Rifat Ata Mustafa Hikmet

Anisotropic gels and plasticized networks formed by liquid crystal molecules

Abstract Two component mixtures containing liquid crystalline diacrylates and liquid crystal molecules with no reactive groups have been made. Upon photopolymeriz-ation of the mixtures anisotropic networks containing liquid crystal molecules which were not chemically attached to the networks were created. In these systems the behaviour of the liquid crystal molecules was found to be dominated by the network molecules. Even when the liquid crystal molecules in these networks were heated above their isotropic transition temperature they remained partly oriented, thus contributing to the anisotropy of the system.

Synthesis and photopolymerization of cholesteric liquid crystalline diacrylates

Abstract The synthesis of chiral liquid crystalline diacrylates is described. By mixing one of them with non-chiral liquid crystalline diacrylates. cholesteric phases are obtained of which the wavelength of reflection can be chosen by the composition of the mixture. After isothermal photopolymerization of the mixture a crosslinked network is obtained with thermally stable properties.

Ferroelectric liquid crystal gels Network stabilized ferroelectric display

Abstract Liquid crystal (LC) mixtures of a reactive diacrylate and a commercial ferroelectric liquid crystal (FLC) mixture were produced. The mixtures were brought into cells provided with orientation layers in which various orientations such as uniaxial and twisted orientations, could be induced. When the desired orientation had been obtained, the polymerization of the reactive molecules was induced creating a three-dimensional anisotropic network containing the FLC molecules which were not chemically attached to the network (ferroelectric gels). The presence of the anisotropic network was found to have a large effect on various properties of the FLC molecules. For example, complex orientations (for example, 180° twisted) obtained by polymerization (formation of the gel) in the nematic phase remained unchanged when the gel was cooled to the FLC phase. Likewise uniaxial orientation, which cannot be realised in thick cells containing the pure FLC, could be realised when ferroelectric gels were used. For th...

Lyotropic Liquid‐Crystalline Behavior in Disc‐Shaped Compounds Incorporating the 3,3′‐Di(acylamino)‐2,2′‐bipyridine Unit

An extended‐core liquid crystal (LC) based on the 3,3′‐di(acylamino)‐2,2′‐bipyridine unit is investigated with respect to its behavior in alkane solvents. It is demonstrated that the lyotropic mesomorphism observed in dodecane can be tuned by varying the concentration of the sample, and the columns can be switched in an electric field. The Figure shows the structure of the columnar nematic phase present in dodecane containing 15.7 wt.‐% LC.

Anisotropic networks obtained by in situ cationic polymerization of liquid-crystalline divinyl ethers

Abstract Anisotropic networks were obtained by photoinduced cationic polymerization of liquid-crystalline (LC) divinyl ethers. For this purpose two new LC divinyl ethers were synthesized. Transition temperatures, phase behaviour and the temperature dependence of the order parameter for the two systems were determined before polymerization. Subsequently the polymerization of the system was induced upon obtaining long-range uniaxial orientation of the molecules in the nematic state. In this way highly oriented networks were obtained. These anisotropic networks showed temperature-stable optical properties. Thermomechanical properties were found also to be anisotropic and highly dependent on relaxation mechanisms effective within the networks.

Structure of cholesteric gels and their electrically induced light scattering and colour changes

Abstract Networks with a helical structure containing large amounts of low mass liquid crystal molecules (gels) were produced by photopolymerization of a nematic diacrylate in the presence of low mass cholesteric mixtures. Prior to polymerization the systems selectively reflected a band of circularly polarized light. Upon polymerization, the gels obtained gave rise to two reflection peaks which were subsequently associated with polymer rich and polymer poor phases. At high network concentrations (>40 per cent w/w) one of the peaks did not appear, indicating that the behaviour of the molecules which were not chemically attached to the network was totally dominated by the network. From the gap required for the suppression of the rotation of the director, it was estimated that the molecules, which were not crosslinked, were confined in layers thinner than 85 nm. The gels were very transparent and did not give rise to excess light scattering as compared with the monomeric state. Upon application of an electri...

Switchable mirrors of chiral liquid crystal gels

Chiral nematic liquid crystal compositions containing mono- and di-acrylates in mixtures with non-reactive liquid crystal molecules were produced. Defect-free planar macroscopic alignment of the molecules was induced between uniaxially rubbed substrates. Polymerization of the acrylate groups was initiated photochemically leading to the formation of a new class of chiral gels consisting of a lightly cross-linked network containing non-reactive molecules. In this way, the chiral pitch and, hence, the reflection colour became permanently fixed. Under the influence of an electric field the gels could be switched to the homeotropic state and reverted to the planar defect-free state upon removal of the electric field. Depending on the composition of the gel, either the position of the reflection band remained the same while its magnitude and/or width decreased, or its position shifted to smaller wavelengths with increasing electric field. Some of the gels showed sharp switching characteristics with a hysteresis...

Energy transfer in hybrid quantum dot light-emitting diodes

Energy transfer in a host-guest system consisting of a blue-emitting poly(2,7-spirofluorene) (PSF) donor and red-emitting CdSe∕ZnS core shell quantum dots (QDs) as acceptor is investigated in solid films, using time-resolved optical spectroscopy, and in electroluminescent diodes. In the QD:PSF composite films, the Forster radius for energy transfer is found to be 4–6nm. In electroluminescent devices lacking an electron transport layer, the electroluminescence (EL) spectrum of the QD:PSF polymer composite is similar to the photoluminescence (PL), giving evidence for energy transfer from PSF to the QDs. The addition of an electron transport layer between the emitting layer and the cathode results in a significant change in the EL spectrum and a considerable improved device performance, providing almost pure monochromatic emission at 630nm with a luminance efficiency of 0.32cd∕A. The change in spectrum signifies that the electron transport layer changes the dominant pathway for QD emission from energy transf...

Anisotropic polymerization shrinkage behaviour of liquid-crystalline diacrylates

Abstract Anisotropic polymerization shrinkage of liquid-crystalline acrylates has been studied. In order to investigate the changes occurring on the microscopic scale, X-ray diffraction was combined with density measurements. As a result it was shown that at high polymerization temperatures the polymerization shrinkage in the direction of the molecular orientation was higher than the shrinkage in the directions perpendicular to the molecular orientation. Subsequently the shrinkage obtained on the microscopic scale was compared with the results obtained on the macroscopic scale. The behaviour was interpreted in terms of the molecular structure and the packing density within the systems.

Dielectric relaxation of liquid crystal molecules in anisotropic confinements

Abstract The dielectric relaxation of the liquid crystal 4-n-pentyl-4′-cyanobiphenyl (K15) in the presence of an anisotropic network has been studied. Anisotropic networks containing K15 molecules were prepared by in situ polymerisation of liquid-crystalline diacrylate molecules in a mixture containing K15. By changing the network concentration, the effect of the network molecules on the behaviour of the K15 molecules, which were not chemically attached to the network, was investigated. With increasing network concentration it was found that the mean relaxation times of K15 molecules shifted to lower temperatures and that their distribution became broader. The activation energy associated with the relaxation, however, remained almost constant before showing some increase at high network concentrations.