We review the current state of knowledge of phase separation and phase equilibria in porous materials. Our emphasis is on fundamental studies of simple fluids (composed of small, neutral molecules) and well-characterized materials. While theoretical and molecular simulation studies are stressed, we also survey experimental investigations that are fundamental in nature. Following a brief survey of the most useful theoretical and simulation methods, we describe the nature of gas‐liquid (capillary condensation), layering, liquid‐liquid and freezing/melting transitions. In each case studies for simple pore geometries, and also more complex ones where available, are discussed. While a reasonably good understanding is available for phase equilibria of pure adsorbates in simple pore geometries, there is a need to extend the models to more complex pore geometries that include effects of chemical and geometrical heterogeneity and connectivity. In addition, with the exception of liquid‐liquid equilibria, little work has been done so far on phase separation for mixtures in porous media.

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Phase separation in confined systems

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

/pdf/effects-of-confinement-on-freezing-and-melting-37p9d56e3k.pdf

Effects of confinement on freezing and melting.

A review of experimental work on freezing and melting in confinement is presented. A range of systems, from metal oxide gels to porous glasses to novel nanoporous materials, is discussed. Features such as melting-point depression, hysteresis between freezing and melting, modifications to bulk solid structure and solid-solid transitions are reviewed for substances such as helium, organic fluids, water and metals. Recent work with well characterized assemblies of cylindrical pores like MCM-41 and graphitic microfibres with slit pores has suggested that the macroscopic picture of melting and freezing breaks down in pores of molecular dimensions. Applications of the surface force apparatus to the study of freezing and melting phenomena in confinement are discussed in some detail. This instrument is unique in allowing the study of conditions in a single pore, without the complications of pore blockage and connectivity effects. The results have confirmed the classical picture of melting-point depression in larger pores, and allowed the direct observation of capillary condensation of solid from vapour. Other results include the measurement of solvation forces across apparently fluid films below the bulk melting point and a solid-like response to shear of films above the bulk melting point. These somewhat contradictory findings highlight the difficulty of using bulk concepts to define the phase state of a substance confined to nanoscale pores.

https://iopscience.iop.org/article/10.1088/0953-8984/13/11/201/pdf

Confinement effects on freezing and melting

An overview of polymer dispersed liquid crystal (PDLC) materials, their physical properties, and potential applications in the optic and electrooptic industry is presented. These optoelectronic materials have unique properties which are expected to expand liquid crystal technology into new display and light shutter applications. Recent research by small and large industrial and university laboratories on device physics and chemistry has provided substantial progress towards the commercialization of these materials. Work to date on such features as response times, switching voltage, and contrast as well as material preparation procedures and unique optical characteristics will be reviewed. These materials are also of interest to basic physics because of new kinds of physical phenomena brought on by the confinement of a nematic liquid crystal to small submicron-size droplets. Enhanced surface-to-volume ratio, large nematic deformations and problems associated with molecular anchoring at a polymer w...

Polymer dispersed liquid crystals for display application

The response times and operating voltages of light shutters formed from polymer dispersed liquid crystals (PDLCs) have been studied experimentally and the results compared with calculations based on non-sperhically shaped nematic droplet models. The experiments were performed on light shutters with elongated and uniformly aligned droplets where the relaxation time and voltage response were measured. It is shown that the droplet shape can be a dominant factor, particularly for the relaxation time, and the data are compared with equations derived in terms of the aspect ratio of the droplet l = a/b, where a and b are the lengths of the semi-major and semi-minor axes, respectively, of the elongated droplet. It is further demonstrated that the electric field inside the droplet can be considerably smaller than the applied field, due to the conductivity and dielectric properties of the polymer and liquid crystal materials. These data are used to obtain values for the ratio of the conductivities of the p...

Response times and voltages for PDLC light shutters

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

Randomly constrained orientational order in porous glass.

The invention provides a new liquid crystal display material prepared by the phase separation of a homogeneous solution of liquid crystal and thermoplastic resin to create in situ microdroplets of liquid crystal within a set transparent resin matrix. The new display material is characterized by ease of fabrication and the ability to be reworked by simple heating and cooling of the thermoplastic resin containing microdroplets of liquid crystal. It has been discovered that such a material fabricated from a thermoplastic resin and a liquid crystal having a liquid crystalline phase to isotropic phase transition temperature above the softening temperature of the resin exhibits a reversible field-independent memory.

Liquid crystal display material comprising a liquid crystal dispersion in a thermoplastic resin

Submicrometer-size droplets of monomeric liquid crystals dispersed in a solid polymer matrix are studied by deuterium NMR. Phase-separation processes are used to achieve droplets of a size comparable and less than the magnetic coherence length \ensuremath{\xi}\ensuremath{\sim}0.5 \ensuremath{\mu}m, of the NMR magnetic field, 4.7 T. Selectively deuterated (4'-pentoxy- and 4'-methoxy-) 4-cyanobiphenyl compounds (5OCB-${d}_{2}$) and (1OCB-${d}_{3}$) deuterated in the \ensuremath{\alpha} and methyl positions, respectively, provide for two different NMR measurement time scales to examine effects of the self-diffusion of the molecules confined to droplets. It is found that the line shape of the spectra depends on several factors which include self-diffusion, size, shape of the droplets, and type of anchoring on the droplet walls. Theoretical director configurations calculated for spherical and nonspherical droplets in the presence of an external field were used to simulate NMR spectra in the presence of self-diffusion. Effects due to external fields present during sample preparation are discussed.

Deuterium NMR of polymer dispersed liquid crystals.

The invention provides a low loss liquid crystal modulator for coloring and shaping a light beam (IB) wherein liquid crystal light modulators (2B, 2G, 2R) provide continuous variation in color and intensity output by controlled scattering; the liquid crystal light modulators (2B, 2G, 2R) comprise addressable sheets of light-wave-length size liquid crystal occlusions in a transparent matrix (21); the modulators (2B, 2G, 2R) employ nematic liquid crystal exhibiting positive dielectric anisotropy; the modulator is employed in varying and coloring output beams (OB) to allow for colored data or video transmission; images produced may vary in time as is required for, e.g., color television; the modulator is employed in color projectors for both data and video projection, for color printing, and for colored stage illumination. Low loss transmission of the modulators (2B, 2G, 2R) provides for bright images which can be viewed under high ambient illumination.

Low loss liquid crystal modulator for coloring and shaping a light beam

An oriented ferroelectric liquid crystal in the chiral smectic C phase with the smectic planes normal to bounding plates of a cell is interspersed with polymer which stabilizes and freezes the arrangement of the liquid crystal. The polymer which stabilizes the liquid crystal is formed by a method which includes introducing a mixture of a monomer and a ferroelectric liquid crystal between cell walls when the liquid crystal is in the isotropic phase, cooling the liquid crystal to a chiral nematic or smectic phase, and polymerizing the monomer. The cooling of the mixture can be done in the presence of a magnetic or electric field.

Slobodan Zumer

Papers

Randomly constrained orientational order in porous glass.

Liquid crystal display material comprising a liquid crystal dispersion in a thermoplastic resin

Deuterium NMR of polymer dispersed liquid crystals.

Low loss liquid crystal modulator for coloring and shaping a light beam

Method of making a ferroelectric liquid crystal cell including a cooling step