Interlamellar tie chains

The formation of the microstructure involves the folding of linear segments of polymer chains in an orderly manner to form a crystalline lamellae, which tends to organize into a spherulite structure. The SCB hinder the formation of spherulite. However, the volume of spherulite/axialites increases if the branched segments participate in their formation [59]. Heterogeneity due to MW and SCB leads to segregation of PE molecules on solidification [59-65], The low MW species are accumulated in the peripheral parts of the spherulite/axialites [63]. The low-MW segregated material is brittle due to a low concentration of interlamellar tie chains [65] and... 

The concentration of interlamellar tie chains in a given sample depends on molar mass, which determines the spatial distribution of the chains, and long period, i.e. the sum of crystal and amorphous layer thicknesses. Figure 7.33 shows two very... 

The number of interlamellar tie chains must be sufficiently high to prevent early brittle fracture. 

Fig. 4. Molecular model of a stack of parallel lamellae of the spherulitic structure A, interlamellar tie molecule B, boundary layer between two mosaic blocks C, chain end in the amorphous surface layer (c ilium) D, thickness of the crystalline core of the lamella E, linear vacancy caused by the chain end in the crystal lattice L. long period I, thickness of the amorphous layer (Peterlir ).

Semicrystalline polymers contain liquid-like amorphous and ordered crystalline phases. When solidified from the pure melt, these polymers show a spherulitic structure in which crystalline lamellae composed of folded chain crystallites radiate from the center of the spherulite in such a way that a constant long period or crystallinity is apvproximately maintained. The amorphous regions reside in the interlamellar regions in the form of tie chains, whose ends are attached to adjacent lamellae loop chains, whose ends are attached to the same lamella cilia chains with only one end attached to a lamella (or dangling chain ends), and floating chains which are not attached to any lamellae. This hierarchical structure is illustrated in Figure 1. 

FIGURE 4.6 Compromise model showing folded-chain lamellae tied together by interlamellar amorphous chains. 

Interlamellar connections cannot be directly imaged therefore, their existence and properties have to be inferred fiom material properties. For the sake of simplicity, no distinction is drawn here between the effects of ionic and covalent links. Unless otherwise noted, the term tie chain is used generically to encompass all types of interciystallite connections. 

Interlamellar connections are cmcial to the mechanical properties of polyethylene because they transmit forces between crystallites. Tie chains determine or influence a variety of mechanical properties, such as ductility, toughness, and modulus. Without the benefit of interlamellar links, polyethylene would be a brittle material with little physical strength. 

Figure 5.88 is an illustration of two-dimensional defects in the form of surfaces and grain boundaries. They either terminate a crystal or separate it from the three-dimensional defects. In polymer crystals, these surfaces and grain boundaries are rarely clean terminations of single-crystalline domains, as one would expect from the unit cell descriptions in Sect. 5.1. The surfaces may contain folds or chain ends and may be traversed by tie molecules to other crystals and cilia and loose loops that enter the amorphous areas, as is illustrated in Figs. 5.87 and 2.98. The properties of a polycrystalline sample are largely determined by the cohesion achieved across such surfaces and the mechanical properties of the interlamellar material, the amorphous defects. 

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