Reversed curvatures

Techniques cannot be used for every structural element. The shape of the part must permit removal of the winding mandrel after final curing. Reversed curvatures should be eliminated whenever possible, since it is difficult to wind them and hold the filaments under tension. In order to meet this problem, fusible, expandable, and multiparty mandrels are often required. 

Whilst the early reports of intermediate phases concerned systems with reversed curvature [73-76] these were for surfactants where some residual short range order in the polar groups was probably present. There are few definitive reports of fully molten intermediate phases with reversed curvatures. In fact the pattern of how intermediate phases replace the normal bicontinuous cubic phase as alkyl chain size increase only became recognized as systematic studies on homologous series were carried out [37, 66], Here it has required a combination of microscopy, multinuclear NMR and X-ray diffraction to elucidate the structures. Such studies on reversed phases have yet to be carried out, particularly where small variations in alkyl chain structure are made. 

When finally cured, the molding is removed from the mold (in many cases, the mold may itself be dismantled, to facilitate de-molding, especially where reverse curvature is involved). The molding can be trimmed, finished and fitted out as necessary. A number of moldings can be bonded to each other. 

As an example of the effects of an amphiphilic drug on the structure of surfactant self-assemblies. Figure 1.4 shows part of the phase diagram of monoolein, water, lidocaine base and licocaine-HCl (21). As can be seen, the cubic phase (c) formed by the monoolein-water system transforms into a lamellar liquid crystalline phase on addition of lidocaine-HCl, whereas it transforms into a reversed hexagonal or reversed micellar phase on addition of the lidocaine base. Based on X-ray data, it was inferred that the cubic phase of the monoolein-water system had a slightly reversed curvature (critical packing parameter about 1.2). Thus, on addition of the... 

Figure 5.23 Warping of plate of gel dried by evaporation from upper surface. Plate warps upward initially then reverses curvature after top surface becomes dry. (Adapted from Ref. 36.)...

FIGURE 3-14. Performance Characteristics of Motors. For typical squirrel-cage induction motors. Points (I) show the speed-torque limits during initial acceleration. Area is the maximum torque range recommended for short-term overloads. Point is the reverse-curvature point on the speed-torque curve and is the point of minimum accelerating torque. 

One generally cannot wind reverse curvature. To wind a reverse curvature, wind the exact shape on a positive dummy mandrel insert and then remove the insert and place the fiber. 

---