High-performance Polyimide Fibers Research Articles

Fabrication of high performance polyimide fibers by dry-jet wet spinning technology

High-performance co-polyimide (co-PI) fibers derived from 3,3`4,4`-biphenyldianhydride (BPDA), p-phenylenediamine (p-PDA) and 2-(4-aminophenyl)-5-amino-benzimidazole (BIA) were prepared via the wet spinning and dry-jet wet spinning method, respectively. The distinctions between wet spinning and dry-jet wet spinning processes on the structures and properties of co-PI fibers were systematically investigated. The co-PI fibers prepared by dry-jet wet spinning exhibited smoother surface morphology and higher mechanical properties than those of co-PI prepared by wet spinning. Wide-angle X-ray diffraction (WAXD) results showed that the co-PI fibers prepared by dry-jet wet spinning exhibited highly oriented structure along the fiber direction along with a low degree of lateral packing order in the transverse direction. Small-angle X-ray scattering (SAXS) revealed that the radius, length, misorientation and internal surface roughness of the microvoids in the co-PI fibers prepared by the dry-jet wet spinning are lower than those in co-PI fibers prepared by wet spinning. Moreover, the effect of air layer height, as well as the heat treatment on the structure and properties of fibers during the dry-jet wet spinning process, was also studied. The tensile strength and modulus of co-PI fibers prepared by dry-jet wet spinning reached 2.72 GPa and 114.29 GPa, respectively, with an air layer height of 10 mm and a heat stretching temperature of 440°C.

Preparation and Interfacial Properties of Hydroxyl-Containing Polyimide Fibers

Developing polyimide (PI) fibers with excellent interfacial adhesion and high mechanical properties for the PI fiber-reinforced polymer matrix composites (PFRPs) industry has been challenging. In this work, 4,4'-diamino-(1,1'-biphenyl)-3,3'-diol (HAB) diamine was introduced into the rigid molecular chains, and the high-performance PI fibers, presenting an interfacial shear strength (IFSS) value of 46.33 MPa, tensile strength of 2.62 GPa, and modulus of 100.15 GPa, were successfully manufactured when the content of HAB in mixed diamines was 30 mol %. Fourier transform infrared (FTIR) spectroscopy identified the presence of intermolecular H-bonding interactions, and 2D small-angle X-ray scattering indicated that the introduction of HAB moiety contributed to reducing the radii of microvoids in the fibers, which were considered to be the key factors leading to a significant enhancement in the mechanical properties of the fibers. X-ray photoelectron spectroscopy (XPS) and the static contact angle intuitively illustrated that the synthetic fiber surface contained active hydroxyl groups. The IFSS value of PI fiber/epoxy resin composites (PI/EPs) was 56.47 MPa when the content of HAB reached 70 mol %. Failure morphologies confirmed that the interfacial adhesion of PI/EPs was enhanced owing to the surface activity of PI fibers. Consequently, this study provides an effective strategy to the long-standing problems of high mechanical performances and poor surface activity for traditional PI fibers used in the PFRPs industry.

Preparation of polyimide-metal complexes with tunable Sol-Gel transition by Introduction of Zn(II)-Coordination

Developing fully rigid-chain polyimides with reversible sol-gel/gel-sol transitions is of great interest for designing high-performance materials. To date, few approaches have been employed to construct polyimide gels other than hydrogen bonding. Here, we report a polyimide-Zn(II) complex with thermo-responsive sol-gel/gel-sol transition behavior by introducing supramolecular coordination interactions. The tunable dynamic sol-gel behaviors of polyimide-Zn(II) complexes were confirmed using rheological methods. Microstructure analysis shows that coordination interactions occur between the benzimidazole molecules and Zn(II) ions. As a result, the apparent viscosity of the polyimide-Zn(II) complex even rises approximately four orders of magnitude higher than that of the PI-neat oligomer solution. Based on the 2D-correlation spectra and X-ray diffraction results, we deduced that the formation of dense chain stacking caused by coordination effects plays an essential part in the reversible sol-gel transition. As a preliminary experiment, this work gives a novel strategy to construct polyimide gels, showing application prospects in the fabrication of porous polyimide membranes, high-performance polyimide fibers, and aerogels, etc.

