
FIG. 3 is an image of POM at various temperatures during the OBZ-AC thermal crosslinking curing process in example 1.
FIG. 4 is a DSC profile of OBZ-AC prepared in example 1.
FIG. 5 is a SAXS curve of OBZ-PES (b) obtained after crosslinking curing at 273℃for 60min for OBZ-AC (a) prepared in example 1 and example 2.
FIG. 6 is an isothermal curing graph of example 1 in which OBZ-AC was heated at 273℃for (a) 1min (b) 3min (c) 6min (d) 17min (e) 30min (f) 60 min.
FIG. 7 is the FT-IR spectra of (a) BZ-COOH, (b) OBZ-COOH, (c) OBZ-AC prepared in example 1 and (d) OBZ-PES prepared in example 2.
FIG. 8 is a FTIR spectrum of OBZ-AC prepared in example 1 after curing at different temperatures for 2 hours.
FIG. 9 is a solid 13 CNMR of OBZ-PES prepared in example 2.
FIG. 10 is a graph of the Thermal Conductivity (TC) of OBZ-COOH, OBZ-AC prepared in example 1 and OBZ-PES prepared in example 2.
FIG. 11 is a DMA curve of the OBZ-PES prepared in example 2.
FIG. 12 is a TGA curve of OBZ-COOH prepared in example 1, OBZ-AC, and OBZ-PES prepared in example 2.
Detailed Description
The invention will be further illustrated with reference to specific examples. But is not limited thereto.
Meanwhile, the experimental methods described in the following examples are all conventional methods unless otherwise specified; the reagents, materials, and devices are commercially available unless otherwise specified.
Example 1
Synthesis of benzoxazine monomer (BZ-COOH, wherein R 1 is- (CH 2)11CH3)
In a 100mL flask, 4-hydroxybenzoic acid (5 mmol,0.69 g), dodecylamine (5 mmol,0.945 g) and paraformaldehyde (12.5 mmol,0.3877 g) were mixed in 10mL dioxane and heated to reflux at 90℃for 24h (magnetic stirring). The mixture was then allowed to cool at room temperature, and then 100mL of deionized water was poured into the above mixture in a 500 mL beaker to give a pale yellow precipitate. After the product is filtered, the product is further washed by deionized water for 3 times, and is dried at 60 ℃ to obtain the benzoxazine monomer. The yield thereof was found to be 90%.
Synthesis of Oligobenzoxazines (OBZ-COOH, wherein R 1 is- (CH 2)11CH3)
BZ-COOH (0.90 g), t-butylphenol (0.10 g) and tetrahydrofuran (5 mL, dried in advance in an oven at about 60 ℃ C. For 12 hours) were added to a 100mL flask. And ring-opening polymerization is carried out by taking p-n-butylphenol as an initiator. The mixture was then poured into an aluminum mold, which was placed in an air circulation oven and polymerized at 150℃for 3h. The crude product was cooled to room temperature, dissolved in n-hexane, and precipitated by dropping in methanol. The precipitate was filtered and dried overnight in vacuo to give the orange product in 77% yield.
FIG. 1 is a SEC chromatogram of BZ-COOH and OBZ-COOH. In comparison with BZ-COOH, the GPC trace of OBZ-COOH shifted to the high molecular weight region, indicating that ring opening polymerization of the benzoxazine monomer occurred, forming a benzoxazine oligomer. In addition, it represents a bimodal curve. The average molecular weight (Mn) of the OBZ-COOH was 1200g/mol, and the polydispersity index (PDI) was 1.40.
Synthesis of polybenzoxazine oligomer, dodecylamino acetoxy polybenzoxazine (OBZ-AC)
OBZ-COOH (4 mmol), acetic anhydride (0.47 g,4.7 mmol), aqueous H 2SO4 (0.49 mL,0.005 mmol) and dimethylacetamide DMAC (5 mL) were mixed thoroughly and dispersed uniformly, and refluxed at 110℃for 2 hours (under mechanical stirring, nitrogen atmosphere). And then dried in a vacuum oven at 80℃for 24 hours to give dodecylamino acetoxy polybenzoxazine (OBZ-AC).
The yield was 90% calculated using OBZ-COOH as starting material.
FIG. 2 shows the 1H NMR spectra of (A) BZ-COOH, (B) OBZ-COOH and (C) OBZ-AC prepared in this example.
