Wen Ling Zhang

Graphene oxide coated core–shell structured polystyrene microspheres and their electrorheological characteristics under applied electric field

Core–shell structured polystyrene (PS)–graphene oxide (GO) microspherical particles were synthesized by adsorbing the GO sheets on the PS surface through a strong π–π stacking interaction. As core materials, monodispersed PS microspheres were prepared using a dispersion polymerization, while the shell part of GO was synthesized by a modified Hummers method. Morphology of the composite particles was studied by both scanning electron microscopy and transmission electron microscopy, while their structure and chemical components were examined viaX-ray diffraction and Fourier-transform infrared spectroscopy, respectively. All the data confirmed the coexistence of PS and GO with the expected core–shell structure of the composite. In addition, for the study on the electroresponsive behavior, the composite was dispersed in silicone oil and its electrorheological (ER) characteristics were examined via both an optical microscope and a rotational rheometer which was equipped with a high voltage source. Without an electric field, it behaved like a fluid, however, when an external electric field is present, the particles became polarized and demonstrated typical chain-like ER structures.

Silica-Graphene Oxide Hybrid Composite Particles and Their Electroresponsive Characteristics

Silica-graphene oxide (Si-GO) hybrid composite particles were prepared by the hydrolysis of tetraethyl orthosilicate (TEOS) in the presence of hydrophilic GO obtained from a modified Hummers method. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images provided visible evidence of the silica nanoparticles grafted on the surface of GO, resulting in Si-GO hybrid composite particles. Energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) spectra indicated the coexistence of silica and GO in the composite particles. The Si-GO hybrid composite particles showed better thermal stability than that of GO according to thermogravimetric analysis (TGA). The electrorheological (ER) characteristics of the Si-GO hybrid composite based ER fluid were examined further by optical microscopy and a rotational rheometer in controlled shear rate mode under various electric field strengths. Shear stress curves were fitted using both conventional Bingham model and a constitutive Cho-Choi-Jhon model. The polarizability and relaxation time of the ER fluid from dielectric spectra measured using an LCR meter showed a good correlation with its ER characteristics.

Electrorheology of Graphene Oxide

Novel polarizable graphene oxide (GO) particles with oxidized groups on their edge and basal planes were prepared by a modified Hummers method, and their electro-responsive electrorheological (ER) characteristics when dispersed in silicone oil were examined with and without an electric field applied. The fibrillation phenomenon of this GO-based electro-responsive fluid was also observed via an optical microscope under an applied electric field. Both flow curves and dielectric spectra of the ER fluid were measured using a rotational rheometer and a LCR meter, respectively. Its viscoelastic properties of both storage and loss moduli were also examined using a vertical oscillation rheometer equipped with a high voltage generator, finding that the GO-based smart ER system behaves as a viscoelastic material under an applied electric field.

Pickering emulsion-fabricated polystyrene–graphene oxide microspheres and their electrorheology

Graphene oxide (GO) was used as a solid surfactant, and core–shell structured polystyrene (PS)–GO microspheres were fabricated using the Pickering emulsion polymerization method. The resulting electro-responsive ‘smart’ microspheres were dispersed in silicone oil and their electrorheological characteristics were measured using a rotational rheometer under an applied electric field and correlated with their dielectric properties. The GO was found to impart electro-responsive properties to the polymer composite suspension under an applied electric field.

Facile and fast synthesis of polyaniline-coated poly(glycidyl methacrylate) core-shell microspheres and their electro-responsive characteristics.

Electro-responsive core-shell structured particles were fabricated in two steps. In the first step, a spherical and monodisperse poly(glycidyl methacrylate) (PGMA) core was prepared by dispersion polymerization with an epoxy group, which was then functionalized with an amine functional group (ami-PGMA) via an epoxide-amine reaction with ethylenediamine. In the second step, a conducting polyaniline (PANI) shell was grafted onto the ami-PGMA surface via the in situ polymerization of an aniline monomer with a uniform thickness. The epoxy group on the PGMA microspheres provided a simple and fast way to react with amine functional groups without the need for a further swelling or grafting process. The morphology of the core-shell structure was confirmed by scanning election microscopy and transmission electron microscopy. The electrorheological properties of the PGMA/PANI particles-based suspension were examined using a Couette-type rotational rheometer under an applied electric field. The shear stress curves were fitted to the Cho-Choi-Jhon (CCJ) model of the rheological equation of state.

