Calorimetric study of octylcyanobiphenyl liquid crystal confined to a controlled-pore glass.
TL;DR: 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.
Abstract: 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.
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TL;DR: Both simple and more complex adsorbates that are confined in various environments (slit or cylindrical pores and also disordered porous materials) are considered and how confinement affects the glass transition is addressed.
Abstract: We present a review of experimental, theoretical, and molecular simulation studies of confinement effects on freezing and melting We consider both simple and more complex adsorbates that are confined in various environments (slit or cylindrical pores and also disordered porous materials) The most commonly used molecular simulation, theoretical and experimental methods are first presented We also provide a brief description of the most widely used porous materials The current state of knowledge on the effects of confinement on structure and freezing temperature, and the appearance of new surface-driven and confinement-driven phases are then discussed We also address how confinement affects the glass transition
640 citations
TL;DR: In this paper, a review of spatially confined, non-equilibrium physics in nanoporous media is presented. And a particular emphasis is put on texture formation upon crystallisation in nanopore-confined condensed matter, a topic both of high fundamental interest and of increasing nanotechnological importance.
Abstract: Spatial confinement in nanoporous media affects the structure, thermodynamics and mobility of molecular soft matter often markedly. This article reviews thermodynamic equilibrium phenomena, such as physisorption, capillary condensation, crystallisation, self-diffusion, and structural phase transitions as well as selected aspects of the emerging field of spatially confined, non-equilibrium physics, i.e. the rheology of liquids, capillarity-driven flow phenomena, and imbibition front broadening in nanoporous materials. The observations in the nanoscale systems are related to the corresponding bulk phenomenologies. The complexity of the confined molecular species is varied from simple building blocks, like noble gas atoms, normal alkanes and alcohols to liquid crystals, polymers, ionic liquids, proteins and water. Mostly, experiments with mesoporous solids of alumina, gold, carbon, silica, and silicon with pore diameters ranging from a few up to 50 nm are presented. The observed peculiarities of nanopore-confined condensed matter are also discussed with regard to applications. A particular emphasis is put on texture formation upon crystallisation in nanoporous media, a topic both of high fundamental interest and of increasing nanotechnological importance, e.g. for the synthesis of organic/inorganic hybrid materials by melt infiltration, the usage of nanoporous solids in crystal nucleation or in template-assisted electrochemical deposition of nano structures.
246 citations
TL;DR: In this paper, a review of spatially confined, non-equilibrium physics in nanoporous media is presented. And a particular emphasis is put on texture formation upon crystallisation in nanopore-confined condensed matter, a topic both of high fundamental interest and of increasing nanotechnological importance.
Abstract: Spatial confinement in nanoporous media affects the structure, thermodynamics and mobility of molecular soft matter often markedly. This article reviews thermodynamic equilibrium phenomena, such as physisorption, capillary condensation, crystallisation, self-diffusion, and structural phase transitions as well as selected aspects of the emerging field of spatially confined, non-equilibrium physics, i.e. the rheology of liquids, capillarity-driven flow phenomena, and imbibition front broadening in nanoporous materials. The observations in the nanoscale systems are related to the corresponding bulk phenomenologies. The complexity of the confined molecular species is varied from simple building blocks, like noble gas atoms, normal alkanes and alcohols to liquid crystals, polymers, ionic liquids, proteins and water. Mostly, experiments with mesoporous solids of alumina, carbon, gold, silica, and silicon having pore diameters ranging from a few up to 50 nanometers are presented. The observed peculiarities of nanopore-confined condensed matter are also discussed with regard to applications. A particular emphasis is put on texture formation upon crystallisation in nanoporous media, a topic both of high fundamental interest and of increasing nanotechnological importance, e.g., for the synthesis of organic/inorganic hybrid materials by melt infiltration, the usage of nanoporous solids in crystal nucleation or in template-assisted electrochemical deposition of nano structures.
238 citations
TL;DR: In this paper, the electrocaloric effect (ECE) was studied as a function of the magnitude of the electric field step E in the vicinity of the critical point in several bulk relaxor ferroelectric ceramic systems.
Abstract: The electrocaloric effect (ECE), i.e., the conversion of electric energy into heat, is of great importance for application in new generation cooling or heating devices that would be friendlier to the environment. Here, utilizing direct measurements of the ECE change of the temperature ΔT via a high resolution calorimeter, we study the ECE as a function of the magnitude of the electric-field step E in the vicinity of the critical point in several bulk relaxor ferroelectric ceramic systems. Relatively large ΔT of ∼2 to 3 K were obtained at modest fields of 90 kV/cm, even in the case of ceramic materials. The effective responsivity ΔT/E as a function of the electric field shows a characteristic peak near the critical point, which demonstrates the importance of proximity to the critical point for the enhancement of the electrocaloric effect. Experimental results are in good agreement with the theoretical calculations based on the spherical random-bond random-field model.