In FIG. 2A, the characteristic formants of Ar-CH 2 -N (a) and O-CH 2 -N (b) appear at 3.99ppm and 4.91ppm, respectively, indicating oxazine ring formation. The peak at 0.84ppm (d) is attributed to the proton of-CH 3 in n-dodecylamine, the integral ratio is 1.97:1.98:3.09, consistent with the theoretical value of 2:2:3. This means a successful synthesis of BZ-COOH benzoxazine monomers. Peaks from 6.77ppm (f) to 7.66ppm (g) belong to aromatic protons. After the ring-opening polymerization, the peak at 4.91ppm (B) of O-CH 2 -N completely disappeared, and the peak at 3.99ppm (a) was broadened and shifted to 3.55ppm, as shown in FIG. 2B, indicating that the BZ-COOH ring-opening polymerization was successful.
In FIG. 2C, the new peak at 2.01ppm (j) is due to the methyl proton of the acetoxy group. Based on the integrated values of the 2.01ppm peak and the 7.98ppm peak, the percent conversion of OBZ-COOH to OBZ-AC was calculated as:
Wherein P is the integral value of the peak value of methyl protons at 2.01ppm, respectively, and R is the integral value of the peak value of phenol hydroxyl protons at 7.98ppm, respectively. The calculated conversion of OBZ-COOH was 84.5%.
In fig. 2B and C, the substituent R is dodecyl.
FIG. 3 is a POM image at various temperatures during the OBZ-AC thermal crosslinking curing process of this example. The test method is as follows: the dodecylamine acetoxy polybenzoxazine (OBZ-AC) (4 mmol), zn (CH 3COO)2 (0.04 mmol) and dimethylacetamide DMAC (5 mL) prepared in this example were thoroughly mixed and dispersed uniformly, and refluxed for 3h under an inert atmosphere of nitrogen at 140 ℃ after which the mixture was poured into an aluminum mold and dried at 80 ℃ for 24h in a vacuum oven, placed on a polarizing microscope hot stand at a heating rate of 10 ℃ per minute, and the POM image of the resulting product was tested when the temperature was raised from room temperature to 160, 180, 200, 220, 240, 260, 273 or 280 ℃, observed that no liquid crystal structure was observed when heated at low temperature, and that lamellar structure appeared when heated at 160 ℃, indicating the initiation of copolymerization.
FIG. 4 is a DSC thermogram of OBZ-AC prepared in this example. The Tonset and Tmax of the exothermic peaks were 151 ℃ and 239 ℃, respectively. The exothermic temperature is in the range of 151-340 c due to the occurrence of the esterification crosslinking reaction.
FIG. 6 is an isothermal curing graph of the OBZ-AC of this example heated at 273 ℃ (a) 1min (b) 3min (c) 6min (d) 17min (e) 30min (f) 60 min. The test method is as follows: dodecylamine acetoxy polybenzoxazine (OBZ-AC) (4 mmol), zn (CH 3COO)2 (0.04 mmol) and dimethylacetamide DMAC (5 mL) prepared in this example were thoroughly mixed and dispersed uniformly, and mechanically stirred under an inert atmosphere of nitrogen at 140℃for reflux for 3h, then the mixture was poured into an aluminum mold and dried in a vacuum oven at 80℃for 24 h.a polarizing microscope was heated from room temperature to 273℃at a heating rate of 10℃per minute, the above-mentioned dried mixture was directly placed on the hot stage of a polarizing microscope at 273℃for different times (a) 1min (b) 3min (c) 6min (d) 17min (e) 30min (f) 60min, and an isothermal curing pattern was tested at the initial stage after curing 1min (a) and 3min (b) without the appearance of bright birefringent images, indicating isotropic phase, but when the reaction proceeded to 6min, a clear birefringent image (c) indicating the formation of a liquid crystal structure was observed.
FIG. 8 is a FTIR spectrum of the OBZ-AC of this example after curing at different temperatures for 2 hours. The test method is as follows: dodecylamine acetoxy polybenzoxazine (OBZ-AC) (4 mmol), zn (CH 3COO)2 (0.04 mmol) and dimethylacetamide DMAC (5 mL) prepared in this example were thoroughly mixed and dispersed well, and refluxed under nitrogen under an inert atmosphere at 140℃for 3h after which the mixture was poured into an aluminum mould and dried in a vacuum oven at 80℃for 24h after which it was placed in an air circulation oven at a heating rate of 10℃per minute and warmed up to different temperatures (120, 140, 160, 180, 200 or 220 ℃) from room temperature and then incubated for 2h after cooling to room temperature, the FTIR spectrum of the test sample was reduced in peak intensity at 1775cm -1 for acetoxyC=O with increasing temperature while, due to the action of the ester group C=O, a new peak was developed and added at 1749cm -1, these changes indicated that the esterification reaction was taking place, the peak at 1775cm -1 was almost disappeared after the sample was cured at 200℃for 2h, indicating that the esterification reaction of OBZ-AC was completed.
