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Due to their unique anisotropic morphology and properties, graphene-based materials have received extensive attention in the field of smart materials. Recent studies show that graphene-based materials have potential application as a dispersed phase to develop high-performance electrorheological (ER) fluids, a kind of smart suspension whose viscosity and viscoelastic properties can be adjusted by external electric fields. However, pure graphene is not suitable for use as the dispersed phase of ER fluids due to the electric short circuit caused by its high electrical conductivity under electric fields. However, graphene oxide (GO) and graphene-based composites are suitable for use as the dispersed phase of ER fluids and show significantly enhanced property. In this review, we look critically at the latest developments of ER fluids based on GO and graphene-based composites, including their preparation, electrically tunable ER property, and dispersed stability. The mechanism behind enhanced ER property is discussed according to dielectric spectrum analysis. Finally, we also propose the remaining challenges and possible developments for the future outlook in this field. [ABSTRACT FROM AUTHOR]

Abstract: A facile thermal oxidation method in air is suggested to treat chemically synthesized conducting polyaniline (PANI) nanotubes in order controllably to decrease the conductivity. The structure, morphology and conductivity were characterized, and the polarization and electrorheological (ER) properties of nanotubes when dispersed in insulating oil were measured as a function of the thermal-oxidative time. Results showed that the appropriate thermal oxidation treatment did not destroy the nanotube morphology but induced the deprotonation and oxidation of PANI backbone. As a result, the conductivity was decreased and the polarization properties were changed. The nanotubes obtained after thermal oxidation at 200 °C for 4 h demonstrated the optimal ER performances. The ER activity decreased as the thermal oxidation time increased, and this could be explained by the change of polarization properties. [Copyright &y& Elsevier]

Abstract: Using a chemical oxidative polymerization, polypyrrole (PPy) nanofibers were synthesized. After further thermo-oxidative treatment in air, the conductivity of PPy nanofibers was adjusted to a suitable level for use as a non-conventional nanofiber-based electrorheological (ER) suspension. Under electric fields, rheological properties of thermo-oxidative PPy nanofiber suspension were characterized. It showed that the nanofiber suspension possessed notable ER effect and low current density. Especially, the yield stress and shear modulus of nanofiber suspension were stronger than that of conventional granular suspension at the same volume fraction though the off-field viscosity of former was lower than that of latter. The ER effect and current density of thermo-oxidative PPy nanofiber suspension depended on the thermo-oxidative time and the nanofibers obtained after treatment for 3–5h at 240°C exhibited the optimal ER performances. It also showed that the thermo-oxidative PPy nanofiber suspension could maintain good ER properties within a wide operating temperature range of 25–115°C. [Copyright &y& Elsevier]

Abstract: Nitrogen-enriched carbonaceous nanotubes (N-CTs) were prepared by the heat treatment of conducting polyaniline (PANI) nanotubes and then were used as new carbonaceous electrorheological (ER) fluids. Characterization showed that the nanotubular morphology of the original PANI was preserved after heat treatment, whereas the chemical structure and conductivity were changed significantly depending on the heat treatment temperatures. Under electric fields, the rheological properties of the N-CT suspensions prepared by the ultrasonic dispersion of the N-CTs in silicone oil were measured. This showed that the N-CT suspensions possessed versatile ER performance including high ER efficiency, good dispersion stability, and temperature stability. Especially, compared to the corresponding heat treated granular PANI suspensions, the N-CT suspensions showed better dispersion stability and higher ER effect. Furthermore, the ER effect of N-CT suspensions could be adjusted by varying heat treatment temperatures and the N-CTs obtained at around 600°C exhibited the maximum ER effect. This could be explained by the polarization response, which originated from the regular change of conductivity of N-CTs as a function of heat treatment temperatures. [Copyright &y& Elsevier]

Abstract: Using a modified oxidative polymerization, the nano-fibrous polyaniline with 200nm diameter and several micrometer lengths was synthesized on a large scale and then was applied as a new electrorheological (ER) fluid. Compared to conventional granular polyaniline ER fluid, the nano-fibrous polyaniline ER fluid exhibited distinctly improved suspended stability. Under electric fields, the nano-fibrous PANI ER fluid also exhibited larger ER effect. Its shear stresses are about 1.2–1.5 times as high as those of the granular PANI ER fluid. At the same time, the shear stress of nano-fibrous PANI ER fluid could maintain a stable level and even an increase for the wide shear rate regions from 0.1s−1 to 1000s−1 under various electric fields. In addition, the dynamic experiment showed that the shear modulus of nano-fibrous polyaniline ER fluid under electric field was higher than that of the granular polyaniline fluid, which also confirmed the larger ER effect. [Copyright &y& Elsevier]

By using different conductivity of polyaniline as filler, a kind of poly(ionic liquid)/polyaniline composite particles was synthesized to investigate the influence of dielectric polarization rate difference between filler and matrix on the electrorheological response and flow stability of composite-based electrorheological fluids under simultaneous effect of shear and electric fields. The composite particles were prepared by a post ion-exchange procedure and then treated by ammonia or hydrazine to obtain different conductivity of polyaniline. Their electrorheological response was measured by dispersing these composite particles in insulating carrier liquid under electric fields. It showed that the composite particles treated by ammonia had the strongest electrorheological response and most stable flow behavior in a broad shear rate region from 0.5 s−1 to 1000 s−1. By using dielectric spectroscopy, it found that the enhanced electrorheological response with stable flow depended on the matching degree of the dielectric polarization rates between poly(ionic liquid) matrix and polyaniline filler. The closer their polarization rates are, the more stable the flow curves are. These results are helpful to design optimal composite-based electrorheological materials with enhanced and stable ER performance. [ABSTRACT FROM AUTHOR]

