Deux methodes photothermiques a l'etude d'une preparation de contact d'un melange binaire de cristaux liquides

The photopyroelectric (PPE) method is proposed as a sensitive technique to study a binary mixture of liquid crystals in a contact preparation. The photothermal signal is generated while scanning the contact preparation. The crystal-smectic A, smectic A-nematic, and nematic-isotropic interphase boundaries are detected. The displacement of these boundaries due to the variation of the temperature is monitored. The study of these displacements allows us to draw the complete temperature-concentration phase diagram of a binary mixtures. This revised version was published online in August 2006 with corrections to the Cover Date.     

Pressure effect on the nematic-isotropic transition in dilaterally substituted nematogens

The effect of pressure on the liquid crystal properties of two new dilaterally substituted nematogens has been studied. The method employed involves measurement of the thermal pressure variation of a sample under isochoric conditions. The pressure-temperature phase diagrams were determined. As for unsubstituted compounds, the nematic phase is stabilized upon application of pressure. The values of the (?T/?P)^NI slopes of the clearing lines for these nematogens are, however, significantly higher than those generally characterizing rod-shaped nematics. The entropy separation method was used to estimate the constant volume and the volume-dependent terms of the entropy changes at the nematic-isotropic transition. The values of these contributions were determined on the basis of thermobarometric data showing that the transition entropy is strongly volume-dependent. This suggests that the higher values of the (?T/?P)^NI slopes observed for these compounds can be related to the rearrangement of the lateral flexible chains in the nematic phase when the pressure increases, leading to a decrease of the excluded free volume caused by the bulky core of the molecules.     

Second harmonic generation from alternating langmuir-blodgett films

We report here the investigation of the second harmonic generation from a non-centrosymmetric alternating Langmuir-Blodgett film of ω-tricosenoic acid and highly polarisable merocyanine dye. Results for films containing one, two and three layers of the merocyanine dye are described.     

Photopyroelectric investigation of binary mixtures of liquid crystals in contact preparations

The photopyroelectric (PPE) method is proposed as a sensitive technique to study a binary mixture of liquid crystals in a contact preparation. The photothermal signal is generated while scanning the contact preparation. The crystal (K)/smectic-A (S _A), smectic-A/nematic (N), and nematic/isotropic (I) interphase boundaries are detected. The displacement of these boundaries due to the variation of the temperature is monitored. Received: 13 Oktober 1998 / Accepted: 7 July 1999 / Published online: 3 December 1999