Polymer crystallization fringed micelle model

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. 

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. 

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... 

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...

Fig. 2.4. The fringed-micelle model of semicrystalline polymers. The solid consists of an intimate mixture of ordered crystals and randomly structured amorphous regions. The molecuiar iength is considerably greater than the length of a crystal. A molecule thus passes through several crystals and several amorphous regions. The integrity of the two-phase solid is thus maintained by the long molecules.

The random arrangement of adjacent crystals in the fringed-micelle model (see Fig. 2.4), does not accord with evidence obtained from microscopy. The electron microscope shows the crystals in polyethylene to be very thin twisted lamellae laid one upon another (Fig. 1.7). The lamellae are too small to be observed with the light microscope. Light microscopic examination of thin films or sections between crossed polarizers reveals complex polyhedral objects known as spherulites (see Fig. 2.5). They are in fact a complex ordered aggregation of the sub-microscopic crystals. In polyethylene the crystal lamellae are about 10 nm thick. They are separated one from another by thin lamellae of amorphous polymer of about the same thickness. The lateral... 

The fringed micelle model was called into question by the discovery of polymer single crystals grown from solution [8]. Linear polyethylene, for example, forms single crystal lamellae with lateral dimensions of the order of 10-20 nm and thickness of lOnm. Electron diffraction shows that the molecular chains are oriented approximately normal to the... 

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