Rigid core

Fig. 2. The generic molecular structure of calamitic liquid crystals illustrating the semi-rigid core fragments, the positions of the end-groups (C and A), linking groups (B) and, possibly, laterally substituted groups (L)...

A lattice model of uniaxial smectics, formed by molecules with flexible tails, was recently suggested by Dowell [29]. It was shown that differences in the steric (hard-repulsive) packing of rigid cores and flexible tails - as a function of tail chain flexibility - can stabilize different types of smectic A phases. These results explain the fact that virtually all molecules that form smectic phases (with only a few exceptions [la, 4]) have one or more flexible tail chains. Furthermore, as the chain tails are shortened, the smectic phase disappears, replaced by the nematic phase (Fig. 1). 

Fig. 2a-f. Mesogenic molecules with differing degrees of polar and sterical asymmetry a symmetric molecule with rigid core and two hydrocarbon tails b terminally polar molecule (arrow indicates the permanent dipole) c swallow-tailed (hiforked) molecule d hanana shaped molecule e terminally fluorinated molecule f polyphilic molecule (hatched areas correspond to the fluorinated fragment)... 

Compound 129, an intensely blue compound, was obtained by direct functionalization of DDTOMe 20b with LDA and chloro(chloromethyldimethylsilane) at one of the terminal positions followed by conversion of chloride to NCS using sodium thiocyanate in acetone (Scheme 12). The other isothiocyanates were prepared via Stille coupling of mono or dibrominated rigid cores with appropriate thienyl or phenyl stannanes followed by conversion of the chlorine atom of Si(CH3)2CH2Cl attached to a phenyl group into isothiocyanate occurred in rather low yield. 

Figure 8.5 gives the structure of the molecular subject of this classic study, decyloxybenzylideneaminomethylbutylcinnamate (DOBAMBC, 2). DOBAMBC possesses the archetypal FLC molecular structural features A rigid core with two flexible tails, one of which possesses a stereogenic center. A classic theoretical treatment of the SmC phase from 1978 by Durand et al. suggested that a zigzag conformation of the LC molecules is important.9... 

Mesogen composed of molecules each having an elongated rigid core with several flexible chains attached to the end(s). 

Mesogen composed of molecules each with an elongated rigid core and a branched flexible chain, with branches of about the same length, attached at each end. 

Because of the variation in flexibility, the terms film and sheet vary with materials. For PVC, films have a thickness of 6 mil (0.15 mm) and less while sheets are thicker than this. While PVC is relatively rigid with a tensile modulus greater than about 690 MPa (105 psi), thin films are easily folded. Films are generally shipped as rolls with the PVC rolled around a central rigid core. Sheets are generally shipped as flat layered sheets. 

Parameters required for the description of polyesters I are taken from a recent paper (Abe, A, J. Am. Chem. Soc. 1984, 106, 14) which dealt with the dipole moments of dialkyl esters of dicarboxylic acids. Since the ester groups are all assumed to be in the trans configuration, short-range interactions between consecutive rigid cores are unimportant. As for the rotation around the O-C — C-C bond, the six-state scheme (termed model I in the above paper) is employed. The statistical weight parameter a representing the relative importance of the reversed ester conformations with respect to the normal ones is set equal to unity. The three-state scheme (termed model II) proposed alternatively in the above reference is examined for chains with n = 5 and 6 for comparison. In this model the C O /CC eclipsed form is assumed to be intrinsically more stable than the C 0 /CH form a stabilzation energy E( 1) of 5.0 kj mol-1 is adopted. 

Mesophases Induced by Association of Complementary Molecular Components. A common type of molecular species that form thermotropic liquid crystals possesses an axial rigid core fitted with flexible chains at each end. One may then imagine splitting the central core into two complementary halves e and 3, whose association would generate the mesogenic supermolecule, as schematically represented in Figure 38. 

Hairy rigid rod polymers, in which flexible side chains are attached to a rigid core, present attractive properties [9.155]. A supramolecular version of such materials may be the triple helical supramolecular species described above (Figure 41), which presents the features of a hairy cylinder. Similarly, the components 176 and 177 bearing long R chains yield self-assembled, supramolecular hairy rigid rods. 

The nematic phase (N) is the least ordered, and hence the most fluid liquid crystal phase. The order in this type of LC phases is based on a rigid and anisometric (in most cases rod-shaped or disc-shaped) molecular architecture. Such molecules tend to minimize the excluded volume between them, and this leads to long range orientational order. For rod-like molecules the ratio between molecular length and its broadness determines the stability of the nematic phase with respect to the isotropic liquid state and the stability rises with increase of this ratio. In most cases the rigid cores are combined with flexible chains, typically alkyl chains, which hinder crystallization and in this way retain fluidity despite of the onset of order. 

The general design concept for liquid crystalline crown ethers discussed in this section consists of a rigid core with terminal chains and a laterally attached crown ether unit (Fig. 3). 

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