Properties Of Polyimide Fibers Research Articles

Influence of temperature and imidization method on the structure and properties of polyimide fibers prepared by wet spinning

Influence of temperature and imidization method on the structure and properties of polyimide fibers prepared by wet spinning

Structures and properties of polyimide fibers prepared via gel spinning induced by chemical imidization

A series of 3,3′,4,4′-biphenyldianhydride (BPDA)/p-phenylene diamine (PDA) polyimide (PI) fibers were prepared through a novel gel spinning method induced by chemical imidization. The effects of draft ratios, solvent content and thermal treatment temperature on the aggregation structure and properties of PI fibers in the gel spinning were systematically investigated by experiment and molecular dynamics (MD) simulation. The mechanical properties of the fibers are significantly improved with increasing draft ratio due to the development of the molecular orientation along the fiber direction during large-ratio drafting process. The MD simulation results indicate that the nascent gel fiber with more solvent content has larger interchain space and weaker interchain interaction and molecular chains tend to be arranged along the drafting direction. The highly oriented PI fibers are completely imidized after thermal treatment at 320 °C, reaching the optimum tensile strength of 1.43 GPa and modulus of 121 GPa. The surface and internal defects of the highly oriented PI fiber gradually increased with increasing the thermal treatment temperature, resulting in a slight decrease in mechanical properties. In a word, the fiber orientation is carried out before the thermal imidization in the gel spinning process, which can greatly increase fiber draft ratio and reduce the subsequent thermal treatment temperature and energy consumption.

Effect of waterborne epoxy resin sizing on the surface and properties of high‐strength‐high‐modulus polyimide fibers

AbstractTo improve the interface adhesion between high‐strength‐high‐modulus polyimide (PI) fibers and epoxy resin (EP) matrix, waterborne epoxy resin (WEP) sizing agents are used to modify the surface of PI fibers. The sizing treatment of PI fibers was carried out by the self‐made sizing machine, and the effect of WEP sizing concentrations on the structure and properties of PI fibers were discussed in detail. The surface roughness, activated carbon percentage, and surface energy of WEP‐sized PI fibers increased with increasing WEP sizing agent contents. While, the mechanical and thermal properties of WEP‐sized PI fibers decreased slightly with the increment of WEP sizing agent concentrations. The optimum WEP sizing concentration was recognized to be 2 wt%. The interfacial shear strength and the interlinear shear strength values of PI fibers and epoxy resin (PI/EP) composites were improved by 175.22 and 25.31%, respectively. Moreover, the mechanism of surface characteristic improvement with WEP sizing treatment were proposed, contributing to the enhancement of mechanical interlocking and chemical interaction between fibers and matrix.

Preparation and Properties of a Thermoplastic Partially Crystalline Polyimide in the Oriented State

Polyamido acids derived from 4,4′-bis(4-aminophenoxy)diphenyl (diamine) and 1,3-bis(3′,4-dicarboxyphenoxy)benzene dianhydride were synthesized in two amide solvents: dimethylformamide and N-methylpyrrolidone. Fibers with different extrusion ratios were obtained by the coagulation method. These fibers were converted to polyimide fibers by heat treatment. The influence of the solvent used in the polyamido acid synthesis and of the extrusion ratio on the structure and properties of polyimide fibers was examined. An increase in the extrusion ratio from 1 to 2 leads to an increase in the strength and elongation at break of the polyimide fibers.

Ecofriendly Preparation of Cellulose Nanocrystal-Coated Polyimide Fiber: Strategy for Improved Wettability.

Polyimide (PI) fibers pose potential problems in applications. Their low surface activity causes poor interfacial wettability, easy agglomeration in aqueous solutions, and poor dispersibility. Therefore, this work proposes a method of surface modification of alkali-treated PI fibers with cellulose nanocrystals (CNCs) under the combined catalytic action of a Lewis acid and a crosslinker. The dispersion degree of PI fibers in aqueous solution before and after CNC modification and the contact angle of the PI fiber paper are measured. The results show that the wettability of the PI fibers improved. Furthermore, the structure and properties of PI fibers before and after CNC treatment are characterized via scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis. The pore-size distribution of the PI-fiber paper is measured by a porous-material pore-size analyzer. Compared with the original PI fibers, the oxygen content of the fiber surfaces increases after CNC treatment because of the esterification reaction and crosslinking that occur on the surfaces. The increase in the number of oxygen-containing polar groups and the increased surface roughness of the PI fiber improve its wettability. The contact angle of the PI fiber paper in deionized water is reduced by 14.9° and that in ethanol by 4.8°; the fiber dispersion degree is increased by 45%. These results indicate that the fibers have remarkably improved hydrophilicity and dispersion in the aqueous phase. Therefore, the method developed herein is to prepare high-performance organic fibers and corresponding composite materials.

