Mingtao Qiao

Hyperbranched polysiloxane (HBPSi)-based polyimide films with ultralow dielectric permittivity, desirable mechanical and thermal properties

Low-dielectric polyimide (PI) is on high demand in the next generation of high-density and high-speed integrated circuits. The introduction of fluorine or pores into PIs has been reported to efficiently obtain low-dielectric properties, but unavoidably deteriorate the mechanical and/or thermal properties. Therefore, it is a great challenge for PI to decrease its Dk and simultaneously maintain its mechanical and thermal properties. Herein, a series of robust PI films were fabricated by copolymerizing amine-functionalized hyperbranched polysiloxane (HBPSi) with pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA). The outstanding dielectric properties were achieved in a 35 wt% HBPSi PI film, which exhibits a Dk as low as 2.24 (1 MHz), mainly owing to the enhanced free volume and dielectric confinement effect afforded by the bulky HBPSi. Meanwhile, 35 wt% HBPSi PI demonstrates remarkable thermal stability and admirable mechanical properties, with the glass transition temperature of 388 °C, 5% weight loss temperature in argon flow up to 554 °C, a tensile strength of 80.6 MPa, elongation at break of 13.7% and a tensile modulus of 1.36 GPa. It also demonstrates conspicuous film homogeneity and planarity with the surface roughness as low as 0.42 nm and good moisture resistance with water uptake less than 1.5%. The prominent comprehensive properties make HBPSi PI a strong candidate for the future interlayer dielectrics.

Fabrication of electromagnetic Fe3O4@polyaniline nanofibers with high aspect ratio

Electromagnetic Fe3O4@polyaniline (PANI) nanofibers with high aspect ratio were realized by combining magnetic-field-induced (MFI) self-assembly and in situ surface polymerization of aniline. The key to fabrication is to induce chaining of the amino-Fe3O4 microspheres during the PANI coating process, which allows additional deposited PANI to warp entire chains into nanofibers. Hydrogen bonds are thought to be the driving force that makes aniline form a PANI coating shell instead of irregular sheets. Compared to the Fe3O4@PANI microspheres, higher magnetization saturation value and conductivity value were achieved in the Fe3O4@PANI nanofibers, which hold high promise for potential applications in microwave absorbents, electromagnetic shielding coatings, and other usage associated with conventional electromagnetic techniques.

Atomic oxygen resistance of polyimide/silicon hybrid thin films with different compositions and architectures

Silicon (Si)-containing polyimides (PIs) with superior atomic oxygen (AO) resistance are promising materials for space applications. Here, in this study, we present the synthesis and characterization of eight Si-containing PI thin films and evaluate their AO durability. The resulting PI films exhibited high thermal stability and preferable AO resistance but showed slightly reduced mechanical performance relative to pristine PI. The highest optical transparency at 550 nm was observed for PI/octaaminopropylsilsesquioxane, while the lowest value was observed for PI/silica (SiO2) hybrids. X-Ray photoelectron spectroscopic study suggested that the topmost surface of PI was degraded at the early stage and an SiO2 inert protective layer was finally formed on the surface of hybrid films after AO exposure. It is found that Si-containing units of higher oxidation states and with higher Si/O molar ratio are favorable to improve the AO resistance. Dispersion of Si at molecular level contributes to improving anti-AO property as well as optical transparency of the prepared films. The characterization of scanning electron microscopy indicated a continuous SiO2 protective layer was crucial to prevent AO from eroding the bulk matrix.

Application of yolk–shell Fe3O4@N-doped carbon nanochains as highly effective microwave-absorption material

Yolk–shell Fe3O4@N-doped carbon nanochains, intended for application as a novel microwave-absorption material, have been constructed by a three-step method. Magnetic-field-induced distillation-precipitation polymerization was used to synthesize nanochains with a one-dimensional (1D) structure. Then, a polypyrrole shell was uniformly applied to the surface of the nanochains through oxidant-directed vapor-phase polymerization, and finally the pyrolysis process was completed. The obtained products were characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), and thermogravimetric analyses (TGA) to confirm the compositions. The morphology and microstructure were observed using an optical microscope, scanning electron microscope (SEM), and transmission electron microscope (TEM). The N2 absorption–desorption isotherms indicate a Brunauer–Emmett–Teller (BET) specific surface area of 74 m2/g and a pore width of 5–30 nm. Investigations of the microwave absorption performance indicate that paraffin-based composites loaded with 20 wt.% yolk–shell Fe3O4@N-doped carbon nanochains possess a minimum reflection loss of −63.09 dB (11.91 GHz) and an effective absorption bandwidth of 5.34 GHz at a matching layer thickness of 3.1 mm. In addition, by tailoring the layer thicknesses, the effective absorption frequency bands can be made to cover most of the C, X, and Ku bands. By offering the advantages of stronger absorption, broad absorption bandwidth, low loading, thin layers, and intrinsic light weight, yolk–shell Fe3O4@N-doped carbon nanochains will be excellent candidates for practical application to microwave absorption. An analysis of the microwave absorption mechanism reveals that the excellent microwave absorption performance can be explained by the quarter-wavelength cancellation theory, good impedance matching, intense conductive loss, multiple reflections and scatterings, dielectric loss, magnetic loss, and microwave plasma loss.

