This research was undertaken to develop state-of-the-art interfacially polymerized (IP) defect-free thin-film composite (TFC) membranes and understand their structure-function-performance relationships. Recent research showed the presence of defects in interfacially polymerized commercial membranes which potentially deter performance in liquid separations and render the membranes inadequate for gas separations. Firstly, a modified method (named KRO1) was developed to fabricate interfacially polymerized defect-free TFCs using m-phenylene diamine (MPD) and trimesoyl chloride (TMC). The systematic study revealed the ability to heal defects in-situ by tweaking the reaction time along with considerably improving the membrane crosslinking by controlling the organic solution temperature. The two discoveries were combined to produce highly crosslinked, defect-free MPD-TMC polyamide membranes which showed exceptional performance for separating H2 from CO2. Permeance and pure-gas selectivity of the membrane increased with temperature. H2 permeance of 350 GPU and H2/CO2 selectivity of ~100 at 140 °C were obtained, the highest reported performance for this application using polymeric materials to date. Secondly, the membranes produced using KRO1 were tested for reverse-osmosis (RO) performance which revealed significantly improved boron rejection compared to commercial membranes reaching a maximum of 99% at 15.5 bar feed pressure at pH 10. The study also unveiled direct correlations between membrane crosslinking and salt separation performance in addition to the membrane surface roughness. Thirdly, this was followed by replacing the conventional IP TMC monomer with a large, rigid and contorted tetra-acyl chloride (TripTaC) monomer to enhance the performance of IP TFCs. The fabricated TFCs showed considerable performance boosts especially for separating of small solutes from organic solvents such as methanol. A rise in H2 permeance was also observed compared to the conventional MPD-TMC TFCs while reaching a maximum H2/CO2 selectivity of 9 at 22 °C. Finally, the research was completed by showing the potential of KRO1 for fabrication of defect-free TFCs using a range of aqueous diamine monomers. KRO1 enabled defect-free gas properties for all monomers used showing exceptional performance for separating H2-CO2 and O2-N2 mixtures. It was further shown that the formulation could also improve the RO separation of interfacially polymerized polyamide TFCs beyond those shown by commercially available TFCs.

The subject of the doctoral thesis is the study of moisture resistance of macro-defect-free (MDF) composites based on calcium-aluminate cement and polyvinylalcohol used as polymer. MDF cements are perspective materials providing unique properties relative to traditional cement pastes with great potential to incoming constructional utilization. But it is known that MDF cements exhibit sensitivity to water, with swelling and reduction of strength. Therefore the work is aimed in monitoring of an organic polymer effect on the moisture resistance of MDF materials and phase changes. The effects of three different organotitanate cross-linking agents on the properties were investigated. The organotitanate-modified MDF cement samples exhibited improved moisture resistance as compared to the standard samples when exposed to 100 % relative humidity or water at laboratory conditions. The influence of composition at different curing on the properties of model MDF cement samples was observed. The characterization of MDF samples was based especially on test of flexural strength, XRD, DTA/TGA and EGA, EDS and scanning electron microscopy analyses.

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The subject of the thesis is to improve the moisture resistance of MDF (macro-defect-free) composites prepared on the basis of aluminous cement and polyvinyl alcohol. These materials are characterized by the absence of defects in the structure and high mechanical performance. These features make MDF composites a promising type of material that could be used in the future for construction purposes. The basic deficiency of MDF composites is insufficient resistance to water or moisture, accompanied by a significant decrease in strength. Therefore, this work deals with increasing of moisture resistance using organotitanate agent and modification of the polymer. Two types of polyvinyl alcohols differing in hydrolysis degree and degree of the polymerization has been modified. Based on the results of the measurement the effect of storage conditions on the properties and structure of prepared MDF composites was observed. The characterization of MDF composites were realized by measurement of flexural strength in bending, scanning electron microscopy with EDS analysis, TG-DTA-EGA, water absorption and consequently the porosity of all prepared MDF composites.

Thesis (M.Sc. (Physics)) -- University of Limpopo, 2022 / The gas sensing performance of two-dimensional (2D) hexagonal boron nitride nanosheets (h-BNNSs) has being studied by means of computational and experimental methods. The structural, stability and vacancies properties of both defect free and defected 2D h-BNNSs were studied using the classical molecular dynamics (MD) approach. The calculations were performed in the NVT Evans and NPT hoover ensembles using the Tersoff potentials with the Verlet leapfrog algorithm to obtain reliable structural properties and energies for defect free, boron (B) and nitrogen (N) vacancies. B and N defect energies were calculated relative to the bulk defect free total energies, and the results suggest that N vacancy is the most stable vacancy as compared to the B vacancy. The radial distribution functions and structure factors were used to predict the most probable structural form. Mean square displacements suggests the mobility of B and N atoms in the system is increasing with an increase in the surface area of the nanosheets. Results obtained are compared with the bulk defect free h-BNNSs. Experimentally, 2D h-BNNSs were synthesised using the wet chemical reaction method through chemical vapour deposition (CVD) catalyst free approach. The X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR), Raman Spectroscopy (RM), UV-visible Spectroscopy (UV-VIS), dynamic light scattering (DLS), Energy Dispersion Spectroscopy (EDS) and Brunauer-Emmett Teller (BET) were adopted to attain the structural properties of the nanosheets. Each spectroscopic technique affirmed unique features about the surface morphology of h BNNSs. The crystallinity of the nanosheets with the stacking of the B and N vii honeycomb lattice was validated by the XRD, while the TEM disclosed the specimen orientations and chemical compositions of phases with the number of layers of a planar honeycomb BN sheet, the EDS express the atoms present in the samples and BET validated the surface area of the materials. The FTIR, RM, DLS and the UV-vis expressed the formation of the in-plane, out-of-plane h-BN vibrations and, the nature of the surface with the thickness, particles stability together with the optical properties of the nanosheets. From TEM, FTIR, RS and BET the material fabricated at 800°C showed different morphologies, large number of disordering together with high surface area, which enhances the sensing properties of the nanosheets. However, with an increase in temperature the sensitivity of the nanosheets was found to decrease. Additionally, the UV-vis results, confirmed a lower energy band gap of 4.79, 4.55 and 4.70 eV for materials fabricated at 800, 900 and 1000 °C, that improved the semiconducting properties of the materials, which in return enhanced the sensing properties of the nanosheets. The gas sensing properties of the 2D h BNNSs were also investigated on hydrogen sulphide (H2S) and carbon monoxide (CO). The fabricated sensor based on 800 – 900 °C h-BNNSs showed good sensitivity towards ppm of H2S at 250 °C. The excellent gas sensing properties could be attributed to high surface area, small crystallite size, defect/disordering of h BNNSs. Overall, the h-BNNSs were found to be more sensitive to H2S over CO. / University of Limpopo (UL) Mintek Council for Scientific and Industrial Research (CSIR) Center for High Performance Computing (CHPC)