Example 2
Synthesis of highly thermally conductive aromatic ester-crosslinked liquid crystalline polybenzoxazines, namely aromatic ester-based polybenzoxazines [1] (OBZ-PES)
Dodecylamine acetoxy polybenzoxazine (OBZ-AC) (4 mmol) prepared in example 1, zn (CH 3COO)2 (0.04 mmol) and dimethylacetamide DMAC (5 mL) were thoroughly mixed and dispersed uniformly, and mechanically stirred under reflux for 3h under an inert atmosphere of nitrogen at 140 ℃ then the mixture was poured into an aluminum mold, dried for 24h at 80 ℃ in a vacuum oven then placed in an air circulation program oven at a heating rate of 10 ℃/min from room temperature to 273 ℃ and then incubated for 1h to give OBZ-PES.
FIG. 5 shows SAXS spectra of OBZ-AC (a) of the present example and OBZ-PES (b) prepared in the present example. In the spectrum 5a, no clear peak could be observed due to its amorphous nature. After curing, both types of peaks can be seen in fig. 5 b. The broad peak at q1=0.9 nm -1、q3＝12.36nm-1、q4＝14.22nm-1 is a short-range ordered lamellar arrangement. The peak at q2=6.7 nm -1 corresponds to a long range ordered lamellar arrangement of the smectic C phase structure. The interlayer spacing was calculated to be 2.3nm according to the bragg equation (d=2pi/q). Curing and crosslinking for 60min at 273 ℃ to form a liquid crystal structure.
FIG. 7 (a) is an FTIR spectrum of a benzoxazine monomer prepared in example 1, wherein the peak at 925cm -1 corresponds to the oxazine ring. The FTIR spectrum of the oligobenzoxazine prepared in example 1 is shown in FIG. 7 (b), the peak intensity at 1465cm -1 corresponding to the tetra-substituted benzene ring increases, and the new peak at 3545cm -1 corresponds to the phenolic hydroxyl group. The FTIR spectrum of the polybenzoxazine oligomer prepared in example 1 is shown in FIG. 7 (c), and the success of the substitution of hydroxyl group with acetoxy group is shown by the appearance of a new peak at 1759cm -1 and the disappearance of the peak at 3545cm -1. The FTIR spectrum of the OBZ-PES prepared in this example is shown in FIG. 7 (d), and the crosslinking reaction of OBZ-AC is represented by the peak at 1759cm -1 which disappeared due to the acetate carbonyl group and the new peak at 1749cm -1 which formed due to the polyester carbonyl group.
The solid phase 13 CNMR spectrum of OBZ-PES confirmed the formation of aromatic ester bonds (FIG. 9). The new peak at 194.82ppm (f) was due to carbon resonance of the aromatic ester linkage, indicating successful esterification.
FIG. 10 shows the Thermal Conductivity (TC) of OBZ-COOH, OBZ-AC prepared in example 1 and OBZ-PES prepared in this example. TC of the OBZ-PES was as high as 0.284W/mK, 21% and 16% higher than OBZ-COOH and OBZ-AC, respectively. The high thermal conductivity of the OBZ-PES is due to its liquid crystal structure. This ordered structure facilitates its thermal conductivity. The low TC of OBZ-COOH and OBZ-AC is due to the lack of ordered arrangement and the amorphous nature of the molecular chains.
The DMA curve of the OBZ-PES prepared in this example is shown in FIG. 11. From the ratio peaks of the G ', G' and tan delta curves, the glass transition temperatures were 248 ℃, 269 ℃ and 290 ℃, respectively. In general, the stiffness and crosslink density of the polymer chains can significantly affect the Tg value of the thermoset polymer. The high crosslink density and the presence of rigid aryl ester units of the polymers of the present invention result in a high Tg.
FIG. 12 shows the TGA curves of OBZ-COOH prepared in example 1, OBZ-AC and OBZ-PES prepared in this example. Compared to OBZ-COOH, OBZ-AC has better thermal stability, while cross-linked OBZ-PES has the highest thermal stability. The 5% weight loss temperature (Td 5) and 10% weight loss temperature (Td 10) of the OBZ-PES were up to 383℃and 410℃respectively. This is due to its high crosslink density and high aromatic ester bond energy. In addition, the coke rate of OBZ-PES was 49%, which was far higher than 16% of OBZ-COOH and 17% of OBZ-AC. A significant enhancement of thermal properties by aryl ester cross-linking can clearly be observed.