Preparation of High-performance Polyimide Fibers with Wholly Rigid Structures Containing Benzobisoxazole Moieties

Preparation of High-performance Polyimide Fibers with Wholly Rigid Structures Containing Benzobisoxazole Moieties

Comparison of high-performance polyimide copolymer fibers containing pyrimidine moieties based on coplanar structures

The effects of molecular coplanar structure on crystallinity, orientation, and overall performances of polyimide (PI) copolymer fibers, especially mechanical properties, were investigated. In detail, three diamines with similar structures, including 4,4′-diaminobenzophenone (DABP), 4,4′-diaminobenzanilide (DABA), and benzidine (Bz), were introduced as third monomers into the classical heterocyclic 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA)/2,5-bis(4-aminophenyl)pyrimidine (PRM) system, eventually the high-performance PI fibers being fabricated successfully via dry-jet wet spinning process. The conformation simulations illustrated the different coplanar levels found in three similar diamines: DABP (worst), DABA (better), and Bz (best). Thus, the BPDA/Bz dimer showed excellent coplanar structures, and the dihedral angle of the farthest benzene rings was only 4.42°. The wide-angle X-ray diffraction patterns indicated that the crystallinity and orientation of fibers increased with the increase of the coplanarity degree and molecular packing. Interestingly, the Bz-PI fiber possessed the strongest mechanical properties, i.e., fracture strength of 4.01 GPa and initial modulus of 137.34 GPa, which was primarily contributed by optimal molecular coplanarity and highest orientation.

Preparation and properties of high-performance polyimide copolymer fibers containing rigid pyrimidine and benzoxazole moieties with hydrogen bonding

The utilization of 5-amino-2-(2′-hydroxy-4′-aminophenyl)-benzoxazole (HBOA) as third monomer was beneficial for the introduction of OH-type H-bonding into polyimide (PI) matrix through the copolymerization with 2,5-bis(4-aminophenyl)pyrimidine (PRM) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA). And high-performance PI fibers were successfully fabricated via the dry-jet wet spinning process. The FTIR spectra confirmed that the strong interactions of intramolecular and intermolecular H-bonding existed simultaneously in PI fibers. Moreover, the glass transition temperatures (Tg) of all fibers were within 276–318 °C, and the temperatures at 5% weight loss (T5%) ranged from 574 °C to 604 °C, revealing the remarkable thermal stability of PI fibers. The two-dimensional wide-angle X-ray diffraction (2D WAXD) results illustrated a U-shaped change in the crystallinity (from 32.1% to 12.2% until 33.4%) of PI fibers with increasing HBOA content, which was also reflected in the density variation of fibers. The two-dimensional small-angle X-ray scattering (2D SAXS) patterns demonstrated that the micro-defects in all co-PI fibers profitably decreased in comparison with those in homo-PI fibers. Furthermore, the orientation factor of fibers presented an improvement from 0.843 to 0.899 after incorporating HBOA moiety. When the molar ratio of PRM/HBOA was 7/3, the co-PI fiber exhibited optimum mechanical properties with tensile strength and modulus as high as 4.15 GPa and 125.93 GPa, respectively. Interestingly, the variation in fiber orientation was in line with the same trend as that of mechanical properties of PI fibers, indicating orientation was also a crucial factor to improve the mechanical properties. The relationship between structure and mechanical property was also investigated.

