Slow relaxation effects at the second-order nematic to lamellar smectic phase transition in micellar liquid crystals.

Abstract

The second-order nematic to lamellar-smectic phase transition is studied in the micellar systems decylammonium chloride--${\mathrm{NH}}_{4}$Cl--water and cesium perfluoro-octanoate--${\mathrm{D}}_{2}$O, by measuring the electric conductivity of aligned samples and the density as functions of temperature. In the nematic range, for temperatures higher than about 1 \ifmmode^\circ\else\textdegree\fi{}C above the transition, the data on cooling and heating coincide within experimental error. Closer to the lamellar-smectic phase significant differences occur between the values observed on heating and cooling. For the density, the difference between the two values in the smectic range is several ${10}^{\mathrm{\ensuremath{-}}4}$ g/${\mathrm{cm}}^{3}$. The processes responsible for the observed behavior have long relaxation times, estimated in the order of several hours to several days. A simple rearrangement of the micelles into layers should have relaxation times comparable to the relaxation times of the nematic to smectic-A phase transition in thermotropic liquid crystals; however, no such relaxation effect has been reported in thermotropic liquid crystals, even at very fast heating and cooling rates. Accordingly, the slow relaxation suggests significant change in the micellar structure at the nematic to lamellar-smectic phase transition, probably a divergence in the aggregation number, i.e., a transition to continuous lamellae. The observed long relaxation times reflect the slow approach to an equilibrium distribution of defects after the formation of the lamellae.