2D columnar mesophase

Table I. Effect of Chemical Structure on the Temperature Transitions of Polyorganosiloxanes exhibiting 2D-columnar Mesophase ...

Figure 2. Effect of the chemical structure of organic side groups on the structural parameter of 2D-columnar mesophase.

Figure 4. Self-organization of POCS-4 and POCS -5 into a 2D-columnar mesophase. Computer simulation of molecular conformation of PMCS-4 (a, b). Ordering on molecular level (c). Ordering of cycles on intermolecular level (d).

Dimensionai Monolayered Mesophase. General characterization of the phase behavior of homopolymers, copolymers and oligomers with decamethylhexa-siloxane cycles (PMCS-6) obtained by means of DSC and X-ray studies is summarized in Tables II - III. A comparison of this data with corresponding data presented in Table I reveals marked differences between the factors responsible for mesophase behavior in PMCS-6 and POCS, which exhibited 2D-columnar mesophase. 

Fig. 38 2D-lattices of columnar mesophases (cuts perpendicular to the column long axes) and their plane groups... 

A molecular chain of PDES contains no mesogenic elements still two columnar mesophases Xi and X2 exist above the melting point of the crystalline state. In cross-linked PDES elastomers stretching can induce an amorphous-mesophase transition. With increasing stress the transition shifts to higher temperature. Kinetically this transition follows a 2D growth model. 

The classification of polymer mesophases have been considered in many reports (P-70). On analyzing the published data there arises much controversy in the identification. In order to avoid confusion in terminology it is necessary to define the basic terms relating to polymer columnar mesophases before further discussion. Columnar polymer systems have both a correlation of the centers of gravity and molecular orientation, but have mesomorphic properties due to the conformational disorder both of the polymer back-bone and side chains. The structural unit of the polymeric columnar mesophase is a macromolecule. In columnar phases macromolecules form regular 2D-periodic arrays. The two-dimensional symmetry of the column packing and the parameters of two-dimensional lattice are strongly dependent on the form and dimensions of the cross-section of a polymer molecule. 

It should be noted that 2D-mesophases formed by long side chains in a series of comb-like polymers (7) can not be attributed to columnar mesophases due to the lack of properties inherent to columnar mesophases. By the same reason sanidic mesophases (72) including 2D-ordered one are not to be classified as columnar ones. 

Liquid crystals (LCs) forming 2D columnar structures are promising materials for photovoltaic applications. Typically, homeotropic alignment of columnar mesophases occurs in thin films confined between two substrates. However, there is a significant interest in generating the perpendicular orientation of the self-assembled columns in open film. Recently, spontaneous formation of macroscopic home-otropically aligned LC monodomains has been reported on nanopatterned surfaces fabricated by means of friction transfer of poly(tetrafluoroethylene). Figure 26(a) shows starlike... 

Depending on temperature, transitions between distinct types of LC phases can occur.3 All transitions between various liquid crystal phases with 0D, ID, or 2D periodicity (nematic, smectic, and columnar phases) and between these liquid crystal phases and the isotropic liquid state are reversible with nearly no hysteresis. However, due to the kinetic nature of crystallization, strong hysteresis can occur for the transition to solid crystalline phases (overcooling), which allows liquid crystal phases to be observed below the melting point, and these phases are termed monotropic (monotropic phases are shown in parenthesis). Some overcooling could also be found for mesophases with 3D order, namely cubic phases. The order-disorder transition from the liquid crystalline phases to the isotropic liquid state (assigned as clearing temperature) is used as a measure of the stability of the LC phase considered.4... 

This anisotropic phase is of intermediate viscosity to discrete micellar and bicontinuous cubic phases. The standard picture of a hexagonal mesophase consists of a dense packing of cylindrical micelles, arranged on a 2D hexagonal lattice. It is often identified by a characteristic fan texture in the optical microscope, due to focal conic domains of columns. This mesophases is the archetypal columnar (rod) lyotropic mesophase. 

Figure 3.18. 2D-WAXS difFractograms of C42 (HBC-C12), C78 (67), C96 (42a) and C132 (49) in the mesophase state showing the well-ordered, columnar structure. Reproduced by permission of The Royal Society of Chemistry.

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