Facile fabrication of self-assembled PMMA/graphene oxide composite particles and their electroresponsive properties

Core–shell-structured poly(methyl methacrylate) (PMMA)/graphene oxide (GO) composite particles were prepared using a facile process, in which GO was adsorbed spontaneously onto a microspherical PMMA surface when hydrophobic microspheres were dispersed in deionized (DI) water stabilized by amphiphilic GO under ultrasonication. The fabricated composite was characterized by SEM, TEM, FT-IR, and thermogravimetric analysis. Results showed that the particle surface could be wrapped with GO without the need for surfactants. In addition, electrorheological behavior of the chain-forming process of the PMMA/GO composite particles was observed by optical microscopy under an applied electric field. Both shear stress and shear viscosity related to the strength of the applied electric field were measured using a rotational rheometer. The proposed Cho–Choi–Jhon model was used to describe their ER performances for the entire shear rate region. Moreover, the response of the shear stress to an imposed square voltage at a fixed shear rate was also examined.

Core–shell structured graphene oxide-adsorbed anisotropic poly(methyl methacrylate) microparticles and their electrorheology

This paper reports the facile preparation of graphene oxide (GO)-adsorbed snowman-like anisotropic poly(methyl methacrylate) (SPMMA) microspheres along with their electro-responsive electrorheological (ER) performance when dispersed in silicone oil. GO was prepared using a modified Hummers method, whereas monodispersed SPMMA particles were fabricated using a seed emulsion polymerization procedure. GO was adsorbed on the surface of the SPMMA particles with the aid of a cationic surfactant cetyl trimethylammonium bromide through a negative–positive electrostatic attraction. The surface morphology, chemical components, thermal stability and electrical conductivity of the obtained core–shell structured GO–SPMMA particles were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis and resistivity meter, respectively. The ER characteristics of the obtained particles were examined using an optical microscope and a rotational rheometer. This novel ER fluid exhibited typical ER properties of solid-like behavior under an applied electric field.

Fabrication of semiconducting polyaniline-wrapped halloysite nanotube composite and its electrorheology

An organic/inorganic polyaniline-wrapped halloysite nanotube (PANI/HNT) composite was prepared by the in situ polymerization of aniline in the presence of a HNT dispersion. The physical properties of the resulting PANI/HNT composite were characterized by scanning electron microscopy, thermogravimetric analysis, and Fourier-transform infrared spectroscopy. This organic/inorganic composite particle was then dispersed in silicone oil as an electrorheological (ER) fluid, and its ER properties were examined using a Couette-type rotational rheometer equipped with a high-voltage generator. This novel halloysite composite-based ER fluid exhibited typical ER properties under an applied electric field.

Pickering emulsion polymerization of core–shell-structured polyaniline@SiO2 nanoparticles and their electrorheological response

We synthesized semiconducting polyaniline (PANI) nanoparticles through a solid-stabilized Pickering emulsion route using silica nanoparticles. Specific morphologies of the silica nanoparticle wrapped PANI particles were observed using both scanning electron microscope and transmission electron microscope, which showed the emulsifiability of silica nanoparticles in the emulsion system. Electrorheological (ER) behavior of this novel particle-based ER fluid dispersed in silicone oil was measured by a controlled shear stress rheometer and analyzed with a flow curve equation of Cho-Choi-Jhon model, which fitted well the flow curves measured in the exposed electric field.

Electrorheological activity generation by graphene oxide coating on low-dielectric silica particles

A recent challenge in the field of electrorheology is to generate or to enhance the electrorheological (ER) activity of an inactive or lowly active suspension using core–shell structured particles. Here we illustrate the application of graphene oxide (GO) adsorbed onto the silica particles to make the core–shell structure. The suspension exhibited significant changes in shear viscosity upon the application of an external electric field. The suspension of the core–shell particles also exhibited an occurrence of ER activity enhancement more than an order of magnitude, compared to that of pure silica suspension. The yield stress of this system scales as τy ∝ E1.5. The dielectric relaxation study revealed that the ER activity was due to the fast occurrence of polarization in the GO–Si suspension. The suspension showed good sensitivity and stability under the electric field. These findings open up the possibilities for design of smart suspensions with high ER responses.