194 citations
TL;DR: In this paper, the Widom line and the critical line for the paraelectric to ferroelectric transformations in the composition-temperature electric field (PMN-PT) phase diagram were studied.
Abstract: The giant electromechanical response in ferroelectric relaxors such as $\mathrm{Pb}({\mathrm{Mg}}_{1∕3}{\mathrm{Nb}}_{2∕3}){\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}$ (PMN-PT) is of great importance for a number of ultrasonic and medical applications as well as in telecommunications. On the basis of the dielectric, heat capacity, and piezoelectric investigations on PMN-PT crystals of various PT compositions and bias fields, we have recently shown the existence of a line of critical points for the paraelectric to ferroelectric transformations in the composition-temperature-electric field $(x\text{\ensuremath{-}}T\text{\ensuremath{-}}E)$ phase diagram. Here, we show the piezobehavior in more detail and present a theoretical evaluation of the Widom line and the critical line. This line effectively terminates a surface of first order transitions. Above this line, supercritical evolution has been observed. On approaching the critical point, both the enthalpy cost to induce the intermediate monoclinic states and thus the barrier for polarization rotations decrease significantly. The maximum of the piezoelectric response is not at $E=0$, but at the critical field values. It is shown that the critical fluctuations in the proximity of the critical points are directly responsible for the observed enhancement of the electromechanical response in the PMN-PT system. In view of the large electric field dependence of the dielectric constant near the critical point, these systems may also be important as electric field tunable elements.
133 citations
References
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TL;DR: 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, which cannot currently detect the tricritical point suggested by numerical work but have not approached any fundamental sensitivity limits.
Abstract: 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
15 citations
01 Feb 2001
TL;DR: This article studied the effects of both orientational and translational disorder on the nematic-smectic-A (NA) phase transition, using the Landau-de Gennes treatment.
Abstract: We study the effects of both orientational and translational disorder on the nematic–smectic-A (NA) phase transition, using the Landau–de Gennes treatment. The only influence of random-field orientational disorder is to slightly increase the NA transition temperature, whereas the translational randomness has four important effects. (i) The NA transition temperature is decreased by the randomness. (ii) The bulk second-order NA phase transition is transformed into a first-order one. (iii) There is a small degree of translational order even in the high-temperature phase. (iv) The strict long-range smectic-A order is converted into a quasi-long-range order by any finite amount of disorder.
15 citations
TL;DR: In this paper, the authors present details of calculations of the core structure of a screw dislocation in a liquid crystal, using the phenomenological Landau-de Gennes free energy functional.
Abstract: We present details of calculations of the core structure of a screw dislocation in a smectic A liquid crystal, using the phenomenological Landau-de Gennes free energy functional. The order parameter frustration created by topological constraints far from the dislocation core is resolved in one of three qualitatively different ways. The three types of dislocation core solution are the DT (double twist), CL (classical), and BP (broken polar symmetry) solutions, respectively. The stability requirements for these structures are discussed, as a function of temperature, smectic elastic properties, and coupling between smectic and nematic order. The effect of possible inhomogeneity between left- and right-handed conformers is also examined.
15 citations
TL;DR: This work numerically determine the critical condition for the surface-driven formation and depinning of the dislocation and shows how the value of the positional anchoring strength at the surface can be determined.
Abstract: Using a Landau‐de Gennes approach, we model the formation of an edge dislocation in a smectic-A cell initially in the bookshelf structure. The driving force is the mismatch between the layer thickness in a bulk smectic-A liquid crystal and that imposed by confining plates. The core structure of the dislocation is calculated taking into account spatial variations of the smectic translational order parameter. We numerically determine the critical condition for the surface-driven formation and depinning of the dislocation. By exploiting this phenomenon, we show how the value of the positional anchoring strength at the surface can be determined.
12 citations
TL;DR: In this article, the fluctuation-induced renormalization of the tracer order parameter is calculated and a classic tricritical behavior is expected. But the renormalizations become immeasurably small in the case of improper ferroelastics.
Abstract: Fluctuations of order parameters Q near a tricritical point coupled with strain e( varies as eQ2) lead to fluctuation-induced first-order phase transitions. Anisotropic fluctuations with strong correlations along a 'hard' direction show a much reduced first-order behaviour. In the case of improper ferroelastics the fluctuation-related renormalizations become immeasurably small and a classic tricritical behaviour is expected. Experimental observations of such fluctuations are facilitated if the fluctuating order parameter couples bi-quadratically with a second, non-critical order parameter which is used as tracer. The fluctuation-induced renormalization of the tracer order parameter is calculated.
8 citations