Structure of Bulk Polymers

Most polymers are partially crystalline. The degree of crystallinity of polymers may, however, range very widely from 0 percent for noncrystallizable polymers, through intermediate crystallinities, up to nearly 100 percent for polytetrafluo-roethylene and linear polyethylene. A direct evidence of the crystallinity in polymers is obtained from x-ray diffraction studies. The x-ray patterns of many crystalline polymers show both sharp features characteristic of ordered regions (called crystallites) and diffuse features characteristic of a molecularly disordered phase 

Lamellae are thin, flat platelets on the order of 100 to 200 A (0.01-0.02 micron) thick and several microns in lateral dimensions, while polymer molecules 

Lamellae are thin, at platelets on the order of 100 to 200 A (0.01-0.02 micron) thick and several microns in lateral dimensions, while polymer molecules are generally on the order of 1000 to 10,000 A long. Since the polymer chain axis is perpendicular to the plane of the lamellae, as revealed by electron diffraction, the polymer molecules must therefore be folded back and forth within the crystal. This arrangement has been shown to be sterically possible. In polyethylene, for example, the molecules can fold in snch a way that only abont ve chain carbon atoms are required for the fold, that is, for the chain to reverse its direction. Each molecnle folds np and down in a regular fashion to establish a fold plane. As illustrated in Fig. 2.17(a), a single fold plane may contain many polymer chains. The height of the fold plane is known as the fold period. It corresponds to the thickness of the lamellae. 

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