Noncovalent Functionalization of Graphene Nanoplatelets and Their Applications in Supercapacitors.

We demonstrate a simple noncovalent functionalization technique, which involves graphite exfoliation and subsequent coating of the resulting graphene nanoplatelets (GNPs) with trimellitic anhydride (TMA), using a thermomechanical exfoliation process. TMA adsorbs on the surface of the GNPs, resulting in a reduction of the specific surface area to 312 ± 9 m2/g compared to 410 ± 12 m2/g for the unmodified GNPs. Detailed imaging, thermogravimetric, and X-ray diffraction analysis showed that the modified GNPs (TMA-GNPs) maintain similar structure to the unmodified GNPs. The presence of functional groups, confirmed by X-ray photoelectron spectroscopy analysis, caused an increase in the surface energy from 45.6 mJ/m2 for the GNPs to 57.9 mJ/m2 for TMA-GNPs. The resulting coated TMA-GNPs form stable dispersions in water while maintaining their inherent conductive properties, thus enabling applications, such as the manufacture of conductive films and supercapacitors. As a proof-of-concept, electrodes for supercapacitors are prepared from concentrated aqueous dispersions of the functionalized GNPs. Electrochemical characterization of the supercapacitors using electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic charge/discharge tests showed a specific capacitance of 22.2 F/cm3 at a scan rate of 1 mV/s from cyclic voltammetry and 17.3 F/cm3 at a current density of 1 A/g from galvanostatic charge/discharge tests, with a 90% capacitance retention after 10,000 cycles.

Adhesive Wearable Sensors for Electroencephalography from Hairy Scalp.

Electroencephalography has garnered interest for applications in mobile healthcare, human-machine interfaces, and Internet of Things. Conventional electroencephalography relies on wet and dry electrodes. Despite favorable interface impedance of wet electrodes and skin, the application of a large amount of gel at their interface with skin limits the electroencephalography spatial resolution, increases the risk of shorting between electrodes, and makes them unsuited for long-term mobile recording. In contrast, dry electrodes are better suited for long-term recordings but susceptible to motion artifacts. In addition, both wet and dry electrodes are non-adhesive to the hairy scalp and mechanical support, or chemical adhesives are used to hold them in place. Herein, a conical microstructure array (CMSA) based sensor made of carbon nanotube-polydimethylsiloxane composite is reported. The CMSA sensor is fabricated using the innovative, cost-effective, and scalable method of viscosity-controlled dip-pull process. The sensor adheres to the hairy scalp by generating negative pressure in its conical microstructures when it is pressed against scalp. Aided by the application of a trace amount of gel, CMSA sensor establishes good electrical contact with the skin, enabling its applications in mobile electroencephalography over extended periods. Notably, the signal quality of CMSA sensors is comparable to that of medical-grade wet gel electrodes.

Achieving high yield of graphene nanoplatelets in poloxamer-assisted ultrasonication of graphite in water.

The role of surfactant (Pluronic® F 127) concentration on the yield and morphological characteristics of graphene nanoplatelets (GNPs) produced from the sonication of aqueous graphene suspensions is investigated in this work. By employing a wide surfactant concentration range (0.1-15 wt%) and sonication power densities up to 420 W L-1 we identify two graphene exfoliation regimes: the first occurs at low sonication power densities (<340 W L-1) and produces GNPs with sizes 200-300 nm, aspect ratios between 70 and 100, and concentrations up 1 mg mL-1. In that regime, the surfactant concentration has no effect on the exfoliation results. In the second exfoliation regime (>340 W L-1), surfactant concentrations greater than 10 wt% produce dramatic increases in GNP yields, namely up to 3.0 mg mL-1, and overall larger GNPs (350-500 nm) with smaller aspect ratios (5-60). We attribute these changes to the onset of a more energy intensive mechanism, termed cleavage. Cleavage involves the separation of graphite clusters in sub-bulk multi-layered graphene entities, as opposed to exfoliation, which involves the separation of individual or few-layer GNPs. Choosing an exfoliation regime by tuning simple process parameters enables control over the yield, size and morphology of the produced GNPs.

Manipulating the structure of medium-chain-length polyhydroxyalkanoate (MCL-PHA) to enhance thermal properties and crystallization kinetics.

We investigated the effects of the structure of medium-chain-length polyhydroxyalkanoates (MCL-PHAs) on their thermal properties and crystallization kinetics. The predominantly homopolymeric poly(3-hydroxydecanoate), P(3HD)-98, and the poly(3-hydroxydodecanoate), P(3HDD), exhibited sharp crystallization peaks upon cooling, with the latter exhibiting faster crystallization rates. A chemical modification strategy involving reaction with dicumyl peroxide and triallyl trimesate coagent was implemented to introduce branching and enhance the crystallization kinetics of P(3HD-98). Increases in the exothermic crystallization temperature by 8 °C and in the overall crystallinity of the P(3HD)-98 were observed upon chemical modification. The Avrami crystallization kinetic parameters obtained by fitting the isothermal crystallization data revealed a significant increase in the crystallization rate of the modified P(3HD)-98.

Correlation between the length reduction of carbon nanotubes and the electrical percolation threshold of melt compounded polyolefin composites.

The objectives of this work are to quantify the degree of multiwalled carbon nanotube (MWCNT) length reduction upon melt compounding and to demonstrate unambiguously that the length reduction is mainly responsible for the increase in electrical percolation threshold of the resulting composites. Polyolefin matrices of varying viscosities and different functional groups are melt compounded with MWCNTs. A simple method is developed to solubilize the polymer matrix and isolate the MWCNTs, enabling detailed imaging analysis. In spite of the perceived strength of the MWCNTs, the results demonstrate that the shear forces developed during melt mixing are sufficient to cause significant nanotube breakage and length reduction. Breakage is promoted when higher MWCNT contents are used, due to increased probability of particle collisions. Furthermore, the higher shear forces transmitted to the nanotubes in the presence of higher matrix viscosities and functional groups that promote interfacial interactions, shift the nanotube distribution toward smaller sizes. The length reduction of the MWCNTs causes significant increases in the percolation threshold, due to the loss of interconnectivity, which results in fewer conductive pathways. These findings are validated by comparing the experimental percolation threshold values with those predicted by the improved interparticle distance theoretical model.

A noncovalent compatibilization approach to improve the filler dispersion and properties of polyethylene/graphene composites.

Graphene was prepared by low temperature vacuum-assisted thermal exfoliation of graphite oxide. The resulting thermally reduced graphene oxide (TRGO) had a specific surface area of 586 m(2)/g and consisted of a mixture of single-layered and multilayered graphene. The TRGO was added to maleated linear low-density polyethylene LLDPE and to its derivatives with pyridine aromatic groups by melt compounding. The LLDPE/TRGO composites exhibited very low electrical percolation thresholds, between 0.5 and 0.9 vol %, depending on the matrix viscosity and the type of functional groups. The dispersion of the TRGO in the compatibilized composites was improved significantly, due to enhanced noncovalent interactions between the aromatic moieties grafted onto the polymer matrix and the filler. Better dispersion resulted in a slight increase in the rheological and electrical percolation thresholds, and to significant improvements in mechanical properties and thermal conductivity, compared to the noncompatibilized composites. The presence of high surface area nanoplatelets within the polymer also resulted in a substantially improved thermal stability. Compared to their counterparts containing multiwalled carbon nanotubes, LLDPE/TRGO composites had lower percolation thresholds. Therefore, lower amounts of TRGO were sufficient to impart electrical conductivity and modulus improvements, without compromising the ductility of the composites.