Morphology-dependent electrochemical supercapacitors in multi-dimensional polyaniline nanostructures

Although some polyaniline (PANI) morphologies have been reported, their synthesis with regular structures in multiple dimensions has met with limited success. Here, well-defined PANI spheres, roses, cloud-like and rhombic plates, layered flowers, columns, blocks, and dendrites were prepared by employing static surfactant systems in a 0.010 M HCl solution at room temperature. The acquisition of these multi-dimensional (MD) nanostructures was a result of the fact that aniline and the newly formed PANI molecules were subjected to a synergistic effect of soft templates and self-assembly processes. Detailed electrochemical measurements were performed to investigate the capacitance of these MD nanostructures. PANI layered flowers possessed the highest specific capacitance of 272 F g−1 at the current density of 1.0 A g−1 due to their morphologies. Additionally, two factors including the surfactant dosage and the pH value were evaluated to discern their impacts on the PANI morphologies. The method presented herein renders the possibility for fabricating regular MD PANI nanostructures. And these nanostructures also have potential to be applied in supercapacitor electrodes and energy storage.

Fabricating and Tailoring Polyaniline (PANI) Nanofibers with High Aspect Ratio in a Low‐Acid Environment in a Magnetic Field

In a 0.010 m HCl solution, we successfully transformed irregular polyaniline (PANI) agglomerates into uniform PANI nanofibers with a diameter of 46-145 nm and a characteristic length on the order of several microns by the addition of superparamagnetic Fe3 O4 microspheres in a magnetic field. The PANI morphological evolution showed that the PANI nanofibers stemmed from the PANI coating shell synthesized on the surface of the Fe3 O4 microsphere chains. It was found that the magnetic field could optimize the PANI nanofibers with a narrow diameter size distribution, and effectively suppressed secondary growth. When compared with other microspheres (like silica and polystyrene), only the use of superparamagnetic Fe3 O4 microspheres resulted in the appearance of PANI nanofibers. Attempts to form these high-quality PANI nanofibers in other concentrations of HCl solution were unsuccessful. This deficiency was largely attributed to the inappropriate quantity of aniline cations.

Preparation of polyaniline (PANI)-coated Fe3O4 microsphere chains and PANI chain-like hollow spheres without using surfactants

We successfully coated polyaniline (PANI) onto amino-Fe3O4 microsphere chains to form PANI-coated Fe3O4 microsphere chain (PFMC) composites without using any surfactants. Chaining the amino-Fe3O4 microspheres as templates was realized via a magnetic-field-induced (MFI) assembly process. The hydrogen bonding formed between amino-Fe3O4 microspheres and aniline molecules was the driving force of aniline polymerization on the surface of the microspheres rather than in solution. After the Fe3O4 microspheres cores were removed, PANI chain-like hollow spheres (PCHM) were obtained. The length and PANI shell thickness of PFMC composites and corresponding PCHM could be effectively tuned by employing different dosages of aniline. It was found that the PANI shell thickness d1, average interparticle separation d2, and PANI loading yield were linearly increased with increasing aniline dosage in a certain range. This effective method not only supports a simple approach to the PFMC composites and PCHM, but also demonstrates that a PANI coating shell can be easily formed via solely electrostatic interactions without the aid of surfactants.

Amino-Fe3O4 Microspheres Directed Synthesis of a Series of Polyaniline Hierarchical Nanostructures with Different Wettability.

We demonstrated polyaniline (PANI) dimensional transformation by adding trace amino-Fe3O4 microspheres to aniline polymerization. Different PANI nanostructures (i.e., flowers, tentacles, and nanofibers) could be produced by controlling the nucleation position and number on the surface of Fe3O4 microspheres, where hydrogen bonding were spontaneously formed between amino groups of Fe3O4 microspheres and aniline molecules. By additionally introducing an external magnetic field, PANI towers were obtained. These PANI nanostructures displayed distinctly different surface wettability in the range from hydrophobicity to hydrophilicity, which was ascribed to the synergistic effect of their dimension, hierarchy, and size. Therefore, the dimension and property of PANI nanostructures can be largely rationalized and predicted by adjusting the PANI nucleation and growth. Using PANI as a model system, the strategies presented here provide insight into the general scheme of dimension and structure control for other conducting polymers.

Novel Reusable Porous Polyimide Fibers for Hot-oil Adsorption

The development of oil sorbents with high thermal stability, adsorption capacity, reusability and recoverability is of great significance for hot oil leakage protection, especially for oil spillage of oil refinery, petrochemical industry and cars. In our work, highly efficient hot oil adsorption of polyimide (PI) fibers with excellent thermal stability was successfully prepared by a facile electrospinning method followed by post-treatment. The corresponding morphologies, structures and oil adsorption properties of as-prepared PI fibers at different temperatures were analyzed and characterized. Results showed that PI fibers presented a stable morphology and pore structure at 200°C. The oil adsorption capacity of porous PI fibers for hot motor oil (200°C) was about 57.4gg-1, higher than that of PI fibers (32.7gg-1) with non-porous structure for the motor oil at room temperature. Even after ten adsorption cycles, porous PI fibers still maintained a comparable oil sorption capacity (oil retention of 4.2%). The obtained porous PI fibers exhibited excellent hot oil adsorption capacity, reusability and recoverability, which would broaden the application of electrospun fibers in oil spill cleanup and further provide a versatile platform for exploring the technologies of nanofibers in hot oil adsorption field.