Mechano-reversible physical aging of elastic oriented syndiotactic polypropylene

Syndiotactic polypropylene (sPP), obtained at 0°C in the trans-planar mesophase, was drawn at room temperature up to λ = 6, and left at room temperature for 1 year with fixed or relaxed ends. Data analysis allowed the clarification of the structure of the crystalline phases and their transformations during the aging. In both oriented samples similar structural changes were observed, although they were due to different aging mechanisms. The physical aging led to the crystallization in both samples of an oriented helical form, due to a partial transformation of the mesophase and of the amorphous phase. In the oriented sample aged with fixed ends, a small fraction of the crystalline trans-planar form III became stable even by releasing the tension after 1 year. This last sample did not undergo the large shrinkage, always observed by unloaded sPP after drawing, and therefore was no more elastic. Also, the sample aged with free ends for 1 year showed a reduced elasticity in terms of both dissipated energy and permanent set. However, after a new deformation up to λ = 6, the fiber recovered its previous elasticity. Indeed a mechano-reversibility was apparent for the oriented elastic sample of sPP aged at room temperature with free ends, leading to a renewed elasticity