Different from classic polyelectrolytes, poly(ionic liquid)s (PILs) bearing hydrophobic counterions show high electrorheological (ER) effect in dry state. This opens a way to develop new generation of anhydrous polyelectrolyte ER fluids. However, PILs with linear backbone are limited in practical applications due to low glass transition temperature. Crosslinking is an effective method to overcome this problem but the structure-property relationship in crosslinked PIL ER system is not fully understood. Herein, a family of self-crosslinked poly(bis-vinylimidazolium bis(trifluoromethane sulfonylimide))-based PILs with different length of alkyl spacer (P[C n DVIM][TFSI]) were synthesized. The influence of alkyl spacer length on ER effect was studied and the influence mechanism was analyzed by combination of rheology, dielectric spectroscopy and X-ray scattering measurements. It shows that as alkyl spacer length increases, the ER effect increases but the working temperature range becomes narrow. This is related to the fact that the length variation of alkyl spacer alters the mesh size of crosslinking network of P[C n DVIM][TFSI], which further influences the transport dynamic of mobile counterions and ion motion-induced interfacial polarization. Self-crosslinked poly(ionic liquid) particles with different length of alkyl spacer are synthesized, which show increased ER effect but narrowed working temperature range with increasing alkyl spacer length because increasing the mesh size of crosslinking network decreases the potential energy barrier for ion transport and ion motion-induced interfacial polarization but easily causes current leakage at high temperature. Image 1 • Self-crosslinked PILs with different length of alkyl spacer are synthesized. • As spacer length increases, ER effect of PILs increases but working temperature range becomes narrow. • Spacer length influence is because mesh size of crosslinking network influences ion transport and interfacial polarization. [ABSTRACT FROM AUTHOR]

A series of random co-poly(ionic liquid)s (co-PILs) neutralized with mixed counterions of organic bis(trifluoromethane sulfonyl)imide ([TFSI]) and inorganic tetrafluoroborate ([BF 4 ]) of different mole ratio had been synthesized for use as electrorheological (ER) particles. The temperature-modulated ER effect of the co-PIL particles had been investigated and compared with that of homo-poly(ionic liquid)s neutralized with single organic [TFSI] counteranions. This work demonstrated that the ER effectiveness and temperature effect of electrorheological fluid based on neat organic counteranions-typed PILs can be significantly improved by a simple substitution of inorganic counteranions with a suitable mole ratio. The higher electrorheological effectiveness with suppressed leakage current density and enhanced temperature stability originated from that the substitution of inorganic counteranions for organic counteranions could systematically elevate the glass transition temperature and modulate the activation energy of ion movement in PILs matrix. Linear polymeric ionic liquids (PILs) neutralized with mixed counteranions of organic ([TFSI]) and inorganic ([BF 4 ]) could exhibit higher ER effectiveness and broader working temperatures range than the analogous PILs neutralized with single organic counteranions. [Display omitted] • PILs containing organic and inorganic counterions of various mole ratio was prepared for use as electrorheological particles. • Partial replacement of [TFSI] by [BF 4 ] could improve the electrorheological effectiveness and thermal stability of PILs. • Partial replacement of [TFSI] by [BF 4 ] could modulate the glass temperature and ionic shifting activation energy of PILs. [ABSTRACT FROM AUTHOR]

Electrorheological (ER) fluids are a kind of smart materials whose viscosity can be adjusted by electric fields, which have wide application in the field of electrical-mechanical conversion. However, the practical application of ER fluids is still limited by insufficient performance. Polyelectrolyte-based ER materials have relatively high ER effect due to their large polarizability, good dispersion stability due to their low density, and small wear due to their low hardness, which are frequently studied as one of the most promising ER materials. This review introduces the research progress of ER materials, especially polyelectrolyte-based ER materials. The synthesis methods and ER properties of three types of polyelectrolyte ER materials are introduced, including traditional polyelectrolytes, polyether polymer electrolytes and recently developed poly(ionic liquid) (PIL) polyelectrolytes. In particular, the advantages of poly(ionic liquid)s in the development of high-performance anhydrous polyelectrolyte ER materials are highlighted, and the relationship between the intrinsic structure and ER property of PIL ER materials is discussed in detail in order to guide the material design. Finally, we propose the remaining challenges and possible developments in this field for the future outlook. This review introduces the research progress of electrorheological (ER) materials, especially polyelectrolyte-based ER materials including traditional polyelectrolytes, polyether polymer electrolytes and recently developed poly(ionic liquid) polyelectrolytes. In particular, the advantages of poly(ionic liquid)s in the development of high-performance anhydrous polyelectrolyte ER materials and the relationship between the intrinsic structure and ER property of PIL ER materials are highlighted. [Display omitted] [ABSTRACT FROM AUTHOR]