Hexatic smectics

LS. In the LS phase the molecules are oriented normal to the surface in a hexagonal unit cell. It is identified with the hexatic smectic BH phase. Chains can rotate and have axial symmetry due to their lack of tilt. Cai and Rice developed a density functional model for the tilting transition between the L2 and LS phases [202]. Calculations with this model show that amphiphile-surface interactions play an important role in determining the tilt their conclusions support the lack of tilt found in fluorinated amphiphiles [203]. 

Note There are three types of hexatic smectic mesophases. smectic B (SmB), smectic F (SmF) and smectic I (SmI). Here, the term hexatic may be omitted because it is implicit for this group of smectic mesophases. 

Hexatic smectic mesophase in which the director is perpendicular to the layers with the long-range hexagonal bond-orientational order. 

Hexatic smectic mesophase the structure of which may be regarded as a C-centered monoclinic cell with a hexagonal packing of the molecules with the director tilted, with respect to the layer normals, towards the sides of the hexagons. 

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... 

In the crystalline version of smectic B, there is positional as well as bond-orientational order, and this ordering is called hexagonal, as opposed to hexatic. In the tilted version of the hexatic smectic B, there are two different directions the tilt has been found to take with respect to the hexagonal bond orientation, namely toward nearest and toward next nearest neighbors, producing the two different smectics I and F (see Fig. 10-27). Similarly, there are two different tilted crystalline hexagonal phases, smectics J and G. Even in smectic crystals, the molecules retain freedom to rotate cooperatively about theii axes, hence smectic ciystals are not as solid as most simple crystals. Nature is wonderfully creative in contriving... 

Aside from exhibiting a rich range of conventional smectic phases, members of the m.OnO.m family also exhibit novel modulated hexatic phases [155] in which the tilted hexatic smectic monolayers have a periodic modulation in their smectic structure, analogous to the Sm C ribbon phase. Two such phases have been identified and have been termed the Sml and Sm2 phases. The Sml phase is exhibited by members of the m.OnO.m family with... 

Fig. 9.12. Wide angle X-ray scattering patterns produced by correlations between molecules in the plane of the layers, (a) The diffuse ring of a smectic A phase, characteristic of short range correlations, (b) Appearance of six diffuse traces in the case of a hexatic smectic phase. Intensity modulation is characteristic of long range bond order (i.e. a subtle orientational order), whereas the diffuse nature of the traces indicates liquid-type long range order...

In lyotropics, phases with a stmcture comparable to the ones of the thermotropic hexatic smectic phases exist, as pointed out by Smith et al. [26]. Again, the surfactant molecules show a bond-orientational order within the layers and can either be parallel (lamellar Lp) [27] or tilted (lamellar Lp-) [28] with respect to the layer normal k. These phases are called gel-like rather than liquid crystalline, due to their dramatically increased viscosity compared to the lamellar L phase. This increased... 

Fig. 3.5 a Long-range orientational and positional order as found for a crystal, b Hexatic smectic phase with long-range orientational but only short-range positional order (redrawn after [24])... 

The principal physical difference between hexatic and three-dimensional crystal phases is their ability to support or not to support a shear parallel to smectic layers. Like a fiuid, the hexatic smectic Bh phase cannot support such a shear [8, 9]. The relevant order parameter... 

The smectic phase has at least two unique directions, the director and the layer normal. Eleven types of smectic mesophases have been identified, ranging from smectic A to smectic K. They differ in the arrangement of the mesogens within each individual layer. For example, in the smectic-A phase, these two directions are collinear (orientational order), whereas in the smectic-C phase the director could make an angle with the layer normal (positional order). In addition to orientational order and positional order which we have just mentioned, there is another order called bond orientation order. There are several smectic phases which contain this order and they are called hexatic smectics. 

SmA chiral orthogonal smectic A SmB / chiral orthogonal hexatic smectic SmBjfgx B phase (SmB preferred)... 

SmA-C transitions, ferroelectric 481 SmA-hexatic smectic B transition 291 f SmA-SmC transition 65, 289 f, 326,478 SmA-TGBA-cholesteric, multicritical behavior 368 SmA-SmA critical point 302 SmAj/SmA fluctuations 384 SmC -SmA-TGB, multicritical behavior 368 SmC/C phases, flow/viscosity 470 f small angle neutron scattering (SANS) 684 smart pixels 810,814 smectic mesophase, difftision 590 smectic order parameters, XRD 646 smectic phases 17, 43, 60... 

Tilted Pseudohexagonal G J Monoclinic H K Stacked hexatics Smectic F Smectic I Smectic C SmCi SmC2 SmCd SmC ... 

Synthesis of Alkyl 4-Alkoxybiphenyl-4 -carboxylates Hexatic Smectic B Materials (e.g., 650BC)... 

One of the most examined hexatic smectic B materials is hexyl 4-n-pentyloxybiphenyl-4 -carboxylate (650BC). This material and related homologues can be prepared by the scheme shown in Fig. 29 [14, 28, 40]. The synthesis uses bromo alkoxybiphenyls as useful intermediates. These intermediate compounds can be prepared as described in... 

Synthesis of 4-Alkylphenyl 4-Alkylbiphenyl-4 -carboxylates Smectic C, Smectic I, Hexatic Smectic B Materials... 

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