Example 3
Synthesis of benzoxazine monomer (BZ-COOH 2 in which R 1 is-CH 3)
In a 100mL flask, 4-hydroxybenzoic acid (5 mmol,0.69 g), methylamine (5 mmol,0.155 g) and paraformaldehyde (12.5 mmol,0.3877 g) were mixed in 10mL dioxane and heated to reflux at 90℃for 24h (magnetic stirring). The mixture was then allowed to cool at room temperature, and then 100mL of deionized water was poured into the above mixture in a 500 mL beaker to give a pale yellow precipitate. After the product is filtered, the product is further washed by deionized water for 3 times, and is dried at 60 ℃ to obtain the benzoxazine monomer. The yield thereof was found to be 87%.
Synthesis of Oligobenzoxazines (wherein R 1 is-CH 3, abbreviated as OBZ-COOH 2)
BZ-COOH (0.90 g), t-butylphenol (0.10 g) and tetrahydrofuran (5 mL, dried in advance in an oven at about 60 ℃ C. For 12 hours) were added to a 100mL flask. And ring-opening polymerization is carried out by taking p-n-butylphenol as an initiator. The mixture was then poured into an aluminum mold, which was placed in an air circulation oven and polymerized at 180℃for 3h. The crude product was cooled to room temperature, dissolved in n-hexane, and precipitated by dropping in methanol. The precipitate was filtered and dried overnight in vacuo to give the orange product in 75% yield.
Synthesis of polybenzoxazine oligomer, i.e., methylamino acetoxy polybenzoxazine (OBZ-AC 2)
OBZ-COOH2 (4 mmol), acetic anhydride (0.47 g,4.7 mmol), aqueous H 2SO4 (0.58 mL, 0.006mmol) and dimethylacetamide DMAC (5 mL) prepared above were taken, thoroughly mixed and dispersed uniformly, and refluxed at 130℃for 3 hours (mechanical stirring, nitrogen atmosphere). Then dried in a vacuum oven at 100deg.C for 24h to give methylamino acetoxy polybenzoxazine (OBZ-AC 2).
Example 4
Synthesis of highly thermally conductive aromatic ester-crosslinked liquid crystalline polybenzoxazines, namely aromatic ester-based polybenzoxazines [2] (OBZ-PES 2)
The methylamino acetoxy polybenzoxazine (OBZ-AC 2) (4 mmol) prepared in example 3, zn (CH 3COO)2 (0.04 mmol) and dimethylacetamide DMAC (5 mL) were thoroughly mixed and dispersed uniformly, and mechanically stirred under reflux for 3h under an inert atmosphere of nitrogen at 140 ℃ then the mixture was poured into an aluminum mold and dried in a vacuum oven at 80 ℃ for 24h then placed in an air circulation program oven at a heating rate of 10 ℃/min to 220 ℃ and then incubated for 1h to give OBZ-PES2.
Example 5
Synthesis of benzoxazine monomer (BZ-COOH 3, wherein R 1 is-CH 2CH2 OH)
In a 100mL flask, 4-hydroxybenzoic acid (5 mmol,0.69 g), ethanolamine (5 mmol,0.305 g) and paraformaldehyde (12.5 mmol,0.3877 g) were mixed in 10mL dioxane and heated to reflux at 90℃for 24h (magnetic stirring). The mixture was then allowed to cool at room temperature, and then 100mL of deionized water was poured into the above mixture in a 500mL beaker to give a pale yellow precipitate. After the product is filtered, the product is further washed by deionized water for 3 times, and is dried at 60 ℃ to obtain the benzoxazine monomer. The yield thereof was found to be 83%.
Synthesis of Oligobenzoxazines (wherein R 1 is-CH 2CH2 OH, abbreviated as OBZ-COOH 3)
BZ-COOH (0.90 g), t-butylphenol (0.10 g) and tetrahydrofuran (5 mL, dried in advance in an oven at about 60 ℃ C. For 12 hours) were added to a 100mL flask. And ring-opening polymerization is carried out by taking p-n-butylphenol as an initiator. The mixture was then poured into an aluminum mold, which was placed in an air circulation oven and polymerized at 200℃for 4h. The crude product was cooled to room temperature, dissolved in n-hexane, and precipitated by dropping in methanol. The precipitate was filtered and dried overnight in vacuo to give the orange product in 70% yield.
Synthesis of polybenzoxazine oligomer, 2-aminoethanol acetoxy polybenzoxazine (OBZ-AC 3)
The ethanol group-containing OBZ-COOH3 (4 mmol), acetic anhydride (0.47 g,4.7 mmol), aqueous H 2SO4 (0.68 mL, 0.0075 mmol) and dimethylacetamide DMAC (5 mL) prepared above were taken, thoroughly mixed and dispersed uniformly, and refluxed at 130℃for 3 hours (mechanical stirring, nitrogen atmosphere). Then dried in a vacuum oven at 100deg.C for 24h to give 2-aminoethoxy acetoxy polybenzoxazine (OBZ-AC 3).