Microstructure evolution and properties of polyimide fibers containing trifluoromethyl units

A series of copolyimide (co-PI) fibers containing trifluoromethyl units were successfully obtained on the molecular design of 3,3′,4,4′-biphenyltetracarboxylic dianhydride, p-phenylenediamine, and 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (HFBAPP) via a widely utilized two-step wet-spinning method. Significant variations on the microstructures and properties of the resulting co-PI fibers were observed after the incorporation of HFBAPP moieties. The tensile strength and initial modulus of the fibers decreased from 0.70 GPa to 0.38 GPa and from 69.63 GPa to 9.60 GPa, respectively. However, the dielectric permittivity decreased from 3.62 to 2.85 in the frequency of 10 MHz as a result of the incorporation of trifluoromethyl units. Two-dimensional wide-angle X-ray diffraction showed that the fibers exhibited highly oriented molecular alignments along the fiber direction and low lateral packing degree in the transverse direction. In addition, the co-PI fibers possessed excellent thermal–oxidative stabilities with the 5% weight loss temperature ranging from 532°C to 552°C under nitrogen atmosphere and the glass transition temperature ranging from 312°C to 330°C.

Highly improved mechanical and dielectric properties of paper-based composites with polyimide chopped fiber functionalized by ethylenediamine

In this study, polyimide (PI) chopped fibers were modified by ethylenediamine, and PI paper-based composites were fabricated using the as-modified PI chopped fibers and para-aramid fibrids which served as raw materials through the wet-forming process. The influence of different alkali treatment durations on the properties of PI fibers and composites was investigated. In sharp contrast with the pristine PI fibers, there was a satisfying difference in modified PI fibers including rougher surfaces and improved wettability due to the introduction of hydrophilic groups as confirmed by scanning electron microscope and Fourier transform infrared spectrometer. With increase in the modification time, the tensile index, the tearing index, and the internal bond strength of the composites were improved by 40.6, 27.0, and 103.57%, respectively, due to the hydrogen bonds. Meanwhile, the dielectric strength of the composites increased by 77.2% compared with the unmodified ones. These remarkable changes were mainly attributed to the enhanced interfacial bonding of the composites and decreased porosity in the three-dimensional network structure of the paper. Moreover, thermal stability was kept preferably within a certain range in spite of slight decrease.

The Properties of Polyimide Fibers Modified by Functionalized Muti-Wall Carbon Nanotubes Based on Friedel-Crafts Acylation

Muti-wall carbon nanotubes (MWCNTs) were functionalized by grafting polyimide (PI) on their surface via Friedel-Crafts acylation. The functionalized MWCNTs (f-MWCNTs) showd less damages than unfunctionalized ones. The partially imidized polyamide acid as-spun fibers containing f-MWCNTs were prepared by wet spinning, and the final PI/f-MWCNTs composite fibers were obtained by heat treatment. The tensile strength of the PI based composite fiber containing 1.0 wt% f-MWCNTs was 818.3 MPa and the Young’s modulus was 9.26 GPa, which were about 81% and 88% higher than those of pure PI fiber, respectively. Besides, the thermal stability of PI/f-MWCNTs composite fibers was obviously improved.

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.

Structures and properties of polyimide fibers containing ether units

Copolyimide (co-PI) fibers containing 4,4′-oxydiphthalic anhydride (ODPA) moiety into the 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA)/p-phenylenediamine backbone were prepared via a two-step wet-spinning method. The processability and mechanical properties were improved significantly after the incorporation of ODPA, and the fibers exhibited an optimum tensile strength of 10.94 cN dtex−1 and modulus of 470.52 cN dtex−1 with elongation of 2.75 % at a BPDA/ODPA molar ratio of 7/3. Two-dimensional wide angle X-ray diffraction indicated that highly oriented structures and ordered molecular packing regions were formed in the fibers. Two-dimensional small angle X-ray scattering revealed that the incorporation of ODPA resulted in the reduction in radius, length, misorientation, and internal surface roughness of the microvoids in the fibers simultaneously, which was supposed to be mainly dominated for the drastically improved mechanical properties of PI fibers. Moreover, the co-PI fibers exhibited excellent thermal and thermal-oxidative stability, and the 5 % weight loss temperature was above 572 and 535 °C under nitrogen and air, respectively.

