High-orientation polyimide fiber based on liquid crystal spinning and preparation method thereof

Abstract

The invention relates to a high-orientation polyimide fiber based on liquid crystal spinning and a preparation method of the high-orientation polyimide fiber. Rigid anhydride is used as a raw material to prepare para-diacid diester, and the para-diacid diester and rigid diamine are polymerized to prepare a full-para polyamic acid ester solution with lyotropic liquid crystal characteristics; and then adding an organic base catalyst to prepare a spinning solution, and carrying out high-temperature dry spinning, catalytic cyclization and high-temperature heat treatment to prepare the high-orientation polyimide fiber. According to the invention, an organic base catalyst is introduced into a spinning solution with lyotropic liquid crystal characteristics, and the high orientation of a liquid crystal domain along a fiber axis is realized by virtue of the shearing and fluid stretching effects of spinneret orifices; then, on the basis of high-temperature dry spinning and catalytic cyclization reaction, structural transformation from polyamide acid ester to polyimide is achieved while curing forming of the fiber is promoted, locking of an oriented structure and densification of the interior of the fiber are achieved, and therefore the stability and uniformity of the fiber are guaranteed; the method has great application potential in the field of extreme special environments such as aerospace.

Description

Technical Field

[0001] The invention belongs to the technical field of polymer fibers, and particularly 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 is a significant difference 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 conformation entropy barrier, resulting in difficulty in forming similar highly ordered molecular stacking in solid-state phase transitions.

[0003] As another precursor of polyimide, polyamic acid ester (PAE) is favorable for the formation of lyotropic liquid crystals because of its high molecular chain rigidity and high symmetry of the fully para-positioned polyamic acid ester molecular chain. For example, Neuber et al. esterified PMDA and prepared the para-positioned diacid diester by recrystallization; then reacted with rigid diamine to obtain the fully para-positioned polyamic acid ethyl ester solution, which observed the typical nematic liquid crystal woven structure in the concentration range of 30 wt%~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 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 texture of PAE, causing it to gradually transform from nematic to smectic (Macromolecules, 2019, 52(13): 5054-5066.). However, polyamic acid ester generates polyimide through intramolecular dealcoholization reaction at high temperature. The reaction is accompanied by the escape of small molecular alcohols, which may cause voids and defects inside the fiber and destroy the mechanical properties of the fiber. Although the current research has proved that para-polyamic acid ester solution may exhibit liquid crystal properties, there is no report on the preparation of polyimide fibers by liquid crystal polyimide ester. From polyamic acid ester to polyimide fiber, it is still necessary to solve the following problems: 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 fibers have high orientation, high crystallinity, dense structure and excellent mechanical properties.

[0005] The invention provides a highly oriented polyimide fiber based on liquid crystal spinning, wherein a para-diacid diester is prepared with a rigid acid anhydride as a raw material, and is polymerized with a rigid diamine to obtain a fully para-polyamic acid ester solution having lyotropic liquid crystal properties; an organic base catalyst is subsequently 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; wherein 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-diacid diester monomer to acyl chloride reaction, and then reacting it with diamine in an organic solvent to prepare a fully para-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 concentration to obtain a polyamic acid ester spinning solution;

[0015] (4) preparing highly oriented polyimide primary fibers by dry spinning 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-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: firstly stirring the reaction in an ice-water bath for 6 to 8 hours, and then heating the temperature to 60 to 100° C. to evaporate to dryness under nitrogen purge.

[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: at -10-60°C, for 6-12 hours, and the solid content is controlled at 8-20wt%.

[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 primary 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 statistics of the polymer chains shown 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. It is known that 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 exhibiting liquid crystal properties in concentrated sulfuric acid solution; the para-aramid prepared by the liquid crystal spinning method has high strength and modulus. Fig. 9 As shown, PPTA <r 2 / S 2 > up to 11.96. In the traditional PAA molecular chain, due to the simultaneous presence of para-PAA structures (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 carbon chain of the PAE side chain ester 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, too 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 characteristics, the molecular chain is highly oriented along the fiber axis at the spinneret hole through 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 fibers 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 internal densification of the fiber is achieved, thereby ensuring the stability and uniformity of the fiber. It has great application potential in extreme special environment fields 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 It is 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 8These are pictures of dripping (a) and broken / hairy fibers (b) that appeared during the spinning process of the PAA solution prepared in Comparative Example 1.

