Stress-Induced Phase Transitions in Syndiotactic Propene−Butene Copolymers

A study of the structural transformations occurring during tensile deformation of syndiotactic propylene−butene copolymers (sPPBu) is reported. Melt-crystallized samples of sPPBu copolymers with butene content lower than nearly 70 mol% crystallize in disordered modifications of the helical form I of syndiotactic polypropylene (sPP). The polymorphic transformation of the helical form I into the trans-planar form III, typical of sPP, occurs by stretching sPPBu copolymers with butene content lower than 20 mol%. This transformation is inhibited by the presence of higher butene concentrations. The trans-planar form III transforms back into the C-centered helical form II of sPP by releasing the tension in sPPBu samples with butene contents lower than 4 mol%, and into the helical form I for butene concentrations higher than 4 mol%. For sPPBu copolymers with butene content higher than 20 mol% and lower than 69 mol% only well oriented crystals of the helical form I are obtained by stretching, and no structural transitions occur by releasing the tension. Unoriented samples of copolymers with butene concentration higher than 69 mol% are crystallized in the C-centered helical form I of syndiotactic polybutene (sPB), which transforms by stretching at high deformation into the new form I′ of sPB, characterized by the packing of chains in helical conformation similar to the B-centered form I of sPP. This new form is metastable and transforms back into the more stable C-centered form I of sPB by releasing the tension.