Polyethylene random lamellar structure

Very high molar mass polyethylene (M>2,000,000 g mol-1) crystallises without the, formation of a clear superstructure, sometimes referred to as the random lamellar structure [116]. The great many chain entanglements present in high molar mass polymers obstruct crystallisation and the crystals become small and their orientation less correlated with surrounding crystal lamellae. 

Figure 5.15 Model of structural evolution process in the isothermal crystallization of polyethylene chains. The random chain segments change to the locally regularized trans-form. The stacked lamellar structure is formed with the long period 800 A. The structure changes further to the stacked lamellar structure of 400 A period by inserting the new lamella into the original lamellae. It is noted that the radius of gyration is kept unchanged during this process. (From Reference 46 with permission from the Society of Polymer Science, Japan.)...

The technique was first applied to polyethylene and i-polypropylene revealing itself to be sensitive to crystallinity, lamellar orientation, crystal structure, and crystal defects (55). Thus it penetrates between lamellae attacks lamellar side surfaces more than their fold surfaces, a- more than /3-i-polypropylene and removes dislocation cores preferentially. As with chlorosulfonation, it allows systematic study of samples by cutting them open where desired. But it can also reveal the particular character of external or fracture surfaces, neither of which is usually random. 

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