A highly oriented polyimide fiber based on liquid crystal spinning and its preparation method

Abstract

The present invention relates to a highly oriented polyimide fiber based on liquid crystal spinning and a preparation method thereof. Rigid acid anhydride is used as a raw material to prepare a para-diacid diester, which is polymerized with a rigid diamine to prepare a fully para-positioned polyamic acid ester solution having lyotropic liquid crystal properties; an organic base catalyst is then added to prepare a spinning solution, and the highly oriented polyimide fiber is prepared by high-temperature dry spinning, catalytic cyclization, and high-temperature heat treatment. The present invention introduces an organic base catalyst into the spinning solution having lyotropic liquid crystal properties, and with the help of the shearing and fluid stretching effects of the spinneret, achieves a high degree of orientation of the liquid crystal domain along the fiber axis; then, based on high-temperature dry spinning and catalytic cyclization reactions, the fiber is promoted to achieve a structural transformation from polyamic acid ester to polyimide while being solidified and formed, achieving locking of the orientation structure and densification inside the fiber, thereby ensuring the stability and uniformity of the fiber. It has great application potential in extreme special environments such as aerospace.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer fibers, and in particular relates to a highly oriented polyimide fiber based on liquid crystal spinning and a preparation method thereof. Background Art

[0002] The mechanical properties of fibers are not only affected by their molecular structure and molecular weight, but are also closely related to the orientation, crystallization, and micro-defects of the macromolecular chains. The production process of traditional polyimide (PI) fibers begins with a polyamic acid (PAA) precursor solution. The PAA molecular chains, composed of polar amide bonds and carboxyl groups, exhibit good solubility in conventional polar solvents (such as N,N-dimethylacetamide (DMAc) or N-methylpyrrolidone (NMP). However, there are significant differences between the random coil conformation of PAA and the rigid rod-like structure of PI. Even after multi-stage stretching, it is difficult to overcome the molecular chain conformational entropy barrier, making it difficult to form similar highly ordered molecular stacking during solid-state phase transitions.

[0003] Polyamic acid ester (PAE) is another precursor of polyimide. Due to its high molecular chain rigidity and high symmetry of the fully para-positioned polyamic acid ester molecular chain, it is conducive to the formation of lyotropic liquid crystals. For example, Neuber et al. esterified pyromellitic anhydride (PMDA) and prepared a para-positioned diacid diester by recrystallization. Subsequently, they reacted it with a rigid diamine to obtain a fully para-positioned polyamic acid ethyl ester solution. In this solution, a typical nematic liquid crystal structure was observed in the concentration range of 30 wt% to 50 wt% (Macromolecular Chemistry and Physics, 2002, 203(3): 598-604., Macromolecules, 1992, 25(25): 6784-6790.). Tanaka et al. further extended the side chain ethyl ester group to a C4~C12 alkyl chain. Due to the incompatibility between the aliphatic structure and the aromatic structure, the driving system formed a unique layered structure, which significantly improved the order of the liquid crystal state of PAE, causing it to gradually transform from a nematic to a smectic state (Macromolecules, 2019, 52(13): 5054-5066.). However, polyamic acid ester generates polyimide through an intramolecular dealcoholization reaction at high temperature. The reaction is accompanied by the escape of small molecular alcohols, which may cause voids and defects in the fiber, thereby destroying the mechanical properties of the fiber. Although current research has shown that para-polyamic acid ester solutions may exhibit liquid crystal properties, there has been no report on the preparation of polyimide fibers using liquid crystal polyimide esters. From polyamic acid ester to polyimide fibers, the following still need to be solved: 1) how to match the liquid crystal characteristic parameters (temperature, concentration, shear force) with the fiber forming process; 2) how to avoid the escape of side chain alcohols from damaging the mechanical properties of the fiber. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a highly oriented polyimide fiber based on liquid crystal spinning and a preparation method thereof, which solves the problems of low molecular chain orientation and low crystallinity of traditional polyimide fibers. The prepared polyimide fiber has high orientation, high crystallinity, dense structure and excellent mechanical properties.

[0005] The present invention provides a highly oriented polyimide fiber based on liquid crystal spinning. Rigid acid anhydride is used as a raw material to prepare a para-diacid diester, which is polymerized with a rigid diamine to prepare a fully para-polyamic acid ester solution with lyotropic liquid crystal properties. An organic base catalyst is then added to prepare a spinning solution, and the highly oriented polyimide fiber is prepared through high-temperature dry spinning, catalytic cyclization and high-temperature heat treatment. The high orientation refers to an orientation factor ≥ 0.6; the temperature of the high-temperature dry spinning is 150-250°C; and the process parameters of the high-temperature heat treatment are: a temperature of 350-480°C and a tensile strength of 1-20 cN / dtex.

[0006] Preferably, the rigid anhydride is one of the following structures:

[0007] , , , , .

[0008] Preferably, the rigid diamine is one of the following structures:

[0009] , , , , , , , , , , .

[0010] Preferably, the organic base catalyst includes at least one of quinoline, isoquinoline, imidazole, 1,4-diazabicyclo[2.2.2]octane, picoline, and dimethylimidazole.