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

[0041] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it 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 within the scope limited by the appended claims of the application equally.

[0042] Example 1

[0043] Step (1): Under nitrogen deoxygenation protection, add 21.81 g (0.1 mol) of pyromellitic dianhydride (PMDA) into 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, and the remaining viscous solid is added to a Soxhlet extractor and extracted with butyl acetate to obtain a para-diacid diester. The obtained para-diacid diester is recrystallized twice in water and acetone and a third time in anhydrous cyclohexane, and vacuum dried to obtain a pure para-PMDE-C2.

[0045] 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 being fully dissolved under mechanical stirring, 5 mL of triethylamine and 10 mL of trimethylsilyl chloride were added as catalysts, and 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 to 0°C and stirred for reaction for 2 h, then the temperature was slowly raised to 30°C and reacted for 3 h to obtain a full 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 is added to the para-PAE-C2 solution prepared above and stirred to prepare a spinning solution having lyotropic liquid crystal properties. The spinning solution is subjected to a standing degassing treatment for 1 hour at room temperature and filtered through a metal filter to remove impurities and undissolved particles. The filtered spinning solution is transported to a spinning device (such as Figure 5 The fiber was extruded through a spinning tunnel with a circular cross-section through a spinneret, the hot air temperature of the tunnel was 190°C, the atmosphere was nitrogen, and the winding speed was 110 m / min. The solvent gradually evaporated from the fiber through the hot air, and finally almost completely cyclized para-PAE-C2 nascent fibers were obtained; the nascent fibers were then cyclized and heat treated at 450°C and a tensile strength of 3.8 cN / dtex, and the prepared fibers had 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 into 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 is subjected to rotary evaporation in a rotary evaporator to remove most of the butanol solvent, and the remaining viscous solid is added to a Soxhlet extractor to extract with butyl acetate to obtain the para-diacid diester. The para-diacid diester is recrystallized twice in water and acetone and a third time in anhydrous cyclohexane, and vacuum dried to obtain pure para-dibutyl phthalate (para-PMDE-C4).

[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, and heat to 80-90°C. After reflux for 20 min, remove 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.

[0051] Step (4): Under nitrogen atmosphere, add 0.1 mol of para-diester dichloride DMAc solution to a 500 mL three-necked flask, stir mechanically until fully dissolved, 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), such as 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 the mixture was uniformly stirred to obtain a spinning solution having lyotropic liquid crystal properties. Spinning was performed in the same manner as step (3) of Example 1 to obtain almost completely cyclized primary fibers. The primary fibers were then cyclized and heat treated at 460°C and 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 into 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, and the remaining viscous solid is added to a Soxhlet extractor to extract with butyl acetate to obtain the para-diacid diester. The obtained para-diacid diester is recrystallized twice in water and acetone and a third time in anhydrous cyclohexane, and 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 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 DMAc solution of para-diester dichloride to a 500 mL three-necked flask, add 5 mL of triethylamine and 10 mL of trimethylsilyl chloride as catalysts after full dissolution under mechanical stirring, add 0.1 mol of PDA and continue to add DMAc to adjust the solid content to 35 wt%. Under nitrogen protection, 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-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 carried out in the same manner as step (3) of Example 1 to obtain almost completely cyclized primary fibers. The primary fibers were then cyclized and heat treated at 460°C and a tensile strength of 5.2 cN / dtex to obtain fibers with 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 into 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 180°C, the boiling point of octanol, 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, and the remaining viscous solid is added to a Soxhlet extractor to extract with butyl acetate to obtain the para-diacid diester. The obtained para-diacid diester is recrystallized twice in water and acetone and a third time in anhydrous cyclohexane, and 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, and heat to 80-90°C. After reflux for 20 min, remove 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.

[0063] Step (4): Under nitrogen atmosphere, add 0.1 mol of DMAc solution of para-diester dichloride to a 500 mL three-necked flask, add 5 mL of triethylamine and 10 mL of trimethylsilyl chloride as catalysts after full dissolution under mechanical stirring, add 0.1 mol of PDA and continue to add DMAc to adjust the solid content to 40 wt%. Under nitrogen protection, 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 (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 carried out in the same manner as step (3) of Example 1 to obtain almost completely cyclized primary fibers. The primary fibers were then cyclized and heat treated at 440°C and a tensile strength of 4.9 cN / dtex to obtain fibers with 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 a dodecanol solution of esterified dodecyl phthalate (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, and the remaining viscous solid is added to a Soxhlet extractor to extract with butyl acetate to obtain the para-diacid diester. The obtained para-diacid diester is recrystallized twice in water and acetone and a third time in anhydrous cyclohexane, and 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, and heat to 80-90°C. After reflux for 20 min, remove 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.

