Hydrocarbon main-chain liquid-crystalline

Hydrocarbon Main-Chain Liquid Crystalline Polymers... 

Polymer A showed a crystalline X-ray diffraction pattern and an endothermic peak at 31-42 C in differential scanning thermograms. The polymer had birefringence below 90 C under a polarizing microscope. It is of interest to speculate as follows. Both of the hydrocarbon side chains and the polysaccharide main chains are crystalline and the polymer A exhibits a unique two-stage melting process of both crystalline regions. The polymer possibly forms a liquid-crystalline mesophase between these two transition temperatures. 

The phase behavior of a synthetic lecithin, dipalmitoyllecithin, as analyzed by Chapman and co-workers (5), is diagrammed in Figure 3. The main features are the same as in the phase diagram of egg lecithin a mixture of numerous homologs. As a consequence of the variation in fatty acid chain length, the chain melting point is lowered which means that the critical temperature for formation of liquid crystalline phases is reduced. This temperature is about 42 °C for dipalmitoyllecithin, and, if the lamellar liquid crystal is cooled below this temperature, a so-called gel phase is formed. The hydrocarbon chains in the lipid bilayers of this phase are extended, and they can be regarded as crystalline. The gel phase and the transitions between ordered and disordered chains are considered separately. 

Phase transition is an important property of membranes. Below the phase transition temperature, lipids are tilted and highly ordered. They are in their solid or "gel" state. Increasing the temperature leads to a pre-transition, characterized by periodic undulations and straightening of the hydrocarbon chain. Further increase of the temperature causes the main phase transition. Above the main phase transition temperature, lipids are fluid or "liquid crystalline." Figure 3 shows the phase diagram for the interaction of water with a lipid as well as its inferred arrangements in a model membrane (5). Phase transitions in membranes and membrane models have been extensively studied by spectroscopic techniques and by differential scanning calorimetry. 

Cho et al. described the synthesis and polymerization of 4,8-cyclododeca-dien-l-yl-(4 -methoxy-4-biphenyl) terephthalate VIII [54,55]. Polymerization was carried out with WCl4(OAr)2/PbEt4. The double bonds in the polymer backbone were subsequently hydrogenated with H2/Pd(C), leading to a SCLCP with a fully saturated hydrocarbon backbone. This polymer system had a very flexible polymer backbone but a stiff connection between the main chain and the mesogenic unit. The distance between two adjacent side chains was about 12 methylene units. This very flexible main chain allowed the polymer to organize into a LC mesophase. Both polymers - the unsaturated and the saturated -showed smectic liquid crystalline mesophases with almost the same transition temperatures (see Table 5). 

The lamellar gel-lamellar liquid-crystalline (L - L ) phase transition, frequently also referred to as (chain-)melting, order-disorder, solid-fluid, or main transition, is the major energetic event in the lipid bilayers and takes place with a large enthalpy change. It is associated with rotameric disordering of the hydrocarbon chains, increased headgroup hydration, and increased... 

Membrane lipids, and particularly cholesterol, are instrumental not only in the control of diffusion across biological membranes but also in the determination of the activity of membrane-bound enzymes, their modulation by hormones and other agents, and the determination of membrane fluidity (for original references, see [4,6]). It is generally accepted that incorporation of cholesterol in a lipid bilayer membrane tends to decrease significantly the permeability of these membranes to water. Movement of water across these membranes occurs primarily by dissolution in the membrane matrix. The decrease in the rate of water transport as a result of cholesterol incorporation is due mainly to a decrease in membrane fluidity. As a general rule, it is found that the presence of cholesterol in membranes or the incorporation of cholesterol into dispersions composed of phosphatidylserine or ganglioside lead to a decrease in the fluidity of the hydrocarbon chains of lipid membranes which are in the liquid-crystalline state [4,20]. 

Most of the reported structural transformations of PC lipid bilayers with respect to environmental conditions such as temperature, pressure, pH, etc., are associated with isomerizations of the constituent lipid molecules (mostly 14—24 carbon acyl chains). For instance, the transition from the ordered gel to fluid liquid crystalline phase, called the main phase transition, has been related to the melting of the hydrocarbon chains in the gel phase, phospholipids with all trans alkyl chains are present, whereas in the disordered liquid crystalline phase the most populated conformational states correspond to gauche forms in the alkyl chains. 

As described earlier, thermotropic, liquid crystalline polymers can be obtained by inserting flexible units in the main chain. As shown in Figure 8, these units can be hydrocarbon ... 

For more than two thousand years liquid-crystalline phases of soaps have been used for cleaning. The physical structure of these phases was revealed only recently. This was mainly due to the opinion that soap molecules should be regarded as rigid rods. In an infrared absorption spectroscopy study of anhydrous soaps, Chapman (1958) concluded that a high-temperature phase transition was caused by melting of the chains. Later Luzzati and co-workers (1960) conclusively demonstrated the liquid nature of the hydrocarbon chains as a fundamental feature in the structure of liquid-crystalline phases. They were also able to determine the structures of the most common liquid-crystalline phases. 

The nature of the headgroup plays a crucial role in determining the phase transition, for two main reasons. One is the electrostatic interaction among head-groups at the bilayer surface, which may differently affect the stability of the bilayer structure. The other is the influence of the headgroup on the alignment of the hydrocarbon chains. For example, protonated pyridinium salts show a simple single phase transition from the solid crystalline state to an isotropic liquid, while methylated pyridinium salts exhibit solid crystalline-solid crystalline, solid crystalline-liquid crystalline, and liquid crystalline-isotropic liquid transitions [77],... 

---