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  • Author or Editor: J. M. Buisine x
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For liquid crystals exhibiting several fluid or viscous phases, no thermodilatometric method exists for small samples. By simply adding to the basic equipment for the investigation of liquid crystals a Michelson interferometer objective and a photomultiplier, dilatometric studies can be performed from photometric measurements on little droplets of fluid materials. The volume of the samples is about 0.1 cubic millimeter. Introducing the notion of useful radius, data for the isobaric thermal expansion coefficient and volume changes at the phase transitions are deduced from experiments. Efficiency and sensitivity tests are performed on two well known fluid compounds: mercury and silicone oil. The method is then applied to the determination of the thermodilatometric data of two pure liquid crystal compounds — pentyl et octyl cyano biphenyles — and of a commercial nematic liquid crystal mixture.

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Modulated differential scanning calorimetry

Transitions between monotropic and enantiotropic phases of a cholesteric liquid crystal

Journal of Thermal Analysis and Calorimetry
Authors: F. Roussel and J. M. Buisine

The Modulated Differential Calorimetry (MDSC) is applied to the determination of the reversibility in the cholesteryl chloride, which presents a cholesteric monotropic phase between the isotropic and crystalline states. The experimental modulation parameters that govern this method i.e. frequency, amplitude and heating/cooling rate, are determined. MDSC curves and complementary thermomicroscopical observations assign melting, crystallization and liquid cholesteric transition as ‘non reversing’, and clarification as ‘reversing’.

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Liquid crystals exhibit successively several fluids and/or viscous phases and therefore microscopical methods play an important role for their characterization. No really efficient routine dilatometric method exist for rather small samples. By simple addition of a Michelson's interferometer objective and a photo-multiplier to the basic equipment for the investigation of liquid crystals — i.e. a polarizing microscope associated to a heating and cooling stage — dilatometric investigations as well as studies of phase transitions are getting accessible. In fact, when a droplet of liquid crystal, set up in a spherical stamp, is lightened through the interferometer with a quasi monochromatic source, equal thickness circular fringes are observable on the surface of the sample. A thermal expansion of the droplet leads to displacements of these fringes. The principle for the measurement of the volume changes consists by recording, versus the temperature, the variations of the light intensity of the central fringe. The volume of the sample, at given temperatures, is determined from photomicrographs of the whole interference feature. This volume is lower than 0.1 mm3. Experiments on the pure compound octyl-cyanobiphenyl show that the method is convenient for the detection of the phase transition, even if the transformation is weakly first order asS A -N. Tested with a commercial ternary mixture the expansivity of phases and the transitions volumes changes can be deduced from experiments.

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By thermobarometric analysis, we have investigated gels and coacervates of polyphosphates systems (x MO-y P2O5-z CaO-n H2O, whereM=Na or Mg) and studied their physico-chemical transformations until 1000 bars. In every case, the same general shape is obtained for the thermobarograms. A strong pressure increment is first observed bounded to the expansion of the sample under temperature effect. Then, between 100 and 110‡C, a free water release is detected by an important and progressive decrement of pressure issue from a strong volume decrement. At higher temperature, the hydrolysis of the systems is turned into pressure increments. Such experiments are the first showing that, by thermobarometric analysis, physico-chemical transformations can be easily studied. Moreover, for the first time, a negative volume change at a transformation obtained by increasing the temperature has been able to be observed by thermobarometric analysis.

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