Phase Separation Of Liquid Crystal Research Articles

Bulk metallic glasses — a new engineering material

The development of new metallic alloys which form bulk glasses at low cooling rates has led to significant advances in the study of undercooled liquid metals and the glass transition in metallic systems. For the first time it has become possible to carry out measurements of thermophysical properties such as specific heat, viscosity, atomic diffusion, and crystal nucleation rates for liquid alloys over the full temperature range of the undercooled melt. These new materials exhibit a rich variety of phenomena, such as liquid crystal phase separation and nanocrystalline phase formation, and have interesting potential as engineering materials.

The morphology and performance of holographic transmission gratings recorded in polymer dispersed liquid crystals

Holographic transmission gratings are formed by the anisotropic visible laser radiation curing of a multifunctional acrylate monomer blended with the liquid crystal (LC) mixture E7. This results in an anisotropic spatial distribution of phase-separated LC droplets within the photochemically cured polymer matrix. The morphology of thin films (5–20 μm) containing the gratings is examined by low-voltage, high-resolution scanning electron microscopy and transmission electron microscopy. Low concentrations of E7 (16% LC) coupled with rapid curing kinetics result in the formation of narrow LC-rich Bragg lamellae without well defined boundaries. These LC-rich lamellae, with approximate widths of 100 nm, are composed of small droplets measuring 20–50 nm in diameter. Increasing the concentration of LC in the prepolymer mixture results in larger lamellae (canals) of LC-rich material. Films formed from a mixture containing the highest LC concentration (34% LC) exhibited lamellae approximately 200–250 nm wide that are separated by polymer lamellae that also possess a small fraction of phase-separated LC droplets. The other variable examined in detail, laser writing intensity, has little effect on the morphologies exhibited in these films. The morphology is related to the performance (diffraction efficiency, transmission, switching times and fields) through a simple model.

A Calorimetric Study of Polymer-Dispersed Liquid Crystals: Cure Energetics and Kinetics

Abstract Polymer-dispersed liquid crystal (PDLC) films are formed by the phase separation of liquid crystal (LC) microdroplets from a polymer matrix during cross-linking (cure) of the matrix. Thus, studics of the cure energetics and kinetics of PDLCs provide insights into the mechanisms of their formation. We have carried out such studies for PDLCs cured in two different ways. We have measured the heat released during cure (ΔQ cure) and the time constant for the cure process (T cure) as a function of cure temperature (T cure) for two thermally-cured and four UV-cured systems. The heat of cure for both types of systems exhibits similar behavior: ΔQ cure goes through a broad maximum at a value of T cure where the degree of matrix cure is greatest. On the other hand, the behavior of the cure time constant differs for the two systems: T cure for thermally-cured PDLCs decreases monotonically with temperature, whereas that for UV-cured PDLCs exhibits a minimum. Both quantities are of importance for optimization...

Cryomicroscopy as a support technique for calorimetric measurements by DSC for the study of the kinetic parameters of crystallization in aqueous solutions: Part 1. Nucleation in the water—,2-propanediol system

Cryomicroscopy is shown to be complementary to calorimetry for qualitative and quantitative studies of crystallization kinetic parameters of aqueous solutions. The binary system water—,2-propanediol is investigated as an example, using the Johnson—Avrami theoretical model under isothermal conditions. The effect of nucleation on crystallization parameters is discussed, comparing calorimetric measurements with direct observations by cryomicroscopy. The experimental results depend strongly upon the thermal history of the sample prior to the isothermal experiment. The density of nuclei formed during warming after vitrification in 42.5% w/w 1,2-propanediol is inversely proportional to the warming rate, i.e. proportional to the time spent in the homogeneous nucleation thermal domain. The results support the hypothesis of a kinetic component for liquid—crystal phase separation during homogeneous nucleation. Assuming an Arrhenius dependence for the crystallization constant, K = K 0 exp(- E RT ), K 0 is shown to vary as the square root of the nucleus density, in agreement with the discoidal form of the crystals; the activation energy E is shown to be independent of nucleus density.

Cure Parameters and Phase Behavior of An Ultraviolet-Cured Polymer-Dispersed Liquid Crystal

Abstract Using calorimetry and scanning electron microscopy we have investigated the formation energetics and kinetics as well as the phase behavior of an ultraviolet-cured polymer-dispersed liquid crystal (PDLC) system. The purpose of this work was to determine those cure conditions which lead to liquid crystal microdroplet formation. We have found that a range of cure temperatures exists in which the degree of polymer cure is highest. In this range phase separation of liquid crystal from the polymer is maximized, and the best droplet microstructure occurs. Samples cured in the optimum temperature range also have the highest nematic-isotropic transition temperature. Results of calorimetric and electron microscopic studies are presented.

Phase Separation of Liquid Crystals in Polymers

Abstract New optoelectronic materials based on polymer dispersed liquid crystals (PDLC) show great potential for application in displays, temperature sensors, optical computing and for solar energy control. We report liquid crystal, termoset or thermoplastic materials. PDLC materials may be formed by several different processes. The liquid crystal may be dissolved in low molecular weight polymer precursors, in a thermoplastic melt or with a thermoplastic in a common solvent. Subsequent polymerization, cooling of the polymer melt or solvent evaporation lead to liquid crystal immiscibility, droplet formation and growth, and polymer gelation. The optoelectronic properties of these materials are affected by the droplet morphology. Specific examples are presented for each of these processes and it is demonstrated how the droplet morphology and density, and thus device performance, can be controlled by each method. The thermoplestics are suitable for forming films by a variety of techniques. A range of polymers...