Crystalline polymers fringed-micelle model

Flgure 2.1 Conformational differences of polymer chains in the amorphous and crystalline states. Fringed micelle model. Parallel and coiled lines represent, respectively, portions of chains in the crystalline and the amorphous regions. 

Fig. 2.5. Chain molecules in (A) amorphous, (B) crystalline and (C) semicrystalline regions of polymers (fringed micelle model).

If the ordered, crystalline regions are cross sections of bundles of chains and the chains go from one bundle to the next (although not necessarily in the same plane), this is the older fringe-micelle model. If the emerging chains repeatedly fold buck and reenter the same bundle in this or a different plane, this is the folded-chain model. In either case the mechanical deformation behavior of such complex structures is varied and difficult to unravel unambiguously on a molecular or microscopic scale. In many respects the behavior of crystalline polymers is like that of two-ph ise systems as predicted by the fringed-micelle- model illustrated in Figure 7, in which there is a distinct crystalline phase embedded in an amorphous phase (134). 

Figure 8 Fringe-micelle model of crystalline polymers. (PramRef. 131,)...

The traditional model used to explain the properties of the (partly) crystalline polymers is the "fringed micelle model" of Hermann et al. (1930). While the coexistence of small crystallites and amorphous regions in this model is assumed to be such that polymer chains are perfectly ordered over distances corresponding to the dimensions of the crystallites, the same polymer chains include also disordered segments belonging to the amorphous regions, which lead to a composite single-phase structure (Fig. 2.10). 

In the present concept of the structure of crystalline polymers there is only room for the fringed micelle model when polymers of low crystallinity are concerned. For polymers of intermediate degrees of crystallinity, a structure involving "paracrystals" and discrete amorphous regions seems probable. For highly crystalline polymers there is no experimental evidence whatsoever of the existence of discrete amorphous regions. Here the fringed micelle model has to be rejected, whereas the paracrystallinity model is acceptable. 

Whatever the cause of this small amount of crystalline phase, the dimensions of a crystallite (ca. 100 A) are much smaller than a chain length, and it is likely that a given chain will go through two or more crystallites, which will then be connected by one or more covalent links. This situation is similar to the one known in polymer physics as the fringed micelle model (see Ref. 12, p. 187), and is sketched on Fig. 9. This has consequences on the behavior of films upon stretching (see Section II.D.3). 

While the fringe-micelle model for crystalline polymers has not been fashionable for some time, it may have some utility in modeling stress transfer and faUure mechanisms. In any event, a fringe micelle model is a primitive form of more general composites models which attempt to model the behavior of crystalliiM polymers using the same techniques as for filled S5rstems or fiber reinforced plastics. The ESR studies may serve to provide valuable insight into the validity of such models for... 

FIGURE 40.1 Fringed-micelle model of a linear polymer with semi-crystalline structure. 

Some cotton cellulose is noncrystalline or amorphous in the sense of lacking definite crystalline form. One reason is that cotton cellulose has a broad molecular weight distribution, making high-crystalline perfection impossible. The small crystallites constitute deviations from ideal crystals that are infinite arrays. The remaining amorphous character of most polymers is often thought to arise from the fringed micelle model of the solid structure. In... 

For convenience, we can regard an isotropic semicrystalline polymer as being made up of an isotropic polycrystalline phase and an isotropic amorphous phase, as shown in Fig. 6, which is purely diagrammatic (it shows the classical fringed micelle model for simplicity). From a geometrical standpoint, we cannot discriminate a priori between two continuous interpenetrating phases, a dispersion of a crystalline phase in an amorphous phase, or of an amorphous phase in a crystalline phase. The distinction may depend upon the volume fractions. 

Fig. 10.11 Illustration of the metastable polymer conformation in the crystalline regions. From left to right are the fringed-micelle model, the lamellar crystal with adjacent chain folding, the switchboard model and the variable-cluster model...

An early conceptual theory to account for these differences in x for semi-crystalline polymers was the fringed micelle model, the intermediate portions referred to above being the fringes. An individual polymer chain molecule may pass through several micelles and/or re-enter the same one, and a given micelle may contain contributions from just one, or from several, molecules. 

Fig. 1-28 Schematic diagram of the fringed micelle model of polymer amorphous-crystalline structure [44,45].

While much has been learned about the crystalline structure of polymers, the exact shape and structure of crystalline regions is still under intense study as increasing the degree of crystallinity leads to improved thermo-mechanical properties (Table 4.4). Relations between crystalline structure and mechanical response will be discussed in more detail later. The first interpretation of crystalline structure was suggested by x-ray diffraction studies and is known as the fringed micelle model . The Bragg... 

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