Structural changes during annealing of the crystalline helical form of syndiotactic polypropylene

A sample of syndiotactic polypropylene was quenched from the melt at 0°C and kept at this temperature for 1 min. Upon heating to room temperature, it shows the disordered crystalline form I, with the chains in helical conformation. In addition, it also contains a fraction of trans-planar mesophase, formed during the short time at 0°C. It was annealed at increasing temperatures, up to 120°C, and the structural changes analyzed by x-rays and FTIR measurements. Annealing up to 90°C results in a progressive decrease in the trans-planar mesophase, and also the melting of the more disordered and smaller helical crystals. In this range, the structural parameters such as crystallinity and crystal perfection smoothly increase, whereas a steeper increase is observed after 90°C when the disordered phases have disappeared. Afterwards, the formation of the more ordered form I, characterized by a peak at 18.8° of 2θ in the x-ray diffractogram, becomes possible. The content of the mesophase was determined by measurements of sorption of a vapor at low activity; it was 30% in the initial sample. This value decreased up to the annealing temperature of 90°C, where the value of the crystallinity was found coincident with the fraction of impermeable phase derived by sorption. In addition, transport properties confirm that the disappearance of the mesophase allows the formation of the ordered crystalline form I.