Porous Polyimide and Carbon Nanotubes: Solvent Vapor–Induced Transformation in the Nanochannels of Anodic Aluminum Oxide Templates

Polyimides (PIs) have attracted wide attention because of their exceptional thermal stability and applications in areas such as printed circuit boards and multichip modules. It remains a great challenge, however, to control the morphologies and properties of PI‐based nanomaterials, especially porous PI‐based nanotubes. In this work, a versatile method to fabricate porous PI nanotubes via the template method is developed, with a solvent vapor–induced transformation process. First, polyamic acid (PAA) solutions are used as precursors and infiltrated into the nanochannels of anodic aluminum oxide templates, forming PAA nanotubes. After the samples are annealed in tetrahydrofuran or dimethylformamide vapors, depression of the PAA tubes occurs, causing the formation of pores. The porous PAA nanotubes can be further converted to porous PI and carbon nanotubes by imidization and carbonization processes, respectively. The pore sizes of the porous nanomaterials can be controlled by changing the type of the annealing solvent and the solvent annealing time.     

Fabrication and electrochemical characterization of polyimide-derived carbon nanofibers for self-standing supercapacitor electrode materials

We report the electrochemical performance of aromatic polyimide (PI)-based carbon nanofibers (CNFs), which were fabricated by electrospinning, imidization, and carbonization process of poly(amic acid) (PAA) as an aromatic PI precursor. For the purpose, PAA solution was electrospun into nanofibers, which were then converted into CNFs via one-step (PAA-CNFs) or two-step heat treatment (PI-CNFs) of imidization and carbonization. The FTIR and Raman spectra demonstrated a successful structural evolution from PAA nanofibers to PI nanofibers to CNFs at the molecular level. The SEM images revealed that the average diameter of the nanofibers decreased noticeably via imidization and carbonization, while it decreased slightly with increasing the carbonization temperature from 800 °C to 1000 °C. In case of PI-CNF carbonized at 1000 °C, a porous structure was developed on the surface of nanofibers. The electrical conductivity of PI-CNFs, which was even higher than that of PAA-CNFs, increased significantly from 0.41 to 2.50 S/cm with increasing the carbonization temperature. From cyclic voltammetry and galvanostatic charge/discharge tests, PI-CNF carbonized at 1000 °C was evaluated to have a maximum electrochemical performance of specific capacitance of ~126.3 F/g, energy density of ~12.2 Wh/kg, and power density of ~160 W/kg, in addition to an excellent operational stability. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47846.     

Three-dimensional printing of high-performance polyimide by direct ink writing of hydrogel precursor

Three-dimensional (3D) printing of all-aromatic polyimides (PIs) is attracting extensive attention due to the advantages of excellent comprehensive performances and complex structures. It still remains challenges because of some drawbacks, such as poor solubility and infusibility, and the environmental unfriendly preparation. Here, polyamide acid salt (PAAS) hydrogels were prepared by the reaction of polyamide acid (PAA) with organic amines in water. PAAS hydrogels had remarkable rheological properties, which made it applied in direct ink writing 3D printing. The printed objects were finally converted into high-performances PI objects with prominent mechanical properties and thermal stability. Not only so, the advantages also include simple preparation, less organic solvent, low thermal imidization temperature (250°C), controllable macrostructure and porous microstructure, and the present 3D printed PI by PAAS was expected to significantly promote the application potentials. In this study, thermal conductivity (100°C) of the PI objects after structure design and 3D printing was reduced from 0.102 W m⁻¹ K⁻¹ to 0.061 W m⁻¹ K⁻¹, and density was reduced from 0.4562 g cm⁻³ to 0.2731 g cm⁻³.     

Preparation of Iron Oxide Particle-Decorated Lignin-Based Carbon Nanofibers as Electrode Material for Pseudocapacitor

Iron oxide particle-decorated lignin-based carbon nanofibers (IO-LCNFs) were fabricated from organic mixtures containing acetic acid lignin (AAL) together with ferric acetylacetonate (Fe(acac)₃) via electrospinning followed by stabilization in air and carbonization in nitrogen. After the addition of Fe(acac)₃, IO-LCNFs showed different morphologies: Non-fused IO-LCNFs were obtained with diameters of 400–500 nm; iron oxide nanoparticles with diameters of 30–60 nm were exposed outside and well-distributed when sufficient amounts of Fe(acac)₃ were added. These carbon nanofibers were then used as electrode material for pseudocapacitor. It was found that the iron oxide particles enhanced the resulting electrochemical properties via reversible redox reactions. IO-LCNFs made from the composite nanofibers with mass ratio of AAL/Fe(acac)₃ of 80/20 [i.e., IO-LCNFs (80/20)] exhibited the highest specific capacitance, 72.1 F g^?1, at current density of 500 mA g^?1.     

