We present a calorimetric study of the phase behavior of octylcyanobiphenyl (8CB) liquid crystal confined to a controlled-pore glass (CPG). We used CPG matrices with characteristic void diameters ranging from 400 to 20 nm. In bulk we obtain weakly first-order isotropic to nematic (I-N) phase transition and nearly continuous character of the nematic to smectic-A (N-SmA) phase transition. In all CPG matrices the I-N transition remains weakly first order, while the N-SmA one becomes progressively suppressed with decreasing CPG pore radius. With decreased pore diameters both phase transition temperatures monotonously decrease following similar trends, but increasing the stability range of the N phase. The heat-capacity response at the weakly first order I-N and continuous N-SmA phase transitions gradually approaches the tricritical-like and three-dimensional XY behavior, respectively. The main observed features were explained using a bicomponent single pore type phenomenological model.

Calorimetric study of octylcyanobiphenyl liquid crystal confined to a controlled-pore glass.

Different kind of measurements were performed on the liquid crystal nonyloxycyanobiphenyl (9OCB) to carry out a study of the molecular dynamics in the smectic A (SmA), nematic (N), and isotropic (I) phases as well as an exhaustive analysis of both the SmA-to-N and N-to-I phase transitions. For the dynamic study, broadband dielectric spectroscopy (102 to 1.8 × 109 Hz) was used. Two orientations (parallel and perpendicular) of the molecular director with regard to the probing electric field were investigated. From this study, the static dielectric permittivity was obtained in both alignments and, in addition, the molecular motions that contribute to each one were discussed. The static dielectric data together with specific heat and volumetric determinations were analyzed, proving that both phase transitions are weakly first order, displaying a nearly tricritical behavior. However, the width of metastable regions seems to be dependent on the physical magnitude, although specific heat and volumetric determina...

Critical behavior and scaling relationships at the SmA(d)-N and N-I transitions in nonyloxycyanobiphenyl (9OCB).

We report optical birefringence data by two different methods with high temperature resolution for octylcyanobiphenyl (8CB) near the smectic-A to nematic (Sm-A-N) phase transition temperature T(AN). Within the resolution of our experiments, we find that the Sm-A-N phase transition is continuous. For a possible discontinuity in the orientational order parameter S(T) at T(AN), we arrive at an upper limit of 0.0002, which is substantially smaller than other estimates in literature, but consistent with the value of 0.00008 derived from the upper limit of the latent heat from high-resolution adiabatic scanning calorimetry (ASC), which is itself consistent with the Halperin-Lubensky-Ma theory. The temperature derivative of the order parameter exhibits a power law divergence with a critical exponent that is consistent with the value α = 0.31 ± 0.03 for the specific heat capacity obtained by ASC.

High-resolution birefringence investigation of octylcyanobiphenyl (8CB): an upper bound on the discontinuity at the smectic-A to nematic phase transition.

A detailed study has been performed for mixtures of octyloxycyanobiphenyl (8OCB) and nonyloxycyanobiphenyl (9OCB) liquid crystals and nine of their mixtures by means of high-resolution adiabatic scanning calorimetry. The isotropic to nematic transitions are weakly first order with latent heat values in the range usually encountered for this transition in other liquid crystals. With the exception of pure 8OCB, for which only an upper limit of $1.8\text{ }\text{J}\text{ }{\text{kg}}^{\ensuremath{-}1}$ for the latent heat could be established, finite latent heats have been obtained for the nematic to smectic-$A$ transition of all the mixtures and of pure 9OCB. The concentration dependence of their latent heats could be well fitted with a crossover function consistent with a mean-field free-energy expression that has a nonzero cubic term induced by the Halperin-Lubensky-Ma (HLM) coupling between the smectic-$A$ order parameter and the orientational director fluctuations. Clearly first-order transitions with measurable latent heats are found for mole fractions of 9OCB in the mixtures where the effective critical exponent for the specific-heat capacity has substantially lower values than the tricritical one (0.5). This is qualitatively different from what has been observed so far in other liquid-crystal systems and yields strong experimental evidence from a calorimetric experiment for the HLM coupling between the smectic-$A$ order parameter and the director orientation fluctuations.

Order of phase transitions and tricriticality in mixtures of octyloxycyanobiphenyl and nonyloxycyanobiphenyl liquid crystals: a high-resolution study by adiabatic scanning calorimetry.

On the nature of the smectic mesophase in 8CB + 10CB binary mixtures. Evidences for a smectic A to nematic tricritical point

What is the order of the nematic--smectic-[ital A] ([ital NA]) transition The answer to this question has flip-flopped over the years as ever more sophisticated theories and ever more careful experiments have addressed the issue. The Landau theory predicts that the transition can be either first or second order, depending on material parameters. de Gennes and McMillan [Solid State Commun. 10, 753 (1972); Phys. Rev. A 14, 1238 (1971)] showed that nematic fluctuations would drive the transition to first order as the temperature of the [ital NA] transition approached that of the nematic-isotropic transition. Halperin, Lubensky, and Ma [Phys. Rev. Lett. 32, 292 (1974)] (HLM) then argued that the effect of nematic fluctuations is more subtle and concluded that the transition is always at least weakly first order. Monte Carlo simulations indicate, however, that for a large enough nematic range, the transition becomes second order. We investigate the order of the [ital NA] transition experimentally by measuring the capillary length (ratio of the surface tension to the latent heat) near an apparent tricritical point in a binary liquid-crystal mixture. Our measurements confirm the existence of the extra free-energy term predicted by the HLM theory and yield, as a by-product, surface-tensionmore » measurements of the [ital NA] interface. Although we cannot currently detect the tricritical point suggested by numerical work, we have not approached any fundamental sensitivity limits either.« less

Nancy Tamblyn

Papers

Investigating the nematic-smectic-A transition by capillary-length measurements near an apparent tricritical point.