Pitch smectic phases

Chiral Smectic. In much the same way as a chiral compound forms the chiral nematic phase instead of the nematic phase, a compound with a chiral center forms a chiral smectic C phase rather than a smectic C phase. In a chiral smectic CHquid crystal, the angle the director is tilted away from the normal to the layers is constant, but the direction of the tilt rotates around the layer normal in going from one layer to the next. This is shown in Figure 10. The distance over which the director rotates completely around the layer normal is called the pitch, and can be as small as 250 nm and as large as desired. If the molecule contains a permanent dipole moment transverse to the long molecular axis, then the chiral smectic phase is ferroelectric. Therefore a device utilizing this phase can be intrinsically bistable, paving the way for important appHcations. 

We should state that our suggested model is not completely justified yet, but it remains the only one that can explain the whole ensemble of properties observed for the isotropic smectic phase of P8 M and related polymers. Keeping that in mind, we will refer hereafter to the mesophase as the isoSm phase, which most probably has the uitrashort pitch TGB A structure. 

The chiral side chain polymers derived from asymmetric esters of terephthalic acid and hydroquinone can form (in a broad temperature range, including ambient temperature) an unusual mesophase (the isotropic smectic phase, IsoSm ) characterized by high transparency and optical isotropy within the visible wavelength range, combined with a hidden layered smectic ordering and some elements of helical superstructure at shorter dimensions of 10 to 250 nm. The short-pitch TGB A model seems to be the most adequate for the mesophase structure. 

A similar reversal of handedness takes place as the composition is varied.The inverse pitch shows a nearly linear relationship with composition around the nematic point, but there are significant departures when one of the components has a smectic phase at a lower tem-perature. 44 The anomaly may again be attributed to smectic-like short-range order. 

The pitch of the helix depends on concentration c of a dopant for small c Po ac and a is called helical twisting power of the dopant [15]. However, with increasing c the dependence becomes nonlinear and the heUx handedness can even change sign (the case of cholesteryl chloride dopant in p-butoxybenzyli-dene-p -butylaniline, BBBA, see Fig. 4.24). The same chiral, locally nematic phase with a short pitch in the range of 0.1-1 pm is traditionally called cholesteric phase because, at first, it has been found in cholesteryl esters. Such short-pitch phases manifest some properties of layered (smectic) phases. 

Both cholesteric and smectic mesophases are layered. In the former case, the periodicity arises from a natural twist to the director field, and in the latter, from a center-of-mass correlation in one dimension. There are many types of smectic phases distinguished by their symmetry and order. The set of field-induced phenomena is quite different for these two materials, owing primarily to the very different layer compressibility. That is, the cholesteric pitch can be unwound by an external field, whereas the smectic layering is typically too strong to be altered significantly. However, because of the common layered structure, there are also strong similarities. 

Figure 13.14. Dependence of the pitch of a chiral nematic liquid crystal near the transition to a smectic phase.

The sense of the pitch of the chiral smectic C mesophase is controlled by the same molecular factors as that of the cholesteric phase. If a chiral nematic forms a tilted smectic phase (C, H, I ) on cooling, either directly or through an intermediate smectic A phase, the smectic phase has the same sense of rotation as the chiral nematic phase. 

A helical director field also occurs in the chiral smectic-C phase and those smectic phases where the director is tilted with respect to the layer normal (Figure 1.13(c)). In these cases, the pitch axis is parallel to the layer normal and the director inclined with respect to the pitch axis. Very complicated defect structures can occur in the temperature range between the cholesteric (or isotropic) phase and a smectic phase. The incompatibility between a cholesteric-like helical director field (with the director perpendicular to the pitch axis) and a smectic layer structure (with the layer normal parallel or almost parallel to the director) leads to the appearance of grain boundaries which in turn consist of a regular lattice of screw dislocations. The resulting structures of twist grain boundary phases are currently extensively studied. The state of the art in this topical field is summarized in Chapter 10. 

Twist grain boundary (TGB) phases [l]-[4] usually appear in the temperature range between a cholesteric N phase with short pitch and a smectic phase, typically SmA or SmC. In particular, they are expected to appear close to a N /SmA/SmC triple point [5]. One of their remarkable properties is the selective reflection of circularly polarized light [2], [3]. This feature shows that the director field has a helical structure similar to the cholesteric phase. On the other hand. X-ray investigations of TGB phases indicate a layer structure as occurring in smectic phases [6]. Chirality of the system is an essential precondition for the occurrence of TGB phases. In mixtures of... 

A variety of effects can occur in the TGB phases due to the influence of an electric field. The coupling between the director and the field may be due to the dielectric anisotropy Sa, or due to the dependence of the smectic tilt angle on the electric field (electroclinic effect), or due to the spontaneous polarization. In contrast to the typical behavior of smectic phases, a small electric field cannot only result in a reorientation of the director, but also in a reorientation of the smectic layers [138], Higher fields can cause a reorientation of the pitch axis, helical unwinding [139], [140], a shift of the wavelength of selective refiection [141], or field-induced phase transitions [103], [141]. 

SmC chiral C phase SmC infinite pitch smectic C - ferroelectric... 

Certain chiral nematic liquid crystals (or mixtures) are found to exhibit thermo-chromism, that is, temperature dependent selective reflection here the pitch length of such materials or mixtures is often found to have an inverse relationship with temperature and can depend on the nature of the material s mesomorphism. Thermochrom-ism is found to be at its most spectacular in materials which display chiral nematic phases and underlying smectic phases this is exemplified for a material with an I-N -SmA sequence (on cooling) and is shown schematically in Fig. 2. 

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