Electrospun polyimide nanofibers and their applications

Aromatic polyimides (PIs) are high-performance polymers with rigid heterocyclic imide rings and aromatic benzene rings in their macromolecular backbones. Owing to excellent mechanical properties and thermal stability, as well as readily adjustable molecular structures, PIs have been widely adopted for many applications related to electronics, aerospace, automobile, and other industries. In recent years, PI fibers prepared by electrospinning of polyamic acid (PAA) precursor nanofibers followed by imidization (commonly known as electrospun PI nanofibers) have attracted growing interests. Herein, the preparation, evaluation, and application of electrospun PI nanofibers are reviewed. PI polymers and the electrospinning technique are introduced first followed by the preparation of electrospun nanofibers of homo-PI, co-PI, blend-PI, and PI composite. Subsequently, the mechanical and thermal properties of electrospun PI nanofibers are presented; in particular, the mechanical properties of individual electrospun PI nanofibers are highlighted. Thereafter, various applications of electrospun PI nanofibers are outlined, including reinforcement of composites, Li-ion battery separators, fuel cell proton exchange membranes, sensors, microelectronics, high-temperature filtration media, super-hydrophobic PI nanofibers, and PI-based carbon nanofibers. In the final section of conclusions and perspectives, future research endeavors and high-value applications of electrospun PI nanofibers are discussed.     

Carbon nanonodules fewer than ten graphenes thick grown on aligned amorphous carbon nanofibers

A novel structure of carbon nanonodules containing fewer than 10 layers graphene has grown on amorphous carbon nanofibers by carbonization-induced self-assembly. It is found that a successive processes containing pre-oxidation in air at 220 °C and carbonization in a high vacuum (1 × 10⁻⁴ Pa) at 750 °C are necessary for the fabrication of the carbon nanonodules. Possible mechanism for the evolution of amorphous nanofibers to carbon nanonodules is presented. It is also found that the temperature of the collector during electrospinning of the fiber and the pressure of carbonization are critical factors for growth of the nanonodules. With these mechanisms, carbon nanonodules can be selectively grown on the prepared amorphous carbon nanofibers using pre-oxidation and carbonization of an electrospun glycerol–polyacrylonitrile fiber.     

静电纺丝制备聚丙烯腈纳米碳纤维

利用静电纺丝制备连续的聚丙烯腈纳米碳纤维;介绍了静电纺丝的原理、影响静电纺丝的主要因素以及制备纳米碳纤维、纳米活性炭纤维、纳米碳纤维复合材料的方法和原理;分析了静电纺丝产率低,难以得到单向平铺的纤维等问题,影响静电纺丝的参数主要有溶液特性、纺丝工艺参数、纺丝环境参数。由静电纺丝得到纳米聚丙烯腈纤维,然后再经预氧化和碳化制备纳米碳纤维,或把纳米纤维预氧化,经活化、碳化制备纳米活性炭纤维。并指出纳米碳纤维具有巨大的潜在应用空间。     

Preparation and characterization of silica/polyimide nanocomposite films based on water‐soluble poly(amic acid) ammonium salt

In this article, a series of new silica/polyimide (SiO₂/PI) nanocomposite films with high dielectric constant (>4.0), low dielectric loss (<0.0325), high breakdown strength (288.8 kV mm⁻¹), and high volume resistivity (2.498 × 10¹⁴ Ω m) were prepared by the hydrolysis of tetraethyl orthosilicate in water‐soluble poly(amic acid) ammonium salt (PAAS). The chemical structure of nanocomposite films compared with the traditional pure PI was confirmed by Fourier transform infrared spectroscopy and X‐ray diffraction patterns. The results indicated that both the PAAS and the polyamide acid (PAA) material were effectively converted into the corresponding PI material through the thermal imidization and the amorphous SiO₂ was embedded in the nanocomposite films without structural changes. Thermal stability of the nanocomposite films was increased though mechanical property was generally decreased with increasing the mass fraction of SiO₂. All the nanocomposite films exhibited an almost single‐step thermal decomposition behavior and the average decomposition temperature was about 615°C. It was concluded that the effective dispersion of SiO₂ particles in PI matrix vigorously improved the comprehensive performance of the SiO₂/PI nanocomposite films and expanded their applications in the electronic and environment‐friendly industries. POLYM. COMPOS., 38:774–781, 2017. © 2015 Society of Plastics Engineers     

