| Abstract
| - An analysis of the mechanical properties and a detailed structural study of samples ofsyndiotactic polypropylene (sPP) are presented in order to clarify the origin of the elastic behavior ofsPP. Samples of sPP having different stereoregularity and showing different polymorphic behavior havebeen analyzed. The present study allows to establish that the elastic properties of unoriented specimensof 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 valuesless than 10% are attained for these fibers. The elastic properties of sPP mainly originate from a reversiblecrystal−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 transforms into thecrystalline form with chains in the trans-planar conformation (form III) by stretching. Upon release ofthe stress, the trans-planar form transforms again into the helical form. The recovery of the dimensionof the fibers is associated with the reversible strain of the chain conformation and, hence, of the crystallinelattice. From this analysis it is possible to conclude that, while the driving force which induces the recoveryof the initial dimensions in common elastomers upon the release of the stress is mainly entropic, in thecase of sPP it is basically linked to the enthalpy gain achieved when the sample is relaxed, which involvesa crystal−crystal phase transition from the metastable form III to the more stable helical modification(form II).
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