Introduction to Liquid Crystalline Phases

Although the liquid crystalline phase of most polybibenzoates usually undergoes a rapid transformation into a three-dimensional crystal, the introduction of oxygen atoms in the spacer of polybibenzoates has been used to prevent or to slow down this transformation. The dynamic mechanical behavior of polybibenzoates with 2, 3, or 4 oxyethylene groups in the spacer (PDEB, PTEB, and PTTB, respectively) is determined by the composition of the spacer [24], as discussed in this section. 

Hopefully with this brief introduction the reader will be able to appreciate fully the chapters which follow and which have been written by experts in the field. The complexity and beauty of the liquid crystalline phase has attracted many able scientists and the applications of liquid crystals in the electronics industry have provided a secure funding base for the subject. This is therefore still a field which is expanding rapidly and many research avenues remain to be explored by newcomers. Perhaps after reading these volumes of Structure and Bonding you will be tempted to join this exciting endeavour. 

The introduction of liquid crystals as stabilizing elements for emulsions occurred in 1969 when it was found that the sudden stabilization at emulsifier concentrations in excess of 2.5% of a water—p-xylene emulsion by a commercial octa(ethylene glycol) nonylphenyl ether was due to the formation of a liquid crystalline phase in the emulsion (26). Later investigations confirmed the strong stabilizing action of these structures (27). 

The surfactant association structures have a long history of research ranging from the McBaln introduction of the aqueous micellar concept(1.) over the interpretation of mlcelllzatlon as a critical phenomenon — — to the analysis of the structure of lyotropic liquid crystals(A) and the comprehensive picture of the phase relations in water/surfactant/amphlphile systems.These studies have emphasized the relation between the association structures in isotropic liquid solutions and the liquid crystalline phases. Parallel extensive investigations in crystalline/ liquid crystalline lipid structureshave provided important insight in the mechanisms of the associations. 

Gangadhara et al. have linked the cyanobiphenyl mesogen via a dicarbox-imide-group to an oxanorbornene ring system Vl-n, n=2-8 (see Fig. 4). Polymerization was carried out with Schrock type initiator 4. The dicarboximide linkage probably hindered the formation of LC phases even the introduction of relatively long spacers between the polymer backbone and the mesogen did not lead to liquid crystalline monomers or polymers [41]. 

Solubilization can be defined as the preparation of a thermodynamically stable isotropic solution of a substance normally insoluble or very slightly soluble in a given solvent by the introduction of an additional amphiphilic component or components. The amphiphilic components (surfactants) must be introduced at a concentration at or above their critical micelle concentrations. Simple micellar systems (and reverse micellar) as well as liquid crystalline phases and vesicles referred to above are all capable of solubilization. In liquid crystalline phases and vesicles, a ternary system is formed on incorporation of the solubilizate and thus these anisotropic systems are not strictly in accordance with the definition given above. 

The saturated aqueous solution of surfactant at 25°C is in equilibrium with a liquid crystalline phase which contains about 25 wt% water. This phase dispersed in solution is the same as the phase formed by water vapor sorption into initially dry surfactant, according to nmr spectroscopy (which virtually eliminates the possibility, mentioned in the Introduction, of a complicating triple point in the two-component system). This hydrated liquid crystal is probably lamellar, to judge by the similarities in texture with lamellar liquid crystals of phospholipids and water (36). It is not uncommon for surfactants for form liquid crystalline phases by absorption of water, or hydrocarbon, or both (37). Moreover the true solubility of many other surfactants (particularly alkyl aryl sulfonates) in water, in salt water, and in hydrocarbon is small, sometimes as small as 0.003 wt% in water, below the Krafft point (38,39). Hence the present finding of liquid crystalline phase in equilibrium with isotropic aqueous solution at surfactant levels above 0.1 wt% may be representative of broad classes of surfactants, including some of interest in connection with... 

The need to add either salt or a long-chain alcohol in order to prepare nematic phases from classical surfactants (e.g., SDS-decanol-water [154] decylammonium chloride-ammonium chloride-water [237]) may be understood in terms of this model. These additives shift the liquid crystalline phases away from their optimum concentration ranges and leading to a buildup of frustration. This frustration is resolved by the introduction of local structural fluctuations that destabilize the mesophases and lead to finite micelles of the appropriate sizes [177]. For instance, decanol limits the domains of existence of the mesophases for the system SDS-decanol-water [238]. The addition of NH4CI to the decylammonium chloride-water system suppresses the hexagonal phase and enhances the stability of the discotic phase [239]. 

---