Mesophases in General

Compared to other biomolecular systems, lipid bilayer membranes and lyotropic lipid mesophases in general have been shown to respond most sensitively to hydrostatic pressure. The methods used in the high pressure studies have mainly included X-ray and neutron diffraction, fluorescence, IR and Raman spectroscopy, light transmission and volumetric measurements. Only a small amount of work has been performed using NMR techniques combined with high-pressure, a field which was pioneered by Jonas and co-workers " although the method is very powerful, non-invasive and allows the study of a series of structural and dynamic properties of the systems in detail and with atomic resolution. 

Cholesteric mesophases in general resemble the nematic mesophase when observed by both small-angle X-ray diffraction, SAXD, and WAXD. The smectic mesophase, in contrast, produces both diffuse rings at 4--5 A and sharp rings at a distance generally, but not necessarily, equal to the repeat length of the monomer unit, between 15 and 50 A. 

Two simple thermodynamic considerations are suggested upon examination of Fig. 8. The first is that at temperatures below Tml the free energy of the bulk mesophase G m is in general bound to be lower than Gl, the free energy of the amorphous. In the limit of Class II mesophases, since AHml = 0, we will have Gm = Gl at T = 0 K while Gm < Gl at temperatures 0 < T < Tml since it is Sm > Sl at temperatures low enough as compared to Tml (Sect. 3.1). In the case of Class I mesophases AHml > 0. he., mesophases are enthalpically stabilized with respect to the liquid state, while Sm < Sl, so it will be Gm < Gl at temperatures T, with 0 < T < Tml- Note that the above consideration will in... 

For spin-f nuclei, dipolar interactions may be modulated by intramolecular (DF, reorientation etc.) and/or intermolecular (TD) processes. In general, the intra- and inter-molecular processes can produce quite different Tj frequency dispersion curves. In practice, NMR field cycling experiments are often needed to extend the frequency domain from those employed in conventional spectrometers to a lower frequency range (i.e., the kHz regime) for unambiguous separation (and identification) of different relaxation mechanisms. The proton spin relaxation by anisotropic TD in various mesophases has been considered by Zumer and Vilfan.131 133,159 In the nematic phase, Zumer and Vilfan found the following expression for T ... 

The recommendations embodied in this document are concerned with the terminology relating to the structure of crystalline polymers and the process of macromolecular crystallization. The document is limited to systems exhibiting crystallinity in the classical sense of three-dimensionally periodic regularity. The recommendations deal primarily with crystal structures that are comprised of essentially rectilinear, parallel-packed polymer chains, and secondarily, with those composed of so-called globular macromolecules. Since the latter are biological in nature, they are not covered in detail here. In general, macromolecular systems with mesophases are also omitted, but crystalline polymers with conformational disorder are included. 

In general, GUDC is known to produce a mesophase during dissolution of cholesterol(CHL) in the presence of lecithin(EL) in vitro, eind then increases the dissolution rate of CHL ty the mesophase formation. However, the effect is less than that of GCDC in vivo. 

The crystallization of 3D-ordered crystalline phases from thermotropic mesophases, envisaged as stable pre-crystalline partially ordered intermediates, is an additional interesting issue which should be considered with care experimentally, theoretically, and with appropriate simulation approaches. Depending upon the nature of the mesophase it can be seen as a crystal-crystal transition or, for conformationally disordered, columnar mesophases, it approaches a true crystallization process. It is quite clear that the preexisting order will play a major role for example if the mesophase is chain-extended, bundle equilibria and chain-folding should not play any role. Indeed available experimental evidence supports this idea. Mechanistic and kinetic features should in general differ widely from the standard chain-folded crystallization processes yielding thin lamellar structures. In a number of cases (polyphosphazenes, polysiloxanes, see below) the crystalline polymorphs obtained from the chain-extended precursor differ from those obtained from solution. 

Composition of Parent Pitch. Once the chemical composition of the carbonizing system moves away from the comparative simplicity of polynuclear aromatic hydrocarbons to that of industrial pitches, then the pyrolysis chemistry incorporates effects caused by the presence of heteroatoms (0, N and S) and alkyl and naphthenic groups. In general terms, the system becomes more Reactive creating higher concentrations of radicals detectable by ESR. This in turn, leads to enhanced cross-linkages and polymerization of molecular constituents of any mesophase which is formed, and this causes enhanced viscosity and a reduction in size of optical texture. 

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