Mesophase motion

Mesophase in which non-vibratory molecular motion is frozen by supercooling a mesophase stable at a higher temperature. 

An important characteristic of the cubic mesophases, either non-amphiphilic or amphiphilic, is that because of the fairly free thermal rotational motions of their constituent units, they typically give high resolution NMR spectra. In this respect they behave like amorphous liquids and quite differently from conventional solids or from mesophases such as Mi, M2, or G in which rotation of the units is more severely restricted. 

The micelles of this size and shape (i.e., in sufficient close proximity) must be concentrated sufficiently to result in the formation of the mesophase lattice at the prevailing temperature. At some higher temperature, this lattice will break down to give the amorphous liquid by the disintegrating effect of the increased thermal motion. 

It is interesting that work on the internal motions of the molecules that produce lyotropic mesophases is more advanced. This is mainly because of the importance of the microscopic properties of these systems in solubilization and interfacial problems, problems which are encountered in industry as well as in cell membrane biology. The structural and functional roles of lipid molecules in biomembranes are much discussed investigations of the physicochemical properties of lipid media thus might provide orientations for biological studies. Moreover, the findings on the flexibility of the paraffinic chains in lyotropic mesophases might also be relevant to similar problems in thermotropic mesophases. 

Discussion. We can now propose a coarse description of the paraffinic medium in a lamellar lyotropic mesophase (potassium laurate-water). Fast translational diffusion, with D 10"6 at 90 °C, occurs while the chain conformation changes. The characteristic times of the chain deformations are distributed up to 3.10"6 sec at 90 °C. Presence of the soap-water interface and of neighboring molecules limits the number of conformations accessible to the chains. These findings confirm the concept of the paraffinic medium as an anisotropic liquid. One must also compare the frequencies of the slowest deformation mode (106 Hz) and of the local diffusive jump (109 Hz). When one molecule wants to slip by the side of another, the way has to be free. If the swinging motions of the molecules, or their slowest deformation modes, were uncorrelated, the molecules would have to wait about 10"6 sec between two diffusive jumps. The rapid diffusion could then be understood if the slow motions were collective motions in the lamellae. In this respect, the slow motions could depend on the macroscopic structure (lamellar or cylindrical, for example)... 

The appearance of a sharp central peak in the deuteron NMR spectrum for unaligned lyotropic liquid crystalline samples has been observed by several authors (30, 33, 34). This has been interpreted in terms of phase inhomogenities (35) or isotropic motion (36). However, recently Wennerstrom et al. showed for a lamellar amphiphile-water mesophase that this could be referred to double quantum transitions (37). It is expected that double quantum transition peaks appear in NMR spectra... 

A number of characteristic temperatures are important in LC polymer work. The glass transition temperature, Tg, is that temperature below which segmental motion of the main chain of the polymer does not occur, although motions (e.g., rotation) of side-groups may occur. The isotropization or clearing temperature, 7j, is the temperature at which the polymer enters the isotropic melt from one of its mesophases and the birefringence of the mesophase disappears. Temperatures are often quoted more specifically defining where phase transitions occur. For example, 7, would be the temperature where the nematic phase enters the isotropic melt. In this case, of course, TNI is the same as Tr... 

The relative magnitudes of the Tg values in the two-phase region should be related to the development of microstructure in mesophase pitch, as discussed earlier. Tg will also be a useful parameter in defining the maximum temperature at which fibres can be oxidized without molecular motion causing some decrease in extent of preferred orientation. 

Fine deformed microstructures with strong preferred orientations could be produced by a single stroke of the wire probe (25), as illustrated by Figure 17. A vertical section made on a specimen quenched immediately after deformation confirmed that the underlying structure was fibrous with tt and 2tt wedge disclinations (30). The relaxation or coarsening after deformation indicated that the mesophase was sufficiently fluid for disclination motion... 

SCLCPs combine liquid crystalline properties and polymeric behavior in one material. If the mesogenic unit is fixed directly to the polymer main chain, the motion of the liquid crystalline side chain is coupled with the motion of the polymer backbone, preventing the formation of a LC mesophase. Therefore, Finkelmann and Ringsdorf proposed that the introduction of a flexible spacer between the main chain and the mesogenic unit would decouple their motions, allowing the mesogenic moiety to build up an orientational order [29,30]. 

Podgornik R, Strey HH, Gawrisch K, Rau DC, Rupprecht A, Parsegian VA. Bond orientational order, molecular motion, and free energy of high-density DNA mesophases. Proc. Natl. Acad. Sci. U.S.A. 1996 93 4261 266. 

K by the forcible pressure swing adsorption method (ca. 13 MPa). The adsorbed methane molecules are located in the pocket-like narrow corners of the necks of the ID channel [20]. Because the thermal motion of the pseudo-spherical methane molecules seems to be effectively suppressed in its translation mode but rotation is allowed, the forcible adsorption of methane gas produces an inclusion plastic crystal [20], which can be regarded as a mesophase between the fluid and solid state of the phase of a guest incorporated in a crystal host the guest molecules are randomly oriented, but their alignment follows the crystal periodicity. 

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