Graphene nanoplatelets (GNP) with thickness of 6 ~ 8 nm and lateral dimension of 5 μm (M5) and 25 μm (M25) have been used to prepare epoxy composites. Epoxy composites were fabricated initially by shear mixing to investigate the effects of filler type on the structure and properties of composites. The complex viscosity of GNP-epoxy mixture was found to increase by almost three orders of magnitude going from the neat epoxy to the 8 wt.% loading, leading to difficulties in their processing. Scanning electron microscopy of the composites showed that both fillers aggregated at high loadings with the M25 buckling more easily due to its larger diameter, which compromises its aspect ratio advantage over M5, resulting in only slightly better mechanical performance. Polarized Raman spectroscopy revealed that both M5 and M25 were randomly distributed in the epoxy matrix, After adding M5 and M25 fillers, the storage modulus increase with the filler loadings, however, the glass transition temperature (Tg) drops slightly after initial incorporation, then rises with further filler addition attributed to the pin effects of filler aggregations. In terms of electrical property, M25 has lower percolation (1 wt.%) than M5 composites due to its bigger aspect ratio, which enable M25 to form a conductive network more efficiently. Furthermore, M25 composites also have slightly better thermal and mechanical properties over that of M5 composites. However, the difference is not significant considering the aspect ratio of M25 is five times of that of M5. The reason is that the aggregation and buckling of M25 compromise its advantage over M5. Due to the better performance of M25 as filler, M25/epoxy composites were prepared by shear mixing, solvent compounding and three-roll mill. Samples made by solvent compounding display the lowest percolation threshold (0.5 wt.%), related to its relatively uniform dispersion of M25 in matrix, resulting in higher thermal conductivity and better mechanical properties. Water uptake in a water bath at 50 °C took 75 days to be saturated. Higher loaded samples have lower diffusion coefficient because of the barrier effects of GNP fillers, but have higher maximum water absorbed, which is owing to filler aggregation. Properties test of aged and unaged specimens show thermal conductivity of the aged was enhanced due to water’s higher thermal conductivity than epoxy resin matrix, while electrical performance was compromised due to the swelling effects caused by absorbed water. The mechanical properties of aged samples also dropped slightly due to plasticization effects.

Made available in DSpace on 2016-06-02T19:19:58Z (GMT). No. of bitstreams: 1 MELLO_Waldomiro_2014.pdf: 2920239 bytes, checksum: a0ba2a82065620b41788d178ed20df21 (MD5) Previous issue date: 2014-11-29 / In this work we have compared the way graphene oxide nanoplatelets are nanostructured in bilayers of poly(diallyldimethylammonium chloride) (PDDA) and reduced graphene oxide stabilized in poly(sodium 4-styrenesulfonate) (GPSS), called as (PDDA-GPSS)n, with n representing the number of deposited bilayers. LbL (layer-by-layer) films were fabricated by the self-assembly technique throughout the dipping, spray and dynamic methodologies, available at Laboratory FINEP1, UFSCar, campus Sorocaba. When compared with other bottom-up strategies employed in the build-up of nanostructured ultrathin films, the LbL technique is simple, cheap and easy to handle, beside the incorporation of distinct materials in the film structure, not limited to the form and size of substrates. The times used for the polyelectrolytes in the dipping and dynamic methods were based on the kinetic growth of the (PDDA-GPSS)8 films. In the spray methodology the time used was based initially in the literature and further confirmed by experimental data considering pressure and spray time. All nanostructures formed were characterized by UV-vis spectroscopy, atomic force microscopy, scanning electron microscopy, profilometry and impedance spectroscopy. In all cases it was observed a linear growth of the LbL structures, pointing that the same amount of material was adsorbed at each deposition step. Films obtained from the dynamic methodology indicated higher amount of material adsorbed in the nanostructures, and by an adequate control of the parameters used in the self-assembly methods by physical adsorption one can make a fine tuning regarding to the spontaneous aggregation of the nanoplatelets at solid interfaces. / Neste trabalho comparamos a forma com a qual as nanofolhas de grafeno são nanoestruturadas em bicamadas de poli(cloreto de dialildimetilamônio) (PDDA) e grafeno estabilizado em poli(estireno sulfonato de sódio) (GPSS), designados por (PDDA-GPSS)n sendo n o número de bicamadas depositadas. Os filmes foram fabricados com a técnica de automontagem por adsorção física (LbL, do inglês Layer-by-Layer), utilizando os métodos de imersão, spray e dinâmico, disponíveis no Laboratório Finep1 da UFSCar, campus Sorocaba. Comparada a outras estratégias bottom-up empregadas na construção de filmes ultrafinos nanoestruturados, a técnica LbL é simples, barata e de fácil aplicação, além de permitir a incorporação de diferentes materiais, e não estar limitada quanto à forma e o tamanho dos substratos. Os tempos para a aplicação dos polieletrólitos por imersão e pelo método dinâmico foram estabelecidos com base nas cinéticas de crescimento de filmes (PDDA-GPSS)8. Para o método de spray os tempos foram estabelecidos inicialmente com base na literatura, e depois confirmados em ensaios de crescimento, tendo como variáveis a pressão de ar e o tempo de spray. As nanoestruturas foram caracterizadas por espectroscopia na região do ultravioleta e do visível, microscopia de força atômica, microscopia eletrônica de varredura, perfilometria e espectroscopia de impedância. Em todos os casos foi observado crescimento linear das estruturas LbL, indicando mesma quantidade de material adsorvido a cada etapa de deposição. Os filmes obtidos pelo método dinâmico indicaram maior quantidade de material agregado nas nanoestruturas, e pelo controle adequado nos parâmetros utilizados nos métodos de automontagem por adsorção física podemos realizar um ajuste fino em relação à agregação espontânea de nanofolhas de grafeno em interfaces sólidas