Temperature and orientation induced polymorphic behavior of syndiotactic polypropylene

Syndiotactic polypropylene (sPP) was rapidly quenched from the melt to room temperature, obtaining films in the disordered form I, with the chains in helical conformation. A significant portion of chains in the trans-planar mesophase was found, as evidenced by X-ray diffraction and FTIR spectroscopy. The quenched films were drawn to a draw ratio of 6 at 25, 60, and 90°C and analyzed before and after releasing the tension. A large shrinkage was observed unhooking the fibers, and this parameter, expressed as (lmax - l f)/lmax, almost linearly decreased on increasing the drawing temperature. The crystalline phase composition was investigated by diffraction techniques in both the fixed and relaxed fibers. The fully extended fibers drawn at all the investigated temperatures contain a significant amount of the trans-planar mesophase while the fraction of the crystalline trans-planar form III decreases on increasing the drawing temperature and it is absent in the fiber drawn at 90°C. Upon relaxation, we found evidence that the trans-planar mesophase in the fixed fiber crystallizes into form II while form III in the fixed fibers tends to transform into the trans-planar mesophase. An infrared and dynamic-mechanical analysis on the fibers was also performed to confirm the structural results. The analysis evidenced that on increasing the drawing temperature, the mesophase in the fixed fibers more and more transforms into the helical form II. On the other hand trans-planar chains remain in the amorphous fraction of all the relaxed fibers. The experimental results allow one to better clarify the role of the trans-planar mesophase in phase transitions of sPP and, taking into account also literature data, to propose a more detailed scheme of transformations between different polymorphs of this polymer.