Polymer fringed micelle

Figure III 32. Morphology of a semi-crystalline polymer (fringed micelle structure).

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

Figure 3.6. Two-dimensional representation of molecules in a crystalline polymer according to the fringed micelle theory showing ordered regions (crystallites) embedded in an amorphous matrix.

The fringed micelle theory has been less favoured recently following research on the subject of polymer single crystals. This work has led to the suggestion that polymer crystallisation takes place by single molecules folding themselves at intervals of about 10 nm to form lamellae as shown in Figure 3.3b. These lamellae appear to be the fundamental structures of crystalline polymers. 

Figure 3.3 Arrangements of molecules in crystalline polymers according to (a) fringed micelle and (b) lamellae theories...

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 fringed micelle picture is not particularly suitable for describing synthetic polymers crystallized from solution or melt. However, the fibrils of many nat-... 

Fig. 7. Schematic representation of the fringed micelle model of crastalline polymers... 

However, with naturally occurring macromolecules, such as cellulose, the older fringed micelle concept is believed to apply. This represents a crystalline polymer, (it would be more correct to speak of semicrystalline or partially crystalline polymers since a material consisting of chain molecules can never be completely ordered), made up of ordered (crystalline) domains interspersed with disordered (amorphous) domains, so that each polymer chain passes through several crystalline and amorphous regions (Figure 4). 

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). 

A second important event was the development by Hosemann (1950) of a theory by which the X-ray patterns are explained in a completely different way, namely, in terms of statistical disorder. In this concept, the paracrystallinity model (Fig. 2.11), the so-called amorphous regions appear to be the same as small defect sites. A randomised amorphous phase is not required to explain polymer behaviour. Several phenomena, such as creep, recrystallisation and fracture, are better explained by motions of dislocations (as in solid state physics) than by the traditional fringed micelle model. 

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. 

It is now generally accepted that the morphology of a polymer depends on the contributions of three different macro-conformations (a) the random coil or irregularly folded molecule as found in the glassy state, (b) the folded chain, as found in lamellar structures and (c) the extended chain. The fringed micelle (d) may be seen as mixture of (a), (b) and (c) (see Fig. 2.12) with paracrystallinity as an extreme. 

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). 

Fig. 1.11 Diagrams of polymer crystallinity models (a) fringed micelles, (b) a chain-folded lamella.

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