Glaura G. Silva

Supercapacitor Operating At 200 Degrees Celsius

The operating temperatures of current electrochemical energy storage devices are limited due to electrolyte degradation and separator instability at higher temperatures. Here we demonstrate that a tailored mixture of materials can facilitate operation of supercapacitors at record temperatures, as high as 200°C. Composite electrolyte/separator structures made from naturally occurring clay and room temperature ionic liquids, with graphitic carbon electrodes, show stable supercapacitor performance at 200°C with good cyclic stability. Free standing films of such high temperature composite electrolyte systems can become versatile functional membranes in several high temperature energy conversion and storage applications.

Observation of Dynamic Strain Hardening in Polymer Nanocomposites

Most materials respond either elastically or inelastically to applied stress, while repeated loading can result in mechanical fatigue. Conversely, bones and other biomechanical tissues have the ability to strengthen when subjected to recurring elastic stress. The cyclic compressive loading of vertically aligned carbon nanotube/poly(dimethylsiloxane) nanocomposites has revealed a self-stiffening response previously unseen in synthetic materials. This behavior results in a permanent increase in stiffness that continues until the dynamic stress is removed and resumes when it is reapplied. The effect is also specific to dynamic loads, similar to the localized self-strengthening that occurs in biological structures. These observations help to elucidate the complex interactions between matrix materials and nanostructures, and control over this mechanism could lead to the development of adaptable structural materials and active, load-bearing artificial connective tissues.

Long-term behavior of epoxy/graphene-based composites determined by dynamic mechanical analysis

Graphene oxide and reduced graphene oxide were obtained from graphite and reduction of graphene oxide, respectively, and functionalized with 4,4′-Methylenebis (phenyl isocyanate) isocyanate. Epoxy/graphene-based composites were prepared by dispersing the as-prepared carbon materials in epoxy resin based on diglycidyl ether of bisphenol A followed by curing with triethylenetetramine. The efficiency of the functionalization of the carbon materials was followed by Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoeletronic spectroscopy, and atomic force microscopy. The epoxy-based composites were characterized by dynamic mechanical analysis at frequency and temperature sweeps in order to evaluate the long-term behavior of the composites by using the time temperature superposition principles and the master curves of multi frequency. All composites presented better working temperature range and durability as compared with the pure epoxy network. However, those prepared with graphene oxide and its reduced form functionalized with isocyanate groups presented better mechanical performance and long-term durability, probably because of the better filler-matrix interactions achieved in theses system. The results obtained suggest that the presence of isocyanate groups is the key role for achieving good mechanical performance, and the reduction step of graphene oxide is not of paramount importance for achieving good mechanical response.

Dual-responsive and super absorbing thermally cross-linked hydrogel based on methacrylate substituted polyphosphazene

Stimuli-responsive hydrogels have played a crucial role in biomaterials being extensively studied for applications such as tissue engineering and drug delivery. Here, we report on the synthesis and characterization of a new hydrogel based on methacrylate substituted polyphosphazene (PMAPhos). The hydrogel was obtained spontaneously after the polyphosphazene substitution reaction and its mechanical properties were improved after annealing. By annealing the hydrogel in the presence of a 6% w/w methacrylic acid solution, we could obtain a poly(methacrylic acid) (PMAA) branched and slightly cross-linked polyphosphazene network, which showed ultra-high absorbency being able to absorb distilled water, at maximum, as much as 870 times of its own weight. Moreover, PMAPhos showed dual-responsive behaviour responding to both pH and temperature changes. Unlike many other similar systems, the hydrogel structure collapsed not only at lower but also at higher pHs. The gel volume phase transition as a function of temperature was determined by both differential scanning calorimetry (DSC) and by swelling experiments. However, after the hydrogel annealing, the thermal response was suppressed and overridden by the pH-response behaviour of the PMAA branches. Hydrogel morphology and swelling behaviour were also analyzed directly by Cryo-transmission electron microscopy, which revealed the fibrous polymer network. Remarkable features of this new hydrogel are: the biodegradation offered by the polyphosphazene backbone and simplicity of preparation where no co-polymerizations were used to reach the dual-responsive behaviour. Besides, the particular pH-responsive behaviour found for this hydrogel is a useful characteristic for applications aiming for controlled delivery of substances in specific sites of the human body.

Layer-by-layer assembled films of multi-walled carbon nanotubes with chitosan and cellulose nanocrystals

Chitosan solutions and cellulose nanocrystal suspensions were used to produce highly stable aqueous dispersions of multi-walled carbon nanotubes (MWCNTs). The different MWCNT dispersions, presenting positive and negative charges, were used to prepare multilayered hybrid thin films through electrostatic layer-by-layer (LBL) self-assembly. The MWCNTs are well dispersed and homogeneously distributed on each layer of chitosan and cellulose nanocrystals of the films. The nanotubes are densely packed in each multilayer, forming a random network. The surface of the LBL film exhibited a uniform and relatively smooth surface with a mean roughness value of ∼5.8±0.4nm. Electrochemical characterization revealed a decrease in two orders of magnitude in the film resistance as the number of bilayers increased from 5 to 20, which is a consequence of an increase in the amount of conductive material (MWCNT). The thin films with up to 20 bilayers exhibited transmittance higher than 90% in the visible range. The results presented in this work demonstrate the viability of the LBL technique for the deposition of active materials using the biopolymer pair chitosan/cellulose nanocrystals. The obtained films can be employed for the design of transparent and biocompatible carbon nanostructured based electrodes.