[0011] The present invention also provides a method for preparing highly oriented polyimide fibers based on liquid crystal spinning, comprising the following steps:

[0012] (1) Heating a rigid acid anhydride in an organic alcohol solvent to obtain a diacid diester mixture, and then preparing a pure para-diacid diester monomer by recrystallization;

[0013] (2) subjecting the above-mentioned pure para-position diacid diester monomer to acyl chloride reaction, and then reacting it with diamine in an organic solvent to prepare a fully para-position polyamic acid ester solution;

[0014] (3) adding an organic base catalyst to the polyamic acid ester solution, and adjusting the solid content to 20 wt % to 60 wt % by heating and concentrating to obtain a polyamic acid ester spinning solution;

[0015] (4) preparing highly oriented polyimide spun fibers by a dry spinning process using the polyamic acid ester spinning solution;

[0016] (5) Finally, high-temperature heat treatment is performed to obtain highly oriented polyimide fibers based on liquid crystal spinning.

[0017] Preferably, the organic alcohol solvent in step (1) is any one of C2 to C14 saturated fatty alcohols.

[0018] Preferably, the heating process parameters in step (1) are: heating to 60-100° C. and reflux for 2-20 hours.

[0019] Preferably, the chlorination reagent used in the chlorination reaction in step (2) is oxalyl chloride or thionyl chloride, and the molar ratio of oxalyl chloride or thionyl chloride to the carboxyl group in the para-diacid diester monomer is 1 to 5:1; the solvent used in the chlorination reaction is at least one of dichloromethane, ethyl acetate, and tetrahydrofuran.

[0020] Preferably, the acyl chloride reaction in step (2) is as follows: first, stirring the reaction in an ice-water bath for 6 to 8 hours, and then heating the temperature to 60 to 100° C. under nitrogen purge to evaporate to dryness.

[0021] Preferably, the organic solvent in step (2) is one of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, m-cresol, γ-butyrolactone, and p-chlorophenol.

[0022] Preferably, the reaction conditions with diamine in step (2) are: reaction at -10-60°C for 6-12 hours, and the solid content is controlled at 8-20 wt%.

[0023] Preferably, the amount of the organic base catalyst in step (3) is 0.01% to 10% of the mass of the polyamic acid ester.

[0024] Preferably, the process parameters of the dry spinning in step (4) are: a tunnel temperature of a gradient temperature of 150-250° C.; and a spinning speed of 100-300 m / min.

[0025] Preferably, the process parameters of the high-temperature heat treatment in step (5) are: temperature of 350-480°C, and tensile strength of 1-20 cN / dtex.

[0026] Preferably, the cyclization degree of the polyimide nascent fibers in step (4) is greater than 70%, and the total fiber orientation degree is greater than or equal to 0.5.

[0027] Preferably, the orientation degree of the polyimide fiber in step (5) is ≥0.6.

[0028] Through Materials Studio (MS) software, Figures 1 to 4 The conformational statistical properties of the polymer chains were quantitatively analyzed by calculating the mean square end-to-end distance ( <r 2 >), mean square radius of rotation ( 2 >) and their ratios, systematically evaluating the rigidity and flexibility of the molecular chain. <r 2 / S 2 >The ratio is 6, while a rigid chain will lead to a higher ratio, and vice versa. The known poly (p-phenylene terephthalamide) (PPTA) has a conjugated effect between the benzene ring and the amide bond, which restricts the rotation of the molecular chain, forming a nearly fully rigid linear structure and showing liquid crystal properties in concentrated sulfuric acid solution; the para-aramid prepared by the liquid crystal spinning method has high strength and modulus. Figure 9 As shown, PPTA <r 2 / S 2 > As high as 11.96. In the traditional PAA molecular chain, due to the presence of para-PAA, <r 2 / S 2 >=10.01) and meta-PAA (meta-PAA, <r 2 / S 2 >=8.94), its molecular chain rigidity and regularity are low, making it difficult for it to present a liquid crystal state. It is worth noting that as the length of the PAE side chain ester carbon chain increases (C2→C 10 ), molecular chain <r 2 / S 2 >The ratio increased from 10.74 to 11.88 (para-PAE-C 10 ), the molecular chain rigidity is significantly increased, proving that the formation of lyotropic liquid crystal phase can be promoted by regulating the side group structure, providing theoretical support for the liquid crystal spinning in the present invention. However, excessively long side chains may have a strong disturbance on the main chain, such as para-PAE-C 12 of <r 2 / S 2 >The value is 10.45, which reduces the rigidity of the molecular chain.

[0029] Beneficial effects ​

[0030] (1) The present invention synthesizes fully para-polyamic acid ester through molecular structure design. Based on its lyotropic liquid crystal properties, the molecular chain is highly oriented along the fiber axis at the spinneret under the synergistic effect of temperature field, shear field and external force field. And with the synergistic effect of solvent volatilization and catalytic cyclization in the dry spinning channel, the transformation of polyamide ester to polyimide structure and the densification of the fiber are achieved, avoiding the generation of defects such as internal voids in the fiber caused by the escape of ester groups in the high-temperature solid state. Finally, after further high-temperature heat treatment, the fiber orientation and crystallinity are further improved, thereby realizing the preparation of highly oriented polyimide fibers.

