Hurova D. Features of the structure of solid molecular compounds: Nitrogen 14N2 and 15N2 and polymers

The dissertation presents the results of the structural studies of individual molecular compounds in crystalline and amorphous states. From the analysis of the intensities of X-ray diffractograms of solid 14N2 and 15N2 (solid nitrogen) in the orientationally ordered phase, the temperature dependences of the mean square displacement of nitrogen molecules from the lattice site and the orientational order parameter were obtained. Using the radial distribution function construction method and quantum mechanical calculations, changes in the structure of the poly (4,4'-oxydiphenylene-pyromellitimide) polymer film under the external influence (uniaxial stretching and comprehensive compression) are investigated. The structural characteristics of composites based on epoxy resin with carbon nanostructures impurities (copolymers, single- and multi-walled carbon nanotubes, graphene oxide) are studing using the X-ray diffraction method and optical microscopy. The introduction provides a rationale for the relevance of the dissertation topic in both fundamental and applied aspects. The purpose, tasks, objects and methods of research are determined. The scientific novelty and practical significance of the obtained results are formulated. Information about publications, personal contribution of the researcher and results approval of the dissertation is provided. The first chapter presents a review of the literature. The properties of molecular compounds, including cryocrystals and polymers, are examined. Molecular crystals are characterized by a transition from an orientationally disordered state to an orientationally ordered state (order-disorder). In the low-temperature phase, the molecules are oriented along the corresponding crystallographic axis and exhibit translational and librational vibrations. The study of solid nitrogen 14N2, as well as theoretical and experimental work on the determination of the orientational order parameter is considered. Previously conducted structural and mechanical studies of polyimide films under external influences are presented. Works on the study of composites with the addition of carbon nanostructures are given. At the moment, much attention is paid to the analysis of the mechanical properties of these composites, and there is a small number of works on the study of these structure. In the second chapter, the methodology of conducting X-ray structural studies of crystalline and amorphous compounds, as well as optical studies of epoxy resin with carbon nanostructure additives, is described in detail. The chapter also outlines the methodology for performing low-temperature experiments (obtaining in situ samples in an X-ray cryostat). The third chapter is dedicated to structural studies and the analysis of X-ray reflection intensities from 14N2 and 15N2 in the orientationally ordered phase. A novel approach for describing the translational and librational vibrations of nitrogen molecules from experimental X-ray data is presented. The temperature dependence of the mean square displacement of 14N2 and 15N2 molecules from the lattice site in the orientationally ordered phase was obtained from the X-ray data. The temperature dependence of the orientational order parameter in the low-temperature 15N2 phase was constructed. It was determined that the temperature at which 15N2 molecules begin active reorientational movement is about 45 K. The fourth chapter focuses on the study of structural changes in 4,4'-oxydiphenylene-pyromellitimide polymer films under uniaxial tension and comprehensive compression. Using the radial distribution function method and quantum mechanical calculations (DFT method), it was shown that under uniaxial tension, the molecular configuration transforms due to changes in the angles between the two aromatic rings. Comprehensive compression doesn’t lead to noticeable changes in the geometry of the molecules but results in the mutual ordering of polymer chains. The fifth chapter is dedicated to the study of composites based on epoxy resins with impurities of copolymer, single- and multi-walled carbon nanotubes, and graphene oxide. From the obtained X-ray data, it was established that the addition of carbon nanostructure impurities (~1% by weight) doesn’t lead to the formation of a crystalline phase. The addition of single- and multi-walled carbon nanotubes influence on intermolecular interactions between the matrix molecules, diffraction from the carbon nanotubes "phase" isn’t observed. The addition of graphene oxide to the epoxy matrix leads to the multiphase substance formation, the value of the short-range order region for graphene oxide is about 20 Å. Optical studies of the composite with graphene oxide impurities indicate an uneven distribution of graphene clusters in the sample, which may be related both to the peculiarities of the sample preparation and to the conduct of mechanical testing