[0069] Step (4): Under nitrogen atmosphere, add 0.1 mol of DMAc solution of para-diester dichloride to a 500 mL three-necked flask, add 5 mL of triethylamine and 10 mL of trimethylsilyl chloride as catalysts after full dissolution under mechanical stirring, add 0.1 mol of PDA and continue to add DMAc to adjust the solid content to 45 wt%. Under nitrogen protection, control the system temperature to 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% of DBU was added to the para-PAE-C12 solution prepared above, and the mixture was uniformly stirred to obtain a spinning solution having lyotropic liquid crystal properties. Spinning was performed in the same manner as 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 fibers with an orientation factor of 0.67.

[0071] Example 6

[0072] Step (1): Under nitrogen atmosphere, 0.1 mol of DMAc solution of the product of step (2) of Example 1 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 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 to 0°C and stirred for 12 h, then the temperature was 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 is subjected to the same catalyst and spinning process as in step (5) of Example 5 to prepare almost completely cyclized spun fibers; the spun fibers are 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 DMAc solution of the product of step (2) of Example 1 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 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 to 0°C and stirred for 14 h, then the temperature was 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 is subjected to the same catalyst and spinning process as in step (5) of Example 5 to prepare almost completely cyclized spun fibers; the spun fibers are then cyclized and heat treated at 460°C and 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, add 45.8 g (0.1 mol) of p-phenylene-diphenyltrimethylol dianhydride (TAHQ) into a 100 mL three-necked flask, add 46 mL of anhydrous butanol to adjust the solid content to 40 wt%, heat to 120°C in an oil bath, reflux the evaporated butanol through a condenser, and stir 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, and the remaining viscous solid is added to a Soxhlet extractor to extract with butyl acetate to obtain the para-diacid diester. The obtained para-diacid diester is recrystallized twice in water and acetone and a third time in anhydrous cyclohexane, and 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 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.

[0081] Step (4): Under nitrogen atmosphere, add 0.1 mol of para-diester dichloride DMAc solution to a 500 mL three-necked flask, stir mechanically to fully dissolve, add 5 mL of triethylamine and 10 mL of trimethylsilyl chloride as catalysts, add 0.1 mol of TFMB and continue to add DMAc to adjust the solid content to 30 wt%. Under nitrogen protection, 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 (para-TEHQ-PAE-TFMB-C4) solution.

[0082] Step (5): A 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 (e.g. 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 rises to above 120°C, the texture disappears and the solvent evaporates to form a solid film, 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 primary fibers; then the primary fibers were 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, wherein meta-PAA and para-PAA were protected.

[0085] Step (2): The polyamic acid solution prepared above is spun using the same catalyst and spinning process as in Example 2. The spinnability of the solution is low due to the strong interaction force and strong rigidity of the molecular chains. Figure 8 As shown, a large amount of dripping, broken fibers and the like 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, add 44.42 g (0.1 mol) of hexafluorodianhydride (6FDA) into a 250 mL three-necked flask, add 45 mL of anhydrous ethanol to adjust the solid content to 40 wt%, heat to 80 ° C in an oil bath, reflux the evaporated ethanol through a condenser, and mechanically stir 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, and the remaining viscous solid is added to a Soxhlet extractor to extract with butyl acetate to obtain the para-diacid diester. The obtained para-diacid diester is recrystallized twice in water and acetone and a third time in anhydrous cyclohexane, and vacuum dried 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 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.

[0090] Step (4): Under nitrogen atmosphere, add 0.1 mol of para-diester dichloride DMAc solution to a 500 mL three-necked flask, add 5 mL of triethylamine and 10 mL of trimethylsilyl chloride as catalysts after full dissolution under mechanical stirring, add 0.1 mol of 1,3-bis(4'-aminophenoxy)benzoate and continue to add DMAc to adjust the solid content to 15 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) which does not show liquid crystal properties.

