摘要:

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.

ISSN:0267-8292    

DOI:10.1080/026782998207037

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The structure of 2,2-difluorobiphenyl in the solid, crystalline phase has been determined by X-ray diffraction. In this phase the molecules are all in a single conformation having the two fluorine nuclei in a syn -arrangment, with the two ring normals at 58 to one another. The structure of the same molecule, but dissolved in a liquid crystalline solvent has been investigated by NMR spectroscopy. In the liquid crystalline phase there is rotation about the inter-ring bond through an angle phi, with a probability distribution P(phi) which has an absolute maximum at the syn-form with phi about 51. There is also a second maximum in P(phi) at about 130, corresponding to the anti-form. The syn- and anti-forms are present in the approximate ratio 0.58:0.42.

ISSN:0267-8292    

DOI:10.1080/026782998207046

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The results of dielectric studies of two bi-component mixtures of well separated members (fourth and twelfth) of the series of 5-n-alkyl-2-(4-isothiocyanatophenyl)-1,3-dioxanes (nDBTs) are presented. Pure nDBTs exhibit the smectic A phase only, whereas the mixtures of well separated members of the series create a nematic gap. It was found that molecular rotations around the short axes are hindered by a lower activation barrier in the smectic A than in the nematic phase of the mixture, similarly to the situation for many pure substances with the same phase sequence. In the case of the large concentration of the longer component, a crystalline phase is created in which the short component performs uniaxial rotations in cages formed by the long molecules of the second component.

ISSN:0267-8292    

DOI:10.1080/026782998207055

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

This study investigates the optical response of liquid crystal cells to a low frequency square wave voltage of 0.1 Hz. It is found that there are three physical phenomena that dominate the overall properties of the device. The first is the discharging effect whereby the effective voltage over the liquid crystal layer decreases as a function of time; this occurs due to mobile ions being present within the liquid crystal material. The second is the charging-up of the cell where the effective voltage increases with time; this is attributed to charge separation taking place within the polyimide layer upon application of the d.c. voltage component. The third effect is cell asymmetry whereby the effective voltage depends upon the polarity of the externally applied field; this is the result of a locked-in d.c. holding voltage being present within the cell layers. These three effects are analysed in some detail with the view of developing a liquid crystal cell capable of being driven with a low frequency square wave voltage. A model of a liquid crystal cell in which the liquid crystal material can dissolve impurity ions from the alignment layers and in which the ions can then become re-adsorbed into the polyimide layer is deduced.

ISSN:0267-8292    

DOI:10.1080/026782998207064

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Three series of ferroelectric liquid crystals have been synthesized, having three ester groups in the mesogenic core and one ester group in the chiral chain. The ester groups introduced into the core decrease the temperature of crystallization, which results in a broad temperature range ferroelectric smectic C phase in homologues with a long non-chiral chain. These phases exhibit a high spontaneous polarization and relatively short pitch of the helical structure.

ISSN:0267-8292    

DOI:10.1080/026782998207073

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We have studied the phase retardation of linearly polarized light in a hybrid nematic liquid crystal cell. For a certain range of directions of the applied magnetic field the phase retardation is found to change non-monotonically with the magnetic induction. The observed behaviour is described rather well by the standard Frank elastic theory. The corrections resulting from subsurface deformations, which are characteristic both for second order elasticity approach and for surface field theory, are also considered. The analysis of the experimental data suggests that the presence of distortions in the zero-field director configuration is the necessary condition for the non-monotonic phase retardation, which implies that such an experiment could be used for the detection of misalignment of the effective pretilts in a nematic cell.

ISSN:0267-8292    

DOI:10.1080/026782998207082

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

This paper presents a theoretical examination of a nematic cell bounded by bistable weakly anchored surfaces. Switching between the two resultant equilibrium states is achieved by means of an electric field. In setting up the problem we identify a parameter whose measurement is important to the development of any device which relies upon weak anchoring. The results of this work are discussed in relation to the measurement of this constant.

ISSN:0267-8292    

DOI:10.1080/026782998207091

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

In several smectic phases the long molecules are tilted towards the layer planes. The molecules in the layers of smectic C phases have a preferred tilt direction with a C2 rotation axis perpendicular to the tilt plane. If the molecules have a brick-like shape, a configuration possessing the D2h symmetry is also possible. For molecules shaped like chevrons or bananas, the existence of a smectic phase with the symmetry Cv was recently reported. We consider different in-plane configurations of smectics using a geometrical approach based on the 'scaled particle theory'. Varying the geometrical parameters of hard rod particles, a phase diagram for several smectic configurations is predicted. Depending on the particle shape, phases with dipolar order (C2,Cv) and quadrupolar order (D2h) can be stable.

ISSN:0267-8292    

DOI:10.1080/026782998207109

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Several new laterally substituted liquid crystalline compounds have been synthesized. They have the same main core which contains four rings (two aromatic, two alicyclic) with two lateral substituents introduced on the same side of one of the inner rings. One of the substituents is a 4-X-benzyloxy group (X=CH3,CN,Cl) and the other is a hexyloxy chain. The presence of the lateral aromatic substituent makes these compounds deviate markedly from the classical rod-shape. However, a wide enantiotropic nematic phase is present for all the compounds. The order parameters of the chain and the para -disubstituted aromatic rings were obtained by using a 2D 13C NMR technique with variable angle spinning. The temperature dependence of the order parameters was estimated using 13C chemical shifts with slow spinning of the sample parallel to the magnetic field. The results indicate that the two lateral substituents are more or less folded back along the mesogenic core. Thus, the flexible lateral chain is found to be roughly aligned with the molecular long axis, whereas the para axis of the less flexible aromatic branch makes a considerable angle with the molecular long axis imposed by the core, substantially increasing the mean width of the molecule. The core ordering does not seem to be influenced by the type and position of the substituents. The folding back of the lateral chain and the substantial tilt of the lateral aromatic branch with respect to the core main axis are confirmed by the X-ray structure of a parent compound.

ISSN:0267-8292    

DOI:10.1080/026782998207118

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor