Smectic B phase

The transition from smectic A to smectic B phase is characterized by tire development of a sixfold modulation of density witliin tire smectic layers ( hexatic ordering), which can be seen from x-ray diffraction experiments where a sixfold symmetry of diffuse scattering appears. This sixfold symmetry reflects tire bond orientational order. An appropriate order parameter to describe tlie SmA-SmB phase transition is tlien [18,19 and 20]... 

The positional order of the molecules within the smectic layers disappears when the smectic B phase is heated to the smectic A phase. Likewise, the one-dimensional positional order of the smectic M phase is lost in the transition to the nematic phase. AH of the transitions given in this example are reversible upon heating and cooling they are therefore enantiotropic. When a given Hquid crystal phase can only be obtained by changing the temperature in one direction (ie, the mesophase occurs below the soHd to isotropic Hquid transition due to supercooling), then it is monotropic. An example of this is the smectic A phase of cholesteryl nonanoate [1182-66-7] (4), which occurs only if the chiral nematic phase is cooled (21). The transitions are aH reversible as long as crystals of the soHd phase do not form. 

Thermotropic liquid-crystalline properties of different metal alkanesulfonates are studied by microscopy and X-ray diffraction [59]. Sodium soaps show smectic polymorphism of smectic A and smectic B phases. Ammonium soaps only show smectic A phases but polymorphism in the crystalline state. Calcium soaps show columnar mesophases. In Figs. 32 and 33 some textures and x-ray diffraction patterns are depicted. 

FIG. 32 Textures of Sodium Pentadecane 1-Sulfonate, (a), Crystalline phase at 80°C (b), smectic B phase at 113°C (c), smectic A phase at 250°C. Sample window 1.4 mm2, crossed Nicols. 

The distribution of the intermolecular vector is also of value in distinguishing between smectic A and smectic B phases with the latter having long range bond orientational order [23, 24]. At the local level we can define a bond orientational order parameter, PeCn) for molecule i at position q by [25]... 

Fig. 8. The distance dependence of the bond orientational correlation function gs (r ) found for the mesogen GB(4.4, 20.0, 1, 1) in the smectic A (.) and the smectic B (—) phases...

C = crystalline phase N = nematic phase Sa = smectic A phase Sb = smectic B phase I = isotropic phase. 

In 1986, Walz and Haase [148] presented the crystal structure of the mesogenic hydrocarbon compound l,2-bis-(4 -pentylcyclohexyl)ethane. The compound exhibits a smectic B phase over a remarkably broad range of temperature. To our knowledge, this is the only crystal structure determination of a mesogenic hydrocarbon compound up to now. Since this compound does not contain any polar groups, the arrangement in the crystalline state is... 

An even more dramatic example of the potential lack of selectivity afforded to the Norrish II reactions of ketones by supposedly very ordered systems than that described in the 76 systems is provided by neat samples of the mesomorphic alkanophenones (81) [278]. These molecules are capable of existing in nematic and smectic B mesophases (see Figure 16) as shown in Scheme 42. The instability of the monotropic smectic B phase of 81a and smectic B phase of 81b did not allow their photoreactions to be examined these smectic phases became solids soon after the initiation of irradiation. 

However, the monotropic nematic phase of 81a was sufficiently stable to allow its Norrish II reactions to be examined. Similarly, Norrish II product ratios from irradiation of the smectic B phases of 81c and 81d were easily measured. Although the packing of molecules in the solid phases of the 81 homologues is unknown, inferential evidence supports their being layered also. 

SmB SmC SmC SmCA SmCPA SmCPp SmCo, SmIA SmX UCST XRD Smectic B phase Smectic C phase (synclinic tilted smectic C phase) Chiral (synclinic tilted) smectic C phase Chiral anticlinic tilted (antiferroelectric switching) SmC phase Antiferroelectric switching polar smectic C phase Ferroelectric switching polar smectic C phase Chiral smectic C alpha phase Chiral antiferroelectric switching smectic I phase Smectic phase with unknown structure Upper critical solution temperature X-ray diffraction... 

Fig. 3.3 Phase diagram of diacylphosphatidylcholines as suggested by X-ray, optical birefringence, and electron microscopy. Lj corresponds to the smectic A state and tp, Lf, Pp to the smectic B phase of thermotropic liquid crystals. The shaded area is a region of coexistence. (Reprinted from Fig. 2 of ref. 15 with permission from Wiley-VCH.)...

Most of the compounds (96-105) collated in Table 3.9 exhibit a nematic phase as well as an unwanted smectic B phase. However, only the compounds linked directly, e.g. (96-99,102 and or linked by the... 

The data collated in the Tables 3.14-3.16 reveals that the presence of a trans carbon-carbon double bond in a limited number of specific positions gives rise to a higher clearing point and a higher 33/ 11 ratio than those of the corresponding material without a double bond in the terminal chain. Smectic phases, especially the ordered smectic B phase, are strongly suppressed. This is... 

Butyl stearate has an enantiotropic smectic B phase from 14... 27 °C [727],... 

All polymers showed semicrystalline behavior, as demonstrated by WAXD measurements. DSC thermograms were presented, but LC properties were found to be difficult to determine due to the concomitant thermally-induced crosslinking of the unsaturated polymers upon melting. In some cases smectic B phases were identified by POM. The melting temperatures of the saturated analogues were lower than for the corresponding unsaturated derivatives. Optical textures of the saturated polymers showed lancets in the background, typical of a solid-like smectic phase [89]. 

Within a smectic layer, a state of order called hexatic can exist that is intermediate between the liquid order of the smectic A, and the crystalline order of crystalhne smectic B. This state of order characterizes the hexatic smectic-B phase. Flexatic order consists of long-range bond orientational order, but no long-range positional order. This is illustrated... 

The original Gay-Beme potential forms a nematic phase and a Smectic B phase, which is more solid like than liquid like. Ellipsoidal bodies do usually not form smectic A phases because they can easily diffuse from one layer to another layer. However, if one increases the side by side attraction it becomes possible to form smectic A phases [6]. When one calculates transport coefficients very long simulation runs are required. Therefore one sometimes re-places the Lennard-Jones core by a purely repulsive 1/r core in order to decrease the range of the potential. Thereby one decreases the number of interactions, so that the simulations become faster. The Gay-Beme potential can be generalised to biaxial bodies by forming a string of oblate ellipsoids the axes of which are parallel to each other and perpendicular to the line joining their centres of mass [35]. One can also introduce an ellipsoidal core where the three axis are different [38]. 

To summarize our results on side-chain all-hydrocarbon LC polymers [I], we found that they displayed Smectic E phases at lower temperatures and Smectic B phases at higher temperatures. The corresponding LC monomers exhibited only Smectic B phases. The presence of tails seems to enhance and stabilize liquid crystallinity. The monomers and polymers that do not have tails are not liquid crystalline. All of the substituted biphenyl hydrocarbon liquid crystals found in the literature are disubstituted. Examples of monosubstituted biphenyl hydrocarbon liquid crystals have not been found. The results obtained from this work support the idea that disubstitution of the biphenyl moiety is necessary to obtain a liquid crystalline phase. 

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