Abstract
An analysis of the mechanical properties and a detailed structural study of samples of syndiotactic polypropylene (sPP) are presented in order to clarify the origin of the elastic behavior of sPP. Samples of sPP having different stereoregularity and showing different polymorphic behavior have been analyzed. The present study allows to establish that the elastic properties of unoriented specimens of sPP are poor, since the initial dimensions are only partially recovered upon the release of the stress. Good elastic properties are, instead, shown by sPP fibers, which have been previously oriented (for instance, uniaxially drawn at 400% and at 600% their initial length and then stress-relaxed). Tension set values less than 10% are attained for these fibers. The elastic properties of sPP mainly originate from a reversible crystal−crystal phase transition which occurs during the stretching and when the tension is removed. The stable crystalline modification of sPP with chains in the helical conformation transfo...
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