Elastic properties and polymorphic behavior of fibers of syndiotactic polypropylene at different temperatures

An analysis of the elastic properties of syndiotactic polypropylene at different temperatures is reported. Fibers stretched at low temperatures are in the trans-planar form III, which transforms into the helical form II upon the release of the tension, whereas fibers stretched at high temperatures are in the stable helical form I. The mechanical analysis of these fibers has shown that only at room temperatures the specimens show good elastic properties. The elastic recovery of the samples is in part lost also at room temperature in annealed fibers, which are in the stable form I, in both the stretched and unstrained states. When the structural transition from the trans-planar form III into the helical form occurs upon removing the tension, a nearly total elastic recovery is observed. Only a partial elastic recovery is instead observed when this transition is absent. The relative amount of crystalline phase in unoriented films and in stretched and stress-relaxed fibers is rather constant (approximate to40-43%) and does not depend on the stretching temperature in the range 25-80 degreesC. These data indicate that the elastic behavior of syndiotactic polypropylene is in part linked to the enthalpy gain achieved when the tension is removed, due to the metastability, in the unstrained state, of the trans-planar form, which transforms into the stable helical form. On the other hand, also an entropic factor must play a role, due to the entangled amorphous chains which experience a reversible conformational transition between the disordered (coil) and extended conformations during the stretching and relaxing process.