[0031] (2) The present invention solves the problem of insufficient mechanical properties caused by limited orientation and crystallinity in traditional polyimide fibers. In the high-temperature channel of dry spinning, by means of solvent evaporation and catalytic cyclization reaction, the orientation structure is locked and the fiber is densified, thereby ensuring the stability and uniformity of the fiber. It has great application potential in extreme special environments such as aerospace. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a polyimide molecular structure synthesized from pyromellitic anhydride (PMDA) and p-phenylenediamine (PDA) as monomers.

[0033] Figure 2 It is the molecular structure of poly(p-phenylene terephthalamide) (PPTA).

[0034] Figure 3 is the molecular formula of meta-polyamic acid (meta-PAA) and para-polyamic acid (para-PAA) synthesized from PMDA and PDA monomers in Comparative Example 1.

[0035] Figure 4 The molecular formula of the para-PAE (para-PAE-Cx) synthesized from PMDA and PDA monomers in Examples 1 to 5.

[0036] Figure 5 Schematic diagram of dry spinning equipment.

[0037] Figure 6 This is a picture of the para-PAE-C4 solution prepared in Example 2 under a polarizing microscope.

[0038] Figure 7 Hot stage polarizing microscope images of the para-TEHQ-PAE-TFMB-C4 spinning solution prepared in Example 8 at different temperatures.

[0039] Figure 8Pictures of dripping (a) and broken / hairy yarns (b) appearing during the spinning process of the PAA solution prepared in Comparative Example 1.

[0040] Figure 9 Molecular conformation data parameters calculated for simulation. DETAILED DESCRIPTION

[0041] Below in conjunction with specific embodiment, further set forth the present invention.Should be understood that these embodiments are only used to illustrate the present invention and are not used in limiting the scope of the present invention.In addition, should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall equally within the scope limited by the appended claims of the application.

[0042] Example 1

[0043] Step (1): Under nitrogen deoxygenation protection, add 21.81 g (0.1 mol) of pyromellitic dianhydride (PMDA) to a 100 mL three-necked flask, add 22 mL of anhydrous ethanol to adjust the solid content to 20 wt%, heat to 80°C in an oil bath, reflux the evaporated ethanol through a condenser, and stir for 3 hours to obtain an ethanol solution of esterified diethyl phthalate (PMDE-C2).

[0044] Step (2): The prepared ethanol solution of PMDE-C2 is subjected to rotary evaporation in a rotary evaporator to remove most of the ethanol solvent. The remaining viscous solid is added to a Soxhlet extractor and extracted with butyl acetate to obtain the para-diacid diester. The obtained para-diacid diester is recrystallized twice from water and acetone and a third time from anhydrous cyclohexane, and then vacuum dried to obtain pure para-PMDE-C2.

[0045] Step (3): Under nitrogen atmosphere, 0.1 mol of the product of step (2) in DMAc solution was added to a 500 mL three-necked flask. After being fully dissolved under mechanical stirring, 5 mL of triethylamine and 10 mL of trimethylsilyl chloride were added as catalysts. 0.1 mol of p-phenylenediamine (PDA) was added and DMAc was continued to be added to adjust the solid content to 20 wt%. Under nitrogen protection, the system temperature was controlled at 0°C and stirred for 2 h. The reaction was then slowly heated to 30°C and reacted for 3 h to obtain a fully para-polyamide ester solution (para-PAE-C2) with a solid content of about 30 wt%, which exhibited certain liquid crystal properties above 50°C.

[0046] Step (4): 0.1 wt% of isoquinoline was added to the para-PAE-C2 solution prepared above and stirred to prepare a spinning solution with lyotropic liquid crystal properties. The solution was allowed to stand for 1 hour at room temperature for degassing and then filtered through a metal filter to remove impurities and undissolved particles. The filtered spinning solution was transported to a spinning device (such as Figure 5 The fibers were extruded through a circular spinneret (as shown in the figure), where the hot air temperature was 190°C, the atmosphere was nitrogen, and the take-up speed was 110 m / min. The hot air gradually evaporated the solvent from the fibers, ultimately producing nearly completely cyclized para-PAE-C2 spun fibers. The spun fibers were then cyclized and heat-treated at 450°C and a tensile strength of 3.8 cN / dtex, resulting in a fiber with an orientation factor of 0.76 and a breaking strength of 4.5 cN / dtex.

[0047] Example 2

[0048] Step (1): Under nitrogen deoxygenation protection, add 21.81 g (0.1 mol) of PMDA to a 100 mL three-necked flask, add 22 mL of anhydrous butanol to adjust the solid content to 25 wt%, heat to 120 ° C in an oil bath, reflux the evaporated butanol through a condenser, and stir for 5 hours to obtain a butanol solution of esterified dibutyl phthalate (PMDE-C4).