Synergistic “Anchor” Effect of Carbon Nanotubes and Silica: A Facile and Efficient Double-Nanocomposite System To Reinforce High-Performance Polyimide Fibers

Double nanofillers are becoming an important and effective strategy to reinforce and functionalize polymeric materials, and maximizing the strength of nanofillers to compensate for their weaknesses remains of great interest in the field of nanocomposite. Herein, we report a facile and efficient double-nanofiller strategy to reinforce polyimide fibers. After grafting 3-aminopropyltriethoxysilane (APTES) onto fluorinated multiwalled carbon nanotubes (F-MWCNTs), the functionalized MWCNTs, APTES@F-MWCNTs, reacted with silica sol via the reactions of Si–OH groups. Because of the formation of chemical bonds, dispersibility of MWCNTs was improved by the assistance of a grafted silica sol; MWCNTs prevent the silica sol from diffusing into a coagulation bath and being lost of silica during wet-spinning. Synergistic “anchor” effect of carbon nanotubes and silica results in a considerable improvement of mechanical properties (44.4%) of polyimide fibers and an increase in the Tg value from 348 to 381 °C.

Surface modification optimization for high-performance polyimide fibers

In this paper, two types of polyimide (PI) fibers, namely, PI-1 and PI-2, were modified using various treatments, including silane coupling agent KH-550, SiO2 sol–gel, and ozone treatments. The surface properties of the PI fibers and the adhesion behavior of PI fiber/epoxy composites were evaluated. The chemical compositions and surface morphologies of the PI fibers were characterized by x-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy. The results indicated that the interfacial properties of the PI fibers were enhanced after treatment with these three methods. In particular, the silane coupling agent exhibited the highest efficiency for surface modification.

High-tenacity and high-modulus polyimide fibers containing benzimidazole and pyrimidine units

Polyimide fibers (PI) combine high thermal stability, good chemical and UV resistance. However, most commercial PI fibers including P84®, Kermel®, Yilun® and Suplon® do not exhibit sufficiently high mechanical properties. Finding an appropriate method that is commercially available to prepare PI fibers with a high tenacity and modulus is required urgently for making large scale manufacturing attractive. Herein, we describe an approach to preparing high-performance PI fibers by introducing the heterocyclic pyrimidine and benzimidazole units into PI backbones. PI fibers prepared by this way exhibited a higher tenacity and modulus compared to those previously reported PIs and even Kevlar 49® fibers due to the high molecular weight, strong hydrogen bonding interaction and highly oriented crystalline structure. In addition, the detailed structural analysis based on WAXD and SAXS has revealed that the stress-induced crystallization and more compact morphology formed in PI fibers by the post hot-drawing process. Accordingly, as the drawing ratio was set to 3.8, an optimum tenacity and modulus of the PI fiber reached to 2.3N/tex and 71.8N/tex, respectively, indicating that the resultant fibers have a great potential application as the reinforcing fiber in advanced composites.

Preparation of high-performance polyimide fibers via a partial pre-imidization process

In this work, a series of polyimide fibers (PI) based on 3,3′,4,4′-biphenyltetracarboxylic anhydride (BPDA)/p-phenylenediamine (PDA) were prepared through a partial pre-imidization process, and the properties of these fibers were investigated in detail. The pre-imidization degree (pre-ID) was successfully controlled by adjusting the amount of dehydration reagents, which was confirmed by Fourier Transform Infrared. The scanning electron microscope images show that the addition of dehydration reagents in the poly(amic acid) spinning dope improves the shaping of the precursor fibers, which is related to the thermodynamic phase separation in the coagulation bath. The WAXD patterns indicate that the formed partial rigid-rod and oriented PI chains in the pre-imidization process are beneficial for forming a more ordered and regular crystalline structure in the post-heat-drawing process, thus improving their mechanical properties, which exhibit an optimum tensile strength and modulus of 1.7 GPa and 95.0 GPa, respectively. Meanwhile, the prepared PI fibers exhibit excellent thermal stabilities with the 5 wt% (T5%) weight loss temperature ranging from 530 to 568 °C depending on the pre-imidization degree. This work provides a simple and facile approach to preparing high-performance PI fibers through the partial pre-imidization process.

