Lamellar conformation

A different approach [23, 32] considers AF in Eq. (2.2) to consist of a part which must be defined with respect to infinite chain length plus an entropy of localization [33] due to the pairing of chain ends which becomes important in the case of closely stacked lamellar crystals [34]. It amounts to — Kin (Cp) per molecule, where C is a constant related to the flexibility of the chains in the melt, and which arises due to the conformations of the finite chain. Hence ... 

It is now possible to obtain the average lamellar thickness L. Assuming that the folds are incorporated into the lamellar crystal after a proper conformational straightening, and attributing a 0.127 nm axial advancement to each... 

Lipid assemblies of the lamellar type, such as lipid bilayers, can feature a true phase transition in which the topology does not change. Upon cooling, the bilayer goes from the fluid phase to the gel phase. In the fluid phase, the acyl chains are disordered, in the sense that there is enough free volume around the chains to allow for chain conformation variations. In the gel phase, the acyl chains are more densely packed and believed to be ordered in an all-trans (straight) configuration. For very pure systems, at temperatures below this sharp gel-to-liquid phase transition, there are several other states and distinct transitions detectable (pre-transition, ripple phase, etc.). These phases will not be reviewed here. In biomembranes, many type of lipids (and other molecules) occur, and it is known that for this reason the gel-to-liquid phase transition is... 

Fig-1 Mean-field predication of the morphologies for conformationally symmetric diblock melts. Phases are labeled as S (bcc spheres), C (hexagonal cylinders), G (bicontin-uous la 3 d. cubic), L (lamellar)./a is the volume fraction... 

Unlike the bulk morphology, block copolymer thin films are often characterized by thickness-dependent highly oriented domains, as a result of surface and interfacial energy minimization [115,116]. For example, in the simplest composition-symmetric (ID lamellae) coil-coil thin films, the overall trend when t>Lo is for the lamellae to be oriented parallel to the plane of the film [115]. Under symmetric boundary conditions, frustration cannot be avoided if t is not commensurate with L0 in a confined film and the lamellar period deviates from the bulk value by compressing the chain conformation [117]. Under asymmetric boundary conditions, an incomplete top layer composed of islands and holes of height Lo forms as in the incommensurate case [118]. However, it has also been observed that microdomains can reorient such that they are perpendicular to the surface [ 119], or they can take mixed orientations to relieve the constraint [66]. 

Pressure was applied in this study to fine tune the lipid chain-lengths and conformation and to select specific lamellar phases. For example, the phospholipid bilayer thickness increases by 1 A/kbar in the liquid-crystalline phase, and up to six gel phases have been found in fully hydrated DPPC dispersions in the pressure-temperature phase space up to 15 kbar and 80 °C, respectively. NMR spectral parameters were used to detect structural and dynamic changes upon incorporation of the polypeptide into the lipid bilayers. 

With these factors in mind, a new method to evaluate the conformation of an amphiphilic molecule at the site of interest was Introduced. The method is built on the fact that the determination of Interlayer spacings of a lamellar liquid crystal using low angle X-ray diffraction methods in combination with density measurements will provide sufficient information to calculate the cross-sectional areas occupied by each amphiphlle (19). 

Employing x-ray methods, Flaim and Friberg (2 ) studied the conformation of C21-DA as the acid or monosoap in a lamellar liquid crystalline matrix. At low water concentrations, the conformations are the same (extended). At somewhat higher water... 

Figure 17. Schematic lepiesentation of a lamellar polyethylene single ciystal. (a) and (b) show enlaiged features of its stnicture in (o) the zigzag conformation of the macromolecular chain inside the crystal in (b) a hypothetical model for the regular folding, drawn according to the calculation reported in ref. 224a.

X-ray diffraction techniques can be used to establish the structure of crystalline polymers. Measurements are typically made on crystalline lamellar platelets grown from dilute solution, fibres or stretched films. Such methods have been applied to several different inorganic polymers. For example, based on measurements on stretched samples of silicone rubber, poly(dimethylsiloxane) (Me2SiO) has been shown to possess a helical conformation (Figure 8.4). ... 

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