Causa, Andrea (2015) Nanoparticle-Induced Morphologies in Multiphase Polymer Systems. [Tesi di dottorato]

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Abstract

Introducing nanoscale fillers into polymer matrices has recently emerged as a simple, cost-effective way to enhance the performances of the host materials. A mere capitalization of the filler properties to produce "nano-filled polymers", however, appears rather reductive. An undoubtedly more ambitious goal is the design of "genuine nanocomposites", based on a clever use of nanoparticles to impart new physical characteristics and behaviors that are absent in the unfilled matrix. In the present dissertation, the opportunity of pursuing such an approach to manipulate the phase-separated morphology of immiscible polymer blends is investigated from both a fundamental and a technological point of view. Due to the thermodynamic immiscibility of most polymers, phase separation typically occurs in polymer blending processes, such as melt compounding and solvent casting. The resulting segregated morphologies are determined by the mutual interactions experienced by the different fluid and solid phases during processing. In the first part of the study, the physical mechanisms that govern the melt-state microstructural evolutions of polymer blends in the presence of nanoparticles are elucidated through the combination of viscoelastic measurements and morphological analyses. It is highlighted that, when the mobility of the nanoparticles is inhibited due to their anchoring at a polymer/polymer interface, the assembly of the filler is hindered until interface saturation happens. Above this threshold, the same dynamics as in monophasic melts are recovered. Even so, plate-like particles provided with sufficient bending stiffness are found to induce bulk morphological modifications without the need of interfacial jamming. Among others, the stabilization of irregularly-shaped polymer domains and clustering phenomena, which affect the phase inversion of the blend, have been observed. The second part of the study is focused on the formation of films of semi-crystalline polymers through solvent casting. The precipitation and crystallization of the polymers and, eventually, the surface morphology and texture of the resulting flat systems are governed by a complex interplay among wetting, solvent evaporation and, in case of polymer blends, phase separation processes. In addition, all these phenomena are ultimately related to the interactions among the constituents of the starting polymer solution and the casting substrate. Incorporating nanoparticles to the pristine solution significantly affects the evolution of the polymer phases and the final blend morphology, the filler assembly dynamics being in turn influenced by the changing properties of the suspending medium during solvent evaporation. Nanoparticles can induce either a coarsening or a refinement of the phase domains, depending on their state of dispersion in the solution and the structures they form in the course of the casting process. The acquired fundamental knowledge on the nanoparticle-induced morphological modifications in multiphase systems is ultimately capitalized for technologically relevant targets, such as the production of heat-resistant formulations based on recycled plastics and of biodegradable and biocompatible polymer films with prescribed topography and texture.

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