Full Off-Specular and Specular Reflectometry for Soft Thin Film Analysis

Specular reflectivity (SR) and off-specular neutron scattering (OSS) are non-destructive scattering techniques, which, through deuteration (exchange of hydrogen atoms with deuterium), give a high contrast even among very similar chemical species and are therefore highly suitable for investigations of soft thin films. While readily available software exists for analysis of SR, there are no such existing tools for OSS. Through a combination of both, the former yielding a density profile in the direction normal to the sample surface and the latter yielding an in-plane lateral structure at each of the buried interfaces, one can obtain detailed information of buried morphology on length scales ranging from angstroms to 10 micrometres. OSS analysis was developed to analyze buried interfaces in absolute scale, with only structural parameters being free. Moreover, the SR and OSS are intricately linked and have to be fitted to the data simultaneously, constraining the model to a high degree. The OSS was applied to investigation of soft interfaces as model systems.The stability and instability of thin polymer layers have been studied extensively in the last decades, both as their applications as coatings have become part of our daily life, and in fundamental soft matter research. The mechanism of immiscible polymer film decomposition was investigated by preparing bilayers of deuterated polystyrene (d-PS) and protonated poly(methyl methacrylate) (h-PMMA) of different thicknesses on Si substrates. The in-plane lateral cutoff length was measured at the PS/PMMA interface as a function of d-PS thickness. For preannealed films with thickness of dPS below 250 angstroms, the result follows the prediction of the capillary waves cutoff length. For thicker films, spontaneous nucleation of the top layer, with a clear OSS signature, was observed. This was also the case for the non-preannealed samples, showing that preannealing causes irreversible adsorption on the substrate and thus stabilizes the layer.The bilayers were then measured also as a function of annealing time, measuring the nucleation of the top layer from the beginning. The measurements show that the breakup starts immediately after the start of annealing. For a sufficiently thick PMMA layer, the expected PS/PMMA interfacial width was measured. Different cutoff lengths were measured at different regimes of nucleation. Furthermore, a thick/thin/thick PMMA/PS/PMMA trilayer was prepared on an Si substrate, isolating the influence of the substrate to study the hypothesized spinodal dewetting of the thin layer. The roughnesses of the interfaces of the thin layer grow with the same rate and their peaks and valleys are supposed to oscillate in anti-phase. As the simulation subsequently showed, the difference between the in-phase and anti-phase oscillations is too small to be measured with the current instruments. The minimum detectable amount of material would be 3 stacked repeating units of PMMA/PS/PMMA, where the results of the simulations show a noticeable difference. Such a system was measured and is qualitatively compared to the simulations.Doctorat en Sciencesinfo:eu-repo/semantics/nonPublishe

2D reflectometry for the investigation of polymer interfaces: Off-specular neutron scattering

Specular and off-specular neutron reflectometry have been used in a combined approach to study thin polymer films. Our goal in this work is to illustrate the power of the off-specular scattering technique to probe the properties of the buried interface of immiscible polymer bilayers of deuterated polystyrene and protonated poly(methyl methacrylate) (h-PMMA). The diffuse scattering stemming from these systems is discussed in relation to thermal fluctuations at the polymer/polymer interface, providing a means to extract in-plane correlation lengths from buried interfaces. In addition the onset of hole formation in the top layer is evidenced by the diffuse scattering, not easily detectable by specular reflection alone.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Combined specular and off-specular reflectometry: Elucidating the complex structure of soft buried interfaces

Neutron specular reflectometry (SR) and off-specular scattering (OSS) are non­destructive techniques which, through deuteration, give a high contrast even among chemically identical species and are therefore highly suitable for investigations of soft-matter thin films. Through a combination of these two techniques, the former yielding a density profile in the direction normal to the sample surface and the latter yielding a depth-resolved in-plane lateral structure, one can obtain quite detailed information on buried morphology on length scales ranging from the order of ångströms to ∼10 μm. This is illustrated via quantitative evaluation of data on SR and OSS collected in time-of-flight (ToF) measurements of a set of films composed of immiscible polymer layers, protonated poly(methyl methacrylate) and deuterated polystyrene, undergoing a decomposition process upon annealing. Joint SR and OSS data analysis was performed by the use of a quick and robust originally developed algorithm including a common absolute-scale normalization of both types of scattering, which are intricately linked, constraining the model to a high degree. This, particularly, makes it possible to distinguish readily between different dewetting scenarios driven either by the nucleation and growth of defects (holes, protrusions etc.) or by thermal fluctuations in the buried interface between layers. Finally, the 2D OSS maps of particular cases are presented in different spaces and qualitative differences are explained, allowing also the qualitative differentiation of the in-plane structure of long-range order, the correlated roughness and bulk defects by a simple inspection of the scattering maps prior to quantitative fits.SCOPUS: ar.jinfo:eu-repo/semantics/publishe