Smectic side-chain polymers

The optical microscopy or TOA of the smectic side-chain polymers shows the important effect that viscosity has on the behaviour of these systems, i.e. the existence of T and the fluid region, the rate of growth of texture on cooling and the immobility of the texture below r. Since smectic materials will be more viscous than the nematic phases, this suggests again that the materials should be useful for the storage of electro-optic information if this information can be written on an acceptable timescale. 

Scherowsky, G.. Kiihnpast, K., and Springer, J., Three switching states in a chiral smectic side-chain polymer, Makromol. Chem. Rapid Commun., 12, 381-385 (1991). 

Smectic side chain polymers show an oblate equDibrium conformation of the polymer melt where the polymer backbone is partially confined between the smectic layers independent of the attachment geometry. For Sa polymers the confinement depends not only on the smectic order parameter but also on the t3q)e of Sa phase structure. For the monolayer phase structure Sai the anisotropy of the radii of gyration is R /R 0.3 while for the less densely packed partially bUayer structure Saci the confinement of the backbone is less pronounced (R /R 0.7) [63,64, 69-71]. 

Smectic side-chain polymers prefer locally oblate chain conformations, independent of the spacer length or attachment geometry. Analogous to oblate nematic polydomain elastomers, biaxial mechanical stretching or uniaxial compression can be used to orient Sa polydomain elastomers. This achieves a simultaneous orientaticai of the director and the smectic layer normal in a uniform homeotropic fashion [74],... 

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. 

The steric frustrations have also been detected in LC polymers [66-68]. For example, the smectic A phase with a local two-dimensional lattice was found by Endres et al. [67] for combined main chain/side chain polymers containing no terminal dipoles, but with repeating units of laterally branched mesogens. A frustrated bilayer smectic phase was observed by Watanabe et al. [68] in main-chain polymers with two odd numbered spacers sufficiently differing in their length (Fig. 7). 

Fig. 17a-c. Sketches of the molecular arrangements for the smectic structure with alternating layer-to-layer tilt a conventional and chevron smectic C layering in low molecular mass mesogens b ferroelectric hilayer chevron structures for achiral side-chain polymers c antiferroelectric hilayer chevron structures for achiral side-chain polymers. Arrows indicate the macroscopic polarization in the direction of the molecular tilt... 

As sparse as the dataset describing mainchain nematic LCP blends with conventional polymers is, it is rich compared to the almost non-existent data on the blending of other types of LCPs-side chain polymers, flexible spacer polymers, smectics, etc. 

The systematic synthesis of non amphiphilic l.c.-side chain polymers and detailed physico-chemical investigations are discussed. The phase behavior and structure ofnematic, cholesteric and smectic polymers are described. Their optical properties and the state of order of cholesteric and nematic polymers are analysed in comparison to conventional low molar mass liquid crystals. The phase transition into the glassy state and optical characterization of the anisotropic glasses having liquid crystalline structures are examined. 

Table 2. Examples for some nematic side chain polymers (g = glassy s = smectic n = nematic i = isotropic)...

With these three different examples it has been demonstrated that the systematics observed for the polymorphism of m-l.c. s is also valid for the side chain polymers, provided that a flexible spacer connects the rigid mesogenic moieties to the polymer main chain. Deviations from this behavior are observed, when the mesogenic moieties are directly linked to the backbone. Under these conditions, normally no liquid crystalline behavior is to be expected, according to the model considerations mentioned in Chap. 2.1. Some examples, however, proved l.c. properties for such systems, which are characterized by two striking properties Very high glass transition temperatures and only smectic structures even in case of short substituents... 

Fig. 31. Schematic model of the smectic structure of these liquid crystalline side chain polymers...

As already mentioned in Chap. 2.2. one of the most obvious features of the l.c. side chain polymers is their ability to become glassy. The glass transition can be observed by cooling nematic, cholesteric and smectic polymers depending on the chemical constitution of the system and is indicated e.g. by a bend in the V(T) curves as schematically shown in Fig. 8. Two questions are of interest which will be discussed in this chapter ... 

AMORPHOUS, OPTICALLY ISOTROPIC MESOPHASE OF CHIRAL SIDE-CHAIN POLYMERS WITH A HIDDEN LAYER STRUCTURE—THE ISOTROPIC SMECTIC PHASE... 

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. 

Kilian, D., Kozlovsky, M. V., and Haase, W. Dielectric measurements on the isotropic smectic phase of dyed side-chain polymers. Liquid Crystals 26, 705 (1999). 

Thermotropic liquid crystals can then be furflier subdivided into high molecular mass, main and side-chain polymers [10] and low molecular mass, the latter class of compounds being one of the areas of this review. The phases exhibited by the low molecular mass molecules are then properly described with reference to the symmetry and/or supramolecular geometry of the phases, which are briefly introduced here and are discussed in more detail further below. Thus, the most disordered mesophase is the nematic (N), which is found for calamitic molecules (N), discoidal molecules (Nq) and columnar aggregates (Nc), among others. The more ordered lamellar or smectic phases (S) [11, 12] are commonly shown by calamitic molecules, and there exists a variety of such phases distinguished by a subscripted letter (e. g. Sa, Sb)- Columnar phases (often, if incorrectly, referred to as discotic phases) may be formed from stacks of disc-like molecules, or from... 

The various orientations and conformations mesogenic groups can adopt in a layer has been studied extensively for low molar mass smectic materials, and the classification and terminology of smectic systems is entirely based on these studies. However, low molar mass smectic compound or smectic LC-side chain polymers do, of course, not allow one to elucidate the role spacers play in the layer structures of LC-main chain polymers. Therefore, poly(ester-imide)s, po-... 

An initial approach to supramolecular H-bonded mesogenic polymer complexes involves a polyacrylate with 4-oxybenzoic acid moieties via a hexamethylene spacer 29 [26]. The 1 1 complexation of the side chain of the polymer and stilbazole 3 n - 2) (nematic, 168-216 °C) results in the formation of an extended supramolecular mesogen in the side chain (Fig. 11). Side-chain polymer complex 30 exhibits a nematic phase up to 252 °C, which shows that a significantly stabilized mesophase is achieved by the complexation of two different components. Liquid-crystalline properties have been examined for the series of complexes formed between polyacrylates and trans-4-alkoxy-4 -stilbazoles [33, 78]. Figure 12 shows transition temperatures against the carbon number of the alkyl chain for the series of complexes 31 [33]. They exhibit thermally stable smectic liquid-crystalline phases. For example, smectic E, B, and A phases are observed until 192 °C after the glass transition at 38 °C for the complex with m = 6 [78a]. 

Another type of supramolecular side-chain polymers (shown in Fig. lOB) is obtained by the complexation of functionalized mesogenic molecules with polymer backbones. The hydrogen-bonded complex of 37 is obtained by mixing poly(4-vinylpyridine) or poly(4-vinylpyridine-co-styrene) with mesogenic compounds terminated by a carboxylic acid moiety through a flexible spacer [84-88]. The hydrogen bonding between imidazole and carboxylic acid is also useful for the formation of supramolecular complexes. Polymeric complex 38 exhibits a smectic A phase from 10 to 65 °C [89]. 

The interaction of carboxylic acids affects the molecular order of side-chain polymer liquid crystals [130]. Copolymerization of an acrylate containing a cyanobiphenyl unit through butyl group and acrylic acid leads to the induction of a smectic A phase. The intra- and intermolecular H-bonds of carboxylic groups stabilize layered structures. 

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