| Περίληψη: | The objective of the present doctoral dissertation is the development and application of advanced computational tools for the prediction of the structural properties of Carbon Nanotube-based polymer membranes, as well as the transport properties of water molecules diffusing within these nanocomposites. Molecular Dynamics simulations were used up to their computational limits to achieve this study, followed by a detailed geometric analysis involving Monte Carlo integration. The present study can be divided in three parts. In the first part, the structural and conformational properties of poly (methyl methacrylate) (PMMA) nanocomposite systems are studied, with embedded Carbon Nanotubes (CNTs) of several diameters and loading, at several temperatures. The mobility of water was also studied in the pure and nanocomposite membranes. In the second part, the free volume as well as the volume which is accessible to several small diffusants (such as water molecules) is studied in the CNT-based nanocomposites, based on a geometric analysis method. In the final part, the geometric analysis is modified and applied to aerosol nanoparticles, aiming at the study of their phase state.
Molecular dynamics (MD) is utilized to simulate a model PMMA system containing randomly dispersed CNTs. Our objective is to determine the equilibrium structure and dynamic behaviour of PMMA chains at the interface with a carbon nanotube. The effect of CNTs with various diameters and concentrations in the host PMMA matrix on the equilibrium squared radius-of-gyration and squared end-to-end distance of PMMA chains is investigated. The density, structure, and conformation of PMMA both axially and perpendicular to the CNT surface are examined. MD simulations reveal that the presence of CNTs causes a small reduction in the size of polymer chains, which becomes more pronounced as the concentration (volume fraction) and diameter of CNTs in the nanocomposite increase. A comprehensive analysis of the conformations of adsorbed PMMA chains in terms of trains, loops, and tails, as well as their statistical properties is also provided. Importantly, PMMA chains tend to penetrate significantly inside the CNTs through their faces; as a result of CNT filling by PMMA chains, the region near the CNT mouths is characterized by a significantly higher polymer mass density (by nearly 45 %) than the bulk of the nanocomposite. Additional simulation results for local and terminal relaxation in PMMA-CNT nanocomposites reveal that, due to the strong PMMA-CNT attractive forces, all relaxation times in the interfacial region are significantly lengthened compared to the bulk, and the same is true for the diffusive (translational) motion of the chains. The density profile that develops in the vicinity of CNTs (both axially and radially) appears to significantly slow down PMMA dynamics at all length scales. How this affects the nanocomposite's glass transition temperature is also analysed.
The simulations of the CNT-based polymer nanocomposites were repeated in the presence of a few water molecules. The high densities of the polymer in the interfacial region around the CNT mouths drastically reduce the diffusivity of small penetrants (in our study, water molecules) in the nanocomposite membrane due to the extremely long times required for these small molecules to diffuse through such a dense interfacial layer before reaching the interior of the nanotubes, where they can travel extremely fast. According to the simulations, the time required for a confined water molecule to escape from the closed mouths of a CNT may be many orders of magnitude longer than the time required for the same molecule to pass through the CNT pore. This research demonstrates the importance of completely avoiding (or at least minimizing) the penetration of polymer chains into the CNT pores through the tube mouths for the efficient transport of small to moderate-sized molecules in model CNT-based polymer membranes, as this provides the greatest resistance to their flow.
In the second part of this Thesis, a geometric analysis is applied to the previously studied systems. Using Delaunay tessellation followed by Monte Carlo integration, the clusters of sites where a hard-sphere penetrant of radius equal to a few Angstroms can reside in model carbon nanotube - atactic poly(methyl-methacrylate) (CNT-PMMA) nanocomposite microstructures are determined, and their dependence on penetrant size and temperature is analysed. The starting configurations for the geometric analysis were formed by cooling to lower temperatures and re-equilibrating completely equilibrated atomic structures at a higher temperature using long molecular dynamics simulations. Because the tetrahedra produced by the Delaunay tessellation are irregular in space, an analytical calculation of free volume is difficult; hence, Monte Carlo integration was used. The unoccupied volume and the volume accessible to a spherical penetrant of a particular radius was estimated inside each tetrahedron by taking into consideration the space filled by polymer and CNT atoms. Afterwards, the distribution of the volume and size of the related cavities was determined. Finally, the network of clusters formed was quantified and likely diffusion pathways for the studied penetrant were determined, by identifying neighbouring clusters of tetrahedra that are mutually accessible to a given penetrant using a connectivity algorithm very similar to that proposed by Greenfield and Theodorou [Macromolecules, 1993, 26, 5461-5471].
In the last part of this Thesis, the geometric analysis was modified to study aerosol nanoparticles. The microstructures of aerosol nanoparticles were examined using Delaunay Tessellation and Monte Carlo integration to determine the relationship between their free volume and their state of matter. The investigated nanoparticles include water, cis-pinonic acid, and inorganic ions (such as sulfate and ammonium). The effects of relative humidity and organic content on the free and accessible volume to several penetrating particles was explored. The majority of water-sized molecule-accessible holes are located in the middle and outer domains of the nanoparticles, which are dominated by organic molecules. On the other hand, no cavities could be found in the inorganic areas that may host any existent penetrant. At high humidity levels, the free volume in the interfacial areas between organic and inorganic substances diminishes, indicating full separation of organic molecules and inorganic ions, the latter exhibiting a preference to be organized at the core of the nanoparticle. It was discovered that cis-pinonic acid forms a single island in the outer region of the nanoparticle with the same density as pure bulk cis-pinonic acid, suggesting the existence of an amorphous, soft solid phase in the outer region of the nanoparticle. The inorganic mass, on the other hand, forms a single continuous island with a density comparable to that of ammonium sulfate, suggesting the presence of a rigid solid phase at the particle's core.
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