[0049] Step (2): The PMDE-C4 butanol solution obtained is subjected to rotary evaporation in a rotary evaporator to remove most of the butanol solvent. The remaining viscous solid is added to a Soxhlet extractor and extracted with butyl acetate to obtain the para-diacid diester. The obtained para-diacid diester is recrystallized twice from water and acetone and a third time from anhydrous cyclohexane, and then vacuum dried to obtain pure para-PMDE-C4 dibutyl ester.

[0050] Step (3): Add 10 g of para-PMDE-C4 to 50 mL of THF and 25.38 g (0.2 mol) of oxalyl chloride. Heat to 80-90°C. After reflux for 20 min, remove the THF and excess oxalyl chloride by vacuum distillation. Add 10 mL of toluene to the solution and continue distillation under vacuum to remove the remaining oxalyl chloride to obtain the para-chlorinated diester product.

[0051] Step (4): Under nitrogen atmosphere, add 0.1 mol of para-diester dichloride in DMAc to a 500 mL three-necked flask. After fully dissolving under mechanical stirring, add 5 mL of triethylamine and 10 mL of trimethylsilyl chloride as catalysts. Add 0.1 mol of PDA and continue to add DMAc to adjust the solid content to 30 wt%. Under nitrogen atmosphere, control the system temperature to 0°C and stir for 2 h. Then slowly raise the temperature to 30°C and react for 3 h to obtain a polyamide ester solution (para-PAE-C4). Figure 6 As shown, it exhibits certain lyotropic liquid crystal properties.

[0052] Step (5): 0.2 wt% of 1,4-diazabicyclo[2.2.2]octane (DBU) was added to the para-PAE-C4 solution prepared above and stirred uniformly to obtain a spinning solution having lyotropic liquid crystal properties. The spinning was carried out in the same manner as in step (3) of Example 1 to obtain almost completely cyclized spun fibers. The spun fibers were then cyclized and heat-treated at 460°C under a tensile strength of 5.6 cN / dtex to obtain fibers with an orientation factor of 0.70 and a breaking strength of 6.0 cN / dtex.

[0053] Example 3

[0054] Step (1): Under nitrogen deoxygenation protection, add 21.81 g (0.1 mol) of PMDA to a 100 mL three-necked flask, add 22 mL of anhydrous hexanol to adjust the solid content to 35 wt%, heat in an oil bath to the boiling point of hexanol 160°C, reflux the evaporated hexanol through a condenser, and stir for 8 hours to obtain an hexanol solution of esterified dihexyl phthalate (PMDE-C6).

[0055] Step (2): The obtained diacid diester hexanol solution is subjected to rotary evaporation in a rotary evaporator to remove most of the hexanol solvent. The remaining viscous solid is added to a Soxhlet extractor and extracted with butyl acetate to obtain the para-diacid diester. The obtained para-diacid diester is recrystallized twice from water and acetone and a third time from anhydrous cyclohexane, and then vacuum dried to obtain pure para-diacid dihexyl ester (para-PMDE-C6).

[0056] Step (3): Add 10 g of the diacid diester to 50 mL of THF and 25.38 g (0.2 mol) of oxalyl chloride, and heat to 80-90°C. After reflux for 20 min, remove the THF and excess oxalyl chloride by vacuum distillation. Add 10 mL of toluene to the solution, and continue distillation under vacuum to remove the residual oxalyl chloride to obtain the para-acyl chloride diester product.

[0057] Step (4): Under nitrogen atmosphere, add 0.1 mol of para-diester dichloride in DMAc to a 500 mL three-necked flask. After sufficient dissolution under mechanical stirring, add 5 mL of triethylamine and 10 mL of trimethylsilyl chloride as catalysts. Add 0.1 mol of PDA and continue to add DMAc to adjust the solid content to 35 wt%. Under nitrogen atmosphere, control the system temperature at 0°C and stir for 2 h. Then slowly raise the temperature to 30°C and react for 3 h to obtain a polyamide ester solution (para-PAE-C6).

[0058] Step (5): 0.3 wt% of DBU was added to the para-PAE-C6 solution prepared above and the mixture was uniformly stirred to obtain a spinning solution having lyotropic liquid crystal properties. The spinning was performed in the same manner as in step (3) of Example 1 to obtain almost completely cyclized spun fibers. The spun fibers were then cyclized and heat-treated at 460°C under a tensile strength of 5.2 cN / dtex. The prepared fibers had an orientation factor of 0.69 and a breaking strength of 5.8 cN / dtex.

[0059] Example 4

[0060] Step (1): Under nitrogen deoxygenation protection, add 21.81 g (0.1 mol) of PMDA to a 100 mL three-necked flask, add 22 mL of anhydrous octanol to adjust the solid content to 35 wt%, heat in an oil bath to the boiling point of octanol 180°C, reflux the evaporated octanol through a condenser, and stir for 10 hours to obtain an octanol solution of esterified dioctyl phthalate (PMDE-C8).

