Wenge Zheng

Tough Graphene−Polymer Microcellular Foams for Electromagnetic Interference Shielding

Functional polymethylmethacrylate (PMMA)/graphene nanocomposite microcellular foams were prepared by blending of PMMA with graphene sheets followed by foaming with subcritical CO(2) as an environmentally benign foaming agent. The addition of graphene sheets endows the insulating PMMA foams with high electrical conductivity and improved electromagnetic interference (EMI) shielding efficiency with microwave absorption as the dominant EMI shielding mechanism. Interestingly, because of the presence of the numerous microcellular cells, the graphene-PMMA foam exhibits greatly improved ductility and tensile toughness compared to its bulk counterpart. This work provides a promising methodology to fabricate tough and lightweight graphene-PMMA nanocomposite microcellular foams with superior electrical and EMI shielding properties by simultaneously combining the functionality and reinforcement of the graphene sheets and the toughening effect of the microcellular cells.

Lightweight, Multifunctional Polyetherimide/Graphene@Fe3O4 Composite Foams for Shielding of Electromagnetic Pollution

Novel high-performance polyetherimide (PEI)/graphene@Fe3O4 (G@Fe3O4) composite foams with flexible character and low density of about 0.28-0.4 g/cm(3) have been developed by using a phase separation method. The obtained PEI/G@Fe3O4 foam with G@Fe3O4 loading of 10 wt % exhibited excellent specific EMI shielding effectiveness (EMI SE) of ~41.5 dB/(g/cm(3)) at 8-12 GHz. Moreover, most the applied microwave was verified to be absorbed rather than being reflected back, resulting from the improved impedance matching, electromagnetic wave attenuation, as well as multiple reflections. Meanwhile, the resulting foams also possessed a superparamagnetic behavior and low thermal conductiviy of 0.042-0.071 W/(m K). This technique is fast, highly reproducible, and scalable, which may facilitate the commercialization of such composite foams and generalize the use of them as EMI shielding materials in the fields of spacecraft and aircraft.

Vacuum-assisted synthesis of graphene from thermal exfoliation and reduction of graphite oxide

We report a vacuum-assisted method for thermal exfoliation and in situreduction of graphite oxide in large quantity at a temperature as low as 135 °C. The resulting graphene sheets contain only few-layered sheets with an average thickness of 0.9 nm, and their specific surface area (758 m2 g−1) is comparable to that of conventional graphene generated at 1050 °C at atmospheric pressure (700 m2 g−1). The in situ thermal reduction during the exfoliation process was confirmed by the increased C/O atomic ratio compared to that of graphite oxide. The restoration of the graphitic sp2 network makes it highly efficient in improving the electrical conductivity of polymers at a low graphene loading.

Melt blending in situ enhances the interaction between polystyrene and graphene through π-π stacking.

The effect of melt blending on the interaction between graphene and polystyrene (PS) matrix has been investigated in this paper. The interaction between graphene and PS was significantly enhanced by melt blending, which led to an increased amount of PS-functional graphene (PSFG) exhibiting good solubility in some solvents. The PS chains on PSFG could effectively prevent the graphene sheets from aggregating and the prepared PS/PSFG composites exhibited a homogeneous dispersion and an improved electrical property. The mechanism of melt blending on this enhanced interaction was attributed to the formation of π-π stacking during the melt blending. Moreover, the formation of chemical bonding during melt blending may have also enhanced the interaction.

Synthesis of graphene by low-temperature exfoliation and reduction of graphite oxide under ambient atmosphere

We firstly report a facile approach to produce few-layered graphene sheets by low-temperature (130 °C) exfoliation and reduction of graphite oxide under ambient atmosphere with the aid of HCl. The obtained graphene materials exhibited high BET specific surface area (∼500 m2 g−1) and excellent electrical conductivity (∼1200 S m−1).

Compressible Graphene-Coated Polymer Foams with Ultralow Density for Adjustable Electromagnetic Interference (EMI) Shielding

The fabrication of low-density and compressible polymer/graphene composite (PGC) foams for adjustable electromagnetic interference (EMI) shielding remains a daunting challenge. Herein, ultralightweight and compressible PGC foams have been developed by simple solution dip-coating of graphene on commercial polyurethane (PU) sponges with highly porous network structure. The resultant PU/graphene (PUG) foams had a density as low as ∼0.027-0.030 g/cm(3) and possessed good comprehensive EMI shielding performance together with an absorption-dominant mechanism, possibly due to both conductive dissipation and multiple reflections and scattering of EM waves by the inside 3D conductive graphene network. Moreover, by taking advantage of their remarkable compressibility, the shielding performance of the PUG foams could be simply adjusted through a simple mechanical compression, showing promise for adjustable EMI shielding. We believe that the strategy for fabricating PGC foams through a simple dip-coating method could potentially promote the large-scale production of lightweight foam materials for EMI shielding.

