Microscopical study of nanometric particles adhesion / Estudo microscopico de adesão de particulas nanometricas

AUTOR(ES)
DATA DE PUBLICAÇÃO

2009

RESUMO

The adhesion of nanometric phases is a topic of increasing scientific interest due to its importance for the production and properties of new nanostructured materials. This study explores the following hypothesis: immiscible and wettable particles carrying charges with same signal can adhere provided that they are mixed in aqueous dispersion and dried out. This hypothesis is detailed in the following mechanism: first, during the drying process the particles are attracted and placed in close contact due to the capillary adhesion forces. Second, the evenly charged particles in the resulting dry solid are counterbalanced by counter-ions from the serum, forming ionic bridges between the surfaces, that account for electrostatic adhesion. To verify this hypothesis, pairs of many nanoparticles types were mixed in aqueous medium and dried out. The resulting samples were analyzed by electron and probe microscopy. The energy filtered transmission electron microscopy (EFTEM) was extensively explored thanks to a new experimental protocol that allows the analyses of thick particles and aggregates (>500 nm). In every case, the different phases remain in contact with each other after drying, showing that electrostatic forces are able to unite evenly charged particles. In the case of calcium montmorillonite and styrene-acrylic latex, Ca ions are located in domains in which the clay and the latex are superimposed, evidencing ionic bridge formation. The contribution of the electrostatic energy to the adhesion of charged interfaces was calculated using a model of three parallel surfaces, in which a positively charged surface is in between two negatively charged surfaces. Calculations have shown that in conditions analogous to the styrene-acrylic latex and montmorillonite interface, the electrostatic contribution in the interface can reach values as high as -200 kJ.mol and these results are verified using experimental data from mechanical testing. The electrostatic adhesion model was used to prepare PET and clay nanocomposite materials, obtaining materials with tensile strength higher than the pristine polymer. The final section of the thesis describes self-propelled nanomotors synthesis. These are doublets made of a silica particle adhered to a platinum particle. Platinum surface catalyses the hydrogen peroxide decomposition in aqueous solution creating a surface tension gradient that propels the dimers, producing non-Brownian motion

ASSUNTO(S)

adhesion microscopy nanoparticulas microscopia nanoparticles adesão

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