Meijun Ma

Superior Mechanical Properties of Epoxy Composites Reinforced by 3D Interconnected Graphene Skeleton

Epoxy-based composites reinforced by three-dimensional graphene skeleton (3DGS) were fabricated in resin transfer molding method with respect to the difficulty in good dispersion and arrangement of graphene sheets in composites by directly mixing graphene and epoxy. 3DGS was synthesized in the process of self-assembly and reduction with poly(amidoamine) dendrimers. In the formation of 3DGS, graphene sheets were in good dispersion and ordered state, which resulted in exceptional mechanical properties and thermal stability for epoxy composites. For 3DGS/epoxy composites, the tensile and compressive strengths significantly increased by 120.9% and 148.3%, respectively, as well as the glass transition temperature, which increased by a notable 19 °C, unlike the thermal exfoliation graphene/epoxy composites via direct-mixing route, which increased by only 0.20 wt % content of fillers. Relative to the graphene/epoxy composites in direct-mixing method mentioned in literature, the increase in tensile and compressive strengths of 3DGS/epoxy composites was at least twofold and sevenfold, respectively. It can be expected that 3DGS, which comes from preforming graphene sheets orderly and dispersedly, would replace graphene nanosheets in polymer nanocomposite reinforcement and endow composites with unique structure and some unexpected performance.

Improving dielectric properties of poly(vinylidene fluoride) composites: effects of surface functionalization of exfoliated graphene

To investigate the effects of surface functionalization of exfoliated graphene (EG) on the crystalline form of β-phase and dielectric properties of poly(vinylidene fluoride) (PVDF), we prepared PVDF-based composites reinforced by different functionalized EG. The X-ray photoelectron spectroscopy results indicated that a wide variety of chemical functional groups such as C–OH, C–O–C, C=O, COOH and C–F could be introduced on the surface of modified EG. As confirmed by results of Fourier transform infrared spectrum and X-ray diffraction, the β-phase PVDF can be produced in the composites with the incorporation of functionalized EG. In the frequency ranging from 102 to 107 Hz, the dielectric permittivity of PVDF composites shows an obvious increase owing to a variation of the carbonyl group (C=O) content. Among all the composites, the EG grafted with polymethyl methacrylate/PVDF composite has the highest dielectric permittivity and dielectric loss.

Synergetic improvement of mechanical properties and surface activities in γ-irradiated carbon fibers revealed by radial positioning spectroscopy and mechanical model

The relationship between microstructures, surface activities, and mechanical properties of γ-irradiated carbon fibers has been evaluated quantitatively. X-ray photoelectron spectroscopy combined with argon ion sputtering indicated that the outer-surface part (∼10 nm) was functionalized and disordered by the grafting reaction; this led to an increase in the surface activity and loss of mechanical properties of γ-irradiated carbon fibers. The degree of covalent cross-linking between subsurface layers of graphene (∼1.5 μm) showed a more notable increase than that of the core (∼4 μm); this indicated that the sub-surface was mainly responsible for improvement in the tensile strength of γ-irradiated carbon fibers. Increases of 15.5% (argon) and 13.3% (epoxy chloropropane) in tensile strength were achieved. Moreover, interfacial shear strength of a single fiber in matrix increased by 19.15% (argon) and 75.03% (epoxy chloropropane). Therefore, this spatially resolved study paved a meaningful way to understand the relationship among microstructures, surface activities, and mechanical properties of γ-irradiated carbon fibers.

Enhancing Dielectric Properties of Poly(vinylidene fluoride)-Based Hybrid Nanocomposites by Synergic Employment of Hydroxylated BaTiO3 and Silanized Graphene

ABSTRACT The poly(vinylidene fluoride)-based hybrid composites with a blended BaTiO3–exfoliated graphene as filler were fabricated with the efficient method of solution casting and hot pressing and their dielectric properties were investigated in detail. In advance, both of the nanoparticles are functionalized to improve the interfacial bonding between fillers and matrix. Synergistic effects arising from blending zero-dimensional BaTiO3 and two-dimensional exfoliated grapheneare identified by the improved distribution of nanoparticles in poly(vinylidene fluoride) host and mitigating the concomitant increase in dielectric loss while maintaining the high dielectric constant values of the composites. GRAPHICAL ABSTRACT