Structural investigation of the pressure induced effects in liquid crystals

Liquid crystal phases are characterized by a long range orientational order. Numerous studies on liquid crystals under hydrostatic pressure display interesting pressure induced phenomena which seem to indicate that this long range order is disturbed. It is shown for example that re-entrant phases appear and that phase transition temperature shifts are commonly observed. These pressure induced effects result from the packing properties exacerbated by the pressure induction of the molar volume. Despite numerous developments, the structural modifications which accompany these pressure induced effects, as the phase transition temperature shifts, have hardly been investigated. We provide, using neutron diffraction, the physical reasons for these temperature shifts. We report here on the very few structural studies of the influence of hydrostatic pressure on the stability of a liquid crystal phase. This study is carried out using two specifically designed neutron pressure cells reaching pressure values up to 120 MPa. The liquid crystal system is described in terms of pressure-induced correlation lengths and layer spacing, which are the relevant parameters to account for the phase structure. It will be shown that the structural investigation is particularly noteworthy in the lamella phase since the characteristic lengths can be tremendously modified under pressure, underlining a correlated change of dynamics. In the case of high molecular weight liquid crystals (side-liquid crystal polymers), it will be shown that the re-entrant nematic-smectic transition is unchanged with respect to the pressure, indicating that the pressure induced reduction of the specific volume concerns the polymer component only