Národní úložiště šedé literatury Nalezeno 2 záznamů.  Hledání trvalo 0.00 vteřin. 
Cellular polymer nanocomposites
Zárybnická, Klára ; Crosby, Alfred (oponent) ; Lehocký,, Marián (oponent) ; Jančář, Josef (vedoucí práce)
This dissertation thesis deals with the preparation and characterization of polymer nanocomposite foams with a focus on means to control their structure at multiple length scales and application in 3D printing in their fabrication. The aim of this work is to investigate polymer nanocomposite with hierarchical structure – from the nano-, through the micro to macro scale. The structural properties of polymer nanocomposites prepared from glassy polymers by the solvent-casting method were investigated in the first part of the work. It has been shown that the difference in the solubility parameters of the polymer and the solvent plays a crucial role. This finding has been verified for systems containing various nanoparticles, polymers, and solvents. With the knowledge of the general principles controlling the structure of nanocomposites, impact polystyrene filled with nanosilica was investigated in greater detail. These nanocomposites were used for the preparation of nanocomposite foams. The porous structure was achieved using a thermal chemical blowing agent azodicarbonamide. The filaments were extruded and the material was processed by 3D printing into the required shapes and foamed. The result was a hierarchical system with the organization of the structure from nano (organization of nanoparticles), through micro (two-component polymer blend structure and foam structure) to macro scale (foam structure and 3D printed design). The effect of nanoparticles on the structure and the thermal and mechanical properties of polymeric foams were observed. The nanoparticles operate as a nucleating agent in the formation of the foam. Pores are easily formed on their surface so that with the content of nanoparticles in the system smaller pores have been formed, which helped to make the foam fine and homogeneous. The presence of nanoparticles changed the surface energy of the blowing agent grains, thanks to which it decomposed at lower temperatures and foaming was even faster. At the same time, nanoparticles have the potential to reinforce foam walls and thus improve mechanical properties. 3D printing is a popular and widespread technique, due to its simplicity it is in many laboratories and test institutions, therefore the demand for filaments with special properties is growing. The material developed in this dissertation is essentially a finished and characterized product that could contribute to the satisfaction of this claim.
Magnetically assembled nanoparticle structures and their effect on mechanical response of polymer nanocomposites
Zbončák, Marek ; Khúnová,, Viera (oponent) ; Crosby, Alfred (oponent) ; Jančář, Josef (vedoucí práce)
Magnetically directed self-assembly in polymer nanocomposites is studied in this dissertation thesis. Structuring of the polymer nanocomposites by application of relatively weak external magnetic fields (B=0-50 mT) has been proven to be convenient method for the control of their nano- and microstructure. The effect of the field strength, particle loading, viscosity and assembling time on the resulted structure was studied in different systems such as photopolymer, polyurethane or colloidally dispersed magnetic nanoparticles in acetone with a small amount of dissolved polymer. Self-assembled structures – without application of the external magnetic field exhibit a multi-step aggregation into nanoparticle assemblies with a complex shape. By the calculation of interaction energies between the nanoparticles, magnetic interactions were attributed to be mainly responsible for the aggregation in self-assembled systems. With an increasing magnetic field, magnetic nanoparticles are rapidly arranged into high aspect ratio one-dimensional particle chains with a homogenous orientation in the bulk polymer matrix. After prolonged assembling time, the structures gradually grow from small submicro structures to large microscopic superstructures. This method exhibits large potential to be used for controlled creation of wide variety of structures in polymer nanocomposites suitable for technological applications and/or for fundamental studies. Magnetically structured polymer nanocomposites show significant directional anisotropy of composite’s stiffness at the temperatures above glass transition of the system while there is no effect on the mechanical response in glassy state. Longitudinally oriented structures exhibit much stronger effect on the composite’s stiffness. Reinforcing effectivity exhibits temperature dependent course with a maximum obtained approximately 60 °C above glass transition. The structure of magnetically assembled polymer nanocomposites was described by multi-level hierarchic model of material. Micromechanics was used to address the orientation dependent reinforcement and temperature dependent stiffness of the hybrid nanoparticle-polymer structures. Load carrying capability, deformation and non-zero stiffness of the hybrid structures were attributed to be responsible for the reinforcement of the polymer nanocomposites. The presence of polymer bridges between nanoparticles transmitting the stress through the magnetic structures is proposed to be essential for the mechanical properties of polymer nanocomposites and for stiffness of the hybrid structures.

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