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

Neutron specular reflectometry (SR) and off-specular scattering (OSS) are non\uaddestructive 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 \ue5ngstr\uf6ms to ∼10 \ub5m. 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 fit

Engineering short, strong hydrogen bonds in urea di-carboxylic acid complexes

A series of seven 2:1 molecular complexes of urea (U) and methyl ureas with di-carboxylic acids (A) are reported, along with the results of their study by variable temperature diffraction. These all contain short, strong O–H⋯O hydrogen bonds and a recurring acid⋯amide heterodimer forming U–A–U synthons. Despite differences in the degree of saturation of the linking C–C groups of the di-carboxylic acids and the single or double methyl substitution of one of the N atoms of the urea, the packing arrangements are remarkably similar in five of the complexes; the exceptions being N-methylurea oxalic acid and N,N-dimethylurea fumaric acid. The five similar molecular complexes all show contraction of one unit cell parameter on increasing temperature due to rearrangements of the weaker interactions which hold together the U–A–U units. The strength of the short, strong O–H⋯O hydrogen bond is shown to be linked both to the length of the connecting bridge between the carboxylic acid groups of the acid, and to the ΔpKa values between the two components. ©, 2014, The Royal Society of Chemistry

4-Phenoxyphenol: A Porous Molecular Material

4-Phenoxyphenol is a simple organic molecule that crystallizes as a porous material with channels running throughout the structure. The channels are constructed by a 6-fold hydrogen bonded ring and can host solvent molecules incorporated during crystal growth, with a minimum channel diameter of 5.8-5.9 angstrom; each channel usually contains a single solvent molecule per unit cell. The hydrogen bonded ring shows surprising flexibility, being able both to breathe and to sustain its crystalline integrity even when grown with empty pores. This is particularly surprising given that the remainder of the interactions within the crystal structure are C-H center dot center dot center dot pi interactions and are weak in nature. It is also possible to grow "dry" porous 4-phenoxyphenol crystals by using a bulky solvent in the recrystallization. © 2012, American Chemical Society

Temperature dependent solid-state proton migration in dimethylurea-oxalic acid complexes

The phenomenon of solid-state proton migration within molecular complexes containing short hydrogen bonds is investigated in two dimethylurea-oxalic acid complexes. Extensive characterisation by both X-ray and neutron diffraction shows that proton migration along the hydrogen bond can be induced in these complexes as a function of temperature. This emphasises the subtle features of the hydrogen bond potential well in such short hydrogen bonded complexes, both intrinsically and in the effect of the local crystalline environment. Based on these findings, the synthesis and analysis of a series of solid-state molecular complexes is shown to be a potential route to designing materials with tuneable proton migration effects. © 2012, Royal Society of Chemistry