X-ray Diffraction and Tomography Studies of Functional Organic Fibres
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- Institutt for fysikk 
Structural characterisations of materials belong to the primary interests of basic and applied research, since they provide insights into the interactions of molecules and atoms, establish correlations with desired properties and point towards improving materials. One of the widely used tools for such structural investigation is X-ray diffraction. Its discovery nearly a century ago opened up a new era for visualising the internal nanostructures of different materials in both two and three dimensions and revolutionised our understanding to different materials properties. In this thesis, results of fibre diffraction studies by different X-ray based techniques, combined with some computer-aided modelling, on variety of synthetic and natural polymeric fibres comprising high performance, natural organics and semiconducting liquid-crystalline fibres, are presented and their potentials and results in structure determination are discussed. Fibres are ubiquitous in everyday life and a wide variety of objects which we are everyday in contact with them such as clothes, wooden chairs, high performance tires, sailing ropes, etc are consisted of fibres. It is well know that the properties and functions of many polymeric fibres at real working conditions are often strongly related to their structure at the micro- and nano level. Therefore, understanding the molecular packing/arrangement of these nanostructures, their orientation, size, shape, and their response to the applied external strain is important for understanding the performance of the fibres in new and challenging technological applications. In this thesis using different X-ray phase contrast imaging techniques, namely Talbot grating-based imaging and Coherent Diffractive Imaging (CDI) of Ptychography the effect of humidity on a piece of cotton fabric and single silk and wool fibres has been investigated at different length scales, respectively. In ptychography experiment, we resolve internal and surface nanostructures of both silk and wool fibres in 3D. In addition, we show quantitative detailed information about the spatial density variations in the form of detailed maps of the size, shape and orientation distributions of the nanopores inside the silk fibre at both dry and humid conditions. We found that for both silk and wool, fibres swell anisotropically in humid conditions. To the best of our knowledge, this is the first in situ experiment performed with this nano imaging tool. We also have used another phase imaging technique, namely Talbot grating-based imaging, to investigate the applicability of Talbot imaging for studying humidity transport in a piece of weaved cotton fabric, with a resolution much lower in the ptychography experiment, in the micrometer range. We have shown that with having access to three different image modalities from this technique, namely absorption-, phase- and dark-field images, this technique is a suitable non-destructive tool for tracking (studying) the dewetting process in the weakly scattering materials at different in situ conditions and act as a complementary techniques to the ptychography imaging technique in term of allowing to image a bigger sample (i.e. larger field of view). We found that in the dry and wet situations, the dark-field and phase images respectively give the “best” image in terms of resolving many micron-sized details in the sample. In addition, results of Small-Angle X-ray Scattering (SAXS) experiment on a set of high performance fibres (PPTA) under axially compressive and tensile strain have been presented and discussed. The strain values both in heat treated and as-spun Twarons have been calculated using elestica loop method and a two dimensional heuristic function for fitting the 2D scattering patterns of them has been used. We show that exerted tensile and compressive strain affect the nanostructures of these fibres. In fact, when exceeding a certain amount of strain, irreversible destruction of the original nanostructure takes place, leading to qualitative changes of the SAXS pattern. Finally, the molecular packing, arrangements, orientations and interatomic distances of two sets of slightly different semiconducting liquid-crystalline extruded fibres have been obtained using Wide-Angle X-ray Scattering (WAXS) and scanning WAXS techniques. Particularly for the F8TBT polymeric fibre, structural data are provided for the first time, and information about the nearest-neighbour packing distances and coherence length of this macromolecule has been provided. For the second set of samples in this category, three discotic columnar hydrazone based fibres with six, eight and ten carbons of achiral alkoxy chains attached to their outer peripheries has been investigated using a multi-technique strategy combining mainly WAXS and Solid-State Nuclear Magnetic Resonance (SSNMR) and detailed information about the columnar structures, their unit cell type and dimensions has been deduced. In this study, for the first time the co-existence of two different packing morphologies in a columnar compound has been reported. I expect that the result of this PhD study will make some of structural and physical properties of these functional materials and their dependence to the different humidity levels more clear, thus providing information that leads to improved understanding and subsequently better fibre based products.