[0061] Step (2): The obtained PMDE-C8 octanol solution is subjected to rotary evaporation in a rotary evaporator to remove most of the octanol solvent. The remaining viscous solid is added to a Soxhlet extractor and extracted with butyl acetate to obtain the para-diacid diester. The obtained para-diacid diester is recrystallized twice from water and acetone and a third time from anhydrous cyclohexane, and then vacuum dried to obtain the pure para-diacid diester (para-PMDE-C8).

[0062] Step (3): Add 10 g of para-PMDE-C8 to 50 mL of THF and 25.38 g (0.2 mol) of oxalyl chloride. Heat to 80-90°C. After reflux for 20 min, remove the THF and excess oxalyl chloride by vacuum distillation. Add 10 mL of toluene to the solution and continue distillation under vacuum to remove the remaining oxalyl chloride to obtain the para-chlorinated diester product.

[0063] Step (4): Under nitrogen atmosphere, add 0.1 mol of para-diester dichloride in DMAc to a 500 mL three-necked flask. After sufficient dissolution under mechanical stirring, add 5 mL of triethylamine and 10 mL of trimethylsilyl chloride as catalysts. Add 0.1 mol of PDA and continue to add DMAc to adjust the solid content to 40 wt%. Under nitrogen atmosphere, control the system temperature at 0°C and stir for 2 h. Then slowly raise the temperature to 30°C and react for 3 h to obtain a polyamide ester (para-PAE-C8) solution.

[0064] Step (5): 0.5 wt% of DBU was added to the para-PAE-C8 solution prepared above and the mixture was uniformly stirred to obtain a spinning solution having lyotropic liquid crystal properties. The spinning was performed in the same manner as in step (3) of Example 1 to obtain almost completely cyclized spun fibers. The spun fibers were then cyclized and heat-treated at 440°C under a tensile strength of 4.9 cN / dtex. The prepared fibers had an orientation factor of 0.71 and a breaking strength of 5.3 cN / dtex.

[0065] Example 5

[0066] Step (1): Under nitrogen deoxygenation protection, add 21.81 g (0.1 mol) of PMDA to a 100 mL three-necked flask, add 22 mL of anhydrous dodecanol to adjust the solid content to 40 wt%, heat to the boiling point of the alcohol in an oil bath, reflux the evaporated ethanol through a condenser, and stir for 3 hours to obtain the esterified dodecanol solution of PMDE-C12.

[0067] Step (2): The obtained PMDE-C12 dodecanol solution is subjected to rotary evaporation in a rotary evaporator to remove most of the dodecanol solvent. The remaining viscous solid is added to a Soxhlet extractor and extracted with butyl acetate to obtain the para-diacid diester. The obtained para-diacid diester is recrystallized twice from water and acetone and a third time from anhydrous cyclohexane, and then vacuum dried to obtain the pure para-diacid diester (para-PMDE-C12).

[0068] Step (3): Add 10 g of para-PMDE-C12 to 50 mL of THF and 25.38 g (0.2 mol) of oxalyl chloride. Heat to 80-90°C. After reflux for 20 min, remove the THF and excess oxalyl chloride by vacuum distillation. Add 10 mL of toluene to the solution and continue distillation under vacuum to remove the remaining oxalyl chloride, yielding the para-chlorinated diester product.

[0069] Step (4): Under nitrogen atmosphere, add 0.1 mol of para-diester dichloride in DMAc to a 500 mL three-necked flask. After sufficient dissolution under mechanical stirring, add 5 mL of triethylamine and 10 mL of trimethylsilyl chloride as catalysts. Add 0.1 mol of PDA and continue to add DMAc to adjust the solid content to 45 wt%. Under nitrogen atmosphere, control the system temperature at 0°C and stir for 8 h. Then slowly raise the temperature to 30°C and react for 3 h to obtain a polyamide ester (para-PAE-C12) solution.

[0070] Step (5): 0.5 wt% DBU was added to the para-PAE-C12 solution prepared above and stirred uniformly to obtain a spinning solution having lyotropic liquid crystal properties. The spinning was performed in the same manner as in step (3) of Example 1 to obtain almost completely cyclized spun fibers. The spun fibers were then cyclized and heat-treated at 440°C to obtain a fiber with an orientation factor of 0.67.

[0071] Example 6

[0072] Step (1): Under nitrogen atmosphere, 0.1 mol of the DMAc solution of the product of step (2) of Example 1 was added to a 500 mL three-necked flask. After sufficient dissolution under mechanical stirring, 5 mL of triethylamine and 10 mL of trimethylsilyl chloride were added as catalysts. 0.1 mol of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFMB) was added, and DMAc was continued to be added to adjust the solid content to 30 wt%. Under nitrogen atmosphere, the system temperature was controlled at 0°C and stirred for 12 h. The temperature was then slowly raised to 30°C and reacted for 3 h to obtain a polyamide ester solution (para-PAE-TFMB-C4).

[0073] Step (2): The para-PAE-TFMB-C4 solution prepared above was used with the same catalyst and spinning process as in step (5) of Example 5 to prepare almost completely cyclized spun fibers; the spun fibers were then cyclized and heat-treated at 410°C and a tensile strength of 5.8 cN / dtex to prepare fibers with an orientation factor of 0.80 and a breaking strength of 6.3 cN / dtex.

