The principal goal of this thesis is the generation of hierarchical lightweight materials with desired mechanical properties. The work in this thesis contributes toward this objective in different levels, assembling nanoparticles using microfluidic bubbles as templates, exploring the ability to modify the surface and tailor the structure and mechanical properties of these shelled bubbles, and incorporating these nanoparticle- shelled bubbles in composites observing the effect of the hierarchical assembly on the overall macroscopic properties.
Chapter 1 describes the challenges present on the generation of lightweight materials with high stiffness, strength and toughness. It summarizes the current approaches of generating syntactic foams, using hollow microparticles and polymers in composites, and the limitations present when the hollow microparticles used are generated using conventional bulk methods. Chapter 1 presents microfluidics as a method to generate nanoparticle-shelled bubbles and its benefits over conventional bulk methods, and summarizes the goals and approaches of this thesis.
Chapter 2 presents the potential of nanoparticle assemblies using microfluidic bubbles. It shows that a precise control over the conditions during the microfluidic generation of gas-in-oil-in-water (G/O/W) bubbles enables the generation of ultra-stable nanoparticle-shelled bubbles that withstand drying without breaking or compromising their structure. This finding allows for a deeper exploration of the potential of these nanoparticle assemblies, for example, this chapter also describes the assembly of these
nanoparticle-shelled bubbles at interfaces in hexagonal close packed arrays or in fractal- like structures when the surface is partially modified. A deep fundamental study of the behavior at the interface of these bubbles is also presented.
Chapter 3 proposes the use of these nanoparticle-shelled bubbles as building blocks for the generation of lightweight materials. It describes the ability to tailor the structure of these shelled bubbles using thermal treatments that provides the shelled bubbles with different mechanical properties, optimal for targeted applications. Therefore this chapter presents the potential of these nanoparticle-shelled bubbles and provides an in depth study of the structure-property relation, necessary for the practical application of these materials. In Chapter 3 the mechanical properties of shelled bubbles are studied using nanoindentation and in situ compression tests.
Chapter 4, complementing Chapter 3, uses simulation work to explain the mechanical behavior of the different nanoparticle-shelled bubbles described in the previous chapter. As-assembled bubbles can be modeled using an elastic-perfectly plastic material, while calcined and sintered bubbles display an elastic behavior. The differences in the secondary cracking events observed in situ are explained by FEA using a Drucker- Prager material model for the as-assembled bubbles that incorporates pressure sensitivity and plastic dilation. In addition to the material models the simulation work of this chapter reveals that the geometry of the shells have a significant impact on the mechanical behavior. The modeling of the shelled bubbles in this chapter provides invaluable information for designing purposes.
Chapter 5 presents the preparation and mechanical characterization of syntactic foams using the previously studied nanoparticle-shelled bubbles, calcined (at 1000 ºC) and sintered (at 1200 ºC), as fillers. This chapter addresses the goal of generating three dimensional assembly of the nanoparticle-shelled bubbles in composites. A great emphasis is placed on enhancing the interfacial strength between fillers and matrix since it is a current challenge that can easily compromise the mechanical properties of the composite. The mechanical properties of composites made using porous and rough calcined bubbles display better mechanical properties than composites made with smooth and non-porous sintered bubbles despite the fact that individual sintered bubbles present much better properties than calcined bubbles (see Chapter 3 and Chapter 4). The roughness and porosity of the calcined bubble shells by means of increasing the surface area and contact area with the polymeric matrix, greatly enhances the interfacial strength of the composite. These results show that hierarchically assembled nanoparticles using microfluidic bubbles as templates, gave us the opportunity to make rough and porous bubbles to generate lightweight materials with high mechanical properties.
Some fundamental questions and challenges are still open to be resolved and explored. For example, it will be important to quantify the interfacial strength of the nanoparticle-shelled bubbles in the polymer composites to better understand the effect that roughness and porosity of the bubbles can impart. Several studies have suggested methods to measure the interfacial strength in particulate composites by determining the debonding stress by acoustic emission experiments,151 or more commonly by measuring
the macroscopic tensile properties of the composite and determining the interfacial strength with semi-empirical equations.152
It will be of interest to explore the potential of the microfluidic generation of bubbles, further exploring the generation of non-spherical bubbles, which could result in anisotropic materials when arranged in three dimensional regular structures. Platelet shaped bubbles can be formed by compression of A/O/W compound bubbles before complete solvent evaporation as shown the preliminary result in Figure 6.1. The compression is performed between two flat substrates. The final aspect ratio is controlled using spacers of different sizes.
Figure 6.1 SEM image of a platelet bubble
Other fundamental questions remain to be answered in the two-dimensional assembly of bubbles at interfaces, in which a controlled Janus boundary could lead to interesting arrangements for practical applications.
We have observed that the interfacial strength of the bubble/polymer composites notably increases when roughness and porosity are present in the bubbles. Therefore, it
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will be important to explore the potential of this finding. For example, study different ways to control the porosity and roughness of the bubble shells using different nanoparticle sizes to form the shell, or different materials, that when thermally processed will melt at different temperatures. Another way to enhance the interfacial strength will be to combine methods of polymer infiltration through the nanopoarticles forming the shell using for example temperature and vacuum that will increase the interfacial area between nanoparticles and polymer.
Another interesting study will be to perform finite element analysis of the bubble- polymer composites. Simulations can provide a detailed independent analysis of the effects of the structural variables of the composite (volume fraction of bubbles, mechanical properties of constituents, polydispersity on the geometry and/or mechanical properties of the fillers, etc.) on the overall mechanical response of the composite. This understanding will be critical for designing purposes in the generation of functional lightweight materials.