Lamellar ribbon

Twisting of the lamellar ribbons along the radial direction is responsible for the banding superimposed on the Maltese cross in Fig. 4.12. From the spacing of the bands, the period of the twist can be calculated and is found to depend on crystallization conditions. 

The growth of polymer spherulites involves the segregation of noncrystal-lizable material into the regions between the lamellar ribbons. The components that arc not incorporated into the crystallites include additives like oxidation stabilizers, catalyst residues, and so on. as well as comonomer units or branches. The spherulite structures and interspherulitic boundaries are held together primarily by polymer molecules which run between the twisted lamellar subunits and the spherulites themselves. Slow crystallization at low degrees of supercooling... 

Fig. 5.5 Schematic hierarchy of microscopic stnictuie of PE blend sphtaulites consisting of semicrystalline lamellar ribbons. Green in the composite image indicates D-HDPE, and red indicates H-LLDPE. The dotted rectangular box in the molecular stmcture indicates the unit cell of crystalline PE, with the unit cell parameters (a, b, c) as indicated [28]...

Figwe 42 Image of an S-shaped helicoid with a left-handed twist. The direction of the S-shape is linked to the sense of twist by the direction of the surface stresses so that it appears that the lamellar ribbon scoops the melt during its growth. " ... 

Crystallization of polymers from the melt usually results in lamella formation. However, the detailed organization of these lamellas in the larger morphological entities usually observed can be complex and is found for the pol)nners studied in detail to date to depend on molecular weight and degree of undercooling of the melt. A common morphological entity obtained, when crystallization is carried out from the melt, is the spherulite the substructure of these can be fibrous or made up of lamellar ribbons. 

Structural data, indicating the existence of columnar and even disc-like mesophases for the anhydrous alkali, alkaline earth, and cadmium salts of long-chain fatty acids were first reported by Skoulios et al. [9]. For example the potassium ssdts can be lamellar, ribbon-like or disc-like in a three dimensional lattice [33], see Fig. 1. 

Figure 3.69 Sketch of an undislocated lamellar ribbon (left), a lamella with one left-handed topological screw dislocation of strength b = Ic (center) and a lamella with four left-handed topological giant screw dislocations of the same strength (right). As illustrated, the dislocations have generated a left-hand twist, but no spiral growth. From Schultz and Kinloch [107] with permission from Elsevier.

Nylon crystallites consist of sheets of chains that are hydrogen-bonded to their neighbors. On a supermolecular scale, crystallites have a lamellar structure, that is they are many times longer and broader than they are thick. When nylon crystallizes from an isotropic molten state, it generally forms spherulites, which consist of ribbon-like lamellae radiating in all directions... 

Figure 5.16 Cryo-transmission electron micrograph of (a, b) helical ribbons and (c, d) multi-lamellar tubules in aqueous dispersions of A-dodecanoyl-L-serine (28) at pH 6.4 (a-c) and 4.9 (d). Reprinted with permission from Ref. 79. Copyright 2001 by the American Chemical Society.

Hence, there are two options for molecules with unlike end-chains either the incompatible chains mix which retains the lamellar structure, or these chains segregate and layer modulation with formation of ribbon phases (columnar 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.

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