[0074] Example 7

[0075] Step (1): Under nitrogen atmosphere, 0.1 mol of the product of step (2) of Example 1 was added to a 500 mL three-necked flask in DMAc solution. After sufficient dissolution under mechanical stirring, 5 mL of triethylamine and 10 mL of trimethylsilyl chloride were added as catalysts. 0.1 mol of octyl benzyl adipate (BOA) was added, and DMAc was continued to be added to adjust the solid content to 15 wt%. Under nitrogen protection, the system temperature was controlled at 0°C and stirred for 14 h. The temperature was then slowly raised to 30°C and reacted for 5 h to obtain a polyamide ester solution (para-PAE-BOA-C4).

[0076] Step (2): The para-PAE-BOA-C4 solution prepared above was treated with the same catalyst and spinning process as in step (5) of Example 5 to prepare almost completely cyclized spun fibers; the spun fibers were then cyclized and heat-treated at 460°C under a tensile strength of 6.8 cN / dtex to prepare fibers with an orientation factor of 0.85 and a breaking strength of 7.2 cN / dtex.

[0077] Example 8

[0078] Step (1): Under nitrogen deoxygenation protection, 45.8 g (0.1 mol) of p-phenylene trimellitic dianhydride (TAHQ) was added to a 100 mL three-necked flask, and 46 mL of anhydrous butanol was added to adjust the solid content to 40 wt%. The mixture was heated to 120 ° C in an oil bath, and the evaporated butanol was refluxed through a condenser. The mixture was stirred for 3 hours to obtain a butanol solution of esterified dibutyl phthalate (TEHQ).

[0079] Step (2): The obtained TEHQ butanol solution is subjected to rotary evaporation in a rotary evaporator to remove most of the butanol solvent. The remaining viscous solid is added to a Soxhlet extractor and extracted with butyl acetate to obtain the para-diacid diester. The obtained para-diacid diester is recrystallized twice from water and acetone and a third time from anhydrous cyclohexane, and then vacuum dried to obtain the pure para-diacid diester (para-TEHQ).

[0080] Step (3): Add 10 g of para-TEHQ to 50 mL of THF and 25.38 g (0.2 mol) of oxalyl chloride, and heat to 80-90°C. After reflux for 20 min, remove the THF and excess oxalyl chloride by vacuum distillation. Add 10 mL of toluene to the solution, and continue distillation under vacuum to remove the residual oxalyl chloride to obtain the para-chlorinated diester product.

[0081] Step (4): Under nitrogen atmosphere, a 0.1 mol solution of para-diester dichloride in DMAc was added to a 500 mL three-necked flask. After sufficient dissolution under mechanical stirring, 5 mL of triethylamine and 10 mL of trimethylsilyl chloride were added as catalysts. 0.1 mol of TFMB was added, and DMAc was continued to adjust the solid content to 30 wt%. Under nitrogen atmosphere, the system temperature was controlled at 0°C and stirred for 2 h. The temperature was then slowly raised to 30°C and reacted for 3 h to obtain a polyamide ester (para-TEHQ-PAE-TFMB-C4) solution.

[0082] Step (5): The para-TEHQ-PAE-TFMB-C4 solution with a solid content of 15 wt% was scraped to form a film, and its phase change process was observed under a hot stage polarizing microscope at a temperature program of 10 ° C / min (such as Figure 7 As shown): At room temperature, it is an isotropic uniform solution; when the temperature is raised to 80°C, a striped texture of alternating light and dark appears, and the texture characteristics change significantly with increasing temperature; when the temperature is raised to above 120°C, the texture disappears and a solid film is formed as the solvent evaporates, which means that it has obvious liquid crystal properties. The para-TEHQ-PAE-TFMB-C4 solution with a solid content of 30 wt% prepared above was used with the same catalyst and spinning process as in step (4) of Example 1 to prepare almost completely cyclized spun fibers; the spun fibers were then cyclized and heat-treated at 380°C and a tensile strength of 3.5 cN / dtex to prepare fibers with an orientation factor of 0.81 and a breaking strength of 3.9 cN / dtex.

[0083] Comparative Example 1

[0084] Step (1): Under nitrogen deoxygenation protection, DMAc and 10.81 g (0.1 mol) of PDA were added to a 100 mL three-necked flask, and the mixture was mechanically stirred at room temperature for 30 min until PDA was completely dissolved in DMAc. 0.1 mol of PMDA was weighed and slowly added to the three-necked flask while stirring. The remaining DMAc was added to adjust the solid content of the system to 15 wt%. The three-necked flask was reacted at -10 ° C under nitrogen protection for 10 h to obtain a polyamic acid (PAA) solution, in which meta-polyamic acid (meta-PAA) and para-polyamic acid (para-PAA) were protected.

[0085] Step (2): The polyamic acid solution prepared above was spun using the same catalyst and spinning process as in Example 2. The spinnability of the solution was low due to the strong interaction force and strong rigidity of the molecular chain. Figure 8 As shown, a large number of dripping and broken fibers occurred, and the prepared primary fibers were also brittle and lost their application value.

