Zhongyuan Huang

Chemically modified graphene/polyimide composite films based on utilization of covalent bonding and oriented distribution.

Herein, we have developed a rather simple composite fabrication approach to achieving molecular-level dispersion and planar orientation of chemically modified graphene (CMG) in the thermosetting polyimide (PI) matrix as well as realizing strong adhesion at the interfacial regions between reinforcing filler and matrix. The covalent adhesion of CMG to PI matrix and oriented distribution of CMG were carefully confirmed and analyzed by detailed investigations. Combination of covalent bonding and oriented distribution could enlarge the effectiveness of CMG in the matrix. Efficient stress transfer was found at the CMG/PI interfaces. Significant improvements in the mechanical performances, thermal stability, electrical conductivity, and hydrophobic behavior were achieved by addition of only a small amount of CMG. Furthermore, it is noteworthy that the hydrophilic-to-hydrophobic transition and the electrical percolation were observed at only 0.2 wt % CMG in this composite system. This facile methodology is believed to afford broad application potential in graphene-based polymer nanocomposites, especially other types of high-performance thermosetting systems.

Investigation on Tribological Properties of Fluorinated Polyimide

The 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) was used to synthesize polyimide to introduce different amounts of fluoride to the main chain of polyimide. The infrared spectra indicate that the imidizing process was almost complete and fluorinated monomer was formed in the structure. Fluoride-containing polyimide showed better thermal stability, higher tensile strength, and lower surface energy than neat polyimide. The increase of the fluoride monomer ratio contributed to the tribological properties of polyimide. The friction coefficient decreased with the increase of the fluoride monomer ratio. Surface-free energy and friction heat can alter the physical state of polymer sliding surfaces, and had a great effect on the tribological behaviors. Abrasive wear was designed and executed in this work. The wear rate decreased with the increase of the fluoride monomer ratio.

Tribological Behaviors of Fluorinated Polyimides at Different Temperatures

4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA)-based copolyimides were synthesized and the tribological properties of the copolyimides with different heat histories were studied at different temperatures. Fluoride-containing polyimide (PI) showed better thermal stability, decreased friction coefficients, and postponed the consequence of friction variation, which depended on temperature, than nonfluorinated PI. Thermal treatments seemed to increase the friction coefficients of copolyimides, and reduced the tensile strengths of the materials. The effects of applied load (P) and sliding speed (V) on tribological behaviors of thermally treated copolyimides were also examined and the variations of friction coefficient depending on PV values were investigated for clear understanding of its relationship between PV value and friction coefficient with different thermal treating time. Distortions of net structures of the chains and molecular motion contributed to variations of tribological properties of thermal treated copolyimides.

Surface Modification by Air-Plasma Treatment and Its Effect on the Tribological Behavior of Hybrid Fabric–Polyphenylene Sulfide Composites

Untreated and air-plasma-treated hybrid polytetrafluoroethylene (PTFE)/poly(p-phenylene terephthalamide) (Kevlar) fabrics were used to prepare hybrid fabric–polyphenylene sulfide (PPS) composites by dip-coating in a PPS suspension followed by hot pressing at 295°C for 5 minutes. The tribological properties of the resulting composites were evaluated on a MPX-2000A friction and wear tester with an end-face contact mode. To investigate the surface changes, untreated and treated hybrid fabrics were characterized by using a single fiber tensile test, fabric bonding property test, Fourier transform infrared spectroscopy, scanning electron microscopy, and water contact angle measurements. The results indicated that the air-plasma treatment could significantly improve the tribological behavior of the hybrid fabric–PPS composites. Moreover, the impact of the air-plasma treatment power and time on the tribological properties of the hybrid fabric–PPS composites was systematically studied. An optimum condition of the air-plasma treatment for our equipment was 120 W and 10 minutes. Active carbonyl (C˭O) groups were introduced on the surface after the treatment, coupled with an increase in the surface roughness, both of which improved the tribological properties of the hybrid fabric–PPS composites by strengthening the bonding property among the hybrid fabric, PPS, and substrate. On the other hand, excessive air-plasma treatment of the hybrid fabric weakened the tribological properties of the hybrid fabric–PPS composites.

An Investigation into the Migration of Segments in Crosslinked Fluorinated Polyimide Adhesive

Crosslinked fluorinated polyimides (CFPI) were successfully synthesized to study and explore the effect of cross-linkage on the migration of fluorinated segments and on the adhesion strength. Characterization by dynamic thermomechanical analysis (DMA) and thermo gravimetric analysis (TGA) confirmed good thermal properties of CFPI. X-ray photoelectron spectroscopy (XPS) results showed that the ratio of fluorinated component (6FDA-ODA) concentration of the surface to the bulk decreased with the crosslink density. The water contact angle of CFPI was lower than that of non-crosslinked fluorinated polyimide, indicating that the migration of fluorinated groups to the surface was reduced by the presence of cross-linkage. Therefore, CFPI, with no fluorine segregation on the surface, exhibited excellent wetting of adherent surfaces and adhesion strength, which was proved by lap shear strength (LSS) measurements and scanning electron microscopy.

Effect of Surface-Treated Carbon Nanotubes on the Mechanical and Tribological Performances of Phenolic Resin

Reinforced phenolic resin (PF) was prepared by in situ polymerization of different fractions of carbon nanotubes (CNTs) that had undergone oxidation. The compressive strength and hardness of the material, according to mechanical property testing, was improved by the modified CNTs. The tribological properties of PF composites were investigated by a block-on-ring friction and wear tester. The results indicated that CNTs reinforced PF showed lower friction coefficient and higher wear resistance, compared with pure PF. The morphologies of the worn surfaces, debris, and transfer films were observed by scanning electron microscopy (SEM) and optical microscopy (OM). A continuous and thinner transfer film formed during the friction test of the composites led to the significant improvement of the tribological properties.

Synthesis and Characterization of a Fluorinated Phenolic Resin/phenolic Resin Blend

ABSTRACT Fluorine atoms were introduced into the molecular chain of a phenolic resin (PR) by a two-step acid synthesis process to improve its thermal and hydrophobic properties. The successful synthesis of a fluorinated phenolic resin/phenolic resin blend (F-PR/PR) was proven by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance 19F (19F-NMR). The thermal properties of F-PR/PR were analyzed by differential scanning calorimetry (DSC) and thermo gravimetric analysis (TGA) and the results indicate that the F-PR/PR had good thermal stability. The hydrophobic properties of PR were effectively modified as demonstrated by the water drop contact angles. The flexural and tribological properties of F-PR/PR were also investigated. The fluorine atom, by virtue of its electronegativity, size and bond strength with carbon, can be used to create composites from F-PR/PR with remarkable properties. For comparison, two other resin blends were also synthesized.