Graphitic carbon nanofibers developed from bundles of aligned electrospun polyacrylonitrile nanofibers containing phosphoric acid

Graphitic carbon nanofibers (GCNFs) with diameters of approximately 300 nm were developed using bundles of aligned electrospun polyacrylonitrile (PAN) nanofibers containing phosphoric acid (PA) as the innovative precursors through thermal treatments of stabilization, carbonization, and graphitization. The morphological, structural, and mechanical properties of GCNFs were systematically characterized and/or evaluated. The GCNFs made from the electrospun PAN precursor nanofibers containing 1.5 wt.% of PA exhibited mechanical strength that was 62.3% higher than that of the GCNFs made from the precursor nanofibers without PA. The molecules of PA in the electrospun PAN precursor nanofibers initiated the cyclization and induced the aromatization during stabilization, as indicated by the FT-IR and TGA results. The stabilized PAN nanofibers possessed regularly oriented ladder structures, which facilitated the further formation of ordered graphitic structures in GCNFs during carbonization and graphitization, as indicated by the TEM, XRD, and Raman results.     

Solution processed low‐k dielectric core‐shell nanoparticles for additive manufacturing of microwave devices

This study introduces a new core‐shell structured polytetrafluoroethylene (PTFE)/polyimide (PI) nanoparticle for additive manufacturing of microwave substrates. Materials were synthesized using a solution processed method through the electrostatic interaction between PTFE with negative potential and poly(amic acid) salt (PAAS, a PI precursor) with positive potential followed by the thermal imidization of PAAS. Microscopic studies by transmission electron microscopy, scanning electron microscopy, and atomic force microscopy confirmed the formation of core‐shell nanoparticles, a porous material network, and a reduction of surface roughness upon imidization. In addition to excellent high temperature stability (<0.4% weight loss at 500 °C), the synthesized materials showed improved particle‐to‐particle adhesion and particle‐to‐substrate adhesion compared to PTFE alone, and good dielectric properties measured at 7.2 GHz utilizing a cavity perturbation technique. The materials consisting of 5% to 35% of PI exhibited low relative permittivity (?′) of 2.14 to 2.38 and loss tangent (tan δ) of 0.001 to 0.0018, which make them well suited for use in additive manufacturing. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 45335.     

Processing of continuous poly(amide‐imide) nanofibers by electrospinning

Continuous poly(amide‐imide) nanofibers were fabricated using a novel electrospinning method with rotating and re‐collecting cylindrical collectors. The nanofilaments were modified using various post‐treatments, i.e. glycerol treatment and thermal imidization under tension, for possible application as high‐performance reinforcements. Morphological and mechanical properties of continuous poly(amide‐imide) nanofibers prepared by the electrospinning process and various post‐treatments were measured. Severe adhesion between individual nanofibers within fiber bundles was inhibited through surface treatment of the electrospun nanofiber bundles by spraying with glycerol. The morphological and mechanical properties of the continuous poly(amide‐imide) nanofibers and thermal stability were improved using thermal imidization at high temperature under tension. The morphological and mechanical properties of the continuous electrospun nanofibers were improved significantly by post‐treatments after electrospinning because uniform and complete thermal imidization occurred through the core region of the nanofibers. Copyright © 2009 Society of Chemical Industry     

Epoxy Resin Nanofibers Prepared Using Electrospun Core/Sheath Nanofibers as Templates

A strategy is described to prepare epoxy resin nanofibers by combining coaxial electrospinning and traditional hot‐curing processes. Core/sheath nanofibers with diameters of 480 ± 80 nm are prepared at flow rates of 0.1 and 2 mL h^?1 for the core (20% w/v EP and 6% w/v curing agent in ethanol/acetone) and sheath (10% PVP in ethanol) fluids. After the curing of the nanofibers and selective removal of the sheath PVP, EP nanofibers with an average diameter of 210 ± 60 nm are obtained. ATR‐FTIR analysis shows that the EP nanofibers display no obvious difference compared with an EP film cast from the core solution. The method presented allows to develop functional EP nanoproducts and to prepare heat‐cured resin nanofibers.