[0086] Comparative Example 2

[0087] Step (1): Under nitrogen deoxygenation protection, 44.42 g (0.1 mol) of hexafluorodianhydride (6FDA) was added to a 250 mL three-necked flask, and 45 mL of anhydrous ethanol was added to adjust the solid content to 40 wt%. The mixture was heated to 80 °C in an oil bath, and the evaporated ethanol was refluxed through a condenser. The mixture was mechanically stirred for 3 hours to obtain an ethanol solution of diethyl hexafluorodicarboxylate (6FDE) after esterification.

[0088] Step (2): The obtained diacid diester ethanol solution is subjected to rotary evaporation in a rotary evaporator to remove most of the ethanol solvent. The remaining viscous solid is added to a Soxhlet extractor and extracted with butyl acetate to obtain the para-diacid diester. The obtained para-diacid diester is recrystallized twice from water and acetone and a third time from anhydrous cyclohexane, and then dried under vacuum to obtain the pure para-diacid diester (para-6FDE).

[0089] Step (3): Add 40 g of the diacid diester to 100 mL of THF and 25.38 g (0.2 mol) of oxalyl chloride, and heat to 80-90°C. After reflux for 20 min, remove the THF and excess oxalyl chloride by vacuum distillation. Add 10 mL of toluene to the solution, and continue distillation under vacuum to remove the residual oxalyl chloride to obtain the para-acylated diester product.

[0090] Step (4): Under nitrogen, a 0.1 mol solution of para-diester dichloride in DMAc was added to a 500 mL three-necked flask. After sufficient dissolution under mechanical stirring, 5 mL of triethylamine and 10 mL of trimethylsilyl chloride were added as catalysts. 0.1 mol of 1,3-bis(4'-aminophenoxy)benzoate was added, and DMAc was further added to adjust the solid content to 15 wt%. Under nitrogen, the system temperature was controlled at 0°C and stirred for 2 h. The temperature was then slowly raised to 30°C and reacted for 3 h to obtain a polyamide ester solution (para-PAE) which did not exhibit liquid crystal properties.

[0091] Step (5): 0.1 wt% of isoquinoline was added to the para-PAE solution prepared above and stirred uniformly to obtain a transparent and uniform spinning solution. Spinning was performed in the same manner as in step (3) of Example 1 to obtain partially cyclized spun fibers. The spun fibers were then cyclized and heat-treated at 350°C under a tensile strength of 2.2 cN / dtex. The prepared fibers had an orientation factor of 0.2 and a breaking strength of 2.5 cN / dtex.

[0092] Comparative Example 3

[0093] Step (1): Under nitrogen deoxygenation protection, add 21.81 g (0.1 mol) of pyromellitic dianhydride (PMDA) to a 100 mL three-necked flask, add 22 mL of anhydrous ethanol to adjust the solid content to 20 wt%, heat to 80°C in an oil bath, reflux the evaporated ethanol through a condenser, and stir for 3 hours to obtain an ethanol solution of esterified diethyl phthalate (PMDE-C2).

[0094] Step (2): The PMDE-C2 solution prepared above was subjected to rotary evaporation to remove the ethanol solvent, yielding a brown solid of diethyl phthalate. 20 g of diethyl phthalate was added to 50 mL of THF and 25.38 g (0.2 mol) of oxalyl chloride, and the mixture was heated to 80-90°C. After reflux for 120 min, the THF and excess oxalyl chloride were removed by vacuum distillation. 10 ml of toluene was added to the solution, and the solution was further distilled under vacuum to remove the residual oxalyl chloride, yielding the diethyl phthalate product.

[0095] Step (3): Under nitrogen atmosphere, 0.1 mol of the DMAc solution of the product of step (2) was added to a 500 mL three-necked flask. After sufficient dissolution under mechanical stirring, 5 mL of triethylamine and 10 mL of trimethylsilyl chloride were added as catalysts. 0.1 mol of p-phenylenediamine (PDA) was added and DMAc was continued to be added to adjust the solid content to 20 wt%. Under nitrogen protection, the system temperature was controlled at 0°C and stirred for 2 h. The reaction was then slowly heated to 30°C and reacted for 3 h to obtain a polyamic acid ester solution (PAE-C2), which was a transparent, uniform solution without liquid crystal properties.

[0096] Step (4): The prepared PAE-C2 spinning solution is subjected to a 2-hour static degassing treatment at room temperature and filtered through a metal filter to remove impurities and undissolved particles. The filtered spinning solution is transported to the spinning equipment and extruded through a spinneret with a circular cross-section into a spinning tunnel. The tunnel is 10 meters long, the hot air temperature of the tunnel is 185°C, the atmosphere is nitrogen, and the winding speed is 120 m / min. The solvent is gradually evaporated from the fiber by the action of the hot air, and the partially cyclized PAE-C2 nascent fiber is finally obtained; the nascent fiber is then cyclized and heat-treated at 450°C and a tensile strength of 1.0 cN / dtex. The prepared fiber has an orientation factor of ≤0.2, a breaking strength of 1.5 cN / dtex, and is relatively brittle.

