Ribbon-like aggregates

As pointed out above, the semicrystalline polymer can be considered as a two-phase composite of amorphous regions sandwiched between hard crystalline lamellae (Fig. 4.2(a)). Crystal lamellae ( c) are normally 10-25 nm thick and have transverse dimensions of 0.1-1 pm while the amorphous layer thickness, a, is 5-10 nm. As mentioned in the previous section, melt-crystallized polymers generally exhibit a spherulitic morphology in which ribbon-like lamellae are arranged radially in the polycrystalline aggregate (Bassett, 1981). Since the indentation process involves plastic yielding under the stress field of the indenter, microhardness is correlated to the modes of deformation of the semicrystalline polymers (see Chapter 2). These... 

Fig. 17 Supramolecular architectures from self-assembly of rod-coil macrocycles, a Body-centered orthorhombic structures from ribbon-like aggregates b body-centered tetragonal structures from barrel-like aggregates...

Fig. 3. —Diagrammatic Representation of Stacks of Cellulose Chains and Their Possible Aggregation.94 [Each cellulose chain is ribbon-like and approximately oval in cross-section (labeled) the view is down the ribbon. Note that the stacks are labeled as sheets in the drawing, after the original authors, b = fiber and chain axis, which is perpendicular to the plane of this diagram and therefore not shown a and c are the other edges of the Meyer-Misch cell.]...

Fig. 6 Cryo-TEM micrographs show the shape of the aggregates in the course of the vesicle-to-micelle transformation in the MGO/CTAB mixture. (A) Coexistence of globular micelles, elongated micelles that are probably ribbon-like, and vesicles (lipid mole fraction 0.47). (B) Perforated vesicles (lipid mole fraction 0.64). Reproduced from Ref. [30] with permission of the American Chemical Society.

Formation of H-bonding between synthetic polymers is used as a way to improve compatibility in blends. Interesting examples of switching from closed to open assemblies are the reversible transformation of the structure of folic acid 3 into linear ribbon-like aggregates (by salt concentration changes),and of cyclic octapep-tides into linear chains (by UV radiation). 

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