Development of form II crystallinity in oriented syndiotactic polypropylene: Role of the trans-planar mesophase

Films of syndiotactic polypropylene (sPP) elongated at room temperature up to a draw ratio λ = 6 showed the trans-planar form III with some trans-planar mesophase. Upon releasing tension, we obtained fibers with a high content of trans-planar mesophase and a small fraction of helical form II crystals. Released fibers were annealed at increasing temperatures up to 130°C, and the polymorphic behavior was investigated by X-ray and FTIR. Between 60 and 80°C, an increase of the helical forms I and II, both of them highly oriented, occurs, while the trans-planar mesophase practically disappears. Successive annealing at higher temperature brings further increase of both helical modifications until, around 130°C, significant percentages of form II melt and recrystallize in form I. The unrelaxed λ = 6 fiber was also annealed at 100°C, developing significant form II fractions along with form III and some form I. Two more sets of fibers were obtained under conditions in which the trans-planar mesophase was not present. In both cases, upon annealing at constant strain, no formation of the helical form II was observed and only a mixture of the trans-planar form III and the helical form I developed. The described observations suggest that, in oriented sPP samples, upon relaxing or annealing at temperatures below 100°C, the pseudohexagonal trans-planar mesophase plays a mayor role as precursor for crystallization of the chiral form II. This appears consistent with recently proposed analyses of chiral crystallization of helical polymers.