Polymorphic Transitions Induced by Annealing in Stretched Fibers of Syndiotactic Polypropylene

A study of the polymorphic transitions induced by annealing of fibers of syndiotactic polypropylene initially in the trans-planar form III and in the mesomorphic form is reported. Both the trans-planar form III and the mesomorphic form transform by annealing at high temperatures into a mixture of crystals of the antichiral helical form I and the isochiral helical form II. Annealing of the trans-planar mesomorphic form produces simultaneous formation of the isochiral form II and the antichiral form I, already at low temperatures (40-60 °C). The relative amounts of forms I and II increase with increasing the annealing temperature and fibers containing similar amounts of crystals of forms I and II are obtained at high temperature (120-130 °C). Fibers of the crystalline trans-planar form III transform mainly into the antichiral form I by annealing up to 80 °C. At higher temperatures also the isochiral form II develops, and fibers in a mixture of similar amounts of crystals of both helical forms are obtained at 120-130 °C. These results indicate that the formation of the isochiral helical form II is not necessarily linked to the presence of the trans-planar mesomorphic form, as suggested in the literature, but it can be obtained also starting from the crystalline form III. These experimental data are not consistent with the hypothesis reported in the literature that intermediate mesophases are necessarily precursors of the crystallization of the isochiral structure of syndiotactic polypropylene.