Polymorphism of oriented syndiotactic polypropylene

Two syndiotactic polypropylene (sPP) samples were obtained by quenching the melt in a bath at 0 degrees C, for 1 min (sample 1A) and 3 days (sample 1B). In these conditions sample 1A crystallized in the disordered form I, having the chains in helical conformation, whereas sample 1B formed a mesophase with the chains in trans-planar conformation, as evident by X-rays and FTIR analysis. Samples 1A and 1B were drawn at room temperature, obtaining fibers with a draw ratio between 4 and 7, which were analyzed under tension. The fibers, with a draw ratio 4 and 5, obtained from sample 1A, show a well oriented helical form and a fraction of oriented trans-planar form. When the draw ratio reaches the value of 6 and 7, the crystalline form III with chains in trans-planar conformation is obtained. At variance, the fibers obtained from sample 1B show a progressive orientation of the trans-planar mesophase, and the appearance of form III at draw ratio of 6 and 7. Therefore, the fibers drawn at lambda = 6 and 7, coming from the two samples 1A and 1B, are very similar, as shown by X-rays and FTIR analysis. Despite this similarity, upon releasing the tension, they undergo different transformations, going toward the oriented helical form or the oriented trans-planar mesophase, respectively.