Microstructures and electrothermal characterization of aromatic poly(azomethine ether)-derived carbon films

We report the microstructural evolution and electrothermal properties of aromatic poly(azomethine ether) (PAME)-derived carbon films, which were fabricated by a facile spin-coating and following carbonization at different temperatures of 300–1,000°C. For the purpose, poly[3-(4-nitrilophenoxy)phenylenenitrilomethine-1,3-phenylenemethine] (mPAME) with a high residue of ~56.4 wt% after carbonization at 1,000°C was synthesized for a polymeric precursor for carbon films. The X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray diffraction analyses revealed that the molecular structures of mPAME films changed into an intrinsically nitrogen-doped graphitic structure, dominantly at the carbonization temperatures of 800–100°C. The electrical conductivity increased considerably from ~10⁻⁷ S/cm for mPAME-derived films fabricated at 300–700°C to ~10⁰ S/cm for the film carbonized at 800°C to ~10¹ S/cm for the films carbonized at 900–1,000°C. Accordingly, mPAME-derived carbon films, which were carbonized at 900–1,000°C, exhibited excellent electrothermal performance, such as rapid temperature responsiveness, high maximum temperatures, and high electric power efficiency to relatively low applied voltages of 5–13 V.     

Preparation of carbon fiber web from electrostatic spinning of PMDA-ODA poly(amic acid) solution

Pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA) were copolymerized in a tetrahydrofuran (THF)/methanol (MeOH) mixed solvent to form a 12 wt.% poly(amic acid) (PAA) solution. It was electrostatically spun at 13-15 kV to form a PAA web of fine fibers with diameters less than 2-3 μm. The PAA web was heated to 150-250 °C to induce cyclization, transforming the PAA web into polyimide (PI) web. Then, the PI web was heat-treated at 700, 800, 900, 1000 and 2200 °C. The carbonization yield decreased monotonically from 64% at 700 °C to 53% at 1000 °C. The electrical conductivity of carbonized PI webs also increased with increasing heat treatment temperature, exhibiting 2.5 and 5.3 S/cm at 1000 and 2200 °C, respectively. The tensile strength and modulus of the carbonized web were 5.0 and 73.9 MPa, respectively.     

Properties of carbon nanofibers prepared from electrospun polyimide

A solution of a polyimide (PI, Matrimid® 5218) in dimethylacetamide was electrospun, and carbonization of the electrospun nonwoven fabrics produced carbon nanofiber fabrics. The effects of iron(III) acetylacetonate (AAI) on carbonization and the resulting morphology were also investigated. The carbonization behavior of the nonwoven fabrics was examined by X‐ray diffraction and Raman spectroscopy. AAI promoted carbonization of the nonwoven fabrics and increased the carbon yield. Addition of 3 wt % AAI increased the crystal dimension of electrospun PI nanofibers from 1.06 to 4.18 nm and decreased the integrated intensity ratio from 3.37 to 1.83 when heat treated at 1200°C. Scanning electron microscopy images of the carbonized nonwoven fabrics showed that AAI remained as particles within the fibers after carbonization. In addition, transmission electron microscopy observations revealed that turbostratic‐oriented graphite layers were observed around the particles even at 1200°C, which have been reported only on carbonization of rigid‐chain solvent insoluble PI materials under tension. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97:165–170, 2005     

Synthesis and structure of carbon belts made of carbon nanofibers supported on carbon foams

Two-dimensional carbon belts (CBs) made of carbon nanofibers (CNFs) supported on a carbon foam (CFoam) substrate have been synthesized by a procedure involving carbonization of polyamic acid (PAA)/Ni(NO₃)₂ solution impregnated polyurethane foam in flowing H₂ at 700 °C and catalytic chemical vapor deposition (CCVD) using C₂H₄ as a carbon source and SO₂ as a promoter. The CBs, which are hundreds of micrometers in length, several micrometers in width and tens of nanometers in thickness, are made of CNFs with a low degree of graphitization that array with an orientation roughly parallel to the longitudinal axis of the CBs. The results show that the mass ratio of Ni to PAA, a H₂ atmosphere in carbonization and SO₂ in CCVD process are the three key factors governing the growth of the CBs.     

