Amorphous material/polymers/regions structure

Interestingly, the amorphous regions within the spherulite confer some flexibility onto the material while crystalline platelets give the material strength, just as in the case with largely amorphous materials. This theme of amorphous flexibility and crystalline strength (and brittleness) is a central idea in polymer structure-property relationships. 

Post-failure studies of the fracture surface morphology of bulk semi-crystalline polymers are more difficult than those of amorphous materials due to the more complex multiphase structure associated with semi-crystalline materials However, it was shown that some fractographic details point to the formation of a stress whitened region ahead of a notch prior to final fracture of the material. In particular, stress-whitened regions were easily visible in semi-crystalline polymers such as LDPE and HDPE The resulting, macroscopically apparently brittle fracture... 

Polymers containing crystalline structures pack more closely and attain higher densities than amorphous materials. Crystallinity contributes to the high densities of crystalline poly (tetrafluoroethylene) (ca. 2.2gcm ) and poly (vinylidene chloride) (ca 1.7gcm ). The conformation of molecules in the crystalline structures also affects the density. Polyethylene adopts a planar conformation while polypropylene molecules have a helical conformation in the crystalline regions. Helices require more space than planar forms, resulting in a lower density for polypropylene than polyethylene (Brydson, 1999). 

Example 11.5 draws attention to the fact that the crystalline and noncrystalline regions, particularly the crystalline regions, are likely to be highly anisotropic and that this must be taken into account in applying the models. A further complication is that, for many polymer samples, the structures illustrated schematically in fig. 11.9 are too simplified. Structures such as those shown in fig. 11.10, which allow for amorphous material both... 

Characterization of polymer orientation is most often accomplished via X-ray techniques which are suited to crystalline and paracrystalline regions (i-d). However, semicrystalline polymers present a complex system of crystalline, amorphous, and intermediate pluses ( -d) and complete characterization of semicrystalline polymers can only be achieved by application of a variety of techniques sensitive to particular aspects of orientation. As discussed by Desper (4), one must determine the degree of orientation of the individual phases in semicrystalline polymers in order to develop an understanding of structure-property relationships. Although the amorphous regions of oriented and unoriented semicrystalline polymers are primarily responsible for the environmental stress cracking behaviour and transport properties of the polymers, few techniques are available to examine the state of the amorphous material at the submicroscopic level. 

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