Short-range order smectic

Multilamellar bilayers in the fluid phase are also ordered in the sense that they are smectic liquid crystals. Of great interest is the range of molecular order this is long-range in the sense that the molecules are confined to two dimensions there is also some kind of short-range order in molecular orientations and conformations, but the range of this latter ordering is not known at present. 

Short range order in liquid-like systems as well as long range order in crystalline domains are reflected in WAXS-patterns very dearly. Some examples of calculated X-ray patterns from PTFE (Phase I), a smectic LC-phase and even a PE melt, show that our model covers a wide range of macromolecular structures running the whole scale from crystalline systems over mesophases up to polymer melts. The range of intra- and intermolecular order can be estimated fairly well with the help of density correlation functions. 

Liquid crystals can be in the smectic, nematic, or isotropic states. In the smectic liquid crystalline state there is a long-range order in the direction of the long axis of the molecules. These molecules may be in single- or bilayer conformation, have molecular axis normal or tilted to the plane of the layer, and frozen or melted chains. In the nematic liquid crystalline state the molecules are aligned side by side but not in specific layers. The isotropic liquid crystalline state is more or less a liquid state, but where clusters with short-range order persist (Small, 1986, pp. 49-51). 

A14N NMR study of order fluctuations in the isotropic phase of liquid crystals has been reported. (209) The experimental data for the isotropic phases of -azoxyanisole and of diethylazoxy benzoate are accounted for in terms of short range order fluctuations of the nematic and of the smectic types respectively. 

The liquid crystalline phase is called a mesopliase and is intermediate between solid and liquid. In this mesopliase the molecules show liquid-like long-range behaviour, i.e. are essentially disordered, but also some crystal-like aspects of short-range order. The type of long-range order in the mesopliase may vary. If the molecules align themselves as layers this is described as a smectic phase if the alignment is as parallel threads it is described as a nematic phase. 

An important source of error in these calculations is the neglect of short-range order. In particular, the theory fails for the bend and twist elastic constants when smectic-like short-range order is present in the nematic liquid crystal. Under such circumstances these two constants exhibit a critical divergence as the temperature approaches the smectic-nematic point and the light scattering also shows a marked temperature dependence. The present treatment is then inadequate and more elaborate models have been proposed. The phenomenological theory of this aspect of the problem will be discussed in chapter 5. 

A similar reversal of handedness takes place as the composition is varied.The inverse pitch shows a nearly linear relationship with composition around the nematic point, but there are significant departures when one of the components has a smectic phase at a lower tem-perature. 44 The anomaly may again be attributed to smectic-like short-range order. 

To discuss the critical behaviour of the twist and bend elastic constants in the nematic phase, we observe that the Frank free energy expression should include the contribution due to smectic short-range order ... 

Fig. 5.5.2. Temperature dependence of the susceptibility Of) and the correlation lengths if and for smectic A short-range order in the nematic phase of butoxybenzylidene-octylaniline. Open circles were deduced from X-ray scattering, and filled circles from light scattering. The smectic density wavevector remains...

The narrower a range of the nematic phase in a homological series of different compounds (as an example see Fig. 6.1 la) the stronger are first order features of the N-SmA transition. In some sense, the SmA phase feels the proximity of the isotropic phase. In other words, we may say that, in the isotropic phase, there are traces of both nematic and smectic A short-range order. 

The main effect of a central longitudinal dipole moment in SC and GB molecules is to stabilize smectic layering relative to the nematic phase, when compared with the nonpolar case, due to favourable antiferroelectric short range order [225,227,228]. As the strength of the dipole increases the nematic... 

Phases possessing only liquid-like short-range order within their layers smectic- (orthogonal) and C (tilted). 

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