Specific Heat near the Nematic-Smectic-ATricritical Point

Abstract

Specific-heat measurements near the nematic-smectic-^4. transitions of the octyloxy, nonyloxy, and decyloxy members of the 4-w-pentyl-phenylthiol-4'-alkoxybenzoate (rcS5) homologous series reveal the presence of critical-tricritical crossover behavior upon increasing n f with n t ~9. The results are consistent with Alben's extension of de Gennes's superfluid analogy to include superf!uid-He 3 -He 4 -like tricritical behavior.The question of the existence of a nematicsmectic-^4 tricritical point has motivated interest among experimentalists for several years. Since the theoretical discovery by Kobayashi 1 and McMillan 2 that coupling between the nematic and smectic order parameters may drive this transition first order, there have been several experiments that strongly suggest such a point does exist on phase diagrams of temperature versus pressure 3 or concentration. 4 * 5 On the theoretical side, de Gennes 6 has written a Landau theory exhibiting coupling between the smectic order parameter and the nematic director. Based on this analogy, Halperin, Lubensky, and Ma 7 ' 8 have made calculations which predicted that the nematic-smectic-A transition should always be first order, thus ruling out conventional tricritical behavior. In this Letter we report specificheat and birefringence measurements which indicate that coupling to the nematic order parameter, in the manner of Kobayashi 1 and McMillan, 2 appears to produce a conventional tricritical point. Specifically, we have made measurements on three members of the nS5 homologous series, 9 namely, SS5, 5S5, and TUS5.The specific-heat data were taken on a highresolution ac microcalorimeter described elsewhere. 10 ' 11 The BS5 data were published previously. 10 The data for all three homologs are shown in Fig. 1, where the most striking feature is seen to be the profound increase in the strength of the anomaly as n increases from B to IU. This may be understood as a consequence of the decreasing nematic temperature range, as seen in Fig. 2, and the consequent increase in contribution from coupling between the nematic and smectic order parameters. In the microscopic theory this coupling leads to first-order behavior for T NA /T NI >0.87. 2 Experimentally 4 this occurs at a somewhat higher value of T NA /T NI (i.e., for a 500 a O 100 r F .

High-pressure phase transitions in liquid crystals

“…In the mentioned paper we have also presented some results of the use of continuous pressure variations in decreasing and increasing modes used as inducing variables in an isothermal investigation of 4-n-pentyl-phenylthiol-.._4'-decycloxybenzoate liquid crystal (10S5). It is known from the literature that the 10S5 liquid crystal passes from isotropic liquid phase to the crystalline solid phase through the nematic and three smectic phases and when heating under near-atmospheric pressure the solid phase goes directly to the smetic-A phase [5,6]. The pressure-conlrolled investigations confirmed non-symmetric behaviour of the transitions during isothermal compressions and decompressions over the pressure interval from 0.1 to 300 MPa [4].…”

Birefringence study of the nematic‐smectic a phase transition ofN-(4-n-pentyloxybenzylidene)-4-n-alkylaniline (50.m)

“…In general, the short-range smectic A order in the nematic phase just above the transition point, TNA has been discussed in terms of a Landau model for the N-SmA transition (Brisbin, DeHoff, Lockhart and Johnson, 1979;D'Humikres and Ltger, 1975;Halperin, Lubensky and Shang-Keng Ma, 1974). According to this treatment, the temperature dependence of the birefringence is expected to be proportional to ( T -TNA)?.…”

Nematic–lamellar tricritical behavior and structure of the lamellar phase in the ammonium pentadecafluorooctanoate (APFO)/water system

“…2 The phase behavior is essentially universaP irrespective of the structure of the polar head group. The dominant features are that all of the phases are translationally ordered and the phase transitions are associated with a dramatic change in the topology of the aggregate.…”