Infoscience

Thesis

Functionally graded polymer nanocomposites using magnetic fields

Functionally Graded Materials (FGM) are a class of materials in which the composition and the properties gradually change over the volume. The present PhD project aims to generate a full understanding of a novel synthetic strategy for polymeric FGMs based on the magnetophoretic motion of magnetic nanofillers in a polymer matrix under the application of an external magnetic field gradient. Due to its high rate, cost–effectiveness and low–toxicity, UV polymerization was considered as the first curing option for the magnetic nanocomposites. The curability problem deriving from the presence of opaque particles was tackled by studying two different systems based on the same hyperbranched acrylated polymer matrix (HBP): one containing bare Fe3O4 nanoparticles whereas the other loaded with Fe3O4@SiO2 core–shell nanoparticles. The employment of Fe3O4@SiO2 NPs for the synthesis of magnetic polymeric films showed to be effective towards the inclusion of 10–times higher particle volume fractions compared to formulations including bare Fe3O4. By adding Fe3O4@SiO2 nanoparticles to the UV–curable matrix, the motion of the magnetic responsive filler was induced upon the application of magnetic field gradients generated by permanent magnets. The process was speeded up through the employment of a simple set–up composed of two block magnets in repulsion configuration and through the functionalization of the particle surfaces with an acrylated silane. From the experimental analysis of the photocuring process, the residual stresses arising during the photopolymerization of functionally graded composite coatings based on the UV–curable HBP matrix and Fe3O4@SiO2 nanoparticles were evaluated with a Finite Element Method approach. These coatings were able to consistently decrease the residual stresses at the coating–substrate interface compared to those encountered either in composites with homogeneous compositions or in the pure polymer, provided that reinforcing fillers were concentrated at the coating–substrate interface. If instead nanoparticles were concentrated at the free surface, graded nanocomposite coatings could reduce by as much as 40% the interfacial stress arising from thermal variations, as demonstrated through numerical simulations. Nanoindentation tests were carried out on the surface of polymer nanocomposites exhibiting either graded or homogeneous distributions of Fe3O4@SiO2 nanoparticles in the UV–curable matrix. It was experimentally shown how only at relatively small indentation depths, large increases in modulus and hardnesswere obtained for graded composites with respect to their homogeneous counterparts. Through a Material Point Method approach, experimental nanoindentation tests were successfully simulated. Finally, nanocomposite systems based on epoxy matrices were considered. In particular, composites with gradients in permittivity were synthesized through the application of a magnetic field to suspensions of high–permittivity Fe3O4@TiO2 particles in an epoxy resin. The application of the magnetic force not only induced the movement of the particles, but also caused their alignment in high aspect ratio structures, resulting in rather steep permittivity gradients. Numerical simulations clarified how the as–synthesized materials, employed as insulators, have the potential to efficiently reduce the electric field stress at the electrode–insulator interface and have intrinsic durability advantages over their homogeneous counterparts.

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