Example 6
Synthesis of highly thermally conductive aromatic ester-crosslinked liquid crystalline polybenzoxazines, namely aromatic ester-based polybenzoxazines [3] (OBZ-PES 3)
The 2-aminoethanol-based acetoxy polybenzoxazine (OBZ-AC 3) (4 mmol) prepared in example 5, zn (CH 3COO)2 (0.04 mmol) and N, N-dimethylformamide DMF (5 mL) were thoroughly mixed and dispersed uniformly, and mechanically stirred under reflux for 3 hours under nitrogen in an inert atmosphere at 140 ℃ then the mixture was poured into an aluminum mold and dried for 24 hours at 80 ℃ in a vacuum oven then placed in an air circulation program oven at a heating rate of 10 ℃ C./min and heated to 240 ℃ C., followed by heat preservation for 1 hour to obtain OBZ-PES3.
Example 7
Synthesis of benzoxazine monomer (BZ-COOH 4, wherein R 1 is-CH 2-CH＝CH2)
In a 100mL flask, 4-hydroxybenzoic acid (5 mmol,0.69 g), acrylamide (5 mmol, 0.284 g) and paraformaldehyde (12.5 mmol,0.3877 g) were mixed in 10mL dioxane and heated to reflux at 90℃for 24h (magnetic stirring). The mixture was then allowed to cool at room temperature, and then 100mL of deionized water was poured into the above mixture in a 500 mL beaker to give a pale yellow precipitate. After the product is filtered, the product is further washed by deionized water for 3 times, and is dried at 60 ℃ to obtain the benzoxazine monomer. The yield thereof was found to be 87%.
Oligobenzoxazines (wherein R 1 is-CH 2-CH＝CH2, OBZ-COOH4 for short)
BZ-COOH (0.90 g), t-butylphenol (0.10 g) and toluene (5 mL, dried in an oven at about 80℃for 12 hours in advance) were added to a 100mL flask. And ring-opening polymerization is carried out by taking p-n-butylphenol as an initiator. The mixture was then poured into aluminum molds. The mold was placed in an air circulation oven and polymerized for 3 hours at 175 ℃. The crude product was cooled to room temperature, dissolved in n-hexane, and precipitated by dropping in methanol. The precipitate was filtered and dried overnight in vacuo to give the orange product in 65% yield.
Synthesis of allylamine acetoxy polybenzoxazine (OBZ-AC 4)
The allyl-containing OBZ-COOH4 (4 mmol), acetic anhydride (0.47 g,4.7 mmol), aqueous H 2SO4 (0.68 mL, 0.0071 mmol) and dimethylacetamide DMAC (5 mL) prepared above were thoroughly mixed and dispersed uniformly, and refluxed at 145℃for 5 hours (mechanical stirring, nitrogen atmosphere). Then dried in a vacuum oven at 120deg.C for 24h to give allylamine-based acetoxy polybenzoxazine (OBZ-AC 4).
Example 8
Synthesis of highly thermally conductive aromatic ester-crosslinked liquid crystalline polybenzoxazines, namely aromatic ester-based polybenzoxazines [4] (OBZ-PES 4)
Allylacetoxy polybenzoxazine (OBZ-AC 4) (4 mmol) prepared in example 7, zn (CH 3COO)2 (0.04 mmol) and N-methylpyrrolidone NMP (5 mL) were thoroughly mixed and dispersed uniformly, mechanically stirred under nitrogen at 140℃for reflux for 3h, then the mixture was poured into an aluminum mold, dried at 80℃for 24h in a vacuum oven, then placed in an air circulation program oven at a heating rate of 10℃per minute, heated to 233℃and then kept for 1h to obtain OBZ-PES4.
Example 9
Synthesis of benzoxazine monomer (BZ-COOH 5, wherein R 1 is furfuryl)
In a 100mL flask, 4-hydroxybenzoic acid (5 mmol,0.69 g), furfuryl amine (5 mmol,0.406 g) and paraformaldehyde (12.5 mmol,0.3877 g) were mixed in 10mL dioxane and heated to reflux at 90℃for 24h (magnetic stirring). The mixture was then allowed to cool at room temperature, and then 100mL of deionized water was poured into the above mixture in a 500 mL beaker to give a pale yellow precipitate. After the product is filtered, the product is further washed by deionized water for 3 times, and is dried at 60 ℃ to obtain the benzoxazine monomer. The yield thereof was found to be 79%.
