Wide- and small-angle X-ray scattering (WAXS / SAXS)

X-ray scattering lines of the DMO are devoted to the structural investigation of the various mesomorphic states of matter encountered with temperature. Their use is preceded by an initial observation of samples by Polarizing Optical Microscopy, which confirmed the existence of mesophases, and by a DSC analysis that listed all mesomorphic ranges. X-ray scattering experiments performed as a function of temperature then clarify the nature of the phases, give access to the geometry of the structure and inform upon their symmetry. The combination with volumetric data from dilatometry then opens the door to the modeling of the molecular organizations and to advances in the understanding of mesophases structures and of their transitions.

Small-Angle X-ray Scattering line in temperature

The linear X-ray beams of the four diffraction lines are produced by sealed-tube generators equipped with focusing bent quartz monochromators. All lines are equipped with image plate detection (Storage Phosphor Screens from GE Healthcare and Storm 820 scanner from Molecular Dynamics), but the main line also offers real-time detection by using a multi-channel gas detector Inel CPS120. All systems are measured with this last set-up accessing to Bragg distances from 70 to 2 Å and to temperatures between 20 and 200°C. These ranges are sufficient in most cases, but periodicities greater than 70 Å are occasionally encountered and require resorting to our SAXS line, which covers the 350 to 25 Å Bragg distances interval. The whole optical path of the two further lines is under vacuum and this peculiarity allows the detection of very weak reflections located between 80 3.3 Å. These lines are moreover equipped with ovens with controlled temperature up to 300°C.
Our lines allow acquiring patterns of “powders” (i.e. of non-oriented samples that can be liquids) contained in capillaries or sealed cells. More structural information can be obtained from samples oriented in a field or by an interface. Ovens with a transverse magnetic field and in grazing incidence of a droplet were developed for this purpose. Even more efficient alignment methods become possible with the access to a synchrotron facility. In particular GIWAXS on thin films turned out to repeatedly complete information from scattering on bulk samples with our laboratory lines.

Example of powder WAXS pattern. This measurement, performed in a hexagonal
columnar mesophase at 200°C, was taken from: B. Alameddine, O. F. Aebischer, B. Heinrich,
D. Guillon, B. Donnio, T. A. Jenny, “Influence of linear and branched perfluoroalkylated side
chains on the π–π stacking behaviour of hexa-peri-hexabenzocoronene and thermotropic
properties”, Supramolecular Chemistry, (2014), 26, 125-137. dhk stands for (hk) reflections
from the bi-dimensional lattice, hF and hH are the scattering maxima from lateral distances
between molten fluorinated chains and between molten alkyl chains, h0 and 2h0 are distances
between mesogens and mesogen dimers, piled into columns; ordinates and abscissa : detector
counting and Bragg angle for a wavelength of 1.5405 Å.

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