RESUMONeste trabalho foram preparadas dispersões aquosas de óxido de grafeno (GO) e óxido de grafeno reduzido (rGO), em diferentes tempos de oxidação, com o objetivo de verificar a influência do tempo de oxidação em relação às características das folhas (flakes) finais. As dispersões de GO foram preparadas pelo método de Hummers modificado utilizando os seguintes tempos de oxidação: 4 h; 1 dia; 3 dias; 7 dias e 10 dias. Em seguida, as dispersões de GO foram submetidas ao tratamento de redução com ácido ascórbico, obtendo-se o rGO. As caracterizações das amostras foram realizadas por meio das análises de espectroscopia Raman, espectroscopia de fotoelétrons excitados por raios X (XPS), microscopia eletrônica de varredura (MEV) e difração de raios X (DRX). A espectroscopia Raman mostrou a variação da relação de intensidade entre as bandas D e G (I D /I G ) à medida que o tempo de oxidação foi aumentado. A análise de XPS exibiu a alteração dos percentuais de carbono (C) e oxigênio (O). Através de análise no MEV foi correlacionada a morfologia dos flakes com o aumento do tempo de oxidação. E por fim, com o auxílio de DRX, foi verificada a variação da distância interplanar.Palavras-chave: óxido de grafeno, óxido de grafeno reduzido, tempo de oxidação. ABSTRACTIn this work aqueous dispersions of graphene oxide (GO) and reduced graphene oxide (rGO) were prepared in different oxidation times in order to verify the influence of the oxidation time on the characteristics of the final flakes. GO dispersions were prepared by the modified Hummers method, using the following oxidation times: 4 h; 1 day; 3 days; 7 days and 10 days. Afterwards the dispersions of GO were subjected to the reduction treatment with ascorbic acid, obtaining the rGO. Samples were characterized by Raman spectroscopy, Xray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and X-ray diffraction (XRD). Raman spectroscopy showed the variation of the intensity ratio between the D and G bands (I D / I G ) as the oxidation time was increased. The XPS analysis indicated the alteration of the percentages of carbon (C) and oxygen (O). The SEM analysis correlated the morphology of the flakes with the increase of the oxidation time. Finally, with the aid of XRD, the variation of the interplanar distance was verified.

Graphene oxide (GO) incorporation in natural fiber composites has recently defined a novel class of materials with enhanced properties for applications, including ballistic armors. In the present work, the performance of a 0.5 vol % GO-incorporated epoxy matrix composite reinforced with 30 vol % fabric made of ramie fibers was investigated by stand-alone ballistic tests against the threat of a 0.22 lead projectile. Composite characterization was also performed by Fourier-transform infrared spectroscopy, thermal analysis and X-ray diffraction. Ballistic tests disclosed an absorbed energy of 130 J, which is higher than those reported for other natural fabrics epoxy composite, 74–97 J, as well as plain Kevlar (synthetic aramid fabric), 100 J, with the same thickness. This is attributed to the improved adhesion between the ramie fabric and the composite matrix due to the GO—incorporated epoxy. The onset of thermal degradation above 300 °C indicates a relatively higher working temperature as compared to common natural fiber polymer composites. DSC peaks show a low amount of heat absorbed or release due to glass transition endothermic (113–121 °C) and volatile release exothermic (~132 °C) events. The 1030 cm−1 prominent FTIR band, associated with GO bands between epoxy chains and graphene oxide groups, suggested an effective distribution of GO throughout the composite matrix. As expected, XRD of the 30 vol % ramie fabric-reinforced GO-incorporated epoxy matrix composite confirmed the displacement of the (0 0 1) peak of GO by 8° due to intercalation of epoxy chains into the spacing between GO layers. By improving the adhesion to the ramie fabric and enhancing the thermal stability of the epoxy matrix, as well as by superior absorption energy from projectile penetration, the GO may contribute to the composite effective ballistic performance.

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RESUMODispersões de óxido de grafeno (GO) e de óxido de grafeno reduzido (rGO) foram produzidas, respectivamente, por via química através do método de Hummers modificado e por redução do GO com adição de polímero PSS (poli (4-estirenosulfonato de sódio)) e ácido ascórbico. Amostras de GO e rGO/PSS foram caracterizadas pelas seguintes técnicas: microscopia óptica, microscopia eletrônica de varredura (MEV), espectroscopia Raman e espectroscopia de fotoelétrons excitados por raios X (XPS). A partir das dispersões foram depositados filmes finos em substratos de vidro pelos seguintes métodos: spray, dip coating e spin coating. Os filmes produzidos foram, em seguida, tratados termicamente para melhoria da aderência nos substratos e, especificamente para os filmes de GO, para redução, de modo a formar o óxido de grafeno reduzido termicamente (trGO). Os filmes foram caracterizados quanto a espessura, morfologia, transmitância e resistência de folha. Por meio das caracterizações realizadas a partir das dispersões foi possível observar folhas de grafeno pela microscopia óptica, indicando o sucesso do método de produção empregado; por meio do XPS, determinou-se a composição das amostras de GO e rGO e confirmou-se que a redução com ácido ascórbico foi efetiva; as transmitâncias obtidas mostraram-se dentro da faixa indicada para aplicação e as resistividades de algumas amostras, alcançaram valores próximos a de materiais tradicionalmente aplicados como filmes transparentes e condutores. Palavras-chave: óxido de grafeno reduzido; filmes transparentes e condutores; métodos de deposição. ABSTRACTDispersions of graphene oxide (GO) and reduced graphene oxide (rGO) were respectively chemically produced by the modified Hummers method and by reducing GO with the addition of PSS polymer (poly (4-styrenesulfonate sodium)) and ascorbic acid. Samples of GO and rGO/PSS were characterized by the following techniques: optical microscopy, scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). From the dispersions thin films were deposited on glass substrates by the following methods: spray, dip coating and spin coating. The films produced were then thermally treated to improve adhesion on substrates and, specifically for GO films, for reduction, so as to form thermally reduced graphene oxide (trGO). The films were characterized as thickness, morphology, transmittance and sheet resistance. By means of the characterizations made from the dispersions it was possible to observe graphene sheets by optical microscopy, indicating the success of the production method employed; by means of the XPS, the composition of the samples of GO and rGO was determined and confirmed that the reduction with ascorbic acid was effective; the obtained transmittances were within the range indicated for application and