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.

Structures and properties of polyimide fibers containing fluorine groups

The fluorine groups were successfully incorporated into the polymer backbone, and the structure–property relationship of the resulting polyimide fibers were systematically investigated.

Structure and properties of polyimide fibers containing benzimidazole and Amide Units

ABSTRACTCo‐polyimide (co‐PI) fibers with outstanding mechanical properties were fabricated via thermal imidization of polyamic acids, derived from a new design of combining the amide and benzimidazole diamine monomers, 4‐amino‐N‐(4‐aminophenyl)benzamide (DABA) and 2‐(4‐aminophenyl)‐5‐aminobenzimidazole (BIA), with 3,3′,4,4′‐biphenyltetracarboxylic dianhydride (BPDA). The crystalline structure and micromorphology of the prepared co‐PI fibers were investigated by synchrotron wide‐angle X‐ray diffraction (WAXD) and small‐angle X‐ray scattering (SAXS). The two‐dimensional WAXD spectra imply that the co‐PI fibers possess a structure between smectic‐like and three‐dimensionally ordered crystalline phase, and all the obtained fibers are highly oriented along the fiber axis. SAXS patterns exhibit a pair of meridional scattering streaks for the homo‐PI (BPDA/BIA) fiber, suggesting the presence of periodic lamellar structure. The incorporation of DABA into the polymer chains destroyed the lamellar structure but led to smaller size of microvoids upon increasing DABA moiety, based on SAXS analysis. The co‐PI fibers, with the molar ratio of BIA/DABA being 7/3, exhibited the optimum tensile strength and modulus of 1.96 and 108.3 GPa, respectively, attributed to the well‐defined ordered and dense structure. The chemical structure and molecular packing significantly affected the thermal stability of fibers, resulting in the different glass transition temperatures (Tg) from 350 to 380 °C. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 183–191

The properties, morphology and structure of BPDA/ PPD/ BOA polyimide fibers

AbstractA family of random copoly(amic acid)s (coPAAs) containing the 2-(4- aminophenyl)-5-aminobenzoxazole (BOA) moiety were synthesized in DMAc, followed by dry-jet wet spinning process into as-spun PAA fibers. The modified polyimide (PI) fibers were obtained from PAA fibers after imidized and drawn in a furnace. The mechanical properties of fibers were improved by incorporating BOA into BPDA/PPD backbone. SEM photo showed that the cross-section of each stage fibers was round and voids free. Besides, the “skin-core” and microfibrillar structure were not observed. The thermal properties of PI fibers were also investigated. The results showed that the PI fibers owned excellent thermal stability, moreover, the dimensional stability and the microphase separation of fibers were improved by heat-drawn stage. The Tg were found to be around 290°C by TMA and DMA. The X-ray (WAXD and SAXS) experiments indicated that the longitudinal stacks, the molecular orientation and the structural homogeneity of PI fiber were improved in the preparation process of fibers.

Preparation and properties of polyimide fibers

Polyimides were synthesized from 4,4′-diamino diphenyl methane and pyromellitic dianhydride using low-temperature solution polycondensation. Solutions of these polyamic acids in dimethyl-formamide (DMF) were spun into fibers by the wet spinning technique using a mixture of DMF and water as coagulants. Various spinning parameters such as dope concentration, bath composition, and jet stretch were standardized to get polyimide fibers with optimum properties. It was observed that fibers spun at higher jet stretch did not cyclize satisfactorily. Higher dope concentrations gave fibers with better properties. Cyclodehydrated fibers were hot-drawn at 300°C. Fibers with a tenacity of 380 mN/tex, an extension at break of 10%, and initial modulus of 4060 mN/tex were obtained. Mechanical properties of fibers at elevated temperatures, i.e., 100 and 200°C were also measured. Heat aging at 100, 200, and 300°C was carried out for 10 hr. This resulted in an increase in the initial modulus of fibers. However, a 28% decrease in tenacity was observed when the fibers were heat-aged at 300°C. The dynamic thermogravimetry in air showed that fibers were stable up to 400°C. The activation energy of decomposition, calculated from these thermograms in the temperature range 540–610°C was 101 kJ/mole.