Yanhui Chen

Ultra-low gas permeability and efficient reinforcement of cellulose nanocomposite films by well-aligned graphene oxide nanosheets

Cellulose is often considered to be an ideal candidate for biodegradable packaging films, but its main weakness is its poor gas barrier performance. We used a simple, efficient, low cost, recyclable, non-toxic and environmentally friendly processing solvent (an aqueous solution of NaOH/urea) to fabricate graphene oxide nanosheet (GONS)/regenerated cellulose (RC) nanocomposite films with an ultra-low O2 permeability and high mechanical performance. Transmission electron microscopy and two-dimensional wide-angle X-ray diffraction measurements showed that the GONSs were fully exfoliated, homogeneously dispersed and highly aligned along the surface of the cellulose nanocomposite films. Rheological and Fourier transform infrared spectroscopy measurements demonstrated the existence of strong H-bonding interactions between the GONSs and the cellulose matrix. A significant improvement in the barrier properties of the regenerated cellulose nanocomposite films was achieved. The O2 permeability coefficient was reduced by about 1000 times relative to the pure regenerated cellulose film at a low GONS loading of 1.64 vol%. The tensile strength and Youngs modulus of the regenerated cellulose nanocomposite films were enhanced by about 67 and 68%, respectively, compared with the RC film. The theoretical simulation results of the Cussler and Halpin–Tsai models consistently confirmed that the GONSs tended to align parallel to the film surface; this was probably induced by gravitational forces and further consolidated by hot pressing. The work presented here indicates that this simple and environmentally friendly method is an effective strategy to design highly aligned nanofillers in polymer nanocomposite films. The cellulose nanocomposite films obtained have excellent potential as packaging materials for protecting perishable goods susceptible to O2 degradation.

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.

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.

Polypropylene films with high barrier performance via crystal morphology manipulation

In this work, a simple and cost-effective method is proposed to prepare iPP films with high barrier properties. By adding a very small amount of β-nucleating agent, three types of β-crystal morphologies, namely, β-spherulites, β-transcrystals and β-“flower”-like agglomerates are sequentially obtained in iPP with the increasing processing temperature. Among β-nucleated iPP films, the one with β-“flower”-like agglomerates demonstrates the best water vapor and oxygen barrier performances, whose water permeability coefficient is decreased by 50.3% and oxygen permeability coefficient by 67.5%, compared to pure iPP films. Refined crystals, sufficient connection within the crystallites and lamellae parallel to the gas flow direction in iPP with β-“flower”-like agglomerates may force the gas molecules to take longer and more tortuous pathway, thus resulting in excellent barrier properties. Our work provides a new idea to prepare iPP films with high barrier properties by simply manipulating their crystal morphology.

Precession electron diffraction assisted orientation mapping of gradient nanostructure in a Ni-based superalloy

Surface mechanical grinding of a Ni-based superalloy can introduce a gradient microstructure in the surface layer with a grain size from nanoscale to microscale. In-depth investigation of the crystal orientation distribution of the surface nanostructured layer is more often, however, not an easy work by using the scanning electron microscope (SEM) based electron backscatter diffraction (EBSD) method due to its sensitivity to lattice distortions and spatial resolution limitation. Here we use a newly developed precession electron diffraction (PED) technique coupled with transmission electron microscopy (TEM) to investigate the microstructural and crystallographic characteristics of the surface gradient nanostructure, with particular emphasis on the topmost nanocrystalline layer. A strong shear texture and a minor Copper texture were identified according to orientation analyses of the 1.6 pm thick near-surface nanocrystalline layer. The PED technique is proved to be practical for two dimensional orientation mapping of severely deformed microstructures at the nanoscale.

Temperature-dependent β-crystal growth in isotactic polypropylene with β-nucleating agent after shear flow

In our current work, the effect of the shear temperature on the growth of β-crystal in isotactic polypropylene (iPP) with β-nucleating agent is investigated by means of in situ two-dimensional wide-angle X-ray diffraction (2D-WAXD). At low shear temperatures, the formed shear-induced oriented precursors are hard to relax back to random coiled state due to the weak mobility of molecular chains. Therefore, plenty of oriented α-crystals are induced by shear-induced oriented precursors, while β-crystal is greatly depressed. As the shear temperature increases, oriented β-crystal gradually increases along with the decrease of α-crystal. It is deduced that the shear temperature at which the content of β-crystal increases to the (maximum) value found in quiescent crystallization is almost the same as that at which the accelerating effect of flow on crystallization kinetics is completely erased. Our work manifests its significance in regulating β-crystal and thus in the structure and property manipulation of iPP.