A. Golemme

Electrooptic properties of polymer dispersed liquid crystals

Abstract An investigation of the electrooptic properties of polymer dispersed liquid crystals (PDLC) is presented. These materials are light modulating systems. They show a reversible optical response from an opaque state to a highly transmitting state under the action of an appropriate electric field which aligns the liquid crystal director. The switching voltage required to establish such an electric field has been monitored as a function of (i) the starting materials used for the preparation of the PDLCs, (ii) the ageing (curing time) of the PDLC cells. Other physical properties, such as the electrical resistivity and the dielectric constant of the materials, have been measured. The correlations between these properties have been studied. The PDLC switching voltage appears to be strongly correlated with the resistivity. Our data suggest that ionic impurities play a dominant role with respect to the electrooptic response of PDLC films.

Phases and phase transitions in a nematic lyotropic system with a biaxial phase

Abstract NMR spectroscopy and optical microscopy have been used to study phase transitions and structure in nematic lyotropic mesophases formed by potassium laurate, decylammonium hydrochloride and water. The different mesophases obtained in a well-defined composition range have been characterized, by deuterium NMR, following the evolution of the D2O spectra as a function of temperature and of the orientation of the samples with respect to the magnetic field. Wide ranges of biaxiality have been found and the asymmetry parameter of the averaged electrical field gradient tensor on the deuterium of the D2O molecule has been determined. The presence of the different mesophases has always been confirmed by observing oriented samples under the polarizing optical microscope.

Investigation of ribbon lyotropic structures by deuterium magnetic resonance

Deuterium magnetic resonance is used to study the shape and size of ribbon‐like aggregates as well as the contrasting molecular ordering which is shown to occur in different regions of the aggregate. Elongated ribbon‐like aggregate structures are formed from a ternary mixture of potassium palmitate/potassium laurate/water. Deuterium spectral patterns from various selectively deuterated sites of the potassium palmitate compound are analyzed in terms of a simple model for the ribbon structure. The results indicate that the order parameter S of the terminal methyl position is enhanced by a factor of 2 in the curved cylindrical edge of the ribbon over its value in the uncurved central lamellar‐like region. The size of the ribbon model is also determined. At lower temperature, the ribbons transform into cylindrical aggregates on a rectangular lattice. Deuterium magnetic resonance of water is shown to be sensitive to the details of the lattice structure.

Molecular distribution among regions of different curvature in aggregates of a lyotropic ternary system: A NMR measurement

A lyotropic phase consisting of elongated ribbon aggregates on a rectangular lattice is induced when a small amount (∼1 molu2009%) of potassium laurate (KL) is added to a potassium palmitate (KP)/water mixture. By selective deuteration it is shown that the KL and KP molecules are not uniformly distributed throughout the ribbon aggregate. The less abundant KL molecules with the shorter hydrocarbon chain are more concentrated at the edges of the ribbons where there is high curvature, whereas the more abundant KP molecules are more concentrated in the central lamellar region of the aggregates. A quantitative measurement is described. The deuterium spectra also show how the KP and KL lipids are distributed among coexisting phases when an uneven distribution also occurs. The KP molecules prefer the lamellar structures, whereas the KL molecules prefer the aggregate structures.

Photorefractive chiral smectic A phases

We present results of the photorefractive properties of a chiral smectic A liquid-crystalline phase. In our approach, the mesophase is sandwiched between two photoconducting layers. In contrast to the effect due to the dielectric anisotropy typical of nematic phases, the refractive-index modulation is due to the electroclinic effect. We measured the grating spacing and the field dependence of the gain coefficient and obtained a net two-beam coupling gain of 600 cm−1 at 35 V/μm.

Dynamic behavior of water in lyotropic ternary mixtures

Abstract The diffusion of water molecules in lyotropic mixtures of potassium palmitate/potassium laurate/water was investigated using pulsed-field gradient nuclear magnetic resonance (PFG-NMR). The self-diffusion constant of water as a function of temperature was measured in two samples having different compositions and found to be sensitive to the structures of the different mesophases. Deuterium nuclear magnetic resonance (D-NMR) experiments were also performed on a mixture containing D2O, in a temperature rang where a rectangular mesophase (RBa ) coexists with a lamellar (La ) mesophase. The diffusion data, combined with the D-NMR spectral profiles allowed for the study of the size of the mixed mesophase domains.

Studies of Structure and Molecular Distribution in Lyotropic Ribbon Aggregates

Abstract A lyotropic phase consisting of elongated ribbon aggregates on a rectangular lattice is formed with a ternary mixture of potassium palmitate (KP), potassium laurate (KL) and water. Deuterium NMR and x-ray studies of molecular factors which affect the formation of the aggregate system are reviewed. By selective deuteration of the lipids it is shown that the KL and KP molecules are not uniformly distributed throughout the ribbon aggregate. The less abundant KL molecules with the shorter hydrocarbon chain are more concentrated at the edges of the ribbons where there is high curvature whereas the more abundant KP molecules are more concentrated in the central lamellar region of the aggregate. The shape of the ribbon and molecular ordering within the ribbon are also reviewed.

PHASE SEPARATION KINETICS IN MIXTURES OF POLYMERS AND LIQUID CRYSTALS

Light scattering has been used to study phase separation kinetics in mixtures containing liquid crystals and epoxy resins. In the samples studied, phase separation was induced by the polymerization of the resins with an appropriate curing agent. Experiments were carried out at different compositions and at different temperatures. The results show that the kinetic mechanism of phase separation is composition dependent. For high liquid crystal content the data are in qualitative agreement with existing theories describing spinodal decomposition; at lower concentrations the mechanism is different. The physical properties of the resulting materials are independent of the decomposition mechanism. The data have also been analysed considering the scaling behaviour expected for late stages of phase separation in polyinduced meric mixtures. Samples obtained in a narrow concentration range, where the two kinetic mechanisms overlap, exhibit peculiar physical properties.

Phase separation in polymer dispersed liquid crystals with two different reacting monomers

We have studied the phase separation process in a mixture of a liquid crystal material and two different monomers undergoing a polymerization process. Phase equilibrium was theoretically analysed using a Van der Waals free energy density approach. Results indicate that the more slowly reacting monomer will be found at higher concentration at the droplet boundary in the fully cured polymer dispersed liquid crystal. Experimental evidence, which includes the observation of director configurations and the measurement of transmission in the on-state, is presented.

Water ordering in a MTAB/decanol/water lyotropic system

Abstract Lyotropic mesophases consisting of myristyltrimethylammonium bromide and water or MTAB, water and decanol have been investigated. 2H NMR studies from both the binary and the ternary systems are reported in which the quadrupolar splittings from D2O and decanol (α-deuteriated) have been analysed. 17O NMR, 81Br NMR and optical microscopy have also been used. In the concentration and temperature ranges studied, the binary system exhibits a cylindrical phase, while for the ternary system a discotic phase is found. An anomalous behaviour of the quadrupolar splitting of water is observed: it vanishes at a particular temperature, where optical microscopy does not show any phase transition and the quadrupolar splitting of deuteriated decanol does not exhibit any discontinuity. We found that the change in the order of water does not depend on a variation of micellar structure. We have interpreted this behaviour in terms of the existence of two different sites occupied by water molecules at the interface, w...