     

Preparation of high temperature resistant Ag/PI/Cu composite nano particles inserted with PI insulating layer

Polyimide (PI)@copper (Cu) composite nano particles have been successfully synthesized from poly(amic acid) triethylamine salts (PAAS) and Cu(II) ions via a one-step high-temperature induction/imidization route. The formation of PI@Cu nano particles has been investigated by the stoichiometric ratio of PAAS and Cu ion. The resulting products, formed stable shell-core structures, exhibited the uniform core-size and thick shell layer. Additionally, the multi-layer structure, Ag@PI@Cu, was successfully prepared via a post process of PI@Cu nanoparticles. The morphology of the formed “Sunflower-mode” structure, with the pistil of Cu, the sunflower seed of PI, and the petal of Ag, was also characterized by SEM and TEM. Both electrical resistivity and thermal conductivity of nano particles were measured. The coefficient of heat conduction of Ag@PI@Cu is even 255 times, 754 times, 3081 times, and 1310 times as large as PI@Cu in 50 °C, 100 °C, 150 °C, and 200 °C, respectively. The resistance of both nano particles is that the result of Rs_PI@Cu and Rs_Ag@PI@Cu is 11.0*10⁹ Ω and 0.11 Ω, respectively, and also the difference between them is more than 10¹².     

Low Temperature Carbothermal Reduction Synthesis of ZrC Nanofibers via Cyclized Electrospun PVP/Zr(OPr)4 Hybrid

The fabrication capability of zirconium carbide (ZrC) nanofibers by a novel polymeric solution was examined using electrospinning method. The electrospinnable solution was prepared from the reaction of zirconium n‐propoxide (Zr(OPr)₄) with acetylacetone and acetic acid followed by the addition of polyvinylpyrrolidone (PVP) solution. By utilizing thermal and microstructural analyses such as differential scanning calorimetry–thermogravimetry (DSC–TG), field emission scanning electron microscopy (FE‐SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD), and Brunauer–Emmett–Teller (BET), the effect of heat treatment type on the morphology and crystallinity of as‐spun PVP/Zr(OPr)₄ hybrid fibers was examined. The results showed that direct carbonization treatment of as‐spun fibers under argon atmosphere led to spherical ZrC aggregates in lack of fibrillar morphology, whereas carbonization coupled with cyclization could be recognized as the unique template to govern the morphology and crystallinity of ZrC nanofibers. Carbonization of the cyclized fibers at 1550°C in flowing argon atmosphere produced the thick, fragmented rosary‐like fibers with a diameter of 357 nm, while through a 100°C decrease in carbonization temperature to 1450°C, the thin, smooth, long, and uniform ZrC nanofibers with 176 nm diameter and a medium surface area of 23 m²/g were obtained.     

Fabrication and characterization of carbon nanofibers with a multiple tubular porous structure via electrospinning

Carbon nanofibers with a multiple tubular porous structure were prepared via electrospinning from a polymer blend solution of polyacrylonitrile (PAN) and polylactide (PLA) followed by carbonization. The electrospun composite nanofibers underwent pre-oxidization and carbonization, which selectively eliminated PLA phases and transformed the continuous PAN phase into carbon, thereby porous structure formed in the carbon nanofibers. The morphologies of as-spun, pre-oxidized and carbonized nanofibers were studied by scanning electron microscope (SEM) and transmission electron microscopy (TEM). It was found that carbon nanofibers with an average diameter about 250 nm and a multiple tubular porous structure were obtained. The chemical changes during thermal treatment were studied by Fourier transform infrared spectrometer (FTIR), Raman spectra, differential thermal analysis (DTA) and thermogravimetric analysis (TG). The results showed that PLA phases were effectively removed and the continuous PAN phase was completely carbonized. The obtained carbon nanofibers had more disordered non-graphitized structures than non-porous nanofibers.     

Influence of iron (III) acetylacetonate on structure and electrical conductivity of Fe3O4/carbon composite nanofibers

Fe₃O₄/carbon composite nanofibers were prepared by electrospinning polyacrylonitrile (PAN), iron (III) acetylacetonate (AAI) and dimethyl formamide (DMF) compound solutions, followed by stabilization and carbonization processes. Emphasis was put on the influence of AAI on reactions during stabilization and carbonization. The effect of Fe₃O₄ on catalytic graphitization and electrical conductivity was also studied. Experimental results show that AAI has participated in the reactions and structural changes of PAN during stabilization and carbonization, and is evidenced to promote the processes. Fe₃O₄ nanoparticles exhibit catalytic effect on carbonization process that promote graphitization by a catalytic effect at low AAI content and inhibit the formation of graphitized layers when AAI content is excessive. Therefore, there exists an optimum AAI content (C_o) where composite nanofibers show the maximum graphitization degree and electrical conductivity. With proper amount of AAI addition, Fe₃O₄/carbon composite nanofibers showing high graphite degree and electrical conductivity could be achieved.     