Characterizing intrinsic charges in top gated bilayer graphene device by Raman spectroscopy

D. L. Mafra, P. Gava, L. M. Malard, R. S. Borges, G. G. Silva, J. A. Leon, F. Plentz, F. Mauri, M. A. Pimenta Departamento de F́ısica, Universidade Federal de Minas Gerais, 30123-970, Belo Horizonte, Brazil. IMPMC, Universit Paris 6 et 7, CNRS, IPGP, Paris, France. Departamento de Qúımica, Universidade Federal de Minas Gerais, 30123-970, Belo Horizonte, Brazil. ∗ These authors contributed equally to this work.

Facile Graphene Oxide Preparation by Microwave-Assisted Acid Method

Few-layered graphene oxide (GO) was prepared using a fast and energy-saving method by microwave-assisted acid technique. The oxygenated groups existing on the GO surface were determined using UV-Vis, X-ray photoelectron and Fourier-transform infrared spectroscopies. An oxygenated group percentage of 30% in mass for the GO was observed by thermogravimetric analysis. The reduced few-layered graphene oxide (rGO) film annealed at 110 °C deposited onto a silicon/silica wafer showed expanded graphite-like structure with 0.70 nm between the rGO sheets, as determined by X-ray diffraction. This rGO film exhibited a relatively high electrical conductivity value of 7.36 × 102 S m-1 confirming the good restoration of the π-conjugated system. The prepared GO sample exhibited good stability in water from pH 4 to 12, as determined by its zeta potential, and contained 5 to 9 layers, as determined by atomic force microscopy (AFM) and transmission electron microscopy (TEM).

NANOCOMPÓSITOS DE POLIURETANA TERMOPLÁSTICA E NANOTUBOS DE CARBONO DE PAREDES MÚLTIPLAS PARA DISSIPAÇÃO ELETROSTÁTICA

Polyurethane/multi-walled carbon nanotube (MWCNT) nanocomposites have been prepared with nanotube concentrations between 0.01 wt% and 1 wt%. MWCNT as-synthesized samples with ~74 nm diameter and ~7 μm length were introduced by solution processing in the polyurethane matrix. Scanning electron microscopy (SEM) images demonstrated good dispersion and adhesion of the CNTs to the polymeric matrix. The C=O stretching band showed evidence of perturbation of the hydrogen interaction between urethanic moieties in the nanocomposites as compared to pure TPU. Differential scanning calorimetry and positron anihilation lifetime spectroscopy measurements allowed the detection of glass transition displacement with carbon nanotube addition. Furthermore, the electrical conductivity of the nanocomposites was significantly increased with the addition of CNT.

Cross-linking effect on thermal, conducting and electrochemical properties of an elastomeric polymer electrolyte

Abstract Complexing salts, like lithium perchlorate, form interesting polymer electrolytes with terpolymers containing branched ether derivatives, which can be cross-linked, depending on the allyl glycidyl ether group content. Using thermal and electrochemical techniques, the effect of polymer cross-linking upon the properties of the resulting electrolytes has been estimated. It is shown that increasing the salt concentration intensifies the stiffness of the terpolymer network or the linear/branched one and the glass transition temperature consequently rises. Both electrolyte systems investigated in this work present good thermal stabilities up to 270 °C and ionic conductivities of 10 −4 S cm −1 at 30 °C with 8 wt.% of LiClO 4 . Cross-linking does not severely affect the electrolyte conductivity but very significantly enlarges the electroactivity window (from 3 to 5 V) and improves the mechanical stability. The thermal dependence of the conductivity revealed a particular conduction mechanism attributed to the presence of short mobile chains due to the high polydispersity of the terpolymer. Thermal, electrical and special electrochemical properties suggest the inclusion of this material into the available polymer electrolytes for developing new electrochemical applications.

THEORETICAL STUDY OF SOLVENT AND TEMPERATURE EFFECTS ON THE BEHAVIOUR OF POLY(ETHYLENE OXIDE) (PEO)

Molecular mechanics and molecular dynamics simulations were applied in order to study the behaviour of poly ethylene . . oxide PEO in different temperatures and solvents. It was found that over the temperature range of 50-500 K the equilibrium structure of PEO is folded. Both kinetic and potential energies increase with temperature. The behaviour of PEO . . in two different solvents - CHCl « s 5 and H O « s 80 - was found to be similar, with the folded structure observed 32 in equilibrium. The solvation energy calculated using the GBrSA model yielded essentially the same value in CHCl and 3