Liquid analogous crystalline state

Attaching non amphiphilic or amphiphilic liquid crystalline molecules as side chains to linear, branched or crosslinkedpolymers yields liquid crystal (l.c.) side chain polymers, which can exhibit the liquid crystalline state analogously to the conventional low molar mass liquid crystals. The l.c.-side chain polymers combine the specific, anisotropic properties of the liquid crystalline state with the specific properties of polymers. 

To function properly, cell membranes must exist primarily in the liquid-crystalline state. Model studies using synthetic lipids have demonstrated that the presence of a (9Z)-olefinic fatty acyl side chain in a diacylphospholipid lowers its gel-liquid phase temperature (T ) by some 50° C relative to its fully saturated analog (4). The presence of the A fatty acyl side chain lowers the T even more and constitutes an important adaptation to chilling for plants. The regulation of the response to chilling temperatures has been studied in some detail in various organisms (4). 

If the concentration of surfactant becomes high enough, surfactant structures often develop long-range order, and hence they become liquid crystalline. They are lyotropic liquid crystals, because the transition to the liquid-crystalline state is induced by concentration changes. Surfactant solutions can form nematic and smectic-A liquid-crystalline phases analogous to those discussed in Chapter 10. In addition, hexagonal and cubic phases are common in surfactant solutions. 

These results were confirmed by Thompson et al., who investigated the thermal and mesomorphic behavior of Fc materials and those of analogous compounds in which Fc was replaced by a benzene or a cyclohexane ring (see, for example, 51-53).They observed that Fc destabilizes the liquid-crystalline state. Indeed, whereas 51 and 52 show enantiotropic SmC and SmA or SmC and N phases, respectively, 53 displays only a monotropic N phase. This behavior indicates that Fc plays the role of a bulky unit, again as a consequence of its three-dimensional structure. 

Conventional principles and methods concern the synthetic routes for macromo-lecular networks and the realization of the liquid crystalline state by mesogenic monomer tmits. Network chemistry has to consider the reactivity and functionality of the monomer tuiits. In most cases, this excludes ionic polymerization techniques and reduces utihzable methods to radical polymerization and polymer analog reactions for side chain networks, and to polycondensation or polyadditirai reactions for main chain elastomers. The chemistry of the crosslinking process and the chemical constitution of the crosslinker have to be adapted to the polymerization process. Applying photo-chemistry of suitable functional mmiomer units opens an additional, versatile pathway to build up the network structure. 

Maier and Saupe [13] developed a statistical theory to describe the liquid crystalline state and the molecular ordering for the nematic phase. In analogy to the treatment of ordering phenomena in ferromagnetics or ferroelectrics, this theory describes the intermolecular orientational forces by a mean field method. Each individual molecule feels a nematic potential D = f (0, S, V) which depends on the momentaneous angle 6 between its long axis and the optic axis, the order parameter S and the molar volume V. S is then given by... 

K diverges in the vicinity of the N-SmA-SmC point where Cx vanishes. The type II condition is thus expected to be fulfilled close to the N-SmA-SmC point. In presence of chirality, this is precisely the place where a liquid crystal analog of the Abrikosov flux phase should show up. The structure of this new liquid crystalline state will be described in subsequent section. 

The structure for different liquid crystalline phases of small molecules, that is the packing of the molecules, is in principle well established and depicted in many textbooks dealing with the liquid crystalline state. Structural models for liquid crystalline side-chain polymers in different liquid crystalline phases have also been proposed analogous to the packing of small molecules but were challenged by other research workers. 

At low temperatures the bend in the V(T) curves indicates a glass transition. It is of interest, whether this transition can be described in analogy to amorphous polymers on the basis of the concept of using one internal order parameter and whether the liquid crystalline structure is maintained in the glassy state. This will be discussed in Chap. 2.3.4. 

The potential for novel phase behaviour in rod-coil block copolymers is illustrated by the recent work of Thomas and co-workers on poly(hexyl iso-cyanate)(PHIC)-PS rod-coil diblock copolymers (Chen etal. 1996). PHIC, which adopts a helical conformation in the solid state, has a long persistence length (50-60 A) (Bur and Fetters 1976) and can form lyotropic liquid crystal phases in solution (Aharoni 1980). The polymer studied by Thomas and co-workers has a short PS block attached to a long PHIC block. A number of morphologies were reported—wavy lamellar, zigzag and arrowhead structures—where the rod block is tilted with respect to the layers, and there are different alternations of tilt between domains (Chen et al. 1996) (Fig. 2.37). These structures are analogous to tilted smectic thermotropic liquid crystalline phases (Chen et al. 1996). 

---