Chengen He

Non-covalently modified graphene sheets by imidazolium ionic liquids for multifunctional polymer nanocomposites

Chemical reduction of graphite oxide (GO) to produce graphene nanosheets often results in irreversible agglomeration and precipitation. Herein, stable well-dispersed graphene sheets in solvents were obtained by simultaneous functionalization and reduction of GO under alkaline conditions, in the presence of sodium borohydride and imidazolium ionic liquids (Imi-ILs) containing two vinyl-benzyl groups. In this case, positively charged imidazolium groups of Imi-ILs underwent ion-exchange with negatively charged GO sheets and were linked to their edges, while Imi-ILs were non-covalently attached onto the large surfaces of graphene through π–π and/or cation–π stacking interactions. The vinyl-benzyl reactive sites were then copolymerized in situ with methyl methacrylate to fabricate graphene/poly(methyl methacrylate) (PMMA) composites. Functionalized graphene sheets were uniformly dispersed in the PMMA matrix and contributed to large increases in storage modulus (+58.3%) and glass transition temperature (+19.2 °C) at 2.08 vol.% loading. High electrical conductivity was also achieved at graphene loading levels beyond 1 vol.% (ca. 2.55 Sm−1) with a low percolation threshold (0.25 vol.%) for the composites. Hence, a general methodology which facilitates the development of a multifunctional advanced material has been successfully established. This can be extended to other vinyl polymer-based composites containing graphene.

Graphene-Enabled Superior and Tunable Photomechanical Actuation in Liquid Crystalline Elastomer Nanocomposites.

Programmable photoactuation enabled by graphene: Graphene sheets aligned in liquid crystalline elastomers are capable of absorbing near-infrared light. They thereafter act as nanoheaters and provide thermally conductive pathways to trigger the nematic-to-isotropic transition of elastomers, leading to macroscopic mechanical deformation of nanocomposites. Large strain, high actuation force, high initial sensitivity, fast reversible response, and long cyclability are concurrently achieved in nanocomposites.

Judicious selection of bifunctional molecules to chemically modify graphene for improving nanomechanical and thermal properties of polymer composites

Covalently-functionalised graphene (FG) was successfully obtained by grafting m-isopropenyl-α, α′-dimethyl benzyl isocyanate (m-TMI) to graphene oxide (GO) followed by the chemical and solvothermal reduction of GO. The FG sheets were hydrophobic and stable in polar solvents such as N,N-dimethylformamide. The reactive vinyl-benzyl groups of m-TMI attached to FG copolymerised with methyl methacrylate to produce graphene/poly(methyl methacrylate) (PMMA) composites. The FG sheets were well dispersed in PMMA and formed strong interfacial bonds with the matrix, contributing to large increases in elastic modulus (+72.9%) and indentation hardness (+51.2%) at 1% loading by weight. The incorporation of FG into PMMA changed its elastic-plastic behaviour; hence, a decrease in the plasticity index and an increase in recovery resistance were observed for the resulting composites due to the increased portion related to the elastic work. The onset decomposition temperature and glass transition temperature of neat PMMA increased by 100 °C and 12.7 °C, respectively, by the addition of 1 wt% FG. Herein, in situ copolymerisation of monomers and well-suspended FG promotes the exfoliation of graphene associated with strong chemical bonding with the polymer matrix. This report provides a promising and facile method for fabricating high-performance polymeric composites.

Simultaneous improvement in the flame resistance and thermal conductivity of epoxy/Al2O3 composites by incorporating polymeric flame retardant-functionalized graphene

Fire hazards related to polymer-based thermally conductive composites (PTCs) used in electronic equipment are a significant, but often neglected, risk. Here, we offer a solution by incorporating flame retardant-functionalized graphene (PFR-fRGO) into PTCs using a procedure that improves both their flame resistance and thermal conductivity. Briefly, PFR-fRGO was prepared by covalently grafting a polyphosphoramide oligomer (PDMPD) onto the surface of graphene, which was then introduced in situ into epoxy resin/Al2O3 (EP/Al2O3) composites. As expected, the incorporation of PFR-fRGO not only increased the thermal conduction paths by weakening the settlement of microparticles, but also reduced the interfacial thermal resistance by enhancing interfacial interactions, both of which resulted in an enhancement of the thermal conductivity of the ternary composites. The resultant EP/Al2O3/PFR-fRGO composite exhibited a superior flame retarding ability with dramatic decreases being seen in the high peak heat release rate (PHRR), the total heat release (THR) and the total smoke production (TSP), i.e. 53%, 37% and 57%, respectively, when compared to pure epoxy resin. Additionally, a synergistic flame retarding effect was found in the ternary composite compared to the EP/PFR-fRGO and EP/Al2O3 composites. The remarkable enhancement in flame retardancy was mainly attributed to the catalytic charring effect of PFR-fRGO and the template effect of Al2O3, both of which resulted in the formation of a high strength, thermally stable protective layer in the condensed phase that is able to retard the permeation of heat and volatile degradation products during combustion, slow down the heat release rate and protect the underlying polymer.

