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
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TL;DR: These findings are explained by a Landau type of model where the liquid crystal is confined to independent pore segments, suggesting that the glassylike orientational order is to a large extent locally determined.
Abstract: Magnetic resonance and calorimetric studies of 4'-n-pentyl-4-cyanobiphenyl confined to Vycor glass, a random network of pores with average diameter ∼70 A, are reported. Under such severely constrained conditions, the nematic-to-isotropic transition is replaced by a continuous evolution of orientational order in the pores. Our findings are explained by a Landau type of model where the liquid crystal is confined to independent pore segments, suggesting that the glassylike orientational order is to a large extent locally determined
157 citations
TL;DR: In this paper, high-resolution calorimetric studies have been made of the liquid crystal phase transitions for several dispersions of 70-Angstrom-diam silica spheres (aerosil) in octylcyanobiphenyl (8CB) as a function of silica density.
Abstract: High-resolution calorimetric studies have been made of the liquid crystal phase transitions for several dispersions of 70-{Angstrom}-diam silica spheres (aerosil) in octylcyanobiphenyl (8CB) as a function of silica density {rho}{sub S}. The excess specific heat peaks associated with the nematic-isotropic (N-I) and the nematic{endash}smectic-{ital A} (N-SmA) transitions both exhibit shifts to lower temperatures, decreases in the specific heat maximum values, and decreases in the transition enthalpies as {rho}{sub S} is increased. Two distinct regimes of {rho}{sub S}-dependent behaviors are observed with a crossover between them at {rho}{sub S}{congruent}0.1thinspgthinspcm{sup {minus}3}. For lower silica densities, sharp second-order C{sub p} peaks are observed at the N-SmA transitions, characterized by effective critical exponents that decrease monotonically with {rho}{sub S} from the pure 8CB value toward the three-dimensional {ital XY} value, and two closely spaced but distinct first-order C{sub p} features are observed at the N-I transition. For higher silica densities, both the N-SmA and the N-I transitions exhibit a single rounded C{sub p} peak, shifting in temperature and decreasing in total enthalpy in a manner similar to that observed in 8CB+aerogel systems. Small angle x-ray scattering data are qualitatively aerogel-like and yield temperature-independent mass-fractal dimensionalities for aerosil aggregates that differ for samples with silicamore » densities above and below the crossover density. {copyright} {ital 1998} {ital The American Physical Society}« less
153 citations
TL;DR: Light scattering and precision calorimetry show that the nematic ordering of octylcyanobiphenyl (8CB) filling the connected network of pores of a silica aerogel does not occur via the first-order phase transition characteristic of the bulk.
Abstract: Light scattering and precision calorimetry show that the nematic ordering of octylcyanobiphenyl (8CB) filling the connected network of pores of a silica aerogel does not occur via the first-order phase transition characteristic of the bulk. Rather, ordering is continuous with an orientational correlation length never increasing beyond the aerogel pore size. The heat-capacity anomly of the second-order nematic--smectic-A phase transition seen in the bulk is absent or greatly broadened in the aerogel.
151 citations
TL;DR: In this paper, a high-resolution computerized calorimeter capable of fully automatic operation in either ac or relaxation modes is described, with emphasis on a new version of the relaxation technique in which the heater power is ramped linearly in time.
Abstract: A high-resolution computerized calorimeter capable of fully automatic operation in either ac or relaxation modes is described. Emphasis is given to a new version of the relaxation technique in which the heater power is ramped linearly in time. This improvement results in superior performance and convenience in studying both first- and second-order phase transitions and allows quantitative evaluation of latent heats as well as pretransitional heat capacity variations. Examples are given for the use of this calorimeter in the study of liquid crystal phase transitions.
113 citations
TL;DR: Novel effects regarding specific-heat amplitudes, transition-temperature shifts, and broadening and rounding of phase transitions are found, strongly dependent on the director orientation and on the order of the phase transition.
Abstract: Using an ac calorimetry technique we measured the specific heat at the nematic-isotropic, smectic-A--nematic, and smectic-A--isotropic phase transitions, for the alkylcyanobiphenyl liquid crystal nCB series confined to the 0.2-\ensuremath{\mu}m-diam pores of Anopore membranes. Studies were also performed as a function of nematic director alignment within the pores. Novel effects regarding specific-heat amplitudes, transition-temperature shifts, and broadening and rounding of phase transitions are found. They are strongly dependent on the director orientation and on the order of the phase transition.
101 citations