Grazing-incidence small-angle X-ray or neutron scattering of thin polymer films reveals information about the ordering and preferential orientations of the phase-separated microdomains within the films. The grazing-incidence geometry enhances the surface sensitivity; however, the scattering has to be treated within the framework of the distorted-wave Born approximation. In this work, the case of thin films with lamellar mesostructure is studied, where the orientation of the lamellae is either perpendicular or parallel to the film interfaces. For perpendicular lamellae, Bragg rods are found, which are extended along the film normal, whereas for parallel lamellae, peaks along the film normal appear. The positions of the maxima present in the latter case are explained by accounting for refraction at the film surface and reflection at the film-substrate interface. The results are relevant for thin films of lamellar diblock copolymers.
Grazing incidence small-angle scattering and electron microscopy have been used to show for the first time that nonspherical nanoparticles can assemble into highly ordered body-centered tetragonal mesocrystals. Energy models accounting for the directionality and magnitude of the van der Waals and dipolar interactions as a function of the degree of truncation of the nanocubes illustrated the importance of the directional dipolar forces for the formation of the initial nanocube clusters and the dominance of the van der Waals multibody interactions in the dense packed arrays.
Using tapping mode atomic force microscopy, we studied the influence of molar mass (13.9−183 kg/mol) on the lamellar orientation at the surface of thin films of symmetric polystyrene−polybutadiene
diblock copolymers prepared via spin-coating and by slow solvent-casting. The ratio between film thickness
and lamellar thickness was varied between 0.5 and 10. Whereas for film thicknesses between 1 and 10
times the lamellar thickness, a lamellar orientation parallel to the film surface is preferred for low molar
masses (below ∼55 kg/mol), high molar mass samples (above ∼90 kg/mol) rather form lamellae oriented
perpendicular to the surface. For film thicknesses equal to the lamellar thickness, the films do not exhibit
any texture, whereas for film thicknesses equal to half the lamellar thickness, a weak surface structure
could be observed. This is consistent with symmetric wetting, i.e., the same block adsorbs at both film
interfaces, but their selectivity is only weak. Thus, entropic contributions like chain stretching along a
wall and the enrichment of free chain ends at the interface become important, and both parallel and
perpendicular orientations of lamellae in thin films can occur.
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