Crystalline Fluid

Biological membranes provide the essential barrier between cells and the organelles of which cells are composed. Cellular membranes are complicated extensive biomolecular sheetlike structures, mostly fonned by lipid molecules held together by cooperative nonco-valent interactions. A membrane is not a static structure, but rather a complex dynamical two-dimensional liquid crystalline fluid mosaic of oriented proteins and lipids. A number of experimental approaches can be used to investigate and characterize biological membranes. However, the complexity of membranes is such that experimental data remain very difficult to interpret at the microscopic level. In recent years, computational studies of membranes based on detailed atomic models, as summarized in Chapter 21, have greatly increased the ability to interpret experimental data, yielding a much-improved picture of the structure and dynamics of lipid bilayers and the relationship of those properties to membrane function [21]. 

Membranes are composed of phospholipids and proteins. The fatty acid composition of the phospholipids in a membrane influences how it is affected by the cold. In general, as the temperature of a cell is lowered the lipids in the membrane bilayer undergo a phase transition from a liquid crystalline (fluid) state to a gel (more solid) state. The temperature at which this transition takes place is very narrow for phospholipids composed of a simple mixture of fatty acids, but is quite broad for the phospholipids in cellular membranes. It is usually implied from various methods... 

Examples of the temperature-dependence of diffusion in select silicate, oxide and carbonate systems are shown in Figure 23a for volume diffusion and Figure 23b for grain boundary diffusion. A comparison of diffusivities and activation data compiled in Table 2 (see Appendix) leads to the following general observations regarding diffusion behavior in crystalline-fluid (gas) systems. 

One of the most unique aspects of liquid crystalline fluids is the fact that most of the orientation ajjd texture generated during flow is maintained in the solid state . Hence, the flow characteristics (i.e. the rheology) are directly related to the properties of injection-molded specimens or fibers. We shall, therefore, review some of what has been reported on the rheology of liquid crystalline fluids in an effort to identify any behavior... 

Our intent, as mentioned earlier, is not to review all the studies concerned with liquid crystalline fluids but to compare their properties with flexible chain polymers, interpret their properties in terms of the domain structure, and look for correlations between flow characteristics and processing conditions. We first examine the behavior of liquid crystalline copolyesters in steady shear flow and in small strain dynamic oscillatory flow. 

Typically the viscosity of liquid crystalline fluids is highly dependent on shear rate over many orders of magnitude of y. This shear dependence of viscosity is illustrated by the data in Figure 7 for a 60 mole % PHB/PET copolyester. In this figure, data... 

Some researchers consider the viscosity versus shear rate curve to consist of three regions. At low y, n depends on 7" because of the presence of yield stresses. In region II there is a plateau where n is nearly independent of y. Finally, in region III, n depends on y to some power (n) between -1.0 and 0 which is similar to flexible chain isotropic systems. It is our contention that yield stresses (Oq ) are not common features of pure liquid crystalline fluids. For sey r l different lyotropic systems, values of no are observed at low y. It is possible that only... 

Normal stress differences generated in shear flow are common features of isotropic macromolecular fluids.. They are associated with fluid elasticity and are related to a number of phenomena such as elastic recoil, die swell, orientation, and melt fracture. In fact, the ratio, N /2o, is related to the ultimate recoverable shear strain which can be measured in a cone-and-plate device on cessation of flow. Liquid crystalline fluids also exhibit positive values of N (see Figure 9) and in some cases negative values of Vail... 

Although heat treatment leads to improved properties, no further drawing steps are required to generate highly oriented systems. Hence, the orientation which is found in fiber and injection molded specimens must arise during flow and be maintained during the solidification process. The final physical properties must therefore be directly related to the rheological properties of the liquid crystalline fluids. 

Intrinsic proteins can also modify the order of phospholipid hydrocarbon chains. When cytochrome oxidase is reconstituted into vesicles of dimyristoylphosphatidylcholine, deuterated at the fatty acid methyl end, it is seen that Avq varies with the protein/ lipid ratios. Experiments performed with the lipid in the liquid-crystalline (fluid) state show (Fig. 9.24) that lipid order decreases as the protein concentration in the bilayer increases (Kang etal., 1979). This is attributed to the rough irregular protein surface in contact with the acyl chains. 

Doi theory for liquid crystalline fluids, LCF (Doraiswamy and Metzner 1986). The kinetics of the distribution function, f, is given by... 

On the other hand, the processing of liquid-crystalline polymers has received considerable interests. One of the most unique aspects of liquid crystalline fluids is the rigid-rod constituents capability to orient during processing. Furthermore, these orientations and the resultant texture could be maintained during the solidification process. Hence, the rheological properties of spinning dopes are directly correlated to the final mechanical properties of fibers. 

The outcome, though more or less strongly disordered can still be referred to the parent crystal lattice, i.e. the liquid is quasi crystalline. However, if the molecules can form short range hydrogen bonds, based on atoms of low atomic number such as N, O, F, S the quasi crystalline fluid generally rearranges its molecules so as to decrease the enthalpy by... 

Aggregation in colloidal systems can be introduced by various mechanism. Attractive interactions between the colloids is the most prominent example. Another possibility is to confine the colloids in one phase of a phase separating mixture, e.g., in the isotropic phase of a liquid crystalline fluid that is undergoing the isotropic-to-nematic transition [52, 53]. This unusual soft solid consists of a foam hke structure, where the bubbles are filled with liquid crystal in the nematic phase and the colloids are confined in the walls separating the bubbles [54]. 

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