High-performance polyimide nanofiber membranes prepared by electrospinning

Polyimide (PI) nanofiber membranes were successfully prepared from a PI/N-methyl-2-pyrrolidone (NMP) solution via electrospinning. This technique simply and facilely produced efficient high-performance PI fibers. The morphology, surface wettability, thermal stability, mechanical properties, and filtration performance of the as-prepared PI nanofiber membranes were characterized in detail. The membranes exhibited smooth and hydrophobic surfaces. The nanofibers were well distributed in the membranes with fiber diameters in the range of 140–400 nm. All the PI nanofiber membranes showed excellent thermostability, and their initial decomposition temperature ( Td) and heat resistance temperature ( THRI) exceeded 544.4°C and 198.8°C, respectively. The PI nanofiber membranes also possessed reasonable mechanical properties with a tensile strength and a Young’s modulus reaching 10.5 and 927.6 MPa, respectively. Regarding the filtration performance, the developed nanofiber membranes achieved the best filtration efficiency of 90.4%. Such electrospun PI nanofiber membranes can be a promising candidate for hot gas filtration.

High-performance polyimide fibers derived from wholly rigid-rod monomers

Insufficient mechanical properties are one of the major obstacles for the wide application of polyimide (PI) fibers, especially utilized as reinforcement in structural composites. In this work, the copolymerization of two rigid-rod diamines, 2,5-bis(4-aminophenyl)-pyrimidine (PRM) and p-phenylenediamine (PPD), with the 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) was developed to fabricate PI fibers with high strength and high modulus. The effects of chemical structure (diamine molar ratio) on the molecular packing behavior, micromorphology as well as properties of the resulting co-PI fibers have been investigated in detail. Wide-angle X-ray diffraction spectra showed that co-PI fibers exhibited higher oriented structures with lower lateral packing order degrees in relative to both homo-BPDA/PRM and homo-BPDA/PPD PI fibers, and the Hermans’ orientation degrees reached 0.97 for the two samples with PRM/PPD molar ratio of 5/5 and 4/6. Small-angle X-ray scattering (2D SAXS) patterns indicated that the co-PI fiber with the PRM molar content of 50% showed the densest inner structure. Accordingly, this co-PI fiber presented a higher mechanical performance (tensile strength of 3.11 GPa and modulus of 144.2 GPa) as compared to those PI fibers reported previously. Meanwhile, these wholly rigid-rod PI fibers exhibited excellent thermal stabilities with the 5 and 10% weight loss temperatures ranging from 548 to 565 °C and from 572 to 589 °C, respectively. The prominent comprehensive properties make these PI fibers strong candidates for the future fiber-reinforced polymer composites.

Tensile modulus enhancement and mechanism of polyimide fibers by post-thermal treatment induced microvoid evolution

The correlation between mechanical property and the microstructure of two-step wet spinning polyimide (PI) fibers after thermal annealing was investigated. In-situ wide-angle X-ray diffraction (WAXD) and ex-situ small angle X-ray scattering (SAXS) methods were utilized to study the liquid crystal-like structure orientation and microvoids structure (the length and the orientation angle of microvoids) for PI fibers during annealing process. The effects of annealing temperature and time on the microstructure were systematically studied. The results reveal that the liquid crystal-like structure and the orientation degree keep nearly the same after annealing. Both of the molecular chain relaxation and the evolution of aligned microvoids are attributed to the change in tensile modulus. In low pre-drawing fiber, a slight decrease in microvoid length and an obvious rise in microvoid orientation angle lead to improved tensile modulus after annealing. In highly pre-drawn fiber, the tensile modulus decreases first due to de-orientation of polymer chains and then increase owing to disordering microvoids with decreasing size. Our work demonstrates the relationship between microvoids and mechanical properties in annealed PI fibers which provides a new avenue to access high performance PI fiber by facile annealing method and a deep insight into the structure and properties of high performance PI fibers.