Carbon nanofibers from renewable bioresources (lignin) and a recycled commodity polymer [poly(ethylene terephthalate)]

Utilizing inexpensive biorenewable and waste raw materials for the production of carbon nanofibers can pave the way for lowering their manufacturing cost. In this research, lignin is combined with recycled poly(ethylene terephthalate) (PET) to fabricate precursor fibers via electrospinning. The process is optimized using the Design of Experiments statistical methodology and fibers with minimum average diameter equal to 191 ± 60 nm are prepared. Investigation with Attenuated Total Reflection – Fourier Transform Infrared Spectroscopy reveals the lignin structural changes induced by the solvent (trifluoroacetic acid), which is used for the preparation of homogeneous solutions of lignin and PET in various concentrations, while it gives an indication of the blending of the two electrospun polymers. The good miscibility between lignin and PET is also confirmed with Differential Scanning Calorimetry. The subsequent carbonization of the precursor fibrous mats results in a fibrous carbon structure with average fiber diameters similar to those of the precursor fibers. The successful transformation into carbon nanofibers is affirmed by Energy Dispersive X‐ray Spectroscopy. The Carbon content of these nanofibers amounts to 94.3%. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43936.     

Development of carbon nanofibers from aligned electrospun polyacrylonitrile nanofiber bundles and characterization of their microstructural, electrical, and mechanical properties

Carbon nanofibers with diameters of 200-300 nm were developed through stabilization and carbonization of aligned electrospun polyacrylonitrile (PAN) nanofiber bundles. Prior to the oxidative stabilization in air, the electrospun PAN nanofiber bundle was tightly wrapped onto a glass rod, so that tension existed during the stabilization. We also investigated several carbonization procedures by varying final carbonization temperatures in the range from 1000 to 2200 °C. The study revealed that: (1) with increase of the final carbonization temperature, the carbon nanofibers became more graphitic and structurally ordered; (2) the carbon nanofiber bundles possessed anisotropic electrical conductivities, and the differences between the parallel and perpendicular directions to the bundle axes were over 20 times; and (3) the tensile strengths and Young's moduli of the prepared carbon nanofiber bundles were in the ranges of 300-600 MPa and 40-60 GPa, respectively.     

Functional Copolymer/Organo-MMT Nanoarchitectures: XXVI. Fabrication and Characterization of Electrospun Nanofibers from PCL/ODA-MMT and Copolymer-g-PLA/ Ag-MMT Blends

We synthesized poly(?-caprolactone)/octadecyl amine-montmorillonite clay nanocomposite as a matrix polymer by solution intercalative method and new amphiphilic poly(maleic anhydrde-alt-1-octadecene)-g-poly(L-lactic acid)/Ag⁺-montmorillonite clay nanocomposite as a partner polymer by interlamellar graft copolymerization of lactic acid onto anhydride copolymer in the presence of silver salt of montmorillonite clay as catalyst-nanofiller. Novel polymer nanofibers were fabricated by electrospinning of matrix/partner blends with different volume ratios. The nanocomposites and nanofibers were investigated by Fourier transform infrared spectroscopy, thermal gravimetric analysis–differential scanning calorimetry, and scanning electron microscope–transmission electron microscope methods. The diameters, morphologies, and thermal behavior of fibers were strongly depended on the partner-polymer nanocomposites loadings. The fabricated biocompatible and biodegradable nanofibers can be utilized for biomedical and filtration applications.     

Nanofibers from recycle waste expanded polystyrene using natural solvent

Summary A new recycling technique has been developed. In this method, EPS (expanded polystyrene), generally called Styrofoam, is dissolved with natural solvent, d-limonene and electrospun. This method can economically produce the nanofibers. The electrospinning process produces a nonwoven mat of long polymer fibers with diameters in the range of 10–500 nm and high surface areas per unit mass. PS (Polystyrene) polymer dissolved in different solvents such as THF (Tetrahydrofuran), DMF (Dimethylformaide), and DMAc (Dimethylacetamide) etc. may all be electrospun into nanofibers. These solvents cause environmental problem and difficulty of process handling. Natural solvent, d-limonene is used for dissolving PS. PS nanofibers are produced with PS solution using d-Limonene. This paper describes the use of polystyrene (PS) nanofibers electrospun from recycled EPS solution dissolved in d-limonene. The electrospun polystyrene nanofiber diameters vary from 300 to 900 nm, with an average diameter of about 700 nm.