[0091] Step (5): 0.1 wt% of isoquinoline was added to the para-PAE solution prepared above, and the mixture was stirred uniformly to obtain a transparent and uniform spinning solution. The spinning was carried out in the same manner as step (3) of Example 1 to obtain partially cyclized spun fibers. The spun fibers were then cyclized and heat treated at 350°C and a tensile strength of 2.2 cN / dtex to obtain fibers with 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) into 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 is subjected to rotary evaporation in a rotary evaporator to remove the ethanol solvent to obtain a brown solid of diethyl phthalate. 20 g of diethyl phthalate is added to 50 mL of THF and 25.38 g (0.2 mol) of oxalyl chloride, and heated to 80-90°C. After reflux for 120 min, THF and excess oxalyl chloride are removed by vacuum distillation. 10 ml of toluene is added to the solution, and the distillation is continued under vacuum to remove the residual oxalyl chloride to obtain the diethyl phthalate product.

[0095] Step (3): Under nitrogen atmosphere, add 0.1 mol of DMAc solution of the product of step (2) into a 500 mL three-necked flask, add 5 mL of triethylamine and 10 mL of trimethylsilyl chloride as catalysts after full dissolution under mechanical stirring, add 0.1 mol of p-phenylenediamine (PDA) and continue to add DMAc to adjust the solid content to 20 wt%. Under nitrogen protection, 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 polyamic acid ester solution (PAE-C2), which is a transparent and 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, 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 hot air, and the partially cyclized PAE-C2 primary fiber is finally obtained; then the primary fiber is 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 spinning was carried out in the same step as step (4) of comparative example 3 to finally obtain almost completely cyclized PAE-C2 nascent fibers. 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 and a breaking strength of 1.7 cN / dtex. The fibers 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 the above embodiment 1 is used as a spinning solution, and is subjected to a static degassing treatment at room temperature for 1 hour, and filtered through a metal filter to remove impurities and undissolved particles. The filtered spinning solution is transported to a spinning device, extruded through a spinneret with a circular cross-section into a spinning tunnel, the hot air temperature of the tunnel is 190°C, the atmosphere is nitrogen, and the winding speed is 110 m / min. The solvent is gradually evaporated from the fiber by the action of the hot air, and the partially cyclized para-PAE-C2 primary fiber is finally obtained. The primary fiber is then cyclized and heat-treated at 450°C and a tensile strength of 2.0 cN / dtex. The prepared fiber has a large number of hairy and broken fibers, and the orientation factor = 0.6, which loses its application value.

Claims

1. A highly oriented polyimide fiber based on liquid crystal spinning, characterized in that: A para-diacid diester is prepared with rigid acid anhydride as a raw material, and is polymerized with a rigid diamine to obtain 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 a highly oriented polyimide fiber is prepared by high-temperature dry spinning, catalytic cyclization and high-temperature heat treatment; wherein the high orientation refers to an orientation factor ≥ 0.6; the temperature of the high-temperature dry spinning is 150~250℃; the process parameters of the high-temperature heat treatment are: temperature of 350~480℃, and tensile strength of 1~20 cN / dtex.

2. The highly oriented polyimide fiber according to claim 1, characterized in that: The rigid anhydride is one of the following structures: ， ， ， ， 。 3. The highly oriented polyimide fiber according to claim 1, characterized in that: The rigid diamine is one of the following structures: ， ， ， ， ， ， ， ， ， ， 。 4. The highly oriented polyimide fiber according to claim 1, characterized in that: The organic base catalyst includes at least one of quinoline, isoquinoline, imidazole, 1,4-diazabicyclo[2.2.2]octane, picoline and dimethylimidazole.

5. A method for preparing highly oriented polyimide fibers based on liquid crystal spinning as claimed in any one of claims 1 to 4, 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-diacid diester monomer to acyl chloride reaction, and then reacting it with diamine in an organic solvent to prepare a fully para-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 highly oriented polyimide primary fibers by dry spinning the polyamic acid ester spinning solution; (5) Finally, high-temperature heat treatment is performed to obtain highly oriented polyimide fibers based on liquid crystal spinning.

6. The preparation method according to claim 5, characterized in that: The organic alcohol solvent in step (1) is any one of C2-C14 saturated fatty alcohols.

7. The preparation method according to claim 5, characterized in that: The chlorination agent used in the chlorination reaction in step (2) is one of oxalyl chloride and thionyl chloride, and the molar ratio of oxalyl chloride and 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.

8. The preparation method according to claim 5, characterized in that: The process parameters of the dry spinning in step (4) are: the spinning speed is 100-300 m / min.

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

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