Polyimide/graphene composite foam sheets with ultrahigh thermostability for electromagnetic interference shielding

Herein Kapton-type aromatic polyimide (PI) composite foams with reduced graphene oxide (rGO) content ranging from 1 to 16 wt% have been fabricated via a three-step method: in situ polymerization, nonsolvent induced phase separation and thermal imidization, and used for electromagnetic interference shielding. The resultant PI/16 wt% rGO foam with low density of 0.28 g cm−3 and thickness of 0.8 mm exhibited an effective EMI shielding effectiveness of 17–21 dB in X band (8–12 GHz). Additionally, the thermostability of the foams was also significantly enhanced, for the 5% weight loss temperature it was improved from 508 °C for the pure PI foam to 520 °C for the PI/1 wt% rGO foam, and consequently, to 581 °C for the PI/16 wt% rGO foam. Even with the high rGO content (16 wt%), the composite foam was fairly flexible. Tensile testing revealed that the PI/16 wt% rGO foam possessed a tensile strength of 11.4 MPa and an elongation at break of 9.6%, respectively.

Ultrasonication-assisted direct functionalization of graphene with macromolecules

Poly(vinyl alcohol) (PVA) functionalized graphene (f-G) was prepared by ultrasonication of pristine graphene (p-G) in a PVA aqueous solution. PVA macroradicals formed by sonochemical degradation of the PVA solution were successfully trapped by graphene and grafted onto its surface. This was confirmed by transmission electron microscopy, atomic-force microscopy and 1H NMR measurements. The content of PVA on graphene was estimated to be ∼35%. The f-G could be well dispersed in the PVA matrix by a simple solution mixing and casting procedure. Due to the effective load transfer between f-G and PVA matrix, the mechanical properties of the f-G/PVA films were significantly improved. Compared with the p-G/PVA films, a 12.6% increase in tensile strength and a 15.6% improvement of Youngs modulus were achieved by addition of only 0.3 wt% f-G. Moreover, our simple ultrasonication technique could enable us to functionalize graphene with other polymers.

How a bio-based epoxy monomer enhanced the properties of diglycidyl ether of bisphenol A (DGEBA)/graphene composites

A bio-based epoxy monomer (GA-II) was synthesized from renewable gallic acid. The aromatic group contained made it capable of being absorbed onto the surface of graphene via strong π–π interactions, which was proven by Raman spectra and UV spectra. The GA-II anchored graphene was easily homogeneously dispersed in the epoxy resin. After solidification, the graphene/epoxy composites demonstrated superior performances in terms of good mechanical properties, excellent thermal conductivity, as well as high electrical conductivity. With the addition of only 2 wt% GA-II/graphene, the tensile strength, tensile modulus, flexural strength and flexural modulus of the composites were improved by 27%, 47%, 9% and 21%, respectively. The thermal and electrical conductivities were also improved by 12-fold (from 0.15 to 1.8 W m−1 K−1) and 8 orders (from 7.0 × 10−15 to 3.28 × 10−5 s cm−1), respectively. This work provided us with an environmentally friendly agent with high efficiency for graphene dispersion and demonstrated an efficient method for fabricating epoxy/graphene composites with superior properties.

The Effects of Exfoliated Nano-clay on the Extrusion Microcellular Foaming of Amorphous and Crystalline Nylon

This article demonstrates the effect of exfoliated nano-clay on the microcellular foam processing of amorphous and crystalline nylon. Amorphous and crystalline nylon 6 nanocomposites are prepared using a twin-screw extruder. The exfoliated nanocomposite structures are characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Nylon and its nanocomposites are foamed in extrusion using supercritical CO2. Subsequently, the cell morphologies of nylon and its nanocomposite foams are investigated. It appeared that the nano-clay not only enhanced cell nucleation, but also suppressed cell deterioration during the microcellular foaming of nylon.