Wentao Zhai

Facile Preparation of Lightweight Microcellular Polyetherimide/Graphene Composite Foams for Electromagnetic Interference Shielding

We report a facile approach to produce lightweight microcellular polyetherimide (PEI)/graphene nanocomposite foams with a density of about 0.3 g/cm3 by a phase separation process. It was observed that the strong extensional flow generated during cell growth induced the enrichment and orientation of graphene on cell walls. This action decreased the electrical conductivity percolation from 0.21 vol % for PEI/graphene nanocomposite to 0.18 vol % for PEI/graphene foam. Furthermore, the foaming process significantly increased the specific electromagnetic interference (EMI) shielding effectiveness from 17 to 44 dB/(g/cm3). In addition, PEI/graphene nanocomposite foams possessed low thermal conductivity of 0.065-0.037 W/m·K even at 200 °C and high Youngs modulus of 180-290 MPa.

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.

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.

A Study of the Crystallization, Melting, and Foaming Behaviors of Polylactic Acid in Compressed CO2

The crystallization and melting behaviors of linear polylactic acid (PLA) treated by compressed CO2 was investigated. The isothermal crystallization test indicated that while PLA exhibited very low crystallization kinetics under atmospheric pressure, CO2 exposure significantly increased PLA’s crystallization rate; a high crystallinity of 16.5% was achieved after CO2 treatment for only 1 min at 100 °C and 6.89 MPa. One melting peak could be found in the DSC curve, and this exhibited a slight dependency on treatment times, temperatures, and pressures. PLA samples tended to foam during the gas release process, and a foaming window as a function of time and temperature was established. Based on the foaming window, crystallinity, and cell morphology, it was found that foaming clearly reduced the needed time for PLA’s crystallization equilibrium.

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.

Enhanced thermoelectric property by the construction of a nanocomposite 3D interconnected architecture consisting of graphene nanolayers sandwiched by polypyrrole nanowires

A new strategy, i.e. interfacial adsorption-soft template polymerization, is developed to enhance polymer thermoelectric property. The obtained nanocomposite 3D interconnected architecture consisting of reduced graphene oxide (rGO) nanolayers sandwiched by polypyrrole (PPy) nanowires is directly confirmed by scanning and transmission electron microscopies. Moreover, the nanocomposites reveal significantly enhanced thermoelectric performance. At rGO:PPy ratio of 50 wt%, the nanocomposite power factor reaches ∼476.1 times that of pure PPy nanowires. Our results suggest that a greatly enhanced thermoelectric property for polymer nanocomposites can be achieved by a complex morphology design.

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.

Morphology and thermoelectric properties of graphene nanosheets enwrapped with polypyrrole

With the help of sodium dodecyl sulfate (SDS), uniform polypyrrole (PPy) coatings were conveniently grown on both sides of reduced graphene oxide (rGO) nanosheet surfaces via a template-directed in situ polymerization. The rGO/PPy composites exhibited greatly enhanced thermoelectric performance with a power factor at room temperature of up to 3.01 μW m−1 K−2, which is 84 times greater than that of the pure PPy.