Fabrication and Thermal Dissipation Properties of Carbon Nanofibers Derived from Electrospun Poly(Amic Acid) Carboxylate Salt Nanofibers

Polyimides (PIs) possess excellent mechanical properties, thermal stability, and chemical resistance and can be converted to carbon materials by thermal carbonization. The preparation of carbon nanomaterials by carbonizing PI‐based nanomaterials, however, has been less studied. In this work, the fabrication of PI nanofibers is investigated using electrospinning and their transformation to carbon nanofibers. Poly(amic acid) carboxylate salts (PAASs) solutions are first electrospun to form PAAS nanofibers. After the imidization and carbonization processes, PI and carbon nanofibers can then be obtained, respectively. The Raman spectra reveal that the carbon nanofibers are partially graphitized by the carbonization process. The diameters of the PI nanofibers are observed to be smaller than those of the PAAS nanofibers because of the formation of the more densely packed structures after the imidization processes; the diameters of the carbon nanofibers remain similar to those of the PI nanofibers after the carbonization process. The thermal dissipation behaviors of the PI and carbon nanofibers are also examined. The infrared images indicate that the transfer rates of thermal energy for the carbon nanofibers are higher than those for the PI nanofibers, due to the better thermal conductivity of carbon caused by the covalent sp² bonding between carbon atoms.     

静电纺聚酰亚胺非织造布的制备与表征

以N,N-二甲基乙酰胺(DMAc)为溶剂,3,3',4,4'-二苯醚四甲酸二酐(ODPA)和4,4'-二氨基二苯醚(ODA)为单体,利用高压静电纺丝技术,制备了聚酰胺酸(PAA)和聚酰亚胺(PI)非织造布,并采用扫描电镜(SEM)对PAA及PI非织造布的表面形态进行表征,研究了PI非织造布的力学性能。结果表明:经300℃热亚胺化处理得到的PI非织造布,纤维平均直径减小到500nm以下,纤维的带状形貌与PAA明显不同,并且出现了收缩、弯曲等现象。静电纺丝法制得的PI非织造布的力学性能仍然比较优越。     

The production of porosity in carbon nanofibers by the catalytic action of Ni nanoparticles in low temperature activation

Carbon nanofibers (CNFs) containing Ni nanoparticles were synthesized by carbonization of electrospun polyacrylonitrile nanofibers containing NiCl₂ followed by low-temperature activation (∼325 °C) in an oxygen atmosphere. The surface area of activated CNFs with 0.11 wt.% of the Ni oxide nanoparticles was 654 m²/g with increasing nanopores, which is significantly higher than the value for pure CNFs (30 m²/g). It was confirmed that the addition of trace amounts of Ni nanoparticles effectively produces a porous structure due to their catalytic role, which can increase the specific adsorption capacity of the activated CNFs without structural deformation.     

Comparative study of interphase evolution in polysiloxane resin-derived matrix containing carbon micro and nanofibers during thermal treatment

The work presents the results of research on composite materials made of silicon-containing polymer-derived ceramic matrix composites (PDC-Cs) and nanocomposites (PDC-NCs). Carbon micro and nanofibers (CFs and CNFs) were used as reinforcements. The interactions between carbon micro and nanofibers and polysiloxane matrix, as well as interphase evolution mechanism in composite samples during their heating to 1000 °C were studied. CF/resin and CNF/resin composites were prepared via liquid precursor infiltration process of unidirectionally aligned fibers. After heating to 700 °C–800 °C, decomposition of the resin in the presence of CNFs led to the formation of fiber/organic-inorganic composites with pseudo-plastic properties and improved oxidation resistance compared to as-prepared fiber/resin composites. The most favourable mechanical properties and oxidation resistance were obtained for composites and nanocomposites containing the maximum amount of carbon nanoparticles precipitated in the SiOC matrix during the heat treatment at 800 °C. The precipitated carbon phase improves fiber/matrix adhesion of composites.     

