Structural analysis of random propylene-ethylene copolymers

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Abstract
Structural properties of random propylene ethylene copolymers with low ethylene contents between 0 % and 11 mol % were investigated using mainly X-ray techniques. One of the essential issues of this paper is that a cocrystallization of the comonomer units does not take place. This is based on the following observations: The long period and the thickness of the crystalline lamellae decrease with increasing ethylene content. The electron density difference between crystalline and amorphous phase, however, as determined with the invariant, does not show any changes. The heat of fusion, normalized to the degree of crystallinity, does not decrease. This leads to the conclusion that the observed changes are not due to an incorporation of the ethylene units as defects, but to a reduced extension of the crystalline regions, now confined by the ethylene units which interrupt the helical chain conformation. The γ-modification, detectable by means of wide-angle X-ray diffractometry from an ethylene content of 5 mol % on, partially converted into the α-modification on annealing.

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