Tensile properties of highly syndiotactic polypropylene

The tensile properties and uniaxial drawing of highly syndiotactic polypropylene (s-PP) have been studied. The results were compared to those of isotactic polypropylene (i-PP) having a comparable molecular weight, molecular-weight distribution and stereoregularity. S-PP and i-PP of different initial morphologies, prepared by crystallization from the melt and solution, were drawn by solid-state coextrusion and tensile drawing. The structure and properties of drawn films have been characterized by wide- and small-angle Xray diffraction, stress-strain behaviour, etc. The maximum achievable draw ratio (DR) for s-PP was 5-10, depending on the initial morphologies, and significantly lower than that for i-PP (DR less than or equal to 30). The poor ductility of s-PP was ascribed to the absence of a crystalline relaxation, which was clearly observed in the highly drawable i-PP by dynamic mechanical tests. The tensile properties of drawn films increased with DR for each of the PPs. At a given DR, however, the modulus was remarkably lower for s-PP than for i-PP, yet the tensile strengths were not significantly different. The maximum tensile modulus and strength of s-PP achieved in this work were 3.0 and 0.33 GPa, respectively. These values were remarkably lower than those (20 and 0.60 GPa, respectively) achieved for an i-PP having comparable molecular characteristics, reflecting the low crystal modulus, drawability and crystallinity of s-PP compared to those of i-PP. The structural change on drawing of s-PP was also discussed.