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.     

Fabrication and characterization of in situ synthesized iron oxide‐modified polyimide nanoweb by needleless electrospinning

A series of poly(amic acid) (PAA) solutions were prepared by sol–gel condensation of 4,4′‐oxydianiline (ODA) and 4,4′‐oxydiphthalic anhydride (ODPA), containing various wt % (5, 10, 15) of an iron oxide precursor, that is, tris(acetylacetonato)iron(III) complex. The resulting PAA solutions were electrospun at 78 kV and collected as webs of nonwoven nanofibers of diameter ～60–70 nm and subsequently converted to iron oxide‐modified polyimide (PI) nanofibers by slow thermal imidization. Aminopropyl triethoxysilane (APTES) and tetraethoxyorthosilicate (TEOS) were used as coupling agent and silica precursor, respectively, to enhance the compatibility between organic polymer matrix and inorganic moieties. SEM images reveal smooth and defect‐free surface morphologies of the nanofibers. Superparamagnetic properties of the nanofibers were revealed by vibrating sample magnetometer (VSM). FT‐infrared spectroscopy (IR), powder XRD, thermogravimetric analysis, and differential scanning calorimetry were employed to systematically characterize material structural properties, thermal stabilities, etc. Nanowebs showed excellent thermal stability around 446°C, with a glass transition temperature around 270°C. The above study demonstrates a good example for fabrication of highly thermally stable bead‐free nanofiber webs by needleless electrospinning. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40432.     

Biomimetic synthesis and characterization of carbon nanofiber/hydroxyapatite composite scaffolds

Three dimensional electrospun carbon nanofiber (CNF)/hydroxyapatite (HAp) composites were biomimetically synthesized in simulated body fluid (SBF). The CNFs with diameter of ∼250 nm were first fabricated from electrospun polyacrylonitrile precursor nanofibers by stabilization at 280 °C for 2 h, followed by carbonization at 1200 °C. The morphology, structure and water contact angle (WCA) of the CNFs and CNF/HAp composites were characterized. The pristine CNFs were hydrophobic with a WCA of 139.6°, resulting in the HAp growth only on the very outer layer fibers of the CNF mat. Treatment in NaOH aq. solutions introduced carboxylic groups onto the CNFs surfaces, and hence making the CNFs hydrophilic. In the SBF, the surface activated CNFs bonded with Ca²⁺ to form nuclei, which then easily induced the growth of HAp crystals on the CNFs throughout the CNF mat. The fracture strength of the CNF/HAp composite with a CNF content of 41.3% reached 67.3 MPa. Such CNF/HAp composites with strong interfacial bondings and high mechanical strength can be potentially useful in the field of bone tissue engineering.     

Preparation of porous carbon nanofibers derived from graphene oxide/polyacrylonitrile composites as electrochemical electrode materials

Porous carbon nanofibers (CNFs) derived from graphene oxide (GO) were prepared from the carbonization of electrospun polyacrylonitrile nanofibers with up to 15 wt.% GO at 1200 °C, followed by a low-temperature activation. The activated CNFs with reduced GOs (r-GO) revealed a specific surface area and adsorption capacity of 631 m²/g and 191.2 F/g, respectively, which are significantly higher than those of pure CNFs (16 m²/g and 3.1 F/g). It is believed that rough interfaces between r-GO and CNFs introduce oxygen pathways during activation, help to produce large amounts of all types of pores compared to pure activated CNFs.     

Preparation of well‐controlled porous carbon nanofiber materials by varying the compatibility of polymer blends