Simultaneous polymerization enabled the facile fabrication of S-doped carbons with tunable mesoporosity for high-capacitance supercapacitors

A cationic polymerization of 2-thiophenemethanol (ThM) and a sol–gel polycondensation of tetraethylorthosilicate (TEOS) were simultaneously catalyzed by trifluoroacetic acid in a single process step to produce poly(2-thiophenemethanol)/silica (PThM/SiO2) composites. S-Doped mesoporous carbon (S-MC) materials were then achieved by high-temperature carbonization of PThM/SiO2 under an inert atmosphere and subsequent etching off SiO2 in hydrofluoric acid. This in situ crafting process allows us to tailor the porosity of S-MC in the range of 6 to 30 nm. The specific surface area (278–650 m2 g−1) and pore volume (0.15–0.67 cm3 g−1) increase with increasing the feed ratio of TEOS to ThM. Both the specific surface area and pore volume of S-MC are also higher than those of the un-doped mesoporous carbon (MC) materials using furfuryl alcohol as the starting monomer. The S-MC electrodes thus show larger specific capacitance (Cs) values (252 F g−1 at 25 mV s−1 and 125 F g−1 at 0.5 A g−1) compared to the un-doped MC electrode (203 F g−1 at 25 mV s−1 and 110 F g−1 at 0.5 A g−1). The retention of initial Cs for S-MC is 66%, higher than 53% for MC after a 20-fold increase in the scan rate. After 1000 charge/discharge cycles, the Cs retention for S-MC is 97%, also higher than that of MC (93%). As expected, the S-MC electrodes exhibit larger Cs, higher rate performance, and better cycling stability, compared to the MC counterparts and those fabricated in the absence of TEOS by identical experimental processes. Excellent performance can be contributed to the mesoporous morphology in combination with active doping of rich S heteroatoms.

Nanostructures: Graphene‐Enabled Superior and Tunable Photomechanical Actuation in Liquid Crystalline Elastomer Nanocomposites (Adv. Mater. 41/2015)

Actuated materials undergo shape and dimension changes in response to external stimuli and thus convert various forms of input energies into mechanical work. On page 6376, Y. Yang, Z. Lin, and co-workers report novel near-infrared responsive composites consisting of uniformly aligned graphene in a liquid-crystalline elastomer matrix. Large deformation amplitude, high driving force, fast reversible response, and long cyclability can be achieved by tuning the characteristics of graphene in these nanocomposites. Such materials have potential applications in robotics, artificial muscles, switches, motors, sensors, and micromechanical systems.

Solid polymer electrolyte based on ionic bond or covalent bond functionalized silica nanoparticles

Although various types of nanoparticle have been ubiquitously employed as additives to promote the practical performances of composite polymer electrolytes (CPEs) in lithium-ion batteries, the influence of the type of chemical bond between the core and canopy of the modified nanoparticle on the properties of CPEs has rarely been investigated. Herein, two types of nanoparticle additive, namely, ionic bond modified nanoparticles (IBNs) and covalent bond modified nanoparticles (CBNs), were prepared conveniently based on nanosilica with different particle sizes in order to optimize the overall performance of the electrolyte. Furthermore, the CPEs were fabricated by doping IBNs or CBNs as well as lithium salts within a poly(ethylene oxide) matrix and their electrochemical properties were investigated. The dramatic enhancement of the ionic conductivity of the CPEs resulted from the addition of fillers into the system, and the improvement became more significant when the fillers were IBNs that used the smaller silica nanoparticle as the core segment, due to the increased chain mobility, as estimated by the smaller Tg value. Moreover, a broad electrochemical stability window was obtained in the presence of IBNs, and the lithium-ion transference number of the system containing lithium bis(trifluoromethanesulfonimide), which has large anions in the structure, was almost two times higher than the CPEs using lithium perchlorate as the lithium source. Therefore, the synergistic effects of the filler structures and the electrolyte compositions are the key factors to improve the electrochemical performances of CPEs.

Carbon-Based Polyaniline Nanocomposites for Supercapacitors

Abstract Polyaniline (PANI) has been used as a popular pseudo-capacitive material for supercapacitors due to its higher capacitance compared to carbon-based electric double-layer capacitors. However, PAN I exhibits relatively low conductivity, poor cycle stability, and hence limited usage in high power capability due to its large volumetric change. The integration of PANI with carbonaceous nanomaterials has been highly efficient in improving the above disadvantages. This chapter reports recent developments of carbon-based PANI composites for supercapacitors. Various methods, including chemical oxidative polymerization, electrochemical polymerization, interfacial polymerization, Pickering emulsion polymerization, electrodeposition, electrospinning, solution mixing, self-assembly, and chemical grafting are demonstrated, respectively. The relationship between the device performance and nanostructures of PANI and its composites is then discussed. General guidelines for rational design and optimal fabrication of carbon-based PANI composites for supercapacitors are further provided. Critical challenges and potential perspectives regarding the composites for supercapacitors are finally presented.

Low-temperature baroplastic processing of graphene-based polymer composites by pressure-induced flow

Two-stage emulsion polymerization was employed to synthesize nanoparticles consisting of a low glass transition temperature core of poly(n-butyl acrylate) (PBA) and a glassy poly(methyl methylacrylate) (PMMA) shell. Incorporation of graphene oxide (GO) into the PBA-PMMA latex produced GO/PBA-PMMA composites after demulsification and graphene/PBA-PMMA composites after chemical reduction of GO. The as-prepared powdery materials were processed into thin films by compression molding at room temperature as the result of a pressure-induced mixing mechanism of microphase-separated baroplastics. The presence of oxygen-containing groups for GO sheets contributed to better dispersion and stronger interface with the matrix, thereby showing greater reinforcement efficiency toward polymers compared to graphene sheets. In addition, both Youngs modulus and yield strength for all materials increased with applied pressure and processing time due to better flowability, processability and cohesion at higher pressure and longer time. Low-temperature processing under pressure is of significance for energy conservation, recyclability and environmental protection during plastic processing.