The recent emergence of molecular films as candidates for functional electronic materials has prompted numerous investigations of the underlying mechanisms responsible for their structure and ...formation. This review describes the role of epitaxy in molecular organization on crystalline substrates. A much‐needed grammar of epitaxy is presented that classifies the various modes of epitaxy according to transformation matrices that relate the overlayer lattice to the substrate lattice. The different modes of epitaxy can be organized hierarchically to reflect the balance of overlayer–substrate and molecule–molecule energies. In the case of molecular overlayers, the mismatch of overlayer and substrate symmetries commonly leads to coincident epitaxy in which some of the overlayer lattice points do not reside on substrate lattice points. Analyses of numerous reported epitaxial molecular films reveal that coincidence is quite common even though, based on overlayer–substrate interface energies alone, not as energetically favorable as commensurism. The prevalence of coincidence can be attributed to overlayer elastic constants, associated with molecule–molecule interactions within the overlayer, that are larger than the elastic constants of the overlayer–substrate interface. This condition facilitates prediction of the epitaxial configuration and overlayer structure through simple and comparatively efficient geometric modeling that does not require the input of potential energies, while revealing the role of phase coherence between the overlayer and substrate lattices.
A classification scheme for epitaxial crystalline overlayers on crystalline substratesis described that is particularly useful for understanding the structure and formation of molecular films. Geometrical lattice match calculations useful for the confirmation and prediction of epitaxial relationships are also presented. The Figure shows the commensurate structure of Cu‐Pc on MoS2.
The mechanical properties of many composites are determined in part by the chemical structure and bonding at the interface between constituents in the microstructure. The study of these interfaces in ...molecular crystal – polymer composites is difficult using traditional techniques such as electron microscopy or X-ray scattering because of weak or detrimental interactions between the probe and materials. Here, the interface between acetaminophen and a poly(ester urethane) copolymer is analyzed using ellipsometry, infrared spectroscopy, and neutron reflectometry. These materials were chosen for their relevance to pharmaceutical tablets and plastic-bonded explosives. The acetaminophen was shown to dissolve into the polymer coating and creates an interphase region between the two materials; this mixing is almost certainly produced by typical formulation conditions, and likely affects mechanical response of the composite. Additionally, reflectometry shows that plasticizing the polymer alters this interphase region. These techniques can be applied to similar composites to reveal the relation between formulation conditions, constituent interface microstructure characteristics, and bulk mechanical response.
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The study of high performance composites such as plastic-bonded explosives under extreme conditions often requires innovative experimental techniques. Here, static synchrotron X-ray phase-contrast ...imaging (PCI) of simulated explosive materials has been performed at high speed in an effort to determine feasibility of imaging material response to dynamic, high-strain rate events (102–107s−1). The microstructure of pristine materials, idealized composites and simulated explosive composites has been characterized with synchrotron PCI at the Advanced Photon Source. High spatial resolution (2μm) of the microstructure was achieved with 5μs exposures, and features such as interfaces, cracks, voids, and bubbles were clearly observed. The likelihood of obtaining sufficient phase information at even faster exposures (e.g., 0.2–0.5μs) is shown to be high.
Crack initiation and propagation is a common concern for molecular composites such as plastic bonded explosives (PBXs) and pharmaceutical tablets. Under compressive stresses, cracks form at contacts ...between crystals and propagate along crystal-binder interfaces, causing composite failure. To investigate this process, crystal-binder interfaces have been characterised and their mechanical properties tested. Here, samples were created with interfaces representative of those in PBXs and characterised with surface energy measurements and neutron reflectometry (NR). Nanoindentation was performed to simulate the deformation and cracking that occurs at crystal-crystal contacts through the binder. NR revealed that use of a plasticiser disrupts typical crystal-binder intermixing and results in a mechanically weaker interface. During nanoindentation, a plasticised binder was observed by atomic force microscopy to delaminate around indentation impressions, whereas a non-plasticised binder did not. Differences in interfacial adhesion and incompatible strain, dictated by the elastic-plastic film compliances, were used to explain the contrasting delamination behaviours.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The mechanical and chemical response of energetic materials is controlled by a convolution of deformation mechanisms that span length scales and evolve during impact. Traditional methods use ...continuum measurements to infer the microstructural response whereas advances in synchrotron capabilities and diagnostics are providing new, unique opportunities to interrogate materials in real time and in situ. Experiments have been performed on a new gas-gun system (IMPact system for Ultrafast Synchrotron Experiments) using single X-ray bunch phase contrast imaging (PCI) and Laue diffraction at the Advanced Photon Source (APS). The low absorption of molecular materials maximizes x-ray beam penetration, allowing measurements in transmission using the brilliance currently available at APS Sector 32. The transmission geometry makes it possible to observe both average lattice response and spatially heterogeneous, continuum response (1-4 um spatial resolution over ~2 × 2 mm area, 80 ps exposure, 153 ns frame-rate) in energetic materials ranging from single crystals to plastic-bonded composites. The current work describes our progress developing and using these diagnostics to observe deformation mechanisms relevant to explosives and the first experiments performed with explosives on IMPULSE at APS.
The onset of plastic deformation was investigated using nanoindentation in single crystals of the explosive cyclotrimethylene trinitramine (RDX). Cleavage and habit planes were tested revealing a ...range of yielding behaviors. Smooth habit planes of unprocessed single crystals exhibited distinct yield points near the theoretical shear strength; planes produced by cleavage yielded at lower applied stresses. Cumulative probability distributions of maximum shear stresses at yield were used to illustrate the representative yielding behavior for samples prepared by the different methods. A statistically significant difference was observed for cleavage and habit planes. This suggested that structural defects, such as dislocations from growth and sample preparation, were being probed and nanoindentation can be used to correlate the mechanical response of organic molecular crystals with defect density. This capability may help explain the observed range of measurement differences in fundamental properties in this class of materials, such as sensitivity to the initiation of detonation in explosives, and disparate tablet integrity and stability responses in polymorphs of some pharmaceutical materials.
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