PhD Thesis Defence Presentations - Panagiotis Mermigkis

The structural and conformational properties of poly (methyl methacrylate) (PMMA) nanocomposite systems are studied through Molecular Dynamics simulations, with embedded Carbon Nanotubes (CNTs) of several diameters and loading, at several temperatures. 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. 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 high densities of the polymer in the interfacial region around the CNT mouths drastically reduce the diffusivity of small penetrants 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. Afterwards, 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 are determined, and their dependence on penetrant size and temperature is analysed. 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. In the last part of this Thesis, the geometric analysis was modified to study aerosol nanoparticles, in order 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. 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.