Lamellar structures, packing behavior

Fig. 3 Geometric models representing packing behavior of micelles, inverse micelles, and lamellar structures. Lipid molecules with domains that occupy a large lateral distance compared to adjacent domains of the molecule will conform to corresponding shapes and packing behavior. Micelles and inverse micelles approximate cone shape, whereas lamellar systems resemble cylinders. (Adapted from Refs. l)...

In a solvent, block copolymer phase behavior is controlled by the interaction between the segments of the polsrmers and the solvent molecules as well as the interaction between the segments of the two blocks. If the solvent is unfavorable for one block, this can lead to micelle formation in dilute solution. The phase behavior of concentrated solutions can be mapped onto that of block copolymer melts (97). Lamellar, hexagonal-packed cylinder, micellar cubic, and bicontinu-ous cubic structures have all been observed (these are all lyotropic liquid crystal phases, similar to those observed for nonionic surfactants). This is illustrated by representative phase diagrams for Pluronic triblocks in Figure 6. 

One type of lipid that is dominant in biological interfaces is lecithin, and lecithin-water systems have therefore been examined extensively by different physical techniques. Small s binary system (3) for egg lecithin-water is presented in Figure 2. The lamellar phase is formed over a large composition range, and, at very low water content, the phase behavior is quite complex. Their structures as proposed by Luzzati and co-workers (4) are either lamellar with different hydrocarbon chain packings or based on rods both types are discussed below. 

Under defined conditions, the toughness is also driven by the content and spatial distribution of the -nucleating agent. The increase in fracture resistance is more pronounced in PP homopolymers than in random or rubber-modified copolymers. In the case of sequential copolymers, the molecular architecture inhibits a maximization of the amount of the /1-phase in heterophasic systems, the rubber phase mainly controls the fracture behavior. The performance of -nucleated grades has been explained in terms of smaller spherulitic size, lower packing density and favorable lamellar arrangement of the /3-modification (towards the cross-hatched structure of the non-nucleated resin) which induce a higher mobility of both crystalline and amorphous phases. 

Figure 16.1 Relationship between molecular shape, aggregate structure in dilute dispersions, phase behavior and packing parameter. Micellar phase (L,), cubic micellar phase (I), hexagonal phase (H), bicontinuous cubic phase (Q), La lamellar phase. Subscripts I and II indicate normal and inverted phases, respectively. From M. Scarzello, Aggregation Properties of Amphiphilic DNA-Carriers for Cene Delivery, Ph. D. Thesis University of Groningen, p 6, 2006...

The imposition of shear flow can have quite dramatic consequences on the structure and phase behavior of complex fluids. Steady shearing of binary amphiphilic systems can lead to a completely new phase of densely packed onionlike vesicles [140]. Shear flow also strongly affects the stability of the lamellar phase [141-145]. We want to discuss here the role of shear in the microemulsion-to-lamellar transition. 

---