Dynamics and Structures of Nanoparticles in Polymer Melts

Abstract

The possibility of combining the physical properties of different components to obtain materials with new structural or functional features is what really makes polymer nanocomposites attractive. Actually, to fully exploit the potential of such a class of materials, the morphological and structural implications stemming from the nanometric sizes of the filler have to be taken into account. In the present dissertation, the physical mechanisms governing the nanoparticle dynamics and connectivity in polymer melts are investigated from a fundamental point of view. For such a purpose, the linear viscoelastic response of polymer nanocomposites has been deeply studied and morphological analyses have been used as support. As thermodynamically out-of-equilibrium materials, if provided by sufficient energy, polymer nanocomposites evolve to achieve equilibrium. In this process, the nanoparticles rearrange themselves in the melt and typically aggregate, dictating the ultimate microstructure of the material from which its macroscopic performances depend. In the first part of the study, the key role of the mobility of nanoparticles on their dynamics of assembly has been highlighted. The investigation then focuses on the rheological implications of the superstructures formed by the nanoparticles when they are free to rearrange in the host polymer medium. A simple two-phase model, recently proposed in the literature, has been used for such a purpose. First, the ability of the model in recognizing the elasticity of filler networks too weak to be identified by conventional viscoelastic analyses has been demonstrated. More importantly, the generalization of the model has been proposed and validated, proving that it is able to describe the linear viscoelasticity of polymer nanocomposites differing among them both in the nature of the nanoparticles and in the affinity between the polymer and filler phases. A further test of the robustness of the approach is given by verifying the possibility of satisfactorily describing the viscoelastic response of other complex fluids, such as nanocomposites based on biphasic polymer matrices. In the end, the gained fundamental knowledge has been exploited for practical purposes. The study is targeted to the investigation of the effect of material- and process-related factors on the filler state of dispersion in the final polymer nanocomposite.