Strut phases

Exotic phases are found at compositions between lamellae and hexagonal cylinders (see Figs. 12-19 and 12-20). Some examples of the morphologies of these phases are shown in Fig. 12-22 these include cubic strut phases, tetragonal and rhombohedral mesh phases, and rectangular ribbon phases. 

Figure 12.22 Types of phases that occur between hexagonal and lamellar phases (a) Strut phase, the left image is the minimal P surface over which surfactant is draped, and the right is the topology of the two strut networks on either side of the minimal surface (b) tetragonal mesh phase, (c) ribbon phases containing cylinders with ellipsoidal cross sections.

The other phases are less exotic. The mesh phases consists of lamellae with ordered holes, while ribbon phases are deformed cylinders on a rectangular lattice (see Fig. 12-22). These phases can are usually type I phases with the tails inside the deformed cylinders or inside the hole-filled lamellae, but they could also be inverse, type II, phases. Type II mesh and ribbon phases seem not to have been reported much type II strut phases are common for two-tailed lipids, such as those in cell membranes. In fact, type II strut phases evidently serve biological functions, since they have been found to exist in cellular structures such as the endoplasmic reticulum and the mitochondrion (Seddon 1996). 

Cubic strut phases are common in the phase diagrams of two-tailed surfactants. These surfactants have a relatively high value of the vfaolc parameter, because the volume-to-length ratio v/i(. of the double tail is twice that of a single tail. A high value of v/aoic is consistent with the formation of type II bicontinuous and other inverse phases, such as the inverse hexagonal phase in Fig. 12-24. 

Figure 12.25 Phase diagram of didodecyldimethylammonium bromide (DDAB) in water and styrene at 20°C. The phases include an oil-rich isotropic phase L2, lamellar phases, and five distinct cubic strut phases, including the G, D, P, C(P), and an unknown phase C5. Above the cubic phases are regions of two- and three-phase coexistence. (From Strom and Anderson 1992, reprinted with permission from Langmuir 8 691. Copyright 1992, American Chemical Society.)...

Atomic force microscopy and attenuated total reflection infrared spectroscopy were used to study the changes occurring in the micromorphology of a single strut of flexible polyurethane foam. A mathematical model of the deformation and orientation in the rubbery phase, but which takes account of the harder domains, is presented which may be successfully used to predict the shapes of the stress-strain curves for solid polyurethane elastomers with different hard phase contents. It may also be used for low density polyethylene at different temperatures. Yield and rubber crosslink density are given as explanations of departure from ideal elastic behaviour. 17 refs. 

Finally, at the Texas Heart Institute, we are developing a concept device that will consist of a catheter deployed micropump in the descending aorta and fixated with metallic struts to the aortic wall. It is designed to accelerate blood flow in the descending aorta to unload the heart. Its power will be supplied through a transarterial power cable to an outside power source. Though still in the conceptual phase only, it may be truly implantable in the cath lab without surgery. 

The resistance to fluid flow is a measure of the physical structure of the foam. In order to control the flow through a foam, ceU size, degree of reticulation, density, and other physical factors must be controlled. The control of these physical factors, however, is achieved through the chemistry and the process by which the foam is made. The strength of the bulk polymer is measured by the tensile test described above, but it is clear that the tensile strengths of the individual bars and struts that form the boundaries of an individual cell determine, in part, the qualities of the cells that develop. A highly branched or cross-linked polymer molecule will possess certain tensile and elongation properties that define the cells. The process is also a critical part of the fluid flow formula, mostly due to kinetic factors. As discussed above, the addition of a polyol and/or water to a prepolymer initiates reactions that produce CO2 and cause a mass to polymerize. The juxtaposition of these two reactions defines the quality of the foam produced. Temperature is the primary factor that controls these reactions. Another factor is the emulsification of the prepolymer or isocyanate phase with the polyol or water. 

Ultrafasi electron diffraction and tran krt complex strut ures - From gas phase to crystallography... 

The limiting case of porous plastics are the so-called reticular porous plastics which lack cell walls, and the entire polymer phase is concentrated in the cell struts ... 

The fourth type, an open dodecahedron, may be observed open-cell foams, provided the viscosity of the starting polymer phase is high enough. If one sixth of alt walls, or more, have been broken, the resultant plastic foam will be an entirely open-cell foam, i.e. feature the so-called through -porosity. The fifth morphological type (called web structure) is a minimum surface dodecahedron having all the material of the cell walls drained off to the struts. 

The cells of the structure can be either closed or open. The former type is like a foam, in that the matrix can geometrically be compared with the continuous phase of the foam, which consists of thin lamellae and Plateau borders. Here we have thicker lamellae or walls, and beams (struts, ribs) where two lamellae meet if the cells are filled with gas we can call the system a solid foam. Open cells occur when the lamellae contain holes now we speak of a sponge. Some types of sponge structures merely consist of beams. Most plant tissues (see, e.g., Figure 9.4) consist of closed cells that are mainly filled with an aqueous liquid. 

PS with P(S-b-B), Mesh-and-strut morphology consisting mostly of hyperbolic interfaces, found (K-resin) between the lamellar and spherical phases were observed... 

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