Phases frustrated phase

Thermotropic liquid crystals, 15 86-98 bent-core, 15 98 discotic phases of, 15 96 frustrated phases of, 15 94-96 metallomesogens, 15 97 nematic liquid crystals, 15 86-92 smectic liquid crystals, 15 92-94 Thermotropic mesophases, 20 79 Thermotropic polycarbonates, 19 804 Thermotropic polyesters, liquid-crystalline, 20 34... 

Frustrated Phases. Chiral molecules normally form chiral phases, hui in some cases this is dune in an interesting way. For example, it is not... 

Finally, we have attempted to evaluate the possible impact of an intermediate liquid crystalline phase and the possibility of transfer of helical hand information from the melt to the crystal throughout this process. Assuming that the melt is structured, the melt of chiral but racemic polyolefins would be made of stretches of helical stems that are equally partitioned between left- and right-handed helices. Formation of antichiral structures (such as in a iPP) could be interpreted as indicating a possible transfer of information (but the problem of the sequence of helical hands would still remain). This analysis is, however, ruined by the observation that many of these polymers also form chiral structures (frustrated p phase of iPP, Form III of iPBul). For the achiral poly(5-methyl-pentene-l), the chiral, frustrated phase is actually the more stable one, and can be obtained by melting and recrystallization of a less stable antichiral phase. 

Keywords Blue phase Frustration Double twist Electrooptic effect Photonic crystal... 

Recent progress in material science, notably with the development of new materials exhibiting blue phases, has generated a renewed interest in the incorporation of the functional properties with the unique structure of frustrated phases. Synthesis of a monosubstituted ferrocene-based chiral Schiff s base derivative which exhibits TGBA and blue phases has been reported [17] (Fig. 9). Other metallomesogens leading to blue phases have been found for palladium complexes [18] (Fig. 10). Optically active materials incorporating... 

Keywords Liquid crystals, self-organizing systems. Chirality, Frustrated phases. Superconductivity, Abrikosov flux phases... 

Molecular shape anisotropy plays a very important role in determining whether or not liquid crystal phases will be formed, and indeed, when they are formed, which particular modification will be generated in preference to other mesophases [3, 4]. Molecules that form liquid crystals are usually carbon based, but they need not be. They often have preferred shapes, based on rods, bananas, boards or discs. In the following account, the properties of some unusual rod-like organic systems which have asymmetric structures thereby making them optically active and chiral will be described. In particular, the unusual self-organizing properties and the structures of the frustrated phases of the (R), (S), and racemic forms of 1-methylheptyl 4 -(4-n-... 

Fig. 6. However, these two structures are incompatible with one another and cannot co-exist and the molecules still fill space uniformly without forming defects. The matter is resolved by the formation of a periodic ordering of screw dislocations which enables a quasi-helical structure to co-exist with a layered structure. This is achieved by having small blocks/sheets of molecules, which have a local smectic structure, being rotated with respect to one another by a set of screw dislocations, thereby forming a helical structure [15]. As the macroscopic helix is formed with the aid of screw dislocations, the dislocations themselves must be periodic. It is predicted that rows of screw dislocations in the lattice will form grain boundaries in the phase, see Fig. 7, and hence this structurally frustrated phase, which was theoretically predicted by Renn and Lubensky [15], was called the twist grain boundary (TGB).

Fig. 4.9 Structure of polar smectic A phases Aj, A2 and Ad and a frustrated phase Amod...

The metastable /3-phase of iPP was imaged in another study, where epitaxial crystallization was found to result in a biaxial orientation that could not be achieved mechanically because of the p a transition that occurs during orientation. A lateral periodicity of 1.9 nm was found in the (110) face, corresponding to the distance between three chains, and is indicative of the frustrated packing of the /3-phase of iPP. Variability in the image suggested the possibility of two distinct frustrated phases existing in the samples (92). 

Most experiments on LC elastomers have so far used single crystal elastomers made via the two-step crosslinking process, which involves stretching in the LC state. There is increasing evidence that this situation represents a special thermodynamic state - smectic elastomers made in such a way are well aligned but their layer positions are frustrated due to the random crosslink distribution. Evidently, there is room for experiments on nematic and smectic elastomer samples oriented in different ways, for example by photo-crosslinking. In such a way, any memory of the aligning procedure imprinted in the samples will be avoided (at least partially) and new features of phases and phase transitions could be revealed. 

For a detailed discussion of the frustrated phases, such as blue, TGB, and chiral columnar phases, see the chapters by Bock, Crooker, Kitzerow, and Pieranski. 

Fig. 39.6 Transition from inactive toluene soivate phase of 5 to inactive compieteiy desoivated phase through active frustrated phase anaiyzed with subsequent PXRD changes and gas sorption isotherm for O2, N2, CH4, and CO2 (Reproduced with permission from Ref. [27a]. Copyright 2006, American Chemical Society)...

Another class of frustrated phase results from the frustration between bend or twist deformations in smectic phases (Section 5.6) and the tendency to form a layered structure. Twisted grain boundary phases are frustrated smectic phases and both SmA and SmC versions have been observed. The phases are denoted TGBA and TGBC respectively and are formed by chiral mesogens. The phases are macroscopically chiral and result from arrays of screw dislocations (i.e. defects in lattice order) which lead to a twist in the director between grains of layers, i.e. to a helical rotation of layers. 

These frustrated phases were called blue phases because when they were first observed microscopically by Coates and Gray [28] they appeared blue. Their strong blue color is due to the selective reflection of light. Other materials were later discovered which exhibited blue phases where the selective reflection was in the red or green region of the spectrum. Experimentally it was found, however, that the helical pitch length... 

Fig. 9.22 (a) X-ray pattern of oriented smectic phase observed in BPCO3-011 fibers at 200 °C. The fiber specimen was prepared by pulling up the isotropic melt and its axis is placed in the vertical direction. It includes several other inner reflections except for the usual layer reflections, indicating the frustrated phase, (b) Tentative structural model of frustrated smectic phase. For convenience, the polymer chains in an all-trans conformation are illustrated... 

When collisional effects are negligible the second component in (3.8) vanishes, and J becomes a constant. The solution obtained in this case is used in Eq. (3.4). If collisions occur they change J and frustrate the vibrational phase simultaneously. Nevertheless, the processes are usually considered to be statistically independent ... 

Physically the independence reflects the fact that dephasing is performed by weak long-range interactions, and rotational relaxation results mainly from short-range, repulsive forces. In other words the rotational state is changed solely when the distance between molecules becomes rather short, while the phase is frustrated in all cases and the contribution of frontal collisions is not so significant. 

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