Synthesis of Oligobenzoxazines (wherein R1 is furfuryl, OBZ-COOH5 for short):
BZ-COOH (0.90 g), t-butylphenol (0.10 g) and tetrahydrofuran (5 mL, dried in advance in an oven at about 80℃for 12 hours) were added to a 100mL flask. And ring-opening polymerization is carried out by taking p-n-butylphenol as an initiator. The mixture was then poured into aluminum molds, which were placed in an air circulation oven and polymerized for 5 hours at 193 ℃. The crude product was cooled to room temperature, dissolved in n-hexane, and precipitated by dropping in methanol. The precipitate was filtered and dried overnight in vacuo to give the orange product in 72% yield.
Synthesis of furfuryl-amino-acetoxy-polybenzoxazine (OBZ-AC 5)
The furfuryl group-containing OBZ-COOH5 (4 mmol), acetic anhydride (0.47 g,4.7 mmol), aqueous H 2SO4 (0.70 mL,0.0072 mmol) and dimethylacetamide DMAC (8 mL) prepared above were taken, thoroughly mixed and dispersed uniformly, and refluxed at 150℃for 5 hours (mechanical stirring, nitrogen atmosphere). Then dried in a vacuum oven at 120℃for 24 hours to give furfuryl-amino-acetoxy-polybenzoxazine (OBZ-AC 5).
Example 10
Synthesis of highly thermally conductive aromatic ester-crosslinked liquid crystalline polybenzoxazines, namely aromatic ester-based polybenzoxazines [5] (OBZ-PES 5)
The furfuryl acetoxy polybenzoxazine (OBZ-AC 5) (4 mmol) prepared in example 9, zn (CH 3COO)2 (0.04 mmol) and dimethylacetamide DMAC (5 mL) were thoroughly mixed and dispersed uniformly, and mechanically stirred under reflux for 3h under nitrogen in an inert atmosphere at 140 ℃ then the mixture was poured into an aluminum mold and dried in a vacuum oven at 80 ℃ for 24h then placed in an air circulation program oven at a heating rate of 10 ℃/min to 206 ℃ and then incubated for 1h to give OBZ-PES5.
Example 11
Synthesis of benzoxazine monomer (BZ-COOH 6 for short, wherein R 1 is phenyl)
In a 100mL flask, 4-hydroxybenzoic acid (5 mmol,0.69 g), aniline (5 mmol, 0.4636 g) and paraformaldehyde (12.5 mmol,0.3877 g) were mixed in 10mL dioxane and heated to reflux at 90℃for 24h (magnetic stirring). The mixture was then allowed to cool at room temperature, and then 100mL of deionized water was poured into the above mixture in a 500mL beaker to give a pale yellow precipitate. After the product is filtered, the product is further washed by deionized water for 3 times, and is dried at 60 ℃ to obtain the benzoxazine monomer. The yield thereof was found to be 84%.
Synthesis of Oligobenzoxazines (wherein R1 is phenyl, abbreviated as OBZ-COOH 6)
BZ-COOH (0.90 g), t-butylphenol (0.10 g) and tetrahydrofuran (5 mL, dried in advance in an oven at about 80℃for 12 hours) were added to a 100mL flask. And ring-opening polymerization is carried out by taking p-n-butylphenol as an initiator. The mixture was then poured into an aluminum mold, which was placed in an air circulation oven and polymerized at 190℃for 3h. The crude product was cooled to room temperature, dissolved in n-hexane, and precipitated by dropping in methanol. The precipitate was filtered and dried overnight in vacuo to give the orange product in 71.5% yield.
Synthesis of anilinoacetoxy polybenzoxazine (OBZ-AC 6)
The anilino group-containing OBZ-COOH6 (4 mmol), acetic anhydride (0.47 g,4.7 mmol), aqueous H 2SO4 (0.78 mL,0.008 mmol) and dimethylacetamide DMAC (10 mL) prepared above were taken, thoroughly mixed and dispersed uniformly, and refluxed at 150℃for 5 hours (mechanical stirring, nitrogen atmosphere). Then dried in a vacuum oven at 130℃for 24 hours to give anilinoacetoxypolybenzoxazine (OBZ-AC 6).
