The fringed-micelle model

The fringed-micelle model was an early attempt to inter-relate long molecules, small crystals and a sea of amorphous material. It was proposed in 1930, by Hermans and others, to explain the structure of gelatin and was subsequently applied to natural rubber. It is now believed to be incorrect as the basic model for polymer crystallites, but it is worth describing for historical reasons and because it may be a good approximation to the true structure in special cases. The essentials of the model are illustrated in fig. 5.2. 

The model suggests the occurrence of fibrillar crystallites, which can grow both parallel and perpendicular to the chain axes. Each chain 

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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). 

It has been suggested that glass transition is an important physicochemical event that controls the phase transition process of starch (Biliaderis, 1998). According to Biliaderis (1998), the "fringe-micelle" model (Fig. 5.16) does not permit assignment of a definite Tg for most starches. This is because the change in heat capacity during phase... 

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

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). 

The fringed micelle model gives an extremely simple interpretation of the "degree of crystallinity" in terms of fractions of well-defined crystalline and amorphous regions. Many excellent correlations have evolved from this model through the years, so that it has long been popular. 

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). 

If the fringed micelle model (see Section II.C.5.a) is applicable, large values of, which would correspond to distances between crystallites greater than the length of the chain connecting them, imply breaking of covalent bonds this may place the limit for fracture of the sample. 

By contrast, total molecular architecture, involving the conjunction of crystalline and amorphous parts, has proved to be much less amenable to investigation. For a long time structural interpretations were based on the fringed-micelle model in which molecules are supposed to wander through... 

FIGURE 8-51 Schematic diagram depicting the fringed micelle model. 

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 6 The fringed micelle model of polymer microstructure...

The most obvious question that needs to be answered about polymer crystallites is question (i) of section 5.1, How can long molecules give rise to small crystallites . Two principal types of answer have been given they lead to the fringed-micelle model and the chain-folded model for polymer crystallites. A further type of crystallite, the chain-extended crystal, can also occur when samples are prepared in special ways. These three types of crystallite are considered in the following sections. 

Shortly afterwards (in 1957) Fischer showed by electron microscopy that the crystallites in melt-grown spherulites of polyethylene and nylon were most likely to be lamellar rather than fibrillar, as would be expected from the fringed-micelle model. It is now accepted that chain-folded lamellar crystallites play an important part in the structure of most ordinary... 

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... 

Fig. 2.11 A pictorial rendering of the fringed-micelle model (from Bryant (1947) courtesy of Wiley).

The fringed micelle picture is not particularly suitable for describing synthetic polymers crystallized from solution or melt. However, the fibrils of many natural substances, such as cellulose and proteins (collagen, silk), consist of bundles of macromolecules in a parallel alignment, compatible with the fringed micelle model. For synthetic polymers, however, it is more often found that they crystallize such that the macromolecules fold with an essentially constant length, leading to a lamellar-type crystallite structure (switchboard-model. Fig. 1.11). 

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. 

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