Origin of the elastic behavior of syndiotactic polypropylene

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 transforms into the crystalline form with chains in the trans-planar conformation (form III) by stretching. Upon release of the stress, the trans-planar form transforms again into the helical form. The recovery of the dimension of the fibers is associated with the reversible strain of the chain conformation and, hence, of the crystalline lattice. From this analysis it is possible to conclude that, while the driving force which induces the recovery of the initial dimensions in common elastomers upon the release of the stress is mainly entropic, in the case of sPP it is basically linked to the enthalpy gain achieved when the sample is relaxed, which involves a crystal-crystal phase transition from the metastable form III to the more stable helical modification (form II).