Example 12
Synthesis of highly thermally conductive aromatic ester-crosslinked liquid crystalline polybenzoxazines, namely aromatic ester-based polybenzoxazines [6] (OBZ-PES 6)
Anilinoacetoxy polybenzoxazine (OBZ-AC 6) (4 mmol) prepared in example 11, zn (CH 3COO)2 (0.04 mmol) and dimethylacetamide DMAC (5 mL) were thoroughly mixed and dispersed uniformly, and mechanically stirred under reflux for 3h under an inert atmosphere of nitrogen at 140 ℃ then the mixture was poured into an aluminum mold and dried in a vacuum oven at 80 ℃ for 24h then placed in an air circulation program oven at a heating rate of 10 ℃/min to 245 ℃ and then incubated for 1h to obtain OBZ-PES6.
Test examples
The temperature range of formation of the liquid crystal structure during curing of the polybenzoxazine oligomer of the examples, and the TC, tg, T d5、Td10, scorch rate of the oligomeric polybenzoxazine, polybenzoxazine oligomer, aromatic ester-crosslinked liquid crystal polybenzoxazine prepared in the examples were tested.
The temperature range in which the liquid crystal structure is formed means: the temperature at which the formation of the liquid crystal structure starts to be carried out to a temperature at which the liquid crystal structure is no longer changed.
The test method of the temperature range of the liquid crystal structure formation is as follows: the polybenzoxazine oligomer sample (4 mmol), zn (CH 3COO)2 (0.04 mmol) and dimethylacetamide DMAC (5 mL) were thoroughly mixed and dispersed uniformly, and refluxed under mechanical stirring under nitrogen at 140℃for 3 hours, then the mixture was poured into an aluminum mold, dried in a vacuum oven at 80℃for 24 hours, the sample was heated on a hot stage of a polarizing microscope at a heating rate of 10℃per minute, and the formation of liquid crystal structures at different temperatures was examined from room temperature to different temperatures, thereby obtaining a temperature range in which the liquid crystal structures were formed.
The results are shown in Table 1.
TABLE 1 Performance data for different samples
As can be seen from Table 1, the temperature range at which the liquid crystal is present during curing of the polybenzoxazine oligomer prepared in example 1 is 160℃to 273 ℃. The liquid crystal structure is an ordered structure formed by self-crosslinking to form an aromatic ester bond. The liquid crystal of the present invention exists in a temperature range exceeding that of a conventional liquid crystal, which means that the polymer of the present invention does not lose a liquid crystal structure upon polymerization like a conventional polymer, and has a higher thermal conductivity than a conventional liquid crystal polymer. The glass transition temperature (Tg is more than or equal to 260 ℃) is far higher than that of the common liquid crystal copolymer. Therefore, the heat resistance of the thermosetting resin designed and synthesized by the invention is superior to that of common resin, and the requirement of high heat resistance can be met.
The above-described embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above-described embodiments. Other changes, modifications, substitutions, and simplifications without departing from the spirit and principles of the invention are equivalent substitution methods, and belong to the scope of the invention.
The liquid crystal polybenzoxazines synthesized according to the invention are useful in integrated circuits, electronics, LED lighting and liquid crystal displays.

Claims (11)

1. A polybenzoxazine oligomer having the structure of formula I:
wherein R 1 is selected from one of the following ：-CH3,-(CH2)11CH3,-CH2CH2OH,-CH2-CH＝CH2,
R 2 is selected from one of the following: -CH 2CH3,-CH3;
n is 3-6.
2. The method for preparing the polybenzoxazine oligomer according to claim 1, comprising the steps of:
(1) Dissolving p-hydroxybenzoic acid, amine compounds and paraformaldehyde in a low-polarity solvent, and carrying out reflux reaction to obtain benzoxazine monomers;
Wherein R 1 and R 1 in the compound of formula I have the same meaning;
the low-polarity solvent is one or more of toluene, xylene, ethanol or dioxane;
(2) Dissolving benzoxazine monomer and initiator in solvent, and performing ring-opening polymerization reaction to obtain oligomeric benzoxazine; then adding anhydride, acid catalyst and solvent, fully mixing and dispersing uniformly, and obtaining the polybenzoxazine oligomer (I) through acylation reaction and drying.
3. The method of preparing a polybenzoxazine oligomer according to claim 2, wherein in step (1) one or more of the following conditions are included:
i. The structural formula of the amine compound is R 1-NH2; wherein R 1 and R 1 in the compound of formula I have the same meaning;
ii. The molar ratio of the parahydroxybenzoic acid to the amine compound to the paraformaldehyde is 1:2:1-1:2:6;
iii, the volume ratio of the molar quantity of the parahydroxybenzoic acid to the low-polarity solvent is 1:1-1:20 mol/L;
iv, the reflux reaction temperature is 80-110 ℃, and the reflux reaction time is 10-24 hours;
v, the post-treatment method of the reaction liquid obtained by the reflux reaction comprises the following steps: deionized water is added into the reaction liquid, and then the benzoxazine monomer is obtained through filtration, washing and drying.
