Structural investigation of the pressure induced effects in liquid crystals

Abstract

Liquid crystal phases are characterized by a long range orientational order. Numerousstudies on liquid crystals under hydrostatic pressure display interesting pressure inducedphenomena which seem to indicate that this long range order is disturbed. It is shown forexample that re-entrant phases appear and that phase transition temperatureshifts are commonly observed. These pressure induced effects result from thepacking properties exacerbated by the pressure induction of the molar volume.Despite numerous developments, the structural modifications which accompany thesepressure induced effects, as the phase transition temperature shifts, have hardly beeninvestigated. We provide, using neutron diffraction, the physical reasons for thesetemperature shifts. We report here on the very few structural studies of the influence ofhydrostatic pressure on the stability of a liquid crystal phase. This study is carried outusing two specifically designed neutron pressure cells reaching pressure values up to120 MPa. The liquid crystal system is described in terms of pressure-induced correlationlengths and layer spacing, which are the relevant parameters to account for the phasestructure. It will be shown that the structural investigation is particularly noteworthy inthe lamella phase since the characteristic lengths can be tremendously modified underpressure, underlining a correlated change of dynamics. In the case of high molecular weightliquid crystals (side-liquid crystal polymers), it will be shown that the re-entrantnematic–smectic transition is unchanged with respect to the pressure, indicating that thepressure induced reduction of the specific volume concerns the polymer componentonly.