[0097] Comparative Example 4

[0098] 0.1 wt% of isoquinoline was introduced into the polyamic acid ester solution (PAE-C2) prepared in the above comparative example 3 and prepared into a uniform spinning solution by stirring, and the spinning was carried out in the same step as step (4) of comparative example 3, and finally almost completely cyclized PAE-C2 nascent fibers were obtained; the nascent fibers were then cyclized and heat-treated at 450°C and a tensile strength of 1.0 cN / dtex. The prepared fibers had an orientation factor of ≈0.2, a breaking strength of 1.7 cN / dtex, and were relatively brittle.

[0099] Comparative Example 5

[0100] The fully para-polyamide ester solution (para-PAE-C2) with a solid content of about 30 wt% prepared in step (3) of Example 1 was used as a spinning solution. The solution was allowed to stand for 1 hour at room temperature for degassing and filtered through a metal filter to remove impurities and undissolved particles. The filtered spinning solution was transported to a spinning device and extruded through a spinneret with a circular cross-section into a spinning tunnel. The hot air temperature of the tunnel was 190°C, the atmosphere was nitrogen, and the winding speed was 110 m / min. The solvent was gradually evaporated from the fiber by the hot air, and the partially cyclized para-PAE-C2 spun fiber was finally obtained. The spun fiber was then cyclized and heat-treated at 450°C and a tensile strength of 2.0 cN / dtex. The prepared fiber had a large amount of fuzz and broken fibers, and the orientation factor was 0.6, which was no longer suitable for application.

Claims

1. A highly oriented polyimide fiber based on liquid crystal spinning, characterized by: A para-diacid diester is prepared using rigid acid anhydride as a raw material, and then polymerized with a rigid diamine to produce a fully para-polyamic acid ester solution with lyotropic liquid crystal properties. An organic base catalyst is then added to prepare a spinning solution, and highly oriented polyimide fibers are produced through high-temperature dry spinning, catalytic cyclization, and high-temperature heat treatment. The high orientation refers to an orientation factor ≥ 0.6; the temperature of the high-temperature dry spinning is 150-250°C; the process parameters of the high-temperature heat treatment are: temperature 350-480°C, and tensile strength 1-20 cN / dtex; The rigid acid anhydride is one of pyromellitic anhydride and p-phenylene-diphenyltrimethylol dianhydride; The rigid diamine is one of p-phenylenediamine and 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl; The organic base catalyst includes at least one of quinoline, isoquinoline, imidazole, 1,4-diazabicyclo[2.2.2]octane, picoline, and dimethylimidazole; the amount of the organic base catalyst is 0.01% to 10% of the mass of the polyamic acid ester; The preparation method of the spinning solution comprises the following steps: (1) Heating a rigid acid anhydride in an organic alcohol solvent to obtain a diacid diester mixture, and then preparing a pure para-diacid diester monomer by recrystallization; (2) subjecting the above-mentioned pure para-position diacid diester monomer to acyl chloride reaction, and then reacting it with diamine in an organic solvent to prepare a fully para-position polyamic acid ester solution; (3) An organic base catalyst is added to the polyamic acid ester solution, and the solid content is adjusted to 20 wt % to 60 wt % by heating and concentration to obtain a polyamic acid ester spinning solution.

2. A method for preparing highly oriented polyimide fibers based on liquid crystal spinning according to claim 1, comprising the following steps: (1) Heating a rigid acid anhydride in an organic alcohol solvent to obtain a diacid diester mixture, and then preparing a pure para-diacid diester monomer by recrystallization; (2) subjecting the above-mentioned pure para-position diacid diester monomer to acyl chloride reaction, and then reacting it with diamine in an organic solvent to prepare a fully para-position polyamic acid ester solution; (3) adding an organic base catalyst to the polyamic acid ester solution, and adjusting the solid content to 20 wt % to 60 wt % by heating and concentrating to obtain a polyamic acid ester spinning solution; (4) preparing the polyamic acid ester spinning solution through a dry spinning process to obtain highly oriented polyimide primary fibers; wherein, The process parameters of dry spinning are: spinning speed 100~300 m / min; (5) Finally, high-temperature heat treatment is performed to obtain highly oriented polyimide fibers based on liquid crystal spinning.

3. The preparation method according to claim 2, wherein: The organic alcohol solvent in step (1) is any one of C2 to C14 saturated fatty alcohols.

4. The preparation method according to claim 2, wherein: The chlorination reagent used in the chlorination reaction in step (2) is one of oxalyl chloride and thionyl chloride, and the molar ratio of oxalyl chloride to the carboxyl group in the para-diacid diester monomer is 1 to 5:1; the solvent used in the chlorination reaction is at least one of dichloromethane, ethyl acetate, and tetrahydrofuran.

5. The preparation method according to claim 2, wherein: The cyclization degree of the polyimide nascent fibers in step (4) is greater than 70%, and the total fiber orientation degree is greater than or equal to 0.5.

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