4. The method of preparing a polybenzoxazine oligomer according to claim 2, wherein in step (2) one or more of the following conditions are included:
i. the solvent is tetrahydrofuran or toluene; the volume ratio of the mass of the benzoxazine monomer to the solvent is 0.1-1 g/mL;
ii. The initiator is Benzoyl Peroxide (BPO), 2' -Azobisisobutyronitrile (AIBN), tert-butylphenol (TBP) or p-n-butylphenol; the addition amount of the initiator is 0.1-15% of the mass of the benzoxazine monomer;
iii, the ring-opening polymerization reaction temperature is 130-200 ℃ and the reaction time is 2-5 h;
The post-treatment method of the crude product obtained by ring-opening polymerization reaction comprises the following steps: the crude product is cooled to room temperature, then is dissolved in normal hexane, is added dropwise into methanol for precipitation, and is filtered and dried to obtain the oligobenzoxazine.
5. The method of preparing a polybenzoxazine oligomer according to claim 2, wherein in step (2) one or more of the following conditions are included:
i. The anhydride is acetic anhydride or propionic anhydride; the molar ratio of the anhydride to the oligobenzoxazine is 1:1-2:1;
ii. The acid catalyst is an aqueous solution of an acid, wherein the acid is sulfuric acid, sulfonic acid or hydrochloric acid; the concentration of the aqueous solution of the acid is 0.005-0.05 mol/L; the molar ratio of the acid catalyst to the oligobenzoxazine is 1:500-1:820;
iii, the solvent is dimethylacetamide; the volume ratio of the molar quantity of the oligobenzoxazine to the solvent is 0.2-1 mol/L;
iv, the acylation reaction temperature is 100-150 ℃, the acylation reaction time is 2-6 h, and the acylation reaction is carried out in inert atmosphere under stirring condition; the inert atmosphere is nitrogen or argon;
v, drying is vacuum drying, the drying temperature is 60-130 ℃, and the drying time is 20-30 h.
6. The high-thermal-conductivity aromatic ester crosslinked liquid crystal polybenzoxazine is characterized by having a structure shown in the following formula II:
wherein R 1 is selected from one of the following ：-CH3,-(CH2)11CH3,-CH2CH2OH,-CH2-CH＝CH2,
7. The method for preparing a high thermal conductivity aromatic ester crosslinked liquid crystal polybenzoxazine according to claim 6, comprising the steps of: dissolving the polybenzoxazine oligomer (I) and a catalyst in an organic solvent, and heating, crosslinking and curing to obtain high-thermal-conductivity aromatic ester crosslinked liquid crystal polybenzoxazine;
wherein R 1 is selected from one of the following ：-CH3,-(CH2)11CH3,-CH2CH2OH,-CH2-CH＝CH2,
R 2 is selected from one of the following: -CH 2CH3,-CH3;
n is 3-6.
8. The method for preparing a high thermal conductivity aromatic ester crosslinked liquid crystal polybenzoxazine according to claim 7, including one or more of the following conditions:
i. the catalyst is calcium acetate, sodium acetate, manganese acetate or zinc acetate; the mass ratio of the catalyst to the polybenzoxazine oligomer (I) is 0.01:1-0.05:1;
ii. The organic solvent is dimethylacetamide, N-dimethylformamide or N-methylpyrrolidone; the ratio of the molar amount of the polybenzoxazine oligomer (I) to the volume of the organic solvent is 0.5 to 1mol/L.
9. The method for preparing a high thermal conductivity aromatic ester crosslinked liquid crystal polybenzoxazine according to claim 7, further comprising the steps of reflux stirring and drying before the heat crosslinking curing.
10. The method for preparing high thermal conductivity aromatic ester crosslinked liquid crystal polybenzoxazine according to claim 9, wherein the reflux stirring temperature is 110-180 ℃, the reflux stirring time is 2-6 h, and the reflux stirring is performed in inert atmosphere under stirring conditions; the inert atmosphere is nitrogen or argon; the drying is vacuum drying, the drying temperature is 60-100 ℃, and the drying time is 20-30 h.
11. The method for preparing a high thermal conductivity aromatic ester crosslinked liquid crystal polybenzoxazine according to claim 7, wherein the heat crosslinking curing is performed in air; the heating crosslinking curing method comprises the following steps: heating to 160-273 ℃ at a heating rate of 8-12 ℃/min, and then preserving heat for 0-2 h; or the heat crosslinking curing method comprises the following steps: directly preserving the temperature at 160-273 ℃ for 6 min-2 h.
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