Defects in polymer crystals

To understand the importance of defects in polymer crystals, one must distinguish structure-insensitive properties from structure-sensitive properties. For crystals of small molecules and rigid macromolecules (see Fig. 1.6), the structure-insensitive properties often can be derived directly from the ideal crystal structure as summarized in Fig. 5.80. The density, for example, can be calculated from the unit cell dimensions (see Sect. 5.1). The polymeric materials in form of flexible macromolecules are, in... 

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. 

Sumpter BG, Noid DW, Wunderlich B (1992) Computational Experiments on the Motion and Generation of Defects in Polymer Crystals. Macromolecules 25 7247-7255. Peterlin A (1971) Molecular Model of Drawing Polyethylene and Polypropylene. J Mater Sci 6 490-508. 

In general, however, identification of the crystal cell is only part of the problem of characterizing the structure of crystalline polymers. Crystals are never perfect and the units cells do not infinitely duplicate through space even when they are grown very carefully from dilute solution using low molecular mass materials. As with the organic crystals considered in Chapter 3, a variety of defects can be observed and are associated with chain ends, kinks in the chain and jogs (defects where the chains do not lie exactly parallel). The presence of molecular (point) defects in polymer crystals is indicated by an expansion of the unit cell as has been shown by comparison of branched and linear chain polyethylene. The c parameter remains constant, but the a and b directions are expanded for the branched polymer crystals. Both methyl and... 

The presence of molecular (point) defects in polymer crystals may be indicated by an expansion of the unit cell. The unit cell parameters of branched polyethylene have been extensively studied and compared with those of linear polyethylene (Fig. 7.9). The c parameter remains constant and the branched polymer crystals are expanded in the a (mostly) and b directions. Both methyl and ethyl branches cause... 

The consequences of the computed defect properties for polymer crystal behavior have been discussed in the literature cited and will not be gone into here. We suffice to conclude that it is possible to devise a computational strategy that allows the energy minimization of chain assemblies large enought to permit realistic and practical simulation of the energies and packings of defects in polymer crystals. 

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