Chain folded 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). 

Fig. 8. Schematic representation of the folded chain model of crystalline domains in thermoplastics. a Lamella crystallized from dilute solution (multiple/adjacent re-entry) and b Lamella crystallized from melt (single entry/far distance re-entry)...

Figure 18.1. Folded-chain model for crystallinity. Adapted from Rosen (1993). Reproduced by permission of John Wiley Sons, Ltd.

Figure 2.7 Folded-chain models for single crystals, (a) Regular adjacent reentry (b) nonregular random reentry.

The Folded-Chain Model This led to the folded-chain model, illustrated in Figure 6.10 (41). Ideally the molecules fold back and forth with hairpin turns. W ile adjacent reentry has been generally confirmed by small-angle neutron scattering and infrared studies for single crystals, the present understanding of bulk crystallized polymers indicates a much more complex situation (see below). 

The proposal that transverse sectioning of natural cellulose fibres can be used to test theories on the structure of microfibrils has been examined theoretically. It was concluded that fibre-sectioning experiments described in the literature do not disprove the folded-chain model, and it was suggested that clearly divided sections exist along the axis of the microfibril at intervals of 200 A. This cannot be explained in terms of a fringed micelle model. However, it is possible that cellulose II has a folded-chain conformation, since a single molecule of cellulose can be folded back and forth in the (101) plane to form a sheet-like structure that fits into the unit cell. A cellulose molecule can achieve a sharp U-tum in the... 

Fig. 4. 30 Folded chain model for a crystalline lamellae in polymers.

A schematic visualization of a spherulite is given in Fig, 4,36. Here the spherical nature is apparent and it is to be noted that the individual fi-brils/lamellae grow radially. The individual fibrils have a folded chain structure and the chain traverses both crystalline regions and amorphous regions as illustrated in Fig, 4,31 of the folded chain model. 

Explain the folded chain model for crystallinity. The fringed micelle model. 

This folded-chain model has been well substantiated for single polymer crystals. The lamellae are about 50-60 carbon atoms thick, with about five carbon atoms in a direct reentry fold. The atoms in a fold, whether direct or indirect reentry, can never be part of a crystal lattice. 

Fig. 14. The irregularly folded chain model of Privalko and Lipatov from Fig. 2 of ref. 44. is the mean square end-to-end distance and is the fold length.

Figure 3.12 compares the chain re-enhy in folded-chain and switchboard models. In the folded-chain model, the polymer chains reenter the crystalline phase right next to where they left. On the other hand, in the switchboard model, the polymer chains do not have to follow adjacent re-entry. Instead, they can re-enter the crystalline phase randomly. The switchboard model requires minimum movement and re-organization of polymer chains. 

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