Smectic C phases

In the SmC phase the longitudinal molecular axes are tilted from the smectic layer 

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Detailed x-ray diffraction studies on polar liquid crystals have demonstrated tire existence of multiple smectic A and smectic C phases [M, 15 and 16]. The first evidence for a smectic A-smectic A phase transition was provided by tire optical microscopy observations of Sigaud etal [17] on binary mixtures of two smectogens. Different stmctures exist due to tire competing effects of dipolar interactions (which can lead to alternating head-tail or interdigitated stmctures) and steric effects (which lead to a layer period equal to tire molecular lengtli). These... 

As witli tlie nematic phase, a chiral version of tlie smectic C phase has been observed and is denoted SniC. In tliis phase, tlie director rotates around tlie cone generated by tlie tilt angle [9,32]. This phase is helielectric, i.e. tlie spontaneous polarization induced by dipolar ordering (transverse to tlie molecular long axis) rotates around a helix. However, if tlie helix is unwound by external forces such as surface interactions, or electric fields or by compensating tlie pitch in a mixture, so tliat it becomes infinite, tlie phase becomes ferroelectric. This is tlie basis of ferroelectric liquid crystal displays (section C2.2.4.4). If tliere is an alternation in polarization direction between layers tlie phase can be ferrielectric or antiferroelectric. A smectic A phase foniied by chiral molecules is sometimes denoted SiiiA, altliough, due to the untilted symmetry of tlie phase, it is not itself chiral. This notation is strictly incorrect because tlie asterisk should be used to indicate the chirality of tlie phase and not tliat of tlie constituent molecules. 

Leadbetter A J 1979 Structural studies of nematic, smectic A and smectic C phases The Moiecuiar Physics of Liquid Crystais ed G R Luckhurst and G W Gray (London Academic)... 

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. 

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

Ito et al. [152] described the crystal structure of 4-[(S)-2-methylbutyl]phe-nyl 4 -hexylbiphenyl-4-carboxylate which shows a smectic A phase and a cholesteric phase. The molecules are arranged in a tilted smectic-like layer structure. Within the layers, the long molecular axes are tilted (30°). However, the compound exhibits no smectic C phase. 

Smectic A and C phases are characterized by a translational order in one dimension and a liquid-like positional order in two others. In the smectic A phase the molecules are oriented on average in the direction perpendicular to the layers, whereas in the smectic C phase the director is tilted with respect to the layer normal. A simple model of the smectic A phase has been proposed by McMillan [8] and Kobayashi [9] by extending the Maier-Saupe approach for the case of one-dimensional density modulation. The corresponding mean field, single particle potential can be expanded in a Fourier series retaining only the leading term ... 

These compounds show a nematic and smectic C phase for relatively short chain lengths (n 7-10) and a hexagonal (columnar) phase for longer chains. 

Similar behavior has also been observed for polymerizations of small amounts of various monomers in ordered LC phases (30). The rate of polymerization is enhanced considerably for a non-mesogenic diacrylate in the smectic C phase and is more than three times that observed in the isotropic phase of the same LC and over six times that observed for polymerization in an isotropic solvent. Similar results were observed for a variety of mesogenic and other non-mesogenic monomers (31). Interestingly, the mechanisms behind this rate enhancement is not the same for all monomers and is highly dependent on the segregation behavior. 

Samples for infra-red absorption measurement were introduced between two rubbed nylon coated calcium fluoride substrates spaced 10 pm apart. To insure proper parallel alignment, samples were cooled at 0.02° C/minute from the isotropic to the Smectic C phase. The alignment was then checked using polarizing microscopy. Polarized IR spectra (32 scans per spectrum) were obtained using an FTIR spectrometer (IFS-66 Bruker, Pillerica, MA) equipped with a wire grid polarizer at a resolution of 2 cm"1. 

The phase behavior is changed considerably upon addition of PPDA monomer to the FLC as shown in Figure 2. The reduced transition temperatures for the LC phases, i.e. the transition temperature for the pure FLC subtracted from that of the FLC/monomer (or polymer) mixtures, are plotted as a function of the concentration before and after polymerization. Before polymerization the reduced transition temperatures decrease almost linearly for the first order isotropic to smectic A transition, as would be expected. The reduced temperatures for the transition from the smectic A to the smectic C phase for the monomer/FLC mixtures also decrease linearly with concentration, but the decrease is considerably more pronounced. This decrease continues until the LC is saturated in monomer (about 13 wt%). 

To determine if this phenomenon is isolated to amorphous monomers, a liquid crystalline diacrylate (C6M) was polymerized in W7,W82 at temperatures corresponding to the two smectic phases as well as the isotropic phase. The polymerization rate for C6M is plotted as a function of time for representative temperatures in Figure 6. Again, the polymerization shows marked acceleration in the ordered smectic C phase and occurs much faster than the isotropic polymerization. As seen in the HDDA polymerizations, the smectic A rate also lies between the rates of the other two polymerization temperatures. 

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