The relationships between the compatibility in binary polymer blends and the pore sizes of carbon nanofibers (CNFs) prepared from the blends were investigated. Compatibility was determined by the difference between the solubility parameters of each polymer in the polymer blends. Porous CNFs were prepared by an electrospinning and carbonization process using binary polymer blends, consisting of polyacrylonitrile (PAN) as the carbonizing polymer and poly(acrylic acid) (PAA), poly(ethylene glycol), poly(methyl methacrylate) or polystyrene (PS) as the pyrolyzing polymer. The pore size of the CNFs increased with increasing difference in solubility parameter. The CNFs prepared using the PAN/PAA blend, which had the smallest solubility parameter difference, exhibited a pore size of 1.66 nm compared to 18.24 nm for the CNFs prepared using the PAN/PS blend. The prepared CNF webs with controlled meso‐sized pores showed a stable cycle performance in cyclic voltammetry measurements and improved impedance characteristics. This method focusing on the compatibility in polymer blends was simple to apply and effective for controlling the pore sizes and surface area of CNFs for application as electrode materials in energy storage systems. © 2013 Society of Chemical Industry     

Effect of carbonization temperature on electrical conductivity of carbon papers prepared from petroleum pitch-coated glass fibers

Carbon papers were prepared wet-laid process of pitch-coated carbonized glass fibers, and the electrical conductivity of conductive carbon paper was investigated based on the structural and morphology of pitch-coated carbonized glass fibers prepared by different carbonization temperature. The electrical conductivity of the carbon paper which made of pitch-coated carbonized glass fibers was decreased at 900 °C and 1000 °C of carbonization temperature of pitch-coated glass fibers. This is due to the low stacked crystalline structure of pitch-coated glass fibers which is resulting from a basal carbon loss, in situ gasification and pyrolysis of low molecular compounds of coated pitch, at high carbonization temperature.     

Thermal imidization of poly(amic acid) precursors on surface-modified poly(tetrafluoroethylene) films via graft copolymerization with glycidyl methacrylate

A novel method for preparing composites of polyimides (PI) laminated to poly(tetrafluoroethylene) (PTFE) films is reported. PI/PTFE composites were developed through thermal imidization of poly(amic acid) (PAA) precursors on surface-modified PTFE films. Surface modification of PTFE films was carried out via Ar plasma pretreatment of the films, followed by UV-induced graft copolymerization with glycidyl methacrylate (GMA). The surface composition and topography of the graft copolymerized PTFE films and the delaminated PI and PTFE surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. The adhesion strengths of the PI (imidized PAA) on the GMA graft copolymerized PTFE films were evaluated as a function of various thermal imidization schedules. The adhesion reliability of the PI/PTFE composites was tested by a series of hydrothermal cycles. The development of strong Tpeel adhesion strengths of about 8 N/cm with excellent reliability for the PI/PTFE composites was attributable to the synergistic effect of coupling the curing of the epoxide functional groups of the grafted GMA chains with the imidization process of the PAA and the fact that the GMA chains were covalently tethered onto the PTFE surface. The PI/PTFE composites delaminated via cohesive failure inside the PTFE substrates. The delaminated PI film with a covalently adhered 'rough' PTFE surface layer exhibited a water contact angle as high as 140°.     

Preparation and properties of core–shell silver/polyimide nanocomposites

A new series of core–shell structured silver/polyimide (PI) nanocomposites was prepared by in situ polymerization followed by the chemical imidization of poly(amic acid) (PAA, precursor of PI) at a low temperature. The TEM images showed that the silver cores of the nanocomposites were encapsulated with homogeneous shells with thickness of 4 and 8 nm at silver contents of 90 and 60 %, respectively. The shell thickness was controlled by varying the content of PAA. FTIR spectroscopic analysis indicated that the imide ring formation occurred after the chemical imidization. The Ag/PI nanocomposites showed excellent thermal stability and exhibited only 10 % weight loss at 300 °C in the air. Moreover, percolation was observed at silver weight fractions close to the critical value, and the maximum dielectric permittivity of the nanocomposites was 120, which is about 40 times higher than that of pristine PI.     

Length changes in PAN fibres during their pyrolysis to carbon fibres

A systematic study of the shrinkage taking place during the carbonization of polyacrylonitrile (PAN) fibres is reported. Shrinkage occurs from 200°C to 1000°C, the extent of which has been found to depend on (a) the time of preoxidation, (b) the type of oxidizing gas and (c) the carbonization conditions. The carbonization shrinkage is found to be independent of length changes during the preoxidation but can decrease from the usual 23% to about 2% by applying tension during the carbonization. A reaction is proposed to explain the shrinkage between 600°C and 800°C. The strength as well as Young's modulus of carbon fibres of under-oxidized fibres can be improved by applying tension during the carbonization. It is further predicted that the mechanical properties of carbon fibres preoxidized to the optimal level would decrease if carbonized under excessive tension.     

