摘要:

We have observed by polarizing microscopy a novel order transition inside the nematic phase and definitely below the clearing point of trans -4-hexylcyclohexane-1-carboxylic acid (C). The substance is known to exhibit the phase sequence Cr 32 C SmB 47 C N 96 C I (where Cr crystal, SmB smectic B, N nematic, I isotropic). The order transition, very smooth, was recognized both on cooling and on heating of the sample, consisting of a sandwich cell made by two glass plates unidirectionally rubbed, with a gap around 1mum. The nematic 'subphase' below the order transition is better ordered (with smaller and more regular domains) than the other 'subphase' above this transition. The data are compared with those obtained for 4- n -heptyloxybenzoic acid (HOBA) and already discussed in terms of a surface transition, and are interpreted as due to the presence of smectic cybotactic clusters. To enhance the detection sensitivity, we applied a method of image processing recently introduced by us and also able to reveal soft structural changes in the image frame. Since the method sensitivity is at least 10 times higher with respect to standard techniques, the order transition, even though very smooth (especially on heating), was easily detected, without the necessity for special cell treatment to align the director perfectly in the high temperature nematic phase. 6

ISSN:0267-8292    

DOI:10.1080/026782998205903

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A new Landau-type phenomenological free energy function to describe the phase diagram of uniaxial anisotropic materials is proposed. The resulting phase diagram includes crystalline, smectic A, nematic and isotropic states, as well as a theoretically possible plastic solid state. Depending on the parameters of the free energy, smectic A phase may or may not be present for a given material. The obtained topologies of the temperature-pressure phase diagram qualitatively agree with results from computer simulations of model liquid crystalline materials.

ISSN:0267-8292    

DOI:10.1080/026782998205912

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The layer structure that occurs in an achiral smectic C liquid crystal device has been investigated as a function of temperature using the small angle X-ray scattering facility at the Synchrotron Radiation Source, Daresbury UK. The material studied shows a direct phase transition from the nematic to the smectic C phase. The layer structure proposed on the basis of the diffraction data is relatively complex, containing regions with chevron, quasi-bookshelf and curved structures. A rationale for the formation of the structure is presented, relying on both the phase transition characteristics of the system and the anisotropic layer elasticity in the smectic C phase. Qualitative analysis indicates that the layer constant A is greater than A 21, i.e. layer flexing is easier perpendicular to the plane of the director than parallel to it. It is also demonstrated that the surface chevron angle is several degrees different from the tilt angle of the smectic C phase at temperatures well below the smectic C to nematic phase transition. 12

ISSN:0267-8292    

DOI:10.1080/026782998205921

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Several new fluorinated liquid crystal compounds, 4- n -propylbicyclohexylbenzyl fluoroalkyl ethers (C2BFE) and a 4- n -propylcyclohexylbenzyl 2-perfluoropropoxy-2,3,3,3-tetrafluoropropyl ether (C1BFE), were synthesized and their phase transition behaviours and electro-optical properties investigated by DSC, polarizing optical microscopy, Abbe refractometry and other physical methods. All the C2BFE compounds showed a layered mesophase like SmB, while C2BFE had no mesophase. All the fluorinated compounds have good solubility in ZLI-1132 (more than 15 wt %) and can be used as a component for lowering the refractive anisotropy (Delta n) of the nematic solvent ZLI-1132.

ISSN:0267-8292    

DOI:10.1080/026782998205930

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor