Dilatometry is a specific technique of the DMO that completes the structural investigation of mesophases beforehand discovered and characterized by Polarizing Optical Microscopy, DSC and X-ray scattering. With the notable exception of the nematic phase, mesophases generally exhibit long-range correlated structures owed to the chemical bonding of non-miscible molecular segments, e.g. a conjugated segment connected to an aliphatic chain. The amphipatic segregation process (i.e. the phase separation at the molecular level) yields vast periodic domains of each species with different electronic densities, so that the structures emerging from the domains regular alternation can be elucidated by X-ray scattering. The deeper understanding of mesophase structures and phase transitions however requires insight in the organizations within segregated domains, which is realized by combining geometrical parameters gained from X-rays and volumetric data from dilatometry.
The principle of the analysis is based on the comparison between the geometrical features of segments and the ratio of the molecular volume to the characteristic size of the structure, i.e. the layer thickness, the cross-section of columns or the lattice volume, depending on the one-, two- or three-dimensional nature of the mesophase. The molecular volume can be directly measured with our dilatometry set-up, providing that the quantity of product required by the experiment is available (approximately 0.6 g, and since this is a non-destructive experiment, the sample is almost entirely recovered after the measurement) and providing a good solvent to solubilize the compound. Otherwise, less accurate molecular volume values are obtained by adding individual segment volumes. In practice, these volumes proved to vary by less than 1% between the various mesophases and the isotropic liquid. This pseudo-empirical rule (some fragments could be measured beforehand) allowed creating with time a rich library of various molecular fragment volumes that provides crucial reference data needed for the structural analysis.
In addition dilatometry completes information from DSC regarding the thermodynamics of phase transitions and the technique is particularly adapted to characterize glass transitions. Our set-up directly measures the sample volume as a function of temperature (varied between 30 and 195°C), the accuracy being 0.1 % on the absolute volume and 0.01 % on volume variations.
Example of dilatometry curve, taken from: W. Dobbs, B. Heinrich, C. Bourgogne, B. Donnio, E. Terazzi, M.-E. Bonnet, F. Stock, P. Erbacher, A.-L. Bolcato-Bellemin, L. Douce, “Mesomorphic Imidazolium Salts: New Vectors for Efficient siRNA Transfection”, J. Am. Chem. Soc., (2009), 131, 13338–13346. Vmol is the molecular volume, T is the temperature, Cr is a crystalline phase, Cub is a cubic mesophase and Colh is a hexagonal columnar mesophase. The inset view reveals the volume expansion associated to the phase transition, as the linear fit of the Vmol variation in the center of the cubic domain is subtracted from the experimental curve.
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