Effect of post-curing on the mechanical properties of carbonized phenolic resins

After curing, phenol-formaldehyde resins were post-cured at 160°C, 230°C, and 300°C in air for several hours, and then those post-cured samples were carbonized at 1000°C. The effect of post-curing on the physical properties and microstructure of the carbonized phenolic resin is reported in this article. The purpose of post-curing was to improve the mechanical properties of the carbonized resins. The post-curing process promoted the crosslinking reaction and the evolution of gases. The cured resin post-cured at a higher post-curing temperature (300°C) had a significantly higher weight loss, greater linear shrinkage and lower density than others samples. During carbonization the post-curing process not only decreased the weight loss but also limited the shrinkage. Post-curing also promoted the formation of carbon basal planes and the chemical densification in structures of the final carbonized resins. The increase in post-curing temperature and time had the effect of reducing the linear shrinkage of the resin during carbonization. The TGA thermal analysis showed that the post-cured resins improved the total weight loss more than 15 wt% over the unpost-cured resin. The carbonized resins developed from the post-cured resins had a greater flexural modulus by about 10–50% and improved the linear shrinkage by about 10% over that developed from unpost-cured resin.     

Curing behavior,thermal, and mechanical properties of epoxy/polyamic acid based on 4,4′-biphtalic dianhydride and 3,3′-dihydroxybenzidine

Different synthesis routes were studied to obtain 4,4′-biphtalic dianhydride/3,3′-dihydroxybenzidine polyimide precursors (polyamic acids [PAAs]) with different inherent viscosities (IVs) and imidization degrees. The synthesized PAAs were introduced as a thermoplastic modifier into an epoxy (EP) resin. Different loadings of PAA were used to investigate the curing behavior, heat resistance, and mechanical properties. The onset curing temperature of the EP by adding 20 wt% PAA diminished by around 15°C. Thermogravimetric analysis revealed that the initial and 10 wt% weight loss temperature for EP with 5 wt% PAA improved by 13°C and 7.7%, respectively. Further, the results of tensile and plane-strain fracture toughness tests indicated that as the amount of PAA increased, the strength and toughness of EP decreased. These improvements were due to the high heat resistance and mechanical properties of PI precursor introduced into the EP, which formed a three-dimensional structure together. The interlaminar shear strength (ILSS) of the system experienced a reduction; however, after adding 2 phr nanosilica to the system containing PAA with average IV and imidization degree, ILSS showed 4.4% increment.     

The retarding effects and structural evolution of a bio-based high-performance polyimide during thermal imidization

The thermal imidization evolution of a bio-based high-performance polyimide, namely adenine-containing polyimide (API), was investigated by thermogravimetric analysis (TGA), in situ Fourier transform infrared spectroscopy (in situ FTIR), and wide-angle X-ray diffraction (WAXD), in contrast to an adenine-free 4,4′-oxydiphthalic anhydride (ODPA)/4,4′-oxydianiline (ODA) PI. The influence derived from adenine was focused. At precursor stage of API (polyamic acid, PAA), the H-bonding interaction of PAA–PAA type as well as the especial interaction between the secondary amine of adenine and solvent (dimethylacetamide, DMAc) was discovered. Structural evolution of API was traced by in situ FTIR and multistage WAXD from PAA stage to PI stage. Compared with OPI, the retarding effects were found in the process of thermal imidization of API, partly due to the formation of H-bonding derived from the extra secondary amine of adenine moieties, which complicated the H-bonding form in API. Finally, a hypothesis of evolution of thermal imidization process about API molecule was proposed in contrast with adenine-free ODPA ODA PI. Compared with the consistency of both API and ODPA ODA PI in PAA stage, API possessed a more delicate thermal imidization process. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46953.