Surface modification of high-performance polyimide fibers by oxygen plasma treatment

In order to improve the interfacial compatibility between the fibers and epoxy resins, polyimide (PI) fibers were modified using oxygen plasma with various treatment powers. The properties of PI fibers before and after surface modification were comparatively characterized according to their chemical composition, surface morphology, surface free energy, single filament tensile strength, and interfacial shear strength (IFSS). Most of the fiber properties, including the ratio of oxygen to nitrogen, oxygen concentration, surface free energy, and IFSS firstly increased with the plasma power, and then decreased when the plasma power was higher than 120 W. With a plasma power of 120 W, the oxygen concentration and the ratio of oxygen to carbon atoms (O/C) was 26% (22% for untreated fiber) and 0.38 (0.28 for untreated fiber), respectively. Meanwhile, the values of surface free energy and IFSS were 89% and 30% higher than those for untreated fiber, respectively. Furthermore, the values of single filament tensile strength for modified fibers were only 2.3% lower than those for pristine one. These results indicated that the compatibility between PI fibers and epoxy resins was greatly improved without compromising mechanical properties.

Preparation of high-performance polyimide fibers containing benzimidazole and benzoxazole units

A series of aromatic copolyimide (co-PI) fibers containing benzimidazole and benzoxazole were prepared by introducing 5-amino-2-(4-aminobenzene)benzoxazole (BOA) and 2-(4-aminophenyl)-5-aminobenzimidazole (BIA) into the rigid 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA)/p-phenylenediamine (p-PDA) backbone via a typical two-step wet-spinning method. The incorporated BOA and BIA moieties in the polymer chains resulted in significant changes on the comprehensive performances of the resulting PI fibers. The optimum tensile strength was 2.26 GPa with the BIA/BOA molar ratio of 3/1, whereas the optimum initial modulus was 145.0 GPa with the BIA/BOA molar ratio of 1/3. The drastically improved mechanical properties were originated from the hydrogen-bonding intermolecular interactions by BIA moieties and high molecular orientation by BOA moieties. FT-IR results confirmed the presence of hydrogen-bonding interaction in the fibers with high imidization degree. Two-dimensional wide-angle X-ray diffraction (2D WAXD) indicated the existence of highly oriented structures along the fiber axial direction, while this feature varied with different diamine ratios. SEM observations showed that the fibers were obtained with homogeneous and uniform structures at various diamine ratios. Meanwhile, the co-PI fibers exhibited excellent thermal properties with 5 % weight loss temperature ranging from 563 to 570 °C in nitrogen and glass transition temperature ranging from 308 to 321 °C.

Effect of pre-imidization on the structures and properties of polyimide fibers

The pre-imidization process was adopted in preparing high-performance polyimide fibers, resulting in the effectively improved mechanical properties of the resultant polyimide fibers.

Correlation between hydrogen‐bonding interaction and mechanical properties of polyimide fibers

AbstractNovel co‐polymerization polyimide (PI) fibers based on 4,4′‐oxydianiline (ODA)‐pyromellitic dianhydride (PMDA) were prepared. 2‐(4‐Aminophenyl)‐5‐aminobenzimidazole (PABZ) containing the NH group was introduced into the structure of the fibers as the proton donor. The results of Fourier transform infrared (FTIR) and dynamic mechanical analysis (DMA) showed that hydrogen bonding occured between the NH group and chains, which strongly enhanced interchain interaction. This hydrogen bonding interaction increased the tensile strength and initial modulus of the PI fibers up to 2.5 times and 26 times, respectively, compared to those of homo‐PI PMDA‐ODA fibers with no hydrogen‐bonding interaction because of the absence of proton donors after the imidization process. In the mean time, glass transition temperature (Tg) of the modified PI fibers was found to be 410–440°C, which was higher than that of the homo‐PI PMDA‐ODA fibers. From the result, a novel access to molecular design and manufacture of high performance PI fibers with good properties could be provided. Copyright © 2009 John Wiley & Sons, Ltd.