Trung Dung Dao

Properties of Graphene/Shape Memory Thermoplastic Polyurethane Composites Actuating by Various Methods

Shape memory behavior of crystalline shape memory polyurethane (SPU) reinforced with graphene, which utilizes melting temperature as a shape recovery temperature, was examined with various external actuating stimuli such as direct heating, resistive heating, and infrared (IR) heating. Compatibility of graphene with crystalline SPU was adjusted by altering the structure of the hard segment of the SPU, by changing the structure of the graphene, and by changing the preparation method of the graphene/SPU composite. The SPU made of aromatic 4,4′-diphenylmethane diisocyanate (MSPU) exhibited better compatibility with graphene, having an aromatic structure, compared to that made of the aliphatic hexamethylene diisocyanate. The finely dispersed graphene effectively reinforced MSPU, improved shape recovery of MSPU, and served effectively as a filler, triggering shape recovery by resistive or IR heating. Compatibility was enhanced when the graphene was modified with methanol. This improved shape recovery by direct heating, but worsened the conductivity of the composite, and consequently the efficiency of resistive heating for shape recovery also declined. Graphene modified with methanol was more effective than pristine graphene in terms of shape recovery by IR heating.

Alumina-coated graphene nanosheet and its composite of acrylic rubber

A graphene was coated with a thin alumina layer to prepare a novel nanosheet which had high thermal conductivity but low electrical conductivity. The nanosheet with minimal aggregation was prepared effectively by first coating it with aluminum tri-sec-butoxide in anhydrous dimethylformamide, followed by rapid calcination in an inert atmosphere after the hydrolysis of the alkoxide. The morphology observed by scanning electron microscopy and elemental mapping by energy-dispersive X-ray spectrometry showed that the alumina layer coated on the graphene surface was uniform and ultra-thin. Thermogravimetry demonstrated that the uniformly coated alumina protective layer substantially improved the thermal stability of the graphene and that the electrically-insulative alumina layer effectively reduced the electrical conductivity of the graphene. The enhanced polar nature of surface as well as the increased surface roughness due to the coated alumina improved the dispersion of the graphene in the polar acrylic rubber matrix and the interaction at the interface. This led to an effective improvement of the thermal conductivity but marginal increase in electrical conductivity by the filler. Tensile modulus increased drastically to as high as 470% for the composite reinforced with the 5 phr (about 2.5 vol%) loading of the alumina-coated graphene.

Solid-state functionalization of graphene with amino acids toward water-dispersity: implications on a composite with polyaniline and its characteristics as a supercapacitor electrode material

A new and facile modification method to endow graphene with water-dispersity is reported. The remanent epoxide group on graphene prepared by the thermal reduction of graphite oxide is effectively utilized for the modification reaction with the amine group of potassium 6-aminocaproate using a simple process of heating the well-mixed solid state mixture. The graphene modified to have negatively charged carboxylate moieties disperses easily and stably in water. A graphene/polyaniline (PANI) composite is prepared efficiently in an aqueous system by the simple physical mixing of aqueous dispersions of negatively charged graphene and positively charged micron-size PANI particles, followed by drying. The composites exhibit higher energy storage capacities as supercapacitor electrodes compared to those of either graphene or PANI themselves. The synergistic effect is most evident at a graphene/PANI weight ratio of 20/80. The composite exhibits more than twice the specific capacitance of either PANI or graphene. This suggests that the scaffold structure of hydrophilic graphene enclosing PANI particles reduces not only the contact resistance at the electrode/electrolyte interface but also the diffusion length for effective charge transfer.

Compatibility of functionalized graphene with polyethylene and its copolymers

The compatibility of polyethylene (PE) with a functionalized graphene sheet (FGS), which was prepared by the thermal reduction of graphite oxide, was examined in this study. The dispersion of the FGS in the PE was improved as the molecular weight of the PE was decreased. The PE copolymers containing polar comonomers such as maleic anhydride or acrylic acid exhibited better compatibility with FGS than the PE homopolymers. The compatibility of the FGS with PE copolymers containing small amounts of comonomers, which have a solubility parameter slightly larger [up to approximately 0.5 (J/cm3)1/2] than that of PE itself, was better than the compatibility of both the PE and PE copolymers containing larger amounts of comonomers. The morphology, electric conductivity, and tensile properties of FGS/PE copolymer/PE nanocomposites showed that the copolymers with a solubility parameter slightly larger than that of PE effectively served as a compatibilizer in FGS/PE nanocomposites.

Graphene functionalized with poly(vinyl alcohol) as a Pickering stabilizer for suspension polymerization of poly(methyl methacrylate)

Two types of thermally reduced graphenes (TRGs) having different lateral sizes were non-covalently modified with poly(vinyl alcohol) to endow water-dispersibility. The modified TRGs were examined as Pickering stabilizers for the suspension polymerization of methyl methacrylate (MMA). They were effective graphene-based Pickering stabilizers for the system with almost all of the polymerized composite microparticles having a regular spherical shape. The particle size of the composite microparticles was tunable by the size or the amount of modified TRG used as stabilizer. The almost perfect core-shell structure of the composite microparticles effectively enhanced the thermal stability of the core PMMA. In addition, when the core-shell microparticles were compression molded into a monolith, the obtained composite exhibited an ultra-low percolation threshold of electrical conductivity of around 0.04vol%.

Shape memory polyurethane nanocomposites with a functionalized graphene

The roles of graphene in shape memory polyurethane nanocomposite (SMPUN) as a reinforcing filler, as a fixed structure for memorizing a specified original shape, and as a conductive filler to actuate shape recovery by resistive heating, were examined. The effectiveness of each role was modulated by functionalizing graphene with a hydroxyl group through oxidation with H2O2. The reinforcing effect of graphene and its role as a fixed structure were enhanced by the hydroxyl groups due to the increased grafting of polyurethane chains on graphene during the in situ preparation of SMPUN. However, the oxidation reduced the conductivity of SMPUN, resulting in the deterioration of resistive heating.